Orthogonal frequency-division multiplexing (OFDM) is a digital multi-carrier modulation technique that partitions the available bandwidth into multiple orthogonal subcarriers. Each subcarrier is modulated with a conventional modulation scheme at a low symbol rate, maintaining high data rates over the entire bandwidth. OFDM has advantages over single-carrier schemes in coping with severe channel conditions without complex equalization filters. It is used widely in digital television and audio broadcasting, wireless networks including WiFi, and mobile phone networks including 4G LTE.
IRJET- Orthogonal Frequency Division Multiplexing (OFDM) based Uplink Multipl...IRJET Journal
This document discusses Orthogonal Frequency Division Multiplexing (OFDM) for uplink transmission over different channel conditions. It begins by introducing OFDM and how it splits data across multiple narrowband carriers to mitigate intersymbol interference. It then discusses using QPSK modulation with OFDM. Next, it analyzes OFDM performance over additive white Gaussian noise (AWGN) channels and different fading channels like Rayleigh and Rician fading. It finds that BER decreases with increasing SNR in AWGN channels, and the amount of fading impacts BER in fading channels. Simulation results using MATLAB show the effect of different channels on OFDM signals.
Popular Interview Wireless Question with AnswerVARUN KUMAR
Favourable propagation refers to the orthogonality among vector-valued wireless channels that can maximize total system throughput. It occurs when the interference terms in the channel capacity equation, which involve the product of different channel coefficients, approach zero. Time division duplexing (TDD) is more complex than frequency division duplexing (FDD) due to hardware mismatches across base stations and user equipment for uplink and downlink channels. Channel reciprocity can be achieved through FDD if the uplink and downlink carrier frequencies are nearly equal, but not if they are significantly different.
This document discusses OFDM (Orthogonal Frequency Division Multiplexing), including its basic idea of using multiple narrowband subcarriers instead of a single wideband carrier. OFDM has advantages like being robust to multipath interference and narrowband interference. It is sensitive to issues like frequency offset and phase noise. OFDM uses orthogonal subcarriers to maximize spectral efficiency and allows overlapping bands. Guard intervals and cyclic prefixes help mitigate inter-symbol interference. OFDM is used in standards like DSL, 802.11a, DAB and DVB. Simulation results show the theoretical and simulated bit error rates for OFDM with BPSK modulation.
- The document discusses wireless channel propagation and fading. It covers topics like large-scale fading (path loss and shadowing), small-scale fading (time-selective and frequency-selective fading), and statistical characterization of fading channels.
- Small-scale fading is caused by multipath propagation and results in rapid fluctuations in the strength of the received signal over short periods of time or travel distances. It can be time-selective or frequency-selective depending on delay spread and Doppler spread.
- Common distributions for modeling fading amplitudes are Rayleigh for non-line-of-sight environments and Rician when there is a dominant line-of-sight path. The document presents models for generating both Rayleigh and Rician fading
This document summarizes a student project on OFDM transmitters and receivers. It includes an introduction to OFDM that describes its use of orthogonal subcarriers. It also compares single carrier modulation to multi-carrier modulation using OFDM. The document outlines the basic OFDM transmitter and receiver block diagrams. It discusses the constellation mapper, IFFT block, cyclic prefix, and design approaches for these blocks. Simulation results are presented comparing transmitted and received signals. BER performance is evaluated for different modulation schemes like QPSK and QAM. The document concludes that OFDM provides high bandwidth efficiency and overcomes interference through the IFFT and cyclic prefix.
This document discusses Orthogonal Frequency Division Multiple Access (OFDMA) and related topics. It provides information on:
- The structure and principles of OFDM symbols, including how they are constructed from orthogonal subcarriers modulated by parallel data streams.
- Challenges caused by carrier frequency offsets and sampling time offsets at the receiver, and how symbol synchronization is needed.
- Channel estimation techniques in OFDMA systems, including least squares estimation, block-type and comb-type pilot structures, and linear/second-order interpolation methods.
- How OFDMA exploits multiuser diversity by adaptively allocating subcarriers to users experiencing favorable channel conditions.
This document provides an overview of Orthogonal Frequency Division Multiplexing (OFDM), Orthogonal Frequency Division Multiple Access (OFDMA) and Scalable OFDMA (SOFDMA). It describes how OFDM divides available bandwidth into multiple orthogonal subcarriers to combat multipath interference in wireless channels. OFDMA further divides subcarriers to enable multiple access by multiple users. SOFDMA, defined in IEEE 802.16e, allows dynamic allocation of subcarriers for scalable bandwidth allocation to users.
What is the main difference between single carrier and ofdm yahoo! answersen_maruf78
Single carrier (SC) transmission has a lower peak-to-average power ratio than orthogonal frequency-division multiplexing (OFDM). SC frequency-division multiple access (SC-FDMA) divides the frequency band into subsets, assigning each user a different subset, whereas OFDM divides data into multiple parallel streams and assigns each stream a unique subcarrier. Both SC-FDMA and OFDM use guard intervals and fast Fourier transforms to combat interference, but SC-FDMA applies these on the transmitter side while OFDM applies them on the receiver side. SC-FDMA has a lower peak-to-average power ratio than OFDM, making it more suitable for applications with power constraints like uplink mobile communications.
IRJET- Orthogonal Frequency Division Multiplexing (OFDM) based Uplink Multipl...IRJET Journal
This document discusses Orthogonal Frequency Division Multiplexing (OFDM) for uplink transmission over different channel conditions. It begins by introducing OFDM and how it splits data across multiple narrowband carriers to mitigate intersymbol interference. It then discusses using QPSK modulation with OFDM. Next, it analyzes OFDM performance over additive white Gaussian noise (AWGN) channels and different fading channels like Rayleigh and Rician fading. It finds that BER decreases with increasing SNR in AWGN channels, and the amount of fading impacts BER in fading channels. Simulation results using MATLAB show the effect of different channels on OFDM signals.
Popular Interview Wireless Question with AnswerVARUN KUMAR
Favourable propagation refers to the orthogonality among vector-valued wireless channels that can maximize total system throughput. It occurs when the interference terms in the channel capacity equation, which involve the product of different channel coefficients, approach zero. Time division duplexing (TDD) is more complex than frequency division duplexing (FDD) due to hardware mismatches across base stations and user equipment for uplink and downlink channels. Channel reciprocity can be achieved through FDD if the uplink and downlink carrier frequencies are nearly equal, but not if they are significantly different.
This document discusses OFDM (Orthogonal Frequency Division Multiplexing), including its basic idea of using multiple narrowband subcarriers instead of a single wideband carrier. OFDM has advantages like being robust to multipath interference and narrowband interference. It is sensitive to issues like frequency offset and phase noise. OFDM uses orthogonal subcarriers to maximize spectral efficiency and allows overlapping bands. Guard intervals and cyclic prefixes help mitigate inter-symbol interference. OFDM is used in standards like DSL, 802.11a, DAB and DVB. Simulation results show the theoretical and simulated bit error rates for OFDM with BPSK modulation.
- The document discusses wireless channel propagation and fading. It covers topics like large-scale fading (path loss and shadowing), small-scale fading (time-selective and frequency-selective fading), and statistical characterization of fading channels.
- Small-scale fading is caused by multipath propagation and results in rapid fluctuations in the strength of the received signal over short periods of time or travel distances. It can be time-selective or frequency-selective depending on delay spread and Doppler spread.
- Common distributions for modeling fading amplitudes are Rayleigh for non-line-of-sight environments and Rician when there is a dominant line-of-sight path. The document presents models for generating both Rayleigh and Rician fading
This document summarizes a student project on OFDM transmitters and receivers. It includes an introduction to OFDM that describes its use of orthogonal subcarriers. It also compares single carrier modulation to multi-carrier modulation using OFDM. The document outlines the basic OFDM transmitter and receiver block diagrams. It discusses the constellation mapper, IFFT block, cyclic prefix, and design approaches for these blocks. Simulation results are presented comparing transmitted and received signals. BER performance is evaluated for different modulation schemes like QPSK and QAM. The document concludes that OFDM provides high bandwidth efficiency and overcomes interference through the IFFT and cyclic prefix.
This document discusses Orthogonal Frequency Division Multiple Access (OFDMA) and related topics. It provides information on:
- The structure and principles of OFDM symbols, including how they are constructed from orthogonal subcarriers modulated by parallel data streams.
- Challenges caused by carrier frequency offsets and sampling time offsets at the receiver, and how symbol synchronization is needed.
- Channel estimation techniques in OFDMA systems, including least squares estimation, block-type and comb-type pilot structures, and linear/second-order interpolation methods.
- How OFDMA exploits multiuser diversity by adaptively allocating subcarriers to users experiencing favorable channel conditions.
This document provides an overview of Orthogonal Frequency Division Multiplexing (OFDM), Orthogonal Frequency Division Multiple Access (OFDMA) and Scalable OFDMA (SOFDMA). It describes how OFDM divides available bandwidth into multiple orthogonal subcarriers to combat multipath interference in wireless channels. OFDMA further divides subcarriers to enable multiple access by multiple users. SOFDMA, defined in IEEE 802.16e, allows dynamic allocation of subcarriers for scalable bandwidth allocation to users.
What is the main difference between single carrier and ofdm yahoo! answersen_maruf78
Single carrier (SC) transmission has a lower peak-to-average power ratio than orthogonal frequency-division multiplexing (OFDM). SC frequency-division multiple access (SC-FDMA) divides the frequency band into subsets, assigning each user a different subset, whereas OFDM divides data into multiple parallel streams and assigns each stream a unique subcarrier. Both SC-FDMA and OFDM use guard intervals and fast Fourier transforms to combat interference, but SC-FDMA applies these on the transmitter side while OFDM applies them on the receiver side. SC-FDMA has a lower peak-to-average power ratio than OFDM, making it more suitable for applications with power constraints like uplink mobile communications.
Deterministic MIMO Channel Capacity
• CSI is Known to the Transmitter Side
• CSI is Not Available at the Transmitter Side
Channel Capacity of Random MIMO Channels
This document summarizes design issues in OFDM systems. It discusses the OFDM modem block diagram and signal description. It then focuses on key receiver design issues like time and frequency synchronization through techniques like frequency offset correction, carrier phase tracking, and channel equalization. It also addresses signal dynamic range issues and how they are handled in standards like IEEE 802.11a/g. The document provides illustrations of effects of different impairments and solutions used in practical OFDM systems.
This document provides information about Quadrature Amplitude Modulation (QAM). It begins with an objective to analyze the bit error rate performance of M-QAM modulation over fading channels. It then introduces QAM, explaining that it modulates data onto carriers using both amplitude and phase variations. The document outlines the basic working principles of QAM modulation and demodulation. It discusses constellation diagrams and provides examples of 16-QAM, 64-QAM, and 256-QAM constellations. The document notes some advantages of QAM, such as high data rates, and some disadvantages, such as increased susceptibility to noise in higher order QAM. It concludes that QAM is widely used in radio communications due to its ability to increase data
This document provides an overview of linear modulation techniques used in communication systems. It begins by defining modulation as the process of varying one waveform (the carrier) according to the characteristics of another waveform (the message signal). The key parameters that can be varied for a sinusoidal carrier are amplitude, phase, and frequency.
The document then discusses the need for modulation, noting that it allows information to be transmitted efficiently by converting baseband signals into narrowband signals suitable for transmission. Modulation provides benefits like ease of radiation from antennas, efficient use of channel bandwidth, multiplexing of multiple signals, and frequency assignment for different stations. It can also improve the signal-to-noise ratio at the receiver under some schemes.
The
PAPR REDUCTION OF OFDM SIGNAL BY USING COMBINED HADAMARD AND MODIFIED MEU-LAW...IJCNCJournal
Orthogonal frequency division multiplexing (OFDM) is a technique which gives high quality of service (QOS) to the users by mitigating the fading signals as well as high data rates in multimedia services. However, the peak-to-average power ratio (PAPR) is a technical challenge that reduces the efficiency of RF power amplifiers. In this paper, we propose the combined Hadamard transform and modified meu-law companding transform method in order to lessen the effects of the peak-to-average power ratio of the
OFDM signal. Simulation results show that the proposed scheme reduces PAPR compared to other companding techniques as well as the Hadamard transform technique when used on its own.
Analog and digital modulation formats of optical fiber communication within a...IAEME Publication
The document discusses various analog and digital modulation formats used in optical fiber communication systems for transmitting data at rates up to and beyond 100 Gb/s. It provides a classification of analog modulation techniques such as AM/IM, FM/IM, and PM and digital modulation techniques such as OOK, BPSK, DPSK, QPSK, DQPSK, RZ-DQPSK, RZ-DPSK-3ASK and PM-QPSK. For each technique, it summarizes advantages, disadvantages, spectral efficiency, tolerance to impairments, and application areas. It concludes that advanced digital modulation formats like PM-QPSK provide higher spectral efficiency and tolerance to transmission impairments compared to analog modulation, enabling multi-Tb/
Multiplexing generally refers to independent signals, those produced by different sources. SO it is a question of how to share the spectrum with these users. In OFDM the question of multiplexing is applied to independent signals but these independent signals are a sub-set of the one main signal.
In OFDM the signal itself is first split into independent channels, modulated by data and then re-multiplexed to create the OFDM carrier.
OFDM is a combination of modulation and multiplexing.
A Wideband Bandpass Filter by Integrating a Section of High Pass HMSIW with a...fanfan he
This document presents a wideband bandpass filter design based on integrating a half mode substrate integrated waveguide (HMSIW) high pass filter with a microstrip lowpass filter. The HMSIW provides the lower cutoff frequency of 7.1 GHz, while the microstrip lowpass filter provides the upper cutoff of 16.6 GHz, resulting in a predicted passband of 7.1-16.6 GHz (74% bandwidth). A prototype was fabricated on a dielectric substrate and measured results showed good agreement with simulations, with an insertion loss of less than 1 dB across the passband and greater than 30 dB rejection outside the passband.
Multiplexing is a technique where multiple users can use the same medium simultaneously with minimal interference. There are four main types of multiplexing: space division, frequency division, time division, and code division.
Time division multiplexing involves all senders using the same frequency but transmitting at different time intervals with guard spaces between transmissions to avoid interference. Precise synchronization is required between users.
Frequency shift keying and phase shift keying are digital modulation techniques used to convert digital data to analog signals for transmission. In frequency shift keying, two different frequencies represent binary 1 and 0, while in phase shift keying a 180 degree phase shift represents a change between 1 and 0.
Performance and Analysis of OFDM Signal Using Matlab SimulinkIJMER
Multi-carrier modulation is an attractive technique for fourth generation .OFDM is based on
multicarrier modulation technique. In OFDM system the bit stream is divided into many different sub
channels. An efficient and distortionless scheme for peak power reduction in OFDM is proposed. In this
paper, a set of mapping where the actual transmit signal is selected. From this set of signal reduced
PAPR. Simulation results are shown. The lowest PAPR is compared with conventional work. It is also
compared BER to SNR and best result is achieved.
This document discusses modeling wireless communication systems using MATLAB. It covers:
- Characterizing the wireless channel from physics of propagation to multi-path fading channels.
- Statistical characterization of channels including Doppler spectrum, delay spread, coherence time and bandwidth.
- Simulating multi-path fading channels in MATLAB and modeling path loss.
- Explaining how multi-path propagation leads to signal distortion and is equivalent to undesired filtering.
RF testing has remained hype for most of us. But seriously it is not so. It can be very interesting and one can develop a lot of interest in this if given an opportunity.
In this paper, authors have started with the some basic concepts of radio engineering which we studied in engineering and built upon these concepts to use in practical applications.
We have also described the basic principles of Signal Analyzer and Signal Generator which are the most common test tools used for any radio testing.
Design Ofdm System And Remove Nonlinear Distortion In OFDM Signal At Transmit...Rupesh Sharma
although OFDM seems to be a solution to keep up with
the demand of increasing data rates, it has some drawbacks.
Sensitivity to high PAPR is the most significant of these
drawbacks. The main objective of this paper was to investigate
and document the effects of PAPR on the performance of OFDM
based digital communications under different channel conditions.
A step-by-step approach was adopted in order to achieve the
objective of this paper. The first step is to provide a basic
background on the principles of OFDM. The reasons for the
PAPR and a theoretical analysis of these effects on OFDM
systems are documented. The OFDM system has a high peak-toaverage
power ratio (PAPR) that can cause unwanted saturation
in the power amplifiers, leading to in-band distortion and out-ofband
radiation. To be able to observe the system behavior, the
simulation results for different channel models are presented in
graphical form. Next, the simulation results obtained in this work
are compared to the simulation results reported in related studies
Orthogonal frequency-division multiplexing (OFDM)
[1] is a method of encoding digital data on multiple carrier
frequencies. OFDM[1] has developed into a popular scheme
for wideband digital communication, whether wireless or
over copper wires, used in applications such as digital television
and audio broadcasting, DSL Internet access, wireless networks,
powerline networks, and 4G mobile communications. In the
Several wireless standards such as IEEE 802.11a[2] and
HiperLAN2[3].The orthogonality of the subcarriers is no longer
maintained which results in ICI (Inter carrier Interference)[4]
.ICI reduction techniques achieve a better SNR and BER in
OFDM at zero phase noise variance . This technique will use a
large number of closely spaced orthogonal subcarriers to avoid
phase noise. It provides high data rates with sufficient robustness
to radio channel damages. A major problem in OFDM is carrier
frequency offset error between the transmitted and received
signals. Due to this the orthogonality of the subcarriers is no
longer maintained which results in ICI (Inter carrier
Interference). In this paper, we used the ICI self-cancellation
technique and reduced the ICI and improved the BER and SNR
we are also calculate the SNR=15db and 20db at different phase
noise variance.
The document discusses OFDM (Orthogonal Frequency Division Multiplexing) for wireless communication. It introduces OFDM as a modulation technique that divides the available bandwidth into multiple orthogonal subcarriers. This allows for overlapping subchannels and improves spectral efficiency compared to conventional FDM. The document then covers OFDM system modeling, generation of subcarriers, fading effects, use of guard times and cyclic extensions to mitigate multipath interference, windowing techniques, and factors to consider when choosing OFDM system parameters.
This document discusses VLSI implementation of orthogonal frequency division multiplexing (OFDM). It provides background on OFDM, explaining that it consists of multiple closely spaced carriers that can achieve high data transmission rates with wide bandwidths. The document outlines the key components of an OFDM transceiver including scrambling, interleaving, constellation mapping, IFFT/FFT processing, and parallel-to-serial conversion. It describes the hardware implementation of these components and advantages of OFDM such as robustness to interference and insensitivity to timing errors.
This document discusses frequency coordination between UMTS and GSM systems operating at 900 MHz. It analyzes the interference between UMTS900 and GSM900 networks that will need to coexist during the transition from GSM to UMTS spectrum. Laboratory tests were conducted using commercial UMTS and GSM equipment to characterize transmitter and receiver performance under interference conditions and determine the required guard band between the networks based on acceptable sensitivity degradation levels. The limiting factor was found to be interference from GSM mobile stations to UMTS base stations, and a guard band of 4.2 MHz would allow satisfactory system performance.
This document analyzes frequency coordination between UMTS900 and GSM900 systems operating at 900 MHz. It summarizes lab tests conducted on commercial UMTS900 and GSM900 equipment to measure interference levels and assess the impact of mutual interference. The limiting factor is found to be interference from GSM mobile stations to UMTS Node B receivers. With a frequency offset of 2.2 MHz or more, satisfactory system performance can be achieved even when as little as 4.2 MHz of GSM spectrum is cleared for UMTS use.
4g LTE and LTE-A for mobile broadband-notePei-Che Chang
This document discusses the basic principles of OFDM (Orthogonal Frequency Division Multiplexing) transmission. It covers several key topics:
1) OFDM uses multiple subcarriers to transmit data in parallel. The subcarriers are spaced closely together with minimal spacing between them.
2) OFDM modulation and demodulation can be implemented efficiently using IDFT/DFT (IFFT/FFT) processing.
3) Cyclic prefixes are added to combat inter-symbol interference from multipath channels. This preserves subcarrier orthogonality.
4) With a cyclic prefix, the channel appears flat on each subcarrier, allowing one-tap frequency domain equalization. Channel estimation is done using reference symbols.
Performance Analysis of OFDM in Combating Multipath FadingIOSR Journals
Mobile Communication system has been on high rampage for high data transmission over wireless
medium with various challenges caused by the transmission Channel. OFDM is been discovered in recent years
to deal with this problems because of its ability to elegantly cope with multipath interference. This paper
investigates the performance of different modulation schemes using M-ary Phase Shift Keying (M-PSK) and Mary
Quadrature Amplitude Modulation (M-QAM) in information transmission with OFDM technique over Ideal
channel AWGN and worst channel Rayleigh Fading channel in terms of Bits Error Rate (BER). Analysis was
made for different types of modulation schemes BPSK, QPSK, 4-QAM and 16-QAM gray coded bit mapping.
Also, a feasibility of OFDM been used to combat multipath fading was analyzed with comparison between a
single carrier technique and OFDM multicarrier technique. Variation between SNR results with respect to BER
is plotted to show the trade off differences between the modulation schemes with the result showing that OFDM
allows data transmission with minimal error over fading channel than a Single Carrier
Deterministic MIMO Channel Capacity
• CSI is Known to the Transmitter Side
• CSI is Not Available at the Transmitter Side
Channel Capacity of Random MIMO Channels
This document summarizes design issues in OFDM systems. It discusses the OFDM modem block diagram and signal description. It then focuses on key receiver design issues like time and frequency synchronization through techniques like frequency offset correction, carrier phase tracking, and channel equalization. It also addresses signal dynamic range issues and how they are handled in standards like IEEE 802.11a/g. The document provides illustrations of effects of different impairments and solutions used in practical OFDM systems.
This document provides information about Quadrature Amplitude Modulation (QAM). It begins with an objective to analyze the bit error rate performance of M-QAM modulation over fading channels. It then introduces QAM, explaining that it modulates data onto carriers using both amplitude and phase variations. The document outlines the basic working principles of QAM modulation and demodulation. It discusses constellation diagrams and provides examples of 16-QAM, 64-QAM, and 256-QAM constellations. The document notes some advantages of QAM, such as high data rates, and some disadvantages, such as increased susceptibility to noise in higher order QAM. It concludes that QAM is widely used in radio communications due to its ability to increase data
This document provides an overview of linear modulation techniques used in communication systems. It begins by defining modulation as the process of varying one waveform (the carrier) according to the characteristics of another waveform (the message signal). The key parameters that can be varied for a sinusoidal carrier are amplitude, phase, and frequency.
The document then discusses the need for modulation, noting that it allows information to be transmitted efficiently by converting baseband signals into narrowband signals suitable for transmission. Modulation provides benefits like ease of radiation from antennas, efficient use of channel bandwidth, multiplexing of multiple signals, and frequency assignment for different stations. It can also improve the signal-to-noise ratio at the receiver under some schemes.
The
PAPR REDUCTION OF OFDM SIGNAL BY USING COMBINED HADAMARD AND MODIFIED MEU-LAW...IJCNCJournal
Orthogonal frequency division multiplexing (OFDM) is a technique which gives high quality of service (QOS) to the users by mitigating the fading signals as well as high data rates in multimedia services. However, the peak-to-average power ratio (PAPR) is a technical challenge that reduces the efficiency of RF power amplifiers. In this paper, we propose the combined Hadamard transform and modified meu-law companding transform method in order to lessen the effects of the peak-to-average power ratio of the
OFDM signal. Simulation results show that the proposed scheme reduces PAPR compared to other companding techniques as well as the Hadamard transform technique when used on its own.
Analog and digital modulation formats of optical fiber communication within a...IAEME Publication
The document discusses various analog and digital modulation formats used in optical fiber communication systems for transmitting data at rates up to and beyond 100 Gb/s. It provides a classification of analog modulation techniques such as AM/IM, FM/IM, and PM and digital modulation techniques such as OOK, BPSK, DPSK, QPSK, DQPSK, RZ-DQPSK, RZ-DPSK-3ASK and PM-QPSK. For each technique, it summarizes advantages, disadvantages, spectral efficiency, tolerance to impairments, and application areas. It concludes that advanced digital modulation formats like PM-QPSK provide higher spectral efficiency and tolerance to transmission impairments compared to analog modulation, enabling multi-Tb/
Multiplexing generally refers to independent signals, those produced by different sources. SO it is a question of how to share the spectrum with these users. In OFDM the question of multiplexing is applied to independent signals but these independent signals are a sub-set of the one main signal.
In OFDM the signal itself is first split into independent channels, modulated by data and then re-multiplexed to create the OFDM carrier.
OFDM is a combination of modulation and multiplexing.
A Wideband Bandpass Filter by Integrating a Section of High Pass HMSIW with a...fanfan he
This document presents a wideband bandpass filter design based on integrating a half mode substrate integrated waveguide (HMSIW) high pass filter with a microstrip lowpass filter. The HMSIW provides the lower cutoff frequency of 7.1 GHz, while the microstrip lowpass filter provides the upper cutoff of 16.6 GHz, resulting in a predicted passband of 7.1-16.6 GHz (74% bandwidth). A prototype was fabricated on a dielectric substrate and measured results showed good agreement with simulations, with an insertion loss of less than 1 dB across the passband and greater than 30 dB rejection outside the passband.
Multiplexing is a technique where multiple users can use the same medium simultaneously with minimal interference. There are four main types of multiplexing: space division, frequency division, time division, and code division.
Time division multiplexing involves all senders using the same frequency but transmitting at different time intervals with guard spaces between transmissions to avoid interference. Precise synchronization is required between users.
Frequency shift keying and phase shift keying are digital modulation techniques used to convert digital data to analog signals for transmission. In frequency shift keying, two different frequencies represent binary 1 and 0, while in phase shift keying a 180 degree phase shift represents a change between 1 and 0.
Performance and Analysis of OFDM Signal Using Matlab SimulinkIJMER
Multi-carrier modulation is an attractive technique for fourth generation .OFDM is based on
multicarrier modulation technique. In OFDM system the bit stream is divided into many different sub
channels. An efficient and distortionless scheme for peak power reduction in OFDM is proposed. In this
paper, a set of mapping where the actual transmit signal is selected. From this set of signal reduced
PAPR. Simulation results are shown. The lowest PAPR is compared with conventional work. It is also
compared BER to SNR and best result is achieved.
This document discusses modeling wireless communication systems using MATLAB. It covers:
- Characterizing the wireless channel from physics of propagation to multi-path fading channels.
- Statistical characterization of channels including Doppler spectrum, delay spread, coherence time and bandwidth.
- Simulating multi-path fading channels in MATLAB and modeling path loss.
- Explaining how multi-path propagation leads to signal distortion and is equivalent to undesired filtering.
RF testing has remained hype for most of us. But seriously it is not so. It can be very interesting and one can develop a lot of interest in this if given an opportunity.
In this paper, authors have started with the some basic concepts of radio engineering which we studied in engineering and built upon these concepts to use in practical applications.
We have also described the basic principles of Signal Analyzer and Signal Generator which are the most common test tools used for any radio testing.
Design Ofdm System And Remove Nonlinear Distortion In OFDM Signal At Transmit...Rupesh Sharma
although OFDM seems to be a solution to keep up with
the demand of increasing data rates, it has some drawbacks.
Sensitivity to high PAPR is the most significant of these
drawbacks. The main objective of this paper was to investigate
and document the effects of PAPR on the performance of OFDM
based digital communications under different channel conditions.
A step-by-step approach was adopted in order to achieve the
objective of this paper. The first step is to provide a basic
background on the principles of OFDM. The reasons for the
PAPR and a theoretical analysis of these effects on OFDM
systems are documented. The OFDM system has a high peak-toaverage
power ratio (PAPR) that can cause unwanted saturation
in the power amplifiers, leading to in-band distortion and out-ofband
radiation. To be able to observe the system behavior, the
simulation results for different channel models are presented in
graphical form. Next, the simulation results obtained in this work
are compared to the simulation results reported in related studies
Orthogonal frequency-division multiplexing (OFDM)
[1] is a method of encoding digital data on multiple carrier
frequencies. OFDM[1] has developed into a popular scheme
for wideband digital communication, whether wireless or
over copper wires, used in applications such as digital television
and audio broadcasting, DSL Internet access, wireless networks,
powerline networks, and 4G mobile communications. In the
Several wireless standards such as IEEE 802.11a[2] and
HiperLAN2[3].The orthogonality of the subcarriers is no longer
maintained which results in ICI (Inter carrier Interference)[4]
.ICI reduction techniques achieve a better SNR and BER in
OFDM at zero phase noise variance . This technique will use a
large number of closely spaced orthogonal subcarriers to avoid
phase noise. It provides high data rates with sufficient robustness
to radio channel damages. A major problem in OFDM is carrier
frequency offset error between the transmitted and received
signals. Due to this the orthogonality of the subcarriers is no
longer maintained which results in ICI (Inter carrier
Interference). In this paper, we used the ICI self-cancellation
technique and reduced the ICI and improved the BER and SNR
we are also calculate the SNR=15db and 20db at different phase
noise variance.
The document discusses OFDM (Orthogonal Frequency Division Multiplexing) for wireless communication. It introduces OFDM as a modulation technique that divides the available bandwidth into multiple orthogonal subcarriers. This allows for overlapping subchannels and improves spectral efficiency compared to conventional FDM. The document then covers OFDM system modeling, generation of subcarriers, fading effects, use of guard times and cyclic extensions to mitigate multipath interference, windowing techniques, and factors to consider when choosing OFDM system parameters.
This document discusses VLSI implementation of orthogonal frequency division multiplexing (OFDM). It provides background on OFDM, explaining that it consists of multiple closely spaced carriers that can achieve high data transmission rates with wide bandwidths. The document outlines the key components of an OFDM transceiver including scrambling, interleaving, constellation mapping, IFFT/FFT processing, and parallel-to-serial conversion. It describes the hardware implementation of these components and advantages of OFDM such as robustness to interference and insensitivity to timing errors.
This document discusses frequency coordination between UMTS and GSM systems operating at 900 MHz. It analyzes the interference between UMTS900 and GSM900 networks that will need to coexist during the transition from GSM to UMTS spectrum. Laboratory tests were conducted using commercial UMTS and GSM equipment to characterize transmitter and receiver performance under interference conditions and determine the required guard band between the networks based on acceptable sensitivity degradation levels. The limiting factor was found to be interference from GSM mobile stations to UMTS base stations, and a guard band of 4.2 MHz would allow satisfactory system performance.
This document analyzes frequency coordination between UMTS900 and GSM900 systems operating at 900 MHz. It summarizes lab tests conducted on commercial UMTS900 and GSM900 equipment to measure interference levels and assess the impact of mutual interference. The limiting factor is found to be interference from GSM mobile stations to UMTS Node B receivers. With a frequency offset of 2.2 MHz or more, satisfactory system performance can be achieved even when as little as 4.2 MHz of GSM spectrum is cleared for UMTS use.
4g LTE and LTE-A for mobile broadband-notePei-Che Chang
This document discusses the basic principles of OFDM (Orthogonal Frequency Division Multiplexing) transmission. It covers several key topics:
1) OFDM uses multiple subcarriers to transmit data in parallel. The subcarriers are spaced closely together with minimal spacing between them.
2) OFDM modulation and demodulation can be implemented efficiently using IDFT/DFT (IFFT/FFT) processing.
3) Cyclic prefixes are added to combat inter-symbol interference from multipath channels. This preserves subcarrier orthogonality.
4) With a cyclic prefix, the channel appears flat on each subcarrier, allowing one-tap frequency domain equalization. Channel estimation is done using reference symbols.
Performance Analysis of OFDM in Combating Multipath FadingIOSR Journals
Mobile Communication system has been on high rampage for high data transmission over wireless
medium with various challenges caused by the transmission Channel. OFDM is been discovered in recent years
to deal with this problems because of its ability to elegantly cope with multipath interference. This paper
investigates the performance of different modulation schemes using M-ary Phase Shift Keying (M-PSK) and Mary
Quadrature Amplitude Modulation (M-QAM) in information transmission with OFDM technique over Ideal
channel AWGN and worst channel Rayleigh Fading channel in terms of Bits Error Rate (BER). Analysis was
made for different types of modulation schemes BPSK, QPSK, 4-QAM and 16-QAM gray coded bit mapping.
Also, a feasibility of OFDM been used to combat multipath fading was analyzed with comparison between a
single carrier technique and OFDM multicarrier technique. Variation between SNR results with respect to BER
is plotted to show the trade off differences between the modulation schemes with the result showing that OFDM
allows data transmission with minimal error over fading channel than a Single Carrier
Performance Analysis of OFDM in Combating Multipath FadingIOSR Journals
Abstract: Mobile Communication system has been on high rampage for high data transmission over wireless medium with various challenges caused by the transmission Channel. OFDM is been discovered in recent years to deal with this problems because of its ability to elegantly cope with multipath interference. This paper investigates the performance of different modulation schemes using M-ary Phase Shift Keying (M-PSK) and M-ary Quadrature Amplitude Modulation (M-QAM) in information transmission with OFDM technique over Ideal channel AWGN and worst channel Rayleigh Fading channel in terms of Bits Error Rate (BER). Analysis was made for different types of modulation schemes BPSK, QPSK, 4-QAM and 16-QAM gray coded bit mapping. Also, a feasibility of OFDM been used to combat multipath fading was analyzed with comparison between a single carrier technique and OFDM multicarrier technique. Variation between SNR results with respect to BER is plotted to show the trade off differences between the modulation schemes with the result showing that OFDM allows data transmission with minimal error over fading channel than a Single Carrier. Keywords: OFDM, Single Carrier, AWGN, Rayleigh fading, BER, M-ary PSK, M-ary QAM
Comparative Analysis of PAPR Reduction Techniques in OFDM Using Precoding Tec...IJSRD
In this modern era, Orthogonal Frequency Division Multiplexing (OFDM) has been proved to be an explicit promising technique for wired and wireless systems because of its several advantages like high spectral efficiency, robustness against frequency selective fading, relatively simple receiver implementation etc. Besides having a number of advantages OFDM suffers from few disadvantages like high Peak to Average Power Ratio (PAPR), Intercarrier Interference (ICI), Intersymbol Interference (ISI) etc. These detrimental effects, if not compensated properly and timely, can result in system performance degradation. This paper mainly concentrates on reduction of PAPR.A comparisons have been made between various precoding techniques against conventional OFDM.
Comparative Analysis of PAPR Reduction Techniques in OFDM Using Precoding Tec...IJSRD
In this modern era, Orthogonal Frequency Division Multiplexing (OFDM) has been proved to be an explicit promising technique for wired and wireless systems because of its several advantages like high spectral efficiency, robustness against frequency selective fading, relatively simple receiver implementation etc. Besides having a number of advantages OFDM suffers from few disadvantages like high Peak to Average Power Ratio (PAPR), Intercarrier Interference (ICI), Intersymbol Interference (ISI) etc. These detrimental effects, if not compensated properly and timely, can result in system performance degradation. This paper mainly concentrates on reduction of PAPR.A comparisons have been made between various precoding techniques against conventional OFDM.
Performance analysis of DWT based OFDM over FFT based OFDM and implementing o...VLSICS Design
Growth in technology has led to unprecedented demand for high speed architectures for complex signal processing applications. In 4G wireless communication systems, bandwidth is a precious commodity, and service providers are continuously met with the challenge of accommodating more users with in a limited allocated bandwidth. To increase data rate of wireless medium with higher performance, OFDM (orthogonal frequency division multiplexing) is used. Recently DWT (Discrete wavelet transforms) is adopted in place of FFT (Fast Fourier transform) for frequency translation. Modulation schemes such as 16-QAM, 32-QAM, 64-QAM and 128-QAM (Quadrature amplitude modulation) have been used in the developed OFDM system for both DWT and FFT based model. In this paper we propose a DWT-IDWT based OFDM transmitter and receiver that achieve better performance in terms SNR and BER for AWGN channel. It proves all the wavelet families better over the IFFT-FFT implementation. The OFDM model is developed using Simulink, various test cases have been considered to verify its performance. The DWTOFDM using Lifting Scheme architecture is implemented on FPGA optimizing hardware, speed & cost. The wavelet filter used for this is Daubechies (9, 7) with N=2. The RTL code is written in Verilog-HDL and simulated in Modelsim. The design is then synthesized in Xilinx and implemented on Virtex5 FPGA board and the results were validated using ChipScope.
Performance analysis of DWT based OFDM over FFT based OFDM and implementing o...VLSICS Design
Growth in technology has led to unprecedented demand for high speed architectures for complex signal processing applications. In 4G wireless communication systems, bandwidth is a precious commodity, and service providers are continuously met with the challenge of accommodating more users with in a limited allocated bandwidth. To increase data rate of wireless medium with higher performance, OFDM (orthogonal frequency division multiplexing) is used. Recently DWT (Discrete wavelet transforms) is adopted in place of FFT (Fast Fourier transform) for frequency translation. Modulation schemes such as 16-QAM, 32-QAM, 64-QAM and 128-QAM (Quadrature amplitude modulation) have been used in the developed OFDM system for both DWT and FFT based model. In this paper we propose a DWT-IDWT based OFDM transmitter and receiver that achieve better performance in terms SNR and BER for AWGN channel. It proves all the wavelet families better over the IFFT-FFT implementation. The OFDM model is developed using Simulink, various test cases have been considered to verify its performance. The DWTOFDM using Lifting Scheme architecture is implemented on FPGA optimizing hardware, speed & cost. The wavelet filter used for this is Daubechies (9, 7) with N=2. The RTL code is written in Verilog-HDL and simulated in Modelsim. The design is then synthesized in Xilinx and implemented on Virtex5 FPGA board and the results were validated using ChipScope.
Ofdm mimo and v-blast algorithm-key to high speed wireless communicationIAEME Publication
This document provides an overview of OFDM-MIMO technology for high-speed wireless communication. It discusses how OFDM divides a high-rate data stream into parallel lower-rate substreams that are transmitted over multiple carriers. MIMO is then used to further improve data rates and quality of service by exploiting multiple antennas at the transmitter and receiver. The document also describes V-BLAST, an algorithm used in MIMO systems to separate simultaneously transmitted signals at the receiver through ordered cancellation.
Ofdm mimo and v-blast algorithm-key to high speed wireless communicationIAEME Publication
This document provides an overview of OFDM-MIMO technology for high-speed wireless communication. It discusses how OFDM divides a high-rate data stream into parallel lower-rate substreams that are transmitted over multiple carriers. MIMO is then used to further improve data rates and quality of service by exploiting multiple antennas at the transmitter and receiver. The document also describes V-BLAST, an algorithm used in MIMO systems to separate simultaneously transmitted signals at the receiver through ordered cancellation.
A modified design of acf operation for reducing papr of ofdm signalijngnjournal
Next generation wireless communication technology long term evolution (LTE) has implemented
orthogonal frequency division multiplexing (OFDM) technique as a strong candidate for radio access
systems. It has several attributes such as providing robustness to multipath fading & impulse noise,
eliminating intersymbol interference (ISI), inter carrier interference (ICI) & the need for equalizers. The
major challenging issue of OFDM technique is the high peak to average power ratio (PAPR) which is
defined as the ratio of the peak power to the average power of the OFDM signal. A trade-off is
necessary for reducing PAPR with increasing bit error rate (BER), computational complexity or data rate
loss etc. In this paper, a moderately modified design of amplitude clipping & filtering operation (ACF) is
proposed and implemented which shows the significant improvement in case of PAPR reduction for both
quadrature phase shift keying (QPSK) & quadrature amplitude modulation (QAM) while increasing slight
BER match up to to an existing method.
A Peak to Average Power Ratio (PAPR) Reduction in OFDM SystemsIRJET Journal
This document discusses peak-to-average power ratio (PAPR) reduction techniques for orthogonal frequency division multiplexing (OFDM) systems. It begins with an introduction to OFDM and the problem of high PAPR values in OFDM signals. It then describes the clipping and filtering method and parabolic peak cancellation method for PAPR reduction. It analyzes these techniques by evaluating complementary cumulative distribution function (CCDF) curves and bit error rate (BER) with the goal of minimizing PAPR while maintaining acceptable BER. Power amplifier nonlinearity is also discussed as a key factor affected by high PAPR OFDM signals.
A Modified Design Of Acf Operation For Reducing Papr Of Ofdm Signaljosephjonse
Next generation wireless communication technology long term evolution (LTE) has implemented orthogonal frequency division multiplexing (OFDM) technique as a strong candidate for radio access systems. It has several attributes such as providing robustness to multipath fading & impulse noise, eliminating intersymbol interference (ISI), inter carrier interference (ICI) & the need for equalizers. The major challenging issue of OFDM technique is the high peak to average power ratio (PAPR) which is defined as the ratio of the peak power to the average power of the OFDM signal. A trade-off is necessary for reducing PAPR with increasing bit error rate (BER), computational complexity or data rate loss etc. In this paper, a moderately modified design of amplitude clipping & filtering operation (ACF) is proposed and implemented which shows the significant improvement in case of PAPR reduction for both quadrature phase shift keying (QPSK) & quadrature amplitude modulation (QAM) while increasing slight BER match up to to an existing method.
This document provides an overview of OFDMA (Orthogonal Frequency Division Multiple Access), a modulation and multiple access scheme used in broadband wireless systems. It discusses how OFDMA combines OFDM modulation with TDMA and FDMA to allow multiple users to transmit simultaneously on different sub-channels. Key advantages of OFDMA include improved signal-to-noise ratio from power concentration and the ability to use adaptive modulation and error correction coding tailored to each user's channel conditions. The document also covers implementation considerations for OFDMA such as synchronization, channel estimation, and addressing challenges from phase noise and power amplifier nonlinearity.
COMPARISON OF BER AND NUMBER OF ERRORS WITH DIFFERENT MODULATION TECHNIQUES I...Sukhvinder Singh Malik
This paper provides analysis of BER and Number of Errors for MIMO-OFDM wireless communication system by using different modulation techniques. Wireless designers constantly seek to improve the spectrum efficiency/capacity, coverage of wireless networks, and link reliability. So the performances of the wireless communication systems can be enhanced by using multiple transmit and receive antennas, which is generally referred to as the MIMO technique. Here analysis will be carried out for an OFDM wireless communication system using different modulation techniques and considering the effect and the wireless channel like AWGN, fading. Performance results will be evaluated numerically and graphically using the plots of BER versus SNR and plots of number of errors versus SNR.
OFDM allows tightly packed carriers to convey information orthogonally and with high bandwidth efficiency
Objectives Description:
Concepts
Basic idea
Introduction to OFDM
Implementation
Advantages and Drawbacks.
FDMA
Single Mode Optical Fiber in Rof System Using DWDMIJERA Editor
Performance analysis was carried out to find the effect of crosstalk in a WDM system. Firstly, analysis of BER
was carried out without crosstalk. Then analysis of BER with crosstalk was done. Using equation for crosstalk,
number of channels was plotted using matlab. System parameters were optimized for a particular crosstalk.
Objective of the thesis work
Performance Analysis is carried out to find the effect of crosstalk due to optical cross connect in a DWDM
system considering a WDM based optical cross connect (OXC). An analysis is carried out to find the amount of
crosstalk due to OXC. The bit error rate performance degradation due to crosstalk is evaluated for OXC
parameter and number of wavelengths per fiber. The optimum parameters such as optimum number of channels
and hops are determined.
Comparative performance analysis of different modulation techniques for papr ...IJCNCJournal
One of the most important multi-carrier tran
smission techniques used in the latest wireless com
munication
arena is known as Orthogonal Frequency Division Mul
tiplexing (OFDM). It has several characteristics
such as providing greater immunity to multipath fad
ing & impulse noise, eliminating Inter Symbol
Interference (ISI) & Inter Carrier Interference (IC
I) using a guard interval known as Cyclic Prefix (C
P). A
regular difficulty of OFDM signal is high peak to a
verage power ratio (PAPR) which is defined as the r
atio
of the peak power to the average power of OFDM Sign
al. An improved design of amplitude clipping &
filtering technique of us previously reduced signif
icant amount of PAPR with slightly increase bit err
or rate
(BER) compare to an existing method in case of Quad
rature Phase Shift Keying (QPSK) & Quadrature
Amplitude Modulation (QAM). This paper investigates
a comparative performance analysis of the differen
t
higher order modulation techniques on that design.
This document describes a simulator designed to analyze bit error rates using orthogonal frequency division multiplexing (OFDM) under different modulation schemes and channel conditions. The simulator was implemented in MATLAB and allows users to choose modulation types, channel types (AWGN, Rayleigh, Rician), and other parameters. It then generates plots of bit error rate versus signal-to-noise ratio for performance analysis. Screenshots of the user interface are provided along with sample output plots and discussion of the simulator design and capabilities.
This document provides an overview of Orthogonal Frequency Division Multiplexing (OFDM). It defines OFDM as a digital modulation technique that splits a high-rate data stream into multiple lower-rate streams and transmits them simultaneously over a number of subcarriers. The main advantages of OFDM are high spectral efficiency, resilience to radio interference and multipath fading. Common applications of OFDM include wireless networks, Wi-Fi, WiMAX and digital audio/video broadcasting.
Digital Twins Computer Networking Paper Presentation.pptxaryanpankaj78
A Digital Twin in computer networking is a virtual representation of a physical network, used to simulate, analyze, and optimize network performance and reliability. It leverages real-time data to enhance network management, predict issues, and improve decision-making processes.
Build the Next Generation of Apps with the Einstein 1 Platform.
Rejoignez Philippe Ozil pour une session de workshops qui vous guidera à travers les détails de la plateforme Einstein 1, l'importance des données pour la création d'applications d'intelligence artificielle et les différents outils et technologies que Salesforce propose pour vous apporter tous les bénéfices de l'IA.
Road construction is not as easy as it seems to be, it includes various steps and it starts with its designing and
structure including the traffic volume consideration. Then base layer is done by bulldozers and levelers and after
base surface coating has to be done. For giving road a smooth surface with flexibility, Asphalt concrete is used.
Asphalt requires an aggregate sub base material layer, and then a base layer to be put into first place. Asphalt road
construction is formulated to support the heavy traffic load and climatic conditions. It is 100% recyclable and
saving non renewable natural resources.
With the advancement of technology, Asphalt technology gives assurance about the good drainage system and with
skid resistance it can be used where safety is necessary such as outsidethe schools.
The largest use of Asphalt is for making asphalt concrete for road surfaces. It is widely used in airports around the
world due to the sturdiness and ability to be repaired quickly, it is widely used for runways dedicated to aircraft
landing and taking off. Asphalt is normally stored and transported at 150’C or 300’F temperature
Blood finder application project report (1).pdfKamal Acharya
Blood Finder is an emergency time app where a user can search for the blood banks as
well as the registered blood donors around Mumbai. This application also provide an
opportunity for the user of this application to become a registered donor for this user have
to enroll for the donor request from the application itself. If the admin wish to make user
a registered donor, with some of the formalities with the organization it can be done.
Specialization of this application is that the user will not have to register on sign-in for
searching the blood banks and blood donors it can be just done by installing the
application to the mobile.
The purpose of making this application is to save the user’s time for searching blood of
needed blood group during the time of the emergency.
This is an android application developed in Java and XML with the connectivity of
SQLite database. This application will provide most of basic functionality required for an
emergency time application. All the details of Blood banks and Blood donors are stored
in the database i.e. SQLite.
This application allowed the user to get all the information regarding blood banks and
blood donors such as Name, Number, Address, Blood Group, rather than searching it on
the different websites and wasting the precious time. This application is effective and
user friendly.
Prediction of Electrical Energy Efficiency Using Information on Consumer's Ac...PriyankaKilaniya
Energy efficiency has been important since the latter part of the last century. The main object of this survey is to determine the energy efficiency knowledge among consumers. Two separate districts in Bangladesh are selected to conduct the survey on households and showrooms about the energy and seller also. The survey uses the data to find some regression equations from which it is easy to predict energy efficiency knowledge. The data is analyzed and calculated based on five important criteria. The initial target was to find some factors that help predict a person's energy efficiency knowledge. From the survey, it is found that the energy efficiency awareness among the people of our country is very low. Relationships between household energy use behaviors are estimated using a unique dataset of about 40 households and 20 showrooms in Bangladesh's Chapainawabganj and Bagerhat districts. Knowledge of energy consumption and energy efficiency technology options is found to be associated with household use of energy conservation practices. Household characteristics also influence household energy use behavior. Younger household cohorts are more likely to adopt energy-efficient technologies and energy conservation practices and place primary importance on energy saving for environmental reasons. Education also influences attitudes toward energy conservation in Bangladesh. Low-education households indicate they primarily save electricity for the environment while high-education households indicate they are motivated by environmental concerns.
Applications of artificial Intelligence in Mechanical Engineering.pdfAtif Razi
Historically, mechanical engineering has relied heavily on human expertise and empirical methods to solve complex problems. With the introduction of computer-aided design (CAD) and finite element analysis (FEA), the field took its first steps towards digitization. These tools allowed engineers to simulate and analyze mechanical systems with greater accuracy and efficiency. However, the sheer volume of data generated by modern engineering systems and the increasing complexity of these systems have necessitated more advanced analytical tools, paving the way for AI.
AI offers the capability to process vast amounts of data, identify patterns, and make predictions with a level of speed and accuracy unattainable by traditional methods. This has profound implications for mechanical engineering, enabling more efficient design processes, predictive maintenance strategies, and optimized manufacturing operations. AI-driven tools can learn from historical data, adapt to new information, and continuously improve their performance, making them invaluable in tackling the multifaceted challenges of modern mechanical engineering.
DEEP LEARNING FOR SMART GRID INTRUSION DETECTION: A HYBRID CNN-LSTM-BASED MODELijaia
As digital technology becomes more deeply embedded in power systems, protecting the communication
networks of Smart Grids (SG) has emerged as a critical concern. Distributed Network Protocol 3 (DNP3)
represents a multi-tiered application layer protocol extensively utilized in Supervisory Control and Data
Acquisition (SCADA)-based smart grids to facilitate real-time data gathering and control functionalities.
Robust Intrusion Detection Systems (IDS) are necessary for early threat detection and mitigation because
of the interconnection of these networks, which makes them vulnerable to a variety of cyberattacks. To
solve this issue, this paper develops a hybrid Deep Learning (DL) model specifically designed for intrusion
detection in smart grids. The proposed approach is a combination of the Convolutional Neural Network
(CNN) and the Long-Short-Term Memory algorithms (LSTM). We employed a recent intrusion detection
dataset (DNP3), which focuses on unauthorized commands and Denial of Service (DoS) cyberattacks, to
train and test our model. The results of our experiments show that our CNN-LSTM method is much better
at finding smart grid intrusions than other deep learning algorithms used for classification. In addition,
our proposed approach improves accuracy, precision, recall, and F1 score, achieving a high detection
accuracy rate of 99.50%.
Generative AI Use cases applications solutions and implementation.pdfmahaffeycheryld
Generative AI solutions encompass a range of capabilities from content creation to complex problem-solving across industries. Implementing generative AI involves identifying specific business needs, developing tailored AI models using techniques like GANs and VAEs, and integrating these models into existing workflows. Data quality and continuous model refinement are crucial for effective implementation. Businesses must also consider ethical implications and ensure transparency in AI decision-making. Generative AI's implementation aims to enhance efficiency, creativity, and innovation by leveraging autonomous generation and sophisticated learning algorithms to meet diverse business challenges.
https://www.leewayhertz.com/generative-ai-use-cases-and-applications/
Open Channel Flow: fluid flow with a free surfaceIndrajeet sahu
Open Channel Flow: This topic focuses on fluid flow with a free surface, such as in rivers, canals, and drainage ditches. Key concepts include the classification of flow types (steady vs. unsteady, uniform vs. non-uniform), hydraulic radius, flow resistance, Manning's equation, critical flow conditions, and energy and momentum principles. It also covers flow measurement techniques, gradually varied flow analysis, and the design of open channels. Understanding these principles is vital for effective water resource management and engineering applications.
5G Radio Network Througput Problem Analysis HCIA.pdf
Orthogonal frequency
1. Orthogonal frequency-division multiplexing (OFDM) is a method of encoding digital data on multiple carrier
frequencies.OFDM has developed into a popular scheme for wideband digital communication,used in applications
such as digital television and audio broadcasting, DSL Internetaccess,wireless networks,powerline networks,
and 4G mobile communications.
OFDM is a frequency-division multiplexing (FDM) scheme used as a digital multi-carrier modulation method.A large
number ofcloselyspaced orthogonal sub-carrier signals are used to carry data[1]
on several parallel data streams or
channels.Each sub-carrier is modulated with a conventional modulation scheme (such as quadrature amplitude
modulation or phase-shiftkeying) at a low symbol rate, maintaining total data rates similar to conventional single-
carrier modulation schemes in the same bandwidth.
The primaryadvantage of OFDM over single-carrier schemes is its abilityto cope with severe channel conditions (for
example,attenuation ofhigh frequencies in a long copper wire, narrowband interference and frequency-
selective fading due to multipath) withoutcomplexequalization filters.Channel equalization is simplified because
OFDM may be viewed as using manyslowlymodulated narrowband signals rather than one rapidly
modulated wideband signal.The low symbol rate makes the use of a guard interval between symbols affordable,
making itpossible to eliminate intersymbol interference (ISI) and utilize echoes and time-spreading (on analogue TV
these are visible as ghosting and blurring,respectively) to achieve a diversity gain,i.e. a signal-to-noise
ratio improvement.This mechanism also facilitates the design of single frequencynetworks (SFNs),where several
adjacenttransmitters send the same signal simultaneouslyat the same frequency,as the signals from multiple distant
transmitters maybe combined constructively,rather than interfering as would typically occur in a traditional single-
carrier system.
_____________________________________________________________________________________________
Orthogonal functions[edit]
Main article: Orthogonal functions
By using integral calculus,itis common to use the following to define the inner product of two functions f and g:
Here we introduce a nonnegative weightfunction w(x) in the definition ofthis inner product. In simple cases, w(x)
= 1.
We say that these functions are orthogonal (with respectto this inner product) if the value of this integral is
zero:
Note that two functions which are orthogonal with respectto one inner product, aren't necessarily
orthogonal with respectto another inner product.
We write the norms with respectto this inner productand the weightfunction as
The members ofa setof functions {{{1}}}are:
_________________________________________________________________________________
2. orthogonal on the closed interval [a, b] if
orthonormal on the interval [a, b] if
where
is the "Kronecker delta" function. In other words,any two of them are orthogonal,and
the norm of each is 1 in the case of the orthonormal sequence.See in particular
theorthogonal polynomials.
Examples[edit]
The vectors (1, 3, 2)T
, (3, −1, 0)T
, (1, 3, −5)T
are orthogonal to each other, since
(1)(3) + (3)(−1) + (2)(0) = 0, (3)(1) + (−1)(3) + (0)(−5) = 0, and (1)(1) + (3)(3) +
(2)(−5) = 0.
The vectors (1, 0, 1, 0, ...)T
and (0, 1, 0, 1, ...)T
are orthogonal to each other. The dot
productof these vectors is 0. We can then make the generalization to consider the
vectors in Z2
n
:
for some positive integer a,and for 1 ≤ k ≤ a − 1, these vectors are orthogonal,for example (1, 0, 0, 1, 0, 0,
1, 0)T
, (0, 1, 0, 0, 1, 0, 0, 1)T
, (0, 0, 1, 0, 0, 1, 0, 0)T
are orthogonal.
A subcarrier is a sideband ofa radio frequency carrier wave, which is modulated to send additional
information.Examples include the provision ofcolour in a black and white television system or the
provision of stereo in a monophonic radio broadcast.There is no physical difference between a carrier
and a subcarrier;the "sub" implies thatit has been derived from a carrier, which has been amplitude
modulated bya steady signal and has a constantfrequency relation to it.
----------------------------------------------------------------------------------------------------------------------------------
3. The following listis a summaryof existing OFDM based standards and products.For further details,see
the Usage section atthe end of the article.
Cable
ADSL and VDSL broadband access via POTS copper wiring
DVB-C2, an enhanced version of the DVB-C digital cable TV standard
Power line communication (PLC)
ITU-T G.hn, a standard which provides high-speed local area networking ofexisting home wiring (power lines,
phone lines and coaxial cables)[2]
TrailBlazer telephone line modems
Multimedia over Coax Alliance (MoCA) home networking
DOCSIS 3.1 Broadband delivery
Wireless[edit]
The wireless LAN (WLAN) radio interfaces IEEE 802.11a,g, n, ac and HIPERLAN/2
The digital radio systems DAB/EUREKA 147, DAB+, Digital Radio Mondiale,HD Radio,T-DMB and ISDB-TSB
The terrestrial digital TV systems DVB-Tand ISDB-T
The terrestrial mobile TV systems DVB-H,T-DMB, ISDB-T and MediaFLO forward link
The wireless personal area network (PAN) ultra-wideband (UWB) IEEE 802.15.3a implementation suggested
by WiMedia Alliance
The OFDM based multiple access technologyOFDMA is also used in several 4G and pre-4G cellular
networks and mobile broadband standards:
The mobilitymode of the wireless MAN/broadband wireless access (BWA) standard IEEE 802.16e (or Mobile-
WiMAX)
The mobile broadband wireless access (MBWA) standard IEEE 802.20
The downlink ofthe 3GPP Long Term Evolution (LTE) fourth generation mobile broadband standard.The radio
interface was formerly named High Speed OFDM PacketAccess (HSOPA), now named Evolved UMTS
Terrestrial Radio Access (E-UTRA)
Summary of advantages[edit]
High spectral efficiency as compared to other double sideband modulation schemes, spread spectrum,etc.
Can easilyadapt to severe channel conditions withoutcomplextime-domain equalization.
Robustagainstnarrow-band co-channel interference
Robustagainst intersymbol interference (ISI) and fading caused by multipath propagation
Efficient implementation using fastFourier transform (FFT)
Low sensitivity to time synchronization errors
Tuned sub-channel receiver filters are not required (unlike conventional FDM)
Facilitates single frequencynetworks (SFNs) (i.e.transmitter macrodiversity)
Summary of disadvantages[edit]
Sensitive to Doppler shift
4. Sensitive to frequency synchronization problems
High peak-to-average-power ratio (PAPR), requiring linear transmitter circuitry,which suffers from poor power
efficiency
Loss ofefficiency caused by cyclic prefix/guard interval
Characteristics and principles of operation[edit]
Orthogonality[edit]
Conceptually,OFDM is a specialized FDM, the additional constraintbeing thatall carrier signals are orthogonal to
one another.
In OFDM, the sub-carrier frequencies are chosen so thatthe sub-carriers are orthogonal to each other, meaning
that cross-talk between the sub-channels is eliminated and inter-carrier guard bands are notrequired.This greatly
simplifies the design ofboth the transmitter and the receiver; unlike conventional FDM, a separate filter for each sub-
channel is notrequired.
The orthogonalityrequires thatthe sub-carrier spacing is Hertz, where TU seconds is the useful symbol
duration (the receiver side window size),and k is a positive integer, typically equal to 1. Therefore, with N sub-
carriers,the total passband bandwidth will be B ≈ N·Δf (Hz).
The orthogonalityalso allows high spectral efficiency,with a total symbol rate near the Nyquistrate for the equivalent
baseband signal (i.e.near halfthe Nyquist rate for the double-side band physical passband signal).Almost the whole
available frequency band can be utilized. OFDM generallyhas a nearly 'white' spectrum,giving it benign
electromagnetic interference properties with respectto other co-channel users.
A simple example:A useful symbol duration TU = 1 ms would require a sub-carrier spacing
of (or an integer multiple ofthat) for orthogonality. N = 1,000 sub-carriers would result
in a total passband bandwidth of NΔf = 1 MHz. For this symbol time,the required bandwidth in theory
according to Nyquistis N=1/2TU = 0.5 MHz (i.e., halfof the achieved bandwidth required byour scheme).If a
guard interval is applied (see below),Nyquistbandwidth requirementwould be even lower. The FFT would
resultin N = 1,000 samples per symbol.If no guard interval was applied, this would resultin a base band
complexvalued signal with a sample rate of 1 MHz, which would require a baseband bandwidth of0.5 MHz
according to Nyquist.However, the passband RFsignal is produced bymultiplying the baseband signal with
a carrier waveform (i.e., double-sideband quadrature amplitude-modulation) resulting in a passband
bandwidth of1 MHz. A single-side band (SSB) or vestigial sideband (VSB) modulation scheme would
achieve almosthalfthatbandwidth for the same symbol rate (i.e., twice as high spectral efficiency for the
same symbol alphabetlength).It is however more sensitive to multipath interference.
OFDM requires very accurate frequency synchronization between the receiver and the transmitter;with
frequency deviation the sub-carriers will no longer be orthogonal,causing inter-carrier interference (ICI) (i.e.,
cross-talk between the sub-carriers).Frequencyoffsets are typically caused by mismatched transmitter and
receiver oscillators,or byDoppler shift due to movement.While Doppler shiftalone may be compensated for by
the receiver, the situation is worsened when combined with multipath,as reflections will appear atvarious
frequency offsets,which is much harder to correct. This effect typically worsens as speed increases,[3]
and is an
importantfactor limiting the use of OFDM in high-speed vehicles.In order to mitigate ICI in such scenarios,one
can shape each sub-carrier in order to minimize the interference resulting in a non-orthogonal subcarriers
overlapping.[4]
For example,a low-complexityscheme referred to as WCP-OFDM (Weighted Cyclic Prefix
Orthogonal Frequency-Division Multiplexing) consists in using shortfilters atthe transmitter outputin order to
5. perform a potentiallynon-rectangular pulse shaping and a near perfect reconstruction using a single-tap per
subcarrier equalization.[5]
Other ICIsuppression techniques usuallyincrease drasticallythe receiver complexity.[6]
Implementation using the FFT algorithm[edit]
The orthogonalityallows for efficient modulator and demodulator implementation using the FFT algorithm on the
receiver side,and inverse FFT on the sender side.Although the principles and some ofthe benefits have been
known since the 1960s,OFDM is popular for wideband communications todayby way of low-costdigital signal
processingcomponents thatcan efficiently calculate the FFT.
The time to compute the inverse-FFT or FFT transform has to take less than the time for each symbol.,[7]
which
for example for DVB-T (FFT 8k) means the computation has to be done in 896 µs or less.
For an 8192-pointFFT this may be approximated to:[7][clarification needed]
[7]
MIPS = Million instructions per second
The computational demand approximatelyscales linearlywith FFT size so a double size FFT needs double
the amountof time and vice versa.[7]
As a comparison an Intel Pentium III CPU at 1.266 GHz is able to
calculate a 8 192 point FFT in 576 µs using FFTW.[8]
Intel Pentium M at 1.6 GHz does it in 387 µs.[9]
Intel
Core Duo at 3.0 GHz does it in96.8 µs.[10]
Guard interval for elimination of intersymbol interference[edit]
One key principle ofOFDM is that since low symbol rate modulation schemes (i.e.,where the symbols are
relatively long compared to the channel time characteristics) suffer less from intersymbol
interference caused by multipath propagation,itis advantageous to transmita number of low-rate streams
in parallel instead ofa single high-rate stream.Since the duration of each symbol is long,it is feasible to
inserta guard interval between the OFDM symbols,thus eliminating the intersymbol interference.
The guard interval also eliminates the need for a pulse-shaping filter,and it reduces the sensitivity to time
synchronization problems.
A simple example:Ifone sends a million symbols per second using conventional single-carrier modulation
over a wireless channel,then the duration of each symbol would be one microsecond or less.This imposes
severe constraints on synchronization and necessitates the removal ofmultipath interference.If the same
million symbols per second are spread among one thousand sub-channels,the duration of each symbol can
be longer by a factor of a thousand (i.e., one millisecond) for orthogonalitywith approximatelythe same
bandwidth.Assume thata guard interval of 1/8 of the symbol length is inserted between each symbol.
Intersymbol interference can be avoided if the multipath time-spreading (the time between the reception of
the first and the lastecho) is shorter than the guard interval (i.e., 125 microseconds).This corresponds to a
maximum difference of37.5 kilometers between the lengths ofthe paths.
6. The cyclic prefix, which is transmitted during the guard interval, consists ofthe end of the OFDM
symbol copied into the guard interval, and the guard interval is transmitted followed bythe OFDM
symbol.The reason thatthe guard interval consists ofa copy of the end of the OFDM symbol is so that
the receiver will integrate over an integer number ofsinusoid cycles for each of the multipaths when it
performs OFDM demodulation with the FFT. In some standards such as Ultrawideband,in the interest
of transmitted power,cyclic prefix is skipped and nothing is sentduring the guard interval. The receiver
will then have to mimic the cyclic prefix functionalityby copying the end part of the OFDM symbol and
adding it to the beginning portion.
Simplified equalization[edit]
The effects of frequency-selective channel conditions,for example fading caused by multipath
propagation,can be considered as constant(flat) over an OFDM sub-channel ifthe sub-channel is
sufficientlynarrow-banded (i.e.,if the number ofsub-channels is sufficientlylarge).This makes
frequency domain equalization possible atthe receiver, which is far simpler than the time-domain
equalization used in conventional single-carrier modulation.In OFDM, the equalizer only has to
multiplyeach detected sub-carrier (each Fourier coefficient) in each OFDM symbol by a constant
complexnumber,or a rarely changed value.
Our example:The OFDM equalization in the above numerical example would require one complexvalued
multiplication per subcarrier and symbol (i.e., complexmultiplications per OFDM symbol;i.e.,
one million multiplications per second,atthe receiver). The FFT algorithm requires
[this is imprecise:over half of these complexmultiplications are trivial,i.e. = to 1 and are not implementedin
software or HW]. complex-valued multiplications per OFDM symbol (i.e., 10 million multiplications per
second),at both the receiver and transmitter side.This should be compared with the corresponding one
million symbols/second single-carrier modulation case mentioned in the example,where the equalization of
125 microseconds time-spreading using a FIR filter would require,in a naive implementation,125
multiplications per symbol (i.e.,125 million multiplications per second).FFT techniques can be used to
reduce the number ofmultiplications for an FIR filter based time-domain equalizer to a number comparable
with OFDM, at the costof delay between reception and decoding which also becomes comparable with
OFDM.
If differential modulation such as DPSK or DQPSK is applied to each sub-carrier,equalization can
be completelyomitted,since these non-coherentschemes are insensitive to slowlychanging
amplitude and phase distortion.
In a sense,improvements in FIR equalization using FFTs or partial FFTs leads mathematically
closer to OFDM,[citation needed]
but the OFDM technique is easier to understand and implement,and the
sub-channels can be independentlyadapted in other ways than varying equalization coefficients,
such as switching between different QAM constellation patterns and error-correction schemes to
match individual sub-channel noise and interference characteristics.[clarification needed]
Some of the sub-carriers in some ofthe OFDM symbols maycarry pilotsignals for measurement
of the channel conditions[11][12]
(i.e., the equalizer gain and phase shiftfor each sub-carrier).Pilot
signals and training symbols (preambles) mayalso be used for time synchronization (to avoid
intersymbol interference,ISI) and frequency synchronization (to avoid inter-carrier interference,
ICI, caused by Doppler shift).
7. OFDM was initiallyused for wired and stationarywireless communications.However,with an
increasing number ofapplications operating in highlymobile environments,the effect of dispersive
fading caused by a combination ofmulti-path propagation and doppler shift is more significant.
Over the lastdecade,research has been done on how to equalize OFDM transmission over
doubly selective channels.[13][14][15]
Channel coding and interleaving[edit]
OFDM is invariably used in conjunction with channel coding (forward error correction),and almost
always uses frequencyand/or time interleaving.
Frequency (subcarrier) interleaving increases resistance to frequency-selective channel conditions
such as fading.For example,when a part of the channel bandwidth fades,frequencyinterleaving
ensures thatthe bit errors that would resultfrom those subcarriers in the faded part of the
bandwidth are spread outin the bit-stream rather than being concentrated.Similarly,time
interleaving ensures thatbits that are originallyclose together in the bit-stream are transmitted far
apart in time, thus mitigating againstsevere fading as would happen when travelling athigh
speed.
However, time interleaving is of little benefitin slowlyfading channels,such as for stationary
reception,and frequency interleaving offers little to no benefit for narrowband channels thatsuffer
from flat-fading (where the whole channel bandwidth fades atthe same time).
The reason why interleaving is used on OFDM is to attemptto spread the errors out in the bit-
stream thatis presented to the error correction decoder,because when such decoders are
presented with a high concentration of errors the decoder is unable to correct all the bit errors,
and a burstof uncorrected errors occurs.A similar design ofaudio data encoding makes compact
disc (CD) playback robust.
A classical type of error correction coding used with OFDM-based systems is convolutional
coding,often concatenated with Reed-Solomon coding.Usually,additional interleaving (on top of
the time and frequency interleaving mentioned above) in between the two layers of coding is
implemented.The choice for Reed-Solomon coding as the outer error correction code is based on
the observation that the Viterbi decoder used for inner convolutional decoding produces short
error bursts when there is a high concentration oferrors,and Reed-Solomon codes are inherently
well-suited to correcting bursts oferrors.
Newer systems,however, usuallynow adoptnear-optimal types of error correction codes that use
the turbo decoding principle,where the decoder iterates towards the desired solution.Examples
of such error correction coding types include turbo codes and LDPC codes,which perform close
to the Shannon limit for the Additive White Gaussian Noise (AWGN) channel.Some systems that
have implemented these codes have concatenated them with either Reed-Solomon (for example
on the MediaFLO system) or BCH codes(on the DVB-S2 system) to improve upon an error
floor inherentto these codes athigh signal-to-noise ratios.[16]
Adaptive transmission[edit]
The resilience to severe channel conditions can be further enhanced ifinformation aboutthe
channel is sentover a return-channel.Based on this feedback information,adaptivemodulation,
channel coding and power allocation maybe applied across all sub-carriers,or individuallyto
8. each sub-carrier.In the latter case,if a particular range of frequencies suffers from interference or
attenuation,the carriers within that range can be disabled or made to run slower by applying more
robustmodulation or error coding to those sub-carriers.
The term discrete multitone modulation (DMT) denotes OFDM based communication systems that
adaptthe transmission to the channel conditions individuallyfor each sub-carrier,by means ofso-
called bit-loading.Examples are ADSL and VDSL.
The upstream and downstream speeds can be varied by allocating either more or fewer carriers
for each purpose.Some forms of rate-adaptive DSL use this feature in real time,so that the
bitrate is adapted to the co-channel interference and bandwidth is allocated to whichever
subscriber needs itmost.
OFDM extended with multiple access[edit]
OFDM in its primaryform is considered as a digital modulation technique,and nota multi-
user channel access method,since itis utilized for transferring one bit stream over one
communication channel using one sequence ofOFDM symbols.However,OFDM can be
combined with multiple access using time,frequencyor coding separation ofthe users.
In orthogonal frequency-division multiple access (OFDMA), frequency-division multiple access is
achieved by assigning differentOFDM sub-channels to differentusers.OFDMA supports
differentiated quality of service by assigning differentnumber ofsub-carriers to differentusers in a
similar fashion as in CDMA, and thus complexpacket scheduling orMedia Access
Control schemes can be avoided. OFDMA is used in:
the mobilitymode of the IEEE 802.16 Wireless MAN standard,commonlyreferred to as
WiMAX,
the IEEE 802.20 mobile Wireless MAN standard,commonlyreferred to as MBWA,
the 3GPP Long Term Evolution (LTE) fourth generation mobile broadband standard
downlink.The radio interface was formerlynamed High Speed OFDM Packet Access
(HSOPA), now named Evolved UMTS Terrestrial Radio Access (E-UTRA).
the now defunct Qualcomm/3GPP2 Ultra Mobile Broadband (UMB) project, intended as a
successor of CDMA2000, but replaced by LTE.
OFDMA is also a candidate access method for the IEEE 802.22 Wireless Regional Area
Networks (WRAN). The projectaims atdesigning the first cognitive radio based standard
operating in the VHF-low UHF spectrum (TV spectrum).
In Multi-carrier code division multiple access (MC-CDMA), also known as OFDM-CDMA, OFDM is
combined with CDMA spread spectrum communication for coding separation ofthe users.Co-
channel interference can be mitigated,meaning thatmanual fixed channel allocation (FCA)
frequency planning is simplified,or complexdynamic channel allocation (DCA) schemes are
avoided.
Space diversity[edit]
In OFDM based wide area broadcasting,receivers can benefitfrom receiving signals from several
spatiallydispersed transmitters simultaneously,since transmitters will onlydestructivelyinterfere
with each other on a limited number ofsub-carriers,whereas in general theywill actually reinforce
9. coverage over a wide area. This is very beneficial in many countries,as itpermits the operation of
national single-frequencynetworks (SFN), where many transmitters send the same signal
simultaneouslyover the same channel frequency.SFNs utilise the available spectrum more
effectively than conventional multi-frequencybroadcastnetworks (MFN), where program content
is replicated on different carrier frequencies.SFNs also resultin a diversity gain in receivers
situated midwaybetween the transmitters.The coverage area is increased and the outage
probabilitydecreased in comparison to an MFN, due to increased received signal strength
averaged over all sub-carriers.
Although the guard interval only contains redundantdata,which means thatit reduces the
capacity, some OFDM-based systems,such as some ofthe broadcasting systems,deliberately
use a long guard interval in order to allow the transmitters to be spaced farther apart in an SFN,
and longer guard intervals allow larger SFN cell-sizes.A rule of thumb for the maximum distance
between transmitters in an SFN is equal to the distance a signal travels during the guard interval
— for instance,a guard interval of 200 microseconds would allow transmitters to be spaced 60 km
apart.
A single frequency network is a form of transmitter macrodiversity.The concept can be further
utilized in dynamic single-frequency networks (DSFN),where the SFN grouping is changed from
timeslotto timeslot.
OFDM may be combined with other forms of space diversity, for example antenna
arrays and MIMO channels.This is done in the IEEE 802.11 Wireless LAN standards.
Linear transmitter power amplifier[edit]
An OFDM signal exhibits a high peak-to-average power ratio (PAPR) because the independent
phases ofthe sub-carriers mean thatthey will often combine constructively.Handling this high
PAPR requires:
A high-resolution digital-to-analogue converter (DAC) in the transmitter
A high-resolution analogue-to-digital converter (ADC) in the receiver
A linear signal chain
Any non-linearityin the signal chain will cause intermodulation distortion that
Raises the noise floor
May cause inter-carrier interference
Generates out-of-band spurious radiation
The linearity requirementis demanding,especiallyfor transmitter RFoutput circuitry where
amplifiers are often designed to be non-linear in order to minimise power consumption.In practical
OFDM systems a small amountofpeak clipping is allowed to limitthe PAPR in a judicious trade-
off againstthe above consequences.However,the transmitter outputfilter which is required to
reduce out-of-band spurs to legal levels has the effect of restoring peak levels that were clipped,
so clipping is notan effective way to reduce PAPR.
Although the spectral efficiency of OFDM is attractive for both terrestrial and space
communications,the high PAPR requirements have so far limited OFDM applications to terrestrial
systems.
10. The crestfactor CF (in dB) for an OFDM system with n uncorrelated sub-carriers is[17]
CF = 10 log ( n ) + CFc ...
where CFc is the crest factor (in dB) for each sub-carrier.(CFc is 3.01 dB for the sine waves used
for BPSK and QPSK modulation).
For example,the DVB-T signal in 2K mode is composed of1705 sub-carriers thatare each
QPSK-modulated,giving a crestfactor of 35.32 dB.[17]
Many crestfactor reduction techniques have been developed.
The dynamic range required for an FM receiver is 120 dB while DAB only require about 90
dB.[18]
As a comparison,each extra bit per sample increases the dynamic range with6 dB.
Efficiency comparison between single carrier and multicarrier[edit]
The performance ofany communication system can be measured in terms ofits power efficiency
and bandwidth efficiency. The power efficiency describes the abilityof communication system to
preserve bit error rate (BER) of the transmitted signal atlow power levels. Bandwidth
efficiency reflects how efficiently the allocated bandwidth is utilized and is defined as the
throughputdata rate per Hertz in a given bandwidth.If the large number of subcarriers are used,
the bandwidth efficiencyof multicarrier system such as OFDMwith using optical fiber channel is
defined as[19]
Factor 2 is because oftwo polarization states in the fiber.
where is the symbol rate in giga symbol per second (Gsps),and is the
bandwidth ofOFDM signal.
There is saving of bandwidth by using Multicarrier modulation with orthogonal frequencydivision
multiplexing .So the bandwidth for multicarrier system is less in comparison with single carrier
system and hence bandwidth efficiencyof multicarrier system is larger than single carrier s ystem.
S.No.
Transmission
Type
M in
M-
QAM
No. of
Subcarriers
Bit
rate
Fiber
length
Power at the
receiver (at
BER of 10−9
)
Bandwidth
efficiency
1. single carrier 64 1
10
Gbit/s
20 km -37.3 dBm 6.0000
2. multicarrier 64 128
10
20 km -36.3 dBm 10.6022
11. Gbit/s
There is only 1 dBm increase in receiver power,but we get 76.7% improvementin bandwidth
efficiency with using multicarrier transmission technique.
Characteristics and principles of operation[edit]
Orthogonality[edit]
Conceptually,OFDM is a specialized FDM, the additional constraintbeing thatall carrier signals are orthogonal to
one another.
In OFDM, the sub-carrier frequencies are chosen so thatthe sub-carriers are orthogonal to each other, meaning
that cross-talk between the sub-channels is eliminated and inter-carrier guard bands are notrequired.This greatly
simplifies the design ofboth the transmitter and the receiver; unlike conventional FDM, a separate filter for each sub-
channel is notrequired.
The orthogonalityrequires thatthe sub-carrier spacing is Hertz, where TU seconds is the useful symbol
duration (the receiver side window size),and k is a positive integer, typically equal to 1. Therefore, with N sub-
carriers,the total passband bandwidth will be B ≈ N·Δf (Hz).
The orthogonalityalso allows high spectral efficiency,with a total symbol rate near the Nyquistrate for the equivalent
baseband signal (i.e.near halfthe Nyquist rate for the double-side band physical passband signal).Almostthe whole
available frequency band can be utilized. OFDM generallyhas a nearly 'white' spectrum,giving it benign
electromagnetic interference properties with respectto other co-channel users.
A simple example:A useful symbol duration TU = 1 ms would require a sub-carrier spacing
of (or an integer multiple ofthat) for orthogonality. N = 1,000 sub-carriers would result
in a total passband bandwidth of NΔf = 1 MHz. For this symbol time,the required bandwidth in theory
according to Nyquistis N=1/2TU = 0.5 MHz (i.e., halfof the achieved bandwidth required byour scheme).If a
guard interval is applied (see below),Nyquistbandwidth requirementwould be even lower. The FFT would
resultin N = 1,000 samples per symbol.If no guard interval was applied,this would resultin a base band
complexvalued signal with a sample rate of 1 MHz, which would require a baseband bandwidth of0.5 MHz
according to Nyquist.However, the passband RFsignal is produced bymultiplying the baseband signal with
a carrier waveform (i.e., double-sideband quadrature amplitude-modulation) resulting in a passband
bandwidth of1 MHz. A single-side band (SSB) or vestigial sideband (VSB) modulation scheme would
achieve almosthalfthatbandwidth for the same symbol rate (i.e., twice as high spectral efficiency for the
same symbol alphabetlength).It is however more sensitive to multipath interference.
OFDM requires very accurate frequency synchronization between the receiver and the transmitter;with
frequency deviation the sub-carriers will no longer be orthogonal,causing inter-carrier interference (ICI) (i.e.,
cross-talk between the sub-carriers).Frequencyoffsets are typically caused by mismatched transmitter and
receiver oscillators,or byDoppler shift due to movement.While Doppler shiftalone may be compensated for by
the receiver, the situation is worsened when combined with multipath,as reflections will appear atvarious
frequency offsets,which is much harder to correct. This effect typically worsens as speed increases,[3]
and is an
importantfactor limiting the use of OFDM in high-speed vehicles.In order to mitigate ICI in such scenarios,one
can shape each sub-carrier in order to minimize the interference resulting in a non-orthogonal subcarriers
overlapping.[4]
For example,a low-complexityscheme referred to as WCP-OFDM (Weighted Cyclic Prefix
12. Orthogonal Frequency-Division Multiplexing) consists in using shortfilters atthe transmitter outputin order to
perform a potentiallynon-rectangular pulse shaping and a near perfect reconstruction using a single-tap per
subcarrier equalization.[5]
Other ICIsuppression techniques usuallyincrease drasticallythe receiver complexity.[6]
Implementation using the FFT algorithm[edit]
The orthogonalityallows for efficient modulator and demodulator implementation using the FFT algorithm on the
receiver side,and inverse FFT on the sender side.Although the principles and some ofthe benefits have been
known since the 1960s,OFDM is popular for wideband communications todayby way of low-costdigital signal
processingcomponents thatcan efficiently calculate the FFT.
The time to compute the inverse-FFT or FFT transform has to take less than the time for each symbol.,[7]
which
for example for DVB-T (FFT 8k) means the computation has to be done in 896 µs or less.
For an 8192-pointFFT this may be approximated to:[7][clarification needed]
[7]
MIPS = Million instructions per second
The computational demand approximatelyscales linearlywith FFT size so a double size FFT needs double
the amountof time and vice versa.[7]
As a comparison an Intel Pentium III CPU at 1.266 GHz is able to
calculate a 8 192 point FFT in 576 µs using FFTW.[8]
Intel Pentium M at 1.6 GHz does it in 387 µs.[9]
Intel
Core Duo at 3.0 GHz does it in96.8 µs.[10]
Guard interval for elimination of intersymbol interference[edit]
One key principle ofOFDM is that since low symbol rate modulation schemes (i.e.,where the symbols are
relatively long compared to the channel time characteristics) suffer less from intersymbol
interference caused by multipath propagation,itis advantageous to transmita number of low-rate streams
in parallel instead ofa single high-rate stream.Since the duration of each symbol is long,it is feasible to
inserta guard interval between the OFDM symbols,thus eliminating the intersymbol interference.
The guard interval also eliminates the need for a pulse-shaping filter,and it reduces the sensitivity to time
synchronization problems.
A simple example:Ifone sends a million symbols per second using conventional single-carrier modulation
over a wireless channel,then the duration of each symbol would be one microsecond or less.This imposes
severe constraints on synchronization and necessitates the removal ofmultipath interference.If the same
million symbols per second are spread among one thousand sub-channels,the duration of each symbol can
be longer by a factor of a thousand (i.e., one millisecond) for orthogonalitywith approximatelythe same
bandwidth.Assume thata guard interval of 1/8 of the symbol length is inserted between each symbol.
Intersymbol interference can be avoided if the multipath time-spreading (the time between the reception of
the first and the lastecho) is shorter than the guard interval (i.e., 125 microseconds).This corresponds to a
maximum difference of37.5 kilometers between the lengths ofthe paths.
13. The cyclic prefix, which is transmitted during the guard interval, consists ofthe end of the OFDM
symbol copied into the guard interval, and the guard interval is transmitted followed bythe OFDM
symbol.The reason thatthe guard interval consists ofa copy of the end of the OFDM symbol is so that
the receiver will integrate over an integer number ofsinusoid cycles for each of the multipaths when it
performs OFDM demodulation with the FFT. In some standards such as Ultrawideband,in the interest
of transmitted power,cyclic prefix is skipped and nothing is sentduring the guard interval. The receiver
will then have to mimic the cyclic prefix functionalityby copying the end part of the OFDM symbol and
adding it to the beginning portion.
Simplified equalization[edit]
The effects of frequency-selective channel conditions,for example fading caused by multipath
propagation,can be considered as constant(flat) over an OFDM sub-channel ifthe sub-channel is
sufficientlynarrow-banded (i.e.,if the number ofsub-channels is sufficientlylarge).This makes
frequency domain equalization possible atthe receiver, which is far simpler than the time-domain
equalization used in conventional single-carrier modulation.In OFDM, the equalizer only has to
multiplyeach detected sub-carrier (each Fourier coefficient) in each OFDM symbol by a constant
complexnumber,or a rarely changed value.
Our example:The OFDM equalization in the above numerical example would require one complexvalued
multiplication per subcarrier and symbol (i.e., complexmultiplications per OFDM symbol;i.e.,
one million multiplications per second,atthe receiver). The FFT algorithm requires
[this is imprecise:over half of these complexmultiplications are trivial,i.e. = to 1 and are not implementedin
software or HW]. complex-valued multiplications per OFDM symbol (i.e., 10 million multiplications per
second),at both the receiver and transmitter side.This should be compared with the corresponding one
million symbols/second single-carrier modulation case mentioned in the example,where the equalization of
125 microseconds time-spreading using a FIR filter would require,in a naive implementation,125
multiplications per symbol (i.e.,125 million multiplications per second).FFT techniques can be used to
reduce the number ofmultiplications for an FIR filter based time-domain equalizer to a number comparable
with OFDM, at the costof delay between reception and decoding which also becomes comparable with
OFDM.
If differential modulation such as DPSK or DQPSK is applied to each sub-carrier,equalization can
be completelyomitted,since these non-coherentschemes are insensitive to slowlychanging
amplitude and phase distortion.
In a sense,improvements in FIR equalization using FFTs or partial FFTs leads mathematically
closer to OFDM,[citation needed]
but the OFDM technique is easier to understand and implement,and the
sub-channels can be independentlyadapted in other ways than varying equalization coefficients,
such as switching between different QAM constellation patterns and error-correction schemes to
match individual sub-channel noise and interference characteristics.[clarification needed]
Some of the sub-carriers in some ofthe OFDM symbols maycarry pilotsignals for measurement
of the channel conditions[11][12]
(i.e., the equalizer gain and phase shiftfor each sub-carrier).Pilot
signals and training symbols (preambles) mayalso be used for time synchronization (to avoid
intersymbol interference,ISI) and frequency synchronization (to avoid inter-carrier interference,
ICI, caused by Doppler shift).
14. OFDM was initiallyused for wired and stationarywireless communications.However,with an
increasing number ofapplications operating in highlymobile environments,the effect of dispersive
fading caused by a combination ofmulti-path propagation and doppler shift is more significant.
Over the lastdecade,research has been done on how to equalize OFDM transmission over
doubly selective channels.[13][14][15]
Channel coding and interleaving[edit]
OFDM is invariably used in conjunction with channel coding (forward error correction),and almost
always uses frequencyand/or time interleaving.
Frequency (subcarrier) interleaving increases resistance to frequency-selective channel conditions
such as fading.For example,when a part of the channel bandwidth fades,frequencyinterleaving
ensures thatthe bit errors that would resultfrom those subcarriers in the faded part of the
bandwidth are spread outin the bit-stream rather than being concentrated.Similarly,time
interleaving ensures thatbits that are originallyclose together in the bit-stream are transmitted far
apart in time, thus mitigating againstsevere fading as would happen when travelling athigh
speed.
However, time interleaving is of little benefitin slowlyfading channels,such as for stationary
reception,and frequency interleaving offers little to no benefit for narrowband channels thatsuffer
from flat-fading (where the whole channel bandwidth fades atthe same time).
The reason why interleaving is used on OFDM is to attemptto spread the errors out in the bit-
stream thatis presented to the error correction decoder,because when such decoders are
presented with a high concentration of errors the decoder is unable to correct all the bit errors,
and a burstof uncorrected errors occurs.A similar design ofaudio data encoding makes compact
disc (CD) playback robust.
A classical type of error correction coding used with OFDM-based systems is convolutional
coding,often concatenated with Reed-Solomon coding.Usually,additional interleaving (on top of
the time and frequency interleaving mentioned above) in between the two layers of coding is
implemented.The choice for Reed-Solomon coding as the outer error correction code is based on
the observation that the Viterbi decoder used for inner convolutional decoding produces short
error bursts when there is a high concentration oferrors,and Reed-Solomon codes are inherently
well-suited to correcting bursts oferrors.
Newer systems,however,usuallynow adoptnear-optimal types of error correction codes that use
the turbo decoding principle,where the decoder iterates towards the desired solution.Examples
of such error correction coding types include turbo codes and LDPC codes,which perform close
to the Shannon limit for the Additive White Gaussian Noise (AWGN) channel.Some systems that
have implemented these codes have concatenated them with either Reed-Solomon (for example
on the MediaFLO system) or BCH codes(on the DVB-S2 system) to improve upon an error
floor inherentto these codes athigh signal-to-noise ratios.[16]
Adaptive transmission[edit]
The resilience to severe channel conditions can be further enhanced ifinformation aboutthe
channel is sentover a return-channel.Based on this feedback information,adaptivemodulation,
channel coding and power allocation maybe applied across all sub-carriers,or individuallyto
15. each sub-carrier.In the latter case,if a particular range of frequencies suffers from interference or
attenuation,the carriers within that range can be disabled or made to run slower by applying more
robustmodulation or error coding to those sub-carriers.
The term discrete multitone modulation (DMT) denotes OFDM based communication systems that
adaptthe transmission to the channel conditions individuallyfor each sub-carrier,by means ofso-
called bit-loading.Examples are ADSL and VDSL.
The upstream and downstream speeds can be varied by allocating either more or fewer carriers
for each purpose.Some forms of rate-adaptive DSL use this feature in real time,so that the
bitrate is adapted to the co-channel interference and bandwidth is allocated to whichever
subscriber needs itmost.
OFDM extended with multiple access[edit]
OFDM in its primaryform is considered as a digital modulation technique,and nota multi-
user channel access method,since itis utilized for transferring one bit stream over one
communication channel using one sequence ofOFDM symbols.However,OFDM can be
combined with multiple access using time,frequencyor coding separation ofthe users.
In orthogonal frequency-division multiple access (OFDMA), frequency-division multiple access is
achieved by assigning differentOFDM sub-channels to differentusers.OFDMA supports
differentiated quality of service by assigning differentnumber ofsub-carriers to differentusers in a
similar fashion as in CDMA, and thus complexpacket scheduling orMedia Access
Control schemes can be avoided. OFDMA is used in:
the mobilitymode of the IEEE 802.16 Wireless MAN standard,commonlyreferred to as
WiMAX,
the IEEE 802.20 mobile Wireless MAN standard,commonlyreferred to as MBWA,
the 3GPP Long Term Evolution (LTE) fourth generation mobile broadband standard
downlink.The radio interface was formerlynamed High Speed OFDM Packet Access
(HSOPA), now named Evolved UMTS Terrestrial Radio Access (E-UTRA).
the now defunct Qualcomm/3GPP2 Ultra Mobile Broadband (UMB) project, intended as a
successor of CDMA2000, but replaced by LTE.
OFDMA is also a candidate access method for the IEEE 802.22 Wireless Regional Area
Networks (WRAN). The projectaims atdesigning the first cognitive radio based standard
operating in the VHF-low UHF spectrum (TV spectrum).
In Multi-carrier code division multiple access (MC-CDMA), also known as OFDM-CDMA, OFDM is
combined with CDMA spread spectrum communication for coding separation ofthe users.Co-
channel interference can be mitigated,meaning thatmanual fixed channel allocation (FCA)
frequency planning is simplified,or complexdynamic channel allocation (DCA) schemes are
avoided.
Space diversity[edit]
In OFDM based wide area broadcasting,receivers can benefitfrom receiving signals from several
spatiallydispersed transmitters simultaneously,since transmitters will onlydestructivelyinterfere
with each other on a limited number ofsub-carriers,whereas in general theywill actually reinforce
16. coverage over a wide area. This is very beneficial in many countries,as itpermits the operation of
national single-frequencynetworks (SFN), where many transmitters send the same signal
simultaneouslyover the same channel frequency.SFNs utilise the available spectrum more
effectively than conventional multi-frequencybroadcastnetworks (MFN), where program content
is replicated on different carrier frequencies.SFNs also resultin a diversity gain in receivers
situated midwaybetween the transmitters.The coverage area is increased and the outage
probabilitydecreased in comparison to an MFN, due to increased received signal strength
averaged over all sub-carriers.
Although the guard interval only contains redundantdata,which means thatit reduces the
capacity, some OFDM-based systems,such as some ofthe broadcasting systems,deliberately
use a long guard interval in order to allow the transmitters to be spaced farther apart in an SFN,
and longer guard intervals allow larger SFN cell-sizes.A rule of thumb for the maximum distance
between transmitters in an SFN is equal to the distance a signal travels during the guard interval
— for instance,a guard interval of 200 microseconds would allow transmitters to be spaced 60 km
apart.
A single frequency network is a form of transmitter macrodiversity.The concept can be further
utilized in dynamic single-frequency networks (DSFN),where the SFN grouping is changed from
timeslotto timeslot.
OFDM may be combined with other forms of space diversity, for example antenna
arrays and MIMO channels.This is done in the IEEE 802.11 Wireless LAN standards.
Linear transmitter power amplifier[edit]
An OFDM signal exhibits a high peak-to-average power ratio (PAPR) because the independent
phases ofthe sub-carriers mean thatthey will often combine constructively.Handling this high
PAPR requires:
A high-resolution digital-to-analogue converter (DAC) in the transmitter
A high-resolution analogue-to-digital converter (ADC) in the receiver
A linear signal chain
Any non-linearityin the signal chain will cause intermodulation distortion that
Raises the noise floor
May cause inter-carrier interference
Generates out-of-band spurious radiation
The linearity requirementis demanding,especiallyfor transmitter RFoutput circuitry where
amplifiers are often designed to be non-linear in order to minimise power consumption.In practical
OFDM systems a small amountofpeak clipping is allowed to limitthe PAPR in a judicious trade-
off againstthe above consequences.However,the transmitter outputfilter which is required to
reduce out-of-band spurs to legal levels has the effect of restoring peak levels that were clipped,
so clipping is notan effective way to reduce PAPR.
Although the spectral efficiency of OFDM is attractive for both terrestrial and space
communications,the high PAPR requirements have so far limited OFDM applications to terrestrial
systems.
17. The crestfactor CF (in dB) for an OFDM system with n uncorrelated sub-carriers is[17]
CF = 10 log ( n ) + CFc ...
where CFc is the crest factor (in dB) for each sub-carrier.(CFc is 3.01 dB for the sine waves used
for BPSK and QPSK modulation).
For example,the DVB-T signal in 2K mode is composed of1705 sub-carriers thatare each
QPSK-modulated,giving a crestfactor of 35.32 dB.[17]
Many crestfactor reduction techniques have been developed.
The dynamic range required for an FM receiver is 120 dB while DAB only require about 90
dB.[18]
As a comparison,each extra bit per sample increases the dynamic range with6 dB.
Efficiency comparison between single carrier and multicarrier[edit]
The performance ofany communication system can be measured in terms ofits power efficiency
and bandwidth efficiency. The power efficiency describes the abilityof communication system to
preserve bit error rate (BER) of the transmitted signal atlow power levels. Bandwidth
efficiency reflects how efficiently the allocated bandwidth is utilized and is defined as the
throughputdata rate per Hertz in a given bandwidth.If the large number of subcarriers are used,
the bandwidth efficiencyof multicarrier system such as OFDMwith using optical fiber channel is
defined as[19]
Factor 2 is because oftwo polarization states in the fiber.
where is the symbol rate in giga symbol per second (Gsps),and is the
bandwidth ofOFDM signal.
There is saving of bandwidth by using Multicarrier modulation with orthogonal frequencydivision
multiplexing .So the bandwidth for multicarrier system is less in comparison with single carrier
system and hence bandwidth efficiencyof multicarrier system is larger than single carrier s ystem.
S.No.
Transmission
Type
M in
M-
QAM
No. of
Subcarriers
Bit
rate
Fiber
length
Power at the
receiver (at
BER of 10−9
)
Bandwidth
efficiency
1. single carrier 64 1
10
Gbit/s
20 km -37.3 dBm 6.0000
2. multicarrier 64 128
10
20 km -36.3 dBm 10.6022
18. Gbit/s
There is only 1 dBm increase in receiver power,but we get 76.7% improvementin bandwidth
efficiency with using multicarrier transmission technique.
Characteristics and principles of operation[edit]
Orthogonality[edit]
Conceptually,OFDM is a specialized FDM, the additional constraintbeing thatall carrier signals are orthogonal to
one another.
In OFDM, the sub-carrier frequencies are chosen so thatthe sub-carriers are orthogonal to each other, meaning
that cross-talk between the sub-channels is eliminated and inter-carrier guard bands are notrequired.This greatly
simplifies the design ofboth the transmitter and the receiver; unlike conventional FDM, a separate filter for each sub-
channel is notrequired.
The orthogonalityrequires thatthe sub-carrier spacing is Hertz, where TU seconds is the useful symbol
duration (the receiver side window size),and k is a positive integer, typically equal to 1. Therefore, with N sub-
carriers,the total passband bandwidth will be B ≈ N·Δf (Hz).
The orthogonalityalso allows high spectral efficiency,with a total symbol rate near the Nyquistrate for the equivalent
baseband signal (i.e.near halfthe Nyquist rate for the double-side band physical passband signal).Almostthe whole
available frequency band can be utilized. OFDM generallyhas a nearly 'white' spectrum,giving it benign
electromagnetic interference properties with respectto other co-channel users.
A simple example:A useful symbol duration TU = 1 ms would require a sub-carrier spacing
of (or an integer multiple ofthat) for orthogonality. N = 1,000 sub-carriers would result
in a total passband bandwidth of NΔf = 1 MHz. For this symbol time,the required bandwidth in theory
according to Nyquistis N=1/2TU = 0.5 MHz (i.e., halfof the achieved bandwidth required byour scheme).If a
guard interval is applied (see below),Nyquistbandwidth requirementwould be even lower. The FFT would
resultin N = 1,000 samples per symbol.If no guard interval was applied,this would resultin a base band
complexvalued signal with a sample rate of 1 MHz, which would require a baseband bandwidth of0.5 MHz
according to Nyquist.However, the passband RFsignal is produced bymultiplying the baseband signal with
a carrier waveform (i.e., double-sideband quadrature amplitude-modulation) resulting in a passband
bandwidth of1 MHz. A single-side band (SSB) or vestigial sideband (VSB) modulation scheme would
achieve almosthalfthatbandwidth for the same symbol rate (i.e., twice as high spectral efficiency for the
same symbol alphabetlength).It is however more sensitive to multipath interference.
OFDM requires very accurate frequency synchronization between the receiver and the transmitter;with
frequency deviation the sub-carriers will no longer be orthogonal,causing inter-carrier interference (ICI) (i.e.,
cross-talk between the sub-carriers).Frequencyoffsets are typically caused by mismatched transmitter and
receiver oscillators,or byDoppler shift due to movement.While Doppler shiftalone may be compensated for by
the receiver, the situation is worsened when combined with multipath,as reflections will appear atvarious
frequency offsets,which is much harder to correct. This effect typically worsens as speed increases,[3]
and is an
importantfactor limiting the use of OFDM in high-speed vehicles.In order to mitigate ICI in such scenarios,one
can shape each sub-carrier in order to minimize the interference resulting in a non-orthogonal subcarriers
overlapping.[4]
For example,a low-complexityscheme referred to as WCP-OFDM (Weighted Cyclic Prefix
19. Orthogonal Frequency-Division Multiplexing) consists in using shortfilters atthe transmitter outputin order to
perform a potentiallynon-rectangular pulse shaping and a near perfect reconstruction using a single-tap per
subcarrier equalization.[5]
Other ICIsuppression techniques usuallyincrease drasticallythe receiver complexity.[6]
Implementation using the FFT algorithm[edit]
The orthogonalityallows for efficient modulator and demodulator implementation using the FFT algorithm on the
receiver side,and inverse FFT on the sender side.Although the principles and some ofthe benefits have been
known since the 1960s,OFDM is popular for wideband communications todayby way of low-costdigital signal
processingcomponents thatcan efficiently calculate the FFT.
The time to compute the inverse-FFT or FFT transform has to take less than the time for each symbol.,[7]
which
for example for DVB-T (FFT 8k) means the computation has to be done in 896 µs or less.
For an 8192-pointFFT this may be approximated to:[7][clarification needed]
[7]
MIPS = Million instructions per second
The computational demand approximatelyscales linearlywith FFT size so a double size FFT needs double
the amountof time and vice versa.[7]
As a comparison an Intel Pentium III CPU at 1.266 GHz is able to
calculate a 8 192 point FFT in 576 µs using FFTW.[8]
Intel Pentium M at 1.6 GHz does it in 387 µs.[9]
Intel
Core Duo at 3.0 GHz does it in96.8 µs.[10]
Guard interval for elimination of intersymbol interference[edit]
One key principle ofOFDM is that since low symbol rate modulation schemes (i.e.,where the symbols are
relatively long compared to the channel time characteristics) suffer less from intersymbol
interference caused by multipath propagation,itis advantageous to transmita number of low-rate streams
in parallel instead ofa single high-rate stream.Since the duration of each symbol is long,it is feasible to
inserta guard interval between the OFDM symbols,thus eliminating the intersymbol interference.
The guard interval also eliminates the need for a pulse-shaping filter,and it reduces the sensitivity to time
synchronization problems.
A simple example:Ifone sends a million symbols per second using conventional single-carrier modulation
over a wireless channel,then the duration of each symbol would be one microsecond or less.This imposes
severe constraints on synchronization and necessitates the removal ofmultipath interference.If the same
million symbols per second are spread among one thousand sub-channels,the duration of each symbol can
be longer by a factor of a thousand (i.e., one millisecond) for orthogonalitywith approximatelythe same
bandwidth.Assume thata guard interval of 1/8 of the symbol length is inserted between each symbol.
Intersymbol interference can be avoided if the multipath time-spreading (the time between the reception of
the first and the lastecho) is shorter than the guard interval (i.e., 125 microseconds).This corresponds to a
maximum difference of37.5 kilometers between the lengths ofthe paths.
20. The cyclic prefix, which is transmitted during the guard interval, consists ofthe end of the OFDM
symbol copied into the guard interval, and the guard interval is transmitted followed bythe OFDM
symbol.The reason thatthe guard interval consists ofa copy of the end of the OFDM symbol is so that
the receiver will integrate over an integer number ofsinusoid cycles for each of the multipaths when it
performs OFDM demodulation with the FFT. In some standards such as Ultrawideband,in the interest
of transmitted power,cyclic prefix is skipped and nothing is sentduring the guard interval. The receiver
will then have to mimic the cyclic prefix functionalityby copying the end part of the OFDM symbol and
adding it to the beginning portion.
Simplified equalization[edit]
The effects of frequency-selective channel conditions,for example fading caused by multipath
propagation,can be considered as constant(flat) over an OFDM sub-channel ifthe sub-channel is
sufficientlynarrow-banded (i.e.,if the number ofsub-channels is sufficientlylarge).This makes
frequency domain equalization possible atthe receiver, which is far simpler than the time-domain
equalization used in conventional single-carrier modulation.In OFDM, the equalizer only has to
multiplyeach detected sub-carrier (each Fourier coefficient) in each OFDM symbol by a constant
complexnumber,or a rarely changed value.
Our example:The OFDM equalization in the above numerical example would require one complexvalued
multiplication per subcarrier and symbol (i.e., complexmultiplications per OFDM symbol;i.e.,
one million multiplications per second,atthe receiver). The FFT algorithm requires
[this is imprecise:over half of these complexmultiplications are trivial,i.e. = to 1 and are not implementedin
software or HW]. complex-valued multiplications per OFDM symbol (i.e., 10 million multiplications per
second),at both the receiver and transmitter side.This should be compared with the corresponding one
million symbols/second single-carrier modulation case mentioned in the example,where the equalization of
125 microseconds time-spreading using a FIR filter would require,in a naive implementation,125
multiplications per symbol (i.e., 125 million multiplications per second).FFT techniques can be used to
reduce the number ofmultiplications for an FIR filter based time-domain equalizer to a number comparable
with OFDM, at the costof delay between reception and decoding which also becomes comparable with
OFDM.
If differential modulation such as DPSK or DQPSK is applied to each sub-carrier,equalization can
be completelyomitted,since these non-coherentschemes are insensitive to slowlychanging
amplitude and phase distortion.
In a sense,improvements in FIR equalization using FFTs or partial FFTs leads mathematically
closer to OFDM,[citation needed]
but the OFDM technique is easier to understand and implement,and the
sub-channels can be independentlyadapted in other ways than varying equalization coefficients,
such as switching between different QAM constellation patterns and error-correction schemes to
match individual sub-channel noise and interference characteristics.[clarification needed]
Some of the sub-carriers in some ofthe OFDM symbols maycarry pilotsignals for measurement
of the channel conditions[11][12]
(i.e., the equalizer gain and phase shiftfor each sub-carrier).Pilot
signals and training symbols (preambles) mayalso be used for time synchronization (to avoid
intersymbol interference,ISI) and frequency synchronization (to avoid inter-carrier interference,
ICI, caused by Doppler shift).
21. OFDM was initiallyused for wired and stationarywireless communications.However,with an
increasing number ofapplications operating in highlymobile environments,the effect of dispersive
fading caused by a combination ofmulti-path propagation and doppler shift is more significant.
Over the lastdecade,research has been done on how to equalize OFDM transmission over
doubly selective channels.[13][14][15]
Channel coding and interleaving[edit]
OFDM is invariably used in conjunction with channel coding (forward error correction),and almost
always uses frequencyand/or time interleaving.
Frequency (subcarrier) interleaving increases resistance to frequency-selective channel conditions
such as fading.For example,when a part of the channel bandwidth fades,frequencyinterleaving
ensures thatthe bit errors that would resultfrom those subcarriers in the faded part of the
bandwidth are spread outin the bit-stream rather than being concentrated.Similarly,time
interleaving ensures thatbits that are originallyclose together in the bit-stream are transmitted far
apart in time, thus mitigating againstsevere fading as would happen when travelling athigh
speed.
However, time interleaving is of little benefitin slowlyfading channels,such as for stationary
reception,and frequency interleaving offers little to no benefit for narrowband channels thatsuffer
from flat-fading (where the whole channel bandwidth fades atthe same time).
The reason why interleaving is used on OFDM is to attemptto spread the errors out in the bit-
stream thatis presented to the error correction decoder,because when such decoders are
presented with a high concentration of errors the decoder is unable to correct all the bit errors,
and a burstof uncorrected errors occurs.A similar design ofaudio data encoding makes compact
disc (CD) playback robust.
A classical type of error correction coding used with OFDM-based systems is convolutional
coding,often concatenated with Reed-Solomon coding.Usually,additional interleaving (on top of
the time and frequency interleaving mentioned above) in between the two layers of coding is
implemented.The choice for Reed-Solomon coding as the outer error correction code is based on
the observation that the Viterbi decoder used for inner convolutional decoding produces short
error bursts when there is a high concentration oferrors,and Reed-Solomon codes are inherently
well-suited to correcting bursts oferrors.
Newer systems,however,usuallynow adoptnear-optimal types of error correction codes that use
the turbo decoding principle,where the decoder iterates towards the desired solution.Examples
of such error correction coding types include turbo codes and LDPC codes,which perform close
to the Shannon limit for the Additive White Gaussian Noise (AWGN) channel.Some systems that
have implemented these codes have concatenated them with either Reed-Solomon (for example
on the MediaFLO system) or BCH codes(on the DVB-S2 system) to improve upon an error
floor inherentto these codes athigh signal-to-noise ratios.[16]
Adaptive transmission[edit]
The resilience to severe channel conditions can be further enhanced ifinformation aboutthe
channel is sentover a return-channel.Based on this feedback information,adaptivemodulation,
channel coding and power allocation maybe applied across all sub-carriers,or individuallyto
22. each sub-carrier.In the latter case,if a particular range of frequencies suffers from interference or
attenuation,the carriers within that range can be disabled or made to run slower by applying more
robustmodulation or error coding to those sub-carriers.
The term discrete multitone modulation (DMT) denotes OFDM based communication systems that
adaptthe transmission to the channel conditions individuallyfor each sub-carrier,by means ofso-
called bit-loading.Examples are ADSL and VDSL.
The upstream and downstream speeds can be varied by allocating either more or fewer carriers
for each purpose.Some forms ofrate-adaptive DSL use this feature in real time,so that the
bitrate is adapted to the co-channel interference and bandwidth is allocated to whichever
subscriber needs itmost.
OFDM extended with multiple access[edit]
OFDM in its primaryform is considered as a digital modulation technique,and nota multi-
user channel access method,since itis utilized for transferring one bit stream over one
communication channel using one sequence ofOFDM symbols.However,OFDM can be
combined with multiple access using time,frequencyor coding separation ofthe users.
In orthogonal frequency-division multiple access (OFDMA), frequency-division multiple access is
achieved by assigning differentOFDM sub-channels to different users.OFDMA supports
differentiated quality of service by assigning differentnumber ofsub-carriers to differentusers in a
similar fashion as in CDMA, and thus complexpacket scheduling orMedia Access
Control schemes can be avoided. OFDMA is used in:
the mobilitymode of the IEEE 802.16 Wireless MAN standard,commonlyreferred to as
WiMAX,
the IEEE 802.20 mobile Wireless MAN standard,commonlyreferred to as MBWA,
the 3GPP Long Term Evolution (LTE) fourth generation mobile broadband standard
downlink.The radio interface was formerlynamed High Speed OFDM Packet Access
(HSOPA), now named Evolved UMTS Terrestrial Radio Access (E-UTRA).
the now defunct Qualcomm/3GPP2 Ultra Mobile Broadband (UMB) project, intended as a
successor ofCDMA2000, but replaced by LTE.
OFDMA is also a candidate access method for the IEEE 802.22 Wireless Regional Area
Networks (WRAN). The projectaims atdesigning the first cognitive radio based standard
operating in the VHF-low UHF spectrum (TV spectrum).
In Multi-carrier code division multiple access (MC-CDMA), also known as OFDM-CDMA, OFDM is
combined with CDMA spread spectrum communication for coding separation ofthe users.Co-
channel interference can be mitigated,meaning thatmanual fixed channel allocation (FCA)
frequency planning is simplified,or complexdynamic channel allocation (DCA) schemes are
avoided.
Space diversity[edit]
In OFDM based wide area broadcasting,receivers can benefitfrom receiving signals from several
spatiallydispersed transmitters simultaneously,since transmitters will onlydestructivelyinterfere
with each other on a limited number ofsub-carriers,whereas in general theywill actually reinforce
23. coverage over a wide area. This is very beneficial in many countries,as itpermits the operation of
national single-frequencynetworks (SFN), where many transmitters send the same signal
simultaneouslyover the same channel frequency.SFNs utilise the available spectrum more
effectively than conventional multi-frequencybroadcastnetworks (MFN), where program content
is replicated on different carrier frequencies.SFNs also resultin a diversity gain in receivers
situated midwaybetween the transmitters.The coverage area is increased and the outage
probabilitydecreased in comparison to an MFN, due to increased received signal strength
averaged over all sub-carriers.
Although the guard interval only contains redundantdata,which means thatit reduces the
capacity, some OFDM-based systems,such as some ofthe broadcasting systems,deliberately
use a long guard interval in order to allow the transmitters to be spaced farther apart in an SFN,
and longer guard intervals allow larger SFN cell-sizes.A rule of thumb for the maximum distance
between transmitters in an SFN is equal to the distance a signal travels during the guard interval
— for instance,a guard interval of 200 microseconds would allow transmitters to be spaced 60 km
apart.
A single frequency network is a form of transmitter macrodiversity.The concept can be further
utilized in dynamic single-frequency networks (DSFN),where the SFN grouping is changed from
timeslotto timeslot.
OFDM may be combined with other forms of space diversity, for example antenna
arrays and MIMO channels.This is done in the IEEE 802.11 Wireless LAN standards.
Linear transmitter power amplifier[edit]
An OFDM signal exhibits a high peak-to-average power ratio (PAPR) because the independent
phases ofthe sub-carriers mean thatthey will often combine constructively.Handling this high
PAPR requires:
A high-resolution digital-to-analogue converter (DAC) in the transmitter
A high-resolution analogue-to-digital converter (ADC) in the receiver
A linear signal chain
Any non-linearityin the signal chain will cause intermodulation distortion that
Raises the noise floor
May cause inter-carrier interference
Generates out-of-band spurious radiation
The linearity requirementis demanding,especiallyfor transmitter RFoutput circuitry where
amplifiers are often designed to be non-linear in order to minimise power consumption.In practical
OFDM systems a small amountofpeak clipping is allowed to limitthe PAPR in a judicious trade-
off againstthe above consequences.However,the transmitter outputfilter which is required to
reduce out-of-band spurs to legal levels has the effect of restoring peak levels that were clipped,
so clipping is notan effective way to reduce PAPR.
Although the spectral efficiency of OFDM is attractive for both terrestrial and space
communications,the high PAPR requirements have so far limited OFDM applications to terrestrial
systems.
24. The crestfactor CF (in dB) for an OFDM system with n uncorrelated sub-carriers is[17]
CF = 10 log ( n ) + CFc ...
where CFc is the crest factor (in dB) for each sub-carrier.(CFc is 3.01 dB for the sine waves used
for BPSK and QPSK modulation).
For example,the DVB-T signal in 2K mode is composed of1705 sub-carriers thatare each
QPSK-modulated,giving a crestfactor of 35.32 dB.[17]
Many crestfactor reduction techniques have been developed.
The dynamic range required for an FM receiver is 120 dB while DAB only require about 90
dB.[18]
As a comparison,each extra bit per sample increases the dynamic range with6 dB.
Efficiency comparison between single carrier and multicarrier[edit]
The performance ofany communication system can be measured in terms ofits power efficiency
and bandwidth efficiency. The power efficiency describes the abilityof communication system to
preserve bit error rate (BER) of the transmitted signal atlow power levels. Bandwidth
efficiency reflects how efficiently the allocated bandwidth is utilized and is defined as the
throughputdata rate per Hertz in a given bandwidth.If the large number of subcarriers are used,
the bandwidth efficiencyof multicarrier system such as OFDMwith using optical fiber channel is
defined as[19]
Factor 2 is because oftwo polarization states in the fiber.
where is the symbol rate in giga symbol per second (Gsps),and is the
bandwidth ofOFDM signal.
There is saving of bandwidth by using Multicarrier modulation with orthogonal frequencydivision
multiplexing .So the bandwidth for multicarrier system is less in comparison with single carrier
system and hence bandwidth efficiencyof multicarrier system is larger than single carrier system.
S.No.
Transmission
Type
M in
M-
QAM
No. of
Subcarriers
Bit
rate
Fiber
length
Power at the
receiver (at
BER of 10−9
)
Bandwidth
efficiency
1. single carrier 64 1
10
Gbit/s
20 km -37.3 dBm 6.0000
2. multicarrier 64 128
10
20 km -36.3 dBm 10.6022
25. Gbit/s
There is only 1 dBm increase in receiver power,but we get 76.7% improvementin bandwidth
efficiency with using multicarrier transmission technique.
This section describes a simple idealized OFDM system model suitable for a time-
invariant AWGN channel.
An OFDM carrier signal is the sum ofa number of orthogonal sub-carriers,with baseband data on each sub-carrier
being independentlymodulated commonlyusing some type of quadrature amplitude modulation (QAM) or phase-shift
keying (PSK). This composite baseband signal is typicallyused to modulate a main RF carrier.
is a serial stream ofbinary digits.By inverse multiplexing,these are firstdemultiplexed into parallel streams,
and each one mapped to a (possiblycomplex) symbol stream using some modulation constellation (QAM, PSK, etc.).
Note that the constellations maybe different, so some streams maycarry a higher bit-rate than others.
An inverse FFT is computed on each setof symbols,giving a setof complex time-domain samples.These samples
are then quadrature-mixed to passband in the standard way.The real and imaginarycomponents are firstconverted
to the analogue domain using digital-to-analogue converters (DACs);the analogue signals are then used to
modulatecosine and sine waves atthe carrier frequency, , respectively. These signals are then summed to give
the transmission signal, .
Receiver
26. The receiver picks up the signal , which is then quadrature-mixed down to baseband using cosine and sine
waves at the carrier frequency. This also creates signals centered on , so low-pass filters are used to reject
these.The baseband signals are then sampled and digitised using analog-to-digital converters (ADCs),and a
forward FFT is used to convert back to the frequency domain.
This returns parallel streams,each ofwhich is converted to a binary stream using an appropriate symbol detector.
These streams are then re-combined into a serial stream, ,which is an estimate ofthe original binarystream at
the transmitter.
Mathematical description
If sub-carriers are used,and each sub-carrier is modulated using alternative symbols,the OFDM symbol
alphabetconsists of combined symbols.
The low-pass equivalent OFDM signal is expressed as:
where are the data symbols, is the number ofsub-carriers,and is the OFDM symbol time.The
sub-carrier spacing of makes them orthogonal over each symbol period;this property is expressed as:
where denotes the complexconjugate operator and is the Kronecker delta.
To avoid intersymbol interference in multipath fading channels,a guard interval of length is inserted
prior to the OFDM block. During this interval, a cyclic prefix is transmitted such thatthe signal in the
interval equals the signal in the interval . The OFDM signal with cyclic prefix
is thus:
The low-pass signal above can be either real or complex-valued.Real-valued low-pass equivalent
signals are typicallytransmitted atbaseband—wireline applications such as DSL use this approach.
For wireless applications,the low-pass signal is typicallycomplex-valued;in which case,the
transmitted signal is up-converted to a carrier frequency . In general, the transmitted signal can be
represented as: