Brief Introduction to Spread spectrum Techniques
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There are two main types of spread spectrum multiple access techniques: frequency hopped multiple access (FHMA) and direct sequence multiple access (DSMA). FHMA involves transmitting digital data on different carrier frequencies in a pseudorandom pattern, while DSMA multiplies a message signal by a pseudorandom spreading signal with a much higher chip rate. Code division multiple access (CDMA) is another name for DSMA. Both techniques allow multiple users to transmit simultaneously on the same frequency band.
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Spread spectrum modulation
This document provides an overview of spread spectrum technologies including frequency hopping spread spectrum (FHSS) and direct sequence spread spectrum (DSSS). It defines spread spectrum, describes how data is modulated at different rates using DSSS, explains FHSS and DSSS in detail, and lists factors that impact wireless signal performance. Key aspects covered include the FCC regulations for unlicensed use of spread spectrum in the 2.4GHz band, how DSSS spreads and encodes data across multiple frequencies, and how FHSS rapidly hops between frequencies to transmit information.
Spread spectrum
Spread spectrum communication uses wideband noise-like signals that are hard to detect, intercept, or jam. It spreads data over multiple frequencies. There are two main techniques: direct sequence spread spectrum multiplies a data signal by a pseudorandom code, and frequency hopping spread spectrum modulates a narrowband carrier that hops between frequencies. Spread spectrum provides benefits like resistance to interference and jamming, better signal quality, and inherent security. It finds applications in wireless networks, Bluetooth, and CDMA cellular systems.
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This document discusses multiplexing and multiple access techniques used in communication networks. It describes multiplexing as transmitting multiple signals over a single medium, such as frequency division multiplexing (FDM) and time division multiplexing (TDM). Multiple access allows several stations to transmit simultaneously into the same network using techniques like frequency division multiple access (FDMA), time division multiple access (TDMA), and code division multiple access (CDMA). FDMA divides the available bandwidth into frequency slots, TDMA divides transmission time into time slots, and CDMA assigns unique codes to signals.
Frequency Division Multiple Access (FDMA)
Frequency Division Multiple Access (FDMA) is a channel access method where the available bandwidth is divided into multiple non-overlapping frequency bands and each user is assigned a specific frequency band. Each user can transmit or receive independently in its assigned frequency band without interference from other users. FDMA requires expensive bandpass filters for each frequency band and has strict linearity requirements for the transmission medium. The number of channels in an FDMA system is calculated by dividing the total available bandwidth minus the guard bands by the bandwidth of each individual channel.
1. introduction to wireless communication
Wireless communication transfers information between points without a wired connection. A transmitter sends electromagnetic waves through the air medium to a receiver. Radio propagation allows signals to travel from the transmitter in three modes: direct, ground wave, and sky wave. Multipath propagation occurs when signals reach the receiver along multiple paths, which can cause constructive or destructive interference. Fading describes signal attenuation over time due to factors like multipath delay spread, Doppler spread from mobility, rain, obstacles, and frequency. Shadowing also reduces signal strength from blockage by objects along the transmission path.
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2.2 frequency division multiple access
Frequency Division Multiple Access (FDMA) is a multiple access technique that assigns individual frequency bands or channels to each user. FDMA avoids interference by ensuring each user is allocated a unique frequency band. While FDMA allows multiple users to access the channel simultaneously and has low inter-symbol interference, it requires tight RF filtering to minimize adjacent channel interference and duplexers, increasing costs. Non-linear effects from power amplifiers can also cause intermodulation frequencies, interfering with adjacent channels. The capacity of an FDMA system is calculated based on the total spectrum allocation, guard band allocation, and channel bandwidth.
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Frequency Division Multiple Access (FDMA)
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Wireless communication transfers information between points without a wired connection. A transmitter sends electromagnetic waves through the air medium to a receiver. Radio propagation allows signals to travel from the transmitter in three modes: direct, ground wave, and sky wave. Multipath propagation occurs when signals reach the receiver along multiple paths, which can cause constructive or destructive interference. Fading describes signal attenuation over time due to factors like multipath delay spread, Doppler spread from mobility, rain, obstacles, and frequency. Shadowing also reduces signal strength from blockage by objects along the transmission path.
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2.2 frequency division multiple access
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2. wireless propagation models free space propagation
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3. free space path loss model part 1
Frii's equation models free space path loss and predicts the amount of power (Pr) that can be received by an antenna at a distance D from the transmitting antenna. The equation is:
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Where Pt is the transmitted power, Gt and Gr are the gains of the transmitting and receiving antennas, λ is the wavelength, D is the distance between antennas, and L accounts for transmission losses. According to the model, received power decays quadratically with distance, and factors like antenna gain and wavelength affect reception levels.
2.3 time division multiple access
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Tdm & fdm
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Gmsk
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IS-95 Cdma
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This document provides an overview of multiple access techniques for wireless communications, including FDMA, TDMA, CDMA, and various spread spectrum techniques. It describes the basic principles of each technique, including how they divide up frequency bands, time slots, or code channels to allow multiple users to access a shared channel simultaneously. Examples of wireless networks that use each technique are also provided.
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Frii's equation models free space path loss and predicts the amount of power (Pr) that can be received by an antenna at a distance D from the transmitting antenna. The equation is:
Pr = Pt * Gt * Gr * λ2 / (4πD)2 * L
Where Pt is the transmitted power, Gt and Gr are the gains of the transmitting and receiving antennas, λ is the wavelength, D is the distance between antennas, and L accounts for transmission losses. According to the model, received power decays quadratically with distance, and factors like antenna gain and wavelength affect reception levels.
2.3 time division multiple access
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Gmsk
Gaussian Minimum Shift Keying (GMSK) is a form of continuous-phase frequency shift keying that uses a Gaussian filter to generate a constant envelope signal. It provides better spectral efficiency than MSK through bandwidth reduction while maintaining low intersymbol interference. GMSK is used widely in wireless technologies like GSM and CDPD due to its power efficiency and good bit error rate performance compared to other modulation schemes. While more spectrally efficient than MSK, GMSK also has slightly higher error rates and requires more complex receivers.
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IS-95 CDMA is an air interface standard that uses code division multiple access (CDMA). It employs various techniques to improve system capacity and performance, including bandwidth recycling, power control, soft handoffs, diversity combining, and variable rate vocoding. Key aspects of IS-95 include the use of quadrature phase shift keying modulation at a 1.2288 Mcps chip rate, forward error correction coding, and multiple logical channels (pilot, sync, paging, traffic) defined using orthogonal Walsh codes.
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Multiple access techniques for wireless communication
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The document discusses the evolution of wireless communication technology. It covers topics like the electromagnetic spectrum, the need for modulation to increase communication range and allow multiplexing. The Shannon capacity theorem is discussed as it relates to the maximum data rate of a channel based on its bandwidth and signal-to-noise ratio. Spread spectrum techniques like CDMA are introduced which spread signals across frequency bands to make them resistant to noise and interference. Multiple access schemes to allow multiple users like FDMA, TDMA, and CDMA are covered. OFDM is summarized as a technique that uses multiple subcarriers to increase spectral efficiency. Finally, the generations of mobile communication standards are listed.
Lect13 multiple access
Multiple access techniques allow multiple users to share a channel by dividing the available bandwidth through frequency, time, or code division. Frequency division multiple access (FDMA) allocates unique frequency bands to each user. Time division multiple access (TDMA) divides the channel into time slots and allocates slots to users. Code division multiple access (CDMA) spreads each signal over the entire bandwidth using unique codes. Common examples are GSM which uses FDMA and TDMA, and CDMA cellular networks. Bluetooth and Wi-Fi both use frequency hopping spread spectrum (FHSS) to avoid interference.
Bandwidth Utilization Multiplexing and Spectrum Spreading
This document provides an overview of bandwidth utilization techniques including multiplexing and spread spectrum. It discusses multiplexing techniques like frequency-division multiplexing (FDM), wavelength-division multiplexing (WDM), and time-division multiplexing (TDM). It also covers spread spectrum techniques such as frequency hopping spread spectrum (FHSS) and direct sequence spread spectrum (DSSS). The key advantages of these techniques are that they allow for more efficient use of bandwidth and increase resistance to interference and jamming.
COMMUNICATION TECHNIQUES.pptx
The document discusses multiple access techniques for satellites, including FDMA, TDMA, and CDMA. FDMA assigns different frequency bands to signals to avoid interference. TDMA assigns fixed time slots to each channel. CDMA assigns a unique code to each channel and allows signals to occupy the full bandwidth simultaneously. CDMA provides the highest spectrum efficiency but also the highest costs due to complex equipment requirements.
TDMA, FDMA, and CDMA
What are the differences between FDMA, CDMA and TDMA?
How they work?
Adv and DisAdv of them.
Real life applications etc
WCN U5.pptx
The document discusses multiple access schemes used to allow multiple users to share limited wireless spectrum or communication channels simultaneously. It describes several schemes including FDMA, TDMA, CDMA, OFDMA, and SDMA. FDMA divides the channel into frequency bands and assigns each user an individual band. TDMA divides the channel into time slots and assigns each user slots. CDMA spreads each user's signal over the entire channel using unique codes. OFDMA and SDMA combine these techniques with OFDM and spatial separation of users.
multiple access techniques used in wireless communication
This document discusses multiple access techniques for wireless communication. It describes frequency division duplexing (FDD) and time division duplexing (TDD) for sharing radio spectrum. The main multiple access techniques are described as frequency division multiple access (FDMA), time division multiple access (TDMA), and code division multiple access (CDMA). FDMA allocates different frequency bands to each user, TDMA divides the available time into time slots and allocates one slot per user, and CDMA uses pseudo-random codes to distinguish users transmitting simultaneously on the same frequency. Common cellular systems like AMPS, GSM, and IS-95 are cited as examples.
multiple access techniques for wireless communication
This document discusses multiple access techniques for wireless communication. It describes three main techniques: frequency division multiple access (FDMA), time division multiple access (TDMA), and code division multiple access (CDMA). FDMA allocates different frequency bands to different users. TDMA divides the available bandwidth into time slots and allocates slots to users. CDMA spreads user signals using unique codes and allows simultaneous transmission. Common cellular systems that use these techniques include AMPS (FDMA), GSM (TDMA), and IS-95 (CDMA).
Cdma, speard spectrum
CDMA uses direct sequence spread spectrum technology where each user is assigned a unique pseudorandom noise (PN) code and spreads their data signal using this code. Users transmit simultaneously over the same frequency band and are separated by their unique PN codes. CDMA capacity depends on factors like processing gain, signal-to-noise ratio, and voice activity. The network architecture includes mobile stations, base stations, base station controllers, mobile switching centers, and various registers for authentication, location, and equipment identification. Spread spectrum techniques like CDMA and FHSS provide advantages such as reduced interference, better quality voice/data, security, and resistance to jamming.
Multiplexing
This document discusses different types of multiplexing including frequency division multiplexing (FDM), time division multiplexing (TDM), and code division multiple access (CDMA). FDM divides the spectrum into logical channels and allocates each user an exclusive frequency band. TDM divides the shared channel among users by assigning time slots. CDMA allows multiple users to occupy the entire channel bandwidth simultaneously by encoding each user's signal differently.
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Brief Introduction to Spread spectrum Techniques
- 1. Spread Spectrum Multiple Access There are two main types of spread spectrumThere are two main types of spread spectrum multiple access techniques; a) Frequency hopped multiple access (FH)a) Frequency hopped multiple access (FH) b) Direct sequence multiple access (DS). Direct l i l i l ll d dsequence multiple access is also called code division multiple access (CDMA) and DSSS
- 4. Frequency Hopped Multiple Access (FHMA) • Is a digital multiple access system in which the carrier frequencies of the individual users are varied in a pseudorandom fashion within a wideband channelpseudorandom fashion within a wideband channel. • The digital data is broken into uniform sized bursts which are transmitted on different carrier frequenciesfrequencies. • instantaneous bandwidth of any one transmission burst is much smaller than the total spread bandwidth. • The pseudorandom change of the carrier frequencies of the user randomizes the occupancy of a specificof the user randomizes the occupancy of a specific channel at any given time.
- 5. • If the rate of change of the carrier frequency is greater than the symbol rate then the system is referred to as a fast frequency hopping system. If h h l h l h l h• If the channel changes at a rate less than or equal to the symbol rate, it is called slow frequency hopping. • FHMA systems often employ energy efficient constant• FHMA systems often employ energy efficient constant envelope modulation. • provides a level of security, especially when largep y, p y g number of channels are used • signal is somewhat immune to fading, since error control coding and interleaving can be used to protect the frequency hopped signal against deep fades which may occasionally occur during the hopping sequencemay occasionally occur during the hopping sequence
- 9. • the narrowband message signal is multiplied by a very large bandwidth signal called the spreading signal. • The spreading signal is a pseudo‐noise code sequence that has a chip rate which is orders of magnitudes greater thanhas a chip rate which is orders of magnitudes greater than the data rate of the message • All users in a CDMA system use the same carrier frequency and may transmit simultaneously. • Each user has its own pseudorandom codeword which is approximately orthogonal to all other code wordsapproximately orthogonal to all other code words • The receiver performs a time correlation operation to detect only the specific desired codeword.y p • All other codewords appear as noise due to decorrelation.
- 10. The features of CDMA • Many users of a CDMA system share the same frequency • Either TDD or FDD may be used• Either TDD or FDD may be used. • Unlike TDMA or FDMA, CDMA has a soft capacity limit. • Increasing the number of users in a CDMA system raises the noise floor in a linear manner. Thus, there is no absolute limit on the number of users in CDMA. Multipath fading may be substantially reduced because the signal is spread over a large spectrum. • If the spread spectrum bandwidth is greater than the• If the spread spectrum bandwidth is greater than the • coherence bandwidth of the channel, the inherent frequency diversity will mitigate the effects of small‐ l f discale fading.
- 11. • The chip duration is very short and usually much less than the channel delay spread.than the channel delay spread. • Since PN sequences have low autocorrelation, multipath which is delayed by more than a chip will appear as noiseappear as noise. • A RAKE receiver can be used to improve reception by collecting time delayed versions of the required signal. Si CDMA h l ll i• Since CDMA uses co‐channel cells, it can use macroscopic spatial diversity to provide soft handoff. • Soft handoff is performed by the MSC, which can simultaneously monitor a particular user from two or more base stations • The MSC may chose the best version of the signal aty g any time without switching frequencies. • Self‐jamming is a problem in CDMA system. Self‐ jamming arises from the fact that the spreadingjamming arises from the fact that the spreading sequences of different users are not exactly orthogonal