Study paper on Scalable OFDM Access (SOFDMA)

1.0    Overview

This paper provides an overview of Orthogonal Frequency Div...
NLOS

                                         LOS
                                                                       ...
Figure 2 .Multi path Fading Channel.

2.2.2   Delay Spread

The received radio signal from a transmitter consists of typic...
data. The idea of multiple carrier transmission is used for combating the hostility of such
channels. OFDM is a special fo...
FDMA Spectrum

                                                                          Frequency


                     ...
Transmitted                             Channel Orthogonalization
   signal
                                 QAM          ...
OFDMA provides multiplexing operation of data streams from multiple users onto the
downlink sub-channels and uplink multip...
Pilot Subcarriers
                                                                    Data subcarriers

                  ...
4.0     Scalable OFDMA (SOFDMA)

Scalable OFDMA is the OFDMA mode is used in Mobile Wi-MAX defined in IEEE
802.16e. Scalab...
and UL, as well as other OFDMA default features such as a variety of sub-carriers
allocation and diversity schemes. Discus...
5.0      Advantages and Disadvantages of SOFDMA System

5.1      Advantages

5.1.1. Combating ISI and Reducing ICI

When s...
5.2.2      Peak-to-Average Power Ratio (PAPR)

Peak to Average Power Ratio (PAPR) is proportional to the number of sub-car...
FDM       Frequency Division Multiplexing
FFT       Fast Fourier Transform
FDD       Frequency Division Duplexing
H-ARQ   ...
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Ofdma Basics

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Ofdma Basics

  1. 1. Study paper on Scalable OFDM Access (SOFDMA) 1.0 Overview This paper provides an overview of Orthogonal Frequency Division Multiplexing (OFDM), Orthogonal Frequency Division Multiplexing Access (OFDMA) and the Scalable OFDMA (SOFDMA) and its advantages. It is assumed that readers already have some background in wireless communication. SOFDMA is the air interface defined for portable/mobile Wi-MAX systems by IEEE in IEEE 802.16e(2005) standard. This paper starts with the description of the wireless communication channel and the challenges faced in non-line-of-sight (NLOS) links. It then describes the features of orthogonal frequency division multiplexing (OFDM), orthogonal frequency division multiple access (OFDMA), and Scalable OFDMA (SOFDMA). Wireline and wireless, fixed and mobile communications or networking technologies have chosen OFDM techniques to achieve higher data rate (which is what is needed in the broadband). Examples of such technologies are: ADSL, Wi-Fi (802.11a/g), DAB, Wi-MAX. This technology is also being considered for 802.11n, 802.20 and 4G. In this paper references are made for the use of SOFDMA in Wi-MAX system defined in IEEE 802.16e (2005). 2.0 Wireless Channel Impairments Wireless channel is always very unpredictable with challenging propagation situations. It is very different from wireline channel. In an ideal radio channel, the received signal would consist only a direct path signal which would be a reconstruction of the transmitted signal. However, in a real radio channel, the signal would be modified during transmission in the channel. The unique characteristic of wireless channels is the multipath. There are other serious impairments also present to the channel, namely propagation path loss, shadow fading, Doppler spread, time dispersion or delay spread, etc. 2.1 Multi path Propagation Effects There may or may not be a direct line-of-sight path between the transmitter and the receiver and electromagnetic waves from the transmit antenna travel via several different paths until they reach a receiver. The propagation through these multiple paths are referred to as multi path propagation as shown in Figure-1.
  2. 2. NLOS LOS Mobile Base Station Station NLOS Figure 1: Multipath Phenomenon LOS = Line of Sight NLOS = Non Line of Sight Multi path presents a challenge for any communication system and results in additional complexity of system design. The length of each path is different and so the signals coming to a receiver over each path experiences different time delays, resulting in “delay spread”. The wireless channel is thus characterized by the delay spread which depends on the terrain type, environment (e.g. urban, suburban, rural), and other factors. 2.2.1 Channel Fading Frequency selective and Time Selective Fading. In radio transmissions, the channel spectral response is not flat. In the frequency domain large delay spreads translate into frequency-selective fading. Signals on some frequencies arrive at the receiver in phase while signals at some other frequencies arrive out of phase. This results in frequency selective fading as shown in Fig. 2. NLOS channels may also vary in time significantly, due to moving transceivers in mobile communications. Also time variation of NLOS channels is caused by other moving objects in the paths of signals. This results in time selective fading as shown in Fig. 2.
  3. 3. Figure 2 .Multi path Fading Channel. 2.2.2 Delay Spread The received radio signal from a transmitter consists of typically a direct signal, plus reflected signals. The reflected signals arrive at a later time than the direct signal because of the extra path length, giving rise to a slightly different arrival time of the transmitted pulse, thus spreading the received energy. Delay spread is the time spread between the arrival of the first and last multi path signal seen by the receiver. In a digital system, the delay spread can lead to Inter-Symbol Interference (ISI). This is due to the delayed multi path signal overlapping with the following symbols. Fig.3 shows ISI due to delay spread on the received signal. As the transmitted bit rate is increased the amount of inter symbol interference also increases. Figure 3. Explanation of Inter Symbol Interference 3.0 OFDM Basics The nature of WLAN applications demand high data rates. The systems have to deal with unpredictable wireless channels at high data rate. The channel distortions at high data rate is significant and it is not possible to recover the data with a simple receiver. A Complex receiver is needed to correctly estimate the channel to recover the originally transmitted
  4. 4. data. The idea of multiple carrier transmission is used for combating the hostility of such channels. OFDM is a special form of multi carrier transmission. To understand how OFDM, OFDMA and SOFDMA works, it is useful to start with FDM (Frequency Division Multiplexing). 3.1 FDM (Frequency Division Multiplexing) In FDM system, signals from multiple transmitters are transmitted simultaneously (at the same time slot) over multiple frequencies. Each frequency range (sub-carrier) is modulated separately by different data stream and a spacing (guard band) is placed between sub-carriers to avoid signal overlap. …… Frequency Figure 4 (a): FDM (Frequency Division Multiplexing) 3.2 OFDM (Orthogonal Frequency Division Multiplexing ) OFDM is a multiplexing technique that divides the bandwidth into multiple frequency sub carriers. OFDM also uses multiple sub-carriers but the sub-carriers are closely spaced to each other without causing interference, removing guard bands between adjacent sub- carriers. Here all the sub carriers are orthogonal to each other. Two periodic signals are orthogonal when the integral of their product, over one period, is equal to zero. The use of OFDM results in bandwidth saving as seen in the figure 3. …… Frequency Figure 4 (b): OFDM (Orthogonal Frequency Division Multiplexing)
  5. 5. FDMA Spectrum Frequency Saved bandwidth OFDM Spectrum Frequency Figure 4 (c) : Bandwidth comparison of OFDMA and FDMA Figure 5 below gives an example of orthogonal sub carriers in OFDM system. A B C D E Power (W) Carrier Frequency (Hz) Figure 5: Example of orthogonal sub carriers in OFDM system. 3.3 OFDM Principal A block diagram of an OFDM Transmitter is as shown in figure 6. In this system, a very high rate data stream is divided into multiple parallel low rate data streams ( this increases symbol duration).
  6. 6. Transmitted Channel Orthogonalization signal QAM OFDMA Sub-carrier FEC Symbol IFFT Carrier Modulation allocation Mapping Radio Channel Received Pre-FEC Channel De-Orthogonalization Demodulatio signal Figure 6: Block diagram of a OFDM modulator. Each smaller data stream is then mapped to individual data sub-carrier and modulated using some sorts of PSK (Phase Shift Keying) or QAM (Quadrature Amplitude Modulation). i.e. BPSK, QPSK, 16-QAM, 64-QAM. The term “OFDM” is frequently followed by the number that depicts the potential number of sub carriers (including guard-band sub carriers) in the signal (e.g. OFDM-256). The use of OFDM results in higher spectral efficiency. Besides this an OFDM system such as Wi-MAX is more resilient in NLOS environment. It can efficiently overcome interference and frequency-selective fading caused by multi path because equalizing is done on a subset of sub-carriers instead of a single broader carrier. The effect of ISI (Inter Symbol Interference) is suppressed by virtue of a longer symbol period of the parallel OFDM sub-carriers than a single carrier system and the use of cyclic prefix (CP)- explained later. 3.4 OFDMA (Orthogonal Frequency Division Multiple Access). OFDM in its primary form is considered as a digital modulation technique and not a multi user channel access technique. It is utilized for transferring one bit stream over one communication channel using one sequence of OFDM symbols. However, OFDM can be combined with multiple accesses using time, frequency or coding separation of the users. OFDMA employs multiple closely spaced sub-carriers. The sub-carriers are divided into groups of sub-carriers. Each group is named a sub-channel. The sub-carriers that form a sub-channel need not be adjacent …… Frequency Figure 7: Orthogonal Frequency Division Multiplexing Access Sub-carriers with the same color represent a sub-channel.
  7. 7. OFDMA provides multiplexing operation of data streams from multiple users onto the downlink sub-channels and uplink multiple access by means of uplink sub-channels. The multiple access is achieved by assigning different OFDM sub - channels to different users. In the downlink, a sub-channel may be intended for different receivers. In the uplink, a transmitter may be assigned one or more sub-channels. 3.4.1. The Cyclic Prefix The increased symbol duration in OFDMA improves the delay spread while the Inter Symbol Interference (ISI) is completely eliminated by introduction of a Cyclic Prefix (some data). CP is a repetition of the last samples of the data portion that is appended at the beginning of the data payload. The ISI is completely eliminated as long as the CP duration is longer than the channel delay spread. A drawback of the CP is that it introduces overhead, which effectively reduces bandwidth efficiency. Since OFDM signal power spectrum has a sharp fall off at the edge of channel, a larger fraction of the allocated channel bandwidth can be utilized for data transmission which compensates the loss in efficiency due to the cyclic prefix. The concept of CP in OFDMA is explained in figure 8. Total Symbol Period Ts Cyclic Prefix Data Payload Tg Tu Useful Symbol Tg Period Figure 8: Cyclic Prefix 3.4.2. OFDMA symbol Structure and sub channelization. Both OFDM and OFDMA symbols are structured in similar way. In OFDMA each symbol consists of sub-channels that carry data sub-carriers carrying information, pilot sub-carriers as reference frequencies and for various estimation purposes, DC sub- carrier as the center frequency, and guard sub-carriers or guard bands for keeping the space between OFDMA signals as shown in figure 9..
  8. 8. Pilot Subcarriers Data subcarriers DC Subcarriers Guard subcarriers … … … … Channel Figure 9: Structure of sub-carriers in OFDMA 3.4.3. Sub-channelization Active (data and pilot) sub-carriers are grouped into subsets of sub-carriers called sub- channels Sub-channelization defines sub-channels that can be allocated to subscriber stations (SSs) depending on their channel conditions and data requirements. Using sub- channelization, within the same time slot, a BS can allocate more transmit power to SSs with lower SNR and less power to user devices with higher SNR. Sub-channelization also enables the BS to allocate higher power to sub-channels assigned to indoor SSs resulting in better in-building coverage. Sub-channelization in the uplink can save a user device transmit power because it can concentrate power only on certain sub-channel(s) allocated to it. This power-saving feature is particularly useful for battery-powered user devices. The concept of sub-channelization is explained in figure 10. OFDM OFDMA Sub- carriers Sub- channe Time Time Figure 10 : Sub-Channelization in OFDM and OFDMA
  9. 9. 4.0 Scalable OFDMA (SOFDMA) Scalable OFDMA is the OFDMA mode is used in Mobile Wi-MAX defined in IEEE 802.16e. Scalability is supported by adjusting the size of FFT size while fixing the sub- carrier frequency spacing in 10.94 kHz. It supports channel bandwidths ranging from 1.25 MHz to 20 MHz. SOFDMA adds scalability to OFDMA. With bandwidth scalability, Mobile Wi-MAX technology can comply with various frequency regulations worldwide. 4.1 Basic Principals In SOFDMA, • Sub-carrier spacing is independent of bandwidth. • The number of sub-carriers scales with bandwidth. • The smallest unit of bandwidth allocation, based on the concept of sub-channels, is fixed and independent of bandwidth and other modes of operation. • The number of sub-channels scales with bandwidth and the capacity of each individual sub-channel remain constant. 4.2 OFDMA Scalable parameters Smaller FFT size is given to lower bandwidth channels, while larger FFT size to wider channels. By making the sub-carrier frequency spacing constant, SOFDMA reduces system complexity of smaller channels and improves performance of wider channels. In order to keep optimal sub-carrier spacing, the FFT size should scale with the bandwidth. This concept is introduced in Scalable OFDMA (SOFDMA).This results in the property that the number of sub-channels scales with FFT/bandwidth. Various scalable parameters in SOFDMA along with the fixed parameters are given in the table below. PARAMETERS VALUES System Channel Bandwidth (MHz) 1.25 5 10 20 Sampling frequency Fp in MHz 1.4 5.6 11.2 22.4 FFT Size (Nfft) 128 512 1024 2048 Number of Sub-Channels 2 8 16 32 Sub-Carrier Frequency Spacing 10.94KHz Useful symbol Time ( Tb=1/f) 91.4 microsecond Guard Time (Tg=Tb/8) 11.4 microsecond OFDMA symbol duration (Ts=Tb+Tg) 102.9 microsecond Number of OFDMA Symbols (5ms Frame) 48 In addition to variable FFT sizes, SOFDMA supports features such as Advanced Modulation and Coding (AMC), Hybrid Automatic Repeat Request (H-ARQ), high- efficiency uplink sub-channel structures, Multiple-Input-Multiple Output (MIMO) in DL
  10. 10. and UL, as well as other OFDMA default features such as a variety of sub-carriers allocation and diversity schemes. Discussing these features is beyond of the scope of the paper. 4.3 SOFDMA frame structure There are two types of frame structure, FDD and TDD. TDD has many advantages over the FDD. The figure below explains the OFDM frame structure for a TDD implementation. OFDM Symbol Number 0 1 3 5 7 9 … … N-1 0 … … … M-1 1 FCH Coded symbol write order Burst 1 UL Sub- MAP DL Burs#2 channel (conl) Burst 2 Logical s-1 DL Number s MAP DL Burs#1 Burst 3 s+1 DL Burs#4 ACK- DL Burs#3 Burst 4 CH Preamble DL Burs#5 UL DL Burs#6 Burst 5 MAP Ranging DL Burs#7 Fast feedback (CQICH) N Downlink Subframe Uplink Subframe Guard Figure 11: OFDMA frame Structure TDD enables adjustment of the DL/UL ratio to effectively support the asymmetric DL/UL traffic, while FDD DL/UL always have fixed and equal DL and UL bandwidths. Recommended number of DL/UL OFDM symbols can flexibly realize a range of asymmetric DL/UL traffic ratio. For a 10MHz Bandwidth, number of OFDM symbols in UL and DL are as given in the table below. Description Base Station Values Number of OFDM symbols in DL/UL for (35:12),(34:13),(33:14),(32:15),(31:16), a 10MHz Bandwidth (30:17), (29:18), (28:19), (27:20), (26:21),
  11. 11. 5.0 Advantages and Disadvantages of SOFDMA System 5.1 Advantages 5.1.1. Combating ISI and Reducing ICI When signal passes through a time-dispersive channel, the orthogonality of the signal can be lost. CP helps to maintain orthogonality between the sub carriers. Initially guard interval-empty space between two OFDM symbols served as a buffer for the multi path reflection. But the empty guard time introduces Inter Carrier Interference (ICI) that is crosstalk between different sub carriers. A better solution is cyclic extension of OFDM symbol or CP. It ensures that the delayed replicas of the OFDM symbols will always have a complete symbol within the FFT interval (often referred as FFT window). At the receiver side, CP is removed before any processing starts. As long as the length of CP interval is larger than maximum expected delay spread, all reflections of previous symbols are removed and orthogonality is restored. 5.1.2. Spectral Efficiency In the case of OFDM, a better spectral efficiency is achieved by maintaining orthogonality between the sub-carriers. 5.1.3. Some Other Benefits of OFDM System - Low cost due to its simplicity. - It is possible to significantly enhance the capacity by adapting the data rate per sub-carrier according to SNR of that particular sub-carrier. - OFDM is more resistant to frequency selective fading than single carrier systems. - OFDM can be used for high-speed multimedia applications with lower service cost. - Smart antennas can be integrated with OFDM. MIMO systems and space-time coding can be realized on OFDM and all the benefits of MIMO systems can be obtained easily. Adaptive modulation and tone/power allocation are also realizable on OFDM. 5.2 Disadvantage of SOFDMA system 5.2.1. Strict Synchronization Requirement OFDMA is highly sensitive to time and frequency synchronization errors. Demodulation of an OFDM signal with an offset in the frequency can lead to a high bit error rate.
  12. 12. 5.2.2 Peak-to-Average Power Ratio (PAPR) Peak to Average Power Ratio (PAPR) is proportional to the number of sub-carriers used for OFDM systems. An OFDM system with large number of sub-carriers will thus have a very large PAPR when the sub-carriers add up coherently. Large PAPR of a system makes the implementation of Digital-to-Analog Converter (DAC) and Analog-to-Digital Converter (ADC) to be extremely difficult. The design of RF amplifier also becomes increasingly difficult as the PAPR increases. 6.0 Conclusion It may be concluded that OFDMA is a very efficient technique for broadband data transmission over radio frequencies. It can be implemented digitally with simplicity and at low cost. Therefore, it is being adopted in almost all the new wireless technologies. 7.0 References 1) http//www.wimaxforum.org/ 2) IEEE 802.16e standards 3) Various internet sites on Wi-Max 8.0 ABBREVIATIONS & ACRONYMS AAS Adaptive Antenna System ADSL Asymmetric Digital Subscribers Line AMC Adaptive Modulation and coding CDMA Code Division Multiple Access CP Cyclic Prefix DAB Digital Audio Broadcasting DL Downlink BPSK Bipolar Phase Shift Keying DPSK Differential Phase Shift Keying FEC Forward Error Correction
  13. 13. FDM Frequency Division Multiplexing FFT Fast Fourier Transform FDD Frequency Division Duplexing H-ARQ Hybrid Automatic Repeat Request ICI Inter Carrier Interference IEEE Institute of Electrical and Electronics Engineers IFFT Inverse Fast Fourier Transform ISI Inter Symbol Interference LOS Line-of-Sight MIMO Multiple Input- Multiple Output NLOS Non-Line-of-Sight OFDM Orthogonal Frequency Division Multiplexing OFDMA Orthogonal Frequency Division Multiple Access PSK Phase Shift Keying PAPR Peak to Average Power Ratio QAM Quadrature Amplitude Modulation QPSK Quadrature Phase-Shift Keying SNR Signal-to-Noise Ratio S-OFDMA Scalable Orthogonal Frequency Division Multiple Access SS Subscriber Station TDD Time Division Duplex UL Uplink Wi-MAX Worldwide Interoperability Microwave access WLAN Wireless Local Area Network

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