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Performance Evaluation of OFDM Technique for High Speed Communication Applications

by

Mukul Kabra

A Thesis Submitted in Partial Fulfillment of the Requirements for the Degree of

Master of Technology in Information and Communication Technology to Dhirubhai Ambani Institute of Information and Communication Technology Gandhinagar, India

May, 2005

DA-IICT

Declaration This is to certify that (i) the thesis comprises my original work towards the degree of Master of Technology in Information and Communication Technology at DA-IICT and has not been submitted elsewhere for a degree, (ii) due acknowledgement has been made in the text to all other material used.

Signature of Student (Mukul Kabra)

Certificate

This is to certify that the thesis work entitled “Performance Evaluation of OFDM Technique for High Speed Communication Applications” has been carried out by Mukul Kabra (200311034) for the degree of Master of Technology in Information and Communication Technology at this Institute under my supervision.

Thesis Supervisor (Prof. S.L. Maskara)

i

Acknowledgements

I would like to thank my advisor Prof. S. L. Maskara for his invaluable advice, patience, and support during the work on this thesis. It has been a pleasure as well as a privilege to have had the opportunity to work with him. I would like to thank my committee members Prof. V. P. Sinha, Prof. V. K. Chakka for their expert help and valuable feedback on the thesis. I wish to convey warmest thanks to my parents and my sister Shruti, for providing encouragement and support during work on this thesis. I would like to thank my parents for the inspiration and support they have provided throughout my life. I would like to thank Alok, Sudhir, Gangadhar, Kunal for the many hours of white board discussion, and support. I would also like to thank Dharmendra, Awkash, Krutarth for their support as friends and for sharing my excitement of discoveries made during this thesis. I would particularly like to thank Prof. S.L. Maskara for his motivational discussions about research and publications. Finally, I would like to thank my computer for only crashing seriously once during the writing of this thesis.

ii

Contents Page No.

1

Abstract

v

List of Principal Symbols and Acronyms

vi

List of Figures

viii

List of Tables

xi

Introduction 1.1 1.2 1.3 1.4 1.5

2

3

1 Orthogonal Frequency Division Multiplexing and its Potential……. 1 Motivation………………………………………………………….. 3 Objectives…………………………………………………………... 4 Approach to work…………………………………………………... 5 Thesis Organization………………………………………………… 5

OFDM Principles and Review

6

2.1 2.2 2.3 2.4 2.5 2.6

Orthogonal Frequency Division Multiplexing Basics……………… Theory of OFDM…………………………………………………… Advances in Digital Signal Processing……………………………... Fading channel Characteristics……………………………………... OFDM System……………………………………………………… Major Applications………………………………………………….

6 11 14 15

2.7 Literature Survey and Review of OFDM…………………………...

29

Simulation of OFDM in Wireless LAN application

36

3.1 3.2 3.3 3.4

Wireless Local Area Network Description…………………………. Channel Models for Wireless LAN………………………………… Simulation of OFDM based Wireless LAN………………………... Wireless LAN standards…………………………………………….

36 37 40 47

3.5 Performance Results………………………………………………...

58

iii

21 28

4

Performance of OFDM in Digital Audio Broadcasting 4.1 Digital Audio Broadcasting………………………………………... 4.2 Channel models for DAB………………………………………….. 4.3 Simulation of Digital Audio Broadcasting System………………... 4.4 Performance Results………………………………………………..

66 66 67 68 74

5

Study of OFDM Application in Asymmetric Digital Subscriber Lines 5.1 Digital Subscriber Lines…………………………………………… 5.2 Application of OFDM in Asymmetric Digital Subscriber Lines….. 5.3 Implementation of OFDM based ADSL…………………………... 5.4 Performance Results……………………………………………….

78 78 80 81 90

6

92 Summary and Conclusion Discussion of Results……………………………………………… 92 Conclusion…………………………………………………………. 93 References Appendix A

95 98

iv

Abstract The Internet revolution has created the need for wireless technologies that can deliver data at high speeds in a spectrally efficient manner. However, supporting such high data rates with sufficient robustness to radio channel impairments requires careful selection of modulation techniques. The demand for high-speed mobile wireless communications is rapidly growing. OFDM technology promises to be a key technique for achieving the high data capacity and spectral efficiency requirements for wireless communication systems of the near future. Orthogonal frequency division multiplexing (OFDM) is a special case of multicarrier transmission, where a single data stream is transmitted over a number of lower rate subcarriers. OFDM is currently being used in Europe for digital audio and video broadcasting. The IEEE standardization group decided to select OFDM as the basis for their new 5-GHz standard, targeting a range of data stream from 6 up to 54 Mbps. This standard was the first one to use OFDM in packet-based communications, while the use of OFDM until now was limited to continuous transmission systems. OFDM is also being considered as a serious candidate for fourth generation cellular systems. In this project, transmitter, channel and receiver were simulated with various parameters, to evaluate the performance and different possibilities in the implementation. Also, some considerations about forward error correction coding, interleaving, synchronization and channel estimation are given to improve the system performance.

v

List of Principal Symbols and Acronyms φ(t) ∆f φh(τ) τrms

BC Bd hb(t,τ) KMOD N n(t) Td TS TFFT Xi(t) Xk(f) 2G 3G 4G ADC ADSL ASC AP BER BPSK CCK COFDM CP CSI DA DAB DAC DBLAST DD DDS DFT DSL DVB DVB-T DSSS EGC FEC FEQ FFT FHSS

Subcarrier in time domain Frequency spacing between orthogonal subcarriers Channel correlation function RMS delay spread Coherence bandwidth Doppler spread of the channel Baseband representation of time varying channel Normalization factor for different constellations Number of IFFT point Additive white Gaussian noise in time domain Coherence time OFDM symbol duration FFT duration Transmitted ith OFDM signal in time domain kth subcarrier of OFDM signal in frequency domain 2nd Generation 3rd Generation 4th Generation Analog-to-Digital Converter Asymmetric Digital Subscriber Lines Antenna Selection Combining Access Point Bit Error Rate Binary Phase Shift Keying Complementary Code Keying Coded Orthogonal Frequency Division Multiplexing Cyclic Prefix Channel State Information Data Aided Algorithms Digital Audio Broadcasting Digital-to-Analog Converter Diagonal Bell Labs Layered Space-Time Decision Directed Direct Digital Synthesis Discrete Fourier Transform Digital Subscriber Line Digital Video Broadcasting Digital Video Broadcasting - Terrestrial Direct Sequence Spread Spectrum Equal Gain Combining Forward Error Correction Frequency domain equalizer Fast Fourier Transform Frequency Hopping Spread Spectrum vi

FM HDSL HDTV HiperLAN ICI IDFT IFFT IR ISI LOS LMS LMMSE MAN MARC MIMO MMAC MMSE MRC MSE NDA NLOS OFDM PAPR POTS PRS PBCC PHY QAM QoS QPSK RF RMS SNR SSC STC TEQ VBLAST VDSL WLAN W-OFDM WSS

Frequency Modulation High-bit-rate Digital Subscriber Lines High Definition TeleVision High Performance Radio Local Area Networks Inter Carrier Interference Inverse Discrete Fourier Transform Inverse Fast Fourier Transform Infra Red Inter Symbol Interference Line-Of-Sight Least Mean Square Linear Minimum Mean Squared Error Metropolitan Area Networks Maximum Average (Signal-to-Noise) Ratio Combining Multiple Input Multiple Output Multimedia Mobile Access Communications Systems Minimum Mean Squared Error Maximum Ratio Combining Mean Squared Error Non Data-Aided Algorithms Non Line-Of-Sight Orthogonal Frequency Division Multiplexing Peak-to-Average Power Ratio Plain Old Telephone System Phase reference symbol for DAB Packet Binary Convolutional Coding Physical Layer Quadrature Amplitude Modulation Quality of Service Quadrature Phase Shift Keying Radio Frequency Root-Mean-Square Signal-to-Noise Ratio Subcarrier Selection Combining Space-Time Coding Time domain Equalizer Vertical Bell Labs LAyered Space-Time Very-high-speed Digital Subscriber Lines Wireless Local Area Networks Wideband Orthogonal Frequency Division Multiplexing Wide Sense Stationary

vii

List of Figures Page No. Figure 2.1

Comparison of OFDM with FDM for spectral efficiency

7

Figure 2.2

Time domain construction of an OFDM signal

9

Figure 2.3

Frequency response of the subcarriers in a 5 tone OFDM signal

11

Figure 2.4

Multipath Propagation (Tx- Transmitter, Rx- Receiver)

15

Figure 2.5

Fading channel manifestations

17

Figure 2.6

Complete transmission model of OFDM system

21

Figure 2.7

Constellation diagram for 64 QAM

23

Figure 2.8

Baseband implementation of OFDM scheme

24

Figure 2.9

Effect of Cyclic Prefix in OFDM

25

Figure 2.10

RF up conversion techniques

26

Figure 2.11

Pilot subcarrier arrangement in OFDM symbol

28

Figure 3.1

Impulse response of ETSI channel model A to E

39

Figure 3.2

Data Scrambler

41

Figure 3.3

Convolutional Encoder (Constraint length k = 7)

42

Figure 3.4

Effect of Convolutional Coding and soft Viterbi Decoding

43

Figure 3.5

(a) Transmitted Constellation with QPSK modulation, 128 subcarriers (before normalization), (b) Received Constellation with 6 dB SNR in AWGN channel

Figure 3.6

44

(a) Transmitted Constellation with 16-QAM modulation (before normalization), (b) Received Constellation with 12dB SNR in AWGN channel

44

Figure 3.7

Transmitted waveforms of OFDM signal, 64 QAM Modulation

45

Figure 3.8

PSD of the transmitted OFDM signal 64 subcarrier (48 data, 4 pilot and 12 null subcarriers) with 64QAM Modulation

Figure 3.9

(a) Received Constellation without equalization in 24 Mbps mode, (b) with equalization, in Channel model A with 12dB SNR

Figure 3.10

45 46

(a) Received Constellation without equalization in 24 Mbps mode, (b) with equalization, in Channel model E with 12dB SNR

viii

46

Figure 3.11

(a) Received Constellation without equalization in 54 Mbps mode, (b) with equalization, in Channel model A with 24dB SNR

46

Figure 3.12

PPDU Frame format for the IEEE 802.11a standard

49

Figure 3.13

OFDM training symbol structure (from IEEE 802.11a standard)

50

Figure 3.14

Coding and interleaving performance of 12 Mbps mode (a) in non fading channel and (b) ETSI channel model A

52

Figure 3.15

Subcarrier frequency allocation

53

Figure 3.16

Inputs and Outputs of IFFT

53

Figure 3.17

OFDM frame with cyclic extension and windowing for two receptions of the FFT period

Figure 3.18

54

(a) Auto-Correlation curve and Cross-Correlation peaks for ideal case (No noise, no multipath), (b) Auto-Correlation curve and Cross-Correlation peaks for SNR=10dB and multipath delay spread 60 ns and no frequency offset

Figure 3.19

56

SNR requirements for CP length of 100, 200, 400 and 800 ns in channel model A (max delay spread = 300 ns), v=50 km/hr for different modulation schemes

Figure 3.20

59

Comparison of SNR requirements for number of subcarriers 64, 128 and 256 with 16 QAM Modulation and CP = 800 ns (a) in channel model E, (b) in JTC channel model C, (c) AWGN channel

Figure 3.21

Effect of FEC and Interleaving at different data rates in frequency selective fading channel model A

Figure 3.22

61

The IEEE 802.11a standard for Data rate of (a) 6 Mbps, (b) 12 Mbps and (c) 24 Mbps in ETSI channels A to E

Figure 3.23

60

62

The IEEE 802.11a standard for Data rate of (a) 36 Mbps, (b) 48 Mbps and (c) 54 Mbps in ETSI channels A to E

63

Figure 3.24

Performance in ETSI Channel model A for different data rates

64

Figure 3.25

Performance of modulation schemes in (a) AWGN channel (b) in JTC channel model A without coding and interleaving

Figure 3.26

65

BER performance of OFDM with different modulation schemes in (a) JTC channel model B and C without coding and interleaving

ix

65

Figure 4.1

Impulse response of Urban Area

68

Figure 4.2

Effect of selective channel on the transmitted OFDM signal

68

Figure 4.3

Block diagram of a DAB transmitter

70

Figure 4.4

Structure of the DAB transmission frame

70

Figure 4.5

OFDM signal after D/A converter in time and frequency domain

74

Figure 4.6

OFDM signal after LPF in time and frequency domain

74

Figure 4.7

DAB Transmitted signal (a) in time domain (b) in frequency domain, for mode 1 having 2048 subcarriers

75

Figure 4.8

Rayleigh fading Envelope and phase response at 100 km/hr speed

75

Figure 4.9

BER v/s SNR Curves for Rayleigh fading channels at different speed for transmission mode 1

76

Figure 4.10

Performance for different fading channels for transmission mode-1

77

Figure 4.11

Performance for different fading channels for transmission mode-2

77

Figure 5.1

ADSL Frequency Plan

80

Figure 5.2

Block diagram of a ADSL Transceiver using DMT

82

Figure 5.3

(a) SNR (b) bit allocation for given SNR at each subcarrier for CSA Loop-1

Figure 5.4

83

The fast and interleaved bit streams are merged together and are thereafter transmitted to the tone order

Figure 5.5

84

The constellation points for odd number of bits. (a) 8-QAM and (b) 32-QAM

85

Figure 5.6

Generation of real signal from IFFT

86

Figure 5.7

Channel impulse response for CSA Loop – 1 in time domain before and after channel shortening

88

Figure 5.8

Frequency response of CSA Loop-1

89

Figure 5.9

The adaptive LMS algorithm that is used in FEQ

89

Figure 5.10

Effect of frequency domain equalization on constellations

91

x

List of Tables Page No. Table 2.1

Evaluation of the FFT Complexity

15

Table 2.2

Evaluation of the FFT Complexity

15

Table 2.3

Typical Delay Spread

19

Table 3.1

Channel model for HIPERLAN/2 at 5GHz

38

Table 3.2

Delay profile for channel A to E

38

Table 3.3

JTC Channel Model Parameters for Indoor Office Areas

39

Table 3.4

Physical Layer parameters for WLAN simulation

40

Table 3.5

Modulation-dependent normalization factor KMOD

43

Table 3.6

Rate dependent parameter for the IEEE 802.11a standard

48

Table 3.7

Physical layer parameters for the IEEE 802.11a standard

48

Table 3.8

SNR comparison of different length of cyclic prefix

59

Table 3.9

SNR comparison of different no. of subcarriers for BER of 10-4

59

Table 3.10

SNR comparison for different data rates for the IEEE 802.11a

61

Table 3.11

SNR comparison for various modulation schemes

65

Table 4.1

Delay profile of different channels for DAB

67

Table 4.2

Definition of the parameters for transmission modes I, II and III

71

Table 4.3

SNR requirements for Transmission Mode 1 and 2 for BER 10-4

77

Table 5.1

High speed data communication standards

78

Table 5.2

Physical Layer Parameters for DMT (OFDM) based ADSL

81

Table 5.3.

Simulation results for different CSA Loop channels

91

Table A.1

BPSK encoding table

98

Table A.2

QPSK encoding table

98

Table A.3

16 QAM encoding table

98

Table A.4

64 QAM encoding table

98

xi

Chapter 1

Introduction 1.1

Orthogonal Frequency Division Multiplexing and its Potential

The Orthogonal Frequency Division Multiplexing (OFDM) technique has the potential of enhancing data rates in a band-limited channel in general and under fading condition in particular. In OFDM, multiple subcarriers, each with relatively smaller bandwidth are used to transmit the user’s information requiring relatively higher bandwidth. Instead of transmitting high data rates on a single carrier requiring high bandwidth, in OFDM, the high data rate signal is split into many low data rate streams, which are then transmitted on multiple closely spaced orthogonal carriers. In fact the bandwidth of each subcarrier should be made small compared with the coherence bandwidth of fading channel. In other words, the symbol period of a sub-stream is made large compared to the delay spread of the time dispersive radio channel. Further, the choice of orthogonal subcarriers to each other makes the adjacent subcarriers spacing minimal. Such kind of signal processing results in considerable reduction of the overall bandwidth compared to a single carrier system. OFDM therefore supports higher data rates over channels with limited bandwidth. Implementation of OFDM transmitter and receiver is considerably simplified because of use of Inverse Discrete Fourier Transform (IDFT) using digital signal processing techniques. These features have resulted in wide range applications of OFDM, both in wireline as well as wireless communication systems. Orthogonality of frequencies in OFDM makes it possible to arrange the subcarriers in such a way that the sidebands of the individual carriers overlap and still the signals are received at the receiver without being interfered by ICI. The receiver acts as a bank of demodulators, translating each subcarrier down to DC, with the resulting signal integrated over a symbol period to recover raw data. If the other subcarriers are all down converted to the baseband frequencies, that in the time domain they have a whole number of cycles in a symbol period TS, then the integration process results in zero contribution from all other subcarriers. Thus, the subcarriers are orthogonal (i.e. linearly independent) if the carrier spacing is a multiple of 1/ TS [1].

1

Although OFDM has only recently been gaining interest from telecommunications industry, it has a long history of existence. It is reported that OFDM based systems were in existence during the Second World War. In US military, several high frequency military systems such as KINEPLEX, ANDEFT and KATHRYN had used OFDM. In December 1966, Robert W. Chang [1] outlined a theoretical way to transmit simultaneous data stream through linear band limited channel without Inter Symbol Interference (ISI) and Inter Carrier Interference (ICI). Subsequently, he obtained the first US patent on OFDM in 1970. Around the same time, Saltzberg [2] performed an analysis of the performance of the OFDM system. Until this time, we needed a large number of subcarrier oscillators to perform parallel modulations and demodulations. A major breakthrough in the history of OFDM came in 1971, when Weinstein and Ebert [3] used Discrete Fourier Transform (DFT) to perform baseband modulation and demodulation focusing on efficient processing. This eliminated the need for bank of subcarrier oscillators, thus paving the way for easier, more useful and efficient implementation of the system. All the proposals until this time used guard spaces in frequency domain and a raised cosine windowing in time domain to combat ISI and ICI. Another milestone for OFDM history was when Peled and Ruiz [4] introduced Cyclic Prefix (CP) or cyclic extension in 1980. This solved the problem of maintaining orthogonal characteristics of the transmitted signals at severe transmission conditions. The generic idea that they placed was to use cyclic extension of OFDM symbols instead of using empty guard spaces in frequency domain. This effectively turns the channel as performing cyclic convolution, which provides orthogonality over dispersive channels, when CP is longer than the channel impulse response [5]. It is obvious that introducing CP causes loss of signal energy proportional to length of CP compared to symbol length, but, on the other hand, it facilitates a zero ICI advantage which pays off. By this time, inclusion of FFT and CP in OFDM system and substantial advancements in Digital Signal Processing (DSP) technology made OFDM an important part of telecommunications landscape. In the 1990s, OFDM was exploited for wideband data communications over mobile radio FM channels, High-bit-rate Digital Subscriber Lines (HDSL at 1.6Mbps), Asymmetric Digital Subscriber Lines (ADSL up to 6Mbps) and Very-high-speed Digital Subscriber Lines (VDSL at 100Mbps). Digital Audio Broadcasting (DAB) was the first commercial use of OFDM technology. By 1992, DAB was proposed and the standard was formulated by ETSI in 1994 but this service came to 2

reality in 1995 in UK and Sweden. Digital Video Broadcasting (DVB) along with HighDefinition TeleVision (HDTV) terrestrial broadcasting standard was published in 1995. Several Wireless Local Area Network (WLAN) standards adopted OFDM on their physical layers. Development of European WLAN standard High Performance LAN (HiperLAN) started in 1995. HiperLAN/2 was defined in June 1999 [15], which adopts OFDM in physical layer. Recently IEEE 802.11a [14] in USA has also adopted OFDM in their PHY layer. The primary applications of OFDM are in multimedia push technology, wireless LAN and xDSLs. It has also been suggested for power line communications systems [18] due to its resilience to time dispersive channels and narrow band interferers. Perhaps of even greater importance is the emergence of this technology as a competitor for future 4th Generations (4G) wireless systems. These systems, expected to emerge by the year 2010, promise to at last deliver on the wireless Nirvana of anywhere, anytime, anything communications. Should OFDM gain prominence in this arena, and telecom giants are banking on just this scenario, then OFDM will become the technology of choice in most wireless links worldwide.

1.2

Motivation

Wireless communication has gained momentum in the last decade of 20th century with the success of 2nd Generations (2G) of digital cellular mobile services. With the advent of 3rd Generation (3G) wireless systems, it is expected that higher mobility with reasonable data rate (up to 2Mbps) can be provided to meet the current user needs. But, 3G is not the end of the tunnel; ever increasing user demands have drawn the industry to search for better solutions to support data rates of the range of tens of Mbps. Naturally dealing with ever unpredictable wireless channel at high data rate communications is not an easy task. Hostile wireless channel has always been proved as a bottleneck for high-speed wireless systems. This motivated the researchers towards finding a better solution for combating all the odds of wireless channels; thus, the idea of multi-carrier transmission has surfaced recently to be used for future generations of wireless systems. At high data rates, the channel distortion to the data is very significant, and it is somewhat impossible to recover the transmitted data with a simple receiver. A very complex receiver structure is needed which makes use of computationally extensive equalization and channel estimation algorithms, such that the estimations can be used with the received data to recover the originally transmitted data. OFDM can drastically simplify the equalization problem by 3

turning the frequency selective channel to a flat channel. A simple one-tap equalizer is needed to estimate the channel and recover the data. Future telecommunication systems must be spectrally efficient to support a number of high data rate users. OFDM uses the available spectrum very efficiently, which is very useful for multimedia communications. Orthogonal Frequency Division Multiplexing promises a higher user data rate and great resilience to severe signal fading effects of the wireless channel at a reasonable level of implementation complexity. It has been taken as the primary physical layer technology in high data rate Wireless LAN/MAN standards. IEEE 802.11a and HiperLAN/2 have the capability to operate in a range of a few tens of meters in typical office space environment. In the upcoming standard IEEE 802.20, which is targeted at achieving data rate of greater than 1 Mbps at 250 kmph, OFDM is one of the potential candidates. The industry has not offered any interface yet. Many companies are still researching and developing, over all for the receiver, which is the key part of the system. The standard does not give rules about it and its implementation is up to the designer. Pure OFDM or hybrid OFDM will be most likely the choice for physical layer multiple access technique in the future generation of telecommunication systems. Thus we see that there is a strong possibility that next generation wireless era belongs to OFDM technology.

1.3

Objectives

This thesis investigates the effectiveness of Orthogonal Frequency Division Multiplexing (OFDM) as a modulation technique for wireless and wireline applications. The main aim was to evaluate the performance of OFDM as a modulation technique for high data rate applications. Several of the main factors effecting the performance of a OFDM system, were measured including multipath delay spread, channel noise, modulation scheme, error control coding, interleaving, distortion of the signal (clipping), and time and frequency synchronization requirements. Objective of this thesis is to acquire a sound understanding of OFDM technology and to evaluate the performance of some of the important applications of OFDM like WLAN, DAB and DSL by simulating the transmitter, fading channel, as encountered in practical scenarios and the receiver. This thesis basically finds the performance of OFDM in practical selective channels for various applications.

4

1.4

Approach to Work

Performance of OFDM has been studied for various communication applications through extensive simulation. To compare the performance with different parameter variations, different Matlab scripts have been written and simulation have been done for each block of transmitter, receiver and channel, to access the performance of OFDM system. For practical measurements, data has been taken from a stored text or image or sound file and transmitted using some OFDM application. After applying the channel effect to the transmitted signal, the data is recovered at the receiver and again written into the file for comparison of input to output. Bit error rate versus SNR has been taken as the main performance evaluation criterion in different channel environments for different applications. A file having size of tens of kilobytes has been read so that at least BER of 10-5 can be measured with acceptable simulation time.

1.5

Thesis Organization

The report is structured as follows. Introduction to the OFDM basics and analytical model of OFDM has been discussed in chapter 2. In the same chapter different impairments of radio channel for high data rates have been discussed, a general OFDM system have been explained with basic block diagram, literature survey and review of the OFDM have been given, which includes OFDM implementation issues and recent work done for the same. In chapter 3, basic WLAN description, different channel models for WLAN with general OFDM parameters for physical layer have been considered and results are shown. WLAN standard, IEEE 802.11a and HIPERLAN/2 have been simulated and results have been obtained as specific example of application of OFDM. Next chapter considers Digital Audio Broadcasting as another applications of OFDM. General description, channel models for DAB and simulation of physical layer have been given. Finally, the performance results for different modes have been given and discussed. Chapter 5 includes one wireline application, Asymmetric Digital subscriber Lines, for providing high data rate with twisted pair. This chapter basically provides a study of ADSL as application of OFDM and simulation of ADSL standard. Chapter 6 concludes the report by discussing the result obtained and key research issues related to the development of OFDM wireless system aimed at high data rate.

5

Chapter 2

OFDM and Background and Review 2.1

Orthogonal Frequency Division Multiplexing Basics

OFDM is a modulation scheme that allows digital data to be efficiently and reliably transmitted over a radio channel, even in multipath environments. OFDM transmits data using a large number of narrow bandwidth carriers. These carriers are regularly spaced in frequency, forming a block of spectrum. The frequency spacing and time synchronisation of the carriers is chosen in such a way that the carriers are orthogonal, meaning that they do not cause interference to each other, despite the overlapping of subcarriers with each other in the frequency domain. The name ‘OFDM’ is derived from the fact that the digital data is sent using many carriers, each of a different frequency (Frequency Division Multiplexing) and these carriers are orthogonal to each other, hence Orthogonal Frequency Division Multiplexing. OFDM is a special form of FDM. In conventional broadcasting, each radio station transmits on a different frequency, effectively using FDM to maintain a separation between the stations. There is however no coordination or synchronization between each of these stations. With an OFDM transmission such as DAB, the information signals from multiple stations is combined into a single multiplexed stream of data. This data is then transmitted using an OFDM ensemble that is made up from a dense packing of many subcarriers. All the subcarriers within the OFDM signal are time and frequency synchronised to each other, allowing the interference between subcarriers to be carefully controlled. These multiple subcarriers overlap in the frequency domain, but do not cause Inter-Carrier Interference (ICI) due to the orthogonal nature of the modulation. Typically, with FDM, the transmission signals need to have a large frequency guard-band between channels to prevent interference. This lowers the overall spectral efficiency. However with OFDM, the orthogonal packing of the subcarriers greatly reduces this guard band, improving the spectral efficiency as can be seen from the Figure 2.1.

6

Conventional FDM

Orthogonal FDM

Figure 2.1 Comparison of OFDM with FDM for spectral efficiency Each of the carriers in a FDM transmission can use an analogue or digital modulation scheme. There is no synchronisation between the transmission and so one station could transmit using FM and another in digital using FSK. In a single OFDM transmission all the subcarriers are synchronised to each other, restricting the transmission to digital modulation schemes. OFDM technique is symbol based, and can be treated as a large number of low bit rate carriers transmitting in parallel. All these carriers transmit in unison using synchronised time and frequency, forming a single block of spectrum. This is to ensure that the orthogonal nature of the structure is maintained. Since these multiple carriers form a single OFDM transmission, they are commonly referred to as ‘subcarriers’, with the term of ‘carrier’ reserved for describing the RF carrier mixing the signal from base band. There are several ways of looking at what makes the subcarriers in an OFDM signal orthogonal and why this prevents interference between them. Coded Orthogonal Frequency Division Multiplexing (COFDM) is the same as OFDM except that forward error correction is applied to the signal before transmission. Orthogonality . Orthogonality is a property that allows multiple information signals to be transmitted perfectly over a common channel and detected, without interference because orthogonal signals can be treated as mutually independent of each other. Loss of orthogonality results in blurring between these information signals and degradation in communications. Many 7

common multiplexing schemes are inherently orthogonal. Time Division Multiplexing (TDM) allows transmission of multiple information signals over a single channel by assigning unique time slots to each separate information signal. During each time slot only the signal from a single source is transmitted preventing any interference between the multiple information sources. Because of this TDM is orthogonal in nature. In the frequency domain most FDM systems are orthogonal as each of the separate transmission signals are well spaced out in frequency preventing interference. Although these methods are orthogonal the term OFDM has been reserved for a special form of FDM. The subcarriers in an OFDM signal are spaced as close as is theoretically possible while maintain orthogonality between them (see Figure 2.2). It was Chang [1] and Saltzburg [2] who realized that if the subchannel spacing (∆f) is equal to the reciprocal of symbol period (Ts), then the modulated signals would be orthogonal and could readily be separated by correlation using a conventional matched filter or correlator. Let ∆ f = 1 / TS (where TS is the “useful” symbol period over which the receiver

integrates the demodulated signal). Then k

th

carrier (at baseband) is written as:

e j 2πkΛ f t , 0