Low-Density Parity-Check Coded Orthogonal Frequency Division Multiplexing Modulation System Design for Wireless Communication Applications

Trellis coded modulation (TCM) invented by G. Ungerboeck (1976) combined coding and modulation using constellation expansion and mapping by set partitioning to achieve substantial coding gains. Low-Density Parity-Check (LDPC) code invented by Gallager (1962) and rediscovered by Mackay and Neal recently (1996) are shown to have near-Shannon-limit performance. Combining with orthogonal frequency division multiplexing (OFDM) modulation technique using efficient coded modulation scheme, performance of LDPC coded OFDM modulation can be better than traditional concatenated Reed-Solomon with convoluational coded OFDM and commonly used LDPC coded OFDM bit-interleaved coded modulation (BICM) schemes. In this thesis, we investigate efficient LDPC coded OFDM modulation schemes for fixed broadband wireless access (FBWA) communication system. Based on the parameters of IEEE 802.16a-2003 standard OFDM-256 PHY layer specification, we combine LDPC codes with several multilevel quadrature amplitude (QAM) modulations. A set of efficient LDPC coded modulation schemes with different constellation sizes is investigated in this thesis. The bit error rate (BER) performance of these schemes over additive white Gaussian noise (AWGN) and multipath fading channels is simulated and discussed. We investigate several LDPC coded OFDM modulation schemes, including MultiLevel Coding (MLC), Bit-Interleaved Coded Modulation (BICM), LDPC Coded Modulation (LCM), and Reed-Solomon with LCM (RS-LCM) schemes. Among them, we choose LCM and RS-LCM schemes to evaluate their performance by simulations, and BICM scheme for comparison. RS-LCM scheme has coding gains by 0.3~0.8 dB at a BER (Bit Error Rate) of 10-5 over conventional LDPC coded modulation BICM scheme, while does not suffer from error floors like that in LCM scheme at low bit error rates when low rate component codes are applied. RS-LCM scheme with little extra complexity by the use of Reed-Solomon code can make the transmission of LDPC coded OFDM system more efficient and less vulnerable to multipath fading effect.Abstract iii Acknowledgements v Table of Contents vi List of Tables x Chapter 1 Introduction 1 1.1 Overview of Wireless Communication System 1 1.2 Trends of Wireless Communication System 1 1.3 Thesis Organization 3 Chapter 2 LDPC Coded OFDM System 4 2.1 Low-Density Parity-Check (LDPC) Codes 4 2.1.1 History of LDPC Codes 4 2.1.2 Representation of LDPC Codes: Tanner Graphs 4 2.1.3 Regular LDPC Codes 6 2.1.4 Irregular LDPC Codes 7 2.1.5 Encoding of LDPC Codes 7 2.1.6 Decoding of LDPC Codes – Sum-Product Algorithm 8 2.1.7 Generation of Likelihood Ratio 11 2.1.8 Applications of LDPC Codes 12 2.2 Orthogonal Frequency Division Multiplexing Technique 13 2.2.1 The Fundamental Theory of OFDM 13 2.2.2 Signal Representation of OFDM Using IDFT/DFT 17 2.2.3 Parallel Data Transmission and Multiple Carriers 17 2.2.4 OFDM Discrete-time Model 18 2.2.5 Guard Interval and Cyclic Prefix 20 2.2.6 Carrier Recovery 21 2.2.6.1 Channel Estimation and Equalization 21 2.2.6.2 Channel Estimation Based on Block-Type Pilot Arrangement 22 2.2.6.3 Channel Estimation Based on Comb-Type Pilot Arrangement 22 2.2.7 High Peak-to-Average Power Ratio Problem 23 2.2.8 Applications of OFDM 24 2.3 LDPC Coded OFDM System Overview 25 Chapter 3 System Description 27 3.1 The Standard IEEE 802.16/WiMAX Forum 27 3.2 Overview of 802.16a OFDM-256 PHY 30 3.2.1 OFDM Symbol Constitution 30 3.2.2 Channel Coding 32 3.2.2.1 Randomization 33 3.2.2.2 Forward Error Coding (FEC) 33 3.2.2.3 Interleaving / De-interleaving 35 3.2.3 Modulation 36 3.2.3.1 Data Modulation / Demodulation 36 3.2.3.2 Pilot Modulation 37 3.3 Wireless Channel Modeling 38 3.3.1 Modeling of General Wireless Channels 39 3.3.2 Channel Impulse Response Used in Simulations 41 Chapter 4 Spectral Efficient Coded Modulation Schemes 43 4.1 Relative Work 43 4.1.1 Trellis Coded Modulation (TCM) 43 4.1.2 Multilevel Coding (MLC) Scheme 47 4.1.3 Bit-Interleaved Coded Modulation (BICM) Scheme 51 4.2 LDPC Coded Modulation BICM Scheme 52 4.3 LDPC Coded Modulation (LCM) Scheme 54 4.4 Reed-Solomon Code with LCM (RS-LCM) Scheme 57 Chapter 5 Simulation Results and Discussion 59 5.1 Simulation Results of Original RS-CC Coded OFDM-256 59 5.2 Simulation Results of LDPC BICM Scheme 62 5.3 Simulation Results of LCM Scheme 65 5.4 Simulation Results of RS-LCM Scheme 72 5.5 Simulation Results Summary 73 Chapter 6 Conclusion and Future Work 75 6.1 Conclusion 75 6.2 Future Work 75 References 7