LDPC-based space-time coded OFDM systems over correlated fading channels: performance analysis and receiver design

Abstract

We consider the performance analysis and the receiver design of low-density parity-check (LDPC) code based space-time coded (STC) orthogonal frequency-division multiplexing (OFDM) system over correlated frequency-and time-selective fading channels. I. SYSTEM DESCRIPTIONConsider an STC-OFDM system with K subcarriers, N transmitter antennas and M receiver antennas, signaling through frequency-and time-selective fading channels. Each STC code word spans P adjacent OFDM words (or slots); and each OFDM word consists of (Nh') STC symbols. It is assumed that the fading processes associated with different transmitter-receiver antenna pairs are uncorrelated; and for a particular transmitter-receiver antenna pair, the fading process is correlated in both frequency and time. PERFORMANCE ANALYSIS AND DESIGN PRINCIPLESBy numerically evaluating the outage channel capacity of the STC-OFDM systems, we find that the frequency selectivity and the time selectivity are equivalent in terms of their impact on the outage capacity; and the product of their order, denoted by L, ultimately affects the outage capacity.By analyzing the Chernoff upper bound of the pairwise error probability (PEP) of the STC-OFDM systems, we conclude that the highest possible diversity order the STC-OFDM system can provide is ( N M L ) , i.e., the product of the number of transmitter antennas, the number of receiver antennas and the diversity order of selective-fading channels.Moreover, we argue that systematic coding design is less helpful in the system considered here, due to the assumption that the fading channel profiles (e.g., the delay profile or the Doppler frequency) is known only at the receiver. Instead, we propose that two general principles should be met in choosing STC codes in order to robustly exploit the rich diversity resources in this system, namely, large eflective length [l] and ideal interleaving. LDPC-BASED STC-OFDM SYSTEMSLow-density parity-check (LDPC) codes are the class of linear block codes characterized by "sparse" parity-check matrices [2]. The LDPC codes have the following advantages for the STC-OFDM system considered here: ( I ) The minimum distance of binary LDPC codes increases linearly with the block-length with probability close to 1 [a], which in turn 'increases the effective length of the LDPC-based STC's. (2) No extra interleaver is required because the parity-check matrix of LDPC codes is randomly generated. (3) The decoding complexity of LDPC codes is usually low and is highly parallelizable. (4) For a given block-length and code rate, a "good" LDPC code can be easily constructed. ( 5 )LDPC codes do not typically show an error floor, which is suitable for short-frame applications.At the transmitter of an LDPC-based STC-OFDM system, the information bits are first encoded by an LDPC encoder, and then the binary LDPC coded bits are modulated into STC symbols, which are transmitted from N antennas and over P adjacent OFDM words. IV. TURBO RECEIVERWe consider the receiver design for LDPC-based STC-OFDM systems in unknown ...

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