Lowering the Error Floor of Optimized Short-Block-Length LDPC-Coded OFDM via Spreading

In this paper, short-block-length low-density parity-check (LDPC) codes are optimized and introduced, along with Walsh-Hadamard spreading, to orthogonal frequency-division multiplexing (OFDM) systems. Prior research has shown that lowering the threshold associated with LDPC codes results in higher error floors. The error floor becomes more pronounced for short-block-length LDPC codes that have been proposed for many wireless standards. In this paper, we show that the use of spreading along with code optimization and girth conditioning not only improves the performance but also lowers the error floor for short-length LDPC codes. Density evolution and differential evolution methods are used during the optimization phase of the codes for the frequency-uncorrelated Rayleigh fading channel. Performance results are obtained for extended irregular repeat-accumulate codes and progressive edge-growth algorithm generated codes. Next, the performances of LDPC-coded OFDM and spread OFDM systems in correlated Rayleigh fading channels are presented. A channel statistic, namely the number of faded bits per OFDM symbol, is theoretically derived, and it is demonstrated that the distribution of this channel statistic can be used to determine how close the correlated channel performance is going to be to that of the uncorrelated fading channel.