Optimized LDPC Codes for OFDM and Spread OFDM in Correlated Channels

This paper analyzes optimized short block length low-density parity-check (LDPC) codes in spread OFDM systems that operate in frequency correlated Rayleigh fading channels. Extended irregular repeat-accumulate (eIRA) codes and codes constructed with progressive edge-growth (PEG) algorithm are optimized using density evolution and differential evolution techniques and any error floors are improved using girth conditioning. It is shown here that error floors in LDPC coded OFDM is lowered by introducing spreading on top of channel coding. It is also demonstrated that by using the distribution of the number of faded bits per OFDM symbol, the similarity in performance between codes in uncorrelated and correlated Rayleigh fading channels can be determined a priori.