Turbo codes for OFDM with antenna diversity

Abstract

OFDM with diversity and coding has been proposed as an effective means for achieving high-rates in wireless environments. Turbo codes have been shown to give near-capacity performance in additive white Gaussian noise (AWGN) channels and is being considered to enhance mobile wireless channel performance. In this paper, we present simulation results of an OFDM system with turbo coding. Comparisons with systems employing convolutional and Reed-Solomon (RS) codes are made. We first study diversity, interleaving and soft decoding for convolutional codes. The same structure is employed for turbo codes of similar complexity and varying word size. Even with a low constraint length, convolutional codes have the potential to outperform RS codes, provided that the interleaver and soft decoder are properly designed. We evaluate Turbo code performance under slow fading conditions and study the effects of changing word size. Increasing word size theoretically provides better interleaving between the two component codes. However, this advantage is less clear when the fading rate is significantly lower than the symbol rate, which is typical of the high data-rate system considered here. Under such conditions, the advantage of using two component convolutional codes in turbo codes is limited. A single convolutional code with higher constraint length may be a better choice.