Exploiting Space-Time-Frequency Diversity With MIMO-OFDM for Underwater Acoustic Communications

Underwater acoustic (UWA) channels exhibit time-varying fading statistics, thus a coded modulation scheme optimally designed for a specific model (e.g., Rayleigh fading) will perform poorly when the channel statistics change. Exploiting diversity via coded modulation is a robust approach to improve the reliability of the acoustic link in a variety of channel conditions. Two coded modulation schemes drawn from the terrestrial radio literature are compared in terms of their bit error rate (BER). The first scheme combines trellis coded modulation (TCM) based on an 8-phase-shift keying (8-PSK) signal set and symbol interleaving. The second scheme is based on bit-interleaved coded modulation (BICM), which includes a convolutional encoder, a bit interleaver, and a 16-quadrature-amplitude-modulation (16-QAM) signal set. These schemes, which are designed to have the same bit rate and decoding complexity, are tested under two scenarios. In the first scenario, a single-input-multiple-output (SIMO) system is implemented by means of orthogonal frequency-division multiplexing (OFDM) modulation. In the second scenario, a multiple-input-multiple-output (MIMO) system is implemented and each of the coded modulation scheme is coupled with a 3/4-rate space-time block code (STBC) before applying OFDM. Analyzing both simulated and experimental data, the following results, which also hold for terrestrial radio, are confirmed: coded modulation schemes emphasizing higher Hamming distance (such as BICM) yield a lower error rate when spatial diversity is very limited (first scenario). On the other hand, coded modulation schemes emphasizing higher free Euclidean distance (such as TCM) demonstrate a lower error rate when spatial diversity is sufficiently high (second scenario).