Multiinput multioutput OFDM for shallow-water UWA communications

Abstract

We investigate performance of turbo coded multiple-input multiple-output (MIMO)-OFDM systems with layered space time (LST) architectures for underwater acoustic (UWA) channels by using simulations and results from the AUVfest experiment performed in June 2007. MIMO systems have been shown to be promising in the sense that they increase the reliable transmission rates significantly without consuming additional bandwidth and power. Robustness of OFDM systems with cyclic prefix or zero padding to ISI channels are also well known; so the combination of MIMO techniques and OFDM can be regarded as a promising technology for shallow water UWA communications which is characterized by severe bandwidth limitations and long intersymbol interference (ISI) spans. The paper reviews necessary components of a MIMO-OFDM communication system, including, time and frequency synchronization, channel estimation, and tracking of the varying channel parameters. Modifications necessary to make the system suitable for UWA channels are summarized. Results of the AUVFest 2007 experiment are very promising; for instance, 2×2 MIMO-OFDM can reach up to 60 Kbps transmission for a bandwidth of 16 KHz with simple receiver structures for a range of 2000 m. In addition to the coherent system, differential and unitary space-time coded MIMO-OFDM scenarios are also considered.