DSP implementation of SISO and MIMO OFDM acoustic modems

Significant progress has been made recently on the use of multicarrier modulation in the form of orthogonal frequency division multiplexing (OFDM) for high data rate underwater acoustic communications. In this paper, we present implementation results of OFDM acoustic modems under both single-input single-output and multi-input multi-output (two transmitters and two receivers) settings. The data rates are 3.2 kb/s and 6.4 kb/s for SISO and MIMO systems, respectively, with QPSK modulation, rate-1/2 channel coding, and signal bandwidth of 6 kHz. Real-time operation has been achieved with impressive margins, using a floating point TMS320C6713 DSP development board. Specifically, with convolutional coding, the processing time for each OFDM block of duration 210 ms is about 38 ms and 77 ms for SISO and MIMO settings, respectively, with the DSP running at 225 MHz. With nonbinary low-density parity-check (LDPC) coding, which gains about 2 dB in error performance relative to convolutional coding, the processing time per block increases to 50 ms and 101 ms for SISO and MIMO settings, respectively. We have also implemented the OFDM acoustic modems using a fixed-point TMS320C6416 DSP development board, where the DSP core runs at 1 GHz. The per-block processing time reduces by two thirds with negligible performance degradation.