Performance analysis of an underwater acoustic communications time diversity receiver

One method for introducing time diversity into a decision feedback equalizer (DFE) spatial diversity communications receiver operating in an underwater acoustic environment is presented and analyzed. Time diversity refers to the repetition of transmitted packets and the subsequent batch processing of these packets by an appropriately modified multichannel receiver. To overcome the effects of the time-varying nature of the channel, the modifications made to the spatial diversity-based multichannel DFE (SD-DFE) include appropriate Doppler shift compensation for each received packet and determination of the proper alignment among the time-displaced packets to allow for successful equalization. The proposed time diversity DFE (TD-DFE) receiver is shown to achieve a high rate of packet success under a variety of channel conditions. More significantly, the TD-DFE algorithm is able to equalize packets when a comparable SD-DFE might fail. The packet-to-packet coherence of a series of transmissions is analyzed to provide insight into how the characteristics of the channel affect the performance of the TD-DFE. This analysis illustrates why a time diversity-based equalization approach can be more robust for the underwater acoustic channel as compared to a standard TD-DFE algorithm at the possible expense of reducing throughput.