Achieving Delay Diversity in Asynchronous Underwater Acoustic (UWA) Cooperative Communication Systems

Abstract

In cooperative UWA systems, due to the low speedof sound, a node can experience significant time delays amongthe signals received from geographically separated nodes. Oneway to combat the asynchronism issues is to employ orthogonalfrequency division multiplexing (OFDM)-based transmissions atthe source node by preceding every OFDM block with anextremely long cyclic prefix (CP) which reduces the transmissionrates dramatically. One may increase the OFDM block lengthaccordingly to compensate for the rate loss which also degradesthe performance due to the significantly time-varying nature ofUWA channels. In this paper, we develop a new OFDM-basedscheme to combat the asynchronism problem in cooperativeUWA systems without adding a long CP (in the order of thelong relative delays) at the transmitter. By adding a muchmore manageable (short) CP at the source, we obtain a delaydiversity structure at the destination for effective processing andexploitation of spatial diversity by utilizing a low complexityViterbi decoder at the destination, e.g., for a binary phase shiftkeying (BPSK) modulated system, we need a two-state Viterbidecoder. We provide pairwise error probability (PEP) analysisof the system for both time-invariant and block fading channelsshowing that the system achieves full spatial diversity. We findthrough extensive simulations that the proposed scheme offers asignificantly improved error rate performance for time-varyingchannels (typical in UWA communications) compared to theexisting approaches.