Rake Receiver for an Underwater Acoustic Communication

Abstract

Code division multiple access (CDMA) is a proper candidate for multiple access scheme of centralized underwater acoustic communication network (UAN), which has huge propagation delay and limited bandwidth. We discuss a coherent rake receiver in the shallow water environment modelled by three-path Rayleigh fading channel based on Jake’s model. BPSK modulated signal centered at 25kHz is spreaded by Walsh code and quadrature spreaded by pseudo noise (PN) code. The chip rate is fixed at 9,600 chips per second. Channel estimation is accomplished by averaging the despreaded symbol of the pilot channel over pre-defined interval called channel estimation length. The computer simulation results show that 2dB more Eb/No is required to achieve the theoretical bit error rate (BER) when spreading factor (SF) is 8, and BER is more degraded as the pilot gain is more and more larger than 0.5. For larger SF, the required channel estimation length is shortened and the sensitivity of BER for the channel estimation length and pilot gain goes down. The Eb/No gap to the theoretical BER is reduced up to 0.8dB when SF is 32.