Broadband FIR Beamformer for Underwater Aoustic Communication

Beamforming for underwater acoustic communication (UAC) is affected by the broadband feature of UAC signal, which has relatively low center frequency compared with the signal bandwidth. The narrow-band assumption does not hold good in the UAC. (Unrealistic assumption) In this paper, we present computer simulation results of a broadband FIR beamformer for UAC using the baseband equivalent array signal model for phase-coherent UAC. We consider a vertical uniform linear array with 8 omni-directional elements and UAC system with carrier frequency 25 KHz and symbol rate 5 KHz. Pulse shaping filter (PSF) is the raised cosine filter with roll-off factor 0.25. Hence bandwidth to carrier frequency ratio is 0.25. The sensor spacing is half of the wavelength corresponds to carrier frequency. For simplicity, we assume that there is no signal fading and phase synchronization is perfect. We have considered two propagating paths: direct and surface reflected ones. The surface reflected path is regarded as interference signal. The signal to noise ratio (SNR) and signal to interference ratio (SIR) are fixed to 6 dB and OdB, respectively. The results show that the broadband FIR beamformer achieves optimum signal to interference and noise ratio (SINR) and outperforms the conventional narrowband beamformer by 0.5 dB of SINR when the inter-tap delay is a quarter of the symbol interval and the order of FIR filter is one. The broadband FIR beamformer performance is degraded when the FIR filter order is increased above the optimum value. The inter-tap delay should be small enough and it is related to the optimum FIR filter order.