Underwater Acoustic QPSK Receiver Implementation and Its Test Results at the Very Shallow Water

Abstract

In this paper, we describe an DSP based implemented QPSK receiver prototype for 25kHz carrier frequency and 5 kHz symbol rate with excess bandwidth 0.35, and its test results at the very shallow water of the South Sea of Korea. The implemented digital receiver prototype for underwater communication is composed of analog and digital signal processing parts. The analog part, composed of an implemented omni-directional acoustic sensor and a bi-directional deriving unit, is in charge of acquiring the acoustic signal and of converting the acoustic signal into an electric signal. The sampled passband signal should be converted into a baseband signal and then the baseband signal is further processed to enhance the signal quality and reduce decision error by synchronizer, equalizer, and Viterbi decoder. The joint synchronizer detects the packet synchronization and estimates the coarse symbol timing and phase offset at the same time. After synchronization, the received baseband signal is decimated to take 4 samples per symbol which is the input of channel equalizer. The channel equalizer improves the quality of signal by reducing the multipath interference and tracking the phase and frequency offsets. The Viterbi decoder decides the transmitted information using the output of equalizer. The implemented QPSK receiver prototype is tested the BER performance by transmitting 160pixel*100pixel*8bit/pixel images with horizontal transmission distances of 1km, 2km and 3km at the very shallow water (approximately 25m water depth). Test result shows that BER is 0, 2.3x10-5, and 1.8x10-3 for the 1km, 2km and 3km transmission distance, respectively.