An adaptive, soft-decision multiuser receiver for underwater acoustical channels


An underwater acoustic local area network (ALAN) is being deployed which permits two-way data telemetry between many high-rate, ocean-bottom sensors and a central, surface-deployed receiver in the 10- 40kHz vertical acoustical channel [1]. The oceanbottom nodes initiate the transmission process by requesting data channel time slots through a common narrowband request channel. Request packets overlap in time and frequency in this scenario, and the throughput and average transmission delay rely heavily on the successful resolution of the request channel collisions. This paper describes the design and performance of a request channel receiver capable of resolving collisions between several asynchronous and cochannel packets. The receiver algorithm differs from: standard capture schemes (by demodulating the data from both strong and weak transmitters), conventional spread-spectrum receivers (by overcoming the near-far problem), and existing multiple-access demodulation techniques (by adapting to the number of interfering signals, and the unknown phase, Doppler, amplitude and timing of each signal in the collision). The receiver demodulates the collided packets by decision-directed techniques, it differs from known decision-directed multiple-access receivers [2] [3] [4] through a novel method of estimating the interference for each user which minimizes error propagation due to inaccurate tentative decisions [5] [6]. An in-water experiment illustrates that this technique is extremely desirable for collision resolution in underwater acoustic local area networks, and also for underwater autonomous vehicles with both sidescan sonar as well as acoustic telemetry links.