Multiuser navigation and telemetry performance issues in shallow water environments

Achieving robust and reliable acoustic position fixing and telemetry performance in shallow water is complicated by the highly dispersive nature of the medium. This dispersion manifests as both time and frequency spread by virtue of the acoustic multipath phenomenon, which presents at a macroscopic (geometry influenced) and microscopic (forward scattering) scale. Platform dynamics in conjunction with the intrinsic variability of ocean propagation and forward scatter mechanisms lead to variations in both the macroscopic and microscopic behaviour of multipath. It is both the extent and the variability of shallow water multipath which makes reliable acoustic measurement and telemetry so challenging in these environments. Signal design has a key role to play, both in terms of waveform time-bandwidth product, and waveform intrinsic correlation properties in such environments. However, in practical shallow water channels, multipath and Doppler conspire to undermine favorable correlation properties, and so good signal design must go hand in hand with other techniques and methods to address the de-correlative effects of Doppler and which mitigate multipath and multi-user interference in both navigation and telemetry applications. This paper explores the technical issues, trade offs and associated computational complexity associated with the practical implementation of these techniques as a means to improve shallow water detection, navigation and telemetry decoding performance for dynamic point to point data links and multiuser data links. It is shown that whilst computational complexity can quickly scale for applications requiring robust waveforms, with high Doppler tolerance and multi-user capabilities, the associated real time processing overhead is within the capabilities of current generation fixed point digital signal processing devices (DSP) when supported by an efficient, scalable signal processing architecture. In order to demonstrate this capability, field trials w- ere performed using a high performance transceiver signal processing platform, in conjunction with enhanced wideband waveform designs, Doppler tolerant receiver processing, and adaptive multi-element spatial-temporal beamforming to demonstrate robust, multi-user detection, navigation (range Doppler-bearing) and data telemetry in shallow water between dynamic parties at variable data rates up to several kbps. Results are presented summarizing the performance of the system when operating in a shallow water channel (20 m) under, noisy, multi-aspect, dynamic conditions (+/-5kts) with range depth ratio varying from approximately 1:1 to 50:1 out to the geographical limitations of the test site (1.5 km). Metrics for detection, navigation parameter estimation and telemetry command decoding performance are presented and discussed. The results demonstrated high detection (95%) and associated telemetry command decoding success rate (94%) evaluated over the two day test period. This included transmissions received at poor aspect and transmissions received at low signal to (platform) noise). The high detection rate coupled with associated improvements in parameter estimation fidelity, notably range, Doppler and bearing estimation, demonstrates a particularly robust solution for shallow water navigation and position fixing applications. Associated telemetry streaming performance, demonstrated characteristic range-data rate trade off with rates of several kbps achieved under good to moderate channel conditions at good signal to noise, with lower data rate modes providing high reliability performance under reduced signal to noise and more challenging channel conditions. It is concluded that with appropriate signal design and supportive signal processing, high reliability multi-user, position fixing, navigation and telemetry is achievable for dynamic platforms in shallow water over extended ranges. The signal processing overhead, although not trivial, is well within the capability