APPLYING DIVERSITY TO MITIGATE INTERFERENCE IN UNDERWATER ACOUSTIC COMMUNICATION NETWORKS

Many acoustic channels suffer from interference which is neither narrowband nor impulsive. This relatively long duration partial band interference can be particularly detrimental to system performance. In undersea networks, many dropped messages are lost due to partial band interference which corrupts different portions of the received signal depending on the relative position of the interferers, information source and receivers due to the slow speed of propagation. A survey of recent work in interference mitigation and orthogonal frequency division multiplexing (OFDM) provides motivation to develop a spatial diversity receiver for use in underwater networks. The spatial diversity receiver for underwater communications identifies portions of the signal suffering from interference on different receivers, removes these portions of the signal and then optimally combines the remaining clean portions of the signal. The gain of the spatial diversity combining strategy is investigated as a function of signal to interference ratio (SIR), signal to noise ratio (SNR), interference bandwidth and time duration. Analytic results of performance for receivers on additive white Gaussian noise channels suffering time orthogonal interference demonstrate the effectiveness of the spatial diversity combining strategy as compared to conventional maximum ratio combining. Simulation results on time invariant channels confirm the effectiveness of the algorithm under more complex channel conditions. Comparison of the spatial diversity receiver performance to a recently developed single receiver parameterized interference cancellation algorithm is made using experimental data collected on the Atlantic Undersea Test and Evaluation Center (AUTEC) network which consists of multiple distributed cabled hydrophones. Both techniques are effective strategies for combating interference but approach the problem of interference in fundamentally different ways: the spatial diversity receiver blanks the interference while the cancellation algorithm coherently removes it from the desired signal. Spatial diversity reconstruction is effective and realizes the most gain at low SIR and moderate SNR while parameterized interference cancellation is most effective at moderate SIRs and SNRs. The two approaches are complimentary and an effective multi-channel receiver strategy would be to adaptively utilize both techniques