Abstract
Time reversal (TR) can achieve temporal and spatial focusing by exploiting spatial diversity in complex underwater environments with significant multipath. This property makes TR useful for underwater acoustic (UWA) communications. Conventional TR is realized by performing equal gain combining (EGC) on the single element TR output signals of each element of the vertical receive array (VRA). However, in the actual environment, the signal-to-noise ratio (SNR) and the received noise power of each element are different, which leads to the reduction of the focusing gain. This paper proposes a time reversal maximum ratio combining (TR-MRC) method to process the received signals of the VRA, so that a higher output SNR can be obtained. The theoretical derivation of the TR-MRC weight coefficients indicates that the weight coefficients are only related to the input noise power of each element, and are not affected by the multipath structure. The correctness of the derivation is demonstrated with the experimental data of the long-range UWA communications conducted in the South China Sea. In addition, the experimental results illustrate that compared to the conventional TR, TR-MRC can provide better performance in terms of output SNR and bit error rate (BER) in UWA communications.
Highlights
Underwater acoustic (UWA) channels are considered one of the most challenging wireless communication channels [1,2]
It can be seen from Equation (5) that after Time reversal (TR) combining, all the multipath signals received by the vertical receive array (VRA) are coherently superimposed to achieve spatial focusing, while the noises are incoherently superimposed, so the output signal-to-noise ratio (SNR) is improved
The weight coefficients calculated by the method proposed in 1t0his paper are basically consistent with those calculated by the conventional method, and1t2hose weight coefficients have a reciprocal relationship with the received noise power illu14strated in Figure 5, indicating that the time reversal maximum ratio combining (TR-MRC) weight coefficients are only related to the16received noise power of each array element while having nothing to do with the multip1a8th
Summary
Underwater acoustic (UWA) channels are considered one of the most challenging wireless communication channels [1,2]. The output signals of each receiver are combined using the EGC method to obtain the final output signal: r(t) = ∑ ri(t) i=1 It can be seen from Equation (5) that after TR combining, all the multipath signals received by the VRA are coherently superimposed to achieve spatial focusing, while the noises are incoherently superimposed, so the output SNR is improved. In the actual marine environment, the SNRs and noise power of the TR outputs of the receivers at different depths are different In this case, the spatial focusing gain obtained by combining the TR output signals of each element using the EGC method is not the best.
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