The performance of noncoherent M -ary frequencyshift keying (MFSK) with dual-branch postdetection switch and stay combining (SSC) in two-wave with diffuse power (TWDP) fading is studied. A closed-form expression for the average bit error rate (BER) of noncoherent MFSK with dual-branch postdetection SSC is derived. Monte carlo simulation results are presented to verify the validity of the obtained analytical expressions. The effects of TWDP fading parameters on the relative performances of postdetection and predetection SSC receivers are studied. It is shown that postdetection SSC has superior performance over predetection SSC.Index Terms-Diversity combining, fading channels, noncoherent frequency shift keying, predetection diversity, postdetection diversity, Rayleigh fading, Rician fading, two-wave with diffuse power fading.

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We analyze the optimal location of underwater acoustic sensor nodes, with respect to the capacity of the wireless links between the nodes, in a multi-hop hierarchical tree structure based on the information each node has to transfer between underwater levels. A novel algorithm is proposed to calculate the optimal distances between sensor nodes for the proposed hierarchical network. Numerical results for different channel conditions; such as an ideal channel, an imperfect channel with a certain BER, and a channel with interference, are provided. In addition, our algorithm is used to obtain the optimal frequency for each level in the tree shaped network for the above-mentioned conditions.