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The issue of channel state information at the transmitter is investigated using MIMO channel measurements and by deriving expressions for ergodic and outage capacity in a Rayleigh fading channel. Expressions for bit error rates in Rayleigh fading channels are also presented for orthogonal space time block codes and for beamforming where the bit error rates in the beamforming case follow from the distribution of the largest eigenvalue to Wishart matrices. We demonstrate by measurements that Rayleigh fading is a valid assumption in non line of sight channels, although a Nakagami-m distribution showed to be a more appropriate distribution model in both line of sight and NLOS environments. It was also demonstrated that channel state information at the transmitter is less useful in high-SNR scenarios but is more useful in line of sight channels compared to non line of sight environments.

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Abstract-This paper introduces the underwater acoustic modem as implemented within the UAN -Underwater Acoustic Network project. The low power modem has implemented turbo equalization algorithms in addition to variable spread rate direct sequence spread spectrum signaling. The network layer implemented on the modem support automatic network discovery, multi hop routing and support for mobile nodes, and when expanded with a single board computer via serial line it supports IP connectivity end-to-end. Experimental results from sea trials are presented.