Experimental performance of concatenated coding in shallow-water channels

Abstract

In a series of phase coherent underwater acoustic communication experiments near the New England continental shelfbreak, the coding gain realized by a concatenated outer block code and trellis coded modulation (TCM) inner code was measured and compared to ideal asymptotic limits. Viterbi decoding of the inner code was fully integrated into the adaptive decision feedback equalizer structure. Coding gain is typically not realized until after decision delays of several code constraint lengths. Delayed decisions for the tap update algorithm, however, degrades equalization performance. Two specific techniques were employed to mitigate this conflict, including linear extrapolation of the carrier phase correction and a unique evolution of the tap weights for each state of the Viterbi trellis. Coding gains were realized for both eight-level phase shift keyed (8 coding states) and 16 quadrature amplitude shift keyed (128 coding states) waveforms. Bit error rate as a function of output signal-to-noise ratio was computed to demonstrate practical, as opposed to asymptotic, coding gains. The extent to which coding increased the system performance envelope will then be discussed.