Characterization of temporal fluctuations of broadband channel impulse functions within communication packets

Abstract

Temporal fluctuations of an acoustic channel can be characterized by the amplitude and phase fluctuation statistics of individual multipath arrivals, and the temporal coherence of the channel. At low frequencies where the temporal coherence time is much longer than the duration of the probing pulse, the measurements of the fluctuation statistics and channel coherence functions were straightforward and can be found in previous publications. As frequency increases to tens of kHz or above, the coherence time can be shorter than the packet length used in acoustic communications (acomms). Under such circumstances, the channel coherence can impede the equalizer performance and measurement of the temporal evolution of the channel within the duration of a packet (the intra-packet coherence) is critical for modeling the acomms performance. In this paper we report the measurements of intra-packet coherence and multipath fluctuation statistics from two at-sea experiments using M-sequence signals having sequence length much shorter than the acomms packet length. Initial analysis results associated with fixed–fixed and moving-drifting source–receiver configurations will be presented. [Work supported by ONR.]