Atmospheric multipath resolution using spread spectrum acoustic signals

A method for measurement of multipath propagation is presented and applied to short-range outdoor sound propagation. The method employs spread-spectrum signals, in which a sinusoidal carrier is modulated by a pseudorandom maximal-length sequence. Spectrum spreading is determined by the chip rate, or the rate of carrier modulation; faster chip rates result in broader spectra. Deconvolution of received acoustic signals with respect to transmitted signals provides a filtered version of the channel impulse response, which in turn yields time delay and amplitude estimates for each propagation path. This method was tested in an outdoor sound propagation experiment in the presence of 8 m/s wind. Multiple spread-spectrum signals were transmitted from a loudspeaker and received by a three-axis microphone array. Deconvolution of the received signals clearly resolved a direct wavefront and a ground-reflected wavefront, while correlation processing failed to resolve the two propagation paths. Measured path lengths and arrival directions for these wavefronts were confirmed by a three-dimensional ray-tracing model that incorporated sound-speed and wind-velocity profiles determined from meteorological measurements. The temporal resolution of the impulse response measurements is shown to be primarily limited by the system bandwidth employed in the deconvolution operation, rather than by the carrier frequency and chip rate.