Channel sounding for acoustic communications : techniques and shallow-water examples

This report deals with channel soundings, which are measurements of the time-varying impulse response of a propagation medium. The treatment focuses on underwater acoustic channels, the characterization of which is becoming increasingly important to support developments in the fast growing field of underwater communications and networking. The benefits of channel sounding include increased understanding of channel physics and modem performance, validation of channel models, and support for the establishment of standard test channels. Channel models are of crucial importance in communication transceivers and channel simulators. The report creates awareness of the risk of measurement errors related to properties of the probe signal, such as aliasing and delay-Doppler coupling. Channel parameters are defined and numerous shallow-water example soundings are reviewed. There are many conclusions, the most important one perhaps being that there is no typical or average acoustic channel. The variation in statistical properties, delay spread, and Doppler spread is immense. This variation is not only found between geographical areas and seasons, but also on smaller scales. For instance, the examples show that a wind burst or a passing ship can completely alter the scattering properties of a channel. Doppler spectra are examined for wideband and narrowband waveforms. In agreement with our previous work, the basic shape of measured spectra appears to be well characterized by stretched and compressed exponentials. The spectral width increases with the frequency, something that should be kept in mind upon applying narrowband tools and models to broadband waveforms. An empirical relationship is established that reduces the stretched-exponential spectrum to a single parameter. The present report is useful in several ways. It can be used as a guide for channel soundings at sea, including straightforward signal processing and computation of channel parameters. It also helps to recognize measurement errors and other pitfalls. Parameterization of Doppler spectra is useful for stochastic channel modeling. Most importantly, however, the collection of example soundings emphasizes the wide variety of acoustic propagation channels. It illustrates the challenge to devise communication systems that are efficient and robust to the environment, as well as channel simulators that faithfully mimic these environments. On the other hand, the set of channels in this report is selected so as to demonstrate the diversity as a function of area, season, weather, and local disturbances. One is unlikely to encounter all these channels for a given application or mission.