Localization of OFDM Sources Using a Single Hydrophone in Rician Fading Channels

This paper investigates range-depth localization of an orthogonal frequency-division multiplexing (OFDM) source using a single hydrophone, which serves as a portable scheme suitable for small underwater platforms. We consider a typical shallow-water propagation model, where the rich position information embedded in multipath delays and amplitudes offers the potential to achieve the above purpose. Specifically, we exploit the frequency-domain input-output relationship of pilot subcarriers to construct a measurement model and incorporate joint processing of multiple OFDM symbols to enhance localization performance. To characterize random fluctuations in path gains caused by scattering from the rough surface and bottom, we introduce the Rician fading model. Under this assumption, each complex path amplitude comprises a deterministic component associated with the source position and a random perturbation. To fully leverage the parametric model, we adopt the stochastic maximum likelihood principle. The resulting high-dimensional optimization problem is iteratively solved by the developed expectation-maximization-type algorithm. In addition, a stable initialization scheme is designed to strive for convergence to the global optimum. To quantitatively evaluate the sensing capability of the integrated system, we derive the Cramér-Rao bound on the mean square error of the localization result and elucidate the performance gain from the joint processing. For completeness, we also present a tailored communication processing chain for symbol detection. Simulations and laboratory experiments demonstrate the superiority of the proposed algorithm over existing matched field processing-based positioning techniques.