MIMO-OTH Radar: Signal Model for Arbitrary Placement and Signals With Non-Point Targets

Taking into account the existence of multipath ionospheric propagation (MIP), a typical phenomenon that skywave over-the-horizon (OTH) radar may encounter, this paper develops the received signal model for a non-point target for multiple-input multiple-output skywave over-the-horizon (MIMO-OTH) radar using the most well accepted physics-based model of the ionosphere. The signal model describes the ionospheric state, the number of propagation paths between a radar antenna and the target center, as well as the correlation between reflection coefficients. It is shown that varying system parameters, such as antenna positions and signal frequencies, can result in causing the model to change from a case with highly correlated reflection coefficients, for some paths, to a case with virtually uncorrelated reflection coefficients for these same paths. Conditions are presented that describe when the highly correlated reflection coefficient case applies and when the virtually uncorrelated reflection coefficient case applies. Then, the proposed model is used to solve an example target detection problem. Some cases are shown where the performance may improve when more paths are present. The maximum possible diversity is computed and shown to increase with the number of paths which partially explains these observations. It is shown in the studied example that even if the receive antennas are closely spaced, full diversity gain may be obtained due to MIP.