Communications System Performance and Design in the Presence of Radar Interference

Increasing demands for spectrum have necessitated the coexistence of communications and radar systems within the same band. This paper investigates how an unaltered radar system affects the performance of a communications receiver. For a single-carrier communications system, it is shown that a low power radar signal can be treated as Gaussian noise while a strong radar signal can be subtracted off the received signal, but in doing so one of the two signal dimensions is lost. Complex-valued constellation design problems are next proposed, with the goal of either minimizing the error rate under a power constraint, or maximizing the transmission rate under both error rate and power constraints. Numerically, the designed constellation is shaped as a concentric hexagon for weak radar interference while it morphs into an uneven pulse amplitude modulation for strong interference. A multi-carrier orthogonal frequency division multiplexing communications system is lastly considered. Due to the radar interference, the received signal becomes correlated over time and across carriers. To reduce the complexity of the optimal receiver, several suboptimal decoders are analyzed, among which the one that discards the correlations between subcarriers is numerically found to perform close to the optimal one.