The Degrees-of-Freedom in Monostatic ISAC Channels: NLoS Exploitation vs. Reduction

Abstract: The degrees of freedom (DoFs) attained in monostatic integrated sensing and communications (ISAC) are analyzed. Specifically, monostatic sensing aims for extracting target-orientation information from the line of sight (LoS) channel between the transmitter and the target, since the Non-LoS (NLoS) paths only contain clutter or interference. By contrast, in wireless communications, typically, both the LoS and NLoS paths are exploited for achieving diversity or multiplexing gains. Hence, we shed light on the NLoS exploitation vs. reduction tradeoffs in a monostatic ISAC scenario. In particular, we optimize the transmit power of each signal path to maximize the communication rate, while guaranteeing the sensing performance for the target. The non-convex problem formulated is firstly solved in closed form for a single-NLoS-link scenario, then we harness the popular successive convex approximation (SCA) method for a general multiple-NLoS-link scenario. Our simulation results characterize the fundamental performance tradeoffs between sensing and communication, demonstrating that the available DoFs in the ISAC channel should be efficiently exploited in a way that is distinctly different from that of communication-only scenarios.