A nullspace-based precoder with subspace expansion for radar/communications coexistence

In this paper we consider spectrum sharing between a communication system, modeled as MIMO interference channel (MIMO IFC), and MIMO radar. We first derive a zero-forcing precoder to eliminate the interference at the communication receivers. We show that this precoder is equivalent to orthogonal projection matrix onto the null space of an effective interference channel and show that this choice of precoder will degrade the radar target direction estimation performance. We show that the radar performance can be improved at the cost of non-zero interference at communication users by projecting the radar signal onto a subspace which is an expanded version of the aforementioned null space. We propose two approaches to smoothly expand the projected subspace and study the tradeoff between radar performance and interference at the communication users through simulation.