Joint User Pairing and Beamforming Design for NOMA-Aided CFMM-ISAC Systems

Integrated sensing and communication (ISAC) systems with cell-free massive multiple-input-multiple-output (CFMM) frameworks can offer significant performance improvements for both sensing and communication functions. However, there is an obstacle to the system supporting an extremely large number of users in explosive terminal access scenarios. In this article, we propose a novel nonorthogonal multiple access (NOMA)-aided CFMM-ISAC architecture to enhance the connectivity and efficiency of the system. First, we consider a commonly used single-radar sensing scenario and jointly design user pairing and beamforming to maximize the minimum achievable communication rate while ensuring the sensing requirement. To solve the nonconvexity of the formulated optimization problem, the binary variables denoting the feasible pairing of users are first relaxed into continuous variables. An iterative optimization programming algorithm is then developed using the semidefinite relaxation (SDR), rank-one constraint omission (RCO), and successive convex approximation (SCA) approaches. To obtain feasible initial points of the iterative algorithm, we propose an initial point construction algorithm. Based on the obtained user pairing strategy, a corrective beamforming scheme is devised. We further extend our proposed algorithms to a generalized multiradar sensing scenario. Simulation results validate that the proposed joint user pairing and beamforming design achieves noticeable performance improvements compared to the existing methods.