Abstract

In wideband joint communications and sensing (JCAS) systems, the waveforms are often designed and optimized over the entire bandwidth. This significantly limits the degrees of freedom in beamforming and causes severe communications performance loss, especially under a strict radar sensing constraint. In this work, we consider a downlink of a wideband multiuser (MU) multiple-input multiple-output (MIMO) system. We propose a subcarrier allocation strategy incorporated in an efficient JCAS design to improve the communications–sensing performance tradeoff of the system. The main idea is to employ a subset of subcarriers for radar sensing while enabling communications over the entire bandwidth. This creates dedicated subcarriers for communications without any interference from the radar function, whose sensing accuracy can still be guaranteed through strict design constraints. To realize the idea, we formulate the sum rate maximization problem and propose a three-step solution based on successive convex approximation and Riemannian manifold optimization. We further propose efficient initialization methods to reduce complexity and improve the convergence of the employed optimization schemes. Our simulation results for a 32 × 4 MU-MIMO system show that the proposed scheme offers 70% improvement in the system sum rate while guaranteeing the same radar beampattern mean square error and requiring less radio frequency chains compared with the conventional JCAS design over the entire bandwidth.