In conventional joint communications and sensing (JCAS) designs for multi-carrier multiple-input multiple-output (MIMO) systems, the dual-functional waveforms are often optimized for the whole frequency band, resulting in limited communications--sensing performance tradeoff. To overcome the limitation, we propose employing a subset of subcarriers for JCAS, while the communications function is performed over all the subcarriers. This offers more degrees of freedom to enhance the communications performance under a given sensing accuracy. We first formulate the rate maximization under the sensing accuracy constraint to optimize the beamformers and JCAS subcarriers. The problem is solved via Riemannian manifold optimization and closed-form solutions. Numerical results for an 8x4 MIMO system with 64 subcarriers show that compared to the conventional subcarrier sharing scheme, the proposed scheme employing 16 JCAS subcarriers offers 60% improvement in the achievable communications rate at the signal-to-noise ratio of 10 dB. Meanwhile, this scheme generates the sensing beampattern with the same quality as the conventional JCAS design.

翻译：在传统多载波多输入多输出（MIMO）系统的联合通信与感知（JCAS）设计中，双功能波形通常针对整个频段进行优化，导致通信与感知性能的权衡受限。为克服这一局限，我们提出采用子载波子集进行JCAS，而通信功能在所有子载波上执行。这为在给定感知精度下提升通信性能提供了更多自由度。我们首先在感知精度约束下建立速率最大化问题，以优化波束成形器和JCAS子载波。该问题通过黎曼流形优化和闭式解求解。针对具有64个子载波的8x4 MIMO系统的数值结果表明，与传统子载波共享方案相比，所提出的采用16个JCAS子载波的方案在信噪比为10 dB时，可实现通信速率提升60%。同时，该方案生成的感知波束方向图质量与传统JCAS设计相当。