This paper studies the transmit beamforming in a downlink integrated sensing and communication (ISAC) system, where a base station (BS) equipped with a uniform linear array (ULA) sends combined information-bearing and dedicated radar signals to simultaneously perform downlink multiuser communication and radar target sensing. Under this setup, we maximize the radar sensing performance (in terms of minimizing the beampattern matching errors or maximizing the minimum weighted beampattern gains), subject to the communication users' minimum signal-to-interference-plus-noise ratio (SINR) requirements and the BS's transmit power constraints. In particular, we consider two types of communication receivers, namely Type-I and Type-II receivers, which do not have and do have the capability of cancelling the interference from the {\emph{a-priori}} known dedicated radar signals, respectively. Under both Type-I and Type-II receivers, the beampattern matching and minimum weighted beampattern gain maximization problems are globally optimally solved via applying the semidefinite relaxation (SDR) technique together with the rigorous proof of the tightness of SDR for both Type-I and Type-II receivers under the two design criteria. It is shown that at the optimality, radar signals are not required with Type-I receivers under some specific conditions, while radar signals are always needed to enhance the performance with Type-II receivers. Numerical results show that the minimum weighted beampattern gain maximization leads to significantly higher beampattern gains at the worst-case sensing angles with a much lower computational complexity than the beampattern matching design. We show that by exploiting the capability of canceling the interference caused by the radar signals, the case with Type-II receivers results in better sensing performance than that with Type-I receivers and other conventional designs.

翻译：本文研究了下行链路集成感知与通信（ISAC）系统中配备均匀线性阵列（ULA）的基站（BS）发送融合信息承载信号与专用雷达信号，以同时实现下行多用户通信与雷达目标感知的发射波束赋形问题。在此框架下，我们在满足通信用户最小信干噪比（SINR）需求及BS发射功率约束的条件下，最大化雷达感知性能（即最小化波束方向图匹配误差或最大化最小加权波束方向图增益）。具体考虑两类通信接收机：类型-I接收机不具备消除已知专用雷达信号干扰的能力，而类型-II接收机具备此能力。针对两类接收机下的波束方向图匹配与最小加权波束方向图增益最大化问题，通过应用半定松弛（SDR）技术并严格证明两类接收机在两种设计准则下SDR的紧性，获得了全局最优解。结果表明：在最优条件下，类型-I接收机在某些特定场景中无需发射雷达信号，而类型-II接收机则始终需要雷达信号以增强性能。数值实验显示：相较于波束方向图匹配设计，最小加权波束方向图增益最大化能够在最差感知角度上获得显著更高的波束方向图增益，且计算复杂度更低。通过利用消除雷达信号干扰的能力，类型-II接收机相比类型-I接收机及其他传统设计能实现更优的感知性能。