This paper investigates radar-assisted user acquisition for downlink multi-user multiple-input multiple-output (MIMO) transmission using Orthogonal Frequency Division Multiplexing (OFDM) signals. Specifically, we formulate a concise mathematical model for the user acquisition problem, where each user is characterized by its delay and beamspace response. Therefore, we propose a two-stage method for user acquisition, where the Multiple Signal Classification (MUSIC) algorithm is adopted for delay estimation, and then a least absolute shrinkage and selection operator (LASSO) is applied for estimating the user response in the beamspace. Furthermore, we also provide a comprehensive performance analysis of the considered problem based on the pair-wise error probability (PEP). Particularly, we show that the rank and the geometric mean of non-zero eigenvalues of the squared beamspace difference matrix determines the user acquisition performance. More importantly, we reveal that simultaneously probing multiple beams outperforms concentrating power on a specific beam direction in each time slot under the power constraint, when only limited OFDM symbols are transmitted. Our numerical results confirm our conclusions and also demonstrate a promising acquisition performance of the proposed two-stage method.

翻译：本文研究利用正交频分复用（OFDM）信号进行下行多用户多输入多输出（MIMO）传输的雷达辅助用户捕获问题。具体而言，我们为所述用户捕获问题建立了一个简洁的数学模型，其中每个用户由其时延和波束空间响应表征。为此，我们提出了一种两阶段的用户捕获方法：首先采用多重信号分类（MUSIC）算法进行时延估计，随后应用最小绝对收缩与选择算子（LASSO）来估计用户在波束空间中的响应。此外，我们还基于成对错误概率（PEP）对所研究问题进行了全面的性能分析。特别地，我们证明了平方波束空间差矩阵的秩及其非零特征值的几何平均值决定了用户捕获性能。更重要的是，我们揭示出在功率约束下，当仅传输有限数量的OFDM符号时，在每个时隙中同时探测多个波束的性能优于将功率集中于特定波束方向。数值结果验证了我们的结论，并证明了所提两阶段方法具有优异的捕获性能。