As the demand for wireless connectivity continues to soar, the fifth generation and beyond wireless networks are exploring new ways to efficiently utilize the wireless spectrum and reduce hardware costs. One such approach is the integration of sensing and communications (ISAC) paradigms to jointly access the spectrum. Recent ISAC studies have focused on upper millimeter-wave and low terahertz bands to exploit ultrawide bandwidths. At these frequencies, hybrid beamformers that employ fewer radio-frequency chains are employed to offset expensive hardware but at the cost of lower multiplexing gains. Wideband hybrid beamforming also suffers from the beam-split effect arising from the subcarrier-independent (SI) analog beamformers. To overcome these limitations, this paper introduces a spatial path index modulation (SPIM) ISAC architecture, which transmits additional information bits via modulating the spatial paths between the base station and communications users. We design the SPIM-ISAC beamformers by first estimating both radar and communications parameters by developing beam-split-aware algorithms. Then, we propose to employ a family of hybrid beamforming techniques such as hybrid, SI, and subcarrier-dependent analog-only, and beam-split-aware beamformers. Numerical experiments demonstrate that the proposed SPIM-ISAC approach exhibits significantly improved spectral efficiency performance in the presence of beam-split than that of even fully digital non-SPIM beamformers.

翻译：随着无线连接需求的持续增长，第五代及后续无线网络正在探索高效利用无线频谱并降低硬件成本的新方法。其中一种方法是采用感知与通信一体化（ISAC）范式，实现频谱的联合接入。近期ISAC研究聚焦于毫米波高频段与太赫兹低频段，以充分利用超宽带特性。在这些频段中，采用射频链路数更少的混合波束赋形器虽能降低硬件成本，但会牺牲复用增益。此外，宽带混合波束赋形还面临由子载波无关（SI）模拟波束赋形器引起的波束分裂效应。为突破上述限制，本文提出一种空间路径索引调制（SPIM）ISAC架构，通过调制基站与通信用户之间的空间路径来传输额外信息比特。我们首先设计波束分裂感知算法，同时估计雷达与通信参数，进而构建SPIM-ISAC波束赋形器。随后，我们提出采用混合波束赋形、子载波无关模拟波束赋形、子载波相关纯模拟波束赋形及波束分裂感知波束赋形等一系列混合波束赋形技术。数值实验表明，所提出的SPIM-ISAC方法在存在波束分裂的情况下，其频谱效率性能显著优于全数字非SPIM波束赋形器。