Wideband Near-Field Integrated Sensing and Communications: A hybrid precoding perspective

Integrated sensing and communications (ISAC) is a promising technology in solving the exacerbated spectrum congestion problem. Future developments of ISAC are expected to provide high-data-rate communications and high-resolution sensing, which demand a large frequency bandwidth and a large-scale (XL) antenna array. This has a two-fold consequence. The electromagnetic (EM) properties change from far-field uniform planar wave (UPW) to near-field uniform spherical wave (USW) propagation, and the waveform must now follow the wideband signal model in lieu of the commonly assumed narrow-band signal structure. The consideration of near-field wideband ISAC has recently prompted a reevaluation of the design techniques, including channel modeling and parameter estimation as well as precoding technologies. In this article, we review the principles of near-field wideband sensing and communications and describe the progress made in devising suitable precoding techniques. Various approaches to ISAC system design, including fully digital and hybrid analog‒digital precoders, are discussed along with their respective benefits and limitations. The efficacy of the design is judged by the achievable accuracy in target range and angular direction estimates, the power focusing, and the communication quality of service (QoS). We provide at the end a perspective on possible future research and trends in this area.