Near-Field Automotive Joint Radar-Communications with Spatial Path Index Modulation

We consider a terahertz (THz)-band joint radar-communications (JRC) scenario, wherein the radar targets and the communication user are in the near-field. In THz wireless systems, the transmission range is short so that the signal wavefront is spherical in near-field. Further- more, large THz antenna arrays suffer from beam-squint arising from its ultra-wide bandwidth. To compensate the loss due to beam-squint, we devise a technique using spatial path index modulation (SPIM) that also improves the spectral efficiency (SE) performance of the overall system. Specifically, SPIM allows the transmission of additional information bits to the receiver via modulating the indices of the spatial paths. To this end, we design hybrid analog/digital beam- formers for the near-field JRC scenario by generating multiple beams toward both radar targets and the communications users. Numerical experiments demonstrate that the proposed approach exhibits signifi- cant SE performance even higher than that of the use of fully digital beamformers without SPIM in the presence of near-field beam-squint.