The recent development of integrated sensing and communications (ISAC) technology offers new opportunities to meet high-throughput and low-latency communication as well as high-resolution localization requirements in vehicular networks. However, considering the limited transmit power of the road site units (RSUs) and the relatively small radar cross section (RCS) of vehicles with random reflection coefficients, the power of echo signals may be too weak to be utilized for effective target detection and tracking. Moreover, high-frequency signals usually suffer from large fading loss when penetrating vehicles, which seriously degrades the quality of communication services inside the vehicles. To handle this issue, we propose a novel sensing-assisted communication mechanism by employing an intelligent omni-surface (IOS) on the surface of vehicles to enhance both sensing and communication (S&C) performance. To this end, we first propose a two-stage ISAC protocol, including the joint S&C stage and the communication-only stage, to fulfill more efficient communication performance improvements benefited from sensing. The achievable communication rate maximization problem is formulated by jointly optimizing the transmit beamforming, the IOS phase shifts, and the duration of the joint S&C stage. However, solving this ISAC optimization problem is highly non-trivial since inaccurate estimation and measurement information renders the achievable rate lack of closed-form expression. To handle this issue, we first derive a closed-form expression of the achievable rate under uncertain location information, and then unveil a sufficient and necessary condition for the existence of the joint S&C stage to offer useful insights for practical system design. Moreover, two typical scenarios including interference-limited and noise-limited cases are analyzed.

翻译：集成感知与通信（ISAC）技术的近期发展为满足车联网中高吞吐量、低时延通信以及高分辨率定位需求提供了新机遇。然而，考虑到路侧单元（RSU）发射功率受限、车辆具有随机反射系数且其雷达散射截面（RCS）相对较小，回波信号功率可能过弱而无法有效用于目标探测与跟踪。此外，高频信号穿透车辆时通常面临严重衰落损耗，这会显著降低车内通信服务质量。为解决该问题，本文提出一种新颖的感知赋能通信机制：在车辆表面部署智能全向表面（IOS）以同时提升感知与通信（S&C）性能。为此，我们首先设计包含联合S&C阶段与纯通信阶段的两阶段ISAC协议，从而通过感知实现更高效的通信性能提升。通过联合优化发射波束赋形、IOS相位偏移及联合S&C阶段持续时间，建立可达通信速率最大化问题。然而，由于非精确估计和测量信息导致可达速率缺乏闭合表达式，求解该ISAC优化问题极具挑战性。针对这一难题，我们首先推导出位置信息不确定条件下可达速率的闭合表达式，进而揭示联合S&C阶段存在的充要条件，为实际系统设计提供有益启示。此外，本文还分析了干扰受限与噪声受限两种典型场景。