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：近年来的集成感知与通信（ISAC）技术发展为满足车联网中高吞吐量、低延迟通信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）有限的发射功率和具有随机反射系数的车辆相对较小的雷达散射截面（RCS），回波信号功率可能过弱而无法有效用于目标检测与跟踪。同时，高频信号穿透车辆时通常面临严重衰落损耗，这将显著降低车内通信服务质量。针对这一问题，我们在车辆表面部署智能全向表面（IOS），提出了一种新型感知辅助通信机制，以同时增强感知与通信（S&C）性能。为此，我们首先设计了一种两阶段ISAC协议，包含联合S&C阶段和纯通信阶段，通过感知能力实现更高效的通信性能提升。通过联合优化发射波束成形、IOS相位偏移和联合S&C阶段持续时间，建立了可达通信速率最大化问题。然而，由于不精确的估计与测量信息导致可达速率缺乏闭式表达式，该ISAC优化问题的求解极具挑战性。针对该问题，我们首先推导了位置信息不确定条件下可达速率的闭式表达式，继而揭示了联合S&C阶段存在的充要条件，为实际系统设计提供重要指导。同时，分别对干扰受限和噪声受限两种典型场景进行了分析。