An ISAC-based Beam Alignment Approach for Enhancing Terahertz Network Coverage

Terahertz (THz) communication can provide high-capacity data transmissions benefiting from the ultra-broad bandwidth, but suffers from severe propagation loss and line-of-sight (LoS) blockage that limits the network coverage. To compensate for the loss, narrow beams are required. But they in turn bring in THz narrow beam alignment challenges in mobile networks, resulting in poor link connection and severe latency that degrades the network performance. This paper exploits integrated sensing and communication (ISAC) technology to reduce the THz beam misalignment caused by LoS blockage and user mobility. In line with the 5G-specified beam management, we propose a joint reference signal (RS) and the synchronization signal block (SSB)-based sensing scheme to predict the need for beam switches, and signalling in advance to prevent beam misalignment, named the joint SSB and RS-based sensing (JSRS) scheme. By reducing the impact of imperfect sensing, we further provide a time-frequency sensing resource allocation scheme that minimizes the beam misalignment probability. Considering the sensing trade-offs and THz features, we provide stochastic geometry-based expressions for coverage probability and network throughput, revealing design insights into ISAC-THz network deployment and resource allocation. In urban use cases tested, numerical results show that the JSRS scheme averagely reduces 80% probability of beam misalignment, and enhances the coverage probability by about 75%, compared to the network with 5G-required positioning ability. Despite the imperfect sensing accuracy, JSRS scheme can achieve comparable performance to that of the ideal-sensing case. By exploiting the angular perception ability of directional beams, JSRS scheme helps mitigate the challenge of narrow beam management to enhance the network coverage and throughput, showing the great potential of ISAC-aided THz communication.