On the Performance of Rate Splitting Multiple Access for ISAC in Device-to-Multi-Device IoT Communications

In this paper, we analyze the performance of rate splitting multiple access (RSMA) technique for a multi-device communication system applying integrated sensing and communication (ISAC) to alleviate the problem of overlapping spectrum of radar signal and communication frequency bands. The system includes a cooperative access point (AP) which serves as a sensing node and a decode-and-forward (DF) relay to support the communication between a mobile device (MD) and multiple Internet-of-Things devices (IoDs). Assuming Nakagami fading channels, we provide an extensive analytical framework to evaluate the dual functionalities of the system considering various scenarios with different assumptions on blocklength, channel state information (CSI), and successive interference cancellation (SIC). In other words, we consider both infinite and finite blocklength transmissions under practical impairments including imperfect CSI and SIC. We investigate the outage probability (OP), and ergodic sum rate assuming infinite blocklength, while the block error rate (BLER), and goodput are analyzed in the finite blocklength regime. The closed-form and asymptotic expressions for the OP and BLER are presented. In addition, to evaluate the sensing performance, we derive the closed-form expressions of the false alarm and detection probabilities. Through the simulation results, we validate our analysis and delve into the impacts of various system parameters including transmit power, Nakagami shaping parameter, CSI error, SIC imperfection, the number of devices, and sensing threshold. Further, we observe that the proposed RSMA-based ISAC system provides higher ergodic sum rates compared to non-orthogonal multiple access (NOMA) both in the presence and absence of practical impairments.