Average BLER Analysis of Ambient Backscatter RSMA Systems for URLLC Short-Packet Communications

Rate-splitting multiple access (RSMA) has emerged as a promising approach for ultra-reliable and low-latency communications (URLLC), addressing stringent delay and reliability requirements in next-generation networks. This study investigates downlink RSMA for URLLC short-packet communication (SPC) by analyzing the average block error rate (BLER) in an RSMA-enabled symbiotic backscatter system. A base station serving two users via RSMA is considered, while a backscatter device passively assists the transmission, improving overall efficiency. Closed-form expressions are derived for approximate BLERs and the overall system throughput, and validated through Monte Carlo simulations. Results indicate that RSMA achieves about one order-of-magnitude lower BLER than NOMA at rho_S = 30 dB and approximately 15-20% higher throughput in the considered short-packet regime. The impact of power allocation and blocklength on reliability is also investigated, providing design insights for URLLC optimization. These findings demonstrate that RSMA offers improved reliability and fairness under envisioned URLLC short-packet scenarios, while the inclusion of passive backscatter enhances energy efficiency without additional transmit power.