Rate splitting multiple access (RSMA) is one of the most promising techniques for ultra-reliable and low-latency communications (URLLC) with stringent requirements on delay and reliability of multiple access. To fully explore the delay performance enhancement brought by uplink RSMA to URLLC, in this paper, we evaluate the performance of two-user uplink RSMA and propose the corresponding blocklength minimization problem. We analyze the impact of finite blocklength (FBL) code on the achievable rate region and the effective throughput of uplink RSMA. On this basis, we propose the problem of minimizing the blocklength for uplink RSMA with power allocation under constrained reliability and effective throughput. Then, we present an alternating optimization method to solve this non-convex problem. Simulation results show that different from the infinite blocklength (IBL) regime, the achievable rate region of the uplink RSMA is not always larger than that of uplink non-orthogonal multiple access (NOMA) in the FBL regime. But with the help of our proposed blocklength minimization scheme, uplink RSMA can achieve the same achievable rate with a smaller blocklength compared to uplink NOMA, frequency division multiple access (FDMA), and time division multiple access (TDMA) without the need for time sharing in the FBL regime, showing the potential of uplink RSMA to achieve low delay for URLLC.

翻译：速率分割多址接入（RSMA）是为满足多址接入时延与可靠性严苛要求而实现超可靠低时延通信（URLLC）最具前景的技术之一。为充分挖掘上行RSMA为URLLC带来的时延性能增益，本文评估了两用户上行RSMA的性能并提出了相应的块长度最小化问题。我们分析了有限块长度编码对上行RSMA可达速率域与有效吞吐量的影响，在此基础上构建了可靠性与有效吞吐量约束下结合功率分配的上行RSMA块长度最小化问题，并提出一种交替优化方法求解该非凸问题。仿真结果表明：与无限块长度场景不同，在有限块长度场景下上行RSMA的可达速率域并非始终优于上行非正交多址接入。但通过本文提出的块长度最小化方案，上行RSMA在有限块长度场景下无需时分共享即可相比上行非正交多址接入、频分多址接入与时分多址接入以更小的块长度实现相同的可达速率，这证明了上行RSMA在实现URLLC低时延方面的潜力。