  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.
