Mid Sweden University

In this work, we address the problem of jointly optimizing the transmit power and blocklength of a two-users scenario for orthogonal multiple access (OMA) and non-orthogonal (NOMA) access schemes. We formulate an optimization problem to obtain the energy-optimal blocklength and transmit power under ultra-reliable and low-latency communications (URLLC) reliability and latency constraints. The aim is to minimize the energy consumption at short-blocklength regime. Due to the problem's complexity, we decompose it into two sub-problems for the OMA case, where the base station (BS) employs a 2D search for strong user, and the bisection method for the weak user. On the contrary, we find the sufficient transmit power conditions for NOMA to obtain the feasible solution. Our results show that the minimum required energy increases with the reliability requirement for both the OMA and NOMA, while NOMA consumes more energy than OMA for the same reliability target. Moreover, the results indicate that NOMA reduces latency due to better blocklength utilization compared to OMA when the channel gain disparity between the users is small.