Channel Blocklength Minimization in MU-MISO Nonorthogonal Multiple Access for URLLC Services

This article investigates the channel blocklength minimization problem in a multiuser multiinput single-output (MU-MISO) nonorthogonal multiple access (NOMA) system with beamforming and imperfect channel state information at the transmitter to satisfy the stringent quality of service (QoS) requirements of ultrareliable and low-latency communications (URLLC). We jointly optimize the channel blocklength and beamforming vector to minimize the used channel blocklength in a mission-critical Internet-of-Things scenario. To solve the formulated problem, we first propose a bisection method to obtain the blocklength. The formulated problem is further transformed into a min-max problem, and the minimum error probability is obtained by performing the successive convex approach algorithm. Finally, we update the blocklength according to the obtained error probability. The simulation results show that the minimum blocklength in the MISO NOMA is lower than that in MISO orthogonal multiple access and demonstrate the superiority of the proposed scheme in latency reduction for URLLC.