Power allocation for BER minimization in an uplink MUSA scenario

Multi-user shared access (MUSA) is one of the most promising non orthogonal multiple access (NOMA) schemes to accommodate the stringent requirements of massive machine type communications (mMTC). MUSA adopts a grant-free access strategy and uses non-orthogonal spreading sequences to accommodate a large number of users in the same radio resources. The inter-user interference is subsequently handled using successive interference cancellation (SIC) algorithm, which allows a low decoding complexity. However, sequential decoding can be affected by error propagation when the power difference between simultaneous transmissions is not significant. In this paper, we propose a power allocation algorithm in order to avoid the error propagation problem by guaranteeing sufficient power difference between concurrent received signals. We formulate an optimization problem which aims at minimizing the system average bit error rate. We show that the optimization problem has an optimal global solution and an algorithm is proposed for its resolution. Simulation results show the impact of performing power allocation for SIC decoding on the bit error rate.