Abstract
Using wireless uplink non-orthogonal multiple access (NOMA), the throughput of an uplink random access system can be significantly improved. In this letter, assuming that each user only knows the channel response from itself to the base station, we study an uplink NOMA random access system which involves multiple power levels. For the signal decoding of a specific power level, we consider the effect of collisions happened at the lower and higher power levels. Our goal is to optimize the probability that a user selects a power level to transmit signals, in order to maximize the system throughput. We theoretically prove that the system throughput maximization problem is a convex problem and propose methods to find its optimal solution. Furthermore, we also prove that the optimal probability selecting the higher power level should be lower than or equal to that selecting the lower one. It is shown from simulation results that the proposed uplink NOMA system is more robust to the increase of users than a conventional orthogonal multiple access system.