Fundamentals of Power Allocation Strategies for Downlink Multi-user NOMA with Target Rates
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For a downlink multi-user non-orthogonal multiple access (NOMA) system with successive interference cancellation receivers, the fundamental relationship between the user channel gains, target rates, and power allocation is investigated. It is shown that the power allocation strategy is completely derived as functions of the users' target rates and the successive decoding order, and not of the channel gains, so long as the target rates have the same ordering as the user channel gains. It is proven that the total irremovable interference at each receiver has a fundamental upper-limit that is a function of the target rates, and the outage probability is one when this limit is exceeded. The concept of well-behaved power allocation strategies is defined and its properties are derived. It is shown that any power allocation strategy that has outage performance equal to orthogonal multiple access (OMA) is always well-behaved. Furthermore, it is proven that there always exists a well-behaved power allocation strategy that has improved outage probability performance over OMA. A specific well-behaved strategy is derived to demonstrate the improvement of NOMA over OMA for K users. Simulations validate the theoretical results, and show that the best performance gains are obtained by the users with weaker channels.
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