Outage Performance of CDF-Based Scheduling in Downlink and Uplink NOMA Systems

The scheduling strategy is a vital precondition for achieving remarkable performance benefits offered by non-orthogonal multiple-access (NOMA) schemes. In this article, the cumulative distribution function (CDF)-based scheduling for NOMA (CS-NOMA) networks with randomly deployed users is investigated. We consider two practical successive interference cancellation (SIC) restrictions, namely imperfect SIC and SIC power difference constraint, in both downlink and uplink transmissions. Exact analytical expressions for the outage probabilities of the two scheduled users are derived in both fixed power allocation (FPA) and cognitive-radio-inspired power allocation (CPA) scenarios. To get more insights, high signal to noise ratio (SNR) approximations or bounds of the outage probabilities are derived, and then be utilized to analyze the achieved diversity orders. Assuming the number of near and far users are respectively represented as K and B. Results reveal that, in downlink transmission, the far user can achieve a diversity order of B in both FPA and CPA policies, while the near user's diversity order will be reduced from K (in FPA) to min {K, B} (in CPA). However, in uplink transmission, the achieved diversity orders of the near and far users will be increased from zeros to K and min {K, B}, respectively. Simulation results are provided to validate the accuracy of the analytical expressions.