On the Achievable Rate Region of NOMA Under Outage Probability Constraints

In this letter, we study the outage achievable rate region of non-orthogonal multiple access (NOMA), which is defined as the achievable rate region when all users are required to satisfy their outage probability constraints. First, we formally define the outage achievable rate region of NOMA assuming a slow fading channel model. Then, we introduce a successive interference cancelation (SIC) order under the assumption of statistical channel information. Under this assumption, we find that the outage capacity region of broadcast channel (BC) is also epsilon-achievable by NOMA, which requires the use of SIC, though the definition of outage in NOMA is different from that in BC. Combining the converse result, we get a full characterization of the outage achievable rate region of NOMA, which also shows the optimality of the SIC order used here. Then, we give some discussions about power allocation and the practical insights derived from the characterization. Finally, numerical simulations are provided.