Signal Superposition in NOMA with Proper and Improper Gaussian Signaling

—Recent findings in a single-cell two-user networkhave shown that a higher network throughput is achieved byusing a common message to be decoded by both users andconveying partial information for both users, rather than theentire information for one of the two users. The latter isessentially the conventional non-orthogonal multiple access(NOMA), which performs better than orthogonal multipleaccess (OMA) only under users’ highly dissimilar channelconditions. Unlike NOMA, the former performs consistentlybetter than OMA. This paper generalizes such signallingstrategy to a general multi-cell multiuser network, whichleads to a new NOMA approach (called n-NOMA), in whicheach pair of users decodes a message that conveys partialinformation for one of them only. Unlike the conventionalNOMA, whose performance is highly dependent on theusers’ pairing strategy, the proposed n-NOMA consistentlyoutperforms both NOMA and OMA schemes. Both properand improper Gaussian signaling is considered for all theconcerned schemes and it is shown that the latter is clearlymore advantageous than the former.Index Terms—Signal superposition, orthogonal multipleaccess (OMA), non-orthogonal multiple access (NOMA),improper Gaussian signaling (IGS), max-min throughput optimization