Phase Rotated Non-Orthogonal Multiple Access for 3-User Superposition Signals

Abstract

Non-orthogonal multiple access (NOMA) is one of the most promising techniques to achieve high spectral efficiency for 5G. In the NOMA downlink, a transmitter allocates different power levels to users and linearly combines user signals using the square roots of the power levels. A constellation point of the combined signal can be very close to other points, which degrades bit error rate (BER) performance severely. To overcome such degradation, the proposed NOMA system rotates phases of the user signals and then linearly combines the resultant signals. On the receiver side, multiuser detection (MUD) is introduced so as to improve BER performance more than successive interference cancellation (SIC), which the conventional scheme employs. Since the proposed scheme with phase rotation (PR) for 2-user signal transmission has been thoroughly investigated, this paper considers 3-user transmission in the proposed NOMA scheme and proposes one method to optimize two angles of PR, whereas another method previously proposed by the authors can control only one angle. Computer simulations under the 3-user condition demonstrate that the proposed scheme employing PR and joint MUD and SIC for signal detection is much superior to other schemes in downlink BER performance and that the two optimization methods bring about almost the same BER.