Signal Space Diversity Based Non-Orthogonal Multiple Access System Employing $M$-Ary PSK

The power domain non-orthogonal multiple access (PD-NOMA) performs reasonably well under perfect successive interference cancellation (SIC) conditions. However, under imperfect SIC, the average sum rate (ASR) and outage performance of PD-NOMA diminish drastically. This work introduces a new signal space diversity (SSD) based PD-NOMA system to mitigate this sudden decrease in performance. In the proposed system, users' information will be multiplexed in the in-phase and quadrature parts of the rotated constellation points of the
$M$
-ary phase-shift keying (PSK) modulation symbol. The proposed system is capable of circumventing the necessity of SICs, leading to no power allocation constraints and reducing the complexity and delay at the receiver. The ASR and outage probability (OP) analytical expressions are evaluated under the Nakagami-m fading channel. It is noticed from the analysis that the proposed system is superior to the PD-NOMA system under imperfect SIC and exhibits comparable performance with that under perfect SIC. Furthermore, the analytical expressions for symbol error rate (SER) and average SER (ASER) are determined for
$M$
-ary PSK modulation scheme, and the optimal angle that minimizes the SER is determined for any value of
$M$
.