Optimal Rotated QPSK Constellation for a Reliable SOMA Powerline Communication System in Nakagami-m Noise Environment

An
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-branch receive diversity powerline communication (PLC) system with two users and the transmitter employing a power-domain non-orthogonal multiple access (PD-NOMA) along with a rotated quadrature phase-shift keying (QPSK) modulation scheme, generally known as semi-orthogonal multiple access (SOMA) for data transmission is considered in this paper. The rotation of the data constellation and a proposed pre-processing at the receiver end for each user eliminate the usage of the additional successive-interference cancellation (SIC) hardware generally used in PD-NOMA PLC systems. Furthermore, optimal receiver structures are proposed using which closed-form expressions of the symbol error probability (SEP) are derived for the users using a characteristic function (c.f.) approach. The optimal rotation of the QPSK constellation minimizing the obtained SEP of the system is also derived. The diversity order of the system and the optimal angle of rotation at high average signal-to-noise ratio (SNR) values are found to be dependent only on the number of diversity branches. Moreover, the general dependence of the optimal rotation angle on various system parameters is corroborated by numerical results which provide various insights into the design of a reliable and energy-efficient PD-SOMA PLC system without the use of SIC hardware. The optimal rotation angle of the QPSK constellation also leads to novel optimal constellations for the users which are different than the conventional constellation utilized for data transmission in digital communication systems.