Transmission Performance of Superposed Modulation Using QPSK and 1024-QAM in Downlink NOMA

Non-orthogonal multiple access (NOMA) that can improve spectrum efficiency is a potential candidate for 5th-generation (5G) mobile systems. Higher-order quadrature amplitude modulation (QAM) schemes such as 256- and 1024-QAM are expected to increase data rates within a limited bandwidth in 5G mobile systems. In this paper, we propose an orthogonal frequency division multiplexing (OFDM)-based superposed modulation using quadrature amplitude modulation (QPSK) and 1024-QAM for downlink NOMA and investigate the transmission performance under a typical additive white Gaussian noise (AWGN) channel. We determine the bit error rate (BER) performance of OFDM-based superposed QPSK and 1024-QAM signals in AWGN channels for two types of power allocation sets using link-level simulations. We also investigate the relationship between the power allocation factor for 1024-QAM and the E
_b
/N
₀
values required to meet a BER of 10
^-2
for both QPSK and 1024-QAM. This work provides an effective solution for introducing downlink NOMA using OFDM-based QPSK and 1024-QAM in 5G systems and beyond.