Linear Precoding Techniques for OFDM-Based NOMA over Frequency-Selective Fading Channels

Non-orthogonal multiple access (NOMA) is one of the potential candidates for future radio access to address the increasing demands of mobile traffic. NOMA-based systems employ orthogonal frequency division multiplexing (OFDM) for multicarrier modulation which induces peak-to-average power ratio (PAPR). High PAPR makes conventional NOMA systems spectrally and energy inefficient. Thus, in this paper, precoded NOMA system is proposed to overcome the PAPR problem in downlink NOMA. Different unitary transforms such as Walsh Hadamard transform (WHT), Zadoff chu transform (ZCT), and discrete Hartley transform are applied as linear precoding techniques to overcome the PAPR problem in NOMA. Moreover, a new precoding technique -transform is also proposed which is a hybrid combination of WHT and ZCT. Link-level performance of power domain NOMA is analysed in terms of PAPR, spectral efficiency, bit error rate, and computational complexity. Simulation results show that in the presence of linear precoding techniques, performance of classic NOMA systems improve at the cost of slight increment in complexity.