Design and Performance Analysis of NOMA-OFDM With Time-Domain Interleaving

This letter presents an improved non-orthogonal multiple access (NOMA) design using time-domain interleaving (TDI). The frequency diversity inherent in TDI is leveraged to improve the bit error rate (BER) of NOMA by mitigating the detrimental effects of channel frequency selectivity. The instantaneous signal-to-interference-plus-noise ratio (SINR) of the proposed system is derived and used to evaluate the BER using minimum-mean-square-error (MMSE) equalization semi-analytically with spatial diversity over frequency-selective fading channels for an arbitrary number of users. Moreover, a tight analytic lower-bound (LB) is derived to enable efficient BER evaluation. The obtained analytical results, corroborated by Monte Carlo simulation, demonstrate that TDI can provide significant BER improvement to NOMA by resolving the performance degradation caused by severe fading and multi-user interference. The synergy of receiver spatial diversity and TDI enabled the low-complexity zero-forcing (ZF) equalizer to offer BER that is comparable to MMSE, which is drastically different from the single-input single-output (SISO) case.