On Kurtosis of Spreading Sequences for NOMA

In this paper, a new metric for design of spreading sequences in non-orthogonal multiple access (NOMA) systems is introduced. We show that while achieving Welch bound equality (WBE) in designing spreading sequences is important, the kurtosis of the cross-correlations should also be considered as another design metric. For example, we show that despite having the most uniform and lowest cross-correlations behavior, the performance of NOMA transmission based on Grassmannian codes is not superior to other spreading-based NOMA schemes. We also demonstrate that spreading sequences with proper Kurtosis behavior can provide more accurate signal-to-interference-plus-noise-ratio (SINR) measurement at the receiver. Accurate SINR measurement is important for successive interference cancellation (SIC) receivers that rely on accurate ordering of users' signals for interference cancellation; avoiding extra processing and latency. Based on presented evaluation results, the main contribution of the paper lies in that in design of a spreading sequence for NOMA, we should also consider the Kurtosis measure of the sequence.