Non-orthogonal multiple access (NOMA) scheme with Gaussian input signals, which have no constellation constraint on the input alphabets, has been investigated intensively, whereas the research on finite input constellations is relatively few. This paper focuses on the power allocation in practical NOMA application with finite-alphabet inputs. We propose a practical power allocation scheme for downlink NOMA scenario based on the merit of maximizing the minimum Euclidean distance (MMED) by adopting the constellation-constrained (CC) capacity. Applying the nearest neighbor approximation and Jensen's inequality, we prove that optimal CC capacity can be achieved under the MMED criterion at high signal-to-noise ratio (SNR). Instead of updating power allocation coefficient instantaneously, a preset fixed power factor α MMED derived from the MMED criterion is preferred and thus reduces the complexity of overall system implementation. We take two cases into consideration, namely, Gaussian broadcast channel (GBC) and fading broadcast channel (FBC). We model close channel conditions, which are challenging for conventional NOMA as the GBC scenario, and model near-far effect as well Rayleigh fading as the FBC scenario. In these two cases, we show that the optimal performance can be guaranteed at high SNR without the knowledge of channel state information (CSI). We study the power coefficient pairs for the most commonly used Mary quadrature amplitude modulation (M-QAM) constellations based on the MMED criterion. We explore the relationship between the power allocation coefficients and the sizes of constellations. The numerical simulations on CC sum capacity and capacity regions are provided to validate our analysis.INDEX TERMS Constellation constrained capacity, finite-alphabet inputs, minimum Euclidean distance, non-orthogonal multiple access.

In this study, the blind despreading of long-code direct sequence spread spectrum signals (LC-DSSS) is considered. Qiu et al. presented a heuristic segmentation-based method to estimate the spreading code of LC-DSSS signals, which divided the received signal into far shorter temporal windows than the symbol duration, then spreading code was reconstructed from these short windows by computing the related covariance matrices. However, Qiu et al. did not consider the criterion of determining the segmentation length. By combining the matrix perturbation theory and run length property, the authors propose a run-length method for determining the segmentation length based on the segmentation idea, and this method can also be applied to long-code direct-sequence code division multiple access (LC-DS-CDMA) signals. Monte Carlo simulations for both LC-DSSS signals and LC-DS-CDMA signals are presented to demonstrate the effectiveness of the proposed method.

A downlink non-orthogonal multiple access with QPSK input constellations based on virtual channel optimisation is presented to address the similar channel conditions scenario. The novel idea is to construct a uniquely decodable sum constellation by maximising the minimum Euclidean distance among the points of the superposed constellation. An optimal angle difference is derived to make the proposed scheme easy to implement. With a fixed angle pair setup, a non-orthogonal multiple access system without the knowledge of channel state information is proposed. Simulation results are provided to demonstrate the performance of the proposed scheme.

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