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In this paper, a non‐orthogonal multiple access (NOMA) scheme is considered for downlink millimeter‐wave (mmWave) transmissions over beamspace massive multiple‐input multiple‐output (MIMO) channels. To improve the NOMA detection and reduce its complexity, it is assumed that users are grouped in clusters. The major benefit of the NOMA depends on the power allocation (PA) among users. In this context, to ensure the quality of service (QoS) of users, a threshold rate for each user is defined. The aim of this work is to propose a new PA strategy that takes into account the pre‐defined threshold rates and maximizes the achievable sum rate. Thereafter, considering the Karush‐Kuhn‐Tucker (KKT) conditions, a closed‐form solution for the PA issue under perfect and imperfect channel state information (CSI) assumptions is derived. Through simulation results, it is shown that the proposed scheme outperforms the conventional orthogonal multiple access techniques in terms of spectral efficiency (SE) and energy efficiency (EE). In addition, the proposed robust PA scheme for the imperfect CSI case reduces the performance loss due to the channel uncertainty. [ABSTRACT FROM AUTHOR]

The hybrid beamforming can significantly improve the spectral and energy efficiencies of the millimeter‐wave (mmWave) massive multiple input multiple output (MIMO) systems without obvious performance loss. However, the fundamental limitation of existing hybrid beamforming is that they include high‐resolution analog‐to‐digital converters (ADCs) which consume high power. To overcome this fundamental limitation, in this paper, spectrum and energy efficient mmWave massive MIMO transmission scheme is proposed that jointly optimizes the hybrid combiner and the number of ADC bits. Particularly, the hybrid combiner is presented as a combination of three parts: the analog combiner matrix, the quantization matrix, and the digital baseband matrix which is based on the Kalman filter. Unlike most existing works that rely on sumrate optimization to design the hybrid combiner, in this work, the mean square error (MSE) is chosen as the main performance metric to describe the reliability of the system. Simulations results confirm that the proposed Kalman based hybrid combiner exhibits the best performance in terms of spectral and energy efficiencies compared to minimum mean square error (MMSE) and zero forcing (ZF) receivers. [ABSTRACT FROM AUTHOR]

The combination of Linear Dispersion Space-Time Block Codes (LD-STBCs) and Multiband Orthogonal Frequency Division Multiplexing Ultra-WideBand (MB-OFDM UWB) technologies has been proposed in the literature to improve the error performance and data rate. Recently, it has been shown that the LD codes allow the repetitions of data symbols conjugates. They cause a transformation of the Multiple-Input Multiple-Output (MIMO) channel that requires a Widely Linear (WL) filtering. However, proposed LD-STBC MB-OFDM UWB system does not take this issue into account. In this paper, we propose a Widely Linear Minimum Mean Square Error (WLMMSE) channel estimator for LD-STBC MB-OFDM UWB systems. The proposed channel estimator proceeds to WL estimation instead of linear one. Via simulation results, we prove that the LD-STBCs provide significant performance gain for MB-OFDM UWB system compared to Single-Input Single-Output (SISO) configuration. We show also that the proposed WLMMSE channel estimator outperforms the LMMSE one. This performance improvement is observed for both proper and improper signal constellations. Additionally, the computational complexity of our proposal is comparable with that of LMMSE estimator. Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]