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Channel estimation is challenging for millimeterwave (mmWave) massive MIMO with hybrid precoding, since the number of radio frequency (RF) chains is much smaller than that of antennas. Conventional compressive sensing based channel estimation schemes suffer from severe resolution loss due to the channel angle quantization. To improve the channel estimation accuracy, we propose an iterative reweight (IR)-based superresolution channel estimation scheme in this paper. By optimizing an objective function through the gradient descent method, the proposed scheme can iteratively move the estimated angle of arrivals/departures (AoAs/AoDs) towards the optimal solutions, and finally realize the super-resolution channel estimation. In the optimization, a weight parameter is used to control the tradeoff between the sparsity and the data fitting error. In addition, a singular value decomposition (SVD)-based preconditioning is developed to reduce the computational complexity of the proposed scheme. Simulation results verify the better performance of the proposed scheme than conventional solutions.Index Terms-Millimeter-wave (mmWave), massive MIMO, hybrid precoding, angle of arrival (AoA), angle of departure (AoD), super-resolution channel estimation.

Non-orthogonal multiple access (NOMA) has been regarded as one of the promising key technologies for future 5G systems. In the uplink grant-free NOMA schemes, dynamic scheduling is not required, which can significantly reduce the signaling overhead and transmission latency. However, user activity has to be detected in grant-free NOMA systems, which is challenging in practice. In this paper, by exploiting the inherent structured sparsity of user activity naturally existing in NOMA systems, we propose a low-complexity multi-user detector based on structured compressive sensing (SCS) to realize joint user activity and data detection. Particularly, we propose a structured iterative support detection (SISD) algorithm by exploiting such structured sparsity, which is able to jointly detect user activity and transmitted data in several continuous time slots. Simulation results show that the proposed scheme can achieve better performance than conventional solutions.Index Terms-5G, non-orthogonal multiple access (NOMA), multi-user detection (MUD), structured compressive sensing (SC-S).

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Millimeter-wave (mmWave) massive multiple-input multiple-output (MIMO) has already been considered as a promising solution to meet the requirement of the higher data rate for the future Internet of Things (IoTs). Hybrid precoding is an effective solution for the mmWave massive MIMO systems to significantly decrease the number of radio frequency (RF) chains without an apparent sumrate loss. However, the current literature on hybrid precoding considers either the high-resolution (HR) phase shifters (PSs) with high power consumption or the impractical narrowband mmWave channel model. To this end, in this paper, we investigate an energy-efficient hybrid precoding scheme using one-bit PSs for practical frequency-selective wideband mmWave massive MIMO systems. Specifically, we provide the energy consumption analysis to reveal the fact that the energy consumed by the one-bit PSs is much lower than that by the HR-PSs, and the array gain loss incurred by using one-bit PSs is minimal. Moreover, motivated by the cross-entropy optimization (CEO) algorithm evolved for machine learning, we propose the CEO-based hybrid precoding scheme to maximize the achievable sum-rate of the considered system. In the CEO-based hybrid precoding, we update the probability distributions of the elements in the hybrid precoder to minimize the cross-entropy between the two probability distributions so that we can generate the final solution close to the optimal one. Furthermore, we extend the proposed CEO-based hybrid precoding scheme from the case with one-bit PSs to the general case with HR-PSs to show that our solution can also be applied in other scenarios. The performance evaluation demonstrates that our proposed scheme can obtain near-optimal sum-rate and considerably higher energy efficiency than some existing solutions.