Multiuser Beamforming Scheme Research Articles

Structured channel covariance estimation from limited samples for large antenna arrays

In massive multiuser multiple antenna systems, the knowledge of the users’ channel covariance matrix is crucial for minimum mean square error channel estimation in the uplink and it plays an important role in several multiuser beamforming schemes in the downlink. Due to the large number of base station antennas, accurate covariance estimation is challenging especially in the case where the number of samples is limited and thus comparable to the channel vector dimension. As a result, the standard sample covariance estimator may yield a too large estimation error which in turn may yield significant system performance degradation with respect to the ideal channel covariance knowledge case. To address such problem, we propose a method based on a parametric representation of the channel angular scattering function. The proposed parametric representation includes a discrete specular component which is addressed using the well-known MUltiple SIgnal Classification (MUSIC) method, and a diffuse scattering component, which is modeled as the superposition of suitable dictionary functions. To obtain the representation parameters, we propose two methods, where the first solves a nonnegative least-squares problem and the second maximizes the likelihood function using expectation–maximization. Our simulation results show that the proposed methods outperform the state of the art with respect to various estimation quality metrics and different sample sizes.

A Novel Multiuser Beamforming System With Reduced Complexity and Beam Optimizations

Analog beamforming is widely used in the incoming 5G/B5G communications systems to leverage against power consumption by significantly reducing the number of RF chains. In this paper, we propose a new multiuser beamforming scheme that allows the number of RF chains to be smaller than the number of users. In addition, we propose the beamforming algorithms to minimize the transmit power and weighted symbol error rates for the proposed scheme. For both high- and low-SNR regimes, the proposed beamforming designs provide simple closed-form solutions. The simulation results show that with half the number of RF chains and, thus, lower hardware cost, the proposed scheme outperforms the conventional zero-forcing beamforming systems in several interested parameter settings. Meanwhile, the performance degradation due to the quantization effect of the proposed system is less pronounced than that of the conventional beamforming systems.

Hybrid Precoding Design Achieving Fully Digital Performance for Millimeter Wave Communications

As a promising candidate for millimeter wave (mmWave) multiple-input and multiple-output (MIMO) communications, hybrid precoding techniques can reap the benefit of large antenna arrays, yet with only limited number of radio frequency (RF) chains. In this paper, we investigate the problem of achieving the same performance of the fully digital system with hybrid precoding. Specifically, for the single user MIMO system, we propose a closed form hybrid precoding design that can achieve the optimal fully digital performance for both frequency-flat and frequency-selective channels, and only requires the number of RF chains to equal the number of paths of the channel. The design for the case with even less RF chains is also given. Furthermore, for the multiuser (MU) system with single antenna at each mobile terminal (MT), two MU beamforming schemes are considered, which are the directional beamforming and zero-forcing. We show that for both schemes, the fully digital performance can be achieved with our proposed hybrid precoding designs with the number of RF chains no less than the sum number of channel paths from the base station to all the selected MTs. Numerical results are provided to validate our analytical results and show the performance gain of the proposed hybrid precoding designs compared to other benchmark schemes.

Joint Beamformer Design for Wireless Fronthaul and Access Links in C-RANs

This paper presents a joint design framework of fronthaul and access links in cloud radio access networks, wherein the fronthaul data delivery between the central processor (CP) and small-cell base stations (SBSs) is carried over wireless links, which is more cost effective and flexible than the conventional wired fronthaul solutions. In this framework, the coordinated beamforming scheme is adopted by the SBSs to serve the users cooperatively, which not only reduces the bandwidth requirement over the fronthaul links, but also eliminates the inter-cell interference in the access links. To further exploit the spatial diversity over the fronthaul links, the multi-user beamforming scheme is adopted by the CP to deliver the data of all users to their serving SBSs simultaneously. This non-convex and combinatorial optimization problem is reformulated to a unified beamformer design problem using the $\ell _{0}/\ell _{1}$ -norm framework, which is still difficult to obtain the optimal solutions. Therefore, we first propose an algorithm based on the difference of convex (DC) programming scheme to find the suboptimal solutions, whereby the formulated problem is transformed to the standard DC programming problem and solved iteratively using the convex-concave procedure algorithm. We then propose another algorithm based on the successive convex approximation and weighted minimum-mean-squared-error approaches, and solve the transformed problem using the block coordinate update scheme. The pros and cons of these two algorithms are discussed, and simulation results are provided to demonstrate the performance gain of our schemes over other benchmark schemes.

Multiuser Beamforming with Limited Feedback for FDD Massive MIMO Systems

This paper discusses the multiuser beamforming in FDD massive MIMO systems. It first introduces the feature of FDD massive MIMO systems to implement multiuser beamforming schemes. After that, considering the realistic implementation of multiuser beamforming scheme in FDD massive MIMO systems, it introduces the knowledge of channel quantization. In the main part of the paper, we introduce two traditional multiuser beamforming schemes and analyse their merits and demerits. Based on these, we propose a novel multiuser beamforming scheme to flexibly combine the merits of the traditional beamforming schemes. In the final part of the paper, we give some simulation results to compare the beamforming schemes mentioned in the paper. These simulation results show the superiority of the proposed beamforming scheme.