A survey on hybrid beamforming techniques in 5G : architecture and system model perspectives

The increasing wireless data traffic demands have driven the need to explore suitable spectrum regions for meeting the projected requirements. In the light of this, millimeter wave (mmWave) communication has received considerable attention from the research community. Typically, in fifth generation (5G) wireless networks, mmWave massive multiple-input multiple-output (MIMO) communications is realized by the hybrid transceivers which combine high dimensional analog phase shifters and power amplifiers with lower-dimensional digital signal processing units. This hybrid beamforming design reduces the cost and power consumption which is aligned with an energy-efficient design vision of 5G. In this paper, we track the progress in hybrid beamforming for massive MIMO communications in the context of system models of the hybrid transceivers' structures, the digital and analog beamforming matrices with the possible antenna configuration scenarios and the hybrid beamforming in heterogeneous wireless networks. We extend the scope of the discussion by including resource management issues in hybrid beamforming. We explore the suitability of hybrid beamforming methods, both, existing and proposed till first quarter of 2017, and identify the exciting future challenges in this domain

Index Modulation Techniques for Energy-efficient Transmission in Large-scale MIMO Systems

This thesis exploits index modulation techniques to design energy- and spectrum-efficient system models to operate in future wireless networks. In this respect, index modulation techniques are studied considering two different media: mapping the information onto the frequency indices of multicarrier systems, and onto the antenna array indices of a platform that comprises multiple antennas. The index modulation techniques in wideband communication scenarios considering orthogonal and generalized frequency division multiplexing systems are studied first. Single cell multiuser networks are considered while developing the system models that exploit the index modulation on the subcarriers of the multicarrier systems. Instead of actively modulating all the subcarriers, a subset is selected according to the index modulation bits. As a result, there are subcarriers that remain idle during the data transmission phase and the activation pattern of the subcarriers convey additional information. The transceivers for the orthogonal and generalized frequency division multiplexing systems with index modulation are both designed considering the uplink and downlink transmission phases with a linear combiner and precoder in order to reduce the system complexity. In the developed system models, channel state information is required only at the base station. The linear combiner is designed adopting minimum mean square error method to mitigate the inter-user-interference. The proposed system models offer a flexible design as the parameters are independent of each other. The parameters can be adjusted to design the system in favor of the energy efficiency, spectrum efficiency, peak-to-average power ratio, or error performance. Then, the index modulation techniques are studied for large-scale multiple-input multiple-output systems that operate in millimeter wave bands. In order to overcome the drawbacks of transmission in millimeter wave frequencies, channel properties should be taken in to account while envisaging the wireless communication network. The large-scale multiple-input multiple-output systems increase the degrees of freedom in the spatial domain. This feature can be exploited to focus the transmit power directly onto the intended receiver terminal to cope with the severe path-loss. However, scaling up the number of hardware elements results in excessive power consumption. Hybrid architectures provide a remedy by shifting a part of the signal processing to the analog domain. In this way, the number of bulky and high power consuming hardware elements can be reduced. However, there will be a performance degradation as a consequence of renouncing the fully digital signal processing. Index modulation techniques can be combined with the hybrid system architecture to compensate the loss in spectrum efficiency to further increase the data rates. A user terminal architecture is designed that employs analog beamforming together with spatial modulation where a part of the information bits is mapped onto the indices of the antenna arrays. The system is comprised a switching stage that allocates the user terminal antennas on the phase shifter groups to minimize the spatial correlation, and a phase shifting stage that maximizes the beamforming gain to combat the path-loss. A computationally efficient optimization algorithm is developed to configure the system. The flexibility of the architecture enables optimization of the hybrid transceiver at any signal-to-noise ratio values. A base station is designed in which hybrid beamforming together with spatial modulation is employed. The analog beamformer is designed to point the transmit beam only in the direction of the intended user terminal to mitigate leakage of the transmit power to other directions. The analog beamformer to transmit the signal is chosen based on the spatial modulation bits. The digital precoder is designed to eliminate the inter-user-interference by exploiting the zero-forcing method. The base station computes the hybrid beamformers and the digital combiners, and only feeds back the digital combiners of each antenna array-user pair to the related user terminals. Thus, a low complexity user architecture is sufficient to achieve a higher performance. The developed optimization framework for the energy efficiency jointly optimizes the number of served users and the total transmit power by utilizing the derived upper bound of the achievable rate. The proposed transceiver architectures provide a more energy-efficient system model compared to the hybrid systems in which the spatial modulation technique is not exploited. This thesis develops low-complexity system models that operate in narrowband and wideband channel environments to meet the energy and spectrum efficiency demands of future wireless networks. It is corroborated in the thesis that adopting index modulation techniques both in the systems improves the system performance in various aspects.:1 Introduction 1 1.1 Motivation 1 1.2 Overview and Contribution 2 1.3 Outline 9 2 Preliminaries and Fundamentals 13 2.1 Multicarrier Systems 13 2.2 Large-scale Multiple Input Multiple Output Systems 17 2.3 Index Modulation Techniques 19 2.4 Single Cell Multiuser Networks 22 3 Multicarrier Systems with Index Modulation 27 3.1 Orthogonal Frequency Division Multiplexing 28 3.2 Generalized Frequency Division Multiplexing 40 3.3 Summary 52 4 Hybrid Beamforming with Spatial Modulation 55 4.1 Uplink Transmission 56 4.2 Downlink Transmission 74 4.3 Summary 106 5 Conclusion and Outlook 109 5.1 Conclusion 109 5.2 Outlook 111 A Quantization Error Derivations 113 B On the Achievable Rate of Gaussian Mixtures 115 B.1 The Conditional Density Function 115 B.2 Tight Bounds on the Differential Entropy 116 B.3 A Bound on the Achievable Rate 118 C Multiuser MIMO Downlink without Spatial Modulation 121 Bibliograph

Low complexity precoding schemes for massive MIMO systems

PhD ThesisIn order to deal with the challenges of the exponentially growing communication traffic and spectrum bands with wider bandwidth, massive MIMO technology was been proposed, which employs an unprecedented number of base station antennas simultaneously to serve a smaller number of user terminals in the same channel. Although the very large antenna arrays for massive multiple-input multiple-output (MIMO) systems lead to unprecedented data throughputs and beamforming gains to meet these data traffic demands, they also lead to prohibitively high energy consumption and hardware complexity. In terms of precoding schemes, the conventional linear precoding entirely processes the complex signals in the digital domain and then upconverts to the carrier frequency after passing through radio frequency (RF) chains, which can achieve near-optimal performance with the large antenna arrays. However, it is infeasible because with fully digital precoding, every antenna element needs to be coupled with one RF chain, including the digital-toanalog convertors, mixers and filters, which is accountable for excessively high hardware cost and power consumption. This thesis focuses on the design and analysis of low complexity precoding schemes. The novel contributions in this thesis are presented in three sections. First, a low complexity hybrid precoding scheme is proposed for the downlink transmission of massive multi-user MIMO systems with a finite dimensional channel model. By analysing the structure of the channel model, the beamsteering codebooks are combined with extracting the phase of the conjugate transpose of the fast fading matrix to design the RF precoder, which thereby harvests the large array gain achieved by an unprecedented number of base station antennas. Then a baseband precoder is designed based on the equivalent channel with zero forcing (ZF) precoding. In addition, a tight upper bound on the spectral efficiency is derived and the performance of hybrid precoding is investigated. Second, based on successive refinement, a new iterative hybrid precoding scheme is proposed with a sub-connected architecture for mmWave MIMO systems.In each iteration, the first step is to design the RF precoder and the second step is to design the baseband precoder. The RF precoder is regarded as an input to update the baseband precoder until the stopping criterion is triggered. Phase extraction is used to obtain the RF precoder and then the baseband precoder is optimized by the orthogonal property. This algorithm effectively optimizes the hybrid precoders and reduces the hardware complexity with sub-connected architecture. A closed-form expression of upper bound for the spectral efficiency is derived and the energy efficiency and the complexity of the proposed hybrid precoding scheme are analyzed. Finally, the use of low-resolution digital-to-analog converters (DACs) for each antenna and RF chain is considered. Moreover, in a more practical scenario, the hardware mismatch between the uplink and the downlink for the channel matrix is a focus, where the downlink is not the transpose of the uplink in time-division duplex mode. The impact of one-bit DACs on linear precoding is studied for the massive MIMO systems with hardware mismatch. Using the Bussgang theorem and random matrix theorem, a closed-form expression for the signal to quantization, interference and noise ratio with consideration of hardware mismatch and one-bit ZF precoding is derived, which can be used to derive the achiev- able rate. Then a performance approximation is also derived in the high signal-to-noise ratio (SNR) region, which is related to the ratio of the number of base station antennas and the number of mobile users , and the statistics of the circuit gains at the base station. In conclusion, analytical and numerical results show that the proposed techniques are able to achieve close-to-optimal performances with low hardware complexity, thus the low complexity precoding schemes can be valid candidates for practical implementations of modern communication systems

Channel Estimation and Hybrid Precoding for Frequency Selective Multiuser mmWave MIMO Systems

© 2018 IEEE. This version of the article has been accepted for publication, after peer review. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works. The Version of Record is available online at: https://doi.org/10.1109/JSTSP.2018.2819130[Abstract]: Configuring the hybrid precoders and combiners in a millimeter wave multiuser multiple-input multiple-output system is challenging in frequency selective channels. In this paper, we develop a system that uses compressive estimation on the uplink to configure precoders and combiners for the downlink. In the first step, the base station (BS) simultaneously estimates the channels from all the mobile stations on each subcarrier. To reduce the number of measurements required, compressed sensing techniques are developed that exploit common support on the different subcarriers. In the second step, exploiting reciprocity and the channel estimates the BS designs hybrid precoders and combiners. Two algorithms are developed for this purpose, with different performance and complexity tradeoffs: First, a factorization of the purely digital solution; and second, an iterative hybrid design. Extensive numerical experiments evaluate the proposed solutions comparing to the state-of-the-art strategies, and illustrating design tradeoffs in overhead, complexity, and performance.This work was supported in part by Xunta de Galicia (ED431C 2016-045, ED341D R2016/012, ED431G/01, ED431G/04), in part by AEI of Spain (TEC2015-69648-REDC, TEC2016-75067-C4-1-R, TEC2016- 75103-C2-2-R), in part by ERDF funds (AEI/FEDER, EU), and in part by the National Science Foundation under Grant 1702800.Xunta de Galicia; ED431C 2016/045Xunta de Galicia; ED341D R2016/012Xunta de Galicia; ED431G/01Xunta de Galicia; ED431G/0