What will 5G be? What it will not be is an incremental advance on 4G. The previous four generations of cellular technology have each been a major paradigm shift that has broken backward compatibility. Indeed, 5G will need to be a paradigm shift that includes very high carrier frequencies with massive bandwidths, extreme base station and device densities, and unprecedented numbers of antennas. However, unlike the previous four generations, it will also be highly integrative: tying any new 5G air interface and spectrum together with LTE and WiFi to provide universal high-rate coverage and a seamless user experience. To support this, the core network will also have to reach unprecedented levels of flexibility and intelligence, spectrum regulation will need to be rethought and improved, and energy and cost efficiencies will become even more critical considerations. This paper discusses all of these topics, identifying key challenges for future research and preliminary 5G standardization activities, while providing a comprehensive overview of the current literature, and in particular of the papers appearing in this special issue.

What Will 5G Be

The adoption of a reconfigurable intelligent surface (RIS) for downlink multi-user communication from a multi-antenna base station is investigated in this paper. We develop energy-efficient designs for both the transmit power allocation and the phase shifts of the surface reflecting elements subject to individual link budget guarantees for the mobile users. This leads to non-convex design optimization problems for which to tackle we propose two computationally affordable approaches, capitalizing on alternating maximization, gradient descent search, and sequential fractional programming. Specifically, one algorithm employs gradient descent for obtaining the RIS phase coefficients, and fractional programming for optimal transmit power allocation. Instead, the second algorithm employs sequential fractional programming for the optimization of the RIS phase shifts. In addition, a realistic power consumption model for RIS-based systems is presented, and the performance of the proposed methods is analyzed in a realistic outdoor environment. In particular, our results show that the proposed RIS-based resource allocation methods are able to provide up to 300% higher energy efficiency in comparison with the use of regular multi-antenna amplify-and-forward relaying.

Reconfigurable Intelligent Surfaces for Energy Efficiency in Wireless Communication

A Cell-Free Massive MIMO (multiple-input multiple-output) system comprises a very large number of distributed access points (APs), which simultaneously serve a much smaller number of users over the same time/frequency resources based on directly measured channel characteristics. The APs and users have only one antenna each. The APs acquire channel state information through time-division duplex operation and the reception of uplink pilot signals transmitted by the users. The APs perform multiplexing/de-multiplexing through conjugate beamforming on the downlink and matched filtering on the uplink. Closed-form expressions for individual user uplink and downlink throughputs lead to max–min power control algorithms. Max–min power control ensures uniformly good service throughout the area of coverage. A pilot assignment algorithm helps to mitigate the effects of pilot contamination, but power control is far more important in that regard. Cell-Free Massive MIMO has considerably improved performance with respect to a conventional small-cell scheme, whereby each user is served by a dedicated AP, in terms of both 95%-likely per-user throughput and immunity to shadow fading spatial correlation. Under uncorrelated shadow fading conditions, the cell-free scheme provides nearly fivefold improvement in 95%-likely per-user throughput over the small-cell scheme, and tenfold improvement when shadow fading is correlated.

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Cell-Free Massive MIMO Versus Small Cells

A Cell-Free Massive MIMO (multiple-input multiple-output) system comprises a very large number of distributed access points (APs)which simultaneously serve a much smaller number of users over the same time/frequency resources based on directly measured channel characteristics. The APs and users have only one antenna each. The APs acquire channel state information through time-division duplex operation and the reception of uplink pilot signals transmitted by the users. The APs perform multiplexing/de-multiplexing through conjugate beamforming on the downlink and matched filtering on the uplink. Closed-form expressions for individual user uplink and downlink throughputs lead to max-min power control algorithms. Max-min power control ensures uniformly good service throughout the area of coverage. A pilot assignment algorithm helps to mitigate the effects of pilot contamination, but power control is far more important in that regard. 
Cell-Free Massive MIMO has considerably improved performance with respect to a conventional small-cell scheme, whereby each user is served by a dedicated AP, in terms of both 95%-likely per-user throughput and immunity to shadow fading spatial correlation. Under uncorrelated shadow fading conditions, the cell-free scheme provides nearly 5-fold improvement in 95%-likely per-user throughput over the small-cell scheme, and 10-fold improvement when shadow fading is correlated.

Cell-Free Massive MIMO versus Small Cells

The exponential growth and availability of data in all forms is the main booster to the continuing evolution in the communications industry. The popularization of traffic-intensive applications including high definition video, 3-D visualization, augmented reality, wearable devices, and cloud computing defines a new era of mobile communications. The immense amount of traffic generated by today’s customers requires a paradigm shift in all aspects of mobile networks. Ultradense network (UDN) is one of the leading ideas in this racetrack. In UDNs, the access nodes and/or the number of communication links per unit area are densified. In this paper, we provide a survey-style introduction to dense small cell networks. Moreover, we summarize and compare some of the recent achievements and research findings. We discuss the modeling techniques and the performance metrics widely used to model problems in UDN. Also, we present the enabling technologies for network densification in order to understand the state-of-the-art. We consider many research directions in this survey, namely, user association, interference management, energy efficiency, spectrum sharing, resource management, scheduling, backhauling, propagation modeling, and the economics of UDN deployment. Finally, we discuss the challenges and open problems to the researchers in the field or newcomers who aim to conduct research in this interesting and active area of research.

Ultra-Dense Networks: A Survey

Providing uniformly high capacity in cellular systems is challenging due to fading, path loss, and interference. A partial solution to this problem is the deployment of distributed antenna systems, where transmission points are distributed throughout the cell using coax cable or fiber, instead of being centrally located on a single tower. This article reviews how distributed antenna systems are evolving to provide higher performance on the downlink in cellular systems. Research trends in distributed antennas for the downlink of cellular systems are described along with current progress on their integration into commercial wireless cellular standards. A key observation is that distributed antenna systems are tightly integrated into the cellular architecture, and incorporate physical layer technologies like MIMO communication and multiuser MIMO to provide higher data rates.

A current perspective on distributed antenna systems for the downlink of cellular systems

In this paper two schemes of distributed antenna selection, Pathloss-based Antenna Selection (P-AS) and Energy-efficient Antenna Selection (E-AS), are studied for the purpose of improving energy efficiency in Distributed Antenna Systems (DAS). Path loss information is exploited in P-AS, whilst the energy consumption of antenna nodes is taken into account in E-AS for the Pre-coding Matrix Index (PMI) selection and resource scheduling. The performance of the two algorithms is evaluated by means of numerical simulations based on two different DAS topology models of node layouts; a homogeneous topology model where distributed directional antennas are deployed at equidistant positions on the cell edges, and a heterogeneous topology model where omni-directional antennas are positioned at the cell centers. Simulation results show that both P-AS and E-AS outperform the Single Frequency Network (SFN) where all the transmission antennas transmit the same signal continuously in terms of energy efficiency. When compared to SFN, energy efficiency is improved by more than 50% for both schemes and for both homogeneous and heterogeneous layouts. In the case of the homogeneous topology model, P-AS scheme improves the energy efficiency by 54% at a rather low throughput loss of 7%, whilst E-AS improves energy efficiency considerably by nearly 70% without any loss of throughput.

Antenna selection schemes for energy efficiency in distributed antenna systems

Multiuser diversity is achieved by maximum C/I scheduler in both fast and slow fading scenarios. However, fairness among multiple users is not guaranteed in slow fading channel because time and frequency resource are always occupied by the user with largest signal to interference and noise ratio(SINR). Opportunistic beamforming using dumb antennas is a multiple antennas transmit technique to increase the fluctuation rate and dynamic range of effective channel coefficients in slow fading environment. In this paper, we propose a method to improve the fairness of maximum C/I scheduler with the technique of dumb antennas. The theoretical analysis of users' scheduling probability shows that the fairness of maximum C/I scheduler can be greatly improved by dumb antennas in slow fading channels without significant loss in cell spectrum efficiency. The engineering issues of its application in the downlink transmission of 3GPP LTE system are discussed. The numerical results of simulation with practical LTE configurations and assumptions also verify our proposal.

Fairness Improvement of Maximum C/I Scheduler by Dumb Antennas in Slow Fading Channel

Distributed antenna systems(DAS) are techniques to improve the coverage and capacity of wireless system by reducing large scale fading of wireless channels. It is especially effective in noise limited scenarios, e.g. uplink transmission. In this paper, we propose a DAS layout where multiple sectorized antennas are located at the corners of hexagonal sectors. These distributed antennas are connected with a central unit within a sector via optical fibers. Only the signals received by distributed antennas within a sector are jointly processed. The power consumption and spectrum efficiency of several uplink transmission schemes in the proposed DAS layout are evaluated and compared with those in centralized antenna systems(CAS) in multi-cell environment. Compared with the baseline in the CAS layout, user equipment(UE) in the proposed DAS layout can spare the power consumption by as much as 16.25dB suppose the same spectrum efficiency threshold. Or equivalently, average and 5% outage spectrum efficiency are improved by as much as 226% and 209% respectively with same power consumption.

Energy-Efficient Uplink Transmission in Sectorized Distributed Antenna Systems

Distributed antenna systems (DAS) are traditionally deployed for the coverage purpose, where multiple antennas connected with a common source are geographically separated within the cell. In order to provide high rate services in the 4G wireless system, DAS should be further enhanced with the help of advanced MIMO techniques. In this paper, we start with reviewing the typical scenarios for DAS, which are widely discussed in standardization. Then, the benefits of DAS with single antenna selection (SAS) are analyzed. Based on this, a DAS transmission mode compatible with 3GPP Long Time Evolution (LTE) Release 8 standard is proposed. According to the system level simulation, the cell average and cell edge throughput can be improved by 35.0% and 82.3% respectively, compared with LTE Release 8 system. Moreover, the total transmitted power can be reduced by 75%.

Jiayin Zhang

Papers

A current perspective on distributed antenna systems for the downlink of cellular systems

Antenna selection schemes for energy efficiency in distributed antenna systems

Fairness Improvement of Maximum C/I Scheduler by Dumb Antennas in Slow Fading Channel

Energy-Efficient Uplink Transmission in Sectorized Distributed Antenna Systems

Distributed Antenna Systems and Their Applications in 4G Wireless Systems