Fifth-generation (5G) cellular networks will almost certainly operate in the high-bandwidth, underutilized millimeter-wave (mmWave) frequency spectrum, which offers the potentiality of high-capacity wireless transmission of multi-gigabit-per-second (Gbps) data rates. Despite the enormous available bandwidth potential, mmWave signal transmissions suffer from fundamental technical challenges like severe path loss, sensitivity to blockage, directivity, and narrow beamwidth, due to its short wavelengths. To effectively support system design and deployment, accurate channel modeling comprising several 5G technologies and scenarios is essential. This survey provides a comprehensive overview of several emerging technologies for 5G systems, such as massive multiple-input multiple-output (MIMO) technologies, multiple access technologies, hybrid analog-digital precoding and combining, non-orthogonal multiple access (NOMA), cell-free massive MIMO, and simultaneous wireless information and power transfer (SWIPT) technologies. These technologies induce distinct propagation characteristics and establish specific requirements on 5G channel modeling. To tackle these challenges, we first provide a survey of existing solutions and standards and discuss the radio-frequency (RF) spectrum and regulatory issues for mmWave communications. Second, we compared existing wireless communication techniques like sub-6-GHz WiFi and sub-6 GHz 4G LTE over mmWave communications which come with benefits comprising narrow beam, high signal quality, large capacity data transmission, and strong detection potential. Third, we describe the fundamental propagation characteristics of the mmWave band and survey the existing channel models for mmWave communications. Fourth, we track evolution and advancements in hybrid beamforming for massive MIMO systems in terms of system models of hybrid precoding architectures, hybrid analog and digital precoding/combining matrices, with the potential antenna configuration scenarios and mmWave channel estimation (CE) techniques. Fifth, we extend the scope of the discussion by including multiple access technologies for mmWave systems such as non-orthogonal multiple access (NOMA) and space-division multiple access (SDMA), with limited RF chains at the base station. Lastly, we explore the integration of SWIPT in mmWave massive MIMO systems, with limited RF chains, to realize spectrally and energy-efficient communications.

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A Comprehensive Survey on Millimeter Wave Communications for Fifth-Generation Wireless Networks: Feasibility and Challenges

Wireless engineers and business planners commonly raise the question on where, when, and how millimeter-wave (mmWave) will be used in 5G and beyond. Since the next generation network is not just a new radio access standard, but instead an integration of networks for vertical markets with diverse applications, answers to the question depend on scenarios and use cases to be deployed. This paper gives four 5G mmWave deployment examples and describes in chronological order the scenarios and use cases of their probable deployment, including expected system architectures and hardware prototypes. The paper starts with 28 GHz outdoor backhauling for fixed wireless access and moving hotspots, which will be demonstrated at the PyeongChang winter Olympic games in 2018. The second deployment example is a 60 GHz unlicensed indoor access system at the Tokyo-Narita airport, which is combined with Mobile Edge Computing (MEC) to enable ultra-high speed content download with low latency. The third example is mmWave mesh network to be used as a micro Radio Access Network ({\\mu}-RAN), for cost-effective backhauling of small-cell Base Stations (BSs) in dense urban scenarios. The last example is mmWave based Vehicular-to-Vehicular (V2V) and Vehicular-to-Everything (V2X) communications system, which enables automated driving by exchanging High Definition (HD) dynamic map information between cars and Roadside Units (RSUs). For 5G and beyond, mmWave and MEC will play important roles for a diverse set of applications that require both ultra-high data rate and low latency communications.

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Where, When, and How mmWave is Used in 5G and Beyond

Recently, mobile networking systems have been designed with more complexity of infrastructure and higher diversity of associated devices and resources, as well as more dynamical formations of networks, due to the fast development of current Internet and mobile communication industry. In such emerging mobile heterogeneous networks (HetNets), there are a large number of technical challenges focusing on the efficient organization, management, maintenance, and optimization, over the complicated system resources. In particular, HetNets have attracted great interest from academia and industry in deploying more effective solutions based on artificial intelligence (AI) techniques, e.g., machine learning, bio-inspired algorithms, fuzzy neural network, and so on, because AI techniques can naturally handle the problems of large-scale complex systems, such as HetNets towards more intelligent and automatic-evolving ones. In this paper, we discuss the state-of-the-art AI-based techniques for evolving the smarter HetNets infrastructure and systems, focusing on the research issues of self-configuration, self-healing, and self-optimization, respectively. A detailed taxonomy of the related AI-based techniques of HetNets is also shown by discussing the pros and cons for various AI-based techniques for different problems in HetNets. Opening research issues and pending challenges are concluded as well, which can provide guidelines for future research work.

Artificial Intelligence-Based Techniques for Emerging Heterogeneous Network: State of the Arts, Opportunities, and Challenges

Conventional cellular systems are designed to ensure ubiquitous coverage with an always present wireless channel irrespective of the spatial and temporal demand of service. This approach raises several problems due to the tight coupling between network and data access points, as well as the paradigm shift towards data-oriented services, heterogeneous deployments and network densification. A logical separation between control and data planes is seen as a promising solution that could overcome these issues, by providing data services under the umbrella of a coverage layer. This article presents a holistic survey of existing literature on the control-data separation architecture (CDSA) for cellular radio access networks. As a starting point, we discuss the fundamentals, concepts, and general structure of the CDSA. Then, we point out limitations of the conventional architecture in futuristic deployment scenarios. In addition, we present and critically discuss the work that has been done to investigate potential benefits of the CDSA, as well as its technical challenges and enabling technologies. Finally, an overview of standardisation proposals related to this research vision is provided.

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Control-Data Separation Architecture for Cellular Radio Access Networks: A Survey and Outlook

For heterogeneous network, which has been viewed as one pioneering technology for making cellular networks be evolved into 5G systems, reducing energy consumption by dynamically switching off base stations (BSs) has attracted increasing attention recently. With aiming at optimization on energy saving only or another energy-related performance tradeoffs, several BS switch-off strategies have been proposed from different design perspectives, such as random, distance-aware, load-aware, and auction-based strategies. Furthermore, work has been done to consider joint design for BS switch-off strategy and another strategies, such as user association, resource allocation, and physical-layer interference cancellation strategies. Finally, there have been research results about this topic in emerging cloud radio access networks. In this paper, we take an overview on these technologies and present the state of the art on each aspect. Some challenges that need to be solved in this research filed for future work are also described.

Survey of Strategies for Switching Off Base Stations in Heterogeneous Networks for Greener 5G Systems

Millimeter-wave (mmw) frequency bands, especially 60 GHz unlicensed band, are considered as a promising solution for gigabit short range wireless communication systems. IEEE standard 802.11ad, also known as WiGig, is standardized for the usage of the 60 GHz unlicensed band for wireless local area networks (WLANs). By using this mmw WLAN, multi-Gbps rate can be achieved to support bandwidth-intensive multimedia applications. Exhaustive search along with beamforming (BF) is usually used to overcome 60 GHz channel propagation loss and accomplish data transmissions in such mmw WLANs. Because of its short range transmission with a high susceptibility to path blocking, multiple number of mmw access points (APs) should be used to fully cover a typical target environment for future high capacity multi-Gbps WLANs. Therefore, coordination among mmw APs is highly needed to overcome packet collisions resulting from un-coordinated exhaustive search BF and to increase the total capacity of mmw WLANs. In this paper, we firstly give the current status of mmw WLANs with our developed WiGig AP prototype. Then, we highlight the great need for coordinated transmissions among mmw APs as a key enabler for future high capacity mmw WLANs. Two different types of coordinated mmw WLAN architecture are introduced. One is the distributed antenna type architecture to realize centralized coordination, while the other is an autonomous coordination with the assistance of legacy Wi-Fi signaling. Moreover, two heterogeneous network (HetNet) architectures are also introduced to efficiently extend the coordinated mmw WLANs to be used for future 5 Generation (5G) cellular networks.

Millimeter-Wave Wireless LAN and Its Extension toward 5G Heterogeneous Networks

This paper introduces a concept of cloud cooperated heterogeneous cellular network (C-HetNet) where small power pico base stations (BSs) overlaid on a macrocell are connected to cloud radio access network (C-RAN) to operate cooperatively with the macro BS. Since the macro BS assists operation of the pico BSs, it is easy to deploy multi-band HetNet where e.g. macro BS is operated at 2GHz band and pico BSs at 3GHz or 60GHz band to expand operation bandwidth. Moreover, cell structure of pico BSs are controlled dynamically to track hotspot users via beam steering and cooperative transmission among pico BSs. In this paper, several numerical simulations show effectiveness of the proposed architecture, where 1,000 times system rate than that of current single band cellular network is achieved by using the proposed architecture.

Cloud cooperated heterogeneous cellular networks

The mobile traffic explosion predicted to be increased by 1000 times in the next 10 years has become a remarkable issue due to the proliferation of smart devices such as smart phones or tablets. Energy consumption of information processing is also becoming an economic issue for operators. It is a critical task to design the next generation cellular networks (5G) to be both spectral/energy efficient. This paper considers a future C-RAN based cloud cooperated HetNet which enables global resource optimization among smallcells. The architecture allows optimal user association for data offloading as well as dynamic ON/OFF of smallcell BSs in adaptation to daily data traffic. A joint optimization on both user association and dynamic ON/OFF scheme of BSs to maximizing the system rate over consumed energy of the network investigated in the paper reveals that without sacrificing the system's power resource, our proposed approach can effectively deactivate unnecessary BSs and attain the target 1000× system rate gain in the case of mm-wave smallcells.

Dynamic cell activation and user association for green 5G heterogeneous cellular networks

In conventional cellular systems, transmission rate degrades at cell-edge because of inter-cell interference and pathloss. This problem is called “cell-edge problem”. To solve this problem, Coordinated Multi-Point (CoMP) technique has been proposed recently. CoMP can convert inter-cell interference signals from neighbor base stations (BSs) to desired signals. However, CoMP requires accurate synchronization among cooperative BSs as well as Channel State Information (CSI) between target user and all cooperative BSs. For practical realization of CoMP, new BS architecture in which a BS unit is connected to multiple Remote Radio Heads (RRHs) located apart through optical fiber has been proposed. Even using this architecture, however, CoMP can only be realized within the predefined connected RRHs (CoMP cluster). Therefore, cluster-edge users cannot experience throughput improvement by means of CoMP. This paper proposes a novel BS architecture called shared RRH network, in which each RRH is additionally connected to multiple BS units. As flexible clustering is made possible by this architecture, semi-dynamic clustering using geometrically overlapped cluster patterns allocated with orthogonal resources can be achieved which alleviates the cluster-edge problem. Simulation with parameters based on the 3GPP is setup for performance evaluation of the proposed system. Numerical results show that semi-dynamic CoMP using shared RRHs can significantly improve the system performance both at cell-inner and cell-edge compared with the conventional RRH systems, which confirms the effectiveness of the proposed network.

Shared Remote Radio Head architecture to realize semi-dynamic clustering in CoMP cellular networks

5G communication networks will bring enhanced mobile broadband services to users and vertical markets supporting very wide range requirements from context-dependent applications. To support such applications in 5G, not only the conventional area traffic capacity but network energy efficiency is also a critical factor since energy consumption of information processing is also becoming an economic issue for operators. Dealing with this problem, our project considers a C-RAN based cloud cooperated HetNet architecture which enables global resource optimization among smallcells to maximize objective functions of interest e.g. energy efficiency. On the other hand, a dynamic traffic model based on that the network can dynamically adapt to the variation in a cost-effective way is also crucial for the design of 5G. This paper develops such traffic model based on realistic measurement data in metropolitan Tokyo. Based on the developed model, a on-demand cell activation / deactivation jointly with user association mechanism is applied to maximize the network's energy efficiency defined as the system rate over the total consumed energy. Numerical results show the effectiveness of the proposed algorithm against dynamic variation of hourly traffic while improving users' satisfaction as compared to conventional homogeneous network. The paper also confirms the superiority of mm-wave smallcell HetNet against conventional microwave HetNet in terms of energy efficiency in bps/W.

Roya E. Rezagah

Papers

Millimeter-Wave Wireless LAN and Its Extension toward 5G Heterogeneous Networks

Cloud cooperated heterogeneous cellular networks

Dynamic cell activation and user association for green 5G heterogeneous cellular networks

Shared Remote Radio Head architecture to realize semi-dynamic clustering in CoMP cellular networks

Practical evaluation of on-demand smallcell ON/OFF based on traffic model for 5G cellular networks