Massive multiple-input multiple-output MIMO is one of themost promising technologies for the next generation of wirelesscommunication networks because it has the potential to providegame-changing improvements in spectral efficiency SE and energyefficiency EE. This monograph summarizes many years ofresearch insights in a clear and self-contained way and providesthe reader with the necessary knowledge and mathematical toolsto carry out independent research in this area. Starting froma rigorous definition of Massive MIMO, the monograph coversthe important aspects of channel estimation, SE, EE, hardwareefficiency HE, and various practical deployment considerations.From the beginning, a very general, yet tractable, canonical systemmodel with spatial channel correlation is introduced. This modelis used to realistically assess the SE and EE, and is later extendedto also include the impact of hardware impairments. Owing tothis rigorous modeling approach, a lot of classic "wisdom" aboutMassive MIMO, based on too simplistic system models, is shownto be questionable.

https://www.nowpublishers.com/article/DownloadSummary/SIG-093

Massive MIMO Networks: Spectral, Energy, and Hardware Efficiency

The 3GPP has raised the need to revisit the design of next generations of cellular networks in order to make them capable and efficient to provide M2M services. One of the key challenges that has been identified is the need to enhance the operation of the random access channel of LTE and LTE-A. The current mechanism to request access to the system is known to suffer from congestion and overloading in the presence of a huge number of devices. For this reason, different research groups around the globe are working towards the design of more efficient ways of managing the access to these networks in such circumstances. This paper aims to provide a survey of the alternatives that have been proposed over the last years to improve the operation of the random access channel of LTE and LTE-A. A comprehensive discussion of the different alternatives is provided, identifying strengths and weaknesses of each one of them, while drawing future trends to steer the efforts over the same shooting line. In addition, while existing literature has been focused on the performance in terms of delay, the energy efficiency of the access mechanism of LTE will play a key role in the deployment of M2M networks. For this reason, a comprehensive performance evaluation of the energy efficiency of the random access mechanism of LTE is provided in this paper. The aim of this computer-based simulation study is to set a baseline performance upon which new and more energy-efficient mechanisms can be designed in the near future.

Is the Random Access Channel of LTE and LTE-A Suitable for M2M Communications? A Survey of Alternatives

This monograph presents a unified framework for energy efficiency maximization in wireless networks via fractional programming theory. The definition of energy efficiency is introduced, with reference to single-user and multi-user wireless networks, and it is observed how the problem of resource allocation for energy efficiency optimization is naturally cast as a fractional program. An extensive review of the state-of-the-art in energy efficiency optimization by fractional programming is provided, with reference to centralized and distributed resource allocation schemes. A solid background on fractional programming theory is provided. The key-notion of generalized concavity is presented and its strong connection with fractional functions described. A taxonomy of fractional problems is introduced, and for each class of fractional problem, general solution algorithms are described, discussing their complexity and convergence properties. The described theoretical and algorithmic framework is applied to solve energy efficiency maximization problems in practical wireless networks. A general system and signal model is developed which encompasses many relevant special cases, such as one-hop and two-hop heterogeneous networks, multi-cell networks, small-cell networks, device-to-device systems, cognitive radio systems, and hardware-impaired networks, wherein multiple-antennas and multiple subcarriers are possibly employed. Energy-efficient resource allocation algorithms are developed, considering both centralized, cooperative schemes, as well as distributed approaches for self-organizing networks. Finally, some remarks on future lines of research are given, stating some open problems that remain to be studied. It is shown how the described framework is general enough to be extended in these directions, proving useful in tackling future challenges that may arise in the design of energy-efficient future wireless networks.

/pdf/energy-efficiency-in-wireless-networks-via-fractional-yuqemf824y.pdf

Energy Efficiency in Wireless Networks via Fractional Programming Theory

A new class of low-complexity, yet energy-efficient Multiple-Input Multiple-Output (MIMO) transmission techniques, namely, the family of Spatial Modulation (SM) aided MIMOs (SM-MIMO), has emerged. These systems are capable of exploiting the spatial dimensions (i.e., the antenna indices) as an additional dimension invoked for transmitting information, apart from the traditional Amplitude and Phase Modulation (APM). SM is capable of efficiently operating in diverse MIMO configurations in the context of future communication systems. It constitutes a promising transmission candidate for large-scale MIMO design and for the indoor optical wireless communication while relying on a single-Radio Frequency (RF) chain. Moreover, SM may be also viewed as an entirely new hybrid modulation scheme, which is still in its infancy. This paper aims for providing a general survey of the SM design framework as well as of its intrinsic limits. In particular, we focus our attention on the associated transceiver design, on spatial constellation optimization, on link adaptation techniques, on distributed/cooperative protocol design issues, and on their meritorious variants.

/pdf/design-guidelines-for-spatial-modulation-4di7fcwiu9.pdf

Design Guidelines for Spatial Modulation

The fifth generation (5G) mobile networks are envisioned to support the deluge of data traffic with reduced energy consumption and improved quality of service (QoS) provision. To this end, key enabling technologies, such as heterogeneous networks (HetNets), massive multiple-input multiple-output (MIMO), and millimeter wave (mmWave) techniques, have been identified to bring 5G to fruition. Regardless of the technology adopted, a user association mechanism is needed to determine whether a user is associated with a particular base station (BS) before data transmission commences. User association plays a pivotal role in enhancing the load balancing, the spectrum efficiency, and the energy efficiency of networks. The emerging 5G networks introduce numerous challenges and opportunities for the design of sophisticated user association mechanisms. Hence, substantial research efforts are dedicated to the issues of user association in HetNets, massive MIMO networks, mmWave networks, and energy harvesting networks. We introduce a taxonomy as a framework for systematically studying the existing user association algorithms. Based on the proposed taxonomy, we then proceed to present an extensive overview of the state-of-the-art in user association algorithms conceived for HetNets, massive MIMO, mmWave, and energy harvesting networks. Finally, we summarize the challenges as well as opportunities of user association in 5G and provide design guidelines and potential solutions for sophisticated user association mechanisms.

/pdf/user-association-in-5g-networks-a-survey-and-an-outlook-4zdf9nh1ox.pdf

User Association in 5G Networks: A Survey and an Outlook

Reducing energy consumption in wireless communications has attracted increasing attention recently. Advanced physical layer techniques such as multiple-input multiple-output (MIMO) and orthogonal frequency division multiplexing (OFDM), cognitive radio, network coding, cooperative communication, etc.; new network architectures such as heterogeneous networks, distributed antennas, multi-hop cellulars, etc.; as well as radio and network resource management schemes such as various cross-layer optimization algorithms, dynamic power saving, multiple radio access technologies coordination, etc. have been proposed to address this issue. In this article, we overview these technologies and present the state-of-the-art on each aspect. Some challenges that need to be solved in the area are also described.

A survey of energy-efficient wireless communications

To support the heterogeneous demands for the network and mobile user equipment (UE), multiple radio access technologies (RATs), operating with different system configurations and resources, have evolved and now coexist. Recently, with data traffic exponentially increasing, there has been a significant expansion in wireless network infrastructure, raising a justifiable concern over the concomitant drastic increase in energy consumption. Thus, energy-efficient design in multi-RAT networks is becoming increasingly important. In this paper, we consider resource allocation that maximizes the energy efficiency (EE) for orthogonal frequency division multiple access (OFDMA) in multi-RAT networks. To this end, we present the optimal resource allocation problem for parallel transmission utilizing multiple RATs. Since the formulated problem is NP-hard, a modified problem is proposed, for which a near-optimal resource allocation algorithm is developed by exploiting the intrinsic quasiconcavity of the problem. To reduce the computational complexity, we develop a low-complexity suboptimal allocation strategy based on joint iterative subcarrier and waterfilling power allocation over multiple RATs. Simulation results show that the proposed algorithms can achieve a higher EE compared to a conventional spectral-efficiency-based approach and can obtain performance comparable with the optimal solution, but with much less complexity.

Energy-Efficient Resource Allocation for OFDMA-Based Multi-RAT Networks

Recently, due to the growing concern over the ever-increasing energy consumption, much attention has been given to the energy-efficient design of wireless communication systems. Various strategies for improving the energy efficiency, which is defined as the transmitted bits per total energy consumed (in bits/Joule), have been proposed. In this paper, we analyze the energy efficiency of clustered cooperative beamforming where distributed relay nodes construct a virtual Multiple-Input Single-Output beamforming system with a maximum transmit power constraint, and compare its performance with that of direct communications. All the energy consumption overheads incurred are taken into account in the energy efficiency analysis assuming a constant circuit-power consumption model. We then study the number of relay nodes that maximizes the efficiency for a given outage constraint. In addition, we propose a mode switching algorithm which optimally chooses between using cooperative beamforming or direct communications for transmission. We show that cooperative beamforming significantly outperforms direct communications in energy efficiency, as well as in spectral efficiency, even though a much larger amount of circuit power is consumed.

Energy-efficient cooperative beamforming in clustered wireless networks

Recently, much attention has been given to heterogeneous networks where a mobile device is capable of accessing multiple random access networks (RANs), where each RAN operates on a different carrier frequency with a different transmission bandwidth, using multi-radio platforms embedded in the device. In this letter, we investigate the efficient use of multiple radio access technologies (RATs) to improve the energy efficiency in cooperative networks. Considering the circuit power consumption of each RAT, we study energy-efficient best-select relaying and show that the proposed scheme significantly outperforms traditional best-select relaying as well as direct communications.

Energy-Efficient Cooperative Relaying in Heterogeneous Radio Access Networks

Energy efficiency (EE) is an important objective in uplink wireless communications due to the limited capacity of batteries in user equipment (UE). In this letter, we propose a scheduling metric to balance the tradeoff between EE and fairness. We consider the scenario in which the access point (AP) serves a subset of UEs when available resources cannot support all of the UEs' quality-of-service (QoS) requirements. Based on the scheduling metric, we propose a suboptimal joint scheduling and resource allocation algorithm that maximizes the system EE. Simulation results show that the proposed algorithm can achieve a good tradeoff between the system EE and fairness.

Gubong Lim

Papers

A survey of energy-efficient wireless communications

Energy-Efficient Resource Allocation for OFDMA-Based Multi-RAT Networks

Energy-efficient cooperative beamforming in clustered wireless networks

Energy-Efficient Cooperative Relaying in Heterogeneous Radio Access Networks

Energy-Efficient Scheduling and Resource Allocation in Uplink OFDMA Systems