SPECIAL SECTION ON GREEN COMMUNICATIONS AND NETWORKING FOR 5G

Date of current version December 31, 2018.

Digital Object Identifier 10.1109/ACCESS.2018.2885258

EDITORIAL

IEEE ACCESS SPECIAL SECTION EDITORIAL:GREEN COMMUNICATIONS ANDNETWORKING FOR 5G

The fifth-generation (5G) communication systems willinclude very high carrier frequencies with massive band-widths, extreme base station and device densities, andunprecedented numbers of antennas. However, unlike theprevious four generations, high speed data transfer and ubiq-uitous connectivity among various types of devices will beprovided by integration of LTE and Wi-Fi.

The development of green and energy-efficient 5G com-munication systems has recently become an important topicin communications. It is envisioned in such cases to becomenecessary to meet the growing demands for high data ratewith limited resources. This Special Section has 42 acceptedarticles, and a brief summary about each article is presentedas follows.

For heterogeneous networks, which have been viewed asa pioneering technology for making cellular networks evolveinto 5G systems, reducing energy consumption by dynami-cally switching off base stations (BSs) has recently attractedincreased attention. In the invited article by Zhao et al., ‘‘Sur-vey of Strategies for Switching Off Base Stations in Het-erogeneous Networks for Greener 5G Systems,’’ the authorstake an overview on these technologies and present the stateof the art on each aspect. Some challenges that need to besolved in this research field for future work are also described.With the aim of optimization of energy saving only or otherenergy-related performance tradeoffs, several BS switch-offstrategies have been proposed from different design per-spectives, such as random, distance-aware, load-aware, andauction based strategies. Furthermore, work has been doneto consider joint design for BS switch-off strategy and otherstrategies, such as user association, resource allocation, andphysical-layer interference cancellation strategies. Finally,there have been research results about this topic in emergingcloud radio access networks.

In recent years, there has been a growing interest ingreen cellular networks for reducing the energy dissipatedby communications and networking devices, including theBSs and battery-powered user terminals (UTs). In the articleby Zou et al., ‘‘Cognitive Network Cooperation for GreenCellular Networks,’’ the authors investigate the joint employ-ment of cognition and cooperation techniques invoked for

improving the energy efficiency of cellular networks. In orderto illustrate the energy efficiency benefits of using both cog-nition and cooperation, the authors study the cooperationbetween television stations (TVSs) and BSs in transmittingto UTs relying on an opportunistic exploitation of the TVspectrum, where the unused TV spectral band is utilized inan opportunistic way, depending on whether it is detected tobe idle (or not).

Antenna polarization is an effective resource to beexploited for space-limited devices to support a large net-work payload. However, channel modeling with regard toantenna polarization is still in its early stages. In the article bySu et al., ‘‘ChannelModel for PolarizedMIMOSystemswithPower Radiation Pattern Concern,’’ the authors assess severalexisting channel models that support antenna polarization,and the advantages and disadvantages of the existing modelsare clearly summarized. To maintain the advantages (e.g.,supporting the MIMO array extension and considering theantenna radiation pattern) and overcome the disadvantages(e.g., reducing modeling complexity but preserving accu-racy), the authors propose a 3-D channel model for polarizedmultiple input multiple output (CM3D-PMIMO) systems.In wireless sensor networks (WSNs), the benefits of

exploiting the sink mobility to prolong network lifetimehave been well recognized. In physical environments, allkinds of obstacles could exist in the sensing field. Therefore,a research challenge is how to efficiently dispatch the mobilesink to find an obstacle-avoiding shortest route. In the articleby Xie et al. ‘‘Cluster-Based Routing for the Mobile Sinkin Wireless Sensor Networks with Obstacles,’’ the authorspresent an energy-efficient routing mechanism based on thecluster-based method for the mobile sink in WSNs withobstacles. According to the cluster-based method, the nodesselected as cluster heads collect data from their cluster mem-bers and transfer the data collected to the mobile sink.

Tomeet the demands of an increased capacity, an improveddata rate, and a better quality of the service of the next-generation networks, there is a need to adopt energy-efficientarchitectures. Along with these requirements, it is also oursocial responsibility to reduce the carbon footprint by reduc-ing the power consumption in a wireless network. In the

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article by Jha et al., ‘‘Power Optimization in 5G Networks:A Step towards GrEEn Communication,’’ the authors havesurveyed various techniques for the power optimization ofthe upcoming 5G networks. The primary focus is on the useof relays and small cells to improve the energy efficiencyof the network, and the various scenarios of relaying forthe next-generation networks have been discussed. Alongwith this, the importance of simultaneous wireless power andinformation transfer, massive MIMO, and millimeter waveshave been analyzed for 5G networks.

The Single-carrier Frequency-Division Multiple Access(SC-FDMA) communication scheme is the key techniquein green communications and networking for 5G for theuplink of SC-FDMA multiuser systems due to frequencydifferences introducing access interference. The precision ofthe traditional parallel and serial interference cancellationalgorithms is not high, and the number of required iterationsfor achieving satisfactory results is large. In the article byBao et al., ‘‘A Multiuser Detection Algorithm in the UplinkSC-FDMA System for Green Communication Network’’, theauthors propose a new algorithm for the optimal weightedparallel interference cancellation (PIC) to eliminate multipleaccess interference. This method has higher precision thantraditional PIC and the number of interference eliminationiterations required is low.

Non-orthogonal multiple access (NOMA) has been shownin the literature to have a better performance than OMA interms of sum channel capacity. In the article by Liu et al.,‘‘On the Capacity Comparison between MIMO-NOMA andMIMO-OMA,’’ the authors provide the proof to demonstratethat the MIMO-NOMA is strictly better than MIMO-OMAin terms of sum channel capacity (except for the case whereonly one user is being communicated to), i.e., for any rate pairachieved by MIMO-OMA, there is a power split for whichMIMO-NOMA can achieve rate pairs that are strictly larger.

The main object of green communication is to reduce asmuch as possible the energy consumption of the communi-cation system. In the article by Zhao et al., ‘‘A Green Dis-tributed Signal Reconstruction Algorithm in Wireless Sen-sor Networks,’’ the authors have investigated the green dis-tributed nonlinear state estimation problem in WSNs, whichwill be seamlessly integrated with the forthcoming 5G com-munication system. In order to develop a distributed recon-struction algorithm to fuse the random linear measurementsfrom the nodes in WSNs, the pseudo-measurement (PM)embedded in the cubature Kalman filter (CKF) is formulatedinto information form, and then the derived information filteris combined with the consensus filter, while the square-rootversion is further developed to improve the performance andstrengthen power saving capability.

Energy consumption is one of the constraints in WSNs.Routing protocols are the hot areas to address quality-of-service (QoS) related issues, viz., energy consumption, net-work lifetime, network scalability, and packet overhead. Theproblem is to identify the dead nodes and to choose anothersuitable path so that the data transmission becomes smoother

and less energy gets conserved. In order to resolve theseissues, in the article by Ahmed et al., ‘‘Energy EfficientDirection Based PDORP Routing Protocol for Wireless Sen-sor Networks), the authors propose directional transmission-based energy aware routing protocol named PDORP. Theproposed protocol PDORP has the characteristics of bothpower efficient gathering sensor information system anddynamic source routing (DSR) routing protocols. The per-formance analysis, comparison through a hybridizationapproach of the proposed routing protocol, provides betterresults comprising of less bit error rate, less delay, less energyconsumption, and better throughput, which leads to betterQoS and prolongs the lifetime of the network.

Tree networks are widely applied in sensor networks ofInternet of Things (IoTs). In the article by Feng et al., ‘‘AnEfficient Tree-Based Self-Organizing Protocol for Internetof Things), the authors propose an efficient tree-based self-organizing protocol (ETSP) for sensor networks of IoTs.In ETSP, all nodes are divided into two kinds: networknodes and non-network nodes. Network nodes can broad-cast packets to their neighboring nodes. Non-network nodescollect the broadcasted packets and determine whether tojoin the network. The topology is adjusted dynamically tobalance energy consumption and prolong network lifetime.Simulation results show that our proposed protocol can con-struct a reliable tree-based network quickly.With the networkscale increasing, the self-organization time, average hop, andpacket loss ratio will not increase more.

Since compressive sensing (CS) theory has come into theworld, it has been widely applied in many fields includingthe 5G system, cloud monitoring, image coding, and so on.In the article by Chen et al., ‘‘A Review of CompressiveSensing in Information Security Field,’’ the authors reviewCS in information security field from two aspects: theoreticalsecurity and application security for the first time. Moreover,the CS applied in image cipher is one of the most widespreadapplications, as its characteristics of dimensional reductionand random projection can be utilized and integrated intoimage cryptosystems, which can achieve simultaneous com-pression and encryption of an image or multiple images.

The rapid growth of mobile devices and modern com-munication applications result in the explosive demand forwireless data. Communication networks have to face severalchallenges such as spectrum sharing and energy scarcity todeal with the dramatic increase in wireless data, which shiftthe focus of research directions to 5G networks. In the articleby Jo et al., ‘‘Cooperative Wireless Energy Harvesting andSpectrum Sharing in 5G Networks,’’ the authors propose anovel best cooperativemechanism (BCM) for wireless energyharvesting and spectrum sharing in 5G networks. Data trans-fer and energy harvesting are finished in the designed timeslotmode. In the proposed BCM, secondary users (SUs) harvestenergy from both ambient signals and primary user’s (PU’s)signals.

Energy-efficiency is an important issue to address formm-wave full-duplex relaying systems, which highlights the

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green evolution of 5G wireless communications. Existingfull-duplex relaying related research is spectral efficiency-oriented, which is not efficient in cutting carbon foot-print. In the article by Wei et al., ‘‘Energy-Efficiencyof Millimeter-Wave Full-Duplex Relaying Systems: Chal-lenges and Solutions,’’ the authors address the criticalenergy-efficiency challenges in implementing full-duplexrelaying in mm-wave systems, including low drain effi-ciency of power amplifier, high circuit power consump-tion, and additional power required by full-duplex relays tomitigate self-interference. Some energy-efficiency-orientedfuture research is also envisaged for mm-wave full-duplexrelaying systems.

To achieve this mobile cloud computing service, aftermoving the computing capacity away from end devices todata centers (DCs), in the article by Ning et al., ‘‘Location-Recommendation-Aware Virtual Network Embedding inEnergy-Efficient Optical-Wireless Hybrid Networks Sup-porting 5G Models,’’ the authors propose the convergedinfrastructure integrating optical metro and ubiquitous wire-less access technologies in accordance with the 5G model.To minimize energy consumption, the authors propose anovel design framework of location-recommendation-awarevirtual network embedding. This design framework deter-mines the interdependency among user groups so that we canembed the virtual networks owned by user groups with a highinterdependence into the same DC of the substrate optical-wireless hybrid infrastructure. Thus, the adviser can locallypush the corresponding products to user groups at a singleDC instead of consuming a large amount of energy to buildinter-DC paths.

With the explosive development of communication tech-nologies, more customer friendly services have beendesigned for the next generation of cellular technology,that is, 5G communication. In the article by Jin et al.,‘‘Multi-label Classification Methods for Green Comput-ing and Application for Mobile Medical Recommenda-tions,’’ the authors propose two label selection methodsfor multi-label classification: clustering-based sampling andfrequency-based sampling. The authors apply their proposedmulti-label classification methods as an innovative 5G appli-cation to predict doctor labels for doctor recommendations.The authors perform experiments on real-world data sets. Theexperimental results show that our methods achieve the state-of-the-art performance compared with baselines.

In a multipath communication scenario, it is often relevantto estimate the directions and relative delays of each mul-tipath signal. In the article by Liu et al., ‘‘Joint Estimationof DOA and TDOA of Multiple Reflections in Mobile Com-munications,’’ the authors present an effect algorithm for thesimultaneous estimation of these parameters by re-iterativesuper-resolution, beamforming, and multiple signal classi-fication (MUSIC)-like searching techniques. The algorithmfirst separates and estimates direction of arrival (DOA) of themultipath signals. Then, time difference of arrival (TDOA)for the incident signals are mapped into phase shifts in the

frequency domain. At the same time, the pairing of the esti-mated DOA and TDOA is automatically determined. Com-puter simulations illustrating the performance of the proposedalgorithm with the Cramer–Rao bound are included.

With the rapid increase of mobile connected devices andbase stations, the mobile and wireless industry annuallycauses a significant carbon emission, i.e., in total 2% ofthe worldwide and the fastest growing sector. As a result,the green aspect of the 5G networks is of the utmost con-cern. Energy harvesting communication has recently gaineda great attention as a promising technology for 5G wirelessnetworks. In the article by Duong et al., ‘‘Secure 5G Wire-less Communications: A Joint Relay Selection and Wire-less Power Transfer Approach,’’ the authors investigate thesecrecy performance of an energy harvesting relay system,where a legitimate source communicates with a legitimatedestination via the assistance of multiple trusted relays. In theconsidered system, the source and relays deploy the time-switching-based radio frequency energy harvesting techniqueto harvest energy from a multi-antenna beacon.

Massive MIMO has been extensively studied and con-sidered as a key enabling technology for the 5G wire-less communication systems, due to its potential to achievehigh energy efficiency and spectral efficiency. In the arti-cle by Chen et al., ‘‘Original Symbol Phase Rotated SecureTransmission against Powerful Massive MIMO Eavesdrop-per,’’ motivated by the need to protect against eavesdrop-pers equipped with powerful large antenna arrays, whichhas received scarce attention in the literature, a physicallayer security approach called original symbol phase rotated(OSPR) secure transmission scheme is proposed. The authorsshow that when the BS has a large enough, but finite num-ber of antennas, the proposed OSPR scheme can achieve aconsiderable security performance in that the eavesdropperis unable to recover most of the original symbols, even withunlimited antennas.

Massive MIMO technology is one of the promising tech-nologies in 5Gwireless communication networks. Among thevarious signal-processing problems, the two dimension (2-D)beamforming for link reliability enhancement has attracted alot of attention and the beamforming technique is required tooperate in both azimuth and elevation directions. It is wellknown that the performance of the beamforming relies on theestimate accuracy of DOA algorithm. As a result, to estimatethe elevation and azimuth angles efficiently and accuratelyis a vital problem. In the article by Lan et al., ‘‘A RAREAlgorithm for 2D DOA Estimation Based on Nested Arrayin Massive MIMO System,’’ the authors propose a low com-putational complexity rank reduce (RARE) algorithm basedon 2-D nested array for DOA estimation in massive multiple-input multiple-output system. By utilizing the difference co-array of the 2-D nested array, one can obtain the order of N2

virtual sensors with only N actual sensors, which can enableus to detect a greater number of sources than sensors.

Low-power green communication has become the one ofthe most important and promising research topics for energy

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efficiency communications. We investigated the energyefficiency green communication in 3-D directional sensornetworks. In the article by Han et al., ‘‘An Energy Effi-ciency Node Scheduling Model for Spatial-Temporal Cov-erage Optimization in 3D Directional Sensor Networks,’’the authors address this problem of maximizing the spatial-temporal coverage by scheduling sensors’ activity after theyhave been deployed. Experimental results show that com-pared with the grid-based elementary region generationscheme, the position-based scheme significantly reduces thealgorithmic complexity and is time consuming while gener-ating elementary regions in network.

The primary goal of this research is the optimization ofdata transmissions and connections between 5G base sta-tions as well as the improvement of access technologies andtransmission methods in consideration of Massive MIMO,a key technology in 5G networks. In the article by Wu et al.,‘‘Interference Reduction by Millimeter Wave Technology for5G-Based Green Communications,’’ the authors use 5G mil-limeter wave (mmWave), due to its high directivity and sensi-tivity to blockage, to enhance the connection system. Becausethe number of macro-cells will affect the communicationquality and the computational complexity, this research espe-cially focuses on three factors of a network: Delay, Capacityand Path Loss, and attempts to figure out the most energy-efficient BS densities for 5G-based green communications.

Self-managed access points (APs) with growing intelli-gence can optimize their own performances but pose poten-tial negative impacts on others without energy efficiency.In the article by Jiang et al., ‘‘Interference-Aware Coordi-nated Power Allocation in AutonomousWi-Fi Environment,’’the authors focus on modeling the coordinated interactionamong interest-independent and self-configured APs, andconduct the power allocation case study in the autonomousWi-Fi scenario. Specifically, the authors build a coordinationWi-Fi platform (CWP), a public platform for APs interactingwith each other. CWP is deployed in a real office environ-ment, where coordination interactions between VAs can bringa maximum 40-Mb/s throughput improvement with the Nashbargaining-based power control in the multi-AP experiments.

As the mobile operators strive to accommodate the increas-ing load and capacity requirements, direct communicationamong users’ devices, namely, device-to-device (D2D) com-munication emerges as an advantageous solution for cellulartraffic offloading. However, the formation and the opera-tion of D2D networks are challenging, as D2D peers mustsatisfy various network-related constraints. In the article byDatsika et al., ‘‘Green Cooperative Device-to-Device Com-munication: A Social-Aware Perspective,’’ the authors outlinethe challenges that appear in D2D cooperative networkingandmedium access control (MAC) design under the influenceof social characteristics and the energy consumption concernsthat arise in modern D2D networking scenarios. Consideringthe users’ social ties, we present an energy efficient social-aware cooperative D2D MAC protocol as a paradigm ofsocial information inclusion in the green D2D MAC design.

Last, we discuss the practical issues of the adoption of socialawareness in green D2D cooperation, which may affect theD2D performance in realistic scenarios.

Mobile edge computing (MEC) is a promising paradigmto provide cloud-computing capabilities in close proximity tomobile devices in 5G networks. In the article by Zhang et al.,‘‘Energy-efficient Offloading for Mobile Edge Computingin 5G Heterogeneous Networks,’’ the authors study energy-efficient computation offloading (EECO) mechanisms forMEC in 5G heterogeneous networks. We formulate an opti-mization problem to minimize the energy consumption ofthe offloading system, where the energy cost of both taskcomputing and file transmission is taken into consideration.Incorporating the multi-access characteristics of the 5G het-erogeneous network, we then design an EECO scheme, whichjointly optimizes offloading and radio resource allocation toobtain the minimal energy consumption under the latencyconstraints.

As an emerging and promising technology, massiveMIMOusually adopts tens of or even hundreds of antennas to simul-taneously serve tens of mobile terminals in time divisionduplex (TDD) mode. In the article by Xu et al., ‘‘Joint Param-eter Selection for Massive MIMO: An Energy-Efficient Per-spective,’’ the authors investigate the energy-efficientmassiveMIMO design problem in both single-cell and multi-cellscenarios with the aim of selecting appropriate parameters tomaximize the overall system energy utilization. To this end,a more realistic parameter selection model is established byconsidering the channel estimation error and pilot contami-nation. For effective solving, we divide the original probleminto multiple sub-problems according to the setting of pilotreusing factor, and employ the proposed alternative optimiza-tion plus bisection searching (AO-BS) algorithm to solve eachsub-problem. Through numerical simulations, it is observedthat the proposed AO-BS algorithm converges rapidly and isalmost able to achieve the globally optimal solution.

Due to the recent interest in energy efficient cellular net-works, many mobile operators are adopting an environmen-tally friendly management of their cellular networks. Themain focus is on the radio access networks (RANs) sincearound 60% of the energy is consumed by the BSs. In thearticle by Sboui et al., ‘‘On Green Cognitive Radio CellularNetworks: Dynamic Spectrum and Operation Management,’’the authors study a profit maximization problem relatedto cognitive radio cellular networks in an environmentallyfriendly framework. The objective of the primary networkand the secondary network is to maximize their profits whilerespecting a certain carbon dioxide (CO2) emission thresh-old. In this article, the primary network can switch off some ofits BSs powered bymicrogrids, and hence leases the spectrumin the corresponding cells, to reduce its footprint.

The tremendously large number of increasing Internet pro-tocol (IP) packets call for QoS guaranteed packets trans-mission with low-delay, high throughput and high energyefficiency (defined as the transmitted bits per unit energyconsumption) in the 5G networks. For this motivation,

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wavelength division multiplexing (WDM) networks and thenext generation of wireless technologies are two major meth-ods in the wired and wireless networks, respectively. In thearticle by Wen et al., ‘‘(Cross-Networks Energy EfficiencyTradeoff: From Wired Networks to Wireless Networks,’’ theauthors formulate a joint optimization model to enhance theperformance of energy efficiency in 5G. Specifically, each BSis equipped with a set of parallel tunable lasers for simulta-neous transmission of multiple packets from the uplink usersin the cell as well as the data center networks (DCNs). Theauthors propose a novel joint cross-networks scheduling androuting (JCNSR) algorithm according to the wireless channelquality of users, user data rate, and the topology constraint.

Interference alignment (IA), which was originally pro-posed for K-user/node interference channel, has attractedconsiderable attention in recent years. The basic idea of IAis to align the interference imposed on the receiver intoa subspace having the lowest possible dimensions so thatthe interference-free dimensions can be maximized for thedesired signal. In the article by Lv et al., ‘‘Wireless Infor-mation and Energy Transfer in Multi-Cluster MIMO UplinkNetworks Through Opportunistic Interference Alignment,’’the authors consider a K-cluster (K ≥ 2) simultaneous wire-less information and power transfer network, where S nodes(S ≥ 2) are selected from N nodes within each cluster forthe uplink information transmission and the remaining N− Sidle nodes are dedicated to energy harvesting. Based on theintra-cluster performance aware (ICPA) philosophy, a pair ofopportunistic interference alignment (OIA) schemes, namely,the coarse ICPA-OIA (C-ICPA-OIA) and the refined ICPA-OIA (R-ICPA-OIA), is proposed for balancing the sum rateperformance achieved and the energy harvested.

Though the energy consumed by the idle mobile resourcesis insignificant in comparison to the cloud counterpart,such consumption has drastic impacts on the mobiledevices causing unnecessary battery drains. In the article byLiu et al., ‘‘Mobilouds: An Energy EfficientMCCCollabora-tive Framework with Extended Mobile Participation for NextGenerationNetworks,’’ the authors proposeMobiloudswhichencompass a multi-tier processing architecture with variouslevels of process cluster capacities and a software applica-tion to manage energy efficient utilization of such processclusters. The proposed Mobilouds framework encourages themobile device participation in the Mobile Cloud Computing(MCC) collaborative execution, and thereby reduces the pres-ence of idle mobile resources and utilizes such idle resourcesin the actual task execution.

As the next generation network architecture, software-defined networking (SDN) has exciting application prospects.Compared with the traditional networks, the SDN has manyadvantages to support traffic engineering due to its distin-guishing characteristics, such as isolation of control andforwarding, global centralized control, and programmabil-ity of network behavior. This article focuses on the trafficengineering technology based on the SDN. In the articleby Wan et al., ‘‘Traffic Engineering in Software-Defined

Networking: Measurement and Management,’’ first, theauthors propose a reference framework for traffic engineeringin the SDN, which consists of two parts, traffic measurementand traffic management. Traffic measurement is responsiblefor monitoring and analyzing real-time network traffic, as aprerequisite for traffic management. In the proposed frame-work, technologies related to traffic measurement includenetwork parameters measurement, a general measurementframework, and traffic analysis and prediction; technologiesrelated to traffic management include traffic load balancing,QoS-guarantee scheduling, energy-saving scheduling, andtraffic management for the hybrid IP/SDN.

Energy efficiency is critical to fully achieve the huge poten-tials of D2D communications with limited battery capacity.In the article by Dong et al., ‘‘Iterative Energy-Efficient Sta-ble Matching Approach for Context-Aware Resource Alloca-tion in D2D Communications,’’ the authors consider the two-stage energy efficiency optimization problem, which consistsof a joint spectrum and power allocation problem in the firststage, and a context-aware D2D peer selection problem inthe second stage. First, the preferences of user equipmentare defined as the maximum achievable energy efficiency.An iterative power allocation algorithm is proposed to opti-mize energy efficiency under a specific match, which isdeveloped by exploiting nonlinear fractional programmingand Lagrange dual decomposition. Second, the authors pro-pose an iterative matching algorithm which produces a stablematch based on the fixed preferences and then dynamicallyupdates the preferences according to the latest matchingresults.

WSNs, as key components of the Internet of Things, areemerging as promising platforms that enable a wide rangeof applications in both military and civilian domains. It isdesirable for alarm packets to be forwarded to the sink asquickly as possible in wireless sensor networks. In the arti-cle by Li et al., ‘‘FFSC: An Energy Efficiency Communica-tions Approach for Delay Minimizing in Internet of Things,’’the authors initially analyze the theory of the relationshipsbetween network configurations and network lifetime as wellas transmission delay. Then, they propose an approximateoptimization approach to minimize the end-to-end delay witha reduced complexity of configuration under the conditionthat the network lifetime remains greater than the specifiedtarget value. Both the comprehensive theoretical analysisand the experimental results indicate that the performanceof FFSC is better than the methods proposed by previousstudies.

The MIMO broadcast channels have been investigatedextensively during the last two decades. By assuming per-fect channel state information at transmitter, the capacityregion can be achieved by dirty-paper coding, in whichall users share the same frequency-time resource. In thearticle by Chen et al., ‘‘Beamforming for Combating Inter-cluster and Intra-cluster Interference in Hybrid NOMA Sys-tems,’’ the authors consider downlink beamforming (BF)for hybrid NOMA systems, in order to combat inter and

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intra cluster interference. First, to minimize the inter andintra-cluster interference, the projection hybrid NOMA(PH-NOMA) BF algorithm is introduced, by combiningconventional zero-forcing BF (ZFBF) and hybrid NOMA(H-NOMA) precoding. Second, to further reduce the overallinterference, two user pairing algorithms, termed projection-based pairing algorithm (PBPA) and inversion-based pair-ing algorithm (IBPA), are also proposed, by adopting theproperties of quasi-degradation developed previously. Conse-quently, the proposed BF algorithm is obtained by combiningPH-NOMA and PBPA/IBPA.

With the new physical layer design and subsequent newmedium access control functions under more demand onquality of services and user experience, in the article byDeng et al., ‘‘On Quality-of-Service Provisioning in IEEE802.11ax WLANs,’’ the authors first survey the QoS supportin legacy 802.11. Then, the authors summarize the IEEE802.11ax standardization activities in progress and presentan overview of current perspectives and expected featureson medium access control protocol design to better supportQoS and user experience in 802.11ax. The authors present themotivation behind the design, explain design principles, andidentify new research challenges. To better satisfy customerneeds on high bandwidth and low latency, emerging LTELicensed-AssistedAccess (LTE-LAA) and its impacts to QoSprovisioning in IEEE 802.11ax are further addressed giventhe collaboration between cellular andWLANs, and given thetrend of 5G cellular over unlicensed bands.

Heterogeneous networks (HetNets) supported with relaysand device to device communication can be considered asa promising solution to realize the ambitious targets of thefuture fifth generation networks in terms of energy efficiencyand capacity. HetNets necessitate optimal power allocation,spectrum resource allocation, and cell selection to meet thequality of service requirements. In the article by Ali et al.,‘‘Energy Efficient Resource Allocation in D2D-Assisted Het-erogeneous Networks with Relays), the authors formulate anenergy efficiency maximization problem in terms of resourceallocation and cell selection for HetNets, where the objectiveis to maximize the network throughput per unit networkpower consumption. The performance of ε-optimal solutionobtained by outer approximation algorithm (OAA) method isshown for different network parameters, such as number ofusers, required date rate, and capacity of network.

The filter bank multicarrier with offset quadrature ampli-tude modulation (FBMC/OQAM) is being studied by manyresearchers as a key enabler for the 5G air interface.In the article by Wang et al., ‘‘Hybrid PAPR ReductionScheme for FBMC/OQAM Systems Based on Multi DataBlock PTS and TR methods,’’ the authors propose a hybridpeak-to-average power ratio (PAPR) reduction scheme forFBMC/OQAM signals by utilizing multi data block partialtransmit sequence (PTS) and tone reservation (TR). In thehybrid PTS-TR scheme, the data blocks signal is dividedinto several segments and the number of data blocks in eachsegment is determined by the overlapping factor. Simulation

results show that the M-hybrid scheme can achieve about0.2-dB PAPR performance better than the hybrid PTS-TRscheme.

As a key technology for 5G, physical-layer network coding(PNC) supplies a powerful platform by leveraging the broad-cast nature of wireless media. However, the symbol error rate(SER) of PNC is not well investigated, which would seriouslyinfluence a user’s quality of experience due to packet loss inwireless environment. In the article by Ning et al., ‘‘TowardEfficient 5G Transmission: SER Performance Analysis forAsynchronous Physical-layer Network Coding,’’ consideringboth phase and symbol misalignments, the authors performanalysis on SER of asynchronous PNC. By assuming partof the information is known to the relay, the authors derivethe lower bound for SER. Afterward, through applying theconcept of error vector and eliminating redundant terms,the authors derive the upper bound for SER. Both the lowerand upper bounds are applicable to either multiuser detection-based network coding or belief propagation-based maximuma posteriori decoding.

In order to satisfy the various requirements of futurenetwork services, 5G wireless network are proposed andare becoming a hot topic in academic and industrial fields.Location-based services are widely used with the develop-ment of wireless communication and mobile Internet tech-nology. A number of popular spatial-temporal cloaking tech-nologies have been proposed, and the number of users in ananonymizing spatial region (ASR) is uncontrollable. In thearticle by Bai et al., ‘‘A Location Cloaking Algorithm BasedOn Combinatorial Optimization For Location-based Servicesin 5G Networks,’’ based on the semi-trusted Server archi-tecture, the authors propose a location cloaking algorithm(LCA) based on combinatorial optimization. First, the semi-trusted server architecture divides the information of mobileusers into three parts, so adversaries are unable to obtain thelocation and identity at the same time. Then it utilizes thespatial k-anonymity algorithm LCA to hide the real locationof the user, which controls the number of real users to aroundk in the anonymous result sets (Aset), so that both the numberof users in ASR and the area of ASR are minimized, therebyimproving query precision and decreasing the resource con-sumption.

Over the last decade, mobile communications have beenwitnessing a noteworthy increase of data traffic demandthat is causing enormous energy consumption in cellularnetworks. The reduction of their fossil fuel consumptionin addition to the huge energy bills paid by mobile oper-ators is considered as the most important challenges forthe next-generation cellular networks. Although most of theproposed studies were focusing on individual physical layerpower optimizations, there is a growing necessity to meet thegreen objective of fifth-generation cellular networks whilerespecting the user’s quality of service. In the article byGhazzai et al., ‘‘Next-Generation Environment-Aware Cel-lular Networks: Modern Green Techniques and Implemen-tation Challenges,’’ the authors investigate four important

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techniques that could be exploited separately or together inorder to enable wireless operators to achieve significant eco-nomic benefits and environmental savings: 1) the base stationsleeping strategy; 2) the optimized energy procurement fromthe smart grid; 3) the base station energy sharing; and 4) thegreen networking collaboration between competitive mobileoperators. The presented simulation results measure the gainthat could be obtained using these techniques compared withthat of traditional scenarios. Finally, this article discusses theissues and challenges related to the implementations of thesetechniques in real environments.

Millimeter-wave (mmWave) transmission is regarded as akey technology for 5G networks. In the article by Cui et al.,‘‘Vehicle Positioning Using 5G Millimeter-Wave Systems,’’the authors examine vehicle positioning using 5G mmWavesignals. Both a correlation receiver and an energy detector areconsidered for timing estimation. Further, fixed and dynamicthresholds for energy detection are examined. It is shown thata correlation receiver can provide excellent ranging accuracybut has high computational complexity, whereas an energydetector has low computational complexity and providesgood ranging accuracy.

With the rapid development of network technology andcloud computing, more and more organizations and usersoutsource their data into the cloud server. In order to protectdata privacy, the sensitive data has to be encrypted whichincreases the heavy computational overhead and brings greatchallenges to resource-constraint devices. In the article byZhou et al., ‘‘Secure Index for Resource-constraint MobileDevices in Cloud Computing,’’ the authors propose secureindex based on counting Bloom filter (CBF) for rankedmultiple keywords search. In the proposed scheme, severalalgorithms are designed to maintain and look up CBF, whilea pruning algorithm is used to delete the repeated items forsaving the space. Besides, the relevance scores are encryptedby the Paillier cryptosystem. It ensures that the same rele-vance scores are encrypted into different bits, which can resistthe statistical analyses on the cipher text of the relevancescores. Moreover, since the Paillier cryptosystem supportshomomorphic addition of cipher text without the knowledgeof the private key, the major computing work in ranking couldbe moved from user side to the cloud server side.

In forthcoming years, the valuable information hidden inthe deluge of data will be extracted and utilized in every fieldto improve quality and efficiency. However, the bottleneck ofrealizing this magnificent vista of future intelligent lives liesin how to satisfy the practical demands to transmit huge datavolume through efficient wireless communication in diversescenarios. Herein, multi-scenario wireless communicationtriggers critical problems in wireless channel modeling and

soundings for 5G IoT, which by far, are under-studied. In thearticle by Fan et al., ‘‘Smart Channel Sounder for 5G IoT:From Wireless Big Data to Active Communication,’’ theauthors introduce a general wireless channel model and itsmultiple up-to-date corresponding channel sounding meth-ods for future 5G IoT wireless communications. Throughadopting the perspective of wireless big data excavation,the smart channel sounder transforms the traditional passivewireless communication scheme into an active expectation-guaranteed wireless communication scheme, which helpsachieve efficient and green communication. To demonstratethe validity and efficiency of this smart sounder scheme, wemake a compatible prototype testified in multiple scenarios.The multiple real-scenario experiments demonstrate that thesmart sounder can function effectively, especially in thosescenarios where traditional channel state information (CSI)is not available or imperfect.

Finally, we would like to thank the authors who submit-ted their high quality manuscripts to this Special Section.We would like to acknowledge the contribution of the review-ers who have participated in the review process, and providedhelpful comments and suggestions to the authors to improvetheir manuscripts. We especially thank Professor MichaelPecht, the Editor-in-Chief of IEEE ACCESS, for his adviceand strong support during the process of putting together thisSpecial Section. We also hope that the readers will enjoyreading the articles included in this Special Section.

GUANGJIE HAN, Guest EditorHohai University

Nanjing 210098, China

LIANGTIAN WAN, Guest EditorNanyang Technological University

Singapore 639798

JOEL J. P. C. RODRIGUES, Guest EditorNational Institute of Telecommunications (Inatel) Brazil

Instituto de Telecomunicações PortugalUniversity of Fortaleza (UNIFOR), Brazil

37540-000 Santa Rita do Sapucaí, Brazil

HONGYI WU, Guest EditorUniversity of Louisiana at Lafayette

Lafayette, LA 70503, USA

DAQIANG ZHANG, Guest EditorTongji University

Shanghai 200092, China

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GUANGJIE HAN (S’03–M’05–SM’18) received the Ph.D. degree from Northeastern Uni-versity, Shenyang, China, in 2004. From 2004 to 2005, he was the Production Manager ofZTE Company. From 2005 to 2006, he was the Key Account Manager of Huawei Company.In 2008, he was a Post-Doctoral Researcher with the Department of Computer Science,Chonnam National University, Gwangju, South Korea. From 2010 to 2011, he was a VisitingResearch Scholar with Osaka University, Suita, Japan. In 2017, he was a Visiting Professorwith the City University of Hong Kong, Hong Kong. He is currently a Professor with theDepartment of Information and Communication System, Hohai University, Changzhou, China,and a Distinguished Professor with the Dalian University of Technology, Dalian, China. Hehas authored over 298 papers published in international conference proceedings and journals,including the IEEE COMST, IEEE TII, IEEE TMC, IEEE TVT, IEEE TIE, IEEE TPDS,IEEE TETC, IEEE IoT JOURNAL, IEEE TETCI, IEEE TCC, IEEE SYSTEMS JOURNAL, IEEESENSORS, IEEE WIRELESS COMMUNICATIONS, IEEE Communications, IEEE Network, and so

on. He holds 120 patents. His current H-index is 29 and i10-index is 71 in Google Citation (Google Scholar). The total citationof his papers by other people is more than 3603. His current research interests include the Internet of Things, industrial Internet,mobile computing, artificial intelligence, and security.Dr. Han is a member of ACM. He has served as a reviewer for more than 60 journals. He was a recipient of the ComManTel

2014, ComComAP 2014, Chinacom 2014, and Qshine 2016 Best Paper Awards. He has served as a co-chair for more than50 international conferences/workshops and as a Technical Program Committee member for more than 150 conferences. Hehas served on the editorial boards of up to 16 international journals, including the IEEE Network, the IEEE SYSTEMS JOURNAL,the IEEE ACCESS, the IEEE/CCA JAS, Telecommunication Systems, and so on. He has guest edited a number of special issuesof IEEE Journals and Magazines, including the IEEE Communications Magazine, the IEEE WIRELESS COMMUNICATIONS,the IEEE TRANSACTIONS ON INDUSTRIAL INFORMATICS, Computer Networks, and so on.

LIANGTIAN WAN received the B.S. and Ph.D. degrees from the College of Informationand Communication Engineering, Harbin Engineering University, Harbin, China, in 2011 and2015, respectively. He is currently a Research Fellow with the School of Electrical andElectrical Engineering, Nanyang Technological University, Singapore. He has around 30 paperspublished or under review in refereed international journals (e.g., the IEEE TRANSACTIONSON SIGNAL PROCESSING, the IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, the IEEETRANSACTIONS ON INDUSTRIAL INFORMATICS, the IEEE SYSTEMS JOURNAL, and the IEEEACCESS) and in conferences (e.g., ICASSP, VTC-Spring, and ACM Mobihoc). He holds twopatents.Dr. Wan has been serving as an Associate Editor for the IEEE ACCESS, as the Co-Editor-

in-Chief (Special Issues Editor) for EAI Endorsed Transactions on Industrial Networks andIntelligent, as a TPCmember of several international conferences and as a reviewer of more than20 journals [e.g., the IEEE Communications Magazine, the IEEE Network Magazine, the IEEE

TRANSACTIONS ON VEHICULAR TECHNOLOGY, the IEEE SYSTEMS JOURNAL, the IEEE SENSORS JOURNAL, IET Radar, Sonar& Navigation, the Journal of Network and Computer Applications,Microprocessors and Microsystems (Elsevier), and Securityand Communication Networks (Wiley)].

JOEL J. P. C. RODRIGUES (S’01–M’06–SM’06) received the five-year B.Sc. degree(licentiate) in informatics engineering from the University of Coimbra, Portugal, the M.Sc.and Ph.D. degrees in informatics engineering from the University of Beira Interior, Covilhã,Portugal, the Habilitation in computer science and engineering from the University of HauteAlsace, France, and the Academic Title of Aggregated Professor in informatics engineeringfrom UBI. He is a Professor and a Senior Researcher with the National Institute of Telecom-munications, Brazil, and a Senior Researcher with the Instituto de Telecomunicações, Portugal.He has been a Visiting Professor at the University of Fortaleza, Brazil. He has authored or co-authored over 650 papers in refereed international journals and conferences, three books, andtwo patents. His main research interests include e-health, sensor networks and the IoT, vehicularcommunications, and mobile and ubiquitous computing. He is a member of many internationalTPCs and has participated in several international conferences. He had been awarded severalOutstanding Leadership and Outstanding Service Awards by IEEE Communications Society

and several best papers awards. He is a Licensed Professional

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Engineer (as a Senior Member), a member of the Internet Society, and a Senior Member of ACM. He is the Leader of theInternet of Things Research Group (CNPq), the Director of the Conference Development–IEEE ComSoc Board of Governors,IEEE Distinguished Lecturer, the Technical Activities Committee Chair of the IEEE ComSoc Latin America Region Board,the President of the Scientific Council at ParkUrbis–Covilhã Science and Technology Park, the Past-Chair of the IEEE ComSocTechnical Committee on eHealth, the Past-Chair of the IEEE ComSoc Technical Committee on Communications Software,a Steering CommitteeMember of the IEEE Life Sciences Technical Community and the Publications Co-Chair, and theMemberRepresentative of the IEEE Communications Society on the IEEE Biometrics Council. He has been the General Chair and TPCChair of many international conferences, including IEEE ICC, IEEEGLOBECOM, IEEEHEALTHCOM, and IEEE LatinCom.He is the Editor-in-Chief of two international journals and editorial board member of several highly-reputed journals.

HONGYI WU received the B.S. degree in scientific instruments from Zhejiang University,Hangzhou, China, in 1996, and the M.S. degree in electrical engineering and Ph.D. degree incomputer science from the State University of New York at Buffalo, in 2000 and 2002, respec-tively. He is the Batten Chair in cybersecurity and the Director of the Center for CybersecurityEducation and Research, Old Dominion University (ODU), where he is also a Professor with theDepartment of Electrical and Computer Engineering. Before joiningODU, hewas anAlfred andHelen Lamson Endowed Professor at the Center for Advanced Computer Studies, Universityof Louisiana at Lafayette (UL Lafayette). His research focuses on networked cyber-physicalsystems for security, safety, and emergency management applications, where the devices areoften light-weight, with extremely limited computing power, storage space, communicationbandwidth, and battery supply. He received the NSF CAREER Award in 2004 and the ULLafayette Distinguished Professor Award in 2011.

DAQIANG ZHANG received the joint supervised Ph.D. degree in computer science from Shanghai Jiao Tong Universityand Hong Kong Polytechnic University. From 2011 to 2012, he held a postdoctoral position with the Institute Mines-SudParisTélécom, France. He is an Associate Professor with the School of Software Engineering, Tongji University. He isa Senior Member of CCF.

His research includes mobile computing, distributed computing, and wireless sensor networks. He has published morethan 80 papers in major journals and international conferences in those above areas. He has got papers published in ACMTransactions on Embedded Computing, the IEEE TRANSACTIONS ON PARALLEL AND DISTRIBUTED SYSTEMS, the IEEETRANSACTIONS ON COMPUTERS, the IEEE TRANSACTIONS ON EMERGING TOPICS IN COMPUTING, the IEEE TRANSACTIONS ON

INDUSTRIAL INFORMATICS, the IEEE WIRELESS COMMUNICATIONS, the IEEE Network, the IEEE SYSTEMS JOURNAL, the IEEECOMMUNICATIONS, the IEEE COMMUNICATIONS LETTER, UBICOMP, MASS, GLOBECOM, HPCC, ICPP, ICC, and WCNC.He received the ‘‘mostly downloaded paper up to 31 March, 2014’’ at ACM/Springer MONET, the ‘‘most viewed paper up to31 December, 2013’’ at Elsevier PMC, and the Best Paper Award from ACCV’2009 and IEEE UIC’2012.

He was the Guest Editor of the IEEE SYSTEMS JOURNAL, the IEEE ACCESS, Computer Networks (Elsevier), the Journal ofUniversal Computer Science, and ACM/Springer MONET. He is an Editor of Telecommunication Systems (Springer), EuropeanTransactions on Telecommunications (Wiley),KSII Transactions on Internet and Information Systems (Korea Society of InternetInformation), and New Review of Hypermedia and Multimedia (Taylor & Francis).

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