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Review

Device-to-Device Communication in Cellular Networks: A Survey

Pimmy Gandotra, Rakesh Kumar Jha n

Shri Mata Vaishno Devi University, Block C, Room No. - 104, Katra Kakrayal, Dist. - Udhampur 182320, Jammu and Kashmir, India

a r t i c l e i n f o

Article history:Received 16 January 2016Received in revised form5 May 2016Accepted 4 June 2016

Keywords:Mobile network operators (MNOs)Peer discoveryUltra dense networks (UDNs)Millimeter wave (mmWave)Next generation networks (NGNs)Device-to-Device (D2D) communication

x.doi.org/10.1016/j.jnca.2016.06.00445/& 2016 Elsevier Ltd. All rights reserved.

esponding author.ail address: [email protected] (R.K. Jha

e cite this article as: Gandotra, P., JhComputer Applications (2016), http:/

a b s t r a c t

A constant need to increase the network capacity for meeting the growing demands of the subscribershas led to the evolution of cellular communication networks from the first generation (1G) to the fifthgeneration (5G). There will be billions of connected devices in the near future. Such a large number ofconnections are expected to be heterogeneous in nature, demanding higher data rates, lesser delays,enhanced system capacity and superior throughput. The available spectrum resources are limited andneed to be flexibly used by the mobile network operators (MNOs) to cope with the rising demands. Anemerging facilitator of the upcoming high data rate demanding next generation networks (NGNs) isdevice-to-device (D2D) communication. An extensive survey on device-to-device (D2D) communicationhas been presented in this paper, including the plus points it offers; the key open issues associated with itlike peer discovery, resource allocation etc, demanding special attention of the research community;some of its integrant technologies like millimeter wave D2D (mmWave), ultra dense networks (UDNs),cognitive D2D, handover procedure in D2D and its numerous use cases. Architecture is suggested aimingto fulfill all the subscriber demands in an optimal manner. The Appendix mentions some ongoingstandardization activities and research projects of D2D communication.

& 2016 Elsevier Ltd. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.1. Contributions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2. The roadmap to device-to-device (D2D) communication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22.1. First generation (1G) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.2. Second generation (2G) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.3. Third generation (3G) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.4. Fourth generation (4G) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.5. Fifth generation (5G) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

3. Outline of device-to-device (D2D) communicaion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44. Integrant features of D2D . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

4.1. Millimeter wave D2D communication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64.2. Cooperative D2D communication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74.3. Handover in device-to-device communication. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74.4. Hybrid automatic repeat request (HARQ) operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84.5. D2D ultra dense networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84.6. Cognitive D2D . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94.7. Network coding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

5. Key open challenges in D2D . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105.1. Peer discovery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105.2. Resource allocation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

5.2.1. Network model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
5.3. Power control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
).

a, R.K., Device-to-Device Communication in Cellular Networks: A Survey. Journal of Network/dx.doi.org/10.1016/j.jnca.2016.06.004i

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5.4. Interference management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145.5. Security . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

6. Application areas of D2D communication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15Appendix A. Standardization activities for D2D . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16Appendix B. Ongoing projects on D2D . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16Appendix C. Abbreviations used in paper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

1. Introduction

Today the number of hand-held devices is drastically increas-ing, with a rising demand for higher data rate applications. In or-der to meet the needs of the next generation applications, thepresent data rates need a refinement. The fifth generation (5G)networks are expected and will have to fulfill these rising de-mands. A competent technology of the next generation networks(NGNs) is Device-to-Device (D2D) Communication, which is ex-pected to play an indispensable role in the approaching era ofwireless communication. The use of D2D communication did notgain much importance in the previous generations of wirelesscommunication, but in 5G networks, it is expected to be a vitalpart. The rising trends (Astely et al., 2013) pave way for thisemerging technology. With the introduction of device-to-device(D2D) communication, direct transmission between devices ispossible. This is expected to improve the reliability of the linkbetween the devices, enhance spectral efficiency and system ca-pacity (Chai et al., 2013), with reduced latency within the net-works. Such a technique is essential for fulfilling the chief goals ofthe mobile network operators (MNOs).

D2D communication allows communication between two de-vices, without the participation of the Base Station (BS), or theevolved NodeB (eNB). Proximate devices can directly communicatewith each other by establishing direct links. Due to the smalldistance between the D2D users, it supports power saving withinthe network, which is not possible in case of conventional cellularcommunication. It promises improvement in energy efficiency,throughput and reduce delay. It has the potential to effectivelyoffload traffic from the network core. Hence, it is a very flexibletechnique of communication, within the cellular networks.

Qualcomm's FlashLinQ (Wu et al., 2010) was the first endeavortowards the implementation of device to device (D2D) commu-nication in cellular networks. It takes advantage of orthogonalfrequency division multiple access (OFDMA) in conjunction withdistributed scheduling for peer discovery, link management andsynchronization of timings. Another organization involved in ex-amining D2D communication in cellular networks is 3GPP (ThirdGeneration Partnership Project) (3GPP, 2013a,, 2014a,, 2013b). D2Dcommunication is under investigation by the 3GPP as ProximityServices (ProSe). It is expected to function as a public safety net-work feature in Release 12 of 3GPP. The task of standardization ofdevice-to-device communication and the ongoing projects arebriefly discussed in Appendix A and B. A next generation networkscenario, supporting device-to-device (D2D) communication alongwith some general use cases is depicted in Fig. 1. The most popularuse cases of D2D include public safety services, cellular offloading,vehicle-to-vehicle (V2V) communication, content distribution.

In spite of the numerous benefits offered by device-to-device(D2D) communication, a number of concerns are involved with itsimplementation. When sharing the same resources, interferencebetween the cellular users and D2D users needs to be controlled.For this, numerous interference management algorithms havebeen proposed in literature. Other concerns include peer discovery

Please cite this article as: Gandotra, P., Jha, R.K., Device-to-Device Coand Computer Applications (2016), http://dx.doi.org/10.1016/j.jnca.20

and mode selection, power control for the devices, radio resourceallocation and security of the communication.

1.1. Contributions

Existing surveys (Liu et al., 2014; Asadi et al., 2014) on device-to-device (D2D) communication provide an extensive literature onthe various issues in D2D communication. The authors in Liu et al.(2014) comprehensively describe the state-of-the-art researchwork. on D2D communication in LTE-Advanced networks. In Asadiet al. (2014), the literature available on D2D communication ispresented as Inband D2D and Outband D2D. This survey, on theother hand, draws upon the growing need for switching towardsthe device-to-device (D2D) technology. Architecture for device-to-device (D2D) communication has been proposed, which clearlydepicts the scenario of the next generation networks (NGNs) andis the prime focus of this survey. It aims to aid the cellular net-works in near future by allocating resources optimally to the D2Dusers in the network and the cellular users as well, with the use ofsectored antennas at the base station (BS). Such architecture hasthe potential to efficiently serve the rising demands of the sub-scribers and meet the requirements of the network operators.Additionally, a mathematical analysis has been discussed, which isthe basis of any resource allocation technique, for analyzing net-work throughput. Number of features can be integrated with D2Dcommunication, to enhance its utility in existing cellular systems.These have been discussed in this survey. A number of challengesexist, pertaining to the implementation of device-to-device (D2D)communication. Few important algorithms in relation to theseissues have been discussed. Thus, focus of this survey is to briefabout different aspects of D2D communication.

The organization of the survey is as follows: Following the in-troduction, a roadmap to D2D communication has been presentedin Section II. An overview of device-to-device (D2D) communica-tion has been presented in Section III. The various features whichcan be integrated with D2D communication to further enhancetheir utility and performance in cellular networks are discussed inSection IV. Incorporating D2D communication in existing cellularnetworks engenders a number of challenges, which have beendiscussed in Section V. Architecture has been proposed in thissection, to overcome the issue of radio resource management.Since the architecture uses sectored antennas at the base station,interference between D2D users and cellular users within thenetworks is overcome to a large extent. In the next generationnetworks, a number of applications are expected to be supportedby D2D communication, and are discussed in Section VI. Lastly, thepaper concludes in Section VII.

2. The roadmap to device-to-device (D2D) communication

Telegraphy was demonstrated by Joseph Henry and Samuel F.B.Morse, in 1832. In 1864, James Clerk Maxwell postulated wirelesspropagation, which was verified and demonstrated by Heinrich

mmunication in Cellular Networks: A Survey. Journal of Network16.06.004i




P. Gandotra, R.K. Jha / Journal of Network and Computer Applications ∎ (∎∎∎∎) ∎∎∎–∎∎∎ 3

Hertz in 1880 and 1887, respectively. Marconi and Popov startedexperiments with the radio-telegraph shortly thereafter, andMarconi patented a complete wireless system in 1897. Thesemarked the advent of wireless communication.

The very first wireless networks were discovered during thepre-industrial age. These primarily were based on Line of Sight(LOS) transmissions. These networks were then replaced by thetelegraph, and later by the telephone. After the invention of thetelephone, Marconi demonstrated the first radio transmission.Thereafter, radio technology rapidly gained importance as trans-missions over very large distances was possible with better qual-ity, low cost and less power.

Initially, only analog data transmission took place. Then therewas a shift towards digital data transmission. The generations ofwireless networks have evolved from first generation (1G) to fifthgeneration (5G). A brief overview of the generations in connectionto D2D communication has been given in this section.

2.1. First generation (1G)

This generation of wireless communication came into existence inthe early 1980s and supported data rates up to 2.8 Kbps. Thesenetworks were circuit switched. The analog cellular technology wasreferred to as Analog Mobile Phone Service (AMPS) and it usedFrequency Division Multiplexing (FDM). These were completely in-secure networks and required large power consumption. Also,quality of calls was very low. Due to less number of subscribersduring this era, the need for direct transmission was never felt.

2.2. Second generation (2G)

It is digital cellular. It came into existence in the late 1990s. Thefirst second generation (2G) system was Global System for Mobile

Table 1Comparison of D2D with wlan and bluetooth.

Feature considered Bluetooth Wlan

Pairing Require manual pairing Require user deQuality of Service (QoS) No hard QoS guarantee No hard QoS guSpectrum Unlicensed UnlicensedStandardization Bluetooth SIG IEEE 802.11Maximum Data rate 25 Mb/s 54 Mb/sModulation Technique GFSK DSSSMax.Transmission Distance 10–100 m 32 mForward Error Correction ARQ, FEC (MAC) ARQ, FEC (PHYMax Transmit Power 4 dBm 15 dBmPricing Free of cost Free of cost

Fig. 1. A General Scenario supporting device-to-device (D2D) communication.


(GSM). It supported a maximum data rate of up to 64 kbps. Othertechnologies included in it are Code Division Multiple Access(CDMA) and IS-95. It provided services like email and short mes-sage service (SMS) (Santhi et al., 2003; Halonen et al., 2003). Thesenetworks are more secure against eavesdropping, as compared tothe 1G network. This generation could not handle complex datalike videos.

Between 2G and 3G came another generation, 2.5G. Data ratesof up to 200 kbps were supported in 2.5G. Technologies includedGeneral Packet Radio Service (GPRS) and Enhanced Data Rate forGSM Evolution (EDGE). No direct communication was introducedin wireless communication till this period.

2.3. Third generation (3G)

Data rates supported by the third generation networks are up to2 Mbps. These came in late 2000 and support services with improvedvoice quality and help maintain better Quality of Service (QoS). Thetechnologies supported by 3G include Wideband Code Division Mul-tiple Access (WCDMA), Universal Mobile Telecommunication System(UMTS), and Code Division Multiple Access (CDMA) 2000. Technolo-gies like Evolution-Data Optimized (EVDO), High Speed Uplink/Downlink Packet Access (HSUPA/HSDPA) form a part of 3.5G andprovide improved data rates in comparison to 3G. Though 3G is moreadvantageous than 2G, it requires more power than 2G networks andis costlier than 2G in terms of the plans it offers. In this generation,WLAN and Bluetooth gained popularity and allowed direct commu-nication between devices. These techniques function in the unlicensedband, like in the industrial, scientific and medical (ISM) band, notmeeting the Quality of Service (QoS) requirements of the networkefficiently. Licensed band is more capable of handling the interferenceissue, thereby meeting the QoS needs of the cellular networks. As aresult, interference management is possible with the help of a centralcontrolling entity in the cellular network (the base station). With D2Dcommunication underlaying cellular networks, direct transmissionbetween devices is possible in the licensed spectrum. Thus, D2Dcommunication in cellular networks was introduced in the next gen-erations. A comparison of Bluetooth andWLAN technologies with D2Dcommunication has been shown in Table 1.

2.4. Fourth generation (4G)

Further enhancement in data rates are provided by 4G net-works. These provide a system completely based on internetprotocol (IP). Applications supported by 4G networks includeMultimedia Messaging Service (MMS), Digital Video Broadcasting(DVB), HDTV, video chatting etc. Technologies include Long TermEvolution Advanced (LTE-A) and Mobile Worldwide Interoper-ability for Microwave Access (WiMAX). 4G networks are referredto as MAGIC: Mobile multimedia, Anytime anywhere, Global Mo-bility Support, Integrated wireless solution and Customized

D2D communication

fined settings for access points Base station assisted or device assistedarantee Provides hard QoS guarantees

Licensed, Unlicensed3GPP Release 125–10 Gb/sSC-FDMA (Uplink), OFDMA (Downlink)Up to 500 m

) Low Density Parity Check codes (LDPC)24 dBmOperator decides the cost





GrowthinTraffic

(Exabytesp erm

o nth)

2014 201520

40

60

80

100

120

140

2016 2017 2018 2019Years

160

180

59.9

72.4

88.4

109

135.5

168

Fig. 3. Rising Trends in D2D (Cisco, 2014).


Personal Service. Long Term Evolution-Advanced (LTE-A) in-troduced device-to-device (D2D) communication in cellularnetworks.

2.5. Fifth generation (5G)

The fifth generation (5G) of wireless communication is the nextgeneration networks. 4G systems will soon be replaced by 5G in orderto fulfill the increasing demands of the subscribers for higher datarates and support numerous applications. It includes various enhancedtechnologies like Beam Division Multiple Access (BDMA) and Non-and quasi-orthogonal or Filter Bank multi carrier (FBMC) multipleaccess. 5G is the result of an aggregation of numerous technologieslike, mmWave communication, Massive MIMO, Cognitive Radio Net-works (CRNs), Visible light communication (VLC). The first four gen-erations were completely dependent upon the base station (BS), thuscalled network centric. But 5G is heading towards device-centric ap-proach, i.e. network setup and managed by the devices themselves.Device-to-Device (D2D) Communication is being considered as anessential component of the 5G networks. It is expected to result in anenhanced system capacity, increased spectral efficiency, betterthroughput and reduced latency. An overview of the eras of wirelesscommunication and the services supported by them is depicted inFig. 2. A detailed overview of the evolution of generations of wirelesscommunication has been given in Gupta and Jha (2015).

There has been a drastic growth in traffic over the years, andwill continue in the years to come, as depicted in Fig. 3 (Cisco,2014). This results in overloading at the base station (BS). Due tothis mounting load on the base station (BS), there is an increase inthe demand for power. To overcome this need for high power,some amount of traffic needs to be offloaded from the base stationand here D2D communication plays a crucial role. Since D2Dcommunication allows devices to communicate with each otherwithout traversing the base station, load on the base station ishighly reduced.

Fig. 2. Generations of wireless communication.


3. Outline of device-to-device (D2D) communicaion

Taking into consideration the architecture perspective, D2Dcommunication networks appear to be similar to Mobile Ad-hocNetworks (MANETs) and Cognitive Radio Network (CRN). MANETsis a collection of mobile nodes which form a temporary networkwithout the assistance of any centralized administrator. These aregenerally multi hop networks. A number of challenges are faced byMANETs which prevent them from providing the required Qualityof Service (QoS) guarantees. These challenges include unreliablewireless channel, contention of the wireless channel, and lack of acentralized control. The nodes in MANETs suffer from severe re-source constraints. In case of cognitive radio networks (CRNs),spectrum sensing is a big challenge (Bansal et al., 2007). The CRphysical layer aspects also have to be addressed, in order to exploitits utility completely. Other issues in CRNs include white spacedetection, collision avoidance, and synchronization.

In comparison to MANETs and CRNs, D2D networks can beeither base station (BS) controlled or device controlled. When basestation controlled, issues in MANETs and CRN can be overcome byD2D communication. The users in cognitive radio networks (CRNs)(Cheng et al., 2012) are identified as primary or secondary, whichis possible in D2D communication also. But, cognitive sensing andautonomous functioning of CRNs is not supported in D2D com-munication. The challenges of QoS provisioning existing in MAN-ETs are overcome by D2D communication. A brief comparison ofD2D communication and MANETs has been depicted in Table 2.Due to the benefits offered, device-to-device (D2D) communica-tion is being looked upon as an effective technique to meet therising user demands. It supports development of new applicationsand data offloading which is a significant contribution of thistechnique.

The fifth generation (5G) cellular networks, with Device-to-Device (D2D) Communication enabled within is considered astwo-tier networks. The two tiers in these networks are referred toas the macro cell tier and the device tier. Conventional cellularcommunication is supported by the macro cell tier, while D2Dcommunication is supported by the device tier. These cellularnetworks thus are similar to the existing networks. The differencelies in the fact that faithful services can be achieved by the devicesat the cell edges and those in the congested areas within the cell.As devices in the device tier allow direct D2D communication, thebase station may have a partial control or a full control over thecommunication between the devices. Thus, device to device (D2D)communication in the device tier is categorized into four differenttypes (Tehrani et al., 2014):





Table 2Manets V/S D2D Communiacation.

MANETs D2D Communication

Multi-hop Networks One-hop networksNo QoS guarantee QoS guaranteeNo improvement in Spectral efficiencysupported

Improvement in spectral efficiencysupported

Less security guarantee Better security guaranteeNo centralized control Centrally control by the base station

(either fully or partially)Manual connectivity Seamless association, subject to ful-

fillment of distance constraintNo handover phenomenon Handover phenomenon is possiblePoor Resource utilization; power con-straint based resource utilization

Efficient resource utilization

Fig. 5. Direct communication between devices with operator controlled linkestablishment.


(1) Device relaying with controlled link establishment from the operatorDevices at the cell edges or in poor coverage areas are capable ofcommunicating with the base station (BS) by relaying informationthrough other devices. All tasks of establishing the communica-tion between the devices are handled by the base station (BS). Thebattery life of the devices is enhanced this way. The architecture isas shown in Fig. 4.

(2) Direct communication between devices with controlled link es-tablishment by the operatorTwo devices communicate directly with each other, withcontrol links provided by the base station. Though direct, thecommunication is entirely managed by the base station. Sincein (1) and (2), a central controlling entity, i.e. the base station(BS) is present, interference management is possible. Thearchitecture is as shown in Fig. 5.

(3) Device relaying with controlled link establishment from the de-viceTwo devices communicate via relays, within the cellular net-works. Resource allocation, setting up of call, interferencemanagement, all is managed by the devices themselves, in adistributive fashion. Control of the base station is missing. Thearchitecture is as shown in Fig. 6.

(4) Direct communication between devices (Direct d2d) with con-trolled link establishment by the device

Fig. 4. Relaying Devices with controlled link establishment from the operator.


Devices communicate directly, without aid from the base station(BS). Call setup and management are handled by the devicesthemselves, as in (3). The architecture is as shown in Fig. 7.

The two-tier cellular network architecture is advantageous overthe conventional cellular architecture. The benefits offered are asfollows:

1. One hop communication: the devices can communicate witheach other through a single hop. Lesser resources are then re-quired for the communication, resulting in an efficient utiliza-tion of the spectrum. Since proximity users directly

Fig. 6. Relaying device with device controlled link establishment.





Fig. 7. Direct communication between devices (Direct D2D) with device controlledlink establishment.

Fig. 8. D2D integrant features.


communicate with each other in D2D communication, latency isgreatly reduced. These are desirable aspects in a cellular net-work. The mobile network operators are also benefitted bythese aspects of D2D communication.

2. Spectrum Reusability: with D2D communication in cellular net-works, same spectrum is shared by the D2D users as well as thecellular users. This supports spectrum reusability, thereby im-proving the spectrum reuse ratio.

3. Optimization of Power Levels: since D2D links exist betweenproximate devices, over a small distance, transmission power isless. This enhances the battery life of the devices. As a result,higher energy efficiency can be achieved with D2D commu-nication in cellular networks.

4. Improved Coverage Area: as discussed in (1) and (3), D2Dcommunication is possible with relays. This supports commu-nication over greater ranges, thus increasing the overall cover-age area.

Optimal density of D2D users in a network is demonstratedin Liu et al. (2012). In spite of the number of advantages that areoffered by D2D communication over the conventional cellularcommunication, some limitations exist. The authors in Hakolaet al. (2010) discuss about possibility of use of D2D communicationwithin the cellular systems. Feasibility of D2D communication isdetermined by the distance restriction. Another concern is theinterference, which may be between the users of the same tier ordifferent tiers. In cases of base-station assisted D2D communica-tion ((1) and (2)), the BS essentially acts as a central controllingentity and can overcome interference problem to some extent. Thebase station (BS) manages spectrum allocation and aids in avoid-ing interference among the devices. In device-assisted D2D com-munication ((3) and (4)), there is no central controlling entity.These communication techniques are more challenging than theother two. For optimum performance of D2D communication inthe cellular networks, smart interference management schemes,supporting optimal resource allocation need to be designed. Aconsiderable amount of literature is available in this context, andoffers a wide range of opportunities for the researchers to furtherexplore these areas.


Prior to direct transmission of information between the de-vices, they need to find each other. Device discovery can be pos-sible by a periodic broadcasting of the device identity. Distanceconstraint is generally considered, for D2D pair formation. Peerdiscovery and mode selection is an open research issue in device-to-device (D2D) communication. For any cellular network, a majorconcern is security. When exchanging information through relays,as in (3), network security must be assured. This can be madepossible by ‘closed access’, where a list of trusted devices is pre-pared by every device belonging to the device tier. If a device,under the relay scenario, does not find some devices in its own list,it communicates in the macro cell tier. The issue of security, withmachine-to-machine (M2M) communication taken as reference, isdiscussed in Cha et al. (2009), Yue et al. (2013), Perrig et al.(2004), Zhou et al. (2008) and Muraleedharan and Lisa (2006). Thebase station has the capability to authenticate the devices that areacting as relays, and use encryption to maintain privacy for theinformation of devices.

4. Integrant features of D2D

Originally, the concept of device-to-device (D2D) communica-tion was used for sensor networks, ad hoc networks and meshnetworks. The devices communicated in a distributive fashion, inthe industrial, scientific, medical (ISM) band, in the absences ofany controlling entity. Nowadays, however, in LTE-A and the nextgeneration networks (NGNs), D2D communication is gaining po-pularity for use in the licensed band. Formation of direct links isuseful for the improvement in the overall network performance,and also to the devices in terms of energy efficiency and com-plexity. A number of features of 5G networks can be integratedwith device-to-device (D2D) communication (Fig. 8.), which actsas an enabler for D2D communication in the existing cellularnetworks. Some of these have been briefly listed below.

4.1. Millimeter wave D2D communication

A promising technology of the future 5G networks is millimeterwave (mmWave) communication, providing multi-gigabits-per-second to the user equipments (UEs). It operates over a wide fre-quency band of 30 GHz to 300 GHz. Efficient utilization of thebandwidth is feasible by enabling device-to-device (D2D) com-munication in the next generation networks. Using D2D commu-nications in mmWave cellular networks, a number of direct con-current links can be supported, resulting in an improved networkcapacity. Also, simultaneous connections can be supported inmmWave networks due to the highly directional antennas andhigh propagation loss in mmWave communication. A schedulingmechanism for downloading of popular content in mmWave small






cells, exploiting D2D transmissions has been proposed in Niu et al.(2015), resulting in an overall improvement in transmission effi-ciency. As per the simulation results, the proposed scheme resultsin reduced latency and enhanced throughput. This clearly revealsthe benefits of D2D communication at mmWave frequencies.

In mmWave 5G cellular networks, two types of D2D commu-nications are possible - local D2D communication and global D2Dcommunication. In local D2D communications, if the LOS path isblocked, then a path is developed between the two devices asso-ciated with the same base station, with the help of relays or di-rectly. In global D2D communications, devices associated withdifferent BSs are connected through the backbone networks, viahopping. But, D2D connections in mmWave networks can sufferinterference (Qiao et al., 2015). This is possible in case there aremultiple D2D communications within the cellular network (localD2D communications).

Coexistence of local and global D2D communication in thenetwork results in interference between local D2D communica-tions and between B2B/D2B (Base station to base station/ Device toBase station) communications. Due to the highly directional natureof mmWave communication, high data rate B2B communicationsare supported in the cellular networks. Since mmWave commu-nication use directional antennas, resource sharing schemes musttake into account the directional interference as well for suchscenarios, for efficient spectrum utilization. Although use of di-rectional antennas is advantageous in terms of enhanced networkcapacity and spatial reuse, there are certain challenges also asso-ciated with it. Generally, problems arise in case of neighbor dis-covery, like deafness problem, and tend to promote research inthis field. The problem related to blockage and directionality inmmWave communication has been solved using cross-layermodeling and design techniques (Singh et al., 2007). A majorproblem for mmWave propagation is unavailability of a standardchannel model.

4.2. Cooperative D2D communication

Cooperative communication is a focal technology in the cellularnetworks today. For D2D communication, their impact is expectedto be remarkable. When the D2D pairs are far away from eachother, the direct link between the users is not good enough forcommunication (Cao et al., 2015). Here is where cooperation plays

Fig. 9. Cooperative D2D communication; R serves as Relay for D2D


an essential role. Cooperation aids in improving the quality of D2Dcommunication for data offloading between the UEs. It enablesinterference reduction as well as increased network coverage. Inorder to depict cooperative networking, a scenario of cooperativecommunication has been shown in Fig. 9.

In case of cooperative D2D communication, the networkadaptively decides the communication mode as underlay, overlayor cooperative relay mode on the basis of channel quality and datarate requirement. Selection of relay is an important issue in co-operative D2D networks. Since a large number of relays can beused, the relay selection needs to be optimum and efficient. Relayselection methods have been proposed in Wang et al. (2012).These algorithms help the BS to choose the best of all the relays. Incase of relay selection, generally the BS is assumed to play a pas-sive role because using centralized methods in the selection pro-cess increases load on the BS. Selecting relays in a distributivemanner eliminates relays that are not proper.

Cooperation is based on social reciprocity, and trust, discussedin Chen et al. (2015). The authors have evaluated an efficient D2Dcooperation strategy by proposing a game-theoretic approach. Acooperative multi-hop D2D scenario is discussed in Carpio et al.(2015), which results in boosting of data rate. Though cooperationcontributes towards improving system performance and QoS, buta large amount of UE power is also consumed. This needs to beoptimized. In literature, cooperation among D2D users and alsobetween D2D users and cellular users is widely studied.

4.3. Handover in device-to-device communication

When devices are undergoing D2D communication, they enterinto the neighbor cells at some point or the other. When the twoUEs are in close proximity, they undergo a joint handover. Undercertain circumstances, the devices may not be in proximity or oneof them may get handed over to some neighboring cell, resultingin a half handover. Very less literature is available on handover ofD2D communication.

A basic and effective handover algorithm is the handover de-cision method (Chen et al., 2015), involving the use of a number ofvariables referred to as, Handover Margin (HOM), Time to Trigger(TTT) timer, LTE threshold (LTEth), D2D threshold (D2Dth) and Timeto Trigger of D2D (TTTD).

HOM is a constant variable representing a threshold of the

communication in both the scenarios, supporting cooperation.





Fig. 10. Regular handover scenario, (a) before handover; (b) after handover fromcell1 to cell2.

Fig. 11. HARQ process.


difference between the strength of the received signal to thesource eNB and the strength of the received signal to the targeteNBs. The strength of the received signal is called reference signalreceiving power (RSRP), in an LTE system. It is ensured by thevalue of HOM that the target eNB is the most appropriate forProximity Services (ProSe). Value of TTT is the time interval re-quired to satisfy HOM condition, as stated in Chen et al. (2015).Once TTT condition is satisfied, then the handover action can besuccessfully completed. Different values of TTT can be used byProSe UEs. Unnecessary handovers, called “Ping-Pong effect” canbe reduced by HOM and TTT. LTE Threshold (LTEth) is a constantvariable which represents whether the basic services can be pro-vided by the source eNB to the ProSe UEs or not. D2D threshold(D2Dth) is used to check the radio signal strength of D2D quality.The conditions for triggering handover are

> + ( )RSRP RSRP HOM 1T S

> ( )HOTrigger TTT 2

Here, RSRPT and RSRPS are the values of RSRP from target andsource eNBs, respectively. THE HOTrigger is the handover triggertimer which turns on as soon as (1) is satisfied. The handoverdecision is made by the eNB provided all requisite conditions aresatisfied. On the basis of the D2D handover decision method, ajoint or a half handover procedure can be selected, or even nohandover. In case of joint handover, a collective handover of all thedevices takes place to the target eNB, while in case of half hand-over; one of the UEs is handed over to the target eNB while theother remains connected to the source eNB. When handover oc-curs, there is exchange of some unnecessary control overhead aswell, between the devices. A general handover scenario has beendepicted in Fig. 10, representing handover of UE1 from one basestation to another (BS1 to BS2). Mobility management solutionshave been provided in Yilmaz et al. (2014) where two schemes forsmart mobility management have been proposed: D2D-awarehandover and D2D-triggered handover. The simulation results il-lustrate that these schemes reduce end-to-end latency in the D2Dcommunication and reduce signaling overhead as well, within thenetwork. Vertical and horizontal handover are efficient for redu-cing energy consumption in heterogeneous networks (Radwanand Rodriguez, 2015).

4.4. Hybrid automatic repeat request (HARQ) operation

Automatic Repeat Request (ARQ) retransmission and forwarderror correction are combined in HARQ. It tends to make D2Dcommunication more robust. In D2D communication, two types ofHARQ exist- Direct and Indirect (Mumtaz et al., 2014). In case ofindirect HARQ, an ACK/NACK is sent by the D2D receiver to theeNB which is then further relayed to the D2D transmitter. Reusingof uplink and downlink channels is possible with indirect HARQ.The D2D receiver directly sends an ACK/NACK to the D2D trans-mitter, in case of direct HARQ. It can be used either in-coverage oran out of coverage scenario.

A cellular HARQ phenomenon has been depicted in Fig. 11. Thefigure shows multicasting of packets by the BS to the UEs in thenetwork. The HARQ feedback message provides the receivingstatus of the packets, at the UEs. Depending on whether a packet isreceived or not, an acknowledgement/negative acknowledgement(ACK/NACK) is sent by the UEs. In case the BS receives a NACK, itretransmits the packet. This technique consumes a large amount ofenergy and involves significant signaling overhead. A compressedHARQ mechanism has been proposed in Du et al. (2012), in anetwork with underlay D2D communication, which provides


better results in terms of signaling overhead. This mechanism ishighly efficient for multicast services and performs better thanconventional D2D multicast. The authors of Miao et al. (2014)propose a cross layer design based on HARQ. Three types of HARQhave been discussed in this design: Type I HARQ, Type II HARQ andType III HARQ. Using HARQ and cross layer optimization effectivelyresults in improving the D2D transmission rate, and throughput.Thus, incorporation of HARQ in D2D communication results in anefficient error correction within the network.

4.5. D2D ultra dense networks

An important concern of operators today is offloading cellulardata. With the growth in the use of smart phones and tablets, thecore and access networks tend to overload. Traffic offloading isnecessary in such a scenario, so as to free up the loaded path byproviding alternate paths to the traffic. In 3GPP system





Table 3Comparison of 3GPP offloading solutions.

S No Feature SIPTO LIPA D2D Communication

1. Definition Offloads selected IP traffic to the internet locally,as well as at macrocellular access networks

Allows offloading traffic directly to a local net-work, which is connected to the same H(eNB), asthe UE

Offloads traffic at the radio access network,as well as the core network

2. Qos Not maintained Not maintained Maintained3. Offload Points At or above eNBs At or above eNBs Data Offload points positioned at mobile

terminals4. 3GPP Release Rel 10 Rel 10 Rel 12

Fig. 12. Ultra dense networks with D2D communication.


architecture (Release 10), two offloading solutions have beenprovided, which are Local IP Access (LIPA) and Selected IP TrafficOffload (SIPTO) (3GPP, 2011). Another offloading technique from3GPP is device-to-device (D2D) communication. D2D offloadingcapability is more advantageous in comparison to LIPA and SIPTO,as D2D offloading avoids radio congestion as well, apart fromoffloading the core network. This results in an enhancement innetwork capacity. The offloading solutions from 3GPP have beencompared in Table 3. A detail of these techniques is providedin Yang et al. (2013).

Apart from these techniques, small cells provide an efficientmeans for offloading the traffic, and aid the other offloadingtechniques as well. With cell size getting smaller over the gen-erations, there is less competition among the users for resources,yielding a substantial increase in spectrum efficiency. Small cellsinclude picocells, microcells and femtocells, varying in the cellsizes and transmission power. Deployment of a large number oflow power small cell base station (SBSs) results in ultradensenetworks (UDNs). Such a deployment helps in frequency reuse andcontrols interference. Ultra Dense networks (UDNs) have morenumber of nodes (UEs) per unit area. UDN has recently been ac-cepted as an important enabling technology for enhancing thenetwork capacity.

D2D along with small cells (Malandrino et al., 2014), both play akey role in offloading traffic from the eNB. D2D mainly focuses onoffloading proximity services while hot-spot traffic is offloaded bythe small cells. Integration of these two technologies results in Ultra-dense 5G deployments, as shown in Fig. 12. UDNs as an importantcomponent of the next generation networks (NGNs) have been dis-cussed in Baldemair et al. (2015). It is expected to enable higher datarates and lower delays within the network. Working of UDN in themmWave band will result in a contiguous bandwidth of about 2 GHz.The system level performance of UDN has been evaluated in Chenet al. (2015). The simulation results show increase in QoS with in-creasing number of SBSs. However, there are very high chances ofinterference between the macro-cell links, the D2D links and thesmall cell links. This problem can further worsen if the D2D links arefrom different cells. Also, deployment of SBSs is a big challenge. Allthese aspects need to be critically addressed.

4.6. Cognitive D2D

Cognitive communication has played an essential role in im-provement of spectrum efficiency by enabling the use of vacantbands by secondary users without causing any hindrance to theprimary users. Cognitive Radio Networks (CRNs) offer a class ofnetworks that have the ability to change their operating para-meters, on the basis of their interaction with the surroundingenvironment. Unused part of the spectrum can be utilized by CRNsby spectrum sensing. Consensus based algorithms for cooperativespectrum sensing has been given in Ejaz et al. (2013).

The use of cognitive D2D reduces the burden of frequency al-location on the operator. Additionally, sensing and reusing of ISM


band resources is possible with cognitive D2D. The ongoingcommunication within the ISM band remains unaffected. In in-band D2D communication (discussed in next section), cognitivespectrum access (CSA) can result in efficient resource utilizationand interference management. Two spectrum access techniques,D2D-unaware spectrum access and D2D-aware spectrum access,have been discussed in Sakr et al. (2015). The CSA scheme can beoptimized by finest selection of the network design parameters. ACR-assisted D2D communication in cellular networks has beeninvestigated in Khoshkholgh et al. (2015), in which, the UEs accessthe spectrum by a mixed underlay/overlay sharing of spectrum.Cognitive and energy harvesting-based D2D communication hasbeen modeled in Sakr et al. (2015). Its proposed model is evaluatedon the basis of the stochastic geometry, which shows that theoverall QoS of the cellular network improves with cognitive D2Dcommunication, when network parameters are tuned carefully.The use of D2D communication for vehicular communication isdiscussed in Mumtaz et al. (2015), with the use of cognitive radiofor offloading vehicular traffic. The results show reduction in





D2D Communication

Inband Outband

Underlay Overlay Controlled Autonomous

Fig. 13. Types of D2D Communication (Asadi et al., 2014).


transmission delay. A combination of D2D and cognitive commu-nication makes D2D communication very diverse (Liu et al., 2014).

4.7. Network coding

A potential technique for the overall throughput improvementof a network is network coding. The transmitting nodes, withnetwork coding, tend to combine the packets before transmission.This reduces the amount of routing information. Network codingin D2D communication helps in reducing power consumption,interference, etc. (Wu et al., 2015). It also provides security andcommunication efficiency. This has been discussed in Pahlevaniet al. (2014), using the protocols: CORE and PlayNCool. Due to itsunique advantages, network coding enables throughput im-provement, delay reduction and energy efficiency in the D2Dcommunication. Though there are number of advantages of net-work coding, but it requires a large amount of resources (both timeand radio) for decoding the data received at the D2D node. Ad-ditionally, since a number of packets are combined, uniqueness ofthe coefficients cannot be guaranteed. As a result, this remains anopen research field for the researchers.

The above mentioned are a few features which can be used inconjunction with D2D communication. Once successfully im-plemented, these will result in numerous advantages to the serviceproviders, as well as the subscribers. The overall utility of thecellular networks will be greatly enhanced, as presented in thepreceding discussion. Issues related to the above mentioned fea-tures of wireless networks require attention and further research.

5. Key open challenges in D2D

Device-to-Device (D2D) communication may use the licensedspectrum (in band) or the unlicensed spectrum (out band) fordirect link formation (Asadi et al., 2014). Inband D2D commu-nication is categorized as underlay and overlay. Underlay D2Dcommunication allows set up of direct links and cellular links inthe cellular spectrum. In overlay D2D, on the other hand, a dedi-cated portion of the available spectrum is used for Device-to-De-vice (D2D) communication, with rest of the spectrum used forcellular communication. As out band D2D communication exploitsthe unlicensed spectrum for the formation of direct links, it iscategorized as autonomous and controlled. When controlled, theradio interfaces in D2D are managed by the eNB, while in auton-omous, these are coordinated by the user equipments (UEs)themselves. Interference between D2D users and cellular users isno issue in out band D2D, but coordination of the communicationin the unlicensed band requires a second radio interface (like, Wi-Fi Direct (W. Alliance, 2010), Bluetooth (Bluetooth, 2001), ZigBee(Alliance, 2006)). The categorization of D2D communication hasbeen depicted in Fig. 13. To utilize the limited available spectrumin the most efficient manner, one must know where to use whichcategory of D2D communication.

For implementing D2D communication in cellular networks, anumber of key issues need to be addressed. To obtain completeadvantage of Device to Device (D2D) communication, overcomingthese issues efficiently, is important. Some of these are listed be-low, and available literature considers in band as well as out bandD2D.

5.1. Peer discovery

Since D2D communication is gaining popularity, identifyingefficient means of discovering proximate users has become ne-cessary. The process of peer discovery should be efficient, so thatD2D links are discovered and established quickly. It is also


important for ensuring optimum throughput, efficiency and re-source allocation within the system. Setting up of direct links re-quires devices to discover each other first. Once discovered, directlinks are set up, and then occurs transmission over those links.Researchers are working on different approaches for device dis-covery. In Lee et al. (2016), spatial correlation of wireless channelsis considered for low power peer discovery. The simulation resultsshow that peers can be discovered with very low power con-sumption. It provides a very accurate method of peer discovery.Peer discovery techniques can be restricted discovery and opendiscovery (Feng et al., 2014). In case of restricted discovery, the UEscannot be detected without their prior explicit permission. Thisthus maintains user privacy. In case of open discovery, UEs can bedetected during the duration for which they lie in proximity ofother UEs. From the perspective of the network, device discoverycan be controlled either tightly by the base station, or lightly(Fodor et al., 2012; Lei et al., 2012).

The authors in Maciel et al. (2015) propose a neighbor dis-covery technique which is based on power vectors, and consider-ing a time-variant channel. It is a low complexity algorithm, wherethe probability of a false detection is close to zero. Energy requiredto support D2D communication is high. For an energy efficientnetwork, a device discovery technique is proposed in Fodor et al.(2012). A social-aware peer discovery scheme has been proposedin Zhang et al. (2015). The scheme enhances the network perfor-mance by improving the data delivery ratio, exploiting the socialinformation only. An effective network-assisted technique for de-vice discovery has been proposed in Nguyen et al. (2014) for thesupport of device-to-device communication in LTE networks. Theresults show that the probability of device discovery is quite highin this technique, for a certain discovery interval.

In Tang et al. (2014), the authors propose neighbor discoverywith the use of a sounding reference signal (SRS) channel. Theuplink transmissions of cellular users play an essential role infinding the neighbors. Neighbor discovery under unknown chan-nel statistics is also considered. A review to various techniques fordevice discovery is given in (Zou et al., 2014). These includeBluetooth discovery, Wi-Fi (Ad Hoc) Device Discovery, IrDA DeviceDiscovery. Request based discovery, Direct Discovery, Requestbased Discovery, Packet and Signature-based Discovery and Net-work-Assisted discovery. A summary of the various peer discoverymethods is given in Table 4.

On completion of peer discovery, session setup takes place. Forthe setting up of sessions, two methods have been developed; IPbased detection and dedicated D2D signaling. The existing litera-ture mainly focuses on single cell scenarios, for device discoveryand session setup. Works on multi cell scenario is more beneficialas it supports efficient resource utilization. Device-to-Device(D2D) discovery and session set up is a very challenging job, sinceit needs cooperation from the adjacent base-stations (BS).





Table 4Methods of peer discovery.

Reference Method of discovery

Lee et al. (2016) Low power DiscoveryFeng et al. (2014) � Restricted Discovery

� Open DiscoveryFodor et al. (2012) Energy efficient device discoveryMaciel et al. (2015) Discovery based on power vectorsZhang et al. (2015) Social aware peer discoveryNguyen et al. (2014) Network-assisted discoveryTang et al. (2014) Sound Referencing Signal for neighbor discoveryZou et al. (2014) � Bluetooth Discovery

� Wi-Fi Device Discovery� Wi-Fi Direct Device Discovery� IrDA Device Discovery� Network Assisted Discovery� Packet and Signature-based Discovery� Request Based Discovery� Direct Discovery


5.2. Resource allocation

After device discovery, availability of resources is important forenabling communication over the direct links. Radio resource allo-cation is thus important for enhancing the spectral efficiency of D2Dcommunication, underlaying cellular communication. Resource allo-cation strategies in D2D communication can be centralized or dis-tributed. Centralized techniques (Zhou et al., 2008) cause complexityin case of large networks while distributed techniques (Feng et al.,2014) tend to decrease the device complexity. The distributed tech-niques improve the scalability of the D2D links. Hybrid solutions alsocan be provided and are an area of open research. A number of dif-ferent techniques are available under the literature survey. For ob-taining maximum throughput, D2D communication can operate in anumber of modes. These can be:

Silent Mode: In this mode, the D2D devices stay silent andcannot transmit because of lack of resources. Spectrum reuse, asa result, is not possible.Dedicated Mode: In this mode, some of the available resourcesare dedicated for the D2D users, to be used for directtransmission.Reuse Mode: In this mode, uplink or downlink resources of thecellular users are reused by the D2D users.Cellular Mode: In this, conventional communication occurs,through the eNBs and D2D data is transmitted.

An improvement in the spectrum efficiency can be achieved bythe use of reuse mode. Interference management is better with thededicated and cellular modes. However, these two modes maybeinefficient to maximize the overall network throughput. The de-cision for resource sharing is made by the base station. When theD2D links and cellular links reuse the same resources, it is referredto as non-orthogonal sharing, and when they do not share thesame resources, it is referred to as orthogonal sharing. Better re-source utilization efficiency is achieved by non-orthogonalsharing.

Considering the spectrum efficiency of LTE-A networks, a re-source allocation strategy is proposed in Phunchongharn et al.(2013), for minimization of the transmission length of D2D links.An NP-complete (Garey and Johnson, 1979) problem is formulatedas a Mixed Integer Programming. A low complexity column gen-eration method solves the resource allocation problem in D2Dcommunication. Another technique for resource allocation isprovided in Yu et al. (2011), maximizing throughput of the net-work. The cellular services are given the higher priority over theD2D communication. For evaluating a single-cell scenario, a


system with D2D communication underlaying cellular commu-nication is considered. Resource sharing as orthogonal sharing,non-orthogonal sharing and cellular operation is discussed.In Zhou et al. (2013), optimal resource utilization is achieved bycluster partitioning. In Feng et al. (2013), a method is proposed foroverall throughput improvement of the system, along with en-hancement in spectral efficiency through power allocation andadmission control. A Heuristic Location Dependent Resource Al-location Algorithm has been proposed in Botsov et al. (2014). It iscustomized to vehicle-to-vehicle (V2V) communication, aiming togive prime priority to safety of V2V communication. Resourcepooling has been proposed in Fodor et al. (2012).

A semi-persistent resource sharing algorithm has been pro-posed in Liu et al. (2013), in which inter-cell and intra-cell sce-narios have been considered. This algorithm improves the overallthroughput of the network. Non-orthogonal resource sharing isdiscussed in Gjendemsjo et al. (2006), and Chandrasekhar and She(2008), considering the maximum transmit power constraint. Theauthors of Wang et al. (2011) propose another optimal resourceallocation technique that is able to significantly improve the sumthroughput of D2D as well as cellular communication in a net-work. In order to improve the overall network throughput anduser satisfaction ratio, the authors of Chen et al. (2015) introduce atime-division scheduling (TDS) algorithm for efficient utilizationand allocation of resources, using non-orthogonal sharing mode.Based on the improved proportional fairness algorithm, the au-thors in Zheng et al. (2015) propose adaptive time division sche-duling algorithm, in which D2D pairs are adaptively allocated tothe timeslots, unlike (Chen et al., 2015). A brief overview of someof the mentioned algorithms has been given in Table 5.

On the basis of various algorithms discussed so far, we haveobserved that compared to other well known schemes, Feng et al.(2013), Zheng et al. (2015) and Chen et al. (2015) have providedthe best performance. The D2D access rate, throughput gain, fair-ness and user satisfaction ratio have been maximized in thesealgorithms, which is desirable from the user perspective as well asthe service provider.

5.2.1. Network modelA single cell scenario, with the base-station (BS) at the centre, a

D2D pair and cellular users is considered, as shown in Fig. 14, withD2D communication underlaying cellular communication. The usersthat are capable of carrying out direct D2D communication areidentified by the base station. The location information of all usersand the channel state information (CSI) are provided to the basestation (BS) through the global positioning system (GPS) receiveravailable on the user equipments (UEs). There are high chances ofpotential interference among the users, as depicted by the interferingsignals, in Fig. 14. A D2D link exists between and the D2D transmitter(Dtx) and D2D receiver (Drx,), in accordance with the distance con-straint, Drd0, where D is the distance between Dtx and Drx and d0 isthe maximum distance for direct communication.

As an assumption, it is considered that each cellular user isallotted equal number of resource blocks (RBs). The RB which isallocated to a particular user is shared by a single D2D pair, so as toavoid interference among the D2D pairs. A single pair can shareresources of multiple cellular users in the network. The network isassumed to contain m number of cellular users, n number of D2Dpairs and k number of resource blocks. Let the channel gain be-tween base station and a cellular user be given by gbcu(i), channelgain between D2D transmitter and receiver gd(j), the gain of in-terference link from base station to a Drx be denoted by gbd(j), andgain of interference link between a Dtx to a cellular user be gd(j)cu(i).These channel gains tend to contain distant dependent path loss aswell as shadowing path loss. The base station is assumed to allo-cate resource blocks, for efficient utilization of cellular resources.





Table 5Resource allocation algorithms for device-to-device communication.

REFERENCE ALGORITHM DESCRIPTION OBJECTIVE

Fodor et al. (2012) Resource Pooling Support reuse of resources between D2D and cellularusers, taking advantage of hop gain

To achieve increased throughput, power saving andhigher spectrum efficiency

Garey and John-son (1979)

Column generation method A heuristic algorithm that detects maximum number ofactive D2D links which are capable of transmitting si-multaneously in every time slot, satisfying the accesspattern constraints

With increasing power consumption, reduction intransmission length of D2D connections for densenetworks

Yu et al. (2011) Resource sharing in traditionalcellular and direct D2Dcommunication

Transmissions in orthogonal, non-orthogonal and cel-lular resource sharing modes are optimized in order tomaximize the overall sum rate

Optimization of sum rate, taking into considerationresource allocation and power control, and also ad-hering to transmit power/energy constraints

Zhou et al. (2013) Cluster partitioning and relayselection

An intra-cluster D2D retransmission scheme in whichcooperative relays are adaptively selected through mul-ticast retransmissions

To achieve optimal resource utilization

Feng et al. (2013) Admission control and powerallocation

A maximum weight bipartite technique is used to findsuitable D2D pair for each cellular user; optimum poweris allocated to D2D pairs and their cellular partners

Improvement in spectral efficiency, with enhancedsystem throughput

Botsov et al.(2014)

Heuristic Location DependentResource Allocation algorithm

Persistent resource allocation applied to the networkconsidered, with fixed reservation of resources

Feasible for QoS controlled and services demandingstrict reliability. This algorithm aims at reduction inthe signaling overhead and interference of the net-work under consideration

Liu et al. (2013) Uplink Semi-Persistent Schedul-ing Resource Reuse Algorithm

The D2D users reuse the UL semi persistent resources forminimum interference. The algorithm takes into con-sideration inter cell as well as intra cell interferencebetween D2D and cellular links

Improvement in overall system throughput and re-duction in the interference among D2D links

Chen et al. (2015) Time Division Scheduling (TDS)Algorithm

The entire scheduling period of the base station is di-vided into n equal number of time slots. A location dis-persion principle is used to allocate the D2D pairs in abalanced number, to the slots

A significant improvement in system throughput,along with high D2D user satisfaction ratio

Zheng et al.(2015)

Adaptive Time Division Sche-duling Algorithm

D2D pairs are adaptively allocated to a series of time-slots, using improved proportional fairness algorithm;resources of cellular users are allocated to the D2D pairsassigned to the timeslots

High system throughput and better fairness

Fig. 14. Network model.


The transmission power of the BS and Dtx are given by PBS andPDD, respectively

When jth D2D pair, shares kth RB with ith cellular user, j € n and i€ m, the SINRs at the cellular user and Drx can be represented by

₰ ( )( )

=ή + ( )

( )( )

( )

P g

P g 3

BS bcu jcu i d j

DD d j cu i


₰ =ή + ( )

( )( )

( )

P g

P g 4d j

DD d j

BS bd j

On the other hand, when a cellular user does not share its RBwith any pair, then the its SINR is given by

₰ ( )=ή ( )( )

P g

5cu i

BS bcu i

When a cellular user shares RB, the data rate of ith cellular userafter and before sharing of resources is given respectively by,

Ɽ ₰ ( )= ⁎ ( ) ( )( ) ( ) ( )BW log 6cu i d j cu i d j

Ɽ ₰( )( )= ⁎ ( )( ) BW log 7cu i cu i

where BW denotes the bandwidth of a single RB.When a D2D pair shares a RB with a cellular user, the channel

rate is given by

Ɽ ₰( )( )= ⁎ + ( )( ) BW log 1 8d j d j

When ith cellular user shares kth resource block with jth D2Dpair, it suffers interference from Dtx, resulting in a decrease intransmission rate. This decrement in the data rate is representedby

Ɽ Ɽ Ɽ ( ) ( )∆ = − ( )( ) ( ) 9d j cu i cu i d j

There is an increment in the overall system throughput, afterthe sharing of resources, since rate of the D2D pair increases muchmore, compared to the decrement in the cellular user rate. Thisthroughput increment is represented by






Ɽ Ɽ Ɽ

Ɽ Ɽ

( ) ( )Δ = + −

= − Δ ( )

( ) ( ) ( ) ( )

( ) ( )

T

10

cu i d j d j cu i d j cu i

d j d j

The optimization problem is formulated as

ᴛ Ɽ Ɽ Ɽ∑ ∑ ( )∆ = ( + − )( )= =

( ) ( ) ( )Warg max11

mk

l

j

n

k j d j cu i d j cu i1 1

,

subject to

₰ ≥ ( )SINR 12cuiDj cut

₰ ≥ ( )SINR 13Dj Dt

(12) and (13) are defining the threshold SINR values for cellularuser and D2D users, respectively. Thus, throughput maximizationis accomplished by the above analysis, in cellular networks withunderlaying D2D communication. The condition for minimumnumber of RBs required for the jth D2D pair to maximizethroughput is stated as

where Ɽmin is the minimum data rate required by the D2Dusers, for optimal resource sharing between the users.

The various symbols used in the network model have beensummarized in Table 6.

For the next generation networks, in order to meet the risingdemands and requirements of the mobile network operators, ar-chitecture has been proposed (as depicted in Fig. 15.). Essentialnetwork requirements are expected to be met efficiently, throughthis proposed architecture for resource allocation.

With increasing number of subscribers and the rising demandfor high data rates, the numbers of channels assigned to a cellbecome insufficient to support all the users. A need is felt to have agreater number of channels per unit coverage area of the cell. As aresult, cell splitting is preferred in such scenarios. Splitting cellsinto sectors, with the use of directional antennas at the base sta-tion (BS) enhances their capability to handle more number ofconversations at the same time. Cell sectoring is very useful forincreasing the system subscriber handling capacity. Each sectorthen operates with its own set of frequency channels. With sec-toring, the co-channel cell interference is greatly reduced, andconsiderable improvement in SINR is achieved. This is the reasonfor using sectored antennas at the base station, for the proposedmodel.

Table 6Symbols used in network model.

Symbol Meaning

gbcu(i) Channel gain between BS and CUgd(j) Channel gain between D2D pairgbd(j) Channel gain of interference link between BS and D2Drx

gd(j)cu(i) Channel gain of interference link between BS and D2Dtx

₰ ( ) ( )cu i d j SINR at ith CU on sharing RB with jth D2D pair

₰ ( )d j SINR at jth D2D pair on sharing RB with ith CU

₰ ( )cu i SINR of ith CU not sharing any RB with D2D pairⱤcu(i)d(j) Data rate of ith CU after sharing RB with jth D2D pairⱤcu(i) Data rate of ith CU before sharing RBⱤd(j) Data rate of jth D2D pair

( )Δ¯ ( )T d jcu i Throughput increment on RB sharing

ή Noise Power Density₦min(j) Minimum number of RBs required for the jth D2D pair for max-

imizing throughput.Ɽmin Minimum data rate required by the D2D users


In the proposed model, a scenario with a single cell is con-sidered. The cell is divided into three 120° sectors, as is expectedwith the use of a sectored antenna. Each sector can have anynumber of users. The primary aim is to offload traffic from the basestation and bring about an efficient device-to-device (D2D) com-munication with optimal resource allocation. The UEs that areclose to the BS are served by the BS only, that is, those UEs willoperate in the cellular mode. The UEs that are far from the BS, incongested areas or at the cell edge and are in proximity, commu-nicate through D2D links. The formation of direct links betweenuser equipments (UEs) is dependent on the distance betweenthem, which must be less than or equal to the threshold distance,d0. Thus, D2D link formation occurs when distance between anytwo UEs, m and n is such that d(m, n)rd0. When once the distanceconstraint is met, the UE acts as a cellular user (CUE) or D2D user(DUE). The architecture aims at maximizing throughput, mini-mizing latency, enhancing system capacity, and efficiently utilizingthe licensed spectrum through optimal resource allocation.

5.3. Power control

Setting the optimum transmission power for reusing the fre-quency is an area of interest for the researchers. It is particularlyimportant in case of uplink transmissions because of the near-fareffect and co-channel interference. Once a maximum power levelis allocated to the D2D users, then the Quality of Service (QoS) ofthe cellular users is maintained in the network. Controlling powereffectively mitigates interference in cellular networks. For D2Dunder laid cellular networks, there has been a considerable in-terest in power control methods. A limit is set upon the powerlevel of the D2D transmitter and its reuse partner (the cellularuser), in order to maximize the overall system throughput. This isexpressed as

₰ ₰( ) ( )( ) ( )( ) = + + + ( )P P max log log, arg 1 1 15iCU

jDD

P P cu i d j, 2 2iCU

jD D2

subject to (12) and (13), along with (16) and (17), given as

≤ ( )P P 16iCU

CUm

≤ ( )P P 17jD D

mDD2

where P and PCUm

mDD set the maximum limits to the power of the

cellular user and D2D transmitter, respectively.To regulate the SINR degradation of cellular users, statistical

power control schemes have been discussed in Yu et al. (2009a,,2009b), for different channel models. Some power control techniquesare introduced in Wen et al. (2012) and Wang and Wang (2013),through which improved performance can be achieved. A new dis-tributed scheme for power control has been proposed in Fodor andReider (2011) in which a D2D underlay scenario is considered. Thetechnique aims at minimization of the overall power consumption ofthe network, considering the optimal SINR target which is achievedwith the use of Augmented Lagrangian Penalty Function (ALPF)method. Solving of Eq. (17) needs accessible full channel matrix. Analgorithm with low complexity has been proposed in Wang et al.(2014), based on game-theory, for selection of source and controllingpower. It uses stackelberg game model to show the impact of im-provement in D2D transmission quality.

Lee et al. (2015) develop centralized and distributed algorithmsfor power control in a D2D network underlaying cellular network.A near optimal scheme for power control or rate control de-pending upon the condition of the channel is proposed in Songet al. (2015), thus reducing computational complexity. For themaximization of energy efficiency in the network, the authors





Fig. 15. Proposed Architecture.

Fig. 16. An interference scenario in D2D underlaid cellular network.


of Jiang et al. (2015) propose an iterative joint resource allocationand power control technique. A penalty function approach adop-ted. In order to improve the quality of D2D communication un-derlaying cellular networks, an auction based power allocationapproach is investigated in Xu et al. (2014). It is a low complexityalgorithm, using a reverse iterative combinatorial auction andprovides high system efficiency. Many other power control algo-rithms exist in literature. There is still ongoing research in thiscontext as controlling power levels is essential for managing in-terference between the D2D users and cellular users.

5.4. Interference management

Enabling D2D links within a cellular network pose a big threatof interference to the cellular links in the network. D2D links cancause interference between cellular users and D2D users, resultingin an increase in intra-cell interference. Inter-cell interference isalso possible with D2D communication underlaying cellularcommunication. Interference can be mitigated through mode se-lection, optimum resource allocation, power control. Setting themaximum transmit power limits of the D2D transmitter is an ef-fective technique of limiting the interference between DUEs andCUEs. A general scenario of interference in D2D under laid cellularnetworks is depicted in Fig. 16.

A very critical term related to interference avoidance is modeselection. Generally, distance between the D2D users and cellularusers is considered for mode selection (Overlay/underlay) (Wenet al., 2012). Also, distance between cellular user and the BS is animportant parameter for selection of the mode in the network,thus avoiding interference. In Jänis et al. (2009), MIMO transmis-sion schemes are introduced for interference avoidance, resultingin a great enhancement of D2D SINR.

Due to interfering signals, the received contain three compo-nents:

= +

+ ( )

Received Signal Desired signal Outside interference signal

D D interference signal2 18

Interference at the receiver must be minimized so that a highervalue of SINR is achieved. This can be achieved by modulation andcoding scheme (MCS), which supports error-free reception of in-formation. The D2D interference signals can be reduced, but


interference from outside sources is hard to avoid (Eq. (18)).The authors of Zhou et al. (2015) take into consideration a D2D

underlaying communication network for interference cancellation,along with the transmission powers for maximizing the utility ofthe network. Significant gains are enjoyed by the users in terms ofspectral efficiency. In Wang et al. (2012), authors propose a novelinterference coordination scheme for improving system through-put and efficient resource utilization in a multicast D2D network.The authors of Guo et al. (2015) concentrate on managing inter-ference between D2D users and cellular users by discussing therange of an interference suppression area (ISA) which classifies thestrength of the interference between the cellular and D2D usersand influences the system performance. Adequate adjustment ofthe range of ISA can help achieve optimal system performance.Interference management using network coding is discussedin Wang et al. (2015). In a cellular system with users undergoingcellular communication, along with D2D multicast





INTERFERENCEMANAGEMENT SCHEMES

INTERFERENCEAVOIDANCE

INTERFERENCECANCELLATION

INTERFERENCECOORDINATION

Fig. 17. Interference management schemes in D2D.


communication, both sharing the same spectrum, the interferencescenarios are evaluated in Wang et al. (2012). Interference in sucha scenario can be mitigated by power control, followed by optimalresource allocation. Thus, different approaches are adopted bydifferent researchers for interference mitigation between D2Dlinks and cellular links, and can be categorized as interferenceavoidance schemes; interference cancellation schemes, or inter-ference coordination schemes, as shown in Fig. 17. The authorsin Noura and Nordin (2016) provide a comprehensive survey oninterference management in D2D communication.

5.5. Security

Prior to the acceptance and implementation of the D2D tech-nique in cellular network, security needs to be well addressed. Thechannels are vulnerable to a number of security attacks likeeavesdropping, message modification, and node impersonation. Toprevent these, cryptographic solutions can be used to encrypt theinformation before transmission. The security schemes providedby the cellular operators can be used by the D2D users if they areunder their coverage. But, users outside the coverage of the op-erators can’t be secured. In this case, security signals may bepassed on through relays. Since relays are highly susceptible tomalicious attacks, like eavesdropping attack, free riding attack,denial of service attack (Osanaiye et al., 2016), designing securityschemes for D2D communication is an important challenge to beaddressed.

To make D2D communication secure, physical layer securityplays a key role (Zhu et al., 2014). Incorporating security attributesin D2D communication, at the physical layer is beneficial. Forproviding physical layer security, the received SINR at the eaves-dropper need to be minimized. Beamforming techniques enhancesecurity in cellular networks. Under a typical attack on the on-going D2D communication, if Alice transmits some information ‘x’to the Bob, and it is captured by the intruder, then the signal re-ceived at the bob is

ᴎ= + ( )Y P H x 19bob alice ab b

and at the intruder is

ᴎ= + ( )Y P H x 20I alice ae i

where Y ,bob YI correspond to the received signals at the bob andintruder, respectively; Palice is the transmit power of the Alice; Hab,Hae are Gaussian random variables for modeling the scalar chan-nels, and Ɽb, Ɽi represent noise at the bob and intruder. Theseequations model attack on a single hop D2D communication in acellular network. These are used for secrecy rate maximization in aD2D communication network. Presence of noise components inthe received signal prevents desirable information from reachingthe Bob. Cha et al. (2009), Yue et al. (2013), Perrig et al. (2004),Zhou et al. (2008) and Muraleedharan and Lisa (2006) discussabout the security concerns in D2D communication. A generalarchitecture for securing D2D communication is discussed


in Wang and Yan. (2015). Optimal power can be allocated to theAlice and Bob, to prevent eavesdropping. The communicationoverhead and the key generation time needs to be taken into ac-count while designing the security algorithms.

To assure true benefits of D2D communication in cellular net-works, the above listed issues need special attention of the re-search community.

6. Application areas of D2D communication

In view of the current and future wireless traffic scenario, anumber of use cases of device-to-device (D2D) communicationhave been proposed by the researchers. D2D communication canbe carried out by direct link establishment between sender andreceiver, or with D2D users acting as relays within the networks.The most important application of device-to-device (D2D) com-munication is cellular offloading (Bao et al., 2010), which results inan increased network capacity. Others applications include mul-ticasting (Zhou et al., 2013), video dissemination (Golrezaei et al.,2012), and M2M communication (Pratas and Popovski, 2013).M2M communications will be highly benefitted by D2D commu-nication. It is technology-independent, unlike D2D communica-tion, which is dependent upon the technology. In case of emer-gency communications (public safety communication), D2D com-munication is expected to play an essential role. It has the abilityto assure public protection and disaster relief (PPDR) and nationalsecurity and public safety services (NSPS) (Fodor et al., 2014). Forexample, in case of a natural calamity, like an earthquake, con-ventional cellular networks can get damaged. In such a case, awireless network can be setup between terminals, using D2Dcommunication.

D2D communication, upon integration with Internet of Things(IoT) shall support important applications. This will result in atruly interconnected wireless network. A typical application forsuch a scenario is Vehicle-to-Vehicle (V2V) communication, in theInternet of Vehicles (IoV). This application is particularly im-portant in case of collision avoidance systems, like in coordinatingbraking systems among the vehicles.

Other possible use cases of D2D communication are multiuserMIMO (MU-MIMO) enhancement, cooperative relaying, and vir-tual MIMO. With D2D communication, paired users can exchangethe information about the channel status directly. In this way,channel status information can be fed by the terminals to basestations and improve the performance of MU-MIMO (Li et al.,2012).

Further D2D use cases include location-aware services, socialnetworking, smart grids (Pratas and Popovski, 2013; Fey et al.,2012), e-health, smart city etc. A few use cases of D2D commu-nication have also been addressed in Lei et al. (2012). Thus, a widevariety of applications are offered for the next generation net-works by device-to-device (D2D) communication.

7. Conclusion

In this paper, an extensive survey on device-to-device (D2D)communication has been performed. This emerging technology isexpected to solve the various tribulations of the mobile networkoperators (MNOs), efficiently satisfying all the demands of thesubscribers. A complete overview about the different types of D2Dcommunication and the supported architectures has been broughtup. A number of features can be used in conjunction with D2Dcommunication, to enhance the functionality of cellular networks.Some challenges related to the implementation of device-to-de-vice (D2D) communication have been brought up in this survey,





Table 7Standarization of D2D communication.

ORGANIZATION STANDARD

IEEE IEEE 802.15.4 g (SUN)IEEE 802.15.8IEEE 802.16n

QualComm FlashLinQ3GPP ProSe (Release 12)


and various algorithms for dealing with them have been discussed.Architecture has been proposed in the survey, for optimal resourceallocation to the D2D users underlaying cellular networks. This isimportant to ensure efficient communication in the existing cel-lular networks. Some use cases have been quoted, where D2Dcommunication will play a crucial role. Thus, D2D communicationis an integral technology of the future networks, motivating theresearchers to overcome the associated challenges in order tocompletely take advantage of its utility.

Table 8D2D related ongoing projects.

Research Project/Organsization

Objective

METIS D2D Increasing coverage, offloading backhaul, improv-ing spectrum usage, enabling new services

CODEC Resource allocation in D2D communicationWiFiUS D2D communication at millimeter frequencies

Appendix A. Standardization activities for D2D

Device-to-device communication is widely being accepted bythe mobile stakeholders and they believe it to be a big success inwireless technology. Qualcomm, LTE-A and IEEE 802.15.4 g (SUN)are at present involved in the standardization activities of D2Dcommunication over the licensed band.

IEEEE 802.15.4 g was first released as an amendment to the lowrate WPAN in April 2012. It supports three different modulationtechniques, FSK, DSSS and OFDM. Maximum data rate supported isupto 200 kbps with a maximum range of about 2-3 km. SUN ishighly energy efficient, which is a very attracting feature. OtherIEEE standards include IEEE 802.15.8 (for PHY/MAC specification ofD2D) and IEEE 802.16n.

D2D communications with and without infrastructure arebeing studied by 3GPP. Proximity-based Services (ProSe) andGroup Communicaton System Enablers for LTE (GCSE_LTE) arediscussed in 3GPP (2013a,, 2014a,, 2013b). D2D ProSe considersvarious aspects of D2D communication, including one-to-one,one-to-many and one-hop relay and also addresses switching ofmode between D2D mode and cellular mode. ProSe includesworking on identifying UEs in proximity (peer discovery, and es-tablishing direct links between them, so as to enable commu-nication between them either directly or through a locally routedpath via the eNB.

Three different use cases are being studied by 3GPP that reflectthe main market drivers for ProSe� Local commercial advertisement: this sends advertisements to

passing devices automatically.� Network offloading: This helps in avoiding congestion in the

network, by enabling traffic to pass through direct links.� Public Safety Communication: In case of absence of network

coverage, public safety communication is supported.

The study related to feasibility of ProSe started in 2011 by the3GPP Technical Specification Group (TSG) Service and System As-pect (SA), and these also defined the technical requirements in2012. In the documents of Release 12, TS22.278 and TS22.115, thetechnical specifications were written. In Release 13, Radio AccessNetwork related activities of ProSe were expected to be included.Presently, the work on RAN1 (Radio Layer1) is in the middle of itsfeasibility stage along with compilation of proposals for solutionsbut also evaluation models (channel, traffic, mobility) in TR 36.843.The work in RAN2 (Radio Layer 2 and Layer 3-Radio Resource part)is in the first part of the feasibility stage. The work on CT1 (non-access stratum protocols) has not been started yet and will start oncompletion of the work in SA2.

Technical specifications are provided in (3GPP, 2013c) in whichGCSE_LTE refers to the 3GPP architecture based content distribu-tion mechanism and expect the support of an efficient and fastcommunication. Intense research activities and meetings havebeen organized and still being organized by the 3GPP TSG RANWG1. In spite of the ongoing debates, a lot of work on D2D


communication has been done.Initially, D2D communication was proposed in academia for

enabling multihop relays in the cellular networks (Lin and Hsu,2000). Later, it started gaining importance for various use casesand improving spectral efficiency. Qualcomm's FlashLinQ (Wuet al., 2010) was the first attempt towards D2D implementation. Itis a PHY/MAC architecture which is needed for D2D underlayingnetworks. FlashLinQ is an efficient technique that provides peerdiscovery, synchronization of timing and management of link inD2D-enabled cellular networks.

Few researchers have proposed some protocols for device-to-device communication in cellular networks. Protocol stacks forinband and outband D2D communication have been introducedin Raghothaman et al. (2013) and WiFi Direct and LTE D2D in ac-tion (2013), respectively. In Raghothaman et al. (2013), the mod-ifications in terms of architecture and protocol have been given,that need to be made to the existing cellular networks. An im-portant architectural modification involves addition of a D2Dserver in or out of the core network, along with suitable interfaces.In WiFi Direct and LTE D2D in action (2013), the authors mainlyaim at opportunistic relaying of packets. A performance evaluationof the D2D networks using existing simulators like OPNET (OP-NET), NS3 (OPNET), Omnetþþ (Varga, 2007) is expected to bringabout fruitful results. Android softwares are also being developedand worked upon for D2D communication. A summary of thevarious activities is given in Table 7.

Appendix B. Ongoing projects on D2D

Various ongoing D2D communication projects have been ta-bulated in Table 8.

Appendix C. Abbreviations used in paper

Various abbreviations used in the paper have been listed inTable 9.





Table 9List of abbreviations used in the paper.

Abbreviation Explaination

3GPP 3rd Generation Partnership ProjectACK/NACK Acknowledgement/Negative AcknowledgementALPF Augmented Lagrangian Penalty FunctionARQ Automatic Repeat RequestBDMA Beam Division Multiple AccessBS Base StationCDMA Code Division Multiple AccessCRN Cognitive Radio NetworkCSI Channel State InformationCUE Cellular User EquipmentDSSS Direct Sequence Spread SpectrumDUE D2D User EquipmentDVB Digital Video BroadcastingEDGE Enhanced Data rate for GSM EvolutioneNB Evolved Node BEVDO Evolution-Data OptimizedFBMC Fiber Bank MulitcarrierGFSK Gaussian Frequency Shift KeyingGPRS General Packet Radio ServiceGPS Global Positioning SystemHARQ Hybrid Automatic Repeat RequestHOM Handover MarginHSUPA/HSDPA High Speed Uplink/Downlink Packet AccessIoT Internet of ThingsIoV Internet of VehiclesIS-95 Interim Standard-95ISA Interference Suppression AreaISM Industrial, Scientific, MedicalITU International Telecommunication UnionLOS Line Of SightLTE Long Term EvolutionLTE-A Long Term Evolution-AdvancedM2M Machine-to-MachineMANET Mobile Ad-hoc NetworkMIMO Multiple Input Multiple OutputMMS Multimedia Messaging ServicemmWave Millimeter WaveMNO Mobile Network OperatorsMU-MIMO Multi-User MIMONGN Next Generation NetworksNSPS National Security and Public Safety ServicesOFDMA Orthogonal Frequency Division Multiple AccessPPDR Public Protection and Disaster ReliefProSe Proximity ServiceQoS Quality of ServiceRB Resource BlockRSRP Reference Signal Receiving PowerSC-FDMA Single Carrier Frequency Division Multiple AccessSINR Signal-to-Interference-plus-Noise-RatioTDS Time Division SchedulingTTT Time to TriggerUDN Ultra Dense NetworkUE User EquipmentUMTS Universal Mobile Telecommunications SystemV2V Vehicle-to-vehicleVLC Visible Light CommunicationWCDMA Wideband Code Division Multiple AccessWiMax Wireless interoperability for microwave accessWLAN Wireless Local Area Network


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Miss. Pimmy Gandotra (S'16) received the B.E. degreein Electronics and Communication Engineering fromJammu University, Jammu and Kashmir, India, in 2013.She is currently pursuing the M.Tech. degree in Elec-tronics and Communication Engineering at Shri MataVaishno Devi University, Katra, Jammu and Kashmir,India. Her research interest includes the emergingtechnologies of 5G wireless communication network.Currently she is doing her research work on ResourceAllocation in Device to Device communication. She isworking on Qualnet simulation and Matlab tools forWireless Communication. She is receiving the teaching
assistantship from MHRD. She is a student member of
Institute of Electrical and Electronics Engineers (IEEE).





















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Dr. Rakesh K. Jha (S'10, M'13) is currently an AssistantProfessor in School of Electronics and CommunicationEngineering, Shri Mata Vaishno Devi University, katra,Jammu and Kashmir, India. He is carrying out his re-search in wireless communication, power optimiza-tions, wireless security issues and optical communica-tions. He has done B.Tech. in Electronics and Commu-nication Engineering from Bhopal, India and M.Techfrom NIT Jalandhar, India. Received his Ph.D. degreefrom NIT Surat, India in 2013. He has published morethan 30 International Journal papers and more than 20International Conference papers. His area of interest is
Wireless communication, Optical Fiber Communica-
tion, Computer networks, and Security issues. Dr. Jha's one concept related to


router of Wireless Communication has been accepted by ITU (International Tele-communication Union) in 2010. He has received young scientist author award byITU in Dec 2010. He has received APAN fellowship in 2011 and 2012, and studenttravel grant from COMSNET 2012. He is a senior member of IEEE, GISFI and SIAM,International Association of Engineers (IAENG), ACCS (Advance Computing andCommunication Society) and Association for Computing Machinery (ACM).





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