Smart device to smart device communication

Inband D2D has the following advantages: underlay D2D increases the spectralefficiency of cellular spectrum; and all cellular devices are capable of using InbandD2D communication and QoS

Shahid Mumtaz · Jonathan Rodriguez

Editors

Smart Device to Smart Device

Communication

Smart Device to Smart Device Communication

Shahid Mumtaz Jonathan Rodriguez Editors

Smart Device to Smart Device Communication

123

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The Internet of Things envisages over 5 billion connected devices that will spurthe growth in mobile data traffic to rise exponentially with current predictionssuggesting a 1000x increase over the next decade This foreseen market growthhas urged mobile operators to examine new ways to plan, deploy, and manage theirnetworks for improving coverage, boosting their network’s capacity, and reducingtheir capital and operating expenditures (CAPEX and OPEX) To provide asolution toward meeting new and evermore stringent end-user requirements,mobile stakeholders are already preparing the technology roadmap for next gen-eration networks expected to be deployed by 2020 and beyond, which is collec-tively referred to as ‘‘5G.’’

5G has a broad vision and envisages design targets that include 10-100 x peakrate data rate, 1000 x network capacity, 10 x energy efficiency, and 10-30 x lowerlatency These technologies will encompass all aspects of radio access networkand applications: from wireless network infrastructure and topologies to physicallayer transmission techniques, including spectrum availability, channel modeling,device innovations, and green radio

Taking a step toward this vision, Device-to-Device communication in licensedband is one-key enabler toward a more disruptive and cost-effective communi-cation paradigm A key motivation for D2D connectivity is the potential foroperators to offload traffic from the core network and the framework for a newcommunication paradigm to support social networking through localization Thecurrent ad-hoc mode of communication does not support this functionality due toconfiguration complexity LTE-A, Qualcomm and IEEE 802.15.4g (SUN) arecurrently addressing the standardizing of D2D communication over licensed band

A major breakthrough was achieved in due course when 3GPP (LTE-A release, 12June 2012) agreed on starting a study item for D2D technology

This book, inspired by the Eureka Celtic GREEN-T research initiative, bringstogether academic and industrial stakeholders to identify and discuss technicalchallenges in D2D communications, and their position on the 5G roadmap towardmeeting the 1000x challenge

This book is organized in a well-defined structure as shown by Fig.1, that notonly elaborates on the progress toward D2D technology solutions, but also detailspotential use cases, business models, and real time applications In particular,Chap 1 presents an overview tutorial on D2D communication aspects that

v

includes the extension of the 3GPP SAE architecture to support D2D scenarios;definition of the D2D protocol stack; design aspects on D2D communication; linkadaptation; power control; and channel measurement methods in D2D Moreover,

it will elaborate on the use cases, business, and application opportunities that exist

to outline the market potential for this technology In Chap 2, we provide adetailed analysis of the evolved LTE-A access, core and protocol architecture tosupport D2D communication In addition, a comprehensive literature review oncoexistence issues between D2D and cellular communication is given.Chapter 3explains the node/peer discovery and mode selection for D2D communication inthe LTE-A band In the node discovery section, we explain the existing research ondirect discovery that provides the baseline for the novel FlashLinQ technique Thischapter also reviews and classifies the state-of-the-art research on mode selectionand then introduces a queuing model under busty traffic conditions, and highlightsthe challenges and open issues to serve as guidelines for future research So far, wehave discussed the D2D protocol stack, and its node discovery and mode selectionapproach

After selecting the preferable mode, it is important to control the interferencebetween different D2D pairs, and toward other cellular user In this respect,Chap 4explains interference management in D2D network, characterizing this interferenceand highlighting open challenges on this area Thereafter, Chap 5 explainsthe establishment and maintenance of D2D communications It will elaborate therandom access and the retransmission approach for D2D communications, and willpresent some novel proposals for these schemes

Until now we have addressed potential scenarios for the D2D communicationparadigm, and its position on the 5G roadmap toward enabling cost effectivecommunications for proximity-based services However, inChap 6we will detailpotential use-cases that give us an in-depth analysis for system requirements andarchitectural design Specifically, we elaborate on application areas of D2Dcommunications which include cellular network offloading and coverage exten-sion, proximity-based social networking, and providing national security andpublic safety in infrastructure-less situations So far, we have discussed D2Dconnectivity over licensed band However,Chap 7will present different config-urations of D2D communication, i.e., control is still performed by the base station,but data is transferred locally using unlicensed band This kind of D2D architec-ture helps the operator to free some licensed band for other services Moreover,D2D communication can be viewed as one more layer within a HetNet environ-ment which offloads the traffic from both the small cell and macrocell usinglicensed or un-licensed band, as explained in Chap 7 Hence, in Chap 8 weanalyze the performance of incorporating D2D communication in HetNets; com-parisons will be made against a full small-cell deployment in HetNets in terms ofcapacity and backhaul power consumption The last two chapters will explain thedifferent applications of D2D communication: Chap 9 will explain D2D com-munication in mobile cloud architectures This chapter will introduce the concept

of mobile cloud as an efficient platform for cooperative content distribution byexploiting D2D communication Both energy and spectral efficiency aspects ofcommunications will be taken into account, in addition to the throughputenhancement offered by mobile clouds Similarly, Chap 10 will explain theapplication of D2D communication for smart grids

The editors believe that D2D can offer a palette of interesting colors that canpaint new business opportunities for mobile stakeholders promoting it as a strongcandidate technology for next generation wireless communication system

This book is the first of its kind tackling smart device-to-smart device nications, and its inspiration comes from the editors’ experience at the forefront ofEuropean research on D2D However, the editors would also like to thank not onlythe collaborators that have contributed with chapters toward this book, but also the4TELL Research Group at the Instituto de Telecomunicações –Aveiro that haveprovided valuable comments and contributions toward the compilation of thisbook The editors would also like to acknowledge the Eureka Celtic GREEN-Tthat has progressed the state-of-the-art on this fast evolving subject.

commu-ix

Introduction to D2D Communication 1Shahid Mumtaz and Jonathan Rodriguez

LTE-A Access, Core, and Protocol Architecture for D2D

Communication 23Dimitris Tsolkas, Eirini Liotou, Nikos Passas and Lazaros Merakos

Node/Peer Discovery, Mode Selection, and Signaling for D2D

Communication in LTE-A Band 41Lei Lei and Yiru Kuang

Interference Management in D2D Communication 89Daesik Hong and Seokjung Kim

Establishment and Maintenance of D2D Communication 113Shaoyi Xu

Network Assisted Device-to-Device Communications: Use Cases,

Design Approaches, and Performance Aspects 135Gabor Fodor, Stefano Sorrentino and Shabnam Sultana

Network-Assisted D2D Over WiFi Direct 165Alexander Pyattaev, Olga Galinina, Kerstin Johnsson,

Adam Surak, Roman Florea, Sergey Andreev and Yevgeni Koucheryavy

Device-to-Device Communication in Heterogeneous Networks 219Yusuf A Sambo, Muhammad Z Shakir, Fabien Héliot,

Muhammad A Imran, Shahid Mumtaz and Khalid A Qaraqe

xi

D2D-Based Mobile Clouds for Energy- and Spectral-Efficient

Content Distribution 237Hamidreza Bagheri, Marcos Katz, Frank H P Fitzek,

Daniel E Lucani and Morten V Pedersen

Interdependency Between Mobile and Electricity Distribution

Networks: Outlook and Prospects 281

S Horsmanheimo, N Maskey and L Tuomimäki

Shahid Mumtaz and Jonathan Rodriguez

The first wave of fourth generation systems is finally being deployed over Europe,providing a universal platform for broadband mobile services at any time, anyplace, and anywhere However, mobile traffic is still growing and the need formore sophisticated broadband services will further push the limit on currentstandards to provide even tighter integration between wireless technologiesand higher speeds, requiring a new generation of mobile communications: theso-called 5G The evolution toward 5G is considered to be the convergence ofInternet services with existing mobile networking standards leading to the com-monly used term ‘‘mobile internet’’ over heterogeneous networks (HetNets), withvery high connectivity speeds In addition, green communications seem to play apivotal role in this evolutionary path with key mobile stake holders drivingmomentum toward a greener society through cost-effective design approaches Infact, it is becoming increasingly clear from new emerging services and techno-logical trends that energy and cost per bit reduction, service ubiquity and highspeed connectivity are becoming desirable traits for the next generation networks.Providing a step toward the future wireless world, small cells are envisaged as thevehicle for ubiquitous services providing cost-effective high speed communica-tions However, another popular approach that is gaining much interest by mobilestakeholders is device-to-device (D2D) for connecting client devices in closeproximity The major driving force for D2D connectivity is the inherent flexibilityfor operators to offload traffic from the core network, and represents a real stepforward for operators to reduce the energy and cost per bit, particularly for sup-porting proximity-based services such as social networking

S Mumtaz ( &)  J Rodriguez

Instituto de Telecomunicações, Aveiro, Portugal

1 D2D Versus Ad Hoc Network

Recently, wireless local-area network (WLAN) technologies based on the IEEE802.11 standards (e.g., Wi-Fi, Wi-Fi Direct) and wireless personal-area network(WPAN) technologies (e.g., Bluetooth, Ultra Wideband (UWB) technologies)have been increasingly used because they provide Internet access and local ser-vices with low cost and fast access through the license exempt bands (e.g.,industrial, scientific, and medical (ISM) radio bands) These technologies aredesigned for short distances between sender and receiver and therefore achievevery high data rates with low energy consumption

However, communications on a licensed band of a cellular network can bebetter in terms of interference avoidance under a controlled environment Fem-tocells, relays, and picocells are examples of another kind of technologies thatwork under LTE-A license band Table1provides a detailed comparison of thesetechnologies

Following are the shortcomings of the above-mentioned technologies

(a) As Wi-Fi and Bluetooth work in license exempt band, there is no guaranteethat they will work in all places since there is always the possibility of thepresence of an interfering communication system or other sources ofinterference;

(b) Wi-Fi Direct can be used in all public places in the near future as devicesbecome available, but this technology lacks global synchronization (i.e.,synchronization can be used in wireless systems, generally to enable energy-efficient operations For devices to discover each other, they must rendezvous

in space and time Only in a synchronized system can the discovery periods beboth frequent and of low duty cycle Thus, in practice, devices operatingautonomously without infrastructure support in unlicensed spectrum cansynchronize, but only locally [1]);

(c) On the other hand, femtocells, relays, and picocells are all infrastructure-basednetworks in which traffic goes through a centralized node, such as a FemtoBase Station (FBS), even though the source and destination are close to eachother (SenderUser ? FBS/Pico/Relay ? ReceiverUser);

(d) Femtocells and picocells require a separate backhaul connection and act as aseparate base station which increases the installation and energy costs for themobile operators

It has been argued that the biggest cost challenge faced by wireless serviceproviders today is the backhaul network This infrastructure is very expensive tomaintain, energy consuming, and difficult to scale

A very recent and less tackled topic is D2D, direct terminal-to-terminal (DTT

or T2T), peer-to-peer (P2P), handset-to-handset, mobile-to-mobile (M2M), LTE, or ad hoc communication underlying cellular networks

Inband D2D has the following advantages: underlay D2D increases the spectralefficiency of cellular spectrum; and all cellular devices are capable of using InbandD2D communication and QoS management is easy because the cellular spectrumcan be fully controlled by eNB.

The disadvantages of Inband D2D communication are: cellular resources might

be wasted in overlay D2D; the interference management among D2D and cellulartransmission in underlay is very challenging; power control and interferencemanagement solutions usually resort to high complexity resource allocationmethods; and a user cannot have simultaneous cellular and D2D transmissions

It appears that underlay D2D communication is more popular than overlay.However, allocating dedicated spectrum resources to D2D users is not as efficient

as underlay in term of spectral efficiency We believe that the popularity ofunderlay D2D is due to its higher spectral efficiency

Outband D2D: The D2D communication under this category exploits censed spectrum The motivation behind using outband D2D communication iseliminating the interference issue between D2D and cellular links Using unli-censed spectrum requires an extra interface and usually adopts other wirelesstechnologies, such as Wi-Fi Direct, ZigBee, or Bluetooth Outband D2D is furtherdivided into controlled and autonomous communication

unli-In controlled outband D2D communication, the control of second interface/technology is under cellular network and in autonomous outband D2D commu-nication, cellular network controls all the communication but leaves the D2Dcommunication to the users (second interface/technology in not under cellularcontrol) Since outband D2D communication does not occur on cellular spectrum,there is no interference issue as in Inband D2D However, only cellular deviceswith two wireless interfaces (e.g., LTE and Wi-Fi) can use outband D2D, and thususers can have simultaneous D2D cellular communication

3 D2D-Based Cellular Communication

In D2D communication, devices communicate with each other without diate nodes D2D communication uses cellular spectrum (license band) supported

interme-by a cellular infrastructure and promises three types of gain: (a) the proximity ofuser equipments (UE) may allow for extremely high bit rates, low delays, and lowenergy consumption [2,3]; (b) the reuse gain implies that radio resources may besimultaneously used by cellular as well as D2D links, tightening the reuse factor sothat the same spectral resource can be used more than once within the same cell[2]; (c) finally, there is a gain from not having to use both an uplink (UL) and adownlink (DL) resource, as is the case when communicating via the access point inthe cellular mode Moreover, D2D communication may extend the cellular cov-erage and facilitate new types of wireless P2P services D2D is also economicalcommunication because it uses the same pre-existing cellular infrastructure whichincreases network efficiency This increased network efficiency supports moreservices and improves current services and applications

In the past, cellular operators did not consider D2D communication as a method

to enhance the performance of cellular network because the effect of D2D munication is limited to local communication services However, as mobileapplications based on proximity of mobile devices has become increasinglypopular, cellular operators are considering introducing D2D communication intothe cellular networks

com-When operators enable D2D communication in the system, they can see severalbenefits compared to the conventional infrastructure-based communication, such

as improved energy efficiency, increased overall system throughput, and decreasedtraffic load in the network, as shown in Fig.1

4 Current Progress in Standardization

As early as the 3GPP meeting held in June 2011, a study item description on theradio aspects of D2D discovery and communication was submitted by Qualcomm.Meanwhile, a study item description on LTE Direct (LTE-D) was submitted to the3GPP meeting held in August 2011, which proposed the study of the servicerequirement for direct over-the-air LTE D2D discovery and communication Themeeting of November 2011 studied use cases and identified potential requirementsfor a network operator controlled discovery and communication between proxi-mate devices As the finalization of LTE Rel-11, 3GPP initiated the new step onRel-12 and beyond which started as a workshop in June 2012 At that workshop, itwas agreed that the machine type and short-range communication scenarios should

be embraced to give rise to new traffic types As a follow-up to the workshop, itwas agreed to start a study on LTE Proximity Services (ProSe) in the Radio AccessNetwork (RAN) 58 plenary meeting in December 2012 [4] The study on ProSeincludes two parts, namely D2D discovery and D2D communication The mainresults on D2D use cases and potential requirements are captured in [5] and theneeded architectural enhancements to support ProSe in [6] The current work onLTE D2D device discovery and D2D communication mainly focuses on thetechnical details, including discovery signal design, resource allocation andscheduling, synchronization mechanism, etc For IEEE to continue Wi-Fi Directresearch, IEEE 802.11 Infrastructure Service Discovery Study Group has donemuch work on proximate discovery and communication with low energy, longrange (up to 500 m), and large scale (up to 1,000 mobile devices) for mobile socialnetworks since 2010

5 D2D Functional Block in LTE-A SAE Architecture

3GPP technology has the opportunity to become the platform of choice to enableD2D communication The ProSe can be divided into two parts, proximity dis-covery and direct communication, as shown in Fig.2a

With proximity discovery, users can discover other users that are in the imity Discovery mechanisms can be network or user-assisted Proximity discov-ery can be a standalone service to users, for example to enhance social networkingservices and does not need to trigger direct communication Direct communicationrefers to devices communicating directly, without going via a base station, whenthey are within reach of each other Users may initiate direct communicationdirectly without proximity discovery However, proximity discovery is a naturaltrigger for direct communication that can reduce the need for manual interaction

prox-In particular, when direct communication is integrated with a cellular network, it isconsidered natural that it is preceded by network supported proximity discovery.From an architectural view the solution for ProSe involves both UEs, RAN, core

network, and application servers Although different architecture alternatives arestill being evaluated in 3GPP there are some common characteristics that can beidentified In the core, a new network function is proposed that can be added toprovide the ProSe which may be called Proximity Function or ‘‘Proximity Server’’[5] The proximity server would provide the connection between applicationservers and the mobile network It could identify proximity between UEs andinform the application servers about the opportunities D2D sessions could beinitiated from the proximity servers by sending an initiation request to the MME.The MME is then responsible for initiating the D2D radio bearer setup using Udinterface and delivery of IP address for D2D terminating devices Letting theMME handle the connectivity helps D2D communications offer seamless opera-tions to the higher layer protocol stack, and mobility processes between D2D andcellular networking [2].

An alternative solution is to rely on IMS, where the proximity function could beimplemented by an application server that reuses already specified securitymechanisms and interfaces to retrieve location information from the network Thiswould minimize the required architectural changes by reusing the IMS servicecontrol interface to communicate with UEs and other network entities Previously,Doppler et al [2, 7] proposed two mechanisms of D2D connectivity based onSession Initiation Protocol (SIP) and Internet Protocol (IP), which did not rely asheavily on the existing IMS functionality, but proposed a closer integration withthe EPC and RAN

System architecture for D2D communications integrated with the EvolvedPacket System (EPS) is illustrated in Fig.3

In the access network ProSe use the cellular link (Uu) for control plan For dataplan, direct communication would need a new Direct Mobile Communicationinterface (Ud), as shown in Fig.2b

Data

Data Control

Control

Proximity Server

5.1 Protocol Stack and Bearer Management

Figure4a shows a D2D control plan which reuses the LTE-A control protocolstack and Fig.4b introduces the new interface named Ud for data plan which isintegrated into the LTE-A data protocol stack One or more radio bearers are set upfor the transmission of the data plan over the direct path The PHY, MAC, RLC,and PDCP layers for these bearers are terminated at the UEs Each UE is alsosimultaneously connected to an eNB The control plan protocols, namely RRC andNAS, are terminated between each UE and the corresponding eNB and MMErespectively The NAS procedures for service request are altered to include D2Daspects Changes to the RRC protocol are necessary for the establishment andmanagement of the direct path radio bearer In contrast to current LTE-A systems,where the radio bearer is terminated at the UE and the eNB, the endpoints of thedirect path communication are at the two UEs Hence, the RRC configurationprovided by the eNB to both UEs should be compatible with each other RadioLink monitoring, measurement, and handover procedures also need to be altered toaccommodate direct path aspects

The direct path data radio bearer is terminated at the UEs One possibility is thatthe D2D communication works quite autonomously In particular, for the publicsafety use cases it is a requirement that the D2D communication works when there

is no network coverage This may motivate a solution with contention-basedaccess which does not require any centralized resource management However, wewill focus on the case where resource allocation functionality is still retained bythe eNB In this case, there could be a separate MAC entity for D2D in the UE, and

a similar MAC D2D entity in the eNB The functionalities of the D2D MAC in the

Internet

IMS (IP Multimedia Subsystem)

UE1-MME-D2D bearer

permissi

on (IP address

delivery)

Serving gateway

Networking session (SIPsever) D2D Link:

User plan,Link adaptation,HARQ, Power control, Resource allocation.

UE may include data transfer, HARQ, BSR reporting, multiplexing of multipleD2D Logical Channels, and Logical Channel prioritization.

One possible solution to keep D2D communication simple is to only supportunidirectional communication between devices In this case, there would not be anyHARQ, ARQ, or other protocol mechanisms that require bidirectional communi-cation at the MAC or RLC layers Reliability could instead be implemented at higherlayers The functionalities of the MAC D2D entity in the eNB may include resourceallocation and per-TTI scheduling Varying degrees of eNB control can be envi-sioned in the scheduling process, broadly divided into two categories—(a) Full eNBcontrol of per-TTI scheduling or (b) eNB resource grant UE scheduling The directpath may have a simplified RLC, perhaps with no RLC retransmissions In the PDCPfunctionality, UE support of simultaneous separate ciphering for infrastructure anddirect path bearers are required, as well as altered procedures for PDCP sequencepreservation during mobility events between infrastructure and direct path.Once the D2D bearer has been set up between peer devices, the eNB controlsthe radio resources which are used for D2D communication For this purpose, anextended version of the current RRC protocol can be used as illustrated in Fig.3 Itmay also be necessary for D2D users to remain connected with the Internet foradditional services In LTE-A this is achieved if the SAE bearer is preserved andconnectivity to the gateway is maintained It would also be necessary for the RRCprotocol to handle D2D and cellular connections in parallel To maintain the bestconnectivity, the handover from D2D network to cellular network shall take placewhen the cellular connection attains more throughput and lower energy con-sumption than that of D2D

5.2 Security Architecture for D2D

Based on the security architecture defined in [8] and the ProSe high-level referencemodel defined in [5], the ProSe security architecture is as illustrated in Fig.5 As

L1/L2 IP UDP GTP-C

NAS GTP-C

UDP IP L1/L2

MME

S11

MAC RLC PDCP IP App

MAC RLC PDCP IP App

MAC RLC PDCP IP App

MAC RLC PDCP

L1/L2 IP UDP GTP-U

L1/L2 IP UDP GTP-U

L1/L2 IP UDP GTP-U IP

UE1 UE2

PHY PHY PHY

PHY Uu Uu D2D D2D

suggested in [8], there are five main security feature groups Each group isdesigned to protect against certain attacks and achieves certain security goals:

• Network access security (1): it involves the security interactions between theuser and access network, which provides protection against attacks on the radioaccess link;

• Network domain security (2): it enables the secure data/control signal exchangeamong network elements, which provides protection against attacks on wire-linenetwork;

• User domain security (3): secures the access to the mobile station;

• Application domain (4): is an end-to-end security between the application onuser equipment and servers;

• Visibility and configuration security (5): the set of features that control theavailability and configuration of certain security service

We highlight the three changes brought by ProSe, which are

• ProSe Function

This function is a set of software module existing in the core network It interactswith HSS, MME, ProSe application server, and any other network element/modulerelated to ProSe via wire-line It is responsible for recoding user-specific config-uration, authorize node discovery, etc

• ProSe application server and applications

The server existing in the IP network, and has a logical link to the end-userapplications on UEs

• Radio link between two UEs

This radio link is some radio resource in LTE-A frequency band managed bythe network (if available), and dedicated to direct communications between twoUEs

6 D2D Design Aspects

6.1 Modulation Format

The first question on direct communication is on the selection of waveform format.Currently, LTE uses SC-FDMA in the UL and OFDMA in the DL, so the UE isequipped with an SC-FDMA transmitter and an OFDMA receiver If SC-FDMA isused, the D2D UE needs to be equipped with a new SC-FDMA receiver Com-pared to implementing an OFDMA transmitter, implementing SC-FDMA receiver

is more complex since the single carrier transmission requires relatively complexequalization at the receiver However, SC-FDMA transmitter can maintain lowpeak to average power ratio (PARA)

6.2 Frame Structure

Figure6a shows the frame structure for cellular and D2D communication Weassume that all communication is done in FDD band Cellular communicationtakes place in FDD DL while D2D takes place in UL band in FDD and ULsubframe in TDD It is noted that 3GPP has already made the similar workassumption This is because the traffic load of DL is currently much heavier thanthat of UL in cellular networks, which causes inefficient spectrum usage of ULespecially in symmetric radio resource partition systems, such as symmetric FDD.The trend is expected to continue due to the development of the mobile Internet.From this perspective, camping D2D on UL resource is more reasonable especially

in FDD More importantly, the total interference level in UL spectrum is cantly less than that in DL spectrum, which may have substantial impact ontransmission performance Time slots in UL frames are assigned for D2D signalingand for data The signaling occurs between the BS and the D2D and therefore it isseen as cellular communication All CU are able to transmit and receive inde-pendently of other D2D links Most communication sessions are bidirectional Themutual independence makes the D2D communication flexible; larger data transfercan occur simultaneously with short bidirectional exchange of data, without onehaving to wait for the other and less coordination is required from eNB (i.e., lesssignaling by BS)

signifi-6.3 Burst Timing

The timing of the transmissions is one of the most important aspects in D2D network

as well as in conventional cellular systems since it is necessary to synchronize senderand receiver and to manage interference efficiently The requirements on the timing

accuracy in a slotted communication system depend on the specific system design.LTE-A uses frame transmission with long symbol duration (LTE-A symbol length is

67 ls) and cyclic signal extension (LTE-A has 5 and 17 ls defined), which eases thetime synchronization requirements of the frames Given that D2D links are practicalmainly for devices with relatively low mobility speeds, normal cyclic extension ofLTE-A should be more than enough for D2D links In principle, it would be suffi-cient with shorter extensions, but keeping the LTE-symbol compatibility seemsbeneficial from an implementation perspective In addition, a potential multi-hopsolution for D2D communication could benefit from this large time tolerance Theburst timing might originate from the eNB timing when one of the D2D terminatingdevices communicates to the eNB

6.4 Signaling

Signaling is used to manage the resource allocation for all communication devices.The BS should inform D2D devices which frequency band and time slot they aregoing to use Preliminary signaling for D2D communication is performed on thetime slots which are allocated for cellular communications; there is no specificsignaling time slot for D2D communication When a D2D session is initiated,devices may need to signal each other to coordinate and negotiate the send andreceive times Predefined preambles can be used for the start and the end messages.The dynamics of the session play a major role for signaling overhead (i.e., addi-tional signaling is required if links are short-lived due to mobility or resourceavailability)

6.5 Synchronization

In an LTE network-assisted D2D scenario, the two UEs of the D2D pair aresynchronized with the eNB, implying that slot and frame timing as well as fre-quency synchronization is acquired Also, other fundamental system parameters(such as cyclic prefix (CP) length and duplexing mode) are known by the UEs

FDD DL

FDD UL

D2D device CSI

Resource (Dedicated or shared) Link Adapatation/

Therefore, the D2D candidates can be assumed to be synchronized to each otherprior to D2D bearer establishment (e.g., assuming 5 ls CP and 300 m/ls signalpropagation speed, a D2D pair can be assumed to be time synchronized within the

CP up to 1,000–1,500 m distance which is significantly greater than what we canassume for the D2D distance) To maintain time (i.e., OFDM symbol) and fre-quency synchronization in D2D mode (between two subsequent eNB modeselection decision instances), the D2D pair could use reference signals (RS)similar to the LTE demodulation reference signals (DMRS) For example, for thephysical uplink shared channel (PUSCH), LTE uses DMRS in every slot and asimilar solution can be used for the D2D bearer as well However, we can assumethat the UEs keep synchronizing with their serving eNB and therefore, in practicethere will probably be no need for a specific D2D synchronization mechanism

6.6 Mobility

The mobility range of D2D is limited, due to the limited transmission power Theacceptable ranges of D2D links in environments where D2D coexists with cellularnetwork need to be further evaluated D2D radio should be designed for relativelystationary devices due to its short range However, some limited mobility supportshould be offered The conventional mobility situation is to handover IP connec-tions from cellular networks to D2D networks and vice versa if possible

6.7 Link Adaptation

Link adaptation (LA) targets to make the most of the system efficiency and this can

be achieved by providing self-adaptation of the functional points to the dynamics

of the signal-to-interference-plus-noise-ratios and block error rates (BLER).The main building block of LA is the selection of modulation and codingscheme, but also automated repeat request (ARQ) retransmission can be seen as apart of the LA LA uses channel measurements to select the instantaneous mod-ulation and code rate, but also information about the amount of data in thetransmission buffer can be used to identify when there is no need to maximize thedata rate Hybrid ARQ (HARQ) can increase the efficiency compared to traditionalARQ by combining multiple transmissions

The BLER in D2D mode could vary largely depending on whether a D2D linkoperates on dedicated resources or reuses cellular resources The BLER operationpoint might be higher when compared to cellular links, and higher variationbetween different frames can be expected due to the less, controlled interferenceenvironment For the mapping of SINR to BLER points, the EESM (ExponentialEffective SINR Mapping) system metric is used LA and HARQ can be handleddirectly by D2D devices in the case of D2D communication, as shown in Fig.6b

6.8 HARQ Operation

HARQ combines forward error correction (FEC) and ARQ retransmission As theinterference situation may be quite complex and dynamic for D2D communica-tion, HARQ would make the D2D communication more robust D2D HARQ may

be either indirect or direct In indirect HARQ, D2D receiver first sends ACK/NACK to the eNB and then eNB relays ACK/NACK to the D2D transmitter.Indirect HARQ allows reusing existing LTE DL and UL channels with minimalchanges at the cost of additional overhead and possibly longer feedback delay Indirect HARQ, D2D receiver directly sends ACK/NACK to the transmitter DirectHARQ may be used in either in-coverage or out-of-coverage scenario

6.9 Channel Measurements

The required channel measurements and measurement reporting depends on thedegree to which the network is involved in the resource assignment and LA.Measurements of the received strength of the RS transmitted by eNBs on the DLcan be used to estimate the interference that the D2D transmissions will cause.Therefore, measurement reports of these can be useful to the eNB when it assignsresources for the D2D links In the UL, LTE has two different types of referencesymbols: sounding reference symbols and demodulation reference symbols.The sounding RS are transmitted on a wider bandwidth than the actual datatransmission to obtain channel information for the UL scheduling decisions Thismay also be useful in a D2D scenario, if the network has a strict control of theresource allocation, as will be described herein The demodulation reference sym-bols which are transmitted along the physical resource blocks (PRBs) of the payloadare used in channel decoding, demodulation, equalization, and estimation, and couldserve the same purpose as in D2D However, it remains to be studied whether other

RS would be better suited for D2D communications A possible procedure forchannel exchange information is shown in Fig.7 Here we assume that eNB has fullcontrol over radio resource, which is allocated to D2D users through L1/L2 controlsignaling (e.g., Physical Downlink Control Channel (PDCCH)) Resources areallocated on per-TTI basis which is equal to 1 ms in LTE

Assume UE1 and UE2 have established a D2D connection and UE1 has datawaiting to be transmitted to UE2 The eNB is responsible for resource allocation.First, UE1 notifies the eNB that it has data to be transmitted to UE2 According tothe LTE protocol, UE1 can send a buffer status report (BSR) to the eNB throughthe PUSCH for this purpose If no UL resources are available for the BSRtransmission, UE1 can send a one-bit scheduling request (SR) signaling throughthe Physical Uplink Control Channel (PUCCH) Once the eNB receives the SRfrom UE1, it will allocate a small amount of UL resources for the BSRtransmission

After the eNB receives the notification (e.g., BSR) from UE1, it will allocateresources for the data transmission between UE1 and UE2 The resource allocationalgorithms will be discussed inSect 6.10 In an LTE-A system, the eNB usuallyconsiders the channel status when performing resource allocation For the D2Dcommunications, the eNB can obtain the channel status of D2D links between UE1and UE2 by the periodic or aperiodic channel quality indication (CQI) reports fromUE1 and UE2 through the PUCCH It is assumed that UE1/UE2 can perform theCQI estimation from the received Sounding Reference Signal (SRS) transmitted

by its D2D peer

6.10 Power Control

D2D communication can work in both fixed power scheme and fixed SNR targetscheme In the fixed Tx power case, all users in D2D mode use the same Tx power.This scheme is simple, but it does not work well due to the possible large dynamicrange of the D2D SINR The dynamic range of D2D SINR is dependent on theoverall interference situation With random resource allocation/scheduling for allUEs, there can be significant interference from cellular mode UEs to D2D modeUEs In this case, the dynamic range of D2D SINR is rather large, but withdedicated resources the dynamic range is lower In the fixed SNR target case, theselection of the SNR target will affect the total Tx power and the final SINR

UE1

PDCCH (UL g rant)

PUCCH (SR,CQI)PUSCH (BSR)

PDCCH (UL g

rant) PUCCH (CQI)

directly High SINR target requires more Tx power for D2D users, and the finalSINR for those D2D users can be improved However, there could be a risk ofincreasing the overall interference level to the users of the cellular mode.Other schemes are an (LTE-A) open loop1fraction power control scheme and aclosed loop2power control scheme In both schemes, the maximum allowed power

is set to Pmax= 24 dBm Power control plays an important role in RRM functionwhen D2D and cellular links use overlapping resources

7 Use Cases, Application Business Opportunities,

and Open Issues

The D2D use cases can be divided into two categories The first category is simpleD2D communication in which sender and receiver exchange data with each otherand in the second category D2D users act as a relay to the other users, i.e., forwarddata to and from other users Other use cases for D2D are described as follows.(a) Local Data Service

The services which can possibly benefit from direct communication includeinformation sharing, mobile multiplayer gaming, mobile advertising, streamingservices, social or community services with D2D, and extending D2D concepts to

a mobile relay The common denominator of these applications is that they build

on some local connectivity service facilitating communications between people,machines, and sensors in proximity to each other Such applications are certainlypossible to build over existing and evolving cellular networks without the appli-cations being aware of the underlying technology However, D2D technologiesmay offer advantages to some applications that exploit the physical proximity ofthe communicating parties in terms of latency, battery consumption, or end-userprivacy Irrespective of the underlying technology, we foresee the need for atechnology agnostic communication control layer containing functions such asmobility control, user data routing, proximity detection, and security management.(b) Public Safety

3GPP also defines various use cases and their potential requirement for D2DProSe Basically, these application scenarios can be classified as General and

1 Open loop power control is the ability of the UE transmitter to set its output power to a specific value It is used for setting initial uplink and downlink transmission powers when a UE is accessing the network.

2 Closed loop power control is the ability of the UE transmitter to adjust its initial uplink output power in accordance with one or more Transmit Power Control (TPC) commands received in the downlink, in order to keep the received uplink Signal-to-Interference Ratio (SIR) at a given SIR target.

Public Safety (PS) use cases from subscribers’ perspective [5] From thedeployment perspective, they can be categorized into within network coverage andoutside network coverage Figure8a shows the deployment scenario Use casedescriptions and requirements are captured in [5].

(c) Data Security

The other benefit of D2D communication is the added security that it offers,since in D2D, data are not routed through Internet clouds and hence not storedanywhere but on the specified devices

7.1 Vehicle-to-Vehicle Communication

D2D can be used in Vehicle-to-Vehicle (V2V) communication because of thestrict delay requirement in some traffic safety use cases In particular, for collisionavoidance systems it will be essential to have very low latency, for example tocoordinate braking between vehicles Autonomous safety systems without anyV2V communication need to detect if there are braking cars in front based on, forexample, the observed distance to the car, while V2V communication can providenot only information about the nearest car in front, but also from other cars within

eNBBenefitted user

(a)

the communication range, including meeting traffic Therefore, it can get morecomplete and reliable information, which would be useful to avoid accidents.

By using D2D rather than relying on infrastructure, it is possible to both reduce thelatency and to design a solution that works without cellular network coverage.Another use case where D2D can be useful is in group handover of multipleusers, for example in a bus as shown in Fig.8b By using D2D, the devices in agroup could inform each other about the handover and its parameters, withminimal signaling from the network

7.2 Multiuser Cooperative Communication

in Heterogeneous Network

D2D can be used as an innovative idea of Multiuser cooperative communication(MUCC) Figure8c shows the Multiuser cooperative communication (MUCC)scenario As an example, benefited user (BU) is in an area with poor cellularsignal There is another user in the area with good cellular signal This user mayhelp the benefited user to improve its signal and act as a supporting user (SU).There are two radio paths for the benefitted user: BU to eNB (directly) and SU topicocells directly The BU and SU communicate with each other using LTE-AD2D communication This kind of communication improves the systemthroughput because the system always schedules the best user with the bestchannel quality There are several channel/pipes and the probability that allchannels are deteriorated at the same time is quite small and this will increasereliability Furthermore, any single pipe optimization can be used at the same time(e.g., MIMO) as well MUCC further enhances the performance of this scheme

by using context information (i.e., location information, etc.) of the users

In summary, MUCC will increase reliability, throughput, energy efficiency, andlow delay for the mobile users

7.3 D2D Communications with Network Coding

For example, as one scenario of multi-hop D2D communications, if multiple UEsare requesting the same contents from the eNB, they can first form cooperativeclusters according to the geometry, to achieve a higher energy efficiency andspectrum efficiency during the content distribution In the first step, the eNB willtransmit the contents to the cluster heads In the second step, each cluster head will

in turn multicast the contents to other UEs within the cluster through D2D links.The eNB can stay silent during the second step and hence keep the network energyefficient The application of this multi-hop D2D communication scenario includesvideo streaming of most popular programs, for instance, during world cup, whenmultiple UEs are watching the same football match In such multi-hop D2D

communications, network coding, which is a promising mechanism in cooperativenetworks to improve the throughput, can be applied.

7.4 D2D Communications Energy Saving

As shown in Fig.9, for instance, a user (UE1) is going out to a stadium from hishouse to see a football game Let us assume that, BS allows UE1 and UE2 tocommunicate directly to each other while keeping some control over the D2D link

to limit the interference to the cellular receiver Consider the case where a footballorganizer puts a video server at a stadium location from which spectators candownload the game information (i.e., live score, player statistics, spectatorattendances, etc.) using the D2D connection over LTE-A band At the same time,the cellular network can handle phone calls and Internet data traffic without theadditional load that would be caused by traffic from the video server The D2Doperation itself can be transparent to the user UE1 simply enters a URL; thenetwork would detect traffic to the video server and hand it over to a D2D con-nection The same application could also be enabled by a video server with built-inWLAN AP (Access Point) or Bluetooth However, in that case the user has todefine the WLAN AP or perform Bluetooth pairing which can be tedious espe-cially if a secure connection is required Compared to other local connectivitysolutions (based on for example, Bluetooth or WLAN), the D2D communication

supported by a cellular network offers additional compelling advantages First, thenetwork can advertise local services available within the current cell Thus forautomated service discovery, the devices do not have to constantly scan foravailable WLAN AP or Bluetooth devices This is especially advantageous whenconsidering that the constant scanning of Bluetooth devices or WLAN APs is oftenswitched off to reduce the power consumption Second, the cellular network candistribute encryption keys to both D2D devices so that a secure connection can beestablished without manual pairing of devices or entering encryption keys.

It should be noted that these are only a subset of possible business models andfurther discussion is required to develop additional LTE D2D business models

8 Conclusion

In this chapter, we presented a tutorial on mobile-to-mobile communication toassist the mobile stakeholders to evaluate the benefits of D2D technology D2Dcommunication assisted by a cellular network takes advantage of the proximity ofmobile devices to allow reusing resources between cellular and D2D users, and

takes further advantage of hop gain 3GPP started a study item on proximity-basedservices in D2D in release 12 This chapter explains SAE architecture for D2Dnetwork, which includes the core and access part, the protocol stack for D2Dcommunication and different deployment scenarios, design aspects of D2D com-munication, LA, power control, and channel measurement methods in D2Dcommunication and different building blocks for D2D communication which arenecessary when establishing the D2D session; and moreover elaborated on the usecases, business, and application opportunities that exist.

3 F Gábor et al., Design aspects of network assisted device to device communication IEEE Commun Mag 50(3), 170–177 (2012)

4 R1-132861 Final report of 3GPP TSG RAN WG1 #73 v1.0.0, August 2013

5 3GPP TR 22.803 v12.1.0, Feasibility study for proximity services (ProSe), 2013

6 R1-132115, Discussions on LTE device to device communication, ZTE, May 2013

7 K Doppler, M Rinne, C Wijting, C Ribeiro, K Hugl, Device-to device communication as an underlay to LTE-advanced networks IEEE Commun Mag 47(12), 42–49 (2009)

8 3GPP, TR 33.401, v12.9.0 ‘‘Security Architecture’’, Release-12, Sept 2013

Architecture for D2D Communication

Dimitris Tsolkas, Eirini Liotou, Nikos Passas and Lazaros Merakos

1 Introduction to D2D Communications

The term device-to-device (D2D) communications refers to direct short-rangecommunications between terminals of a mobile network, without the intermediatetransmission to a base station (BS) Differing from conventional approaches, such asBluetooth and WiFi-Direct, D2D communications utilize licensed spectrum withquality of service (QoS) guarantees, while no manual network detection-selection isrequired Compared to the very appealing cognitive radio communications, wheresecondary transmissions are allowed in parallel with cellular (primary) transmis-sions, D2D communications are established by cellular (primary) users, reaping thebenefits of being synchronized and controlled by the BS

The introduction of D2D communications in cellular networks is expected to bebeneficial from a variety of perspectives, shifting the current cellular communi-cations to a more flexible and dynamic state The short distance between D2Dtransmitter and receiver provides better link conditions and, thus, more efficientconnection with lower energy consumption From the network’s perspective, theuse of spectrum and processing resources is reduced, since the intermediatetransmissions to the BS are avoided Moreover, the coexistence of cellular andD2D transmissions in shared spectrum bands can lead to higher spectrum utili-zation, offloading at the same time the cellular network From the operators’ point

D Tsolkas ( &)  E Liotou  N Passas  L Merakos

Department of Informatics and Telecommunications, University of Athens,

of view, new business models, probably with a new charging policy, may bedesigned, without the need for purchasing additional spectrum.

In the standardization field, although direct communications are already offered

by local area networks in unlicensed ISM bands (e.g., WiFi-Direct), D2D munications are absent from most cellular systems For the Long Term Evolution(LTE) system [1], the first standardization efforts have recently begun in Release

com-12, in which D2D communications are mainly examined under the perspective ofproviding new commercial or public safety proximity services (ProSe) [2] Inparallel, academia copes with a series of challenges toward enabling D2D com-munications (referred to as ProSe communications) in licensed spectrum.The remainder of this chapter is organized as follows First, we provide acomprehensive literature review on coexistence issues between D2D cellularcommunications Next, we focus on D2D communication aspects currentlyexamined by 3GPP for integrating D2D communications in future LTE networks.Finally, we propose a scheme for enabling D2D communications in LTE-A net-works by enhancing standardized functionalities at the access network

2 State of the Art on D2D Communications

In the literature, the coexistence of D2D and cellular communications is definedunder two basic spectrum sharing approaches: (i) the spectrum underlay, whereD2D transmissions reuse spectrum portions utilized by cellular transmitters and(ii) the spectrum overlay, where temporary empty spectrum portions are used Acomparison of the two approaches can be found in [3] and [4], in terms oftransmission capacity and throughput, respectively The key challenge in bothcases is the mitigation of the generated interferences To this end, a widelyaccepted choice is the exploitation of the uplink (UL) cellular period, where theonly cellular interference victim is the immobile BS [5 7], shifting the majorinterference problem to the protection of the D2D receivers However, the pro-tection of the D2D receivers is quite challenging, since in both underlay andoverlay approaches the interferences caused by neighboring transmissions (eithercellular or D2D) are far from negligible This is an important concern, consideringthat the current trend is to reduce the cell size for achieving higher spatial networkcapacity This trend poses the need for more research on controlling the inter-cellinterference perceived by D2D receivers in multicellular networks

In the literature, the interference problem is mainly dealt with aware Resource Allocation (RA) and Power Control (PC) schemes, e.g., [8 11].The BS selects appropriate spectrum resources and power levels for the D2Dtransmitters, taking into account information about the interferences among D2Dand cellular nodes An important issue here is how the BS acquires the interferenceinformation To this end, different mechanisms that inform the BS about thechannel conditions between the D2D nodes have been introduced, exploitingmainly periodic measurements guided by the BS, e.g., [9, 12, 13] However,

interference-gathering of the interference information consumes network resources, whilereliability highly correlates with the network traffic and topology changes Addi-tionally, even if accurate information is available at the BS, the D2D RA and PCproblems are complex and hard to optimize Consequently, the design of solutionsthat reduce the need for interference information at the BS is an open challenge.Other approaches in the literature deal with the functional enhancementsrequired to cellular networks in order to enable D2D communications Differentscenarios and challenges in an LTE network are presented in [14,15] Especially

in [15], a detailed classification of D2D communication aspects in LTE networks

is presented, and an abstract description of the signaling needed for D2D resourceallocations and data transmission is described A major problem considered is thepeer discovery, i.e., the problem of finding whether the D2D peers are closeenough to directly communicate The two basic peer discovery approaches are thecentralized and the distributed The distributed approach is considered moreflexible and scalable, since it operates under local-level requirements and thecomplexity is shifted to the end-users However, in modern cellular systems, such

as LTE, this approach can lead to uncontrolled use of the licensed band, imposingthe design of centrally controlled peer discovery schemes A promising peer dis-covery solution has also been proposed by Qualcomm under the term FlashLinQ in[16] In addition to peer discovery, this scheme includes: (i) timing and frequencysynchronization derived from cellular spectrum, (ii) link management, and (iii)channel-aware distributed power, data rate, and link scheduling

Focusing on the LTE system, the integration of D2D communications isthoroughly examined in [17,18], where the D2D connections are mainly used fornetwork performance optimization based on the idea of switching between thecellular and the D2D communication modes This idea has also motivated anumber of other papers in the literature, e.g., [19–21] The strong point of thisapproach is that the cellular communications take advantage of the D2D benefits,while the changes in the transmission mode are totally transparent to the end-user

By contrast, the requirement of avoiding interruption during switching from onemode to the other needs more investigation Another promising approach,described in [22], proposes to enhance the LTE network entities in order to offerextra D2D communications on allocated or empty spectrum portions, indepen-dently of the cellular transmissions The main advantage of this approach is thatthe network can handle both the types of communication separately, making D2Dconnections transparent to the core network In both the approaches, the use oflicensed spectrum by the D2D transmitters calls for designing operator-controlledD2D schemes, and shifts the research interest to more centralized solutions.Parallel to the research effort from academia, 3GPP has recently begun working

on integrating D2D communications in LTE Release 12 The main aspects sidered by 3GPP are provided in the following section

con-3 D2D Communication Aspects in LTE-A Network

3.1 Background in LTE-A Network

The main cellular system that is expected to adopt the D2D communications is theLTE system The architecture of an LTE system (and the current release LTE-Advanced—LTE-A) is divided into two basic subsystems: the Evolved—Uni-versal Mobile Telecommunications System (UMTS) Terrestrial Radio AccessNetwork (E-UTRAN) and the Evolved Packet Core (EPC) (Fig.1) This archi-tecture has been adopted on the Internet avoiding the hierarchical structures andproviding increased scalability and efficiency On the one hand, the EPC subsys-tem is a flat all-IP system designed to support high packet data rates and lowlatency in serving flows On the other hand the E-UTRAN is the access network ofthe LTE system The main entities of E-UTRAN are the base stations—referred to

as eNBs (evolved NodeBs) for the macro-cells and HeNBs (Home-eNBs) for thefemto-cells, and the cellular terminals—referred to as UEs (User Equipments).The communication between eNBs and UEs is organized in frames of 10 ms,while each frame is divided into 10 subframes of 1 ms Referring to transmissionsfrom and to eNBs, there are two basic categories of subframes; the downlink (DL)and the uplink (UL), respectively In the frequency domain, each subframe utilizesscalable bandwidth up to 20 MHz (and up to 100 MHz through the carrieraggregation mechanism) divided into subcarriers of 15 KHz spacing Subcarriersare organized into resource blocks RBs of 180 KHz each, i.e., 12 subcarriers define

an RB, the minimum allocation unit in the network

The introduction of the D2D communications must be done in respect to thisarchitecture, while the need for physical layer backward compatibility imposes theD2D-enabled UEs to utilize for their direct transmissions the current structure ofthe spectrum resources

3.2 D2D Communication Scenarios

Currently, a lot of effort is being made by 3GPP Internet introducing the D2Dcommunications in the next amendments of the LTE system For a better study ofthe problem 3GPP has adopted the following terminologies [2]:

ProSe direct communication: a communication between two or more UEs inproximity that are ProSe-enabled, by means of user plane transmission using E-UTRA technology via a path not traversing any network node

ProSe-enabled UE: a UE that supports ProSe requirements and associatedprocedures Unless explicitly stated otherwise, a ProSe-enabled UE refers both to anon-public safety UE and a public safety UE

ProSe-enabled Public Safety UE: a ProSe-enabled UE that also supports ProSeprocedures and capabilities specific to Public Safety

ProSe-enabled non-public safety UE: a UE that supports ProSe procedures andbut not capabilities specific to public safety.

ProSe direct discovery: a procedure employed by a ProSe-enabled UE to cover other ProSe-enabled UEs in its vicinity by using only the capabilities of thetwo UEs with rel.12 E-UTRA technology

dis-EPC-level ProSe discovery: a process by which the EPC determines theproximity of two ProSe-enabled UEs and informs them of their proximity.Based on this terminology two direct communication modes are proposed: (i)the network independent and (ii) the network authorized mode The first mode ofoperation does not require any network assistance to authorize the connection andcommunication is performed by using only functionality and information availablelocally to the UE(s) This mode is applicable:

• only to preauthorized ProSe-enabled Public Safety UEs,

• regardless of whether the UEs are served by E-UTRAN or not, and

• to both one-to-one and one-to-many direct communication

The second mode of operation for ProSe direct communication always requiresnetwork assistance by the EPC to authorize the connection This mode of operationapplies:

• to ProSe one-to-one direct communication,

• when both UEs are ‘‘served by E-UTRAN,’’ and

• for Public Safety UEs it may apply when only one UE is served by E-UTRAN.For these communication modes and considering the registered public landmobile network (PLMN), the direct communication path and coverage status (incoverage or out of coverage), a number of different possible communicationscenarios are defined as shown in Table1, while a comprehensive illustration ofthese scenarios is provided in Fig.2 However, these scenarios do not cover all thepossible scenarios for direct communication, and 3GPP in working on adding morescenarios especially for the case of group communication

UE

EPC S1

LTE-Uu

3.3 D2D Reference Architecture

For supporting the scenarios defined above enhancements are required to the LTEarchitecture Figure3 depicts this architecture and aims to fulfill the followingrequirements posed by 3GPP

• Enable the operator to control the ProSe discovery feature in its network andauthorize the functionality required for the ProSe discovery functions for eachUE

• Enable the ProSe communication or ProSe-assisted WLAN Direct cation and seamless service continuity when switching user traffic between aninfrastructure paths and a ProSe communication path of the ProSe-enabled UEs

communi-• Enable HPLMN operator to authorize ProSe-enabled UE to use ProSe munication separately for the HPLMN and for roaming in VPLMNs

com-• Enable an authorized third party ProSe application to interact with 3GPP work in order to utilize the ProSe services offered by the network

net-• Be able to control ProSe communication between ProSe-enabled UEs when theUEs are served by a same eNB or different eNBs

• Accommodate the ProSe-related security functions related to privacy, supportfor regulatory functions including Lawful Interception, and authentication uponProSe discovery and ProSe communication

• Enable the operator to authorize and authenticate the third-party applicationsbefore making use of the ProSe feature

• Accommodate for charging by the operators (HPLMN or VPLMN) for zation of the ProSe functionality

utili-As depicted in Fig.3, additional to the entities of the conventional LTEarchitecture, a number of new entities are required as shown in Fig.1 Theseentities are as follows:

Application servers (ProSe App Server) incorporates the ProSe capability forbuilding the application functionality, e.g., in the Public Safety cases they can bespecific agencies (PSAP) or in the commercial cases social media These appli-cations are defined outside the 3GPP architecture but there may be reference pointstoward 3GPP entities The Application server can communicate toward an appli-cation in the UE.

Applications in the UE (ProSe UEs App) use the ProSe capability for buildingthe application functionality An example may be for communication betweenmembers of Public Safety groups or for social media application that requests tofind buddies in proximity

ProSe Function in the network (as part of EPS) defined by 3GPP has a ence point toward the ProSe App Server, the EPC, and the UE The functionality

PLMN A PLMN B

(e)

UE1 PLMN A

UE2 PLMN B

PLMN A PLMN B

(g)

UE1 PLMN A

UE2 PLMN B

Fig 2 3GPP direct communication scenarios [ 2 ]

may include but is not restricted to: interworking via a reference point toward thethird-party applications, authorization and configuration of the UE for discoveryand direct communication, enable the functionality of the EPC-level ProSe dis-covery, and provide functionality for charging (via or outside of EPC, e.g., offlinecharging).

Note that for the interconnection of the new entities and the connection with theconventional LTE entities, seven new interfaces/reference points are defined asPC1, PC2, PC3, PC4, PC5, PC6, and SGi (Fig.3)

4 Proposed D2D Scheme

4.1 System Model

We adopt a ProSe communication scenario where both ProSe-Enabled UEs areconnected to the same PLMN/cell The eNB operates as a D2D controller, and assuch, it is responsible for the following: (i) the D2D RA and PC and (ii) the peerdiscovery and tuning for the D2D peers Potentially, the capability for D2Dtransmission is provided to all UEs of the network; however, hereinafter, tosimplify our description, the UEs that implement our D2D scheme will be referred

to as eUEs (enhanced UEs)

Similar to UEs, eUEs request resources for D2D communications from theeNB For each one of the D2D requests, eNB launches a peer discovery procedure,while only the valid D2D pairs (with successful peer discovery procedure) areconsidered in the D2D RA and PC procedures Differing from the conventional RAprocedure, in the D2D RA one the eNB informs both D2D transmitter and receiverabout the allocation grant, tuning them to the allocated resources However, thistuning requires the eNB to know the identity of the D2D receiver Conventionally,

ProSe APP Server

SGi

ProSe Function

PC4 PC2 PC5 LTE-Uu