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IEEE Communications Magazine December 200942 0163-6804/09/$25.00 2009 IEEE

INTRODUCTIONMajor effort has been put in recent years on thedevelopment of Third Generation PartnershipProject (3GPP) Long Term Evolution (LTE),which provides Evolved Universal MobileTelecommunications System (UMTS) terrestrialradio access (EUTRA) and EUTRA network(EUTRAN) technology for higher data ratesand system capacity, and the System Architec-ture Evolution (SAE) for efficient networkingand cost saving operation.

3GPP has recently defined a further studyitem for LTE-Advanced, which shall preparenew technology components for LTE to meetthe IMT-Advanced requirements, also in thelocal area scenario. IMT-Advanced will offerhigh bandwidths up to 100 MHz for higher datarates, global operation, and economy of scale tosupport a wide range of services. Such futureradio access systems will be scalable in terms ofcarrier bandwidth and carrier frequencies onvarious spectrum bands [1]. This article intro-duces a technology component for LTE-Advanced that has so far not been consideredsufficiently: device-to-device (D2D) communica-tion as an underlay to cellular networks.

In recent years wireless local area networks(WLANs) have become increasingly popular, asthey enable access to the Internet and local ser-vices with low-cost infrastructure, and cheap andfast access to the spectrum in the license exemptbands. However, operation on a licensed band

has benefits, as it can guarantee a planned (inter-ference) environment instead of an uncoordinat-ed one. Hence, it could be more convenient forlocal service providers to make investment deci-sions based on access to the licensed spectrumcompared to the unlicensed spectrum. However,the access should be granted with small enoughexpenses, not comparable to the license fees ofcellular operators.

A cellular operator may offer such a cost effi-cient access to the licensed spectrum enabled byD2D communication as a controlled or con-strained underlay to an IMT-Advanced cellularnetwork, as we earlier proposed in [2]. In thisarticle we present the necessary additions to anLTE-Advanced network to enable D2D sessionsetup and management. We outline a solutionfor a D2D session setup using dedicated signal-ing and automatic handover of network routedtraffic to D2D links between nearby (proximity)devices. Furthermore, we present the interfer-ence coordination mechanisms that enableunderlay D2D communication and presentresults on the achievable D2D throughput in aworst case interference limited local area sce-nario.

The concept of D2D communication as anunderlay to a cellular network, operating on thesame resources, is illustrated in Fig. 1. Besidescellular operation, where user equipment (UE)is served by the network via the base stations,called evolved NodeBs (eNBs) in the LTE archi-tecture, UE units may communicate directly witheach other over the D2D links. The UE in D2Dconnections remains controlled by the eNBs andcontinue cellular operation.

The eNBs can control the resources used forcellular communications and by the D2D link.The eNBs can also set constraints on the trans-mit power of D2D transmitters to limit the inter-ference experienced at the cellular receivers.Resources can also be assigned to D2D links inthe case of a dense LTE network with high net-work load, where a cognitive radio with a cellu-lar network as the primary service would not beable to detect locally unused spectrum (i.e.,white spaces).

In addition, 3GPP has introduced a femtobase station solution. For femto, small isolatedcells can be set up by the consumers themselves,when allowed by the operator, to increase spatialreuse of the operators spectrum. Femtocells

ABSTRACTIn this article device-to-device (D2D) com-

munication underlaying a 3GPP LTE-Advancedcellular network is studied as an enabler of localservices with limited interference impact on theprimary cellular network. The approach of thestudy is a tight integration of D2D communica-tion into an LTE-Advanced network. In particu-lar, we propose mechanisms for D2Dcommunication session setup and managementinvolving procedures in the LTE System Archi-tecture Evolution. Moreover, we present numer-ical results based on system simulations in aninterference limited local area scenario. Ourresults show that D2D communication canincrease the total throughput observed in thecell area.

TOPICS IN RADIO COMMUNICATIONS

Klaus Doppler, Mika Rinne, Carl Wijting, Cssio B. Ribeiro, and Klaus Hugl, Nokia Research Center

Device-to-Device Communication as anUnderlay to LTE-Advanced Networks

DOPPLER LAYOUT 11/18/09 12:03 PM Page 42

IEEE Communications Magazine December 2009 43

require access to the cellular core network (byfixed Internet connectivity), and the radioresources assigned to the femtocell have toremain stable and controlled.

Applications and services based on D2Dcommunications can be manifold. As a motivat-ing example, consider a use case where a localmedia server is set up at a rock concert to offera huge amount of promotional material for visi-tors to download. At the same time, there is aneed to handle phone calls reliably and provideInternet access without congestion that would becaused by the additional load due to the mediaserver. While the cellular network is servingphone calls and Internet connectivity, D2D com-munications can provide an efficient solution forthe local media service. D2D communicationcan be utilized to download promotional materi-al to UE as well as to distribute it among UE.BitTorrent can be used for this, as an example.

D2D operation can be fairly transparent tothe user, who simply enters a request for a uni-form resource locator (URL) at the UE. Thenetwork is able to detect that the request is sentto the local media server, and it therefore handsthe communication over to a D2D connection.Existing technologies such as Bluetooth orWLAN do not provide a satisfactory solution.For example, Bluetooth requires manual devicepairing, and WLAN may require user-definedsettings for the access points. Additionally, unli-censed spectrum operation of Bluetooth andWLAN causes uncertainty as to whether thespectrum and services are truly available. Thisresults in a trial-and-error process users do notappreciate.

This article is organized as follows. We illus-trate how D2D communication can be estab-lished in an LTE-Advanced network having anSAE architecture. The main contribution of thearticle is to draft the functionality and messagingrequired to set up a D2D connection inside theSAE architecture, and also if the session setuptakes place as an application layer signaling tothe Internet beyond the cellular network. Weoutline how the interference of D2D connec-tions to the cellular network can be limited.Both the uplink and downlink period of time-division duplex operation of the cellular networkis addressed because of their different natures inmultiple access interference and power control.

Finally, we present performance assessmentresults showing that D2D can operate efficientlywith minor and controlled degradation of cellu-lar operation. Here, we have chosen the chal-lenges of full load and an interference limitedlocal area scenario. It is evident that the benefitsfrom D2D operation during times of underuti-lized cellular resources and low load are evenmore prominent.

D2D SESSION MANAGEMENT INSYSTEM ARCHITECTURE EVOLUTION

LTE systems with the SAE architecture [3] oper-ate fully in the packet-switched domain usingInternet protocols. SAE replaces a conventionalcircuit-switched connection to the mobile switch-ing center (MSC) and extends the General Packet

Radio Service (GPRS) to the SAE server archi-tecture. Session setup happens in the user planeusing the Session Initiation Protocol (SIP). SAEprovides connectivity to the Internet, where a SIPapplication server (AS) is found by a discoveryprocedure or operator assignment. The SAEarchitecture includes the mobility managemententity (MME) and the packet data network(PDN) gateway, which together take care of theUE context, setting up the SAE bearers, IP tun-nels, and IP connectivity between the UE and theserving PDN gateway. Figure 2 shows sessionsetup by SIP (SIP invite) in the SAE architecture.

After successful session setup, any two ormore devices (UE, or UE and servers) may com-municate over the Internet. However, in peer-to-peer communication between devices in closeproximity, the operator network need not beinvolved in the actual data transport except forsignaling of the session setup, charging, and poli-cy enforcement. Therefore, we propose to intro-duce a D2D communication mode toLTE-Advanced, and describe two options forsession setup and management. In order toenable the D2D communication as an underlayoperation to the LTE network, interferencecoordination is a prerequisite for D2D sessionsetup to take place. Interference coordination isstudied with a feasibility analysis.

D2D SESSION SETUP BYDETECTING D2D TRAFFIC

In the SAE architecture, the node in the net-work that is aware of wide-area (global) IPaddresses is the gateway (serving PDN gateway).

Figure 1. Device-to-device communication between UE1 and UE2 as anunderlay to a cellular network. The red shaded area indicates potential inter-ference from D2D communication.

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IEEE Communications Magazine December 200944

The gateway keeps a routing table that enablesIP routing from/to the Internet. The gateway isable to route IP packets to the proper eNBsserving the active destination UE. Similarly, thegateway receives IP packets from the source UEvia the serving eNB and routes them to theInternet. The IP transport from/to the eNBshappens in IP tunnels set-up by the MME in theInitial Attach of the UE to the gateway.

The gateway is able to detect potential D2Dtraffic since it actually processes the IP headersof the data packets and tunnel headers, so itknows by which eNB the UE is served. PotentialD2D traffic is any flow whose tunnel endpointidentifiers match for both UE units (i.e., thesource and destination IP addresses of the for-ward tunnel, and the destination and sourceaddresses of the reverse tunnel are assigned tothe same eNB), or when the tunnel endpointsare assigned to neighboring eNBs. The gatewaymay earmark packets of a potential D2D trafficflow as depicted in Fig. 3.

As a next step, the eNB requests the UE tomake measurements to check if the D2D devicesare in communication range and if D2D commu-nication actually offers higher throughput thancellular communication (discussed in more detaillater). If the transport conditions for D2D com-munication are favorable, the eNB sets up aD2D radio bearer directly between the two UEunits so that they communicate over D2D com-munication resources. Thus, those IP packetswith an IP source address and an IP destinationaddress of the peer UE having an active D2Dconnection need not be transmitted to the eNBand SAE gateway at all.

D2D communication could also be estab-lished for UE served by different eNBs since thegateway is aware of the potential cell neighbor-hood. This is beneficial to avoid an excessiveamount of traffic routed in the network, whichcould actually be classified as D2D trafficinstead. In cases where the UE is in neighboringcells, the eNBs serving the UE have to coordi-nate D2D measurements and the D2D bearersetup over the X2 interface [3].

The session setup requires the followingsteps:

1)A session is initiated by one of the UE units.2)The gateway detects IP traffic originating

from and destined to UE in the same sub-net and to be tunneled within the sameserving eNB or between eNBs servingneighboring cells.

3)If the traffic fulfills certain criteria (e.g.,data rate), the gateway earmarks the trafficas potential D2D traffic.

4)The eNB(s) request measurements from theUE to check if D2D communication offershigher throughput.

5)If both UE units are D2D capable and D2Dcommunication offers higher throughput,the eNB(s) may set up a D2D bearer.

6)Even if the D2D connection setup is suc-cessful, the eNB(s) still maintain the SAEbearer between the UE and the gateway forcellular communications. Furthermore, theeNB maintains the radio resource controlfor both cellular and D2D communication.

7)The UE will send/receive packets to/fromthe IP address of the peer UE using theD2D connection without the eNB or SAEbeing involved in routing.After the D2D bearer has been established

between the peer devices, it is still necessary forthe eNB to control the radio resources also usedby the D2D communication. The UE may ofcourse continue communication to the Internet,because the SAE bearer is maintained and theconnectivity to the gateway remains. For thisactivity, the UE stays in the LTE_Active stateand RRC_Connected state, respectively.

In addition to checking if D2D offers bene-fits, the decision to establish a D2D connectioncan be based on a defined policy such as limitingthe number of D2D connections to keep theinterference to the cellular network below a tol-erable level.

A handover from a D2D connection to a cel-lular connection is initiated when the cellularconnection achieves higher throughput than theD2D or if one of the policies for D2D connec-tions is violated.

One important property of setting up a D2Dconnection by a trigger of detecting IP trafficbetween nearby devices is that it works for anypeer-to-peer IP traffic without service differenti-ation. Thus, it is blind to the actual service. Sec-ond, it is transparent to the user (i.e., the userdoes not have to select a special option for theD2D session). The D2D UE units are passiveparticipants in the session setup, which is mainlyhandled at the MME and higher layers. Howev-er, the automatic switching between cellular andD2D connection should be reliable and seamlessto guarantee user satisfaction.

D2D SESSION SETUP USINGDEDICATED SAE SIGNALING

A session setup using dedicated SAE signalingavoids the overhead from detecting D2D trafficon the fly from IP packets. For this alternative,we propose to provide a specific address formatto separate a D2D SIP session request from ageneric SIP session request. Furthermore, wepropose to enhance the MME with a light SIPhandler to facilitate session setup.

Figure 2. SIP session initiation in LTE-Advanced with SAE architecture.

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The new address format of D2D SIP couldsimply be [email protected]_keyword,where the former part (username@realm) is awell-known SIP uniform resource indicator(URI), and the latter part (D2D_keyword) is anovel extension to let the SAE handle the localD2D session in a special way. The local exten-sion is easily distinguishable as a D2D addressby a special D2D_keyword such as .direct, .local,.peer, or .short.

In this approach, the application (or the user)at the requesting UE needs to decide whether toprefer initiation of a D2D session or a regularsession. We foresee the following options to aidthis decision: The user explicitly selects a D2D session by

selecting the SIP URI format with a localextension.

The UE software detects the local extensionand encapsulates the SIP invite messageinto a control plane message to the MMEinstead of delivering it to a user planeTCP/IP or UDP/IP port.The D2D session setup is illustrated in Fig. 4,

which presents the peer entities UE1 and UE2with the application layer and IP layer protocolsfor session initiation and connectivity. Further-more, the radio resource control (RRC) proto-col is shown.

In this case UE1 calls UE2 by SIP invite mes-sage using the .D2D_keyword extension with theURI of UE2. This lets the SAE network detectthe preference for a local session, which leads tothe setup of a D2D bearer instead of (or in addi-tion to) SAE bearers. This setup procedure islocal to the radio access network and does notinvolve a SIP server far away in the Internet,which leads to faster session setup.

We propose to encapsulate the SIP invitemessage in an NAS control plane message. Inthe SAE architecture, the MME receives allNAS messages from the served UE. The MMEis in charge of the control plane functions likemobility management, and when needed it alsocommunicates to the home subscriber server(HSS) to get subscriber records and take careof user authentication. We propose to enhancethe MME functionality by a light SIP handlerto keep track of the SIP addresses of the UEinside the tracking area. This can be done atthe initial access, when the UE registers to thenetwork and the MME assigns the temporarymobile subscriber identity (TMSI) to the UEthat is used for all control of the UE in thewide area network. In the registration phasethe HSS needs to be involved, but afterwardthe MME can handle the UE (mobility) con-text by itself.

When receiving an NAS message type for aD2D session, the MME simply passes the SIPmessage contents to the light SIP handler,whereas other NAS messages are treated asusual. The NAS message delivery is defined inthe SAE specifications, and the proposal in thisarticle requires no changes to the transport ofNAS messages as such.

After handling the SIP message and detectingthat the D2D UEs are in the same or neighbor-ing cells, the MME will request a setup of aD2D radio bearer from the serving eNB (orserving eNBs for D2D communication happen-ing in an area of multiple cells). The eNB(s) willthen request measurements from the D2D UEto check if a D2D connection is actually possibleand beneficial (discussed in more detail later). Ifa D2D connection is found to be beneficial, a

Figure 3. The gateway earmarks local traffic to indicate potential D2D traffic to the eNB. The eNB can thencheck if the devices corresponding to these packets can set up a D2D connection.

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D2D bearer is set up and the eNB confirms thesuccessful radio bearer setup to the MME.

This procedure takes place instead of (or inaddition to) the MME setting up SAE bearersand IP tunnels between the eNB(s) and thegateway. Furthermore, a novel MME functioncould assign dedicated IP addresses for D2Dcommunication and hence provide local IP con-nectivity. These IP addresses are preferablyassigned by the MME from its local subnetdomain. The MME may deliver the assignedD2D IP addresses (source addresses and desti-nation addresses of UE1 and UE2, respectively)along with the D2D bearer request message tothe eNB(s) and further from the eNB(s) with thebearer setup messages to the UEs. These IPaddresses provide IP connectivity and let the UEopen regular TCP/IP or UDP/IP ports for D2Dcommunications. Nevertheless, the eNB(s)remain in control of the D2D radio bearer.

INTERFERENCE COORDINATIONWITH THE CELLULAR NETWORK

In the past, several wireless standards haveaddressed the need for D2D operation in thesame band as the base station, access point, or

central controller. Examples of such standardsare Hiperlan2, TETRA, and WLAN based onIEEE 802.11 standards. In these examples D2Dcommunication is assumed to occur on separateresources. For example, in Hiperlan2 a terminalinvolved in D2D communications transmits andreceives in reserved time slots that are differentfrom the time slots used by terminals communi-cating with the infrastructure. This is also thecase in TETRA, designed for authorities, wherededicated frequency channels are specificallyreserved country-wide for D2D communication.

These restrictions limit the interference ofD2D transmissions, particularly in an orthogonalsystem, such as classical time-division multipleaccess (TDMA). However, it leads to inefficientutilization of resources, which is especially truefor the large system bandwidths deployed byLTE-Advanced networks.

WLAN allows UE to sense the radio mediumand transmit only if the channel is free. A draw-back of WLAN is that the access point does nothave the possibility to control the resources usedby the ad hoc D2D links, which could be benefi-cial for coordination to protect the communica-tion with the infrastructure [4].

In this article we propose to facilitate localpeer-to-peer communications by a D2D radiothat operates as an underlay to an LTE-Advanced network. Although an LTE networkcould serve technically as well, this propositionwill be timely during the era of LTE-Advanced.

In order to limit the interference of D2Dconnections to the cellular network, we pro-posed in [2] that the eNB be able to control themaximum transmit power of D2D transmitters.Furthermore, the eNB assigns orthogonal fre-quency-division multiple access (OFDMA)resources to the D2D connections sharing theuplink, downlink, or both uplink and downlinkresources of the cellular network.

One way to coordinate interference betweencellular and D2D communications is to assigndedicated OFDMA resource blocks for D2Dconnections, where the amount of resources maybe dynamically adjusted based on temporarycommunication needs. This option should notcompromise the experienced quality of servicefor cellular users with a low signal-to-interfer-ence-plus-noise ratio (SINR). It is even morechallenging to protect the cellular user frominterference when the cellular and D2D trans-missions are allowed to reuse the sameresources.

INTERFERENCE COORDINATION IN THEUPLINK PERIOD

During cellular uplink transmission, the eNB isthe victim receiver of interference from D2Dtransmitters. Since the UE in D2D connection isstill controlled by the serving eNB, it can limitthe maximum transmit power of the D2D trans-mitters. In particular, it can also utilize the cellu-lar power control information for the devicesinvolved in D2D communications.

The transmit power of the D2D transmitter isreduced by a backoff value from the transmitpower determined by the cellular power controlwhen the D2D transmitter reuses cellular uplink

Figure 4. Signaling graph of a D2D session setup with a dedicated D2D exten-sion in the URI format. The MME is enhanced to facilitate session setup forthe D2D communications by a light SIP handler.

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resources. No backoff is required when the D2Dtransmitter uses dedicated (otherwise free)resources. The eNB can additionally apply powerboosting for the uplink transmission of a cellularuser to ensure that the SINR of the cellularuplink meets the target SINR. The boosting isdependent on the backoff value, as detailed in [2].

INTERFERENCE COORDINATION IN THEDOWNLINK PERIOD

The actual location of cellular receivers in thedownlink period depends on the short-termscheduling decisions of the eNB. Hence, the vic-tim receiver at a time can be any of the servedUE. After setting up a D2D connection, theeNB can set the maximum D2D transmit powerto a predetermined value. The maximum D2Dtransmit power will be higher when a D2D con-nection gets dedicated (free) resources thanwhen it reuses (reserved) cellular resources.

A suitable D2D transmit power limit can befound by long-term observations of the impact ofdifferent D2D power levels on the quality of cel-lular links. In addition, the eNB can ensure thatcellular users scheduled on the same resourceswith D2D connections are well isolated in prop-agation conditions [5]. For example, the eNBmight schedule indoor D2D connections togeth-er with outdoor cellular users.

In addition, the eNB can observe the linkquality feedback from its served UE. If itobserves a degradation in UE link quality, itcan reduce the transmit power of D2D trans-mitters [6].

FEASIBILITY ANALYSISThe feasibility of D2D communications as anunderlay of a cellular network is evaluated bymeans of system simulations. A local area sce-nario was selected and is depicted in Fig. 5a.The scenario is fully loaded and interferencelimited. The layout captures characteristics of an

indoor environment with small rooms, represent-ing stores or offices, a larger open area, andlonger rooms representing corridors or halls, forexample. Nine eNBs serve a single floor ofdimension 100 m 100 m.

In our studies we use the channel and propa-gation models defined in WINNER scenario A1(indoor/office) [7]. Links in the same room havea distance-dependent probability for line of sight(LOS) propagation with the path loss exponent1.87, whereas for non-LOS propagation the pathloss exponent is 3.68. The shadow fading hasstandard deviation 4 dB, and each wall intro-duces an additional attenuation of 5 dB.

We consider a synchronized LTE-Advancedcellular network operating on a 100 MHz bandusing time-division duplex. The 100 MHz band issplit into five 20 MHz subbands. Both cellularUE and D2D UE are assigned to a single sub-band at a time. In order to guarantee a fullyloaded scenario, the number of cellular users isset to 180 (on average 20 per cell), and exactly10 D2D pairs (5 sharing the uplink period and 5sharing the downlink period) are generated inevery cell. The cellular users are uniformly dis-tributed in the scenario, whereas the distancebetween D2D pairs is limited to reflect localcommunication.

We assume a traffic scenario where D2Dusers exchange rich content, such as videos andlarge files. The rich media content may origi-nate, for example, from media servers or directfile exchanges between users. Hence, a fullbuffer traffic model holds for both cellular andD2D connections.

In order to limit the impact on the cellularcommunications, interference coordinationmechanisms described earlier (see also [2]) areused. If the uplink period is reused, a powerbackoff value of 10 dB is applied for D2D trans-missions, in addition to the most recent cellularpower control setting. In the downlink period,the D2D transmitters have a constant transmitpower of 5 dBm when they are reusing

Figure 5. Evaluation scenario and D2D throughput: a) map of downlink SINR without D2D communication (interference limited); tri-angles mark access point locations, lines represent walls; b) D2D throughput as a function of the D2D link distance when choosing thebest sharing option and when relaying all D2D traffic through the network.

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(reserved) cellular downlink resources and 3dBm when they get dedicated (free) resources.These values have been chosen such that theSINR reduction for cellular communications islimited to 3dB. The maximum transmit power ofan indoor femto type of eNB (downlink) andUE (uplink) is equal to 24 dBm.

The D2D pairs are randomly assigned toshare uplink or downlink resources with the cel-lular network. The D2D terminals probe theD2D and cellular link quality with and withoutcellular transmissions in the same cell and reportthe results to the eNB. In this study the eNBassigns the D2D mode for UE peers offering thehighest throughput taking into account theamount of resources each mode will get. TheD2D modes are: D2D links reuse the resources allocated

(reserved) for cellular use. D2D links use dedicated (free) resources

not allocated for cellular use at that time. D2D traffic is relayed through the network

avoiding D2D links.In the case of dedicated resources and for trafficrelayed through the network, D2D pairs areassigned a similar share of resources as cellularusers; for example, a D2D pair gets one fourthof the resources when sharing the subband withthree cellular users.

We assume an ideal link adaptation for alllinks, with a limited set of modulation and cod-ing schemes (MCSs) [8], and an exponentialeffective SINR mapping (EESM) link to systemmapping is used [9].

Figure 5b illustrates the D2D throughput as afunction of the D2D link distance with D2Dpairs located in the same room. Especially forshort distances, the throughput of the directD2D option greatly exceeds the throughputwhen solely relaying D2D traffic through theeNB. For 3 m link distance, the average D2D

throughput in this example is 15 Mb/s comparedto 2.5 Mb/s when all D2D traffic is relayed bythe eNB.

The eNB relaying option was actually chosenfor more than 50 percent of the D2D pairs for aD2D link distance larger than 9 m when sharingthe cellular downlink period and 38 m whensharing the cellular uplink period. The large dif-ference can be explained by the limited transmis-sion power when sharing resources during thedownlink period. The D2D link distance forresources allocated in the downlink period couldbe increased, for example, by utilizing multipleantennas to suppress interference from the eNB,as we have shown in [10, 11]. Dedicatedresources were chosen for less than 10 percentof the D2D pairs because the share of D2Dresources was rather limited in the presence of20 cellular users per cell.

In Fig. 6 we evaluate separately the combinedthroughput of all cellular users, all D2D users,and their sum within a cell. Our results showthat D2D throughput reduces drastically if theD2D peers are not located in the same room.Thus, D2D communication underlaying a cellu-lar network will be limited to local (proximity)traffic (in the same room), and applications uti-lizing D2D communication should be designedaccordingly. Nevertheless, the sum throughput(cellular plus D2D) may increase greatly (up to65 percent compared to the sum traffic when allD2D traffic is relayed by the network) when lim-iting D2D communication to devices in the sameroom. On the other hand, the throughput of thecellular users decreases significantly (by 35 per-cent compared to a scenario without D2D users)if all D2D traffic has to be relayed since theD2D pairs require both UL and DL resources.

The presented simulation results focus on anindoor scenario served by a rather tight networkof indoor femto type eNBs. D2D utilizationincreases when underlaying an outdoor widearea cellular network, as has been shown in [2].

CONCLUSIONSIn this article we propose device-to-device com-munication as an underlay to an LTE-Advancedcellular network. We illustrate the requiredsmall changes (additions) to the SAE proceduresand functions to facilitate D2D session setup.Furthermore, we illustrate mechanisms that con-trol and limit the interference of D2D communi-cations to the cellular network.

In the feasibility analysis we studied D2Dcommunication in a local area cellular network.Even though the cellular network may be inter-ference limited already by the cellular communi-cation alone, the D2D peers are still able to usethe D2D communication opportunity, if they areclose and located in the same room. By allowingD2D communication to underlay the cellularnetwork, the overall throughput in the networkmay increase up to 65 percent compared to acase where all D2D traffic is relayed by the cel-lular network. In this manner D2D communica-tion may also serve cellular traffic, offloading forthe eNBs in addition to its other benefits of fastand light session setup, low transport delay, andhigh instantaneous data rate.

Figure 6. Throughput of cellular users and D2D users in a cell for D2D pairswith and without restriction to be in the same room. The D2D link distancehas been limited to maximum D2D distance.

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ACKNOWLEDGMENT

The authors would like to thank KimmoValkealahti for his support to obtain the systemsimulation results.

REFERENCES[1] ITU-R, Invitation for Submission of Proposals for Candi-

date Radio Interface Technologies for the TerrestrialComponents of the Radio Interface(s) for IMT-Advancedand Invitation to Participate in Their Subsequent Evalu-ation, ITU-R Circular Letter 5/LCCE/2, 2008.

[2] P. Jnis et al., Device-to-Device Communication Under-laying Cellular Communications Systems, Intl. J. Com-mun., Network and Sys. Sci., vol. 2, no. 3, June 2009,pp. 16978.

[3] H. Holma and A. Toskala, LTE for UMTS OFDMA andSC-FDMA Based Radio Access, Wiley, 2009

[4] H. Y. Hsieh and R. Sivakumar, On Using Peer-to-PeerCommunication in Cellular Wireless Data Networks,IEEE Trans. Mobile Comp., vol. 3, no. 1, Jan.-Mar. 2004,pp. 5772.

[5] P. Jnis et al., Interference-Aware Resource Allocationfor Device-to-Device Radio Underlaying Cellular Net-works, IEEE VTC Spring, Barcelona, Spain, Apr. 2009.

[6] J. Axns, A. Furuskr, and P. de Bruin, Method andApparatus for Limiting Peer-to-Peer CommunicationInterference, U.S. Patent WO/2007/055623, May 5,2007.

[7] IST WINNER II D1.1.2 WINNER II Channel Models;https://www.ist-winner.org/deliverables.html

[8] ETSI TS 136211, LTE; Evolved Universal TerrestrialRadio Access (E-UTRA); Physical Channels and Modula-tion, Nov. 2008

[9] K. Brueninghaus et al., Link Performance Models forSystem Level Simulations for Broadband Radio AccessSystems, IEEE Intl. Symp. Personal, Indoor and MobileRadio Commun., Berlin, Germany, Sept. 2005.

[10] A. Osseiran et al., Advances in Device-to-Device Com-munications and Network Coding for IMT-Advanced,ICT Mobile Summit, Santander, Spain, June 2009.

[11] P. Jnis et al., Interference-Avoiding MIMO Schemesfor Device-to-Device Radio Underlaying Cellular Net-works, IEEE Intl. Symp. Personal, Indoor and MobileRadio Commun., Tokyo, Japan, Sept. 2009.

BIOGRAPHIESKLAUS DOPPLER ([email protected]) received hisM.Sc. degree in electrical engineering from Graz Universityof Technology, Austria, in 2003. Currently, he pursues

studies toward a Ph.D. in electrical engineering at HelsinkiUniversity of Technology, Finland. Since 2002 he is a seniorresearcher at Nokia Research Center, Helsinki, Finland. Hisresearch interests include the integration of device-to-device communication and relaying in cellular networks,cognitive radio, and radio resource management. He haspublished more than 20 journal and conference articles.

MIKA RINNE ([email protected]) received his M.Sc.degree from Tampere University of Technology in signalprocessing and computer science, in 1989. Currently heacts as a principal scientist at Nokia Research Center. Hisinterests are in the systems research of protocols and algo-rithms for wireless communications including 3GWCDMA//HSPA, beyond 3G, LTE (E-UTRAN), LTE-Advanced/IMT-Advanced technologies, and cognitive radio.He has authored over 25 international journal and confer-ence articles.

CARL WIJTING ([email protected]) received his M.Sc.degree in electrical engineering (with distinction) fromDelft University of Technology, the Netherlands, in 1998,and his Ph.D. degree in wireless communications from Aal-borg University, Denmark, in 2004. Since 2004 he has beenwith Nokia Research Center, where he is currently researchmanager of the cognitive radio system team. His researchconcentrates on wireless system design and agile use ofradio spectrum. He is (co-)author of over 30 internationaljournal and conference papers.

CASSIO B. RIBEIRO ([email protected]) received hiselectronics engineering degree from the Federal Universityof Rio de Janeiro (UFRJ), Brazil, in 2000, his M.Sc. degree inelectrical engineering from COPPE/UFRJ in 2002, and hisD.Sc. degree at COPPE/UFRJ in 2007. He received a D.Sc.(Tech.) degree (with distinction) from Helsinki University ofTechnology, Finland, in 2008. Since 2007 he has beenworking as a researcher at Nokia Research Center, Finland.His research interests are in the fields of wireless communi-cation, cellular systems, multiple-antenna systems, and sig-nal processing. He co-authored a paper that received thebest paper award at IEEE PIMRC 2005.

KLAUS HUGL ([email protected]) received his Dipl.-Ing.(M.Sc.) and Dr.Techn. (Ph.D.) degrees in electrical engineer-ing with highest honors from the Technische UniversittWien (TU-Wien), Austria, in 1998 and 2002, respectively.Since 2002 he has been with Nokia Research Center, wherehe is currently research manager of the optimized localaccess team. His main areas of research include wirelesssystem design, multi-antenna techniques (MIMO), as wellas 3GPP cellular radio technologies. He has (co-)authoredabout 25 conference and journal papers in wireless com-munications.

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