moving cells

IEEE Communications Magazine • June 201362 0163-6804/13/$25.00 © 2013 IEEE

1 Pico eNBs and RRHsneed dedicated backhaulconnections, whereasinband FRNs utilize thesame carrier frequency asthe UE to communicatewith their donor eNBs. Animportant differencebetween a pico eNB andan RRH is that a picoeNB is a low-power eNBthat can form its own cellsunderlying the macroeNB, while an RRH is apart of the macrocell eNB,and is deployed distribu-tively to either extend cov-erage or improve localcapacity. Home eNBs aredeployed inside residentialor official buildings in anunplanned way to formfemtocells and offload thedata traffic when users areat home or at work.

2 A macro eNB is called adonor eNB when it servesthe backhaul links of RNsas well as UE within itscoverage.

INTRODUCTION

The Third Generation Partnership Project(3GPP) has standardized Long Term Evolution(LTE), which is aimed at improving the spectralefficiency of a cellular network. A more intelli-gent base station, the evolved Node B (eNB),was introduced to simplify the system architec-ture, and minimize the control and user planelatency. By employing advanced multi-antennamultiple-input multiple-output (MIMO) tech-niques, carrier aggregation (CA), and otherschemes, the most recent LTE standard, LTE-

Advanced, can support a downlink speed of upto 1 Gb/s [1].

The capacity offered by macro eNBs in thecurrent LTE-Advanced system is not evenly dis-tributed across their coverage areas; user equip-ment (UE) in the cell center can achieve muchhigher throughput than cell edge UE. The het-erogeneous and small cell networks (HetSNets)solution addresses the capacity needs in certaindata-intensive hotspot areas, and provides aubiquitous user experience. An overview of atypical HetSNets scenario is illustrated in Fig. 1.Different types of low-power nodes, includingpico eNBs, remote radio heads (RRHs), inbandfixed relay nodes (FRNs), moving relay nodes(MRNs), and home eNBs,1 have been intro-duced. These low-power nodes are deployedunderlying macro eNBs to either extend cover-age or increase network capacity locally [2].

The number of users using wireless broadbandservices while riding in public transportation vehi-cles is growing rapidly, due to the high penetra-tion of smart phones, tablets, and increasinglyportable laptops [3]. In recent 3GPP studies, adedicated MRN deployment has been consideredas a cost-effective solution to serve data-intensiveUE inside public transportation vehicles [4].

In this article, we discuss the potential advan-tages as well as the implementation challenges ofdeploying MRNs. Compared to analog repeaters,advanced relay nodes (RNs) in LTE-Advancedsystems can decode and forward data to UE.They can also detect or correct errors beforeforwarding the data to UE. Thus, the receivedsignal qualities at the UE can be enhanced byusing such RNs. Two types of advanced RNs,type 1 and type 2 RNs, have been defined by3GPP. Type 1 RNs are non-transparent RNsthat can form their own cells, and terminate alllayer 2 and layer 3 communication protocols.Type 2 RNs are transparent RNs that replicatethe cell ID of their donor eNB (DeNB).2 TheFRN, which has been standardized in the LTE-Advanced system, is an inband half-duplexdecode-and-forward (DF) type 1 RN, while the

ABSTRACT

In future wireless networks, a significantnumber of users accessing wireless broadbandwill be vehicular (i.e., in public transportationvehicles like buses, trams, or trains). The ThirdGeneration Partnership Project has started toinvestigate how to serve these vehicular userscost-effectively, and several solutions have beenproposed. One promising solution is to deploy amoving relay node (MRN), on a public trans-portation vehicle that forms its own cell insidethe vehicle to serve vehicular users. By properantenna placement, an MRN can reduce or eveneliminate the vehicular penetration loss thataffects communication. Moreover, MRNs canexploit various smart antenna techniques andadvanced signal processing schemes, as they areless limited by size and power than regular userequipment. However, there are also challengesin using MRNs, such as designing efficient inter-ference management techniques as well as prop-er mobility management schemes to exploit thebenefit of group handovers for vehicular UEdevices served by the same MRN. Nevertheless,initial system-level evaluation results indicatethat a dedicated MRN deployment shows greatpotential to improve the vehicular user experi-ence, and thereby can potentially bring signifi-cant benefits to future wireless communicationsystems.

HETEROGENEOUS AND SMALL CELL NETWORKS

Yutao Sui, Chalmers University of Technology

Jaakko Vihriälä, Nokia Siemens Networks

Agisilaos Papadogiannis, Chalmers University of Technology

Mikael Sternad, Uppsala University

Wei Yang and Tommy Svensson, Chalmers University of Technology

Moving Cells: A Promising Solution toBoost Performance for Vehicular Users

SUI LAYOUT_Layout 1 5/30/13 4:18 PM Page 62

IEEE Communications Magazine • June 2013 63

use of other types of RNs in LTE networks hasbeen left for further investigation [5, Ch. 30].Similarly, the MRN proposed in recent 3GPPstudies is also an inband half-duplex DF type 1RN that can form its own cell inside a vehicle[4]. Moreover, MRNs can be shared by severaloperators, a fact that can lower the cost of build-ing the networks [6].

Initially, in 3GPP studies, MRNs weredesigned to serve UE inside well isolated high-speed trains [4]. Recent studies show that MRNscan also effectively serve UE inside public trans-portation vehicles in general (buses, trams, etc.)[7, 8]. There are several advantages of usingMRNs. First, group handover (HO) can be per-formed by regarding the UE served by the sameMRN as a group. Depending on the mobilitymanagement schemes, the signaling at the radionetwork side and probability of HO failure ofthe vehicular UE could be noticeably lowered [4,9]. Second, since MRNs are not as limited bysize and power as regular UE, they can betterexploit various smart antenna techniques andadvanced signal processing schemes. For exam-ple, in a train, several backhaul antennas can beinterconnected to form a cooperative and coor-dinated relay system [10], which can strengthenthe backhaul link by using antenna selectiontechniques. Moreover, by a proper placement ofindoor and outdoor antennas, an MRN can cir-cumvent the vehicular penetration loss (VPL)caused by a well isolated vehicle. Measurementsshow that the VPL can be as high as 25 dB in aminivan at the frequency of 2.4 GHz [11], andhigher VPLs are foreseeable in the vehicles ofinterest. Last but not least, compared to directtransmission from eNBs to in-vehicle UE, betterpropagation conditions — less shadowing andpath loss, and higher line-of-sight (LOS) connec-tion probabilities — can be expected for thebackhaul connections between eNBs and MRNs.

Besides using MRNs, several other solutionsusing existing techniques have also been pro-posed in 3GPP to serve vehicular UE [4]. In thisarticle, we first review the solutions based on theexisting techniques to serve vehicular UE. Thenwe present the benefits of using dedicatedlydeployed MRNs to serve UE inside public trans-portation vehicles. Simulation results show thatMRNs can effectively serve vehicular UE. At theend of the article, we discuss the challenges andopen issues as well as future research directionstoward applying MRNs to practical communica-tion systems.

CURRENT SOLUTIONS TOSERVE VEHICULAR UE

Solutions based on existing system elements,such as optimizing the deployment of macrocells,or using layer 1 repeaters or WiFi access points(APs) to assist serving vehicular UE, have beendiscussed in 3GPP [4]. In this section, we brieflyreview the advantages and disadvantages ofthese solutions. All layer 1 repeaters, WiFi APs,and MRNs communicate with a macro eNB viaradio interfaces, but MRNs also act as regulareNBs for the UE they serve, and can potentiallysupport multiple radio access technologies.

DEDICATED MACRO ENBS

As the route of a public transportation vehicle isusually known, the coverage of macro eNBs canbe tuned along the train lines or highways byusing directive antennas. A trade-off betweencoverage and capacity can be addressed byemploying an umbrella structure. More specifi-cally, high-power eNBs can be used to extendcoverage and handle control signaling withunderlying low-power nodes to improve capacity.For instance, with CA, the primary componentcarrier (PCC) is transmitted by a high-powereNB, but the secondary component carriers(SCCs) can be transmitted by low-power nodes.3By employing sophisticated cross-carrier schedul-ing schemes, most of the control signaling can besent by the high-power eNBs, and the data canbe transmitted by the low-power SCCs. If CA isnot possible, the use of RRHs can be consid-ered. Since RRHs share the same cell ID as themacro eNB, the closest RRH can serve the highspeed vehicle without HO mechanisms beingtriggered.

However, the VPL, which is one of the majorchallenges faced by vehicular UE, cannot bereduced by this solution. Site acquisition, deploy-ment, and maintenance are also challenges foroperators, especially for low-power nodes, dueto the requirements for power supply, dedicatedbackhaul connection, and so on. Moreover, thissolution cannot easily be extended to urban sce-narios.

LAYER 1 REPEATERSLayer 1 repeaters are low-cost analog repeaters,as little signal processing is required comparedto the advanced RNs in LTE. They amplify andforward signals in a given frequency band, andthey can possibly work in a full- duplex mode onthe vehicles of interest, as the vehicles can suffi-ciently isolate the transmit and receive chains.This is an advantage compared to the half-duplexFRNs standardized in LTE.4 Moreover, layer 1repeaters can overcome the VPL caused by wellisolated vehicles, and thereby lower the UEtransmit power.

However, there are several issues with layer 1

3 With CA, up to fourSCCs can be aggregatedwith a PCC, and each ofthe component carriersdefines its own servingcell. Different componentcarriers may be configuredto provide different cover-age. The configuration ofPCC and SCCs is termi-nal-specific: depending onthe system load, UEserved by the same macroeNB can have differentPCC and SCCs assign-ments [5, Ch. 28]

4 A half-duplex RN cantransmit and receive inboth directions but notsimultaneously, whereas afull-duplex RN providesconnection in both direc-tions at the same time. Toachieve full duplexing,good isolation betweentransmit and receivechains is required due toself-interference, such asusing frequency-divisionduplexing (FDD).

Figure 1. An overview of 3GPP heterogeneous and small cell networks.

MacroeNB

Type 2fixedrelaynode

Movingrelay node

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IEEE Communications Magazine • June 201364

repeaters. Compared to advanced DF RNs, thesignal-to-interference-plus-noise ratio (SINR)cannot be improved at layer 1 repeaters, sincethey amplify and forward both the receiveddesired and interference signals.5 Furthermore,since layer 1 repeaters are less controlled by thecore network than the advanced RNs, the HO ofvehicular UE between eNBs still needs to beperformed individually, and advanced interfer-ence management schemes cannot be integratedin them. Thus, layer 1 repeaters may bring moreinterference into the system.

LTE FOR BACKHAUL, WIFI FOR ACCESSUsing WiFi to provide Internet access to datausers on board is fairly common, as most smartphones and laptops are WiFi-capable. Similar tolayer 1 repeaters, wireless backhaul nodes can beplaced on the roof of a vehicle with antennasmounted outside communicating with the cellu-lar network, while providing data access to vehic-ular users with WiFi APs. Backhaul links can befull duplex. WiFi is used for data connections,and regular voice calls can be handled by thecellular network. Hence, VPL can be circum-vented, and group HO can be used for datausers. Furthermore, WiFi-only devices can alsouse the cellular network, which may bring extraincome to service providers.

However, using WiFi technology to provideUE with a seamless experience in the currentcellular network is challenging. Although the802.11x extension of the WiFi standard enablesroaming for WiFi devices, functions such ascharging, authentication, and security stillrequire software and hardware upgrades for theWiFi module to access the universal subscriberidentity module (USIM) in the device [12].Moreover, although the 802.11e amendment ofthe WiFi standard has quality of service (QoS)support, it requires that all the APs are con-trolled by the same central unit [13]. As WiFi

networks operate on the open industrial, scien-tific and medical radio (ISM) bands, the inter-ference in these radio bands cannot becoordinated in the same manner as in the dedi-cated frequency bands owned by operators.Thus, it is difficult for the operators to offer sim-ilar QoS as in their own cellular networks.

DEDICATED MRN DEPLOYMENTIn this section, we discuss the deployment ofdedicated MRNs to serve UE inside public trans-portation vehicles. To motivate the use of MRNsto serve vehicular users, we consider a simpleexample. In a noise limited system, assume thatthe position of a vehicular UE device is known,and we are interested in finding the optimallocation for an FRN to minimize the end-to-endoutage probability (OP). This setup was studiedin [14], and Fig. 2 plots the optimal FRN posi-tion as a function of VPL by using a typicalurban propagation model. The plot indicatesthat if we know the communication is affectedby high VPL, the best solution is to place anFRN as close as possible to the vehicle to com-pensate for the VPL. In practice, as the posi-tions of vehicles are not known beforehand, theuse of MRN is more economical and applicableto serve vehicular UE.

In an interference limited scenario, the MRNcan bring additional benefits. As shown in [8,10], MRNs can operate at much lower transmitpower compared to macro eNBs or other low-power nodes, but still achieve similar or evenbetter performance. As shown in [8], when theVPL is 30 dB, a half-duplex MRN can lower theend-to-end OP by 65 percent on average com-pared to direct transmission at vehicular UE,and even better performance is expected from afull-duplex MRN.

In order to understand the performance of anMRN serving vehicular UE in a more practicalsetup, we resort to system-level simulations. Asemi-static system simulator is used, and the sim-ulated network consists of 19 cells. On averagewe drop 25 macro UE devices per cell, and onerandomly located bus with five UE devicesonboard moving through the cells. A systembandwidth of 10 MHz, corresponding to 50 phys-ical resource blocks (PRBs), is considered. VPLis varied between 0 and 30 dB, and detailed sim-ulation parameters are given in Table 1. Bothhalf-duplex and full-duplex MRNs are consid-ered, and the performance is compared with thebaseline case(i.e., vehicular UE devices areserved directly by macro eNBs). The half-duplexMRN assisted transmission is implemented insuch a way that, in the first transmission timeinterval (TTI), the DeNB sends to the MRN (viabackhaul link), and in the following TTI, theMRN communicates to the UE devices in thevehicle; eNBs communicate to macro UE devicesin both TTIs.

Figure 3 shows the downlink performance ofmacro UE and vehicular UE separately, withvarying VPL. The cumulative distribution func-tion (cdf) of the UE throughput is calculated.The results in Fig. 3 are shown as 95 percent UEthroughput (horizontal axis) vs. 5 percentthroughput (vertical axis, corresponding cell

Figure 2. Optimal FRN position when UE position is known.

UE distance from eNB (km)0.10

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VPL = 30 dBVPL = 10 dBVPL = 0 B

5 Layer 1 repeaters canavoid vehicular UEreceivers operating in anoise limited region, butthey cannot provide thesame SINR improvementas DF RNs.



edge performance). The throughput of vehicularUE served by a half-duplex MRN is roughlyhalved compared to the full-duplex MRN casedue to the half-duplex loss. Nevertheless, asVPL increases, the cell edge performance of thevehicular UE served by an MRN, for both thehalf-duplex and full-duplex cases, is greatlyimproved compared to the baseline case. Theperformance improvement is due to two reasons:• Relaying circumvents VPL.• The MRN is installed on the roof of the

vehicle, and thereby better propagationconditions (i.e., less shadowing and pathloss) are experienced by the backhaul linkcompared to the direct link from macroeNBs to vehicular UE.Regarding the macro UE, the duplex scheme

employed by the MRN (square for full-duplexMRN case and diamond when half-duplex MRNis used) has no significant impact on the 95 per-cent UE throughput. However, there is a notice-able decrease of the 5 percent throughput forthe macro UE when a full-duplex MRN is usedto serve vehicular UE compared to the half-duplex MRN case. This is because with a half-duplex MRN, half of the time there is nobackhaul link connection, and during these TTIsall PRBs are instead used to serve macro UE.Moreover, when the backhaul link of the MRNis active in one cell, it generates interference tothe macro UE in other cells. We also notice thatthere are no significant impacts of differentVPLs on the performance of macro UE.

The results show a drastic increase in celledge performance for the vehicular UE. In thisexample, the number of vehicular UE devices isquite small compared to macro UE devices.With a higher number of vehicular UE devices,the performance would probably be affectedsince increasing the number of PRBs for thebackhaul link would also decrease the macro UEperformance. Nevertheless, we can conclude thatthe use of MRNs deployed on the vehicleimproves performance of the vehicular UE,especially at the cell edge.

CHALLENGES AND OPEN ISSUESThe potential of using MRNs to improve theQoS and throughput of vehicular UE is outlinedin previous sections, but there are also chal-lenges. In this section, we briefly discuss thechallenges of deploying MRNs and possiblesolutions.

From the scenarios studied earlier, we canidentify that the backhaul link is the capacitybottleneck. As the UE speed goes up, the rapidvariations of mobile channels combined withfeedback delays reduce the accuracy of the chan-nel state information (CSI) at eNBs. The CSIfeedback inaccuracy at the eNB limits the use ofadvanced MIMO transmission schemesemployed in LTE systems to further increase thethroughput of the backhaul link, as adaptivetransmit beamforming, link adaptation, andchannel-aware scheduling would be severelycompromised. In FDD LTE systems, the CSI atthe eNB would be outdated by at least 5 ms dueto channel estimation delays, the frame struc-ture, and feedback delays. Although LTE-

Advanced systems offer several diversity-basedschemes for high-mobility UE, more resourcesare then required to be allocated to these UEdevices. This lowers the spectral efficiency of thetime-varying MRN backhaul link compared tothat of an FRN in a similar position.

Using channel predictors, such as Kalman orWiener predictors, could potentially enable theuse of adaptive transmit techniques at vehicularvelocities, as they can predict fading channels inspace 0.2–0.3 wavelengths ahead with adequateaccuracy. This means that 5 ms prediction hori-zons could be attained for low vehicular veloci-ties at 900 MHz carriers but not for frequencyabove 1.5 GHz.6 The scenario with an antennaor antenna array on the roof of a fast-movingbus or tram provides an opportunity as well as achallenge. One innovation is the use of a “pre-dictor antenna” [15]. The vehicle velocity and

Table 1. Simulation parameters.

Parameter Value

Carrier frequency 2.6 GHz

System bandwidth 10 MHz

Number of PRBs 50

Number of cells 19

Intersite distance (ISD) 500 m

Duplex scheme Full-duplex (FDD) and half-duplex

Relaying type In-band, decode-and-forward

Vehicle position Random, uniform distribution

Number of vehicular UE devices 5

Number of macro UE devices/cell 25 (average)

Number of macro UE devices 475 (total)

UE position Random, uniform distribution

VPL 0, 10, 20, 30 dB

Number of receive antennas 2 (RN, UE)

PRB allocation principle Resource (PRB) fair

Frequency domain allocation Random

eNB transmission power 46 dBm

MRN transmission power 10 dBm

Shadowing standard deviation eNB-UE 8 dB

Shadowing standard deviation eNB-MRN 4 dB

Receiver noise figure 8 dB

Traffic model Full buffer

6 A vehicle at a speed of50 km/h moves 0.0694 min 5 ms, which is a bitmore than 0.2 wave-lengths at 900 MHz.



direction would be relatively constant over therequired 5 ms prediction horizon. Channel pre-dictions could then be obtained by the predictorantenna located some distance directly in frontof the antennas used for transmission, as shownin Fig. 4. Channel estimates from this antennacould be used to predict the channel experiencedby the rearward antenna a little later when it hasmoved to this position. We would avoid the needto extrapolate in time from the measurements.

Preliminary results based on measurementcampaigns done in a 20 MHz orthogonal fre-quency-division multiplexing (OFDM) system in2.68 GHz downlinks showed that using a predic-tor antenna can significantly improve the CSIfeedback accuracy at a vehicular velocity [15]. Amaximal normalized correlation above 0.95 wasreported between the signal from the predictorantenna and the signal from the antenna behindit mounted on the vehicle roof, with antenna dis-tances (prediction horizons) of up to three wave-lengths, in both LOS and non-LOS (NLOS)conditions. This suggests that a simple predictorthat uses delayed channel estimates from a pre-dictor antenna can provide useful predictionaccuracy at vehicular velocities. Thus, it opens upthe possibility of using the most advanced MIMOtechniques as well as link adaptation to increasethe throughput over the backhaul links of MRNs.

Another challenge in the deployment ofMRNs is from the aspect of mobility manage-ment. This includes the HO of MRNs betweendifferent DeNBs, and HO of UE between MRNsand eNBs. Previous studies showed that by per-forming group HO, the HO failure probabilitiesof vehicular UE can be lowered significantly byusing MRNs [9]. However, as the LTE systemtoday has no mobility support for MRNs, modi-fication of the current system architecture isneeded to provide efficient yet reliable mobilitymanagement for MRNs [4, 7].

Figure 5 shows the current FRN architectureemployed by the LTE system. A DeNB acts as aproxy, and hides an FRN from mobility manage-ment entities (MMEs) and various gateways

serving the UE. As the time required for FRNattachment and configuring the proxy functional-ity at a DeNB is on the order of minutes [5, Ch.30], the same FRN system architecture cannotbe directly adapted for MRNs. On the radio net-work side, by using MRNs, the signaling over-head can be reduced by performing MRN HOinstead of individual UE HO. However, on thecore network side, the data connection of eachUE device still needs to be handled individuallyat the MME [4]. Although this is not required tobe done at the same time as the HO of UE,depending on the network topologies andwhether the target DeNB is served by the sameMME or MME pool, the disturbance caused bythe HO may be very large [7]. To support MRNmobility, several solutions either based on modi-fications of the current FRN architecture orusing an entirely new architecture have been dis-cussed, and various architectures have beencompared in [4, 7]. An overall comparison ofdifferent mobility management architectures isgiven in [4, Table 6.1], and detailed HO delaysare analyzed in [7]. Moreover, in a HetSNetsdeployment scenario, there could potentially beproblems created by a large group of vehicularUE devices that are handed over into a fixedsmall cell, thereby disturbing its resource alloca-tion. But we are already facing an even worsevariant of this problem today, when lots of indi-vidual vehicular UE devices move through afixed network with small cells. Last but not least,how to avoid UE devices close to but outside thevehicles wrongly attaching to the MRNs alsoneeds consideration.

Furthermore, new challenges regarding inter-ference management arise due to the use ofMRNs. As the distance between an MRN and thevehicular UE served by it is very short, the MRNand the vehicular UE can communicate with eachother using very low power. In addition, the VPLcan further help to dampen the signal of theMRN access link that propagates out from thevehicle. Thus, compared to direct transmission,the use of MRNs generates less interference fromthe access link, for both downlink and uplink, toUE outside the vehicles. This is appreciated in adensely deployed urban scenario where link avail-abilities are usually dependent on interferencerather than coverage. For the backhaul link, how-ever, the problem becomes complicated, as inter-ference is expected both between different MRNbackhaul links, and between MRN backhaul linksand macro UE. The use of predictor antennascan improve CSI accuracy to enable the use ofadvanced interference avoidance and cancellationschemes for the backhaul links. Nevertheless,whether enhancements on the current intercellinterference coordination (ICIC) framework inLTE are needed to support the use of MRNs stillrequires further investigation.

DISCUSSION ANDFUTURE DIRECTIONS

In this article, we present the benefits and chal-lenges of using dedicated MRNs to serve vehicu-lar UE. Compared to serving vehicular UEdirectly from macro eNBs, MRNs can increase

Figure 3. Downlink performance of MRN assisted transmission.

95 % throughput (kb/s)900

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the performance of the UE inside the vehiclesignificantly, especially at the cell edge. This isbecause MRNs can effectively reduce VPL, andcreate better propagation conditions for thebackhaul links compared to the direct linksbetween macro eNBs and vehicular UE. As thebackhaul connections of the MRNs become thebottleneck, we present the idea of using predic-tion antennas to increase the accuracy of theCSI feedback for the backhaul links. Thisimproves the performance of channel-dependentscheduling and enables the use of spatial pro-cessing for the backhaul links of MRNs at highvehicular velocities.

However, there are several challenges faced byusing MRNs, both technically and economically.The use of MRNs brings new tasks regardingmobility and interference management to thecommunity. Several mobility management solu-tions have been proposed with initial evaluationresults, but which one should be adopted in thefuture system still requires further investigation.Moreover, whether new ICIC enhancements areneeded to support the use of MRNs is still anopen issue. From the business logic point of view,on one hand, using MRN can significantlyimprove the user experience, and thereby increasethe loyalty of customers. On the other hand, itwill increase the operation costs of operators.Public transportation companies could potentiallyshare these costs. Also, novel context or location-awareness-based services targeting the needs oftravelers and commuters could evolve into newbusiness models in the near future. Further inves-tigations are needed to answer these broad ques-tions. Nevertheless, the use of MRNs has greatpotential as an efficient solution to the ever grow-ing ubiquitous demand for high-quality mobiledata services by vehicular users.

ACKNOWLEDGMENTThe authors would like to thank the SwedishResearch Council (VR) and the EU FP7 forproviding the research funds. We also appreciatethe contributions from our colleagues in the EUFP7 project INFSO-ICT-247223 ARTIST4G.

REFERENCES[1] A. Ghosh et al., “LTE-Advanced: Next-Generation Wire-

less Broadband Technology,” IEEE Trans. Wireless Com-mun., vol. 17, no. 3, June 2010, pp. 10–22.

[2] A. Damnjanovic et al., “A Survey on 3GPP Heteroge-neous Networks,” IEEE Trans. Wireless Commun., vol.18, no. 3, June 2011, pp. 10–21.

[3] Cisco white paper, “Cisco Visual Networking Index:Global Mobile Data Traffic Forecast Update,2011–2016,” tech. rep., http://www.cisco.com/,accessed Nov. 20th, 2012.

[4] 3GPP TR 36.836, “Technical Specification Group RadioAccess Network; Mobile Relay for Evolved Universal Ter-restrial Radio Access (E-UTRA),” tech. rep.,http://www.3gpp.org/, accessed Nov. 20, 2012.

[5] S. Sesia, I. Toufik, and M. Baker, LTE — The UMTS LongTerm Evolution: From Theory to Practice, 2nd ed.,Wiley, 2011.

[6] 3GPP TR 22.951, “Service Aspects and Requirements forNetwork Sharing,” tech. rep., Sept. 2012,http://www.3gpp.org/, accessed Nov. 20, 2012.

[7] M. Khanfouci, Y. Sui, A. Papadogiannis, and M. Färber,“D3.5-c Moving Relays and Mobility Aspects,” tech.rep., Artist4G deliverable, May. 2012, https://ict-artist4g.eu/projet/deliverables accessed Feb. 14, 2013.

[8] Y. Sui et al., “Performance Comparison of Fixed andMoving Relays under Co-Channel Interference,” Proc.IEEE GLOBECOM Wksps., 2012.

[9] W. Li et al., “Performance Evaluation and Analysis onGroup Mobility of Mobile Relay for LTE-Advanced Sys-tem,” Proc. IEEE VTC 2012-Fall.

[10] V. V. Phan et al., “Providing Enhanced Cellular Cover-age in Public Transportation with Smart Relay Sys-tems,” Proc. IEEE VNC, 2010.

[11] E. Tanghe et al., “Evaluation of Vehicle Penetration Loss atWireless Communication Frequencies,” IEEE Trans. Vehic.Tech., vol. 57, no. 4, July 2008, pp. 2036–41.

[12] 3GPP Std. TS 23.234, “3GPP System to Wireless LocalArea Network (WLAN) Interworking; System Descrip-tion,” Sept. 2012, http://www.3gpp.org/, accessed Nov.20, 2012.

[13] IEEE 802.11 WG, “Wireless LAN Medium Access Con-trol (MAC) and Physical Layer (PHY) Specifications.Amendment 8: Medium Access Control (MAC) Qualityof Service Enhancements,” IEEE 802.11e, Nov. 2005.

[14] Y. Sui, A. Papadogiannis, and T. Svensson, “The Poten-tial of Moving Relays — A Performance Analysis,” Proc.IEEE VTC 2012-Spring.

[15] M. Sternad et al., “Using ’Predictor Antennas’ forLong-Range Prediction of Fast Fading for MovingRelays,” Proc. IEEE WCNC, 2012.

BIOGRAPHIESYUTAO SUI [S’11] ([email protected]) received his B.E.degree in electronic engineering and B.A. degree in Englishfrom Yanbian University, Yanji, China, in 2006, and hisM.Sc. degree in wireless communications from Lund Uni-versity, Sweden, in 2009. He is currently a Ph.D. candidatein the Department of Signals and Systems at Chalmers Uni-versity of Technology, Gothenburg, Sweden. He concen-trates his research on heterogeneous and small cellnetworks, especially moving relays for vehicular user com-munications.

JAAKKO VIHRIÄLÄ ([email protected]) received hisM.Sc. degree from the University of Oulu, Finland, in 1995.He joined Nokia in 1993, and is currently with NokiaSiemens Networks, Oulu. He has been working on DSPalgorithm design and implementation, WCDMA receiverdesign, wireless relaying, and heterogeneous networks.

AGISILAOS PAPADOGIANNIS [S’07, M’10] ([email protected]) received his M.Eng. degree in electrical andcomputer engineering from the University of Patras,Greece, in 2005, his M.Sc. (with distinction) degree in com-

Figure 4. An illustration of deploying an MRN on top of a public transporta-tion vehicle.

0.5 to 3wavelengths

Predictorantenna

MRN

Accesslink

VehicularUE

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Backhaul link

Figure 5. FRN system architecture in LTE release 10.

S1

X2 S1

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http://www.cisco.com/

http://www.3gpp.org/



mailto:[email protected]


https://ict-artist4g.eu/projet/deliverables



munications engineering from the University of York, Unit-ed Kingdom, in 2006, and his Ph.D. degree in electronicsand communications from TELECOM ParisTech, Paris,France, in 2009. From 2006 to 2009 he was a researchengineer at Orange Labs, Paris, working on 4G communi-cations. The focus of his work was on coordinated multi-point (CoMP) and relaying techniques with affordableoverheads in relation to the 3GPP LTE-Advanced stan-dards. From April 2010 to August 2011 he was a researchassociate at the Department of Electronics, University ofYork, working on technologies beyond 4G. From August2011 to February 2013 he was a postdoctoral fellow atthe Department of Signals and Systems, Chalmers Uni-versity of Technology, where he conducted teaching,research, and research supervision in the areas of futureuser-centric and energy-efficient networks. He has partic-ipated in several European (FP7 METIS, FP7 ARTIST4G,FP7 BuNGee, CELTIC WINNER+), nationally funded (VRDynamic Multipoint Wireless Transmission, VINNOVAMAGIC, VINNOVA MORE4G, RCUK UC4G), and industrialresearch projects (4GRADIO, PRICE) in the general area offuture wireless systems. He holds two internationalpatents, and has authored a book and numerous highimpact technical papers.

MIKAEL STERNAD [S’83, M’88, SM’90] ([email protected]) is a professor of automatic control at UppsalaUniversity and received a Ph.D. degree from Uppsala Uni-versity in the same field in 1987. With a background inrobust and adaptive estimation, control, and filtering, heled the national Swedish project Wireless IP and is at pre-sent leading the Dynamic Multipoint Wireless Accessframework project funded by the Swedish Research Coun-cil. He has participated in the EU WINNER, WINNER II, andARTIST4G Integrated Projects, contributing to channel pre-diction and estimation, adaptive transmission, design ofrobust linear precoders, deployment concepts, multipleaccess techniques/concepts, MAC layer design, and overallsystem design. Channel prediction has been one focus areaof research for 15 years. He is also involved in audio and

acoustic MIMO signal processing, room compensation, andsound field control. He co-founded and was chairman(2001–2005) of Dirac Research, a company in this area. Hehas authored over 130 papers, six book chapters, and 10patents.

WEI YANG [S’09] ([email protected]) received his B.E.degree in communication engineering and M.E. degree incommunication and information systems from Beijing Uni-versity of Posts and Telecommunications, China, in 2008and 2011, respectively. He is currently pursuing a Ph.D.degree in electrical engineering at Chalmers University ofTechnology. From July to August 2012, he was a visitingstudent at the Laboratory for Information and DecisionSystems, Massachusetts Institute of Technology, Cam-bridge. He is the recipient of a Student Paper Award at the2012 IEEE International Symposium on Information Theory,Cambridge, Massachusetts. His research interests are in theareas of information and communication theory.

TOMMY SVENSSON [S’98, M’03, SM’10] ([email protected]) is an associate professor in communicationsystems at Chalmers University of Technology, where he isleading the research on air interface and microwave back-haul networking technologies for future wireless systems.He received a Ph.D. in information theory from Chalmersin 2003, and has worked at Ericsson AB with core net-works, radio access networks, and microwave transmis-sion products. He partic ipated in the Europeancollaborative research projects WINNER I, II, WINNER+,and ARTIST4G, which have contributed to the 3GPP LTEstandards, and he is currently active in the EU FP7 METISproject targeting solutions for 2020. His main researchinterests are in design and analysis of physical layer algo-rithms, multiple access, resource allocation, cooperativesystems, and moving relays/cells. He has co-authored twobooks and more than 70 journal and conference papers.He is Chairman of the IEEE Sweden VT/COM/IT chapter,and coordinator of the Communication Engineering Mas-ter’s Program at Chalmers.



