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Abstract Relaying as a means of in-band backhaul has thepotential to extend the coverage of Beyond 3G networks,enabling the expected high data rates of these networks to bedelivered without increasing the density of traditional macrobase stations. Assessing the performance of relaying is notsimple, since traditional metrics fail and the performancedepends strongly on the actual deployment. The currentliterature considers usually very artificial deploymentenvironments and propagation models. This paper shows thecoverage and achievable peak data rates for an urban area incentral London using three-dimensional building data and aray-tracing simulator. The number of relays per sector isdetermined for different scenarios, and compared to a MacroDeployment.

Index Terms LTE, LTE-Advanced, Relaying, Deployment,Coverage, Capacity

I. I NTRODUCTION

Third generation mobile networks such as HSPA (HighSpeed Packed Access) currently experience a huge uptake in

data services such as mobile internet access mainly due tothe availability of simple, affordable, small and plug-and-

play modem devices, attractive flat-rate tariffs, and goodnetwork coverage, at least from leading network operators.However, the capacity of current networks is limited; andindoor coverage for high-speed data access poses achallenge. NGNM (Next Generation Mobile NetworksAlliance) have addressed the requirements on nextgeneration mobile networks [1], which include increasedspectral efficiency and re-use of existing infrastructureincluding cell sites.3GPP Long Term Evolution (LTE), IEEE 802.16m and

Ultra Mobile Broadband (UMB) all address theserequirements with MIMO-OFDMA system concepts. Themost important benchmark for further innovation will beLTE Release 8 [4][5]. It comes very close to fundamentallimits in terms of link-level performance by using advancedspatial processing techniques [3]. However, fundamentallimits are not yet reached, when the system level isconsidered and deployment topologies are taken intoaccount. Within 3GPP [5], discussions on further development of LTE have started in April 2008 in thecontext of LTE-Advanced, which addresses operator requirements and envisages to fulfill requirements set out bythe ITU for newly identified frequency bands.

1 Fabian Diehm is now with the Vodafone Chair, Dresden University of Technology, Germany

The coverage and network capacity could be increased byincreasing the Macro Site density, regardless what air interface is deployed. However, the grid of Macro Sites isalready very dense in urban areas, and it is almost

prohibitive from a cost perspective to increase it further.Micro Sites, which could be deployed on street level (e.g.walls, lamp-posts) have the potential to relieve some of thecosts associated with site deployment. However,

backhauling, one of the main cost elements, would still berequired. Installing backhaul such as fiber or microwave tolamp-posts is also very challenging. The licensed frequency

bands used for radio access are very attractive for backhaulas well access and backhaul could use the sametechnology, antennas etc, and synergies could be achieved ina flexible and modular way.Another expression for these in-band backhauling conceptsis Relaying , which has been discussed in academia for along time [8]. Relaying has found its way into the IEEE802.16j standard [6],[10], and a WiMAX profile for relayingis currently discussed, as well as relaying concepts for IEEE802.16m. Relaying is an integral part of the WINNER air interface, a beyond 3G system concept. The benefits of relaying were shown in [9]. Relaying involving mobileterminals is also studied in the literature, but beyond thescope of this paper. This paper assumed decode-and-forwardrelays because they seem to be the most promisingapproach. Amplify-and-forward relays (repeaters) are notstudied, since they suffer from some issues like transmit-receive attenuation, and noise/interference enhancement.

The key advantage of relaying is that coverage and capacitycan be traded in some way the huge capacity LTE has to

offer in theory would not only be available to few usersunderneath the base station, but also to shadowed andindoor users. From that perspective, it seems to bereasonable to even take a small loss in capacity into account,if fairness of the system can be improved, leading to a morehomogenous user experience.However, relaying also has several drawbacks. Accesscapacity is wasted for the backhaul link, and complexityand delay are added. It is therefore important to assess therelaying performance very carefully to study all trade-offs,

before standardization and deployment are envisaged.

Evaluating the capacity and coverage of a mobilecommunications system which includes relaying is verychallenging, since relay deployment and radio channels haveto be modeled realistically at the one hand and simply at theother. Also, results on the performance of relaying vary

On Coverage and Capacity of Relaying in LTE-Advanced in Example Deployments

Ralf Irmer, Member, IEEE , and Fabian Diehm1

Vodafone Group R&D, The Connection, Newbury, RG142FN, United Kingdom. Emailralf.irmer@vodafone.com

978-1-4244-2644-7/08/$25.00 2008 IEEE

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significantly depending on the scenario and the evaluationmethodology. There are attempts to describe relaydeployments in a semi-analytic way, e.g. in IEEE 802.16j[7] or WINNER [9], but it is very hard for an operator todraw general conclusions.The limitations of published relaying performanceassessments were the starting-point of this paper. A realworld example area in a dense urban environment (centralLondon) is selected instead. Topology, buildings and theradio channel are modeled using the three-dimensional ray-tracing tool Pace3D. Relays are placed in an intelligentway by filling coverage holes.

II. LTE RELAYING

Currently LTE has not developed a concept for relaying, butthe majority of 3GPP partners expressed at the LTEAdvanced workshop in Shenzen (China) in April 2008 thatrelaying is an important technology element to address theLTE-Advanced requirements. To enable an analysis of

potential relaying concepts anyway, some assumptions are

made in this paper on what a relay actually is:

Standardized decode-and-forward radio relay (in- band backhaul) using the same frequency band asthe access link, and the same air interfacetechnology (e.g. LTE).

Low equipment cost: The price of relays asassumed in this study is very low compared toconventional Macro Sites, and hence allowsdeployments with many units to be considered.Low cost is targeted for both the equipment and theactual deployment.

Small form factor and low weight: Relays areexpected to be small and lightweight, enabling easyinstallation and support of units on lampposts or

building walls. The form factor should be similar tomunicipal WiFi radio nodes, which are nowadayscommercially deployed in some cities. For simulation it is assumed that relays are deployed ata height of 5m. .

Low transmission power : The relays used in thisstudy have a maximum transmission power of 30dBm (1 Watt).

Omnidirectional Antennas : For deploymentsimplicity and to address the low-cost requirement,omni-directional relays are assumed in this paper.However, deployed WiFi mesh networks haveshown that even 8-fold sectorization is possible insmall form-factor.

Low operational costs: Using in-band backhaul andlow power transmission (30dBm) relays areexpected to have low operational costs. Air conditioning and other active cooling are notrequired. Avoiding directional antennas, personnelneed no more qualification for deployment than for any street-lamps.

Conforms to radio emission health and safetyregulations: This is particularly important as relays

are deployed at a low height where it cannot beguaranteed that people do not get close to thedevices

To integrate relaying into a standard, a couple of issues haveto be addressed and choices have to be made:

The relaying concept should be applicable to both

TDD (Time Division Duplex) and FDD (FrequencyDivision Duplex) schemes.

Frame structure to accommodate efficient and lowdelay relaying.

Transparent or non-transparent mode has to beselected this affects preamble design

Protocol architecture has to be defined Radio resource management that allows for

spectrally efficient relaying HARQ (Hybrid Automatic Repeat Request)

concept hop-by-hop or end-to-end Control channels and measurement procedures

have to be designed Power control has to be adopted to enable a well-

performing overall system Existing security mechanisms such as

authentication and encryption have to be extendedto relaying, and to be enhanced if necessary

It is beyond the scope of this paper to address these issues, but the coverage and capacity analysis of this study isexpected to give guidance for the development of therelaying concept.

III. DEPLOYMENT IN DENSE URBAN AREA CENTRALLONDON

A. Existing Macro-Cellular 3G Network For this study, an area with an existing 3G/HSPA Macro-Cellular deployment by Vodafone was chosen as a referencecase. The area shown in Figure 1 in central London has asize of approximately 1km 2, but interfering cells outside this

area were considered in this study as well to avoid edgeeffects. Most sites in the examined area have three sectors,and the downtlilt of the antennas has been optimized. Thesite density in this particular area is about 4.1 sites/ km 2.

B. LTE Link Budget For the coverage prediction, it is important to base it onaccurate link budgets, which are influenced by both,environment and system specific parameters. Theenvironment specific parameters are based on a three-dimensional ray-tracing tool, which takes the topology dataand all buildings into account. The used tool was calibrated

by drive-tests. It takes account of antenna patterns, pathlossand shadowing. For indoor penetration loss, 20 dB isassumed as in the NGMN evaluation methodology [2].Indoor penetration loss is applied to buildings according to

TABLE IMAXIMUM PATHLOSS FOR UPLINK BASE CASE

Links

AssumedUser/Relay

Bandwidth(MHz):

Re-quired

SNR (dB):

Maximum

allowablepathloss (dB)

Correspon-ding data

rate (Mbps )

UE -> BS 1.08 3 103.5 0.4UE -> RS 1.08 3 96.5 0.4RS -> BS 1.08 17 120.2 2RS -> RS 1.08 17 113.2 2

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the topography data base. For coverage analysis, only theuplink (UL) direction is considered since it is morechallenging, whereas the average peak rate is calculated for the downlink direction.

1542A

1852D

23009G

358C786A

529,800 530,000 530,200 530,400 530,600

1 8 1

, 0 0 0

1 8 1

, 2 0 0

1 8 1

, 4 0 0

1 8 1

, 6 0 0

529,800 530,000 530,200 530, 400 530,600

1 8 1

, 0 0 0

1 8 1

,2 0 0

1 8 1

,4 0 0

1 8 1

, 6 0 0

Figure 1: Central London current 3G/HSPA Macro Sites

The system specific parameters can be described by SINR-to-data rate mappings. LTE SINR-to-data-rate mappings areused, which include HARQ, adaptive modulation andcoding (AMC) and MIMO schemes up to 2x2 spatialmultiplexing in the downlink and 1x2 SIMO in the uplink. Asimple Pedestrian B channel model with 3 km/h is used inthe SINR-to-data rate mapping to take into accountfrequency-selectivity and fading. This approach is well-suited for the analysis in this paper, but could be extended inthe future by using more elaborate channel models such asSCM-E (see e.g. [2]) and using link-level measurementsfrom LTE equipment. LTE downlink is based on OFDMA,whereas the uplink is based on SC-FDMA (Single Carrier Frequency Division Multiple Access).

For the UL coverage link budgets we assume an SNR of 3dB on both the relay and base station access links to beachieved for users with an assignment of 6 simultaneouslyused LTE resource blocks. This equals to a bandwidth of 1.08 MHz per user if a 10 MHz FDD system with 50available resource blocks is considered. These assumptionstranslate into an UL target user data rate of about 400 kbps.User data rates are calculated by assuming a total overheadof 30% for both, up- and downlink. On the relay links(relays to base stations and relays to other relays) an SNR of 17 dB is assumed. Together with the assumption of 6simultaneously used resource blocks this translates into adata rate of about 2 Mbps. These assumptions and theresulting allowable pathlosses which make up the base case link budgets are given in Table I. The presented pathlossesdo not account for antenna gains (which are taken intoaccount by the ray-tracing tool) and include an indoor

penetration margin of 20dB.

C. Relay Deployment In this study, relays are deployed by the procedureillustrated in Figure 2. Basically, the target area coverage of 90% (including indoors) is tried to be met with the existingMacro Sites (BS) and a minimum amount of additionalRelays (RS) deployed at 5m height on street level. The

Relays fill coverage holes whilst it is ensured that they havegood connectivity to one of the Macro sites directly or viaanother Relay Node. This deployment procedure is donemanually with the assistance of a network planning tool. Allsimulations are carried out at a carrier frequency of 2.6GHz.The network planning strategy in this paper is purelycoverage oriented. The data traffic is assumed to be equally

distributed within the cell. Only the strongest signal at eachlocation is taken into account. It is assumed that inter-cellinterference can be treated with other concepts, such asadvanced scheduling, fractional frequency reuse or radioresource management (RRM). Interference is discussed later in this paper.

Define Link Budgets

Apply BS to UE budgets toexisting macro sites andobserve coverage at 1.5m height

1852D

007'40"W

5 1 3 1 ' 0 0 " N

007'40"W

5 1 3 1 ' 0 0 " N

1852D

007'40"W

5 1 3 1 ' 0 0 " N

007'40"W

5 1 3 1 ' 0 0 " N

Apply BS to RS budgets toexisting macro sites andobserve coverage at 5mheight (single hop coverage)

1852D

Site5

Site69

Site71

007'40"W

5 1 3 1 ' 0 0 " N

007'40"W

5 1 3 1 ' 0 0 " N

Add relays and place themin coverage holes at 1.5m height.Make sure they are not placedin coverage holes at 5m height.

1852D

Site5

Site69

Site71

007'40 "W

5 1 3 1 ' 0 0 " N

007'40"W

5 1 3 1 ' 0 0 " N

Simulate combined Relay/Macro Coverage at 1.5mheight

Optimze placement and simulatecombined Relay/Macro coverageat 1.5m height (apply BS to UEand RS to UE budgets)

If target coverage is notmet and relays can still be

placed at locations withcoverage at 5m height.

Simulate combined Relay and Macrocoverage at 5m height (apply BS to RSand RS to RS budgets) Multi hop case

If target coverage is notmet and relays can not be

placed at locations withcoverage at 5m Height.

Done

Add relays and place them incoverage holes at 1.5m height.Make sure they are not placedin coverage holes at 5m height.

If target coverageis met

Figure 2 Relay Deployment Procedure

IV. DEPLOYMENT DENSITIES TO ACHIEVE COVERAGE FOR TARGET DATA RATE WITH DIFFERENT DEPLOYMENTSTRATEGIES

Figure 3 shows the resulting relay deployment if the abovedescribed relay deployment strategy is applied to achievethe 90% uplink coverage target of 3dB. Additionally to the4.1 Macro Sites per km 2, 44 Relays/ km 2 have to bedeployed i.e. 3.6 relays per sector. This represents the basecase result. Because the results are very sensitive to link

budgets, the assumed link budgets were varied by plus andminus 5dB to study the impact of link variations. Table IIshows the number of relays per sector which are necessaryto achieve the uplink coverage target for different accesslink variations. Depending on the assumptions, between 10and 88 relays are necessary for the investigated dense urbanarea which equals 0.8 to 7.2 relay stations per sector. The

presented numbers underline the strong influence of link budget assumptions. Simulations show also that variationson the macro access link have a greater influence on thenumber of deployed relay nodes.

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Figure 3: Relay placement for 90% Coverage. The colorsindicate current Macro Site only coverage (only red areameets uplink signal strength condition). Macro sites with

three-fold sectorization are depicted by three arrows,whereas relays are depicted by one arrow.

To have a comparison to conventional macro deployments,we also created Greenfield Macro (hexagonal grid)deployments in the same area for the same UL indoor datarate target coverage. In the case of Macro Sites three

possible access link variations (-5/0/+5 dB) result in sitedensities of 17.8/8.9/4.4 sites/km 2, respectively. Hence, inorder to achieve the target coverage, we expect to have theoptions of doubling the Macro Site density or adding 3-4relays per sector, if we assume our base case link budgets.Using link budget variations is a very powerful approach,

since it covers a variety of cases: Dependency on carrier frequencies LTE

deployments are considered from 700 MHz to 2.6GHz in different parts of the world

Use of more elaborative antennas at base station,UE or relay, i.e. 4x4 antennas in the downlink or 1x4 antennas in the uplink. The effect could bemodeled either in the SNIR-to-data rate mapping,or in link budgets.

Different assumptions on target coverage data rate Variation of antenna deployment (i.e. use of remote

radio heads, antenna gain, cable loss)

V. ACHIEVABLE DOWNLINK DATA R ATES AND CAPACITY

An LTE FDD system with 10 MHz bandwidth on the up-and downlink is assumed. Considering a guard band of 1MHz, 50 available LTE resource blocks are assumed. Tocalculate user data rates we take an overhead of 30% intoaccount. Resources used for the BS-RS link are not reusedon the RS-UE link or on the direct BS-UE link, i.e. the

resource split penalty of relaying is taken into account. Inthe case of orthogonal resource reuse of resources withinone cell, none of the resource elements can be reused by anynetwork element within one cell. In the case of full resourcereuse, relays can reuse all resource except the ones used theBS-RS feeder links. In this paper, the resource split isoptimized for both cases to uniformly distribute the capacityover the coverage area, i.e. fairness is prioritized opposed to

maximization of overall capacity. The relaying overhead ismodeled by 5% additional overhead on the feeder link. Thisvalue for additional overhead is motivated measurements incommercial WLAN-based mesh networks.It is impossible to have an accurate modeling of interferencein a frequency reuse one based OFDMA system such asLTE with Relaying, as the LTE Relaying concept does notyet exist. Thus assumptions on the interference and relayingconcepts were made and performance bounds are calculated.Three different cases are considered:

Interference is fully neglected noise limitedenvironment (optimistic case)

Interference is taken into account by aninterference margin (realistic case)

Interference is taken into account by full frequencyreuse one between all Micro BSs and all Relays,considering the SINR values (pessimistic case)

It is up to the LTE relaying concept to find a good trade-off between interference impact and efficient resourceutilization.

Existing Macro:

-5 dB 0 dB +5 dB

TP (Mbps)

Greenfield Macro:

-5 dB 0 dB +5 dB

TP (Mbps)

Relay Deployments:

TP (Mbps) -5 dB 0 dB +5 dB

-5 dB

0 dB

+5 dB

R S - >

U E

BS -> UE

BS -> UE

BS -> UE

18.8 13.5 11.0 20.8 15.2 10.9

20.1 14.7 10.3

22.5 16.9 9.9

25.3 19.3 10.3

22.7 16.8 11.1 25.7 19.7 10.4

23.9 18.1 10.9 26.5 20.5 10.1

21.9 15.6 6.4 25.6 19.0 10.2 25.6 19.0 10.3

12.6 9.5 8.9 17.9 13.6 8.9 24.3 19.1 8.9

Figure 4: Downlink Average Peak Throughput in Mbpsfor different deployments, link budgets and interference

assumptions. The left blue bar represents the noiselimited case, the middle green bar the interference-

margin-based realistic case, and the right brown bar theinterference limited pessimistic case

Figure 4 shows the average DL peak throughput for alldeployments and all interference assumptions. Wecalculated the average peak throughput as the average of the

peak throughputs (all resources assigned to one user) for alltiles in the selected area.

TABLE II NUMBER OF RELAYS PER SECTOR FOR DIFFERENT LINK BUDGET

ASSUMPTIONS

UE BS

-5 dB 0 dB +5 dB

U E

R S

-5 dB 7.2 4.1 1.4

0 dB 4.4 3.6 1.1

+5 dB 3.2 2.3 0.8

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Downlink Peak Data Rates vs Area Coverage

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60

Peak Data Rate (Mbps)

% a

r e a ( P e a k

D a t a

R a t e

> = a b s c

i s s a

)

Max Interference Interference Margin No Interference

Figure 5: Downlink Peak Data Rate for the Base Case

Relay Deployment

Figure 5 shows how the peak data rates are distributed over the selected area for the base case relay deployment for different interference assumptions. The graphs for nointerference and maximum interference are the upper andlower bounds, respectively. We expect a real LTE system to

be somewhere in between these bounds.Table III shows the area downlink capacity for differentdeployments. The existing macro sites (2G/3G), which cannot meet the 90% uplink indoor coverage target (3 dB / 400kbps), has a capacity of 167 Mbps/sqkm is an interferencemargin is considered for inter-sector interference. Thecoverage target can be achieved by doubling the site density(Greenfield Macro Deployment), and the area capacity can

be increased then to 507 Mbps/sqkm. If instead relays areadded to the existing macro sites, the 90% UL coveragetarget can also be met and the overall capacity is 180-283Mbps/sqkm, depending on the resource reuse strategy of therelaying concept. The usage of resources for the BS-RS

feeder link is taken into account in this analysis. The

sensitivity to link budget variations is not shown here, butrelays improve the area capacity in all cases compared to theexisting macro-only deployment despite the feeder link resource use penalty. The reason is that macro sites can usetheir resources in a much more efficient way when relaysare deployed, since more efficient coding, modulation andspatial processing schemes can be utilized by both the BSand the relay.The capacity difference between orthogonal and fullresource reuse is very substantial any relaying conceptshould try to come close to full resource reuse if possible but

provide means to deal with interference.

VI. CONCLUSION

The performance of relaying is debated very intensely in theliterature but most studies use statistical models of

deployment areas. This paper compares macro site and relaydeployments in an example urban area (central London)using ray-tracing simulations and three-dimensional

building data. The uplink data coverage for indoor andoutdoor users can be significantly improved by adding 3-4relays per sector, or alternatively doubling the macro sitedensity. The average peak throughput within the consideredarea is 17.8 Mbps in a 10 MHz FDD LTE system with relay

deployments, but this figure depends strongly on howinterference is handled in the relaying concept. This average

peak throughput is substantially higher than in an LTEdeployment considering only existing macro sites. The areacapacity can also be increased by adding relays if a coveragetarget is taken into account in the deployment strategy. Theresource reuse strategy between base stations and relayswithin one cell has a critical impact on the area capacity.Coverage and capacity can be increased by just deployingmore macro sites. But this is very often just not feasiblefrom economic and technical perspectives relays offer anattractive alternative solution as shown in this paper.

VII. OUTLOOK Many assumptions had to be made to take into account therelaying concept, especially on the resource allocation andinterference. Once a relaying concept is developed for theLTE standard, the established bounds can be narroweddown. This investigation, which is based on well-calibratedray-tracing simulations and three-dimensional building datais valid for a particular area. Simulations with statistical

performance evaluation models could be compared to theapproach taken in this paper. This study assumed verysimple, cheap relays with omni-directional antennas.Sectorized relays might increase capacity and offer flexibility for resource reuse and interference handling, buttrade-offs to cost, size and power consumption increase.

R EFERENCES [1] Next Generation Mobile Networks Beyond HSPA & EVDO,

NGMN Alliance White Paper 3.0. December 2006, www.ngmn.org [2] Next Generation Mobile Networks Radio Access Performance

Evaluation Methodology, NGMN Alliance White Paper, June 2007,www.ngmn.org

[3] P. Mogensen et.al.: LTE Capacity Compared to the ShannonBound, In . Proc. IEEE VTC Spring 2007, pp 1234-1238

[4] E. Dahlman, S. Parkvall, S. Soeld, P. Beming:3G Evolution. HSPAand LTE for Mobile Broadband, Academic Press 2007

[5] www.3gpp.org[6] D. Soldani and S. Dixit: Wireless Relays for Broadband Access.

IEEE Communications Magazine Volume 46, No. 3, pp 58-66, March2008.[7] G. Senarath, W. Tong et.al: Multi-hop Relay System Evaluation

Methodology (Channel Model and Performance Metric), IEEE802.16j-06/013r3, 19.2.2007, www.ieee802.org/16/relay

[8] R. Pabst, et.al.: Relay-based Deployment Concept for Wireless andMobile Broadband Radio, IEEE Communications Magazine,Volume 42, No. 9, pp 80-89, Sept. 2004

[9] S. Redana, K. Doppler et.al.: Final assessment of relaying conceptsfor all CG scenarios under consideration of related WINNER L1 andL2 protocol functions, IST WINNER II deliverable D3.5.3,September 2007, www.ist-winner.org

[10] www.ieee802.org/16/relay

TABLE IIIAREA DOWNLINK CAPACITY IN MBPS FOR ONE SQUARE K ILOMETER , BA SE

CASE LINK BUDGET (0 DB) No

interferenceInterference

marginExisting Macro Sites Only (4.1

sites/sqkm)220 167

Greeefield Macro Deployment toachieve target coverage (8.9 sites/sqkm)

685 507

Existing Macro Sites + Relays (FullResource Reuse)

359 283

Existing Macro Sites + Relays(Orthogonal Resource Reuse)

234 180