The Importance of in-building Solutions In

Comparing 3G and 2GThere are several reasons why the demandfor dedicated in-building coverage is greaterin WCDMA networks than in current second-generation networks:• Less favorable link-budget, especially for

high-bit-rate services, because WCDMAnetworks use higher frequencies thanmost second-generation networks, the in-door coverage from macro cells is often in-ferior due to greater link loss. In addition,greater capacity must be provided forWCDMA than for second-generation sys-tems because stronger signal strength isneeded for high-bit-rate services.

• End-users expect better quality of servicefrom third-generation mobile net-works—a key differentiator of third-gen-eration mobile networks is high bit rates,but most high-bit-rate services will beused by end-users who are stationary; thatis, the majority of end-users will be in-doors in cafés, restaurants, offices, hotels,shopping centers, bars, pubs, subways,gyms, train stations, airports, and homes.Few people remain stationary while out-doors because conditions are seldom ideal(too hot, cold, wet or windy). To attractnew subscribers, operators of third-generation mobile networks should, atvery least, provide the same level of cov-erage as end-users have come to expectfrom existing second-generation net-works. To provide this level of coverage,operators can employ dedicated inbuild-ing coverage in key locations.

• Dedicated in-building coverage helps of-fload the macro network. This has a two-fold effect: (a) dedicated in-building sys-tems offload the macro cells by reducingaverage downlink power levels, which inturn, releases additional system capacityfor the outdoor cell; and (b) in-buildingcells are isolated from the macro network,which results in lower interference levels.In-building cells can thus provide greatercapacity than outdoor cells.

Coverage vs. capacity inCDMA-based systems In TDMA systems, cell coverage and capac-ity are more or less independent. Coveragedepends on such factors as radio frequency(RF) power, sensitivity and interference. Ca-pacity, on the other hand, is determined bythe number of channels provided.

By contrast, in CDMA and WCDMA sys-tems, capacity and coverage in the downlinkare highly interdependent (Figure 1). InCDMA systems, multiple users share thesame base station transmitter, and thus thesame power resource in the downlink (Fig-ure 2). As a consequence, the total cell ca-pacity is affected by path loss to each user.Greater path loss is compensated for by acomparably larger portion of the downlinkresource, which leaves less of the downlinkresource for other users. In essence, the sys-tem can handle more users at low powerthan at high power.

Path loss is determined by a user’s loca-tion in the cell. It is also greatly influenced

90 Ericsson Review No. 2, 2004

The importance of in-building solutions in third-generation networksHans Beijner

Providing good in-building coverage plays an important role in attractingand retaining mobile subscribers. Ordinarily, coverage from the macronetwork extends into buildings but should be complemented by dedicat-ed in-building systems. Greater data capacity and the ability of third-gen-eration networks to provide high-speed data services increase thedemands put on the cellular network. Subscribers have great expecta-tions regarding third-generation services. Therefore, when introduced, thenew services must (at very least) be available everywhere second-genera-tion services can be found. Notwithstanding, many third-generation net-works deployed to date have been designed primarily to provide goodcoverage in outdoor environments—not inside buildings. As a conse-quence, users of early implementations of wideband code-division multi-ple access (WCDMA) rate third-generation services poorly compared tosecond-generation services.

Besides guaranteeing better quality of service (QoS), dedicated in-building systems enable operators to catch roaming subscribers—mostroaming subscribers have automatic public land mobile network (PLMN)selection, which means that if the regular connection is lost, their phoneautomatically selects the best available PLMN. Operators who providededicated in-building coverage in key locations, such as airports and trainstations, can thus catch these high-value roaming subscribers.

The implementation of dedicated in-building coverage in CDMA-basednetworks also offloads the macro system, thereby increasing overall sys-tem capacity. In other words, operators can, with minimum investment,continue to use existing networks to serve a growing number of sub-scribers. This gain in capacity means that operators can put off splittingcells and thereby substantially reduce costs of network expansion. In par-ticular, it is more beneficial to deploy dedicated in-building coverage sys-tems in third-generation CDMA-based networks than in time-division mul-tiple access (TDMA) networks.

Interesting solutions for deploying in-building coverage in WCDMA net-works make use of distributed radio base stations (DRS) and passive dis-tributed antenna systems (DAS). These solutions reduce overall costs bylowering transmission costs, increasing trunking gain, and by sharing RBSequipment.

2G Second-generation3G Third-generationCDMA Code-division multiple accessDAS Distributed antenna systemDRS Distributed radio base stationGSM Global system for mobile com-

municationHSDPA High-speed data packet accessPLMN Public land mobile networkRAN Radio access networkRBS Radio base stationRF Radio frequencyTDMA Time-division mobile networkUE User equipmentWCDMA Wideband CDMAWLAN Wireless local area network

BOX A, TERMS AND ABBREVIATIONS

Ericsson Review No. 2, 2004 91

by whether the user is outdoors or indoors.Users located outdoors and close to the cen-ter of the cell have the least path loss; userswho are inside buildings far from the cen-ter of the cell (cell border) have the great-est path loss. Users inside buildings con-sume a proportionately larger share of thedownlink resource, especially when locatedclose to the cell border. Note also that alarger number of users (indoors and out-doors) is almost always located closer to thecell border (high path loss) than to the cellcenter (Figure 3).

Indoor “black holes”Ordinarily, a well-designed WCDMA sys-tem limits the maximum downlink power

to any given user. A typical limit is 20% oftotal available power. Therefore, in a worst-case scenario with users in buildings in less-than-ideal locations, a cell can serve only afew users.

Uplink vs. downlinkConditions in the uplink are somewhat dif-ferent from those in the downlink. Becauseeach user has his or her own dedicated radioresource (user equipment, UE) in the up-link, capacity and coverage are not directlyinterdependent except for the effect of noise,which has an increasingly negative effect asthe number of users in the cell grows. Evenso, dedicated in-building systems also offerspecific benefits to the uplink. For instance,

Cell radius

WCDMA UL

WCDMA DL

Capacity

TDMA DL and UL

Figure 1Dependency of coverage (WCDMA) in theuplink and downlink compared to TDMA.

High load Low load

Reducedcoverage

Figure 2Dependency of coverage and capacity(load) in CDMA-based systems.

less interference from other cells, whichmeans that an isolated indoor cell can havegreater capacity in the uplink than an out-door macro cell.

By design, traffic is not expected to be bal-anced in the uplink and downlink. The ma-jority of high-speed data traffic occurs in thedownlink. Therefore, most indoor cells arelimited by the capacity of the downlink.

High bit rates requiremore powerThe services offered in WCDMA systemsdetermine the power levels required by theuplink and downlink. High-bit-rate ser-vices require greater power levels from theRBS and mobile equipment than low-bit-rate services. Also, bear in mind that most


Figure 3 Areas A1 and A2 are equal and containthe same number of subscribers, butaverage path loss to users in area A2 ismuch greater than for users in area A1.

BTS transmit power levels

Higher bit rates require higher power levels

UE received power levelsFigure 4Higher bit rates require higher power levels

A1= A2 (km2)A1

A2


high-bit-rate services are used by stationaryusers in indoor environments.

Offloading the macro cellIf most or all indoor users in a cell are servedby dedicated indoor systems, then less aver-age power is required to serve all remainingusers in the macro cell. And as we have seen,less power per user means that a larger num-ber of users can be served.

The actual gain in capacity depends onwhere in the cell the indoor users are locat-ed. The greatest benefits can be derived fromintroducing dedicated in-building systemsfor users with high path loss—that is, forusers in buildings close to the cell border.

Isolated indoor cells yieldgreater capacity per cellCell capacity in CDMA-based systems is de-pendent on • the power needed to serve each user in the

cell; and• interference from other cells—less inter-

ference means greater capacity. Minimum interference is achieved by iso-lating cells from one another. Walls andother structures help isolate dedicated in-door cells, making them less vulnerable tointerference from signals from other cells.Ortogonality is also better in indoor sys-tems. The capacity of dedicated indoor cellsis thus typically two to three times greaterthan that of an ordinary non-isolated macrocell. Indoor cells that do not employ softhandover to other cells have even greater ca-pacity.

Potential capacity gainsFigure 5 shows a model for describing gainsin capacity. According to this model, the ad-dition of in-building systems has the po-tential to increase network capacity up to400 percent. Moreover, cost per subscriber,including the cost of the in-building sys-tem, can be reduced 67%.

In the first case, macro cells provide cov-erage for outdoor and indoor users (12 out-door users and 6 indoor users, Figure 5). Av-erage path loss to each user is high, and as aconsequence, total capacity is relatively low.

In the second case, the indoor users enjoydedicated coverage. The coverage frommacro cells is used mostly by outdoor users.Average path loss per user is thus less, andthe capacity of each macro cell has increased,

which means the macro cells can serve moreusers. The addition of a dedicated cell for in-door users also increases available capacityindoors, because the indoor cell is isolatedfrom the macro cells. The capacity of the in-door cell is thus greater than that of the out-door cells.

The gain in capacity signifies that the net-work can handle a larger number of sub-scribers. Compared to traditional methods,such as cell splitting or increasing the num-ber of frequency channels, the addition ofdedicated in-building coverage is thus acost-effective way of catering for new sub-scribers.

The actual gain in capacity depends onfactors such as number of indoor users servedby dedicated in-building coverage, locationof users and buildings, the traffic profile, andpenetration loss due to building walls andstructures. In many cases, the capacity gainis considerable. Theoretical calculationsshow that capacity in certain parts of a cel-lular network can increase by 200-600%provided some indoor users are served bydedicated cells. Measurements in live net-

Cell 112 612

ExampleCapacity: 12 + 6 + 12 = 30

Cell 2

Figure 5The deployment of dedicated in-building systems can increase totalnetwork capacity up to 400%.

Cell 148 2448

ExampleCapacity: 48 + 24 + 48 = 120

Cell 2

works have confirmed considerable gains incapacity.

Benefits to networks withhigh load in the downlink During initial rollout, system load is usual-ly light and coverage in the uplink anddownlink is more or less balanced. The ben-efits of offloading macro cells via dedicatedin-building systems are thus negligible.However, as load increases and the systembecomes increasingly limited by the down-link, the benefit of dedicated in-buildingcoverage increases.

In-building coverageflattens load peaksIn systems that lack dedicated in-buildingcoverage, path loss to different users can varyconsiderably depending on user location andwhether the user is located indoors or not.This variation in path loss means that sys-tem capacity fluctuates according to userdistribution. Operators need to consider thisaspect and allocate sufficient margins whenplanning their networks. But if indoor usersare served via dedicated indoor cells, therewill be less fluctuation in capacity, and op-erators can allocate smaller margins whenplanning their networks.

HSDPAHigh-speed-data packet access (HSDPA) isa method of transmitting very high datarates in a WCDMA network. The theoreti-cal maximum data capacity from HSDPA is14.4Mbps peak and approximately 2Mbpsaverage (shared by all users in a cell).

The interdependent relationship be-tween coverage and capacity describedabove also holds true for HSDPA, becausethroughput to each user is dependent onradio link quality. That is, total datathroughput suffers when several simulta-neous users are located in an area with poorradio-link quality. Therefore, one could saythat WCDMA networks with HSDPA willbenefit from dedicated in-building cover-age, because it ensures that more users canenjoy high data throughput.

CDMA vs. WCDMAEverything described thus far is valid bothfor WCDMA and narrowband CDMA-based systems, such as CDMA2000 and

IS-95. With respect to indoor coverage,however, narrowband CDMA systems dodiffer from WCDMA systems in that theyuse narrower bandwidth for each radio chan-nel. As a consequence, it is easier to set asidea few of these channels for indoor coverageand thereby offload the channels used foroutdoor coverage. By reserving some avail-able channels for indoor coverage, the in-building system needs not dominate oversignals from the macro network. This is otherwise a problem for operators who im-plement dedicated in-building coverage forWCDMA—it is thus impractical to set asidechannels for this purpose (especially if theoperator has only a few WCDMA channels).

Distributed antennasystemsA distributed antenna system (DAS) is anetwork of antennas distributed throughouta building to provide dedicated in-buildingcoverage. The system can be passive or ac-tive. A passive DAS consists of a network ofcoaxial cables, couplers, and power splittersthat distribute RF signals to antennas placedthroughout the building. One variant of apassive DAS employs radiating coaxial ca-bles instead of discrete antennas. The deci-sion to use discrete antennas or radiating ca-bles is usually based on factors such as build-ing structure and installation constraints.

Ordinarily, an active DAS uses opticaldistribution—the RF signals from the RBSare converted into optical signals by a localinterface unit. The optical signals are dis-tributed through the building via optical ca-bles to several remote units where they areconverted back into RF signals. Antennas orsmall coaxial distribution networks are con-nected to each remote unit to provide cov-erage on each floor.

There are many advantages to a passiveDAS compared to an active DAS. For in-stance, it offers low initial cost and high re-liability. Being a wideband system, the pas-sive DAS solution is also ideal for multi-operator and multi-service systems, includ-ing all kinds of radio-based services, such astrunked radio, cellular systems, and wirelesslocal area networks (WLAN) up to 2.5GHz.The majority of today’s installations of in-building systems are passive DAS solutions.

Benefits of DASThe DAS gives operators a more effective so-lution to providing in-building coverage



than small, distributed indoor base stations(pico RBS). For instance, it provides trunk-ing gains on the radio interface and trans-mission links—every radio channel is thusinstantly available in every part of the build-ing; this, in turn, increases spectrum effi-ciency and makes it possible to cater for localtraffic peaks. The RBS needs only one trans-mission line to feed an entire building. Thismakes for a high degree of transmissiontrunking efficiency and decreases theamount of transmission overhead that is nor-mally transmitted to each RBS.

DAS solutions also make it is easy for op-erators to control and confine coverage with-in a building and to reduce interference toand from the macro network, which in-creases overall network capacity.

DRS solutionA distributed RBS solution (DRS) is an in-teresting solution for implementing in-building coverage for WCDMA (Figure 6).The solution shares one RBS between sev-eral buildings or indoor areas. The RBS isconnected to an optical distribution systemthat converts radio signals into optical sig-nals before distributing them to each build-ing over optical fiber. High-power opticalremote units in each building convert the

optical signals back into RF signals. The sig-nals are then distributed inside the build-ing via a coaxial antenna network. Some ofthe benefits of deploying DRS configura-tions are greater capacity thanks to im-proved trunking, low transmission cost, andlow investment in the radio access network.

Greater capacityTrunking gain refers to an increase in num-ber of channels (or bandwidth), or in otherwords, an increase in capacity. The DRS so-lution provides trunking gains for radio in-terface and transmission—spectrum efficien-cy improves because every radio channel is in-stantly available in each associated building.Moreover, the full capacity of the RBS can bemade available for any building or site, mak-ing it easier for operators to handle local peakloads (for example at restaurants during thelunch hour or when users seek indoor shelterfrom sudden rain showers).

Low transmission costOperators can use statistical multiplexingto reduce transmission costs—many usersshare the same transmission, which is con-centrated to one site. In many markets, darkfiber is inexpensive, making the overall costfor a DRS solution favorable compared toother alternatives.

RRU

FiberFiber

RBS/MU

RRURRU

Figure 6DRS solution using RBS 3402 Main-Remote.

Low RAN investmentThe macro site and the dedicated in-build-ing site share the macro RBS. Operator in-vestments in the radio access network arethus lower than investments in comparablealternatives. Ericsson’s RBS 3402 Main-Remote is an ideal choice for this applica-tion.

Capacity—the wayforwardGiven the benefits to WCDMA networks,it can be argued that dedicated in-buildingcoverage should be introduced early in theevolution of 3G systems. The traditionalnetwork evolution of second-generation sys-tems, such as GSM, can be described as fol-lows:1. Initial roll-out and coverage.2.Cell split, shorter site-to-site distance.3.New spectrum.4.Micro cells, in-building cells.Today, however, early measures are taken tocombat interference. The current networkevolution is thus:1. Initial roll-out and coverage.2.Tight frequency reuse (MRP, FLP).3.New spectrum.4.Micro cells, in-building cells.5.Cell split, shorter site-to-site distance.The idea is to introduce network featuresthat reduce interference instead of splittingcells, which at very least, doubles site costs.

Early introduction forWCDMAFor WCDMA—to capitalize on the bene-fits of offloading the macro cell network—it pays to deploy dedicated in-building cov-erage at an earlier stage than was done insecond-generation systems. In most cases,the initial roll-out is planned to providevoice and high-speed data services to out-door users. Indoor coverage is generallyquite limited and mostly consists of voiceservices. High-speed data services are usu-ally only available in buildings close to thecenter of the cell. Users generally perceivein-building coverage to be weaker than thatfrom existing second-generation networks.

Plans to offer comprehensive, in-build-ing, high-speed data coverage from the ini-tial roll-out of the macro network are notfeasible or practical due to the number ofsites that would be necessary.

Ordinarily, operators plan the initial net-work roll-out to handle relatively lightloads, because this approach yields the mostperformance for the least cost. But as systemload increases, the downlink quickly be-comes a limiting factor, and breaks or holesin coverage occur during peaks in traffic. Indense urban areas, in particular, there is con-siderable risk that a temporary concentra-tion of indoor users in one or more cells couldseverely limit the capacity of those cells. Op-erators should thus take some kind of actionto reduce load and improve coverage. Someof their alternatives include• adding new sites and splitting cells; • adding new frequencies; or• offloading the macro network by intro-

ducing dedicated in-building coveragefor indoor hot-spots.

Each of these alternatives achieves the samegoal of reducing system load and maintain-ing coverage, but the associated costs varyconsiderably. Splitting cells is the most cost-ly alternative because of the need for newsites. Adding new frequencies, or introduc-ing a second carrier, provides greater capac-ity, but still does not guarantee adequate in-building coverage. In urban areas, where themajority of users are located indoors, the im-plementation of dedicated in-building sys-tems can be a cost-effective method of deal-ing with increased load. In summary, thepreferred evolution of WCDMA networksis as follows:1. Initial roll-out and coverage.2. Indoor coverage introduced to offload

macro cells and to cover indoor hot-spots.3.Additional carriers (new spectrum).4.Cell split, micro-cells.The main idea is to keep costs down by wait-ing as long as possible to split cells.

Primary targetsTo realize maximum benefit, operatorsshould first target heavily populated build-ings and underground areas that are close to



cell borders. Underground shopping centersand other underground areas close to the cellcenter might also be priority targets if usersin these areas experience substantial pathloss from the macro cell.

Other obvious targets include buildingswhose capacity requirements exceed the ca-pabilities of a single macro cell. In cases likethis, an in-building system is justifiable re-gardless of where the building is located.Moreover, the in-building system can besplit into several cells to provide as much ca-pacity as needed. Typical examples includeairports, skyscrapers, conference centers,shopping centers, and sports stadiums.

Market experienceJapan

In Japan, the first country to introduceWCDMA, NTT DoCoMo is investing heav-ily in in-building coverage—it plans to haveimplemented more than 3000 in-buildingsites by March 31, 2005, and an additional2000 sites by March 31, 2006.

Vodafone Japan has seen a slow increasein subscribers due to a shortage of compet-itive terminals. Notwithstanding, the oper-ator acknowledges the importance of dedi-cated in-building coverage and plans to im-plement 1000 in-building sites before year-end (fiscal year) 2004. The majority of thesewill be single-operator sites, because opera-tors in Japan still compete by providing cov-erage.

KDDI, which operates a CDMA2000 sys-tem at 800 MHz and has plans to introduceservices at 2GHz, is also implementinglarge-scale, dedicated in-building coverage.

Besides single-operator, in-building sites,numerous multi-operator systems have beeninstalled throughout Japan in railway andsubway stations and in underground shop-ping centers. The Japanese Ministries ofPost and Telecommunications and of Con-struction have subsidized these installationsto help guarantee coverage in the event of amajor accident or natural catastrophe, suchas an earthquake. The sites for second-generation services (approximately 600) arenow also being complemented with cover-age for WCDMA.

SpainOperators in Spain recognize that good in-building coverage plays a big role in at-tracting and retaining subscribers. The pri-mary goal is to complement current second-generation dedicated in-building systemswith third-generation coverage and to im-plement new coverage in key locations.

SwedenOperators in Sweden also recognize the ben-efits of dedicated in-building coverage andare looking for cost-effective solutions.Some have already implemented distributedantenna systems using macro RBSs.

ConclusionOperators cannot ignore the importance andadvantages of providing dedicated in-build-ing coverage in third-generation networks.Users who try new third-generation servicesexpect them to exceed existing (second-generation) services in every respect. This isespecially true for in-building coverage. Asa consequence, operators want dedicated in-building coverage systems in third-generation networks to complement the ex-isting macro network.

Besides providing excellent local in-building coverage, dedicated in-buildingsystems for third-generation CDMA-basednetworks greatly offload the part of themacro network. Therefore, they give opera-tors a cost-effective way of catering for newsubscriber growth.

In all likelihood, distributed antenna sys-tems will dominate as the preferred solutionfor providing in-building coverage. New so-lutions, such as the distributed RBS solu-tion, are also of interest thanks to overalllower costs—lower transmission cost,greater trunking gain, and the ability toshare RBS equipment.

Ericsson In-building Competence Centers inSingapore and Spain have provided valu-able input to this article. In particular, theauthor wants to thank Robert McCrorey,Paul Kattukaran and Danny Tan for theircontributions.

ACKNOWLEDGEMENTS

The Importance of in-building Solutions In

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