UMTS co-siting

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The UMTS-GSM Co-Siting Problem

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Introduction

This paper analyses the different aspects that have to be considered when co-siting for

UMTS and GSM Base Stations (BSs) is planned in a radio site by one or several operators.

There are a lot of motivations that can drive to co-siting solutions: such as political,

environmental, financial or technical. All those are analysed here with a special focus on the

technical issues. Our area of interest is Europe but the situation can be extrapolated to other

areas, if not now probably in the future.

The European Environment

Most Western European Mobile Telecommunications markets have achieved what could be

called a mature stage, with high-quality country wide coverage, 3 or 4 networks in competition,and a high degree of service penetration, frequently over 50% of the population. The technology

used in these networks is GSM (usually also known as 2nd generation) and increasingly also

GPRS (the packet data bearer service based on GSM and usually known as 2,5th generation)

and several networks have already launched services based on this new GSM packet bearer

(analogue networks are decaying remaining with a small number of subscribers).

To support the large amount of GSM subscribers, and the increasing traffic volume caused

by decreasing prices and competition between operators, a huge number of GSM Base Stations

have been deployed in the past years. In Germany, for example, more than 69,000 base stations

have been installed on approx. 54,500 sites as reported by the German regulator in its annual

report for the year 2000 [1], with a yearly increase of over 18,000 sites.

Consequently antennas installed on the tops of buildings and telecommunications towers

have become a part of the city and country landscape and the public starts to notice that these

things can be found everywhere. With the increased presence of the antennas and towers, a


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newfound health concern has spread over the population, provoking in some areas a strong

reaction against the deployment of new sites. In some cases even Base Stations which were on

service had to be disconnected following the pressure on local authorities by the concerned

neighbourhoods. Additionally, the visual impact, or architectural protection laws, have often

become important problems. These problems and the lack of regulations, government guidance

and operator sensitivity in some countries, mean that contracting suitable locations to build new

sites is getting more and more difficult.

In some European countries operators have paid what must be considered as unreasonably

high prices for UMTS licenses. In some cases the financial costs of the payment of the license

fees alone are at a similar level with the yearly turnover of a Mobile Telecommunications

Operator with the result that some operators have encountered financial problems. Consequently

operators in some countries require more flexibility from their respective governments to deploy

the networks and to define ways to launch UMTS networks in the most economically feasible

way.

New rules in the European mobile communication arena have to be defined in order to not

jeopardize this economical engine that has been working full power in Europe over the last

decade. Operators, suppliers, governments and local authorities will have to work together to

take care of the concerns of the population. New policies and information strategies have to be

implemented, taking into account all interests and concerns.

In this environment the co-siting problem between GSM and UMTS is one of the most

interesting challenges in the actual mobile communications market. It is probable that the huge

amount of new UMTS BSs that the industry is planning for the next years, will only be realized

when co-siting is used extensively.

The technical problem and its solutions

In the context of this extremely complicated situation, lets have a look at the technical

issues that have to be considered when planning co-siting of UMTS Base Stations with GSM

Base Stations. In line with the complicated environment that has been depicted, the technical

situation is not any easier. Several hundreds of thousands of Base Stations of 7 major BS

suppliers are in the field all with their own performance parameters radiating different levels of


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spurious emissions. On September 2000 the 3GPP approved a new version of the former ETSI

05.05 Recommendation [2] with crucial changes on the level of spurious emissions that GSM

BSs are allowed to radiate especially on the UMTS receive frequency band.

Why was this change necessary? When the old GSM BSs were developed, no special

consideration was given to UMTS being a future system on the European allocation of

spectrums for this system. The level of spurious emissions in the UMTS receive band was

defined with a general limit which is too high to allow co-siting of the two systems.

But before going ahead, lets recall the 3GPPs definition of spurious emissions so that the

concept is clear: Spurious emissions are emissions which are caused by unwanted transmitter

effects such as harmonics emission, parasitic emission, intermodulation products and frequency

conversion products - but exclude out-of-band emissions. This is measured at the base station

RF output port.

The new spurious emissions requirements apply to a system compliant to the 99 GSM

release but 3GPP also issues a recommendation that refers to Base Stations developed and

constructed following previous recommendations: The requirements in this subclause

[referring to the new spurious emissions limits] should also be applied to BTS built to a

hardware specification forR98 or earlier. For a BTS built to a hardware specification for R98

or earlier, with an 8-PSK capable transceiver installed, the 8-PSK transceiver shall meet the R99

requirement.

It is important to remember that here we are talking about standards which are defined in

recommendations, and as such, they are not mandatory, but the GSM and UMTS

recommendations are accepted by the industry as de facto regulations and all suppliers are

compliant to them to a high degree and also their customers require this compliance. In any

case, it is interesting to notice that 3GPP remarks that the new spurious limits should also be

applied to old BSs and that new BSs shall meet the new requirements.

At this point we know that there is a problem in co-siting and that 3GPP has fixed new

limits for the spurious emissions of the GSM BTSs. But how big is this problem? Quite big, if

we analyse the R98 and earlier spurious emissions limits in the UMTS band and the

characteristics of the UMTS system [3], [4]. Approx. 90 dB isolation is required between the

two systems in order to assure a sensitivity degradation of the UMTS system of less than 0,4 dB


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(a typically accepted level when speaking about radio systems). This is a very large figure if we

consider that even with distances between GSM and UMTS BSs of 200 m. we still have risks of

the GSM BS disturbing the UMTS BS. This value doesnt allow co-siting between GSM and

UMTS, without further investigation, and even points out the troubling situation in which

neighbour GSM BSs deployed in the same area may disturb the UMTS BSs.

This means that the situation requires a closer look with the objective of determining if in

practical applications the problem is as critical as the theory says, and what are the corrective

actions that have to be taken to do co-siting, as it is nowadays a strong necessity to be able to

deploy the UMTS networks.

First, old BSs have to meet the R98 and earlier spurious emissions requirements. This

means that all these BSs are better than the above considered limit to calculate the isolation.

Clearly, each supplier will have different figures, and considering one supplier each combiner

solution will result on a different level of radiated spurious emissions. An operator may have

several BS suppliers with more than one generation of equipment. Each of these should be

investigated and the level of spurious emissions, at least as an indicative level, determined.

It can be difficult to determine the exact level of spurious emissions of a specific hardware

implementation, as this parameter was not usually measured in the production lines. Often

passed-not passed tests have been performed without recording the measured level. Despite

this fact, statistical information could be used for planning, measuring a certain number of BSs

at the production line or selecting some of the deployed ones. This information could be used by

the operator to plan co-siting of UMTS with its own GSM BSs, but is not suitable to define co-

siting rules with other operators, because the risk of having a bad BS cannot be completely

ruled out with the statistical assessment of the spurious emissions.

With this situation, an operator can mainly take two approaches to define the co-siting

rules. First, gather and utilize the information about the spurious emissions of its GSM BSs to

define rules for the case of its own infrastructure, taking the risk of having some problems at a

small number of sites. Second, to make all its GSM BSs where co-siting with UMTS will be

realized, compliant with the new 3GPP spurious emissions requirements.

The first solution will probably result in more complicated co-siting rules, as all BS types

and combiner solutions need to be considered. On the other hand, it would be the cheapest


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solution and is, in our opinion, a practical solution, at least for some suppliers. Its drawback is

that it is not easily applicable in co-siting cases with other operators for two main reasons:

performance figures are usually supplier confidential information, and a second operator will

not be willing to take the risk of its UMTS site being disturbed by another operators GSM

system. One action that could increase the applicability of this approach is having the suppliers

decide on a co-ordinated action to give all operators access to the level of spurious emissions of

the existing GSM BSs, at least for the most deployed BSs or in the relevant configuration of the

countries of interest. But this is also difficult to achieve in part because, as explained, it is not

easy to determine the level of the spurious emissions of the existing GSM BSs.

The second solution would solve the problem especially in the case of different operators

planning co-siting. It is also more expensive as some upgrades may be required for the GSM

BSs depending on the supplier. These solutions are hardware solutions and will be discussed

below. With this solution, a proposal would be that the GSM operator presents the UMTS

operator a certificate that the specific BS is compliant to the R99 spurious emissions

requirements.

A hybrid solution could also be adopted.

Any of these solutions make previous negotiations between the operators necessary.

The alternative to these two approaches is to define co-siting rules to protect the UMTS

sites from the GSM sites based purely on distance between the two systems. However, this

would lead to co-siting rules that would be an important handicap in the deployment of the

UMTS networks, discarding the majority of the existing GSM sites for co-location with UMTS

systems of different operators. And we shall emphasize that the existing GSM sites are usually

constructed there because they are the ones that have the best propagation and construction

characteristics or because they were the only sites that could be acquired.

As promised, lets look into the technical solutions that can be used to upgrade an R98 or

earlier BS to the R99 spurious emission requirements. First, we have been talking about the new

requirements repeatedly, but lets quantify them in comparison with the old ones: the power of

the spurious emissions in the UMTS band allowed for a GSM BS has been fixed 51 dB lower

than in the old requirements. This means that depending on the supplier and combiner solution,

new rejection has to be implemented in the receiving UMTS frequency band in the transmission


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path of the GSM BS. In general, combiner solutions using a diplexer will have better rejection

than those without diplexer. To implement this new rejection, several solutions can be used;

these solutions are sometimes linked with losing some output power at the BS but it will be

typically less than 0,5 dB:

the development of new BS combiners (this solution is usually offered by the

suppliers and is becoming available these days),

the introduction of new filters at the antenna connector of the BS (these filters are

available from several ancillary and antenna suppliers),

the introduction of a external diplexer to combine both the systems, GSM andUMTS, to the same broadband or multi-band antenna, that introduces at the same

time the required additional rejection (again, these filters are available from several

ancillaries and antenna suppliers).

One of the things that becomes clear, looking at the different solutions is that operators

would need to discuss and come to agreements that allow them to deploy the UMTS networks

taking into account the increasingly important environmental conditions. European authorities

and the governments, in co-ordination with the local authorities, should also provide a

determined leadership to integrate all interests always discussing and negotiating with all

parties, including the system suppliers and the citizens.

Summary

After presenting a short analysis of the situation in the European Mobile Communication

environment, a technical exposition of the problems engaged with the co-siting of GSM BSs

and UMTS BSs is given.

The technical problem explained by the spurious emissions levels defined by 3GPP has

been analysed and different solutions have been proposed.

Following the new situation with the introduction of UMTS, new rules and modus operandi

should be defined between all relevant parties to be able to handle changes in the deployment of

new mobile networks which is increasingly complicated.


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MFCMFC--net Outsourcingnet Outsourcing

Gregorio Delgado

Program Manager Network Planning

About MFCMFC--net Outsourcingnet Outsourcing

MFCMFC--net Outsourcingnet Outsourcing is an engineering consulting company providing services for the design

and implementation of wireless communication networks. We are specialized in 3rd Generation

Mobile Communications Systems such as UMTS.

We provide first-class engineering solutions and system development services and we

assist our customers to assure that their communication networks are implemented in the most

efficient way always respecting the important parameters of budgets, time and quality.

MFCMFC-net offers a full range of engineering services that include:

RF Network Design

Access Transmission

Fixed Network Design Optimisation

Training

Project Management and Supervision

Establishing RFI and RFQ for Mobile Network

Supplier Selection and Contract Negotiations

Engineering and Planning Guidelines

Please, consult the pages of our website in order to obtain additional information on ourprojects, qualifications and services: www.mfc-net.com.

References:

[1] Referat fr Presse und ffentichkeitsarbeit. Regtp. Jahresbericht 2000. Marktbeobachtungsdaten der

Regulierungsbehrde fr Telekommunikation und Post. www.regtp.de.

[2] 3GPP TS 05.05 v8.6.0 (2000-09). Radio Transmission and Reception. Release 1999. www.3gpp.org.


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[3] 3G TS 25.104 v3.3.0 (2000-06). UTRA (BS) FDD. Radio Transmission and Reception. Release 1999.

www.3gpp.org

[4] 3G TS 25.104 v3.3.0 (2000-06). UTRA (BS) TDD. Radio Transmission and Reception. Release 1999.

www.3gpp.org

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