nortel fractional reuse planning

7/27/2019 Nortel Fractional Reuse Planning

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GSM WIRELESS NETWORKS

Nortel's Fractional Reuse

Technical Aspects

November 98

"NORTHERN TELECOM AND NORTEL MATRA CELLULAR CONFIDENTIAL:

Information contained in this document is the property of Northern Telecom and/or Nortel Matra

Cellular. Except as specifically authorized in writing by Northern Telecom and Nortel Matra

Cellular, the holder of this document shall keep the information contained herein confidential and

shall protect same in whole or in part from disclosure and dissemination to third parties and use forevaluation, operation and maintenance purposes only".

"You may not reproduce, represent, or download through any means, the information contained

herein in any way or in any form without prior written consent of Northern Telecom and Nortel

Matra Cellular ".


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1. Introduction

Fractional re-use is a Nortel technique for increasing the maximum subscriber capacity that

can be supported by a given spectrum without increasing the number of cell sites. It achieves

this whilst at the same time offering improvements in call quality and dropped call rates

compared with non-hopping 4x12 re-use systems. The technique has been assessed by andremains in use with; France Telecom, Bouygues Telecom, Sunday and various operators in

USA and China. These networks were designed with conventional patterns and fractional re-

use was introduced at a later date.

Nortel has led the way in fractional re-use techniques and their implementation and its

approach offers the highest capacity gain available in the market.

To achieve this increase in BSS network capacity we

increase the number of transceivers that can be installed

in a sector beyond that achievable with simple frequency

re-use patterns. Our reference is the commonly used

4x12 re-use pattern which for many operators representsthe basic limit achievable with acceptable interference

levels.

Fractional re-use brings into play many of the standard

GSM features designed to reduce interference under

various conditions; Frequency hopping, DTX, DRX and

up and downlink power control.

In addition Nortel BTSs offer an interference suppression algorithm which can reduce co-

channel interferers by 4dB.

Since 1992 Nortel has had Synthesised frequency hopping available, in this technique it

results in no impact on the number of transceivers required. Where only base-band hopping is

available to the operator, additional expensive transceivers are required (total of one per

hopping frequency) to produce the same results.

We offer two versions of fractional re-use, 1*3 and 1*1. They each give a similar increase in

capacity but are in use by different operators. 1*1 is applicable in low bandwidth networks to

maximise the number of hopping frequencies available, in large bandwidth networks this

becomes less significant.

1*1 has a particular advantage in frequency planning for the traffic carriers, none is required!

Clearly this reduces the workload, however where large bandwidths are available 1*3 can

offer a better C/I ratio since it is not necessary to hop on all frequencies in every cell to realisethe interference reducing benefits of hopping. Implementing fractional re-use on networks

designed for conventional re-use patterns has given each of our customers significant quality

of service benefits immediately (typically a 20% reduction in dropped call rates for example)

and the opportunity of increasing capacity without adding cells. For a network designed from

the start to use these techniques, the benefits can be expected to be even greater.

Quality

Capacity

Simplicity


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2. TechniqueTwo re-use patterns are used in this technique. The first is used for one transceiver in each

sector and typically carries the BCCH on a 4*12 pattern. This transceiver is non-hopping to

meet the fixed frequency requirement of the Broadcast Control Channel. All subsequent

transceivers in a sector frequency hop.

These frequency hopping transceivers use some of the total number of RF channels available

to the operator, after removing the 12 required for the BCCH carriers. To reduce interference

in the system we install only a fraction of the number of transceivers per sector compared

with the number of frequencies allocated.

We have two fractional re-use alternatives available 1*3 and 1*1.

In 1*3 we use 1/3 of the radio channels as hopping frequencies in each sector and 50%load these frequencies with transceivers.

In 1*1 we take this to the ultimate and use all hoping frequencies in all sectors and 20%load these frequencies with transceivers.

2.1 2.1 WHY IS FRACTIONAL RE-USE ESSENTIAL ?

Most GSM operators are now facing increased market competition and booming subscriber

demand. Therefore they must not only find ways to expand cost effectively their network

capacity, but also continue to improve and provide the highest quality of service. Spectrum

being a limited and expensive resource, the essential initial stage is to optimise the use of this

primary resource. Several solution, called spectrum optimisation techniques, among which

Fractional Re-use, exist on the market. In addition, networks are expanding continuously,

with thousands of BTSs, and operators are keen on finding easy but reliable solutions to

manage their growth and their daily optimisation tasks.

These three major issues are answered with Fractional Re-use :

Unmatched capacity increase : over 100% increase without having to acquire new sites

Perfect end-user quality : after having deployed Fractional Re-use, Nortel customers

have noticed major improvement of their quality matrix Unique engineering flexibility : transceivers are added la carte according to

subscribers demand, with no single modification of the frequency plan, therefore saving

major operational costs.

14*121

2 4

7 5 6

8 910

11 12

B

A

C

D

1*3

2 3 1

1 2 3

2 31

2 3

B

A

C

D

1*11

1 1 1

1 1 1

1 11

1 1

B

A

C

D


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Example : 7.2 MHz Spectrum

4x12 1x3@50% 1x1@20%

BCCH channel 12 12 12

TCH TRX / cell 2 4 5

Total TRX / cell 3 5 6

Erlang / cell 14 27 35

Nortel has pioneered Fractional Re-use technology and has commercial service references for

three years. Operators that, a few years before, were reluctant to deploy Fractional Re-use, are

now eager to understand it and recognised it as today's leading spectrum optimisation

solution. Our leadership is mainly due to :

Leading BTS performances : Synthetised Frequency Hopping, essential to Fractional

Re-use, is standard in our BTS portfolio since 92

All the powerful GSM radio features : VAD (Voice Activity Detection) and DTX

(Discontinuous transmission), dynamic uplink and downlink power control, also

being standard in our BTS portfolio since 92 Voice quality thrust : features such as Interference Cancellation Algorithm or a

leading diversity technique, despite not being essential, improves the performance of

the solution

Engineering expertise : Nortel has three years engineering expertise with Fractional

Re-use, which is, today, an invaluable differentiation factor.

2.2 2.2 WHAT ABOUT 1*1 AND 1*3 PATTERNS ?

Nortel has launched Fractional Re-use technology with 1*3 patterns, and some customers are

now using 1*1 patterns. Both have to be advertised, 1*1 having obviously more marketing

impact.1*1 and 1*3 Fractional Re-use are two alternatives of the same technique and there is no real

difference as far as performance (capacity gain, ...) is concerned between the two. Whereas,

1*1 should be easier to implement in most cases. In fact, these two alternatives allow Nortel

to generalise the well-known benefits of Fractional Re-use to almost all networks as they give

further flexibility to adapt the technique to the concerned environment. The deployment of

one or the other should be made according to operators specific constraints :

4x12 1x3@50% 1x1@20%

S333

42 Erl / site

S666105 Erl / site

S55581 Erl / site

150%

100%

Capacity Gain


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For a small bandwidth networks (up to 9 MHz according to the environment), 1*1Fractional Re-use allow to match the 100% capacity gain, that was not achievable

with 1*3 pattern (getting then full benefits of the Synthesised Frequency Hopping).

For an area where the design constraint have lead to an heterogeneous network(mixture of omnidirectional sites and tridirectional sites,,...), a 1*1 design will be

easier to implement

For large bandwidth network with regular design (from 11 to 13 MHz according tothe environment), 1*3 patterns are in general better adapted

Up to now, Nortel states that we can reach 50% fractional loading with 1*3 Fractional Re-

use. With 1*1 Fractional Re-use, we can get 20% fractional loading , which correspond

roughly to the same capacity gain as the one we get with 1*3 patterns. (Note : For 1*1 , the

fractional loading corresponds to the ratio of traffic frequencies within the site over the

number of non BCCH transceivers within the site).

Interference cancellation algorithm

The ICU (available from V10) is an attractive feature for Fractional Re-use networks. Itcombats uplink interference, and when introduced over a Fractional Re-use network, it

enhances the end user quality, and in some case may even allow to increase on some cells (on

case by case basis, with network performance monitoring) the fractional load and therefore

the capacity. Whereas It do not guarantee a value for the C/I improvement if the network is

asynchronous (as it is today) and offers benefits only in the uplink side. That s why it is only

one of the building block of our future solution.

3. Blossoming Fractional Re-use networks

Nortel has Fractional Re-use in commercial service network world-wide for 3 years. Since the

last 6 months, this solution has raised much interest by our customers and many of them havetested and deployed it successfully.

Omnipoint, Salt lake City, USA, GSM1900

Due to the bowl shape valley of the area, the quality of

the signalling (BCCH) needed to be reinforced to ensure

a high quality of service. The solution proposed by

Nortel to the operator was to implement Fractional Re-

use, with a mix of 1*3 and 1*1 patterns, in order to free

some frequencies for the signalling (BCCH) pattern.

Bellsouth, Charlotte, USA, GSM1900

In order to anticipate their subscriber growth and to reduce their operating costs, Bellsouth

decided to deploy Fractional Re-use technique with 1*3 pattern over their Charlotte network.

In particular, this allowed to smoothly introduce, according to subscriber demand, the third

and fourth transceivers in the concerned cells.

USA - GSM 1900


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Sunday, Hong Kong, GSM1800

In a particularly challenging environment (11

cellular operators, coverage issues), Sunday

deployed a very dense network - 500 BTS over

10km in city centre - to ensure a perfect coverage

quality. In order to improve the service quality andto be enhance their network capacity, they decided

to test Fractional re-use patterns and MRP.

The result of the tests was that Fractional Reuse provided the best compromise between

performance, capacity and flexibility, and they decided to generalised it over their network.

Today the deployment of 1*1 Fractional Re-use has been performed over Kowloon (part of

Hong Kong territories) and is still on going for the rest of the area.

Singtel, Singapore, GSM1800

Singtel has deployed a Fractional Re-use solution with 2*6 pattern over their whole network

in order to enhance the voice quality they provide to their subscribers.

France Telecom Mobile, France, GSM900

After intensive quality tests started in June 97,

France Telecom decided to generalised over the

whole south region the Fractional Re-use

technology with 1*1 pattern. The 1*1 pattern was

more suitable than the 1*3 because of the

network heterogeneity, due to the non regular

topology of Marseille. The generalisation was

initialised during the 98 GSM World Congress

where in particular the Palais des festivals BTSsupported a 1*1 reuse with 20 % fractional

loading.

More than 300 sites over Cannes and Nice have been put in service in February 98. FTM was

particularly attracted by the engineering easiness provided by Fractional Re-use patterns.

Bouygues Telecom, France, GSM1800

Bouygues Telecom has recently conducted Fractional Re-use tests over Paris in order to be

able to increase their network capacity. Results have been very satisfactory, with in particular

quality enhancement, and Bouygues Telecom decided to generalise Fractional Re-use

technology over part of their network.

ESAT Digifone, Ireland, GSM900

ESAT Digifone, the second operator in Ireland, captured 45% market share in 9 months. The

operator chose 1*1 Fractional Re-use, as the very best technical solution , to keep their

performance promises while continuing to welcome new subscribers. 1*1 Fractional Re-use

was deployed in march 98 over the capital city of Dublin and the surrounding region.

Switching the network from its previous configuration to the new solution was accomplished

EUROPE - GSM 900FTM, ESAT DIGIFONE ...

SOUTH EAST ASIA- GSM 1800HONK KONG, SINGAPORE


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overnight, minimising the impact on customer service. The overall network capacity was

improved by 55 % and instantaneous result was an improvement of the quality metrix and

voice quality. ESAT Digifone has now the flexibility to further improve their network

capacity la carte . The technique will be generalised over other cities in Ireland (Cork,...).

PTA China (4 network), GSM900

Fractional Re-use, with 1*3 pattern, has been deployed from

launch in 4 PTA networks, among which Shanxhi and Hebei

that both comprise hundreds of BTS. In Xian city wall -

capital city of Shanxhi-, where the subscriber demand is very

high and where the engineering is a real challenge (taking

into account the Chinese PTA constraints...), S666 BTS with

50% fractional loading are in operation and present high

quality metrix. A recent quality audit (including all standard metrix) performed by PTA over

their 30 networks even ranked Shanxhi Fractional Re-use network 3rd

out of the 30 audited

networks !

4. References

The following table gives the GSM networks where Nortel Fractional Reuse (1*1 or 1*3) is

implemented.

Operator BCCH/TCH

Reuse Pattern

Implementation

date

China Hebei PTA GSM900 4*12/1*3 09/1995

CUTC Daqing GSM900 4*12/1*3 09/1995

Singapore GSM1800 network 4*12/2*6 11/1995

China Shaanxi PTA GSM900 4*12/1*3 12/1995

EAE Saudi Arabia 4*12/1*3 11/1996

FTM Marseilles GSM900 18/1*1 07/1997

China Shaanxi PTA GSM900 4*12/1*3 09/1997

BS Charlotte GSM1900 7*21/ 1*1 09/1997

WW Salt lake City GSM1900 7*21/1*1 12/1997

FTM Nice/Cannes GSM900 7*21/1*1 01/1998

Sunday in HK 19/1*1 02/1998

Esat Digifone Dublin GSM900 16//1*1 03/1998

Bytel Paris, Lyon and Nice GSM1800 7*21/2*6/1*3/1*1 03/1998

China Tianjin PTA GSM1800(Dual band trial) 7*21/1*1 03/1998

China Shaanxi PTA GSM900 4*12/1*3 04/1998Taiwan CHT-LDM GSM1800 Ph.2,3 7*21/1*3 08/1998

Esat Digifone GSM900 4*12/1*1 09/1998

5. TECHNICAL OBJECTIVES

CHINA - GSM 900


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The frequency allocation technique used to design a cellular network is a critical factor

determining the maximum capacity. Traditional regular models cannot offer enough

flexibility to fully benefit of the GSM technology and maximise the spectrum efficiency to

cope with the growing cellular demand.

In urban areas where interference are dominating for fixing the Quality of Service

(QoS), an appropriate application of frequency hopping in conjunction with other standard

GSM features can allow to control their distribution witha greater spectrum efficiency. Thisis based on a statistical optimisation of these interference over the whole frequency band and

a strict control of the load of each RF channel.

By comparison with the most aggressive regular frequency reuse pattern applicable to

GSM, the fractional technique may allow to multiply the offered capacity by 2 for the same

QoS. Bandwidth consuming services such as data, which rely on channels aggregation to

increase the end user rate , are the first ones to leverage this resources increase. In addition, it

offers an extreme flexibility to simplify the TRX count evolution while traffic grow without

having to reconsider the frequency allocation plan.

This improvement on the spectrum efficiency and the network management is

independent of other current investigations on minimisation of interference based on traffic

management and self optimisation of network parameters and keep the door open to further

capacity increase.

This paper introduces the theoretical principles which sustain the fractional reuse

technique as well as the operational and engineering aspects. Results achieved within

networks where Nortel has deployed fractional reuse are then presented.

In Annex are given the results of the tests made on the frequency hopping in a real

network.


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6. PRINCIPLES

GSM specifications impose the necessity to ensure full power and full time

transmission of the BCCH beacon frequency across the cells , in order not to compromise the

measurement process performed by mobiles connected in neighboring cells or currently in

idle or selection mode inside the cell. The BCCH layer, onto which no frequency hopping norother GSM features are thus activated, is protected from interference by applying a large

conventional pattern such as 4/12 or 7/21. These patterns provide sufficient robustness and

call quality. The TCH TRXs, on the contrary can fully benefit from the frequency hopping ,

power control and VAD / DTX GSM features.

The fractional reuse technique relies on a split of the available bandwidth in two separate

segments, one dedicated to the BCCH frame, the other to the remaining TCH traffic frames.

BCCH sub-band : Implementation of a conventional pattern

TCH sub-band : Implementation of the Nortel 1/3 or 1/1 reuse pattern

In a 1/x reuse pattern , the total TCH bandwidth is split between x cells and the cluster made

by those x cells is repeated periodically throughout the network. In a 1x1 network, it means

that all the TRX within any cell use the whole TCH sub band. Introduction of such a tight

reuse pattern would obviously degrade the C/I that the TCH TRXs experience in comparison

with the conventional BCCH pattern if several GSM features were not activated to

compensate this C/I degradation.

7. GSM features

1. Frequency Hopping with a fractional load.

When combined with channel coding and interleaving, frequency hopping in itself enables

to improve the system's robustness against C/I interference. Rayleigh fading is smoothed ,

especially for slow mobiles , and hopping on several frequencies creates a situation of

A1

A2A3B1

B2B3C1

D1

D2D3

C2C3

T1

Conventional pattern Nortel 1/3 Fractional pattern

T3

T2

T2

T2

T2

T3

T3

T3

T1

T1

T1


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jammers diversity : co-channel communications become statistically spread over close and

far distances from the communication. The important factor to keep in mind is that the

Frame Erasure Rate becomes more meaningful than the C/I as soon as Frequency Hopping

is introduced. The C/I becomes variable from one burst to the other and its average value

or its worst case value no more represents the voice quality that will eventually be

achieved in the network. The reason is that the interleaving and code correction capabilities

that the GSM offer allow to recover from errors instantaneously encountered on somebursts :

The gain in FER can be accounted for as much as 2 dB, in case frequency hopping is

performed over 8 frequencies , and 3 dB when performed over 12 frequencies (cf. Annex).

Whereas it may not be possible to activate frequency hopping with a conventional reuse

pattern, fractional reuse make it possible as more frequencies are allocated to each cell.

FER versus SFH at -104 dBm

0,00

2,00

4,00

6,00

8,00

10,00

12,00

14,00

1 2 3 4 5 6 7 8

NUMBER OF FREQUENCIES

1 km/h

3 km/h

5 km/h

10 km/h

50 km/h

Frequency hopping can also be applied with some frequencies being unused during any

given time slot.. This ratio TRX/number of TCH frequencies is often referred to as

"use rate", or "fractional" load, since it depicts the utilization rate of each frequency in

time, within the cell. The number of TRXs provisioned in each cell is smaller than the

number of TCH frequencies for a fractional reuse pattern. Each hopping frequency is then

Samplesfrom RX

Equalisation De-interleavingChanneldecoding

Bad Frame Filter

BER FERRxQualbad burst

Same average nb of errors -> same RxQualDifferent distribution of errors -> different FER

104 bursts


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used a fraction of the time in each BTS. Moreover, random frequency hopping laws

(MAIO,HSN) are chosen to maximize interferer diversity.

2. VAD/DTX Feature

Activation of the Voice Activity Detection and Discontinuous Transmission means that

transmission doe not occur when there is no speech. This reduces the transmission

requirements by over 50%. This feature brings a 3dB gain if a voice activity detection ratioof 0.5 is considered.

3. Power control

Power control activation brings an estimated C/I gain of 1.5 to 2 dB. Intensive simulations

have been conducted within Nortel to come to this figure after demonstration that the

partial path loss compensation algorithm was more efficient that the full path loss approachproposed by the GSM 05 recommendations.

4. Nominal Traffic Effect

Last gain comes from the fact that all the TCH channels can not be fully utilized

simultaneously at nominal traffic load. In fact, Erlang B law application for 3 TRX per cell

configurations shows a traffic load of about 70%, which brings in 1.5 dB gain. This 70%

value represents the trunking efficiency of the network.

5. Optional Feature : Interference Cancellation

Voice quality is highly affected by the interference level on both the downlink and theuplink. This interference is mainly self-generated by the GSM system, i.e., by the BTSs onthe downlink and by the mobile handsets on the uplink. Uplink interference is moredifficult to control as it is not constant in time, and depends on mobile handset location,allocated frequencies and other terminals. Nortel proprietary interference cancellationalgorithms provide a solution against uplink interference.

0

2

4

6

8

10

12

14

0 0, 1 0, 2 0 , 3 0 , 4 0 , 5 0 , 6 0 , 7 0 , 8 0, 9 1

PATHLOSS COMPENSATION

%F

ORFERANDCOM,DBFOR

C/I

FER R12

%COM FER>5%

10% C/ I

NO POWERCONTROL

Optimum POWER CONTROL

1,5 to 2 dB gain


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This feature combines the use of receive diversity and Nortel signal processing technology.By eliminating the dominant source of uplink interference from the expected signal afterprocessing, a much higher voice quality is produced.

f1 f1f1 interferer

XWANTED WANTED

DSPf1 wanted

f1 interferer

f1 wanted

INPUT OUTPUT

Thus, the BTS estimates the channel response associated with the expected signal. Then,the system calculates the characteristics of the interfering signal so as to allow theprocessor to focus the digital beam forming (i.e., the diversity processing) and to cancel theinterfering signal.

Adding up these gains gives an indication of the circumstances under which the transition

from one conventional BCCH/TCH combination to a conventional BCCH / fractional TCH

pattern is "possible", i.e. without loss of call quality. These "acceptable" transitions are

obtained when gains out balance losses. Some examples are given in the table below, along

with the capacity gains they permit, while the same quality is maintained :

BCCHpattern

TCH pattern(use rate)

Capacity gain(with spectrum)

12 3 (@ 50%) +75% (with 6 MHz)

12 1 (@ 15%) + 70% (with 5 MHz)

21 6 (@ 50%) +75% (with 12 MHz)

For small spectrums , such as 5 MHz , Nortel recommends to go directly to 1x1 to maximize

the frequency hopping benefits and the statistical gain obtained from interferer diversity.

8. IMPLEMENTATION

Fractional reuse has been successfully implemented by Nortel over 15 networks throughout

the world. A strong engineering expertise has been acquired which now allows Nortel to

propose clear guidelines or optimisation activities to guarantee success when fractional reuse

is introduced :


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Optional Automatic Frequency planning on BCCH : Nortel CT3100 Radio optimisationtool may be used to build a C/I matrix from radio planning tool predictions or field

measurements and optimize automatically the BCCH plan. Such plan is normally

prepared in advance and activated during the night to minimize loss of service.

Modification of the BSIC neighbouring plan

Optimized Handover settings : Introduction of frequency hopping over a large set offrequencies modify the RxQual distribution, which no more represents accurately the

voice quality. In consequence, RxQual handovers criteria must reflect this evolution

,and handovers on Bad RxQual condition will be minimized. Those parameters are

modified at OMC level during normal operation.

Radio access optimisation : due to the above mentioned RxQual versus RxLev profilemodification, the Minimum Access Field required to allow a mobile to camp on a

new cell can be decreased.

As an example, those Network Optimisation activities conducted in parallel with other

activities of monitoring, troubleshooting, network datafill have been achieved within Esat

commercial network in 6 weeks for 300 cells.

Drive Tests are also carried out at different stages to monitor and qualify the Quality of

Service.

A big advantage offered by 1x1 or 1x3 is that it drastically simplifies the network

densification. Provided that appropriate coupling devices have been installed in the BTS

since the beginning, TRXs will be added in sites without any modification on the radio

planning. For instance, an operator with 5MHz could start with S222 (H2D) and upgrade to

S334 (H2D) when traffic requires it, increasing simply the 1x1 fractional load from 8% to

18% . Compared to other densification techniques like cell splitting or micro cellular , which

require new sites acquisition , and new radio planning activities , fractional reuse offersconsiderable operational savings to the operator.

9. RESULTS

The fractional techniques has been deployed over many Nortel networks and

intensively qualified in China (1*3 in Xian with Shaanxi PTA) and in Ireland (1*1 in Dublin

with Digifone). In the same time, France Telecom has conducted its own qualification

campaign in the south of France and widely deployed the 1*1 in Cannes for the last GSM

world congress.

Shaanxi PTA network initial situation was an existing 1*3 fractional reuse deployedin a 6 MHz spectrum with no optimisation and traffic saturation. Additional 2MHz spectrum

could be obtained and up to 6 TRX/Sector were put in service over more than 10 sites. After

optimisation the drop call rate was reduced from 5.5 % to 1.8% and the successful hand over

rate increased from 80% to 95%. This network is still using very high towers for antennas and

some more improvements are expected when the antennas can move down to more reasonable

heights.


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Digifone network was highly congested with a traditional "4*12 like" reuse pattern in

a 7.2 MHz spectrum. Migration to 1*1 was realised in one night over 300 cells. Today , 1x1

has allowed the addition of more than 250 TRX in Dublin area with some sites having up to 5

TRX/sector. After a few weeks of optimisation, a reduction of congestion was achieved so

that the initial rate of sites having less than 2% blocking was increased from 68% to 97%. The

dropped call rate was also reduced from 2.5% to 1.5% and the Handover failure rate

decreased relatively by 17%. In addition some voice quality tests were performed at busyhours and the most significant impact has been noticed on the downlink where the

excellent/good notes increase from 89% to 94%.

Another test is presently ongoing in Australia with a fractional load of up to 20%.

Results are expected to be available within the coming weeks.

Thanks to the flexibility that fractional reuse offers, all those operators have been able

to tremendously reduce their frequency planning activities. New TRX may be added without

any modification of the frequency plan as long as the fractional load limit is not exceeded.

Moreover, TRXs may easily be withdrawn from some sites to be added in others according to

the observed -non homogeneous- traffic distribution.

10. CONCLUSION

The fractional reuse technique is now a well known solution combining the benefits of

the advanced GSM radio management features with high engineering experience. Pioneered

by Nortel in 95, widely deployed since three years, intensively qualified and enhanced during

several fields campaign , this technique now leverages a high operational experience and

optimised radio algorithms to guarantee that the predicted capacity and QoS performances are

achieved. The impact of fractional reuse on the deployment and maintenance cost of GSM

networks is very significant.

Several measurements have confirmed the predictions and have even sometimes over-

passed them particularly for what regards the impact of frequency hopping over a wide

amount of frequencies. This is why research is continuing in Nortel to keep on improving thistechnique by adapted traffic load management techniques like the cell tiering , which keeps

the door open to further spectrum efficiency improvement by significant factors.

Nortel has been always concerned by offering the best solution to minimise the site

counts, either in rural areas where the S8000 BTS features the best sensitivity , or in high

density areas with the fractional reuse.

For narrow spectrum allocation, fractional 1*1 is definitively considered within

Nortel to be the most appropriate solution.


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11. Annex : SFH Trial

In mid 1997, VIAG Interkom has requested from Nortel to perform a Frequency Hopping

Trial in order to assess the practical benefits which can be expected from the activation of this

BSS feature. The measurement campaign, from preparation phase to completion of post-

processing, has lasted from August 12th to September 19th, 1997.

1. Objective

The main objectives of this trial was to evaluate the impact of frequency hopping over

network quality and assess how this improvement could be converted into a potential cell

range increase.

Measurements have been conducted in a range-limited environment, without any traffic load.

Samples have been recorded in buildings at low-speed, in a cell-edge situation, so as to

reproduce worst-case configurations of a network in commercial service, designed for slow-

moving mobiles and high degree of indoor penetration.

2. Data Collection

The variety of the test equipment used throughout the trial has enabled the collection of

multiple parameters : GSM-specific parameters (RxLev, RxQual, FER) and Voice Quality, in

both Uplink and Downlink paths for most of them.

Data collection has been performed under three operating modes (no hopping, random

hopping and cyclical hopping), each mode being activated consecutively, on the same indoor

measurement paths.

Post-processing tools have produced distribution curves as well as correlation curves, whichallow to understand the immediate impact of frequency hopping upon quality, but also how

the benefits of frequency hopping can be integrated in the network design.

In addition, some specific tests have been conducted in order to assess whether the necessity

for any given GSM call to use a non-hopping BCCH carrier in the early stages of call

establishment could prove to be a bottleneck.


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3. Results

Uplink FER cdf - Test Location 3

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0%

4%

8%

12%

16%

20%

24%

28%

32%

36%

40%

44%

48%

Uplink FER

No FH

Random FH

Cyclical FH

FER cumulative distribution

Clear-cut effect of frequency hopping ->reduction of mean FER No inflexion point -> improvement all across the FER range Enhancement is most noticeable in the critical 0-15% FER range for design

1

1,5

2

2,5

3

3,5

4

VoiceQuality

Overall Voice Quality Results

(Mean values over 6 indoor test locations)

No FH Random FH Cyclical FH

Excellent

Goo

Fair

Uplink Downlink Total

Poor

Downlink effect striking : improvement from Fair/Poor to Good !

Overall effect : improvement from Fair/Poor to Good/Fair


17/17

17

12. Conclusion

Measurements have focused on the FER (Frame Erasure Rate) parameter, which provides a

more meaningful insight on the speech frames quality. In all test locations, activation of

frequency hopping has resulted in improvements of the mean FER value : the value of this

shift depends on the original FER range. A typical test location shows reduction of the mean

FER from 7% to less than 2% ; another test performed in somewhat harsher conditions shows

a reduction from 16% to 8%.

In parallel with the audit of these GSM parameters, assessment of voice quality has been

performed using a dedicated call-generation and MOS-analysis device ; significant

improvement on the Downlink path have been observed, with the mean communication

quality being increased from Fair to Good thanks to frequency hopping. Moreover, the

Qvoice vs. FER correlation indicates that FER trends are consistent with Qvoice trends, which

is a confirmation that the FER parameter is the best representative GSM-parameter for voice

quality.

Mean Uplink FER vs RxLev - 7 indoor test locations

0%

2%

4%

6%

8%

10%

12%

14%

16%

18%

20%

-110

-109

-108

-107

-106

-105

-104

-103

-102

-101

-100

-99

-98

-97

-96

-95

-94

-93

-92

-91

-90

Uplink RxLev (dBm)

No FH

Random FH

Cyclical FH

FER vs. RxLev correlation

Gain in quality :FER reduced from 11% to 6% @ Rx = -106 dBmGain in dB :

Rx reduced from -104 to -107 dBm @ FER = 8%

Warning :low nb of

samples withRx > -98 dBm

nortel fractional reuse planning

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