rf optimisation gsm sudarshan

www.gtllimited.com

GTL Limited

Network EngineeringNetwork Engineering

Training on RF

Optimisation GSMJune 2005June 2005

Presented by : Sudarshan IyengarPresented by : Sudarshan Iyengar

AgendaAgenda

A. Understanding RF Network Cycle

B. Basics of RF Design

C. Why do we need optimization??

D. Optimization Stages

E. Physical and Hardware Optimization

F. Database parameter optimization/ Special Tools

Understanding the RF Network Cycle…Understanding the RF Network Cycle…

RF Network CycleRF Network Cycle

CW Drive Test

Model Tuning

RF Planning

Spreadsheet Design

Link Budget

RF Optimization

Parametric Optimization

Neighbor List

Site Parameters

Frequency Planning

PN Planning

RF Site Survey

RF Drive Test

In-Building Solutions

Traffic Engineering

Expansion Planning

Benchmarking

Downlink / Voice Quality

… … Spreadsheet Design ...Spreadsheet Design ...

Usually done during Initial Network Build

• Link budget to calculate the number of sites.

• Calculations based on

– subscriber density,

– traffic per subscriber,

– expected growth in traffic, etc.

… … CW Drive Test/ Model Tuning...CW Drive Test/ Model Tuning...

Purpose

• Model Tuning is used to

– Accurately allocate the sites.

– To achieve more accurate results from the

prediction/simulation tool deployed.

– Identification of hotspots/special coverage

requirement areas.

– Tuned model can be used as a benchmark for future

expansions.

Model Tuning Process

• Setup consists of Test transmitter for the particular band (GSM 900/1800) – usually 20W

• Antenna – Omni/Panel, cables, accessories.

• One candidate chosen to represent each type of clutter area in the network.

• The clutter types could be urban, suburban, rural, etc.

• The test transmitter is setup on a suitable rooftop.

• Test frequency chosen and transmitted

• Drive test is carried out using receiver or TEMS equipment set to scan mode.



Model Tuning Process

• Data collected – Rxlev samples aggregated over 30-50 m bins.

• The Rxlev measurements are processed and input to the prediction tool.

• Clutter offset and other parameters are corrected.

• Corrections are made to achieve – lowest possible Standard Deviation values.

Thus we have a “tuned model”, which can be applied to other areas which have the same clutter type.

… … RF PlanningRF Planning

• The inputs received from spreadsheet design and model tuning surveys, is used to prepare a “Nominal Cell Plan” aka “Hi Level Design”.

• The HLD has the following details

– Distribution of the sites across the agreed

geographical area.

– Coverage/Capacity objective details.

– Type of antennas to be used, sites where special

hardware(TMA/MHA) is required, etc.

… … RF PlanningRF Planning

• The output of the HLD is “search rings” which is defined for each site to be built in the network.

• Each “search ring” will have

– Nominal site coordinates,

– Search radius and

– Specifications about antenna height requirements

for each site, in order that the site objectives are

reasonably achieved.

• Search rings form a basis for further surveys to be carried out to hunt for site candidates and identify suitable ones.

… … RF Site Survey/Drive TestingRF Site Survey/Drive Testing

• Using the inputs provided by the nominal cell plan, the RF team performs

– Surveys for each search ring in the network to identify the

suitable candidates which can be used for building the sites.

– Candidates identified are ranked on basis of their RF

suitability and other parameters such as structural stability,

line of sight clearance(for Tx), accessibility, costs, etc.

– Drive testing may be carried out in some cases, to assess

the RF suitability

– Once suitable candidate(s) is identified..acquisition

begins!!!

… … RF Planning – The REAL Challenge!!!RF Planning – The REAL Challenge!!!

• Acquisition of ideal candidate poses a real challenge to the network design process.

• More often than not candidates which are lower on priority in terms of RF suitability are the ones which get acquired!!

• Often due to acquisition constraints, search rings need to be modified and sometimes even the nominal plan needs to be changed.

• Thus as an end result the network built is deviated from the one which was originally designed in the nominal plan.@!@!!!!$!

Frequency PlanningFrequency Planning

• GSM works on a frequency reuse pattern.

• As the sites get acquired and the build process starts, the RF planners prepare a ‘frequency plan’ for the network.

• Different techniques available for frequency plan – a) Fixed Plan, b) Hopping Plan – further divided into Baseband Hopping and Synthesized Frequency Hopping

• RF Planners either manually or by the use of an AFP(Automatic Frequency Planner) create a frequency plan for the network.

Frequency PlanningFrequency Planning

• An optimal frequency is critical to ensure good RF performance of the network.

• Spectral challenges

• Limited band allocation

• Fast growth rate of subscribers/ traffic growth

• Tighter reuse patterns

RF Optimization/Parametric OptimizationRF Optimization/Parametric Optimization

• During the network build initial RF optimization is done, to ensure that the sites built are reasonably meeting their objectives.

• During the network build phase it is also ensured that optimal parameter settings are done for all sites to ensure good performance.

• Detailed explanation of the above to follow!!

Traffic Planning/Expansion PlanningTraffic Planning/Expansion Planning

• Two stages for Capacity Planning I) Initial Network Build II) Future Expansion.

1) Initial Capacity Plan

• Spreadsheet design is used.

• The expected traffic is calculated based on a certain amount of traffic assigned per subscriber – say 25 mE.

• The total traffic requirement is traffic per subscriber X total no of subscribers.

• Network capacity is based on a certain GOS – say 2 %.

• Erlang B table used to calculate the no. of TRX, hence no of sites.

Traffic Planning/Expansion PlanningTraffic Planning/Expansion Planning

• Two stages for Capacity Planning I) Initial Network Build II) Future Expansion.

2) Future Expansion

• This can also be done using spreadsheet design methodology, using a figure of expected traffic growth.

• Alternatively TRX additions are done on an ad-hoc basis by studying the traffic trend on a weekly/monthly basis.

• In cases where no further TRX addition is practicable, capacity sites are added in the existing network.

• Separate planning is done for Traffic Channels(TCH) and Access Channels (SDCCH).

Inbuilding SolutionsInbuilding Solutions

• IBS is required in places where indoor coverage requirement is critical and the possibility of providing coverage from outdoor sites is not practicable.

• Usually implemented for places like corporate offices, hotels, hospitals, shopping complexes, etc., where both coverage and capacity is essential.

• IBS implementations may consist of

• Repeaters – Low cost solution for covering a small area with less traffic

• Microcells/Macrocells – Separate BTS sites which can be a single carrier ‘microcell’ or a multi carrier ‘macrocell’, implemented in places where larger area needs to be covered and has higher traffic requirement.

Inbuilding SolutionsInbuilding Solutions

• IBS implementations usally deploy a “passive RF network” using DAS(Distributive Antenna Systems). In some exceptional cases active elements like ‘Leaky Feeders’ might be used.

• Cost of leaky feeder is comparatively very high, hence the requirement needs to be justified!!

• IBS performance also needs to be monitored and optimized as it is critical to the performance of the whole network. A bad performing IBS can skew the statistics of the BSC to which it belongs.

• Special handover algorithms are used for controlling handovers between IBS sites to outdoor network, in order to achieve good performance and for traffic management.

BenchmarkingBenchmarking

• Benchmarking is done for having a comparison of own network with competitor’s network in terms of coverage/voice quality.

• Benchmarking is also done for comparing own network’s performance against certain set KPIs or previously achieved performance targets.

• Special tools like Qvoice equipment is available for voice quality benchmarking.

• For coverage/quality benchmarking could be done using regular drive test and post processing tools like TEMS and DESKCAT

• Network Operator/OEM vendor usually subcontracts this activity to a 3rd party, in order to derive unbiased results from the exercise

• Statistical data from benchmarking can be used as a valuable input to the network optimization process.

• The data is used to identify weak areas in the network, which helps in developing strategies for improving the network performance.

BenchmarkingBenchmarking

Mobile Communications propagation is impacted by :

Path Loss

Reflection

Diffraction

Mobile Communications PropagationMobile Communications PropagationMobile Communications PropagationMobile Communications Propagation

The basic path loss is the transmission loss in free space.

Lfsl = 32.4 + 20 log d(in Kms)+ 20logf(in Mhz)

At 900 Mhz, at a distance of 1km , Loss = 91.5 db

Actual prediction of loss cannot be done on this, since in a mobile environment the mobile will receive signals from several reflections.The above formula is only valid under direct LOS and no reflection conditions.

d

Path LossPath LossPath LossPath Loss

Reflection occurs when a propagating electromagnetic wave impinges upon a surface which has very large dimensions as compared to the wavelength of the propagating wave.

Reflections occur from surface of earth, buildings,walls and water.

The wave is partially absorbed and partially reflected.

Amount of absorption will depend on the reflection coefficient of the reflecting surface.

Reflection coefficient is function of the material properties and depends on wave polarization , angle of incidence and the frequency.

ReflectionReflectionReflectionReflection

Path loss for 2 -ray Model ( over flat conductive surface)

ht hr

dL2ray = 40 log d - ( 20 log ht + 20 log hr )

Analytical formula, only valid for larger distances ( > 10 Km)Loss increases at larger distance at a rate of 40db /dec.

At 900 Mhz, 10,000m distance , ht = 100m, hr = 1.5mLfs = 111.5 db whereas L2ray = 116.5 db

This indicates that in 2 ray path , additional loss of 5 db.


Reflection in actual mobile environment , would be from multiple paths.

So, reflection in mobile communications is Multipath reflection. RSL will be resultant of levels coming from all paths.


Diffraction allows radio signals to propagate around the curved surface of earth and behind obstruction.

ht hrShadow region

RSL drops as the receiver moves deep into the shadow regionHuygen's principle on phenomenon of diffraction

All points on a wave-front can be considered as point sources for the production of secondary wavelets, and that these wavelets combine to produce a new wave-front in the direction of propagation.

Diffraction is caused by the propagation of secondary wavelets into the shadowed region

DiffractionDiffractionDiffractionDiffraction

Hills, Mountains, Buildings will cause knife edge diffraction In a Mobile environment most of the diffraction is knife edge.

Diffraction is of two types in general

Smooth Sphere Diffraction Knife Edge Diffraction

Smooth Sphere Diffraction

Diffraction takes place through almost a flat surface.

Knife Edge Diffraction

DiffractionDiffractionDiffractionDiffraction

Fresnel zone geometry

Area around the LOS within which a diffraction can result into antiphase(180 deg) condition is the first fresnel zone.

If an object is within the fresnel zone or completely blocks the zone, then also energy

will arrive at the receiver but will diffraction loss. In Mobile environment, we are not worried about clearance, but only with the

loss.

ht hr

Calculation of Diffraction LossCalculation of Diffraction LossCalculation of Diffraction LossCalculation of Diffraction Loss

h

d1 d2

Fresnel diffraction parameter (v)

Indicates the position of the object with reference to the fresnel zones ( 0 means , object tip on LOS, 1 means tip on 1st fresnel zone on upper side).

2 ( d1 + d2 ) d1.d2

hv =

From "v" , we can compute the diffraction loss.

( all values in "m" )


Relation of "v" with diffraction loss ( graphical )K

nife

edg

e d

iffra

ctio

n ga

in (

GadB

)

Fresnel diffraction parameter v

-3 -2 -1 0 1 2 3 4 5-30

-25

-20

-15

-10

-5

0

5


Radio wave when impinges on a rough surface , reflected energy is spread out in all directions due to scattering.

This is the reason actual RSL in a mobile environment is often stronger then what is predicted by reflection and diffraction.

Objects such as Lamp posts and trees tend to scatter energy in all directions, thereby providing additional radio energy at the receiver.

ScatteringScatteringScatteringScattering

Formula's described earlier are based on simple models of the radio path.

Formula's don't take care of the type of the terrain of the radio path.

Realistic method of prediction would be to use empirical data of radio wave propagation over various types of terrain and land usage.

Empirical data of this type were collected by Okumura from his comprehensive radio wave propagation measurements

Path Loss PredictionPath Loss PredictionPath Loss PredictionPath Loss Prediction

Okumura developed a set of curves giving the attenuation in excess to FSL in an urban area with base station effective height of 200m and and mobile antenna height of 3m.

These curves give the loss as a function of frequency and distance from base station.

Okumura ModelOkumura ModelOkumura ModelOkumura Model

Path loss for different heights can be calculated by these curves by using the formula.

Path Loss = Lfsl + A(f,d) - G(hte) - G(hre)

G(hte) and G(hre) are the effective Base Station and MS antenna heights

G(hte) = 20 log 1000m > hte > 10mhte200

( )

G(hre) = 10 log hre < 3mhre 3

( )

hre 3

( )G(hre) = 20 log hre < 3m


What is Effective Antenna Height ?

3 km 15 km

hmsl

Average ground level (havg)

hte

hte = Antenna height above msl(hmsl) - average ground level (havg)

( average ground level is calculated within 3 - 15km )


Okumura curves are only applicable for urban areas.

For other terrain's, Okumura has provided correction factors.

The correction factors are provided for 3 types of terrain in the form of curves related to frequency.

Open Area : corresponds to a rural , desert kind of terrain

Quasi Open Area : corresponds to rural , countryside kind of terrain

Suburban


Path Loss(o,q,s) = Lfsl + A(f,d) - G(hte) - G(hre) - G(area)

Path loss for other terrain's


Conclusion

Simplest, best and accurate prediction model but only for specific terrain's.

Slow response to rapid changes in terrain.

Model is fairly good for urban and suburban areas, but not as good in rural areas.

Standard deviations between predicted and measured loss values 10 dB to 14 dB.

Model is more graphical than mathematical, for computation we need formula's not graphs.


Hata model is an empirical formulation of the graphical path loss data provided by Okumura.

Hata presented the urban area propagation loss as a standard formula and supplied correction equations for applications for other situations.

Formula's are designed for computer usage, but they are only rough approximations of Okumura's curves.

Since Terrain types profiles are practically infinite, modeling of the tool used for prediction because essential but taking several measurements and several times.

Hata ModelHata ModelHata ModelHata Model

Urban terrain

L(urban) = 69.55 + 26.16 log fc - 13.82 log hte - a(hre) + ( 44.9 - 6.55 log hte ) log d

fc = frequency in MHz ( 150 - 1000 MHz) hte = BTS antenna height ranging 30m to 200m hre = effective receiver antenna height ranging 1m to 10m d = Transmitter receiver separation distance (1 - 20 km )a(hre ) = correction factor for effective mobile antenna height which

is a function of size of the coverage area in db

Small to medium city

a(hre) = (1.1 log fc - 0.7 ) hre - ( 1.56 log fc - 0.8 ) db

For large city

a(hre) = 3.2 ( log 11.75 hre ) - 4.972


Correction for Suburban & Rural terrain's

Loss for Rural Open Area

Loss for SUBURBAN

L(sub) = L (urban) - 2 [ log (fc/28) ] - 5.42

L(ro) = L (urban) - 4.78 ( log fc) - 18.33log fc - 40.942

Loss for Rural Quasi-Open Area

L(rqo) = L (urban) - 4.78 ( log fc) - 18.33log fc - 35.942


L(urb) = 46.3 + 33.9 log fc - 13.82 log hte - a(hre) + ( 44.9 - 6.55 log hte ) log d +Cm

fc = frequency in MHz ( 1500 - 2000 MHz) hte = BTS antenna height ranging 30m to 200m hre = effective receiver antenna height ranging 1m to 10m d = Transmitter receiver separation distance (1 - 20 km )a(hre ) = correction factor for effective mobile antenna height which

is a function of size of the coverage area in dbCm = Correction factor for city size

a(hre) = (1.1 log fc - 0.7 ) hre - ( 1.56 log fc - 0.8 ) dbCm = 0 db for medium city and suburban centers with moderate tree density.Cm = 3 db for metropolitan centers.

For rural areas , the earlier formula's will apply

COST -231working committee developed an extended version of HATA model for frequencies up to 2 GHz.

COST 231 - Hata ModelCOST 231 - Hata ModelCOST 231 - Hata ModelCOST 231 - Hata Model

Selection of models for predicting path loss for GSM will depend on the cell ranges.

GSM has 3 cell ranges and different prediction model for each

Large Cells

Small Cells

Microcells

Path Loss Predictions for GSMPath Loss Predictions for GSMPath Loss Predictions for GSMPath Loss Predictions for GSM

Antenna is installed above the maximum height of the surrounding roof tops.

Propagation is mainly by diffraction and scattering at roof tops in the vicinity of the mobile i.e. the main rays propagate above the roof tops.

Cell radius is mainly 1 km and normally exceeds 3 km. Hata's model and the COST 231-Hata model can be used to

calculate path loss in such cells.

Large CellsLarge CellsLarge CellsLarge Cells

Antenna is sited above the median but below the maximum height of the surrounding roof tops.

Propagation mechanism is same as large cell Maximum range is typically less than 1 - 3 kms. Hata model cannot be used since it is applicable above 1 km. COST 231-Walfish-Ikegami model is used for radius less than

5kms in urban environment.

Small CellsSmall CellsSmall CellsSmall Cells

Without free LOS between BS and MS

Frequency (f) = 800 - 2000 MHzTransmitter height (hte) = 4 - 50mMobile height (hre) = 1 - 3mDistance (d) = 0.02 - 5 kmHeight of buildings "Hroof" (m)Width of road "w" (m)Building separation "b" (m)Road orientation with respect to the direct radio path Phi (o)Free Space Loss (Lfsl)

Path Loss = Lfsl + Lrst + Lmsd

Lrts : roof-top-to-street diffraction and scatter lossLmsd : multiscreen diffraction loss

COST 231 - Walfish-Ikegami ModelCOST 231 - Walfish-Ikegami ModelCOST 231 - Walfish-Ikegami ModelCOST 231 - Walfish-Ikegami Model

With a free LOS between bs and ms ( Street Canyon )

Path Loss = 42.6 + 20 log(d) + 20 log(f) for d > = 0.020 km

COST 231 - Walfish-Ikegami ModelCOST 231 - Walfish-Ikegami ModelCOST 231 - Walfish-Ikegami ModelCOST 231 - Walfish-Ikegami Model

Additional loss which occurs at 900 MHz when moving into a house on the bottom floor on 1.5m height from the street.

Indoor loss near windows ( < 1m ) is typically 12 db.

Building loss as measured by Finish PTT varies between 37 db and -8db with an average of 18db taken over all floors and buildings.

In our predictions and calculations, as per GSM recommendations we will consider 15db as an average indoor loss.

Indoor LossIndoor LossIndoor LossIndoor Loss

Cell in which the base station antenna is mounted generally below roof top level.

Propagation is determined by diffraction and scattering around buildings ie. the main rays propagate in street canyons.

Microcells have a radius in the region of 200 - 300m . Microcells can be supported by smaller and cheaper BTS's.

MicrocellsMicrocellsMicrocellsMicrocells

With a free LOS between bs and ms ( Street Canyon )

Path Loss (GSM 900 ) = 101.7 + 20 log(d) for d > = 0.020 kmPath Loss (DCS 1800 ) = 107.7 + 20 log(d) for d > = 0.020 km

Propagation loss in microcells increases sharply as the receiver movesout of LOS , (ex : around a street corner ).

20db of loss could be added per street corner, up to two or three corners.

Beyond, this the COST231 - Walfish Ikegami model should be used

Microcells ModelMicrocells ModelMicrocells ModelMicrocells Model

A mobile radio signal envelope has continuos variations. These variations continuously fluctuate the signal level and is referred to as the fading phenomenon.

Fading in mobile environment is of 2 types:

Small Scale Fading

Log-normal Fading

FadingFadingFadingFading

Rapid level fluctuation over a short period or travel distance (approx: half wavelength), so that large-scale path loss may be ignored.

MS antenna is lower in height as compared to surrounding objects, so several mulipath signals arrive with various phases and amplitudes and at certain times almost cancel out each other.

Short - term fading at certain times can be heard as annoying bursts.SS

Time

Average

Small Scale FadingSmall Scale FadingSmall Scale FadingSmall Scale Fading

Observing the Short-term fading with reference to average level

Rayleigh distribution

ro

ro (db) = Average - instantaneous fluctuations

(small-scale fading)

ro ranges in 40 db ( 10 db above and 30 db below the average )

ro follows a Rayleigh-distribution , since generally signals arrive from reflections only, hence small-scale fading is often called Rayleigh-Fading.


Rayleigh distributionAs per Rayleigh distribution increase in fade depth is inversely

proportional to the probability (ex: 10 db fade may occur for 40 % of the time, where the probability of 40db fade would be 10 % )

avg level

min recv level of rcvr

deepest fades ( typically 30 db )

Area of poor quality

Pr

Probability that fade depths will enter area of poor quality is required to be less than 10 %.

Fade Margin


If probability of small-scale fades is more below the minimum required signal level, then this could result in distorted speech.

To ensure this probability is less than 10 % , Transmit Power should be adjusted accordingly to achieve a high fade margin.

Space Diversity is quite effective for this kind of fades.

Rayleigh distribution only occurs when there are all reflected waves and no direct LOS signal. If there is a direct LOS signal present with reflections, then it is Ricean distribution of fading which is less severe , since the direct component is relatively much stronger than reflected waves and will restrict deep fades.


Time

SS

Log-normal Fading

Small -scale signal variation when averaged out is called the local mean and is expressed in log scale of power , and is called Log-normal fading.

Log-normal variation is due to the terrain contour between the bs and ms.

If the terrain is an open area, then the change in signal will be with distance only,

but normally there are obstructions ( buildings, trees etc. ) which cause a rapid variation of signal from its local mean over an area of 5 to 50m

Log-normal FadingLog-normal FadingLog-normal FadingLog-normal Fading

Log -normal fading depths when exceeds the min receive level will result into shadow areas ( since this effect is over an area of 5 - 50m ) . This is also referred to as Coverage Holes.

Remedy for this is to keep an additional fade margin on top of min-rcv -level benchmark when predicting coverage.

This margin is called is log-normal shadow margin.

Log-normal shadow margin is in the range of 3-5db, with standard deviation of the local mean in the range of 4-8 db.

For, urban areas GSM recommends a margin of 5 db ( considering 7db as the deviation), this is to achieve 90% location probability on cell edges.

Log-normal FadingLog-normal FadingLog-normal FadingLog-normal Fading

Time Dispersion is the arrival of signals from multiple paths, but spread in time.

Equalizer in GSM can handle multipaths within a delay spread within 4 bit periods (15us) ( path difference of 4.5 kms ).

Any multipath component arriving after 15us will act as interference.

GSM needs a C/I ratio of 9 db, and the same applies to carrier to multipath (>15us) also. This ratio is known to Primary/Multipath (P/M )

Planning and BTS site selection should consider the location probability of Primary/Multipath ratio below 9 db.

Time Dispersion (Multipath)Time Dispersion (Multipath)Time Dispersion (Multipath)Time Dispersion (Multipath)

Optimization and Countermeasures for Time Dispersion is something very interesting and can result into several issues like distorted

voice, echo and even dropped calls !!

Certain countermeasures when adopted in the planning stage can reduce or eliminate these issues.

If problems occur later on, then optimization needs to be done.

Troubleshooting multipath problems is a big issue in live networks.

Reducing Time Dispersion Issues Reducing Time Dispersion Issues Reducing Time Dispersion Issues Reducing Time Dispersion Issues

Site Location

Identify potential reflectors in the predicted cell area.

Locate sites for BTS near reflectors, this will bring the reflections within the window.


Directional Antenna's(Sectorization)

Using Sectored cells config, with the directional antenna pointing away from the reflector.

Antenna's front-back ratio is a very critical parameter.


Over Water Bodies Time dispersion over water can make the quality worst

Covering the area from the other side of the water body will avoid large path differences between reflected signals.

Side lobes can still result into problems, where handovers should take care off, by properly setting neighbors& parameters

Reducing Time Dispersion IssuesReducing Time Dispersion IssuesReducing Time Dispersion IssuesReducing Time Dispersion Issues

Tilting Antenna's

Tilting Antenna will reduce energy radiated towards the reflector.

Antenna's can be tilted horizontally or vertically.

Horizontal tilt will reduce the coverage to a large extent, hence vertical tilt is the most preferred one.

Reducing Output Power ???

Reduction in output power will reduce the energy from both direct as well reflected signal. Hence, P/M will not change.


The shift in frequency relative to the speed of the mobile phone is Doppler Shift.

fd = v

fd = Shift in frequency in Hzv = speed of the mobile in m/s = wavelength in m

Actual received carrier frequency = fc + fd, when mobile is moving towards the transmitter. Actual received carrier frequency = fc - fd, when mobile is moving away from the transmitter. There is no shift , when the vehicle is moving perpendicular to the angle of arrival of the transmitted signal.

Doppler ShiftDoppler ShiftDoppler ShiftDoppler Shift

Frequency PlanningFrequency PlanningFrequency PlanningFrequency Planning

Objective

Optimum uses of Resources

Reduce Interference

Frequency PlanningFrequency PlanningFrequency PlanningFrequency Planning

F=1

F=2F=3

F=4,8

F=5,9

F=6,10

F=7

F=1

F=2F=3

F=4,8

F=5,9

F=6,10

F=7

F=1

F=2F=3

F=4,8

F=5,9

F=6,10

F=7

F= 1,2,3,4,5,6,7,8,9,10

Clusters

Co-Channel ( Re-use ) Cells

GSM uses concept of cells One cell covers small part of network Network has many cells Frequency used in one cell can be

used in another cells

This is known as Frequency Re-use

Frequency Re-use

A

A

Q = DR

C / I = 9 db

Q = Sqrt ( 3 x N )

Co - Channel Re-use factorCo - Channel Re-use factorCo - Channel Re-use factorCo - Channel Re-use factor

Adjacent ARFCN's should not be used in the same cell

It will have no problems in Downlink*, but will have high risk of uplink interference (due to mandatory uplink power control ).

* If Downlink dynamic power control is not used

- 70 dbm ( C/a = 20 )

- 90 dbm ( C/a = -20 )

5 dbm

33 dbm

Since all the ARFCN's in a cell are frame synched, Timeslot numbers will align on all the ARFCn's

Adjacent-Channel Re-use CriteriaAdjacent-Channel Re-use CriteriaAdjacent-Channel Re-use CriteriaAdjacent-Channel Re-use Criteria

Adjacent ARFCN's can be used in adjacent cells, but as far as possible should be avoided.

As such separation of 200 Khz is sufficient, but taking into consideration the propagation effects, as factor of protection 600 Khz should be used*.

In the worst, Adjacent ARFCN's can also be used in adjacent cells by setting appropriate handover parameters ( discussed later in optimization)

* Practically not possible in most of the networks due to tight reuse

Adjacent-Channel Re-use CriteriaAdjacent-Channel Re-use CriteriaAdjacent-Channel Re-use CriteriaAdjacent-Channel Re-use Criteria

Omnidirectional Cell

BTS

Sectorial Cell

BTS

Low gain Antennas Lesser penetration/directivity Receives Int from all

directions Lower implementation cost

High gain Antennas Higher penetration/directivity Receives Int from lesser

directions Higher implementation cost

Cell ConfigurationCell ConfigurationCell ConfigurationCell Configuration

3,6,9A

A

B

C

3,6,9B

3,6,9C

Receives Interference from all directions

Interference in Omni-CellsInterference in Omni-CellsInterference in Omni-CellsInterference in Omni-Cells

A1

A2

A33

69

B1

B2

B3 3

96

C1

C2

C33

69

Receives Interference from lesser directions.

Sectored CellsSectored CellsSectored CellsSectored Cells

Re-use Patterns ensures the optimum separation between Co-Channels.

Re-use pattern is a formation of a cluster with a pattern of frequency

distribution in each cell of the cluster.

Same cluster pattern is then re-used.Preferred Re-use Patterns

Omni - Cells : 3 cell, 7 cell, 12 cell, 14 cell, 19 cells etc

Sector - Cells : 3/9 , 4/12, 7/21

Re-use PatternsRe-use PatternsRe-use PatternsRe-use Patterns

A1

A2A3 B1

B3C1

C2C3

A1

A2A3 B1

B2B3C1

C2C3

A1

A2A3 B1

B2B3C1

C2C3

A1

A2A3 B1

B2B3C1

C2C3

A1

B1

B2B3

A1

A2A3

B2

C1

C2C3 C2C3 C2C3

C2C3

A1

3/9 Re-use Pattern3/9 Re-use Pattern3/9 Re-use Pattern3/9 Re-use Pattern

A1

A2A3 B1

B3C1

C2C3

A1

A2A3 B1

B2B3C1

C2C3

A1

A2A3 B1

B2B3C1

C2C3

A1

A2A3 B1

B2B3C1

C2C3 A1

B1

B2B3

A1

A2A3

B2

C1

C2C3

C2C3 C2C3 C2C3

A1

Using ARFCN's 1to9 , do the channel allocation for the below cells using 3/9 pattern

Exercise !!!Exercise !!!Exercise !!!Exercise !!!

Adjacent Channel Interference is very difficult to avoid within the cluster itself.

1

4

3

2

85

7

96

Frequency Allocation in 3/9 patternsFrequency Allocation in 3/9 patternsFrequency Allocation in 3/9 patternsFrequency Allocation in 3/9 patterns

D1

D3

B1

B3

C1

C2C3 D1

A1

A2A3 B1

B2B3C1

C2C3

B1

B2B3 A1

A2A3C1

C2C3

C1

D1

D2D3

D2D3 B2B3

B2B3

D2 C1

C3

B2

D2D3A1

A2A3

B1

B2B3

C2

D1

D2D3 A1

4/12 Reuse Patterns4/12 Reuse Patterns4/12 Reuse Patterns4/12 Reuse Patterns

Using ARFCN's 61 to72 do the channel allocation for the below cells using 4/12 pattern.

D1

D2D3 C1

C3B1

B2B3

C1

C2C3 D1

D2D3A1

A2A3

A1

A2A3 B1

B2B3C1

C2C3

B1

B2B3 A1

A2A3C1

C2C3

C1

D1

D2D3

B1

B2B3

C2

D1

D2D3 D2D3

B2B3

B2B3

A1

ExerciseExerciseExerciseExercise

1

35

24 6

7

9 1112

10 8

4/12 pattern avoids adjacent channels in adjacent cells

4/12 Pattern Channel Allocation4/12 Pattern Channel Allocation4/12 Pattern Channel Allocation4/12 Pattern Channel Allocation

Larger reuse patterns give reduction in interference

Re-use patterns becomes more effective with sectorial cell configurations.

To implement large patterns ( like 4/12, 7/21) , more channels are required.

So with less resources, the best way to plan is :

1. Use optimum no of channels per cell.2. Thus, increase the pattern size.

Reuse Patterns ConclusionReuse Patterns ConclusionReuse Patterns ConclusionReuse Patterns Conclusion

Critical Factors for good RF NetworkCritical Factors for good RF NetworkCritical Factors for good RF NetworkCritical Factors for good RF Network

• Grid based RF design.

• Maintain standard azimuths while sectorizing cells – This makes frequency plan easier

• Correct choice of antenna type for specific coverage requirements.

• Use of optimal antenna heights – Should be sufficient to cater to the coverage area, but should not exceed the requirement, else it results into large spillovers and interference, making reuse difficult!!

• Use optimal tilt – Electrical tilt as far as possible. In some cases combination of electrical and mechanical tilts

Effect of QOS !

Dissatisfied Customers--- Customers face describes your profit curve--- 1 Dissatisfied customer prevents 10 new

Revenue--- Customer Switchovers--- Less New Customers--- Cost of Dropped Calls--- Cost of Blocked Calls

Quality of ServiceQuality of ServiceQuality of ServiceQuality of Service

Importance of RF OptimizationImportance of RF OptimizationImportance of RF OptimizationImportance of RF Optimization

• RF Optimization is a continuous and iterative process.

• Main Goal – To achieve performance levels to a certain set standard.

• Network subscribers expect wireline/near wireline quality.

• Network subscribers also expect 100 % availability at all given times.

• RF network optimization is a process to try and meet the expectation of subscribers in terms of coverage, QoS, network availability.

• RF optimization also aims to maximize the utility of the available network resources.

• Each operator has a certain set of decided KPIs (Key Performance Indicators) based on which the operator guages the performance of his network.


• RF/Access Network KPIs can be broadly classified into three types

a) Access related KPI

b) Traffic/Resource Usage related KPI

c) Handover related KPI

• Examples of access KPI

a)SDCCH Drop rate b) Call setup success rate

c)SDCCH Blocking, etc.

• Examples of Traffic KPI

a)TCH Drop Rate b) Call success rate

c)TCH Blocking, etc.

• Examples of handover performance KPI

a)Handover Success rate b) Handover failure rate.

c)Handover per cause, per neighbour, etc.


• Apart from the KPIs mentioned earlier the operator may have his own set of custom KPIs which the operator feels is critical to guage the performance of his network.

• RF optimization process drives the effort to achieve and maintain the network performance KPI.

• Optimization can be broadly divided into 3 categories, as follows –

a) Hardware Optimization

b) Physical Optimization

c) Database/Parameter Optimization

• Generally the activities mentioned above are done in parallel. In some cases one may precede the other.

Network Optimization Cycle…Network Optimization Cycle…

Optimization StagesOptimization Stages

RF Planning

Network Rollout

/Build Phase

Nominal Cell Design

RF Fine tuning

Database

parameter optimization

Physical/

Hardware

Optimization

Network Pre –

Optimization

Traffic Optimization

Hardware OptimizationHardware OptimizationHardware OptimizationHardware Optimization

• Hardware Optimization is a process in which ailing network elements which affect the performance of BSS (Access Network) are trouble-shooted.

• The BSS maintenance team attends to hardware issues. However there is a substantial assistance taken from the RF team for isolating the problem to the specific hardware.

• How is hardware optimization done??

• Inputs for the process are

• Drive testing

• OMCR statistics

Hardware Optimization - Hardware Optimization - Typical Hardware ProblemsHardware Optimization - Hardware Optimization - Typical Hardware Problems

• In most cases, hardware failures on a BTS/BSC or any part of the access network – alarms are generated at the OMC, which help in identifying the fault

• In some cases, there are no alarms generated

• Key statistics from OMCR could point towards hardware failures – Typical statistics which indicate such problems are

a) Poor Assignment Success/High Assignment failure rate

b) High TCH/SD RF Loss

c) High handover failure rate

d) Lower call volume/traffic on the cell

• Faulty TRX – One of the most common problems. This can be identified from OMCR statistics as well as drive test. In some cases only a particular timeslot on a TRX could be faulty. Immediate step to be taken is to ‘lock’ the particular timeslot/TRX from the OMC and escalate the fault to the BSS team. For identifying this problem vide drive test, the RF engineer has to go to the site and conduct a timeslot test/make several calls on the particular cell and also test handovers to and from neighbour cells.

• Sleeping TRX/Sleeping Cell – Sometimes certain TRXs/Cells do not take any calls during the day – these are referred to as “sleeping radios” OR “sleeping cells”. Usually this is a temporary problem and gets resolved by performing a ‘Reset’ on the particular site or by doing a ‘Lock – Unlock’ process on the specific TRX/sector.



• Path balance problems – This is also one of the common causes for poor cell performance.

path balance is pegged as an OMCR statistic on a cell basis

General formula is path balance=uplink pathloss – downlink pathloss.

In Motorola OSS pathbalance= pathloss+110.

where pathloss = uplink pathloss – downlink pathloss.

uplink pathloss = actual Ms Txpower – rxlev_ul

downlink pathloss = actual Bs Txpower – rxlev_dl

It is desirable to have the pathloss value as ‘0’ which represents a balanced path. However a deviation of +/- 10 is acceptable


• Path balance problems – If the pathbalance is below 100 or above 120, it indicates that there could be a problem in either downlink or uplink. PB value above 120 represents a weaker uplink and stronger downlink, whereas PB value below 100 would represent a weaker downlink.

If MHA/TMA is used or receive diversity is applicable,an additional 3 dB gain is introduced in the uplink. In such case a deviation of –20 is acceptable, i.e, a PB of 95 would be normal in such case.

• Path Balance – If the PB statistic indicates problem in the downlink/uplink – the RF path should be traced for possible hardware faults. Possible things that could go wrong are –

a) High VSWR due to faulty feeder cable

b) Improper connectorisation

c) Faulty combiner


d) Faulty antenna – improper impedance matching

between

antenna and feeder cable (rare case)

• Processor problems –

• The present BTS equipment architecture is quite robust and with the evolution of VLSI techniques, the different hardware modules have been compacted into single units.

• The current TRXs/TRUs are having inbuilt processing abilities apart from also containing the RF physical channels.

• However in places where older equipment (for e.g. Motorola InCell/Mcell) are still in use, problems with processor (GPROC or MCU), could be encountered.

• These problems are easily identifiable by drive test and usually also show up degradation on OMCR statistics. However in the current scenario these problems have rare occurences.

• BSC/Transcoder Problems – Although the occurrence is rare, there are instances where some part of Transcoder or timeslot on the PCM link go faulty. In such cases, the timeslot mapping needs to be identified and appropriate troubleshooting steps need to be taken. These problems can seldom be identified by drive testing.

• Steps for Hardware Optimization

a) Check from OMCR statistics for indications of hardware faults

b) Check event logs from OMCR to find out if any alarms were generated

c) Conduct call test on the site/cell in question – check for assignment failures, handover failures, from layer 3 messages.


• Steps for Hardware Optimization

d) Isolate the problem to the specific TRX. This can be

done by ‘locking’ the suspicious TRX.

e) Check for downlink receive level on each TRX. In some cases the downlink receive level on a particular TRX may be very low, due to faulty radio.

f) Request VSWR test to be performed if the problem appears to be related to poor path balance.

g) Check for improper connectorization, improper antenna installation. One loose connector could skew the performance of the entire cell!!!

f) If the problem is not isolated to a bad TRX/ other BTS hardware – further investigations needed to check other possible faulty hardware in the BSC/XCDR

Hardware Optimization – Hardware Optimization – Hardware Optimization StepsHardware Optimization – Hardware Optimization – Hardware Optimization Steps

Physical RF OptimizationPhysical RF OptimizationPhysical RF OptimizationPhysical RF Optimization

• A well designed RF is key to good network performance.

• More often than not, the actual network built is deviated from the network designed from the desktop. The variations are

a) Actual site locations are away from the nominal planned locations.

b) It is not practicable to build a grid-based network due to several constraints.

c) Antenna heights may differ from the planned antenna heights.

• Physical RF optimization may be done at several stages of network rollout.


• Physical RF Optimization is an essential requirement during the network build/pre optimization stages. In most cases the OEM vendor is responsible for the network during this phase and he carries out the process to ensure that the actual network is as near good as the desktop designed one.

• The process comprises of conducting a drive test for the entire cluster, which may comprise of one or several BSC areas.

• The drive test results are plotted on a GIS map and deficiencies in coverage/interference problems are identified by plotting Rxlev/Rxqual values.

• Most of the coverage deficiencies are fixed by making changes to antenna heights(rare), bore and tilts.

• At later stages parametric optimization is done to bring the network performance close to desktop design.


• RF optimization is also carried out during network expansion phase, i.e when new site or group of sites are added into the network.

• In many networks RF optimization is also done as a regular process to maintain good network performance.

• RF optimization is helpful in resolving specific coverage problems or interference problems, cell overreach, no dominant server issues, etc.

• Typical thumb rule to follow while carrying out physical RF optimization for resolving coverage or interference issues - Step 1:- Try tilting the antennas.

Step 2:- Try changing the orientation.

Step 3:- Increase or reduce the height iff tilt/reorientation does not solve the problem

Step 4:- Change the antenna type as a last resort.

• The process starts the moment a GSM network goes on air and continues on a day-to-day basis, till the network is operational.

• Under GSM each vendor has hundreds of parameters which can be played with to achieve different performance metrics under different scenarios.

• Usually most of the parameters are enabled with default settings and are always kept unchanged. However there are some specific parameters which control the RF performance which can be changed on a cell or even carrier-level, to achieve specific improvements.

Database/Parameter OptimizationDatabase/Parameter OptimizationDatabase/Parameter OptimizationDatabase/Parameter Optimization

• GSM Features – Before proceeding to database parameters, let us discuss some important GSM features which are commonly being used in current networks.

• GSM networks worldwide are mainly affected by the following types of problems:- 1) Coverage issues, 2) Interference issues, 3)Capacity issues.

• Interference in GSM networks can be reduced significantly by using some special features, as mentioned –

• Frequency Hopping

• DTX and Voice Activity Detection

• Dynamic Power Control

Database/Parameter OptimizationDatabase/Parameter OptimizationDatabase/Parameter OptimizationDatabase/Parameter Optimization

Database Optimization – Frequency HoppingDatabase Optimization – Frequency HoppingDatabase Optimization – Frequency HoppingDatabase Optimization – Frequency Hopping

• Frequency hopping is one of the standardised capacity enhancement features in GSM system. It offers a significant capacity gain without any costly infrastructure requirements.

• Frequency hopping can co-exist with most of the other capacity enhancement features and in many cases it significantly boosts the effect of those features.

• Frequency hopping can be briefly defined as a sequential change of carrier frequency on the radio link between the mobile and the base station.

• When frequency hopping is used, the carrier frequency is changed between each consecutive TDMA frame. This means that for each connection the change of the frequency may happen between every burst.


• At first, the frequency hopping was used in military applications in order to improve the secrecy and to make the system more robust against jamming.

• In cellular network, the frequency hopping also provides some additional benefits such as frequency diversity and interference diversity.

Frequency

Time

F1

F2

F3

Call is transmitted through severalfrequencies in order to • average the interference (interference diversity)• minimise the impact of fading (frequency diversity)



• There are two methods of frequency hopping in GSM, Baseband Frequency Hopping (BB FH) and Synthesised Frequency Hopping (RF FH).

• In the baseband frequency hopping the TRXs operate at fixed frequencies.

• Frequency hopping is generated by switching consecutive bursts in each time slot through different TRXs according to the assigned hopping sequence.

• The number of frequencies to hop over is determined by the number of TRXs


• The first time slot of the BCCH TRX is not allowed to hop, it must be excluded from the hopping sequence.

• This leads to three different hopping groups.

• The first group doesn’t hop and it includes only the BCCH time slot.

• The second group consists of the first time slots of the non-BCCH TRXs.

• The third group includes time slots one through seven from every TRX.

Database Optimization – Baseband HoppingDatabase Optimization – Baseband HoppingDatabase Optimization – Baseband HoppingDatabase Optimization – Baseband Hopping

B

RTSL 0 1 2 3 4 5 6 7

TRX-1

TRX-2

TRX-3

TRX-4

f1 B = BCCH timeslot. It does not hop.

f2

f3

f4

Time slot 0 of TRX-2,-3,-4 hop over f2,f3,f4.

Time slots 1...7 of all TRXs

hop over (f1,f2,f3,f4).

Baseband hopping (BB FH).

Database Optimization – RF HoppingDatabase Optimization – RF HoppingDatabase Optimization – RF HoppingDatabase Optimization – RF Hopping

• In the synthesised frequency hopping all the TRXs except the BCCH TRX change their frequency for every TDMA frame according to the hopping sequence.

• Thus the BCCH TRX doesn’t hop.

• The number of frequencies to hop over is limited to 63, which is the maximum number of frequencies in the Mobile Allocation (MA) list.

BTRX-1

Non-BCCH TRXs are hopping over

the MA-list (f1,f2,f3,...,fn) attached to the cell.

TRX-2

B = BCCH timeslot. TRX does not hop.

f1,

f2,

f3,

fn

f1,

f2,

f3,

fn

. . . .


Synthesised hopping (RF FH).


• The biggest limitation in baseband hopping is that the number of the hopping frequencies is the same as the number of TRXs.

• In synthesised hopping the number of the hopping frequencies can be anything between the number of hopping TRXs and 63.

MSC

BB-FHF1(+ BCCH)

F2

F3

Dig. RF

TRX-3

TRX-1

RF-FH

F1, F2, F3

Dig. RF

TRX-1

TRX-2

BSCTCSM

BCCH

Frequency

Time

F1

F2

F3

MS does not seeany difference

BB-FH is feasible with large configurations RF-FH is viable with smaller configurations

The difference between BB and RF FH.


Database Optimization – RF Hopping – Cell Database Optimization – RF Hopping – Cell AllocationAllocationDatabase Optimization – RF Hopping – Cell Database Optimization – RF Hopping – Cell AllocationAllocation

• The Cell Allocation (CA) is a list of all the frequencies allocated to a cell. The CA is transmitted regularly on the BCCH.

• Usually it is also included in the signaling messages that command the mobile to start using a frequency hopping logical channel. The cell allocation may be different for each cell.

• In PGSM 900 the CA list may include all the 124 available frequencies [GSM 04.08].

• However, the practical limit is 64, since the MA-list can only point to 64 frequencies that are included in the CA list .

Database Optimization – RF Hopping – Mobile Database Optimization – RF Hopping – Mobile AllocationAllocationDatabase Optimization – RF Hopping – Mobile Database Optimization – RF Hopping – Mobile AllocationAllocation

• The MA is a list of hopping frequencies transmitted to a mobile every time it is assigned to a hopping physical channel.

• The MA-list is automatically generated if the baseband hopping is used.

• If the network utilises the RF hopping, the MA-lists have to be generated for each cell by the network planner.

• The MA-list is able to point to 64 of the frequencies defined in the CA list

• However, the BCCH frequency is also included in the CA list, so the practical maximum number of frequencies in the MA-list is 63.

• The frequencies in the MA-list are required to be in increasing order because of the type of signaling used to transfer the MA-list.

Database Optimization – RF Hopping – HSNDatabase Optimization – RF Hopping – HSNDatabase Optimization – RF Hopping – HSNDatabase Optimization – RF Hopping – HSN

• The Hopping Sequence Number (HSN) indicates which hopping sequence of the 64 available is selected.

• The hopping sequence determines the order in which the frequencies in the MA-list are to be used.

• The HSNs 1 - 63 are pseudo random sequences used in the random hopping while the HSN 0 is reserved for a sequential sequence used in the cyclic hopping.

• The hopping sequence algorithm takes HSN and FN as an input and the output of the hopping sequence generation is a Mobile Allocation Index (MAI) which is a number ranging from 0 to the number of frequencies in the MA-list subtracted by one.

• The HSN is a cell specific parameter.

Database Optimization – RF Hopping – MAIODatabase Optimization – RF Hopping – MAIODatabase Optimization – RF Hopping – MAIODatabase Optimization – RF Hopping – MAIO

• When there is more than one TRX in the BTS using the same MA-list the Mobile Allocation Index Offset (MAIO) is used to ensure that each TRX uses always an unique frequency.

• Each hopping TRX is allocated a different MAIO. MAIO is added to MAI when the frequency to be used is determined from the MA-list.

• MAIO and HSN are transmitted to a mobile together with the MA-list.

• The MAIOoffset (Nokia) is a cell specific parameter defining the MAIOTRX for the first hopping TRX in a cell. The MAIOs

for the other hopping TRXs are automatically allocated according to the MAIOstep-parameter

D O C U M E N T T Y P E

T y p e U n i t O r D e p a r t m e n t H e r eT y p e Y o u r N a m e H e r e T y p e D a t e H e r e

G S M H o p p i n g a l g o r i t h m

M A I ( 0 . . . N - 1 ) =

f 1 f 2 f 3 f 4 f Nf N - 1M A

0 1 2 3 N - 1N - 2M A I N D E X( M A I )

T R X - 1 T R X - 2 T R X - 3

F N & H S N

M A I O T R X

T R X - 1 0T R X - 2 1T R X - 3 2

F o r t h i s T D M A f r a m e t h e o u t p u t f r o m t h e a l g o r i t h m i s 1

1

1

+ M A I O T R X MAIOOFFSET ,User definable

These parameters are set automatically


Database Optimization – RF Hopping – MAIO Database Optimization – RF Hopping – MAIO StepStep Database Optimization – RF Hopping – MAIO Database Optimization – RF Hopping – MAIO StepStep

• The MAIOstep is a Nokia specific parameter used in the MAIO

allocation to the TRXs.

• The MAIO for the first hopping TRXs in each cell is defined by the cell specific MAIOoffset parameter

• MAIOs for the other hopping TRXs are assigned by adding the MAIOstep to the MAIO of the previous hopping TRX

• MAIOTRX(N) = MAIOoffset + MAIOstep(n-1)

Sector TRX # HSN MAIO step MAIO offsetl MAIO 1 1 Non-hopping BCCH TRX

2 7 2 0 0 3 2 4 4

2 1 Non-hopping BCCH TRX 2 7 2 6 6 3 8 4 10

3 1 Non-hopping BCCH TRX

2 7 2 12 12 3 14 4 16

MAIO step indicates the difference between the MAIOs of successive TRXs in a cell.

+MAIO step

Example of the use of the MAIO related parameters.


Database Optimization – RF Hopping – Reuse Database Optimization – RF Hopping – Reuse patternspatternsDatabase Optimization – RF Hopping – Reuse Database Optimization – RF Hopping – Reuse patternspatterns

• When RF Hopping is deployed the BCCH layer is planned using the standard 4X3 or 7X3 or an intermediate suitable pattern.

• Maximum protection is assigned while planning to the BCCH layer as it is critical to call setup procedure.

• For the TCH layer there are mainly three types of widely used reuse patterns

• 1X1 – All sectors in the network use a single MA list.

• 1X3 – 3 MA lists are created. Sec A of each cell uses MAL1,

Sec B uses MAL2 and Sec 3 uses MAL3

• Ad-hoc/Mixed SFH – Multiple MA lists are used. Can have as many MA lists as the number of sectors in the network. The reuse is based on fractional loading * with a maximum loading factor of 100 %.

Database Optimization – RF Hopping – Loading Database Optimization – RF Hopping – Loading FactorFactorDatabase Optimization – RF Hopping – Loading Database Optimization – RF Hopping – Loading FactorFactor

• Loading Factor – This is the ratio of no of TRX to the no of hopping frequencies in the MA list

• Loading Factor = No of Hopping TRX/No of Frequencies.

• For eg. Loading factor = 50 % if there are 2 TRX and 4 hopping frequencies.

• Lowest practically achievable loading factor is 33 %for 1X3, 17 % for 1X1 and highest is 100 % .

• Usually 100% loading factor is used in case of ad-hoc RF hopping, for cells with higher configuration (6-6-6), however for lower configuration like (2-2-2) – 50 % loading factor could be used.

• In case of ad-hoc hopping the loading factor can be planned to be specific to the cell configuration.

Database Optimization – DTX & Power ControlDatabase Optimization – DTX & Power ControlDatabase Optimization – DTX & Power ControlDatabase Optimization – DTX & Power Control

• Power control and the DTX are standard GSM features, which are designed to minimise the interference.

• These are mandatory features in the UL, but it is up to the network operator to decide whether to use them or not.

• DTX prevents unnecessary transmissions when there is no need to transfer information

• Power control is used to optimise the transmitted signal strength so that the signal strength at the receiver is still adequate.

• These features can be individually activated for uplink and downlink.

• Operators have been widely using both features in UL direction mainly in order to maximise the battery life in mobiles.


• In a non-hopping network these features provide some quality gain for some users, but this gain cannot be transferred effectively to increased capacity, since the maximum interference experienced by each user is likely to remain the same.

• The power control mechanism doesn’t function optimally because the interference sources are stable causing chain effects where the increase of transmission power of one transmitter causes worse quality in the interfered receiver, which in turn causes the power increase in another transmitter and so on.

• This means that, for example, one mobile located in a coverage limited area may severely limit the possibility of several other transmitters to reduce their power.

• In a random hopping network the quality gain provided by both features can be efficiently exploited to capacity gain because the gain is more equally distributed among the users.

• Since the typical voice activity factor (also called DTX factor) is less than 0.5, DTX effectively cuts the network load in half when it is used.

• The power control works more efficiently because each user has many interference sources. If, one interferer increases its power, the effect on the quality of the connection is not seriously affected. In fact, it is probable that some other interferers are decreasing their powers at the same time. Thus, the system is more stable and chaining effects mentioned earlier do not occur frequently.



GAIN:PC on 1.4 dBDTX on 2.3 dBPC on, DTX on 3.7 dB

GAIN:PC on 1.0 dBDTX on 2.3 dBPC on, DTX on 3.5 dB

Reuse 3/9, TU 3km/h Reuse 3/9, TU 50km/h

C/I improvement

The simulated gain of PC and DTX with FH.


• DTX has some effect on the RXQual distribution.

• Normally the BER is averaged over the duration of one SACCH frame lasting 0.48 seconds and consisting of 104 TDMA frames.

• However, four of these TDMA frames are used for measurements, so that only 100 bursts are actually transmitted and received.

• When DTX is in use and there is no speech activity, only the bursts transmitting the silence descriptor frame (SID-frame) and the SACCH are transmitted.

• When there are periods of no speech activity, the BER is estimated over just the bursts carrying the silence descriptor frame and the SACCH. This includes only 12 bursts over which the BER is averaged (sub quality).


• BER gets averaged much more effectively when DTX is not used yielding to a quality distribution where the proportion of moderate quality values is enhanced.

• The sub quality distribution is wider than the full quality distribution, meaning that more good and bad quality samples are experienced.

• The differences between full and sub quality distributions are largest in frequency hopping networks utilising low frequency allocation reuse, since in that kind of networks the interference situation may be very different from burst to burst.

• A couple of severely interfered bursts may cause very bad quality for the sub quality sample when they happen to occur in the set of 12 bursts over which the sub quality is determined.


• The full quality sample of the same time period has probably only moderate quality deterioration because of the better averaging of BER over 100 bursts.

• In a real network utilising DTX the quality distribution is a mixture of full and sub quality samples.

• The proportions of full and sub samples depend on the speech activity factor also known as the DTX factor.

• The differences in the BER averaging processes cause significant differences in the RXQUAL distributions. These differences should be taken into account when the RXQUAL distributions of networks utilising and not utilising DTX are compared.

1/1 reuse 15 freqs

0.00 %

5.00 %

10.00 %

15.00 %

20.00 %

25.00 %

30.00 %

35.00 %

40.00 %

Q0 Q1 Q2 Q3 Q4 Q5 Q6 Q7

RxQ full

RxQ sub


The distribution of normal RXQual and subRXQual values in a frequency hopping network.


• Power Control – what to optimize??

• The parameters to optimize in case of power control are the window settings.


Downlink Power Control – Typical Rxlev Window settings

Downlink Rxlev (dBm)

BS

Tx

Pow

er

- 75 -95

+ 42

Downlink RxQual

BS

Tx

Pow

er

0 4

+ 42

Downlink Power Control – Typical RxQual Window settings


Uplink Rxlev (dBm)

MS

Tx

Pow

er

- 70 -90

+ 33

Uplink Power Control – Typical Rxlev Window settings

5



• Power Control parameters which can be set

• Downlink/Uplink Rxlev threshold (l_rxlev_dl_p / l_rxlev_ul_p)

• RxQual threshold(l_rxqual_dl_p / l_rxqual_ul_p)

• Power increment/reduction step size (pow_inc_step_size_dl/pow_red_step_size_d)

• Dynamic step adjust algorithm(dyn_step_adj)

• Power Control Features

• Objective is to reduce average interference

• In case of uplink also helps in saving battery power

• Algorithm works on measurement reports sent by the MS every 480 ms (SACCH frame)

• Downlink power control cannot be applied to BCCH carrier

• Uplink power control is mandatory but downlink power control is not mandatory. Feature selectable by the operator.

• For controlling interference in the network the operator uses DTX, Power Control and Frequency Hopping. These features effectively act as combined forces in interference reduction and improved call quality.


Database OptimizationDatabase OptimizationDatabase OptimizationDatabase Optimization

Typical problems which GSM subscribers experience are

• Coverage issues

• Voice quality issues

• Access issues/congestion

• Handover related issues

• Dropped calls

BSS Parameters are broadly classified into the following groups

• Access related parameters

• Call handling/Handover related parameters

• Congestion related parameters


Database Optimization – IDLE Mode Cell Database Optimization – IDLE Mode Cell SelectionSelectionDatabase Optimization – IDLE Mode Cell Database Optimization – IDLE Mode Cell SelectionSelection

• The MS uses a "path loss criterion" parameter C1 to determine whether a cell is suitable to camp on [GSM 03.22]

• C1 depends on 4 parameters:

• 1. Received signal level (suitably averaged)

• 2. The parameter rxLevAccessMin, which is broadcast on the BCCH, and is related to the minimum signal that the operator wants the network to receive when being initially accessed by an MS

• 3. The parameter msTxPwrMaxCCH, which is also broadcast on the BCCH, and is the maximum power that an MS may use when initially accessing the network

• 4. The maximum power of the MS.


Cell Selection in IDLE Mode, based on C1

• Radio Criteria

A = Received Level Average - p1

C1 = (A - Max(B,0))

B = p2 - Maximum RF Power of the Mobile Station

p1 = rxLevelAccessMin

p2 = msTxPowerMaxCCH


Cell Reselection

• In case of reselection from one cell to another in the same location area – the C1 value of target cell must be higher than source cell

• In case of reselection to a target cell in a different location area – the C1 value must be greater than that of the source cell by a database parameter “cell_reselect_hysteresis”

Cell Reselection – C2

• C2 is an option GSM feature which can only be used for cell reselection, it can be enabled or disabled on a cell basis.

• If C2 parameters are not being broadcast the C1 process is used for reselection.

Cell Reselection – C2

• C2= C1 + cell_reselect_offset – temporary offset * H (penalty_time –T) (for penalty_time <31)

• H= 0 if T > penalty_time

• H= 1 if T < penalty_time

• C2= C1 – cell_reselect_offset (for penalty_time= 31)

Why C2??

• Cell Prioritisation

• As a means of encouraging MSs to select some suitable cells in preference to others



Example of C2 usage

• In dualband network-- to give different priorities for different band

• In multilayer-- to give priority to microcell for slow moving traffic

• Any other special case where specific cell required higher priority than the rest

Cell Reselection Strategy

• Positive offset-- encourage MSs to select that cell

• Negative offset-- discourage MSs to select that cell for the duration penalty Time period

Database Optimization – HandoversDatabase Optimization – HandoversDatabase Optimization – HandoversDatabase Optimization – Handovers

Handover

• The handover (HO) process is one of the fundamental principles in cellular mobile radio, maintaining the call in progress whilst the mobile subscriber is moving through the network.

• In idle mode the MS does a cell reselection, whereas in dedicated mode the MS performs a handover.

• Handovers are mainly classified into two types

• A) Inter cell handovers

• B) Intra cell handovers

• Inter cell handovers – further classified as

• Inter BSS – ie between two cells belonging to different BSCs

• Intra BSS – ie between two cells belonging to same BSC

Handover

• Intra cell handovers is the switching of call from one channel/TRX to another TRX within the same cell/BTS. This is an optional feature which can be enabled on a cell basis. Intra cell handovers usually take place when the Rxqual on the source channel deteriorates.

Handover process may be initiated due to the following main reasons

• Radio Criteria

• To maintain receive level/receive quality

• Absolute MS-BS distance

• Power Budget

• Network Criteria

• Traffic load (to manage traffic distribution)


• Handovers also classified as imperative/non-imperative based on the reason for which the process is triggered.

• The cause value contained in the handover recognised message will affect the evaluation process in the BSC.

Handover causes may be prioritized as follows

• 1. Uplink Quality

• 2. Uplink Interference

• 3. Downlink Quality

• 4. Downlink Interference

• 5. Uplink Level

• 6. Downlink Level

• 7. Distance

• 8. Power Budget



Power budget handover

• If an MS on a allocated resource during its measurement reporting process sees another channel that would provide an equal or better quality radio link requiring a lower output power then a handover may be initiated.

• Handovers due to power budget ensure that the MS is always linked to the cell with minimum pathloss though the quality and level thresholds may not be exceeded.

• Handover to the target cell takes place when PBGT> hoMarginPBGT

• PBGT = (msTxPwrMax – Av_Rxlev_DL_HO –(btsTxPwrMax – BTS_TXPWR)) – (msTxPwrMax(n) – Av_Rxlev_NCELL(n))

where n –”nth” adjacent cell which is a handover candidate


Power budget handover

• hoMarginPBGT is a parameter which can be set on a cell to cell basis. Each cell may have a different value for each neighbour cell which is a candidate for power budget handover.

• hoMargin is expressed in dB and is usually set to 4. However this may be reduced if the handover needs to be speeded or increased to 6 or higher to prevent ping-pong or to delay handovers

• In some cases negative homargin may also be used.


Handover Algorithms

• Handover algorithms are used in addition to default parameters to control the handover process

• These algorithms assist in mobility management and are effective in traffic distribution.

• The algorithms have an important role to play in GSM networks which use multi-band or multi-layer architectures.

Handover Algorithms

• In Motorola system there are 7 procedures. These are set by the parameter pbgt_alg_type. The algorithms are briefly defined as follows:-

• Type 1 – Conventional GSM PBGT

• Type 2 – Restricted PBGT for macro cells

• Type 3 – PBGT with Rxlev as qualifier

• Type 4 – PBGT with time in cell as qualifier

• Type 5 – PBGT with delay since neighbour level exceeds threshold as qualifier

• Type 6 – Delayed power budget using dynamic handover margin

• Type 7 – PBGT algorithm to avoid adjacent channel interference


Handover Algorithms

• Of the seven, the most commonly used are Type1, 2, 3 and 7.

• Each handover candidate cell can be defined as one of the seven types of neighbour to the source cell.

Handover per cause

• The handover per cause statistic is a counter-array statistic which counts the reason for each handover event on all cells for which it is enabled.

• This statistic gives important information about the handover performance of the cells and can be used for troubleshooting cells which have high “handover failure rate”.


Handover per neighbour

• This statistic gives the value of no of handover attempts as well as successes for each neighbour cell. This statistic is also helpful in troubleshooting handover performance, it can be used to identify neighbour relations which have a high “handover failure rate”

• The handover per neighbour statistic can also be used for neighbourlist pruning.


Database Optimization – TRHO/Congestion Database Optimization – TRHO/Congestion Related ParametersRelated ParametersDatabase Optimization – TRHO/Congestion Database Optimization – TRHO/Congestion Related ParametersRelated Parameters

TRHO – What does it do??

• TRHO effectively reduces the service area of the congested cells

• Increases service area of under-utilised target cells

• HO is triggered using a special parameter “amhTrhoPbgtMargin” instead of hoMarginPbgt

• General guideline:

• Target cell “Rxlevaccessmin” should be set higher to avoid bad downlink Rxqual after HO

• amhTrhoPbgtMargin must be lower than hoMarginPbgt


TRHO/BSC Parameters

• amhUpperloadthreshold – This parameter determines minimum traffic load threshold at which cell starts to intiate TRHO – default value – 80 %

• amhMaxLoadOfTargetCell – This parameter determines maximum traffic load threshold beyond which target cell will not accept TRHO hand-ins – default value – 60 %

TRHO/BTS Parameters

• amhTrhoPbgtMargin – This parameter is new Pbgt margin when cell exceeds amhUpperloadthresh. It’s the revised power budget margin which replaces the normal Pbgt definition when the Trho criteria are met – default value is –5 dB.


TRHO/Adjacency Parameters

• trhoTargetLevel – This parameter determines the minimum RSSI of the valid target cell candidate reported by the mobile – default is –85 dBm

Directed Retry

• A transition (handover) from SDCCH in one cell to a TCH in another cell during call setup due to unavailability of an empty TCH within the first cell.

• To control traffic distribution between cells to avoid a call rejection.

• Can be used for both MOC and MTC

• Setting guidelines:

• drThreshold should be higher than Rxlevmincell (Rxlevaccessmin); else the improved target cell selection criteria will be ignored.


Congestion Relief

• This procedure is initiated when an MS is assigned to an SDCCH, requires a TCH and none are available.

• Two options are offered for deciding how many handover procedures are actually initiated.

• First Option – The no. of HO procedures initiated is at most the no. of outstanding requests for a TCH.

• Second Option – This allows for initiation of a HO procedure for each MS that meets the modified criteria to support the feature.


Things which normally subscribers normally experience

(common problems) –

• No coverage/poor coverage issues.

• Dropped calls.

• Failed handovers/Dominant server issues.

• Breaks in speech/crackling sound or bad voice quality.

• Access related problems – “Network Busy”.

Often all the above problems are addressed to the RF

optimization team for resolution

RF Optimization – Analysis and troubleshootingRF Optimization – Analysis and troubleshootingRF Optimization – Analysis and troubleshootingRF Optimization – Analysis and troubleshooting

Poor Coverage Issues

• Coverage problems are one of the most concerning issues.

• Subscribers experience a “No network” or “Network Search” scenarios on the fringe area of the cells.

• Mostly these problems are experienced in suburban areas and also in many cases inbuilding coverage problems occur.

• Analysis is simple

• TEMS equipment/test phone displays Rxlev of serving cell and neighbour cells – Generally problem occurs when Rxlev drops below –95 dBm. When the Rxlev drops to –100 dBm or lower the subscriber experiences a “fluctuating single bar” or a “network search” scenario.

• When Rxlev (DL) drops below –95 dBm its very difficult to have successful call setup, as typically the uplink Rxlev would be much lower.

RF Optimization – Poor Coverage IssuesRF Optimization – Poor Coverage IssuesRF Optimization – Poor Coverage IssuesRF Optimization – Poor Coverage Issues

Poor Coverage Issues (Steps to solve the problem)

• Analyze the extent of area which is experiencing a coverage problem

• Can this be solved by physical optimization??

• Possible steps would be to improve the existing serving cell strength by proper antenna orientation or up-tilting the antenna.

• If it is an indoor coverage/limited area coverage issue, this could be resolved by deploying a repeater/micro cell if the traffic requirement in the question area is high.

• In case of rural/suburban cells where the concern is a weak uplink – TMA could be installed.

RF Optimization – Poor Coverage IssuesRF Optimization – Poor Coverage IssuesRF Optimization – Poor Coverage IssuesRF Optimization – Poor Coverage Issues

RF Optimization – Drop Call TroubleshootingRF Optimization – Drop Call TroubleshootingRF Optimization – Drop Call TroubleshootingRF Optimization – Drop Call Troubleshooting

Dropped Calls

• Dropped calls may be attributed to several reasons.

• Usually categorized as –

• Drop during call setup – aka SDCCH Drop.

• Drop during call progress – aka TCH Drop.

• Drop due to failed handovers – with no recovery.

• Call drops may occur due to RF/non RF reasons.

• RF Reasons attributing to dropped calls

• Weak coverage – RL timer times out.

• Interference – low C/I – bad Rxqual – RL timer times out.

• Faulty TRX – resulting in low C/I – call may drop during setup or after TCH assignment – RL timer may/may not time out.

RF Optimization – Drop Call TroubleshootingRF Optimization – Drop Call TroubleshootingRF Optimization – Drop Call TroubleshootingRF Optimization – Drop Call Troubleshooting

Dropped Calls

• Non RF Reasons

• Switch related – MS experiences a “Downlink Disconnect” – abnormal release, usually with a Cause Value.

• CV 47 is a common example – Layer 3 message “DL Disconnect”.

• Non RF related call drops need to be escalated to isolate the fault which could be related to the switch/transcoder or at any point in the Abis/A Interface.

Handover Failures/Problems

• Handover failures may also be attributed to different reasons.

• Usually occur due to RF reasons.

Common RF reasons for handover failures

• Interference – Co BCCH/Co BSIC issue.

• Faulty hardware on target cell.

• Improper neighbourlist definition

Steps to identify and solve Handover issues.

• Use TEMS (layer 3 messages) to identify the cell to which the MS attempts handover and results in a failure

RF Optimization – Handover ProblemsRF Optimization – Handover ProblemsRF Optimization – Handover ProblemsRF Optimization – Handover Problems


• The sequence of layer 3 messages –

• Handover Command

• Handover Access

• Handover Complete

• Handover Failure

• Sometimes the sequence of messages would be

• Handover Command

• Handover Access

• Handover Failure




• The “Handover Command” message contains information about the BCCH and BSIC of the target cell to which the handover was attempted. Check for any possible Co BCCH/Co BSIC interferers.

• Check for possible hardware faults on the target cell.

Neighbourlist problems

• Sometimes handover problems occur due to improper neighbourlist definition.

• Neighbour Rxlevel are reported to be strong, but “Handover Command” does not get initiated.

• Call drags on the source cell and in some situation drops.

• Most common cause is improper definition of “neighbour BSIC/BCCH”


Neighbourlist Problems

• Crosscheck with RF BSC dump to confirm the BCCH/BSIC and other parameters of the target cell.

• Report any inconsistencies to the OMCR personnel.


Traditional RF Optimization

• Traditional RF Optimization involves drive testing for data collection

• Drive testing is periodically done to monitor the network performance

• Mainly two types – a)Long call – continuous data collection

b) Short call – for statistical analysis

• Drive tests provide inputs for optimizing coverage and quality of the network.

• Usage of simulation/coverage prediction tools like PLANET, ASSET, NETPLAN, etc.

• Usually the simulation tool also contains an AFP component or is available as a separate tool.

RF Optimization – Special ToolsRF Optimization – Special ToolsRF Optimization – Special ToolsRF Optimization – Special Tools

Traditional RF Optimization

• Simulation tools work on standard prediction models.

• Variety of standard models could be used – Okumura Hata, Lee’s model, Cost 231, etc.

• Some tools have customized models (e.g Motorola Netplan uses “XLOS” model which works on virtual heights)

• Inputs used by the prediction tool – digital terrain data, clutter data, in some cases clutter heights.

• Model tuning process – a must to achieve near accurate predictions from the tool.


Limitations of Traditional RF Optimization

• Drive test data only simulates traffic generated from on-road subscribers. Indoor traffic is not simulated!!

• Simulation tools work in a GIGO(garbage in garbage out) fashion.

• The model generated by a simulation tool highly depends on the accuracy of the input data (terrain, clutter data,etc.). Clutter/buildings change continuously – needs to be updated often.

• Often tedious and iterative process.

• Optimization works on a trial-and-error basis.

• For good results – its necessary to have detailed local knowledge of city, subscribers, terrain, clutter.



Solution??

• OEM vendors/Third Party vendors have developed special tools which make the optimization process simpler.

• These tools work on “mobile statistics” aka “measurement reports”

• The “measurement reports” are sent by each MS in the network every 480 milliseconds.

• The MRs provide accurate information about the network as seen by the MS

• MRs have information about serving cell and 6 neighbour cells

• This information can be used effectively for generating a “model” which is much more accurate than prediction model generated by standard prediction tools.


Solution??

• MRs are generated by all subscribers on road, inbuilding, fast moving, slow moving, etc.

• Example of tools which work on measurement reports/mobile statistics currently available

• IOS (Intelligent Optimisation Services) – patented by Motorola.

• SCHEMA – GSM Forte.

• Worldwide many GSM operators have opted for these tools and have found satisfactory results.

MOTOROLA IOS

• Motorola Inc has developed a very powerful tool.

• Initially known as IOP (Intelligent Optimization Product)

• Currently Motorola markets it as a service – hence IOS (Intelligent Optimization Service)

• This tool is an integration of

• Powerful collection platform – which connects to BSS and interrogates the BSS for collection of MRs

• Analysis platform which includes integration of CTP(call trace product), Cellopt AFP.

• Cellopt AFP uses interference matrix generated using the MRs

• Easy to use Windows NT GUI

RF Optimization – Special Tools - IOSRF Optimization – Special Tools - IOSRF Optimization – Special Tools - IOSRF Optimization – Special Tools - IOS

MOTOROLA IOS

• Motorola IOS cycles can be run to conduct the following optimization activities

• Hardware Optimization

• Neighbourlist Optimization

• Physical Optimization

• Frequency Plan (fixed, hopping, various patterns)

• Motorola is in process of conducting trials for MVIOS (Multivendor IOS) which supports other OEM vendors like Nokia, Ericsson, etc.

RF Optimization – Special Tools - IOSRF Optimization – Special Tools - IOSRF Optimization – Special Tools - IOSRF Optimization – Special Tools - IOS

RF Optimization – Special Tools – Schema GSM RF Optimization – Special Tools – Schema GSM ForteForteRF Optimization – Special Tools – Schema GSM RF Optimization – Special Tools – Schema GSM ForteForte

SCHEMA – GSM FORTE

• SCHEMA(based in ISRAEL) has developed an efficient product called GSM Forte

• This tool also uses mobile statistics/measurement reports.

• GSM Forte currently supports Nokia, Ericsson and Alcatel. Development is ongoing to support other vendors like Nortel, Siemens, etc.

• GSM Forte also uses “interference matrix” generated from mobile statistics. The product offers –

• Frequency Plan Optimization

• Neighbourlist Optimization

• Database parameter Optimization (will be included in future versions)

SCHEMA – GSM FORTE

• GSM Forte has been widely adopted by several GSM operators across the globe.

• Effective in generating fast and accurate frequency plans and day to day neighbourlist optimization.

• Hutch India – major customer in India.

• Strategic partnership with GTL for providing presales, post sales, tech-support, premium services to its various customers in India and other regions.

• Succesfully implemented trial optimization project and 2 premium services project.!! These projects were independently implemented by us.

RF Optimization – Special Tools – Schema GSM RF Optimization – Special Tools – Schema GSM ForteForteRF Optimization – Special Tools – Schema GSM RF Optimization – Special Tools – Schema GSM ForteForte

THANK YOUTHANK YOUwww.gtllimited.com

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rf optimisation gsm sudarshan

Documents

initial rf optimization

rf team

rf optimisationgsmjune

understanding rf network

test frequency

basics of rf design

network design process

nominal plan