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Reasons for the Need of OptimizationReasons for the Need of Optimization

Network optimization is an iterative process which should improve the quality and performance of a

network and also run the network more efficiently. As in any optimization problem, also in network

optimization, the network will mostly not run optimal from the very beginning. There can be

mentioned several reasons:

• Systematic inaccuracies

• Statistical nature of the involved processes like e.g. traffic and RF propagation

• Dynamical nature of the involved processes like e.g. change of the subscriber’s telephone

behavior (e.g. SMS)

• Wrong (or only too rough) planning assumptions, input data and/or planning models

• Increasing number of subscribers

• Installation errors (for example a wrong cabling: transmitting into cell A, but receiving from

cell B)

• Hardware / software trouble

• ...

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Performance Data MeasurementsPerformance Data Measurements

Performance data measurements can help the network operator for example to localise problem

areas as early as possible and also to verify improvements of the network optimisation.

Concerning radio network optimisation there are related performance data measurements foreseen

by GSM (see: GSM 12.04) and in addition also vendor specific ones.

In general performance data measurements can be run continuously, periodically or sporadically, for

a long time or a short time, observing smaller or greater parts of the network.

The related counters could in principle be actualised continuously during the observation period, but

mostly a scanning method is used. Scanning method means that the system counts the number of

events not continuously but only at particular times. This leads to some uncertainty for the

measurement results. Nevertheless, the error performed can be estimated using statistical methods.

In general, the smaller the scanning interval the higher the precision of the measurement (for

constant observation periods). Typical scanning intervals are 100 ms or 500 ms.

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Performance Data MeasurementsPerformance Data Measurements

Condition(s) for the updating of

the counter value(s)

Counter(s)

Performance data measurement(s)

Scanner

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Drive TestsDrive Tests

Drive tests are performed by the network operator for various reasons:

• To check the coverage in a certain area

• To check the quality of service in a certain area

• To find the answer for customer complaints

• To realise that the network is not properly running

• To verify that the network is properly running

• To verify that certain optimisation steps have been successful

• ...

Drive tests must be well prepared. Before, during and after the drive test the following steps should

be performed:

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Drive TestsDrive Tests

• Make back-up files of the captured data

• Replay the captured data and analyse them

• Find out problem areas and problem events

• Use further post-processing tools to display the captured data more

clearly and to graphically display further values

• Perform statistics and summarise the results

After drive test

• Monitor the test equipment

• Reconnect dropped calls

• Insert notes in the recording file

• Note interesting events separately (e.g. on a piece of paper)

During drive test

• Plan the route where to drive

• Plan the time when to drive

• Determine the MS mode (idle mode/ connected mode) and also the

call strategy (long / short calls)

• Decide which values to focus on (for example: RXQUAL, RXLEV,SQI, ...)

• Select an appropriate test equipment and check the test equipment

• Think of notes which should be inserted later on in the recording file

Before drive test

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Optimization StrategiesOptimization Strategies

Before Optimization:

Clean the hardware

• Performance Data Measurements

• Customer Complaints

• Drive tests

Optimization:

1) Physical parameters

2) Database Parameters

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Optimization of Physical ParametersOptimization of Physical Parameters

Altering antenna tilt:

• to reduce interference

• to limit coverage area

• to improve coverage (e.g. coverage weakness below main lobe)

• to improve in-building penetration

Altering the Antenna tilt must be done very carefully to really improve the situation.

Typical down-tilts are between 0° and 10°, however even higher values (up to 25°) have already

been used.

Altering antenna azimuth:

• to overcome coverage weakness between different sectors

• to reduce interference in certain directions

Increasing or decreasing antenna height:

• to reduce or improve coverage

• to reduce interference

Change of antenna type

• to achieve desired ration characteristics

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Addition / re-movement of TRXs:

• Depending on the real measured traffic load either TRXs can be removed (switched off or

blocked) or must be added. Not really needed TRXs may interfere other cells.

• The number of needed TRXs and also the configuration of the different channels depend on the

offered traffic, and the subscriber behaviour.

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Cell sectorization / cell splitting:

Can be used for:

• Coverage enhancements (since the antenna gain of sectorised antennas is higher than that of

omni directional antennas)

• Interference reduction

• Capacity enhancements, but only if together with the sectorisation also the number of TRXs is

increased (compare Erlang-B loss formula)

Depending on how the splitting is performed:

• it may be a more or less expensive and difficult (time consuming) solution

• coverage weakness between the main lobes may appear

• the capacity will be reduced if the total number of TRXs remains constant

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Implementation of Antenna near Pre-Amplifiers:

Link imbalances are one reason for poor quality, increased call drop rate and increased

handover failure rate. In case of an unbalanced link, the uplink and downlink coverage ranges

differ. Often the downlink range is bigger than the uplink range. This problem can be overcome

by using antenna near preamplifiers which improve the sensitivity and the noise figure of a base

station system. Looking to the link budget: The better the sensitivity of the base station, the

more fare the possible uplink range. In any case, a proper running network requires a balanced

link.

Implementation of Repeaters:

A repeater (see GSM 11.26 (ETS 300 609-4) and GSM 05.05) is a bi-directional (full duplex) RF

amplifier and is used to overcome coverage holes in a base station area. Typical applications of

repeaters are the coverage of problem zones like tunnels, valleys, in buildings, ...

A repeater receives, amplifies and retransmits the downlink signal from a donor base station

into an area with weak or no coverage, and the uplink signal from mobile stations which are

located in such an area. Repeaters extend but do not replace base stations.

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Optimization of Database ParametersOptimization of Database Parameters

Frequency Changes:

• To overcome e.g. sever cases of downlink interference (therefore it is advisable to have some

spare frequencies).

• May influence other areas.

• Re-planning may become necessary.

• In high-density areas often difficult.

Strategies:

• Using spare frequencies in severely interfered regions.

• TCH – BCCH change as temporary solutions in low TCH traffic load areas.

• Re-planning of TCH and BCCH frequencies.

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Frequency Hopping: Cyclic or pseudo random hopping?

Time in TDMA frames

f0

f1

f2

f3

f4

f5

Optimum frequency diversity

due to averaging of Rayleigh fading

⇒ rural, coverage limited areas


Example of cyclic hopping:

HSN = 0

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Frequency Hopping: Example of pseudo random hopping: HSN = 1-63

Co-channel interference averaging

Collision probability: 1/number of hopping frequencies

⇒ interference limited areas (hot spots)

0 1 2 3 4 5 6 7 8 9 10 TDMA Nr.f0

f1

f2

f3

f4

f5

time

0 1 2 3 4 5 6 7 8 9 10f0

f1

f2

f3

f4

f5

Cell1 (e.g. HSN =14) Re-use cell 15 (e.g. HSN =23)


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Radio Link Failure (RLF) / Radio Link Timeout (RLT):

(see also GSM 04.08 and GSM 05.08)

Calls which fail due to radio coverage problems or which suffer under unacceptable voice or data

quality (due to e.g. interference) which cannot be improved by power control or handover are either

released or re-established in a defined way.

The criterion for the detection of a radio link failure by the MS is the success rate of decoding DL-

SACCH messages.

The criterion for the determination of a radio link failure by the BS is either the success rate of

decoding UL-SACCH messages or it is based on RXLEV / RXQUAL measurements.

The MS checks the DL with the help of a radio link (failure) counter running in the MS.

The BS checks the UL with the help of a radio link (failure) counter running in the BS.

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Radio Link Failure (RLF) / Radio Link Timeout (RLT):

The algorithm for the modification of the radio link failure counter S is the following:

Starting value for the Radio Link Failure Counter: Radio_Link_Timeout

In case of successful decoding of SACCH messages: Snew=Sold+2

In case of non-successful decoding of SACCH messages: Snew=Sold-1

value range for S: 0≤ S≤ Radio_Link_Timeout

Radio link failure is detected if: S=0

This algorithm is only running after assignment of a dedicated channel (i.e. in connected mode).

The starting value Radio_Link_Timeout for the MS counter is sent on the BCCH system information

type 3 or on the SACCH system information type 6 in the information element ‘Cell Options’.

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Handover and Power Control:

Reasons for the optimisation of handover parameters:

• To reduce the number of call drops

• To reduce the number of handovers

• To maximise the time duration the MS spends in the best cell

• To improve the speech quality

Handover types: intra- / intercell, BTS, BSC, MSC handovers

Handover causes:

• (Bad) RXQUAL

• (Low) RXLEV

• (far) DISTANCE

• (Power Budget) PBGT

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Thresholds needed for handover evaluation:

Maximum transmission power the BTS may useBS_TXPWR_MAX

Maximum transmission power a MS may use in the adjacent cell (n)MS_TXPWR_MAX(n)

Maximum transmission power a MS may use in the serving cellMS_TXPWR_MAX

Threshold for power budget processHO_MARGIN(n)

Minimum RXLEV required for a MS to be allowed to be handovered to neighbour cell

(n) (incoming HO)

RXLEV_MIN(n)

RXLEV threshold on the downlink for intracell (interference) handoverRXLEV_DL_IH

RXLEV threshold on the uplink for intracell (interference) handoverRXLEV_UL_IH

Threshold for the maximum allowed distance between MS and current BTS (outgoing

HO)

MS_RANGE_MAX

RXQUAL threshold on the downlink for handover process to commence (outgoing HO)L_RXQUAL_DL_H

RXLEV threshold on the downlink for handover process to commence (outgoing HO)L_RXLEV_DL_H

RXQUAL threshold on the uplink for handover process to commence (outgoing HO)L_RXQUAL_UL_H

RXLEV threshold on the uplink for handover process to commence (outgoing HO)L_RXLEV_UL_H

RemarksAbbreviation

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Level triggered handover:

Depending on the measured and averaged RXQUAL_XX and RXLEV_XX values the system (MS

and / or BS) may increase or decrease the output power or may handover the call.

Remarks to the corresponding handover threshold settings:

L_RXLEV_UL_H and L_RXLEV_DL_H should be set some dB (e.g. 5 dB) above the effective (+

diversity gain, + preamplifier) receiver sensitivity limit:

Receiver sensitivity levels due to GSM 05.05:

For DCS 1800 class 1 or class 2MS: -100 dBm

For DCS 1800 class 3 MS: -102 dBm

For GSM 900 small MS: -102dBm

For other GSM 900 MS: -104 dBm

For normal BTS: -104 dBm

Example settings: L_RXLEV_DL_H = –95 dBm

L_RXLEV_UL_H = –102 dBm

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Level triggered handover:

There should be a level hysteresis between the threshold RXLEV_MIN(n) for incoming handover

and the threshold L_RXLEV_XX_H for outgoing handover:

RXLEV_MIN > L_RXLEV_XX_H + 4....10 dB

The size of this hysteresis should be related to the standard deviation of the long term fading

(typically 4...10 dB) and should be large enough to avoid ping-pong handovers and small enough to

allow fast handovers.

Example setting: RXLEV_MIN = -90 dB

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Level triggered Power Control:

Depending on the measured and averaged RX_QUAL and RX_LEV values the system (MS and /

or BS) may increase or decrease the output power or may handover the call.

For the power control and handover threshold settings the following considerations should be taken

into account:

(Upper) RXLEV threshold on the downlink for power reductionU_RXLEV_DL_P

(Lower) RXLEV threshold on the downlink for power increaseL_RXLEV_DL_P

(Upper) RXLEV threshold on the uplink for power reductionU_RXLEV_UL_P

(Lower) RXLEV threshold on the uplink for power increaseL_RXLEV_UL_P

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Level triggered Power Control:

To avoid consecutive power increase or decreases directly after each other the difference between

upper and lower power control thresholds should be large enough (e.g. 10 dB).

To allow the system to perform power control before handover is executed, the lower power control

level thresholds should be about 10 dB above the lower handover level thresholds.

Example settings:

L_RXLEV_DL_H = –95 dBm, L_RXLEV_DL_P = -85 dBm, U_RXLEV_DL_P = -75 dBm

L_RXLEV_UL_H = –102 dBm, L_RXLEV_UL_P = -92 dBm, U_RXLEV_UL_P = -82 dBm

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Power Control Execution:

For the power control execution parameter settings the following considerations should be taken

into account:

Since typically a power increase command is more urgent than a power reduction command, the

power increase step size should be greater than the power reduction step size.

The power increase and power reduction step sizes should be on the one hand small enough to

enable an accurate power control, on the other hand large enough to reduce the number of

necessary power control commands and therefore the signalling load.

Example settings:

POW_INCR_STEP_SIZE = 4 dB

POW_RED_STEP_SIZE = 2 dB

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Quality triggered handover:


and/or BS) may increase or decrease the output power or may handover the call.

Remarks to the corresponding handover threshold settings:

L_RXQUAL_UL_H, L_RXQUAL_DL_H

RXLEV_UL_IH, RXLEV_DL_IH

In case of bad quality (RXQUAL_XX > L_RXQUAL_XX_H) and high signal strength (RXLEV_XX >

RXLEV_XX_IH) at the same time, there is a high probability of the presence of:

cochannel interference, adjacent channel interference, intermodulation problems, intersystem

interference.

Temporary solution: intracell handover

Intracell handover doesn’t help: if frequency hopping is switched on, or if there is only 1 TRX in the

serving cell and the interference is continuous and not bursty.

Examples settings: L_RXQUAL_UL_H = 5, L_RXQUAL_DL_H = 5

RXLEV_UL_IH = -85 dBm , RXLEV_DL_IH = -78 dBm

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Quality triggered Power Control:


and/or BS) may increase or decrease the output power or may handover the call.

Power is increased if the received quality is bad:

RXQUAL_XX > L_RXQAUL_XX_P

Power can be decreased if the received quality is very good:

RXQUAL_XX < U_RXQAUL_XX_P

However, often it is more suitable to control the power decrease by the level criteria and to set

U_RXQAUL_XX_P = 0 or a small value, i.e. to ‘disable’ the power decrease due to good quality.

To make ‘power up before handover’ possible, the following relation between power control and

handover thresholds should be taken into account:

U_RXQUAL_XX_P < L_RXQUAL_XX_P < L_RXQUAL_XX_H

Example settings: U_RXQUAL_XX_P = 0 (or 1)

L_RXQUAL_XX_P = 4

L_RXQUAL_XX_H = 5

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RXQUAL

RXLEV

L_RXQUAL_XX_H

L_RXLEV_XX_H

L_RXLEV_XX_IH

0

0

7

63

Intercell

handover

due to level

No handover action

due to quality or level,

e.g. power budget handover

Intercell handover

due to qualityIntracell handover

due to quality


Handover Thresholds:

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Example settings

L_RXLEV_DL_P = 25

U_RXLEV_DL_P = 35

L_RXQUAL_DL_P = 4

U_RXQUAL_DL_P = 1

RXQUAL

RXLEV

L_RXQUAL_XX_P

U_RXQUAL_XX_P

L_RXLEV_XX_P U_RXLEV_XX_P

0

0

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63

Power

DecreasePower

Increase

Power Decrease

Power Increase

No action due

to power control

POW_RED_STEP_SIZE

Power Increase

L_RXLEV_UL_P = 10

U_RXLEV_UL_P = 15

L_RXQUAL_UL_P = 5

U_RXQUAL_UL_P = 4

POW_RED_STEP_SIZE = 2 dB

POW_INCR_STEP_SIZE = 4 dB


Power Control Thresholds:

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Handover triggered by power budget:

In an interference limited area (e.g. small cells in cities) most of the handovers should be power

budget handovers:

For this type of handover not the level, quality, or distance is the handover cause, since all the

corresponding thresholds are not exceeded in the serving cell, but a neighbour cell offers a better

service (a smaller path loss, see link budget).

Since the power budget hanodver looks for the serving cell with the smallest path loss, this kind of

handover will:

Reduce interference

Prolong MS battery time

The power budget is defined as the difference between the path loss in the serving cell and the

path loss in the neighbour cell:

PBGT(n) = (BS_TXPWR – RXLEV_DL) – ( BS_TXPWR_MAX(n) – RXLEV_DL_NCELL(n))

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Handover triggered by power budget:

Assumption:

BS_TXPWR_MAX – BS_TXPWR_MAX(n) = MS_TXPWR_MAX – MS_TXPWR_MAX(n)

PBGT(n) = RXLEV_DL_NCELL(n) – RXLEV_DL – PWR_C_D + min (MS_TXPWR_MAX,P) – min

(MS_TXPWR_MAX(n),P)

Where PWR_C_D is defined as: BS_TXPWR_MAX – BS_TXPWR

If PBGT(n) > HO_MARGIN(n) the path loss in the serving cell is greater than the path loss in the

neighbour cell + HO_MARGIN so that the neighbour cell is considered as the much better cell.

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Remarks to the setting of the Handover Margin:

• The HO_MARGIN setting should be a compromise between ideal power budget handover (which

requires a small HO_MARGIN value) and a setting to reduce the risk of ping-pong handovers

(which requires a greater HO_MARGIN value).

• A small handover zone increases the risk of ping-pong handovers.

• Usually HO_MARGIN is set symmetrically.

• Asymmetrical HO_MARGIN can be used to influence the size of the handover area and/or to

move the handover area, i.e. to move the cell boundaries.

• Adjusting HO_MARGIN values can therefore also be used to adapt the cell area to the traffic load

or to avoid local interference.

• RXLEV_MIN(n) should be set to such a value that RXLEV_DL_NCELL(n) > RXLEV_MIN(n) in

those areas where PBGT(n) > HO_MARGIN(n) to really allow the power budget handover as soon

as the power budget condition is fulfilled.

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HOM = 0 HOM = 0

HOM = + 6 dB HOM = +6 dB

HOM: Handover margin for

adjacent cell


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Remarks to the pre-processing (averaging) of the measurements needed for power control

and handover decisions:

In general:

• Many measurements should be averaged in case that reliable decisions are necessary (better

statistics).

• Only a few measurements should be averaged in case that fast decisions are necessary.

For level / quality triggered handover / power control decisions:

• To allow the system to ‘power up before handover’ usually the averaging process for the handover

decisions should include more measurements than for power control decisions.

• Usually for level triggered handover decisions more measurement values should be averaged

than for quality triggered handover decisions since quality handovers must be executed quickly if

sudden interference appears.

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BCCH allocation:

Also neighbor cell list is target of optimization process:

Missing neighbor cell ⇒ perhaps call drop

Too many neighbors ⇒ bad statistics, unprecise measurement values, perhaps wrong decisions

In practice: ≈ 6-8 neighbors

12-138

10-1110

6-716

3-432

Number of samples per carrier and

SACCH multiframe

Number of BCCH carriers in BA

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Street corner effect: e.g. 20 dB loss

Fast handover mechanism necessary:

- trigger: uplink measurement receive level below threshold

- short averaging period of measurements

- predefined target cell lists

- small handover margins

- short timer settings

- allow back handover

Fast handover:


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Location Area:

The size of the location area must always be a compromise:

Too big ⇒ perhaps paging overload (PCH overload)

(MS is paged in the whole location area)

Too small ⇒ perhaps too many location updates (AGCH overload)

(MS has to perform location update if location

area is changed)

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Example Drive Test: ExerciseExample Drive Test: Exercise

Exercise:

Discuss the drive test given above:

a) Localize the problem area(s)

b) Suggest counter measures to solve the problem(s)

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Example Drive Test: ExerciseExample Drive Test: Exercise

Exercise:

Discuss the drive test given above:

a) Localize the problem area(s)

b) Suggest counter measures to solve the problem(s)

07 MN1790 Radio Network Optimization

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Transcript of 07 MN1790 Radio Network Optimization