gsm sdcch congestion - optimization

Knowledge Data Base

TCH Drop OptimizationTechnology

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Optimization Knowledge Data Base

TCH Drop OptimizationQuality Assurance

Author Version Date Comment

Quality Assurance 1.0 11.05.2010

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1 PURPOSE................................................................................................................................................ 3

2 PROCESS DESCRIPTION............................................................................................................................ 3

3 KPIS, ANALYSIS & MEASUREMENTS OF THE PROCESS.................................................................................4

4 FLOW DIAGRAM FOR PROCESS................................................................................................................. 5

5 FLOW DIAGRAM POINTS DESCRIPTION.....................................................................................................6

6 Additional Information........................................................................................................................... 13

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1 PURPOSE

The purpose of this procedure is to teach or to help the GSM RF Engineers to effectively optimize the TCH Drop Rate.

There different reason of TCH Drop Rate. In this procedure it tries to explain each one of them and give possible solutions to reduce them.

The document describes the investigation procedure for TCH Drop Optimization.

2 PROCESS DESCRIPTION

The first thing to do is to understand the process of TCH Assignment so in this way we can identify what kind of TCH Drop are we dealing with. First of all we have to identify the possible causes of TCH Drop. This can be checked with the file GT_GSM_BH_STATS.xls. Here you can see all the reasons of the TCH Drop.

The following procedure should be performed for TCH Assignment analysis:

1: For TCH assignment success rate, the first thing, check the TCH Time Congestion.

2: If there is congestion on TCH, it is recommend doing the dimensioning and adding TRU based on carried TCH traffic demand.

3: If there is no congestion on TCH, check the output power of the BTS. If the output power is low, increase the output power.

4: If the output power is ok, check the faulty BTS by extracting BTS error log.

5: If hardware fault found, swap or repair HW.

Perform drivetests to check the coverage and received RxLEV.

6: If no dominant cell or similar signal strengths of a few cells found during drivetests, it is recommended to add BTS.

7: If there is no problem on the dominant cell, check the interference whether co-channel or adjacent channel.

8: Check the disturbance whether it is on SDCCH or target TCH. If disturbance found, improve the frequency plan.

Mostly, the problems of low TCH assignment are TCH availability and interference.

Causes of high rate of call loss drop are as follows:

(1) Interference (network internal interference, external interference, interference from the equipment itself.)

(2) Bad coverage (Blind Zones, Lonely Islands)

(3) Inappropriate handover (planning of adjacent cells, handover parameters.)

(4) Imbalance between uplinks and downlinks (Tower Amplifier, Power Amplifier, Antenna Direction)

(5) Inappropriate Parameter configuration (Counter for radio link failure, number of SACCH multi-frames

(6) Equipment problem (Carrier board, Power Amplifier, Tower Amplifier)

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3 KPIS, ANALYSIS & MEASUREMENTS of the process

% TCH ASSIGNMENT SUCCESS RATE =

Name Data Type Description

TASSATT numeric(12)

Number of assignment attempts

on TCH

TFCASSALL numeric(12)

Assignment complete for all MS

Ericsson Formulas

Total Dropcall DROP_CALL (TNDROP)/(TCASSALL)

DropCall by TA DROP_TCH_ETA (TDISTA)/(TNDROP)

DropCall By Low Signal Strenght DROP_TCH_LSS

(TDISSUL+TDISSDL+TDISSBL)/(TNDROP)

DropCall By LSS in the Uplink DROP_TCH_LSS_UL (TDISSUL)/(TNDROP)

DropCall by LSS in the Downlink DROP_TCH_LSS_DL (TDISSDL)/(TNDROP)

DropCall by Bad Quality DROP_TCH_BQ

(TDISQAUL+TDISQADL+TDISQABL)/(TNDROP)

DropCall By BQ in the UL DROP_TCH_BQ_UL (TDISQAUL)/(TNDROP)

DropCall By BQ in the DL DROP_TCH_BQ_DL (TDISQADL)/(TNDROP)

DropCall By Sudenly Lost Conection DROP_TCH_SLC (TSUDLOS)/(TNDROP)

Counter DescriptionTNDROP Number of dropped connections

TCASSALL Assignment complete for all MSTDISTA Dropped connections at Timming Advance

TDISSXX Dropped connections at Signal StrenghtTDISQAXX Dropped connections at low quality down or up link.TSUDLOS Dropped suddenly lost connections

UL = Uplink, DL = Downlink, BL =Bad Quality and Low Signal

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4 FLOW DIAGRAM FOR PROCESS

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5 FLOW DIAGRAM POINTS DESCRIPTION

The TCH reports can be found in statistics reports server:

\\172.22.12.129\compartido\2008_GSM_Daily_Reports\CELL_REPORTS\GT_GSM_BH_STATS.xls

1. Chose worst performing cells based on “GT_GSM_BH_STATS.xls”: This report shows the busy hour for each cell and has a monthly trend.

2. Is data reliable? Make sure that the information is reliable, so you can be sure that no errors or missing data can be the cause of the high congestion.

Successful assignments show the number of successful TCH allocations at call setup. At unsuccessful assignment, the Assignment Complete message, sent by the MS, was never received by the BTS.

Probable Reason

No dominant serving cell The serving cell cannot cope with the TCH traffic.

Severe congestion on TCH Failing TCH allocation for assignment or handover due to congestion

Low signal strength for call access

The signal strength might be higher on the BCCH than on the TCH.

Interference Disturbance on SDCCH or target TCH

Faulty transceiver Faulty equipment

TCH Drop Analysis

http://www.cellular-planningoptimization.com/2010/02/tch-drop-analysis.html

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1. Radio Link Time-Out

Every time a SACCH message can not be decoded the radio link time-out counter is decreased by 1. If the message can be decoded the counter is incremented by 2. However, the value can not exceed the initial value. The initial value is set by the parameter RLINKT for radio link time-out in themobile station and by RLINKUP for timeout in the BSC. If the mobile moves out of coverage and no measurement reports are received in the BSC, there will be a radio link time-out and the message Channel Release (cause: abnormal release, unspecified) is sent to the mobile station and the SACCH is deactivated in the BTS. A Clear Request message is sent to the MSC. To be sure that the mobilehas stopped transmitting, the BSC now waits RLINKT SACCH periods before the timeslot is released and a new call can be established on the channel.

2. Layer 2 Time-Out

If the BTS never get an acknowledge on a Layer 2 message after the time T200XN200, the BTS will send Error Indication (cause: T200 expired) to the BSC, which will send Channel Release (cause: abnormal release, timer expired) to the mobile station and a Clear Request to the MSC. The SACCH is deactivated and the BSC waits RLINKT SACCH periods before the timeslot is released and a new call can use the channel. This is only valid if the call is in steady state, i.e. not during handover or assignment.

3. Release Indication

When the BTS received a layer 2 DISC frame from the mobile it replies with a Layer 2 UA frame to the mobile station and a Release Indication to the BSC. The system does only react on Release Indication if it is received during a normal disconnection situation. If such a message is received unexpectedly this will usually cause radio link time-out or timer T200 expiration as the mobile station stops the transmitting of measurement reports. It is also possible that the release will be normal depending on when the Release Indication is received.

4. MSC Time-Out

Normal Release:

If the MSC never received a response on a message (e.g. Identity Request) and there is no radio link time-out or layer 2 time-out, the MSC will send a Clear Command to the BSC. The time-out is depending on the message. When receiving Clear Command, the BSC will send a Channel Release (cause: normal release) and then deactivates the SACCH.

Reject (only SDCCH):

If the MSC never receives a response on the first message after Establish Indication, the MSC will send a reject message. If the connection was a Location Update it will be a Location Update Reject (cause: network failure) and if the connection was a mobile originating call (CM Service Request) a CM Service Reject (cause: network failure) will be sent. The MSC will then send a Clear Command to the BSC and the call is cleared by Channel Release (cause: normal release).

5. Assignment to TCH

Before sending an Assignment Command from the BSC at TCH assignment, the following two criterion have to be fulfilled:

a. There must be a TCH channel available, i.e. no congestion

b. The locating algorithm must have received at least one valid measurement report.

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If either of the criterions are not fulfilled, Assignment Command will not be sent and a Channel Release (cause: abnormal release, unspecified) will be sent to the mobile station and a Clear Request to the MSC.

TCH Drop reason

The classifications of TCH Drop Reasons are arranged in the order of priority:

1. ExcessiveTiming Advance

2.Low Signal Strength

3.Bad Quality

4.Sudden Loss of Connection

5.Other Reasons

Excessive Timing Advance

The TCH Drop counters due to Excessive Timing Advance will pegged when the during the time of

disconnection, the last Timing Advance value recorded was higher than the TALIM Parameter. This drop reason is

commonly apparent to isolated or island sites with a wide coverage area.

Action:

Set TALIM to a value close to 63.

Tilt antenna/reduce antenna height/output power, etc. for co-channel cells.

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Low Signal Strength on Down or Uplink or Both Links

The drops counters due to Low Signal Strength will be pegged when the Signal Strength during the last

Measurement Report before the call dropped is below the LOWSSDL and/or LOWSSUL Thresholds. LOWSSDL and

LOWSSUL are BSC Exchange Property parameters which are used only for statistics purposes and do not affect the

behavior of calls. If both UL and DL Signal Strength are below the thresholds, only Drop due to Low SS BL will

pegged. Normally a call is dropped at the border of large rural cell with insufficient coverage. Bad

tunnel coverage cause many dropped calls as well as so called coverage holes. Bad indoor coverage will result in

dropped calls. Building shadowing could be another reason.

Action:

Check coverage plots.

Check output power.

Check power balance and link budget.

Check if Omni site.

Check antenna configuration & type.

Check antenna installation.

Perform drive tests & site survey.

Check TRX/TS with high CONERRCNT.

Solution:

Add a repeater to increase coverage in for example a tunnel. Change to a better antenna (with higher gain) for the

base station. Add a new base station if there are large coverage holes. Block/Deblock TRX.

Poor Quality on Down or Uplink or Both Links

The drops counters due to Bad Quality will be pegged when the Signal Strength during the last

Measurement Report before the call dropped is above the BADQDL and/or BADQUL Thresholds. BADQDL and

BADQUL (expressed in DTQU) are BSC Exchange Property parameter which are used only for statistics purposes and

does not affect the behavior of calls. If both UL and DL Quality are above the thresholds, only Drop due to BAD

Quality BL will pegged.

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Problem on Bad Quality is usually associated with Co-channel Interference on BCCH or TCH.

Faulty MAIO assignment can cause frequency collisions on co-sited cells especially on 1x1 Reuse. External

interference is also one possible cause of problem on quality.

Action:

Check C/I and C/A plots.

Check Frequency Plan (Co-BCCH or Co-BSIC Problem).

Check MAIO, HOP, HSN parameters.

Check FHOP if correctly configured (BB or SY).

Check for External Interference.

Perform drive tests.

Solution:

Change BCCH frequency.

Change BSIC.

Change MAIO, HOP, HSN.

Change FHOP.

Record RIR or on-site Frequency Scanning to identify source of interference.

Use available radio features.

Sudden Loss of Connection

On circuit switch service, when a call is abnormally disconnected, a Clear Message with cause code Call

Control be treated as normal Disconnection is sent to the MSC – named Clear Request Message. Refer to Ericsson

system, the following Urgency condition is checked at that time and the relevant counter is incremented as a

consequence:

Excessive TA

LowSignalStrength

BadQuality

SuddenDrop

As named, stright forward meaning for the dropped call is described for the first three items. However,

Sudden Drop is quite not easy to understand.

Sudden Loss are drops that have not been registered as bad quality, signal strength, timing advance. The term

Sudden Loss is used because if the network cannot establish a connection with the lost MS after a pre-defined

period, the sudden loss counter is incremented if the last reported measurement from the MS does not fulfill any of

the reasons mentioned.

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A connection is marked as Sudden loss if none of the three types of urgency states (that is excessive TA, low

signal strength or bad quality) are indicated and the locating procedure indicates missing measurement results from

the MS.

Drops due to Sudden Loss are drops that have not been registered as low signal strength, excessive timing

advance, bad quality or hardware (other) reasons, and the locating procedure indicates missing measurement

results from the MS.

There are some common scenarios that could lead to Sudden Loss of connections such as very sudden and severe

drops in signal strength, such as when subscribers enter into buildings, elevators, parking garages, etc., very sudden

and severe occurrence of interference, MS runs out of battery during conversation, Handover Lost, BTS HW faults,

Synchronization or A-bis link fault (transmission faults), and MS Faults.

Action:

Check BTS Error Logs, Alarms and Fault Codes.

Check CONERRCNT per TRX and TS.

Check Transmission Link (A-bis).

Check for DIP Slips.

Check LAPD Congestion.

Correlate Handover Lost to Drops due to Sudden Loss

Solution:

Fix Hardware Faults and Alarms.

Reset TRX with high CONERRCNT.

Ensure that Synchronization and A-bis Link are stable.

Change RBLT with high DIP Slips.

Change CONFACT or increase Transmission Capacity

Investigate HO Lost Problem

TCH Drops due to Other Reasons

TCH drops due to Other Reasons are computed by subtracting the sum of drops due to Excessive TA, Low SS, Bad

Quality and Sudden Loss from the Total TCH Drop Counts. Drops due to Other Reasons are generally associated with

hardware problems, transmission link problems on A-bis, Ater or Ainterfaces, and sometimes Handover Lost.

Action:

Check BTS Error Logs.

Check Alarms and Fault Codes.

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Check CONERRCNT per TRX and TS.

Check Transmission Link (A-bis).

Check for DIP Slips.

Correlate Handover Lost to Drops due to Other Reasons

Solution:

Fix Hardware Faults and Alarms.

Reset TRX with high CONERRCNT.

Ensure that Synchronization and A-bis Link are stable.

Change RBLT with high DIP Slips.

Investigate HO Lost Problem

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6 Additional Information

How to Deal with TCH - TCH Interference

A GSM cell has more than one carrier to handle subscriber capacity requirements. Only one of the available

carriers will be the BCH, which will be on continuously; the remaining TCH carriers will only turn on for specific

timeslots when a call is initiated on that channel. During peak hours, the activity on the TCH carriers will be at a

maximum, whereas activity may be zero during off-peak hours. TCH carriers are also reused, and hence can

contribute to co-channel interference, although this interference will not always be present; it will only be present

when these TCH carriers have call activity (not necessarily during peak hours). How do we measure the C/I for these

reused TCH carriers? One approach is to pick the suspected reuse interferer, set up calls on each timeslot (eight

calls), and then drive around in the interfering cell measuring the C/I. This is a tedious process. The easiest way to

solve this measurement problem is to make delta (difference) measurements. As an example, consider two cells,

Cell 1 and Cell 2

TCH-TCH co-channel interference.

Cell 1 has a BCH carrier on ARFCN B1 and Cell 2 has a BCH carrier on ARFCN B2, while the TCH carriers in

both Cell 1 and Cell 2 are on ARFCN T1. Instead of making the C/I measurement on T1, we can make a delta

measurement of B2/B1, which is near to the C/I value of T1 when both the T1 carriers are on air, since the

propagation loss for B2/B1 and T1 is nearly the same. This can be easily done with the GSM receiver, which has a

BCH analyzer display function

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BCH analyzer display generated by GSM receiver. The x-axis shows ARFCNs, and the y-axis shows received power level.

The analyzer allows us to create a user list of ARFCNs for Cell 2 and Cell 1 and then, on the amplitude/time

display, place a marker on B2 and a delta marker on B1. The delta measurement is the same as measuring the C/I on

T1.

Amplitude versus time display showing delta amplitude difference between B1 and B2.

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Using post-processing software, we can also plot the C/I map for TCH-TCH interference

C/I map of TCH-TCH interference measured by the GSM receiver. Points where C/I < 9 dBare indicated by arrows.

gsm sdcch congestion - optimization

Documents

2gapproved date page

gsm rf engineers

tch drop optimizationrevision

fix hardware faults

tch drop rate

missing measurement results

excessive timing advance

bl bad quality