Knowledge Data Base
Technology
TCH Drop Optimization Document ID
Revision
2G Approved
1.0
Date
Page (total)
11/05/2010
Optimization Knowledge Data Base
TCH Drop Optimizat Optimization ion Quality Assurance
Author
Quality Assurance
Version
1.0
Date
11.05.2010
Comment
1 (16)
Knowledge Data Base
Technology
TCH Drop Optimization Document ID
Revision
1.0
2G Approved
Date
11/05/2010
Page (total)
2 (16)
1 PURPOSE............................................................... ........................................ ........................................................................ ................................ ........... ....... .... 3 THE PURPOSE OF THIS PROCEDURE IS TO TEACH OR TO HELP THE GSM RF ENGINEERS TO EFFECTIVELY OPTIMIZE THE TCH DROP RATE......................................................................................3 THERE DIFFERENT REASON REASON OF TCH DROP RATE. IN THIS PROCEDURE IT TRIES TO EXPLAIN EXPLAIN EACH ONE OF THEM AND GIVE POSSIBLE SOLUTIONS TO REDUCE THEM......................................3 2 PROCESS DESCRIPTION............................................................... ......................................... .......................................................... ................. 3 3 KPIS, ANALYSIS & MEASUREMENTS OF THE PROCESS....................................................................4 ............................................................................. .................................... .................................................................................. .................................................................. ......................... ............. ...... ............. ......... ... 5 ........................................................................... ................................... ................................................................................. .................................................................. ......................... .............. ....... ............. ......... ... 5 UL = UPLINK, DL = DOWNLINK, BL =BAD QUALITY AND LOW SIGNAL .............................................5 4 FLOW DIAGRAM FOR PROCESS............................................................... ................................... .......................................... .......... ... 6 5 FLOW DIAGRAM POINTS DESCRIPTION...............................................................................................7 6 ADDITIONAL INFORMATION.................................................................................................................14
Knowledge Data Base
Technology
TCH Drop Optimization Document ID
Revision
2G Approved
1.0
1
Date
Page (total)
11/05/2010
3 (16)
PURPOSE The purpose of this procedure procedure is to teach or to help the GSM GSM RF Engineers to effectively optimize the TCH Drop Rate. There different reason of TCH TCH Drop Rate. In this procedure it tries to explain each each one of them and give possible solutions to reduce them.
The document describes the the investigation procedure for TCH Drop Drop Optimization.
2
PRO PROCES CESS DES DESCRIP CRIPTI TIO ON
The first thing to do is to understand understand the process of TCH Assignment Assignment so in this way we can identify what kind of TCH Drop are we dealing with. First of all we have to identify identify the possible causes causes of TCH TCH Drop. This can be 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 should be performed for TCH Assignment analysis: 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)
Knowledge Data Base
Technology
TCH Drop Optimization Document ID
Revision
2G Approved
Date
1.0
11/05/2010
Page (total)
4 (16)
(5) Inappropriate Parameter configuration (Counter for radio link failure, number of SACCH multi-frames (6) Equipment problem (Carrier board, Power Amplifier, Tower Amplifier)
3
KPIS, KPIS, ANALYS ANALYSIS IS & MEAS MEASURE UREMEN MENTS TS of the the proce process ss % TCH ASSIGNMENT SUCCESS RATE =
Name TASSATT TFCASSALL
Data Type
TFCASSALL * 100 TASSATT
Description Number of assignment attempts on TCH
numeric(12 ) numeric(12 Assignment complete for all MS )
Knowledge Data Base
Technology
TCH Drop Optimization Document ID
Revision
2G Approved
1.0
Date
Page (total)
11/05/2010
5 (16)
Ericsson Formulas
Total Dropcall
DROP_CALL
(TNDROP)/( TNDROP)/(TCASSALL TCASSALL))
DropCall by TA
DROP_TCH_ETA
(TDISTA)/( TDISTA)/(TNDROP TNDROP))
DropCall By Low Signal Stren renght
DROP_TCH TCH_LSS
(TDISSUL+ TDISSUL+TDISSDL+ TDISSDL+TDISSBL )/(TNDROP )/( TNDROP))
DropCall By LSS in the Up Uplink DROP_TCH_LSS_UL (TDISSUL)/( TDISSUL)/(TNDROP TNDROP)) DropCall by LSS in the Do Downlink DROP_TCH_LSS_DL (TDISSDL)/( TDISSDL)/(TNDROP TNDROP)) DropCall by Bad Quality
DROP_TCH_BQ
(TDISQAUL+ TDISQAUL+TDISQADL+ TDISQADL+TDIS QABL)/( QABL)/(TNDROP TNDROP))
DropCall By BQ in the UL
DROP_TCH_BQ_UL
(TDISQAUL)/( TDISQAUL)/(TNDROP TNDROP))
DropCall By BQ in the DL
DROP_TCH_BQ_DL
(TDISQADL)/( TDISQADL)/(TNDROP TNDROP))
DropCall By Sudenly Lost Conection
DROP_TCH_SLC
(TSUDLOS)/( TSUDLOS)/(TNDROP TNDROP))
Counter TNDROP TCASSALL TDISTA TDISSXX TDISQAXX TSUDLOS
Description Number of dropped connections Assignment complete for all MS Dropped connections at Timming Advance Dropped connections at Signal Strenght Dropped connections at low quality down or up link. Dropped suddenly lost connections
UL = Uplink, DL = Downlink, BL =Bad Quality and Low Signal
Knowledge Data Base
Technology
TCH Drop Optimization Document ID
Revision
1.0
4
FLOW FLOW DIAG DIAGRA RAM M FO FOR R PROC PROCES ESS S
2G Approved
Date
11/05/2010
Page (total)
6 (16)
Knowledge Data Base
Technology
TCH Drop Optimization Document ID
Revision
2G Approved
1.0
5
Date
11/05/2010
Page (total)
7 (16)
FLOW FLOW DIAG DIAGRA RAM M POIN POINTS TS DESC DESCRI RIPT PTIO ION N
The TCH reports can be found found in statistics reports server: server: \\172.22.12.129\co \\172.22.12.129\compartido\2008_G mpartido\2008_GSM_Daily_Reports\CELL SM_Daily_Reports\CELL_REPORTS\GT_GSM_BH_ST _REPORTS\GT_GSM_BH_STATS.xls ATS.xls 1. Chose Chose worst worst perfor performin ming g cells cells based based on “GT_GS “GT_GSM_B M_BH_ST H_STATS ATS.xl .xls”: s”: This report report shows the busy hour for each cell and has a monthly trend. 2. Is data reliabl reliable? e? Make sure sure that the inform informatio ation n is reliable, reliable, so you can can be sure that 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 dominant serving serving cell The serving serving cell cannot cannot cope with the TCH traffic. Severe congestion on TCH
Failing Failing TCH allocatio allocation n for assignmen assignmentt or handover due to congestion
Low Low signal signal streng strength th for The signal strength might be higher on call access the BCCH than on the TCH. Interference
Disturbance on SDCCH or target TCH
Faulty tr transceiver
Faulty eq equipment
TCH Drop Analysis
Knowledge Data Base
Technology
TCH Drop Optimization Document ID
Revision
1.0
2G Approved
Date
11/05/2010
Page (total)
8 (16)
1. Radio Link Time-Out
Ever Every y time time a SACC SACCH H mess messag age e can can not not be deco decode ded d the the radi radio o link link time time-o -out ut coun counte terr 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 radio link link time-o time-out ut in themob themobile ile statio station n and by RLINKU RLINKUP P for for timeou timeoutt in the BSC. 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 (cause:: abnorm abnormal al releas release, e, unspec unspecifi ified) ed) is sent sent to the mobile mobile statio station n and the SACCH SACCH is deacti deactivat vated ed in the BTS. A Clear Clear Reques Requestt messag message e is sent to the MSC. MSC. To be sure sure that 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 mess messag age e is rece receiv ived ed unex unexpe pect cted edly ly this this will will usua usuall lly y caus cause e radi radio o link link time time-o -out ut 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:
Knowledge Data Base
Technology
TCH Drop Optimization Document ID
Revision
2G Approved
1.0
Date
Page (total)
11/05/2010
9 (16)
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. 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 classifications of TCH Drop Reasons 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 Para Parame mete ter. r. This This drop drop reas reason on is comm common only ly appa appare rent nt to isol isolat ated ed or isla island nd site sites s with with a wide coverage area. Action:
Knowledge Data Base
Technology
TCH Drop Optimization Document ID
Revision
2G Approved
Date
1.0
11/05/2010
10 (16)
Set TALIM to a value close to 63.
•
•
Page (total)
Tilt antenna/reduce antenna antenna height/output power, etc. for co-channel co-channel cells.
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/ and/or or LOWS LOWSSU SUL L Thre Thresh shol olds ds.. LOWS LOWSSD SDL L and and LOWS LOWSSU SUL L are are BSC BSC Exch Exchan ange ge Prop Proper erty ty parameters which are used only for statistics purposes and do not affect the behavior of calls. If both UL and DL Signal Strength Strength are below the thresholds 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 tunnel coverage coverage cause cause many dropped dropped calls as well as so called called coverage coverage holes. 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 ith
higher
gain)
for
the
base station.
Add
large coverage holes. Block/Deblock TRX. Poor Quality on Down or Uplink or Both Links
a
new
base station if
there
are
Knowledge Data Base
Technology
TCH Drop Optimization Document ID
Revision
2G Approved
1.0
Date
Page (total)
11/05/2010
11 (16)
The drops counters due to Bad Quality will be pegged when the Signal Strength during the last Measurement Measurement Report before the call dropped is above 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. 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.
Knowledge Data Base
Technology
TCH Drop Optimization Document ID
Revision
2G Approved
1.0
Date
Page (total)
11/05/2010
12 (16)
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. 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 streng str ength, th, exc excess essive ive tim timing ing adv advanc ance, e, bad qua qualit lity y or har hardwa dware re (ot (other her)) rea reason sons, s, 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 ver y sud sudden den and severe severe dro drops ps in si signa gnall str streng ength, th, suc such h as whe when n sub subscr scribe ibers rs ent enter er int into o build bu ildin ings gs,, el elev evat ator ors, s, pa park rkin ing g ga gara rage ges, s, et etc. c.,, ve very ry su sudd dden en an and d se seve vere re oc occu curr rren ence ce 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 Reasons are computed by subtracting 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
Knowledge Data Base
Technology
TCH Drop Optimization Document ID
Revision
2G Approved
1.0
Date
11/05/2010
problems on A-bis, Ater or Ainterfaces, and sometimes Handover Lost.
Action:
Check BTS Error Logs. Check Alarms and Fault Codes. 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
Page (total)
13 (16)
Knowledge Data Base
Technology
TCH Drop Optimization Document ID
Revision
1.0
6
2G Approved
Date
11/05/2010
Page (total)
14 (16)
Addi Addittion ional Info Inform rmat atio ion n
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. 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
Knowledge Data Base
Technology
TCH Drop Optimization Document ID
Revision
2G Approved
1.0
Date
11/05/2010
Page (total)
15 (16)
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 versus time display showing delta amplitude difference difference between B1 B1 and B2.
Knowledge Data Base
Technology
TCH Drop Optimization Document ID
Revision
1.0
2G Approved
Date
11/05/2010
Page (total)
16 (16)
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.
