analysis on ffer for cdma

Analysis on Forward FER issue ‐ Lucent

ALCATEL‐LUCENT MANAGED SOLUTIONS INDIA PVT. LTD.

Analysis report on Forward FER issue ‐ Lucent



Problem Definition:‐ Issue observed is spurt of Forward FER all across irrespective of good Ec/Io & MTx.



In the above highlighted patches it is observed that in spite of good Mobile Tx power & Ec/Io values.

Almost 2 to 5% variation is observed in Forward FER values across these areas.

The main reason for Forward Link FER degradation are :‐

1) Weak traffic channel:‐ Mobile Rx Power is High & Ec/Io of serving Pilot is high. 2) Low Ec/Io (Weak Pilot) :‐

• Weak traffic channel:‐ When High Forward FER is accompanied by high Mobile Receive Power & a high Pilot Ec/Io, the lack of quality is due to a weak Forward Traffic Channel.

A strong Pilot channel indicates that the mobile should be in good coverage area & the Forward Traffic channel may be weak if :

a) Forward Power control responds too slowly. (Proprietary forward power control Process may not be responding rapidly enough).

b) Min/Max Traffic channel gain is too Low. c) Correlated interferers present on Forward CDMA Traffic channel. (Improper PN offset

Planning can result in signals arriving in same search window. The mobile cannot distinguish between 2 base stations & may be unable to demodulate the desired signal).

• Low Ec/Io (Weak Pilot Channel):‐ Indicates that the system has been lost. Mobile Receive power may be high or low in this case.

Prior to loss of system, the pilot Ec/Io may begin to degrade. When The Pilot Ec/Io degrades,the forward traffic channel also degrades.

Handoff failure due to missing neighbor can also contribute to Weak pilot Ec/Io condition followed by weak traffic channel.

If we look at above probable reasons of Forward FER, the weak traffic channel in good coverage area is coming out as a major contributor against high FER spots in good coverage area.

Actions planned/Carried out to Analyze The FFER issue:‐

Following steps has been carried out for analysis of the Above problem :‐

The following 3 cell sites in outskirts of MSC 4 Mumbai, where high FFER spots are observed are identified for sample basis testing of the issue.

1) ECHGE technology (RCS 1) 2) DMK Agro (RCS 6) 3) Sameera Elctronics (RCS 53)



Step 1:‐ As a first cut approach the Power control parameters are compared within Lucent,Huawei & ZTE. These are again cross checked with IS 2000 standard for their default values & then the non matching parameters are identified for changes.

The following power control parameter changes has been tried after comparison with Lucent ,Huawei & ZTE.

1) Power control reporting frame count :‐ (PWR_REP_FRAMES)

Lucent = 7 (56 frames) ; Huawei =9 (113 frames) ; ZTE = 7 (56 frames)

The frame count over which FER is measured is small as compared to Huawei. So even if 2 bad frames are received my FER will show as 3.56% & for increasing bad frame count like 5 it will be 8.9%. Value for this parameter can be set from 0‐15. Changed to 9 after referring to standard.

2) PMRM frame reporting delay:‐ (PWR_REP_DELAY)

Lucent = 5 (20 frames) ; Huawei =1 (4 frames) ; ZTE = 1 (4 frames)

This parameter instructs MS in determining theperiod it must wait following transmission of PMRM before it starts frame counting for next PMRM.

For lucent this value looks to be set very high as compared to Huawei/ZTE which may delay the overall power control process in closed loop condition due to wait time is more. This may result in further degradation on FWD link & reflect in high FER. Changed to 1 after referring to standard.

3) Enable periodic based PMRM reporting :‐ (PWR_PERIOD_ENABLE)

Lucent = 1 (PMRM sent periodic) ; Huawei =0 ; ZTE = 0 (No periodic PMRM sent)

This parameter decides whether periodic PMRM should be sent or not. Enabled in Lucent & disabled in Huawei/ZTE.

4) Min Traffic channel Gain increased from 34 to 38 within Lucent default margin.

Post changes the variation in FFER samples observed but no major improvement. As no major improvement observed after above power Control parameter changes hence these are again reverted back to Lucent Default.

Step 2:‐ As a next step the EPBGA/MDA (Enhanced PMRM based gain adjust) algorithm which drives the forward power control parameters has been studied for parameter values. Following is the brief about EPBGA/MDA Algorithm.



Enhanced PMRM Based Gain Adjustment:- Enhanced PMRM (Power Measurement Report Message) Based Gain Adjustment (EPBGA) enhancement controls the forward power control minimum gain based on PMRM report. It is an algorithm designed to reduce drop call rate and improve power control performance. This is an enhanced algorithm to the original PBGA that was released in R22.0. When an MS is in soft handoff, individual handoff legs may have different signal quality. In addition, the forward and reverse link quality on each leg may be uncorrelated. This can be Challenging for the power control algorithm, and in severe cases, can cause the call to drop. The current solution is to use a relatively high minimum gain setting during soft handoff, which reduces the power divergence between different handoff legs, and maintains the aggregate quality of the forward handoff legs at an acceptable level (e.g. low FFER). In order to reduce the minimum gain and maintain link quality at the same time, EPBGA has been introduced. It detects poor forward link signal quality, and raises the forward link minimum gain to compensate for it. When the proper settings are activated in the system, the mobile sends a PMRM every 56 frames to report frame errors on the forward FCH. If the PMRM report indicates poor FFER and weak pilot strength, then the minimum gain value is increased by a fixed delta. Conversely, a consecutive series of good PMRM reports triggers the elevated minimum gain to drop back towards the APXRCV value.

In more detail, when the call is in soft handoff, if the PMRM from the MS indicates FFER above the FFER threshold epbga_ffer and Ec/Io below the Ec/Io threshold epbga_ecio from all handoff legs on the forward link, the BS increases minimum gain setting in use by epbga_min_step. Subsequent reports on poor forward link quality result in further minimum power adjustments. However, the total adjustment away from the APXRCV value of the minimum gain cannot exceed epbga_max_delta. During the period with an elevated minimum gain in use, PMRM reports of FFER below epbga_ffer triggers the minimum gain to recover by epbga_min_delta amount. The number of good PMRM reports necessary to trigger a minimum power down‐adjustment is specified by epbga_good_pmrm. If enough good PMRM reports are received in sequence, the minimum gain will drop to the level of the original APXRCV setting. Mute Detection Algorithm:- Mute Detection Algorithm (MDA) is used in conjunction with EPBGA. It is designed to reduce drop call rate. When the forward link signal for a MS is in a temporary deep fade, it can cause a long sequence of forward frame errors. As per IS‐2000, the MS stops its transmitter after receiving 12 consecutive error frames, in order to preserve the aggregate reverse link quality. If



the forward link fading persists over a long period of time, the call will eventually be dropped. The MDA algorithm detects inactivity in the reverse link and attempts to force the MS out of the mute state by forcing forward power to increase on all handoff legs. With a stronger forward link signal, the communication between the BS and the MS might be restored, hence saving the call. MDA is applied in all handoff states. The parameter setting is listed in Table 2‐9. Our recommendation of this setting is “on”, which is included in Aux_cell_control_1 value “32” for pre‐R29 releases. In R29 and later releases, MDA is turned on whenever EPBGA is on. A gain setting for simplex & 2 or more handoff states has been slightly increased by 1dB to ensure higher traffic channel gain.

Parameter changes are as below:‐

Min Gain (dB) - Simplex…..79) -15.00 80) -15.00 81) -20.00 (was changed to -14 for 79 & 80) - 2-way.....82) -13.00 83) -13.00 84) -15.00 (was changed to -12 for 82 & 83) - 3 or more.85) -11.00 86) -11.00 87) -13.00 (was changed to -10 for 85 & 86) Max Gain (dB) - Simplex...88) -1.00 89) -1.00 90) -1.00 (Set to 1 for 89 & to 0 for 88) - 2 or more.91) -1.00 92) -1.00 93) -1.00 (set to 1 for 92 & to 0 for 91)

The drive comparison is as below.

The improvement observed post changes but was not so considerable.

The hourly service measurements were compared for pre‐post drive test to ensure that there is no degradation in Cell KPI’s like CSSR/DCR/HO fail.



While analyzing the good FER patches only around DMK agro site (RCS 6 ‐ MSC4 Mumbai) we found that the Central Bias algorithm which also having the influence on forward power control is disabled only for this site.

Step 3:‐ So as a next step the enabling/Disabling CB algorithm is tried on identified 3 sites in MSC4 Mumbai after a study of Central Bias Algorithm. Following is the brief about Central Bias Algorithm.

Central Bias Algorithm:- Central Bias Algorithm (Feature ID 8859.11) is introduced in R28 as an enhancement to power control algorithms. It applies to Forward Fundamental Channel (F‐FCH) for both voice and data traffic. When a MS is in Soft Handoff (SHO), the reverse link quality varies among different (SHO) legs, as is the accuracy of Forward power control bits carried by the reverse links. As a result, the F‐FCH power levels of different SHO legs can diverge from each other. A leg can drift to minimum power level if the FPC bit it received has poor quality. Central Bias algorithm targets at balancing the transmit powers of each leg, and control the power drifting of handoff legs. When Central Bias is enabled, in all soft handoff legs of a call, power control will disable down biasing the forward link power, by applying equal adjustment step size for power increase and decrease. In addition, all soft legs will apply a slow adjustment of forward link power, the magnitude of which depends of the difference of its current power to a reference power value. This value is equal to rc3_pref_init or rc4_pref_init, depending on what coding scheme is used.

If the Base Station side receives a series of error frames from the MS, the power control enters “mute” state, where the soft handoff power adjustment reference power value converges from to rc3_pref_init or rc4_pref_init towards pref_mute. If there are no consecutive 2 frames received correctly on the uplink, the power control enters “freeze” state, and pref_mute is used as the reference power value. If the BS does not receive 2 good frames from the MS for another 5 seconds, the power control enters “abort” state, and uses rc3_pref_init ( or rc4_pref_init ) + abort_offset as the reference power value. The magnitude of power adjustment towards its reference value also depends on the convergence factor cb_con_factor. The smaller the factor, the bigger the adjustment. Central Bias is enabled only if all handoff legs have the algorithm enabled. If Central Bias is not enabled for all legs, while EPBGA/MDA is, then EPBGA/MDA will be applied to all handoff legs.

The CB algorithm was found disabled in DMK Agro (RCS 6) site & was enabled at ECHGE Technology (RCS 1) & Sameera Electronics (RCS 53) sites.



So To troubleshoot this it was disabled at ECHGE technology ( RCS 1) site & enabled at DMK Agro (RCS 6) site. Also the target FER settings for EPBGA algorithm was lowered to 3% from 5%. Post changes drive shown sudden improvement in FFER patches around ECHGE Technology site which has the Central Bias algorithm disabled. Some improvement is observed around Sameera Electronics site also but it was marginal.

Following are the set of CB algorithm parameters which were changed. Central Bias Algorithm Related Parameters: Central Bias Enable ............................................+300) _ (set to 3 as per lucent default) Central Bias Reference Power Level Initial Value for RC3 (dB)...+301) ___ (set to -13 as per lucent default) Central Bias Reference Power Level Initial Value for RC4 (dB)...+302) ___ (set to -12 as per lucent default) Central Bias Reference Power Level when MS is Muting (dB).......+303) ___ (set to 000 as per lucent default) Central Bias Convergence Factor.................................+304) ____ (set to 0.75 as per lucent default) Central Bias Reference Power Level Offset in ABORT State (dB)...+305) ___ Enhanced PMRM Base Gain Adjustment (EPBGA) Related Parameters: EPBGA Control Enabled........................................... 306) y EPBGA FFER Threshold (%)........................................ 307) 5 (set to 3 from 5) EPBGA EcIo Threshold (dB)....................................... 308) -1 EPBGA Min Gain Up Adjust (dB)................................... 309) 3 EPBGA Good PMRM Reports (dB).................................... 310) 2 EPBGA Max Delta Absolute (dB)................................... 311) 12 EPBGA Min Gain Delta Absolute (dB).............................. 312) 1



The above observation confirms that the Central bias algorithm is not behaving in designed way & seems to be the main reason for high FFER spots. The sudden improvement in FFER samples around 10% is observed post changes for better than 1% sample category.

When compared the previous 27 days BH stats of the RCS 1,6 & 53 the KPI’s of rcs 6 – DMK agro site where the central bias algorithm was disabled were excellent & above benchmark can be seen in below attached snapshots. This can help us to conclude on impact of disabling the central bias algorithm.

No Parameter change was done on 25th Dec so Peak in DCR is irrespective of change.



Still to leave no chance the service measurements pre‐post drive are compared on hourly basis & no major difference is observed.

Step 4:‐ To conclude more on the above findings we have disabled the central bias algorithm on all the three RCS 1, 6 & 53 in Mumbai MSC 4. The higher Min/Max gain settings of EPBGA algorithm parameters were retained during this time.

In post changes drive the Central Bias algorithm was disabled on all 3 bts ( sameera etx‐53, DMK agro‐6 & ECHGE technology‐1). Which shows 15% improvement in FFER samples for better than 1% sapmles category. In pre‐drive it was only disabled at ECHGE Technology site. Higher min/max gain settings & 5% FER set for rcs 1 & 53.All other KPIS’s like Ec/Io, MTX & MRx were also compared for Pre‐post drive & no major variation is observed in these KPI’s post disabling the CB algorithm.

Step 5:‐ Further to this the Min‐max gain setting & target FER of EPBGA algorithm were also reverted back to Lucent default values & still no major variation is observed in pre‐post KPI’s.

Min Gain (dB) - Simplex….. 79) -14.00 80) -14.00 81) -20.00 (reverted to -15 for 79 & 80) - 2-way.....82) -12.00 83) -12.00 84) -15.00 (reverted to -13 for 82 & 83) - 3 or more.85) -10.00 86) -10.00 87) -13.00 (reverted to -11 for 85 & 86) Max Gain (dB) - Simplex...88) 0.00 89) 1.00 90) -1.00 (Set to -1 for 89 & 88) - 2 or more.91) 0.00 92) 1.00 93) -1.00 (set to -1 for 92 & 91)

EPBGA FFER Threshold (%)........................................ 307) 5 (reverted to 5)



Step 5:‐ In order to have more confirmation on findings to keep enable the CB while lowering the min

gain threshold following parameters changed . But still no better improvement is observed as it was after disabling CB algorithm.

Central Bias Algorithm Related Parameters: Central Bias Enable ............................................+300) 3 Central Bias Reference Power Level Initial Value for RC3 (dB)...+301) -13 (set to -11 & -8) Central Bias Reference Power Level Initial Value for RC4 (dB)...+302) -12 (set to -10 & -7) Central Bias Reference Power Level when MS is Muting (dB).......+303) 000 Central Bias Convergence Factor.................................+304) 0.75 (set to 0.50) Central Bias Reference Power Level Offset in ABORT State (dB)...+305) ___



Summary from above testing:‐

Above observations conclude that the disabling CB showing good result for Forward FER.

The Central Bias algorithm is an enhancement to EPBGA algorithm & improves forward link capacity by lowering the Min gain to still lower values during soft handoff regions.

Till date where testing was carried out were kind of outskirt sites & hence we don’t want to make any decision in a hurry on disabling the Central Bias Algorithm.

To confirm on the findings we would like to carry out the test of disabling CB on cluster of sites in South/Central Mumbai. Below are the attached snapshots of clusters identified. We also like to see the impact on Mobile Tx power & Blocking in these area post disabling the CB algorithm.

Once we receive the go ahead from RCOM planning team we can carry out the testing on below mentioned any 1 cluster of urban/suburban area.

analysis on ffer for cdma

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