Summary of 3G 2G Handover Problems

I. Introduction to D Network

D, a key account of Huawei, is the top three telecom operator in country P. Huawei provides GSM1800 and UMTS2100 networks in Great Manila (also called NCR), the capital of country P. NCR consists of 17 cities and covers three CBD areas: Mania, Makati, and Ortigas. NCR GSM network was provided by Vendor_A from PH1 to PH4. From the beginning of PH5, Huawei swapped all Vendor_A equipment (from the access side to the core network) in NCR. Presently, PH7 GSM project is in progress, and PH8 project is to be started. More than 900 macro BTSs exist in NCR GSM network, and the indoor distributed system consists of more than 200 BTSs. The BTSs implement seamless coverage basically.

The 3G network of D is provided to develop the broadband service and solve the problem with 2G speech service congestion. The project is performed in two phases: PH1 3G network consists of 189 NodeBs, covering CBDs in NCR; PH2 3G network consists of 759 NodeBs, covering the whole NCR. Presently, 189 NodeBs in PH1 are on air, and the survey in PH2 has been complete.

At the CS access side, 3G NodeBs and 2G BTSs have a co-site. 25 2G BSCs are configured, and home BSCs of BTSs are still being tuned (also called re-homing). In PH1 3G network, only one RNC and two subracks are configured. Five 2G MSCs are configured: MSC1H, MSC2H, MSC4H, MSC6H, and MSC7H. 3G core network and 2G MSC3H share a core network.

At the PS side, 2G GPRS network consists of 10 PCUs. 3G and 2G GPRS networks share a system of SGSN+GGSN, and the networking is relatively simple.

II. Comparison of KPIs

Before optimization:

The following table lists the KPIs in 3G 2G CS and PS handover performed in March 2008.

Items

Week 14

Week 15

Week 16

Week 17

3G 2G CS Handover Success Rate

84.02%

85.76%

87.50%

83.34%

3G 2G PS Handover Success Rate

50%

23.08%

38.46%

42.79%

The following table lists the 3G 2G CS Handover Pprepare Success Rates.

Items

Week 14

Week 15

Week 16

Week 17

3G 2G CS Handover Prepare Success Rate

5.6%

7.8%

6.8%

7.6%

After optimization:

The following table lists the KPIs in 3G 2G CS and PS handover performed in October 2008.

Items

Week 37

Week 38

Week 39

Week 40

3G 2G CS Handover Success Rate

92.74%

93.82%

93.09%

94.07%

3G 2G PS Handover Success Rate

80.05%

77.76%

81.93%

78.94%

The following table lists the 3G 2G CS Handover Prepare Success Rate.

Items

Week 37

Week 38

Week 39

Week 40

3G 2G CS Handover Prepare Success Rate

76%

75%

78%

76%

III. Analysis and Handling

1. Find improper data configuration of existing neighbor cells: When 3G 2G handover failed, we first suspected data configuration is improper. As listed in the preceding table, the 3G 2G CS handover prepare success rate is very low, which makes us confirm improper data configuration. At that time, plenty of re-homing work were performed in the 2G network, and the data configuration of neighbor cells loaded on 3G RNCs was planned before months. We obtained the latest 2G engineering parameter table from the 2G RNP. After checking engineering parameters, we found improper LAC and BCCH in some cells. Then we requested the teammates in the core network to check the configuration of neighbor cells in the 3G core network. The configuration of neighbor cells is simple in Huawei core network, and only adjacent LACs are configured. As a result, the data configuration in the core network is proper. After modifying the data of neighbor cells on the RNC, we found the 3G 2G CS handover preparation success rate was still very low (less than 10%) and the handover success rate was still about 85%.

2. Find improper license configuration in the core network: To find a cell that was requested most for handover and failed to complete handover, we performed site test and coordinated RNC and core network engineers for signaling tracing. After the RNC sends a handover request to the MSC3H, the MSC3H sends the handover request to the MSC4H. The MSC4H denies the request. After multiple attempts are performed, The MSC4H denies all attempts. After checking the data configuration in the core network, we found no problem or alarm. After checking licenses on the MSC4H, we found the license for 2G/2G handover was enabled, but the license for 3G 2G handover was disabled. That is, the MSC4H does not support 3G 2G handover. We then checked other MSCs, and found only the MSC4H had no license for 3G 2G handover. After the license is enabled, the 3G 2G CS handover preparation success rate rises to 75%, and the 3G 2G CS handover success rate rises to 90%. The 3G 2G PS handover success rate, however, is still less than 60%.

3. Find missing of neighbor cells and improper cell type configuration: After solving the problem with 3G 2G CS handover, we focused the problem on PS handover. We viewed GSM cells configured on all RNCs, and found some GSM cells and most GSM&GPRS cells. To eleminate the effect of the setting, we modified the type of all 2G cells to GSM&GPRS in the script. After modification, we found 3G 2G PS reselection was still not improved. We then checked the configuration of all 2G neighbor cells, and found only four or five 2G neighbor cells were configured each for most 3G cells. The 2G neighbor cells are much insufficient in CBDs. Therefore, we configured 10 to 15 2G neighbor cells for each 3G cells according to the configuration of 2G neighbor cells for 2G cells. The 3G 2G handover prepare success rate and the 3G 2G handover success rate do not rise explicitly. A new problem, however, occurs: With the numerous GSM cells appear, the 3G 2G handover prepare success rate falls from 75% to a value less than 5%. The handover success rate, however, remains 93%.

4. Modify RelocIuRelCmdTmr from 20s to 36s: We analyzed 3G 2G PS handover data on the RNCs, and found the handover times was very small (about 60 times). We then attempted to modify the 2G handover threshold, for example, modify the 2G RSSI from 95 dBm to 70 dBm, and found the 3G 2G PS handover success rate was still less than 60%. After analysis, we found two causes: IRATHO.FailOutPSUTRAN.PhyChFail and VS.IRATHO.FailOutPSUTRAN.Other. Generally, IRATHO.FailOutPSUTRAN.PhyChFail is derived from 2G congestion or improper 2G data configuration for 3G. For VS.IRATHO.FailOutPSUTRAN.Other, the headquarters suggests tuning the RelocIuRelCmdTmr timer, for example, modify the default value from 20s to 36s. After tuning the timer, we found the number VS.IRATHO.FailOutPSUTRAN. Other causes fell, but the 3G 2G PS handover success rate was not much raised.

5. Export the configuration script from the 2G BSC and check 2G data configuration for 3G: Generally, we checked 2G data configuration for 3G in the engineering parameter table obtained from the 2G RNP. To ensure the accuracy of 2G data configured for 3G, we exported the configuration script from 25 2G BSCs and filtered all the information about 2G cells, such as LAC, BCCH, NCC, and BCC. After analyzing the information, we found the data configuration of about 60 cells was improper. Against the latest 2G data, we found the 2G engineering parameter table was improper. At that time, re-homing was much performed in the 2G network, and data update was not accurate. After modifying the data configuration against the latest data, we found CS handover and PS handover were little improved.

6. Check the TBF setup success rate in the 2G GPRS network: In 3G 2G PS reslection, Atter a Change Request Order message is sent in the 3G network, all 3G flows are complete. After that, the TBF setup process runs in the 2G GPRS network. Therefore, the GPRS setup success rate also affects the 3G 2G PS handover success rate. We obtained the traffic statistics data of 2G GPRS in one month and analyzed the cells related to 3G 2G handover. We found the TBF setup success rate in 23 GPRS cells was 0 and the TBF setup success rate in 113 cells was less than 50%. The TBF setup success rate in most cells is greater than 80%, but the 3G 2G PS handover success rate in these cells is also very low. Obviously, the GPRS TBF setup success rate affects the 3G 2G PS handover success rate. Such a low handover success rate is not caused by only the TBF setup success rate. During checking the data configuration for 2G GPRS, we discussed the configuration problem with the 2G team and found 2G PDTCHs were insufficient, which caused congestion. In addition, we found PCU versions were defective, which caused failed PDCH assignment.

7. Tune the PS inter-RAT handover 2D/2F threshod, the PS inter-RAT handover decision threshold, and the RelocIuRelCmdTmr timer: In the V29 baseline, the inter-RAT handover 2D/2F threshold is set as follows: RSCP: 110/107 dBm; ECNO: 15/13 dB. As a result, when 3G initiates a handover request to 2G, the quality of 3G signals is very poor, which causes handover failure. We modified RSCP to 95/92 dBm and ECNO to 12/10 dB to guarantee the quality of 3G signals in the compressed mode. The PS inter-RAT handover decision threshold is originally set to 16, that is, 3G can be handed over to 2G when the 2G RSSI reaches 95 dBm. For 2G, however, 95 dBm brings very poor quality of signals. After checking 2G DT data in the NCR, we found most coverage levels ranged from 30 dBm to 75 dBm. To ensure good quality of 2G signals when 3G is handed over to 2G, we modified the PS inter-RAT handover threshod to 70 dBm. In addition, we set the RelocIuRelCmdTmr value from 36s to 60s (maximum value). After two days, we found the PS inter-RAT handover success rate rose from 40% to 70%.

8. Tune the value of the T309 timer: In PS handover, After receiving a Cell Change Order From UTRAN message, the UE starts the T309 timer. Before expiration of the T309 timer, If the UE does not access the 2G network and returns to the orignal 3G channel, the UE sends a CCO Failure message to the RNC. After receiving the message, the RNC records a channel failure, which causes 3G 2G handover failure. We modified the value of the T309 timer from 5s to 8s to ensure that the UE accesses the 2G network.

9. Tune the inter-RAT handover failure penalty timer length: To alleviate 2G congestion and control the number of 3G to 2G handovers, we modified the default value from 30s to 60s. If 3G 2G handover failure is caused by 2G overload, the UE does not initiate any 3G 2G handover request in the tuned duration. After several days, we found the PS handover success rate rose to 80%. Now, the problem with PS inter-RAT handover is solved.

10. Analyze 3G 2G handover data in the core network: In 3G 2G handover, five MSCs are involved. After analyzing handover data, we found 3G 2G handover was proper in the 3G MSC3H, and the data handover success rate in the core network was 75%. After analyzing the core network data in one week, we found the number of MSC3H handover requests to MSC6H was 478 (all failed), and the number was 80% of the total number of handover requests. As a result, we located the problem on the MSC6H. After discussing the data configuration with the 2G team at a meeting, we found the 2G handover success rate on the MSC6H was also low (less than 100%). We requested the teammates of the core network to check data, and found LAC configuration was improper. After correction, 2G handover succeeded, and 3G handover still failed. As shown in the captured signaling, the MSC3H sends a handover request to the MSC6H and the MSC6H sends the handover request to the 2G BSC, but the returned acknowledgement message indicates a protocol error. We then checked whether the 2G BSC supported 3G 2G handover, and sent the BAM files of the MSC1H and MSC6H to the headquarters for analysis. The headquarters returned a message, indicating proper data configuration on MSCs.

11. Add the LAC information about five 2G MSCs to 3G Core MSC3H: During the whole tuning process, the 3G 2G CS handover prepare success rate is always less than 10%. To ensure proper data configuration of neighbor cells at the core network side, we added the LAC information (that is, neighbor information) about all five 2G MSCs to 3G Core MSC3H. After adding the information, we found the CS handover prepare success rate was not much improved.

12. Modify the type of 3G cells on 3G MSC3H from SAI to CGI: We checked all parameter settings and data configuration on the 2G MSCs related to handover, and found no problem. The 2G BSCs consist of some new BSCs and old BSCs. We attempted to modify the type of 3G cells on 3G MSC3H from SAI to CGI, and in the next day found the handover success rate in the core network was 75% (normal level). Later, we checked the handover prepare success rate related to the RNC, and found the value rose to 76%.