Internal Use OnlyGSM900 High Call Drop IssueOverview The issue happened after swapping ER/// 2G sites .The ZTE optimization engineer analysed the KPI and found there are too many call drops which are caused by uplink radio link failure. Normally the two reason could cause uplink radio failure 1, GSM Internal same and adjacent frequency interference.2,GSM external interference. For the swapping sites , all the TRX ARFCN are fixed which cant cause the big difference for GSM Internal same and adjacent frequency interference .So the major problem is GSM external interference.AnalysisUplink interference scan

ET E-GSM working uplink frequency band is from 880MHZ to 915MHZ.The scanning result is that from 880-885MHz,the power is -92dBm.Obviously,there is high interference for E-GSM uplink.Radio Interference SourceBased on the given CDMA frequency points, the interference mainly comes from the following two sources. Comes from CDMA800 public base stations. Frequency point: 37; 78; 119. The maximum TX edge frequency: 870 + 0.03*119 + 1.228/2 = 874.184MHz; Comes from military CDMA base stations. Frequency point: 160; 201; 242. The maximum TX edge frequency: 870 + 0.03*242 + 1.228/2 = 878.4884MHz.

The TX bands of these CDMA base stations are adjacent to the RX bands of GSM base stations. As a result, when CDMA base stations transmit, they will cause block interference to the RX of GSM base stationsInterference influence

The CDMA and GSM are working on the adjacent frequency .Every receiver work on the special range of frequency band .When the total power of the range exceed the receiver MAX accepted power ,the receiver will congest .On the other hand ,if there is no hardware filter on the CDMA site ,the scattering frequency will impact the GSM uplink quality.The further analysis shows the bandwidth of the CDMA interference collected was merely 3M and the maximum peak-to-average ratio was 11.46dB. The interfering signals of high peak-to-average ratio raised the noise floor, which caused the RX block inside the devices.

Parameters Mapping& Hardware checking As above analysis ,the external interference raise the call drop times .Before swapping ,the interference must exist .But the same interference make the different influence.Parameters Mapping If the access range changes small ,the receiver will have less interference than before. Some parameters will control the access range, like TRX power ,RxlevMin and so on.

Hardware checkingFor the new swapping sites, they use the new RSU82 S9000 which is higher sensitivity than before. Sensitivity is a very important indicator for the receiver equipment .It means for the target BER ,the receiver could use the Min power .The high sensitivity could decode the weaker signal ,improve the service quality and reduce the call drop times. But when there is external interference ,the sensitivity also have more influence than before and the higher call drop rate than before.

Although changing the high sensitivity S9000 RSU to GUL9016 RSU ,the call drop rate is still higher than before .When use the S9000+hot patch ,the TRX power is 20W and the drop call rate is same as before swapping.(before swapping ,the TRX power is 5W) Swapping solutionThere are two reasons which cause the high call drop rate. No Parameters mapping External Interference & High sensitivity RSU GUL9016ZTE have provided the hot patch for decreasing the interference power and enhance the KPI .After running the hot patch and mapping parameters ,the drop call rate and traffic is normal. Problem cell 775

Problem cell 3425

Problem cell 3426

Problem cell 3427

Problem cell 22635

Problem cell 22636

Problem cell 22637

The KPIs are same as before swapping.Interference Solution We suggest ET contact with the NTRA to check the CDMA filter. For a short time, run the hot patch in the interference area .In the clear area ,dont run the hot patch. The hot patch only could avoid the low and middle interference. For the final solution, ordering the inner hardware filter avoid the low, middle ,high interference .In the clear area, use the normal RSU.

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