Sunday, 14 February 2016

LTE FDD Network Planning & Optimization System-Detach Failure Due to a Wrong TAC

Incident Description (Incident Phenomena)
During an FDD LTE single-site coverage test, the test engineer reported that the CNT could detect PCI signals of three sectors normally when all the three sectors of a base station existed but could not detect PCI signals of the third sector when only the third sector existed. 

 
Problem Cause Analysis
 
Because the problem was special, we first checked the radio parameter configurations of this base station and found that the TAC of the third sector was inconsistent with those of the other two sectors. We suspected that the TAC of the third sector was wrongly configured and modified it. After that, the service of the third sector became normal.
As described in the following part, we tried to analyze the CXT test data to locate the cause of the problem.
1. Analyzing the CXT test data
The three sectors corresponded to PCIs 46, 47, and 3 respectively and the normal TAC was 13312. The test engineer found that when all the three sectors existed, the terminal could detect PCI signals of the third sector and use the FTP downloading service normally, as shown in the following figure.
 
 When only the third sector PCI 3 existed, the TAC of this sector was 48 (0x0030), as shown in the following figure.
 
 Then a TAU REJ message appeared, as shown in the following figure.
 
  At this time, the terminal could not detach normally, that is, it could not use the data service.
 
2. Conducting a test and analyzing the data
To verify that the cause was a wrong TAC configuration, we conducted a simulative test on a testing base station. PCIs of the three sectors of this base station were respectively 0, 169, and 326; the normal TAC was 13312.
The test result was as shown in the following figure.
Then we set the TAC of the first sector to 13568 (another normal TAC), and the TAC was configured in the CN normally. The test result was as shown in the following figure.
The terminal could use the data service normally, as shown in the following figure.
 Then we set the TAC of the first sector to 5, as shown in the following figure.
 


The above analysis showed that because the TAC was wrongly configured, the terminal could not use the data service normally.
 
 Solution
 According to the above analysis, if the TAC is wrongly configured (the TAC is not configured in the CN), the terminal can still detect PCI signals of the cell, but it cannot use the data service under this cell, that is, it cannot access the Internet.
For this kind of problem, it is suggested to check the radio parameters first to keep the radio parameters of the base station consistent.

Budi Prasetyo

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