The customer has a drop call issue. The train RGV1211 call drops in Cell 219 at 07:07 on 03/05/2019.
Signaling analysis
1. The train moves from cell 218 to cell 219. At 07:07:34 on March 5th, the vehicle-mounted terminal initiates a handover from cell 218 to cell 219.
2. After receiving the Handover ACCESS message from the vehicle-mounted terminal, the target cell 219 sends a Handover Detection message to the BSC and sends a PHYINFO message to the vehicle-mounted terminal. However, the target BTS does not receive the normal message from the MS on the FACCH in the 2.1S (the SABME frame during the handover). Therefore, the target BTS reports the message with the cause value ’handover access failure’ to the BSC..
3. The BTS fails to receive the SABME frame from the MS in either of the following cases:
a) the vehicle-mounted device fails to parse the PHY INFO message delivered by the base station;
b) after the vehicle-mounted terminal has received the PHY INFO message, the SABME frame sent by the vehicle-mounted fails to be sent to the base station normally.
4. Normally, even if the vehicle-mounted terminal fails to access the cell in cell 219, the vehicle-mounted terminal needs to send a SABME frame to the source cell to access the source cell to ensure that the service is normal. However, the source cell does not receive the SABME frame from the terminal, so before the handover, the uplink quality and level of the Um interface in the source cell are normal.
5. The vehicle-mounted UE fails to access cell 219 and does not return to cell 218. Therefore, the BSS T3103 (10S) time expires and the ongoing call is released. As a result, call drop occurs.
Traffic statistics analysis
1. The traffic statistics show that cell 219 exists interference for a long time.
TRXConfiguration Analysis
1. As shown in the following table, the configuration of the current carrier is as follows:
2. The frequencies of cells 218 and 319 can be inter modulated to 971 frequencies. Therefore, during the handover from cell 218 to cell 219, the target cell is subjected to third-order intermodulation interference. As a result, the MS fails to decode the PHY INFO message or the target cell fails to parse the SABME frame. As a result, the handover fails.
218(BCCH:963) | 318(BCCH:957) |
219(BCCH:971) | 319(BCCH:955) |
BTS Log Analysis
1. No exception is found in the BTS logs.
Cell 219 interferes with the third-order intermodulation interference of cells 218 and 319. As a result, the vehicle-mounted terminal fails to access the target cell and the vehicle-mounted terminal needs to send a SABME frame to the source cell to establish a link.
However, the signaling shows that the source cell does not receive the SABME frame sent by the vehicle-mounted cell, but the air interface quality of the source cell is normal. In this case, the UE needs to analyze whether the UE sends the SABME frame link set up to the source cell
1. Modify the frequency to exclude the intra-net third-order intermodulation interference. Cell 219 interferes with the third-order intermodulation interference of cell 319 and cell 218. The strength of useful signals at sites near cell 219.
The third-order intermodulation interference affects only the location point in the middle. This problem can be triggered only when the interference signal is slightly different from the normal signal. Therefore, the impact of third-order intermodulation is limited. However, you are advised to change the frequency to remove the third-order intermodulation interference
2. On the vehicle-mounted terminal side, check whether the SABME frame link set up is sent to the source cell after the handover fails.
