• TCH Drop Rate – Parameter Related

The parameter settings on the BSC side and MSC side may affect the TCH call drop rate. You should check the settings of the following parameters for a cell with a high TCH call drop rate. See Case 5: Reduction of Call Drops by Optimizing Handover Parameters and Case 12: Increase in Call Drop Rate Due to Change of TR1N on the MSC Side.
1. SACCH Multi-Frames
This parameter determines whether an uplink radio link is faulty. Each time the BTS fails to decode the measurement report on the SACCH from the MS, the counter decreases by 1. Each time the BTS successfully decodes the measurement report on the SACCH, the counter increases by 2. When the value of this counter is 0, the BTS regards the radio link as faulty. In the traffic measurement, if there are many call drops (M3101A) related to radio link failure, you can infer that the radio propagation conditions are poor. In this case, you can set this parameter to a greater value.
2. Radio Link Timeout
This parameter determines whether a downlink radio link is faulty. Each time the BTS fails to decode the measurement report sent over the SACCH by the MS, the counter decreases by 1. Each time the BTS successfully decodes the measurement report sent over the SACCH, the counter increases by 2. When the value of this parameter is 0, the BTS regards the radio link as faulty. In the traffic measurement, if there are many call drops (M3101A) related to radio link failure, you can infer that the radio propagation conditions are poor. In this case, you can set this parameter to a greater value.

3. RXLEV_ACCESS_MIN
This parameter specifies the minimum receive level of an MS to access the BSS. If this parameter is set to a too small value, some MSs with low receive levels may access the network and call drops are likely to occur. You can set this parameter to a great value to reduce the TCH call drop rate. The counters such as call setup success rate and the counters related to traffic volume, however, are accordingly affected.
4. RACH Min.Access Level
This parameter determines whether an MS can access the network over the RACH. If this parameter is set to a too small value, some MSs with low signal levels may access the network and call drops are likely to occur. You can set this parameter to a great value to reduce the TCH call drop rate. The counters such as call setup success rate and paging success rate, however, are affected.
5. Min DL Power on HO Candidate Cell and Min Access Level Offset
The sum of the values of the two parameters specifies the minimum downlink receive level of a candidate neighboring cell for a handover. If this parameter is set to a too great value, some desired cells may be excluded from the candidate cells; if this parameter is set to a too small value, an unwanted cell may become the candidate cell. Both conditions may lead to the increase of call
drops.
6. Timer T3103 series
Timer T3101 series consists of T3103A, T3103C, and T8. These timers are started to wait for a handover complete message. If the lengths of the timers are set to small values, probably no message is received when timer T3103 series expires. In this case, the BSC considers that the radio link in the originating cell is faulty. Then, the BSC releases the channel in the originating cell. Thus, call drops occur. In the traffic measurement, if many call drops are related to handovers (CM331: Call Drops on Radio Interface in Handover State), you can set this parameter to a greater value. If this parameter is set to a too great value, channel resources are wasted and
TCH congestion occurs.

7. Timer T3109
This parameter specifies the period for waiting for a Release Indication message after the BSC sends a Channel Release message to the BTS. If this parameter is set to a too small value, the link may be released before the Release Indication message is received. As a result, a call drop occurs. You can set this parameter to a greater value to reduce the TCH call drop rate. It is recommended that timer T3109 be set to 1–2 seconds longer than timer Radio Link Timeout.
8. Timer T3111
This parameter specifies the interval between the time that the main signaling link is disconnected and the time that a channel is deactivated. The purpose is to reserve a period of time for repeated link disconnections. If this timer is set to a too small value, a channel may be deactivated too early. Thus, call drops increase.

9. Timers T305 and T308
Timers T305 and T308 are used on the MSC side. Timer T305 specifies the period during which the MSC monitors the on-hook procedure. Timer T308 specifies the period during which the MSC monitors the resource release procedure. You should set the two parameters when adding BSC data. Note that the modification of the data in the timer table does not take effect. If timers T305 and T308 are set to invalid or great values, the MSC clears the call a long time after the MS hangs up. After the T3103 and Radio Link
Timeout timers expire, the number of call drops is increased and thus the TCH call drop rate is significantly affected.

10. TCH Traffic Busy Threshold
If the current channel seizure ratio exceeds the value of this parameter, the BSC preferentially assigns a half-rate channel to a dualrate-enabled call. Otherwise, the BSC assigns a full-rate channel to the dualrate-enabled call. Compared with a full-rate channel, a half-rate channel has weak antiinterference capabilities. Therefore, if a large number of half-rate channels are assigned, the TCH call drop rate increases. It is recommended that this parameter should not be set to a too small value if congestion is unlikely to
occur.

11. Call Reestablishment Forbidden
This parameter specifies whether to allow call reestablishment. In case of burst interference or radio link failure due to blind areas caused by high buildings, call drops occur. In this case, MSs can initiate the call reestablishment procedure to restore communication. To reduce the TCH call drop rate, you can set this parameter to No to allow call reestablishment. In certain conditions, allowing call reestablishment greatly reduces the TCH call drop rate. Call reestablishment lasts for a long time, and therefore some subscribers cannot wait and hang up. This affects user experience.

12. Parameters related to edge handover

When the receive level drops greatly, an edge handover cannot be performed in time in any of the following conditions: The parameter Edge HO UL RX_LEV Threshold or Edge HO DL RX_LEV Threshold is set to a small value; the parameter Inter-cell HO Hysteresis is set to a great value; the parameters Edge HO Watch Time and Edge HO AdjCell Watch Time
are set to great values; the parameters Edge HO Valid Time and Edge HO AdjCell Valid Time are set to great values. As a result, a call drop occurs. To reduce the TCH call drop rate, you can appropriately set these parameters so that edge handovers can be performed in time to avoid call drops.

13. Parameters related to BQ handover
When the signal quality deteriorates, a BQ handover cannot be performed in time in any of the following conditions: The parameters
ULQuaLimitAMRFR, ULQuaLimitAMRHR, UL Qual. Threshold, DLQuaLimitAMRFR, DLQuaLimitAMRHR, and DL Qual. Threshold are
set to great values; the parameter BQ HO Margin is set to a small value; the parameter Inter-cell HO Hysteresis is set to a great value. As a result, call drops occur. To reduce the TCH call drop rate, you should appropriately set these parameters so that BQ handovers can be performed in time to avoid call drops.

14. Parameters related to interference handover
If the parameters RXQUAL1 to RXQUAL12 are set to great values or if the RXLEVOff parameter is set to a great value, strong interference may occur. In this case, if interference handovers are not performed in time, call drops occur. To reduce the TCH call drop rate, you can appropriately set these parameters so that interference handovers can be performed in time to avoid call drops. If the parameters RXQUAL1 to RXQUAL12 are set to small values, the number of handovers due to other causes increases greatly, thus affecting the handover success rate.

15. Parameters related to concentric cell handover
A call at the edge of the overlaid subcell cannot be handed over to the underlaid subcell in any of the following conditions: In the case of a normal concentric cell, the parameters RX_LEV Threshold and RX_LEV Hysteresis are set to great values; in the case of an enhanced concentric cell, the parameter OtoU HO Received Level Threshold is set to a great value. As a result, a call drop is likely to occur. If the Call Drop Ratio on TCH on the TRX in the OverLaid Subcell (RM330a) is high, you can appropriately set these parameters so that calls at the edge of the overlaid subcell can be handed over to the underlaid subcell in time. When a call in the underlaid subcell has interference, the call cannot be handed over to the overlaid subcell if the RX_QUAL for UO HO Allowed parameter is set to Yes and the RX_QUAL Threshold parameter is set to a great value. Thus, a call drop occurs. If the Call Drop Ratio on TCH on the TRX in the Underlaid Subcell (RM330) is high, you can set these parameters properly so that the call can be handed over to the overlaid
subcell at the earliest.

16. Parameters related to power control
If the power control level and quality threshold are set to small values, call drops are likely to occur because of low signal level or bad voice quality.

17. T200 and N200
If the parameters T200 FACCH/F, T200 FACCH/H, N200 of FACCH/Full rate, and N200 of FACCH/Half rate are set to small values, data links are disconnected too early. Thus, all drops are likely to occur. If call drops occur because of T200 expiry, you can increase the values of T200 and N200 properly.

18. Neighboring cell relations
If the neighboring cells configured in the BA2 table are incomplete, call drops are likely to occur in the case of no suitable neighboring cell for handover and progressive deterioration in the voice quality. Neighboring cell relations should be configured completely on the basis of the drive test data and electronic map (for example, Nastar) to minimize the call drops due to no available neighboring cells.

19. MAIO
If frequency hopping (FH) is applied in a cell and the MAIO is set inappropriately (for example, different TRXs serving the same cell have the same MAIO), frequency collision may occur during FH. Thus, the TCH call drop rate increases.

20. Disconnect Handover Protect Timer
This parameter is a software parameter of the BSC. After receiving a DISCONNECT message from an MS, the BSC cannot hand over the MS within the period specified by this parameter. Therefore, the following case can be avoided: After being handed over to the target cell, the MS cannot be put on hook because it does not receive a release acknowledgement message. You are advised to set this parameter properly.

21. TR1N
This parameter should be set on the MSC side. It is used to avoid the retransmission of short messages. When this parameter is set to a too great value, the MSC does not send a CLEAR CMD message if the MS receives a short message during link disconnection. As a result, the MS sends the BTS a DISC message to disconnect layer 2 connection. After receiving the DISC message, the BTS sends a REL_IND message to the BSC. Then, the BSC sends a CLEAR REQ message to the MSC and the number of call drops is incremented by one.

22. Software Parameter 13 and MAX TA
When the parameter Software Parameter 13 is enabled and the parameter MAX TA is set to a too small value, the channel is released when the TA of a call exceeds the MAX TA. In this case, call drops occur. It is recommended that the parameter Software Parameter 13 should not be enabled.

23. Directly Magnifier Site Flag
If a BTS is installed with repeaters, the handover between repeaters can only be asynchronous because the distance between repeaters is long. If synchronous handovers are performed, the handovers may fail and thus many call drops occur. Therefore, when a BTS is installed with repeaters, the parameter Directly Magnifier Site Flag should be set to Yes to avoid asynchronous handovers between cells under the same BTS.