How to Reduce Call Drops in a GSM Network

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How to reduce Call Drops In a GSM Network

posted Oct 13, 2009, 12:53 AM by Gulzar Singh [ updated May 9, 2011, 1:13 PM ]

Meaning

The TCH call drop rate refers to the ratio of call drops to successful TCH seizures after the BSCsuccessfully assigns TCHs to MSs. The TCH call drop rate can be measured from the following

aspects:

TCH call drop rate (including handover)

TCH call drop rate (excluding handover)

The TCH call drop rate, one of the most significant KPIs for telecom operators, is related to

retainability. It indicates the probability of call drops due to various reasons after MSs accessTCHs. A too high TCH call drop rate adversely affects the user's experience.

Recommended Formulas

TCH call drop rate (including handover) = Number of call drops on TCH/(Number of successful

TCH seizures (signaling channel) + Number of successful TCH seizures (TCH) + Number ofsuccessful TCH Seizures in TCH handovers (TCH)) x 100%

TCH call drop rate (excluding handover) = Number of call drops on TCH/Number of successful

TCH seizures (TCH) x 100%

Factors That Affect the TCH Call Drop

RateAccording to user complaints and network optimization experience, the major factorsthat affect the TCH call drop rate are as follows:

Hardware failure


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Transmission problem

Version upgrade

Parameter setting

Intra-network and inter-network interference

Coverage problem

Antenna system problem

Imbalance between uplink and downlink

Repeater problem

1.1 Hardware Failure

When a TRX or a combiner is faulty, seizing the TCH becomes difficult, and thus theTCH call drop rate increases.

1.2 Transmission Problem

The TCH call drop rate increases in the following conditions:

The transmission quality on the A or Abis interface is poor for various reasons.

Transmission links are unstable.

1.3 Version Upgrade

After the BTS version or BSC version is upgraded, the BTS version may beincompatible with the BSC version, and the parameters and algorithms in the newversion may be changed. In this case, the TCH call drop rate increases.

1.4 Parameter Setting

The settings of some parameters on the BSC and MSC sides may affect the TCH calldrop rate. If the following situations occur, the TCH call drop rate may increase:

1. The parameters SACCH Multi-Frames and Radio Link Timeout are set to too smallvalues.


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2. The parameterRXLEV_ACCESS_MIN is set to a too small value.

3. The parameterRACH Min.Access Level is set to a too small value.

4. The parameters Min DL Power on HO Candidate Cell and Min Access Level Offsetare

inappropriately set.

5. The length of timer T3103 (this timer is set to wait for a Handover Completemessage) is set to a too small value.

6. The length of timer T3109 (this timer is set to wait for a Release Indicationmessage) is set to a too small value.

7. The length of timer T3111 (this timer specifies the connection release delay) is setto a too small value.

8. The length of timer T305/T308 is set to an invalid or too great value.

9. The parameterTCH Traffic Busy Thresholdis set to a too small value.

10. The parameterCall Reestablishment Forbidden is set to Yes.

11. The parameters related to edge handover are inappropriately set.

12. The parameters related to BQ handover are inappropriately set.

13. The parameters related to interference handover are inappropriately set.

14. The parameters related to concentric cell handover are inappropriately set.

15. The parameters related to power control are inappropriately set.

16. T200 and N200 are set to too small values.

17. Some neighboring cell relations are not configured.

18. The parameterMAIO is inappropriately set.

19. The parameterDisconnect Handover Protect Timer is set to a too small value.

20. The parameterTR1N is set to a too small value.

21. The parameters Software Parameter 13 and MAX TA are set

to too small values.


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22. If a repeater is used, the parameterDirectly Magnifier Site Flag is set to No.

1.5 Intra-Network and Inter-Network Interference

If inter-network interference and repeater interference exist, or if severe intra-networkinterference occurs because of tight frequency reuse, call drops may occur on TCHsbecause of poor QoS. This adversely affects the TCH call drop rate.

The following types of interference may occur:

1. Inter-network interference from scramblers or privately installed antennas

2. Interference from the CDMA network of China Unicom

3. Repeater interference

4. Inter-modulation interference from BTSs

5. Intra-network co-channel and adjacent-channel interference

1.6 Coverage Problem

The following coverage problems may affect the TCH call drop rate.

1. Discontinuous coverage (blind areas)

The voice quality at the edge of an isolated BTS is poor and calls cannotbe handed over to other cells. In this case, call drops may occur.

In complex terrains such as mountainous regions, the signals are blocked

and thus the transmission is discontinuous, leading to call drops.

2. Poor indoor coverage

Densely distributed buildings and thick walls cause great attenuation and low

indoor signal level, which causes call drops.

3. Cross coverage (isolated BTS)

The serving cell causes cross coverage due to various reasons (such asexcess power). An MS cannot be handed over to another cell due to nosuitable neighboring cells. In this case, the signal level becomes low andthe voice quality of the MS deteriorates. Thus, call drops occur.


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4. Insufficient coverage

If the signal from an antenna is blocked or the BCCH TRX is faulty, call drops

may occur because of discontinuous coverage.

1.7 Antenna System Problem

The following antenna system problems may affect the TCH call drop rate

1. If the transmit antennas of two cells are improperly connected, the uplink signallevel in each cell is much lower than the downlink signal level in the cell. Therefore, calldrops are likely to occur at places far away from the BTS.

2. If a directional cell has main and diversity antennas, the BCCH and SDCCH of thecell may be transmitted from different antennas. If the two antennas have different pitchangles or azimuths, the coverage areas of the two antennas are different. In this case,

the following result may occur: An MS can receive the BCCH signals from one antenna;when a call is made, the MS cannot seize the SDCCH transmitted by the other antennaand thus a call drop occurs.

3. If the feeder is damaged, water leaks in the feeder, or the feeder and the connectorare not securely connected, both the transmit power and receiver sensitivity of theantenna are reduced. Thus, call drops may occur.

1.8 Imbalance Between Uplink and Downlink

The difference between the uplink signal level and the downlink signal level may begreat in the following conditions:

The transmit power of the BTS is high.

The tower mounted amplifier (TMA) or BTS amplifier does notwork properly.

The antenna and the connector are not securely connected.

As a result, call drops may occur at the edge of the BTS coverage area.

1.9 Repeater Problem

If a cell is installed with a repeater, BTS coverage problems may occur in the case thatthe repeater is faulty or that the uplink and downlink gain is inappropriately set.Therefore, the TCH call drop rate increases.

If a wide-frequency repeater is used and the gain is set to a great value, strong


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interference may be caused. As a result, the network quality is adversely affected andthe TCH call drop rate increases.

Documents

How to Reduce Call Drops in a GSM Network