Call drop reason analyzing

1 Equipments problem
The method for equipments problem can refer to the section in call failure analyzing. TRX, CE, vocoders and transmission links are the key points which should be cared for.

2 Call drop caused by overstepping the coverage
Typical case:
MS Rx power is about -100dBm or less;
MS Tx power tends to maximum 23dBm;
The strongest pilot Ec/Io < -15dB or less;
MS Tx_Adj maintains in normal range: 0~-10dB; MS goes into system searching mode after call drop, and can not find system or system signal is very weak and is very easy to be lost.

Analyzing:
In the edge of coverage area, for the forward and reverse links are all very bad, the call drop is one normal phenomenon
When pilot Ec/Io decreases to some extent, the forward link quality will be bad greatly and can not be demodulated well, F-FER will increase very fast. If MS oversteps coverage area too long (more than 5s), MS fading timer expires in 5s, MS will initialize again, that is call drop. MS will go into system searching mode after initialization, because MS has overstepped coverage area, it can not find service system or system signal is very weak and is very easy to be lost.

Note:
The above time is 5 seconds, in fact MS oversteps the coverage area in shorter time (less than 5s) also can cause call drop. In this case usually the reason is BTS call drop mechanism is faster than MS fading timer. When MS oversteps the coverage area, reverse link is also weak and R-FER is also high, then BTS call drop mechanism will be triggered. BTS will release forward link in short time (<5s), when reverse link is too bad. Even if MS returns to coverage area at this time, the call drop will happen also. Because the forward link does not exist, so the call drop must happen, though the pilot has been resumed.

Optimization method:
The ultimate way for this kind of problem is add new BTS or repeater in blind coverage area or poor coverage area.
If adding new BTS is not possible, other methods also can be used to improve coverage, like increase antenna height, select large gain antenna, and adjust antenna azimuth and down tilt. But these methods can not solve problems ultimately, and it should be very careful when change these parameters.