Call Drop reason analyzing and Call drop Caused by Access and Handoff Collision
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.
Call drop caused by access and handoff collision
Typical case:
MS origination call may be failed in poor coverage area in DT test, or call drop will
happen very soon after successful origination. MS Rx level and pilot Ec/Io are all low, and MS Tx power is very high.
As MS moves, MS Rx power will become larger and larger, but pilot Ec/Io is too bad
to satisfy the demodulation requirement;
MS initializes again after call drop and stays in one new strong pilot;
If MS originates successfully in good coverage area and passes the same DT rote, call
drop will not happen;
Analyzing:
If system can not support access handoff (access handoff also need MS to support),
then during MS access procedure, MS can only start handoff after finish access.
The above case is belonging to call drop caused by access and handoff collision. If one
MS starts origination in the edge of coverage area, because this is near to another cell,
it is possible to meet handoff. But our system does not support access handoff, access
and handoff will have collision and access has priority in system. Because access need
some time, so MS Rx power will become higher and higher, but pilot Ec/Io will
become lower and lower. At this time, the objective handoff pilot is becoming strong
interference, when current pilot Ec/Io is lower enough; the forward link will become
very bad and can not be demodulated successfully. The access will be failed. After
access failure, MS will stay in new pilot (It is the objective handoff pilot).
If MS originates successfully in good coverage area and passes the same DT rote, call
drop will not happen, because at this time MS can carry the normal handoff procedure.
Note:
Access handoff can be supported after 5.4 version;
For terminals, 1X handset can support, but 95 handset can not.
Optimization method:
If system and terminals can support access handoff, this kind of problem will not
happen;
Adjust network structure and soft handoff area. Extend soft handoff area in the area
where the above problems are heavy, then MS will firstly handoff to another cell before
access and MS will have enough time to finish access.
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