CDMA Call failure caused by access and handoff collision and
Call failure caused by resource limitation
Call failure caused by access and handoff collision
Typical case:
MS starts origination call in poor coverage area, MS Rx level and pilot Ec/Io are all
low, and MS Tx power is very high.
If MS do not move after origination, the call will be successful;
If MS starts origination call in high speed, its Rx power will increase and Tx power
also increase, the pilot Ec/Io will become bad greatly and the call finally is failed;
After access failure, MS will stay in one new strong pilot;
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 access failure 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 one
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).
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.
Call failure caused by resource limitation
Typical case:
Observe MS received signals in DT test, MS receives BTS ACK message after access,
but does not receive channel assignment message, and finally call is failed;
In this period MS Rx power is in normal state, there is no large fading;
The above call failure does not often happen, usually in busy hour.
The call is normal and seldom meets above problem when traffic is low.
Analyzing:
According to call procedure principle, when BTS receives access request from MS,
firstly BTS will send ACK message and then apply for resource, if resource is available,
BTS will send channel assignment message to MS.
According to above case description, MS has received BTS ACK message but not
received channel assignment messaged.
There are two possible reasons:
1. BTS has sent channel assignment message, but MS does not receive;
2. BTS does not send channel assignment message for resource limitation.
For the first reason, commonly forward link large fading will lead MS can not receive
channel assignment message. According to DT test information, if MS received power
is normal in call procedure and there is no large fading, this kind of reason can be
excluded.
For the second reason, according to DT test, it is very possible. This kind of call failure
does not often happen, usually in busy hour. The normal reason is that system resource
is not enough in busy hour, so BTS rejects access and does not send channel
assignment message.
Note:
There is another reason for the above call failure that is illegal MS. If MS is illegal, it
will send power-up registration message after power up. Because it is illegal,
registration in MSC will be failed. When this MS starts origination call, MSC will
reject to assign resource and MS will not receive channel assignment message;
Another good way for above problem is starting signal tracing for the MS in OMC;
The reasons for resource limitation are a lot of kinds.
They are: CE resource limitation,
vocoder resource limitation, trunk circuit limitation, forward power resource limitation
and so on.
Optimization methods:
Find which kind of resource is not enough and adjust network parameters and topology
structure or network expanding.
If the limitation is physical resource limitation, like CE, vocoders, trunk circuit,
consider expanding related the resource;
If forward power resource is imitated, consider the following methods:
Wireless parameters optimization, check OMC wireless parameters configuration and
all the forward power control parameters;
Adjust network topology structure (including feed parameters and cell power), let free
cell share the traffic of busy cell;
If the busy cell has repeater, change the repeater to BTS, or change free BTS to be the
master BTS of the repeater’
Split cells and increase BTS;
Update to dual carriers;
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