Call failure caused by resource limitation

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;