Drive Test Application in Call Drop Analyzing

1. BTS bad frame
when FER reaches to 90% in 100 frames and this happens 3 times consecutively, BTS will release the links.

Drive Test Application in Call Drop Analyzing
Call drop is the interruption of call and it is caused by BTS or MS releasing traffic channel without user permission.
Call drop mechanism
CDMA system is a closed-loop system, the closed-loop signal link is very necessary between BTS and MS in data or voice transmission. Some important process, like power control and handoff, all need one closed-loop link. If this link is broken for any reason, MS will lose control and need initialization again and go back to idle state. When closed-loop signal link is broken, call drop mechanism will be started. Closed-loop signal link includes forward link and reverse link, any one link is broken can cause call drop.

1 MS drop mechanism

1. MS bad frame
If the mobile station receives N2m consecutive bad frames on the Forward Traffic Channel, it shall disable its transmitter. Thereafter, if the mobile station receives N3m consecutive good frames, the mobile station should re-enable its transmitter.
(Extracted from 95 standard 6.4.4) In 95 standards, N2m is defined as constant 12; N3m is defined as constant 2.

2. MS fading timer
The mobile station shall establish a Forward Traffic Channel fade timer. The timer shall be enabled when the mobile station first enables its transmitter when in the Traffic Channel Initialization Sub state of the Mobile Station Control on the Traffic Channel State. The fade timer shall be reset for T5m seconds whenever N3m consecutive good frames are received on the Forward Traffic Channel. If the timer expires, the mobile station shall disable its transmitter and declare a loss of the Forward Traffic Channel. (Extracted from 95 standard 6.4.4) In 95 standards, N3m is defined as constant 2; T5m is defined as constant 5s.

3. MS acknowledgment failure
If the mobile station has not received an acknowledgement within T1m seconds after transmitting the message, the mobile station shall retransmit the message. (Extracted from 95 standard 6.6.4.1.3.1.1) The mobile has N1m attempts to transmit a message that required acknowledgment. If the mobile does not receive the acknowledgement T1m seconds after the N1m time, the mobile declares an acknowledgment failure. In 95 standards, N1m is defined as constant 3; T1m is defined as constant 0.4s.

2. BTS acknowledgement
There 9 times for BTS acknowledgement, and the waiting time between two acknowledgements is 0.4s.

Integrated analyzing
Call drop ratio can be used to evaluate network integrated performance. Call drop ratio improvement can decrease subscribers’ complaints and increase their satisfaction. For the large scale commercial network, call drop ratio observed from background OMC is more accurate and impersonal; for unloaded or light-load network, it can be got by DT test. In DT test, the call mode adopts “sequence call test”, call drop ratio is defined as: call drop number/total successful call number; if adopts “long call test”, total successful call number is total calling time (s)/90. For urban area, call drop ratio is less than 0.8% means network performance is good; for suburb or roads coverage, this index can be a litter low.