The following can be reason for the dropped call
in CDMA:
In the Forward Link:
1) Improper setting for the overhead channel
power level and the traffic channel power thresholds.
2) Improper hand off parameter setting.
3) Removing the pilot from the active set by the
candidate set before the replacement.
4) Pilot is too weak to lock the code tracking
loop.
5) Distortion of the forward signal by the power
amplifier.
6) Improper control of the forward channel
power.
7) High Frame Error Rate (FER), poor pilot, or
poor RSSI.
In the
Reverse Link:
1) Improper setting of the access parameters,
improper setting of the open loop power control, or improper setting of the
reverse link Eb/No threshold value.
2) A number of signaling conditions cause the
system to revert to the system determination state of the initiated system
setting.
3) Maximized handset power and poor link
quality.
4) Unsuitable pilot during the hand off
operation.
5) Poor performance of the forward link power
control channel.
6) Link imbalance.
7) High FER, frequent change in the transmit
gain, and high transmit power.
Possible Software Problems Causing the Call
Drop/Failure:
1) An updated neighbor list was not received.
After a successful inter-MSC hard hand off, a message carrying an updated
neighbor list of the target cell must be received—if the list is not received,
a call drop occurs. In accordance with the present embodiment, this problem is
solved by applying a supplementary package to the BTS software (i.e., updating
the software) to prevent call drops due to this condition.
2) The guard timer is not properly working.
After a successful forward channel inter-MSC hard hand off, the time until the
associated reverse channel inter-MSC hard hand off occurs is restricted in
order to prevent the ping pong phenomena. In accordance with the present
embodiment, when analyzing the message, if the above-noted time is shorter than
a preset value (e.g., 5 seconds), a supplementary package is applied.
3) An improper message is received while
performing the hard hand off. For example, after receiving the “mobile station
reject order”, the call is dropped. In accordance with the present embodiment,
an improper message of this sort is indicative of a mobile station problem or a
software problem, whereupon appropriate corrective measures are taken.
4) After performing the hard hand off, the call
is not audible and then the call is dropped. In accordance with the present
embodiment, this condition is determined to be a mobile station problem,
whereupon corrective action is effectuated.
Environmental Problem Causing Call Drop/Failure
The cause of the call drop/failure may be an RF
environmental problem, e.g., excessive path loss or delay spread, which results
in low signal strength in the inter-MSC hand off area. When performing hard
hand off under the condition of a prior signal from the serving cell, the
serving cell signal gradually worsens because the mobile station is moving
towards the target cell. In this situation, if the mobile station fails to
receive the signal provided by the serving cell BTS, the hard hand off towards
the target cell cannot be performed, and because the FER increases, the call is
dropped. In accordance with the embodiment, this condition can be remedied by
adjusting the transmit power of the BTS, the antenna direction, and/or the
antenna tilt. The optimized signal strength (Ec/Io) in the service cell of the
inter-MSC hard hand off is -10˜-7 dB.
