Call drop caused by reverse interference and Call drop caused by F-TCH power limitation
Call drop caused by reverse interference
Typical case:
As MS moves, its Rx power and pilot Ec/Io keeps in normal level;
As MS moves, MS Tx power increases and tends to the maximum 23dBm, until call
drop;
As MS moves, MS Tx_Adj increases and it will be positive value, until call drop;
MS re-initializes after call drop and will stay in previous pilot. There is signal in MS,
but origination call is difficult or call drop is very easy to happen after successful
origination.
Analyzing:
In the above case, MS Rx power and pilot Ec/Io are all good, and this means the
forward link is good. But in MS origination procedure MS is increasing transmitting
power until maximum, and this means the reverse link is bad. That is there is reverse
interference. Tx_Adj>0 also means forward is better than reverse.
As MS moves continuously, MS R-FER will increase higher and higher. BTS can not
receive the signal from MS correctly and BTS call drop mechanism is triggered, and
then call drop will happen.
Note:
Call drop caused by reverse interference is very similar to forward and reverse
imbalance, in fact reverse interference is just one of reasons that lead to forward and
reverse imbalance.
For the reverse interference source, there are two kinds: external interference; internal
interference. External interference: other system radio signal may drop into BTS received
bandwidth;
Internal interference: there may be illegal MS which try to access continuously.
Because MS Tx power is limited, the number of illegal MS must be very large.
Optimization method:
External interference: Inform customer and they connect with government to clear;
Internal interference: Usually this case seldom happens, but there is possibility, for
example like repeaters with MS module, which repeaters use MS module for
background observation. But sometimes, if MS is not assigned number, its
origination will be illegal, and then this will become strong interference. The solution
is to close them or assign numbers to them.
Call drop caused by F-TCH power limitation
Typical case:
As MS moves, MS Rx power and pilot Ec/Io is decreasing, but they are still enough to
maintain the link, like: Rx power>-100dBm, Ec/Io >-15dB;
As MS moves, its reverse Tx power is increasing, but it is still enough to maintain the
link;
As MS moves, Tx_Adj can maintain the normal state;
When MS is far away from BTS, F-FER will increase very fast, until call drop;
MS will re-initialize after call drop and stay in the previous pilot.
Analyzing:
Traffic channel gain can be set in background, if the setup value is too low, system can
not assign enough power resource to traffic channel.
In the above case, according to DT test it can be observed that Ec/Io and Rx power are
all higher than threshold (Ec/Io>-15dB, Rx>-100dBm); But when MS is far away from
BTS, F-FER increases very fast until call drop. The reason may be forward traffic
channel gain is too low and the power is not enough. After call drop, MS will re
initialize and stay in previous pilot.
The confirm method is use the instrument which can measure code domain power (like
Viper) to test.
Optimization method:
Check background parameters setup and adjust forward traffic channel maximum gain.
Call drop caused by handoff failure
Handoff failure is one of main reasons for call drop, when MS moves from one cell to
another cell, handoff will happen; if handoff is failed, signal of the current cell will
become worse and worse, F-FER and R-FER will be very high, then call drop will
happen when MS fading timer expires.
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