1. It is useful to declare a “preferred” TRX, in order to favourize TCH allocation on this specific TRX. This option is usually chosen to force GPRS on BCCH TRX first.
2. Regarding the alarm, i don’t know.
3. In your example, the TRX 3 will have a power reduced by 3dB, in order for this cell to have all its TRX providing the same power at BTS output. This is called power leveling.
4. The TCH allocation is favourized by the PREFERENCE MARK, as you stated above. This is a TRX parameter you can change in the OMC-R.
— note : there is few more questions that might interest you :
– “how the BSC maps the logical TRX and the physical TRX ?”. Because the TRX 1 in your cell might not be the 1st TRX in your BTS, physically.
There is an algorithm for this, so the BCCH is usually the TRX with highest EDGE power, etc…
– “is there any priority in the allocation of a TCH, if the PREF MARK is set to 0 on all TRX ?”. will the TCH be allocated on last TRX ? first TRX ? but which TS ? there is an algorithm in the BSC, that will allocate TCH on full rate TRX normal power, then dual rate TRX normal power, then full rate high power, then dual rate high power, and so on.
For PDCH allocation, this is usually done in the reverse order.