There are 2 different things :
1. The interference during dedicated mode are detected thanks to :
– poor RXQUAL (based on Bit Error Rate of current TCH)
– strong RXLEV
There is no measure of the actual C/I, even though there is 1 idle timeslot in the 26-TCH multiframe. Note that in the measurement result message on SACCH UL, there is no field for C/I value.
2. The BTS can measure the UL interference on Idle TCH : when one TS is not busy for CS or PS, the BTS listens to the noise during this timeslot. This is performed on all TRX’s (as far as i recall).