Re: GSM [hlp]
Well, the received signal level on downlink (from BTS to MS) is measured in dBm & it decreases as you go far from the BTS location, at some point, you will have to make a handover (camp on another BTS) due to poor RxLevel (emergency handover). The MS is continuously measuring the Rx singal level from neighbors to the current serving cell. The BSC makes the handover decision based on those measurements on both uplink & downlink.
Most introductory textbooks discuss "Channel Concepts", in terms of Logical & Physical channels.
Each cell has a pre-configured number of TRXs (transceivers) that transmit at certain carrier frequency at a time (or different frequencies if Frequency Hopping is employed). On each carrier frequency, you have 8 time slots (that's TDMA).
GSM planners assign TimeSlot 0 of a certain carrier frequency as BCCH (broadcast control channel), the rest of time slots can be used as TCH (traffic channel). A minimum of one timeslot per cell is allocated for SDCCH (Stand-alone dedicated control channel).
There is plenty of eBooks & documents in this board, search for them, & if u needed extra help, let me know.