Hi RAM,
Frequency is required to Trasnmit and receive signal over the channel.Channels having some frequencies.
uplink and downlink freqencies always different to avoid interference.
In CDMA band( 824-849MHZ : UL
869-894Mhz: DL)
for GSM 900: 890-915 MHZ: UL
935-960Mhz: DL
are using.
There are two technique:
FDD : Frequency division duplexing
TDD: Time division duplexing
In FDD different frequencies for uplink and downlink
In TDD same frequency but this is time shared.