FDMA systems allocate one frequency band continuously (in time) to one specific user, who is the only one transmitting and receiving on this frequencies during his time of transaction. Each radio resource (radio channel) is identified with the carrier frequency and relative bandwidth. In the GSM 900-case the carrier frequencies are in the 900 MHz band and the single channel bandwidth available for one user is 200 kHz.
TDMA systems operate with “time slots”, short periods of time. Each user is assigned to a specific timeslot for his transaction. Within a specific radio channel (frequency channel) several users are served. They share the channel sequentially in time. The time slots are of very short duration, the user, however, perceives a continuous speech stream due to appropriate compression and expansion techniques at transmitter and receiver.
