• GSM 900 uses the circa 900Mhz band
• The frequency band used is 890-915MHz (mobile transmit) and 935-960MHz (base transmit).
• To allow maximum number of users access, each band is subdivided into 124 carrier frequencies spaced 200 kHz apart, using FDMA techniques.
• Each of these carrier frequencies is further subdivided into time slots using TDMA.
• TDMA (Time Division Multiple Access) has 8 time slots (i.e. transmitting for one eighth of the time).
• Hence, one radio channel can support 8 'full rate' traffic.
• A more economical 'half rate' scheme with 16 traffic channels is being introduced.
• TDMA provides each user with the carrier frequency for approximately 0.577ms.
• There is also an extension band of 15 MHz in both directions.
• The peak power of mobile stations depends on the class of mobile.
• Precautions are required to avoid Interference with other users. Power control - 15 steps of 2dB - is provided. The transmitter must be ramped up and down in level in a controlled fashion at the beginning and end of each time slot.
• Careful filtering is also applied.
• Frequency hopping may he optionally employed in order to avoid 'deadspots' and to minimize interference from other signals. The hopping rate is one hop per TDMA frame (4.6µs), or 217 hops per second.
• The method of modulation used Is Gaussian Minimum Shift Keying (GMSK), with a BT value of 0.3 at a gross data rate of 270 kb/s.
• Phase and frequency synchronization must allow for Doppler shift for vehicle speeds up to 250km/h as well as for frequency standard drift, and timing advance to compensate for propagation delay due to round trips for paths, in cells up to 35km radius.