17-3

GSM Theory

The GSM System

The GSM System
The GSM system uses two frequency bands ranging from 890 to 915 MHz and
935to 960MHz. The bands are broken into 125 channels spaced 200 kHz apart.
TheGSM system uses one band to transmit and one to receive. The lower frequency band
(890-915MHz) is used for the Mobile telephone to Base station link; the upper band is for
Base to Mobile.
Channels from each band are used in uplink/downlink channel pairs. The channels in the
channel pair are spaced 45 MHz apart.
The GSM system is time multiplexed, meaning that it is pulsed to allow multiple users
access to the same channel. On each channel there are eight timeslots so that eight users
can be on a channel at the same time. The pulses or timeslots are 576.9 µS long.
The GSM system has the capability of being frequency hopped within the frequency
bands. This allows the system to hop the telephone to another channel and possibly
another timeslot during a telephone call.
The GSM system uses a 0.3 Gaussian Minimum Shift Keying modulation scheme to
modulate the digital data onto the pulsed carrier.
The digital data for one pulse is made up of both voice data and predefined data that is
used for synchronization. The voice data is coded to maximize speech quality and
minimize errors.
Formost of the characteristics mentioned above the GSM system contains many variables.
These variables account for the many screens and fields in the HP/Agilent 8922.