GSM Radio Channel Structure

GSM Radio Channel Structure

This diagram shows that the GSM system uses a single type of radio channel. Each radio channel in the GSM system has a frequency bandwidth of 200 kHz and a data transmission rate of approximately 270 kbps. This example shows that each radio communication channel is divided into 8 time slots (0 through 7). This diagram shows that a simultaneous two-way voice communication session requires at least one radio channel communicates from the base station to the mobile station (called the forward channel) and one channel communicates from the mobile station to the base station (called the reverse channel). This example also shows that some of the radio channel capacity is used to transfer voice (traffic) information and some of the radio channel capacity is used to transfer control messages.

Normal Burst Structures

This figure shows the field structures of the normal burst used in the GSM system. This diagram shows that the field structure is different for the normal burst, synchronization burst, and the frequency correction bursts. The fields transmitted during the normal burst include initial tail bits (ramp-up time), training sequence, flag bits, user data bits, final tail bits, and guard period. This diagram shows that the first 3 bits of the time slot are dedicated to the gradual increase of transmitter power level (ramp-up). For the normal burst, this is followed by the information (user data) bits. The flag bits indicate if the normal burst has been replaced with FACCH signaling information. This diagram shows that some of the bits in the center of the burst are used as training bits (to allow equalizer training). At the end of the transmitted burst there are tail bits (for error protection) and 3 guard period bits that are used during the gradual reduction of the RF transmitter signal (ramp-down).