For the GSM-900 system
, two frequency bands have been made available:
The 25 MHz bands are then divided into 124 pairs of frequency duplex channels with 200 kHz carrier spacing using Frequency Division Multiple Access (FDMA). Since it is not possible for a same cell to use two adjacent channels, the channel spacing can be said to be 200 kHz interleaved. One or more carrier frequencies are assigned to individual Base Station (BS) and a technique known as Time Division Multiple Access (TDMA) is used to split this 200 kHz radio channel into 8 time slots (which creates 8 logical channels). A logical channel is therefore defined by its frequency and the TDMA frame time slot number. By employing eight time slots, each channel transmits the digitized speech in a series of short bursts: a GSM terminal is only ever transmitting for one eighth of the time.
8-slot TDMA together with the 248 physical half-duplex channels corresponds to a total of 1984 logical half-duplex channels. This corresponds to roughly 283 (1984 / 7) logical half-duplex channels per cell. This is because a cell can only use one seventh of the total number of frequencies, see Figure 1.
Figure 1: Typical cellular scheme
Seven sets of frequencies are sufficient to cover an arbitrarily large area, providing that the repeat-distance d is larger than twice the maximum radius r covered by each transmitter.
Each of the frequency channels is segmented into 8 time slots of length 0.577 ms (15/26 ms). The 8 time slots makes up a TDMA frame of length 4.615 ms (120/26 ms). The recurrence of one particular time slot every 4.615 ms makes up one basic channel.
The GSM system distinguishes between traffic channels (used for user data) and control channels (reserved for network management messages). In this overview, we consider only the Traffic Channel/Full-Rate Speech (TCH/FS) used to carry speech at 13 kbps.
TCHs for the uplink and downlink are separated in time by 3
burst periods, so that the mobile does not has to transmit and receive
simultaneously. TCHs are defined using a 26-frame multiframe (i.e. a group of
26 TDMA frames).
The length of a 26-frame multiframe is 120 ms, which is how the
length of a burst period is defined (120 ms / 26 frames / 8 burst periods
per frame).
Out of the 26 frames, 24 are used for traffic, one is used for the Slow
Associated Control Channel (SACCH) and one is currently unused (see
Figure
2).
Figure 2: The TDMA frame structure
Data are transmitted in bursts which are placed within the time slots. The transmission bit rate is 271 kb/s (bit period 3.79 microseconds). To allow for time alignment errors, time dispersion etc, the data burst is slightly shorter than the time slot (148 out of the 156.25 bit periods available within a time slot).
The burst is the transmission quantum of GSM. Its transmission takes place during a time window lasting (576 + 12/13) microseconds, i.e. (156 + 1/4) bit duration. A normal burst contains two packets of 58 bits (57 data bits + 1 stealing bit) surrounding a training sequence of 26 bits. The 26-bit training sequence is of a known pattern that is compared with the received pattern in order to reconstruct the rest of the original signal (multipath equalization). The actual implementation of the equalizer is not specified in the GSM specifications. Three ``tail'' bits are added on each side.
GSM can use slow frequency hopping where the mobile station and the base station transmit each TDMA frame on a different carrier frequency. The frequency hopping algorithm is broadcast on the Broadcast Control Channel. Since multipath fading is dependent on carrier frequency, slow frequency hopping help mitigate the problem. Frequency hopping is an option for each individual cell and a base station is not required to support this feature.