Named after the organization that created the system standards (Groupe Speciale
Mobile) this pan-European digital cellular system has been deployed in Europe
since the early 1990s
[Hodges, 1990].
GSM uses combined TDMA and FDMA with frequency-division duplex for access.
Carriers are spaced at 200 KHz and
support eight TDMA time slots each.
For the up-link the frequency band 890-915 MHz is allocated, while the
down-link uses the band 935-960 MHz.
Each time slot is of duration 577 
s which corresponds to 156.26 bit
periods, including a guard time of 8.25 bit periods.
Eight consecutive time slots form a GSM frame of duration 4.62 ms.
The GSM modulation is Gaussian minimum shift keying with time-bandwidth product of 0.3, i.e., the modulator bandpass has a cut-off frequency of 0.3 times the bit rate. At the bit rate of 270.8 Kbit/s, severe intersymbol interference arises in the cellular environment. To facilitate coherent detection, a 26 bit training sequence is embedded into every time slot. Time diversity is achieved by interleaving over eight frames for speech signals and 20 frames for data communication. Sophisticated error correction coding with varying levels of protection for different outputs of the speech coder is provided. Note that the round-trip delay introduced by the interleaver is on the order of 80 ms for speech signals. GSM provides slow frequency hopping as a further mechanism to improve the efficiency of the interleaver.
