Time-division multiple access: Difference between revisions

Revision as of 19:58, 22 June 2024

Revision as of 19:58, 22 June 2024

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In the GSM system, the synchronization of the mobile phones is achieved by sending timing advance commands from the base station which instruct the mobile phone to transmit earlier and by how much. This compensates for the speed-of-light propagation delay. The mobile phone is not allowed to transmit for its entire time slot; there is a [[guard interval]] at the end of each time slot. As the transmission moves into the guard period, the mobile network adjusts the timing advance to synchronize the transmission.

Initial synchronization of a phone requires even more care. Before a mobile transmits there is no way to know the offset required. For this reason, an entire time slot has to be dedicated to mobiles attempting to contact the network; this is known as the [[random-access channel]] (RACH) in GSM. The mobile attempts to transmit at the beginning of the time slot, as received from the network. If the mobile is located next to the base station, there will be no time delay and this will succeed. If, however, the mobile phone is at just less than 35 km from the base station, the time delay will mean the mobile's transmission arrives at the very end of the time slot. In that case, the mobile will be instructed to transmit its messages starting nearly a whole time slot earlier than would be expected otherwise. Finally, if the mobile is beyond the 35 km cell range in GSM, then the RACH will arrive in a neighbouring time slot and be ignored. It is this feature, rather than limitations of power, that limits the range of a GSM cell to 35 km when no special extension techniques are used. By changing the synchronization between the uplink and downlink at the base station, however, this limitation can be overcome. {{citation needed|date=November 2014}}

=== 3G systems ===