Dynamic time alignment is a technique that allows a radio system base station to receive transmitted signals from mobile radios in an exact time slot, even though not all mobile telephones are the same distance from the base station. Time alignment keeps different mobile radios transmit bursts from colliding or overlapping. Dynamic time alignment is necessary because subscribers are moving, and their radio waves' arrival time at the base station depends on their changing distance from the base station. The greater the distance, the more delay in the signal's arrival time.

The received burst is used by the mobile telephone to determine when to its transmission burst should start. The GSM system has some dedicated protection from transmission burst overlap. Each transmit burst has dedicated guard time of 8.25 bits (30 sec). This allows mobile devices to operate anywhere in a cell within a distance for the cell site of approximately 4.5 km before overlap may occur. When the distance of the mobile device exceeds 4.5 km from the cell site, the transmission timing is advanced to ensure the transmit burst does not overlap with other mobile devices that are operating within that cell's radio coverage area. The transmitter timing can be advanced in 1/2 bit steps to a maximum of 237 sec. This limits the maximum distance a GSM mobile telephone can operate from the cell site to approximately 40 km.

Figure 1.21 shows how the relative transmitter timing in a mobile radio (relative to the...