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Telecom Techniques Guide

GSM

Transmission delay is unavoidable in the radio interface. If the mobile station moves away from the base station during a call, the further distance the more delay. The uplink is as the same.

If the delay is too high, the timeslots of the signal from a certain mobile station and that of the next signal from another mobile station received by the base station will overlap each other, thus causing inter-code interference.

To avoid this, during a call, the measurement report sent from the mobile station to the base station carries a delay value. Moreover, the base station should monitor the time when the call arrives and send an instruction to the mobile station via the downlink channel every 480ms so as to inform the mobile station the time of advance transmission. This time is the TA (timing advance), which ranges between 0~63 (0~233μs  ). The TA value is limited by the timing advance code 0~63bit of the GSM system. Therefore, the maximum coverage distance of the GSM is 35km. Its calculation is as follows:

1/2*3.7 μs /bit*63bit*c=35km

{In the formula, 3.7μs /bit is the duration per bit (156/577); 63bit is the maximum bit number of the time adjustment; c is the light speed (transmission speed of the signal); and ½ indicates that the go and return trip of the signal.}

According to the above description, the distance corresponding to 1bit period is 554m. Influenced by the multi-path propagation and MS synchronization precision, the TA error may reach up to about 3bit (1.6km).

When the MS is in idle mode, the time sequence within the MS can be adjusted via the SCH channel. However, the mobile station does not know how far it is away from the base station. If the distance between the MS and the base station is 30km, the time sequence of the MS will be 100μs  slower than that of the base station.

When the mobile phone sends its first RACH signal, it is already 100μs  later. For there is still another 100μs  of transmission delay, when the signal reaches the base station, the total delay is 200μs . It is very possible that the signal collides with the pulse of the adjacent timeslot around the base station. Therefore, RACH and some other channel access pulses will be shorter than other pulses. Only after receiving the time sequence adjustment signal (TA) from the base station, MS can send pulses of normal length. In this case, the MS needs to send signals by 200μs  in advance.