BTS requires fixed synchronization between both channels, uplink and downlink (i.e. a distance of 3 Burst Periods (BP) or 3 TSs = 1.73 ms) [Why: The reason for this delay is to allow the same TS number to be used in both uplink and downlink directions without requiring the MS to receive and transmit simultaneously In this way, same antenna (transreceiver) can be used for transmission as well as reception]. Mobile Station (MS) has to send its data to BTS 3 timeslots (TS) after it receives data from BTS. So, BTS is configured to receive bursts in a proper time frame.
Now due to propagation delay, the uplink data from MS reaches BTS beyond the stipulated time. So, BTS calculates this delay and asks the MS to send uplink data earlier than that defined by "three time slots" rule.
Timing Advance = 2 times Propagation delay
The reason being 2 times is that there is a round trip propagation delay, BTS-MS-BTS. That is,
- MS receives downlink data from BTS with a delay
- then MS sends uplink data after this delay and it reaches to BTS with an equal additional propagation delay.
So these two delays are added, due to which MS should send uplink data in a way that the downlink and uplink propagation delays are nullified. In other words, MS advances its timing by this amount, with the result that signals from different MS's arriving at the BTS are compensated for propagation delay. This process is called "adaptive frame alignment".
TA can take value from 0 to 63 (expressed in time as 0 to 232 microsecond, in steps of 48/13 microsecond). If TA is known, the distance between MS and BTS can be calculated. Here, each step corresponds to 550 m.
Now due to propagation delay, the uplink data from MS reaches BTS beyond the stipulated time. So, BTS calculates this delay and asks the MS to send uplink data earlier than that defined by "three time slots" rule.
Timing Advance = 2 times Propagation delay
The reason being 2 times is that there is a round trip propagation delay, BTS-MS-BTS. That is,
- MS receives downlink data from BTS with a delay
- then MS sends uplink data after this delay and it reaches to BTS with an equal additional propagation delay.
So these two delays are added, due to which MS should send uplink data in a way that the downlink and uplink propagation delays are nullified. In other words, MS advances its timing by this amount, with the result that signals from different MS's arriving at the BTS are compensated for propagation delay. This process is called "adaptive frame alignment".
TA can take value from 0 to 63 (expressed in time as 0 to 232 microsecond, in steps of 48/13 microsecond). If TA is known, the distance between MS and BTS can be calculated. Here, each step corresponds to 550 m.
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thanx
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