The concept of difference of 3 TS between UL/DL is altogether a different theory.
As far I know it was done to make the circuitry inside MS somewhat simpler (As MS need not have to send and receive simultaneously).
But yes indeed it is one of basis for my two clocks theory:
Just have a look at my logic:
MS adjusts its clock as per the clock(1) of BTS. Under hypothetical condition(assuming no delay)….MS will use clock(1) for listening(RX) and speaking(TX)…….MS will apply a difference of 3TS between TX and RX. Everything is fine here and happy ending.
But in reality..a delay is there….BTS senses this….and he(BTS) tells the MS to advance its timing for speaking a little bit(TA)….MS will now start speaking early(TA)….and BTS starts listening the MS well at the schedule time…But same should be applicable to BTS also….he knows that if is getting the speech of MS delivered late to him so do the MS also….hence BTS also applies a TA on the TS allocated to the particular MS…..
But MS has to apply the concept of speaking(TX) 3 TS later after RX…..Now he is listening to BTS as the clock(1)……..MS cant do any changes wrt to clock(1)…hence MS has to use a separate clock(2) for TX. However the clock(2) will no doubt be derived from clock(1).
Sequence may be
a) get clock(1) from BTS through SCH
b) use this clock(1) for RX
c) apply a difference of 3 TS to this clock(1) for TX . let us say the new time is clock(2) (can say that TX watch is running 3 seconds slower then the RX watch.)
d) But as per the wish of BTS apply a correction of TA to clock(2) and finally use this resulting clock time for transmitting.
May be taken as , a person is wearing wrist watches in both wrists. Let us say he will use the wrist watch of left hand to RX and wrist watch of right hand to speak. The wrist watch of right hand is running slow by a 3 seconds(3TS). But as per the order of BTS the person has to correct the time of right hand TX watch by an amount of TA.
Sorry friends … I think above discussion was not needed indeed. But I have just expressed thoughts of my mind which may not be true.
So, should I conclude that TA will be,
1. calculated by BTS
2. used by MS in UL
3. used by BTS in downlink