05.103GPPRadio subsystem synchronizationTS
The following conventions are adopted in this annex:
‑ the modulating symbol period is denoted T = 48/13 µs;
‑ all timing values are considered for descriptive purposes as real numbers modulo the largest period defined in the system i.e. 3394560000T. When transmitted over the air interface, such a value shall be rounded to the nearest integer multiple of a 1/2 symbol period T/2 and that integer shall be reduced mod some integer multiple of 256 as defined in 3GPP TS 04.08;
‑ the Timing Advance (TA) value, when the distance between the base station and the MS is equal to or less than 35 km, or in the case of GSM 400 is equal to or less than 120 km, represents the estimated two way propagation delay in T units. For the purpose of the calculations in this annex the timing advance values are considered to represent the estimated one way propagation delay in T/2 units which is equivalent to twice the delay in T units.
Assuming that some MS has to perform handover from BTS 0 (the "current" or "old" BTS) to BTS 1 (the "new" BTS), the following quantities are defined.
‑ t0 (resp. t1) denotes the one way line of sight propagation delay between the MS and BTS 0 (resp. BTS 1).
‑ RTD (Real Time Difference) denotes the value of the local system time in BTS 0 minus that of BTS 1.
‑ OTD (Observed Time Difference) denotes the timing difference between BTS 0 and BTS 1 as measured by the MS with the same sign conventions as for RTD.
All these four values are slowly time‑varying due to the MS movement and oscillators drift in the BTS’s, but they are defined here just prior handover execution.
A.1.3 Details of operations
The following relation holds:
OTD = RTD + t1 ‑ t0
Synchronized and pseudo synchronized handovers work as follows:
– for the pseudo synchronized handover, it is assumed that RTD is known to BTS 0 and MS supports the scheme, BTS 0 may order pseudo‑synchronized handover to BTS 1, including RTD in the "HANDOVER COMMAND" message;
– for the synchronized handover, BTS 0 may order synchronized handover to BTS 1, and the MS sets RTD as defined in subclause A.3.3;
– under normal operating conditions, t0 should be closely related to the latest received Timing Advance sent by BTS 0 to the MS; since the MS must have got synch to BTS 1 before performing handover, OTD, RTD and t0 are available to the MS, hence the value of t1 that can be used to set the new Timing Advance parameter without receiving it from BTS 1;
– after successful handover, either synchronized, non‑synchronized or pseudo‑synchronized, the MS shall provide to BTS 1 the value of OTD + t0 in the "HANDOVER COMPLETE" message, allowing BTS 1 to obtain a non biased estimate of RTD given the transmitted OTD + t0 and it is estimated value to t1. In practice, additional processing will be required to mitigate the effects of estimation errors and quantization effects; this matter is left unspecified.