TOAWS, TOAWE and TOA

31 May 2014   Category: Radioaccess

Last time one of my colleagues asked me what was TOAWS, TOAWE and TOA. I knew it related to Frame Protocol and TOA was time of arrival. But I cannot remember any more. So I did some investigation and here are some notes.

These definitions were initiated by Nokia in Tdoc R3-99663 and refined by Ericsson in Tdoc R3-99875 in 1999. Please note, in R3-99875, it mentioned 3GPP TS25.401(V1.2.1) would be updated accordingly. But later “the synchronisation in UTRAN” chapter was seperated as an individual 3GPP TS25.402. So now the official definitions are presented in sub-clause 5 of TS25.402(V11.0.0) while the mechanism is introduced in sub-clause 7 of the same 3GPP technical specification.

These three timers are used for transport channel synchronisation, more specifically, for timing adjustment on Iub/Iur. Figure 10 in TS25.402 illustrates how it works.

TOAWS(Time of Arrival Window Startpoint) and TOAWE(Time of Arrival Window Endpoint) are defined in NBAP messages for Transport bearer Setup/Addition/Reconfiguration.

In the above figure, it’s assumed that the target CFN on air interface is 152 for certain tranport block. Since Layer 1 in Node B needs some processing time(t_proc) to encode & transmit etc. Therefore, this tranport block should arrive in Node B before CFN 152. Node B will send control frame to RNC for timing adjustment with TOA which is the time difference between the TOAWE and when a data frame is received.

• If TBS arrives too early before TOAWS, it will increase the burden of buffer in Node B. Node B will send Timing Adjustment Control frame with positive TOA to RNC;
• If TBS arrives right between TOAWS and TOAWE, timing is perfect. Thus no Timing Adjustment Control frame needed;
• If TBS arrives between TOAWE and LTOA, Node B is still able to process such TBS. But it’s a bit late and dangerous that Node B might not be so quick to process it. So Node B will send Timing Adjustment Control frame with negative TOA to RNC;
• If TBS arrives too late even after LTOA, Node B is unable to process this TBS and it will send Timing Adjustment Control frame with negative TOA to RNC;

After RNC receives Timing Adjustment Control frame with TOA, it will adjust the DL data sending timing accordingly, to synchronize with Node B in this way.

The situation for soft handover is more complicated. Please check Figure 14 in TS25.402 for more details.