procedure. MS1 TAI=0 MS2 TAI=1 Block Access burst (PRACH) Block MS3 TAI=2 MS4 TAI=3 Block Block TA update message for MS1, 2, 3 &4 (1 RLC/MAC control block sent on 4 PTCCH DL) Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Radio and MS-PCUSN Interfaces Chapter 6 In continuous timing advance, the mobile sends a special access burst in an idle slot for the network to derive the necessary timing. In the downlink, the network sends a timing advance via the PACCH, which is transmitted during the idle time slots of the 52 multiframes. The Timing Advance Index (TAI) gives the mobile station the position to send the access burst. The timing advance procedure comprises two parts: The TA is estimated during the initial phase of the data transfer (initial estimation from the access burst). The TA is also continuously updated while the mobile station remains in transfer mode. Initial Timing Advance Estimation (During Access Phase) This is made from the access burst carrying the PRACH. This first estima- tion is sent to the mobile station through the first available message (Packet Immediate Assignment or Packet Resource Assignment). Continuous Update Procedure (During Transfer State) This procedure is carried on one of the allocated PDCH using PRACH UL and PACCH DL. Each mobile station is assigned in the Packet Resource Assignment (for UL or DL packer transfer) a specific PDCH among those allocated and an idle slot (indicated by the TAI) on this PDCH to be used for TA updating. The mobile station sends on its assigned slot a special access burst and receives the TA value on the subsequent TA message. The TAI is coded on 4 bits, creating 16 different positions in groups of 8, 52-time-slot multiframes. In this example, a mobile station transmits an access burst on idle slot 0 of multiframe (n), and receives the TA value on TA message in multiframes (n + 