This is what written in ETSI 05.10 for initial TA estimation..
“When the BTS detects an access burst transmission on RACH it shall measure the delay of this signal relative to the expected signal from an MS at zero distance under static channel conditions. This delay, called the timing
advance, shall be rounded to the nearest symbol period and included in a response from the BTS when applicable.”
k.k..and Now in case of handover,by HO access message target BTS calculates TA and then sends its info to MS via physical msg…