excellent question 🙂 I checked in the books…
as far as I can understand, in the ALU system, the sync HO and the async HO are exactly the same. (suprising !)
I don’t know if they changed it in B10 or something, but right now, both procedures are absolutely identical.
The target BTS will calculate the Timing Advance of the MS thanks to the HO ACCESS messages. Those works like the “CHANNEL REQUEST” : they are sent over “access bursts”. So here you go : TA computation for incoming HO is the same as for channel requests.