Timing Issues on the Radio Air Interface

We said that radio link budgets can be improved by putting handsets into soft handover.
Care must however be taken to maintain time alignment between the serving
Node B and soft handover target Node B. Path delay will be different between the two
serving Node Bs and will change as the user moves. The downlink timing therefore has
to be adjusted from the new serving Node B so that the handset RAKE receiver can
coherently combine the downlink signal from each Node B. The new Node B adjusts
downlink timing in steps of 256 chips (256 × 0.26 μs = 66.56 μs) until a short code lock
is achieved in the handset. If the adjustment is greater than 10 ms, then the downlink
has to be decoded to obtain the system frame number to reclock the second (soft handover)
path with the appropriate delay.