When a MS is performing a SDCCH transfer (call setup, SMS, loc. upd.) and suffering low Rxlev, poor RxQual, or better neighbour, then the BSC will command the MS to perform aHO to a neighbour cell. The MS will resume the SDCCH transfer on a SDCCH subchannel of the target cell.
That’s a SDCCH HO.
in the first cell: outgoing ho
in the target cell : incoming ho