You can go directly from the inner zone (1800) to whatever neighbouring cell (900/1800) – at least for Siemens. The reverse is not true – you always do HO from neighbour to outer zone (900) and immediately to inner (1800). Also for siemens there is no offset for inner complete (only the difference in Pmax – 3dB) and HO margin is one and the same for both zones (HOmargin effective is with 3dB difference). O-ooo I almost forgot no prevention of back HO due to reason “Better cell”.