four values (same, lower, upper, not used). Power Budget Margin This gives in dB how much the signal level of the target cell should exceed that of the source cell to perform a PBGT handover. Fast Moving Threshold This indicates the time (in SACCH frames) that a mobile should stay in a cell's coverage before deciding to hand it over to this lower layer cell. Umbrella Level This is the minimum acceptable level threshold that a mobile should measure to decide whether it is still in the cell's coverage area. At the end, hundreds of parameters exist that handle handover and adjacency. Each one can be tuned separately according to the optimisation need. The parameter tuning section deals with some examples of handover control parameter tuning. Adjacency Optimisation Optimising an adjacency network means deleting the unnecessary relations and adding the missing ones SO that the efficiency of the system increases. If each cell has to handle less adjacency the processing speed gets better. Each cell can have up to 32 adjacents, SO as the network expands, the number of adjacencies increases exponentially. A rational management of adjacencies is the only way to optimise the network. Radio Network Planning and Optimisation Redundant Neighbour Existing site B New site Neighbours Existing site A Missing New site D Neighbour Figure 2.70 Neighbour topology in an expanding network Figure 2.70 shows an explicit example of how adjacencies can become obsolete and at the same time shows whether there is a need to create new adjacencies continuously. When a new site C is added and declared adjacent to existing sites A and B, the relation A-B becomes obsolete and can be deleted. A mobile camping on existing site A and moving towards new site D will perform a handover to a wrong site (C) because the neighbour relationship with the nearest cell (D) is missing. Being on a wrong server means causing interference, using high power due to distance and also means being on a frequency that, initially in the frequency plan, was not expected in