sferring one call from one source cell to Radio Network Optimisation Averaged Averaged Signal Signal Level Level Figure 2.68 (a) Illustration of small window size; (b) illustration of big window size a target one. Such an operation is complex and takes into account a lot of elements such as parameter setting, network conditions, architecture and resource availability. A handover is not possible between two cells unless they are defined as adjacents beforehand. An adjacency definition involves a couple of cells; the relationship should be defined from cell A to cell B and also from cell B to cell A. This requires the usage of some parameters that are explained in the following subsections. The notions of handover and adjacency are quite different in the sense that a handover indicates an action between cells while adjacency indicates the relationship between cells. Handover Algorithm A handover means changing the traffic channel during an ongoing call, either to another timeslot within the same cell or to another cell. The same operation in idle mode is called reselection rather than handover. The network performs radio resource management and triggers handover decisions according to measurements reported by the mobile station and according to predefined parameters and thresholds. The handover process is initiated if the source cell fulfills some criteria and if a better adjacent cell exists to move to. Handovers can be made due to several reasons such as quality, level or interference. Handover causes follow the priority order given below: Interference (UL or DL) UL quality DL quality UL level DL level MS-BS distance Turnaround corner MS Rapid field drop Fast/slow moving MS Better cell (PBGT or umbrella) Radio resource handovers are discussed when the cause is quality, level, interference, power budget or umbrella. Quality Quality handover is issued because the source cell experiences a decrease in quality. Level Level handover is done when the source cell signal level falls. Radio Network Planning and Optimisati