Handoff Detection To initiate the handoff process two issues must be considered: Who initiates the handoff process? How is the need for handoff detected? Making the decision when to effect the handoff must be based on measurement of the linksmade at the MS, at the two BSs, or both. While it is obvious that the measurements can be madeat either the MS or the BSs, it is obvious that the decision to affect their handoff can be madeeither by the network or by the MS. Handoffs are expensive to execute, so unnecessary handoffsshould be avoided. If the handoff criteria are not chosen appropriately, then in the overlappingregion between the two BS coverage area boundaries, the call might be handed back and forthseveral times between them. If the criteria are too conservative, then the call may be lost beforethe handoff can take place. The handoff decision making criteria become even more critical withthe evolution to smaller cell sizes, which is happening to increase the capacity of systems and toreduce power requirements of MSs. Handoff detection is based on link measurement. The measurement process determines theneed for handoff and the target or new channel for transfer. The propagation between the basestation and the MS is made up of the direct line-of-sight path and scattering paths caused byreflections from or diffraction around buildings and terrain. Thus the signal received by the MSat any point consists of a large number of generally horizontally travelling uniform planewaves. The plane wave amplitudes, phases and angles of arrival relative to the direction ofmotion are random. These plane waves interfere and produce a varying field strength patternwith minima and maxima spacing of the order of a quarter-wavelength apart. The MSs receivedsignal fades rapidly and deeply as it moves through this interference pattern. By reciprocity, theBS receiver experiences the same phenomenon as the MS due to the MS motion. The envelopeprocess of this fast-fading phenomenon is Rayleigh –distributed if there is no strong line of sightcomponent, and Rician otherwise. As the MS different scatters and terrain change the planewaves incident on the MS antenna. Therefore, superimposed on the rapid multipath fading areslow variations in the average field strength of the interference pattern due to these newreflection and diffraction paths. 1lhis slower fading phenomenon is called fading. which has along normal distribution. Three measurements are used to determine the quality of a channel: 1. Word error indicator (WEI). Metric that indicates whether the current burst was demodulatedproperly in the MS. 2. Received signal strength indication (RSSI). Measure of received signal strength. The RSSImetric has a large useful dynamic range, typically between 80 to 100 dB. 3. Quality indicator (Q). Estimate of the "eye opening" of a radio signal, which relates to thesignal to interference and noise (S/T) ratio, including the effects of dispersion. QI has a narrowrange (relating to the range of S/1 ratio from 5 dB to 25 dB). Handoff Failures In the link transfer procedure, there are several reasons handoff failures can occur, some ofwhich are:
No channel is available on selected BS. Handoff is denied by the network for reasons such as lack of resources for example no bridge orno suitable channel card; the MS has exceeded some limit on the number of handoffs that maybe attempted in some period of time. It takes the network too long to set up the handoff after it has been initiated. The target link fails in some way during the execution of handoff.