One of the goals of a cellular system is for a user to remain "in touch" even as they move through the system. When a user moves from the coverage area defining one cell into that of another, the system must provide the capability for that user to remain "in touch" even while breaking the connection with one base station and establishing another connection with another base station. This operation is called a handoff. Smaller cells means more frequent handoffs, which requires greater system resources to support and coordinate. Handoff is really a localized form of mobility.2.14
Cellular handoff is done in one of two ways, as shown in Figure 2.3. Hard handoff is when the airlink connection between the mobile and its initially-serving base station are momentarily severed before reconnecting with a new base station. This is the method traditionally used in existing cellular systems, because it requires the least processing by the network providing service. However, it causes a momentary interruption in reception which is sometimes noticeable to the humans engaged in the call being handed off.
The second handoff mode is called soft handoff, in which two base stations are briefly simultaneously connected via the airlink with a mobile during the handoff. As soon as the mobile's RF link with the new base station is acceptable, the initially-serving base station disengages from the mobile. Diversity techniques are employed at both ends of the radio link to ensure a smooth handoff, which is largely undetectable to the humans affected.
Handoffs can be further categorized as being either controlled or assisted by the network or the mobile. A network-controlled handoff is referred to as abase-controlled handoff or BCHO.2.15 Mobile-controlled handoff or MCHO is less commonly used in voice systems, although it is used in CDPD because of the burst mode of transmission employed by CDPD mobiles. Second generation cellular voice systems take advantage of greater intelligence in mobile stations and time-division techniques to perform mobile-assisted handoff or MAHO, in which the mobile participates but does not control the handoff.
Whichever technique is employed, the handoff process is complex. A decision algorithm is used to determine when the handoff should occur, based on factors such as the received power level and signal quality (bit error rate or supervisory tone). Once predetermined threshold values have been exceeded, indicating that the edge of cell coverage has been reached, another decision must be made-where should the mobile next receive service?
The target cell for the handoff is determined by RF measures designed to minimize interference coupled with capacity considerations such as the need for load balancing, availability of idle channels, etc.2.16 All of the decisions for a handoff must be made quickly, because the subscriber could be traveling at highway speeds. This need for rapid handoff decision-making is accentuated by the ever decreasing cell sizes used in urban areas. The requirement for rapid handoffs can only be met with a sufficient level of processing and signalling capability.
The ability of a network to support cell handoffs can be a capacity constraint. Therefore, it is important to avoid unnecessary and undesirable handoffs. The system (network plus mobiles) must distinguish between an actual movement from one cell's coverage area to another and a mobile simply moving to a fringe area, where the RF reception is poor. Smart algorithms involving timers, power control and hysteresis2.17 have been developed to reduce the number of unnecessary handoffs.