Adaptive hard handoff algorithms

The design of hard handoff algorithms based on optimizing the tradeoff between link quality and rate of handoffs is considered. For handoff algorithms based on this criterion, adaptation is precisely defined in terms of remaining on a locus of desirable operating points as system parameters (such as mobile velocity) change. A rule based on a linear cost criterion is used to select desirable operating points. For this rule, it is shown that the optimal handoff algorithm, which is impractical, is easily adapted by fixing a single tradeoff parameter at an appropriate value. The same adaptation property is shown to hold for an easily implementable approximation to the optimal algorithm, the locally optimal (LO) handoff algorithm. This is in contrast to the poor adaptation of hysteresis based approaches which require lookup tables for adaptation. Practical estimators for all relevant system parameters based on a short window of pilot signal strength measurements are also discussed. It is shown that the LO algorithm adapts well when these simple estimators are used. A hysteresis-threshold approximation to the adaptive LO algorithm is also developed.