Praise for Fundamentals of WiMAX

252 Chapter 7 • Networking and Services Aspects of Broadband WirelessReceived Signal StrengthAHandoff ThresholdMinimum Signal Level (MSL)BDistance/TimeBaseStation ABaseStation BFigure 7.13 Handoff detection based on signal strengthTo minimize the dropping probability, the handoff procedures need to be executed quickly,and the ∆ set higher so that the likelihood of the signal’s dropping below the MSL before handoffexecution is minimized. Obviously, setting a large ∆ implies a costlier cellular design withlarger overlap among cells. Being too quick to hand over may also lead to excessive and unnecessaryhandoffs, particularly when there is significant signal fluctuations.Clearly, there is a trade-off between dropping probability and handoff rate. Too few handoffsmay lead to dropped calls, and too many handoffs may cause signaling overload anddegrade service quality. The nature of the trade-off between dropping probability and handoffrate depends on the signal-fluctuation model and the handoff decision algorithm used. For example,Table 7.4 shows the results of a simulation study reported in [67]. The table illustrates thetrade-off between dropping probability and handoff rate for three algorithms, which, respectively,base handoff decisions on (1) instantaneous value of signal level, (2) signal level averagedover past ten samples, and (3) true expected value of signal level. As reported in [67], theseresults are based on Matlab simulations of an MS moving at 20m/s from one BS to another separatedby 1km. A fourth-power exponential decay and lognormal shadow fading with a correlationdistance of 50m 12 is used to model the signal. Signal samples are assumed to be taken every0.5 seconds.12. Defined as the distance at which the signal correlation drops to 0.5.