Presented by
K.Suman Kumar
Kripesh.Ajmera

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ABSTRACT
The steadily growing mobilesubscriber community and their demandfor diversity of service place greatchallenge on the bandwidth utilization,especially in the wireless network part, asradio spectrum is a limited resource.Carefully planned radio usage is criticalfor both system capacity and servicequality. We mainly concentrate on twoaspects of the service provision capabilityof cellular networks in this paper. One iscapacity related that emphasizes the useradmission capability and the other isservice quality that targets the connectioncontinuity. We try to reveal the impact ofhandoff protection mathematically whichis introduced to enhance connectionrobustness on the capacity of cellularmobile systems. Markov approach is usedto analyze the correlation between the useradmission capability and the channelreservation, which is one strategy forhandoff protection, and how the useraccommodation capability is affected bychannel reservation in an ideal trafficmode
l.INTRODUCTION
Mobility is the most importantfeature of a wireless cellularcommunication system. Usually,continuous service is achieved bysupporting handoff (or handover) fromone cell to another. Handoff is the processof changing the channel (frequency, timeslot, spreading code, or combination ofthem) associated with the currentconnection while a call is in progress. It isoften initiated either by crossing a cellboundary or by a deterioration in qualityof the signal in the current channel.Handoff is divided into two broadcategories— hard and soft handoffs. Theyare also characterized by “break beforemake” and “make before break.” In hardhandoffs, current resources are releasedbefore new resources are used; in softhandoffs, both existing and new resourcesare used during the handoff process.Poorly designed handoff schemes tend togenerate very heavy signaling traffic and,thereby, a dramatic decrease in quality ofservice (QoS). (In this chapter, a handoffis assumed to occur only at the cellboundary.) The reason why handoffs arecritical in cellular communication systemsis that neighboring cells are always using adisjoint subset of frequency bands, sonegotiations must take place between themobile station (MS), the current servingbase station (BS), and the next potentialBS. Other related issues, such as decisionmaking and priority strategies duringoverloading, might influence the overallperformance.
TYPES OF HANDOFFS
Handoffs are broadly classified into twocategories—hard and soft handoffs.Usually, the hard handoff can be furtherdivided into two different types—intraandinter cell handoffs. The soft handoffcan also be divided into two differenttypes—multi way soft handoffs and softerhandoffs.A hard handoff is essentially a “breakbefore make” connection. Under thecontrol of the MSC, the BS hands off theMS’s call to another cell and then drop thecall. In a hard handoff, the page link to the priorBS is terminated before or as the user istransferred to the new cell’s BS; the MS islinked to no more than one BS at anygiven time. Hard handoff is primarily usedin FDMA (frequency division multipleaccess) and TDMA (time division multipleaccess), where different frequency rangesare used in adjacent channels in order tominimize channel interference. So whenthe MS moves from one BS to another BS,!it becomes impossible for it tocommunicate with both BSs (sincedifferent frequencies are used).
HANDOFF INITIATION
A hard handoff occurs when the oldconnection is broken before a newconnection is activated. The performanceevaluation of a hard handoff is based onvarious initiation criteria.It is assumed thatthe signal is averaged over time, so thatrapid fluctuations due to the multipathnature of the radio environment can beeliminated. Numerous studies have beendone to determine the shape as well as thelength of the averaging window and theolder measurements may be unreliable.Figure shows a MS moving from one BS(BS1) to another (BS2). The mean signalstrength of BS1 decreases as the MSmoves away from it. Similarly, the meansignal strength of BS2 increases as the MSapproaches it.
Relative Signal Strength with Threshold
This method allows a MS to hand off onlyif the current signal is sufficiently weak(less than threshold) and the other is thestronger of the two. The effect of thethreshold depends on its relative value ascompared to the signal strengths of thetwo BSs at the point at which they areequal.If the threshold is higher than this value,say T1 in Figure, this scheme performsexactly like the relative signal strengthscheme, so the handoff occurs at positionA. If the threshold is lower than this value,say T2 in Figure, the MS would delayhandoff until the current signal levelcrosses the threshold at position B. In thecase of T3, the delay may be so long thatthe MS drifts too far into the new cell.This reduces the quality of thecommunication page link from BS1 and mayresult in a dropped call. In addition, thisresults in additional interference to cochannel users. Thus, this scheme maycreate overlapping cell coverage areas. Athreshold is not used alone in actualpractice because its effectiveness dependson prior knowledge of the crossover signalstrength between the current and candidateBSs.
Relative Signal Strength with Hysteresis
This scheme allows a user to hand off onlyif the new BS is sufficiently stronger (by ahysteresis margin, h in Figure 1.2) than thecurrent one. In this case, the handoffwould occur at point C. This techniqueprevents the so-called ping-pong effect,the repeated handoff between two BSscaused by rapid fluctuations in thereceived signal strengths from both BSs.The first handoff, however, may beunnecessary if the serving BS issufficiently strong.
Relative Signal Strength with Hysteresisand Threshold
This scheme hands a MS over to a new BSonly if the current signal level drops belowa threshold and the target BS is strongerthan the current one by a given hysteresismargin. In Figure, the handoff wouldoccur at point D if the threshold is T3.