effect of handoff strategies

8/8/2019 Effect of handoff strategies

http://slidepdf.com/reader/full/effect-of-handoff-strategies 1/10

CELLULAR SYSTEM CAPACITY-EFFECT OF HANDOFF STRATEGIES

EFFECT OF HANDOFF STRATEGIES

by

K.Suman Kumar (Y7EC318)

[email protected]

Ph.No:8099119724

Kripesh.Ajmera (Y7EC232)

[email protected]

Ph.No:9032073545

DEPARTMENT OF ELECTRONICS AND COMMUNICATIONENGINEERING

KONERU LAKSHMAIAH COLLEGE OF ENGINEERING

GREEN FIELDS, VADDESWARAM




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 user admission capability and the other isservice quality that targets the connectioncontinuity. We try to reveal the impact of handoff protection mathematically whichis introduced to enhance connectionrobustness on the capacity of cellular mobile systems. Markov approach is usedto analyze the correlation between the user admission capability and the channelreservation, which is one strategy for handoff protection, and how the user accommodation capability is affected bychannel reservation in an ideal trafficmodel.

INTRODUCTION

Mobility is the most importantfeature of a wireless cellular communication 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 of them) associated with the currentconnection while a call is in progress. It isoften initiated either by crossing a cell

boundary 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 released

before new resources are used; in softhandoffs, both existing and new resourcesare used during the handoff process.Poorly designed handoff schemes tend to

generate very heavy signaling traffic and,thereby, a dramatic decrease in quality of

service (QoS). (In this chapter, a handoff is assumed to occur only at the cell

boundary.) 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 serving

base station (BS), and the next potentialBS. Other related issues, such as decisionmaking and priority strategies duringoverloading, might influence the overall

performance.

TYPES OF HANDOFFS

Handoffs are broadly classified into twocategories—hard and soft handoffs.Usually, the hard handoff can be further divided into two different types—intra-and inter cell handoffs. The soft handoff can also be divided into two differenttypes—multi way soft handoffs and softer handoffs.

A hard handoff is essentially a “break

before 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 link to the prior BS 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 to

minimize 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 ThresholdThis 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 performs

exactly like the relative signal strengthscheme, so the handoff occurs at position

A. 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 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 actual

practice because its effectiveness dependson prior knowledge of the crossover signalstrength between the current and candidateBSs.

Relative Signal Strength with HysteresisThis 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 handoff would occur at point C. This technique

prevents 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 be

unnecessary if the serving BS issufficiently strong.Relative Signal Strength with Hysteresisand ThresholdThis scheme hands a MS over to a new BSonly if the current signal level drops belowa threshold and the target BS is stronger than the current one by a given hysteresismargin. In Figure, the handoff wouldoccur at point D if the threshold is T3.

Prediction Techniques

Prediction techniques base the handoff decision on the expected future value of the received signal strength. A techniquehas been proposed and simulated toindicate better results, in terms of reduction in the number of unnecessaryhandoffs, than the relative signal strength,

both without and with hysteresis, andthreshold methods.




HANDOFF DECISION

There are numerous methods for performing handoff, at least as many asthe kinds of state information that have

been defined for MSs, as well as the kindsof network entities that maintain the stateinformation . The decision-making processof handoff may be centralized or decentralized (i.e., the handoff decisionmay be made at the MS or network). Fromthe decision process point of view, one canfind at least three different kinds of handoff decisions.

Network-Controlled Handoff

In a network-controlled handoff protocol,the network makes a handoff decision

based on the measurements of the MSs ata number of BSs. In general, the handoff

process (including data transmission,channel switching, and network switching)takes 100–200 ms. Information about thesignal quality for all users is available at asingle point in the network that facilitatesappropriate resource allocation. Network-controlled handoff is used in first-generation analog systems such as AMPS(advanced mobile phone system), TACS(total access communication system), and

NMT (advanced mobile phone system).

Mobile-Assisted Handoff

In a mobile-assisted handoff process, theMS makes measurements and the network makes the decision. In the circuit-switchedGSM (global system mobile), the BScontroller (BSC) is in charge of the radiointerface management. This mainly meansallocation and release of radio channelsand handoff management. The handoff time between handoff decision andexecution in such a circuit-switched GSMis approximately 1 second.

Mobile-Controlled Handoff

In mobile-controlled handoff, each MS iscompletely in control of the handoff

process. This type of handoff has a short

reaction time (on the order of 0.1 second).MS measures the signal strengths from

surrounding BSs and interference levelson all channels. A handoff can be initiatedif the signal strength of the serving BS islower than that of another BS by a certainthreshold.

HANDOFF SCHEMES

In urban mobile cellular radio systems,especially when the cell size becomesrelatively small, the handoff procedure hasa significant impact on system

performance. Blocking probability of originating calls and the forcedtermination probability of ongoing callsare the primary criteria for indicating

performance. In this section, we describeseveral existing traffic models and handoff schemes.

Handoff Schemes in Single TrafficSystemsIn this section, we introduce no priority,

priority, and queuing handoff schemes for a single traffic system such as either avoice or a data system [6–14]. Beforeintroducing these schemes, we assume thata system has many cells, with each havingS channels. The channel holding time hasan exponential distribution with mean rate

both originating and handoff calls aregenerated in a cell according to Poisson

processes, with mean rates O and H,respectively. We assume the system with ahomogeneous cell. We focus our attentionon a single cell (called the marked cell).

Newly generated calls in the marked cellare labeled originating calls (or new calls).A handoff request is generated in themarked cell when a channel holding MSapproaches the marked cell from a

neighboring cell with a signal strength below the handoff threshold Non priorityScheme

In this scheme, all S channels areshared by both originating and handoff request calls. The BS handles a handoff request exactly in the same way as anoriginating call. Both kinds of requests are

blocked if no free channel is available.The system model is shown in figure.With the blocking call cleared (BCC)

policy, we can describe the behavior of a

cell as a (S+ 1) states Markov process.Each state is labeled by an integer i(i = 0,




1,··· ,S), representing the number of channels in use. The state transitiondiagram is shown in Figure. A typicalM/M/S/Sequencing model models thesystem model.

Priority SchemeIn this scheme, priority is given to handoff requests by assigning SR channelsexclusively for handoff calls among the Schannels in a cell. The remaining SC(= S – SR) channels are shared by bothoriginating calls and handoff requests. Anoriginating call is blocked if the number of available channels in the cell is less thanor equal to SR(= S – SC).

Handoff Call Queuing SchemeThis scheme is based on the fact

that adjacent cells in a mobile cellular radio system are overlaid. Thus, there is aconsiderable area (i.e., handoff area)where a call can be handled by BSs inadjacent cells. The time a mobile user spent moving across the handoff area isreferred as the degradation interval. In thisscheme, we assume that the same channel-sharing scheme is used as that of a priorityscheme, except that queuing of handoff requests is allowed. When a MS movesaway from the BS, the received signalstrength decreases, and when it gets lower than a threshold level, the handoff

procedure is initiated.The first-in-first-out (FIFO)

queuing strategy is used and infinite queuesize at the BS is assumed. For a finitequeue size, see the discussion in the nextsection. The duration of a MS in the handoff area depends on system parameterssuch as the moving speed, the direction of the MS, and the cell size.

Originating and Handoff Calls Queuing

SchemeWe consider a system with many cells,each having S channels. In the BS, thereare two queues QHand QOfor handoff requests and originating calls, respectively(Figure). The capacities of QHand QOareMHand MO, respectively. A handoff request is queued in QH if it finds no freechannels on arrival. On the other hand, anoriginating call is queued in QO when onarrival it finds available channels less thanor equal to (S – SC). A request call is

blocked if on arrival its own queue is full.An originating call in the queue is deleted




from the queue when it moves out of thecell before getting a channel. Also, ahandoff request is deleted from the queuewhen it passes

through the handoff area before getting anew channel (i.e., forced termination) or

the conversation is completed before passing through the handoff area. A blocked handoff request call can stillmaintain the communication via thecurrent BS until the received signalstrength goes below the receiver thresholdor the conversation is completed before

passing through the handoff area. A blocked handoff call can repeat trialhandoffs until the received signal strengthgoes below the receiver threshold.However, the capacity of MH of queue

QH is usually large enough so that the blocking probability of handoff requestcalls can be neglected .

TRAFFIC MODEL:

Traffic Model Predictive channel reservation isconsidered to be the handoff protectionstrategy. The Manhattan model representsa typical city environment that ischaracterized by alignment of building

blocks with streets cutting apartneighboring blocks, as illustrated in

Figure. The base station antenna is usuallymounted approximately at the street leveland radio wave follows a line of sight

propagation mode. A pronounced problemis the quick connection disruption when amobile user turns around a street corner and suffers a sharp signal strength pathloss, which is commonly referred to asstreet corner effect.

The average arrival rate of new call intoone cell is and that of handoff is Channelholding time is determined by the dwellingtime an active mobile user spends in a cell.Using the word active, we refer to a user

being engaged in a call connection.

Fig :User Admission




Fig :Channel reservation and cancellation

When an active mobile user isdetected to be approaching cell boundary,its position and speed are monitored tocalculate its remaining time in the currentcell. Once this time falls below athreshold, defined as channel reservationinterval (CRI), it is deemed that theintention of transfer to another cell has

been confirmed, then a channel reservationrequest is issued to the target cell. If thereis idle channel in the target cell, onechannel is immediately reserved, alsoknown as locked in the sense that it istemporarily disabled for other usage in thetarget cell. If the target cell is presently outof free channel, the reservation requestwill be buffered in a queue. When achannel is released in the target cell, the

request queue is examined to see if anyrequest remains unprocessed. The requestat the head of the queue is then assignedthis free channel and leaves the queue. Areleased channel finding the queue emptyremains free until the next channelrequest. It is possible for a mobile user toend its call connection soon after areservation request has been sent out. Inthis case, a reservation cancellationrequest is forwarded to the target cell.Upon receiving a cancellation request, the

target cell releases the locked channel if corresponding reservation request has

been processed, or clears the reservationrequest from the queue if it remainsunprocessed. In our study, CRI is assumedto be accurate enough that call completionis the sole account for a mobile user not toshow up at expiration of CRI. When themobile next channel request. It is possiblefor a mobile user to end its call connectionsoon after a reservation request has beensent out. In this case, a reservationcancellation request is forwarded to thetarget cell. Upon receiving a cancellationrequest, the target cell releases the lockedchannel if corresponding reservationrequest has been processed, or clears thereservation request from the queue if itremains unprocessed.

Termination of a call:

The protocol for terminating a call in thelocal cell is illustrated in Figure 8. When acall U is terminated (either due to thenormal end of the call, or due to handoff

blocking), we first check if the channel being used by U is a foreign channel. If so,we release the foreign channel and returnit to its originally assigned cell. Otherwise,

U is using a local channel-the call is thenterminated and the channel becomes idle.We depict the protocol for handling thescenario when a local channel becomesidle. This scenario arises in the followingsituations:1. Termination of a call in the local cell.2. Handoff from the local cell to a foreigncell without carrying.3. Return of an idle local channel from aforeign cell (when a local channel isreleased in

the foreign cell and returned to the localcell).




USER ACCOMMODATIONCAPACITY :

Markov Approach

In an environment with

homogeneous traffic intensity and uniformchannel allocation, teletraffic performanceof a single cell is analogous to oneanother, so it is possible to focus on onecell to evaluate user accommodationcapacity over the entire system. A Markovmodel is employed for analysis on onecell, as seen in Figure. In this figure, astate is defined to be the sum of activechannels plus the number of reservationrequests pending processing. Assumingchannels allocated to each cell and buffer size limit for request queue, the transitionrates between neighboring states areobtained as follows. Before channeldepletion, both new and handoff calls areentertained, giving their gross arrival rateas the transition rate. In this case, a newcall takes one channel immediately while ahandoff call reserves one channel for later use. However, when all channels have

been occupied, further new call is rejectedand reservation request is queued up to a

maximum length of , resulting a transitionrate equal to the arrival rate of channelreservation requests only. Reservationextends the channel holding time fromCUI to COI. We have assumed negativeexponential distribution for CUI, thechannel holding time without reservation.For ease of analysis, COI is also assumedto be exponentially distributed, the meanvalue of which is obtained in (1)

The expected value can be written as

The average dormant time on state n is

given as

To facilitate the Markov approach, we

approximate the sojourn time of a requestin the queue to be exponentiallydistributed to give out a time-independentrequest-leaving rate that has a value equalto 1/TCRI.Following the Markov chain, the state

probabilities can be obtained as

When a new call comes at a state lower than c, it can be admitted, otherwise, it is




blocked. Then, the new call blocking probability is the sum of the state probabilities from c to c+s .

For a handoff call, a state higher than atthe time of reservation request arrival doesnot necessarily mean handoff failure.

Handoff failure probability can bedecomposed into a series of probabilitiesas in

The component probability on state ngives out the blocking likelihood if areservation request arrives at this state.This latest request will join the queue withanother n-c requests precedent to it.

It is noticed in this group of equations thatcalculation of Pbk and Phf depends on theknowledge of these two probabilitiesthemselves. To solve this problem, Pbk and Phf are given initial values, then theequations are followed to produce a new

pair of Pbk and Phf . The computationcycle continues until these two

probabilities converge to a certain pair of values, which will be the final result.Normalized User Capacity:Let λ denote the gross user arrival rate intoone cell, in which handoff users accountfor a portion of γ. Then, new user andhandoff user have their respective arrivalrate as

Longer CRI protects handoff better so thatlower is observed. When congestionhappens that the target cell has no freechannel upon the receipt of a reservationrequest, long CRI allows more time towait for a channel to be released in thetarget cell, consequently increasing the

possibility of successful reservation .

According to the definition of callincompletion probability, it can be writtenthat

Let Ptr be the transition probability for amobile user to leave its current cell beforecall completion, then the prematuretermination

call incompletion probability as

This probability increases when channelreservation becomes earlier.The normalized user capacity can be

derived as

Although channel reservation tends toimprove the call integrity that contributesto the number of complete servicesessions, it blocks the admission of newusers so that the user group in serviceshrinks




Output Graphs:

Pbk Vs Tcri:

Phf Vs Tcri:

Pic Vs Tcri:

Pic Vs Tcri:

CONCLUSION:

In this paper, to evaluate capacity of cellular systems, both the new user admission capability and the connectioncontinuity should be taken intoconsideration . This paper shows the initialuser acceptance plays a more importantrole in determination of capacity. In

presence of handoff protection, capabilityreduction in the new user admissionoverrides the improved connectioncontinuity. Consequently, the user accommodation capacity is weakened. Asconnection continuity reflects service

quality, this indicates that system capacityand service quality are two conflictingobjectives and tradeoff is inevitable.Using a model where the user mobility can

be accurately predicted, the impact of channel reservation on the system capacityhas been minimized.From the simulated results , it is clear thatchannel reservation will degrade systemcapacity and result in even worse radiousage efficiency in more realisticenvironments, where there are difficulties

for accurate mobility prediction.

effect of handoff strategies

Documents