exact location identification

8/8/2019 Exact Location Identification

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Exact Location Identification in a Mobile Computing NetworkKoushik SinhaDept. of Computer ScienceKaliani Govt. Engg. CollegeWB -741 235 , [email protected]

AbstractIn a mobile communication environment, when-ever there is a call request, the network must knowwhere the intended recipient of the call is currentlylocated. This gives rise to the problem of loca-tion management. Diflerent location management

schemes for mobile networks have been proposed sofar. But whatever be the scheme, it can only iden-tify a location area comprised of a number of cells,or at best a single cell, which may extend over anarea of a few square kilometers. But in case ofsome emergency needs of a mobile user, e.g., incase of fire, medical assistance, accident etc., it isessential to know the exact location of it, to pro-vide immediate services. In such circumstances,it is a prerequisite to identify the exact locationof the particular mobile user, within a radius of afew meters or so . So far, there is no such locationmanagement scheme that is capable of finding theexact location of any user with such an accuracy.In this paper, we propose a simple scheme for de-termining the exact location of a mobile terminal.By our technique, in case of some emergency, themobile user transmits a special distress signal. Inresponse to that, the base station Bo of the cell (inwhich the mobile user currently is), along with twoother adjacent base stations exchange a few mes-sages among themselves and the mobile, and henceit finally computes the exact co-ordinates of the mo-bile user. Next, with the assistance of detailed roadmap, the base station BO, an identify the exact lo-cation. I t has been shown that the technique enablesus to determine the exact location with an accuracyof a few meters only. Moreover, the overhead interms of extra hardware, the number of messagesand the message complexity, as well as the timedelay involved in the identification process are neg-ligibly small.Keywords : Location updation, binding, locationarea, message overhead, message complexity, timestamp, transmission delay1 Introduction

In recent years, wireless networking and mobilecommunication systems are becoming increasingly

Nabanita DasACM UnitIndian Statistical InstituteCalcutta -700 035, Indiandas@isical. c. in

popular throughout the world. Associated withthis, there has been a n ever growing demand forreal-time computation by users for various applica-tions, ranging from medical and military t o indus-trial and civil services, even when they are on move.This has given rise to the concept of mobile com-puting, which is based on the ideas of distributedcomputing with some special considerations due tothe mobility of the users.

The mobile radio communication systems gen-erally assume a cellular architecture, where the re-gion under the control of the network is dividedinto a number of hexagonal cells; the wireless com-munication in each cell is governed by a base sta-tion (BS) [3,10]. The coverage of a typical cellvaries from some kilometers, as in the current cellu-lar networks, to a few meters, as proposed for thefuture Personal Communication Networks (PCN)[20].Each mobile user of the network, has a uniquelogical identifier. The mapping between the logicalidentifier and the physical location of the mobileuser is called the binding. As the users are mobile,the binding changes over time, and there we needlocation management.Location management is a two-step process thatenables the network to find the current attachmentpoint of the mobile user for call delivery. T he firststep is location update. In this stage, the mobileterminal periodically notifies the network of its cur-rent location. The second step is the search, duringwhich the network is queried for the current loca-tion of the mobile terminal to deliver the call.Now the question is when and where the mobileshould send a location updation message. Several

authors have addressed this issue in the literature[l - 9, l l - 15 , 17-19,221. If for each cell crossing, thereis an update message, the signalling traffic due toupdating alone may contribute to an appreciablefraction of the total traffic. Thus location upda-tion will become a major bottleneck in the network.On the contrary, saving location updation cost willincrease the search cost and the latency in the net-work.

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Location upda te schemes are characterized to beeither stat ic or dynamic. In the s tatic or global up-date scheme, the cells at which a mobile updatesare fixed. They are global in the sense tha t all ora group of mobiles always generate their updatemessages from the same set of cells. Th e majordrawback of this scheme is that if a user hops inand out of the reporting cells, the user will gen-erate non-informative update messages, which isundesirable. These methods deliberately leave acontrolled level of uncertainty about the user's ex-act location, which is eliminated by different typesof search strategies.

Under the dynamic update scheme 11, 13 , 211,the update occurs based on the activity of the useronly. I t may not occur at predetermined cells. Intime-based scheme, the mobile generates periodicupdate messages [7]. In distance-based scheme,the user measures the Euclidean distance from thelocation of the last update and generates a newupdate message if the measured distance exceedsa threshold value. In movement-based scheme,the mobile counts the number of cell boundariescrossed and triggers an update message when thisnumber crosses a threshold value [2,7]. A new up-date scheme called the LeZi-Update scheme hasbeen proposed in [4]designed around the well-known universal lossless compression techniques.This scheme tackles the problem of location man-agement from an information-theoretic viewpoint.The update scheme tries to learn the nature ofuser mobility with optimal message exchange. Thishelps in reducing the average paging cost and im-proves the predictive power of the system.

However, none of the above schemes are able tolocate the exact location of a mobile user. Theycan at most identify the location of a mobile withrespect to a single cell, which typically covers anarea of about a few square kilometers. But in cer-tain applications, e.g., medical emergency, distresscall to be attended by the police, military purposesetc., knowledge about the exact location of a mo-bile user is extremely necessary. In this paper, wehave focussed on a particular issue of location'man-agement problem, namely, pinpointing the exactlocation of a mobile user. By this technique, themobile generates a distress call whenever needed,which is received by the corresponding base stationBo, under which the mobile currently is. Bo thencontacts two other adjacent base stations, and eachtransmits some control messages with their localtime stamps to the mobile terminal. Later Bo re-cieves all the three messages retransmitted by themobile in turn with additional time stamps, andhence computes the exact coordinates of the mo-bile within its cell. The process is almost instan-

taneous and requires very little overhead in termsof the messages exchanged and computations in-volved. Moreover it can find the location of a mo-bile with an uncertainty of about a few meters only.The rest of the paper is organized as follows.In Section 2, we present an overview of our pro-posed scheme including the databases used, typesand structures of the messages involved. The al-gorithm for determining the exact location of themobile, along with the necessary overhead in termsof the number of messages and the complexity ofcomputations, is presented in section 3, followed bythe conclusion in section 4.

2 Exact Locat ion Identification SchemeThe conventional ceIlular architecture of mobilecommunication network consists of a wired back-bone network and a wireless network, as has beenshown in Fig.1. Th e entire geographical service

area is divided into a number of cells, each cell be-ing under the control of a Base Station (BS). Ide-ally the cells are assumed to be hexagonal in shape,but in practice, they may be of any arbitrary shape.Several cells are grouped together to form a Loca-tion Area (LA). A number of base stat ions are usu-ally connected to a base station controller (BSC),and a number of BSCs are further connected toa mobile switching center (MSC), either throughwired or wireless connections. We thus have a tree-like hierarchical structure of MSC, BSCs and BSswith a number of wireless-linked mobile users un-der each BS. Each mobile user uses two channels -the uplink channel and the downlink channel - tosend and receive information (data, voice, video)to and from the base station currently servicing it.The channels are allocated either statically or dy-namically to the users. The same channel cannotbe reused within a certain distance owing to possi-ble co-channel interference. Some of the channelsare reserved for special purposes. They are used totransmit and receive specific control signals and theinformation on these channels is called signallinginformation.2.1 An Overview of the Proposed Scheme

Here we assume a cellular architecture wherethe cells may be of arbitrary shape, and each basestation BS maintains a database about its adja-cent base stations, i.e., the base stations servicingthe cells which have common boundary with thecell serviced by BS. The database contains the ge-ographical co-ordinates of the adjacent base sta-tions. Obviously the number of adjacent base sta-tions will be a few only, for hexagonal cells it isalways six.

When a mobile user M is in distress, it trans-mits a special distress message through its uplink

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channel. The master base station Bo, correspond-ing to the current cell of M receives it. This mes-sage is categorized as a highest priority one by Boand is serviced immediately. Bo selects two adja-cent base stations B1 and B2, from its database,to assist it in servicing M . They will be called asthe slave base stations. Next, Bo sends three spe-cial control packets to the mobile M , one directlyto it and two others through B1 and Bz respec-tively. Each of these packets would have varioustime stamps on it indicating the local time inter-vals between the receiving and the sending of thepacket by each station involved in the communica-tion. These time stamps will be used for estimatingthe distance dim between the base station Bi andthe mobile M , for all i, 0 5 i 5 2. The mobile Mwould then be located at the point of intersectionof all the three circles centered at BO,1 and B2 ,with radii dom, dl m and d2m, respectively as shownin Fig. 2, where dol and do2 are the distances ofB1 and B2 respectively, from Bo.

It is, however, desirable that the three base sta-tions involved in the process do not lie on the samestraight line. T he situation is shown in Fig.3. It isevident that in this case, the three circles, as havebeen mentioned above, will intersect a t two distinctpoints, resulting an ambiguity in the exact locationof M . Therefore, we would assume that BOwill se-lect B1 and B2, from its database such that thethree base stations are non-collinear, and thereforecan pinpoint M uniquely, as has been shown in Fig.94.

To implement our technique, we consider a mo-bile computing environment with the following ad-ditional features :a

a

a

a

a

To

Each base s tation maintains a database abou tthe location and identity of the adjacent ones.The special message packets sent by the basestations to Mar e of sufficient strength to reachthe mobile.There are special hardware units at each basestation dedicated to sending and receivingdata packets only for this specific purpose.Channels are kept reserved at each base sta-tion for servicing these emergency calls.The mobile remains stationary during the en-tire process.satisfv the second condition, each base sta-tion may be provided with a booster facility atthe transmitter, such that to service the emer-gency calls only, the base station will switch tothe booster transmitter , and transmits the message

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with higher power such that the coverage is en-hanced to almost double of its normal coverage. Af-ter the transmission, it returns back to its originaltransmission power. Since the messages used forthis purpose a re quite short in length, the increasein power requirement will be negligibly small.The last condition will be true if the speed of themobile user is such that within the time requiredby the whole process its displacement is insignifi-cant . Typically the condition will be satisfied forusers with moderate speed.

2. 2 Distance ComputationWhen a master base station Bo receives a dis-tress call from a mobile M , it immediately sendsou t a packet as a tracking signal to M , and notesit s local time t o . Let, the mobile Mreceive it at itslocal time tom, and retransmit the packet back toBo at a later time instant tbm, attaching this timedifferenceTom(= bm - t o m ) , to the packet. Let BOreceive it at its own local time tb.and M can be calculated from the equation :Hence, the Euclidean distance dom between Bo

where c represents the velocity of light.Next, Bo sends a control packet to B1, at its lo-cal time to l . On receiving this control packet fromBo, B1 immediately retransmits the packet to themobile M , attaching the local time differenceTI be-

tween receiving the packet, and retransmiting i t.As the packet reaches M , it retransmits thepacket to Bo attaching Tim, the time difference be-tween receiving and retransmitting the packet byM.Bo receives this retransmitted signal from M atits local time tbl, say. The Euclidean distance dlmbetween B1 and M can then be calculated from theequation :

Here dol is the distance between Bo and B1, whichis known to Bo, and dom is the distance betweenBo and the mobile M .

Next, Bo repeats the process with B2, hence weget :

tb2 -



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where,t02, and tb2are the sending and receiving times

0 do2 is the distance between Bo and B2,0 T2 is the local time interval between receivingand retransmitting the control packet in B2,

d2m is the distance between B2 and t he mobileM , and0 T2, is the local time interval at B2, betweenthe receiving and retransmitting the controlpacket to M .

Bo then computes do,, d l , and dam from theabove equations, and the exact location of the mo-bile is found by solving the following three simul-taneous equations :

of the control packet in Bo,

2 2( - + (Y - Yo) = do, (4)(5)(6 )

2 2(z - + (Y - 1 ) d l m(z- 2 ) ~(Y - 2 ) ~d i m

where. (ZO,YO) , ( Z 1 , Y l ) and ( Z 2 , Y 2 ) are the geo-graphical co-ordinates of the three base stationsBo, B1 and B2 respectively. These values can beobtained from the database residing with the basestation Bo.From the equations (4), 5) and (6) , we obtaintwo linear equations of the form :

U 1 2 + b1y + c1 = 0U 2 2 + bay + c2 = 0 (7 )(8)

Solving equations (7) and (8) we get one andonly one distinct value of (z , ) , which simultane-ously satisfies equations ( 4 ) , (5) and (6) , providedthe three base stations Bo, B1 and B2 are non-collinear. The z and y co-ordinates so obtainedgive the geographical position of the mobile userM .2.3 Implementation

We assume that each base station, as well as themobiles maintain special high frequency clocks fornoting the time instants of sending and receiving apacket. The clocks need not be synchronized, butthe frequency of each should be known precisely.We will refer to it as the local clock, and hence allthe local time instants at any base station or a mo-bile will refer to the current local clock reading.In order to implement the above scheme, we needto maintain a small database at every base station

and use certain special control signals which havebeen described below.Each base station Bi in the system maintainsa database which contains information about the

base stations adjacent to it. Each record of thisdatabase corresponds to one adjacent base stationBj, and consists of the following four fields :0 1D:The identity of the adjacent base station

0 L0C :The geographical co-ordinates (zj,j ) ofthe base station Bj.0 FREQ:The frequency of the local clock fj usedfor emergency service.0 LIST: A list of other base stations Bk, djacentto Bi, such tha t the three stations Bi, Bj and

B k are non-collinear.Now, whenever there is a distress cull, Bi selects aslave base station B . from its database arbitrarily,and then from the LIST field of the record corre-sponding to Bj, it again selects one B k arbitrarily,to ensure that the three base stations altogetherare non-collinear, and hence can identify the exactlocatation uniquely from equations (7) and (8).

Bj.

2.4 Message TypesThe base stations and the mobile user use dif-ferent types of messages to communicate with eachother. Following is a description of each messagetype and its purpose.0 NORMAL - this indicates that the messagecontains information which can be processedaccording to the normal scheduling algorithmof the base station and the mobile users. Itcan be sent by both the base stations and themobile users.0 EMERGENCY - this is actually the distresscall, sent by the mobile user. The servicingbase station, on receiving the message, imme-diately services it as has been described in thealgorithm in section 3.0 TRACKM - this is the message sent by themaster base station to the mobile, to track it,

in response to the distress call.0 RETRACKM- this is the response from mo-bile to the master base station in response toTRACKM packet.0 EMERG-CTRL - it is sent by the master basestation to the slave stations asking them togenerate tracking signals for the mobile.

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0 TRACKS - it is broadcasted by the slave basestations to track down the distressed mobile.0 RETRACKS - it is the response from the mo-

bile to the slave base stations in response tothe TRACKS packet.All types of messages, mentioned above, exceptthe NORMAL one, have the same priority, whichis higher than the priority of the NORMAL mes-sages.These messages consist of a variable-lengthheader, followed by a variable-length da ta field.The header includes the length of the entire mes-sage, its type, and the identity of the mobile. Eachtime a station (the mobile or the base stations) re-transmits a message, it appends some additionaltags (time stamps, and receiver identities etc.) atthe end of the message, and changes the messagelength accordingly. The formats of different typesof messages are shown in Fig. 4.The variable part of the message consists of a listof typed data items. A similar message structurecan be found in the message-based operating sys-tem, e.g., the Accent Operating System developedfor distributed environments [16].

3 Algorithm for Location Ident i f icat ionHere foMows the outline of the algorithm foridentification of exact location of the mobile M.3.1 Algorithm LOCIDS te p 1 : The distressed mobile user Mini tia tes

the process by sending a message of type EMER-GENCY.S te p 2 : The master base station BO, n re-ceiving a message packet of type EMERGENCY,and sends out a TRACKM message to the mobileM appending its identity master-ID at the end ofthe EMERGENCY message it received. As the re-transmission s tarts , BO otes its local time instantt oStep 3 : BO elects two adjacent base stationsB1 and B2 from its database such tha t BO, 1 and

B2 are non-collinear.

S te p 4 : Bo sends an EMERG-CTRL packetto B1, appending the master-ID and the slave-IDat the end of the EMERGENCY packet. BO lsonotes the local time to 1 at the start of sending thepacket.

,

S te p 5 : When a mobile detects that it has re-ceived a message of type TRACKM, it notes thetime instant t om , next checks the mobile-ID. If it is

not i ts own ID, it simply ignores the packet. Other-wise, it receives the whole packet, and retransmitsit as a RETRACKM message, by attaching the fre-quency of its local clock, and the time differenceTom = tb, - om , at the end of the message, wheretbm is the time instant when it starts retransmis-sion.

S te p 6 :As soon as a base station Bi detects tha tthe type of the message it is receiving is EMERG-CTRL, it immediately notes its local time stampt i , and receives the whole packet. It checks theslave-ID field. If it is its own ID, it retransmitsthe message modifying it into a TRACKS message(i.e., appending the time difference T,, = ti - i atthe end of the message, where ti is the time instantwhen it s tart s retransmission), otherwise it just ig-nores the packet.

Step 7 : As soon as a mobile detects that thetype of the message it is receiving is TRACKM (or TRACKS), it immediately notes its local timestamp t im , receives the whole packet, checks themobile-ID field, if it is its own ID, it retransmits themessage modifying it into a RETRACKM (or RE-TRACKS) message (i.e., appending the frequencyof its clock, and the time differenceTim = tim-&,at the end of the message, where t'im is the timeinstant when it start s retransmission, otherwise itjust ignores the packet.Step 8 : When a base station detects thatthe type of the message it is receiving is a R ETRACKM (or RETRACKS) message, it notes its

current local time, and receives the whole packet.If it is the RETRACKS packet fromB1, the basestation repeats step 4 onwards for the second slavestation B2. Otherwise it computes the exact co-ordinates of the mobile with the help of equations(1-8).

3.2 OverheadIn the algorithm LOCID, a total of nine messagecommunication steps are required to pinpoint theexact location of the mobile M. However, the lengthof these message packets are very small which re-duces the total transaction time. Small messagesize also means low probability of interference and

less channel occupancy.We also need to maintain a small database atevery base station. Since the database consists ofonly four fields per record for each base station,and information about only the adjacent or neigh-boring base stations is maintained, this overhead isalso negligible.

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To record the time instants of sending and re-ceiving the emergency packets, a special hardwaresupport may be useful at each of the base stationsand mobile users. This hardware requirement is,however, rather simple, e.g., it is just a fixed bitcounter with a high frequency clock input, sa y of100 MHz, which just counts the number of inputpulses to this counter between the receiving andsending time instants of the control packet a t thatnode (base station or mobile). Instead of time-stamping the packet with the actual local time,only corresponding counter reading is supplied inthe packet. This would also necessiate only onefield (instead of two) for noting the difference be-tween sending and receiving time instants at themobile as well as the slave base station, thus re-ducing the size of the message header.

For computation of the distances Bo needs theclock frequency of the slave base stations, as wellas the mobile. Here, we have assumed that theclock frequencies of the slave base stations are keptstored in the database of the master base station,whereas that of the mobile is informed through theRETRACKM packet sent by the mobile.3.3 AccuracyTypically, if we assume that the distance be-tween a mobile user Ma nd its servicing base stationBo is 1.5 kilometer, then the time of flight of theelectromagnetic signal (radio wave or microwave)for a round trip (from the base station Bo to Mand back to Bo) is given by

3 x io 3 3 x io3C 3 x lo*j= - - -- seconds= 10psec.If a counter is used a t each of the nodes forrecording the time interval between the sendingand receiving time instants, and the input clockpulse to this counter has a frequency of 100 MHz,then there will be an uncertainty of only one clockpulse in recording this time interval at any node(base station or mobile), amounting to an error of10 ns time in measuring the time of flight t j , i.e.,

an error of 0.1% only. This amounts to an error ofonly 10 x 1 0 - ~ 3 x 108meter = meter in mea-suring a distance. Therefore, from equation ( l ) ,wefind that in computing the distance do,, there maybe an error of 3 meter at maximum.However, while computing dl, and d2, by equa-tion (2) and (3) respectively, the time interval be-tween the receiving and sending of a packet have tobe recorded by the master base station, the slavebase station ( B1 or B2) and also the mobile M.This would cause an error of 30 ns (equivalent to9 meter) in recording the time of flight of the elec-tromagnetic signal in making a round trip from Bo

to B1 (or B2) , then to M , and then back to Bo.Assuming that there is an uncertainty of 3 meterin computing do,, there may be an error of only 12meter in computing dl, or d2,, if all errors are inthe same direction. Since, the exact co-ordinatesof the mobile is the point of intersection of all thethree circles centred at Bo, B1 and B2, with radiido,, dl, and d2, respectively, it is easy to see thatthe inaccuracy in pinpointing the exact location ofMwill be upper bounded by 12 meter only. The un-certainty or error in this estimation can be furtherreduced by using a clock of still higher frequency,leading to a fairly exact identification of the loca-tion of the mobile user M .

Note that the receiving station may need a fewclock cycles to receive the message type field andactivate the emergency service routine. However,this delay is fixed and a priori known by the receiv-ing station, and hence, can easily be taken care ofin recording the actual interval without impairingthe accuracy.4 Conclusion

We have proposed a simple technique for identi-fication of the exact location of a mobile (with anuncertainty of a few meters only) which is basedon message communication under an emergencysituation. This process requires only nine mes-sage communication steps, and very little compu-tational overhead. The length of each of these mes-sages involved in this process is very small, with-out blocking the channel for a long duration. Thetechnique can be successfully employed in locatinga mobile user under a distressed situation by incor-porating a very simple hardware attachment in thesystem. Further improvement of this scheme canbe made by making the system fault-tolerant andreducing the number of messages exchanged.References[l] I. F. Akyildiz and J . S. M. Ho, "Dynamic mo-bile user location update for wireless PCS net-works," Wireless Networks, vol. 1, no. 2, pp.187 - 196, July 1995.[2] I. F. Akyildiz and J. S. M. Ho, "Movement-based location update and selective paging forPCS networks," IEEE/ACM Transactions onNetworking,vol. 4, o. 4, p. 629 - 638, August1996.[3] B. R. Badrinath and T. Imielinski, "Locationmanagement for networks with mobile users,"in Mobile Computing,T. Imielinski and H. F.Korth (eds.), pp. 129 - 152. Massachusetts :Kluwer Academic Publishers, 1996.

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[4] A. Bhattacharya and S. K. Das, "Le-Zi up-date: an information-theoretic framework forpersonal mobility tracking in PCS networks,"Proceedings of the Fifth Annual ACM,/IEEEInternational Conference on Mobile Comput-ing an d Networking, Seattle WA, pp. 1-12 ,August 1999.

[5] A. Bar-Noy, I. Kessler and M. Sidi, "Mobileusers: to update or not to update ?," WirelessNetworks, vol. 1, no. 2, pp. 175 - 185, July1995.[6] A. Bar-Noy and I. Kessler, "Tracking mobileusers in wireless communication networks) "Proceedings IEEE INFOCOM '93, San Fran-sisco, pp. 1232 - 1239, March - April 1993.[7] Y. Birk and Y. Nachman, "Using directionand elapsed-time information to reduce thewireless cost of locating mobile units in cellu-lar networks," Wireless Networks, vol. 1, no.4 , pp. 403 - 412, December 1995.181 T. X. Brown and S. Mohan, " Mobilitymanagement for personal communication sys-tems," IEEE Transactions on Vehicular Tech-nology, vol. 46 , no. 2, pp. 269 - 278, May 1997.[9] S. K. Das and S. K. Sen, "Adaptive locationprediction strategies based on a hierarchicalnetwork model in a cellular mobile environ-ment," The Computer Journal (Special issueon Mobile Computing, pp. 473-486, 1999.

[lo] D. J. Goodman, "Cellular Packet Communi-cations," IEEE Transactions on Communicu-tions, vol. 38 , pp. 1272 - 1280, August 1990.

[l l ] S. K. Das and S. K. Sen, "A new locationupdate strategy for cellular networks and itsimplementation using a genetic algorithm,"Proceedings A CM/IEEE International Con-ference on Mobile Computing and Networking(Mobicom '97),pp. 185 - 194, September 1997.[12] J. S. M. Ho and I. F. Akyildiz, "Mobile userlocation update and paging under delay con-straints," Wireless Networks, vol. 1, no. 4 , pp.41 3 - 425, December 1995.[13] S. J. Kim and C. Y. Lee, "Modeling and anal-ysis of the dynamic location registration and

paging in microcellular systems," IEEE Trans-uctions on Vehicular Technology, vol. 45 , no.1, pp. 82 - 90 , February 1996.[14] G. L. Lyberopoulos, J. G. Markoulidakis, D.V. Polymeros, D. F. Tsirkas and E. D. Sykas,"Intelligent paging strategies for third gen-eration mobile telecommunication systems,"

IEEE Transactions on Vehicular Technology,vol. 44 , no. 3, pp. 543 - 553, August 1995.[15] S. Madhavapeddy, T. K. Basu and A. Roberts,"Adaptive paging algorithms for cellular sys-tems," Technical Report, Bell Northern Re-search, Richardson, Tx, 1994.[16] J. L. Peterson and A. Silberschatz, OperatingSystem Concepts. Massachusetts : Addison-Wesley, 1985.1171 D. Plassmann, "Location management strate-gies for mobile cellular networks of 3rd genera-tion," Proceedings 44th IEEE Vehicular Tech-nology Conference, pp. 649 - 653, June 1994.[18] C. Rose, "Minimizing the average cost of pag-ing and registration: A timer-based method,')Wireless Networks, vol. 2, no. 2, pp. 109 - 116,June 1996.[19] C. Rose and R. Yates, "Minimizing the av-erage cost of paging under delay constraints,"Wireless Networks, vol. 1, no. 2, pp. 211 - 219,July 1995.[20] R. Steele, "Deploying Personal Communica-tion Networks,') IEEE Communications Mag-azine, vol. x, pp. 12 - 15 , September 1990.[21] H. Xie, S. Tabbane and D. Goodman, "Dy-namic location area management and perfor-mance analysis," Proceedings of the 43rd IEEEVehicular Technology Conference, pp. 536 -539, Secaucus, May 1993.[22] R. Yates, C. Rose, S. Rajagopalan and B.Badrinath, "Analysis of a mobile-assistedadaptive location management strategy," M o-bile Networks an d Applications, vol. l , no. 2 ,pp. 105 - 112, October 1996.

- MS Cm -0 s-cell

Fig. ]:The hierarchical architectureofmobile network

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exact location identification

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