Mobile Phone Location Determination and Its Impact on Intelligent Transportation Systems

Mobile Phone Location Determination and Its Impact on Intelligent Transportation Systems

Yilin Zhao,Senior Member,IEEE

Abstract—Research and development on the technologies of locating the mobile(wireless)phone caller have been rapidly gaining momentum around the world.Once these technologies are mature enough to be deployed,they will have significant impact on automotive telematics and modern public transit systems.In this paper,we will discuss why locating mobile phones becomes a hot topic among telecommunications giants,what technologies are being studied and standardized,when we are going to see the actual deployment,and what services they may provide.We will then consider its potential impact on future intelligent trans-portation systems(ITS),including telematics and public transit systems.Many of us have already recognized how important a role the communications systems play in modern transportation. In the near future,if every mobile phone is able to determine its location,advances in our current transportation systems become inevitable.

Index Terms—A-FLT,AOA,assisted-GPS(A-GPS),CDMA, E911,E-OTD,fusion,GSM,intelligent transportation systems (ITS),public transit systems,standards,TDOA,telematics,TOA, wire-less location.

I.I NTRODUCTION

M ANY transportation applications can be supported by centralized location and navigation systems,which uti-lize communications networks,host facilities,and other infra-structures together with the on-board vehicle equipment to lo-cate and navigate[1].These applications include automotive telematics and modern public transit systems.An automotive telematics system typically communicates over a telecommu-nications network while a modern public transit system is an application of the automatic vehicle location(A VL)system. One important application that can be supported by central-ized systems is a wireless-enhanced911(E911)system.The911 telephone number is used in the United States to provide emer-gency assistance for the caller,initially for the line-based(wire-line)telephone system only.Other countries have similar sys-tems,but may use different numbers such as110for China,999 for the United Kingdon,and17for France.The key advantage of an enhanced911over the basic911is that the public safety officer knows the caller’s location and phone number.“Wire-less E911”is a system evolved from the original wireline-based system to the wireless system.Ideally,it would provide the same functionality as the wireline-based implementation.

Manuscript received July1,1999;revised March21,2000.The Associate Editor for this paper was Dr.Shoichi Washino.This paper was presented in part at the Modern Transport in Hong Kong for the21st Century Conference,May 1,1999.

The author is with PCS Research Labs,Motorola,Inc.,Libertyville,IL 60048-5343USA(e-mail:yilin.zhao@https://www.sodocs.net/doc/094719756.html,).

Publisher Item Identifier S1524-9050(00)07345-2.

TABLE I

A CCURACY R EQUIRED FOR L OCATING

M OBILE P

HONES

The U.S.Federal Communications Commission(FCC)has recently made E911a mandatory requirement for wireless communications services such as cellular telephone,wideband (broadband)personal communications services(PCS),and geographic area specialized mobile radio(SMR).This ruling and upcoming service is called wireless E911.The FCC requires that by October1,2001,public safety answering point (PSAP)attendants of wireless communications networks must be able to know a911caller’s phone number for return calls and the location of the caller,so that calls can be routed to an appropriate PSAP and related emergency assistance attendants [2].On September15,1999,the FCC decided to tighten the location accuracy requirement for Phase II implementation from125m in67%of all cases to the new numbers shown in Table I[3].In addition,the FCC requests manufacturers to begin selling and activating location-capable handsets no later than March1,2001.Although certain companies are still petitioning the FCC to relax this new edict,FCC’s action could facilitate the development of many vehicle location and nav-igation applications that use communications infrastructures similar to those used for wireless E911.Besides emergency assistance,it will certainly trigger many location-based services with the mobile phone or wireless network.Therefore,it is not difficult to understand why telecommunications manufacturers and operators have been actively pursuing the technologies to locate the mobile phone.

II.L OCATION T ECHNOLOGIES

There are three most commonly used location technologies: stand-alone,satellite-based,and terrestrial radio-based[4].As examples,a typical stand-alone technology is dead reckoning.A typical satellite-based technology is global positioning system (GPS).A typical terrestrial radio-based technology is the“C”configuration of the Long Range Navigation(LORAN-C) system.For wireless E911,the radio-based(satellite and terrestrial)technologies are the most popular ones.Cellular networks are terrestrial-based communications systems.It is natural to utilize the signals of the network to determine the mobile phone location or to assist the location determination. Research in this area has been very active recently as evidenced

1524–9050/00$10.00?2000IEEE

Fig.1.Location determination by angle of arrival(AOA).

by the new round of publications[1],[4]–[20].In this paper, we will address these radio-based technologies only.The principles behind them are discussed below.For information on technology developers,refer to[21].

Radio-based technology typically uses base stations,satel-lites,or devices emitting radio signals to the mobile receiver to determine the position of its user.Signals can also be emitted from the mobile device to the https://www.sodocs.net/doc/094719756.html,monly studied tech-niques are angle-of-arrival(AOA)positioning,time-of-arrival (TOA)positioning,and time-difference-of-arrival(TDOA) positioning.All these methods require radio transmitters, receivers,or transceivers.In other words,they depend on emit-ting and receiving radio signals to determine the location of an object on which a radio receiver,or a transceiver is attached.To make the position determination,these methods generally have the assumption that one end of the positioning system is fixed and the other end is moveable such as a mobile phone.How-ever,the location determination capability can be either at the fixed end or at the mobile end.Generally,it is up to the system designer to decide where the final location determination capability should reside.For performance improvement,hybrid methods(various combinations of the techniques discussed or with additional techniques)are possible.

The angle-of-arrival(AOA)system determines the mobile phone position based on triangulation(Fig.1).It is also called direction of arrival in some literature.The intersection of two directional lines of bearing defines a unique position,each formed by a radial from a base station to the mobile phone in a two-dimensional space.This technique requires a minimum of two stations(or one pair)to determine a position.If available, more than one pair can be used in practice.Because directional antennas or antenna arrays are required,it is difficult to realize AOA at the mobile phone.

The time-of-arrival(TOA)system determines the mobile phone position based on the intersection of the distance(or range)circles(Fig.2).Since the propagation time of the radio wave is directly proportional to its traversed range,multiplying the speed of light to the time obtains the range from the mobile phone to the communicating base station.Two range mea-surements provide an ambiguous fix and three measurements determine a unique position.The same principle is used by GPS,where the circle becomes the sphere in space and the fourth measurement is required to solve the receiver-clock bias for a three-dimensional solution.The bias is caused by the unsynchronized clocks between the receiver and the satellite.Similarly,for terrestrial-based systems,it also requires precisely synchronized clocks for all transmitters and

receivers.Fig.2.Location determination by time of arrival

(TOA).

Fig.3.Location determination by time difference of arrival(TDOA). Otherwise,a

1-

ZHAO:MOBILE PHONE LOCATION DETERMINATION AND ITS IMPACT ON INTELLIGENT TRANSPORTATION SYSTEMS57 radio signals arriving at a cell site from a caller.For measuring

the signal strength,it employs multiple cell sites to find the loca-

tion.For measuring the signal characteristic patterns,it identi-

fies the unique radio frequency pattern or“signature”of the call

and matches it to a similar pattern stored in its central database.

Because AOA requires the installation of directional antennas

or antenna arrays,TOA and TDOA have been chosen as the cur-

rent standardization choices.Of course,this may change if the

next-generation systems can be equipped with these antennas.

Both TOA and TDOA are time-based measurement technolo-

gies.They can be implemented either based on the forward

(down)link signal or reserved(up)link signal.In addition,the

location determination capability can reside either at the net-

work side or at the mobile phone.In order to“hear”several

base stations or cell sites,the sensitivity of the mobile phone

may need to be increased.For better location accuracy,certain

phones may require higher chip or bit resolution such

as

.These methods also require software modification on the

mobile phone and additional location determination units and

related software in the network.As discussed above,the mobile

phone needs to listen to the signals of at least three base stations

or cell sites.The hearability and geographical locations of these

base stations will affect the availability and the accuracy of the

location determination.

Since the performance of the satellite-based GPS receiver is

getting better and better while the receiver size and price keep

going down,it becomes feasible to develop an assisted GPS

(A-GPS)solution for the mobile phone,which requires soft-

ware and hardware modifications of both the mobile phone and

its communications network.To understand this popular tech-

nology better,we will spend a little bit more space below to

discuss it.

GPS provides an affordable means to determine position,

velocity,and time around the globe[1],[22],[23].The satellite

constellation is developed and maintained by the U.S.De-

partment of Defense.Civilian access is guaranteed through

an agreement with the Department of Transportation.GPS

satellites transmit two carrier frequencies.Typically,only one

is used by civilian receivers.From the perspective of these

civilian receivers on the ground,GPS satellites transmit at

1575.42MHz using code-division multiple-access(CDMA)

technique,which uses a direct-sequence spread-spectrum

(DS-SS)signal at1.023MHz(Mchips/s)with a code period of

1ms.Each satellite’s DS-SS signal is modulated by a50-bit/s

navigation message that includes accurate time and coefficients

(ephemeris)to an equation that describes the satellite’s position

as a function of time.The receiver(more precisely,its antenna)

position determination is based on TOA.

The four main conventional GPS receiver functions are as

follows.

1)Measuring distance from the satellites to the receiver by

determining the pseudoranges(code phases).

2)Extracting the time of arrival of the signal from the con-

tents of the satellite transmitted message.

3)Computing the position of the satellites by evaluating the

ephemeris data at the indicated time of

arrival.

Fig.4.Assisted GPS positioning(BS stands for base station and SMLC stands

for serving mobile location center).

4)Calculating the position of the receiving antenna and the

clock bias of the receiver by using the above data items.

Position errors at the receiver are contributed by the satellite

clock,satellite orbit,ephemeris prediction,ionospheric delay,

tropospheric delay,and selective availability(SA).SA is an ac-

curacy-degradation scheme to reduce the accuracy available to

civilian users to a level within the national security require-

ments of the United States.It decreases the accuracy capability

of autonomous GPS to the100-m(2D-RMS)level,where RMS

stands for root-mean square.To reduce these errors,range and

range-rate corrections can be applied to the raw pseudorange

measurements in order to create a position solution that is accu-

rate to a few meters in open environments.The most important

correction technique is differential GPS(DGPS)[1],[22],[23].

It uses a reference receiver at a surveyed position to send cor-

recting information to a mobile receiver over a communications

link.Note that SA has been turned off since May2000.

In addition to the task of shrinking the GPS antenna to fit a

typical mobile phone,a traditional autonomous GPS receiver

chipset is difficult to embed in the mobile phone for three main

reasons.First,its startup time(from turning on to the initial po-

sition fix)is relatively long due to its long acquisition time of

the navigation message(at least60s to a few minutes).Second,

it is unable to detect weak signals that result from indoor and

urban canyon operations,especially with small cellular sized an-

tennas.Third,its power dissipation is relatively high per fix,pri-

marily due to the long signal acquisition time in an unaided ap-

plication.To deal with these problems,the assisted GPS method

was proposed(Fig.4).

The basic idea of assisted GPS is to establish a GPS reference

network(or a wide-area DGPS network)whose receivers have

clear views of the sky and can operate continuously.This refer-

ence network is also connected with the cellular infrastructure,

and continuously monitors the real-time constellation status and

provides precise data such as satellite visibility,ephemeris and

clock correction,Doppler,and even the pseudorandom noise

code phase for each satellite at a particular epoch time.At the

58IEEE TRANSACTIONS ON INTELLIGENT TRANSPORTATION SYSTEMS,VOL.1,NO.1,MARCH2000

request of the mobile phone or location-based application,the assist data derived from the GPS reference network are trans-mitted to the mobile phone GPS receiver(or sensor)to aid fast startup and to increase the sensor sensitivity.Acquisition time is reduced because the Doppler versus code phase uncertainty space is much smaller than in conventional GPS due to the fact that the search space has been predicted by the reference re-ceiver and network.This allows for rapid search speed and for a much narrower signal search bandwidth which enhances sen-sitivity and reduces mobile receiver power consumption.Once the embedded GPS receiver acquires the available satellite sig-nals,the pseudo-range measurements can be delivered to net-work-based position determination entity(PDE)for position calculation or used internally to compute position in the handset. Additional assisted data,such as DGPS corrections,approx-imate handset location or cell base station(BS)location,and other information such as the satellite almanac,ionospheric delay,universal time coordinated(UTC)offset can be trans-mitted to improve the location accuracy,decrease acquisition time,and allow for handset-based position computation. Several schemes have been proposed in the standards which reduce the number of bits necessary to be exchanged between the handset and the network by using compression techniques such as transmitting only the nonredundant or the changes to parameters instead of the raw parameters themselves.Other satellite systems could be used,such as the Russian GLONASS system,but none of the standards have made provision for anything except GPS and the future GPS Wide Area Aug-mentation System(W AAS)signals.Besides adding a GPS reference network and additional location determination units in the network,the mobile phone must embed,at a minimum,a GPS antenna and RF down-converter circuits,as well as make provision for some form of digital signal processing software or dedicated hardware.

Recent field trials of the assisted GPS system have shown the feasibility of this technology.However,the current implementa-tion has not demonstrated that it can cover every location where voice communication is available.In addition,this solution will not work for legacy phones.

In general,all the radio-based technologies discussed can be affected by interference,blockage,and multipath.It is a great challenge to solve these adverse effects caused by the environ-ment in which we live.

III.L OCATION T ECHNOLOGY B EING S TANDARIZED Telecommunication standards organizations are busy incor-porating the new location technologies into their standards, whether it is GSM,TDMA,cdmaOne,cdma2000,W-CDMA, UMTS,UWC-136,and even Analog.Three main standard organizations involved in second-generation(2G)systems are the European Telecommunications Standards Institute(ETSI), Telecommunications Industry Association(TIA),and the T1 Committee.T1is sponsored by the Alliance for Telecom-munications Industry Solutions(ATIS),which is accredited by the American National Standards Institute(ANSI).For third-generation(3G)systems,the work has been handled by

TABLE II

S TANDARDIZED L OCATION T

ECHNOLOGIES

the Third Generation Partnership Project(3GPP)and3GPP2, respectively.

Listed in Table II are technologies being standardized by the above organizations.However,finding a technology which can achieve the accuracy requirements set by the FCC presents a great challenge.cdma2000,W-CDMA,UMTS,and UWC-136 (3G systems)may adopt and further evolve the location tech-nologies being developed for CDMA and GSM,and are not in-cluded.Interested readers can refer to3GPP and3GPP2Web sites for new development information[24],[25].

Note that A-FLT and E-OTD are described in the following section.LCS SWG stands for LoCation Services Sub-Working Group.Provided in Table II is a sampling of the technologies being considered by different standards organizations based on different multiple-access(MA)techniques.In the following sec-tions,we will discuss these technologies in detail.

A.Time Difference of Arrival(TDOA)

The main TDOA location technologies considered for GSM and CDMA are E-OTD(enhanced observed time difference) and A-FLT(advanced forward link trilateration).In the fol-lowing subsections,we examine each of them more closely. Same as the assisted GPS discussed shortly,when the po-sition is calculated at the network,we call it a network-based MS-assisted TDOA solution.When the position is calculated at the handset,we call it a network-assisted MS-based TDOA so-lution.Note that the“network-based”term used here may not have the same meaning as the one used in the recent rulings of the FCC.In the telecommunication standards literature,handset is often referred to as mobile station(MS).

1)Enhanced Observed Time Difference(E-OTD):E-OTD has been finalized by the GSM standard committees(T1P1.5 and ETSI)in LCS Release98and Release99.Future releases will be handled by3GPP.E-OTD is a TDOA positioning method based on the OTD feature already existing in GSM[26].The MS measures relative time of arrival of the signals(bursts)from several BTSs(Base Transceiver Stations).The position of the MS is determined by trilateration(Fig.5).

There are three basic timing quantities associated with this method:

1)Observed Time Difference(OTD)is the time interval ob-

served by an MS between the reception of signals(bursts) from two different Base Transceiver Stations(BTSs).If we denote as the moment that a burst from the BTS1 is received and as the moment that a burst from the

ZHAO:MOBILE PHONE LOCATION DETERMINATION AND ITS IMPACT ON INTELLIGENT TRANSPORTATION SYSTEMS

59

Fig.5.E-OTD:Hyperbolic Positioning.

BTS 2is received,the OTD value is the time difference,

i.e.,

OTD

.If the two bursts arrive exactly at the same moment,the difference is zero,i.e.,

OTD

.2)Real-Time Difference (RTD)is the relative synchroniza-tion interval in the network between two BTSs.If we de-note as the moment that the BTS 1sends a burst and as the moment that the BTS 2sends a burst,the RTD value is the difference of these moments,i.e.,

RTD

is the speed of light.If the distances to the MS

are the same for both BTSs,

GTD

.These quantities are related by

GTD

RTD

is the bias between the two internal clocks

of the MS and LMU.

4)The distance from MS to BTS (DMB).5)The distance from LMU to BTS

(DLB).

Fig.6.E-OTD:Circular positioning.

These quantities are related by

DMB

MOT

is the speed of light.

There will be one such equation for each BTS.Since there are

three unknowns (MS

position

,and clock

offset

and .The position of the MS is

determined by the intersection of circles centered on the BTSs common to observations made by the MS and LMUs (see Fig.6).

The E-OTD method requires a minimum of three spatially distinct BTSs.All these BTSs must be detectable by the MS.More than three measurements generally produce better loca-tion accuracy .An implementation of the E-OTD method may

require an LMU to BTS ratio

between

and .2)Advanced F orward Link Trilateration (A-FLT):A-FL T has been standardized by the CDMA standard committee (TR45.5)[27].The next LCS release will be handled by 3GPP2.Unlike GSM,CDMA (IS-95)is a time-synchronized system.Therefore,time-difference measurement is easier than GSM.The basic idea of this method is to measure the time difference (phase delay)between CDMA pilot signal pairs.Each pair consists of the serving cell pilot and a neighboring pilot.The time difference is converted to the range information.Finally ,the range data is used to form certain curves at which an intersection is defined for the MS location.

Although the name of this method implies that A-FL T is a handset-based solution,the location can be determined either at the MS or at the network.For an MS-based solution,the MS must determine the time difference of arrival among mul-tiple pilot signals through its searcher.For an MS-assisted solu-tion,Pilot signal measurement message (PSMM)along with the round-trip delay can be used to determine the time difference.Since the basic principle of this method is not much different than TDOA (or E-OTD),we will not discuss it in detail in this paper.

3)Challenges:For TDOA to work properly ,additional location-determination software and hardware must be added to the network.In GSM standards literature,the hardware and

60IEEE TRANSACTIONS ON INTELLIGENT TRANSPORTATION SYSTEMS,VOL.1,NO.1,MARCH 2000

associated element are named as location measurement unit (LMU)and serving mobile location center (SMLC).In CDMA standards literature,the software and associated device are called as position determination entity (PDE).

Many challenges exist.For GSM,one challenge is the un-synchronized nature of the network.Unsynchronized clocks in the network can make the accurate measurements of TDOA very difficult since

1-

to )is expected of the E-OTD

LMU due to the impact of RTD error on the MS location accuracy.We are led to believe that the second-generation LMUs cannot be reused for W-CDMA.Listed in Table III is a summary of the main factors degrading the performance of both the uplink TOA and E-OTD.

TABLE III

E RROR S OURCES IN TOA AND

E-OTD

C.Assisted-GPS (A-GPS)

For classification,as mentioned above when the position is calculated at the network,we call it a network-based MS-as-sisted GPS solution.When the position is calculated at the handset,we call it a network-assisted MS-based GPS solution.Despite the above classification of two assisted GPS solu-tions,their principles are the same.If the GPS receiver does not know its approximate location,it will not be able to deter-mine the visible satellites or estimate the range and Doppler fre-quency of these satellites.It has to search the entire code phase and frequency spaces to locate the visible satellites.For the code phase space,it spans from 0to 1023chips.For the frequency

space,it spans from

4.2kHz.The relative movements between the satellites and receiver make the search even more time-consuming.Therefore,the time-to-first-fix (TTFF)is one important parameter to evaluate the quality of a receiver.For au-tonomous GPS,the present state-of-the-art fix time for an unini-tialized GPS sensor is approximately 60s.Clearly,this is un-acceptable for certain applications such as E911.By transmit-ting assistance data over the cellular network,we can reduce the TTFF of a receiver to a few seconds.It is achieved by signifi-cantly reducing the search window of the code phase and fre-quency space by sending precise measurements of these param-eters to the handset from the network.The reduction in search space allows the receiver to spend its search time focusing on where the signal is expected to be.

1)MS-Assisted GPS:The network-based MS-assisted solution shifts the majority of the traditional GPS receiver functions to the network processor.This method requires an antenna,RF section,and digital processor in the handset for making measurements by generating replica codes and correlating them with the received GPS signals.The network transmits a short assistance message to the mobile station (MS),consisting of time,visible satellite list,satellite signal Doppler and code phase,and their search windows.These parameters help the embedded GPS sensor reduce the GPS acquisition time considerably.These assistance data are valid for a few minutes.The sensor returns to the network from the MS the pseudo-range data processed by the GPS sensor in the handset.

ZHAO:MOBILE PHONE LOCATION DETERMINATION AND ITS IMPACT ON INTELLIGENT TRANSPORTATION SYSTEMS61

After receiving the pseudorange data,the corresponding net-work processor or location server estimates the position of the MS.The differential correction(DGPS)can be applied to the pseudo-range data or final result at the network side to improve the position accuracy.

2)MS-Based GPS:The network-assisted MS-based solu-tion maintains a fully functional GPS receiver in the handset. This requires the same functionality as described in MS-assisted GPS,plus additional means for computing the positions of the satellites and ultimately the position of the MS.This additional handset function generally adds to the handset’s total memory (RAM,ROM)requirements in addition to the extra computing capability such as million instructions per second(MIPS).In the initial startup scenario,more data in the form of the precise satellite orbital elements(ephemeris)must be provided to the MS than for the network-based MS-assisted case.For the case of ephemeris data transmitted to the handset,this data is valid for 2–4h or more and can be updated as necessary over time,thus, once the handset has the data,subsequent updates are rare.Be-sides point-to-point transmission,it also includes using a broad-cast channel to distribute this data efficiently to all handsets in a network.If better position accuracy is required for certain appli-cations,differential correction(DGPS)data must be transmitted to the MS approximately every30s while SA is on.The final position of the MS is generated at the MS itself.The calculated MS location can then be sent to an application outside of the MS if required.

D.Fusion of A-GPS and TDOA/TOA

Many trial results have been reported for the location tech-nologies discussed.As expected,each technology has its own advantages and disadvantages.In general,A-GPS has better accuracy,but worse coverage than TDOA/TOA in buildings and urban canyon areas.For instance,buried inside high-rise buildings,GPS could fail to produce a fix while TDOA/TOA may have very good coverage because more cell towers are available in dense areas.The main problem for TDOA/TOA is that its solution quality depends heavily on the geometric loca-tions of the contributing BSs.For instance,on a long,straight, open-view highway,GPS may have very good accuracy and coverage,but TDOA may fail to produce required solutions because its BSs may simply lie along the same highway.This leads to the idea of integrating these two technologies or another time-based method,TOA,to complement each other. To determine a two-dimentional MS location,the current A-GPS and TDOA/TOA technologies must have at least three satellites and at least three BSs visible all the time,respectively. In reality,this is not always the case.A logical improvement is to integrate the measurements of the A-GPS and TDOA/TOA. In other words,we can view each BS as a pseudo-satellite to supplement the real satellites when the visible satellites are insufficient in number to provide a position fix.As a result, any combination of three of these real/pseudo-satellites will likely generate the location for the MS.Data taken in severe in-building and very dense downtown environments with simultaneous A-GPS and cellular signal measurements indicate the promise of the fusion of the two technologies.

Fusion of assisted-GPS and TDOA/TOA methods not only can deal with the situations where less than three satellites or less than three BSs are available,it can also improve position fix quality and can be used as additional aiding information for the GPS receiver(or sensor).For instance,three visible real/pseudo-satellites may not give us the best geometry.Additional mea-surements will improve the geometry coverage of the MS so the location accuracy will be improved.

IV.L OCATION-B ASED S ERVICES

It is generally believed that location-capable cellular phone and network will be available by October1,2001.Although U.S.FCC’s original intention was to fulfill the role of the mobile phone as a part of the emergency call and assistance system, its rulings will clearly have positive impact on many existing services and will certainly generate more new services which were not available to the general public before.

One market study report has categorized the mobile location services into safety,information,tracking,remote,and billing services,respectively[29].Safety services,especially personal security,are very critical to many countries as evidenced by the U.S.-led mandatory wireless E911ruling.Similar services will also be available to the other counties early in the21st century. Information services include weather,traffic,navigation,and directory assistance.They can dramatically improve the quality of peoples’lives.Tracking services can monitor continuously the location of the vehicle,asset,and people.Through these services,companies could increase their productivity while minimizing the cost for tracking down their goods and proper-ties.People would have less concern over whereabouts of their loved ones.Remote services can provide further convenience as unlocking the car,monitoring the engine,collecting tolls, and guiding precision surveying and farming equipment,etc. Finally,billing services will be able to differentiate a variety of customer services.For instance,home-zone billing could encourage low-mobility subscribers to migrate traffic from conventional wireline-based networks to wireless networks. All these services will generate revenues for many old and new businesses.By the year2005,as estimated by Ovum, there will be a$20billion market for network-based location services and a$2.5billion market for vehicle-based location services worldwide.Similar forecasts have been available from other companies for the world-wide or individual regions,and specific segments of the market.

The Universal Mobile Telecommunication System(UMTS) forum expects about400million mobile subscribers worldwide in the year2000(actual number will be higher)and nearly1800 million subscribers in the year2010[30].For the third-gener-ation mobile radio systems,it plans to support a wide range of services from voice and low-rate to high-rate data services up to at least144kb/s in vehicular,384kb/s in outdoor-to-in-door,and2Mb/s in indoor and picocell environments.Both cir-cuit-switched and packet-oriented services for symmetric and asymmetric traffic will be supported.As estimated by Ovum (Fig.7),the cellular data capacity and services will be expanded very rapidly in the next few years.This will in turn generate many location-based services.

62IEEE TRANSACTIONS ON INTELLIGENT TRANSPORTATION SYSTEMS,VOL.1,NO.1,MARCH

2000

Fig.7.United States and Europe data service subscribers (SMS stands for short message service).

TABLE IV

T ELEMATICS A TTITUDE S

URVEY

V .I MPACT ON T ELEMATICS AND P UBLIC T RANSIT S YSTEMS Location-capable phones and networks will have significant impact on intelligent transportation systems (ITS).In this sec-tion,we discuss two specific applications of ITS:automotive telematics and modern public transit systems.

A typical example of an automotive telematics system is a mayday system [1].It provides vehicle occupants instant con-nection with a service center for emergency assistance or road-side services while automatically reporting the vehicle position.Many people in the United States view this system as their top priority when adding new equipment to their vehicles.It can be expanded to include many other services such as remote door unlocking,remote engine diagnosis,theft detection,notification and stolen-vehicle tracking,airbag deployment notification,au-tomatic route guidance,travel information,and hands-free and voice-activated mobile phone or pager.A recent survey of 300current customers by The Dohring Company has confirmed its popularity (Table IV).Because of this popularity,many auto-mobile manufacturers have been and are bundling it as an orig-inal equipment manufacturer (OEM)unit for new model cars.In the future,this system will be able to add even more safety,security,and fun features,including connecting to the Internet,controlling by enhanced voice recognition,and combining with entertainment equipment.A mayday system uses a cellular phone for voice and data communications and a global positioning system (GPS)receiver for positioning [31].The key features of the mayday system are its human-centered design with a cost-effective location capa-bility and its on-demand wireless communications [1].With a human-centered design,the system can be activated either by the user with a push button or by an emergency event detected by one of the vehicular safety sensors.After the communications channel is established,the user can keep in voice contact with a human operator at the service center.With on-demand com-munications,the system does not need to communicate with the remote host on a regular basis as most automatic vehicle loca-tion (A VL)systems do,so there is a drastic reduction in silent air time and its associated expenses.Future systems will be offered in a single platform [32],and will include many customized ser-vices such as information,entertainment,and wireless Web con-nection [33].

With the current mayday system,the location device and communication device are separate items integrated into one system.Generally,the cellular phone and its transceiver are attached to the vehicle as nonremovable devices.So is the location device.Once the location-capable cellular phone is available,there may be no need for the fixed location and communication devices in the car.All we need could be a

ZHAO:MOBILE PHONE LOCATION DETERMINATION AND ITS IMPACT ON INTELLIGENT TRANSPORTATION SYSTEMS63

portable phone which integrates location and communication functions into one device.This will keep the same telematics functionality while reducing the number of phones for many users and could leave room for other in-vehicle devices.

A typical modern public transit system has the automatic vehicle location(A VL)capability[1].An AVL system tracks the locations of a fleet of vehicles in a particular area and reports the information to a centralized server via a communications network[34].This server can take different forms,such as a dispatch center,a traffic information center,or a transportation management center.For such a system,the location sensors keep updating the dispatch center on the route the vehicle is traversing.The communications network for information transmission can be dedicated radios,satellites channels,or short-range beacons installed along the road.Additional func-tions could be added such as route-by-route transit schedule, en-route information(on-board and at the bus station),transfer management,fare collection registration,passenger counting, vehicle diagnosis,emergency alert,paratransit management, and on-board video surveillance[35]–[37].Due to the avail-ability of centralized communications and its management center,a commonly used location technology is differential GPS(DGPS).Some public transit systems are further assisted by dead-reckoning sensors to complement GPS weakness often encountered in urban canyons,where tall buildings and other human-made landmarks cause satellite signal blockage and reflection.

With the advent of the location-capable phone,the com-plexity of the on-board equipment for the modern public transit system will be further reduced.If the cellular phone location determination could be as accurate as15m or less,there would be no need for any on-board dead-reckoning sensors.If the cellular service could be less costly,there would be no need for any specialized communications network.Furthermore,we could imagine how such a phone could assist the passenger.For instance,before even reaching the bus stop,the phone would be able to display when the next bus would be available based on the approximate time of arrival or distance to the bus stop.If the passenger wishes,it could also remind them individually when they have arrived at a predetermined destination or inform them of attractions and services along the bus route.Many more possibilities exist to serve the passenger better than before. Despite technological advances on the mobile phone loca-tion,there are many other issues that require specific atten-tion before it can be deployed in every cellular network around the world.These include cost recovery,handset development, system overload,and privacy issues.For cost recovery,wireless carriers and their partners need to find out how to make money for providing the location-specific ITS services.For handset de-velopment,manufacturers need to determine what kind of loca-tion solutions need to be implemented and how to minimize the handset cost.For system overload,system developers need to figure out whether these location-specific ITS services can be handled smoothly in the nearly full-loaded network.For privacy issues,it is always a concern when a system can monitor each move of the owner of a handset,whether it is a vehicle used as a location and traffic probe or a cellular phone used as a loca-tion-monitoring and call-tracking device.

In spite of the existence of the above concerns,the mobile phone location activities have been carried forward because of the many benefits that location service can provide to ITS.For instance,it can improve the response time of the emergency ser-vices to accident victims and the severely ill.It can aid in the de-velopment of vehicle crash avoidance and antitheft systems.It can improve the intelligent traffic management and control sys-tems potentially reducing traffic congestion and air pollution.

VI.C ONCLUSIONS

Mobile phone location determination activities have been intensified recently due to the October1,2001deadline. Telecommunications standard organizations are busy incor-porating the new location technologies into their standards, whether it is GSM,UMTS,cdmaOne,cdma2000,W-CDMA, TDMA,or UWC-136.Among the technologies discussed above,TOA,TDOA,and assisted-GPS solutions are the leading contenders for the current communication systems. Once these technologies are finalized in various standards organizations,the location-capable phone will hit the market soon.The location-based services will certainly follow.Besides wireless E911,these services may include location-sensitive billing,location tracking,location-based advertising,and information services such as navigation,weather,and points of interest.Similarly,automotive telematics and public transit systems will benefit,as will the other intelligent transportation systems.As we learned,these systems will be less and less complex while providing more convenient and attractive services.We certainly hope that this will in turn make our transportation systems operate more safely and efficiently,with less congestion,pollution,and environmental impact.

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Yilin Zhao(S’89–M’92–SM’97)received the B.E.

degree in electrical engineering in1982from Dalian

University of Technology,Dalian,China,and the

M.S.E.degree in1986and the Ph.D.degree in1992,

both from the Department of Electrical Engineering

and Computer Science,University of Michigan,Ann

Arbor.

From1982to1984,he was an Instructor and since

1995,has been an Adjunct Professor in the Depart-

ment of Computer Science and Engineering,Dalian

University of Technology.From1987to1991,he was a Teaching Assistant and Research Assistant at the University of Michigan.In 1992,he joined Motorola,Inc.as a Senior Research and Development Engineer. His research interests include intelligent transportation systems(ITS),mobile phone location systems,vehicle location and navigation systems,integrated cir-cuit place-and-route systems,and real-time computer systems.He has delivered ITS tutorials and seminars at many universities,IEEE,SAE,and other interna-tional conferences.

Dr.Zhao is an Associate Editor of the IEEE T RANSACTIONS ON I NTELLIGENT T RANSPORTATION S YSTEMS(ITS),and a Senior Representative of the IEEE Robotics and Automation Society to the IEEE ITS Council.