C O M M U N I C A T I O N S
`
`questions of cost, size, and power con-
`sumption.
`
`NETWORK-BASED GEOLOCATION
`Geolocation technologies that rely exclu-
`sively on wireless networks such as time of
`arrival, time difference of arrival, angle of
`arrival, timing advance, and multipath fin-
`gerprinting offer a shorter time-to-first-fix
`(TTFF) than GPS. They also offer quick
`deployment and continuous tracking capa-
`bility for navigation applications, without
`the added complexity and cost of upgrad-
`ing or replacing handsets. These technolo-
`gies also provide a business opportunity for
`network operators as exclusive providers
`of subscriber-location information.
`On the downside, network-based
`geolocation provides far less accuracy
`than GPS, requires expensive investments
`
`mobile phones. GPS consists of a con-
`stellation of 24 satellites, equally spaced
`in six orbital planes 20,200 kilometers
`above the Earth, that transmit two spe-
`cially coded carrier signals: L1 frequency
`for civilian use, and L2 for military and
`government use.
`
`Assisted-GPS technology
`offers superior accuracy,
`availability, and coverage
`at a reasonable cost.
`
`GPS receivers process the signals to
`compute position in 3D—latitude, lon-
`gitude, and altitude—within a radius of
`10 meters or better. Accuracy has
`increased substantially since the US gov-
`ernment turned off Selective Availability,
`the intentional degradation of GPS sig-
`nals, in May 2000. Because no return
`channel links GPS receivers to satellites,
`any number of users can get their posi-
`tions simultaneously. GPS signals also
`resist interference and jamming.
`To operate properly, however, conven-
`tional GPS receivers need a clear view of
`the skies and signals from at least four
`satellites, requirements that exclude oper-
`ation in buildings or other RF-shadowed
`environments. Further, it takes a GPS
`receiver starting “cold”—without any
`knowledge about the GPS constellation’s
`state—as long as several minutes to
`achieve the mobile station location fix, a
`considerable delay for emergency ser-
`vices. Finally, incorporating GPS receivers
`into trendy, miniature handsets raises
`
`in base-station equipment, and raises pri-
`vacy concerns. For more on network-
`based technologies and their imple-
`mentation, see http://www.cell-loc.com/,
`http://www.geometrix911.com/, http://
`www.trueposition.com/, and http://www.
`uswcorp.com/.
`
`ASSISTED GPS
`Compared to either mobile-station-
`based, stand-alone GPS or network-based
`geolocation, assisted-GPS technology
`offers superior accuracy, availability, and
`coverage at a reasonable cost. As Figure
`1 shows, AGPS consists of
`
`• a wireless handset with a partial
`GPS receiver,
`• an AGPS server with a reference
`GPS receiver that can simultane-
`ously “see” the same satellites as the
`handset, and
`• a wireless network infrastructure
`consisting of base stations and a
`mobile switching center.
`
`Geolocation and
`Assisted GPS
`C urrently in development, numer-
`
`Goran M. Djuknic and Robert E. Richton
`Bell Laboratories, Lucent Technologies
`
`ous geolocation technologies
`can pinpoint a person’s or ob-
`ject’s position on the Earth.
`Knowledge of the spatial distri-
`bution of wireless callers will facilitate the
`planning, design, and operation of next-
`generation broadband wireless networks.
`Mobile users will gain the ability to get
`local traffic information and detailed
`directions to gas stations, restaurants,
`hotels, and other services. Police and res-
`cue teams will be able to quickly and pre-
`cisely locate people who are lost or
`injured but cannot give their precise loca-
`tion. Companies will use geolocation-
`based applications to track personnel,
`vehicles, and other assets.
`The driving force behind the develop-
`ment of this technology is a US Federal
`Communications Commission (FCC)
`mandate stating that by 1 October 2001
`all wireless carriers must provide the
`geolocation of an emergency 911 caller to
`the appropriate public safety answering
`point (see http://www.fcc.gov/e911/).
`Location technologies requiring new,
`modified, or upgraded mobile stations
`must determine the caller’s longitude and
`latitude within 50 meters for 67 percent
`of emergency calls, and within 150 meters
`for 95 percent of the calls. Otherwise, they
`must do so within 100 meters and 300
`meters, respectively, for the same per-
`centage of calls. Currently deployed wire-
`less technology can locate 911 calls within
`an area no smaller than 10 to 15 square
`kilometers.
`
`GLOBAL POSITIONING SYSTEM
`An obvious way to satisfy the FCC
`requirement is to incorporate Global
`Positioning System (GPS) receivers into
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`February 2001
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`

`

`C o m m u n i c a t i o n s
`
`GPS satellites
`
`GPS signal
`
`Assistance
`information
`
`Handset with
`partial GPS receiver
`
`GPS signal
`
`GPS
`receiver
`
`MSC
`
`AGPS
`server
`
`Base
`station
`
`Figure 1. Assisted-GPS concept. The main system components are a wireless handset with
`partial GPS receiver, an AGPS server with reference GPS receiver, and a wireless network
`infrastructure consisting of base stations and a mobile switching center (MSC).
`
`The network can accurately predict the
`GPS signal the handset will receive and
`convey that information to the mobile,
`greatly reducing search space size and
`shortening the TTFF from minutes to a
`second or less. In addition, an AGPS
`receiver in the handset can detect and
`demodulate weaker signals than those
`that conventional GPS receivers require.
`Because the network performs the loca-
`tion calculations, the handset only needs
`to contain a scaled-down GPS receiver.
`By distributing data and processing, as
`well as implementation costs, between the
`network and mobiles, AGPS will opti-
`mize air-interface traffic. It is accurate
`within 50 meters when users are indoors
`and 15 meters when they are outdoors,
`well within federal guidelines and an
`order of magnitude more sensitive than
`conventional GPS. Further, because users
`share data with the network operator,
`AGPS lets them withhold data for privacy
`reasons while the operator can restrict
`assistance to service subscribers.
`
`Reduced search space
`Because an AGPS server can obtain the
`handset’s position from the mobile
`switching center, at least to the level of
`cell and sector, and at the same time mon-
`itor signals from GPS satellites seen by
`
`mobile stations, it can predict the signals
`received by the handset for any given
`time. Specifically, the server can predict
`the Doppler shift due to satellite motion
`of GPS signals received by the handset,
`as well as other signal parameters that
`are a function of the mobile’s location.
`In a typical sector, uncertainty in a
`satellite signal’s predicted time of arrival
`at the mobile is about ±5 µs, which cor-
`responds to ±5 chips of the GPS coarse
`acquisition (C/A) code. Therefore, an
`AGPS server can predict the phase of the
`pseudorandom noise (PRN) sequence
`that the receiver should use to despread
`the C/A signal from a particular satel-
`lite—each GPS satellite transmits a
`unique PRN sequence used for range
`measurements—and communicate that
`prediction to the mobile.
`The search space for the actual
`Doppler shift and PRN phase is thus
`greatly reduced, and the AGPS handset
`receiver can accomplish the task in a frac-
`tion of the time required by conventional
`GPS receivers. Further, the AGPS server
`maintains a connection with the handset
`receiver over the wireless link, so the
`requirement of asking the mobile to make
`specific measurements, collect the results,
`and communicate them back is easily
`met.
`
`After despreading and some additional
`signal processing, an AGPS receiver
`returns back “pseudoranges”—that is,
`ranges measured without taking into
`account the discrepancy between satel-
`lite and receiver clocks—to the AGPS
`server, which then calculates the mobile’s
`location. The mobile can even complete
`the location fix itself without returning
`any data to the server.
`
`Sensitivity assistance
`Sensitivity assistance, also known as
`modulation wipe-off, provides another
`enhancement to detection of GPS signals
`in the handset receiver. The sensitivity-
`assistance message contains predicted
`data bits of the GPS navigation message,
`which are expected to modulate the GPS
`signal of specific satellites at specified
`times. The mobile station receiver can
`therefore remove bit modulation in the
`received GPS signal prior to coherent
`integration. By extending coherent inte-
`gration beyond the 20-ms GPS data-bit
`period—to a second or more when the
`receiver is stationary and to 400 ms
`when it is fast-moving—this approach
`improves receiver sensitivity.
`Sensitivity assistance provides an addi-
`tional 3-to-4-dB improvement in receiver
`sensitivity. Because some of the gain pro-
`vided by the basic assistance—code
`phases and Doppler shift values—is lost
`when integrating the GPS receiver chain
`into a mobile phone, this can prove cru-
`cial to making a practical receiver.
`Achieving optimal performance of sen-
`sitivity assistance in TIA/EIA-95 CDMA
`systems is relatively straightforward
`because base stations and mobiles syn-
`chronize with GPS time. Given that
`global system for mobile communication
`(GSM), time division multiple access
`(TDMA), or advanced mobile phone ser-
`vice (AMPS) systems do not maintain
`such stringent synchronization, imple-
`mentation of sensitivity assistance and
`AGPS technology in general will require
`novel approaches to satisfy the timing
`requirement. The standardized solution
`for GSM and TDMA adds time calibra-
`tion receivers in the field—location mea-
`surement units—that can monitor both
`the wireless-system timing and GPS sig-
`nals used as a timing reference.
`
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`

`Table 1. Advantages and disadvantages of geolocation technologies.
`
`Location technology
`Mobile-station-based
`stand-alone GPS
`
`Pros
`Little or no additional network equipment
`Works with all mobiles
`Privacy not an issue (user controlled)
`Location capability remains in absence of wireless
`coverage or network assistance
`
`Network-based
`systems
`
`AGPS
`
`No added mobile-station complexity or cost
`Works with all mobiles
`Short time to first fix
`Maps and databases increase accuracy of location fix
`Continuous tracking capability for navigation applications
`Business opportunity for network operators as exclusive
`providers of subscriber-location information
`Superior accuracy, availability, and coverage
`Short time to first fix
`Maps and databases increase location accuracy if
`processing done in network
`Minimal impact on battery life
`Implementation cost shared by mobiles and the network
`System evolves with network upgrades
`Location data shared between users and network operator—
`users can withhold data for privacy reasons, and operator
`can restrict assistance to subscribers of service
`Air-interface traffic optimized by distributing data and
`processing between network and mobiles
`
`Cons
`New handsets
`Little or no indoor coverage
`Fails in radio shadows
`Considerable increase in handset cost and complexity
`Additional battery consumption
`Long time to first fix
`System upgrades limited by deployed handset base
`Inferior accuracy
`Additional investments in infrastructure, with very high
`up-front costs
`Difficult network installation and maintenance
`User privacy questionable
`
`Network assistance increases signaling load
`Interoperability between network and mobiles requires
`additional standards, delaying deployment
`New or upgraded handsets needed for initial
`deployment
`
`Hybrid solutions
`Many factors affect the accuracy of
`geolocation technologies, especially terrain
`variations such as hilly versus flat and envi-
`ronmental differences such as urban ver-
`sus suburban versus rural. Other factors,
`like cell size and interference, have smaller
`but noticeable effects. Hybrid approaches
`that use multiple geolocation technologies
`appear to be the most robust solution to
`problems of accuracy and coverage.
`AGPS provides a natural fit for hybrid
`solutions because it uses the wireless net-
`work to supply assistance data to GPS
`receivers in handsets. This feature makes
`it easy to augment the assistance-data
`message with low-accuracy distances
`from handset to base stations measured
`by the network equipment. Such hybrid
`solutions benefit from the high density of
`base stations in dense urban environ-
`ments, which are hostile to GPS signals.
`Conversely, rural environments—where
`base stations are too scarce for network-
`
`based solutions to achieve high accu-
`racy—provide ideal operating conditions
`for AGPS because GPS works well there.
`
`E ven providers who favor mobile-sta-
`
`tion-based solutions view the current
`lack of handsets with location capa-
`bilities as a major obstacle. Proponents
`of network-based solutions regard the
`obstacle as insurmountable.
`Considering the advantages and dis-
`advantages of each approach, summa-
`rized in Table 1, we believe that AGPS,
`augmented with elements from other
`location technologies, is the solution to
`which most wireless systems will ulti-
`mately converge. Such hybrid solutions
`offer superior location accuracy and the
`most potential cost-effectiveness. AGPS
`is also being standardized for all air-
`interfaces, which will prove critical for
`the technology’s widespread deploy-
`ment. ✸
`
`Goran M. Djuknic is a member of the
`technical staff at Lucent Technologies,
`Bell Laboratories. He received a PhD in
`electrical engineering from the City Uni-
`versity of New York. Contact him at
`goran@lucent.com.
`
`Robert E. Richton is a distinguished
`member of the technical staff at
`Lucent Technologies, Bell Laborato-
`ries. He received an MS in physics and
`chemistry from Stevens Institute of
`Technology, Hoboken, N.J. Contact
`him at richton@lucent.com.
`
`Editor: Upkar Varshney, Department of CIS,
`Georgia State University, Atlanta, GA
`30002-4015; voice +1 404 463 9139; fax +1
`404 651 3842; uvarshney@gsu.edu
`
`February 2001
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`125
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