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February 2001 123
COMMUNICATIONS
C
urrently in development, numer-
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
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.
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
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
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
Goran M. Djuknic and Robert E. Richton
Bell Laboratories, Lucent Technologies
Assisted-GPS technology
offers superior accuracy,
availability, and coverage
at a reasonable cost.
124 Computer
Communications
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.
MSC
AGPS
server
GPS
receiver
GPS signal
GPS signal
GPS satellites
Assistance
information
Base
station
Handset with
partial GPS receiver
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).
February 2001 125
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. ✸
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-
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.
Table 1. Advantages and disadvantages of geolocation technologies.
Location technology Pros Cons
Mobile-station-based Little or no additional network equipment New handsets
stand-alone GPS Works with all mobiles Little or no indoor coverage
Privacy not an issue (user controlled) Fails in radio shadows
Location capability remains in absence of wireless Considerable increase in handset cost and complexity
coverage or network assistance Additional battery consumption
Long time to first fix
System upgrades limited by deployed handset base
Network-based No added mobile-station complexity or cost Inferior accuracy
systems Works with all mobiles Additional investments in infrastructure, with very high
Short time to first fix up-front costs
Maps and databases increase accuracy of location fix Difficult network installation and maintenance
Continuous tracking capability for navigation applications User privacy questionable
Business opportunity for network operators as exclusive
providers of subscriber-location information
AGPS Superior accuracy, availability, and coverage Network assistance increases signaling load
Short time to first fix Interoperability between network and mobiles requires
Maps and databases increase location accuracy if additional standards, delaying deployment
processing done in network New or upgraded handsets needed for initial
Minimal impact on battery life deployment
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
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
