Locatability in Mobile Networks
Harsha Nagesh
Integrated Network Research Dept.
Bell Labs, Lucent Technologies
[email protected]
Joint work with Vishy Poosala, Anurag Srivastava, Yuh Jye Chang, Mansoor Alicherry
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Harsha Nagesh
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Talk Outline
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Industry drivers
Cellular Location Tracking Techniques
Design for Locatability
iLocator – Novel Location Service
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Drivers
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GPS infrastructure worth 10 Billion built for smart weapons program
Enhanced-911 for Wireless Networks (E112 in EU)
– FCC ruling to enhance emergency services
– ANI (automatic number identification) across PSTN (wire-line and
–
wireless)
Phase I completed in Oct 2001
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Basic ANI with cell Id/sector information of mobile
– Phase II spread over 4 years, to be completed by 2005
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Each SP has a deployment proposal approved by FCC, including technology
choice and location % coverage of handsets issued
Handset Based - within 50m for 67% of calls, within 150m for 95% of calls
Network Based - within 100m for 67% of calls, within 300m for 95% of calls
TTFF (time-to-first-fix) location is 30s, with first estimate in 7s
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Location Services
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Location services present a significant revenue opportunity
– Expected market size of $7-8B by 2007-08
– Compelling applications: fleet tracker, restaurant finder, buddy alerts, teen-tracking, coupons,
dating, games
What is required to make LBS a reality ?
Global Cellular & LBS Subscribers
Subscribers (M)

2,000
1,800
1,600
1,400
1,200
1,000
800
600
400
200
0
Latin Am LBS
North Am LBS
EMEA LBS
A/P LBS
Total LBS
Total Cellular
2002
2003
2004
2005
2006
2007
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Keys to LBS Takeoff
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Service Providers
– Standards based access to network resident information
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Location, Presence Parlay gateways
– Convergence across wire-line and wireless networks
– Usage based charging
– Locatability – outdoor, indoor, cities, etc
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Application Developers
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Consumers
– Easy deployment across multiple service providers, device technologies
– Compelling applications
– Mass market cell phones with programmability
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Java, Brew, Symbian based
– Fine grained privacy knobs
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Location Tracking Background
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Basic geometric distance between the mobile and a reference point,
with known location
Signals travel with speed of light, c
Three dimensions to location computation
– Physical-layer technique – Angle of Arrival, Time of Arrival, Time
–
–
Difference of Arrival, Hybrid
Computation Model - Network Based, Mobile Based, Hybrid
Synchronization – or lack of among entities involved in location
measurements
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Angle of Arrival (AoA)
A
B
Multi-path effects make AoA less
effective
Base Station
Fixed Known Location
Mobile
TOA
Time of Arrival (ToA)
Mobile
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Time Difference of Arrival (TDoA)
c*dT = sqrt{ (XA - x)2 + (YA - y)2 } - sqrt{ (XB - x)2 + (YB - y)2 }
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dT - Time difference of Signal
- Measured at the mobile, with synchronized transmission at A,B
- Measured at A and B for signal transmitted by mobile
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Multiple pairs of reference points, leads to solving non-linear
hyperbolic equations
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Wireless Networks
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Cell-ID and Cell-ID variants - GSM,
GPRS, and WCDMA
E-OTD (Enhanced Observed Time
difference) - GSM/GPRS
OTDOA (Observed Time Difference
of Arrival) - WCDMA
A-GPS (Wireless Assisted GPS) GSM, GPRS and WCDMA
Hybrid (combinations of A-GPS and
other standards) - GSM, GPRS, and
WCDMA
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CDMA Location Architecture
MSC – Routes 911 calls
with mobile details and
capabilities
 MPC – Stores, retrieves
location data, selects
appropriate PDE, PSAP,
sends routing digits to
MSC
 PDE – Calculates location
based on data from SRS
(Satellite Reference
Service), MPC
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CDMA Link Tri-Lateration
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TDOA technique requires visibility of mobile at 3 or more BS
– Reverse link has visibility problems at 3 BS due to power control
Forward Link TDOA
– Each BS broadcasts the a PN sequence with a known unique offset
– MS uses sliding window correlation to find out pair-wise pilot signal
phase offset differences
– BS uses the phase difference TDOA to locate the mobile
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MSC determines location technique
– Legacy CDMA MS use Enhanced Forward Link Trilateration (EFLT)
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Lower accuracy using Pilot Signal strength (about 244m)
– IS-801 compliant MS use Advanced Forward Link Trilateration
(AFLT), Assisted GPS (AGPS) or Hybrid solutions
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Higher accuracy using Pilot Phase measurements (about 15m) or GPS
assistance
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CDMA Network Design
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Automated design tools to minimize drive tests
Design for a given capacity (blocking probability) maximize coverage
– Forward link pilot Ec/Io for every mobile is above a threshold
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Model the Network as a mesh with nodes representing MS
Optimize capacity-coverage tradeoff points
– Determine the power, tilt, azimuth of each antenna through non-linear
optimization and analytical differentiation
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Capacity-Coverage Intuition
– Power controlled MS is distance (coverage) limited to ensure good signal
reception at the BS which is interference limited
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Ocelot CDMA Capacity-Coverage Tradeoff
Network coverage and network capacity cannot be optimized at the same time
Low Coverage
High Coverage
High Pilot Pollution in Center
=> Large Coverage Hole
Reduced Pilot Pollution in Center:
=> Small Coverage Holes
Large
Antenna Tilt
Small
Cells have equal traffic load
=> High Effective Network Capacity
High Capacity
Traffic load unbalanced
=> Small Effective Network Capacity
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Low Capacity Harsha Nagesh
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Dolphin: Locatability Analysis and Design
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100% geographic coverage does not translate to 100% location coverage
Location tracking using trilateration poses stronger requirements
–
–
–
–
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Must be covered by at least three base stations
Significantly increases interference – pilot pollution
In CDMA this translates to a capacity-coverage-locatability trade-off
Harder than coverage
Non-trivial to assess the locatability of a network
– Drive-by testing is expensive, not flexible, not exhaustive
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CDMA Development Group Specifies detailed test scenarios
Dolphin Software
– Assessing locatability in the current network
– Design keeping in mind capacity – coverage – locatability metrics
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Screenshot of Dolphin
Locatability Assessment
Red: Locatable Region
Grey: Non-locatable Region
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Location Everywhere
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GPS still doesn’t work well indoors and in NY city canyons
Location access everywhere seamlessly through applications
– mobile network, wi-fi hotspots for indoor and outdoor environments
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New Ideas
– Design for location coverage and assess location accuracy
– Push sampled GPS signals indoor to work with standard AGPS mobiles
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Radio-Star is Bell-Labs indoor wireless solution
Extending coverage inside buildings by encapsulating CDMA/GSM in gigabitEthernet frames and using low power antennas
– Explore GPS-repeaters in to fill city GPS canyons
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Hybrid Locatability
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Leverage short range supplementary radio (ex. Bluetooth) to compute
location autonomously
– Distributed computation using both proximity and non-proximity constraints
– Especially useful when not all mobiles are GPS capable or have GPS
–
visibility
Globecom 04 (Vishy Poosala, Harsha Nagesh, Chitra Phadke, Mansoor
Alicherry, Sumesh Philip)
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iLocator – Location Based Service
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A location and presence-based track & alert application offering multiple
services
– Tracks people/events/enterprises and displays their positions on a map and
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–
–
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Alerts when they are in the user’s vicinity
People: Like Instant Messenger, maintains a buddy list and informs about
buddies entering or leaving a configurable distance around the user
Events: Alerts if there is a concert / sale / traffic-jam near user’s location
Enterprises: Display all the preferred restaurants etc. in user’s vicinity
Alert Me: Alert through SMS when the callee becomes available
– e.g. John is calling Mary, Mary is busy or has no coverage, network SMS’es
John when Mary is again able to receive calls
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Provides privacy: users control from the handset who can track them
Supports SMS’ing from within the application
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Value Proposition
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Realize revenues out of the E-911 infrastructure
Prompt more calls and SMSes, complete more calls with Alert Me
The AOL Effect : entice other uses to join the network to interact with
their buddies
– Before inter-carrier location exchange becomes a reality, first mover
advantage for service providers
Convergence: Works across cellular and wifi networks
Mass Market – works on any phone, any location technology, any
network
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Technical Details
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Works on top of any Parlay Network Gateway
Works on any wireless network
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Works with any location-tracking mechanism
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Works on any phone
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– GSM, CDMA, iDEN, TDMA
– HTTP transport needed to support GUI application
– GPS, Triangulation, Cell Number, User-specified
– Adapts to handset capabilities – screen size,
–
–
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stylus, color
Works on any phone through SMSes
JVM needed for map display (Java phones, BREW,
iDEN)
Scales with number of users
Software challenges - strict limit on resident
memory, adaptability to 100’s of handsets, J2ME
implementations, etc
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iLocator Network Diagram and Call Flow
Web pages for
Privacy Policies,
Event registrations
internet
HTTP
4
Map Server
HTTP over air link
5
1
1.
2.
3.
Mobile Network
iLocator Server & Proxy *
CORBA/Parlay APIs
3
2
4.
ISG
Packet-IN
Database
Mobile Network
HLR
5.
iLocator refresh request
Get Locatable Buddies
based on permissions
Eliminate duplicates from
multiple clients and
request ISG for buddy
locations, nearby
enterprises, filter based
on policies (UL and US)
Fetch the map based on
current location and
device parameters
Return map with buddy
information and user
status
MSC
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Intelligent Services Gateway
3rd Party
Applications
Operater
Applications
Content
Providers
Internet/Intranet
Internet/Intranet
MiLife™
MiLife™Intelligent
IntelligentServices
ServicesGateway
Gateway
Provides standard set of APIs and
hides Network complexities
XML GW
ISG Parlay/OSA/PAM API’s
CHAM
PAM
MSG
DIR*
PM
UL
US
CC/UI
FW
ANSI/MAP
ANSI/MAP
SMPP
LDAP
JTAPI
MSC
SMSC
SurePay
HLR
LSS
XML/LIF
MSG
Server
GMLC
SIP
SMTP/IMAP4
Other
Lucent/Non-Lucent Services
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Conclusion
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Design for capacity-coverage-locatability is hard, but important to be E911 compliant fully
Compelling mass market applications that works across service provider
networks and technologies important for LBS to take off
iLocator is a novel service that service providers can leverage to
increase ARPU and lock in customers
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