Context-aware Mobile Systems

Context-Aware Mobile SystemsContext-Aware Mobile Systems

Some IEEE Wireless Communications Standards Some IEEE Wireless Communications Standards

and and

Wireless Network based Positioning of Mobile ClientsWireless Network based Positioning of Mobile Clients

John Aa. SørensenJohn Aa. Sørensen

Lecture Overview

• Some IEEE Wireless Communication Standards.

What is WLAN, WPAN, Zigbee, … ?

• Network Aided Positioning of Mobile Clients, based

on the reference:

Guolin Sun, Jie Chen, Wei Guo, K.J. Ray Liu. ”Signal Processing Techniques in Network-Aided Positioning”. IEEE Signal Processing Magazine, July 2005, pp. 12-23.

Some IEEE Wireless Communicationsstandards

IEEE 802.15.1, Wireless Medium Access Control (MAC) and Physical Layer (PHY) Specifications for Wireless Personal Area Networks (WPANs), 1 Mbit/sec. Bluetooth.

IEEE 802.15.3, Wireless Medium Access Control (MAC) and Physical Layer(PHY) Specifications for High Rate Wireless Personal Area Networks (WPANs).20 Mbit/sec. Images short distance, 2003.

IEEE 802.15.4, Wireless Personal Area Network (WPAN) (250 kbit/sec). Zigbee is interfacing to this, 2003.

IEEE 802.11b, Wireless LAN Medium Access Control and Physical Layer Specifications, 11Mbit/sec. (Wi-Fi), 1999.

IEEE 802.11g, - do - 50 Mbit/dec., 2003.

Guolin Sun, Jie Chen, Wei Guo, K.J. Ray Liu. ”Signal Processing Techniques in Network-Aided Positioning”. IEEE Signal Processing Magazine, July 2005, pp. 12-23.

Grouping of positioning systems:

- Indoor versus Outdoor Systems.- Cellular versus Sensor Network Positioning Design.

Network Aided Positioning of Mobile Clients

Network Aided Positioning of Mobile Clients

Network Aided Positioning of Mobile Clients.A few Fundamental Concepts.

OTD: Observed time difference of arrival

RTD: Relative time difference

TDOA: Time Difference of Arrival

TDOA= OTD – RTD.

AOA: Angle of Arrival.

Network Aided Positioning of Mobile Clients

ij ij ijTDOA OTD RTD

Network Aided Positioning of Mobile Clients

Indoor Outdoor(Positioning by Satellite)

WLAN-- Client-based system design;-- Client-assisted system design.

Sensor network-- Localization with Beacons;-- Localization with moving Beacons;

-- Beacon-free localization.

UWB (Ultrawideband)-- A promising approach for indoor geolocation-- Can achieve very accurate short distance estimation.

GPS (Global Pos. System)-- Requires minimal obstructions -- Long acquisition times (30 secs – 15 mins)-- Has to be synchronous-- High power consumption and high unit cost

A-GPS (Assisted-GPS)-- Much more accurate: Accuracy of 10 – 50 meters, can be used even for indoor positioning;-- Improves acquisition time (< 10 seconds);-- Synchronous or asynchronous; -- More cost effective than GPS;-- Little/no hardware changes required in base stations.

Network Aided Positioning of Mobile Clients

Outdoor Cellular Network Positioning (Standard)

Outdoor Cellular NetworkPositioning (Non-standard)

GSM (with E-OTD)-- Estimation with 50-125 meters of accuracy;-- Slow, about 5 seconds;-- Software change is needed.

GPRS (with A-GPS)-- Handset needs an A-GPS receiver;-- Provides enough accuracy <10m;

Cellular ID-- No air interface needed-- Accuracy depends on sector size-- Accuracy can be improved by hybridization with other methods, such as Cell ID+RTT-- Accuracy can be improved with other methods

Smart Antenna Techniques-- No changes in the handset-- Changes required in each base station-- Not as accurate as A-GPS in most situations-- No change in handset, but special antenna at the Base station is required.

Pattern Matching for Positioning-- Only server Base station required;-- Software solution with hardware modification.

RTT: Round trip time. GPRS: General Packet Radio System.

Network Aided Positioning of Mobile Clients

Smart antennas techniques for localization

Uses Angle ofArrival (AOA)

Network Aided Positioning of Mobile Clients

Pattern matching method for positioning

Use a template of the received signal strengths received by a mobile client,from as many WLAN access points as possible, to ”look up” the specificposition in a table with prestored templates of positions.Requires manual calibration.New architectures: Almost selfcalibrating.

AP1 AP2

AP3AP4

RSSI=(AP1 , AP2 , AP3 , AP4 )

Mobile Client

Pos 1 (AP1,1 AP2,1 AP3,1 AP4,1)

......

Pos N-1

Pos N (AP1,N AP2,N AP3,N AP4,N )

”Best match”

Network Aided Positioning of Mobile Clients

Network Aided Positioning of Mobile Clients

Pattern matching algorithm for location estimation

Estimating the position from receivedpower measurements

Example on blackboard

GPS: Global Positioning System I

The Global Positioning System (GPS) is a satellite-based navigation system made up of a network of 24 satellites placed into orbit by the U.S. Department of Defense.

GPS was originally intended for military applications, but in the 1980s, the government made the system available for civilian use.

GPS works in any weather conditions, anywhere in the world, 24 hours a day.

There are no subscription fees or setup charges to use GPS.

How does GPS Work? I

- GPS satellites circle the earth twice a day in a very precise orbit and transmit signal information to earth.

- GPS receivers take this information and use triangulation to calculate the user's exact location.

- GPS receiver compares the time a signal was transmitted by a satellite with the time it was received. The time difference tells the GPS receiver how far away the satellite is.

- With distance measurements from a few more satellites, the receiver can determine the user's position and display it on the unit's electronic map.

How does GPS Work? II

How does GPS Work? III

- A GPS receiver must be locked on to the signal of at least three satellites to calculate a 2D position (latitude and longi- tude) and track movement.

- With four or more satellites in view, the receiver can determi- ne the user's 3D position (latitude, longitude and altitude).

-Once the user's position has been determined, the GPS unit can calculate other information, such as speed, bearing, track, trip distance, distance to destination, sunrise and sunset time and more.

How does GPS Work? IV

Positioned object.

Latitude angle: . 0 at the equator . 90 Degree at the North Pole.

Reference meridian through Greenwich, England (East of London).

Longitude angle: . 0 degree at Greenwich. . Copenhagen approx. 12.5 Degree East of Greenwich. . New York approx. 75 Degree West of Greenwich.

Altitude: Height above sea level.

N

S

W S

How does GPS Work? V

ta tb

td

The satellite positions are known, together with the transmission times ta, tb and tc, between the satellites and the mobile unit.

Each satellite is on the top point of a cone, with a side length of ta x c.The mobiles position on thesurface of the earth surface, is the intersection between three cones.

Ta x c

Mobile on the surface of the earth. Its position is determined by the intersection of the cones,with side lengths ta, tb, and td.c is the speed of light.

How Accurate is GPS?

GPS receivers are typically accurate to within 15 meters on average.

Differential GPS (DGPS), which corrects GPS signals to within an average of three to five mete-rs. This system con-sists of a network of towers that receive GPS signals and transmit a corrected signal by beacon transmitters. In order to get the corrected signal, users must have a differential beacon receiver and beacon antenna in addition to their GPS.  

The GPS Satellite System

The 24 satellites that make up the GPS space segment are orbiting the earth about 12,000 miles above us. They are constantly moving, making two complete orbits in less than 24 hours. These satellites are travelling at speeds of roughly 7,000 miles an hour.

Facts about the GPS satellites (also called NAVSTAR, the official U.S. Department of Defense name for GPS):• The first GPS satellite was launched in 1978. • A full constellation of 24 satellites was achieved in 1994. • Each satellite is built to last about 10 years. Replacements are constantly being built and launched into orbit. • A GPS satellite weighs approximately 2,000 pounds and is about 17 feet across with the solar panels extended. • Transmitter power is only 50 watts or less.

What is the Signal? I

-GPS satellites transmit two low power radio signals, designated L1 and L2. Civilian GPS uses the L1 frequency of 1575.42 MHz in the UHF band.

- The signals travel by line of sight, meaning they will pass through clouds, glass and plastic but will not go through most solid objects such as buildings and mountains.

- A GPS signal contains three different bits of information : -- a pseudorandom code, -- ephemeris data, and

--- A collection of tables, giving the position of the sun, moon, planets at regular intervals, throughout the year.

-- almanac data.

What is the Signal? II

-The pseudorandom code is simply an I.D. code that identifies which satellite is transmitting information. The number is on the GPS receiver unit's satellite page, as it identifies which satellites it's receiving.

- Ephemeris data, which is constantly transmitted by each satellite, contains important information about the status of the satellite (healthy or unhealthy), current date and time. This part of the signal is essential for determining a position.

-The almanac data tells the GPS receiver where each GPS satellite should be at any time throughout the day. Each satellite transmits almanac data showing the orbital information for that satellite and for every other satellite in the sy- stem.

Sources of GPS Errors I

Multipath originates in reflections from f.inst. metallic surfaces.

Sources of GPS Errors II

• Ionosphere and troposphere delays The satellite signal slows as it passes through the atmosphere. The GPS system uses a built-in model that calculates an average amount of delay to partially correct for this type of error.

• Signal multipath This occurs when the GPS signal is reflected off objects such as tall buildings or large rock surfaces before it reaches the recei- ver. This increases the travel time of the signal, thereby causing errors.

Sources of GPS Errors III

• Receiver clock errors A receiver's built-in clock is not as accurate as the atomic clocks onboard the GPS satellites. Therefore, it may have very slight timing errors.

• Orbital errors Also known as ephemeris errors, these are inaccuracies of the satellite's reported location.

Sources of GPS Errors IV

• Number of satellites visible The more satellites a GPS receiver can "see," the better the accuracy.

Buildings, terrain, electronic interference, or sometimes even dense foliage can block signal reception, causing position errors or possibly no position reading at all. GPS units typically will not work indoors, underwater or underground.

• Satellite geometry/shading

This refers to the relative position of the satellites at any given time. Ideal satellite geometry exists when the satellites are located at wide angles relative to each other. Poor geometry results when the satellites are located in a line or in a tight grouping.

Sources of GPS Errors V

• Intentional degradation of the satellite signal

- Selective Availability (SA) is an intentional degradation of the signal once imposed by the U.S. Department of Defense.

- SA was intended to prevent military adversaries from using the highly accurate GPS signals. The government turned off SA in May 2000, which significantly improved the accuracy of civilian GPS receivers.

References

Todor Cooklev.“Wireless Communication Standards.A Study of IEEE 802.11, 802.15, and 802.16”.IEEE, 2004, ISBN 0-7381-4066-X

Jose A. Gutierrez.Low-Rate Wireless Personal Area Networks.IEEE, 2004, ISBN 0-7381-3557-7

Guolin Sun, Jie Chen, Wei Guo, K.J. Ray Liu. “Signal Processing Technology in Network Aided Positioning”. IEEE Signal Processing Magazine, July, 2005, pp. 12-23.

Per Enge, Pratap Misra.”Special Issue on Global Positioning System”.Proceedings of the IEEE, Vol. 87, No. 1, January 1999.