1 Introduction
There are millions of handheld devices like mobile phones and PDAs in use today. Because of the mobile nature of these devices there are many occasions where it would be nice to know the current location of the device. This location information can be used to provide all kind of value-added services that we could not have dreamed about before. These include new kind of advertising, navigation and information services. By using the location information the applications can better adapt themselves to the environment and provide the user useful information that is relevant to the current situation of the user. Such applications that use the location information are called location aware and the whole concept is called location awareness. Techniques for providing location information have been around for quite a long time, but they haven’t been used much until now. The success of the wireless devices has been the main driver for the rise of the location awareness. Next couple of years are expected to be the breakthrough of the location awareness. Location awareness is a topic of much interest at the moment.
1.1 Location awareness
Location awareness is based on the fact that only some information in the world is relevant for you at the moment. For example if you are hungry and you are in
Location awareness is only a small part of the so-called context awareness. Context awareness means the usage of context information in applications. Context information can include things like identity, spatial, temporal, environmental, social, resource and physiological information. All of these can be used to adapt the application to current environment of the application, for example location, time, light level and people that are near by.
Location awareness is based on the usage of location information. Location information
can be gathered in a number of ways. The most important techniques are presented in the
rest of this Section. The precision of location information is crucial for many applications. It is constrained by the facts like power consumption and the size of the device. Location information is always attached to some entity that is being track. These entities are called located objects. Located objects can be persons, animals, valuable items like cars, deliveries or anything that we want to track.
Proximity awareness means that the PDAs or other handheld devices are aware of the
devices that are near by. For example in the meeting the PDAs of the people attending the
meeting, could automatically change important information or when printing something
the printout would be sent automatically to the nearest printer. Proximity information can
be calculated from the location information.
1.2 Absolute Vs. Relative location awareness
Location awareness can be either relative or absolute. Absolute location awareness means that the actual location is known, for example coordinates of the location or in what place (room, building, city). There are several technologies like GPS that provide the absolute location awareness. Relative location awareness knows what located objects are nearby. This kind of location awareness enables the objects to move around and have the information of the other located objects updated automatically. Relative location aware-ness can be simulated by using an absolute location aware system, if the device can send its location information to the network and receive information about other located objects from the network .
1.3 Receptive Vs. Transmissive localization
Location Awareness in general describes applications in computing and telecommunication, which alter their behavior in dependence of the location of an entity. The latter might be the user of the application, a person the user of the application wants to communicate with, or an object capable of changing location. With the recent thrive in Ubiquitous Computing, dozens of applications demand such location information. It has sufficiently been discussed, that localization indoors has different technological and topological requirements than outdoors. Even if we could derive a probable positioning of one meter, e.g. if we were able to receive GPS even indoors, this left us uncertain if we were at the one or the other side of the wall between two rooms. Thus, a different approach is required in local environments. In the relation of localization and communication, we can distinguish two major categories.
The first category is receptive localization. The position information is distributed ubiquitously, and the mobile device can derive its own location from this information.
Satellite distributed GPS is a typical example. The mobile device can either relate the
derived location to a map independently and provide a local service (without revealing
its own position to any third party), or purposely transmit this location in order to
obtain a value-added service.
The second category is transmissive localization. The position is derived by a fixed
station which either sees the mobile device or receives a beacon from it. The station
can then either transmit the derived location information to the mobile device, or use it
to generate the value-added service for the mobile user. Sub-cell GSM positioning is
the example, where the beacon of the mobile communication channel is also used for
positioning. At cell level, receptive location is possible in this example also, when the
mobile device can identify the very cell it is in.
1.4 Customer needs from location positioning system
Such location positioning applications address common customer needs:
• Where am I (location refinement)
• What is around me (search for points of interest)
• Who is around me (search for people)
• How do I get there (help to reach these locations)
• Let others know where I am (share location information)
1.5 The demand for
Given the mobile nature of people at work and leisure today, Mobile Location Services are attractive to both consumers and business enterprises. There are three value propositions to the end user of MLS, Safety, Convenience, and Productivity —all of which people are willing to pay for.
Safety —The ability to connect to emergency services or roadside assistance from any location, however remote.
Convenience —Find a great restaurant in the neighborhood where you ’re seeing a theater production, without having to first locate a yellow pages directory, pay phone, or paper map.
Productivity —You ’re with customer X, and since customer Y is nearby, why not pay a quick visit?
In addition to three value propositions, there are three major categories of MLS that a service provider could offer:
Business to Consumer, Business to Business, and Call Processing.
Business to Consumer —These services offer convenience and entertainment to consumers on the move.
Examples include finding nearby friends, locating places of interest, or getting driving directions.
Business to Business —These services focus on using location to improve business productivity and efficiency.
Examples include providing increased intelligence and updated data or work orders to mobile work forces such as service fleets and traveling sales people.
Call Processing —These services rely on identifying the location of a mobile caller. Examples include emergency response, roadside service, or location-sensitive call billing plans.
1.6 Detailed target specification / Subdivision Of work
The goal of achieving the location-based service in local area is subdivided in sub goals as explained below,
· To find optimal location technique for the local area and to evaluate it
· Design a prototype for the device for positioning in local area with designing user interface, user device model & giving specification of the device
· To have specification of the whole system along with software simulation
1.7 How can we achieve goal? Move towards….. projectSTHITI
To achieve such a goal, let us imagine how can we develop a complete network that is based on location positioning system. Let’s give name to this imaginable project as projectSTHITI.
The projectSTHITI Platform basically sets up wireless zones within defined areas. These zones are called hot spots. When a wireless user enters a hot spot, his or her device is detected. Content such as advertisements and coupons is generated and displayed on the device. Imagine that the entire mall is a computer screen with clickable buttons. As such, the wireless-device user would be the mouse, and the hot zones would be the buttons. When you enter a hot zone, you click on a button, which delivers content to your cell phone. Even user will also be able to trace his position & can get the location of other interested objects. Using the shopping mall as the example, let's look at the various components of the projectSTHITI Platform:
- Site Manager - The site manager is the heart of the system. This software will allow operators of networks to set up zones in high-traffic areas to detect wireless devices.
- Content Server - Within each store of the mall, there will be a local content server that is connected to the projectSTHITI Platform Network. These servers will have a range that is limited to targeted areas of each store.
- 3G technologies -The technology like CDMA, GSM, which allow electronic devices to communicate with each other, will be used by the projectSTHITI Platform to create the "hot spots."
1.8 Report Structure
Starting with next chapter, the report is organized in the following way; chapter 2 gives an introduction to the use and background for location services. This is to show the reader how important location specific services are expected to be within the area of mobile communication, both on short and long term. The next chapter then discuss various location methods. This is done in order to bring key issues to the reader’s attention. Chapter 4 contains the simulation if TDOA method with four satellites. Chapter 5 contains the discussions of relevant topics in the report. Conclusion and future work are in chapter 6.
A list covering all abbreviations used in the report can be found as very last chapter. Some imagined design of devices are at last.
2 An Introduction to Location Services
2.1 Government Requirements
Some regulations have accelerated the research of location technologies. The Enhanced 911 (E-911) regulative by the US Federal Communications Commission (FCC) pushes location technologies on the
In
Here we will take a closer look at the Enhanced 911 (E-911) standards now.
Understanding how 911 works today requires a cumulative understanding of where it's been. Therefore, this section tackles the story in four stages:
· Before Enhanced 911
· Enhanced 911
· Phase I of wireless 911
· Phase II of wireless 911
2.1.1 Before Enhanced 911
Why 9-1-1?
When designing the first emergency response system, the first question that needed to be answered was which numbers to use. Why was 9-1-1 selected?
When the task force convened to consider such a question, some initial parameters were set. It was going to be a three-digit number, and the first digit could not be "1" or "0."
When this system was originally put in place, most people were using rotary dial phones. For expediency, there was no question that the second and third numbers should be "1." So the real debate centered on which first number to use.
Among the most compelling reasons for settling on "9" was that it would be easy to find in the dark. Simply, locate the last hole on the dial, the "0" and back up one. Or for a touch pad, find the bottom right button, the "#" and move up one.¹
Thus, the emergency 911 code was born.
9-1-1 is the national emergency number dialed when the fastest possible assistance is required for law, fire, or medical assistance, in a life or death situation. When 9-1-1 is dialed, a complex series of advanced telecommunication processes begin to take place.
In the case of a wire line telephone, such as the phone in your home that is connected by wires to the public switch network (the phone network), 9-1-1 dials into a local public safety answering point (PSAP). The PSAP then can dispatch the proper emergency response agency. The 9-1-1 calls include the voice, and other bits of data that are routed to the PSAP.
Routing the call
At this point in 911's history, it was very important that the dispatcher at the PSAP obtain information from the caller about where he or she was and what kind of emergency service was required, then dispatch the correct authorities. As you might imagine, this required some skill in dealing with people under the stress of an emergency. Even then, details about where to send responders could often be hard to get. This breakdown facilitated the development of Enhanced 911.
2.1.2 Enhanced 911
When a call goes out from your phone, your voice isn't the only thing being transmitted in the network. The phone company switch that serves your phone is also sending out an Automatic Number Identification (ANI) signal to the network.
Originally, ANI signaling was designed to assist the phone company in accessing toll charges for long distance calls. With advances in technology, it was eventually employed to aid in relaying needed information to the PSAP for 911 response.
How does it work? Within each call, information containing eight digits is embedded in the signal. These eight digits contain the seven digits of the caller's local number. The eighth digit is called a Numbering Plan Digit (NPD). NPD is basically shorthand for the area code of the originating call. Since most 911 tandems rarely dealt with more than two or three area codes, this was an economical way to relay information with one digit instead of three.
With special equipment, the 911 tandem can read the ANI information and route the callback number to a digital display at the appropriate PSAP. Armed with this ANI information, the PSAP has equipment allowing it to request and receive the caller's physical address or Automatic Location Information (ALI).
With this enhancement, the PSAP is no longer totally dependent on obtaining location and callback information from the caller. Instead, the dispatcher can concentrate on helping the caller through the crisis, while instantly passing along needed information to the correct authorities.
This is how E911 has been working for more than 20 years when dialed from a wireline phone. With the advent of wireless phones and the need for wireless 911, additional challenges present themselves.
2.1.3 Wireless 911 - Phase I
Prior to the FCC order, the effectiveness of calling 911 from your wireless phone had backtracked to the days of relying on the caller to relay the proper information. This presented an additional challenge because now the caller may not even be at a fixed address - he or she could be on the side of the road, in the woods, or in an unfamiliar place. Obtaining accurate location information from the system is critical because getting it from the caller is even more unreliable.
The first phase of sending this information to the correct PSAP is for the callback number and cell tower of origination to be relayed to the PSAP.
So what happens? When a wireless subscriber initiates a call, the closest tower picks up the signal. The wireless service provider's network also has a switching center that works much like the switches on wireline calls. It reads the digits and forwards the calls accordingly.
The wireless service provider must first program its tower to immediately send any 911 call to the appropriate 911 tandem.
First, there is the Pseudo ANI (PANI). This number identifies the cell face (up to three per tower) or just the tower itself. This can narrow down the location of the caller to several hundred square meters at best, but more often several square kilometers. In addition, there is the Wireless Subscriber's callback number, which is sent along the signal. From the 911 tandem, the PANI, callback number and the voice are forwarded on to the appropriate PSAP. In most Phase I deployments, the callback number is part of the ALI response message.
However, number portability presents a challenge to the system. Today, with the explosion of phone numbers needed for wireless service, pagers, fax and Internet access, the use and number of area codes is in abundance. In addition, wireless phones can now roam all over the country.
Remember the eight-digit number used in wireline calls? There were only four Numbering Plan Digits equaling four area codes. This is no longer adequate.
The capability to use the actual area code and a new signaling protocol between the 911 tandem and the PSAP has required new software to be installed in our 911 tandem switches.
2.1.4 Wireless 911 - Phase II
This consists of the:
Network Solutions, Handset Solutions, Hybrid Solution
Phase II of bringing wireless 911 to the public means the PSAP has to receive fairly accurate location information from the wireless subscriber. How accurate? Depending on the technology, anywhere from 50 square meters to 300 square meters.
The responsibility of getting this information from the callers falls to the wireless service providers.
The supported techniques are discussed in the next chapter i. e. Positioning Methods.
2.2 General components
The following figure shows the components of a location-based service.
- The
- The
- Location-Based Service (LBS) Applications: Each of these consists of an application server and a spatial database. The processing center of a location-based service is the application server that handles user interface functions and communicates with the spatial database. Components communicate with each other via application programming interfaces (APIs).
- Location Based Service Components (AGPS = assisted GPS, API = application programming language, BS = base station, GLMC =gateway mobile location center, LBS = location based service, SMS = short message service, GPRS = general packet radio system, WML = wireless markup language )
3 Positioning Methods
3.1 Introduction to positioning methods
There are several different kinds of techniques to get the location information. These techniques include GPS, infrared sensors, mobile phones, badges, electronic keys and even more exotic ones like footstep force profiles. Two approaches to location awareness can be identified. The traditional one is based on tracking the device with the help of GPS, radio bearing, ultrasound, magnetic or infrared tracking. The other approach is to attach passive markers like barcodes or active markers like infrared transmitters to the environment. These markers enable the mobile device to read its location information .The main difference between these approaches is where the intelligence is placed, i.e. is the device smart or is it a simpler one and all the intelligence is put into network.
Because of the varying techniques that are used to provide the location information and lack of standards, the applications are normally quite closely tied to the particular location technique used. In order to be able to use a wide range of techniques easily for creating location aware applications a general location system would be needed. This kind of global system would be helpful for the application providers, since they could easily provide applications, which are not tied to a particular technique like today.
The positioning methods can be divided into three main categories:
· Network based technologies
· Handset based technologies
· Hybrid technology
3.2 Network based technologies
Network based technologies have the advantage that can be used with old mobile terminals. All the required updates for these methods to work, will be the network.One way to locate a caller is to use the network of fixed base stations in a wireless provider's network to triangulate the caller's location. Here's how:
Each station in a carrier's network is outfitted with special radio intercept equipment that receives a signal from any active phone. At any given time, two or more towers are able to compare signals from that phone and locate it based on relative readings. The following are the primary ways wireless carriers can use their network to glean location information.
3.2.1 Cell of Origin (COO)
Mobile phone systems know the position of the phone at the cell level, i.e. on which cell it is. This method is called Cell of Origin (COO). Cells vary in size from a couple of hundreds of meters in cities to tens of kilometers in countryside. Therefore this position information is not very precise. The problem with this method is that it provides the location of the base station that the phone is currently using. However, it might be the case that this base station is not the closest one.
3.2.2 Signal Strength
Measuring signal strength is one way to get the location information. It is based on the assumption that the signal strength is invertionally proportional to the distance. This method requires the signal from three base stations to be received. In reality the changes in the environment including the user affect the signal level, so that the method is not very exact. In cities this method can provide a little bit more precise location information than COO method, but in countryside the precision is much better.
3.2.3 Cell Global Identify – Timing Advance (CGI-TA)
All the required parameters for this method to work are implemented in the network today. The only update that is needed is a mobile positioning center that calculates the position estimate.
The CGI identifies the cell the MS is located in. A cell can be circular or triangular.CGI uses the identity of each cell with coverage area of BTS to locate the user as shown in the figure. It works with existing terminals without modification. It is often complemented with timing advance (TA) information. TA is the measured time between the start of radio frame and a data burst. This information is already built in to the network and the accuracy is decent when the cells are small within a few hundred meters.
3.2.4 Time Difference of Arrival (TDOA)
The TDOA technique works by measuring the exact time of arrival of a handset radio signal at three or more separate cell sites. Because radio waves travel at a fixed and known rate (the speed of light), by calculating the difference in arrival time at pairs of cell sites, it is possible to calculate hyperbolas on which the transmitting device is located. As seen in the figure, measurements at two pairs of cell sites (e.g. sites 1 & 2, and sites 1 & 3) create two intersecting hyperbolas indicating the location of the transmitting device. The TDOA technique typically uses existing receive antennas already present at a cell site.
Each tower in a TDOA system is able to measure the amount of time it takes to receive a phone's signal. They can then translate this information to estimate the distance of the phone from the tower. By cross-referencing this information from other towers in the system, a phone's position is expressed in X and Y coordinates based on longitude and latitude readings.
Using this technique:
- A wireless subscriber can use any handset (digital, analog, TDMA, CDMA, no special add-ons) to make a 911 call.
- The wireless phone's signal is received at various antenna sites. Since each antenna is a (usually) different distance from the caller, the signal arrives at a (very) slightly different time. The technique requires signal timing information from at least three different antenna sites.
- The receivers, synchronized by an atomic clock, send the caller's voice call and timing data on to the mobile switch, where the times are compared and computed to generate a latitude and longitude for the caller.
- The caller's voice call and the latitude and longitude are then sent to the PSAP for use by the dispatcher.
3.2.5 Angle of Arrival (AOA)
The AOA technique determines the direction of arrival of a handset's signal at the cell site. The phase difference of the signal on elements of a calibrated antenna array mounted at the cell site provides the angle of arrival. The intersection of the angles from two or more sites provides the location. The AOA technique determines the direction of arrival of a handset's signal at the cell site. The phase difference of the signal on elements of a calibrated antenna array mounted at the cell site provides the angle of arrival. The intersection of the angles from two or more sites provides the location.
The AOA system uses the antenna arrays at a base station to determine the angle at which a wireless phone's signal arrives at the station. By comparing this angle of arrival data among multiple base stations, the relative location of a wireless phone can also be triangulated. This is also expressed in X and Y coordinates.
Using this technique:
- A wireless subscriber can use any handset (digital, analog, TDMA, CDMA, no special add-ons) to make a 911 call.
- The wireless phone's signal is received at various antenna sites. Each antenna site is equipped with additional gear to detect the compass direction from which the caller's signal is arriving. Generally, at least three sites must receive the handset signal to provide an accurate location.
- The receivers send the caller's voice call and compass data to the mobile switch, where the angles are compared and computed to generate a latitude and longitude for the caller.
- The caller's voice call and the latitude and longitude are then sent to the PSAP for use by the dispatcher.
Handset based technologies have the best accuracy, but needed new or upgraded mobile terminals.
3.3.1 GPS
Another way wireless service providers can bring better location technology to 911 is to use modified handsets that receive Global Positioning System (GPS) signals. GPS technology uses 24 Navstar satellites that broadcast position and time information to location units on the Earth. Like the triangulation method mentioned earlier, the unit uses information from three satellites to fix its position on the Earth. In the case of the modified handset, this information is sent back through the network, ultimately to the PSAP.
Handsets are able to process GPS readings in the following ways:
GPS has problems in the indoor usage and in big cities, because it needs to have a visual contact to the satellites . The precision of the location information can also vary depending on the circumstances were it is used.
Using this technique:
- A wireless subscriber must use a handset specially equipped with GPS circuitry to make a 911 call. Either continuously or when a 911 call is placed, the GPS electronics determine the phone's latitude and longitude using data received from the satellites. Several companies, including SnapTrack and SiRF, offer additional electronics to increase the accuracy and reliability of the GPS fix.
- When the caller dials 911, the voice call and latitude and longitude data are sent to the wireless phone antennas.
- The antennas forward the voice and latitude and longitude data to the carrier's switch.
The switch forwards the voice call and the latitude and longitude to the PSAP for use by the dispatcher, where it's displayed on a map.
3.3.2 Assisted-GPS
The A-GPS technology concept is shown in Figure. The main system components are a wireless handset with partial GPS receiver; A-GPS server with reference GPS receiver that can “see” the same satellites as the handset (DGPS service can be used as well); and wireless network infrastructure, that is, base stations and a mobile switching center (MSC).
Since an A-GPS server can obtain from the MSC the handset’s position (up to the level of cell and sector), and at the same time monitors signals from GPS satellites seen by MS, it can predict the signals received by the handset for any given time. Specifically, it can predict the Doppler shift (due to satellite motion) of GPS signals experienced by the handset receiver, as well as other signal parameters that are a function of the mobile’s location. In a typical sector, the uncertainty in the predicted time of arrival of a satellite signal at the mobile is about ±5 µs, which corresponds to ±5 chips of the C/A spreading code sequence. Therefore, A-GPS server can predict to within ±5 chips the phase of the PRN sequence that the receiver should use to de-spread the C/A signal from a particular satellite, and communicate that prediction to the mobile. The search space for the actual Doppler shift and PRN phase is therefore greatly reduced, and the A-GPS handset receiver can accomplish the task in a small fraction of the time required by conventional GPS receivers. In addition, the A-GPS server maintains a connection with the handset receiver over the wireless link, asking it to make specific measurements, collect the results, and communicate them back. After de-spreading, an A-GPS receiver could pass the PRN phase information back to the A-GPS server, which would then calculate the mobile location coordinates.
To reduce the amount of information sent over the air-interface, a preferred solution is to perform additional signal processing in the handset and return pseudoranges instead. In the recently issued IS-801 (CDMA) and TIA/EIA-136 Rev. C (TDMA) standards, it is even allowed that the mobile completes the location fix. An additional way to help the handset receiver in detecting GPS signals is the socalled sensitivity assistance (or modulation wipe-off). The sensitivity-assistance message contains sets of predicted data bits in the GPS navigation message, which are expected to modulate GPS signal of specific satellites at specified times. Consequently, MS receiver can remove bit modulation in the received GPS signal prior to coherent integration. Receiver sensitivity is therefore improved since coherent integration beyond the 20-ms GPS data-bit period becomes possible, and can be extended to 1 second or more (400 ms in fast moving vehicles). For optimal performance of sensitivity assistance, A-GPS server must communicate to the handset the PRN sequence timing with an accuracy of several microseconds. This is achievable in TIA/EIA-95 CDMA systems since base stations and mobiles are in sync with the GPS time. GSM, TDMA or AMPS systems do not maintain such stringent synchronization, and implementation of sensitivity assistance (and A-GPS technology in general) will require novel approaches to satisfying the timing requirement. The standardized solution for TDMA and GSM is to add time calibration receivers in the field, the so-called Location Measurement Units (LMU), which can monitor both the wireless-system timing and GPS signals used as timing reference. In summary, the most important types of network assistance to handset GPS receivers are reduction of the frequency uncertainty of satellite signals, and provision of ephemeris information.
A-GPS systems use modified handsets that receive GPS signals and then transmit those readings to a computer. This computer then completes the calculation process, relieving the phone of having to process complex location information. With the extra computing strength, the system can use multipath mitigation and signal processing techniques to locate phones indoors, in urban canyons or other places that are a challenge for conventional GPS.
3.4 Hybrid methods
There are also mobile phones that use GPS for getting the location data. By combining
different kind of location techniques, for example GSM network positioning and GPS positioning the availability and precision of the location information can be improved. In the mobile phone based positioning services a special network component called MLC (Mobile Location Center) is used by many of the operators that provide location services . This component can hide the actual technology that is used to provide the location information. This way operators can start to offer new services with the current methods and as new more accurate methods become available they can take them into use.
Various hybrid methods are shown below:
· TDOA & Received Signal Strength (RSS)
· TDOA & AOA
· A-FLT & A-GPS
· E-OTD & A-GPS
Combine highly accurate with highly robust methods, and use multiple inputs to improve both the robustness and coverage. E.g., in A-FLT/A-GPS, A-FLT can extend coverage deep indoors where not enough GPS satellites are visible; in TDOA/AOA, AOA enables operation even when only two BTSs can receive the MS signal.
One example of hybrid method is discussed below:
3.4.1 Enhanced Observed Time Differenced (E-OTD)
This works much like the TDOA, except the reading is made in the reverse. Instead of a tower making time differential readings, the individual wireless phones have special software installed that receives time-synchronized signals from the towers. They then transmit their location back through the system.
3.5 Indoor techniques
3.5.1 Ultrasound
Ultrasound method requires that ultrasound transmitters are installed for example, to the
ceiling. The positioning device has a microphone, which receives the ultrasound information and calculates the position of the device on the basis of the measured traveling time. The precision of ultrasound method is approximately 5 centimeters up to the distance of 20 meters from ultrasound transmitter. There have to be a visual contact between the transmitter and the microphone.
3.5.2 RF and WLAN
RF (Radio Frequency) technique can be used for locating an object in short ranges. The
method is based on observing a reflector or transmitter in the observation field. Usually
the located object also sends some identification information.
WLAN (Wireless LAN) and Bluetooth techniques can also be used for providing location
information. WLAN can provide the location information on the precision of the base
station. This can mean a precision from a couple of meters to tens of meters. Bluetooth
has a range of ten meters. The reflections of signals in indoor usage make the locating
based on measuring the signal traveling times quite difficult.
3.5.3 Infrared
Infrared locating resembles the RF technique. It requires a visual contact between the
transmitter and receiver. The system may include also sensors and other devices that are
devoted to collecting and transmitting the information. Infrared-based tracking systems
have been manufactured for shopping and hospital usage. Usually the implementations
combine RF and infrared technologies.
3.5.4 Others
Pseudolithes are devices that are used to simulate the satellites on the ground. They are
used in places where satellites are not available, for example in buildings and tunnels.
Traditional way of locating is to update the previous position on the basis of traveled
distance and direction. This approach requires some sensors for measuring the distance
and direction. The problem of sensors is that the position is not very exact after a series of
updates.
Location information is also gathered in some places as a side product. For example when using magnetic keys for opening the doors of a building the information is stored some-where. More exotic possibilities include the usage of special purpose tiles that identify the footstep force profiles of the person who enters the room. This identification is done by adding special tiles to entrance points.
4 Simulation of TDOA
This chapter presents the detailed derivation of an algorithm for obtaining an exact solution for the three dimensional location of a mobile given the locations of four fixed stations (like a GPS satellite or a base station in a cell) and the signal time of arrival (TOA) from the mobile to each station. A VHDL model of the algorithm was implemented using the IEEE numeric_std package. The model can be easily synthesized for hardware implementation.
4.1 Introduction
Many organizations are developing competing products to comply with the FCC's E-911 mandate which requires U.S. cellular carriers to provide location information of phone calls, effective October 2001. The accuracy required is 100 meters or better. Many of these products will implement the well known time difference of arrival (TDOA) technique for locating a mobile with varying degrees of accuracy.. Some methods calculate the two dimensional position and others the three dimensional position depending on the degree of simplicity desired. In this paper, a more detailed derivation of a set of equations needed to locate the three dimensional position of a mobile is presented. This detailed derivation will be the basis for implementing a positioning algorithm in C++ and VHDL. The VHDL version will utilize the IEEE numeric_std package so it can be synthesized into an ASIC by anyone seeking a hardware implementation.
4.2 The Derivation
The essence of the TDOA technique is the equation for the distance between two points.
The distance between a mobile and a station is determined indirectly by measuring the time it takes for a signal to reach the station from the mobile. Multiplying the TOA t by the signal velocity c gives us the distance d. From now on, R will be used to represent the distance d since it is the more commonly used notation in TDOA literature.
We need to solve for the three unknowns x, y and z (mobile position). Therefore, equation (1) is expanded to three equations when the specific locations of three satellites i, j and k are given. This requirement can be easily met since GPS satellites broadcast their exact locations.
5. Application of mobile Awareness System
5.1 Application areas
Location aware applications can be provided in multiple areas. The only limiting factor is
the creativity of service providers. Below is a collection of the services that are suggested
in the literature and articles. This gives some kind of picture of the possibilities of location awareness. However, forecasting the future is not an easy task. There will certainly be services that we can not think about yet.
5.1.1 Billing
Location based billing provides the possibility for operators to bill different rates based on the location. This could mean that you get cheaper calls at predefined areas, for example at home and at office. You could also change your predefined areas by sending a message to the operator. Other examples of billing could be automatic billing at theatres, cinemas, trains and busses. These services would be based on proximity awareness.
5.1.2 Safety
Safety area includes many possible applications. The most attention has been given to
the emergency call locating. FCC has ordered that every mobile 911 emergency call in
the United States has to be located within 125m on the 1st of October 2001. This has created a need for the wireless operators to develop and take into use the location tracking services. However, since the schedule is tight there is the risk that operators are
too busy with the emergency calls and ignore the other services. This is not a wise thing
to do since, the actual benefits are coming from other location aware services that they
can implement. Also different kind of push services that provide information or warnings
can be implemented. This could mean, for example, that if there has been a toxic waste
accident all the people in the area could be advised to stay inside.
5.1.3 Information
Information providing has endless possibilities in location aware applications. Location in-formation can be used to help in navigation while traveling or provide information about where is the nearest restaurant or flower shop. Tourist guides that use location information can show background information about the place the tourist currently is. Other information services could include things like traffic warnings or advertisements pushed to the handset.
5.1.4 Tracking
Tracking objects like pets, children or valuable items or deliveries is nowadays quite common. This way they can be found easily when needed. There have been experiments in the office environments of tracking people. One such experiment was the Active Badge, which is presented in next section. Generally the tracking of employees tries to enhance the productivity. GPS positioning has been used for tracking trucks and route optimization for a long time. It has also been used as a navigation tool in outdoor activities like hiking and sailing.
5.1.5 Proximity awareness
Automatic sensing of the devices or persons that are nearby is one application area. This
could mean that incoming calls are directed to the phone closest to you or that printouts
from PDA are automatically delivered to the printer closest to you. Proximity awareness
could also include things like PDAs changing information automatically in meetings or
finding out if your friend is nearby.
5.2 Various Examples
5.2.1 Business Contact Application
Business Connect helps subscribers find hotels, banks, gas stations and other businesses when and where they need to.
Features for Subscribers
Business Connect Application helps your subscribers locate businesses within a specified distance, get directions, and view maps:
Find by category or name
Find Favorites or by distance
Save locations as Favorites
Get step-by-step directions to the business with maps
Call the business without ending the application
Manage personal preferences:
- SMS or email communications
- travel mode (walking or driving)
- route mode (shortest vs. quickest)
- maximum results to show (5, 10, 15)
Benefits to Carriers
Offer a high-value service to your subscribers that brings you revenue
Increases network usage and billable air time
Increases customer satisfaction and loyalty
Minimizes your development and implementation costs
5.2.2 Directions Connect Application
Directions Connect application is an application for wireless carriers and network operators that gives step-by-step directions and color maps to mobile subscribers. An intuitive user interface makes Directions Connect Application easy to use and fosters fast subscriber adoption. With Directions Connect Application, your subscribers get the convenience, safety, and productivity that comes with finding their way more easily. You get increased revenue, customer loyalty, and a market advantage.
Features for Subscribers
· Get detailed step-by-step directions with color maps
· Set mode of transportation –walking or driving
· Get directions to favorites or previously visited places
· Get traffic reports, if available – via Traffic Connect Application
· Send directions to others via SMS or email
· Save routes for later use
Benefits to Carriers
Your subscribers get a convenient, time-saving service relevant to their location. You:
Deliver a high-value, compelling service with revenue potential
Minimize your development and implementation costs
Increase network usage and billable air time
Increase customer satisfaction and loyalty
5.2.3 Entertainment Connect Application
Entertainment Connect Application is an application for wireless carriers and network
operators that lets mobile subscribers find restaurants, theaters, museums and more,
when and where they need to. An intuitive user interface makes Entertainment Connect Application easy to use and fosters fast subscriber adoption. With Entertainment Connect, your subscribers get the convenience and productivity of quickly locating entertainment venues while on the go, and you get increased revenue, customer loyalty, and a market advantage.
Features for Subscribers
· Find entertainment spots quickly
· Search by category, name, or favorite
· Get directions to venues, including steps and maps
· Call the venue without interrupting session
· Send messages and directions to friends via SMS
· Create list of favorites
5.2.5 Friend Connect Application
The ability to easily connect with other people is one of the most compelling attributes of the Internet, and now, wireless communications. The ability to communicate with friends, then go see them in person, adds significant value to basic messaging applications.
Features for Subscribers
• List, add, and edit friends
• Select and show friends by proximity
• Get directions to friends, including steps and maps
• Set up meetings at specific locations
• Send messages and directions to friends via SMS
• Manage privacy and search preferences
Benefits to Carriers
Uses familiar SMS communications
Minimizes your development and implementation costs
Increases network usage and billable air time
Increases customer satisfaction and loyalty
6 Discussion
Different location services require different accuracy of the location estimate .The big issue, in addition to accuracy, is the degree of coverage of the location method. As shown in this report, the location technologies all have different advantages and disadvantages. The best solution will probably be to have combination of CGI+TA, E-OTD and A-GPS implemented in the network. But this is a matter of cost and, as mentioned earlier, the requirements of the different location services the operator choose to offer their customers.
E-TOD has limited coverage in rural areas. But when complemented with CGI+TA (already implemented in the network), good location coverage is still possible in this area. When implementing location services e.g. fleet management, weather reports and emergency services in this area, the location accuracy perhaps do not need to more accurate than up to 1 km. There will be no need for the accuracy of e.g. the A-GPS method, and the E-OTD method complemented with CGI+TA will be appropriate. On the other hand if the operator chooses to implemented location services e.g. route guidance and navigation services, the location accuracy needs to accurate up to 50m .If the E-OTD method is to be used in this area, the density of BTSs needs to increase. If the operator plants to implement services that require this kind of accuracy, the A-GPS method will be preferable in this area. The A-GPS method will give high cost for the consumers, who needs new mobile terminals to use this method. This cost canon the other hand be covered by the operator by subsidiaries.
In the urban areas, the accuracy and coverage is quite good when using E-OTD method. The propagation properties will be the most vital obstacle in this area. But it is assumed that most of the location services available today will not have higher accuracy requirements than the E-OTD method can provide in this area.
According to many industry experts, including recent study reports from CGALIS and LOCUS, a combination of the A-GPS and E-OTD location technique can reach greater level of accuracy in rural and sub-urban areas, but at the same time has a low to medium availability indoors. On the other hand, E-OTD provides high availability and accuracy in dense-urban areas and indoors. Combining the two technologies provides high accuracy and availability in all areas: outdoors, sub-urban, dense-urban and indoors.
The difference in density of BTSs in the urban and rural area also requires different density of LMUs at these areas. The BTSs in the urban area is dense, and requires one LMU per five BTSs. The density of BTSs decreases as you move away from the urban
areas. The rural area requires one LMU per four BTSs. For the E-OTD method to be performed, each of the BTSs involved in the location prediction must have a LMU visible. These LMUs have a high cost for the operator, and it must be taken into account when planning to implement a location technology. Does the operator need the accuracy of the E-OTD method in that particular area, or could a “low cost” method like CGI+TA cover the location accuracy the services requires?
The information about the location of the mobile stations may be used not only to provide a subscriber service, but may also be used for network internal operation such as location assisted hand over. This internal use of the information may lead to higher traffic capacity and improved call completions.
The E-911 regulative by the US Federal Communications Commission (FCC) pushes location technologies in the USA. In Europe a similar regulative is expected. The Commission of European Communities has issued a proposal for a directive on a universal services and users’ rights relating to electronic communications networks. The rollout of new location technology in Norway will probably be marked driven sinse Norway is not a member of the European Union, therefore no carriers have to act in accordance with the upcoming E-112 rules.
6.1 Comparison of location technologies
Table 1. Comparison of location techniques
Method | Accuracy | Coverage | Cost |
| CGI + TA | Limited accuracy Guideline estimate: 100 – 1100 m | Indoor / outdoor : no Limitation | In network: MLCs In handset : no cost |
| TDOA | Better accuracy than CGI + TA, but not as good as A-GPS Guideline estimate: 50 – 200 m | Indoor / Outdoor: no Limitation | In network: MLCs and LMUs In handset: no cost |
| AOA | Guideline estimate: 300 m | Indoor: limited coverage Outdoor: some limitation in case line-of-sight can not be obtained | In network: directional antennas and MLCs In handset: no cost |
| A-GPS | High accuracy guideline estimate: 10-20 m | Indoor: limited coverage Outdoor: some limitation in case line-of-light can not be obtained | In network: MLCs and hardware to provide A-GPS information In handset: additional HW |
Thus, there are so many things to discuss still. Different people suggest different algorithms. At a time I am one of them.
6.2 Leading Contenders by Network Type
6.3 Future work
As we are trying to evaluate TDOA method for local area, at this stage we have already studied all the positioning methods. This is on the basis on the technology, costs and which location services and requirements the operator wants to implement.
We are trying to develop the simulating the model of TDOA and to evaluate the C++ program for the same method which helps us to find the position of the person or any thing. Our ultimate goal will be to provide the ease to user having the best positioning method.
Appendix
Abbreviations
3GPP Third Generation Partnership Project
A-GPA Assisted Global Positioning System
ALI Automatic Location Information
ANI Automatic Number Identification
AOA Angle Of Arrival
API Application Programming Language
ARFC Absolute Radio Frequency Channel
BCCH Broadcast Control Channel
BSC Base Station Controller
BTS Base Transceiver Station
CBC Cell Broadcast Center
CDMA Code Division Multiple Access
CGI-TA Cell Global Identify Timing Tdvance
COO Cell Of Origin
E-112 Enhanced 112
E-911 Enhanced 911
ECR Enhanced Call Routing
E-FLT Enhanced Forward Link Triangulation
E-OTD Enhanced Observed Time Difference
EU European Union
FCC Federal Communication Commission
GMLC Gateway Mobile Location Center
GMSC Gateway Mobile Services Switching Center
GPRS General Packet Radio Service
GPS Global Positioning System
GSM Global System for Mobile telecommunication
gsmSCF GSM Service Control Function
GTD Geometric Time Difference
HLR Home Location Register
IEEE Institute of Electrical and Electronics Engineers
LBS Location Based Services
LCS Location Services
LECS Local Exchange Carrier Switch
LMS Location Measurement Services
LMU Location Measurement Unit
LOCUS Location of Cellular Users for Emergency Services project
LOS Line Of Sight
MLC Mobile Location Center
MPC Mobile Positioning Center
MS Mobile Station
MSC Mobile Services Switching Center
NLOS None Line Of Sight
NPD Numbering Plan Digit
NSS Network Subsystem
OTD Observed Time Difference
PANI Pseudo ANI
PCS Personal Communication Services
PDA Personal Digital Assistance
PLMN Public Land Mobile Network
PPM Parts Per Million
PSAP Public Safety Answering Point
PSTN Public Switched Telephone Network
QoS Quality of Services
RF Radio Frequency
RSS Received Signal Strength
RTD Real Time Difference
SGSN Serving GPRS Support Node
SIM Subscriber Identity Module
SMLC Serving Mobile Location Center
SMR Specialized Mobile Radio
SMS Short Message Service
SMSC Short Message Service Center
TA Timing Advance
TDMA Time Division Multiple Access
AEMS Test Equipment for Mobile Systems
TOA Time Of Arrival
TDOA Time Difference Of Arrival
UL-TOA Uplink Time Of Arrival
UMTS Universal Mobile Telecommunication System
UTM Universal Transverse Mercator
VLR Visitor Location Register
WAP Wireless Application Protocol
WGS World Geodetic Register
WLAN Wireless LAN
WML Wireless Markup Language
XML Extensible Markup Language
TO DOWN LOAD REPORT AND PPT
