Definition: A service provided to a subscriber based on the current geographic location of the MS. Location-based services (LBS) provides service providers the means to deliver personalized services to its subscribers
LBS reflects the convergence of multiple technologies:
Internet - Geographic Information System - Mobile devices
Localization
Localization is based on analysis, evaluation and interpretation of appropriate input parameters. Most of them are related to exploitation of physical characteristics being measurable in a direct or indirect way.
From a physical localization point of view, there are three principle techniques to be distinguished:
1. Signal Strength & Network parameters:
The most basic wireless location technology is given by the radio network setup itself. Each base station is assigned a unique identification number, named CellID. The CellID is received by all mobile phones in its coverage area, thus the position of a target is derived from the coordinates of the base station. Signal strength could be used to reduce the target position. Wave propagation is highly affected by several factors, especially in urban areas, thus signal strength is altered and does not provide a reliable parameter for localization. Cell ID accuracy can be further enhanced by including a measure of Timing Advance (TA) in GSM/GPRS networks or Round Trip Time (RTT) in UMTS networks. TA and RTT use time offset information sent from the Base Transceiver Station (BTS) to adjust a mobile handset’s relative transmit time to correctly align the time at which its signal arrives at the BTS. These measurements can be used to determine the distance from the Mobile Station (MS/UE) to the BTS, further reducing the position error.
2. Triangulation/Trilateration:
In trigonometry and elementary geometry, triangulation is the process of finding a distance to a point by calculating the length of one side of a triangle formed by that point and two other reference points, given measurements of sides and angles of the triangle. Such, trigonometric methods are used for position determination. It can be distinguished as
- Distance-based (tri-)lateration (example: Global Positioning System, GPS), For distance-based lateration, the position of an object is computed by measuring its distance from multiple reference points
- Angle- or direction-based (tri-)angulation (example: AOA-Angle of Arrival, TOA-Time of Arrival,AFLT- Advanced Forward Trilateration, EOTD- Enhanced Observed Time Difference)
3. Proximity:
Proximity is based on the determination of the place of an object that is nearby a well-known reference place. Again, one distinguishes three fundamental sub methods:
- Monitoring and matching the terminal location with the database containing stamped locations(with RSSIs from different Base Stations)
- Monitoring of (WLAN Radio) Access Points. Here, it is evaluated whether a terminal is in the range of one or several APs.
Localization Categories
Different localization principles may be applied to gain position information with respect to an object that is to be tracked. Four different categories can be distinguished:
- Network-based: All necessary measurements are performed by the network (by one or several base stations). The measurements usually are sent to a common location centre being part of the core network. This centre takes over the final computation of the terminals’ positions
- Terminal-based: In the terminal-based localization approach, the terminal accounts for position determination.Disadvantages of terminal-based localization obviously are given by increased terminal complexity. Increased challenges with respect to calculation power and equipment lead to the assumption that this method is only partly applicable for legacy terminals.
- Network-assisted: Similar to terminal-based positioning, network-assisted positioning implies that the final calculation of the terminal’s position is taken over by the terminal.The difference is that possible assistance data is sent by the network. This can be done either on request or in a push-manner.
- Terminal-assisted: This too is a hybrid implementation of the other methods like above. The terminal hereby measures reference signals of incoming base stations and provides feedback reports to the network. The final position computation takes place in a central location centre within the network.
Accuracy increased with adoption of Hybrid techniques.
Types of Positioning Techniques:
So we can classify the existing location positioning techniques currently being deployed by wireless operators over the world as:
Below is explanation for one of each type of positioning techniques.
- Triangulation:
OTDOA (Observed Time Difference of Arrival)
Observed time difference of Arrival is based on the mobile measurements of the relative arrival times of pilot signals from different base stations (Node Bs) in UMTS networks
Signals from at least three Node Bs must be received by the mobile for location determination
Timing synchronization of the different Node Bs is essential. This is obtained by a location measurement unit (LMU) at the BS
Following timing parameters are calculated to compute the final accurate position.
Real Time Difference (RTD): the synchronization difference between the two base stations
Geometric Time Difference (GTD): the propagation time difference between the two base stations
Observed Time Difference (OTD): Time difference measured by the mobile between the receptions of bursts transmitted from the reference BTS and each neighboring BTS in GSM networks
OTDOA with coordinates of BSs and RTD between the Downlink Transmissions of UTRAN BSs provided by LMU at the fixed location which timestamps the UTRAN cell frames to the common GPS timing or measures OTD of all the BSs. Since the position of LMU and all the BSs is known, absolute RTD can be measured for every involved BS. Often, the positioning process is also supported by the RTT measurements from involved base stations
In UMTS NodeB transmissions are synchronously ceased for a short period of time - Idle Period. Terminal can measure neighbor NodeBs during Idle Periods Maximizes the hearability of distant pilots. During the ‘common’ idle period each node B transmits a signal ONLY useful for location estimation, randomly, pseudo-randomly or periodically. OTDOA of these common pilots is measured in the MS for different Node Bs. Positioning is done as in the standard OTDOA algorithm.
Drawbacks:
Added complexity to the network operation
Reduced communication efficiency
- Proximity Based
GSM Fingerprinting/ Database Correlation Method:
RF Fingerprinting offers the simplicity of an received signal strength indication (RSSI)-based lateration approach.
RF Fingerprinting significantly enhances received signal strength (RSS) lateration through the use of RF propagation models based on signal strengths, C/I ratios, cell identifications, time delays, network quality, etc. developed from data gathered in the target or similar environments
Superposition of all propagation paths results in a unique pattern that is specific to a given location. This fingerprint is compared with stored position-patterns from a database to find the most probable location.
Localization using RSSI fingerprinting is based on a database of <location, RSSI>-tuples known a priori that allow extracting the most probable position of a mobile device given its current RSSI-measurements for one or multiple base-stations.
The database is built using a training phase which may be conducted explicitly prior to public deployment or implicitly when the system is already operational. Fingerprinting allows for better estimates because it takes into account the effects that buildings, solid objects or people may have on the signal.
Fingerprints are collected by measurements using a Network planning tool and extensive drive testing as high sampling resolution is needed.
Comparing the input measurement with reference fingerprints using Cost Functions gives the location of the best matching reference fingerprint.
- GPS based techniques
Global Positioning System(GPS)
GPS, also known as Global Navigation Satellite System (GNSS), is used for determining the positions of receivers using signal broadcast by satellites. It was first developed by United States Department of Defense.
GPS satellites orbit the earth every 12 hours at an altitude of approximately 20.200 km. Each satellite contains several high-precision atomic clocks and constantly transmits radio signals using a unique identity code. The GPS localization principle is based on Time of Arrival measurements.
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 to transmit a spread spectrum signal including three different kind of information –
Pseudorandom code- Used to identify transmitting satellite
Almanac data- Informs the GPS receiver where each GPS satellite should be at any time throughout the day.
Ephemeris data- Used for actual positioning since it contains important information about the status of the satellite (healthy or unhealthy), current date and time.
Step 1: The basis of GPS is Triangulation from satellites
Position is calculated from distance measurement to satellites
Step 2: Measuring Distance from Satellite.
Accurate timing is the key to measuring distances to satellite.
Satellite Atomic Clocks- Hence are very accurate.
In receivers- No Atomic Clocks- Hence measurement from fourth satellite is necessary to measure accurate distance.
In receivers- No Atomic Clocks- Hence measurement from fourth satellite is necessary to measure accurate distance.
Step 3: Getting Perfect Timing.
The error information is sent to satellites, to be transmitted along with the timing signals.Satellite positions are continuously monitored
Step 4: Satellites Positions
Both the satellite & receiver are generating pseudo-random codes at exactly the same time.
Signal arrival time= Sync receiver time- Sync of satellite time
Distance= Velocity * Signal arrival time
Drawbacks:
Time To First Fix (TTFF)- 30 sec to 10 min
Weak satellite signal strength indoors & urban environment
High power consumption by GPS receivers.
- Hybrid Technologies
Assisted- Global Positioning System (A-GPS)
A-GPS enhances the startup performance of a GPS satellite-based positioning system. It is used extensively with GPS-capable cellular phones as its development was accelerated by the U.S. FCC's 911 mandate making the location of a cell phone available to emergency call dispatchers.
Reasons for introducing A-GPS:
- Launching a 911 emergency call it would take the receiver several minutes to establish a fix (cold start) and being able to output the caller’s location co-ordinates
- GPS would have a very poor performance in too many situations (indoors, under dense foliage, in (urban) canyons, etc)
- GPS receiver would drain too much power from the handset
- Incorporating a GPS receiver in the handset would be far too expensive and too bulky
An A-GPS receiver can address these problems in several ways, using network elements such as either an assistance server or other data from a network. That assistance generally falls into two categories: a) information used to more quickly acquire satellites, or b) calculations done remotely:
- The assistance server can locate the phone roughly by which cell site it is connected to on the cellular network.
- The assistance server has a good satellite signal, and lots of computation power, so it can compare fragmentary signals relayed to it by cell phones, with the satellite signal it receives directly, and then inform the cell phone or emergency services of the cell phone's position.
- It can supply orbital data and/or almanac for the GPS satellites to the cell phone, enabling the cell phone to lock to the satellites faster in some cases
- The network can provide atomic time (Accurate Time Assistance)
- Simply capturing a brief snapshot of the GPS signal, with approximate time, for the server to later process into a position.
- By having accurate, surveyed coordinates for the cell site towers, it has better knowledge of ionosphere’s conditions and other errors affecting the GPS signal than the cell phone alone, enabling more precise calculation of position.
As an additional benefit, in certain types of A-GPS, both the amount of CPU and programming required for a GPS phone is reduced by offloading most of the work onto the assistance server.
Obtains handsets position from MSC + at same time monitors signals from GPS satellite seen by MS.
Maintains connection with MS- collects measurements & communicates back to MS
A-GPS receiver passes pseudorandom code to server, which helps in calculating MS co-ordinates & hence the distance.Since the calculation of the exact position is done within the network, the handset doesn’t need to be complex and expensive.
Hybrid location technology combines GPS with other location positioning in a way that allows the strengths of one to compensate for the weaknesses of the other to provide a more reliable and robust location solution. Common hybrids are: A-FLT/GPS, E-OTD/GPS and Cell ID/GPS.
Comparison of Location Technologies
The analysis below outlines the comparison between different positioning technologies based on different parameters such as Accuracy, Time to Fix, Coverage, Wireless Standard currently using and additional handset and network requirements for implementation.
From the above analysis we can conclude that the success of LBS depends on choosing the optimal positioning technology weighing the different technical and economic feasibility parameters. The operators like Verizon Wireless have already implemented A-GPS technologies combining the traditional AFLT technique with GPS improving the location accuracy and controlling the huge implementation cost with a better coverage.
In advanced wireless networks such as WiMAX and LTE, a hybrid LBS framework is being adopted on the lines of A-GPS accommodating both MS managed and network managed location.
In the next parts we will cover the LBS Architectural Framework in 3GPP/3GPP2/OMA based standards followed by the handsets(device manufacturers) adoption of these technologies aligned with the use cases and business models for the entire LBS value chain.
- Neil Shah
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