Mobile Location Tracking Techniques - v2uguru by beyondimagination



Abstract 1. Introduction 2. Commercial application of mobile location tracking techniques and issues in mobile communication. Commercial application of mobile positioning Limitation of frequency spectrum and co-channel interference Hand offs 3. Performance 3 Accuracy Reliability Availability Power consumption Hardware size Software size measures of location tracking techniques 2 1


Processing load Cost 4. MS location techniques Network- assisted GPS method Enhanced Observed Time Difference (EOTD) 5. Locating the position of Mobile Station (MS) Forward Link Time Difference of Arrival (TDOA) Evaluation of TDOA method Method suggested by Martin and Hellbrandt Algorithm Evaluation Method suggested by Cheung et al Algorithm 6. Conclusion Figures References Word count 1,313 11 12 13 9 4


Information about the location of a mobile station (MS) can be very useful in providing critical services required during rescuing operations. It can be very useful in providing navigational services. Additionally it can help in tackling issues like co-channel interference and limitations of frequency spectrum. This paper analyses and evaluates mobile location tracking techniques in terms of accuracy, cost and processing load.



Mobile location tracking has become an important technology in mobile communications mainly because of its commercial potential. Besides having commercial potential the information about the location of mobile station is also useful in tackling issues like unnecessary handoffs, limitations of frequency spectrum and co-channel interference. This paper reviews existing mobile location tracking techniques in terms of cost, accuracy and processing loads. It also explains how location tracking of a MS can help in reducing co-channel interferences, increase frequency reuse and avoid unnecessary handoffs.

The paper is organized as follows. Section 2 describes briefly the commercial applications of location tracking and how it can help in tackling issues like hand off, limitations of frequency spectrum and co-channel interference. In section 3 evaluation parameter are briefed upon. In section 4 mobile location tracking techniques are categorized. Section 5 gives an account of some mobile location tracking techniques, which are evaluated in terms of evaluation parameters. The paper concludes with comparative analysis of the mobile location tracking discussed in section 5.




2.1 Commercial application of mobile positioning

Location Tracking of MS has got tremendous commercial applications. It can help in tracing lost and stranded persons. Business establishments can use location tracking of their field staff for directing them to nearest customers. People ca be directed to relevant commercial outlets using location tracking through MS. Service providers can use location based billing which can be used to generate a profile about how many calls are generated from which part of the service area. These are only some of the commercial applications which can be thought of once location tracking becomes possible.

2.2 Limitations of frequency spectrum and Co-channel interference FCC (Federal Commission for Communications) allocated 40MHz of spectrum in the 800MHz band for mobile radio cellular systems. Since each radio channel occupies a bandwidth of 30 kHz, a bandwidth of 40 MHz only provided 40 MHz/30kHz = (40*10exp6)/ (30*10exp3) =1333 radio channels. Since each conversation required a pair of radio channels, one for forward communication and one for reverse communication,

www. there were 133/2=666 pairs of radio cannels available. Since a pair is required for each conversation there can be at the most 666 conversations at a time. Therefore if a given amount of time all 666 pairs were occupied then 667th person had to wait till one of the occupied pair of the radio channel became available. This sometimes resulted in a person waiting for too long to get a conversation going, leading to a poor service scenario. This limitation of frequency spectrum gave rise to frequency reuse and cellular communication concept in which the geographical area to be covered was divided into cells and same frequency radio channels were used repeatedly separated by some distance.

As shown in figure 1 F1 to F7 represents different frequency radio channels. It can be seen in the above figure that these radio channels are being used repeatedly and some geographical distance separates this reuse. This increases the total number of traffic channels available for communication, which in turn increases the system capacity in terms of number of users, which can be served at a time. The minimum distance, which allows the same the same frequency to be reused, depends upon many factors, such as the number of co-channels cells in the vicinity of the centre cell, the type of geographic terrain contour, the antenna height, and the transmitted power at each cell site. But this reuse o same frequency over different geographical locations near to each other gives rise to co-channel interference. For example it can be seen in fig1 that radio channel having the frequency value F2 is being used again separated by cells having frequency F1 and F7. These two radio channels having frequency F2 can interfere with each other as they have same frequency

www. because signals having same frequency have more chance to interfere with each other than signals having different frequencies. This is known as co-channel interference. One way to restrict co-channel interference is to restrict the signals in and around the area where MS is located. Therefore, even if the same frequency channel is being used near by, the two signals are not able to interfere with each other. But in order to supply a localize signal to a MS the exact location of the MS has to be known. When MS moves from one location to a exact location to other the signal has to follow it, which means that the antenna beam should point to the new location, which MS has acquired. This is called power delivery intelligent system, which reduces co-channel interference considerably. One essential condition to implement power delivery intelligent system is the information about the location of MS.

2.3 Hand offs

Handoff comes into the picture in case of user’s who are on the move while connected to the base station. When such a user leaves the current cell and enters a new cell then he has to be assigned a new channel from among the channels, which are assigned to the new cell. This is called handoff. One thing crucial for a handoff is that it should be completed before the user leaves the overlapped region between the previous cells and current cell. Handoffs call for considerable processing load on base station side. So unnecessary handoffs should be avoided. Velocity of a moving MS should be assigned new radio channels or it should be allowed to complete the ongoing call with the currently assigned radio channel e.g. for a MS moving at a very slow speed there is high

www. probability that will complete the current conversation or current call before leaving current cell or overlapped region between current cell and new cell. So there is no need to assign new radio channel and handoff can be avoided. But for a MS moving at a very high speed there is high probability that MS will cross over to new cell before it completes the currently ongoing call. Therefore, handoff is required. Thu information about velocity becomes critical to determine whether handoff should take place or not and to determine velocity information about the location of MS at two time intervals is required.








3.1 ACURACY Accuracy of a mobile location tracking methodology is a measure that defines how close the location measurements are, to the actual location of the mobile station being located. The closer the measured location is to the true location the more accurate the measurements are. In evaluating the accuracy of a positioning methodology, the

www. number of location measurements made is to be considered. A widely used score function in accuracy evaluation of a mobile location tracking techniques is the mean of squared errors (RMSE).

3.2 Availability

Availability is related to converge and capacity. For example GPS (Global Positioning System) signals cannot penetrate thick walls and thus the signals become blocked. Therefore GPS-based positioning cannot be carried beyond this point. But this doesn’t mean that GPS is not good in terms of accuracy and reliability, the only limitation is GPS signal are not available for use in the underground environment. Availability of mobile location tracking techniques also depends upon the planning of the cellular networks. It can be increased by effectively deploying the base stations.

3.2 Reliability Reliability is defined as the ratio of successful positioning attempts out of all attempts made.

3.4 power Consumption This becomes a limiting factor if a mobile location tracking technique is using resources inside a handset solely for the need of positioning or if dedicated hardware has to run to perform measurements or location calculations.


3.5 Hardware size The hardware component required for carrying out the position calculations are to be accommodated in the handset. This may increase the size of the handset. However, user expects the size of their devices to become smaller. Therefore, increased size of handheld devices because of positioning capability may not be liked by the user.

3.6 Software size Some positioning methods require additional software to perform measurements and/or position calculations. If additional software increases the memory requirement then the size of the handset has to be increased to incorporate it.

3.7 Processing Load This is closely related to the size of the software. For real-time applications, the availability of sufficient processing power is imperative, but the need for extra processing power becomes a problem in this ever reducing size of handheld devices.

3.8 Cost Despite good merits in performance, a location tracking technique may be inapplicable if its deployment and operational cost are higher compared with some other methods, although it might not have the same level of performance and technology with respect to cell phones.


www. MS location techniques are categorized into two categories, network- based location tracking and handset-based location tracking techniques. In Network- Based Location Tracking Techniques, the signal transmitted from a mobile station to determine MS location. The main advantage with these techniques lies in their easy implementation, which doesn’t require any software or hardware modification to the handset. Usually, the mobile station is totally passive and doesn’t play any role in the use of legacy handsets. The disadvantages of these methods lie in their lack of accuracy when compared to GPS based positioning. In handset-based location tracking techniques the handset detects the signal transmitted from multiple base stations and/or satellites, and uses these signals to determine its location. The implementation of the techniques normally requires some software or hardware modifications to the handsets, which makes implementation costly and difficult to implement. Some of the handset-based location tracking techniques is:

4.1 Network-assisted GPS method

In this method MS uses the signals acquired from several GPS satellites and auxiliary network information to calculate its location. The mobile is fixed with a GPS Rx and base station feeds the satellite information through the air interface. The information includes a bar code containing satellite ID of the satellite, which is sending the signal. It also contains information about the position of the satellite and the time at

www. which the signal was sent. The handset calculates the difference between the time at which information was sent and the time at which it was received. This time difference is used in calculating the distance of the handset from the satellite. The distance measurement of the handset from three such satellites gives the reference position of the MS.

4.2 Enhanced Observed Time Difference (EOTD)

EOTD positioning technology is based on using MS to monitor a group of base stations. For different signals the mobile measures the Time Difference of Arrival (TDOA) between the serving base station and the neighboring base stations. In GSM (Global System for Mobile communication) this TOA difference is called the Observed Time Difference (OTD) and is one of the required information for handoff. Location Measurements Units (LMUs) in known locations thought the network are used to estimate TOA. Measurements of TOA and OTD are used to estimate the positions of the MS.



For locating a mobile many methodologies have been suggested. Some of them are:

5.1 Forward Link Time Difference of Arrival (TDOA) Method In this method, the mobile station is made to detect signals from at least three base stations. The relative arrival times of the signals from the visible base stations arrival times of the signals from the visible base stations are then used to obtain the location estimates of the MS.

It is assumed that the co-ordinates of the three base stations are known as (x1, y1), (x2, y2) and (x3, y3).

Furthermore, it is assumed that the mobile stations are located at (x, y). Without any loss of generality it can be assumed that base station BS1(x1,y1), base station BS2(x2,y2) and base station BS3(x3,y3) are situated as shown in the figure 2.

If c is the speed of light and t1, t2, t3 are the times taken by the signal to travel from BS1(x1, y1), BS2(x2, y2) and BS3(x3, y3) to the MS respectively. It can be shown that

d2-d1 = c*(t2-t1) = [(x2 +(y-y2)2]0.5 –(x2+y2)0.5 d3-d1 = c*(t3-t1) = [(x-x3)2+(y-y3)2]0.5 – (x2+y2)0.5


Two equations shown above have two unknown x and y. x2, y2 and x3, y3, c. (t2-t1) is known. Therefore, it is possible to reduce the problem of finding the location of the MS to the problem of solving a quadratic equation to determine x and y. 5.1.1 Evaluation of TDOA method

Detection errors, non-line-of-sight multipaths and lack of perfect synchronization amongst the base stations introduce errors in MS location estimates. In urban areas, there is a very high probability that most of the received multipaths will be non-line-of-sight. Synchronization errors and detection errors (measurements errors) are also present. All such impairments cause errors in the location estimation algorithm using TDOA method.

In terms of cost it is expensive since it involves the use of satellites through GPS which is used to assign coordinates to base station. In terms of availability it has some restrictions since GPS signals can’t penetrate through thick walls. Also it calls for incorporating additional features in base stations and mobile stations in order to make GPS technique to work correctly.

5.2 Methods Suggested by Martin and Hell Brandt Algorithm

Another algorithm suggested by Martin and Hell Brandt makes use of field strength data available in mobile communication at different time intervals. In finding the location of MS the service area to be covered is divided into a grid consisting of cells and

www. each cell is given a unique address. This can be done by using GIS (Geographical Information System) technique. After this the average signal power due to mobile station at each cells of the grid is calculated and recorded using a field instrument. This information is stored in a database, which contains average signal power and address of the cell of the grid. To trace the location of a mobile station the signal power received from the mobile station under consideration is compared with the value stored in the database. The square of the difference of the stored value and saved value is calculated for all the values stored in the database. The cell for which this difference is minimum gives the location estimates of the mobile stations.

5.2.1 Evaluation This method is less costly as compared to TDOA and easy to implement since no change is required in the handsets. But interns of accuracy it is not as perfect as TDOA method as average signal power may be same for two difference cells of the grid. This may give wrong location estimates. Remove this lacuna the field test instruments must make precise measurements of average signal power and even the slightest variation should not go unnoticed as this makes basis of the location estimation method. Processing load depends upon the size and the number of cells in which the area to be covered is divided. More the number cells the more accurate system is. But increase the number of cells increase the process load as it increases the number of comparisons to be made.

5.3 Method suggested by Cheung et al Algorithm

www. Another algorithm suggested by Cheung et al makes the use of the received signal strength (RSS) at the base station (BS). It takes into account power received at a particular base station (BS) due to the MS. While calculating this power, the BS takes into account its distance from MS and propagation constant, which can be determined using pathless slope, which in turn is measurable through specific instruments. It then introduces range related parameter, which depends upon the power received, distance of the base station from MS and noise present in the environment. Distance oh the BS from all the MS under consideration is determined using the time taken by signal to reach from MS to BS and speed of light. Both distance and range related parameter are then expressed using co-ordinates of the BS and MS, out of which co-ordinates of the BS are known and those of MS are to be found out. In order to increase the accuracy of the calculation these two measurement equations are laniaries into matrix form as below.


Where the matrix A represents coordinates of all the MS in the vicinity of BS and θ represents the coordinates of BS. Matrix b contains coordinates of MS and range related parameters. The whole problem now reduced to determining A using θ. A Langragian Multiplier 4 is also introduced to increase the accuracy of the calculation. This method is less expensive in terms of implementation as it doesn’t demand any change in the handset. In terms of accuracy simulation results show that measurements done through this method were very close to optimum. But in term of processing loads it increases the burden on processors as it involves the use of matrix.



All the three methods studied in this paper have their relative advantage and disadvantages over each other. While TDOA is more accurate, it is more expensive that other two methods. Among methods based upon received signal strength method in seems to be more accurate as it increases the accuracy by introducing weighted matrix. However it increases the processing burden on the network side. In the method suggested in location of mobile station is obtained by comparing the actual received signal strength with the received signal strength obtained earlier through field instrument record. The

www. accuracy of this method can be increased by collecting the set of mobile location estimates over a period of time and applying an estimation algorithm.





F2 F6 F2 F1




F4 F5 F6 F7

Figure 1 (Figure showing frequency reuse pattern


MS(x, y) MS (x, y)

BS3(x3, y3)


BS1 (x1, y1)

BS2 (x2, y2)


Figure 2 (Figure showing method used in TDOA)



[1] WILLIAM C Y LEE, Mobile Cellular Telecommunications, McGraw-Hill, Singapore, 1995, pp 4-58

[2] MARTIN HELLBRANDT, RUDOLF MATHER, Location Tracking of Mobiles in Cellular Radio Networks, IEEE Transactions on Vehicular Technology

[3] T.S. RAPPAPORT, Wireless Communication: Principles and Practice Prentice Hill Publications, 1996, pp 10-110










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