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Mobile Phone Patent Abstract
A method of measuring the distance between a mobile phone and a
base station based on the round-trip delay (RTD) of a prescribed
message transmitted from the base station to the mobile phone in
a cellular mobile telephone system (CMTS), comprising the steps
of calculating a mean error distance to adjust the distance between
the base station and the mobile phone within the cell boundary of
the base station, measuring the RTD between the base station and
the mobile phone positioned at an arbitrary point within the cell,
calculating the RTD distance between the base station and the mobile
phone based on the RTD measurement by subtracting the mean error
distance from the RTD distance.
Mobile Phone Patent Claims
What is claimed is:
1. A method for measuring a mean error distance to adjust the distance
between a mobile phone and a base station in a cell obtained by
a round-trip delay (RTD) of a prescribed message transmitted from
said base station to said mobile phone in a cellular mobile telephone
system (CMTS), said method comprising the steps of:
determining a plurality of measuring points within the cell of
said base station to be used for obtaining said mean error distance;
measuring respective RTDs between said measuring points and said
base station by sequentially positioning said mobile phone at each
of said measuring points;
calculating respective RTD distances between said measuring points
and said base station based on said RTD measurements;
obtaining respective Global Positioning System(GPS) data for said
measuring points and said base station;
calculating respective GPS distances between said measuring points
and said base station based on said GPS data;
calculating respective error distances by taking the difference
between said respective RTD distances and said GPS distances; and,
obtaining said mean error distance from said respective error distances.
2. The method as defined in claim 1, wherein said step of measuring
the respective RTDs between said measuring points and said base
station further includes the steps of providing said mobile phone
to establish a call connection with said base station and providing
said base station to trace an electronic serial number of said mobile
phone.
3. The method as defined in claim 1, wherein each of said respective
RTDs is represented by a sum of a forward propagation delay by said
prescribed message when transmitted from said base station to said
mobile phone, a reverse propagation delay by said message returning
from said mobile phone to said base station, and a system delay
for said base station to demodulate and analyze said returned message.
4. The method as defined in claim 3, wherein said step of calculating
said respective RTD distances between said measuring points and
said base station further includes the steps of calculating one-way
propagation delay from said measured RTD and calculating one-way
wave distance from said measured RTD.
5. The method as defined in claim 4, wherein said one-way propagation
delay is obtained by subtracting said system delay from said respective
RTD and dividing the remainder by two.
6. The method as defined in claim 4, wherein said one-way wave
distance is obtained by multiplying said one way propagation delay
by the speed of light and the transmission speed of chip unit for
said CMTS.
7. A method for measuring the distance between a mobile phone and
a base station based on the round-trip delay (RTD) of a prescribed
message transmitted from said base station to said mobile phone
in a cellular mobile telephone system (CMTS), said method comprising
the steps of:
calculating a mean error distance to adjust said distance between
said base station and said mobile phone within the cell boundary
of said base station;
measuring the RTD between said base station and said mobile phone
positioned at an arbitrary point within the boundary of said cell;
calculating the RTD distance between said base station and said
mobile phone based on said measured RTD; and,
subtracting said mean error distance from said calculated RTD distance.
8. The method as defined in claim 7, wherein the step of calculating
said mean error distance further includes the steps of:
determining a plurality of measuring points within the cell of
said base station to be used for obtaining said mean error distance;
measuring respective RTDs between said measuring points and said
base station by sequentially positioning said mobile phone at each
of said measuring points;
calculating respective RTD distances between said measuring points
and said base station based on said RTD measurements;
obtaining respective Global Positioning System(GPS) data for said
measuring points and said base station;
calculating respective GPS distances between said measuring points
and said base station based on said GPS data;
calculating respective error distances by taking the difference
between said respective RTD distances and said GPS distances; and,
obtaining said mean error distance from said respective error distances.
9. The method as defined in claim 8, wherein said step of measuring
the respective RTDs between said measuring points and said base
station further includes the steps of providing said mobile phone
to establish a call connection with said base station and providing
said base station to trace an electronic serial number of said mobile
phone.
10. The method as defined in claim 8, wherein each of said respective
RTDs is represented by a sum of a forward propagation delay by said
prescribed message when transmitted from said base station to said
mobile phone, a reverse propagation delay by said message returning
from said mobile phone to said base station, and a system taken
for said base station to demodulate and analyze said returned message.
11. The method as defined in claim 10, wherein said step of calculating
said respective RTD distances between said measuring points and
said base station further includes the steps of calculating one-way
propagation delay from said measured RTD and calculating one-way
wave distance from said measured RTD.
12. The method as defined in claim 11, wherein said one-way propagation
delay is obtained by subtracting said system delay from said respective
RTD and dividing the remainder by two.
13. The method as defined in claim 11, wherein said one-way wave
distance is obtained by multiplying said one way propagation delay
by the speed of light and the transmission speed of chip unit for
said CMTS.
Mobile Phone Patent Description
CLAIM OF PRIORITY
This application makes reference to, incorporates the same herein,
and claims all benefits accruing under 35 U.S.C. Section 119 from
an application for METHOD OF LOCATING A MOBILE PHONE BY MEASURING
THE DISTANCE BETWEEN THE PHONE AND A BASE STATION IN A CELLULAR
MOBILE TELEPHONE SYSTEM filed earlier in the Korean Industrial Property
Office on the 23th day of January 1999 and there duly assigned Ser.
No. 2064/1999.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a cellular mobile telephone system
(CMTS), and more particularly a method for providing more accurate
measurement of the distance between a mobile phone and a base station
by incorporating a round-trip delay (RTD) and the mean error distance
therebetween.
2. Description of the Related Art
A geographic area served by the CMTS is divided into a group of
smaller areas known as cells. Each cell includes a base station
to provide communication with a mobile phone or a portable instrument
within the boundary of a cell. In addition, all the base stations
are controlled by a mobile switching center (MSC) to enable communication
between the cells.
Referring to FIG. 1, a network of the CMTS using the CDMA (Code
Division Multiple Access) technology generally consists of a plurality
of base stations 20, 30, 40 for providing communication with mobile
phones 10 and an MSC 50 for connecting the mobile phones with a
public switched telephone network (PSTN) 60. This type of CMTS is
used to provide additional service to the mobile phone users 20,
30, 40, by locating their positions in relation to the surrounding
base stations. To this end, various methods have been developed
to measure the distance between the mobile phone and the base station.
One method uses the time of arrival (TOA) measurement. Namely,
a mobile phone transmits a prescribed message signal to at least
3 adjacent base stations, thus determining the distance between
the mobile phone and each respective base station using the transmission
time of the message signal. Calculating a radial distance between
the mobile phone and three base stations provides an accurate fix,
as the intersection of the three spheres. However, in this case,
the mobile phone should be provided with the means for transmitting
the prescribed message to the base stations.
Another method uses a round trip delay (RTD) which uses a prescribed
message transmitted from the base station to the mobile phone and
measures the time it takes to complete a round-trip for the prescribed
message to return back to the base station. However, in this case,
radio waves of the message are subjected to various microwave obstructions,
such as buildings, thus the RTD inherently includes a delay resulting
from reflection by such obstructions. This type of delay degrades
the accuracy of locating the exact position of a mobile phone.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method of
obtaining a reliable measurement of the distance between a mobile
phone and a base station based on the RTD measurement adjusted by
a GPS system.
It is another object of the present invention to provide a method
of calculating the mean error distance to adjust the distance between
the mobile phone and the base station obtained by the RTD measurement.
According to an aspect of the present invention, a method of measuring
the mean error distance to adjust the distance between a mobile
phone and a base station obtained by a round-trip delay (RTD) measurement
of a prescribed message transmitted from the base station to the
mobile phone in a cellular mobile telephone system (CMTS), the method
comprises the steps of determining a plurality of measuring points
within the cell boundary of the base station to be used for obtaining
the mean error distance; measuring the respective RTDs between the
measuring points and the base station by sequentially positioning
the mobile phone at each of the measuring points; calculating the
respective RTD distances between the measuring points and the base
station based on the respective RTDs; obtaining the GPS (Global
Positioning System) data of the base station and the measuring points;
calculating the respective GPS distances between the base station
and the measuring points based on the GPS data; and, calculating
the respective error distances based on the difference between the
respective RTD distances and the GPS distances for all measuring
points and obtaining the mean error distance from these respective
error distances.
According to another aspect of the present invention, a method
of measuring the distance between a mobile phone and a base station
in the CMTS, comprises the steps of calculating a mean error distance
to adjust the distance between the mobile phone and the base station
within the cell boundary of the base station; measuring the RTD
between the base station and the mobile phone positioned at an arbitrary
point within the cell; calculating a new and more accurate RTD distance
between the mobile phone and the base station based on the RTD measurement
by subtracting the mean error distance from the measured RTD distance.
The foregoing and other objects, features, and advantages of the
invention will be apparent from the following more particular description
of preferred embodiments as illustrated in the accompanying drawings.
A BRIEF DESCRIPTION OF THE ATTACHED DRAWINGS
FIG. 1 is a block diagram for illustrating the structure of the
conventional network of a CMTS using a CDMA technology;
FIG. 2 is a flow chart for illustrating the steps of obtaining
a mean error distance according to the present invention;
FIG. 3 is a graph for illustrating the distribution of RTD distances
against the GPS distances in a densely populated urban area;
FIG. 4 is a graph for illustrating the distribution of error distances
between the RTD distances and the GPS distances in a densely populated
urban area;
FIG. 5 is a graph for illustrating the distribution of RTD distances
against the GPS distances in an ordinary urban area;
FIG. 6 is a graph for illustrating the distribution of error distances
between the RTD distances and the GPS distances in a densely populated
urban area;
FIG. 7 is a graph for illustrating the distribution of RTD distances
against the GPS distances in a suburban area;
FIG. 8 is a graph for illustrating the distribution of error distances
between the RTD distances and the GPS distances in a suburban area;
FIG. 9 is a graph for illustrating the distribution of RTD distances
against the GPS distances in a rural area;
FIG. 10 is a graph for illustrating the distribution of error distances
between the RTD distances and the GPS distances in a rural area;
FIG. 11 is a graph for illustrating the number of error distances
against the GPS distances and the probability density function thereof
in a densely populated urban area;
FIG. 12 is a graph for illustrating the number of error distances
cumulated against the GPS distances, and the cumulative density
function and the normal cumulative density function thereof in a
densely populated urban area;
FIG. 13 is a graph for illustrating the number of error distances
against the GPS distances and the probability density function thereof
in an ordinary urban area;
FIG. 14 is a graph for illustrating the number of error distances
cumulated against the GPS distances, and the cumulative density
function and the normal cumulative density function thereof in an
ordinary urban area;
FIG. 15 is a graph for illustrating the number of error distances
against the GPS distances and the probability density function thereof
in a suburban area;
FIG. 16 is a graph for illustrating the number of error distances
cumulated against the GPS distances, and the cumulative density
function and normal cumulative density function thereof in a suburban
area;
FIG. 17 is a graph for illustrating the number of error distances
against the GPS distances and the probability density function thereof
in a rural area;
FIG. 18 is a graph for illustrating the number of error distances
cumulated against the GPS distances, and the cumulative density
function and normal cumulative density function thereof in a rural
area; and,
FIG. 19 is a flow chart for illustrating a method of measuring
the distance between a mobile phone and a base station according
to the present invention.
A DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
In the following description, it will be apparent to those skilled
in the art that the present invention may be practiced in other
embodiments that depart from these specific details. For the purpose
of clarity, detailed descriptions of well-known parts are omitted
so as not to obscure the description of the present invention with
unnecessary detail.
Referring to FIG. 2, in step S110, a plurality of measuring points
or reference locations is determined within the cell of a base station
20 which are used for obtaining the error distance between the base
station and a mobile phone. For example, the number of the measuring
point is selected to be at one hundred different locations. In step
S120, the mobile phone 10 is positioned at one of the measuring
positions--namely, the first measuring position--to measure the
RTD between the first measuring point and the base station. The
RTD measurement is processed using a measuring device provided in
the base station and the mobile phone. To this end, as shown in
FIG. 1, the measuring tool of the mobile phone 10 makes the mobile
phone to establish a call connection in the first position while
that of the base station 20 traces the electronic serial number
(ESN) of the mobile phone to measure the RTD. In this case, two
or more mobile phones may be positioned at the first measuring position
and their mean RTD can be obtained to avoid any measurement error.
The measured RTD is transmitted to a separate analysis tool, such
as SCAT 97 and C_MAT, to calculate the one-way propagation distance
between the base station and the mobile phone, namely the RTD distance,
in step S130. In this case, the RTD is defined as the sum of the
forward one-way propagation delay taken by a prescribed message
to be transmitted from the base station 20 to the mobile phone 10,
the reverse one-way propagation delay taken by the return message
to be transmitted from the mobile phone to the receiving antenna
of the base station, and the system delay taken for the base station
to demodulate and analyze the return message received through the
antenna of the base station. The system delay maybe obtained by
positioning the mobile phone in front of the antenna of the base
station and measuring the RTD thereto. Namely, the system delay
is a delay caused by the system; thus, it can be defined by positioning
the mobile phone just in front of the antenna of the base station.
Accordingly, a true RTD may be obtained by dividing the remainder,
after subtracting the system delay from the measured RTD by two
because the measured RTD includes the forward and the reverse one-way
propagation delays as well as the system delay. Thus, the one-way
propagation delay may be expressed by the following Equation 1:
##EQU1##
wherein the one-way propagation delay is calculated using the chip
unit that is about 0.8138 .mu.s in the case of a mobile communication
system having a transmission speed of 1.2288 Mcps. Hence, in such
mobile communication system, the one-way propagation delay can be
determined by the following Equation 2: ##EQU2##
wherein the speed of light is assumed to be 2.998.times.10.sup.8
[m/sec].
Then, in step S140, using the satellite, the GPS data of the mobile
phone 10 and the base station 20 is obtained, transferred to the
analysis tool to calculate the GPS distance between the base station
20 and the mobile phone 10, in step S150. The GPS data is collected
by linking a GPS receiver with a SDM, a terminal data collecting
tool, at the corresponding testing position, and it consists of
the latitude and the longitude distance information in relation
to the base station. Hence, the GPS distance between the base station
and the mobile phone may be expressed by the square root of the
sum of their longitudinal difference squared and the latitudinal
difference squared. However, the GPS has an inherent measurement
error of about 200 m; thus, the maximum measurement error range
can be about 400 m. Accordingly, in step S160, the respective RTD
distances and the GPS distances are obtained for the remaining measuring
points by repeating the previous steps (s110 through s 150).
Meanwhile, the RTD distance between the base station and the mobile
phone has an inherent error because of the possible microwave obstructions,
such as buildings and other structures. Thus, the RTD tends to have
greater error than the corresponding GPS distance. For this reason,
the analysis tool calculates the error distance between the respective
RTD distances and the GPS distances in step S170 to obtain the mean
error distance for a particular base station by taking the difference
between the respective RTD distances and the GPS distances. According
to the present invention, such error distance is obtained for four
base stations, each having a typical geographic morphology as shown
in Table 1.
TABLE 1 Geographic Morphology of Base Ordinary Station Dense Urban
Urban Surburban Rural Area Cell Type 2 Sectors 3 Sectors 3 Sectors
3 Sectors Service Radius 0.7.about.0.8Km 0.8.about.1Km 1.5.about.1.7Km
2.about.2.5Km
In Table 1, the service radius represents the straight distance
between the base station and the mobile phone. The number of measuring
points has been selected to be 100 for the cell boundary of each
base station to measure the relationship between the error distance
and the geographic morphology. Then, the RTDs and the GPS data of
the base station and mobile phone for all measuring points were
obtained to calculate the RTD distances and the GPS distances between
the base station and the mobile station.
FIG. 3 illustrates the distribution of the RTD distances against
the GPS distances and FIG. 4 illustrates the distribution of error
distances between the RTD distances and the GPS distances in a dense
urban area. Likewise, FIG. 5 and FIG. 6 show a similar relationship
between the RTD distances and the GPS distances in an urban area,
respectively; FIG. 7 and FIG. 8 exhibit the same in a suburban area;
and, FIG. 9 and FIG. 10 exhibit the same relationship in a rural
area.
In order to statistically analyze the error distance of each base
station based on the results as shown in FIGS. 3 to 10, the error
distance of each measuring point was regarded as a sample, and the
number of the error distances per 100 m is used to obtain the number
of samples. FIG. 11 illustrates the number of samples and their
probability density function (pdf) against the GPS distances in
the dense urban area, representing the mean error distance as 232
m and the standard deviation as 241 m. In addition, FIG. 12 illustrates
number of error distances cumulated against the GPS distances, the
cumulative density function (cdf) of the samples, and the normal
cumulative density function of the normal distribution of the error
distances in a dense urban area.
Likewise, FIGS. 13 and 14 illustrate similar views in the urban
area, representing the mean error distance as 324 m and the standard
deviation as 203 m; FIGS. 15 and 16 represente the mean error distance
as 293 m and the standard deviation 264 m in the suburban area;
and, FIGS. 17 and 18 represents the mean error distance as 282 m
and the standard deviation as 212 m in the rural area.
As shown in FIGS. 3 to 18, the functionality of the distribution
of the RTD distances to the GPS distances is enhanced in the order
of dense urban<urban<suburban<rural area, and the mean
error distance in the dense urban becomes smaller as the GPS distance
increases. It is also noted that the error distances for each geographic
morphology is relatively distributed around 200 m.about.300 m. If
90% error is allowed for the RTD distance for dense urban, urban,
suburban, and rural areas, respectively, the error distance of 550,
580, 640 and 550 m or below are exhibited. Further, if allowing
99% error, they all have the error distance below 1 Km. Thus, such
mean error distance can be applied to calculate the distance between
the base station and the mobile phone as explained hereinbelow.
Describing the method of calculating the distance between the base
station and the mobile phone in connection with FIG. 19, the RTD
distance and the GPS distance are obtained for the cell boundary
of a base station to calculate the mean error distance in step S210.
Then, the RTD is measured between the base station and the mobile
phone positioned at an arbitrary point in its cell, in step S220.
Then, the RTD distance is calculated from the RTD measurement in
step S230. Finally, the new distance according to the present invention
between the mobile phone and the base station is obtained by subtracting
the mean error distance from the RTD distance, in step S240. Of
course, the mean error distance calculated for a particular base
station may be applied to a mobile phone positioned at any point
in the cell of the base station.
As mentioned in the foregoing, the present invention provides the
CMTS with the means for predetermining the mean error distance based
on the RTD and the GPS data in order to obtain the practically more
accurate distance between the mobile phone and the base station
which is more relatively close to the true distance between them.
While the present invention has been described in connection with
specific embodiments accompanied by the attached drawings, it will
be readily apparent to those skilled in the art that various changes
and modifications may be made thereto without
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