Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
2000-01-24
2003-02-25
Le, Thanh Cong (Department: 2684)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S440000, C455S560000
Reexamination Certificate
active
06526283
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a mobile telecommunication system, and in particular, to a device and method for effectively tracking the location of a mobile telephone in multipath fading characterstics and the non-line-of-sight (NLOS) environment.
2. Description of the Related Art
A mobile telecommunication network allows a registered mobile subscriber to make a call to anyone anywhere and at any time.
FIG. 1
illustrates the configuration of a typical mobile telecommunication network. As shown in
FIG. 1
, the mobile telecommunication network includes a plurality of base stations (BSs)
21
to
24
for providing mobile telecommunication service to a mobile subscriber through a mobile telephone
10
, a base station controller (BSC)
30
for controlling the BSs
21
to
24
, and a mobile switching center (MSC)
50
for connecting the BSC
30
to another BTS or a PSTN (Public Switched Telephone Network).
In a cellular mobile telecommunication network, the whole service area is divided into a plurality of coverage areas having respective base stations (BS) therein. Each BS coverage area is called a “cell.” An MSC controls these BSs so that a subscriber can continue his call without interruption while moving between different cells.
The MSC
50
can reduce the time required for calling a subscriber by locating the cell of the subscriber. In case of an emergency like a fire, or a patient needing first aid treatment, the mobile subscriber should be accurately located. Tracking the location of a mobile subscriber within the boundary of a cell in a mobile telecommunication network is known as “location service.”
A mobile telephone can be located by the mobile telephone itself or through a mobile telecommunication network.
To locate the mobile telephone by itself, the mobile telephone is provided with a GPS (Global Positioning System) receiver to calculate its location in latitude and longitude coordinates based on the location information received from a satellite through the GPS receiver. The requirement of having the extra GPS receiver, however, increases the price and the size of the mobile telephone. Another shortcoming with this method is that the load on the mobile telephone is increased because it has to calculate its location.
As an alternative to locating the mobile telephone by itself, the mobile telephone calculates its location by a trigonometry based on the signals received from at least three BSs. This method also increases the price and the size of the mobile telephone due to the requirement of a separately procured signal receiver. Further, the mobile telephone has a higher load because it has to calculate its location, and sometimes it cannot operate compatibly with other BSs employing a different scheme.
In the case that the mobile telephone network locates the mobile telephone, at least three BSs receive a signal from the mobile telephone, calculate the distances between the BSs and the mobile telephone using the arrival time of the signals at the BSs, then determine the location of the mobile telephone using the trigonometry. This location service is provided generally by a location data processor
40
included in a BSC or in by independently provided device. Upon a request for service about the location of a specific mobile subscriber, i.e., a mobile telephone of a user, the BSC
30
selects the BSs
21
,
22
, and
23
surrounding the mobile telephone for use in the location service, and these selected BSs
21
,
22
, and
23
are ready for communication with the mobile telephone.
The mobile telephone network can calculate the location of the mobile telephone using the time of arrival (TOA) or the time difference of arrival (TDOA).
The TOA method calculates the distance of a mobile telephone and a BS based on the TOA of a signal transmitted from the mobile telephone at the BS. That is, it is assumed that the mobile telephone is located at the intersection point of three circles having the radius of the distances between the BSs and the mobile telephone.
The TDOA method assumes that the TDOAs of a signal transmitted from the mobile telephone at the three BSs define a set of points on a hyperbola, and the mobile telephone is located at the intersection point of at least three hyperbolas. The implementation of this method requires accurate synchronization of each BS, as compared to the TOA method. However, the burden of synchronization is negligible as all the CDMA BSs are already synchronized to one another using their GPS receivers.
As described above, the network tracks the location of a mobile telephone using a specific signal transmitted from the mobile telephone. However, the signal of the mobile telephone is often propagated to a BS in a path longer than the distance between the mobile telephone and the BS due to the multipath fading characteristic and the NLOS effects in a real mobile telecommunication environment. In this case, at least three circles or hyperbolas do not meet at one point but overlap each other over an area. Therefore, the location data processor
40
should detect the most likely point where the mobile telephone is located in the overlap area.
FIG. 2
illustrates a typical TOA method for locating a mobile telephone. As shown in
FIG. 2
, three circles
101
,
102
, and
103
, whose radiuses are the distance between the mobile telephone
10
and at least three BSs
21
,
22
, and
23
, are overlapped across an area indicated as
1
. The mobile telephone
10
is located in the overlap area
1
.
One approach to locating the mobile telephone
10
in the overlap area
1
is to use a common chord, as shown in FIG.
3
. When at least three circles
111
,
112
, and
113
are overlapped over an area without meeting at one point, the mobile telephone
10
is considered to exist at the intersection point of three common chords
114
,
115
, and
116
. The three common chords
114
,
115
, and
116
are defined by the intersection points of the circles
111
,
112
, and
113
. A common chord is defined as a line connecting two points where two circles intersect.
The above method using the common chord is not very accurate in locating the mobile telephone except in the case where the mobile telephone is an approximate equal distance apart from the selected BSs and in a similar propagation environment to each respective BS. In the case that a first mobile telephone
11
is nearer to the first BS
21
, as shown in
FIG. 4
, the procedure will be described by a way of example.
In
FIG. 4
, two circles
131
and
132
are drawn based on the TOAs of a signal transmitted from the first mobile telephone
11
at the first and the second BSs
21
and
22
. A first common chord
133
is defined by the intersection between the circles
131
and
132
. But if the path between the first mobile telephone
11
and the second BS
22
is in an NLOS condition and the path between the first mobile telephone
11
and the first BS
21
is in a line-of-sight LOS) condition, the common chord
133
is positioned far left from the actual location of the mobile telephone
11
.
The effect is the same in the opposite case. If the path between the first mobile telephone
11
and the second BS
22
is in the LOS condition and the path between the first mobile telephone
11
and the first BS
21
is in the NLOS condition, the common chord
133
is also far right from the actual location of the mobile telephone
11
.
As noted from the example shown in
FIG. 4
, the locating method using a common chord involves a huge location error unless the paths between the mobile telephone and each BS have the same propagation environment.
Another method of locating the mobile telephone in the overlap area
1
is to use a least square (LS) scheme. For details of the LS method, see J. J. Caffery and G. L. Stuber, “Subscriber Location in CDMA Cellular Networks”, IEEE Trans. on Vehi. Technol. VT-47, no. 2, pp. 406-415, May 1998, and H. Hashemi, “Pulse Ranging Radiolocation Technique and Its Application to Channel Assignment in Dig
Cha Steve
Cha & Reiter
Cong Le Thanh
Nguyen Thuan T.
Samsung Electronics Co LTD
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