Selection of positioning handover candidates based on path loss

A method, apparatus, and system for the selection of target nodes in locating a receiver is disclosed. The receiver measures the strength of signals transmitted by a number of nodes. The received strength is subtracted from the transmitting power of the nodes to determine the respective path losses. The node with the lowest path loss is selected as the target node. The target node is then used to perform a triangulation for determining the location of the receiver.

BACKGROUND OF THE PRESENT INVENTION 
1. Field of the Invention 
The present invention relates generally to telecommunications systems and 
methods for determining the location of a mobile terminal within a 
wireless network, and specifically to an improved method, system, and 
apparatus for determining nodes which are the likeliest to yield the most 
accurate triangulation results. 
2. Background and Obiects of the Present Invention 
Determining the geographical position of a mobile station within a wireless 
network has recently become important for a wide range of applications. 
For example, positioning services may be used by transport and taxi 
companies to determine the location of their vehicles. In addition, for 
emergency calls, e.g., 911 calls, the exact location of the mobile 
terminal may be extremely important to the outcome of the emergency 
situation. Furthermore, positioning services can be used to determine the 
location of a stolen car, for the detection of home zone calls, which are 
charged at a lower rate, for the detection of hot spots for micro cells, 
or for the subscriber to determine, for example, the nearest gas station, 
restaurant, or hospital. 
Referring now to FIG. 1 of the drawings, an exemplary wireless network, 
such as a Global System for Mobile Communication (GSM) Public Land Mobile 
Network (PLMN) 10, will be described. The PMLN 10 is composed of a 
plurality of areas 12, each with a Mobile Switching Center (MSC) 14 and an 
integrated Visitor Location Register (VLR) 16 therein. The MSC/VLR areas 
12, in turn, include a plurality of Location Areas (LA) 18, which are 
defined as that part of a given MSC/VLR area 12 in which a mobile station 
(MS) (terminal) 20 may move freely without having to send update location 
information to the MSC/VLR area 12 that controls the LA 18. Each Location 
Area 12 is divided into a number of cells 22. Mobile Station (MS) 20 is 
the physical equipment, e.g., a car phone or other portable phone, used by 
mobile subscribers to communicate with the cellular network 10, each 
other, and users outside the subscribed network, both wireline and 
wireless. 
The MSC 14 is in communication with at least one Base Station Controller 
(BSC) 23, which, in turn, is in contact with at least one Base Transceiver 
Station (BTS) 24. The BTS is the physical equipment, illustrated for 
simplicity as a radio tower, that provides radio coverage to the 
geographical part of the cell 22 for which it is responsible. It should be 
understood that the BSC 23 may be connected to several base transceiver 
stations 24, and may be implemented as a stand-alone node or integrated 
with the MSC 14. In either event, the BSC 23 and BTS 24 components, as a 
whole, are generally referred to as a Base Station System (BSS) 25. 
With further reference to FIG. 1, the PLMN Service Area or wireless network 
10 includes a Home Location Register (HLR) 26, which is a database 
maintaining all subscriber information, e.g., user profiles, current 
location information, International Mobile Subscriber Identity (IMSI) 
numbers, and other administrative information. The HLR 26 may be 
co-located with a given MSC 14, integrated with the MSC 14, or 
alternatively can service multiple MSCs 14, the latter of which is 
illustrated in FIG. 1. 
The VLR 16 is a database containing information about all of the Mobile 
Stations 20 currently located within the MSC/VLR area 12. If a MS 20 roams 
into a new MSC/VLR area 12, the VLR 16 connected to that MSC 14 will 
request data about that Mobile Station 20 from the HLR database 26 
(simultaneously informing the HLR 26 about the current location of the MS 
20). Accordingly, if the user of the MS 20 then wants to make a call, the 
local VLR 16 will have the requisite identification information without 
having to reinterrogate the HLR 26. In the aforedescribed manner, the VLR 
and HLR databases 16 and 26, respectively, contain various subscriber 
information associated with a given MS 20. 
Referring now to FIG. 2, the operation of a wireless network 205 performing 
a positioning handover for determining the location of a given MS 200 is 
described. Upon a network positioning request, the Base Station System 
(BSS) (220 and 240) serving the MS 200 generates positioning data, which 
is delivered to the Mobile Switching Center (MSC) 260. This positioning 
data is then forwarded to a Positioning Center (PC) 270 for calculation of 
the geographical location of the MS 200. The location of the MS 200 can 
then be sent to the application 280 that requested the positioning. 
Alternatively, the requesting application 280 could be located within the 
MS 200 itself. The Positioning Center 270 could also be located within the 
MSC 260. 
In order to accurately determine the location of the MS 200, positioning 
data from three separate Base Transceiver Stations (210, 220, and 230) is 
required. This positioning data for GSM systems includes a Timing Advance 
(TA) value, which corresponds to the amount of time in advance that the MS 
200 must send a message in order for the BTS 220 to receive it in the time 
slot allocated to that MS 200. When a message is sent from the MS 200 to 
the BTS 220, there is a propagation delay, which depends on the distance 
between the MS 200 and the BTS 220. TA values are expressed in bit 
periods, and can range from 0 to 63, with each bit period corresponding to 
approximately 550 meters between the MS 200 and the BTS 220. It should be 
understood, however, that any estimate of time, distance, or angle can be 
used, instead of the TA value of GSM systems. 
Once a TA value is determined for one BTS 220, the distance between the MS 
200 and that particular BTS 220 is known, but the actual location is not. 
If, for example, the TA value equals one, the MS 200 could be anywhere 
along a radius of 550 meters. Two TA values from two BTSs, for example, 
BTSs 210 and 220, provide two possible points that the MS 200 could be 
located (where the two radiuses intersect). However, with three TA values 
from three BTSs, e.a., BTSs 210, 220, and 230, the location of the MS 200 
can be determined with a certain degree of accuracy. For example using a 
triangulation algorithm, with knowledge of the three TA values and site 
location data associated with each BTS (210, 220, and 230), the position 
of the mobile station 200 can be determined (with certain accuracy) by the 
Positioning Center 270. 
Therefore, Timing Advance (TA) values are obtained from the original 
(serving) BTS 220 and two neighboring (target) BTSs (210 and 230). In 
order for each target BTS (210 and 230) to determine a TA value, a 
positioning handover to each of the BTSs (210 and 230) must occur. A 
positioning handover is similar to an ordinary asynchronous handover. The 
target BTS, e.g., BTS 210, distinguishes the Positioning Handover from an 
ordinary handover by a new ACTIVATION TYPE in the CHANNEL ACTIVATION 
message. Unlike an ordinary handover, upon reception of a HANDOVER ACCESS 
message from the MS 200, the target BTS 210 only calculates the TA value, 
and does not respond to the mobile station 200, that is, no PHYSICAL 
INFORMATION is sent to the MS 200. Thus, the MS 200 will then return to 
the previous channel allocated by the original BTS 220 after the time 
period defined by the MS's 200 internal counter expires, e.g., 320 
milliseconds. 
If there are more than three BTSs (210, 220, and 230) within the range of 
the MS 200, the serving BSC 240 will have to determine to which two BTSs 
210 and 230 to perform a positioning handover (in order to obtain the TA 
values). In addition, if the serving BTS 220 does not support positioning, 
three target BTSs must be selected. At present, this selection process is 
typically performed by the BSC 240 compiling a mobile assisted handover 
list based on measurements obtained by the MS 200 regarding the signal 
strength of the surrounding BTSs (210, 220 and 230). The BSC 240 then 
selects the two or three BTSs (220 and 230) with the strongest signal 
strength to perform a positioning handover. 
Unfortunately, the selected BTSs (210, 220, and 230) may not be the ideal 
candidates for obtaining positioning data. For example, if the BTSs (210, 
220, and 230) selected for positioning handovers do not surround the 
mobile station 200 to be positioned, the error in the location calculation 
will increase. 
It is therefore an object of the invention to determine the target BTSs 
that are the likeliest to yield the best triangulation results. 
It is also an object of the invention to determine the target BTSs in a 
manner that is computationally simple. 
It is also an object of the invention to determine the target BTSs in a 
manner that requires minimal modifications to existing cellular networks. 
SUMMARY OF THE INVENTION 
The present invention is directed to a method for determining target nodes 
that are the likeliest to yield the best triangulation results by 
determining the strength of signals received at the receiver from various 
nodes, determining the strength of the signals at the nodes, and 
calculating a path loss for each node by subtracting the strength of the 
signal at the receiver from the strength of the signal at the target node. 
The present invention is also directed to a telecommunication system for 
determining the location of a mobile station including Base Transceiver 
Stations for transmitting a signal to the mobile station, path loss 
measurements for measuring the path loss of each Base Transceiver 
Stations, and selection of at least two of the Base Transceiver Stations 
with the lowest path loss. The present invention is also directed to an 
apparatus for selecting target Base Transceiver Stations with memory for 
the output power of Base Transceiver Stations, inputs for receiving a 
measurement report measuring the strength of signals received at a 
receiver, and a way to calculate the path loss of each Base Transceiver 
Station, and for selecting at least two Base Transceiver Stations having 
the lowest path loss.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EXEMPLARY EMBODIMENTS 
The numerous innovative teachings of the present application will be 
described with particular reference to the presently preferred exemplary 
embodiments. However, it should be understood that this class of 
embodiments provides only a few examples of the many advantageous uses of 
the innovative teachings herein. In general, statements made in the 
specification of the present application do not necessarily delimit any of 
the various claimed inventions. Moreover, some statements may apply to 
some inventive features but not to others. 
With reference now to FIG. 3 of the drawings, steps in a sample process for 
determining the optimal base transceiver stations in order to locate a 
Mobile Station 200 within a Location Area 205 is illustrated. Initially, 
after a positioning request is received by the MSC 260 (step 300) from the 
Positioning Center 270, the MSC 260 sends the positioning request to an 
originating (serving) Base Station Controller (BSC) 240 (step 305) if the 
Mobile Station 200 is in a dedicated mode (in use). However, if the MS 200 
is in an idle mode (not in use), the MSC 260 must page the MS 200 and 
setup a call to the MS before forwarding the positioning request to the 
BSC. This call does not activate the ringing tone on the MS 200, and 
therefore, is not noticed by the MS 200. 
The originating BSC 240 then determines which Base Transceiver Station 
(BTS) 220 is currently serving the MS 200 (step 310), and obtains a Timing 
Advance (TA) value (TA1), or other positioning data, from this serving BTS 
220 (step 315), if possible. Thereafter, TA values are obtained from two 
target BTSs (210 and 230) (step 350) by performing a positioning handover 
(step 335). If the serving BTS 220 does not support positioning, an 
additional target BTS (not shown) must be selected. It should be noted 
that other positioning methods based on triangulation can be used instead 
of obtaining TA values, as discussed herein. In addition, positioning of 
the MS 200 can be performed using more than three BTSs (210, 220, and 
230). 
Referring now to FIG. 4, which will be discussed in connection with FIGS. 
2, and 3, the process by which the BSC 240 selects the target BTSs 210 and 
230 (step 320) is described. The MS 200, as in most digital wireless 
systems, utilizes a method for automatically adjusting its transmitting 
power based on the strength of the signal received by the serving BTS 220. 
According to the procedures of Mobile Assisted Hand Over (MAHO), the MS 
200 measures the strength of the received signal from the serving BTS 220, 
as well as a number of BTSs 420a-420f serving each of the six surrounding 
cells 430a-430f, respectively. These measurements are reported to the BSC 
240 in the MS measurement report on the Slow Assisted Control Channel 
(SACCH) about twice every second. 
Upon receiving the measurements in the MS report, the BSC 240 determines 
the path loss of the serving BTS 220 and each neighboring BTS 420a-420f. 
Referring now to FIG. 5, an exemplary illustration of the BSC 240 
determining the path loss of each neighboring BTS 420a-420f is described. 
The column referenced as "Received Signal Strength" is provided by the MS 
200 in the MS measurement report to the BSC 240. The column referenced as 
"Output Power" includes data known, stored, and configured by the BSC 240. 
Those skilled in the art will recognize that the BSC 240 is responsible 
for controlling the output power of the BTSs 220, and 420a-420f. The BSC 
240 can, for example, store data regarding the output power of each BTS 
220, and 420a-420f, respectively, in a database 245. The column entitled 
"Path Loss" represents the path loss of each respective neighboring BTS 
420a-420f. 
The BSC 240 determines the information in the column entitled "Path loss" 
according to the following formula: 
EQU path loss(n)=output power(n)-received signal strength(n) 
where: n=row number. 
The BSC 240 then ranks each neighboring BTS 420a-420f in terms of path 
loss, as illustrated in the column entitled "Rank by path loss." The 
neighboring BTS 420 which reports the lowest path loss is ranked highest. 
The BSC 240 then selects at least two of the neighboring BTSs 420 (at least 
three if the serving BTS 220 does not support positioning) with the lowest 
path loss as the aforedescribed target BTSs 210 and 230 of FIG. 2. In the 
exemplary case of FIG. 5, the BSC 240 would select BTS 420d and BTS 420e, 
to be the target BTSs 210 and 230. 
With reference again to FIG. 3, after step 320 the positioning handover to 
one of the target BTSs 230 (step 322) is accomplished by the serving BSC 
240 sending a new ACTIVATION TYPE in a CHANNEL ACTIVATION message to the 
target BTS 230, which informs the target BTS 230 that a positioning 
handover needs to be performed (step 325). The target BTS 230 then 
acknowledges the CHANNEL ACTIVATION message to the serving BSC 250 (step 
330). 
Thereafter, the BSC 240 sends a command to the MS 200 via the serving BTS 
220 (step 335) to transmit a HANDOVER ACCESS message to the target BTS 230 
(step 340). During the time that the MS 200 is waiting for a response from 
the target BTS 230, e.g., around 320 milliseconds, the target BTS 230 
measures the Timing Advance value (access delay) (TA3) (step 345), using 
access bursts sent by the MS 200, and forwards this positioning data to 
the serving BSC 240 (step 350). A positioning handover can then be 
performed to the other target BTS 210 in the same manner as stated 
hereinbefore. The TA value measured by the target BTS 230 (TA3) is then 
transmitted by the serving BSC 240 to the MSC 260 (step 355), together 
with TA values (TA1 and TA2) obtained from the serving BTS 220 and other 
target BTSs 210. 
Finally, the TA value acquired from the target BTS 230 (TA3), together with 
other TA values (TA1 and TA2) are forwarded to the Positioning Center (PC) 
270 from the MSC 260 (step 360), where the location of the MS 200 is 
determined using the triangulation algorithm (step 365). The PC 270 then 
presents the geographical position of the MS 200 to the requesting 
application (node) 280 (step 370) for further processing. 
As will be recognized by those skilled in the art, the innovative concepts 
described in the present application can be modified and varied over a 
wide range of applications. For example, it should be noted that the 
aforedescribed determination of optimal target Base Transceiver Stations 
can be implemented in any cellular system, and should not be limited to 
GSM systems. For example, in other cellular systems, the Base Station 
Controller function (controlling node) can be implemented within the MSC 
(such as 260) itself. 
In another embodiment, the selection of the Base Transceiver Stations could 
be made by the mobile station. In such an embodiment, each Base 
Transceiver Station would report to the mobile station the transmitting 
strength. The mobile station could then determine the path loss of each 
Base Transceiver Station and select the optimal Base Transceiver Stations. 
It should also be understood that the present invention is not limited to 
the selection of two base transceiver stations. For example, in one 
embodiment, a first base transceiver station can be selected according to 
path loss while any number of other base transceiver stations can be 
selected according to some other criteria. 
Accordingly, the scope of the present invention should not be limited to 
any of the specific exemplary teachings discussed, but is only limited by 
the following claims.