GB2367452A

GB2367452A - System for reducing transmission power of a base station to allow location of a mobile station

Info

Publication number: GB2367452A
Application number: GB0022807A
Authority: GB
Inventors: Brendan Patrick Ludden; Rorie O'neill
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 2000-09-15
Filing date: 2000-09-15
Publication date: 2002-04-03
Also published as: GB0022807D0

Abstract

In order to calculate the location of a mobile telephone, three base transmission stations 10,12,14 may transmit signals to a mobile station 16,18,20,22 and the time of arrival (TOA) of the signals at the mobile station may be used to calculate location in a process called hyperbolic trilateration. When a mobile station 16 is very close to a base station 10 it may be unable to receive signals from more distant stations within a low visibility area R, this is known as the doughnut effect. In order to mitigate this, a desired low visibility area Rr is determined within low visibility area R. A selected number of calls are dropped from base station 10 in order to reduce the total transmission power Ps of the base station and reduce the low visibility area to Rr.

Description

Method and apparatus for powering-down a serving Base Transmission Station (BTS) in a cellular telephone system FIELD OF THE INVENTION The present invention relates to wireless communication systems in general, and more particularly to methods and apparatus for powering-down a serving Base Transmission Station (BTS) in a cellular telephone system.

BACKGROUND OF THE INVENTION Techniques for determining the location of a mobile station (MS) in a cellular telephone system are well known in the art. In one method, known as hyperbolic trilateration, at least three base transmission stations (BTS) at known locations send transmissions to an MS whose location is to be determined. The time of arrival (TOA) of each of the transmissions at the MS is noted according to the MS internal clock, and the time difference of arrival (TDOA) of transmissions from at least two different pairs of BTSs is calculated. Given known BTS transmission times, a hyperbola may be demarcated between each BTS pair by presenting multiple solutions for each transmission's time of flight (TOF).

The intersection between two hyperbolae indicates the estimated location of the MS.

In some cellular telephone systems, such as a Wideband Code Division Multiple Access (W-CDMA) system, the proximity of an MS to a BTS occasionally results in an MS not"seeing"one or both of the other two BTSs needed in order to perform the TDOA measurements. This is due to the"doughnut"effect where the transmission power in the immediate doughnut-shaped area of the BTS is too great to allow inbound transmissions from other BTSs to be received by receivers within the BTS"doughnut." A proposed solution to overcome the doughnut effect and allow receivers within a serving BTS doughnut to see neighboring BTSs involves periodically causing the serving BTS to simply cease transmitting altogether for a short interval, on the order of nanoseconds or milliseconds. Unfortunately, the effect that ceasing transmission for even such a short interval has on some bearer services, such as 2.048 Mb/s high-speed data transmissions, is far more deleterious than it has on other services, such as basic 8 Kb/s speech service. Furthermore, an MS that is not within the serving BTS doughnut does not benefit from the BTS ceasing transmission as it is able to see the neighboring BTSs, and, indeed, only stands to lose from such measures.

SUMMARY OF THE INVENTION The present invention seeks to provide improved methods and apparatus for powering-down a serving Base Transmission Station (BTS) in a cellular telephone system in a manner that overcomes disadvantages of the prior art. The present invention discloses selectively dropping calls currently being served by a BTS in accordance with a predetermined signal (neighboring BTS) to interference (serving BTS) ratio at a particular range from the serving BTS in order to allow MSs within the serving BTSs low-visibility area to see neighboring BTSs, thus minimizing disruption of network performance relative to prior art techniques.

There is thus provided in accordance with a preferred embodiment of the present invention in a cellular telephone system including a serving base transmission station (BTS) transmitting at a power Ps in support of a plurality of calls, where the dropping any of the calls reduces Ps, and at least one neighboring BTS transmitting at a power Pn, the serving BTS having a low-visibility area R at least partially determined by Ps and Po, where a reduction in Ps reduces R, a method of powering-down the serving BTS, the method including the steps of a) determining a desired reduced low-visibility area Rr within the low visibility area R, and b) selectively dropping a number of the plurality of calls sufficient to reduce Ps such that R = Rr while maintaining the serving BTS transmission in support of at least one of the plurality of calls.

Further in accordance with a preferred embodiment of the present invention the determining step includes setting the reduced low-visibility area Rr equivalent to a predetermined region of location accuracy.

Still further in accordance with a preferred embodiment of the present invention the determining step includes identifying a mobile station (MS) within the low-visibility area R, and determining the reduced low-visibility area Rr such that the MS would be located outside of the reduced low-visibility area Rr Additionally in accordance with a preferred embodiment of the present invention the selectively dropping step includes selectively dropping any of the plurality of calls having a low data rate prior to selectively dropping any of the plurality of calls having a high data rate.

Moreover in accordance with a preferred embodiment of the present invention the method further includes the step of reestablishing any of the dropped calls after a predetermined period of elapsed time.

There is also provided in accordance with a preferred embodiment of the present invention a cellular telephone system including a serving base transmission station (BTS) transmitting at a power Ps in support of a plurality of calls, and at least one neighboring BTS transmitting at a power Pn, the serving BTS having a low-visibility area R at least partially determined by Ps and Pn, where a reduction in Ps reduces R, where dropping any of the calls reduces Ps, and where the serving BTS is operative to determine a desired reduced lowvisibility area Rr within the low-visibility area R, and selectively drop a number of the

plurality of calls sufficient to reduce P s such that R = Rr while maintaining the serving BTS transmission in support of at least one of the plurality of calls.

Further in accordance with a preferred embodiment of the present invention the low-visibility area Rr is equivalent to a predetermined region of location accuracy.

Still further in accordance with a preferred embodiment of the present invention the serving BTS is additionally operative to identify a mobile station (MS) within the lowvisibility area R, and determine the reduced low-visibility area Rr such that the MS would be located outside of the reduced low-visibility area Rr Additionally in accordance with a preferred embodiment of the present invention the serving BTS is operative to selectively drop any of the plurality of calls having a low data rate prior to selectively dropping any of the plurality of calls having a high data rate.

Moreover in accordance with a preferred embodiment of the present invention the serving BTS is operative to reestablish any of the dropped calls after a predetermined period of elapsed time.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be understood and appreciated more fully from the following detailed description taken in conjunction with the appended drawings in which: Fig. 1 is a simplified illustration of a system for powering-down a serving Base Transmission Station (BTS) in a cellular telephone system, constructed and operative in accordance with a preferred embodiment of the present invention; and Fig. 2 is a simplified flowchart illustration of a method of operation of the system of Fig. 1, operative in accordance with a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PRESENT INVENTION Reference is now made to Fig. 1 which is a simplified illustration of a system for powering-down a serving Base Transmission Station (BTS) in a cellular telephone system, constructed and operative in accordance with a preferred embodiment of the present invention. In Fig. 1 three base transmission stations (BTS) 10,12, and 14 of a cellular telephone system are shown. BTS 10 is a serving BTS to mobile stations (MS) 16,18, 20, and 22, while BTSs 12 and 14 are neighboring BTSs to the MSs. The transmission area of BTS 10 is represented by a circle 24, while portions of the transmission areas of BTSs 12 and 14 are represented by arc projections 26 and 28 respectively.

Using a known propagation model, such as the Hata model, a low-visibility area R of BTS 10 may be determined given the transmission power Ps of BTS 10 and Pn of BTSs 12 and 14. MSs located within low-visibility area R are unable to receive transmissions from neighboring BTSs due to the"doughnut effect"produced by BTS 10. Ps and Pn are to some extent a function of the number of MS calls each BTS is supporting at any given time as well as the transmission power required to support each call. Thus, dropping a call currently being serviced by BTS 10 will reduce Ps, and a reduction in Ps will result in the reduction of lowvisibility area R as well, such as is shown by a reduced low-visibility area Rr within said lowvisibility area R.

Reference is now made to Fig. 2 which is a simplified flowchart illustration of a preferred method of operation of the system of Fig. 1. In the method of Fig. 2 a desired reduced low-visibility area Rr is determined within low-visibility area R (step 100). One method for determining Rr involves adjusting Rr to an area that is equivalent to a predetermined region of location accuracy used with MS location techniques (step 110). An example of such a region of location accuracy is a current Federal Communications Commission requirement that certain MS location techniques define a region of location accuracy such that an MS is located within the region 67% of the time when the techniques are used. In this example, the size of Rr is set equal to the size of the region of location accuracy. Another method for determining Rr is to identify an MS within low-visibility area R, such as MS 16, using transmission round trip ranging techniques or other known techniques (step 120). Rr may then be determined such that MS 16 would be located outside of reduced low-visibility area Rr (step 130) Once Rr is determined, one or more calls currently being served by BTS 10 are dropped for a short interval, such as a millisecond, and then reestablished. The number of calls dropped is that which is needed to reduce Ps such that R = Rr, and the new low-visibility area Rr is established (steps 140-160). Thus, in contrast to prior art techniques, not all calls currently being served by BTS 10 are dropped.

Calls are preferably selectively dropped sooner or later according to certain predefined criteria. For example, low data rate voice calls on the edge of the BTS 10 transmission area are preferably selected first as they require relatively high transmission power from BTS 10 and there would be relatively little impact from losing several slots on such calls. Conversely, high data rate calls are preferably selected last as such calls would have a greater chance of being dropped altogether even after losing relatively few slots. Calls designated as high priority may also be dropped later rather than sooner without regard to whether they are low or high data rate calls. Emergency calls may cause all non-emergency calls to be dropped by the BTS, obviating Rr as a determinant ofPs.

While the methods and apparatus disclosed herein may or may not have been described with reference to specific hardware or software, the methods and apparatus have been described in a manner sufficient to enable persons of ordinary skill in the art to readily adapt commercially available hardware and software as may be needed to reduce any of the embodiments of the present invention to practice without undue experimentation and using conventional techniques.

While the present invention has been described with reference to a few specific embodiments, the description is intended to be illustrative of the invention as a whole and is not to be construed as limiting the invention to the embodiments shown. It is appreciated that various modifications may occur to those skilled in the art that, while not specifically shown herein, are nevertheless within the true spirit and scope of the invention.

Claims

CLAIMS What is claimed is: 1. In a cellular telephone system including a serving base transmission station (BTS) (10) transmitting at a power Ps in support of a plurality of calls, wherein dropping any of said calls reduces Ps, and at least one neighboring BTS (12,14) transmitting at a power Pn, said serving BTS (10) having a low-visibility area R at least partially determined by Ps and Pn, wherein a reduction in Ps reduces R, a method of powering-down said serving BTS, a method characterized by: a) determining (100) a desired reduced low-visibility area Rr within said lowvisibility area R; and b) selectively dropping (140) a number of said plurality of calls sufficient to reduce Ps such that R = Rr (150) while maintaining said serving BTS (10) transmission in support of at least one of said plurality of calls.
2. A method according to claim 1 wherein said wherein said determining step (100) comprises setting (110) said reduced low-visibility area Rr equivalent to a predetermined region of location accuracy.

3. A method according to claim 1 wherein said determining step (100) comprises: identifying (120) a mobile station (MS) (16) within said low-visibility area R; and determining (130) said reduced low-visibility area Rr such that said MS (16) would be located outside of said reduced low-visibility area Rr 4. A method according to claim 1 wherein said selectively dropping step (140) comprises selectively dropping any of said plurality of calls having a low data rate prior to selectively dropping any of said plurality of calls having a high data rate. 5. A method according to claim 1 and further comprising the step of reestablishing (160) any of said dropped calls after a predetermined period of elapsed time.

6. A cellular telephone system including: a serving base transmission station (BTS) (10) transmitting at a power Ps in support of a plurality of calls; and at least one neighboring BTS (12,14) transmitting at a power Pn, characterized in that said serving BTS (10) has a low-visibility area R at least partially determined by Ps and Pn, wherein a reduction in Ps reduces R, wherein dropping any of said calls reduces Ps, and wherein said serving BTS (10) is operative to: determine (100) a desired reduced low-visibility area Rr within said low-visibility area R; and selectively drop (140) a number of said plurality of calls sufficient to reduce Ps such that R = Rr (150) while maintaining said serving BTS (10) transmission in support of at least one of said plurality of calls.

7. A system according to claim 6 wherein said low-visibility area Rr is equivalent to a predetermined region of location accuracy.

8. A system according to claim 6 wherein said serving BTS (10) is additionally operative to: identify (120) a mobile station (MS) (16) within said low-visibility area R; and determine (130) said reduced low-visibility area Rr such that said MS would be located outside of said reduced low-visibility area Rr 9. A system according to claim 6 wherein said serving BTS (10) is operative to selectively drop any of said plurality of calls having a low data rate prior to selectively dropping any of said plurality of calls having a high data rate.

10. A system according to claim 6 wherein said serving BTS (100 is operative to reestablish (160) any of said dropped calls after a predetermined period of elapsed time. Amendments to the claims have been filed as follows CLAIMS 1. Exhaust emission control equipment for a motor vehicle, comprising a catalytic converter eatable during a heating phase and means for heating air by combustion of fuel components volatilised from fuel in a fuel tank of the vehicle and heating the converter by the heated air, the means for heating air comprising a burner integrated in the converter at an exhaust gas inlet thereof and connected to supply means for supplying to the burner an air flow enriched with the volatilised fuel components and the supply means comprising a flow control valve openable only during the heating phase2. Equipment as claimed in claim 1, the supply means comprising an activated carbon filter for collecting the volatilised fuel components and air feed means for feeding air to the filter to provide a flushing air flow for removal of the components from the filter.
3. Equipment as claimed in claim 2, wherein the filter is housed in a housing having a first inlet connectible by way of a first blocking valve with a vent of the tank, a second inlet communicable by way of a second blocking valve with the ambient atmosphere to form the air feed means and an outlet for the flushing air flow with entrained fuel components.
4. Equipment as claimed in claim 2 or claim 3, comprising a condensing and storing device for receiving the flushing air flow and condensing and storing the fuel components, the device having a relief opening communicating with the ambient atmosphere.
5. Equipment as claimed in claim 3, comprising conveying means connected with the outlet of the housing and operable to convey the flushing air flow directly to the burner.
6. Equipment as claimed in claim 5, comprising a control unit to so control the flow control valve, blocking valves and conveying means that during the heating phase the control valve and second blocking valve are open, the first blocking valve is closed and the conveying means is in operation.
7. Equipment as claimed in claim 4, comprising conveying means connected with the outlet of the housing and with air inlet of the condensing and storing device, the flow control valve being connected with an outlet of the device and an inlet of the burner to permit flow of air charged with fuel components to the burner.
8. Equipment as claimed in claim 7, comprising a control unit to so control the flow control valve, blocking valves and conveying means that during the heating phase the control valve and second blocking valve are open, the first blocking valve is closed and the conveying means is in operation and during engine operation outside the heating phase the control valve is closed, the second blocking valve is open and the pump is placed in operation and taken out of operation at intervals in alternation, the first blocking valve being closed when the pump is operating and open when the pump is not operating.
9. Equipment as claimed in any one of the preceding claims, comprising means for feeding supplementary air to the burner.
10. Equipment as claimed in claim 4, comprising a flushing air flow valve for controlling the flushing air flow from the filter to the device, the device being so connectible at outlet means thereof with a source of underpressure and with the flow control valve that the flushing air flow is produced by underpressure and is fed to the burner by way of the control valve.
11. Equipment as claimed in claim 10, comprising a control unit to so control the flow control valve, blocking valves and flushing air flow valve that during the heating phase the control valve is open and the flushing air valve is closed and during engine operation outside the heating phase the control valve is closed, the second blocking valve is open and the flushing air flow valve is opened and closed at intervals in alternation, the first blocking valve being closed when the flushing air flow valve is open and being open when the flushing air flow valve is closed.
12 Equipment as claimed in claim 10 or claim 11, wherein the underpressure producing the flushing air flow is provided by means for feeding supplementary air to the burner.
13. Equipment as claimed in claim 9 or claim 12, wherein the means for feeding supplementary air comprises a secondary air pump connected with the burner by way of a valve which is open only during the heating phase.
14. Exhaust emission control equipment substantially as hereinbefore described with reference to any one of Figs. 1 to 3 of the accompanying drawings.
15. A motor vehicle with an internal combustion engine and emission control equipment as claimed in any one of the preceding claims.