Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1997-12-10
2001-06-05
Maung, Nay (Department: 2681)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S422100, C455S427000, C455S503000, C342S457000
Reexamination Certificate
active
06243587
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to position detection methods and systems, and more particularly, to a method and system for detecting a position of a mobile device or transmitter, such as a cellular telephone, based on phase differences of transmitted signals of two or more frequencies received at two or more receiving sites.
Prior position detection, or navigation, systems may be generally divided into two categories: passive navigation systems and active navigation systems. In a passive navigation system, a mobile device determines its position based on signals received from transmitters positioned at known locations. In an active navigation system, the mobile device transmits signals which are received by one or more receivers positioned at known locations. The position of the mobile device is then determined based on the received signals and the known position of the receivers.
In the past, passive navigation systems have been generally favored over active navigation systems. One reason for this favoritism is that in active systems each mobile unit must transmit signals to determine its position. Since the number of mobile units in a single active system may be in the millions, these signals may overly congest the active system and may cause the system to malfunction. This problem is exacerbated in situations where the position of the mobile unit needs to be continuously determined with a high degree of accuracy. For example, an airplane needs virtually continuous position determinations due to its high velocity and, therefore, would need to transmit signals virtually continuously. Having a large number of mobile units continuously transmitting such signals could possibly overload, or congest, an active navigation system.
One prior active navigation system was espoused by the GEOSTAR corporation. In the GEOSTAR system, at least one orbiting satellite transmits signals to mobile transponders and receives replies from the mobile transponders. For calculation purposes, the GEOSTAR system assumes that the mobile transponder is on the earth's surface. By measuring the time a signal takes to travel from a satellite to a mobile transponder and back to the satellite (loop propagation delay), the mobile transponder can be determined to lie somewhere on a calculated sphere of appropriate radius. Since the intersection of the calculated sphere and the earth's surface is a circle, the GEOSTAR system thereby locates the mobile transmitter somewhere on the circle. If two satellites are employed, the mobile transponder can be deduced to also lie on a second circle. Since the intersection of these two circles is two points, the GEOSTAR system can therefore locate the mobile transponder at one of the two points.
The GEOSTAR system unfortunately exhibits some significant deficiencies. Firstly, the GEOSTAR system suffers from the problem of congestion as experienced by other prior active navigation systems. Secondly, the GEOSTAR system relies upon loop delay measurements which are typically of questionable reliability or accuracy. For example, the accuracy of loop delay measurements is deleteriously affected by timing errors in the transponder hardware which may be caused by any number of known factors, such as temperature, imprecise manufacturing tolerances and the like. In addition, GEOSTAR systems typically use wideband signal transmissions to assure accurate loop delay measurements. Unfortunately, such wideband signal transmissions occupy a large portion of available bandwidth, thus significantly contributing to the aforedescribed problem of congestion.
Capacity is an issue at the heart of cellular communications systems and satellite-based mobile communications systems. Enough capacity for all users is guaranteed by dividing the service area into a large number of small cells with the ability to re-use the limited number of available radio frequencies again in different cells which are adequately sparated. U.S. Pat. No. 5,619,503 issued to Dent on Apr. 8, 1997 describes improvements to multi-cell or multi-beam communications systems that permit higher capacity by denser frequency re-use—ultimately permitting every frequency channel to be used for a different purpose in every cell or beam. The disclosure of U.S. Pat. No. 5,619,503 is hereby incorporated by reference herein and provides the capacity improvements necessary to allow an active navigation system to succeed. Methods to obtain an initial coarse position estimate are also described and may be used in the current invention.
There is thus a need in the art for a system and method for providing active position determination of a mobile transmitter which increases the accuracy of the position determination.
SUMMARY OF THE INVENTION
This need is met by a method and system in accordance with the present invention wherein a first signal at a first frequency and a second signal at a second frequency are received from a transmitter at first and second receiving stations. Based on phase shifts of the signals received at the first and second receiving stations, a range difference is calculated which locates the transmitter on a first hyperbola having the stations as foci.
In accordance with one aspect of the present invention, a method is provided for determining position of a mobile transmitter, such as a cellular telephone, relative to first and second receiving sites. Either of, or both of, the first and second receiving sites may be a land-based cellular base station or a satellite relay station. A first signal at a first frequency is transmitted by the mobile transmitter and received at the first and second receiving sites, as respective first and second received signals. A second signal at a second frequency is transmitted by the mobile transmitter and received at first and second receiving sites, as respective third and fourth received signals. A first phase difference measurement is made based on the first and second received signals. A second phase difference measurement is made based on the third and fourth received signals. A position of the mobile transmitter is determined based on the first and second phase difference measurements, the first and second frequencies and the first and second known locations. In particular, the first and second phase difference measurement are scaled based on the first and second frequencies and a range difference is determined. For example, a frequency difference may be determined between the first and second frequencies and the difference between the first and second phase difference measurements is scaled by the frequency difference to obtain a range difference.
The first phase difference measurement is preferably performed by determining a first phase shift for the first received signal; determining a second phase shift for the second received signal; and making the first phase difference measurement based on the first and second phase shifts. Similarly, the second phase difference measurement is preferably performed by determining a third phase shift for the third received signal; determining a fourth phase shift for the fourth received signal; and making the second phase difference measurement based on the third and fourth phase shifts.
The calculated range difference places the transmitter on a first hyperbola with foci at the first and second receiving stations. To further define the position of the transmitter, the method may further comprise the steps of calculating third or fourth phase difference measurements for a third receiving site and one of the first or second receiving sites; localizing the transmitter on a second hyperbola related to the third receiving site and the one of the first and second receiving sites; and determining the position of the mobile transmitter based on the intersection of the first and second hyperbolas.
The first frequency may be a random access channel and the second frequency may be a traffic channel. Preferably, at least one of the first and second frequencies is contained in set of frequencies allocated t
Dent Paul Wilkerson
Koorapaty Havish
Ericsson Inc.
Maung Nay
Wood Phillips VanSanten Clark & Mortimer
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