Telecommunications – Transmitter and receiver at separate stations – Distortion – noise – or other interference prevention,...
Reexamination Certificate
1997-12-04
2001-07-10
Urban, Edward F. (Department: 2683)
Telecommunications
Transmitter and receiver at separate stations
Distortion, noise, or other interference prevention,...
C455S067150, C455S067700, C375S343000, C375S346000
Reexamination Certificate
active
06259894
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of signal processing, and more particularly to a method for detecting line-of-sight signals.
BACKGROUND OF THE INVENTION
In wireless communications systems, wireless terminals and base stations are designed to transmit and receive radio frequency (RF) signals that propagate in RF environments. Depending on the type of wireless communications system and the services offered by the wireless system, the wireless terminals and base stations are equipped to perform specific signal-processing functions. For example, some wireless systems are required to identify the geographical location (i.e. geolocation) of the wireless terminals communicating on the system. Such wireless systems have been referred to as geolocation systems. The term geolocation as used herein refers to the point in two- or three-dimensional space defined by a set of coordinates, e.g. longitude and latitude, and/or defined by a vector, i.e. distance and direction, from a known point in space.
Some conventional geolocation systems identify the geolocation of a wireless terminal by determining, at a plurality of locations, the time-of-arrival of the line-of-sight component of a signal transmitted by the wireless terminal. They then process the various times-of-arrival to determine the distance of the wireless terminal from each of, for example, three receiver locations, and from this “distance” information, the geolocation system determines the geolocation of the wireless terminal itself
The line-of-sight component of the transmitted signal is that component of the signal that propagated directly from the wireless terminal to the location at which the signal was received (e.g. a base station) without scattering or reflecting off structures in the RF environment. The term scattering refers to the phenomenon wherein an RF signal, traveling in an RF environment, hits and reflects of structures in the RF environment, thereby causing the RF signal to take random paths through the RF environment. This so-called multipath phenomenon can cause the incoming signal to be composed of several repeated versions of the transmitted signal, each version being a multipath component of the incoming signal.
To determine the time-of-arrival of the line-of-sight component of the transmitted signal, such geolocation systems typically receive the transmitted signal and pass the so-called incoming signal through a matched filter. The matched filter generates a correlation value based on a comparison of the shape of the waveform of the incoming signal to the shape of the waveform of the transmitted signal. The correlation value essentially peaks each time the matched filter determines that the shape of the waveform of the incoming signal is similar to or matches the shape of the waveform of the transmitted signal. Each time the correlation value reaches a peak value, the geolocation system identifies that time as the time-of-arrival of a multipath component of the incoming signal. Since the line-of-sight component of the transmitted signal travels directly to the location of the receiving unit, such conventional geolocation systems assume that the time-of-arrival of the line-of-sight component of the incoming signal is the time at which the correlation value reaches its first peak.
These geolocation systems, however, are hindered by their failure to consider the effects of scattering or the scattering hostility, i.e. the propensity of the RF environment to scatter an RF signal traveling in the RF environment, on the incoming signal. Depending on the scattering hostility of the RF environment in which the incoming signal traveled, the line-of-sight path and thus the line-of-sight component of the incoming signal may never reach the receiver location. For example, there may be a large building in the RF environment that blocks the line-of-sight path between the transmitting unit and the receiving unit, thus preventing the line-of-sight component from ever reaching the receiving unit. When this happens, the time-of-arrival of the first-identified peak of the correlation value may not be the time-of-arrival of the line-of-sight component of the incoming signal, but rather the time-of-arrival of a later-arriving multipath version of the transmitted signal.
When this happens, the identified time-of-arrival of the line-of-sight component will be so-called “time-shifted.” That is, identified time-of-arrival will be the time-of-arrival of the later-arriving multipath component. As a result, the conventional geolocation system will incorrectly assume that the time-of-arrival of the line-of-sight component of the incoming signal arrived at a later time than it would have arrived if it had not been blocked as described above. Thus, the conventional geolocation system would incorrectly calculate the geolocation of the wireless terminal based on a “time-shifted” time-of-arrival.
SUMMARY OF THE INVENTION
According to the principles of the present invention, the time-of-arrival of the line-of-sight component of a received signal is identified with more accuracy than that obtained by merely identifying the time at which the first-arriving component of the incoming signal is received. Instead, the identified time-of-arrival of the first-arriving component of the incoming signal is adjusted by an amount based on at least one parameter of an RF model that characterizes the scattering hostility of the RF environment in which the incoming signal traveled. The resultant “adjusted” time-of-arrival reflects the time at which the line-of-sight component of the incoming signal would have arrived, if the RF environment were scatter-free (e.g. no structures blocking the line-of-sight component of the incoming signal). Thus, such an adjustment to the identified time-of-arrival of the first-arriving component reduces the amount of the “time-shift” due to the scattering hostility in the RF environment in which the incoming signal traveled.
The term parameter as used herein refers to any parameter or so-called dimension that is capable of defining or describing a chaotic process or system (e.g. scattering) in terms of a measure of some aspect of that system. For example, a fractal dimension is a parameter that can be used to define a chaotic system, such as a mountainous landscape, in terms of its self-similarity. The term self-similarity as used herein refers the presence of and/or the number of times a similar shape or pattern repeats itself in the chaotic system. Thus, a fractal dimension that defines a mountainous landscape actually provides a measure of the number of times a particular shape is repeated in the shape of the landscape itself.
Just like a fractal dimension can be used as a parameter to define the shape of a mountainous landscape, so can a fractal dimension be used as a parameter to provide a measure of multipath in an RF environment. In particular, the fractal dimension can be used to characterize the scattering hostility of the RF environment by providing a measure of the number of times a particular shape (i.e. the various multipath components of the incoming signal) is repeated in the waveform of the incoming signal that traveled in the RF environment, By providing a measure of the multipath components of the incoming signal, the fractal dimension actually provides a measure or characterization of the scattering hostility of the RF environment. Thus, according to the principles of the present invention, a set of such fractal dimensions forms an RF model of the RF environment.
In accordance with a feature of the invention, each parameter of the RF model indicates the amount of scattering a given signal would incur in a given region of the RF environment, and thus indicates the relative amount of “time-shift” that would occur if a signal were to travel in each of the regions. For example, a parameter that characterizes a given region as having a greater scattering hostility than another region necessarily indicates that the amount of “time-shift” that would occur in that given region is
Jackson Blane J.
Lucent Technologies - Inc.
Urban Edward F.
LandOfFree
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