Telecommunications – Transmitter and receiver at separate stations – Plural transmitters or receivers
Reexamination Certificate
1998-08-19
2001-06-12
Trost, William G. (Department: 2683)
Telecommunications
Transmitter and receiver at separate stations
Plural transmitters or receivers
C455S456500, C455S457000, C342S450000, C342S457000
Reexamination Certificate
active
06246884
ABSTRACT:
BACKGROUND OF THE INVENTION
I. Field of the Invention
The present invention relates to communication systems. More particularly, the present invention relates to mobile station location within a wireless communication system.
II. Description of the Related Art
Wireless communication systems are becoming commonplace in modern society. One of the most scarce and complex elements of a wireless communication system is the wireless spectral bandwidth over which the wireless link operates. In order to efficiently use the segmented spectral bandwidth, modern commercial cellular communication systems are designed to reuse the allocated spectral resources. Spectral reuse is achieved by allocating the same spectral resources to multiple coverage areas spaced apart by some minimum distance. Depending on the type of system, the spectral resources may be allocated into discrete channels in the time, frequency or code domain or a combination of these. In each case, at some distance away from a first coverage area using a channel, a second coverage area reuses the same channel. The distance between the first and second coverage areas is chosen such that the signal levels emanating from the first coverage area have become sufficiently small at the second coverage area that co-channel interference between the first and second coverage areas is below an acceptable threshold level.
All cellular systems employ a power control mechanism to some extent. By using a power control mechanism, the mobile station changes its transmit signal level to compensate for variations in the path loss between the base station and the mobile station. In an ideal cellular system, the mobile station controls its power level so that within a great majority of the service area, its signal arrives at only one base station with a signal to noise ratio large enough for efficient demodulation.
In wireless communication systems, the actual location of the mobile station is often unknown. Some information about the general location of the mobile station can be easily determined. For example, in a cellular system, each base station defines a physical coverage area. In an ideal cellular system, the coverage areas of adjacent base stations abut one another to create a continuous service area. Mobile stations located within the coverage area of a base station may establish communication with the base station. When a mobile station moves past the outer boundary of the coverage area of a first base station into the coverage area of a second base station, communication with the first base station may become greatly compromised or even impossible due to the decreased signal to noise ratio at which the mobile station signal is received at the first base station. In actual systems, the coverage area of adjacent base stations overlap to some extent. Within the coverage area overlap region, a mobile station may establish communication with either base station or both base stations, depending on the communication techniques used. The coverage area overlap region can be used to execute handoff from one base station to another so that a continuous connection is provided to the mobile station as it moves about within the service area. In this way, the position of a mobile station can be limited to the coverage area corresponding to the base station which is currently providing service to the mobile station. However, base station coverage areas can be relatively large such as several miles in diameter. In many emergency situations, such imprecise location information is nearly useless.
Several techniques have been developed in order to aid in locating a mobile station in a wireless system using one or more location parameters. For example, the angle of arrival, time of arrival and time difference of arrival have all been used as location parameters to determine the location of a mobile station in a wireless system.
FIG. 1
is a diagram illustrating an angle of arrival for a wireless mobile station
102
. The angle of arrival is an indication of the direction from which the signal from the mobile station
102
is received at the base station
100
. The measured angle of arrival designates the location of the mobile station on a line of bearing. The angle of arrival does not provide any information about the distance between the base station
100
and the mobile station
102
.
An interferometer is one means by which the angle of arrival may be measured. An interferometer determines the angle of arrival based on a phase difference of the signal arriving at two or more antennas elements.
A beam forming method can also be used to measure the angle of arrival. A beam forming method determines the angle of arrival based on a best match between the amplitude and phase response of an antenna array and the amplitude and phase signal measurements for each antenna element of the array.
Super-resolution techniques may also be used to determine the angle of arrival. Super-resolution techniques determine the angle of arrival based on a determination of the multipath model and signal statistics, not knowing the signal itself. Academic efforts refining these and other techniques are currently in progress.
The time of arrival determines a circle upon which the mobile station
102
may be found with relation to the base station
100
as shown in FIG.
2
. The radius of the circle is calculated by multiplying the delay between the transmission of a signal from the mobile station
102
and reception of the signal at the base station times the speed of light. (radius=c*delay). The time of arrival does not provide any information concerning the angle of the mobile station
102
with respect to the base station
100
. In a typical prior art system, the base station
100
sends a signal to the mobile station
102
which repeats the signal as soon as it is received. The delay between the transmission and reception of the signal is determined by cross-correlating the transmitted signal and the received signal at a series of time offsets. The cross-correlation exhibits a peak at the time offset equal to the round trip delay or twice of the one-way transmission delay time. The transmitted signal is often called a reference signal. A reference signal is only available when the base station has a priori knowledge concerning the data content of mobile station signal.
FIG. 3
is a diagram illustrating a time difference of arrival determination for a wireless mobile station
102
with respect to a base station
100
A and a base station
100
B. The difference in the time of arrival of a signal received at two base stations determines a hyperbola upon which the mobile station
102
is located. In a typical prior art system, the mobile station
102
transmits a signal which is received by both base stations
100
A and
100
B. Each base station determines the absolute time at which the signal is received. By comparing the difference, a relative distance between the two base stations is determined. For example, by comparing the time difference of arrival, the location of the mobile station
102
is determined to be about 1 kilometer closer to the base station
102
B than the base station
102
A. Alternatively, received signals from base stations
100
A and
100
B are brought to a common processing site where they are cross-correlated to yield directly the difference in propagation time from the mobile station to the base stations.
As a signal propagates between the base station and the mobile station over the wireless channel, it is attenuated by and reflects from objects in the field. At the base station, the various reflected propagations are offset in time from one another due to the differences in the path lengths which the signals travel. For example,
FIG. 4
is a diagram showing three different propagation paths of a signal transmitted by the mobile station and received by the base station. Typically, the first signal to arrive has the largest amplitude and travels a relatively direct path from the mobile station to the base station. However, this is not always the
Karmi Yair
Weiss Anthony
Knobbe Martens Olson & Bear LLP
Sigmaone Communications Corporation
Smith Sheila
Trost William G.
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