Communications: directive radio wave systems and devices (e.g. – Directive – Utilizing correlation techniques
Reexamination Certificate
1999-06-01
2001-12-18
Phan, Dao (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Utilizing correlation techniques
C342S383000
Reexamination Certificate
active
06331837
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to wireless radio-frequency (RF) communication systems. In particular, it relates to isolation systems that separate a desired receive signal from interfering signals in a RF communication system. Interfering signals include all signals received by a receiving antenna that cause co-channel interference.
Some techniques for reducing co-channel interference include frequency separation, time division, orthogonal polarization, and spatial separation. Further reduction of interference requires some type of cancellation. U.S. Pat. No. 5,432,522 shows a canceller that reduces cross-polarization interference in two orthogonally polarized channels. U.S. Pat. No. 5,515,378 shows how adaptive beamforming performed with a phased array provides spatial multiplexing and demultiplexing of wireless communication channels. Each element of the array has an associated electrical signal that is adjusted by a complex-valued weight, then summed to provide an antenna beam pattern having nulls (canceled responses) in a predetermined direction for a particular signal frequency. Transmission of wide-band or multiple-frequency signals causes distortion of the main beam and variance in the location and depth (beam-pattern magnitude) of the nulls.
Beamforming can provide exceptional performance in a fading environment, which is due to the ability of an array to select signals based on the signals directions of arrival. As strong signals are selected for reception, destructive cancellation caused by reflected components arriving at the array elements is mitigated by the placement of nulls. Null placement is also effective in mitigating co-channel interference. However, problems with beamforming include the inability to resolve co-located or closely spaced radio sources unless multipath components of these signals are tracked. In addition, the number of antenna elements limits the number of co-channel interference sources that can be nulled. This is a significant problem because each multipath component arriving at the array is a source of interference. Therefore, a small number of transmitters may provide a large number of interference sources.
SUMMARY OF THE INVENTION
The present invention addresses the lack of available frequency bandwidth allocation for wireless RF communications. Effects of these problems include limited data-transmission capacity, co-channel interference, and limited access to wireless services resulting from increased demand for the services. Substantial improvements in frequency reuse are implemented through techniques of Spatial Interferometry Multiplexing (SIM). SIM is a process of transmitting, receiving, and separating multiple signals that interfere with each other. Applications of this new technique are directed toward, but not limited to, stationary wireless communication systems.
One embodiment of the present invention is a microwave antenna array that receives a plurality of signals in the same frequency channel that are transmitted from a remote location. The antenna array uses a spatial demultiplexer to resolve signals from even co-located sources. Consequently, the spatial demultiplexer provides a frequency reuse improvement of at least several orders of magnitude over the prior art.
A first object of the invention is to provide a new type of spatial demultiplexing technique to resolve closely spaced and co-located sources by using spatial gain distributions of the received signals. A spatial gain distribution is the variation of signal amplitude with respect to space. This results in one antenna receiving a different signal strength relative to another antenna when the antennas are spatially separated. The signal amplitude may be complex valued and, thus, includes phase information. Spatial gain distributions are a natural phenomenon caused by a large number of factors, including multipath fading, shadowing, diffraction, propagation loss, and adsorption. The present invention makes use of spatial gain distributions to enhance interference rejection.
In the present invention, the spatial gain distribution of each receive signal is measured to determine ratios of co-channel interference at the antenna elements. These ratios are used to determine weights in a cancellation (spatial demultiplexing) circuit, which separates the received signals. Adjustments to the spatial gain distribution may be performed by aperture synthesis, beam steering, or interferometric combining of a plurality of signals at either or both the transmit antenna(s) and the receive antenna. Received signals may be separated to recover a single desired signal transmitted by one of a plurality of different transmitters. Received signals may be separated to recover a plurality of desired signals from a multi-element transmitter that generates desired signals having different spatial gain characteristics at the receiving antenna.
Spatial demultiplexing may be performed following beamforming operations in an antenna array. Beamforming changes the effective spatial gain characteristics of received signals “seen” by the array. Thus, the different antenna patterns resulting from different beamforming parameters allow multiple unique inputs to the spatial demultiplexing process.
Another object of the invention is to separate interfering signals received at an antenna array by spatial demultiplexing a plurality of outputs from different beamforming processes. This type of spatial demultiplexing reduces the number of spatially separated antenna elements required to separate interfering received signals.
Spatial gain characteristics of a received signal vary with the location of the receiving antennas, time of reception, and frequency of the transmitted signal. Therefore, another object of the invention is to provide diversity-reception techniques that separate received signals based on space diversity, time diversity, and frequency diversity. Another application of the demultiplexing technique includes separating polarized received signals that have known cross-polarization terms.
Another object of the invention is to provide spatial demultiplexing of multicarrier signals. Benefits of multicarrier protocols include simplified processing, improved bandwidth efficiency, and the ability to transmit large data rates over narrowband signals (which simplifies the spatial demultiplexing process).
Due to their increased size and the problem of preserving a particular orientation with respect to a base station, antenna arrays are not practical for mobile and hand-held units. Typically, an omnidirectional whip-style antenna is used on a mobile unit, and the base station is equipped with an array.
Thus, an object of the present invention is to provide a receiver with a spatial demultiplexing system that uses a single omnidirectional antenna. An embodiment of this invention is demonstrated with a preferred multicarrier protocol known as Carrier Interferometry Multiple Access (CIMA). CIMA provides redundant transmissions on different carrier frequencies. However, this redundancy does not diminish the bandwidth efficiency. In fact, superior bandwidth efficiency is achieved because the CIMA carriers combine in the time domain to produce short impulses. Frequency diversity in the CIMA protocol also enables spatial demultiplexing of the received CIMA signals. Different carriers have different spatial gain distributions (in a multipath fading environment) due to their differences in frequency. Therefore, each transmitted signal has a unique spatial gain distribution represented by the individual amplitudes of its component carriers.
The objects of the present invention recited above; as well as additional objects, are apparent in the description of the preferred embodiments.
REFERENCES:
patent: 4734701 (1988-03-01), Grobert
patent: 4931977 (1990-06-01), Klemes
patent: 5414699 (1995-05-01), Lee
patent: 5471647 (1995-11-01), Gelach
patent: 5515378 (1996-05-01), Roy
patent: 5523526 (1996-06-01), Shattil
patent: 5592490 (1997-01-01), Barratt
patent: 5634199 (1997-0
GenghisComm LLC
Phan Dao
Shattil Steven J.
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