The present invention relates generally to communications systems, and more particularly to signal direction processing for an antenna array.
In many applications it is of interest to determine the direction of arrival of a signal, such as cellular phones and other radio communication system. The determination of a direction of arrival of a received signal from one communications unit can be used to maximize the sensitivity of an antenna array of a second communications system to that direction as well as the directionality of the signal power transmitted in a return transmission. Determining an angle of arrival can also be useful in identifying malicious signals, for example, spoofing or jamming signals intended to disrupt operation of a radio frequency communication system or navigation system. For emergency (e.g., 911) calls from a mobile telephone, direction of arrival can also be used to locate the caller when global positioning system (GPS) data is not available.
In accordance with an example, a system is provided for determining a direction of a signal received at an antenna array. An antenna array includes a plurality of antenna elements, including a reference element. A signal combiner element is configured to combine weighted signals from a subset of the plurality of antenna elements to provide a composite output. An adaptive processing component is configured to determine an optimal set of weights for the subset of the plurality of antenna elements. An angle of arrival search component is configured to find a direction of minimum gain given the optimal set of weights.
In accordance with another example, a method is provided for determining a direction of a signal received at an antenna array. A signal is received at a plurality of antenna elements, including a reference element. Weights are applied to received signals at a subset of the plurality of antenna elements. The weighted signals from a subset of the plurality of antenna elements are combined to provide a composite output. An optimal set of weights are determined for the subset of the plurality of antenna elements as a set of weights that minimize a mean squared error between the composite output and an output of the reference element. An angle of arrival is determined for the signal from the determined optimal set of weights.
In accordance with still another example, a system is provided for determining a direction of a signal received at an antenna array. An antenna array includes a plurality of antenna elements, including a reference element. A signal combiner element is configured to combine weighted signals from a subset of the plurality of antenna elements to provide a composite output. An adaptive processing component is configured to determine an optimal set of weights for the subset of the plurality of antenna elements as a set of weights that minimize a mean squared error between the composite output and an output of the reference element. An angle of arrival search component is configured to find a direction of minimum gain given the optimal set of weights.
Using the systems and methods described herein, it is possible to determine the direction of arrival (DOA) of a signal in space using an antenna array with any conventional element placement. Traditional interferometry techniques for locating signal direction require antenna arrays having very specific element spacing. The inventor has determined that a determination of signal direction can be accomplished more accurately than interferometry techniques by applying minimum mean squared error (MMSE) processing techniques to find a set of array element weights through analysis of the sampled signal space such that the resulting response pattern of the array will have a spatial null in the direction of the signal of interest. The response pattern of the antenna, given these array element weights, can be analyzed to reveal the DOA of the signal of interest.
A signal combiner component 122 is configured to combine the weighted signals from the subset of the plurality of antenna elements to provide a composite output. An adaptive processing component 124 receives the weighted signals and determines an optimal set of weights for the subset of the plurality of antenna elements. When a signal is being received, the weights can be selected to manipulate the directional pattern of the array, for example, to increase gain in the direction of an incoming signal. In the direction finding application of
An angle of arrival search component 126 can then utilize the set of weights to search for a direction of minimum gain, given the set of weights, to determine the direction of the signal. For example, for each of a plurality of angles of arrival, an output value can be calculated, given the response pattern defined by the set of weights and the known values at each of the plurality of elements 112-115. An minimum value in the response pattern can be found by searching a range of possible values iteratively, such that at one or more coarse levels of quantization are searched to narrow the range of possible values before a final fine level of quantization is used to find the angle of arrival with a desired degree of precision. In one implementation, the angle of arrival search component 126 can vary the angle of arrival to be searched in ten degree increments, then one degree increments, then tenth of a degree increments to provide accuracy within a tenth of a degree. It will be appreciated, however, that finer degrees of precision may be available in some applications.
All of the digital baseband signals, other than the signal representing the output of the reference element 215, are then weighted at respective multipliers 252-254 with a set of weights provided from an adaptive processor 258, and combined at an associated signal combiner 260 to provide an output for the antenna array. In the illustrated implementation, the weight set is obtained by using an adaptive beam steering algorithm that seeks to minimize the mean square error between the antenna array output and the output of the reference element 215. The optimal Minimum Mean Square Error (MMSE) weights are given by:
where x is a vector of signal values 262 from the plurality of antenna elements, the superscript * denotes a complex conjugate of a term, and r is the output of the reference element.
$$
where s is the signal, n_{i }is the thermal noise at an i^{th }element, λ is a wavelength of the signal, c is the speed of light in a vacuum, τ_{i }is the delay of the i^{th }element relative to the reference element, and
$$
The cross-correlation matrix, E{
$$
where P_{s }is the signal power and P_{n }is the thermal noise power generated by receivers associated with the plurality of elements.
Returning to
All of the digital baseband signals, other than the signal representing the output of the reference element 415, are then weighted at respective multipliers 452-454 with a set of weights provided from an adaptive processor 458, and combined at an associated signal combiner 460 to provide an output for the antenna array. In the illustrated implementation, the weight set is obtained by using an adaptive beam steering algorithm that seeks to minimize the mean square error between the antenna array output and the output of the reference element 415. In one implementation, the optimal Minimum Mean Square Error (MMSE) weights are given by:
where
Referring back to
$$
where s is the signal, n_{i }is the thermal noise at an i^{th }element, λ is a wavelength of the signal, c is the speed of light in a vacuum, τ_{i }is the delay of the i^{th }element relative to the reference element, P_{s }is the signal power, and
$$
From Eq. 5, the matrix
$$
It will be appreciated that this matrix lacks the thermal noise present in the blind nulling solution of
−
where
Returning to
In view of the foregoing structural and functional features described above, a methodology in accordance with various aspects of the present invention will be better appreciated with reference to
At 508, an optimal set of weights are determined for the subset of the plurality of antenna elements as a set of weights that minimize a mean squared error between the composite output and an output of the reference element. At 510, an angle of arrival for the signal is determined from the determined optimal set of weights. In one implementation, the angle of arrival is determined by finding a direction of minimum gain given the optimal set of weights. For example, a range of possible angles can be searched iteratively, with the iterative search including performing a first search at a coarse quantization level to narrow the range of possible angles and performing a final search at a fine quantization level to determine the angle of arrival.
What has been described above includes exemplary implementations of the present invention. It is, of course, not possible to describe every conceivable combination of components or methodologies for purposes of describing the present invention, but one of ordinary skill in the art will recognize that many further combinations and permutations of the present invention are possible. Accordingly, the present invention is intended to embrace all such alterations, modifications and variations that fall within the spirit and scope of the appended claims.
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Number | Date | Country | |
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20170102445 A1 | Apr 2017 | US |