Abstract:
A method and implementation of wireless communication are disclosed in which wireless signals are exchanged between at least one remote client and a directional antenna array associated with a wireless network, wherein the directional antenna array includes a plurality of antenna elements. A statistical matrix analysis is performed for each of the at least one client and the antenna array, in order to locate angles associated with directions of each client with respect to the antenna array. Values are determined for weighting factors for RF signals of each of the respective plurality of antenna elements, so as to create predetermined phase differences between the signals of the plurality of antenna elements. The predetermined phase differences are used to direct at least one null toward at least one source of interference, so as to avoid signal interference.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation of U.S. patent application Ser. No. 10/225,948 filed Aug. 22, 2002, now U.S. Pat. No. 6,970,722. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The present invention is directed to the field of beamforming, particularly as used with an adaptive antenna array for a wireless telecommunications system, e.g. a wireless local area network (WLAN). In previous-type WLAN systems, it had been sufficient to communicate with wireless clients using one or more omnidirectional antennas. In such a previous-type scheme, wireless clients gain access to the WLAN by operating on different frequency bands and/or time-sharing over the same set of frequency bands.  
         [0003]     As the number of clients in a WLAN increases, with resulting increased demands for WLAN access, it becomes necessary to “manage space,” i.e. spatially isolate communications between clients distributed over a geographic area. To this end, it has become common to employ a directional antenna that can be selectively pointed at clients to allow isolated communications between the clients and the WLAN.  
         [0004]     A common implementation for a directional antenna is to use an adaptive array. Such arrays can be formed of any grouping of antenna elements, such as a dipole, Yagi and patch antennas. These arrays can be one-dimensional, i.e. having linearly-distributed antenna elements. The array can also be two-dimensional, i.e. spread over an area, or three dimensional, i.e. distributed within a volume. Another common type of antenna is a printed array formed by lithographic techniques.  
         [0005]     As the number of clients in a network continues to increase, it becomes increasingly hard to avoid interference between wireless clients, even when using an adaptive antenna array. Also, multipath interference can result from reflections and/or diffraction of the client&#39; signal off metal within the building in which the WLAN operates. For reducing interference, it is possible to provide a narrow beam that can be steered toward a desired client using an array. Alternatively, it is possible to steer a “null” toward a potential interference source, where a “null” is an angular distribution in the array antenna pattern of very low gain signal strength.  
         [0006]     In practice, it is difficult and expensive to form a narrow beam, requiring adaptive arrays with more elements and a high level of precise calibration. However, such arrays are undesirably expensive, due to the level of testing and calibration. Also, with potential sales volumes of several hundred thousand antenna arrays per year, such handling slows down production in addition to adding to the expense, thereby further reducing production efficiency.  
         [0007]     Without calibration and testing, presently available lithographic techniques allow the construction of printed arrays having great precision, having a tolerance of +/−0.003″. An error of 0.005″ in a printed array has been found to produce a small wavelength error of only 0.2% at the 5.0 GHz band. Thus, an array as manufactured would have very desirable performance, except for the expense accounted in testing and calibration.  
       SUMMARY OF THE INVENTION  
       [0008]     The difficulties and drawbacks of previous type schemes are resolved by the method and implementation of wireless communication according to the present invention in which wireless signals are exchanged between at least one remote client and a directional antenna array associated with a wireless network and located at an access point (AP), wherein the directional antenna array includes a plurality of antenna elements. A statistical matrix analysis is performed for each of the at least one client and the antenna array, in order to locate angles associated with directions of each client with respect to the antenna array using either MUSIC, ESPRIT or some other suitable method. Values are determined for weighting factors for RF signals of each of the respective plurality of antenna elements, so as to create predetermined phase differences between the signals of the plurality of antenna elements. The predetermined phase differences are used to direct at least one null toward at least one source of interference, so as to avoid signal interference. (These same weights are used to steer a wide angle, low precision, beam as well.)  
         [0009]     As will be realized, the invention is capable of other and different embodiments and its several details are capable of modifications in various respects, all without departing from the invention. Accordingly, the drawing and description are to be regarded as illustrative and not restrictive. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  depicts an exemplary directional antenna array.  
         [0011]      FIGS. 2A and 2B  respectively illustrate signal reception and broadcasting as performed with an exemplary directional antenna array.  
         [0012]      FIGS. 3A and 3B  respectively illustrate the signal strength distributions for a directional antenna in a perpendicular direction and steered at an angle of 60 degrees.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0013]     In the present invention, signal interference is avoided by the method and implementation of the present invention by steering wide, deep nulls in the direction of interference, e.g. multipath sources or interfering clients and steering rudimentary beams in the desired directions. By creating wide nulls and beams with the present invention, normal manufacturing methods suffice and the positional error of the array can be accommodated, and an uncalibrated antenna array can be employed. In this way, the expensive and time consuming steps of array calibration and testing can be eliminated, thereby considerably reducing expense and increasing efficiency.  
         [0014]     The present invention uses a novel technique of subspace beamforming and wide-null forming using the nominal array manifold to compute suitable weighting factors, for the antenna elements in a steerable, directional antenna array. The present invention can be used with a one-dimensional linear array, or with a two-dimensional or three-dimensional array of arbitrary topology.  
         [0015]     As shown in  FIG. 1 , an antenna array  10  includes a plurality of antenna elements  12  for sending and receiving wireless signals. Each antenna includes an RF converter  14  for converting between baseband electrical signals and radio frequency (RF) wireless signals. Each digital baseband signal is preferably processed using quadrature signals. With quadrature, digital data in the baseband the signal is modulated in two distinct channels (I and Q channels). The I and Q channels are each modulated on carriers of the same frequency, one varying as the cosine and the other varying as the sine of the frequency, so that the channels are 90 degrees out of phase with each other and thus will not interfere. In this way, the baseband signal S is a “symbol,” i.e. a complex signal having components d 1  and d 2  such that: 
 
 S=d   1   +jd   2 , 
 
 where j=√−1 
 
 S=d   1   +jd   2  
 
 where j=√{square root over (−1)} and d 1  and d 2  are the baseband data streams for the I and Q channel respectively. Each of the antenna elements  12  include a multiplier  16  for applying a weighting factor ω 0 , ω 1 , ω 2 , ω 3 , etc. to the outgoing or ingoing RF signal during broadcast mode. The weights ω 0  through ω 3  are complex and are used to create phase differences in the signal, as will be explained in greater detail below. An adder/splitter  18  is used to multiplex the incoming RF signals from the antennas  12 , so as to forward the signals to the network. From the adder/splitter  18 , the signals are directed to the PHY layer, also known as the baseband processor, which takes the “symbols” from the antenna array  10  and converts them to bits that can be processed by the network. In broadcast mode, the adder/splitter  18  simply sends the signal from the PHY to each of the multipliers or each respective antenna. The adder/splitter  18  and the multipliers  16  in combination constitute a beamformer  20  for the antenna array  10 . It is understood that, while only four antenna elements are depicted in  FIG. 1 , any number can be employed without departing from the invention. (Any modulation method can be used as long as one can generate a quadrature signal.) In the process of the present invention, it is necessary to perform a statistical matrix analysis for each client associated with the antenna array  10 . As will be made clear below, the matrix analysis will be used in order to locate beams and nulls associated with the direction of each client with respect to the coordinate system of the antenna array  10 . In this way, the present invention will determine the values for the array weights used in the beamformer, to create phase differences that allow the steering of nulls towards interference sources and beams towards the desired clients. 
 
         [0016]      FIG. 2A  depicts the antenna array  10  with the antenna elements  12  receiving a signal from a client. The client is at a sufficient distance from the array  10  that the signal wavefront can be approximated as a plane wave. For simplicity, the antenna array  10  is shown only in a two-dimensional X-Y plane, though a generalization in a three-dimensional coordinate system can easily be arrived at using the known formulae for depicting electromagnetic propagation. The measured signal strength E at each antenna element is expressed as: 
   {right arrow over (E)}={right arrow over (E)}   o   e   −i(ωt−{right arrow over (k)}·{right arrow over (r)})    
 where {right arrow over (r)} is the observation point (i.e. antenna location) for measuring the field and {right arrow over (k)} is the propagation direction of the wavefront, and {right arrow over (k)}·{right arrow over (r)} is the phase of the measure signal determined by the observation point. The antenna elements  12  are taken to lie along the x-axis of our coordinate system and the array is assumed to be a uniform linear array, so that the signal phase is:  
           k   →     ·     r   →       =         2   ⁢           ⁢   π     λ     ×     cos   ⁡     (   φ   )               
 where λ is the wavelength of the client frequency f such that λ=c/f where c is the speed of light, and φ is the angle of incidence of the signal wavefront. 
 
         [0017]     Each antenna element  12  is separated from each other by a distance d where an element  12  is located at the origin (x=0). Thus, each antenna element  12  will have a phase difference of signal reception such that:  
           for   ⁢           ⁢   x     =   0     ,           ⁢           k   →     ·     r   →       =   0     ;         
           for   ⁢           ⁢   x     =   d     ,           ⁢           k   →     ·     r   →       =         2   ⁢   π   ⁢           ⁢   d     λ     ⁢     cos   ⁡     (   φ   )           ;         
             for   ⁢           ⁢   x     =     2   ⁢           ⁢   d       ,           ⁢           k   →     ·     r   →       =         4   ⁢   π   ⁢           ⁢   d     λ     ⁢     cos   ⁡     (   φ   )           ;       ⁢               
           for   ⁢             ⁢             ⁢   x     =     n   ⁢           ⁢   d       ,           ⁢           k   →     ·     r   →       =         n2π   ⁢           ⁢   d     λ     ⁢     cos   ⁡     (   φ   )           ;         
 
 so that the total received signal strength for an n-element array  10  would be:  
         E   n     ∝     1   +     ⅇ       -   ⅈ     ⁢       2   ⁢           ⁢   π   ⁢           ⁢   d     λ     ⁢   cos   ⁢           ⁢   φ       +     ⅇ       -   ⅈ     ⁢       4   ⁢           ⁢   π   ⁢           ⁢   d     λ     ⁢   cos   ⁢           ⁢   φ       +   …   +     ⅇ       -   ⅈ     ⁢       2   ⁢   n   ⁢           ⁢   π   ⁢           ⁢   d     λ     ⁢   cos   ⁢           ⁢   φ             
 
 Another way of expressing these phases is by defining a new vector called the array manifold defined as  
         a   ⁡     (   φ   )       =       (     1   ,     ⅇ       -   ⅈ     ⁢       2   ⁢           ⁢   π   ⁢           ⁢   d     λ     ⁢   cos   ⁢           ⁢   φ       ,     ⅇ       -   ⅈ     ⁢       4   ⁢           ⁢   π   ⁢           ⁢   d     λ     ⁢   cos   ⁢           ⁢   φ       ,   …   ⁢           ,     ⅇ       -   ⅈ     ⁢       2   ⁢   n   ⁢           ⁢   π   ⁢           ⁢   d     λ     ⁢   cos   ⁢           ⁢   φ         )     .         
 
         [0018]     When the array is used in transmission, as shown in  FIG. 2B , each antenna element  12  is radiating in all directions in the X-Y plane. However, the phase differences between each element  12  are such that the received signal strength E located at an angle φ is the same as E n  shown above.  FIG. 3A  shows the radiation pattern for the antenna array  10  corresponding to the above conditions, where the electric field strength is maximum along the axis (φ=O) and approaches zero for φ=+/−90°.  
         [0019]     In order to transmit a signal toward a client located off-axis, e.g. 60°, it is necessary to adjust the phases of the antenna elements  10  so as to produce a signal maximum centered along φ=60°, as shown in  FIG. 3B . This is accomplished by using the multipliers  16  to apply suitable weighting factors ω 0 , ω 1 , ω 2 , . . . ω n  to each antenna element  12  in an n-element array  10 . This changes the phases of the RF signals transmitted from each antenna element to produce a signal E′ such that:  
         E   ′     =         ω   0     ⁢     ⅇ       -   ⅈ     ⁢           ⁢   0         +       ω   1     ⁢     ⅇ       -   ⅈ     ⁢           ⁢   dcosφ         +   …   +       ω   n     ⁢     ⅇ       -   ⅈ     ⁢           ⁢   ndcosφ               
         or   ⁢           ⁢     E   ′       =       ∑   0   n     ⁢           ⁢       ω   n     ⁢     ⅇ       -   ⅈ     ⁢           ⁢   dcosφ               
 
         [0020]     In order to steer wide, deep nulls toward interference sources, it is necessary to determine weighting factors ω n  such that the radiation distribution is negligible in the direction of interference sources. The first step in the process is to sample the complex baseband signals from each antenna element  12  in the array  12 , so as to obtain “snapshots” of signals from a particular client. This can be done during the initial association of the client to the access point or during subsequent communications with the access point. The sampled signals X for a three element array are expressed in vector form as follows: 
 
 X   T   ={x   0   ,x   1   ,x   3 }.
 
 The sampled signals are used to build up a “covariance matrix” R such that: 
 
 R=XX   H  
 
 i.e., R is the direct product of X and X H , the Hermitian transpose of vector X. For a matrix the Hermitian tranpose is obtained by taking the transpose of the matrix followed by the complex conjugation of each element in the matrix. In the case of a vector, the original vector, if a column vector, is changed into a row vector followed by a complex conjugation of each element in the vector. In the case of a row vector, the transpose results into a column vector. For the purpose of our discussion a non-transposed vector is assumed to be a column vector. In this way, for a three-element antenna array, the covariance matrix is a 3×3 matrix such that:  
               x   0     ⁢     x   0   *               x   0     ⁢     x   1   *               x   0     ⁢     x   2   *                   x   1     ⁢     x   0   *               x   1     ⁢     x   1   *               x   1     ⁢     x   2   *                   x   2     ⁢     x   0   *               x   2     ⁢     x   1   *               x   2     ⁢     x   2   *               
 
 where the values in this matrix and all either auto-correlations or cross-correlations. The covariance matrix R is itself Hermitian, i.e., R=R H , which is to say, if we take the Hermitian transpose of R, we get R back again. 
 
         [0021]     Upon building up the covariance matrix of sampled values from the client signal, the covariance matrix undergoes an “eigen-decomposition” for determining eigenvalues and eigenvectors of the covariance matrix. The equation used for this is given by 
 
 RV   i =λ i   V   i  
 
 where V i  is the i&#39;th eigenvector, R is the covariance matrix and λ i  is the i&#39;th eigenvalue. Of course, it is appreciated that there are as many eigenvalues i as there are rows or columns in the matrix, i.e. for an n×n matrix, there are n eigenvalues. 
 
         [0022]     After the eigen-decomposition is performed, the eigenvalues and eigenvectors are recorded into a table. These eigenvectors are used as weights to produce the steering vector for forming the beam in the direction of the client. Note that one or more eigenvectors corresponding to the largest eigenvalues are used to build the steering vector. In the preferred embodiment, we may assume that the propagation path is reciprocal, and, the same eigenvectors can be used to transmit and receive messages. The array weights, i.e. dominant eigenvectors, recorded in the table are used by the beamformer  20  to steer the energy of the beam. Since the steering only requires calculating the dominant eigenvectors corresponding to the largest eigenvalues, the step of eigen-decomposition is rapid, if one simply calculates the largest eigenvalues and eigenvectors. Thus, it is not necessary to calculate the full eigen-decomposition.  
         [0023]     After computing eigenvalues, it is necessary to determine the direction of arrival of the client signal. Several approaches are known for calculating the direction of arrival, and any could be contemplated without departing from the invention. For example, in one aspect, the array radiation pattern is computed for the dominant eigenvector used as array weights and the signal peak is searched for as a function of angle. In the preferred embodiment, a complimentary projection operator is built from the computed eigenvector. An incident angle is then found corresponding to the maximum distance from the “subspace” defined by the dominant eigenvector and the “array manifold” defined by the separations of antenna elements in the antenna array.  
         [0024]     The dominant eigenvector V is used to generate a matrix A such that: 
 
 A=VV   H  
 
         [0025]     A “projection operator” P for A is found such that: 
 
 P=AA   H  
 
 which when operating on a general vector projects the vector onto the column space of the matrix A. The complimentary projection operator P′ is given as: 
 
 P′=I−P  
 
 where I is the identity matrix. In this way, the complementary projection operation P′, when operating on a general vector, projects the vector onto a space perpendicular to the column space of A. When the projection operator operates on the array manifold the resulting vector will have a maximum when the angle used to compute the array manifold is equal to the angle of incidence. When the complementary projection operator is used there will be a minimum at the angle of incidence. In this way, the incident angle of the client signal can be derived. The computed angle and the eigenvectors constitute the “spatial signature” for the client. These values are saved by the access point to assist in the forming of the steering vectors and determine which clients can access the channel at the same time. 
 
         [0026]     In an alternative embodiment, Capon&#39;s method, MUSIC and ESPRIT, etc. could also be used to compute the angle of incidence.  
         [0027]     The access point housing the array  10  evaluates the spatial signatures and forms nulls in the steering vectors, so that the nulls can be directed toward any nearby clients or other potentially interfering sources. If two or more clients have adequate angular separation from the position of the antenna array  10  as indicated by their spatial signatures, the access point will compute suitable array steering vectors for each client. These steering vectors will then be used for both transmission and reception of messages from each respective client. The nulls are formed by computing an integrated direct product of the array manifold over the angular range needed to control interference, such that:  
       D   =       ∫     θ   1       θ   2       ⁢       A   ⁡     (   φ   )       ⁢       A   T     ⁡     (   φ   )       ⁢           ⁢     ⅆ   φ             
 
 where θ 1  and θ 2  represents the width of the null, e.g. from 40 degrees to 60 degrees. This matrix D is then diagonalized and the eigenvectors used to form a complementary projection operator for the column space spanned of the original integrated matrix formed by the direct product of the array manifold. This complementary projection operator is applied to the steering vector for the client and results in a new steering vector that produces a wide null in the array pattern at the desired position. 
 
         [0028]     The present invention offers simplicity in operating and permits the use of uncalibrated arrays, resulting in reduced manufacturing steps, thereby improving efficiency. Also, by steering nulls, performance is greatly improved. In these ways, the invention offers substantial savings with increased performance.  
         [0029]     As described hereinabove, the present invention provides improvements in efficiency and performance over previous type methods and implementations. However, it will be appreciated that various changes in the details, materials and arrangements of parts which have been herein described and illustrated in order to explain the nature of the invention may be made by those skilled in the area within the principle and scope of the invention will be expressed in the appended claims.