Abstract:
A method is offered of automatically beamforming a radio frequency transmitter having an array antenna. The beamformed signal is transmitted for the benefit of a target communication unit based upon characteristics of a received signal. The method includes the steps of determining a transmit equalizer transfer function and receive equalizer transfer function for each array element of the antenna array based, at least in part, upon application of common input signals and comparison of outputs. The method further includes adaptively filtering a received signal, from a communication unit based, at least in part, upon the determined receive equalizer weights, to provide a receive beamform array. A beamformed signal may then be transmitted to the communication unit based upon the transmit equalizer weights and receive beamform array.

Description:
FIELD OF THE INVENTION 
     The field of the invention relates to beam forming of radio frequency signals and more specifically to adaptive beam forming of radio frequency signals. 
     BACKGROUND OF THE INVENTION 
     Beamformers are known. Such devices may be used to direct radio frequency (RF) energy (emissions) to a specific target at a specific location. Such directed RF emissions (&#34;transmit beamforming&#34;) may be accomplished through the use of directional antenna(s) or through the use of antenna arrays. Where antenna arrays have been used the characteristics of the RF emissions may be influenced by array element positioning or by a mathematical weighting of outputs from array elements. 
     While the process of transmit beamforming may not be difficult, the location to which an RF emission is to be directed may not be readily identifiable. Where the source is a radar transponder, the solution is simplified in that the operator simply selects the direction of transmission and waits for a response. Where, on the other hand, the target is a mobile communication unit then the situation may be considerably more difficult. Transmit beamforming relative to mobile communication units is typically based upon some type of locational feedback from the target. 
     Methodologies of maximizing a receive signal (&#34;receive beamformers&#34;) are also known. Receive beamformers typically receive a signal from an antenna and, through a process of mathematical analysis (or select a set of receive characteristics, maximizing receive signal quality. Where the antenna is a directional antenna the antenna may simply sweep an arc (containing the target) seeking the point of maximum signal strength from a desired target. 
     Antenna arrays may also be configured as receive beamformers through adjustments to physical positioning of array elements, or through adaptive filtering. Changing the positioning of array elements, on the other hand, may lead to unexpected results and loss of signal integrity. Adjustments to positioning of array elements also interferes with reception of RF signals from outside a selected beam area. 
     In general, where signals must be simultaneously received from large numbers of geographically dispersed communication units, physical positioning of antenna element is not practical. Where physical positioning of antenna elements is not practical, receive beamforming may be performed through mathematical analysis of signals received through a multitude of antenna elements. 
     Where receive beamforming is performed through mathematical analysis, the beamformer may exist in a mathematical sense only and may be considered a subset of adaptive filtering (see Adaptive Filter Theory, 2nd ed., Simon Haykin, Prentice Hall, 1991). The receive beamformer, in such case, may be considered as a form of spatial filter attenuating all but selected signals. Since a set of input signals from an antenna array may be received and stored, any number of receive beamformers may operate upon a given set of stored data to produce any number of signals from stored input data. 
     A cellular radiotelephone system is an example of a situation where receive beamforming may be performed through adaptive filtering (adaptive beamforming). Adaptive beamforming in such a system is typically performed at a base site which includes an antenna array and through which a number of simultaneous communication transactions may occur. 
     Adaptive beamforming, in general, may be performed through calculation of a set of antenna array weights. The set of antenna array weights minimizing interference may be calculated using measurements from the array when both a known desired signal and interferers are present. The set of weights may then be used to cancel interference during periods when the desired signal is not known, provided that the location of the sources of interference and the desired signal remain substantially constant. The weights which minimize the interference may be calculated by solving the complex equation as follows: 
     
         Xw=y 
    
     The value, X, is a N×M matrix of array (signal) (simultaneously sampled array outputs), where N is the number of snapshots, and M is the number of antenna elements. ##EQU1## The value, y, is the N×1 vector of the (known) desired signal. ##EQU2## The value w is an adaptive array weight vector (M×1) for all array elements. ##EQU3## Given the weight vector, w, the adaptive output of the beamformer may be computed at any time, t: ##EQU4## 
     While receive beamformers have worked well, an antenna array is typically required as a prerequisite for receive beamforming. Portable communication units (because of size and weight limitations) are typically not equipped with antenna arrays. 
     An alternative to receive beamforming (at a portable) is transmit beamforming at a base site. Transmit beamforming at a base site may allow significant signal energy to be directed to the location of a portable without significantly interfering with reception by another portable. 
     Transmit beamforming, on the other hand, has proved difficult (in practice) because of the difficulty of determining transmit beamform array coefficients. Part of the difficulty of determining transmit coefficients lies in the fact that the coefficients of a receive beamform array used in beamforming a received signal have very little relationship to the coefficients of beamforming a transmitted signal. Phase differences and non-linearities in receive and transmit elements make receive beamform arrays inapplicable to beamforming a transmitted signal. Because of the importance of mobile communications a need exists for a simpler method of beamforming transmitted signals from base sites to portable communication units. 
     SUMMARY OF THE INVENTION 
     A method is offered of automatically beamforming a radio frequency transmitter having an array antenna. The method includes the steps of determining a transmit equalizer transfer function and receive equalizer transfer function for each array element of the antenna array at least in part, upon application of common input signals and comparison of outputs. The method further includes adaptively beamforming a received signal, from a communication unit based, at least in part, upon the determined receive equalizer weights, to provide a receive beamform array. A beamformed signal may then be transmitted to the communication unit based upon the transmit equalizer weights and receive beamform array. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 depicts a communication system, in accordance with the invention. 
     FIG. 2 comprises a block diagram of an apparatus for beamforming a signal, in accordance with the invention. 
     FIG. 3 is a schematic representation of signal flow for calculating transmit differential equalizer weights in accordance with the invention. 
     FIG. 4 depicts a flow chart of transmit beamforming, in accordance with the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The solution to the problem of beamforming a transmitted signal from a base site to a mobile communication unit lies, conceptually, in the development of substantially identical transfer functions for transmit and receive antenna array elements and using a receive beamform array, calculated for a received signal, for transmit beamforming a transmitted signal. Substantially identical transfer functions between transmit and receive array elements may be developed by self-calibration and by calibration of array elements against reference signals. 
     Shown in FIG. 1 is a communication system, generally, (10) in accordance with the invention. Included within such a system (10) is a resource controller (40), transceiver (30), and communication units (22, 23, and 24). The transceiver (30) exchanges communicated signals with communication units (22-24) through an antenna array depicted in FIG. 1 as a single antenna (20). 
     Also included in FIG. 1 is a remote transceiver (25). The remote transceiver (25), in accordance with the invention, is interconnected with the resource controller (40) through use of a data bus (26) (e.g. a &#34;T1&#34; line) for exchange test signals with transceiver 30. (It should be emphasized that the transmitter and receiver of the transceiver (25) must be co-located.) 
     Shown in FIG. 2 is an expanded block diagram of the system (10), including transmit beamforming apparatus in accordance with the invention. As shown (FIG. 2) the antenna array (20, FIG. 1) includes antennas #1-N. As shown each antenna (#1-N) (FIG. 2) has an associated duplex switch (31, 34, or 37), transmitter (33, 36, or 39), and receiver (32, 35, or 38). 
     Turning now to FIG. 4 a flow chart of transmit beamforming under the invention is shown. Reference will be made to the flow chart (FIG. 4) as appropriate in understanding the invention. 
     Each receiver (32, 35, 38) has a receive equalizer (H r   i  (z)) (41, 43, and 45) and a weighting factor (w r   i ) (47, 49, and 51) through which a received signal passes. A summer (54) provides a summation of weighted input signals from the elements of the antenna array (20). The output of the summer (54) is, in turn, applied to a demodulator (55) for decoding of the received signal. 
     Transmitters (33, 36, and 39), likewise, receive an input signal through a modulator (56), weighting factor (48, 50, or 52), and equalizer (42, 44, or 46). The values of the weighting factors for transmit and receive, in accordance with the invention, are complex conjugates (e.g. w r   1  (47)=w t   1  *(48), etc). 
     Transmit and receive equalizers (H r   1  (z) and H t   1  (z), or H r   2  (z) and H t   2  (z), to H r   N  (z) and H t   N  (z)) provide transfer functions which allow for a complex conjugate relationship of transmit and receive characteristics among corresponding transmit and receive elements (w r   i  and w t   i ) of the antenna array (20). A receive beamform array (w r   1  -w r   N ) developed in response to a received signal, in accordance with the invention, is then conjugated to form a transmit beamform array (w t   1  -w t   N ). 
     The order p receive equalizer weights (H r   1  (z), H r   2  (z) . . . H r   N  (z)) are computed by modeling the response needed to force the ith receiver output to match the output of a reference receiver (e.g. #1 receiver) as an all-zero frequency transfer function. The input to the antenna array (20) for calculating receive equalizer weights is the remote transceiver (25, FIG. 1) located at a distance from the array (20). Receive equalizer transfer functions (H r   1  (z), H r   2  (z), to H r   N  (z)) are calculated by solving the vector equation as follows: 
     
         Y.sub.i v.sub.i =y1 
    
     where Y i  is the M×p (M rows, p columns) matrix of outputs, where yi(t) indicates the output of the ith element at time t, of antenna#i: ##EQU5## y1 is the M×1 vector of outputs of the reference antenna #1: 
     
         y1=y1(0)y1(1) . . . y1(M-1), 
    
     and v i  is the equalizer weight vector (p×1) for the ith antenna: 
     
         v.sub.i =v.sub.i (0)v.sub.i (1) . . . v.sub.i (o-1) 
    
     The equation (Y i  v i  =y1) may then be solved (101) by a signal processor (not shown) within the resource controller (40) for v i  using an appropriate least squares method. Given the weight vectors v i , the equalizer transfer functions are given as follows (for all array elements): ##EQU6## 
     The transmit equalizer transfer functions (H t   1  (z), H t   2  (z) . . . H t   N  (z)) are computed using a two-step process. In the first step, of the two-step process, a self-equalizer weight is calculated (103). In the second step, a differential equalizer weight is determined (104) based upon the previously calculated self-equalizer weights. 
     In each step of the two-step process a transmit array element equalizer value is computed by modeling the response needed. In the case of the self-equalizer, a value is calculated to normalize the ith transmitter output to match the input of the ith element. In the case of the differential equalizer a value is calculated to force the output of the ith transmitter to match the output of a reference transmitting element (e.g. element #1). 
     The self-equalizer weight vector (c i ) is calculated by reference to a signal received at the remote transceiver (26) upon application of a set of known, distinct (linearly indendent) input signals to the antenna array (20). The received signal at the remote (r) is a linear combination of the transmitted signals and may be expressed using M transmitted samples for each of the N transmitters and order L models of the transmitters. The self-equalizer weight vector (c i ) may then be determined by solving the equation as follows: 
     
         Xc=r 
    
     where X is the M×NL matrix of inputs to all elements of the array (e.g. X=X 1  X 2  . . . X N ) and, ##EQU7## r is the M×1 vector of outputs of the remote receiver: 
     
         r=r(0)r(1) . . . r(M-1), and 
    
     c is the equalizer weight vector (NL×1) for all array elements: 
     
         c=c.sub.i (0)c.sub.i (1) . . . c.sub.i (L-1) 
    
     the equation (Xc=r) may be solved (103) using an appropriate least squares method. (Note that since X is known, much of the computation needed to find c can be performed once, in advance.) In order for the transmitter outputs to be identical, the inverse of the models of the transmitters could be used. The equalizer transfer functions would therefore be all-pole of order L-1 as follows: ##EQU8## However the transfer function (H t1   i  (z)) is not necessarily stable in that there is no guarantee that the all-zero transmitter models are minimum phase (all zeros are not necessarily within the unit circle). The models are also likely to be less efficient than differential equalizers, since the self-equalizers do not exploit the similarities of outputs between transmitters under conditions of a common input signal. 
     Given the transmitter model weights, c i , differential equalizers can be calculated (104) by simulating the outputs of each transmitter and matching the output of each element to the reference element. Such a process can be depicted in block diagram form by reference to FIG. 3. 
     The simulated generator (50) produces a wideband signal, such as a pseudo noise sequence, which is filtered by both the reference transmit self equalizer transfer function (51) and by the transmit self equalizer transfer function of array element i (52). Once an output is computed (105) the same method can be used as with the receive differential equalizer weights. In this case, the equation to be solved has the form: 
     
         T.sub.i u.sub.i =t.sub.1 
    
     Again, the simulated reference output can be expressed in matrix form as follows: ##EQU9## where t 1  is the M×1 vector of outputs of the simulated reference transmitter #1: 
     
         t.sub.1 =t.sub.1 (0)t.sub.i (1) . . . t.sub.1 (M-1), and 
    
     v i  is the equalizer weight vector (q×1) for the ith antenna: 
     
         u.sub.i =u.sub.i (0)u.sub.i (1) . . . u.sub.i (q-1) 
    
     The equation (T i  u i  =t 1 ), as above, may be solved by an appropriate least squares method. The equalizer transfer functions would therefore be all-pole of order q-1 and determined (105) as follows: ##EQU10## 
     The beneficial affect of calculating the receive transfer function (H r   i  (z)) and the transmit transfer function (H t   i  (z)) lies in the ability of a base site to beamform a transmit signal to a mobile communication unit (22-24) based upon the receive transfer function (H r   i  (z)), the transmit transfer function (H t   i  (z)), and receive beamform coefficients. 
     In accordance with the invention a receive equalizer transfer function and transmit equalizer transfer function for the system (10) is calculated as described above. A communication unit (22) then begins transmitting a signal to the antenna array (10). A receive beamform array is calculated using the receive equalizer transfer function. A transmit beamformed signal may then be beneficially returned to the communication unit using the transmit equalizer transfer function and complex conjugate of the receive beamform array. 
     In another embodiment of the invention the transmit equalizer transfer functions (H t   1  (z), H t   2  (z) . . . H t   N  (z)) are calculated using a single step process. Under such a process the transmit equalizer transfer functions (H t   1  (z), H t   2  (z) . . . H t   N  (z)) are calculated using either self equalizer values, or, differential equalizer values. A transmit beamformed signal may then be created as above. 
     In another embodiment of the invention the receive transfer function (H r   1  (z), H r   2  (z) . . . H r   N  (z)) is calculated by reference to a known signal transmitted by the remote (25). Under the embodiment the transfer function (H r   1  (z), H r   2  (z). . . H r   N  (z)) is computed by modeling the response needed to force the ith receiver output to match the known input to the remote transceiver (25).