Abstract:
A system and method to facilitate wireless testing of a radio frequency (RF) multiple-input, multiple-output (MIMO) signal transceiver device under test (DUT). With the DUT operating in a controlled electromagnetic environment, the tester transmits multiple test signals wirelessly to the DUT. Signal phases of the respective test signals transmitted by the tester are controlled in accordance with feedback signal data from the DUT. Magnitudes of the respective test signals can also be controlled in accordance with such feedback signal data, thereby enabling dynamic optimization of the wireless communication channel condition number k(H).

Description:
BACKGROUND 
       [0001]    The present invention relates to testing of radio frequency (RF) wireless signal transceivers, and in particular, to testing such devices without a need for RF signal cables for conveyance of RF test signals. 
         [0002]    Many of today&#39;s electronic devices use wireless technologies for both connectivity and communications purposes. Because wireless devices transmit and receive electromagnetic energy, and because two or more wireless devices have the potential of interfering with the operations of one another by virtue of their signal frequencies and power spectral densities, these devices and their wireless technologies must adhere to various wireless technology standard specifications. 
         [0003]    When designing such devices, engineers take extraordinary care to ensure that such devices will meet or exceed each of their included wireless technology prescribed standard-based specifications. Furthermore, when these devices are later being manufactured in quantity, they are tested to ensure that manufacturing defects will not cause improper operation, including their adherence to the included wireless technology standard-based specifications. 
         [0004]    For testing these devices following their manufacture and assembly, current wireless device test systems (“testers”) employ a subsystem for analyzing signals received from each device. Such subsystems typically include at least a vector signal generator (VSG) for providing the source signals to be transmitted to the device, and a vector signal analyzer (VSA) for analyzing signals produced by the device. The production of test signals by the VSG and signal analyses performed by the VSA are generally programmable so as to allow each to be used for testing a variety of devices for adherence to a variety of wireless technology standards with differing frequency ranges, bandwidths and signal modulation characteristics. 
         [0005]    Calibration and performance verification testing of a device under test (DUT) are typically done using electrically conductive signal paths, such as RF cables, rather than wireless signal paths, by which a DUT and tester communicate via electromagnetic radiation. Accordingly, the signals between the tester and DUT are conveyed via the conductive signal path rather than being radiated through ambient space. Using such conductive signal paths helps to ensure repeatability and consistency of measurements, and eliminates positioning and orientation of the DUT as a factor in signal conveyance (transmission and reception). 
         [0006]    In the case of a multiple input, multiple output (MIMO) DUT, a signal path must be provided, in some form, for each input/output connection of the DUT. For example, for a MIMO device intended to operate with three antennas, three conductive signal paths, e.g., cables and connections, must be provided for testing. 
         [0007]    However, using conductive signal paths significantly impacts the time needed for testing each DUT due to the need for physically connecting and disconnecting the cables between the DUT and tester. Further, in the case of a MIMO DUT, multiple such connecting and disconnecting actions must be performed, both at the beginning and termination of testing. Further, since the signals being conveyed during testing are not radiated via the ambient space, as they would be in the normally intended use, and the antenna assemblies for the DUT are not in use during such testing, such testing does not simulate real world operation and any performance characteristics attributable to the antennas are not reflected in the test results. 
         [0008]    As an alternative, testing could be done using test signals conveyed via electromagnetic radiation rather than electrical conduction via cables. This would have the benefit of requiring no connecting and disconnecting of test cables, thereby reducing the test time associated with such connections and disconnections. However, the “channel” in which the radiated signals and receiver antennas exist, i.e., the ambient space through which the test signals are radiated and received, is inherently prone to signal interference and errors due to other electromagnetic signals originating elsewhere and permeating the ambient space. Such signals will be received by the DUT antennas and can include multipath signals from each interfering signal source due to signal reflections. Accordingly, the “condition” of the “channel” will typically be poor compared to using individual conductive signal paths, e.g., cables, for each antenna connection. 
         [0009]    One way to prevent, or at least significantly reduce, interference from such extraneous signals, is to isolate the radiated signal interface for the DUT and tester using a shielded enclosure. However, such enclosures have typically not produced comparable measurement accuracy and repeatability. This is particularly true for enclosures that are smaller than the smallest anechoic chambers. Additionally, such enclosures tend to be sensitive to the positioning and orientation of the DUT, as well as to constructive and destructive interference of multipath signals produced within such enclosures. 
         [0010]    Accordingly, it would be desirable to have systems and methods for testing wireless signal transceivers, and particularly wireless MIMO signal transceivers, in which radiated electromagnetic test signals can be used, thereby simulating real world system operation as well as avoiding test time otherwise necessary for connecting and disconnecting test cabling, while maintaining test repeatability and accuracy by avoiding interfering signals due to externally generated signals and multipath signal effects. 
       SUMMARY 
       [0011]    In accordance with the presently claimed invention, a system and method are provided to facilitate wireless testing of a radio frequency (RF) multiple-input, multiple-output (MIMO) signal transceiver device under test (DUT). With the DUT operating in a controlled electromagnetic environment, the tester transmits multiple test signals wirelessly to the DUT. Signal phases of the respective test signals transmitted by the tester are controlled in accordance with feedback signal data from the DUT. Magnitudes of the respective test signals can also be controlled in accordance with such feedback signal data, thereby enabling dynamic optimization of the wireless communication channel condition number k(H). 
         [0012]    In accordance with one embodiment of the presently claimed invention, a system to facilitate wireless testing of a radio frequency (RF) multiple-input, multiple-output (MIMO) signal transceiver device under test (DUT) includes a structure, an electrically conductive signal path, a plurality of antenna arrays and RF signal control circuitry. The structure defines interior and exterior regions and configured to allow placement of a DUT within the interior region and be substantially isolated from electromagnetic radiation originating from the exterior region. The electrically conductive signal path is to couple to the DUT and convey one or more electrical signals between the interior and exterior regions. The plurality of antenna arrays, each of which includes multiple antenna elements, is disposed at least partially within the interior region to radiate respective pluralities of phase-controlled RF test signals. The RF signal control circuitry is coupled to the electrically conductive signal path and the plurality of antenna arrays, and responsive to a plurality of signal data from the DUT related to the respective pluralities of phase-controlled RF test signals and conveyed via the one or more electrical signals, and to a plurality of RF test signals by: replicating each one of the plurality of RF test signals to provide respective pluralities of replica RF test signals; and controlling, in accordance with the plurality of signal data, respective phases of at least a portion of each one of the respective pluralities of replica RF test signals to provide the respective pluralities of phase-controlled RF test signals. 
         [0013]    In accordance with another embodiment of the presently claimed invention, a method of facilitating wireless testing of a radio frequency (RF) multiple-input, multiple-output (MIMO) signal transceiver device under test (DUT) includes providing a structure, an electrically conductive signal path and a plurality of antenna arrays. The structure defines interior and exterior regions and is configured to allow placement of a DUT within the interior region and be substantially isolated from electromagnetic radiation originating from the exterior region. The electrically conductive signal path is to couple to the DUT and convey one or more electrical signals between the interior and exterior regions. The plurality of antenna arrays, each of which includes multiple antenna elements, is disposed at least partially within the interior region to radiate respective pluralities of phase-controlled RF test signals. Further included is responding to a plurality of signal data from the DUT related to the respective pluralities of phase-controlled RF test signals and conveyed via the one or more electrical signals, and to a plurality of RF test signals by: replicating each one of the plurality of RF test signals to provide respective pluralities of replica RF test signals; and controlling, in accordance with the plurality of signal data, respective phases of at least a portion of each one of the respective pluralities of replica RF test signals to provide the respective pluralities of phase-controlled RF test signals. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  depicts a typical operating and possible testing environment for a wireless signal transceiver. 
           [0015]      FIG. 2  depicts a testing environment for a wireless signal transceiver using a conductive test signal path. 
           [0016]      FIG. 3  depicts a testing environment for a MIMO wireless signal transceiver using conductive signal paths and a channel model for such testing environment. 
           [0017]      FIG. 4  depicts a testing environment for a MIMO wireless signal transceiver using radiated electromagnetic signals a channel model for such testing environment. 
           [0018]      FIG. 5  depicts a testing environment in accordance with exemplary embodiments in which a MIMO DUT can be tested using radiated electromagnetic test signals. 
           [0019]      FIG. 6  depicts a testing environment in which a DUT is tested using radiated electromagnetic test signals within a shielded enclosure. 
           [0020]      FIGS. 7 and 8  depict exemplary embodiments of testing environments in which a wireless DUT is tested using radiated electromagnetic test signals in a shielded enclosure with reduced multipath signal effects. 
           [0021]      FIG. 9  depicts a physical representation of a shielded enclosure in accordance with an exemplary embodiment for use in the testing environments of  FIGS. 7 and 8 . 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    The following detailed description is of example embodiments of the presently claimed invention with references to the accompanying drawings. Such description is intended to be illustrative and not limiting with respect to the scope of the present invention. Such embodiments are described in sufficient detail to enable one of ordinary skill in the art to practice the subject invention, and it will be understood that other embodiments may be practiced with some variations without departing from the spirit or scope of the subject invention. 
         [0023]    Throughout the present disclosure, absent a clear indication to the contrary from the context, it will be understood that individual circuit elements as described may be singular or plural in number. For example, the terms “circuit” and “circuitry” may include either a single component or a plurality of components, which are either active and/or passive and are connected or otherwise coupled together (e.g., as one or more integrated circuit chips) to provide the described function. Additionally, the term “signal” may refer to one or more currents, one or more voltages, or a data signal. Within the drawings, like or related elements will have like or related alpha, numeric or alphanumeric designators. Further, while the present invention has been discussed in the context of implementations using discrete electronic circuitry (preferably in the form of one or more integrated circuit chips), the functions of any part of such circuitry may alternatively be implemented using one or more appropriately programmed processors, depending upon the signal frequencies or data rates to be processed. Moreover, to the extent that the figures illustrate diagrams of the functional blocks of various embodiments, the functional blocks are not necessarily indicative of the division between hardware circuitry. 
         [0024]    Referring to  FIG. 1 , a typical operating environment, and ideal testing environment for a wireless signal transceiver (at least in terms of simulating real world operation), would have the tester  100  and DUT  200  communicate wirelessly. Typically, some form of test controller  10 , (e.g., a personal computer) will also be used to exchange testing commands and data via wired signal interfaces  11   a ,  11   b  with the tester  100  and DUT  200 . The tester  100  and DUT  200  each have one (or more for MIMO devices) respective antennas  102 ,  202 , which connect by way of conductive signal connectors  104 ,  204  (e.g., coaxial cable connections, many types of which are well known in the art). Test signals (source and response) are conveyed wirelessly between the tester  100  and DUT  200  via the antennas  102 ,  202 . For example, during a transmit (TX) test of the DUT  200 , electromagnetic signals  203  are radiated from the DUT antenna  202 . Depending upon the directivity of the antenna emission pattern, this signal  203  will radiate in numerous directions, resulting in an incident signal component  203   i  and reflected signal components  203   r  being received by the tester antenna  102 . As discussed above, these reflected signal components  203   r , often the products of multipath signal effects as well as other electromagnetic signals originating elsewhere (not shown), result in constructive and destructive signal interference, thereby preventing reliable and repeatable signal reception and testing results. 
         [0025]    Referring to  FIG. 2 , to avoid such unreliable testing results, a conductive signal path, such as a RF coaxial cable  106 , is used to connect the antenna connectors  104 ,  204  of the tester  100  and DUT  200  to provide a consistent, reliable and repeatable electrically conductive signal path for conveyance of the test signals between the tester  100  and DUT  200 . As discussed above, however, this increases the overall test time due to the time needed for connecting and disconnecting the cable  106  before and after testing. 
         [0026]    Referring to  FIG. 3 , the additional test time for connecting and disconnecting test cabling becomes even longer when testing a MIMO DUT  200   a . In such cases, multiple test cables  106  are needed to connect corresponding tester  104  and DUT  204  connectors to enable conveyance of the RF test signals from the RF signal sources  110  (e.g., VSGs) within the tester  100   a  for reception by the RF signal receivers  210  within the DUT  200   a . For example, in a typical testing environment, the tester for testing MIMO devices will have one or more VSGs  110   a ,  110   b ,  110   n  providing corresponding one or more RF test signals  111   a ,  111   b , . . . ,  111   n  (e.g., packet data signals having variable signal power, packet contents and data rates). Their corresponding test cables  106   a ,  106   b , . . . ,  106   n , connected via respective tester  104   a ,  104   b , . . . ,  104   n  and DUT  204   a ,  204   b , . . . ,  204   n  connectors, convey these signals to provide the received RF test signals  211   a ,  211   b , . . . ,  211   n  for the corresponding RF signal receivers  210   a ,  210   b , . . . ,  210   n  within the DUT  200   a . Accordingly, the additional test time required for connecting and disconnecting these test cables  106  can be increased by a factor n corresponding to the number of test cables  106 . 
         [0027]    As discussed above, using test cables for connecting the tester  100   a  and DUT  200   a  does have the advantage of providing consistent, reliable, and repeatable test connections. As is well known in the art, these test connections  107  can be modeled as a signal channel H characterized by a diagonal matrix  20 , where the diagonal matrix elements  22  correspond to the coefficients h 11 , h 22 , . . . , h nn  for the respective signal channel characteristics (e.g., signal path conductivities or losses for the respective test cables  106 ). 
         [0028]    Referring to  FIG. 4 , in accordance with one or more exemplary embodiments, the conductive, or wired, channel  107  ( FIG. 3 ) is replaced by a wireless channel  107   a  corresponding to a wireless signal interface  106   a  between the tester  100   a  and DUT  200   a . As discussed above, the tester  100   a  and DUT  200   a  communicate test signals  111 ,  211  via respective arrays of antennas  102 ,  202 . In this type of test environment, the signal channel  107   a  is no longer represented by a diagonal matrix  20 , but is instead represented by a matrix  20   a  having one or more non-zero coefficients  24   a ,  24   b  off of the diagonal  22 . As will be readily understood by one skilled in the art, this is due to the multiple wireless signal paths available in the channel  107   a . For example, unlike a cabled signal environment in which, ideally, each DUT connector  204  receives only the signal from its corresponding tester connector  104 . In this wireless channel  107   a , the first DUT antenna  202   a  receives test signals radiated by all of the tester antennas  102   a ,  102   b , . . . ,  102   n , e.g., corresponding to channel H matrix coefficients h 11 , h 12 , . . . , and h 1n . 
         [0029]    In accordance with well known principles, the coefficients h of the channel matrix H correspond to characteristics of the channel  107   a  affecting transmission and reception of the RF test signals. Collectively, these coefficients h define the channel condition number k(H), which is the product of the norm of the H matrix and the norm of the inverse of the H matrix, as represented by the following equation: 
         [0000]        k ( H )=∥ H∥*∥H   −1 ∥
 
         [0030]    The factors affecting these coefficients can alter the channel condition number in ways that can create measurement errors. For example, in a poorly conditioned channel, small errors can cause large errors in the testing results. Where the channel number is low, small errors in the channel can produce small measurements at the receive (RX) antenna. However, where the channel number is high, small errors in the channel can cause large measurement errors at the receive antenna. This channel condition number k(H) is also sensitive to the physical positioning and orientation of the DUT within its testing environment (e.g., a shielded enclosure) and the orientation of its various antennas  204 . Accordingly, even if with no extraneous interfering signals originating elsewhere or arriving via reflections and impinging on the receive antennas  204 , the likelihood of repeatable accurate test results will be low. 
         [0031]    Referring to  FIG. 5 , in accordance with one or more exemplary embodiments, the test signal interface between the tester  100   a  and DUT  200   a  can be wireless. The DUT  200   a  is placed within the interior  301  of a shielded enclosure  300 . Such shielded enclosure  300  can be implemented as a metallic enclosure, e.g., similar in construction or at least in effect to a Faraday cage. This isolates the DUT  200   a  from radiated signals originating from the exterior region  302  of the enclosure  300 . In accordance with exemplary embodiments, the geometry of the enclosure  300  is such that it functions as a closed-ended waveguide. 
         [0032]    Elsewhere, e.g., disposed within or on an opposing interior surface  302  of the enclosure  300 , are multiple (n) antennas arrays  102   a ,  102   b , . . . ,  102   n , each of which radiates multiple phase-controlled RF test signals  103   a ,  103   b , . . . ,  103   n  (discussed in more detail below) originating from the test signal sources  110   a ,  110   b , . . . ,  110   n  within the tester  100   a . Each antenna array includes multiple (M) antenna elements. For example, the first antenna array  102   a  includes m antenna elements  102   aa ,  102   ab , . . .  102   am . Each of these antenna elements  102   aa ,  102   ab , . . . ,  102   am  is driven by a respective phase-controlled RF test signal  131   aa ,  131   ab , . . . ,  131   am  provided by respective RF signal control circuitry  130   a.    
         [0033]    As depicted in the example of the first RF signal control circuitry  130   a , the RF test signal  111   a  from the first RF test signal source  110   a  has its magnitude increased (e.g., amplified) or decreased (e.g., attenuated) by signal magnitude control circuitry  132 . The resulting magnitude-controlled test signal  133  is replicated by signal replication circuitry  134  (e.g., a signal divider). The resulting magnitude-controlled, replicated RF test signals  135   a ,  135   b , . . . ,  135   m  have their respective signal phases controlled (e.g., shifted) by respective phase control circuits  136   a ,  136   b , . . . ,  136   m  to produce magnitude- and phase-controlled signals  131   aa ,  131   ab , . . . ,  131   am  to drive the antenna elements  102   aa ,  102   ab , . . . ,  102   am  of the antenna array  102   a.    
         [0034]    The remaining antenna arrays  102   b , . . . ,  102   n  and their respective antenna elements are driven in a similar manner by corresponding RF signal control circuits  130   b , . . . ,  130   m . This produces corresponding numbers of composite radiated signals  103   a ,  103   b , . . . ,  103   n  for conveyance to and reception by the antennas  202   a ,  202   b , . . . ,  202   n  of the DUT  200   a  in accordance with the channel H matrix, as discussed above. The DUT  200   a  processes its corresponding received test signals  211   a ,  211   b , . . . ,  211   m  and provides one or more feedback signals  201   a  indicative of the characteristics (e.g., magnitudes, relative phases, etc.) of these received signals. These feedback signals  201   a  are provided to control circuitry  138  within the RF signal control circuits  130 . This control circuitry  138  provides control signals  137 ,  139   a ,  139   b , . . . ,  139   m  for the magnitude control circuitry  132  and phase control circuitry  136 . Accordingly, a closed loop control path is provided, thereby enabling gain and phase control of the individual radiated signals from the tester  100   a  for reception by the DUT  200   a . (Alternatively, this control circuitry  130  can be included as part of the tester  100   a .) 
         [0035]    In accordance with well-known channel optimization techniques, the control circuitry  138  uses this feedback data  201   a  from the DUT  200   a  to achieve optimal channel conditions by altering the magnitudes and phases of the radiated signals in such a manner as to minimize the channel condition number k(H), and produce received signals, as measured at each DUT antenna  202 , having approximately equal magnitudes. This will create a communication channel through which the radiated signals produce test results substantially comparable to those produced using conductive signal paths (e.g., RF signal cables). 
         [0036]    This operation by the control circuitry  138  of the RF signal control circuitry  130 , following successive transmissions and channel condition feedback events, will vary the signal magnitude and phase for each antenna array  102   a ,  102   b , . . . ,  102   n  to iteratively achieve an optimized channel condition number k(H). Once such an optimized channel condition number k(H) has been achieved, the corresponding magnitude and phase settings can be retained and the tester  100   a  and DUT  200   a  can continue thereafter in a sequence of tests, just as would be done in a cabled testing environment. 
         [0037]    In practice, a reference DUT can be placed in a test fixture within the shielded enclosure  300  for use in optimizing the channel conditions through the iterative process discussed above. Thereafter, further DUTs of the same design can be successively tested without having to execute channel optimization in every instance, since differences in path loss experienced in the controlled channel environment of the enclosure  300  should be well within normal testing tolerances. 
         [0038]    Referring still to  FIG. 5 , for example, an initial transmission was modeled to produce a channel condition number of 13.8 db, and the magnitudes of the h 11  and h 22  coefficients were −28 db and −28.5 db, respectively. The magnitude matrix for the channel H would be represented as follows: 
         [0000]    
       
         
           
             HdB 
             = 
             
               [ 
               
                 
                   
                     
                       - 
                       28 
                     
                   
                   
                     
                       - 
                       34.2 
                     
                   
                 
                 
                   
                     
                       - 
                       29.8 
                     
                   
                   
                     
                       - 
                       28.5 
                     
                   
                 
               
               ] 
             
           
         
       
       
         
           
             
               k 
                
               
                 ( 
                 H 
                 ) 
               
             
             = 
             
               13.8 
                
               
                   
               
                
               dB 
             
           
         
       
     
         [0039]    After iterative adjustments of magnitude and phase, as discussed above, the channel condition number k(H) was reduced to 2.27 db, and the amplitudes of the h 11  and h 22  coefficients were −0.12 db and −0.18 db, respectively, producing a channel magnitude matrix as follows: 
         [0000]    
       
         
           
             
               H 
               dB 
             
             = 
             
               [ 
               
                 
                   
                     
                       - 
                       0.12 
                     
                   
                   
                     
                       - 
                       13.68 
                     
                   
                 
                 
                   
                     
                       - 
                       15.62 
                     
                   
                   
                     
                       - 
                       0.18 
                     
                   
                 
               
               ] 
             
           
         
       
       
         
           
             
               k 
                
               
                 ( 
                 H 
                 ) 
               
             
             = 
             
               2.27 
                
               
                   
               
                
               dB 
             
           
         
       
     
         [0040]    These results are comparable to those of a cabled testing environment, thereby indicating that such a wireless testing environment can provide test results of comparable accuracy. By eliminating time for connecting and disconnecting cabled signal paths, and factoring in the reduced time for gain and phase adjustments, the overall received signal test time is significantly reduced. 
         [0041]    Referring to  FIG. 6 , influences of multipath signal effects upon the channel condition can be better understood. As discussed above, once disposed within the interior  301  of the enclosure  300 , the DUT  200   a , during transmit testing, radiates an electromagnetic signal  203   a  from each antenna  202   a . This signal  203   a  includes components  203   b ,  203   c  that radiate outwardly and away from the antenna  102   a  of the tester  100   a . However, these signal components  203   b ,  203   c  are reflected off of interior surfaces  304 ,  306  of the enclosure  300  and arrive as reflected signal components  203   br ,  203   cr  to combine, constructively or destructively, depending upon the multipath signal conditions, with the main incident signal component  203   ai . As discussed above, depending upon the constructive and destructive nature of the interference, test results will generally tend to be unreliable and inaccurate for use in proper calibration and performance verification. 
         [0042]    Referring to  FIG. 7 , in accordance with an exemplary embodiment, RF absorbent materials  320   a ,  320   b  are disposed at the reflective surfaces  304 ,  306 . As a result, the reflected signal components  203   br ,  203   cr  are attenuated significantly, thereby producing less interference, either constructively or destructively, with the incident primary signal component  203   ai.    
         [0043]    Additional RF signal control circuitry  150  can be included for use between the antenna array  102   a  mounted within the interior  301  or on the interior surface  302  of the enclosure  300   a  and the tester  100   a . (Alternatively, this additional control circuitry  150  can be included as part of the tester  100   a .) The radiated signals impinging upon the antenna elements  102   aa ,  102   ab , . . . ,  102   am  produce received signals  103   aa ,  103   ab , . . . ,  103   am  with respective signal phases controlled (e.g., shifted) by phase control circuitry  152  having phase control elements  152   a ,  152   b , . . . ,  152   m  controlled in accordance with one or more phase control signals  157   a ,  157   b , . . . ,  157   m  provided by a control system  156 . The resulting phase-controlled signals  153  are combined in a signal combiner  154  to provide the received signal  155   a  for the tester  100   a  and a feedback signal  155   b  for the control system  156 . The control system  156  processes this feedback signal  155   b , as part of a closed loop control network, to adjust, as needed, the respective phases of the composite receive signals  103   aa ,  103   ab , . . . ,  103   am  to minimize the apparent signal path loss associated with the interior region  301  of the enclosure  300   a . This closed loop control network also allows the system to reconfigure the phased array enabled by these antennas  102   a  and phase control circuitry  152  in the event that the positioning or orientation of the DUT  200   a  changes within the enclosure  300   a . As a result, following minimization of the path loss using this feedback loop, accurate and repeatable conveyance of the DUT signal  203   a  to the tester  100   a  using the radiated signal environment within the enclosure  300   a  can be achieved. 
         [0044]    Referring to  FIG. 8 , similar control and improvement in producing accurate and repeatable test results can be achieved for DUT receive signal testing. In this case, the test signal  111   a  provided by the tester  100   a  is replicated by the signal combiner/splitter  154 , and the respective phases of the replicated test signals  153  are adjusted as necessary by the phase control circuitry  152  before being radiated by the antenna elements  102   aa ,  102   ab , . . . ,  102   am . As in the previous case, the reflected signal components  103   br ,  103   cr  are significantly attenuated and result in reduced constructive and destructive interference with the primary incident signal component  103   ai . One or more feedback signals  203   a  from the DUT  200   a  provide the control system  156  with the information necessary for controlling the phases of the replicated test signals  153  to minimize the apparent signal path loss associated with the interior  301  of the enclosure  300   a , thereby establishing consistent and repeatable signal path loss conditions. 
         [0045]    Referring to  FIG. 9 , in accordance with one or more exemplary embodiments, the shielded enclosure  300   b  can be implemented substantially as shown. As discussed above, the DUT can be positioned at one end  301   d  of the interior  301  of the enclosure  300   b , opposite of the interior region  301   b  containing or facing the interior surface  302  on which the tester antenna arrays  102   a ,  102   b , . . . ,  102   n  ( FIG. 5 ) are located. In between is an interior region  301   a  forming a waveguide cavity surrounded by the RF absorbent materials  320 . 
         [0046]    Various other modifications and alterations in the structure and method of operation of this invention will be apparent to those skilled in the art without departing from the scope and the spirit of the invention. Although the invention has been described in connection with specific preferred embodiments, it should be understood that the invention as claimed should not be unduly limited to such specific embodiments. It is intended that the following claims define the scope of the present invention and that structures and methods within the scope of these claims and their equivalents be covered thereby.