Abstract:
Systems and methods for design and testing of RF components are described. One or more RF isolation chambers are used to house MU-MIMO capable devices under test, including wireless access points and client devices. Spatial and angular positioning of the antennas within a chamber and controlled power of the signals into each antenna via RF combiners and RF attenuators to achieve a controllable apparent/virtual angular spread among the respective client device signals is described.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Application No. 62/311,721, titled “Multi User MIMO Testbed and Correlation Control Circuit,” filed on Mar. 22, 2016, the disclosure of which is hereby incorporated by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present disclosure relates to the design and testing and emulation of wireless communication systems and components such as used in wireless communication devices and appliances. 
       BACKGROUND 
       [0003]    Wireless communication has grown to encompass a huge variety of information transactions between electronic machines. These include cellular communications between hand-held units and base stations, wireless communications between peer devices or client-server devices. To enhance the performance of wireless communication systems, multiple input multiple output (MIMO) wireless communication systems can include a plurality of radios at the transmitting or receiving devices of a two-way wireless communication system. Previously, a MIMO station or access point (AP) could only communicate with one device at time on a given frequency channel. Recently, multi user MIMO systems (MU-MIMO) have been developed to further advance the wireless communication capabilities to support simultaneous communication in the same frequency channel with a plurality of devices.  FIG. 1  illustrates an MU-MIMO test system featuring a wireless AP  100  coupled over Ethernet  105  to a computer  120  handling transmit and receive data related to communications over the air (OTA) among several MU-MIMO stations  121 - 123  via AP  100 . AP  100  and all three MU-MIMO stations  121 - 123  can implement the MU-MIMO mechanism according to, for example, 802.11ac or 802.11ax. 
         [0004]    Testing wireless devices in an uncontrolled open air RF environment, such as the one presented in  FIG. 1 , is difficult due to uncontrolled interference and random reflections. MU-MIMO OTA testing is highly challenging because it requires repeatable conditions and specific device positioning in order to achieve repeatable and reliable results. 
         [0005]    Shortcomings of controlled test systems include that they generally are carried out in a “conducted” test fashion, where the antenna of the device under test is removed and an RF cable connected directly to the antenna port of the radio. While this improves repeatability, conducted testing is unsuitable for measuring MU-MIMO gain, since antenna elements are involved in the MU-MIMO beamforming techniques and must be part of the test. 
         [0006]    Therefore, for testing MU-MIMO performance, a better test environment and method are desired. 
       SUMMARY 
       [0007]    Example embodiments described herein have innovative features, no single one of which is indispensable or solely responsible for their desirable attributes. The following description and drawings set forth certain illustrative implementations of the disclosure in detail, which are indicative of several exemplary ways in which the various principles of the disclosure may be carried out. The illustrative examples, however, are not exhaustive of the many possible embodiments of the disclosure. Without limiting the scope of the claims, some of the advantageous features will now be summarized. Other objects, advantages and novel features of the disclosure will be set forth in the following detailed description of the disclosure when considered in conjunction with the drawings, which are intended to illustrate, not limit, the invention. 
         [0008]    In an aspect, the invention is directed to a communication test environment for testing multi-user multiple input multiple output (MU-MIMO) capable devices. The communication test environment comprises a test chamber having electrically isolating walls; at least one MU-MIMO capable access point (AP) disposed in the test chamber; a plurality of repositionable test antennas distributed within said test chamber, each test antenna having a respective line of sight (LOS) to said AP, wherein an angular position of any of said test antennas about said AP is determined according to an Angle of Arrival (AoA), with respect to an arbitrary reference axis that passes through said AP, of RF transmissions from said test antenna to said AP; a plurality of MU-MIMO capable stations, each MU-MIMO capable station coupled to at least one of said test antennas; and a computer coupled to the AP, the computer configured to run a first aggregate throughput test when an MU-MIMO mechanism in the AP and in one or more of the stations is enabled, the computer further configured to run a second aggregate throughput test when said MU-MIMO mechanism in the AP or in said one or more of the stations is disabled. 
         [0009]    In one or more embodiments, the first throughput test comprises sending a first volume of data from the computer simultaneously to said stations via said AP when said MU-MIMO mechanism is enabled, and the second throughput test comprises sending a second volume of data from said computer to said stations via said AP when said MU-MIMO mechanism is disabled. In one or more embodiments, the communication test environment further comprises a positioning platform disposed within said test chamber, on which the AP is mechanically placed, said positioning platform being positionable in response to control signals from said computer or from a second computer to translate said positioning platform, to rotate said platform, or a combination thereof within said test chamber. In one or more embodiments, each said station is coupled to a corresponding at least one RF path extending from its antenna port, through an RF feed-through connector that passes through a test chamber wall, and into said test chamber to connect to at least one of said test antennas. In one or more embodiments, said RF feed-through connector having a first connection point on an exterior side of said walls and a corresponding second connection point on an interior side of said walls, the first and second connection points in electrical communication with one another so as to enable coupling of an exterior and an interior conductor to the first and second connection points, respectively. In one or more embodiments, the communication test environment further comprises an RF attenuator disposed in at least one of the RF paths. 
         [0010]    In one or more embodiments, a first group (N) of said stations is coupled to a first test antenna, via a 1:N RF splitter and a first RF path that extends from a common port of said 1:N RF splitter to said first antenna through a first RF feed-through connector in a first test chamber wall, so as to emulate a first co-located group of MU-MIMO capable stations connected to said AP at an angular position of said first test antenna. In one or more embodiments, a second group (M) of said stations is coupled to a second test antenna, via a 1:M RF splitter and a second RF path that extends from a common port of said 1:M RF splitter to said second antenna through a second RF feed-through connector in the first or a second test chamber wall, so as to emulate a second co-located group of said stations connected to said AP at an angular position of said second test antenna. In one or more embodiments, another station is coupled to a common port of a 1:X splitter, the 1:X splitter having at least first and second RF ports, said first RF port coupled to an RF port of the 1:N splitter via a third RF path, said second RF port coupled to an RF port of the 1:M splitter via a fourth RF path, whereby the third station is coupled to the first and second test antennas. In one or more embodiments, the communication test environment further comprises a first RF attenuator disposed in said third RF path and a second RF attenuator disposed in said fourth RF path. 
         [0011]    In one or more embodiments, at least one of said stations is coupled to a common port of a 1:X splitter, the 1:X splitter having at least first and second RF ports, said first RF port coupled to a first test antenna via a first RF path that passes through a first RF feed-through connector in a first test chamber wall, said second RF port coupled to a second test antenna via a second RF path that passes through a second RF feed-through connector in the first or a second test chamber wall, whereby at least one of said stations is coupled to the first and second test antennas. In one or more embodiments, the communication test environment further comprises a first RF attenuator disposed in said first RF path and a second RF attenuator disposed in said second RF path. 
         [0012]    In one or more embodiments, said test antennas are spatially distributed about said AP so that a difference between the AoA of said RF transmissions from each pair of adjacent test antennas is maximized. In one or more embodiments, the communication test environment further comprises an RF interference generator coupled to at least one antenna disposed in said test chamber. In one or more embodiments, each MU-MIMO capable station is an N×N MU-MIMO capable station having N antenna ports, each of said N antenna ports coupled to a different RF path, each such RF path extending to at least one of said test antennas, wherein N is an integer greater than or equal to 1. 
         [0013]    Another aspect of the invention is directed to a method for testing multi-user multiple input multiple output (MU-MIMO) capable devices. The method comprises placing an MU-MIMO capable access point (AP) in a test chamber having electrically isolating test chamber walls; placing a plurality of repositionable test antennas in said test chamber, wherein each test antenna has a respective line of sight (LOS) to said AP and an angular position of any of said test antennas about said AP is determined according to an Angle of Arrival (AoA) of RF transmissions from said test antenna to said AP with respect to an arbitrary reference axis that passes through said AP; coupling each test antenna to at least one MU-MIMO capable station; coupling a computer to said AP; running a first MU-MIMO aggregate throughput test between said computer and said stations via said AP when an MU-MIMO mechanism in the AP and in one or more of said stations is enabled; and running a non-MU-MIMO aggregate throughput test when said MU-MIMO mechanism in the AP or in said one or more of said stations is disabled. 
         [0014]    In one or more embodiments, the method further comprises determining a MU-MIMO gain by dividing a MU-MIMO aggregate throughput by a non-MU-MIMO aggregate throughput. In one or more embodiments, the method further comprises coupling an antenna port of each station in a first group (N) of stations to a first respective RF path that extends from said antenna port to an RF port of a 1:N RF splitter; and coupling a common port of said 1:N RF splitter to a second RF path that extends from said common port to a first test antenna in said test chamber via an RF feed-through connector in one of said test chamber walls, whereby said first group of stations shares said angular position of said first test antenna and therefore emulates a co-located group of MU-MIMO capable stations coupled to said AP. 
         [0015]    In one or more embodiments, the method further comprises coupling an antenna port of a first station to a first RF path that extends from said antenna port to a common port of an RF splitter; coupling a first RF port of said RF splitter to a second RF path that extends from said first RF port to a first test antenna in said test chamber via a first RF feed-through connector in a first test chamber wall; and coupling a second RF port of said RF splitter to a third RF path that extends from said second RF port to a second test antenna in said test chamber via a second RF feed-through connector in the first or a second test chamber wall, wherein RF signals emitted by said first and second test antennas emulates a virtual AoA of said first station with respect to said AP, said virtual AoA between a first AoA of said first test antenna and a second AoA of said second test antenna. 
         [0016]    In one or more embodiments, the method further comprises disposing a first RF attenuator in said second RF path; disposing a second RF attenuator in said third RF path; adjusting a relative level of attenuation on said first and second attenuators to change a relative power of RF transmissions in said second and third RF paths, whereby said relative level of attenuation controls said virtual AoA of said first station with respect to said AP. In one or more embodiments, the method further comprises spatially distributing said test antennas about said AP so that a difference between the AoA of said RF transmissions from each pair of adjacent test antennas is maximized. In one or more embodiments, the method further comprises electrically coupling an antenna port of one of said wireless stations to an RF path that extends through an RF feed-through connector in one of said test chamber walls and into said test chamber to connect to at least one of said test antennas. 
     
    
     
       IN THE DRAWINGS 
         [0017]    For a fuller understanding of the nature and advantages of the present invention, reference is made to the following detailed description of preferred embodiments and in connection with the accompanying drawings, in which: 
           [0018]      FIG. 1  is a block diagram of an example prior art MU-MIMO test configuration with one MU-MIMO wireless AP and three MU-MIMO stations. 
           [0019]      FIG. 2  illustrates an exemplary MU-MIMO test configuration with two electrically isolated, semi-anechoic chambers, one MU-MIMO wireless AP, and three MU-MIMO stations according to one or more embodiments. 
           [0020]      FIG. 3  is a flowchart which depicts a method for calculating MU-MIMO gain in an MU-MIMO test system according to one or more embodiments. 
           [0021]      FIG. 4  is a depiction of example MU-MIMO aggregate throughput and non-MU-MIMO aggregate throughput results for an MU-MIMO system with three MU-MIMO stations. 
           [0022]      FIGS. 5A and 5B  illustrate an exemplary MU-MIMO test configuration with three MU-MIMO stations and three test antennas according to one or more embodiments. 
           [0023]      FIG. 6  illustrates an exemplary MU-MIMO test configuration with nine MU-MIMO stations and three test antennas according to one or more embodiments. 
           [0024]      FIG. 7  illustrates is a schematic block diagram of an exemplary MU-MIMO test configuration and method according to one or more embodiments. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]      FIG. 2  illustrates an exemplary MU-MIMO test configuration with two electrically isolated, semi-anechoic chambers  201 - 202 , one MU-MIMO capable wireless AP  213 , and three MU-MIMO capable stations  210 - 212  according to one or more embodiments. The AP  213  and stations  210 - 212  are collectively referred to as devices under test (DUT)  210 - 213 , though it is noted that in other embodiments only one of these devices may be a DUT. The DUTs  210 - 213  are placed inside chambers  201 - 202  to isolate the DUTs  210 - 213  from RF transmissions generated by external sources. It is noted that chamber  202  is optional. 
         [0026]    Each of the MU-MIMO stations  210 - 212  establishes a wireless connection with the AP  213  via a respective RF path, which can include any connecting medium, or series of connecting mediums, which allow RF transmissions to be exchanged between two wireless devices. Some examples of connecting mediums include coaxial cables, waveguides, RF feed-through connectors, and isolated over-the-air (OTA) couplings, but those skilled in the art will recognize that other means of coupling RF devices are possible. In  FIG. 2 , the RF paths which connect the AP  213  to each of the MU-MIMO stations  210 - 212  are comprised of OTA coupling between the AP  213  and test antennas  216  within chamber  201 , feed-through connectors  225  in one or more walls of chamber  201  coupled to said test antennas  216 , and electrically shielded coaxial cables  230 . Feed-through connectors  225 , such as SMA, N, or other types of feed-through connectors, are built into the chamber walls and couple RF test signals through the metal walls of chambers  201 - 202 . The feed-through connectors  225  include a first connection point on an exterior side of a chamber  201  wall and a corresponding second connection point on an interior side of said chamber  201  wall. The first and second connection points are in electrical communication with one another so as to enable coupling of an exterior conductor (e.g., coaxial cable  230  outside of the chamber  201 ) and an interior conductor (e.g., a test antenna  216  or a coaxial cable  230  inside the chamber  201 ) to the first and second connection points, respectively. Test signals are coupled via these feed-through connectors  225  and via coaxial cables  230  to the MU-MIMO stations  210 - 212  in chamber  202  (e.g., to an antenna port in each of the MU-MIMO stations  210 - 212 ), completing the RF paths between AP  213  and MU-MIMO stations  210 - 212 . Those skilled in the art will recognize that there are many configurations for the RF paths that AP  213  and MU-MIMO stations  201 - 212  use to establish RF communications sessions. 
         [0027]    The interior walls of chambers  201 - 202  can be covered with an RF-absorber to reduce reflections from the metal walls of chambers  201 - 202 . The RF absorber should be selected so that the reflections resemble real-life indoor RF environments. Examples of RF absorbers include Cummings Microwave LF77 RF absorptive foam. Examples of the construction and/or configuration of one or both of chambers  201 ,  202  can be found in U.S. Patent Application Publication No. 2013/0257468, which are hereby incorporated by reference. 
         [0028]    Reflecting test signals within chambers  201 - 202  create multiple paths through space for transmissions between the AP  213  and MU-MIMO stations  210 - 212 , enhancing MU-MIMO throughput. Filtered Ethernet connection  221  couples Ethernet from computer  232  through the walls of chamber  201  while filtering out external RF signals radiating in frequency bands used during testing. This filter could be designed to couple other types of data interfaces (e.g. USB, HDMI, etc.) through the walls of chamber  201 . Computer  232  can be configured to run one or more tests to measure throughput performance of one or more of the DUTs  201 - 213 . In one example, computer  232  is configured to run a first aggregate throughput test when the MU-MIMO mechanism in AP  213  and some or all of the stations  210 - 212  is enabled and a second aggregate throughput test when the MU-MIMO mechanism in AP  213  and in stations  210 - 212  is disabled. Additional details of these aggregate throughput tests are described below with reference to  FIG. 3 . 
         [0029]    Test configuration  200  includes an optional interference generator  219  coupled via an RF path to an antenna  233  disposed in the test chamber  201 . In other embodiments, the interference generator  219  can be disposed in the test chamber  201  or in a separate electrically isolated chamber. The interference generator  219  can produce RF energy in specific frequency bands and/or in specific patterns for the purpose of disrupting wireless communications. The interference generator  219  can be used to occupy the wireless medium shared by the AP  213  and MU-MIMO stations  210 - 212  (i.e., the OTA coupling in chamber  201 ) in order to model interference from adjacent networks or from random wireless devices, thereby triggering wireless adaptation mechanisms in the AP  213  and MU-MIMO stations  210 - 212 . This interference generator  219  could be programmable and be used to model time-variable RF interference to emulate real-life in-range wireless networks and other sources of interference. 
         [0030]    Attenuators  231  can be placed in at least one of the RF paths between any of the MU-MIMO stations  210 - 212  and the AP  213  (e.g., via electrically shielded coaxial cables  230  in the RF path) to model wireless channel path loss. Each of the attenuators  231  could be programmatically controlled (e.g., via computer  232  or another computer) to model time-variable path loss. 
         [0031]    In some embodiments, each station  210 - 212  is disposed in a separate electrically isolated, semi-anechoic chamber. 
         [0032]    This is only an exemplary configuration. Those skilled in the art recognize that the components in this configuration  200  can be arranged and distributed in different chambers or combined into the same chamber. The example configuration showing in  FIG. 2  supports single input single output (SISO), or 1×1 MU-MIMO capable stations. 2×2 MU-MIMO capable stations can each have 2 antenna ports, requiring that each antenna  216  be replaced by 2 such antennas, each such antenna coupled through a respective RF path to one of the antenna ports of each 2×2 MU-MIMO capable station. In general, an N×N MU-MIMO capable station can have N antenna ports, requiring that each test antenna  216  be replaced by N such antennas, each such test antenna coupled through a respective RF path to one of the antenna ports of each N×N MU-MIMO station. N can be any integer greater than or equal to 1. In one example, each of the N antenna ports of a given N×N MU-MIMO capable station is coupled to a different test antenna. In another example, one or more of the N antenna ports of each N×N MU-MIMO capable station in a plurality of N×N MU-MIMO capable stations are grouped together (e.g., via a splitter as described above) so that they are all coupled to at least one test antenna. In another example, a plurality of N×N MU-MIMO capable stations are coupled to the test antennas, where the integer N is the same or different for each N×N MU-MIMO capable station. 
         [0033]      FIG. 3  is a flowchart that depicts a method for calculating MU-MIMO throughput gain in an MU-MIMO test system, such as the one depicted in  FIG. 2 , according to one or more embodiments. Two throughput tests are run. During the MU-MIMO throughput test  305 , MU-MIMO mechanisms are enabled 310 on AP  213 , MU-MIMO station  210 , MU-MIMO station  211 , and MU-MIMO station  212 . A traffic generator software tool (e.g. iPerf, available at &lt;https://iperf.fr/&gt;) is then used to set up  311  a traffic flow between the computer  232  and MU-MIMO station  210 . Traffic flows are also set up  312  between the computer  232  and MU-MIMO station  211  and set up  313  between the computer  232  and MU-MIMO station  212 . In each traffic flow, a volume of data is sent from the computer  232  to the respective MU-MIMO station  210 - 212 . Throughput is then measured  314  between the computer  232  and each MU-MIMO station  210 - 212 . The throughput measurements between the computer  232  and each MU-MIMO station  210 - 212  are then summed  315  together. The result of this summation is the MU-MIMO aggregate throughput for the MU-MIMO system  200 . 
         [0034]    The first step of the non-MU-MIMO throughput test  306  is to disable  325  the MU-MIMO mechanism on the AP  213 , MU-MIMO station  210 , MU-MIMO station  211 , and MU-MIMO station  212 . Test sequence  326  is the same for both the MU-MIMO throughput test  305  and the non-MU-MIMO throughput test  306 . The throughput measurements between the computer  232  and each MU-MIMO station  210 - 212  are then summed  327  together. The result of this summation is the non-MU-MIMO aggregate throughput for the MU-MIMO system  200 . 
         [0035]    Finally, the MU-MIMO gain is calculated  328 . MU-MIMO gain is equal to the MU-MIMO aggregate throughput divided by the non-MU-MIMO aggregate throughput. For example,  FIG. 4  shows example non-MU-MIMO aggregate throughput  405  and MU-MIMO aggregate throughput  406  results for an MU-MIMO system with three MU-MIMO stations. The MU-MIMO gain for this example system is: 
         [0000]      MU-MIMO gain=935 Mbps/360 Mbps=2.60 
         [0036]    Those skilled in the art recognize that the number of MU-MIMO stations included in an MU-MIMO system can vary. The aggregate throughput is a sum of individual throughput measurements to any number of MU-MIMO stations in an MU-MIMO system. 
         [0037]      FIGS. 5A and 5B  illustrate an exemplary MU-MIMO test configuration with three MU-MIMO stations  505 - 507  and three test antennas  520 - 522  according to one or more embodiments. Test antennas  520 - 522  are distributed in test chamber  523  such that they each have line of sight (LOS) to AP  510 . Having a LOS to AP  510  means that the test signals radiating from test antennas  520 - 522  to the AP  510  (or vice versa) are unobstructed by any objects. The angular position of a test antenna is the angle of arrival at AP  510  of RF transmissions emitted from the test antenna, with respect to an arbitrary reference axis that passes through AP  510  (e.g., reference axis  570  illustrated in  FIG. 5B ). The angular position of a test antenna is also the angle of departure of RF signals emitted from AP  510  and received at the test antenna, with respect to said reference axis. The angle of arrival at the AP and the angle of departure from the AP can be based on one of the imaginary lines that extends between the test antenna (e.g., a center of the test antenna) and the AP. The signal path used to determine the angle of arrival/departure for each antenna  520 - 522  is indicated as dashed lines  550  in  FIGS. 5A and 5B . For example, the angle of arrival and the angle of departure of RF transmissions from/to test antenna  520  is angle  575  with respect to reference axis  570 , as illustrated in  FIG. 5B . In the case of MU-MIMO enabled operation, AP  510  forms a plurality of (in this example 3) simultaneous electromagnetic beams, each beam directed at its target test antenna  520 - 522 . 
         [0038]    Angular spread refers to the total range of angles of arrival/departure possible with given test antenna positions. In example system  511 , the angular spread is 360°. MU-MIMO gain is maximized when the angular spread between each pair of adjacent test antennas  520 - 522  around the AP  510  is maximized. For example, in  FIG. 5A , the angular spread between adjacent test antennas  520  and  522  is 120°. The angular positions and angular spread of the test antennas  520 - 522  around AP  510  affect the MU-MIMO gain of the MU-MIMO system. 
         [0039]    Those skilled in the art will recognize that the number of test antennas  520 - 522  in system  511  as well as the placement of those antennas within the chamber  523  may vary. 
         [0040]    In some embodiments, the test antennas  520 - 522  are distributed in test chamber  523  along a virtual circle  551  about AP  510 . In addition, the test antennas  520 - 522  can be distributed such that the angle between adjacent test antennas (e.g., between antennas  521  and  522 , between antennas  521  and  520 , and between antennas  520  and  522 ) is equal or substantially equal. For example, the angle between adjacent test antennas can be 360°/n, where n is the number of test antennas. Thus, when n=3 (as in  FIGS. 2 and 5 ), the angle between adjacent test antennas can be 120°. When n=4, the angle between adjacent test antenna can be 90°, and so on. In some embodiments, antennas  521 - 523  and AP  510  are co-planar. In other embodiments, antennas  521 - 523  and AP  510  are not co-planar. For example, each antenna  521 - 523  can be disposed at a different height in chamber  523  while maintaining a desired (e.g., maximum) angular separation. In one example, the antennas  521 - 523  are disposed along a virtual sphere about AP  510  while maintaining a desired (e.g., maximum) angular separation. It is noted that the terms virtual circle  551  and virtual sphere are used in the conceptual sense, and their use does not imply the shape or geometry of test chamber  523 . 
         [0041]    In some embodiments, one or more of the test antennas  520 - 522  is repositionable within chamber  523 . Thus, the stations  505 - 507  and the AP  510  can be tested when the test antennas have different angular spreads. 
         [0042]    In some embodiments, AP  510  is disposed on a positioning platform that can rotate or translate horizontally and/or vertically within the chamber  523 . Moving or rotating the AP  510  affects the angle of arrival/departure of signals to/from the antennas  520 - 522  and thus their angular separation. The positioning platform can be controlled by a computer (e.g., computer  232 , discussed above) and it can translate and/or rotate in response to control signals from the computer. 
         [0043]    In some embodiments, the components illustrated in  FIGS. 5A and 5B  are the same, substantially the same, or different than the components illustrated in  FIG. 2 . For example, MU-MIMO stations  505 - 507  can be disposed in an electrically isolated, semi-anechoic chamber, such as chamber  202 . Likewise, the RF paths between the AP  510  and each station  505 - 507  can include cable coupling, RF feed-throughs, attenuators and the like, as discussed above. In addition, the AP  510  can be connected to a computer (e.g., as discussed above) to run one or more tests. 
         [0044]      FIG. 6  illustrates an exemplary MU-MIMO test configuration with nine MU-MIMO stations  610 - 618 , three test antennas  625 - 627 , and an AP  661  according to one embodiment of this invention. The test antennas  625 - 627  and AP  661  are disposed in an electrically-isolating semi-anechoic chamber  650 . In some embodiments, the test antennas  625 - 627  are disposed in chamber  650  in the same manner or in substantially the same manner as test antennas  520 - 522 , described above. AP  661  is coupled to computer  665  via a filtered Ethernet connection built into the wall of chamber  650  (e.g., as discussed above) in order to run one or more tests. 
         [0045]    Test antenna  627  is coupled to the common port on splitter  644  (e.g., a 1:N or a 1:M splitter) through RF feed-through connector  664  in a chamber  650  wall, forming an RF path between the test antenna  627  and the common port on splitter  644 . Two of the RF ports on splitter  644  couple directly to the antenna ports of MU-MIMO stations  616 ,  617 . MU-MIMO station  617  and MU-MIMO station  616  therefore couple to test antenna  627 . Since MU-MIMO stations  616  and  617  radiate test signals through the same test antenna  627 , their transmissions radiate from the same angular position and have the same AoA at the AP  661 , and therefore emulate a co-located group of MU-MIMO capable stations. Since the transmissions from MU-MIMO stations  616  and  617  radiate from the same angular position, the AP  661  treats these stations as being in the same MU-MIMO group, causing the beamforming mechanism on AP  661  to group MU-MIMO stations  616  and  617  into the same MU-MIMO group. An MU-MIMO-capable AP can transmit to or receive from MU-MIMO stations in separate MU-MIMO groups simultaneously, but can only transmit to or receive from one MU-MIMO station in each MU-MIMO group simultaneously. 
         [0046]    A third RF port on splitter  644  couples through attenuator  634  and one of the RF ports on splitter  645  (e.g., a 1:X splitter) and through splitter  645  to the antenna port of MU-MIMO station  618 . In this way, MU-MIMO station  618  is coupled to test antenna  627 . By changing the magnitude of attenuation at attenuator  634 , the power level of transmissions from MU-MIMO station  618  through test antenna  627  can be controlled. A fourth RF port on splitter  644  couples through attenuator  633  and one of the RF ports on splitter  643  and through splitter  643  to the antenna port of MU-MIMO station  615 . In this way, MU-MIMO station  615  is coupled to test antenna  627 . By changing the magnitude of attenuation at attenuator  633 , the power level of transmissions from MU-MIMO station  615  through test antenna  627  can be controlled. 
         [0047]    In the same way that splitter  644  couples directly to MU-MIMO station  617  and MU-MIMO station  616  and to MU-MIMO station  618  through attenuator  645  and to MU-MIMO station  615  through attenuator  633 , so too do splitters  640  and  642  each couple to four MU-MIMO stations. Two RF ports on splitter  640  couple directly to MU-MIMO stations  610  and  611 , one RF port of splitter  640  couples through attenuator  635  and through splitter  645  to MU-MIMO station  618 , and one RF port couples through attenuator  630  and through splitter  641  (e.g., a 1:X splitter) to MU-MIMO station  612 . Since the common port on splitter  640  couples to test antenna  625  via an RF path that extends from the common port on splitter  640  through feed-through connector  662  in chamber  650  wall, MU-MIMO stations  618 ,  610 ,  611 , and  612  couple through splitter  640  to test antenna  625 . Two RF ports on splitter  642  couple directly to MU-MIMO stations  613  and  614 , one RF port couples through attenuator  631  and through splitter  641  to MU-MIMO station  612 , and one RF port couples through attenuator  632  and through splitter  643  to MU-MIMO station  615 . Since the common port on splitter  642  couples to test antenna  626  via an RF path that extends from the common port on splitter  642  through feed-through connector  663  in chamber  650  wall, MU-MIMO stations  612 ,  613 ,  614 , and  615  couple through splitter  642  to test antenna  626 . 
         [0048]    MU-MIMO station  612 ,  615 , and  618  are each coupled to two of the three test antennas  625 - 627 . MU-MIMO station  615  is coupled to the common port of splitter  643  (e.g., a 1:X splitter). A first RF port on splitter  643  couples through attenuator  633  and then couples to said fourth RF port on splitter  644 , forming an RF path between the first RF port on splitter  643  and the RF port on splitter  644 . Since splitter  644  couples through feed-through connector  664 , forming an RF path to test antenna  627 , MU-MIMO station  615  is coupled to test antenna  627 . By changing the magnitude of attenuation at attenuator  633 , the power level of transmissions from MU-MIMO station  615  through test antenna  627  can be controlled. A second RF port on splitter  643  couples through attenuator  632  and then couples to an RF port on splitter  642  forming an RF path between the second RF port on splitter  643  and the RF port on splitter  642 . The common port of splitter  642  couples through feed-through connector  663  in a chamber  650  wall to test antenna  626 , forming an RF path between the common port of splitter  642  and test antenna  626 . In this way, MU-MIMO station  615  is coupled to test antenna  626 . By changing the magnitude of attenuation at attenuator  632 , the power level of transmissions from MU-MIMO station  615  through test antenna  626  can be controlled. In some embodiments, one or both of the first and second RF ports of splitter  643  are directly coupled to test antennas  626 ,  627  via RF paths that include feedthrough connectors  664  and  663  (i.e., without the respective RF path passing through splitter  644  or splitter  642 ), each RF path optionally passing through an attenuator (e.g., one path through attenuator  632  and/or the other path through attenuator  633 ). 
         [0049]    Since MU-MIMO station  615  is coupled to both test antenna  627  and test antenna  626  by respective RF paths that extend from splitter  643  (e.g., a 1:X splitter) to test antennas  626 ,  627 , its test transmissions can radiate through both test antenna  627  and test antenna  626  simultaneously as long as the attenuation at attenuator  633  is not so great as to prevent these test transmissions from reaching test antenna  627  and as long as the attenuation at attenuator  632  is not so great as to prevent these test transmissions from reaching test antenna  626 . Since attenuator  633  controls the power of MU-MIMO station  615 &#39;s transmissions through test antenna  627  and attenuator  632  controls the power of MU-MIMO station  615 &#39;s transmissions through test antenna  626 , changing the relative levels of attenuation on attenuators  633  and  632  therefore changes the relative power of MU-MIMO station  615 &#39;s transmissions from test antennas  627  and  626 . When the relative power of MU-MIMO station  615 &#39;s transmissions through test antennas  627  and  626  is changed, the apparent/virtual angular position of MU-MIMO station  615  at the AP  661  also changes. Controlling the relative level of attenuation of attenuators  633  and  632  therefore controls the apparent/virtual angular position of MU-MIMO station  615  at AP  661 . 
         [0050]    MU-MIMO station  615  is connected through attenuator  633  and through splitter  644  to test antenna  627 , forming an RF path between MU-MIMO station  615  and test antenna  627 , and through attenuator  632  and through splitter  642  to test antenna  626 , forming an RF path between MU-MIMO station  615  and test antenna  626 . MU-MIMO station  618  is connected through attenuator  635  and through splitter  640  to test antenna  625 , forming an RF path between MU-MIMO station  618  and test antenna  625 , and through attenuator  634  and through splitter  644  to test antenna  627 , forming an RF path between MU-MIMO station  618  and test antenna  627 . MU-MIMO station  612  is also connected in the same manner through attenuator  630  and through splitter  640  to test antenna  625 , forming an RF path between MU-MIMO station  612  and test antenna  625 , and through attenuator  631  and through splitter  642  to test antenna  626 , forming an RF path between MU-MIMO station  612  and test antenna  626 . The apparent/virtual angular position of MU-MIMO station  618  at AP  661  can be controlled by varying the relative levels of attenuation of attenuators  635  and  634 , and the apparent/virtual angular position of MU-MIMO station  612  at AP  661  can be controlled by varying the relative levels of attenuation of attenuators  630  and  631 . 
         [0051]    Thus, it can be seen that the angular position of MU-MIMO stations  612 ,  615 , and  618  can be controlled such that they are virtually positioned between the two respective antennas to which they are coupled. 
         [0052]    In some embodiments, one or more of the stations  610 - 618  is/are disposed in an electrically isolated, semi-anechoic chamber. In some embodiments, all stations  610 - 618  are disposed in the same electrically isolated, semi-anechoic chamber. In other embodiments, each station  610 - 618  is disposed in a separate electrically isolated, semi-anechoic chamber. In some embodiments, a first group of stations  610 - 618  is disposed in one electrically isolated, semi-anechoic chamber and a second group of stations is disposed in another electrically isolated, semi-anechoic chamber. 
         [0053]    In  FIG. 7 , computer  730  is coupled via Ethernet connections  740  to the AP  710 , which in turn is coupled to each MU-MIMO station  720 - 722  via separate OTA coupling  770 - 772 . To measure throughput between the computer  730  and each of the MU-MIMO stations  720 - 722  simultaneously, a point-to-multipoint throughput test is implemented. A point-to-multipoint throughput test is a throughput test where a single node in a computer network establishes communication sessions with a plurality of other network nodes and runs traffic between itself and each other node simultaneously. In  FIG. 7 , each of the MU-MIMO stations  720 - 722  uses the computer program iPerf to initialize a single iPerf server session  763 - 765 . Computer  730  then initializes three separate iPerf client sessions  760 - 762 . Each iPerf client session establishes a communication session with one of the iPerf server sessions (iPerf client session  760  establishes communications link  742  with iPerf server  763 , iPerf client session  761  establishes communications link  743  with iPerf server  764 , and iPerf client session  762  establishes communications link  744  with iPerf server  765 ). Finally, traffic flows are run simultaneously between iPerf client session  1   760  and iPerf server  763 , iPerf client session  761  and iPerf server  764 , and iPerf client session  762  and iPerf server  765 . The total aggregate throughput for this system  700  is equal to the sum of the throughput measurements at MU-MIMO stations  720 - 722 . Those skilled in the arts recognize that this is just one configuration and that many other configurations are possible. For example, the number of MU-MIMO stations  720 - 722  is variable and the RF paths between the computer  730 , AP  710 , and MU-MIMO stations  720  could also be reconfigured to use different mediums. 
         [0054]    It is to be appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination. 
         [0055]    Unless otherwise defined, all technical and scientific terms used herein have the same meanings as are commonly understood by one of ordinary skill in the art to which this invention belongs. Although methods similar or equivalent to those described herein can be used in the practice or testing of the present invention, suitable methods are described herein. The present materials, methods, and examples are illustrative only and not intended to be limiting. 
         [0056]    It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. Rather the scope of the present invention is defined by the appended claims and includes both combinations and sub-combinations of the various features described hereinabove as well as variations and modifications thereof, which would occur to persons skilled in the art upon reading the foregoing description.