Abstract:
Described is an access point a plurality of antennas, a plurality of transceivers and a processor. Each of the antennas receives a first signal from each of a plurality of wireless devices. The first signal includes a first identifier of a corresponding wireless device. Each of the transceivers is coupled to each of the antennas. The processor is coupled to each of the transceivers. The processor generates a first communication matrix which includes the first identifier from each of a selected number of the wireless devices. The selected number is no greater than a number of the antennas. The processor utilizes the first communication matrix to resolve multiple wireless communications received from the selected number of the wireless devices within a single time slot over a radio channel.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     The present application relates to and incorporates by reference the entire disclosures of U.S. Application entitled “Wireless Device and Method for Wireless Multiple Access” filed on Mar. 31, 2005 naming Jacob Sharony as inventor, and U.S. Application entitled “System and Method for Wireless Multiple Access” filed on Mar. 31, 2005 naming Jacob Sharony as inventor. 
     
    
     BACKGROUND  
       [0002]     A wireless local area network (WLAN) is a flexible data communications system which may either replace or extend a conventional, wired LAN. The WLAN may provide added functionality and mobility over a distributed environment. That is, the wired LAN transmits data from a first computing device to a further computing device across cables or wires which provide a link to the LAN and any devices connected thereto. The WLAN, however, relies upon radio waves to transfer data between wireless devices. Data is superimposed onto the radio wave through a process called modulation, whereby a carrier wave acts as a transmission medium.  
         [0003]     Exchange of data between the wireless devices over the WLAN has been defined and regulated by standards ratified by the Institute of Electrical and Electronics Engineering (IEEE). These standards include a communication protocol generally known as 802.11, and having several versions, including 802.11a, 802.11b (“Wi-Fi”), 802.11e, 802.11g and 802.11n. Recently, there has been a surge in deployment of 802.11-based wireless infrastructure networks to provide WLAN data sharing and wireless internet access services in public places (e.g., “hot spots”).  
         [0004]     Conventional WLANs utilize a single-in-single-out (“SISO”) cellular sharing architecture, in which data is transferred over a radio channel in a cell. Because the channel is shared by all wireless devices (e.g., mobile units and an access point) within the cell, each device must contend for access to the channel, thus, allowing only one device to transmit on the channel at a given time. Consequently, conventional WLANs present a number of limitations (e.g., delayed transmission times, failed transmission, increased network overhead, limited scalability, etc.).  
         [0005]     In an effort to overcome the limitations of the conventional WLAN, a multiple-in-multiple-out (“MIMO”) shared WLAN architecture has been developed. A MIMO mode uses spatial multiplexing to increase a bit rate and accuracy of data sent between the wireless devices. In the MIMO mode, a single high-speed data stream (e.g., 200 mbps) is divided into several low-speed data streams (e.g., 50 mbps), transmitted to the wireless device (e.g., mobile unit) and recombined into the high-speed data stream for resolving the transmission. However, this high-speed connection is provided only for one-to-one communication (e.g., access point to a single mobile unit) at a given time. In addition, wireless devices operating according to a first version of the 802.11 protocol (e.g., 802.11a, 802.11b, 802.11g, etc.) may not support the high-speed connection without a hardware and/or a software modification(s), which may represent significant costs to operators of the WLAN.  
       SUMMARY OF THE INVENTION  
       [0006]     The present invention relates to an access point which includes a plurality of antennas, a plurality of transceivers and a processor. Each of the antennas receives a first signal from each of a plurality of wireless devices. The first signal includes a first identifier of a corresponding wireless device. Each of the transceivers is coupled to each of the antennas. The processor is coupled to each of the transceivers. The processor generates a first communication matrix which includes the first identifier from each of a selected number of the wireless devices. The selected number is no greater than a number of the antennas. The processor utilizes the first communication matrix to resolve multiple wireless communications received from the selected number of the wireless devices within a single time slot over a radio channel. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]      FIG. 1  shows an exemplary embodiment of a system according to the present invention.  
         [0008]      FIG. 2  shows an exemplary embodiment of a downstream protocol according to the present invention.  
         [0009]      FIG. 3  shows an exemplary embodiment of an upstream protocol according to the present invention.  
         [0010]      FIG. 4  shows an exemplary embodiment of a method according to the present invention.  
         [0011]      FIG. 5  shows a schematic view of an exemplary embodiment of wireless communication of the system according to the present invention.  
         [0012]      FIG. 6  shows an exemplary embodiment of a relationship between an aggregate system throughput and a number of antennas of the system according to the present invention.  
         [0013]      FIG. 7  shows a further exemplary embodiment of the relationship between the aggregate system throughput and the number of antennas of the system according to the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0014]     The present invention may be further understood with reference to the following description and the appended drawings, wherein like elements are referred to with the same reference numerals. The exemplary embodiment of the present invention describes a protocol for providing multiple access to a wireless environment for wireless devices therein. In addition, the protocol of the present invention is preferably compatible with legacy 802.11-based wireless devices using conventional access mechanisms.  
         [0015]      FIG. 1  shows a system  100  according to the present invention. The system  100  may include a WLAN  105  deployed within a space  110 . As understood by those skilled in the art, the space  110  may be either an enclosed environment (e.g., a warehouse, office, home, store, etc.), an open-air environment (e.g., park, etc.) or a combination thereof. The space  110  may be one area or partitioned into more than one area (e.g., an area  115 ). The areas  115  are limited neither in number or dimension. As shown in  FIG. 1 , the space  110  is divided into the areas  115 ( 1 - 3 ).  
         [0016]     The WLAN  105  may include wireless communication devices, such as, an access point (“AP”)  120  and one or more wireless devices (e.g., mobile units (“MUs”)  125 ) wirelessly communicating therewith. The AP  120  may be connected to a server via the WLAN  105 . Though,  FIG. 1  only shows MUs  125 ( 1 - 3 ) within the WLAN  105 , those of skill in the art would understand that the WLAN  105  may include any number and type of MUs (e.g., PDAs, cell phones, scanners, laptops, handheld computers, etc.). Those of skill in the art would further understand that the MU may include a non-mobile unit attached to a wireless device (e.g., a PC with a network interface card).  
         [0017]     Radio frequency (“RF”) signals including data packets may be transmitted between the MUs  125 ( 1 - 3 ) and the AP  120  over a radio channel. As understood by those skilled in the art, the data packets may be transmitted using a modulated RF signal having a common frequency (e.g., 2.4 GHz, 5 GHz). Furthermore, the data packets may include conventional 802.11 packets, such as, authentication, control and data packets. The data packets travel between the AP  120  and the MUs  125 ( 1 - 3 ) along a plurality of paths  130 ( 1 - 6 ) within the space  110 . Though,  FIG. 1  only shows six paths  130 ( 1 - 6 ), those of skill in the art would understand that a number of potential paths is essentially infinite.  
         [0018]     Spatial configuration (e.g., length, direction, etc.) of the paths  130 ( 1 - 6 ) may depend upon one or more factors. These factors include, but are not limited to, a location(s) of the AP  120  and/or the MUs  125 ( 1 - 3 ), a configuration of the space  110  and/or the areas  115 ( 1 - 3 ), a location and/or a shape of an obstruction(s)  135  therein. For example, the path  130 ( 1 ) may pass substantially directly from the MU  125 ( 1 ) to the AP  120 , whereas the path  130 ( 2 ) may reflect from a structure (e.g., a wall). The paths  130 ( 3 - 4 ) between the MU  125 ( 2 ) and the AP  120  may pass from the area  115 ( 2 ) to the area  115 ( 1 ) via an opening (e.g., a doorway  140 ( 1 ), a window, etc.), and may then reflect from one or more structures (e.g., wall(s), obstruction  135 , etc.) in area  115 ( 1 ). The paths  130 ( 5 - 6 ) between the MU  125 ( 3 ) and the AP  120  may pass from the area  115 ( 3 ) to the area  115 ( 1 ) via an opening (e.g., a doorway  140 ( 2 ), a window), and may then reflect from one or more structures (e.g., obstruction  135 , wall(s), etc.). Although, not shown in  FIG. 1 , those of skill in the art would understand that the paths  130 ( 1 - 6 ) may have varied spatial configurations and pass through any of the structures and/or obstructions described.  
         [0019]     The data packets which are transmitted by the MUs  125 ( 1 - 3 ) and/or the AP  120  may differ from the data packets which are received. That is, changes in a length and/or a number of reflections of each of the paths  130 ( 1 - 6 ) may result in variations in attributes of the RF signal, such as, amplitude, phase, arrival time, frequency distribution, etc. Reflective properties of the structures and/or obstructions may further influence the attributes of the signal and the data contained therein. The changes mentioned above are generally referred to as “multi-path fading.” 
         [0020]     According to the present invention, the AP  120  and the MUs  125 ( 1 - 3 ) may utilize a first mode of communication (e.g., 802.11a, 802.11b, 802.11g) and a second mode of communication (e.g., MIMO, 802.11n). To utilize the MIMO mode, the AP  120  may have an architecture including a processor, two or more antennas, two or more receivers and two or more transmitters. Accordingly, each antenna is capable of transmitting and receiving one or more independent signals concurrently and at a substantially common frequency (e.g., the radio channel). The processor of the AP  120  may resolve the wireless communication of the signals received from the MUs  125 ( 1 - 3 ) or further APs.  
         [0021]     Each MU  125  may utilize the MIMO mode using an architecture including a processor, two or more antennas, two or more receivers and one or more transmitters. The antennas and the receivers allow the MU  125  to receive one or more independent signals concurrently and at a substantially common frequency. The transmitter allows the MU  125  to transmit one or more signals to the AP  120 . The processor of the MU  125  may resolve the wireless communication of the received signals from the AP  120  and/or further MUs.  
         [0022]     In a preferred embodiment, the AP  120  includes four antennas, four receivers and four transmitters, and each MU  125  includes four antennas, four receivers and one transmitter. However, those of skill in the art would understand that the AP  120  may include any number of antennas, receivers and transmitters, but, that the number is changed in a 1:1:1 ratio. That is, for any additional antenna, an additional receiver and an additional transmitter may be included. Similarly, the MU  125  may include any number of antennas and receivers, and any change in the number is done according to a 1:1 ratio. The MU  125  may further include any number of transmitters, which would change the ratio of antennas to receivers to transmitters to 1:1:1. However, in a preferred embodiment of the present invention, the MU  125  maintains a single transmitter. In this manner, the protocol described herein may be utilized by wireless devices employing a legacy-802.11 standard (e.g., 802.11a, 802.11b, 802.11g) without significant hardware and/or software modifications. Architectures of the AP  120  and the MU  125  are described in further detail in U.S. patent application Ser. No. 10/738,167, filed on Dec. 17, 2003, entitled “A Spatial Wireless Local Area Network,” the disclosures of which are incorporated herein by reference.  
         [0023]      FIG. 2  shows an exemplary embodiment of wireless communication from the AP  200  to the MUs  210 ( 1 - 4 ), which is typically referred to as “downstream” communication. In this embodiment, the AP  200  may transmit two or more signals from its two or more antennas. As shown in  FIG. 2 , the AP  200  has four antennas, and, correspondingly, transmits four independent signals S 1 -S 4 . The number of signals sent may be directly proportional to the number of antennas (e.g., one independent signal per antenna). Also, in MIMO mode, the AP  200  may transmit the signals S 1 -S 4  concurrently over the radio channel, which will be described in further detail below.  
         [0024]     Due to the multi-path fading and any other factors contributing to signal corruption or degradation, the antennas of each MU  210  receive a signal R 1 -R 4  which differs from the transmitted signals S 1 -S 4 . Those of skill in the art would understand that any or all of the received signals R 1 -R 4  may not differ from the transmitted signals S 1 -S 4 . Accordingly, one or more the received signals R 1 -R 4  may equal one or more of the transmitted signals S 1 -S 4  (e.g., R 1 =S 1 ) In either instance, the received signals R 1 -R 4  may be related to the transmitted signals S 1 -S 4  by a signal-relation equation: R i =Σa ij S j +n i , where a ij  are elements of a transmission matrix and n i  represents a noise level on a receiving channel i.  
         [0025]     Each MU  210  estimates the transmission matrix a ij  using at least a portion of the received signals R 1 -R 4 . In one embodiment, each of the transmitted signals S 1 -S 4  includes a training packet T j , indicative of a transmission channel j used by the AP  200 . The training packet T j  may include a pilot sequence p j  which may be transmitted as a portion of a preamble signal to the transmitted signals S 1 -S 4 . For example, the AP  200  may send one or more training packets T j  in one of a sequence of time slots. Each MU  210  may identify the pilot sequence p j  in each training packet and estimate the transmission matrix a ij  using a matrix equation: a ij =R i /p j . Each MU  210  may then extract the transmitted signal using the signal-relation equation, above. For example, the MU  210 ( 1 ) may receive signals R 1 -R 4  and use pilot sequence p 1 -p 4  to resolve the transmission matrix a ij . The transmission matrix a ij  may then be used in the signal-relation equation to resolve the transmitted signal S 1 . As would be understood by those skilled in the art, the processor of the MU  210  may resolve the transmission matrix a ij  and the transmitted signal S 1  using a software application.  
         [0026]      FIG. 3  shows an exemplary embodiment of communication from the MUs  310 ( 1 - 4 ) to the AP  300 , which is typically referred to as “upstream” communication. As described above, in a preferred embodiment, each MU  310  has one or more transmitters. Thus, each MU  310 ( 1 - 4 ) transmits a signal S 1 -S 4 , respectively, to the AP  300 . Signals R 1 -R 4  received by the AP  300  may differ from the transmitted signals S 1 -S 4  due to, for example, multi-path fading. The received signals R 1 -R 4  are used by the AP  300  in the signal-relation equation: R i =Σa ij S j +n i , which may be the same as that used by the MU  210  in the downstream communication. That is, each of the received signals R 1 -R 4  may include the training packet T j  indicative of the transmission channel j used by the MU  310 . The training packet T j  may further include the pilot sequence p j  which may be transmitted as a portion of a preamble to the transmitted signals S 1 -S 4 . The AP  300  uses the received signals R 1 -R 4  and the pilot sequences p j  to resolve the transmission matrix a ij  with the matrix equation: a ij =R i /p j . The transmitted signals S 1 -S 4  are then resolved using the signal-relation equation.  
         [0027]      FIG. 4  shows an exemplary embodiment of a method  400  according to the present invention. In this embodiment, the method  400  is employed by a receiving station which may be any type of wireless device. For example, in the downstream communication, the MU may employ the method  400 , whereas, in the upstream communication, the AP may employ the method  400 . Thus, the method  400  will be described with respect to a transmitting station and the receiving station. Furthermore, according to the present invention, the receiving station and/or the transmitting station may be operating according to a first mode of communication (e.g., CSMA/CA), but also capable of operating in a second mode of communication (e.g., MIMO). Thus, the method  400  is used by the receiving station as a result of the transmitting station initiating wireless communication in the second mode of communication (e.g., MIMO mode).  
         [0028]     In step  410 , the receiving station receives at least two first signals from the transmitting station. The first signals (e.g., R 1  and R 2 ) are the received versions of at least two second signals (e.g., S 1  and S 2 ) which are transmitted by the transmitting station. As understood by those skilled in the art, the first signals may correspond to a number of transmitting antennas employed by the AP and/or the MU, or a number of MUs transmitting to the AP. The first signals may not contain any data, but may simply include the training packet T j . However, the first signal may be packets (e.g., data packets) which include the training packet T j  and/or the pilot sequence p j  in a preamble thereof.  
         [0029]     In step  420 , the receiving station identifies the pilot sequence p j  included in the training packet T j . Those of skill in the art would understand that the processor in the receiving station or a software application executed thereby may extract the pilot sequence p j  from the training packet T j . Furthermore, the training packet T j  may only include the pilot sequence p j . Thus, in this embodiment, the first signals (e.g., R 1  and R 2 ) may simply be the pilot sequences p 1  and p 2 .  
         [0030]     In step  430 , the receiving station may resolve the transmission matrix a ij  using the matrix equation. As stated above, the transmission matrix a ij  may be estimated as a function of the pilot sequence p j  and the first signals (e.g., R 1  and R 2 ). As with identification of the pilot sequence p j , the processor and/or a software application executed thereby of the receiving station may utilize the matrix equation to resolve the transmission matrix a ij .  
         [0031]     In step  440 , the receiving station may resolve the second signal using the signal-relation equation. As stated above, the second signal is estimated as a function of the transmission matrix a ij , the first signals and the noise n i  on the receiving channel i. Again, the second signal may be resolved by the processor and/or a software application executed thereby of the receiving station.  
         [0032]     In step  450 , the receiving station can begin operating in the second mode of communication. Accordingly, the stations may now transmit and receive signals simultaneously over the share channel. The second mode of communication may increase overall system throughput, reduce corruption and degradation of the data, and allow operators and user of the system to maintain use of legacy 802.11 devices.  
         [0033]      FIG. 5  shows an exemplary embodiment of a system  500  according to the present invention. The system  500  is shown as a schematic timing diagram with phases I-XII representing periods of communication over the channel. In this exemplary embodiment, an AP  505  may be equipped with four antennas  506 - 509 , four receivers and four transmitters. Any number of MUs  510 - n  may be within a communication range of the AP  505 . As shown in  FIG. 5 , each of the MUs may have one or more transmitters, along with four antennas and four receivers. As noted above, those of skill in the art would understand that there is no limitation on the number of antennas, transmitters and receivers on both the AP  505  and the MUs  510 - n . However, it is preferable that the number of antennas, transmitters and receivers of the AP  505  match the number of antennas and receivers of the MUs  510 - n . Furthermore, as noted above, the system  500  may be scaled based on the number of antennas on the AP  505  and/or the number of MUs within the coverage area thereof. Though, the system  500  will be described with respect to the MUs  510 - n  having a single transmitter, those skilled in the art would understand that more than one transmitter may be utilized by the MUs  510 - n.    
         [0034]     In  FIG. 5 , phases I-XII depict an exemplary embodiment of a refresh period (e.g., every 50 ms) with phase I signifying a beginning of the refresh period. Those of skill in the art would understand that the refresh period may have a duration that is inversely proportional to mobility of the MUs  510 - n . For example, an increased mobility of the MUs (e.g., more likely to move in and out of the coverage area of the AP  505 ), may result in a shorter duration of the refresh period. Thus, at an end of the refresh period or at the beginning of a subsequent refresh period, the AP  505  may redetermine which MUs are within the coverage area thereof.  
         [0035]     In phase I, the AP  505  transmits a training packet  535  from each antenna  506 - 509 . As shown in  FIG. 5 , a total of four of the training packets  535  are transmitted in successive predetermined time slots. That is, the AP  505  accesses the channel in a conventional manner according to the first mode communication (e.g., CSMA/CA), and then transmits (e.g., broadcasts) the training packets  535  thereon. In this manner, the AP  505  may guarantee itself the ability to transmit each of the four training packets  535  successively by waiting for a short inter frame space (“SIFS”) between each transmission. As understood by those of skill in the art, the training packets  535  may be received by any MU  510 - n  within the coverage area of the AP  505 . That is, the four training packets  535  are broadcast to all MUs within the coverage area of the AP  505 .  
         [0036]     As described above with reference to the “downstream” communication, each training packet  535  may contain the pilot sequence p j . In an exemplary embodiment, each pilot sequence p j  contains a predetermined set of numbers which corresponds to a number and location of transmitting antennas on the AP  505 . That is, in the embodiment shown in  FIG. 5 , each pilot sequence p j  may contain four numbers. Thus, receipt of the four pilot sequences p j  allows each MU  510 - n  to construct its own transmission matrix a ij , which will be described further below. As shown in  FIG. 5 , each MU  510 - n  within the coverage area of the AP  505  may receive four pilot sequences p 1 -p 4 , each having the predetermined set of four numbers.  
         [0037]     In phase II, each MU  510 - n  receives four of the training packets  535  from the AP  505 . The MUs  510 - n  may then identify the pilot sequence p j  in each training packet  535  and use the predetermined set of numbers contained therein to resolve the transmission matrix a ij . In the embodiment shown in  FIG. 5 , the transmission matrix a ij  may be a four by four matrix. This allows the MUs  510 - n  to estimate the channel for resolving transmissions from the AP  505 . That is, the four numbers in each pilot sequence may be modified (e.g., in amplitude and/or phase) as a result of attenuation and/or multipath fading during transmission of the training packets  535 . Thus, the matrix a constructed by each MU  510 - n  may be different, and will allow each MU  510 - n  to resolve transmissions from the AP  505  addressed for it. As understood by those skilled in the art, every MU  510 - n  does not have to resolve the transmission matrix a ij . For example, if an MU does not desire to transmit on the channel (e.g., no data packets for the AP  505 ), the MU may wait for the subsequent refresh period. However, in a preferred embodiment, each MU  510 - n  which receives the training packets  535  resolves its own transmission matrix a ij .  
         [0038]     After the MUs  510 - n  have resolved the transmission matrix a ij , each of the MUs  510 - n  may decide whether it wants to communicate with the AP  505  according to the second mode of communication (e.g., MIMO mode). As shown in  FIG. 5 , MUs  510 , 520 , 525  and  530  desire to communicate in the MIMO mode. Thus, each of the MUs  510 , 520 , 525  and  530  transmits a control frame to the AP  505 . As understood by those skilled in the art, the control frame may be a request-to-send (“RTS”) frame which is modified to indicate that each of the MUs  510 , 520 , 525  and  530  desires to communicate in the MIMO mode (e.g., MIMO RTS (“MRTS”)  540 ). The MRTS  540  may include a vector with a predetermined set of numbers (e.g., in  FIG. 5 , four numbers). Furthermore, those skilled in the art would understand that the MUs  510 , 520 , 525  and  530  transmit the MRTSs  540  to the AP  505  by gaining access to the channel using the first mode of communication (e.g., CSMA/CA), because the AP  505  has not granted the requests to transmit in the MIMO mode. Furthermore, the AP  505 , at this point, has not received any transmissions from the MUs  510 - n  through which it may estimate the channel (e.g., construct a transmission matrix a ij  for itself).  
         [0039]     One or more the MUs  510 - n  may not desire to transmit in the MIMO mode, but simply intend to communicate according to the first mode. For example, the MU  515  does not transmit the MRTS  540  to the AP  505 , because, for example, it does not have any data packets for the AP  505 . Alternatively, the MU  515  may wish to wait until it has accumulated a predetermined number of data packets before transmitting in the MIMO mode.  
         [0040]     In phase III, the AP  505  receives the MRTS  540  from the MUs  510 , 520 , 535  and  540 , which is similar to the “upstream” communication described above. Although,  FIG. 5  only shows that four of the MUs  510 - n  have requested to communicate in the MIMO mode, those of skill in the art would understand that any number of the MUs  510 - n  may transmit the MRTS  540  to the AP  505 . For example, as shown in  FIG. 5 , if more than four of the MUs  510 - n  had requested to communicate in MIMO mode, the AP  505  may have to determine which of the MUs  510 - n  would be cleared to communicate in the MIMO mode. The AP  505  may invoke a priority scheme based on, for example, bandwidth required and/or application type (e.g., voice, scans, email, etc.). In this manner, the AP  505  may choose four of the MUs  510 - n  with the highest priority to communicate in the MIMO mode. The AP  505  may respond to any number (e.g., 2, 3 . . . n) of requests to communicate in the MIMO mode. Thus, the remaining MUs may communicate in the first mode (e.g., CSMA/CA) when the channel is free, or wait until a subsequent refresh period or MIMO phase.  
         [0041]     Upon receipt of the MRTSs  540 , the AP  505  may use the vectors contained in each to resolve its transmission matrix a ij . That is, the AP  505  has received communications from the MUs which allow it to estimate the channel. Thus, in this embodiment, the AP  505  can now communicate with the four MUs at a first bit rate (e.g., 54 mbps). Alternatively, the AP  505  may communicate with three MUs at a second bit rate (e.g., 72 mbps). In either of these embodiments, each transmitting antenna of the AP  505  may allow for communication at a predefined bit rate. Thus, this bit rate can be varied/divided in any fashion (e.g., based on data type, application, etc.) to partition a bandwidth for the channel.  
         [0042]     Utilizing the transmission matrix a ij  to resolve concurrent transmissions from the MUs, the AP  505  can begin to communicate in the MIMO mode. That is, the AP  505  may transmit control frames  545  concurrently and on the same frequency to each of the MUs  510 , 520 , 525  and  530 . As understood by those skilled in the art, the control frame may be a clear-to-send (“CTS”) frame which is modified to indicate that each of the MUs  510 , 520 , 525  and  530  may begin communicating in the MIMO mode (e.g., MIMO CTS (“MCTS”)  545 ). In a further exemplary embodiment, the MCTS may be broadcast to the MUs  510 - n . However, the broadcast may define which of the MUs  510 - n  is cleared to send in the MIMO mode.  
         [0043]     As shown in  FIG. 5 , the AP  505  is responding to the MRTSs  540  from the MUs  510 , 520 , 525  and  530  to communicate in the MIMO mode. However, the AP  505  may initiate communication in the MIMO mode at the start of the refresh period. That is, the AP  505  may transmit the MCTSs  545  in the phase I to any four of the MUs  510 - n . This may happen if, for example, each of the four MUs receiving the MCTSs  545  in the start of the refresh period maintained its transmission matrix a ij . The four of the MUs  510 - n  may be determined by the AP  505  using, for example, the priority scheme described above. Thus, according to the present invention, one or more of the MUs  510 - n  or the AP  505  may initiate and/or request communication in the MIMO mode.  
         [0044]     In phase IV, the MUs  510 , 520 , 525  and  530  have been cleared to transmit data packets  550  in the MIMO mode. Each of the MUs  510 , 520 , 525  and  530 , may transmit the data packets  550  concurrently to the AP  505 . Using the transmission matrix a ij , the AP  505  can resolve the data packets, as described above with reference to the “upstream” communication.  
         [0045]     In phase V, the AP  505 , communicating in the MIMO mode, may transmit acknowledgment signals (“ACKs”)  555  concurrently to each of the MUs  510 , 520 , 525  and  530  which transmitted the data packets  550 . As understood by those skilled in the art, the MUs  510 , 520 , 525  and  530  may continue transmitting data packets  550  and receiving the ACKS  555  in the MIMO mode for a predetermined amount of time and/or according to a defined protocol.  
         [0046]     In phase VI, the AP  505  transmits data packets  560 , which may have been buffered at, or presently received by, the AP  505  to the MUs  510 , 515 , 520  and n. As shown in  FIG. 5 , the AP  505  is transmitting the data packets  560  in the MIMO mode to the MUs  515  and n which had not requested to transmit in the MIMO mode in phase II or been cleared to transmit in the MIMO mode in phase III. However, as noted above, each MU  510 - n  within the coverage area of the AP  505  receives the training packets  535  and the pilot sequences p j  contained therein. Thus, the MUs  515  and n may resolve the signals from the AP  505  to extract the data packets  560  addressed therefor.  
         [0047]     In phase VII, the MUs  510 , 515 , 520  and n which received the data packets  560  transmit ACKS  565  to the AP  505 , confirming receipt of the data packets  560 . In this embodiment, the MU  515  did not previously request to communicate in the MIMO mode in the phase II. The MU  515  may receive the data packet  560  from the AP  505  transmitting in the MIMO mode, but it may not transmit in the MIMO mode without being cleared to do so by the AP  505 . Thus, as shown in  FIG. 5 , the MU  515  transmits the ACK  565  and an MRTS according to the first mode (e.g., CSMA/CA) requesting that it be allowed to communicate in the MIMO mode. As understood by those skilled in the art, the ACK  565  may be sent separately from the MRTS, or the MRTS may be piggybacked thereon.  
         [0048]     Furthermore, as shown in  FIG. 5 , the MU  530  did not receive the data packet  560  from the AP  505  in phase VI. However, the MU  530  desires to retain the capability to communicate in the MIMO mode. Those of skill in the art would understand that the MU  530  may desire retention of MIMO-capability if, for example, the MU  530  has further data packets to transmit to the AP  505 . In this case, the MU  530  transmits a control frame (e.g., MRTS  570 ) to the AP  505 . The MU  530  may transmit the MRTS  570  in a time slot in which the MUs  510 , 520  and n are transmitting their respective ACKS  565 , because the MU  530  had received the MCTS  545  in phase III.  
         [0049]     In phase VIII, after receiving the ACKs  565  and/or the MRTSs  570 , the AP  505  may transmit further data packets  575 , which may have been buffered at, or presently received by, the AP  505 . As shown in  FIG. 5 , the data packets  575  are transmitted to the MUs  510 , 520 , 525  and  530 . As stated above, the data packets  575  are transmitted concurrently from the AP  505  in a time slot. In phase IX, the MUs  510 , 520 , 525  and  530  which received the data packets  575  concurrently transmit ACKS  580  to the AP  505 , confirming receipt of the data packets  575 .  
         [0050]     In phase X, the AP  505  transmits a control frame (e.g., MCTS  585 ) to each of the MUs  515 , 525 , 530  and n which requested communication in the MIMO mode in phase VII. Also, the MU  525  which may not have requested communication in MIMO mode in phase VII, may have piggybacked a MRTS on the ACK  580  in phase IX. Similarly, the MU n in phase VII may have piggybacked an MRTS on the ACK  565 . Thus, the MUs  515 , 525 , 530  and n are cleared to communicated in the MIMO mode by the AP  505 . In phase XI, the MUs  515 , 525 , 530  and n transmit data packets  590  to the AP  505  concurrently, and, in phase XII, the AP  505  responds with ACKS  595 .  
         [0051]     As understood by those of skill in the art, the AP  505  and the MUs  510 - n  may continue communicating over the channel past the phase XII until and/or after a subsequent refresh period. As discussed above, after the subsequent refresh period is initiated, the AP  505  may again broadcast the training packets in the first mode of communication or in the MIMO mode.  
         [0052]     Furthermore, those skilled in the art would understand that the present invention provides certain advantages over conventional systems. For example, in a conventional MIMO system, an AP communicates only with a single MU, but at an increased bit rate (e.g., 216 mbps). In contrast, the present invention provides for an AP which communicates with two or more MUs at a lower bit rate (e.g., 54 mbps), allowing for compatibility with legacy 802.11 systems which may not be capable of handling the increased bit rate without significant hardware and software modifications. Furthermore, the present invention provides for increased system throughput with minimized overhead, by allowing the AP to communicate with at least two MUs concurrently, and vice-versa.  
         [0053]     As noted above, the AP and/or the MUs may have two or more antennas and receivers.  FIG. 6  shows a graph representing an exemplary relationship between an aggregate throughput and a number of antennas on the AP and the MUs for a system utilizing the present invention. As shown in  FIG. 6 , the aggregate throughput increases in a hyperbolic manner until a saturation point (e.g., 250 antennas, 225 mbps), in which the channel may not be able to support any further transmissions thereon.  FIG. 7  shows a enlarged view of a portion of the graph of  FIG. 6 . In  FIG. 7 , a first ray  700  indicates the exemplary relationship of the graph in  FIG. 6 . A second ray  705  indicates a practical relationship due to anticipated overhead created as a result of the present invention. As the number of antennas is increased, so does the anticipated overhead. However, the anticipated overhead is relatively low considering that, for example, eight MUs may be communicating at the same time and on the same frequency at 54 mbps.  
         [0054]     It will be apparent to those skilled in the art that various modifications may be made in the present invention, without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.