Patent Publication Number: US-11387959-B2

Title: WiFi antenna selection with beamforming

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to Provisional Application No. 62/460,066 filed on Feb. 16, 2017 entitled “ Antenna Selection with Beamforming ”; and to co-pending U.S. application Ser. No. 15/462,903, filed Mar. 19, 2017, entitled “WiFi Antenna Selection with Beamforming”, both of which are incorporated herein by reference in their entirety as if fully set forth herein. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The field of the present invention relates in general to wireless local area networks including wireless access points (WAP) and wireless stations and sounding methods therefore. 
     2. Description of the Related Art 
     Home and office networks, a.k.a. wireless local area networks (WLAN) are established using a device called a Wireless Access Point (WAP). The WAP may include a router. The WAP wirelessly couples all the devices of the home network, e.g. wireless stations such as: computers, printers, televisions, digital video (DVD) players, security cameras and smoke detectors to one another and to the Cable or Subscriber Line through which Internet, video, and television is delivered to the home. Most WAPs implement the IEEE 802.11 standard which is a contention based standard for handling communications among multiple competing devices for a shared wireless communication medium on a selected one of a plurality of communication channels. The frequency range of each communication channel is specified in the corresponding one of the IEEE 802.11 protocols being implemented, e.g. “a”, “b”, “g”, “n”, “ac”, “ad”. Communications follow a hub and spoke model with a WAP at the hub and the spokes corresponding to the wireless links to each ‘client’ device. 
     After selection of a single communication channel for the associated home network, access to the shared communication channel relies on a multiple access methodology identified as Collision Sense Multiple Access (CSMA). CSMA is a distributed random access methodology for sharing a single communication medium, by having a contending communication link back off and retry access a prospective collision on the wireless medium is detected, i.e. if the wireless medium is in use. 
     Communications on the single communication medium are identified as “simplex” meaning, one communication stream from a single source node to one or more target nodes at one time, with all remaining nodes capable of “listening” to the subject transmission. Starting with the IEEE 802.11ac standard and specifically ‘Wave 2’ thereof, discrete communications to more than one target node at the same time may take place using what is called Multi-User (MU) multiple-input multiple-output (MIMO) capability of the WAP. MU capabilities were added to the standard to enable the WAP to communicate with multiple single antenna single stream devices concurrently, thereby increasing the time available for discrete MIMO video links to wireless HDTVs, computers tablets and other high throughput wireless devices the communication capabilities of which rival those of the WAP. 
     To characterize the communication channel between the WAP and each station, a sounding is conducted. An explicit sounding consists of the transmission of a known sequence from the WAP to each associated station, followed by a sounding response from the station characterizing the communication channel between the WAP and itself. The WAP uses the sounding response to focus its antennas in a manner which improves either or both signal strength at the station or downlink throughput thereto. 
     What is needed are improved methods for sounding each communication link between the WAP and its associated stations. 
     SUMMARY 
     The present disclosure provides a method and apparatus for a wireless transceiver, e.g. wireless access point (WAP) or station apparatus to enhance communications with target transceiver(s) over a massive Multiple-input Multiple-output (MIMO) array of antenna greater in number than the number of transmit chains on the transceiver. This surplus of antennas relative to transmit chains, allows the WAP to select an optimal subset of antennas equal in number to the number of transmit chains, for beamforming data communication packets. 
     In an embodiment, a circuit includes a beamform antenna selection circuit coupled to transmit chains. The beamform antenna circuit is to determine a number of possible combinations that the transmit chains may couple to an array of antennas, generate a set of antenna mapping matrices, the number of antenna mapping matrices in the set of antenna mapping matrices being equal to the number of possible combinations, generate, based on the set of antenna mapping matrices, a sounding packet to test the possible combinations, receive sounding feedback, determine a performance for each of the possible combinations based on the sounding feedback, wherein a particular combination has a highest ranked performance; and switchably couple the transmit chain to an antenna of the array of antennas that is associated with the particular combination with the highest ranked performance for transmission of subsequent data communication packets. 
     Aspect of this disclosure may be implemented in hardware, firmware or software. 
     Associated methods and circuits are also claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features and advantages of the present invention will become more apparent to those skilled in the art from the following detailed description in conjunction with the appended drawings in which: 
         FIGS. 1A-D  are drawings of respectively a plan view of a residential wireless local area network (WLAN), a packet diagram of a sounding packet, a system view of sequential soundings and data communication, and a packet exchange view of the sequential soundings to evaluate antenna subsets, and subsequent data communication; 
         FIGS. 2A-C  are hardware views of a transceiver during antenna sounding and data communication and a packet view of those same communication stages respectively; 
         FIGS. 3A-C  are an exploded hardware view of the transmit chain expander circuit, a per antenna tone graph, and a graph of beamforming efficiency for various antenna subsets during transmission of an antenna sounding in accordance with an embodiment of the invention; 
         FIGS. 3D-E  are an exploded hardware view of the transmit chain expander circuit, and a per antenna tone graph during transmission of subsequent data communication packets on the highest ranked beamforming subset of antenna in accordance with an embodiment of the invention; 
         FIGS. 4A-D  are hardware views of massive multiple-input multiple-output (MIMO) antenna arrays with individually steerable antennas in accordance with another embodiment of the invention; 
         FIGS. 5A-C  are overall and detailed circuit diagrams of the WiFi transceiver with massive MIMO beamform antenna subset selection for each link in accordance with an embodiment of the invention; and 
         FIG. 6  is a process flow diagram of processes associated with a massive MIMO transceiver&#39;s beamform antenna subset selection for each link in accordance with an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       FIGS. 1A-D  are Prior Art drawings of respectively a residential wireless local area network (WLAN), a packet diagram of a sounding packet, a system view of sequential soundings and data communication, and a packet exchange view of the sequential soundings and data communication. 
       FIG. 1A  is a plan view of a residential WLAN in a home  101 . The WLAN includes a wireless access point (WAP)  100  with four antennas and two transmit chains. The WAP is shown with a communication link  103  to a targeted station  107 . 
       FIG. 1B  is a packet diagram of a WLAN sounding packet including the sounding field thereof. All WLAN packets whether associated with communicating a sounding or with the communication of user data include a ubiquitous header portion. All WLAN packet headers include various preamble fields with known sequences which allow the receiving station to synchronize reception with packet boundaries and to determine the received channel. What makes a sounding packet a sounding packet is not the sounding field in the header, rather the null data packet announcement (NDPA) packet which precedes the sounding and identifies the receiving station(s) which are requested to share their channel analysis, e.g. CSI, with the transmitter so as to improve its subsequent communications.  FIG. 1B  shows a sounding packet  190  and the representative symbol intervals (SI) required to transmit each field thereof. The header  190 A includes a legacy portion containing the L-STF, L-LTF and L-SIG fields and a very high throughput portion containing the VHT-SIG-A, VHT-STF, VHT-LTF and VHT-SIG-B fields. The legacy (L), long (LTF) and short (STF) training and signal (SIG)  190 B fields are compatible with stations supporting only the IEEE 802.11n or earlier standards. The remaining signal and training fields are intended only for very high throughput, e.g. IEEE 802.11ac compliant devices. The VHT-SIG-A field  190 C contains information on the modulation and coding scheme (MCS) and number of streams of the sounding. The VHT-STF field  190 D is used for automatic gain control (AGC). The VHT-LTF field  190 F, a.k.a. channel estimation or sounding field, contains a long training sequence used for MIMO channel estimation by the receiver. 
       FIG. 1C  is a system view of sequential soundings and data communication. Each sequential sounding  192 A-F is conducted with the two transmit chains of the transceiver coupled to a distinct combination of two out of the four antennas, thus allowing the highest ranked subset of the antenna for a subsequent downlink beamformed data communication link to the targeted station  107  to be determined. Sounding packet  192 A is transmitted on the 1 st  and 2 nd  out of the four antennas, and in response a sounding feedback packet  194 A is received from the targeted station  107 . The sounding feedback packet contains channel state information (CSI) for the channel between the transmitter and receiver. If the receiving station is IEEE 802.11n compliant the sounding feedback is in the form of a channel matrix “H”. If the receiving station is IEEE 802.11ac compliant the sounding feedback is in the form of the actual unitary beamforming matrix V and the per tone diagonal matrix Σ which is directly related to the per tone signal-to-noise ratio (SNR). A second sounding packet  192 B is transmitted on the 1 st  and 3 rd  of the four antennas, and a sounding feedback packet  194 B is received from the targeted station  107 . The third sounding packet  192 C is transmitted on the 1 st  and 4 th  antennas, and a sounding feedback packet  194 C with the resultant channel state information (CSI) is received from the targeted station. The fourth sounding packet  192 D is transmitted on the 2 nd  and 3 rd  antennas, and the sounding feedback packet  194 D is received from the targeted station. The fifth sounding packet  192 E is transmitted on the 2 nd  and 4 th  antenna, and the CSI for that subset of antenna is received in sounding feedback packet  192 E from the targeted station. The sixth and final sounding packet  192 F in the sequential sounding is transmitted on the 3 rd  and 4 th  of the four antennas, and the final sounding feedback packet in the sequence, feedback packet  194 F is received from the targeted station  107 . Once all distinct beamforming candidates, i.e. subsets of antenna have been evaluated by the WAP  100  the highest ranked subset is used for subsequent transmission of communication data packets  196  to the receiving station  107 . Ranking may be based on any one or all of: Received Signal Strength Indication (RSSI), signal-to-noise ratio (SNR) and Channel ranking, for example. The process is repeated by the WAP for each station targeted for downlink communications on the WLAN. 
       FIG. 1D  is a packet view of sequential soundings and data communication shown in  FIG. 1C . The WAP transmits a pair of sounding packets  192 A, i.e. a Null Data Packet Announcement (NDPA) followed by a null data packet (NDP) with the actual sounding field, and in response the targeted station sends back a sounding feedback response packet  194 A. The time and overhead required to send all six soundings  192 - 194 A-F is considerable. All packets including those associated with the sounding, include a header portion shown in crosshatch. Following the sounding, communications resume and downlink communication of user data is sent on the link(s) that have been sounded. These user data packet(s), a.k.a. media access control (MAC) Service Data Unit (MSDU) or Protocol Data Unit (MPDU)  196 A are sent using the associated beamforming matrix determined during the preceding sequence of soundings. The receiving station  107  sends an acknowledgement packet “ACK”  198 A in response. The explicit sounding may be initiated by a WAP or a station. The overhead involved in this sequential approach to selecting a subset of antennas equal to the number of transmit chains of the massive MIMO antenna array is significant. The following  FIGS. 2-6  shown an embodiment of the invention for reducing this overhead by evaluating multiple distinct subsets of the antenna as candidates for subsequent beamformed communications. 
       FIGS. 2A-C  are hardware views of a transceiver during antenna sounding and data communication and a packet view of those same communication stages respectively. 
       FIG. 2A  is a hardware view of a wireless transceiver  200 , e.g. WAP or station, during antenna sounding. The wireless transceiver, e.g. WAP  200 , includes a baseband stage  220 , a combined Analog Front End (AFE) and Radio Frequency (RF) stage  240 , a transmit chain expander circuit  252 , power amplifiers  250 A-B, and the massive MIMO antenna array with the number “N”=4 antenna  276 A-D. Only the transmit chains of the transceiver have been shown for clarity. The transceiver has two transmit chains, formed by a plurality of shared and discrete components coupled to one another in the baseband through RF stages, including: corresponding one of encoder/mappers  227 A-B, spatial mapper  230 , Inverse Discrete Fourier Transform  232 A-B, Digital-to-Analog Converters (DAC)s  242 A-B, filters  244 A-B, upconverters  248 A-B, carrier frequency voltage controlled oscillator  246 , and low voltage amplifiers  249 A-B. Baseband components prior to the IDFT are said to handle transmit processing in the frequency domain, while those after the IDFT are said to handle transmit processing in the time domain. 
     In this case of four antennas and two transmit chains, we&#39;d like to map the two transmit chains to the four antennas, but in such a way that the antenna mapping is different for different tones. While this is conceptually what we want to achieve, it cannot be done directly as with traditional spatial mapping between streams and transmit chains. The goal is to transform the spatial mapping to a form that needs no more than two transmit chains and that connects the output of these two transmit chains to the four antennas using the transmit chain expander circuit that can be easily implemented in the time-domain. For the case under consideration (two transmit chains vs. four antennas), there are six possible ways to map transmit chains to antennas, namely: 1&amp;2, 1&amp;3, 1&amp;4, 2&amp;3, 2&amp;4, and 3&amp;4. If we want to cover all these mappings in our sounding frame, we need six different linearly independent antenna mapping matrices (AMM) e.g. matrices  229 A-F, where each tone in the group test one beamforming antenna combination of Chains=2, MIMO antennas for subsequent beamformed communications. We&#39;ll distribute them equally throughout the toneset  231  of the selected communication channel such that the antenna mapping matrices repeat every six tones and further such that each group of six consecutive tones, e.g. group  231 A, has the same training constellation point. The antenna mapping matrices differ from prior art spatial mapping matrices in that they have a number of rows equal to the number of antenna under test, i.e. 4 antennas in the instant example, rather than a number of rows equal to the number of transmit chains, i.e. 2 chains in the instant example. 
     Next, on the transmit path, the IDFTs transform the frequency domain signals to two time-domain signals output by the IDFTs. These are mapped to the four antennas using a mapping matrix which depends on a linear combination of all frequency-dependent mapping matrices and also exhibits time-dependency. The signals time domain signals can be generated using the two transmit chains that we have available. In the more general case, the number of signals will correspond to the number of available transmit chains. The main question is how to efficiently implement the antenna mapping in the time domain and the transmit chain expander circuit  252  accomplishes this task with a chain combiner circuit  254  and a center frequency shifter circuit  260 . The chain combiner circuit  254  accepts the input from the two chains, and generates outputs equal in number to the number of antenna under test, e.g. 4. Each of the four outputs from the chain combiner is distinct combination of the two transmit chains at the input. The only required operations for the chain combiner circuit  254  are additions and phase rotations of multiple of 90 degrees of the two transmit chains. All of these operations can be executed in the time domain. 
     The second step of the operations performed by the transmit chain expander circuit is performed by the center frequency shifter circuit  260 . This circuit accepts input of the four distinct transmit chain combinations, and multiplies each of those four intermediate outputs with a distinct polynomial sum, e.g. a sum of sinusoids. This too is something that can be implemented directly in the time-domain. All operations can be performed in the time-domain and we get the benefit that multiple possible beamform antenna combinations, i.e. multiple subsets of 2 of the four antennas, can be sounded with a single sounding frame. The crossbar switch  270  couples all the Massive MIMO antenna  276 A-D to the expanded transmit chains during sounding. 
     In an embodiment of the invention the connections could be done in either analog or digital domain. The output the “baseband” block could be analog signals in which case the connections and multiplications are performed by analog blocks. Alternatively, the outputs, combining and multiplications could be done in digital with the final signals then being fed into a discrete set of ADCs. 
     In response to the sounding, the targeted receiving transceiver, e.g. station  107 , performs a normal channel state information (CSI) analysis for each tone just as it would for a standard explicit sounding, in terms of for example: a channel matrix “H”; a beamforming matrix “V”; or a per tone sigma matrix, equivalent to a per tone signal-to-noise (SNR) matrix. The receiver then sends this per tone explicit sounding feedback  295 A in a responsive feedback packet to the transceiver  200 . The transceiver then ranks this feedback tone by tone for each tone in each group, e.g. group  231 A across the entire toneset, and based on any one or all of: Received Signal Strength Indicia (RSSI) or SNR, or channel rank for example, determines which tone in each group, exhibits the highest rank. The transceiver  200  then correlates the highest ranked tone with the corresponding subset of 2 of the 4 antennas that were mapped by the AMM to that tone, and initiates subsequent downlink communications using just the highest ranked subset of antenna for downlink packet transmission, as is shown in  FIG. 2B . 
       FIG. 2B  is a hardware view of the transceiver  200  during subsequent data communication after the sounding in which the optimal, a.k.a. highest ranked, subset of the WAP&#39;s antennas has been determined. During this phase of operations, the WAP transmits downlink user data communication packets to the station  107  via the highest ranked subset of 2 of the transceiver&#39;s four antennas, e.g. antennas  276 B-C. During this phase of operations, the transmit chain expander circuit  252  internally disconnects the two input transmit chains at the low voltage amplifiers  259 A-B from the chain combiner and center frequency shifter and switchably couples the two transmit chains instead to the inputs of a corresponding one of the high voltage power amplifiers  250 A-B. The transmit chain expander circuit, and specifically the crossbar switch portion  270  thereof couples the outputs of the high voltage power amplifiers to a corresponding one of the selected subset of antennas  276 B-C for downlink user data packet transmission to the transceiver  107 . The downlink user data packets are spatially mapped by the spatial mapper of the baseband stage using beamforming matrices having rows equal in number to the number of transceiver chains, e.g. two rows for the example shown. 
     In an embodiment of the invention the CSI sounding feedback  295  obtained in response to the antenna sounding for the highest ranked tones may be used to determine the beamforming matrices for all tones during subsequent transmission of user data packets. In this embodiment of the invention interpolated variants of the beamforming matrices for the highest ranked tones are used for precoding the remaining tones. Alternately, where the number of combinations of subsets of antennas is large, and the number of tones in each group is correspondingly large, beamforming matrix interpolation may not be accurate enough given the tone spread between the highest ranked tone in each group, and another sounding of a traditional nature, a.k.a. a channel sounding, may be required. This sounding will be conducted on all tones, with a traditional spatial mapping matrix with rows equal to the number chains, and with each chain coupled by the transmit chain expander circuit to a corresponding one of the antennas of the highest ranked subset of the massive MIMO antenna array, e.g. the antenna  276 B-C in the example shown. 
       FIG. 2C  is a packet view of the sounding and data communication stages. Only a single sounding  292 A and sounding feedback  294 A is required to determine which of the six combinations of 2 of the 4 antenna is optimal for subsequent beamformed communication link with the targeted receiver. Where the transceiver has 4 chains and 8 antennas, there would be 70 discrete combinations of 4 of the 8 antennas to evaluate, resulting in only one highest ranked tone in each group of 70 tones. In this latter case, an additional sounding  292 B of a traditional nature could be utilized using the selected highest ranked subset of antenna, to determine from the sounding feedback  294 B for each tone the beamforming matrices therefore, without any interpolation. In either case, the sounding related overhead per link is greatly reduced compared to the example shown in  FIGS. 1C-D . Once the beamforming matrices have been determined from sounding feedback, the subsequent user data packet transmissions, e.g. MPDU or MSDU, packets are transmitted on the highest ranked subset of two of the WAP&#39;s four antenna, i.e. antenna  276 B-C to the receiving station  107 . Each user data packet received by the station is acknowledged in an “ACK” packet  298 A. 
       FIGS. 3A-C  are an exploded hardware view of the transmit chain expander circuit, a per antenna tone graph, and a graph of beamforming efficiency for various antenna subsets during transmission of an antenna sounding in accordance with an embodiment of the invention. 
       FIG. 3A  is an exploded hardware view of the transmit chain expander circuit  252  shown in  FIG. 2A  during transmission of an antenna sounding in accordance with an embodiment of the invention. In this embodiment of the invention, the transmit chain expander circuit is positioned in the transmit path after the carrier frequency upconverters  248 A-B for each chain. These upconverters are driven by the carrier frequency voltage controlled oscillator  246 . In the case of an IEEE 802.11ac compliant transceiver in the 5 GHz band, if the selected channel is the lowest 20 MHz channel, then the carrier frequency VCO  246  is tuned to 5.035 GHz. The two transmit chains output by low power amplifiers  249 A-B provide inputs to the transmit chain expander circuit  252  via switches  353 A-B shown as demultiplexers. These switches in conjunction with their crossbar switch counterpart  270  either switch the two transmit chains through the antenna chain combiner and antenna center frequency shifter to all antennas  276 A-D during antenna sounding, or avoid these sub circuits entirely during subsequent transmission of beamformed data packets via power amplifiers  250 A-B and the selected highest ranked subset of two of the four antennas. 
     During antenna sounding the transmit chain combiner circuit  254  accepts the input from the two chains via input switches  353 A-B and generates outputs equal in number to the number of antenna under test, e.g. the four antenna  276 A-D. Each of the four outputs from the chain combiner is a distinct combination of the two transmit chains. The only required operations for the chain combiner circuit  254  are additions and phase rotations of multiple of 90 degrees of the two transmit chains. All of these operations can be executed in the time domain. Specifically, the per antenna chain combiner circuit  254  includes chain combiner circuit blocks  356 A-D equal in number to the number of Massive MIMO antenna. Each circuit block includes in this embodiment of the invention, a multiplier and an adder. The multiplier scales one of the chains with a discrete phase and amplitude scalier alpha, where for example alpha=+−1, +−j, for example. The discretely scaled outputs of the second transmit chain are then added to the signal on the first chain. 
     The second step of the operations performed by the transmit chain expander circuit is performed by the per antenna center frequency shifter circuit  260 . This circuit accepts input of the four distinct transmit chain combinations from the antenna chain combiner at a corresponding one of multipliers  366 A-D. Each of the multipliers multiplies each of those four intermediate outputs with a distinct polynomial sum, e.g. a sum of sinusoids. A subcarrier bandwidth oscillator  363  is shown, coupled to a bank of 1×, 2×, 3×, 4×, 5× frequency scalers  364  is shown along with a DC source  362 . Each adder  365 A-D is shown summing distinct combinations of three of the subcarrier bandwidth scalers. Adder  365 A sums: a DC value from DC source  362 , the subcarrier bandwidth with a unitary frequency scalar, and the subcarrier bandwidth with a 2× frequency scalar. The output of adder  365 A is coupled to an input of multiplier  366 A. 
     The polynomial sums output by the corresponding one of adders  365 A-D to a corresponding one of multipliers  366 A-D are distinct combinations of integer multiples, e.g. 0×,1×,2×,3×,4×,5× of the subcarrier bandwidth which in the case of an IEEE 802.11ac compliant transceiver in one of the 5 GHz band channels is 312.5 kHz. So, the distinct overall center frequency shift on any of the antenna resulting from the injection of polynomial shift sums during sounding corresponds to a small fraction of the overall carrier frequency, e.g. less than 0.01%. The center frequency shifted output from the multipliers  366 A-D is amplified in a corresponding one of amplifiers  369 A-D and the output of each amplifier is coupled via the crossbar output switch  270  and specifically a corresponding one of the multiplexers  372 A-D thereof, to the corresponding one of antennas  276 A-D. In this embodiment of the invention the crossbar output switch is shown as a bank of four multiplexers  272 A-D each of which couples a corresponding one of the Massive MIMO antenna  276 A-D to the expanded transmit chains during sounding. 
     In an alternate embodiment of the invention the transmit chain expander circuit is implemented before upconversion by the upconverters  248 A-B rather than after. 
       FIG. 3B  is a per antenna tone graph of power vs frequency during transmission of an antenna sounding in accordance with an embodiment of the invention. The OFDM tones  367 A on each antenna are shown as discrete rows across a 20 MHz channel with the minute incremental shift in average center frequency  368 A resulting from the antenna center frequency shifter, producing a slight misalignment of the channels transmitted by the 1 st , 2 nd , 3 rd  and 4 th  antenna relative to one another during antenna sounding. 
       FIG. 3C  is a graph of RSSI for various sounded beamforming antenna subsets of two antennas each on corresponding tones in each group. The graph shows the average RSSI determined from sounding feedback for each of six tones  395  within the group of six averaged out across all groups across the OFDM toneset of the selected communication channel. The tone index in each group and the corresponding subset of two antennas is shown. The highest ranked tone is the one in each group with index i+5, and the corresponding antenna subset is antennas 2,3. This graph of beamforming efficiency for various antenna subsets during transmission of an antenna sounding shows that antennas “2,3” are the highest ranked subset of antenna, with the greatest RSSI of any of the other antenna subsets. The highest and lowest ranked antenna subset differ in RSSI by a factor of 4×. These are then highest ranked subset of antenna in the Massive MIMO array for subsequent beamformed transmission of user data packets. 
       FIGS. 3D-E  are an exploded hardware view of the transmit chain expander circuit, and a per antenna tone graph during transmission of subsequent data communication packets on the highest ranked beamforming subset of antenna in accordance with an embodiment of the invention. 
       FIG. 3D  is an exploded hardware view of the transmit chain expander circuit during transmission of subsequent data communication packets on the highest ranked beamforming subset of antenna as determined in the antenna sounding. During this phase of operations, the WAP transmits downlink user data communication packets to the station  107  via the highest ranked subset of 2 of the transceiver&#39;s four antennas, e.g. antennas  276 B-C. During this phase of operations, the transmit chain expander circuit  252  and specifically the input switches  353 A-B disconnect the two transmit chains at the low voltage amplifiers  259 A-B from the chain combiner and center frequency shifter and switchably couples the two transmit chains instead to the inputs of a corresponding one of the high voltage power amplifiers  250 A-B. The transmit chain expander circuit, and specifically the multiplexers  372 A-D of the crossbar switch  270  couple the outputs of the high voltage power amplifiers to a corresponding one of the selected subset of antennas  276 B-C for downlink user data packet transmission to the transceiver  107 . The downlink user data packets are spatially mapped by the spatial mapper of the baseband stage using beamforming matrices having rows equal in number to the number of transceiver chains, e.g. two rows for the example shown. 
       FIG. 3E  is a per antenna tone graph of power vs frequency during transmission of subsequent data communication packets on the highest ranked beamforming subset of antenna, e.g. antenna  276 B-C the 2 nd  and 3 rd  antenna respectively. The OFDM tones  367 B on each antenna  276 B-C are shown as discrete rows across a 20 MHz channel without any of the shift in center frequency exhibited during sounding. Instead the transmitting subset of antenna both have the same center frequency  326 B. 
     In the examples shown above the number of antenna is small, and the number of transmit chains is a significant fraction of the eligible transmit antenna. In an embodiment of the invention where the number of antenna is large and the number of transmit chains is substantially smaller, there will be so many antenna subset combinations to evaluate that more than one sounding may be required. Each antenna sounding will nevertheless provide ranking information for multiple beamforming antenna subset candidates, thus significantly reducing the sounding overhead required to determine the optimal/highest ranked subset of antenna for subsequent transmission of user data packets. 
       FIGS. 4A-D  are hardware views of massive multiple-input multiple-output (MIMO) antenna arrays with individually steerable antennas in accordance with another embodiment of the invention. The Massive MIMO antenna arrays shown in the prior figures individually exhibit generally symmetrical, a.k.a. isotropic, signal strength footprints and can be collectively beamformed via spatial mapping and resultant shifts in phase and amplitude of the transmitted signal to produce regions of constructive interference between the signals transmitted by each of the antenna at a targeted station. By contrast the Massive MIMO antenna arrays shown in  FIGS. 4A-D  have individual antenna each of which exhibits asymmetrical, a.k.a. anisotropic, signal strength footprints. The anisotropic signal strength footprint of each antenna may be either fixed or tunable, without departing from the scope of the claimed invention. 
       FIG. 4A  shows a massive MIMO antenna comprising 4 phased arrays of 4 elements each. Each of the antenna arrays  475 A-D has a distinct anisotropic signal strength footprint,  476 A-D respectively. The spatial orientation of each of antenna arrays  475 A-D is achieved by four individual sub-elements which are spatially biased by means of a corresponding antenna driver  473 A-D which varies the phase and amplitude of the signals output by each antenna&#39;s sub-elements. Each antenna driver  473 A-D may impart a fixed spatial bias to its associated array via fixed changes in the phase and amplitude of an input signal from the corresponding one of multiplexers  372 A-D. In another embodiment of the invention the antenna drivers may be individually tunable in both phase and amplitude of the signals transmitted on each of their associated antenna arrays, thereby adjusting the spatial orientation of the antenna. Where the spatial orientation of each of antennas  475 A-D is fixed, a single antenna sounding may be used to select the optimal or highest ranked subset of antenna for subsequent use in beamformed transmission of user data packets to a targeted receiving station. Where the spatial orientation of each of antennas  475 A-D is tunable, multiple antenna soundings may be required with each sounding providing ranking information for multiple subsets of beamforming candidates. 
       FIG. 4B  shows a massive MIMO antenna comprising 4 Yagi antenna  477 A-D, each of which includes a driven element and one or more parasitic elements. Driven/active and parasitic/passive elements  478 A-B respectively for antenna  477 A are shown. Each of the Yagi antennas  477 A-D has a distinct anisotropic signal strength footprint. The spatial orientation of each Yagi antenna is achieved by one or more parasitic elements the impedance of which is controlled by means of a corresponding antenna driver  474 A-D which varies the impedance of each of the antenna&#39;s parasitic elements to spatially shape the antennas RF signal strength footprint. Each antenna driver  474 A-D may impart a fixed spatial bias to its associated Yagi antenna via fixed impedance settings for the passive elements. The driven element accepts an input signal from the corresponding one of multiplexers  372 A-D. In another embodiment of the invention the antenna drivers may tune the parasitic elements on each of corresponding Yagi antenna, thereby adjusting the spatial orientation of the antenna. Where the spatial orientation of each of antennas  477 A-D is fixed, a single antenna sounding may be used to select the optimal or highest ranked subset of antenna for subsequent use in beamformed transmission of user data packets to a targeted receiving station. Where the spatial orientation of each of antennas  477 A-D is tunable, multiple antenna soundings may be required with each sounding providing ranking information for multiple subsets of beamforming candidates. 
       FIG. 4C  shows a massive MIMO antenna comprising multiple patch antenna affixed to a reflective cylinder  480 . Each set of one or more patch antenna is driven by a sounding signal from a corresponding one of the multiplexers  372 A-D during sounding. Multiplexer  372 A is coupled to patch antennas  479 A. Multiplexer  372 B is coupled to patch antennas  479 B. Multiplexer  372 C is coupled to patch antennas, not shown. Multiplexer  372 D is coupled to patch antennas  479 D. The spatial orientation of each set of patch antennas is governed by their fixed position on the RF reflective cylinder. Thus, no antenna driver is required to impart spatial bias, a.k.a. anisotropic signal strength footprints to each set of patch antenna. A single antenna sounding may be sufficient to select the optimal or highest ranked subset of antenna for subsequent use in beamformed transmission of user data packets to a targeted receiving station. 
       FIG. 4D  shows a massive MIMO antenna comprising multiple antenna  482 A-D each with a corresponding parabolic reflector  481 A-D respectively. Each directional antenna  482 A-D is driven by a sounding signal from a corresponding one of the multiplexers  372 A-D during sounding. The spatial orientation of each antenna is governed by its fixed parabolic reflector. A single antenna sounding may be sufficient to select the optimal or highest ranked subset of antenna for subsequent use in beamformed transmission of user data packets to a targeted receiving station. 
       FIGS. 5A-C  are overall and detailed circuit diagrams of the WiFi transceiver with massive MIMO beamform antenna subset selection for each link in accordance with an embodiment of the invention. 
       FIG. 5A  is a detailed hardware block diagrams of a wireless transceiver in accordance with an embodiment of the current invention in which the transceiver comprises discrete interconnected very large scale integrated circuits (VLSI). Specifically, a VLSI WiFi stage circuit  500  which includes the WiFi transmit and receive baseband and AFE/RF circuits  220 ,  240  respectively; a transmit chain expander VLSI circuit  252  which includes the chain combiner circuit  254  and center frequency shift circuit  260 ; power amplifier circuits  250 A-B; and antennas  276 A-D. The wireless transceiver may be a WAP or a station. The transceiver supports wireless communications on a wireless local area network (WLAN) which network provides associated stations, access to the Internet  501 . 
     The transceiver in this embodiment of the invention is a WAP identified as a 2×2×4 multiple-input multiple-output (MIMO) WAP supporting as many as 2 discrete communication streams over two out of the 4 antennas  276 A-D. The transceiver couples to the Internet  501  via an Ethernet medium access control (EMAC) interface  519  over a cable, fiber, or digital subscriber line (DSL) backbone connection. A packet bus  518  couples the EMAC to the MIMO WiFi baseband  220 , and the analog front end (AFE) and Radio Frequency (RF) stages  240 . 
     In the baseband portion  220  wireless communications transmitted to or received from each user/client/station are processed. The baseband portion is dynamically configurable to support SU-MIMO or MU-MIMO transmission to MU groups of two or more users/stations. The antenna soundings of the current invention are equally applicable to SU-MIMO or MU-MIMO. In the case of MU-MIMO beamform antenna subset evaluation, the feedback from more than one targeted station is evaluated to determine which subset of antenna offers the best beamforming for the targeted stations. The AFE and RF portion  240  handles the upconversion on each of transmit paths and wireless transmission initiated in the baseband. The RF portion also handles the downconversion of the signals received on the receive paths and passes them for further processing to the baseband. 
     TRANSMISSION: The transmit path/chain includes the following discrete and shared components. The WiFi medium access control (WMAC) component  522  includes: hardware queues  522 A for each downlink and uplink communication stream; encryption and decryption circuits  522 B for encrypting and decrypting the downlink and uplink communication streams; medium access circuit  522 C for making the clear channel assessment (CCA), and making exponential random backoff and re-transmission decisions; and a packet processor circuit  522 D for packet processing of the communication streams. The WMAC component has a node table  522 E which lists each node/station on the WLAN, the station&#39;s capabilities, the corresponding encryption key, and the priority associated with its communication traffic. 
     Each sounding or data packet for wireless transmission on the transmit path components to one or more stations is framed in the framer  524 . Next each stream is encoded and scrambled in the encoder and scrambler  526  followed by interleaving and mapping in a corresponding one of the interleaver mappers  528 A-B. Next all transmissions are spatially mapped in the spatial mapper  230  with an antenna mapping matrix (AMM)  229  during antenna sounding, a spatial mapping matrix (SMM) during any channel sounding, or a beamforming matrix “V” for transmission of user data packets subsequent to a sounding. The spatially mapped streams from the spatial mapper are input to Inverse Discrete Fourier Transform (IDFT) components  232 A-B for conversion from the frequency to the time domain and subsequent transmission in the AFT and RF stage  240 . 
     Each IDFT is coupled to a corresponding one of the transmit path/chain components in the AFE/RF stage  240 . Specifically, each IDFT couples to an associated one of the digital-to-analog converters (DAC)  242 A-B for converting the digital transmission to analog, upconverters  248 A-B, coupled to a common channel frequency voltage controlled oscillator (VCO)  246  for upconverting the transmission to the appropriate center frequency of the selected channel(s), filters  549 A-B, e.g. bandpass filters, for controlling the bandwidth of the transmission, and low voltage power amplifiers  249 A-B. During antenna sounding the outputs of the low voltage power amplifiers are coupled through the chain combiner  254  and the center frequency shifter  260  to all the Massive MIMO antenna  276 A-D for concurrent evaluation of multiple subsets or in this case, distinct pairs of antenna in the antenna array. During subsequent transmission of user data packets the outputs of the low voltage power amplifiers are coupled directly through the corresponding one of the high voltage power amplifiers  250 A-B to a highest ranked subset of two of the four antennas. 
     RECEPTION: The receive path/chain includes the following discrete and shared components. Received communications on the transceiver&#39;s array of MIMO antenna are subject to RF processing including downconversion in the AFE-RF stage  240 . There are two receive paths each including the following discrete and shared components: low noise amplifiers (LNA)  580 A-B for amplifying the received signal under control of an analog gain control (AGC)  582  for setting the amount by which the received signal is amplified, filters  581 A-B for bandpass filtering the received signals, downconverters  583 A-B coupled to the VCO  246  for downconverting the received signals, analog-to-digital converters (ADC)  584 A-B for digitizing the downconverted signals. The digital output from each ADC is passed to a corresponding one of the discrete Fourier transform (DFT) components  585 A-B in the baseband portion  220  of the WiFi stage for conversion from the time to the frequency domain. 
     Receive processing in the baseband stage includes the following shared and discrete components: an equalizer  586  to mitigate channel impairments which is coupled to the output of the DFTs  585 A-B. The received streams at the output of the equalizer are subject to demapping and deinterleaving in a corresponding number of the demapper/deinterleavers  587 A-B. Next the received stream(s) are decoded and descrambled in the decoder and descrambler component  588 , followed by de-framing in the deframer  589 . The received communication is then passed to the WMAC component  522  where it is decrypted with the decryption circuit  522 B and placed in the appropriate upstream hardware queue  522 A for upload to the Internet  501 . 
     The transceiver also includes a beamform (BF) Antenna Selection (BFAS) circuit  502  coupled to the plurality of components that form the transmit and receive path as well as to the transmit chain expander circuit  252 . The BFAS controls sounding and selection of the subset of antennas used for transmission of user data packets. The BFAS circuit includes a beamforming antenna sounding circuit  504 , a sounding feedback beamforming antenna evaluation circuit  506 , and a transmit chain expander circuit switch controller  508 . The BFAS circuit  502  is coupled to non-volatile memory/storage  510  containing in an embodiment of the invention, program code  514  for executing the antenna sounding, feedback, and switch control functions on a VLSI processor circuit. The storage also includes antenna test tone records  512 . 
     The beamforming antenna sounding circuit  504  shown in  FIG. 5A  and in exploded hardware view in  FIG. 5B  includes: a beamforming antenna combination tone mapper circuit  504 A, an antenna mapping matrix and training symbol generator circuit  504 B, and a sounding control circuit  504 C. The beamforming antenna combination tone mapper circuit  504 A identifies the transceiver capabilities, determines using combinatorics how many distinct beamformed antenna subsets of the massive MIMO array can be driven with existing Transceiver Chains and maps of OFDM tones to specific beamformed antenna subsets to test in the antenna sounding. The antenna mapping matrix and training symbol generator circuit  504 B determines a linearly Independent Antenna Mapping Matrix (AMM) for each BFAC test tone and a common training symbol for each group of test tones. Antenna test tone records  512  of these AMM, to tone, to antenna subset mappings are stored in storage  510 . The sounding control circuit  504 C injects the antenna mapping matrices into the spatial mapper  230  during the antenna sounding. 
     The sounding feedback beamforming antenna evaluation circuit  506  shown in  FIG. 5A  and in exploded hardware view in  FIG. 5C  includes: a sounding feedback tone ranking circuit  506 A, and a tone-to-beamform antenna subset/combination correlation circuit  506 B. The sounding feedback tone ranking circuit  506 A ranks the sounding feedback for each of the OFDM tones used for sounding antenna combinations/subsets. Ranking may be based on any one or all of: received signal strength indicator (RSSI), received power, signal-to-noise ratio (SNR) or channel rank for example. A highest ranked tone and tone index across all groups of tones involved in the antenna sounding is determined. The tone-to-beamform antenna subset/combination correlation circuit  506 B correlates the highest ranked tone, e.g. the 5 th  tone in each group, across all groups of tones, with the subset of the Massive MIMO antennas with which it is associated using the antenna test tone records  512  in storage. 
     The transmit chain expander circuit switch controller  508  switches the transmit chains through the integrated chain combiner and center frequency shifter circuits  254 ,  260  during antenna sounding. Conversely, once a distinct subset of antenna has been identified the switch controller couples the each transmit chain to the corresponding one of the antenna in the distinct subset via the power amplifiers  250 A-B for example. In this latter switch configuration, an added “normal” channel sounding may be required on the highest ranked subset of antenna to explicitly identify the beamforming matrix for each of the OFDM tones in subsequent transmission of user data packets. 
       FIG. 6  is a process flow diagram of processes associated with a massive MIMO transceiver&#39;s beamform antenna subset selection for each link in accordance with an embodiment of the invention. The first block of processes  600 - 618  are directed to selecting an optimal subset of antennas for beamformed user data packet transmission. In process  600  the transceiver capabilities in terms of the number “N” of Massive MIMO Antennas and the number “T” of transmit chains are identified. Next, in process  604  the number of distinct beamforming antenna subsets/combinations C(N, T) of the Massive MIMO Array that can be driven with existing transmit chains is calculated as: C(N, T)=N!/(T!×(N−T)!) In embodiments of the invention which include Massive MIMO antenna arrays such as those shown in  FIGS. 4A-4B  in which individual antenna are themselves spatially tunable the number of overall antenna subset combinations to test will increase exponentially as S T  where “S” is the number of different spatial directions, e.g. North and South, to which any antenna in the array can be tuned. The number of combinations that are subject to an antenna sounding may be less than the total number of possible combinations, without departing from the scope of the claimed invention. The next process  606  is optional. In process N′ of the Massive MIMO Antennas are selected for the sounding, where N′&gt;T and N′&lt;=N. After the combinatorics calculations have been made control is passed to process  608  in which a plurality of the beamforming Antenna subsets/combinations to test in a single explicit sounding is determined. Next in process  610  at least one OFDM test tone for each of the beamforming antenna subsets/combinations is selected for testing in the explicit sounding. The tones may be grouped as shown in  FIG. 2A . Then in process  612  linearly independent Antenna Mapping Matrix (AMM) are determined for each beamformed antenna subset/combination test tone where AMM row dimension&gt;T and =to N or N′ in the embodiment of the invention where N′ antennas are selected for testing. Next in process  614  the 
     designated beamform antenna subset/combination (BFAC) test tones in the OFDM toneset of an explicit sounding are precoded with corresponding LI AMM. This process is said to take place in the frequency domain since it involves the processing of individual tones or sub-channels each of which has a distinct center frequency. Then in process  616  all of the OFDM tones of the explicit sounding are transformed from the frequency to the time domain in by the IDFT component in each transmit chain. 
     Next, a block  618  of sub-processes  618 A-C is conducted in the time domain, the collective effect of which is to expand the T physical transmit chains into N virtual chains matching in number the number of antenna under test. In process  618 A the explicit sounding signal on all physical transmit chains are combined into “N” distinct combination, one for each of the N antenna under test. Then in process  618 B the transmit chains are switchably coupled to all of the MIMO Antennas under test, e.g. N or N′ antenna. Then in process  618 C each explicit sounding combination is transmitted on a corresponding one of the N or N′ antennas at a distinct center frequency relative to each of the other antenna. Control is then passed to the next block of processes, for sounding feedback processing. 
     In process  630  the sounding feedback for each transmitted tone in the antenna sounding contains channel state information on each received sounding tone including at least one of: a channel matrix “H”, a beamforming matrix “V”, and a Signal-to-Noise Ratio (SNR) matrix. From these the rank of each of the BFAC Test tones across all groups of tones within the sounding toneset may be determined. Ranking may be based any one or all of: RSSI, SNR, channel rank, received power as determined from the sounding feedback. Next in process  632  the subset of T of the N antennas under test associated with highest ranked BFAC test tone, is identified. 
     Control is then passed to decision process  634  in which it is determined if more beamformed antenna subsets need to be sounded. If so control returns to process  608  for further antenna subset soundings. If not control passes to process  636 . In decision process  636  a determination is made as to whether the groups of tones used for the antenna soundings are sufficiently small so that the highest ranked tone and the associated channel state information can be used to determine the beamforming matrices for the lower ranked, a.k.a. gap tones. If so, then no more soundings are necessary and control passes to the user data transmission block of processes  640 - 642 . Alternately, if a traditional/normal channel sounding is required then control is passed to process  638 . In process  638  an explicit channel sounding is transmitted on the highest ranked subset of the antennas to determine from the resultant sounding feedback the beamforming matrix to be used for subsequent user data packet transmission. Control is then also passed to the block of data transmission processes  640 - 642 . 
     In data transmission process  640  each transmit chain is switchably coupled to a corresponding one of the “T” antennas of the identified subset of antennas. Then in process  642  the MPDU or MSDU Data Communication Packets are transmitted on the Identified Subset of Antenna, spatially Mapped across all OFDM tones with SMM and with the same center Frequency for all Antenna. 
     The aforesaid processes repeat themselves intermittently as channel conditions change in order to maintain the efficiency of the WLAN communications. Regular channel soundings may be conducted with greater frequency. 
     EXAMPLES 
     Antenna Selection with Beamforming Antenna selection is performed when the number of physical antennas exceeds the number of transmit or receive chains. In that case, a decision needs to be made about which antennas to connect to the physical transmit or receive paths. In an environment with rich scattering, the selection of the right set of antennas based for example on location and antenna properties is critical to achieve optimal performance. The available antennas may be of different types. One type can be omnidirectional, a.k.a. isotropic. The other type can have a specific spatial pattern, a.k.a. anisotropic. Digital beamforming can be applied in conjunction with these types of antennas. 
     In this invention, a subset of antennas will be selected based on receiver feedback indicating expected performance with beamforming. The receiver will determine its feedback based on reception of a specially designed training signal that allows concurrent estimation of the performance of multiple possible antenna selections. This is achieved by using separate OFDM tones to test different antenna selections. 
     In an embodiment of the invention the selection will require some form of feedback from the receiver side to establish which transmit antennas result in the highest signal at the respective receive antennas. In a system with eight antennas, but only four active transmit paths, a different set of four antennas can be selected for different transmissions. Ideally, the selection will be done intelligently to optimize performance. The antennas could be identical omni-directional antennas. Alternatively, the antennas may each have different spatial coverage resulting in different antennas having different radiation patterns. 
     When there are multiple transmit antennas, the problem becomes how to select a subset of the antennas that results in the best signal at the intended receiver. It may be possible to establish the performance of each transmit antenna to the receive antenna(s) individually but this type of evaluation results in sub-optimal antenna selection when transmission is combined with beamforming. When multi-antenna beamforming is utilized for transmission, the antennas work in coordinated fashion and the best performing beamforming subset of antennas, may not include any of the antenna which exhibit the best performance individually. A beamforming precoding matrix that provides good gain with one subset of antennas will not be optimal for a different antenna subset. The optimal beamforming matrix is specific to the selected subset. 
     Where a wireless transceiver has 4 transmit chains that can be coupled to an optimal subset of 8 antennas there are 70 possible beamforming combinations to test. For each of the four antenna subsets of the eight antennas we apply beamforming to it. The resulting receive power with the calculated beamforming then provides a measure of the strength of the received signal. The received signal strength can vary greatly depending on which four antennas are selected. It is also interesting to observe that in a representative grid layout of the antenna in a 1×8 Gaussian channel, the worst antennas combination, e.g. antennas [4 5 7 8], is the complement of the best antenna combination, e.g. antennas [1 2 3 6] with the two differing by a factor of five in terms of RSSI. 
     Antenna selection with beamforming raises a number of specific issues: a) Antennas needs to be evaluated in groups b) Combining performance of individual antennas can not be used to determine optimal beamforming antennas, and c) a large number of such antenna subset combinations has to be evaluated, e.g. selecting four antennas for beamforming out of eight possible antennas requires the evaluation of 70 possible sets of antennas) 
     A standard mechanism to receive beamforming feedback is through the exchange of sounding frames and sounding responses. Conceptually, each antenna selection will require its own sounding information. From the sounding feedback, the strength of the received signal can be derived. Doing a separate sounding for each antenna combination is not practical, as it requires way too much overhead for the successive soundings of each combination. 
     It should be noted however that we do not need detailed per-tone channel information. A rough estimate of the received signal strength for various antenna selections is sufficient. Ideally, this estimate should still span the entire frequency range to account for possible fading in parts of the spectrum. However, the granularity can be much coarser than what is needed for actual beamforming. A possible approach to getting such feedback is to combine multiple “candidate” antenna subset selections into a single sounding frame. In OFDM, each tone can in principle use a separate spatial mapping matrix. This permits the use of different spatial mapping matrices for different tones. For the first tone, four transmit chains could be mapped to one choice of four (out of e.g. eight) transmit antennas. For the next tone, the four transmit chains could be mapped to a different set of four antennas, and so on. Of course, for each antenna selection, we&#39;ll only receive feedback on a limited number of tones. In practice, the number of different selections will be limited by the desire to repeat each spatial mapping matrix at least a couple of times in the available spectrum width of the selected communication channel(s). In practice, there may be no need to cover all 70 combinations (for the case of four transmit chains and eight antennas). A subset most likely will provide us with a representative sample of the selections. Also, it may be possible to cut the number of antenna sections in half by not explicitly evaluation “complimentary pairs” (such as [4 5 7 8] and [1 2 3 6]). The complement of the worst selection may be a good candidate for the best selection. 
     In an embodiment of the invention, a first combined sounding feedback serves to guide the antenna selection. Once a selection is established, a dedicated channel sounding across all tones may be required for the highest ranked subset of antenna to find a higher-accuracy beamforming matrix for each tone. In another embodiment of the invention in which the beamforming combinations are more limited, a dedicated channel sounding may not be necessary, if the tone spacing between the tones sounding the optimal beamforming subset of antenna is sufficiently small, the channel or beamforming matrix in the sounding feedback may be interpolated across the gap tones. 
     In an embodiment of the invention, the beamforming antenna subset evaluation, can be repeated over time. Typically, the first type of, i.e. the combined antenna subsets and channel sounding (combined) needs to be performed less frequently than the “regular” channel sounding. The main challenge to achieving this in practice is that the relationship between transmit chains and transmit antennas is different for a system with antenna selection. In this case, there is no one-to-one mapping between chains and antennas. By design, the spatial mapping defined in MIMO systems like IEEE 802.11n and IEEE 802.11ac takes streams as input and maps its output to the transmit chains. In the case of antenna selection, the transmit chains are mapped to a subset of transmit antennas using the feedback from a single training sequence. This latter form of mapping happens in the time-domain since outputs of the transmit chains are time-domain signals. As such, it is not conducive to frequency-dependent mapping. In practice, these connections are performed using simple relays. 
     This embodiment of the invention includes a combined spatial mapping in the frequency domain and antenna selection mapping in the time domain that effectively achieves frequency-dependent mapping at the antennas. The antenna selection mapping part of this should remain as simple as possible because of the way it is implemented in practice in the time-domain. 
     In this case of four antennas and two transmit chains, we&#39;d like to map the two transmit chains to the four antennas, but in such a way that the mapping is different for different tones. Conceptually, this can be written as: 
                       Ant   j     ⁡     (   n   )       =       ∑     k   =   0       N   -   1       ⁢       Q   ji     (   k   )       ⁢       X   i     ⁡     (   k   )       ⁢     e     j   ⁢       2   ⁢   π   ⁢           ⁢   kn     N                     (   1   )               
Where:
 
     Ant j (n) is the output of antenna j (j=1, . . . , 4) 
     Q ji   (k)  is the tone-dependent 4×2 spatial mapping matrix 
     X i (k) is the training signal of streams i=1,2 on tone k 
     While this is conceptually what we want to achieve, it can not be done directly as with traditional spatial mapping between streams and transmit chains. The goal is to transform (1) to a form that needs no more than two transmit chains and that connects the output of these two transmit chains to the four antennas using blocks that can be easily implemented in the time-domain. 
     For the case under consideration (two transmit chains vs. four antennas), there are six possible ways to map transmit chains to antennas, namely: 
               [         1           2           ×           ×         ]     ,     [         1           ×           2           ×         ]     ,     [         1           ×           ×           2         ]     ,     [         ×           1           2           ×         ]     ,     [         ×           1           ×           2         ]     ,     [         ×           ×           1           2         ]           
If we want to cover all these mappings in our sounding frame, we need six different spatial mapping matrices. We&#39;ll distribute them equally throughout the spectrum, such that:
 
 Q   ji   (6k+m)   =Q   ji   (m)   (2)
 
i.e.: the matrices repeat every six tones.
 
     With this, we can rewrite (1) as: 
                       Ant   j     ⁡     (   n   )       =       ∑     m   =   0     5     ⁢       ∑     k   =   0         N   /   6     -   1       ⁢       Q   ji     (       6   ⁢   k     +   m     )       ⁢       X   i     ⁡     (       6   ⁢   k     +   m     )       ⁢     e     j   ⁢       2   ⁢     π   ⁡     (       6   ⁢   k     +   m     )       ⁢   n     N                       (   3   )               
This still doesn&#39;t achieve the desired form. As a next step, let&#39;s design the training signal such that X i (6k+m)=X i (6k), i.e. six consecutive tones have the same training constellation point.
 
     With these both assumptions, (3) can be written as: 
                       Ant   j     ⁡     (   n   )       =         (       ∑     m   =   0     5     ⁢       Q   ji     (   m   )       ⁢     e     j   ⁢       2   ⁢   π   ⁢           ⁢   mn     N             )       ︸     M   ji         ⁢       (       ∑     k   =   0         N   /   6     -   1       ⁢       X   i     ⁢     (     6   ⁢   k     )     ⁢     e     j   ⁢       2   ⁢   π   ⁢           ⁢   6   ⁢   kn     N             )       ︸     S   i                   (   4   )               
(4) shows the expression in discrete time. In continuous time, the expression would be:
 
                       Ant   j     ⁡     (   t   )       =         (       ∑     m   =   0     5     ⁢       Q   ji     (   m   )       ⁢     e     j   ⁢           ⁢   2   ⁢   π   ⁢           ⁢   m   ⁢           ⁢   Δ   ⁢           ⁢   f   ⁢           ⁢   t           )       ︸     M   ji         ⁢       (       ∑     k   =   0         N   /   6     -   1       ⁢       X   i     ⁢     (     6   ⁢   k     )     ⁢     e     j   ⁢           ⁢   2   ⁢   π   ⁢           ⁢   6   ⁢   k   ⁢           ⁢   Δ   ⁢           ⁢   f   ⁢           ⁢   t           )       ︸     S   i                   (   5   )               
Analyzing (4) or (5), we see that the desired (but un-implementable) signal (1) has been transformed into a linear combination of two time-domain signals S i . These two time-domain signals are mapped to the four antennas using a new mapping matrix M ji , which depends on a linear combination of all frequency-dependent mapping matrices and also exhibits time-dependency.
 
     The signals S i  can be generated using the two transmit chains that we have available. In the more general case, the number of signals S i  will correspond to the number of available transmit chains. The main question is now how to efficiently implement the antenna mapping in the time domain (the matrix M ji ). To simplify the implementation, we begin by making a convenient choice of mapping matrices as follows: 
                 Q     (   0   )       =     [         1       j           1         -   j             0       0           0       0         ]       ,       Q     (   1   )       =     [         1       j           0       0           1         -   1             0       0         ]       ,       Q     (   2   )       =     [         1       j           0       0           0       0           1       1         ]       ,     
     ⁢       Q     (   3   )       =     [         0       0           1         -   j             1         -   1             0       0         ]       ,       Q     (   4   )       =     [         0       0           1         -   j             0       0           1       1         ]       ,       Q     (   5   )       =     [         0       0           0       0           1         -   1             1       1         ]       ,         
What makes these matrices convenient is that any element is either 0 or identical for all of the matrices. For instance Q 11   (m)  is either 1 or 0 for all m, Q 12   (m)  is either j or 0 for all m. This will simplify the sum over m in (4).
 
     With this choice of matrices, we have: 
               M   ji     =       [           1   +     e     j   ⁢       2   ⁢   π   ⁢           ⁢   n     N         +     e     j   ⁢       2   ⁢   π2n     N                                                                     1   +     e     j   ⁢       2   ⁢   π3   ⁢           ⁢   n     N         +     e     j   ⁢       2   ⁢   π4n     N                                                                       e     j   ⁢       2   ⁢   π   ⁢           ⁢   n     N         +     e     j   ⁢       2   ⁢   π3   ⁢           ⁢   n     N         +     e     j   ⁢       2   ⁢   π5   ⁢           ⁢   n     N                                                                       e     j   ⁢       2   ⁢   π   ⁢           ⁢   2   ⁢   n     N         +     e     j   ⁢       2   ⁢   π4   ⁢           ⁢   n     N         +     e     j   ⁢       2   ⁢   π5   ⁢           ⁢   n     N                 ]     ⁡     [         1       j           1         -   j             1         -   1             1       1         ]             
The first step in the antenna mapping is a fixed 4×2 mapping matrix:
 
                 [         1       j           1         -   j             1         -   1             1       1         ]     ⁡     [           S   1               S   2           ]       =     [             S   1     +     jS   2                   S   1     -     jS   2                   S   1     -     S   2                   S   1     +     S   2             ]           
The intermediate output of this step is a set of four simple linear combinations of the S i . The only required operations are additions and phase rotations of multiple of 90 degrees. All of these operations can be executed very simply in the time domain.
 
     The second step is a multiplication of each of the four intermediate outputs with a sum of sinusoids. This too is something that can be implemented directly in the time-domain. A possible implementation of this is illustrated in  FIG. 3A . The connections between transmit chains and transmit antennas shown in  FIG. 3A  are more complex than what would be needed for straightforward antenna-selection. However, all involved operations can be performed in the time-domain and we get the benefit that multiple possible antennas selections can be sounded with a single sounding frame. 
     Note that the connections could be done in either analog or digital domain. The output of the “baseband” block could be analog signals (as in equation (5)), in which case the connections and multiplications are performed by analog blocks. Alternatively, the outputs, combining and multiplications could be done in digital (as in equation (4)), with the final signals then being fed into a discrete set of ADCs. 
     Process flow for Beamforming Antenna Subset Selection: To determine the best antenna set in combination with beamforming, we&#39;ll essentially go through the following steps: a) Determine set of Spatial Mapping Matrices corresponding to the number of transmit chains and the number of transmit antennas; b) Configure the chains-to-antenna mapping to achieve frequency-dependent mapping; c) transmit frequency-dependent sounding for the selected set of spatial mapping matrices thereby combining different possible beamform antenna selections in a single sounding; d) Identify an optimal selection based on the tone-dependent sounding feedback to the sounding frame; e) select the best beamform antenna subset selection based on received feedback; f) Perform “regular” sounding using the highest ranked antenna subset selection as at antenna selection. 
     This embodiment of the invention selects a subset of antennas to connect to a smaller number of transmit chains. The selection is based on receiver feedback in response to a training signal that is designed to use separate OFDM tones to test different antenna selections. This allows for concurrent estimation of the performance of multiple possible antenna selections. 
     In other embodiments of the invention the aggregated beamforming antenna subset sounding in the frequency domain described and discussed above, may be applied with equal advantage to WAPs or stations with any number of transmit chains and any number of massive MIMO antenna, including but not limited to: 1×2 . . . 1×n, 2×3, 2×4 . . . 2×n, 3×4, 3×n, 4&gt;5 . . . 4×8 . . . 4×n, . . . 8×9, . . . 8×16 . . . 8×n; without departing from the scope of the Claimed invention. 
     The components and processes disclosed herein may be implemented in a combination of software, circuits, hardware, and firmware, integrated with the WAP&#39;s existing transmit and receive path components, and without departing from the scope of the Claimed Invention. 
     The foregoing description of a preferred embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in this art. It is intended that the scope of the invention be defined by the following claims and their equivalents.