Abstract:
A system and method for overriding Carrier-Sense-Multiple-Access/Collision-Avoidance (CSMA/CA) and virtual carrier sense, without harming the traffic that occupies the channel is described herein. Further provided herein are measurements and qualifying criteria for performing the aforementioned channel sharing. The system and method may be based, for example, on opportunistic spatial isolation of nodes from each other and selectively implementing ultra-fast link adaptation.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims benefit from U.S. provisional patent applications Ser. No. 61/844,750 filed on Jul. 10, 2013, Ser. No. 61/861,572 filed on Aug. 2, 2013, Ser. No. 61/845,271 filed on Jul. 11, 2014, Ser. No. 61/876,886 filed on Sep. 12, 2013, and Ser. No. 61/877,012 filed on 12 Sep., 2013, each of which are incorporated herein by reference in their entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to wireless communication, and more specifically to high efficiency Wi-Fi. 
       BACKGROUND OF THE INVENTION 
       [0003]    Prior to setting forth the background of the invention, it may be helpful to set forth definitions of certain terms that will be used hereinafter. 
         [0004]    The term “Wi-Fi” as used herein is defined as any wireless local area network (WLAN) products that are based on the Institute of Electrical and Electronics Engineers (IEEE) 802.11 standards. 
         [0005]    The term “Access Point” or “AP” as used herein is defined as a device that allows wireless devices (also known as User Equipment or “UE”) to connect to a wired network using Wi-Fi, or related standards. The AP usually connects to a router (via a wired network) as a standalone device, but it can also be an integral component of the router itself. 
         [0006]    The term “client” as used herein is defined as any device that has wireless communication capabilities, specifically, the IEEE 802.11 standards. A client may be for example a smart telephone, a laptop, a tablet or a personal computer (PC). 
         [0007]    The notation “STA” as used herein is defined in as an IEEE 802.11 client. 
         [0008]    The term “node” as used herein is defined as general name for both IEEE 802.11 AP and IEEE 802.11 STA. 
         [0009]    The term “serving AP” as used herein is defined in relation to one AP and one STA, wherein the STA is registered to said AP, and said AP and STA are sending and receiving data to and from each other. 
         [0010]    The term “neighboring APs”, nodes or devices may include two (or more) co-frequency APs, nodes or devices that are within each other&#39;s sensitivity range, e.g. at least one of them can receive signals from the other in such as signal-to-noise ratio that allows decoding of signals. 
         [0011]    The term “clear channel assessment range” or “CCA range” as used herein is a range between two IEEE 802.11 nodes, wherein at least one node can receive the other&#39;s transmission at a power level equal or larger than −82 dBm. 
         [0012]    The term “CSMA/CA” stands for Carrier-Sense-Multiple-Access/Collision-Avoidance, representing a requirement to listen before transmitting in a multi-node wireless system that shares a common channel on the basis of first-come-first-served. 
         [0013]    The term “preamble” as used herein describes a certain 802.11 transmitted signal modulation appearing at the beginning of each packet, that when received by other 802.11 nodes, will force them to yield channel access. 
         [0014]    The notation “SINR” stands for Signal-to-Noise and Interference. 
         [0015]    The term “ACK” as used herein, stands for acknowledgement or an acknowledgement signal, as for example defined as the signal transmitted from an IEEE 802.11 receiving node to the IEEE 802.11 node that has transmitted a packet to it, provided the packet was successfully received. 
         [0016]    The term “time division duplex” (TDD) as used herein is referred to in general for systems using the same frequency spectrum for methods of communications in a time division manner such as Wi-Fi systems. 
         [0017]    Implicit feedback is a process used for TDD protocols such as Wi-Fi, where both down and up links share the same spectrum. In the aforementioned process, the uplink channel estimated by the AP, is assumed to be identical to the downlink one—based on reciprocity principle—and is therefore is considered by the AP to represent the channel towards the client/STA 
         [0018]    Explicit feedback is a procedure where AP transmissions are channel estimated by the STA, and then fed back to the AP, providing it with the magnitude of phase and amplitude differences between the signals as transmitted by the AP vis-à-vis as received by the client/STA, allowing it to gauge possible distortions and correct them. 
         [0019]    The term “Legacy Long Training Field” (L-LTF) denotes a field within the IEEE 802.11 Preamble name. 
         [0020]    The term “Very-High-Throughput Long Training Field” (VHT-LTF) denotes a field within the IEEE 802.11 Preamble name Very-High-Throughput Long Training Field. 
         [0021]    Associated STA is defined herein as a STA that is served by a certain AP with a certain Service Set Identifier (SSID). 
         [0022]    Non-associated STA is defined herein as a STA within the range of and non-serving AP and/or its non-serving STAs. 
         [0023]    The acronym “NAV” stands for Network-Allocation-Vector and represents virtual carrier sense mechanism, used by a Wi-Fi transmitting message to broadcast the predicted duration of its transmission, signaling to other nodes how long will the channel be occupied 
         [0024]    The acronym “RTS” stands for Request-To-Send, and represents a message transmitted by one Wi-Fi node to another, probing it for information about its availability to receive data, per the Wi-Fi Alliance protocol. 
         [0025]    The acronym “CTS” stands for Clear-To-Send, and represents a positive response from the said other node to the node originating the RTS, indicating to the requesting node that the channel is clear from its point of view as well. 
         [0026]    The notation “DURATION” is a message embedded in both RTS and CTS, representing a prediction of the future traffic about to be transmitted between two nodes that have captured the channel; other nodes that receive it, must clear the channel as long as the DURATION has nor expired; other nodes that have received the RTS but received the CTS (hidden nodes) will avoid accessing the channel, allowing the receiving node to successfully complete the reception. 
         [0027]    The acronym “FLA” stands for Fast Link Adaptation, and represents processed that reduce transmitting side learning time of the receiver&#39;s SINR. 
         [0028]    The acronym “MCS” stands for Modulation Coding Scheme, mapping SINR to modulation order and code rate. 
         [0029]    The acronym “MRQ” stands for MCS request (Modulation Code Scheme Request). 
         [0030]    The acronym “MSI” stands for MRQ Sequence Identifier, which carries MCS feedback from receiver to transmitter. 
         [0031]    The term “null” as used herein, is a spatial pattern, created by two or more antennas, formed in such a way that minimizes the power level received by a given receiver. A “Rx Null” is a null formed by a receiver&#39;s antennas weight in order to decrease undesired signal level. A “Tx Null” is form by a transmitters antennas weights in order to decrease it undesired transmitted signal at remote receiver&#39;s input. 
         [0032]    The term “nulling AP” as used herein, is a station (client) that is equipped with nulling capabilities. 
         [0033]    The term “nulled AP” as used herein is an Access Point within CCA from a Nulling STA. 
         [0034]    The term “neighbors list” as used herein is a list of neighboring APs and STAs within CCA range which keeps history to be checked. 
         [0035]    The term “close neighbors” as used herein are subgroups in the Neighbors&#39; list that are within each other&#39;s CCA range. 
         [0036]    According to the IEEE 802.11 air protocol, two neighboring APs can download traffic over the same frequency channel to their respective STAs, as long as these APs are not within CCA range of each other. 
         [0037]    When an RTS/CTS procedure is used, an additional condition is introduced. Namely, a legacy AP serving a respective station, must not be within CCA range of the other neighboring AP if the AP is occupying the channel. 
       SUMMARY OF THE INVENTION 
       [0038]    Embodiments of the invention provide a method of overriding CSMA-CA and virtual carrier sense, without harming the traffic that occupies the channel, together with measurements and qualifying criteria for performing said channel simultaneous sharing. Embodiments of the invention are based on opportunistic spatial isolation of nodes from each other and selectively implementing ultra-fast link adaptation. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0039]    The subject matter regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of operation, together with objects, features, and advantages thereof, may best be more fully understood by reference to the following detailed description when read with the accompanying drawings in which: 
           [0040]      FIG. 1A  is a block diagram illustrating co-channel legacy APs deployment in accordance with the prior art; 
           [0041]      FIG. 1B  is a block diagram illustrating a communication node within CCA range of a neighboring node, in accordance with some embodiments of the present invention; 
           [0042]      FIG. 2  is an example of a modified AP turning a two-way null towards a co-channel AP outside immediate CSMA range in accordance with some embodiments of the present invention; 
           [0043]      FIG. 3  show pre-requisites for a nulling based channel reuse in accordance with some embodiments of the present invention; 
           [0044]      FIG. 4  illustrate additional consideration of conflicts with neighbors in accordance with some embodiments of the present invention; 
           [0045]      FIG. 5  illustrates channel reciprocity implicit calibration in accordance with some embodiments of the present invention; 
           [0046]      FIG. 6  illustrate a transmission null preparation process in accordance with some embodiments of the present invention; 
           [0047]      FIG. 7  illustrates additional neighbors&#39; conflict resolution in accordance with some embodiments of the present invention; 
           [0048]      FIG. 8  illustrates reception null preparation—for a single null, in accordance with some embodiments of the present invention; 
           [0049]      FIG. 9  illustrates two reception null separation in accordance with some embodiments of the present invention; 
           [0050]      FIG. 10  is an example of Effective Isotropic Radiated Power (EIRP) modification due&#39;s null steering and null selection in accordance with some embodiments of the present invention; and 
           [0051]      FIG. 11  illustrates ultra-fast link adaptation sequence in accordance with some embodiments of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0052]    In the following description, various aspects of the present invention will be described. For purposes of explanation, specific configurations and details are set forth in order to provide a thorough understanding of the present invention. However, it will also be apparent to one skilled in the art that the present invention may be practiced without the specific details presented herein. Furthermore, well-known features may be omitted or simplified in order not to obscure the present invention. 
         [0053]    Unless specifically stated otherwise, as apparent from the following discussions, it is appreciated that throughout the specification discussions utilizing terms such as “processing,” “computing,” “calculating,” “determining,” or the like, refer to the action and/or processes of a computer or computing system, or similar electronic computing device, that manipulates and/or transforms data represented as physical, such as electronic, quantities within the computing system&#39;s registers and/or memories into other data similarly represented as physical quantities within the computing system&#39;s memories, registers or other such information storage, transmission or display devices. 
         [0054]    The distributed nature of 802.11 channel access protocol facilitated lowering of usage barriers and contributed to its enormous success; some of deficiencies that come along with the protocol&#39;s binary co-existence code of conduct, are recently being readdressed by IEEE efforts to introduce physical layer (PHY) and Media Access Control (MAC) enhancements that will increase spatial capacity; this invention focuses on some scenarios where novel spatial isolation of signaling between adjacent nodes may allow them to use same channel for simultaneous data transmissions. 
         [0055]    According to the 802.11 air protocol, two adjacent APs are exchanging traffic over the same frequency channel with one of its STAs, as long as none of the STAs-APs pairs are with in CCA range of each other; once two transmitting nodes are within CCA range, simultaneous channel access is prohibited. 
         [0056]    In one embodiment, two adjacent APs that are within CCA range of each other may transmit downlink streams to served STAs, as long as these STAs are outside the CCA range of the non-serving AP. This may be made possible for example by one AP tuning a 2-way spatial null towards or in the direction of the other AP. 
         [0057]    It is noted that during the time period in which both APs are transmitting, there is no need to protect one from the other&#39;s signal; the said nulls relevant when the one AP transmits while the other receives; 
         [0058]    In one embodiment scenario, if an AP is receiving ACK from its STA while the other is transmitting at CCA range, jamming must be avoided, so the nulling AP must protect both it&#39;s own ACK reception, as well as the other AP&#39;s ACK reception, by applying a Tx null as well as a Rx null towards the other AP; 
         [0059]    In the aforementioned scenario, if an AP contemplating accessing a channel occupied by another AP is using an RTS/CTS procedure, then the RTS must not be allowed to interfere with a possible ACK arriving at that time, and the CTS must be protected from being jammed by the neighboring AP, so again, a TX and a Rx nulls are applied. 
         [0060]      FIG. 1A  depicts an example of 3 adjacent APs, where AP 1  and AP 2  ( 101 ,  102  respectively) are within the CCA range of each other, meaning that one&#39;s transmission are received by the other at power level higher than −82 dBm, thus requiring them to yield to each other&#39;s preamble; additionally, AP 1 &#39;s other neighbor AP 3 , is outside the CCA range. Thus this pair can access the channel simultaneously, yet not without mutual interference as their receivers experience different boundaries with and without the other&#39;s activity. E.g. when AP 3  is silent, AP 2  may be bounded by AWGN (˜92 dBm in typical Node), while when AP 3  transmits, the boundary will be lower (somewhere between −82 and −92 dBm). Note that such uncertainty, if not accounted for by the link adaptation, may cause an uplink transmission to become un-decodable. 
         [0061]      FIG. 1B  is a block diagram illustrating a communication node  101  within CCA range of a neighboring node  102  having one or more antennas  102 A, in accordance with some embodiments of the present invention. Communication node  101  may include: a plurality of antennas  10 - 1  to  10 -N; a plurality of radio circuits  20 - 1  to  20 -N configured to transmit and receive in compliance with IEEE 802.11 standard; and a baseband processor  30 . 
         [0062]    In operation, communication node  101  is configured to transmit and receive signals within a clear channel assessment (CCA) range of a number N neighboring nodes sharing a co-channel and further communicating in compliance with the IEEE 802.11 standard. 
         [0063]    In operation, baseband processor  30  may be configured to set weights on radio circuits  20 - 1  to  20 -N such that spatial signatures are generated in both downlink and uplink streams or transmissions (the uplink may be define as the direction from the AP to the STA and the uplink being the direction from the STA to the AP) in a way that isolates the communication node from at least one of the N neighboring nodes, in a manner that allows said communication node to access said co-channel, known or recognized by said communication node to be currently used by at least one of the N neighboring nodes, while keeping the transmitted signal level of the communication node as received by said N neighboring nodes, below the CCA signal level. According to some embodiments of the present invention, communication node is a nulling AP and the neighboring nodes are nulled APs, and wherein the APs communicate in compliance with the 802.11 standard. The nulling may be created by the nulling AP, are in both downlink and uplink toward or in the direction of the nulled APs. 
         [0064]      FIG. 2  depicts two APs within CCA range, where AP 2  ( 201 ) is a legacy AP (e.g. older IEEE 802.11 standards such as: a, b, g, n and ac) that has captured the channel first and performs multi-stream downlink transmission to STA 2  ( 202 ); AP* 1  ( 203 ) is an AP modified according to embodiments of the invention; rather than wait for the channel to clear, it performs a 2-way null towards or in the direction of AP 2 , calculating Transmission null ( 205 ) and Reception Null ( 206 ), and serves STA 1  ( 204 ) after verification of various conditions outlined in following paragraphs; the role of the Transmission Null is to protect AP 2  from being jammed by AP* 1  during the period of time in which AP 2  receives ACK 2  from STA 2 ; the role of the Reception Null is to protect AP* 1  from being jammed by AP 2  during the periods in which STA 1  transmits signaling like ACK 1    
         [0065]    Nulling transmitted signals requires that their channels will be known; assuming AP 2  is not performing explicit feedback process with AP* 1 , an implicit feedback is used, based on reciprocity calibration described in  FIG. 5   
         [0066]    As 802.11 multi-stream MIMO does not have an antenna reference signal, nulling can be effectively done only for the one antenna that transmits the preamble (without pre-coding), leaving the others unprotected; such a limited protection does not support multi-stream reception, but may be sufficient for single stream reception, e.g. ACK, and CTS; yet, as MRC reception of said signals is rendered deficient—only one antenna is free from jamming—there is a need to leave margin in the order of 10*log (N) dB (where N is number of AP2 antennas). 
         [0067]    Uplink SINR estimation becomes therefore a requirement; a rough estimate can be done by monitoring AP2&#39;s Downlink MCS; since EIRP is not symmetric, the DL MCS should be compensated for the assumed DL/UP power ratio. 
         [0068]    According to some embodiments of the present invention, the downlink transmit null may be executed while nulling AP transmits a request to send (RTS) to a station about to be served by the nulling AP, and the clear to send (CTS) is received while the uplink receive nulls are executed. Similarly, the nulling AP may send an RTS to a station which the nulling AP is about to serve, and conditions the nulling with reception of clear to send (CTS). 
         [0069]      FIG. 3  addresses the limitation of STAs selection: Since embodiments of the invention limit themselves to the creation of a single null, and as this null is turned to a close by neighboring AP, then STA 2  which is served by said AP 2  neighbor, must be hidden form AP* 1 , or else the Null-base-access is not attempted, and AP* 1  must wait its turn per normal 802.11 access rules; similarly, AP* 1  cannot protect its STA 1  in the way it protects itself, from being jammed by AP 2 , and therefore it makes sure STA 1  is out of AP 2 &#39;s range, via a RTS/CTS exchange with STA 1    
         [0070]    A possible situation where a 3 rd  AP within the range of both said APs, may be blindsided by the process described above, must be addressed. Flowchart  300  summarizes one embodiment: in stage  310  AP* 1  and STA 2  are checked whether they are within a CCA range. In case they are, the nulling attempt is aborted  350 . In a case they are not, AP* 1  issues a RTS to STA 1    320 . Then, in stage  330 , if a CTS signal has been received, the AP* 1  proceeds to nulling  340  and in case a CTS is not received, nulling attempt is aborted  350 . 
         [0071]      FIG. 4  describes such a case where AP* 1    401 , AP 2 ,  402  and AP 3    403  are all within CCA range of each other, and more specifically, AP 3  is outside the energy detection range of AP* 1  for example AP 3  and AP* 1  receive each other at levels lower than −62 dBm but higher than −82 dBm; also assume that AP 3  receives AP 2  at a level equal or higher than it receives AP* 1 ; so when AP* 1  accesses the channel after AP 2  has, updating the original NAV  407  into extended value  408 , AP 3  may be unable to correctly receive AP* 1  Preamble, missing the fact the NAV was extended, and consequently, may access the channel while AP* 1  is still active, risking AP* 1  successful reception of ACK 1 ; such a damage will not occur if STA 1 &#39;s ACK is received at a level higher than AP 3 &#39;s NAV  409 , which constitutes a condition addressed in  FIG. 7 . 
         [0072]    According to some embodiments of the present invention, AP* 1  must consider 3 rd  or 4 th  etc. of neighboring AP s  within all within CCA range of each other, that may miss AP* 1 &#39;s RTS due to current ongoing traffic, consequently accessing the channel while AP* 1  is using it, potentially blocking ACK 1  reception 
         [0073]      FIG. 5  describes a process of achieving nulling AP reciprocity of uplink/downlink, thus enabling implicit feedback for nulling purposes; it is based on calibration of receiving and transmitting circuitries connected to ANT 1  at various Power Amplifier levels, as follows: 
         [0074]    Node 1    501  Calibrates the internal receiving path P R-1    507  versus the internal transmitting path P T-1    508 , registering circuitry biases at various Tx level; Node 1  estimates received signal at ANT 1    505  transmitted from ANT 2    506  of Node 2    502  channel=h 21    503 +P R-1 ; and Converts P R-1  into P* T-1  via said calibration, substitutes h 21 =h 12 , and thus creating reciprocal transmit signal P* T-1 +h 12    504 . 
         [0075]      FIG. 6  describes conditions and setup required for the preparation of an adequate Transmission Null; as indicated above, it is performed and its role is to protect from jamming one of AP 2 &#39;s receiving antennas, in order to allow sufficient ACK 2  reception&#39;s SINR; note that AP 2  assumed to use all N antennas for MRC based reception of ACK 2 , and protecting only 1 of N reduced the max sensitivity by 10*log N dB, which eliminates safe reception of cell-edge STAs and therefore should not be used in for those STAs; as STA 2  is assumed hidden, its expected range from the center is roughly estimated via AP 2 &#39;s downlink MCS observed by AP* 1 ; when Cell Edge situation is suspected AP* 1  reduces its Pout by M 1  dB, for instance by 10*log N dBs (e.g. 6 dB for N=4); identification of Cell Edge is based on observing said AP 2  Down-Link MCS, (e.g. if MCS&lt;4). 
         [0076]    Flowchart  600  summarizes the aforementioned procedure. In stage  610 , AP* 1  monitors AP 2 &#39;s Preamble and channel estimates it. Then, using reciprocity calibration, AP* 1  converts said Rx channel estimates into Q different sets of Tx Null weights  620 . In stage  630  AP* 1  monitors AP 2 &#39;s DL packet MCS. In stage  640 , the value of MCS/data stream rate is checked. In case it is smaller than 4, AP* 1  reduces Pout by DELTA 1  (e.g. 6 dB)  650 . In case not, AP* 1  computes several solutions and proceeds to Null selection  660 . 
         [0077]      FIG. 7  describes the procedure that considers additional neighbors conflict like the one described in  FIG. 4 ; AP* 1  maps the conflicts potential thru history records identifying groups of 3 APs or more that see each other within CCA range, and identifies highest potential interference RSSI s  that may be created by the scenario described in  FIG. 4 . Before attempting to access the channel occupied by AP 2 , it will verify that the STA 1  it is planning to serve will be able to successfully deliver the ACK 1  in the presence of RSSI s . (STA 1  RSSI&gt;highest interfering AP RSSI, otherwise AP 1  does not access the channel). 
         [0078]    According some embodiments, the Nulling AP monitors co-channel neighboring APs, registering those APs whose −62 dBm&gt;RSSI&gt;−82 dBm, labeling them as close-neighbors, and grading them by their RSSI. 
         [0079]    According to some embodiments the Nulling AP identifies within the close neighbors list subgroups of APs that are within CCA range of each other, labeling them channel-sharing subgroups. The neighbors list is a list of neighboring APs and STAs within CCA range which keeps history to be checked. Close neighbors are subgroups in the Neighbors&#39; list that are within each other&#39;s CCA range. 
         [0080]    According some embodiments of the present invention, the identification may be carried out by registering preamble time and NAV periods from different close-neighbors which never overlap each other during the last hour. 
         [0081]    According some embodiments of the present invention, the Nulling AP estimates the SINR of the previous ACK, and compares it with the highest RSSI amongst each of said sharing groups, and conditions the application of nulling procedure by for example the following: ACK RSSI&gt;Highest RSSI+Margin, wherein Margin=minimum required SINR for ACK reception. 
         [0082]    Flowchart  700  summarizes one embodiment of the invention: In stage  710  a background survey of neighboring APs is carried out. In stage  720  AP* 1  monitors neighboring APs received at −82 dBm&lt;RSSI&lt;−62 dBm, forming neighbors List, over a period of time (e.g. few minutes). Then AP* 1  identifies other neighboring APs that might be blindsided by AP 2 &#39;s transmission and miss AP* 1 &#39;s RTS (thus risking ACK 1  safe reception)  730 . The aforementioned identification is based on AP* 1  record keeping of APs groups within range that never transmit simultaneously  740 . Then, in step  750 , AP* 1  lists such neighbors of each possible AP 2  it may select, per their RSSI levels. Before attempting to reuse a channel already occupied by a neighbor AP 2  per an embodiment of the invention procedure, AP* 1  scans said neighbor&#39;s Group I &#39;s RSSI, picking the “highest RSSI”  760 . Then, AP* 1  proceeds to nulling  770 . 
         [0083]      FIG. 8  describes procedure and conditions to set up a Reception Null according to one embodiment. The N antennas of AP* 1  transmit up to N−1 data streams to STA 1  and AP* 1  further tunes 1 Null toward AP 2 &#39;s strongest data stream (i.e., highest RSSI); as the other AP 2  data streams if exist, are not attenuated, the Reception Null is not very deep (e.g. 4-8 dB); with AP 2  RSSI of X, e.g. −70 dBm, and Reception Null depth of Y e.g. 6 dB, STA 1 &#39;s ACK 1 &#39;s RSSI must be larger than X-Y e.g. −76 dBm; (AP probes STA by a RTS with DUR=0 to determine if ACK SINR is strong enough in presence of AP 2  RSSI, ACK RSSI&gt;Highest RSSI+M ACK  (e.g. M ACK =3 dB). 
         [0084]    Flowchart  800  summarizes an embodiment of the procedure of R x  Null preparation of a single null  810 . AP* 1  identifies the data stream having highest RSSI as strongest AP 2 &#39;s data stream  820 . Then AP* 1  sets Rx null on said strongest stream  830 . Then, Residual RSSI is measured  840 . AP* 1  then probes STA 1  and registers ACK 1  SINR  850 . In stage  860  the ACK SINR is checked for sufficiency. In a case it is not sufficient, the nulling attempt is aborted  870 . In case it is sufficient, in stage  880  the ACK signal level is checked whether it is greater than RSSI s +M ACK  (e.g. M ACK =3 dB). In case in is not, the nulling attempt is aborted. In case it is, AP* 1  proceeds to nulling  890 . 
         [0085]    According some embodiments of the present invention, the nulling AP may receive the nulled AP&#39;s preamble and identify the destination address of the STA served by the nulled AP, and validate that said STA is not within CCA range of the nulling AP. 
         [0086]    According some embodiments of the present invention, the nulling is conditioned on the nulling AP&#39;s reception of the MCS transmitted by a nulled AP and identified as high order MCS. Specifically, the high order MCS indicates SNR greater than 10*log(K) dB above MCS-0&#39;s SNR, where K is the number of said nulled AP&#39;s receiving antennas. Whenever the aforementioned condition is not met, or when or if the K is unknown, then the nulling AP&#39;s transmit power is reduced by 6 dB. 
         [0087]    According some embodiments of the present invention, the nulling AP performs channel estimation of the nulled AP&#39;s preamble L-LTF field, calculates Rx null weights and uses reciprocity to create the downlink null. 
         [0088]    According some embodiments of the present invention, the nulling AP has at least three antennas, and calculates a family of Q downlink nulls towards the said preamble of the nulled AP, and selects one of them (where Q is an integer). 
         [0089]    According to some embodiments of the present invention, the served STA is close to Cell Edge, the power projected towards the STA intended to be served by the nulling AP P STA , is maximized via selection of optimum solution out of said Q calculated nulls. 
         [0090]    According to some embodiments of the present invention, a cell edge indication is derived at the nulling AP via estimating the STA&#39;s previously received SINR, for example SINR ACK &lt;5 dB. 
         [0091]    According to some embodiments of the present invention, the Uplink Nulls depth are estimated by Nulling AP applying it to said Beacon transmitted by a nulled AP, and measuring the resultant RSSI after applying said null, labeling it residual RSSI. 
         [0092]    According to some embodiments of the present invention, before uplink null is attempted, the residual RSSI is compared to the previously sent STA ACK signal level received by the Nulling AP and conditions the Null procedure for example by meeting the following: ACK Signal level−Residual RSSI&gt;Margin, where Margin is the minimum SINR required for reception of ACK. 
         [0093]      FIG. 9  describes procedure and conditions to set up two Reception Nulls according to one embodiment. The N antennas of AP* 1  transmit N−2 data streams to STA 1  and AP* 1  further tunes two nulls toward AP 2 &#39;s 2 strongest data streams (e.g., highest RSSI); as the other AP 2  data streams if exist, are not attenuated, the Reception Null is medium (e.g. 8-15 dB); with AP 2  RSSI of X, e.g. −70 dBm, and Reception Null depth of Y e.g. 12 dB, STA 1 &#39;s ACK 1 &#39;s RSSI must be larger than X-Y e.g. −82 dBm, and most cell edge STA 1  will be serviceable with nulling. 
         [0094]    Flowchart  900  summarizes an embodiment of the procedure of R x  Null preparation of two nulls  910 . AP* 1  identifies two data streams having the highest RSSI as two strongest AP 2 &#39;s data stream  920 . Then AP* 1  sets Rx null on said strongest streams  930 . Then, Residual RSSI is measured  940 . AP* 1  then probes STA 1  and registers ACK 1  SINR  950 . In stage  960  the ACK SINR is checked for sufficiency. In a case it is not sufficient; the nulling attempt is aborted  970 . In case it is sufficient, in stage  980  the ACK signal level is checked whether it is greater than RSSI s +M ACK  (e.g. M ACK =3 dB). In case in is not, the nulling attempt is aborted. In case it is, AP* 1  proceeds to nulling  990 . 
         [0095]    According some embodiments of the present invention, the nulling AP performs channel estimation of or on each data stream transmitted by the nulled AP, during the transmission of the VHT-LTF field, and grade their power level from high to low (i.e., RSSI), labeling it as strongest, 2 nd  strongest until N−1. 
         [0096]    According some embodiments of the present invention, the nulling AP creates K−1 uplink nulls towards the K−1 respective strongest data streams, wherein K is the number its receiving antennas. 
         [0097]    According to some embodiments, the uplink nulls depth are estimated by the Nulling AP applying it to the beacon transmitted by a nulled AP, and measuring the resultant RSSI after applying said null, labeling it residual RSSI. 
         [0098]    According to some embodiments, before uplink null is attempted, the residual RSSI is compared to the previously STA ACK signal level received by the Nulling AP and conditions the Null procedure for example by meeting the following: ACK Signal level−Residual Jamming&gt;Margin, where Margin is the minimum SINR required for reception of ACK. 
         [0099]      FIG. 10  demonstrates the impact of transmit nulling ( 1005 ) performed by AP*1  1001  on EIRP received by STA 1    1003  according to one embodiment. It is also noted that the said nulling is done with N antennas transmitting up to N−1 data streams while nulling a single antenna of a neighbor AP 2 ; such nulling would also have impact on the MIMO reception by STA 1 ; for that reason said family of Tx nulls depicted in  FIG. 4  (1. Global optimization without nulling. 2. Projection of global solution onto nulling weights will yield the optimum weights.) such that STA 1  MIMO reception is optimized and protects AP 2 &#39;s reception of ACK; such optimization is done via trial and error, as long as STA 1  is static. 
         [0100]    Stemming from  FIG. 10  is the possibility that while AP* 1  performs Tx nulling toward AP 2 , its radiation pattern may be distorted in some cases reduce SINR experienced by STA 1 ; such SINR reduction—not anticipated by AP* 1  scheduler, may cause MCS mismatch and results in packet error, defeating the purpose to the double channel access; it is therefore important to bring to bear two corrective elements: The need for imminent channel knowledge, and the need to respond to it within the few micro-seconds allowed before transmission go/no-go decision is made. It should be noted that a reduced level of EIRP may require modified modulation. 
         [0101]    Flowchart  1000  summarizes an embodiment of this procedure. In step  1010  Null selection for STA i  is carried out. In step  1020  STA i  is checked whether is static. In case it is static, a null solution from a list of downlink nulls is used and checked if successful transmission. If successful, the solution may be stored for STA i    1060 . Then the stored solution is used and AP* 1  proceeds to nulling AP 2    1070 . In a case that the transmission was not successful, a counter is incremented to a next null on the list  1050  and steps  1030  and  1040  are repeated until transmission is successful so that AP* 1  proceeds to steps  1060  and  1070 . 
         [0102]    In one embodiment of the present invention, the way for AP* 1  to accomplish imminent channel knowledge is to form a Tx beam, then send a RTS to STA 1 , and have it use the MRQ mechanism to inform AP* 1  of its preferred MCS over MSI filed while responding via CTS. 
         [0103]      FIG. 11  is a block diagram  1100  illustrating an embodiment of the present invention according to which AP* 1  is able to imminently transmit the packet with MCS that match the feedback received from STA 1    1107  (assumed to use MRQ/MSI, reporting preferred MCS over CTS). The embodiment of  FIG. 11  may be based on preparing in advance a bank of different modulation version of the packet, stored in the MAC cash memory  1102  of the MAC layer  1101  of AP* 1 . Following an MRQ  1106  MCS request, STA 1  responds with MCS feedback  1105  over MSI field, that once received back by the PHY  1103 , it will be sued to point to the corresponding MCS version, and transmit it to STA 1  via transceiver  1104 . MCS feedback does not exactly match the MCS values stored in the cache memory or other storage, the closest lower MCS version may be used. 
         [0104]    As will be appreciated by one skilled in the art, aspects of the present invention may be embodied as a system, method or an apparatus. Accordingly, aspects of the present invention may take the form of an entirely hardware embodiment, an entirely software embodiment (including firmware, resident software, micro-code, etc.) or an embodiment combining software and hardware aspects that may all generally be referred to herein as a “circuit,” “module” or “system.” 
         [0105]    The aforementioned flowcharts and block diagrams illustrate the architecture, functionality, and operation of possible implementations of systems and methods according to various embodiments of the present invention. In this regard, each block in the flowchart or block diagrams may represent a module, segment, or portion of code, which comprises one or more executable instructions for implementing the specified logical function(s). It should also be noted that, in some alternative implementations, the functions noted in the block may occur out of the order noted in the figures. For example, two blocks shown in succession may, in fact, be executed substantially concurrently, or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved. It will also be noted that each block of the block diagrams and/or flowchart illustration, and combinations of blocks in the block diagrams and/or flowchart illustration, can be implemented by special purpose hardware-based systems that perform the specified functions or acts, or combinations of special purpose hardware and computer instructions. 
         [0106]    In the above description, an embodiment is an example or implementation of embodiments of the invention. The various appearances of “one embodiment,” “an embodiment” or “some embodiments” do not necessarily all refer to the same embodiments. 
         [0107]    Although various features of the invention may be described in the context of a single embodiment, the features may also be provided separately or in any suitable combination. Conversely, although the invention may be described herein in the context of separate embodiments for clarity, the invention may also be implemented in a single embodiment. 
         [0108]    Reference in the specification to “some embodiments”, “an embodiment”, “one embodiment” or “other embodiments” means that a particular feature, structure, or characteristic described in connection with the embodiments is included in at least some embodiments, but not necessarily all embodiments, of the inventions. It will further be recognized that the aspects of the invention described hereinabove may be combined or otherwise coexist in embodiments of the invention. 
         [0109]    The principles and uses of the teachings of the present invention may be better understood with reference to the accompanying description, figures and examples. 
         [0110]    It is to be understood that the details set forth herein do not construe a limitation to an application of the invention. 
         [0111]    Furthermore, it is to be understood that the invention can be carried out or practiced in various ways and that the invention can be implemented in embodiments other than the ones outlined in the description above. 
         [0112]    It is to be understood that the terms “including”, “comprising”, “consisting” and grammatical variants thereof do not preclude the addition of one or more components, features, steps, or integers or groups thereof and that the terms are to be construed as specifying components, features, steps or integers. 
         [0113]    If the specification or claims refer to “an additional” element, that does not preclude there being more than one of the additional element. 
         [0114]    It is to be understood that where the claims or specification refer to “a” or “an” element, such reference is not be construed that there is only one of that element. 
         [0115]    It is to be understood that where the specification states that a component, feature, structure, or characteristic “may”, “might”, “can” or “could” be included, that particular component, feature, structure, or characteristic is not required to be included. 
         [0116]    Where applicable, although state diagrams, flow diagrams or both may be used to describe embodiments, the invention is not limited to those diagrams or to the corresponding descriptions. For example, flow need not move through each illustrated box or state, or in exactly the same order as illustrated and described. 
         [0117]    The term “method” may refer to manners, means, techniques and procedures for accomplishing a given task including, but not limited to, those manners, means, techniques and procedures either known to, or readily developed from known manners, means, techniques and procedures by practitioners of the art to which the invention belongs. 
         [0118]    The descriptions, examples, methods and materials presented in the claims and the specification are not to be construed as limiting but rather as illustrative only. 
         [0119]    Meanings of technical and scientific terms used herein are to be commonly understood as by one of ordinary skill in the art to which the invention belongs, unless otherwise defined. 
         [0120]    The present invention may be implemented in the testing or practice with methods and materials equivalent or similar to those described herein. 
         [0121]    While the invention has been described with respect to a limited number of embodiments, these should not be construed as limitations on the scope of the invention, but rather as exemplifications of some of the preferred embodiments. Other possible variations, modifications, and applications are also within the scope of the invention. Accordingly, the scope of the invention should not be limited by what has thus far been described, but by the appended claims and their legal equivalents.