Abstract:
A method and system for overriding Carrier-Sense-Multiple-Access/Collision-Avoidance (CSMA/CA) without harming the traffic that occupies the channel are provided. The system and method may include for example detecting at a communication node having a plurality of antennas, a preamble transmitted by a co-channel neighboring node operating in compliance with IEEE 802.11 standard within a clear channel assessment (CCA) range of the communication node; and setting uplink transmit weights of the antennas of the communication node, to isolate the communication node from the neighboring node after the neighboring node has transmitted the preamble, to allow the communication node to access the co-channel, by keeping a transmitted signal level of the communication node as received by the neighboring node, below the CCA signal level at one or more of the antennas of said neighboring node. The system may for example implement the method in software running on a baseband processor.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims benefit from U.S. provisional patent application Ser. No. 61/876,886 filed on Sep. 12, 2013, U.S. provisional application Ser. No. 61/946,321 filed on Feb. 28, 2014, and U.S. provisional application Ser. No. 61/946,391 filed on Feb. 28, 2014, 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” or “neighboring nodes” relate to two co-frequency (or co-channel) APs or nodes that are within each other&#39;s sensitivity range, e.g. at least one of them can receive the other in such an signal-to-noise ratio to allows decoding of signals. 
         [0011]    The term “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 Interference and Noise. 
         [0015]    The term “ACK” as used herein, stands for acknowledgement, and is 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 referred to 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]    Associated STA is defined herein as a STA that is served by a certain AP with a certain Service Set Identifier (SSID). 
         [0020]    Non-associated STA is defined herein as a STA within the range of and non-serving AP and/or its non-serving STAs. 
         [0021]    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. 
         [0022]    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. 
         [0023]    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. 
         [0024]    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. 
         [0025]    The acronym “FLA” stands for Fast Link Adaptation, and represents processed that reduce transmitting side learning time of the receiver&#39;s SINR. 
         [0026]    The acronym “MCS” stands for Modulation Coding Scheme, mapping SINR to modulation order and code rate. 
         [0027]    The acronym “MRQ” stands for MCS request (Modulation Code Scheme Request) 
         [0028]    The acronym “MSI” stands for MRQ Sequence Identifier, which carries MCS feedback from receiver to transmitter. 
         [0029]    The term “null” as used herein, is a spatial pattern, created by two or more antennas, formed in such a way that significantly reduces the power level received by a given receiver (e.g., a local minimum). An “Rx Null” is a null formed by a receiver&#39;s antennas weight in order to decrease undesired signal level. A “Tx Null” is formed by transmitter&#39;s antennas weights in order to decrease its undesired transmitted signal at remote receiver&#39;s input. 
         [0030]    The term “Nulling STA” as used herein, is a station (client) that is equipped with nulling capabilities. 
         [0031]    The term “Nulled AP” as used herein is an Access Point within CCA from a Nulling STA. 
         [0032]    The term “Neighbors list” as used herein is a list of neighboring APs and STAs within CCA range which keeps history to be checked. 
         [0033]    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. 
         [0034]    When an RTS/CTS procedure is used, an additional condition is introduced. Namely, a legacy STA receiving the download traffic from its serving AP, must not be within CCA range of the other neighboring AP if said AP is occupying the channel. 
       SUMMARY OF THE INVENTION 
       [0035]    In order to overcome the aforementioned condition, embodiments of a method of overriding CSMA/CA and virtual carrier sense, without harming the traffic that occupies the channel are provided herein. Embodiments of the present invention also provide measurements and qualifying criteria for performing the aforementioned channel simultaneous sharing. A method according to some embodiments of the present invention is based on generating spatial isolation of one node from another. 
         [0036]    In one embodiment of the present invention, a Wi-Fi STA detecting a Preamble from a neighboring AP, and subsequently receiving a RTS from its serving AP, may transmit a CTS message to its serving AP, if said STA can transform the signal received from it by said neighboring AP in such a way that will not harmfully interfere with said neighboring AP&#39;s successful packet ACK by its client STA. 
         [0037]    The aforementioned transformation of the signal received by the neighboring AP, can be performed by the STA via power level reduction, and/or via modifying its overall antennas patterns in a way that produces a Tx null at the one or more of the neighboring APs antennas. 
         [0038]    Additionally, the aforementioned STA may reduce its transmitted power level by 10*log (N), where N being the known or assumed number of said neighboring nulled AP&#39;s antennas, thus compensating for the loss of Maximal Ratio Combining (MRC) gain that could have stemmed from protecting all said N antennas from the STA&#39;s interfering signal 
         [0039]    The aforementioned reduction of transmitted power level by the nulling STA, is not carried out in a case that the nulling STA estimates that said ACK signal is strong enough to be received without errors by the nulled neighboring AP. 
         [0040]    Such estimation may be performed by the STA via registering the MCS sent by said neighboring AP to its served STA, thus deriving the perceived SINR in the neighboring AP&#39;s downlink, then assuming roughly a similar SINR for the corresponding uplink channel. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0041]    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: 
           [0042]      FIG. 1A  is a block diagram illustrating neighboring APs, wherein a STA served by one of said APs is located in the area between said two APs in accordance with some embodiments of the present invention; 
           [0043]      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; 
           [0044]      FIG. 2  is a block diagram illustrating a STA attempting to access the channel occupied by neighboring AP, via placing a null on its transmitting beam, in order to protect the neighboring AP&#39;s traffic from said STA&#39;s interference in accordance with some embodiments of the present invention; 
           [0045]      FIG. 3  is a block diagram illustrating a STA&#39;s transmit null established to enable the neighboring AP&#39;s successful reception of ACK from its respective served STA in accordance with some embodiments of the present invention; 
           [0046]      FIG. 4  is a high level flowchart illustrating a null validation test carried out in accordance with some embodiments of the present invention; 
           [0047]      FIG. 5  is a flowchart illustrating a transmission null calculation in accordance with some embodiments of the present invention; 
           [0048]      FIG. 6  is block diagram illustrating channel reciprocity calibration carried out in accordance with some embodiments of the present invention; 
           [0049]      FIG. 7  is a flowchart illustrating a procedure of checking sufficiency of MCS in accordance with some embodiments of the present invention; 
           [0050]      FIG. 8  is a flowchart illustrating a procedure of cell edge detection and power level adjustment, in accordance with one embodiment of the present invention; 
           [0051]      FIG. 9  is graph diagram comparing different types of nulls in accordance with some embodiments of the present invention; 
           [0052]      FIG. 10  is a block diagram illustrating non-limiting example of calculating a receive null in accordance with some embodiments of the present invention; and 
           [0053]      FIG. 11  is a high level flowchart illustrating a method in accordance with some embodiments of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0054]    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. 
         [0055]    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. 
         [0056]      FIG. 1A  is a block diagram illustrating apparatus with initial conditions wherein a Wi-Fi STA  110  receives a RTS from its serving AP  101 , after a neighboring AP  103  within its CCA range, has occupied the channel and is in process of downloading a packet to its client STA  104 . 
         [0057]      FIG. 1B  is a block diagram illustrating a communication node  110  within CCA range of a neighboring node  103 , in accordance with some embodiments of the present invention. Communication node  110  may include for example a plurality of antennas  10 - 1  to  10 -N, a plurality of radio circuitries  20 - 1  to  20 -N configured to transmit and receive signals via a plurality of antennas  10 - 1  to  10 -N in compliance with the IEEE 802.11 standard, and a baseband processor  30 . Communication node  110  may be configured to transmit and receive signals within a clear channel assessment (CCA) range of neighboring node  103  which has a plurality of antennas and may be configured to transmit and receive signals in a co-channel shared with communication node  110  in compliance with the IEEE 802.11 standard. 
         [0058]    In operation, baseband processor  30  may be configured to set uplink transmit weights at radio circuitries  20 - 1  to  20 -N and adjust the power level of the signal transmitted via antennas  10 - 1  to  10 -N in a way that isolates communication node  110  from neighboring node  103  after neighboring node  103  has transmitted a preamble signal, in a manner that allows communication node  110  to access the co-channel, by keeping a transmitted signal level of communication node  110  as received by neighboring node  103 , below the a signal power level associated with the CCA range at one or more of the antennas  103 A of neighboring node  103 . 
         [0059]    According to some embodiments of the present invention, communication node  110  may be a station configured to be served by its serving access point (e.g., AP 1    101  in  FIG. 1A ) and neighboring node  103  may be an AP which is serving its respective stations (but not serving communication node  110 ). 
         [0060]    According to some embodiments of the present invention, communication node  110  may be a station that has at least two transmitting/receiving antennas, and is configured to set the weights while transmitting towards or in the direction of its serving AP, such that the weights minimize the energy received by neighboring AP&#39;s antennas  103 A, constituting an uplink transmit null towards neighboring AP  103 , wherein station  110  further lowers the power level transmitted by station  110 , by an amount that keeps the estimated power received by neighboring AP  103  below CCA threshold at one or more of the antennas  103 A of neighboring AP  103 . 
         [0061]      FIG. 2  is a block diagram illustrating said STA  210  attempting to access the channel already occupied (e.g. being used for transmitting or receiving) by neighboring AP  203 , via placing a Tx Null  202  that protects the neighboring AP&#39;s  203  receiver, allowing said STA  210  to receive a download packet from its serving AP  201 . 
         [0062]      FIG. 3  is a block diagram illustrating nulling STA  310  sequence of actions as follows: starting with forming Rx Null  303  towards the neighboring AP  304 , minimizing the Preamble signal transmitted by a neighboring AP; using the Rx nulling weights as a basis to calculate a Tx null  302 , which is pointed towards the antenna of the neighboring AP that transmitted the preamble; verifying that said Tx null sufficiently reduces the STA&#39;s interference received by said antennas, allowing the neighboring AP  304  to successfully receive the ACK coming from its served STA  305 ; once sufficient Tx nulling conditions are established by said nulling STA, it issues CTS to is serving AP  301 ; and in case AP  301 &#39;s RTS is accompanied by MRQ, STA  310  adds MCS info to its CTS packet. 
         [0063]    According to some embodiments of the present invention, station STA* 1    310  further lowers the power level of the signals it transmits by an amount that keeps the estimated power received by access point AP 2    304  below CCA threshold. In other words, AP 2  is thus kept beyond CCA range and will not receive the STA&#39;s transmission at a power level equal or higher than −82 dBm, for at least one of the AP 2 &#39;s antennas. 
         [0064]    According to some embodiments of the present invention the uplink transmit null of station STA* 1    310  and the power setting of station STA* 1    310  are performed in order to protect the ACK reception of access point AP 2    304  from being jammed by station STA* 1    310 &#39;s transmission. 
         [0065]    According to some embodiments of the present invention, the uplink transmit null weights of station STA* 1    310  are set prior to responding to serving access point AP 1    301  request to send (RTS) with clear to send (CTS) signals. 
         [0066]    According to some embodiments of the present invention, station STA* 1    310  may monitor a destination address (associated with a destination node) of a packet from transmitting neighboring access point AP 2    304  and if the destination node is recognized within CCA range of station STA* 1   310 , it will not clear to send (CTS). 
         [0067]      FIG. 4  is a high level flowchart  400  illustrating a null validation test  401  carried out by a station/client in order to enable a so-called “friendly” non-interrupting access to a Wi-Fi channel that is already being occupied by a neighboring access point in accordance with some embodiments of the present invention. In a first step, access point AP #   1  sends a request to send RTS and MRQ to station STA* 1    402 . A check whether station STA 2  is located within station STA* 1 &#39;s CCA range is carried out  403 . According to some embodiments of the present invention, STA* 1  maintains or stores a neighbor list of the of STAs within CCA range and checks if the destination address STA 2  of a packet from transmitting neighboring access point AP 2  is in the neighbor list. In a case that station STA 2  is not within station STA* 1 &#39;s CCA range, a check whether access point AP 2  is located within station STA* 1  &#39;s CCA range is carried out  404 . In case it is not within the range, there is no danger of interfering with the communication of access point AP 2  and so a CTS can be transmitted by station STA* 1    408 . In a case that point AP 2  is located within station STA* 1  &#39;s CCA range, station STA* 1  calculates transmit null  405  in a process that will be described in greater detail hereinafter. While calculating the null, a check whether the calculated null is sufficiently deep so as to clear the CCA threshold is carried out  406 . Specifically, a metric named DELTA 1  is calculated being equal to the RSSI level of the preamble signal of access point AP 2  minus transmit null depth minus −82 dBm, namely: AP 2 -Preamble-RSSI−Tx Null Depth−(−82 dBm). In a case that the null is sufficiently deep, transmit null weights are being set  407  and a CTS signal is being transmitted by station STA* 1    408 . In a case that the transmit null is not sufficiently deep, station STA* 1  reduces its transmit power level by a metric named DELTA 1    409 . Then, station STA* 1  intercepts access point AP 2 &#39;s MCS towards station STA 2    410 . A check whether access point AP 2 &#39;s MCS is smaller than 4 is carried out  411 . In a case it is not smaller, a CTS signal is transmitted by station STA* 1    408 . In a case that it is smaller, station STA* 1  further reduces power by DELTA 2  which is equal to 10*log (N), wherein N is the number of access point AP 2 &#39;s antennas known to STA* 1    412  and then goes on to send a CTS signal  408 . 
         [0068]    According to some embodiments of the present invention, station STA* 1    310  may monitor MCS transmitted by neighboring access point AP 2    304  and further reduces transmit power if MCS &lt;4, by a DELTA 2 , wherein DELTA 2 =10*log(N) dB, and N is the known number of said neighboring transmitting AP&#39;s antennas. 
         [0069]    According to some embodiments of the present invention, the transmit uplink null formed by station STA* 1    310  is calculated for example as follows: station STA* 1    310  performs channel estimation and absolute value calculation of the Legacy Long Training Field (L-LTF) field of the Preamble signal transmitted by neighboring access point AP 2    304 , via each of its receiving antennas, and comparing the various said antennas received power level, it registers the highest-power-level as highest received signal strength indication (RSSI); station STA* 1    310  sets weights for the receiving antenna such that total received power of said Preamble signal&#39;s L-LTF is minimized, yielding an estimated receive null weighting matrix W, and registering said minimized-power-level as nulled RSSI; and using reciprocity calibration said station converts said receive null weighting matrix W into a transmit null weighting matrix Ŵ. 
         [0070]    According to some embodiments of the present invention, station STA* 1    310  calculates the difference between highest RSSI and the nulled RSSI, said difference being defined as receive null depth. 
         [0071]      FIG. 5  is a flowchart  500  illustrating a transmission null calculation in accordance with one embodiment of the present invention. The null calculation begins with a step in which station STA* 1  monitors access AP 2 &#39;s Preamble with its N antennas and performs channel estimations S 1 -S N  of the access AP 2 &#39;s antenna that transmits the Preamble, labeling highest S i  power level as “AP 2 -Preamble-RSSI”  501 . Then, station STA* 1  uses the channel estimations to set such weights for its receiving antennas that minimize total received power level, defining said weights Was “Rx Null”, and labeling the total received power level as “Nulled AP 2 −Preamble−RSSI”  502 . “Rx Null Depth” is defined as the difference between “AP 2 -Preamble-RSSI” and “Nulled AP 2 -Preamble-RSSI”  503 . Then, using reciprocity calibration as will be described in greater detail hereinafter in  FIG. 6 , station STA* 1  converts said receive channel estimated weights W into transmit null weights W, labeled “Tx Null”  504 . Then, using long term calibration error estimation, and allowing for weights setting&#39;s precision error, DELTA 3  is calculated, setting “Tx Null Depth”=Rx Null Depth−DELTA 3    505 . Finally, transmit null weights Ŵ, and Tx Null depth, are transferred to the aforementioned Null-Validation-Test depicted herein in  FIG. 4 . 
         [0072]    According to some embodiments of the present invention, station STA* 1    310  estimates the transmit null depth, based on the receive null depth, minus a calculated margin DELTA 3  representing imperfection of receive/transmit null calibration due to aging and temperature variations, and due to limited precision of phase and amplitude setting. 
         [0073]    According to some embodiments of the present invention, aforementioned margin DELTA 3  is determined by a combination of measured phase and amplitude drift between consecutive off-line reciprocity calibration, and a tolerance of phase and amplitude setting precision measured at factory. 
         [0074]    According to some embodiments of the present invention said transmit null depth is compared with the highest RSSI level received by an antenna of station STA* 1    310  and if said highest RSSI minus transmit null depth &gt;−82 dBm, station STA* 1    310  will reduce its transmission power DELTA 1  so that the highest RSSI minus transmit null depth minus DELTA 1  will be lower than −82 dBm. 
         [0075]    According to some embodiments of the present invention, following the RTS and MRQ sent by station&#39;s serving AP, station STA* 1    310  will set up Tx Null, estimate received RTS SINR, and respond to serving access point AP 1    301  with CTS and MCS that corresponds to the aforementioned SINR. 
         [0076]      FIG. 6  is block diagram illustrating channel reciprocity calibration carried out for nulling purposes in accordance with one embodiment of the present invention. Client/Station  601  may include: baseband processor  630 , radio frequency (RF) circuitries  640  and  642 , antennas ANT1 and ANT2. Client/Station  601  calibrates the internal receiving path P R-1    609  versus the internal transmitting path P T-1    610 , registering circuitry biases at various transmit level; Client/Station  601  estimates received signal from ANT 1  channel=h 13    603 +P R-1 , and Converts P R-1  into P *   T-1  via said calibration, substitutes h 31 =h 13 , and thus creating reciprocal transmit signal P *   T-1  h 13 . Similarly, Client/Station  601  Calibrates the internal receiving path P R-2    611  versus the internal transmitting path P T-2    612 , registering circuitry biases at various Tx level; Client/Station  601  estimates received signal from ANT 3  channel=h 32    606 +P R-2 , and Converts P R-2  into P* T-2  via said calibration, substitutes h 32 =h 23 , and thus creating reciprocal transmit signal P* T-2  h 23    605 . 
         [0077]    According to some embodiments of the present invention, station STA* 1    310  may perform an off-line calibration of receive versus transmit nulls, and may calculate a conversion matrix between W and Ŵ. An off-line calibration may be for example a calibration that may be carried out at the factory, prior to operation within a Wi-Fi network, or when the STA ceases regular operation periodically or when required by self or remote indicators. The aforementioned conversion matrix is measured and calculated per frequency subcarrier. 
         [0078]      FIG. 7  is a flowchart  700  illustrating a procedure of checking requirements for transmitting clear to send (CTS) signal for station STA* 1    701  in accordance with one embodiment of the present invention. The procedure starts with setting up the transmit null by station STA* 1    702 . Then, station STA* 1  measures received RTS SINR from serving access point in presence of access point AP 2 &#39;s transmission, and calculates MCS that corresponds to said SINR  703 . Station STA* 1  then responds to access point AP 1 &#39;s MRQ with said calculated MCS  704  and then sends CTS signal  705 . 
         [0079]      FIG. 8  is a flowchart  800  illustrating a procedure of cell edge detection and power adjustment  801  by station STA* 1 , in accordance with one embodiment of the present invention. In step  802 , station STA* 1  verifies access point AP 2 &#39;s MCS and estimates station STA 2 &#39;s cell edge proximity. Then, the MCS is checked to be greater than 4  803 . In a case it is greater than 4, station STA* 1  sends CTS to access point AP 1    805 . In a case it is not greater than 4, station STA* 1  reduces Power Level by a metric named DELTA 2  being equal to 10*log N dB, wherein N=assumed number access point AP 2 &#39;s Rx antennas  804  and only then station STA* 1  sends CTS to access point AP 1    805 . 
         [0080]      FIG. 9  is graph diagram demonstrating nulls created by two antennas and a combiner that tries to set them into equal amplitudes and opposite phases; there can be seen a null depth deteriorations as a function of amplitude inaccuracy (e.g., imbalance) or phases differences inaccuracy (e.g. ≠180 0 ). The smaller are the inaccuracies, the deeper the null is. 
         [0081]      FIG. 10  is a block diagram illustrating a non-limiting example of generating a transmit null in accordance with one embodiment of the present invention. The aim of the null is ANT a  of access point AP 2 &#39;s  1002  which is the antenna that transmits the Preamble signal. In order to effectively null it so that the Preamble is not interfered with, STA* 1    1001  performs channel estimations h- ab  and h- ac . As a result, ANT b  and ANT c  receive S 1  and S 2  Measuring at station STA* 1    310 ; RSSI- b  and RSSI- c  are the power level of ANT a  preamble as received by ANT b  and ANT c , respectively at station STA* 1   1001 ; the highest RSSI is defined as “AP 2 -Preamble-RSSI”. Then, a receive null is being calculated by setting receive weights matrix W so that S=W*H is minimized. Finally, W is defined as a matrix that minimizes S as “Rx Null”. 
         [0082]      FIG. 11  is a high level flowchart illustrating a method  1100  in accordance with some embodiments of the present invention. Method  1100  may include for example detecting at a communication node having a plurality of antennas, a preamble transmitted by a co-channel neighboring node operating in compliance with IEEE 802.11 standard within a clear channel assessment (CCA) range of said communication node  1110 ; and setting uplink transmit weights of said antennas of said communication node and further adjusting transmit power level of said communication node in a way that isolates said communication node from said neighboring node after said neighboring node has transmitted said preamble, in a manner that allows said communication node to access the co-channel, by keeping a transmitted signal level of said communication node as received by said neighboring node, below the CCA signal level at one or more of the antennas of said neighboring node  1120 . 
         [0083]    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.” For example, a baseband processor or other processor may be configured to carry out methods of the present invention by for example executing code or software. 
         [0084]    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. 
         [0085]    In the above description, an embodiment is an example or implementation of the inventions. The various appearances of “one embodiment,” “an embodiment” or “some embodiments” do not necessarily all refer to the same embodiments. 
         [0086]    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. 
         [0087]    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. 
         [0088]    The principles and uses of the teachings of the present invention may be better understood with reference to the accompanying description, figures and examples. 
         [0089]    It is to be understood that the details set forth herein do not construe a limitation to an application of the invention. 
         [0090]    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. 
         [0091]    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. 
         [0092]    If the specification or claims refer to “an additional” element, that does not preclude there being more than one of the additional element. 
         [0093]    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. 
         [0094]    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. 
         [0095]    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. 
         [0096]    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. 
         [0097]    The present invention may be implemented in the testing or practice with methods and materials equivalent or similar to those described herein. 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.