Patent Application
20140313986
The present invention relates generally to the field of radio frequency (RF) 802.11x WiFi systems and methods for enhanced performance using RF beamforming and/or digital signal processing.
Active antenna systems may implement 1-dimensional and 2-dimensional multi-beam base stations that focus transmission and reception into narrow sub-sectors, facilitate reduced interference to neighboring cells, and enable reuse of the radio spectrum at its own cell by activating independent simultaneous co-channel non-overlapping beams.
Base stations may separate transmission and reception by using different frequencies or different time divisions for transmission and reception. For example, cellular protocols, such as GSM (Global System for Mobile Communications), WiMAX (Worldwide Interoperability for Microwave Access), and LTE (Long-Term Evolution), may sync (synchronize) all transmission and receiving channels using time-division. Wi-Fi base stations, which may incorporate a multi-beamforming cluster of co-located, co-channel Wi-Fi access points, may not inherently include such syncing capabilities and may operate inefficiently when in close proximity, due to the nature of the CSMA/CA (Carrier sense multiple access with collision avoidance) property of the Wi-Fi protocol. The CSMA/CA property may require yielding to all first-come Wi-Fi data transmission in order to avoid transmission collisions or jamming. Further, while co-located, co-channel Wi-Fi access points may provide super-isolation of data transmission via RF manipulation methods, additional isolation between access points may be created through signal calibration. Performance may be improved if calibration can occur without significantly impacting network bandwidth.
A plurality of co-located beamforming transceivers may transmit data to at least one user equipment according to a CSMA/CA protocol. The plurality may include a first and a second beamforming transceiver. The first beamforming transceiver may request a calibration signal from the second beamforming transceiver. A processor or transceiver may identify a transmission gap between the user equipment and the plurality of beamforming transceivers. The second beamforming transceiver may transmit a calibration signal during the transmission gap to the first beamforming transceiver.
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 understood by reference to the following detailed description when read with the accompanying drawings in which:
It will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity. Further, where considered appropriate, reference numerals may be repeated among the figures to indicate corresponding or analogous elements.
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.
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's registers and/or memories into other data similarly represented as physical quantities within the computing system's memories, registers or other such information storage, transmission or display devices.
Embodiments of the invention may be described in reference to the IEEE (Institute of Electrical and Electronics Engineer) 802.11 standard for implementing wireless local area networks (WLAN). “802.11xx” may refer to any version of the 802.11 standard, such as 802.11a, 802.11g, or 802.11ac, for example. Versions of the 802.11 standard may operate using a technique called Collision Sense Multiple Access/Collision Avoidance (CSMA/CA), a networking method which aims to prevent transmission collisions before they occur. While embodiments of the invention are described in terms of the 802.11 protocol, other network protocols built on the CSMA/CA concept may be used.
Access points (AP's) using a CSMA/CA wireless network, including IEEE 802.11 WiFi networks, may determine whether a radio channel is clear, prior to broadcasting or transmitting data in the channel. The AP may do this by performing a clear channel assessment (CCA), which includes two functions: listening to received energy on an RF interface (termed “energy detection”), or detecting and decoding an incoming Wi-Fi signal preamble from a nearby AP. A signal preamble may be a signal used to synchronize transmission timing between two devices and may occur at the beginning of every data packet. In a communication standard such as Wi-Fi, a preamble may have a predefined structure and data fields organized in a way that all devices communicating on the standard understand. A CCA is deemed ‘busy’ and thus not available if an AP's receiver can sense radio energy, from another AP, above a CCA sensitivity level or if an AP detects an incoming WiFi signal preamble. The AP may also maintain or store a Network Allocation Vector (NAV), which acts as a countdown timer to when the AP may begin to transmit data. Based on signals from nearby AP's which may indicate the length of a transmitted data packet, an AP's NAV may update the time to transmission, causing further delay to an AP's data transmission. An AP may defer from using the channel to transmit data until both conditions (e.g., CCA deemed ‘busy’ and the NAV timer) have expired.
According to embodiments of the invention, a Multibeam Access Point, which may act as a Wi-Fi base station, may include a cluster or plurality of co-located Wi-Fi access points or transceivers, each access point with independent transmit and receive capabilities. As used herein, transceiver and AP may be used interchangeably as any device having independent transmit and receive functions and capable of acting as a 802.11xx access point. Each access point or transceiver may use directive antennas to focus the radio energy on an azimuth covering an intended user on a user equipment (UE), enabling one or the same radio frequency or frequency channel (e.g., the same or overlapping frequency spectrum) to be used simultaneously or concurrently on a different azimuth beam which points to a different UE. Transceivers or access points may be co-located if, under ordinary usage of the CSMA/CA technique, data transmission from one transceiver prevents simultaneous data transmission from another transceiver on the same channel or frequency. The transceivers' co-location or proximity to each other may cause, for example, RF interference, a busy CCA, or an updated NAV. Co-located transceivers may be clustered or grouped together into one base station that serves UE's in a limited geographical area. Co-located transceivers may share processing tasks or may each have separate processing capabilities. Each access point or transceiver may be coupled to an individual antenna to broadcast or transmit data to a user equipment (UE). The antennas may be arranged in an antenna array. A beamforming antenna may be a directive antenna to focus radio energy on a narrow azimuth covering an intended user on a UE. Broadcasting on a narrow azimuth may enable one or the same frequency channel (e.g., the same or overlapping frequency spectrum) to be used simultaneously or concurrently on a different azimuth beam which points to a different UE.
In order for a multibeam access point to maintain a high capacity and serve multiple UE's simultaneously on the same channel, each transceiver's signal to a UE may require sufficient isolation to prevent interference with signals from other co-located transceivers. For example, one transceiver's data transmission to a UE may leak to or be received by another co-located transceiver, interfering with the co-located transceiver's data transmission with another UE. One remedy is to create RF and antenna isolation between each transceiver. Isolation may be accomplished by applying different techniques such as physical separation of transceiver antennas and manipulating antenna patterns, but leakage may still occur though scattering, multipath, back lobe leakage or a multitude of other paths. Despite design approaches to minimize this leakage, some residual leakage of the second transceiver into the first transceiver will occur. Another remedy may be to nullify or cancel residual leakage signals by analog or digital processing through phase shifting and attenuation adjustments.
This residual leakage may be removed by subtracting a replica of a second transceiver signal from the composite signal (signal of interest plus leakage from second transceiver) received by a first transceiver. The replica of the second transceiver may require the same characteristics as the leakage component of the second transceiver. The characteristics (phase and amplitude) of the leakage version of the signal transmitted by the second transceiver may be determined by calibration.
Calibration may be accomplished by identifying a time period when the radio environment is controlled so that the only signal present is a calibration signal. During this interval (the calibration interval) the characteristic of the leakage signal (phase and amplitude) may be measured. This same phase and amplitude value may then be applied to a replica of the transmitted signal.
A key requirement of calibration may be determining a time when only the calibration signal is present. This may be accomplished by shutting down the operation of the system periodically and entering a calibration period. This is not an operational problem if calibration is need very infrequently (once a month or so). However, an accurate determination of the phase and amplitude adjustment may be required to remove the leakage component may require more frequent calibration, giving rise to the requirement for continuous, minimally disruptive calibration.
The phase shift or attenuation values may be determined by sending or transmitting calibration signals between co-located transceivers and calculating or determining appropriate phase shift and attenuation values that may properly null (e.g. void, cancel, or bring the signal to zero) the leaked signals from each transceiver. Calibration may involve injecting a known signal into the antennas, detecting the resulting output in a manner that measures the characteristics of the antenna, and adjusting the phase shifters and attenuators to achieve the desired results. In order to prevent external signals from interfering with the calibration process, the calibration signals may need to be transmitted during a transmission gap between the multibeam access point and the UE's it is serving. The transmission gap may be a specific time interval where data transmission between co-located transceivers and UE's is paused or ceased. Embodiments of the invention may allow transmission of calibration signals between co-located transceivers without decreasing the capacity or bandwidth of a multibeam access point.
Due to the proximity of the transceivers 102a-d and their antennas 104a-d, a small amount of a signal 119, transmitted by transceiver 104c, may “leak” as a leakage signal 118 into the receiver of transceiver 104a. Leakage signal 118 may be transmitted by one transceiver, such as transceiver 104c, as illustrated, and received by transceiver 104a. The leakage signal 118 may interfere with data transmission between transceiver 104a and its UE 106. Each transceiver may be inadvertently transmitting this leakage signal to each of the other transceivers. In order to null leakage signal 118 and prevent leakage signal 118 from disturbing or interfering with data transmission between transceiver 104a and UE 106, a replica of signal 119 may be subtracted from the total signal received by transceiver 104a. This total signal received by transceiver 104a may, for example, be a signal from UE 106 (e.g., signal 121) and the leakage signal 118. When leakage signal 118 is subtracted from the total signal received by transceiver 104a, signal 121 remains.
A well-known subtraction technique may be used to null the leakage signal 118. For subtraction to be useful, phase and amplitude of the leakage signal 118 may be measured during a calibration interval. Calibration may be performed when the only signal detected by transceiver 104a is the leakage signal 118. All other transceivers may be required not to operate during calibration so that the measurement being made by receiver 116 is only the leakage signal.
The actual calibration signal needs to have characteristics similar the actual signal that is being subtracted. It can be a special short duration signal (as required if the calibration interval is a very short SIFS interval) or it could be an actual WiFi signal (as when calibration is done using the WiFi beacon).
For example, transceiver 104a may request a calibration signal from transceiver 104c. A calibration signal may be a signal with properties (e.g., frequency, phase, amplitude or other similar properties) known or previously agreed upon by each co-located transceivers 104a-d. This may be so that each transceiver 104a-d is able to detect abnormalities or irregularities in received calibration signals and account for these irregularities during calibration. Requests for calibration may occur, for example, through a wire connection 105 between each of the co-located transceivers 104a-d. Processor 114 or controller 108 may identify or define a transmission gap between the multibeam access point 100 (including its transceivers 104a-d) and its serving UE's. The transmission gap may be a time interval scheduled or determined by one of the transceivers (e.g., transceiver 104a) or processors 114, where no data transmission occurs between the transceivers 104a-d and any of the UE's 106. The transmission gap may further be defined to occur during standard data transfers required by a CSMA/CA protocol. Processor 108 or 114 or transceiver 104a may, for example, transmit a timestamp or notification to the other transceivers indicating when the transmission gap may occur. A timestamp may, for example, be a series of characters or bits that describe a particular future time. During the transmission gap, transceiver 104c may transmit a calibration signal to transceiver 104a or to the other transceivers 104b and 104d. The transmission gap may be, for example, a short interframe space (SIFS) or may occur during a beacon preamble. Other transmission gaps may be determined or scheduled.
The calibration sequence may be accomplished by each transceiver 104a-d, one at a time, transmitting a calibration signal while one or all the other transceivers measure the phase and amplitude of the signal. The final values of phase and amplitude may require integration over many measurements to obtain an accurate value. For the SIFS approach, where the individual measurement is less than 10 μsec, thousands of samples may be required. For the beacon approach, where individual measurement is several hundred μsecs, the number of samples may be reduced.
The exact time required is a field tuned adjustment based on the specifics of the installation. However, calibration is a continuing running process, where new calibration data is applied to long term averaged data, slowly adjusting the value. In the following sections, two approaches are described: (a) using SIFS described in
As shown in
After the SIFS or transmission gap has passed, transceiver A 402 may determine whether a calibration signal was received from transceiver C 416 and whether the other co-located transceivers 422 reported no transmission during the SIFS. If these conditions are met, a successful transmission of calibration data or signal has occurred for at least transceiver A 402, and transceiver A 402 may calibrate accordingly. The other co-located transceivers 422 may be calibrated with the received calibration signal from transceiver C 416 if the other co-located transceivers 422 did not receive any data from a UE. If any of the other co-located transceivers 422 were receiving data from a UE during the SIFS, then the received signal from the UE would interfere with the received calibration signal, and the co-located transceivers 422 may not be able to calibrate. If the conditions of operation 412 are not met, transceiver A 402 may continue to wait until another RTS/CTS sequence is initiated with a UE. Transceiver A may continue calibration mode 404 until it has received a sufficient amount or number of successful calibration events. Transceiver A may request a calibration signal from the other co-located transceivers 422, not just transceiver C 416. When transceiver A is finished calibrating, the other co-located transceivers 422 may have also collected or received a substantial amount of successful calibration events, however, the other co-located transceivers 422 may have received fewer successful calibration events due to interference from UE's transmitting to the co-located transceivers 422. If the other co-located transceivers need additional calibration signals, the other co-located transceivers 422 may start the same calibration mode as transceiver A 402.
The SIFS calibration procedure may be executed without any reduction of data flow in the network because it may take advantage of naturally occurring SIFS's. The RTS/CTS sequence may be a normal part of data transfer between transceivers and UE's and may occur at for example about 100 times per second; other rates may be used.
The technique described in
Based on the above estimation, calibrating multiple times per day may be accomplished. Since SIFS calibration may have no impact on channel utilization a multibeam access point may be calibrated continuously as a background function. At approximately 2.7 hours per cycle, about 9 cycles can be accomplished every 24 hours. This may be increased if less than 0.1 seconds of calibration signal or data is required. Other parameters and numbers of equipment may be used.
The calibration procedure described in
Different embodiments are disclosed herein. Features of certain embodiments may be combined with features of other embodiments; thus certain embodiments may be combinations of features of multiple embodiments.
Embodiments of the invention may include an article such as a computer or processor readable non-transitory storage medium, such as for example a memory, a disk drive, or a USB flash memory device encoding, including or storing instructions, e.g., computer-executable instructions, which when executed by a processor or controller, cause the processor or controller to carry out methods disclosed herein.
In various embodiments, computational modules may be implemented by e.g., processors (e.g., a general purpose computer processor or central processing unit executing software), or digital signal processors (DSPs), or other circuitry. The baseband modem may be implanted, for example, as a DSP. A beamforming matrix can be calculated and implemented for example by software running on general purpose processor. Beamformers, gain controllers, switches, combiners, and phase shifters may be implemented, for example using RF circuitries.
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. Different embodiments are disclosed herein. Features of certain embodiments may be combined with features of other embodiments; thus certain embodiments may be combinations of features of multiple embodiments.
This application claims benefit of U.S. Provisional Patent Application No. 61/813,787 filed on Apr. 19, 2013, which is incorporated herein by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61813787 | Apr 2013 | US |
