The 802.11 group of IEEE standards allows stations (e.g., portable computers) to be moved within a facility and connect to a Wireless Local Area Network (WLAN) via Radio Frequency (RF) transmissions to Access Points (AP's) connected to a wired network, referred to as a distribution system. A physical layer in the stations and access points provides low level transmission means by which the stations and access points communicate. Above the physical layer is a Media Access Control (MAC) layer that provides services, such as synchronization, authentication, deauthentication, privacy, association, disassociation, etc.
In operation, when a station comes on-line, synchronization is first established between the physical layers in the station and an access point. The MAC layer then associates and authenticates with that AP.
Typically, in 802.11 stations and access points, the physical layer RF signals are transmitted and received by monopole antennas. A monopole antenna radiates in all directions, generally in a horizontal plane for a vertical oriented element. Monopole antennas are susceptible to effects that degrade the quality of communication between the station and the access points, such as reflection or diffraction of radio wave signals the station and the access points, such as reflection or diffraction of radio wave signals caused by intervening objects, such as walls, desks, people, etc. These objects create multi-path, normal statistical fading, Rayleigh fading, and so forth. As a result, efforts have been made to mitigate signal degradation caused by these effects.
One technique for counteracting the degradation of RF signals is to use two antennas to provide spatial diversity using two antennas spaced some distance apart. The two antennas are coupled to an antenna diversity switch in either or both the stations and access points. The theory behind using two antennas for antenna diversity is that, at any given time, one of the two antennas is likely receiving a signal that is not suffering from the effects of, say, multi-path, and that is the antenna that the station or access point selects via the antenna diversity switch for transceiving signals.
Improvement over simple diversity is provided through a Medium Access Control (MAC) layer antenna steering process for a directional antenna used on the station side of an 802.11 wireless network. The directional antenna provides an improved signal quality in most cases allowing the link to operate at higher data rates.
One embodiment according to the principles of the present invention includes a method or apparatus operating external from a Station Management Entity (SME) and Physical (PHY) layer (e.g., at the MAC layer or in a process in communication with the MAC layer) resident in an 802.11 Network Interface Card in a station. The method or apparatus selects the best directional antenna pattern based on signal quality metrics available from the PHY layer upon reception of frames from the Access Point (AP). The directional antenna may be controlled by a simple two- or three-wire digital interface that drives switches connected to passive or active elements of the directional antenna to cause the directional antenna to form the selected beam pattern. The directional antenna can also be placed in an omni-mode with near equal gain in all directions.
The station surveys the available Access Points by detecting Beacon Frames in omni-directional mode. During synchronization with a particular access point, Beacon frames may be used to perform a search for a “best” antenna direction. The method or apparatus may further include revisiting the omni-directional mode during the reception of the Beacon frame to determine if the advantage of operating in the selected “best” antenna direction is retained. If not, a subsequent search for a “best” antenna direction is performed.
The method or apparatus may also use a series of probe requests to cause a predefined response from an AP. The antenna beam pattern changed between each probe request to determine the best antenna beam pattern. In this way, Beacon frames are not missed should the antenna beam be pointing in a direction away from the AP during the Beacon frame.
The benefits from augmenting the station with a directional antenna are two-fold: (i) improved throughput to individual stations and (ii) ability to support more users in the network. In most RF environments, the signal level received at the station can be improved by orienting a shaped antenna beam in the direction of the strongest signal. The shaped beam provides 3-5 dB additional gain over the omni-directional (“omni”) antennas typically employed. The increased signal level allows the access point and the station to transmit at higher data rates, especially at the outer edge of the coverage area. This improves the throughput to/from that station but also increases the network capacity since the transmission time is reduced. For example, if the access point and the connected stations are able to cut their transmission times in half by employing a higher data rate, the network is able to support twice as many users.
The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.
A description of preferred embodiments of the invention follows.
Directional antennas have traditionally been employed to improve signal quality over line-of-sight RF communications links. The directional antenna uses some form of beam-forming to increase the antenna gain in a particular direction for transmission and reception. The direction may be adjusted or chosen to improve signal quality. In application to the 802.11 wireless access media, the directional antenna provides gain as well as interference rejection and angular diversity. The present invention provides a method to determine the best pointing angle of a directional antenna within the 802.11 MAC layer protocols.
The ability of a directional antenna to provide an increase in signal quality, i.e., Signal-to-Noise Ratio (SNR), is statistical in nature. In some multi-path environments, a directional antenna may provide more than 5 dB of gain, and in others, it may not be better than an omni-directional (“omni”) pattern. Averaging over the whole network coverage area, a system employing an directional antenna might obtain a 10 dB increase in gain about 10% of the time, a 5 dB in gain about 30% of the time, etc. The amount of gain translates into how much data throughput can be increased. For an 802.11b link, for example, the system might need 6 dB of gain to achieve the normally expected maximum 11 Mbps data rate versus the lowest 1 Mbps rate at the edge of the coverage area. For an 802.11a or 802.11g link, the system might need more than 10 dB of gain to achieve the highest data rate of 54 Mbps.
Typically, the control messages (including the Beacon frames) are sent from the Access Point (AP) at the lowest data rate so that all of the stations in the coverage area can correctly receive them. Data frames sent from the access point to a single station can be sent at higher data rates to improve the network efficiency. The means by which the access point decides it can transmit at the higher rates to a specific station is not specified in the 802.11 standards.
Since one objective of the directional antenna is to provide increased throughput for the data frames sent to or from a station, and since most if not all of the antenna gain is used to provide that increase, a station can operate in directional mode following synchronization with a particular access point and have the benefits of the increased throughput. This simplifies the process and keeps the beacon scan time associated with looking for access points consistent with traditional omni antenna equipped stations.
Present technology provides the access points 110 and stations 120 with antenna diversity. The antenna diversity allows the access points 110 and stations 120 with an ability to select one of two antennas to provide transmit and receive duties based on the quality of signal being received. One antenna is selected over another if, in the event of multi-path fading, a signal taking two different paths to the antennas causes signal cancellation to occur at one antenna but not the other. Another example is when interference is caused by two different signals received at the same antenna. Yet another reason for selecting one of the two antennas is due to a changing environment, such as when a station 120c is moved between the third zone 115c and first or second zones 120a, 120b, respectively.
During an antenna search, the second station 120b uses a directive antenna, shown in more detail in
The beam selection search may occur before or after the second station 110b has authenticated and associated with the distribution system 105. Thus, the initial antenna scan may be accomplished within the Media Access Control (MAC) layer. Similarly, beam selection search occurring after the second station 120b has authenticated and associated with the distribution system 105 may be accomplished within the MAC.
The directive antenna array 200a may also be used in an omni-directional mode to provide an omni-directional antenna pattern (not shown). The stations 120 may use an omni-directional pattern prior to sending a transmission for determining whether another station 120 is currently sending a transmission (i.e., Carrier Sense Multiple Access (CSMA)). The stations 120 may also use the selected directional antenna when transmitting to or receiving from the access points 110. In an ‘ad hoc’ network, the stations 120 may revert to an omni-only antenna configuration, since they can receive from any other station 120.
It should be understood that various other forms of directive antenna arrays can be used. Examples include the arrays described in U.S. Pat. No. 6,515,635 issued Feb. 4, 2003, entitled “Adaptive Antenna for Use in Wireless Communication Systems,” and U.S. Patent Publication No. 2002/0036586, published Mar. 28, 2002, entitled “Adaptive Antenna for Use in Wireless Communication System;” the entire teachings of both are incorporated herein by reference.
The directive antenna array 200a provides a directive antenna lobe 300 angled away from antenna elements 205a and 205e. This is an indication that the antenna elements 205a and 205e are in a “reflective” mode, and the antenna elements 205b, 205c, and 205d are in a “transmissive” mode. In other words, the mutual coupling between the active antenna element 206 and the passive antenna elements 205 allows the directive antenna array 200a to scan the directive antenna lobe 300, which, in this case, is directed as shown as a result of the modes in which the passive elements 205 are set. Different mode combinations of passive antenna elements 205 result in different antenna lobe 300 patterns and angles.
Coupled to the ground plane 330 via the inductor 320, the passive antenna element 205a is effectively elongated as shown by the longer representative dashed line 305. This can be viewed as providing a “backboard” for an RF signal coupled to the passive antenna element 205a via mutual coupling with the active antenna element 206. In the case of
It should be understood that alternative coupling techniques may also be used between the passive antenna elements 205 and ground plane 330, such as delay lines and lumped impedances.
The MAC layer 410 includes MAC processes 415 and MAC management 420. The PHY layer 425 includes a convergence layer 430, Direct Sequence Spread Spectrum (DSSS) Physical Layer Convergence Procedure (PLCP) sublayer 435, a DSSS Physical Medium Dependent (PMD) sublayer, which define a PMD Service Access Point (SAP). The operation of each of the components of the MAC and PHY layers 410, 425 is well known in the art. The purpose of introducing the MAC and PHY layers 410, 425 is to provide an understanding as to how an antenna control unit 500 described in reference to
As shown in
In an alternative embodiment, the control unit 500 sends the beam selection control signals 515 to the directional antenna 200a via the PHY layer 425. In such an embodiment, the PHY layer 425 is modified to accommodate a signal feedthrough or support, and the cable 505 extends between the PHY layer 425 and the directional antenna 200a.
The antenna control unit 500, which may be hardware, firmware, or software, is integrated into or alongside the MAC layer 410 and receives indications from the MAC 410 when certain messages are received from the SME 504 or the PHY layer 425. The responses by the antenna control unit 500 to certain SME requests 530 are listed in Table 1.
During initialization of the station 120, the ResetRequest, StartRequest, and ScanRequest cause the antenna control unit 500 to revert to the directional antenna's Omni mode. The JoinRequest triggers the antenna search, which is further illustrated in
Referring now to
One way to determine if the antenna selection should be updated is by monitoring the difference in received signal quality between the directional selection and the omni pattern. This difference, perhaps 4-5 dB, can be recorded when the antenna direction is selected. Thereafter, a predetermined percentage of the Beacon frames may be received using the omni pattern by switching to the omni pattern at known Beacon frame transmission times. The signal quality of these frames are then compared with those received on the directional pattern to check if the signal quality advantage of the directional pattern had degraded (Steps 725 and 730) below a predetermined threshold.
Alternatively, the antenna control may initiate probe requests for determining the best antenna beam. This allows a faster search through the antenna beams 130. Additionally, the probe requests technique eliminates the potential loss of beacon frames that could occur when cycling through the antenna beams 130 on those frames.
Alternatively, antenna directional selection may automatically occur on an event-driven basis, periodically, or randomly.
Depending on the variability of the detected signal and noise levels at the fringes of the coverage area, the process may average multiple signal quality measurements at each antenna direction.
At the point where the antenna search is performed (Step 3), the process may optionally select the omni antenna pattern when signal quality obtained is high enough to support the highest data rate. This occurs when the station is close to the access point.
While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.
This application claims the benefit of U.S. Provisional Application No. 60/479,640, filed Jun. 19, 2003, the entire teachings of which are incorporated herein by reference.
Number | Date | Country | |
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60479640 | Jun 2003 | US |