HK1091077A - Antenna steering for an access point based upon control frames - Google Patents

Description

Antenna steering for an access point based on control frames

Technical Field

The present invention relates to wireless local area networks, and more particularly, to an antenna steering algorithm for an ap operating in a wireless local area network.

Background

There are various standards that allow remote stations, such as portable computers, to move within a Wireless Local Area Network (WLAN) and connect via Radio Frequency (RF) transmissions to an Access Point (AP) that is already connected to a wired network. This wired network is commonly referred to as a wiring system (distribution system). The various standards described above include the IEEE 802.11 standard and its corresponding letter revisions, such as 802.11b and 802.11 g.

A physical layer within the remote station and within the access point provides low-level transmissions to allow the stations to communicate with the access point. Above the physical layer is a Medium Access Control (MAC) layer that provides services such as authentication, de-authentication, privacy, association and disassociation.

In operation, when a remote station goes online, a connection is first established between the station and the physical layer in the ap. The MAC layer is then attached. Generally, for remote stations and access points, physical layer RF signals are transmitted and received using monopole antennas.

A monopole antenna radiates in all directions, and in the case of a vertically oriented component, substantially in a horizontal plane. Monopole antennas are prone to degradation in the quality of communications between remote stations and access points, such as reflection or diffraction of radio wave signals by intervening objects. Intervening objects include walls, tables, and people, for example. These objects produce multipath, normal statistical fading, Rayleigh (Rayleigh) fading, etc. Therefore, there have been efforts to reduce the signal degradation caused by these effects.

One technique for canceling the degradation of the RF signal is to use two antennas to provide diversity. The two antennas are coupled to an antenna diversity switch at one or both of the remote station and the access point. The basic theory for using two antennas for antenna diversity effect is: at any given time, at least one of the antennas is likely to receive a signal that is unaffected by multipath effects. Thus, the antenna is the antenna selected by the remote station or access point to transmit/receive signals via the antenna diversity switch. There remains a need to address the degradation of RF signals between remote stations and an access point in a wireless local area network.

Disclosure of Invention

In view of the foregoing background, it is an object of the present invention to improve communications between an access point and remote stations in a wireless local area network.

An improvement over pure diversity is provided by an antenna steering procedure for an access point (i.e., wireless gateway) in a wireless local area network. Directional antennas improve network throughput and increase the range between access points and remote stations (i.e., wireless user devices). A directional antenna in most cases provides a higher signal-to-noise ratio than an omni-directional antenna, allowing the link to operate at higher data transmission rates.

The antenna steering procedure may be present within a Medium Access Control (MAC) layer of the access point and selects a best or preferred directional antenna arrangement based on a signal quality metric that may be provided by a physical layer upon receiving a signal from a remote station.

In accordance with the principles of the present invention, a preferred direction of the steered access point antenna is determined during a procedure such as registration, authentication, or subsequent exchange of data between the access point and a selected remote station. In one embodiment, this determination is made by software or firmware operating in the access point. The ap antenna control software/firmware may create a database containing the identities of remote stations and the antenna directions associated with the stations to optimize communication performance.

Hardware may be used in conjunction with the inherent diversity selection circuitry of conventional 802.11 equipment to select the preferred directional antenna angle. The ap may utilize the signaling to cause the remote station to transmit a probe response signal, wherein the ap measures the signal quality of the probe response signal. The access point may compare the metric corresponding to the signal received from the remote station in a directional antenna mode to the metric corresponding to the signal received from the remote station in an omni-directional antenna mode to determine whether a new antenna scan should be performed. If the access point determines that a hidden node is present, it may initiate a protection mechanism using request to send/clear to send (RTS/CTS), such as defined by the 802.11 standard.

The addition of a directional antenna to an access point has the dual benefits of: increasing the throughput to individual remote stations and the ability to support more users within the network. In most RF environments, the signal level received by the remote station is improved by having the access point transmit with a shaped antenna beam aimed in the direction of the station. The shaped antenna beam may provide, for example, a gain benefit of 3-5dB over omni-directional antennas conventionally deployed at access points. The increased signal level allows the link between the access point and the remote station to operate at higher data transmission rates, particularly in the out-of-band of the coverage area. The directional antenna steering process resides within an access point to support operation with remote stations.

More particularly, the present invention is directed to a method of operating an access point in a WLAN, the access point including a directional antenna for communicating with at least one remote station in a forward link on the basis of an exchange of packet data including a plurality of control frames and a data frame. The directional antenna includes a plurality of antenna patterns. The method includes receiving a first control frame from the at least one remote station via a first antenna pattern of the directional antenna, transmitting a first data frame to the at least one remote station, and receiving a second control frame from the at least one remote station via a second antenna pattern of the directional antenna.

The method further includes measuring a signal quality of a first control frame received via the first antenna pattern and a signal quality of a second control frame received via the second antenna pattern, and comparing respective measured signal qualities associated with the first and second antenna patterns. The second antenna pattern is selected to transmit a second data frame to the at least one remote station if the measured signal quality associated with the second antenna pattern exceeds the measured signal quality associated with the first antenna pattern by a predetermined threshold.

The transmitting of the first data frame may be performed using a preferred antenna pattern of the directional antenna, and wherein the comparing further comprises comparing respective measured signal qualities associated with the first and second antenna patterns with a signal quality associated with the preferred antenna pattern. The second antenna pattern is selected to transmit a second data frame to the at least one remote station if the measured signal quality associated with the second antenna pattern exceeds the measured signal quality associated with the first and preferred antenna patterns by a predetermined threshold.

The first antenna pattern may include an omnidirectional angle and the second antenna pattern may include a directional angle. Alternatively, the first antenna pattern may include a first orientation angle and the second antenna pattern may include a second orientation angle. The first control frame received may include a clear to send message and the second control frame transmitted may include an acknowledgement message.

Another aspect of the present invention is directed to a method of operating an access point in a WLAN, the access point including a directional antenna for communicating with at least one remote station in a reverse link based on an exchange of packet data including a plurality of control frames and a data frame. The directional antenna includes a plurality of antenna patterns.

The method further includes receiving a first control frame from the at least one remote station via a first antenna pattern of the directional antenna, transmitting a second control frame to the at least one remote station, and receiving a first data frame from the at least one remote station via a second antenna pattern of the directional antenna. The signal quality of the first control frame received via the first antenna pattern and the signal quality of the first data frame received via the second antenna pattern are measured. The respective measured signal qualities associated with the first and second antenna patterns are compared.

The method further includes selecting the second antenna pattern to transmit a second data frame to the at least one remote station when the measured signal quality associated with the second antenna pattern exceeds the measured signal quality associated with the first antenna pattern by a predetermined threshold.

The first antenna pattern may include an omnidirectional angle and the second antenna pattern may include a directional angle. Alternatively, the first antenna pattern may include a first orientation angle and the second antenna pattern may include a second orientation angle. The first control frame received may include a request-to-send message and the second control frame transmitted may include a clear-to-send message.

Another aspect of the present invention is directed to an access point for a Wireless Local Area Network (WLAN) including a directional antenna having a plurality of antenna patterns and a controller coupled to the directional antenna for controlling the directional antenna. The controller may be configured to communicate with at least one remote station as described above in a forward link. Alternatively, the controller may be configured to communicate with at least one remote station as described above in a reverse link.

To further illustrate the above objects, structural features and effects of the present invention, the present invention will be described in detail below with reference to the accompanying drawings.

Drawings

Fig. 1A is a diagram of a Wireless Local Area Network (WLAN) employing the principles of the present invention;

fig. 1B is a diagram of an access point in the WLAN of fig. 1A undergoing antenna scanning;

fig. 2A is a diagram of an access point of fig. 1A with an external directional antenna array;

fig. 2B is a simplified diagram of the access point of fig. 2A in which the directional antenna array is incorporated into an internal PCMCIA card;

FIG. 3A is a diagram of the directional antenna array of FIG. 2A;

FIG. 3B is a diagram of a switch for selecting a state of an antenna element of a directional antenna of FIG. 3A;

FIG. 4 is a block diagram of an access point of FIG. 1A utilizing subsystems, layers and an antenna steering process in accordance with the principles of the present invention;

fig. 5A is a signal diagram that may be optionally used for the antenna steering process of fig. 4;

fig. 5B is an alternative signal diagram for the antenna steering process of fig. 4 as desired;

FIG. 6 is an alternative block diagram to FIG. 4, in which antenna diversity circuitry is used;

fig. 7 is a signal diagram employing a hidden node technique, optionally used in the antenna steering process of fig. 4;

FIG. 8 is a top view of the network of FIG. 1 with bi-directional signaling;

FIG. 9 is a top view of the network of FIG. 1 with antenna beam readings;

fig. 10 is a flow chart of a method of operating an access point of a WLAN based on spatial diversity in accordance with the present invention;

fig. 11 is a flow chart of a method of operating an access point of a WLAN based on sounding signals in accordance with the present invention;

fig. 12 and 13 are flowcharts of a method of operating an access point for a WLAN based on control frames in the forward and reverse links, respectively, in accordance with the present invention; and is

Fig. 14 is a flow chart of a method of operating an access point of a WLAN based on hidden node identification in accordance with the present invention.

Detailed Description

The present invention will now be described in more detail with reference to the appended drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numerals refer to like elements throughout, and accents are used to indicate similar elements in alternative embodiments.

Referring initially to fig. 1A, a Wireless Local Area Network (WLAN)100 having a patching system 105 is illustrated. The access points 110a, 110b, and 110c are connected to the wiring system 105 via a wired connection, such as a wired data network connection. Each of the access points 110a, 110b, and 110c has a respective area 115a, 115b, 115c capable of communicating with the remote stations 120a, 120b, 120c via Radio Frequency (RF) signals. The remote stations 120a, 120b, 120c are equipped with wireless local area network hardware and software to access the patching system 105. In the following description, reference numerals 110, 120, and 115, respectively, are used when a general reference is made to an access point, a remote station, and a zone.

Current technology provides antenna diversity for access point 110 and remote station 120. Antenna diversity allows the access point 110 and the remote station 120 to select one of two antennas to provide transmit and receive operations based on the received signal quality. One reason for choosing one antenna apart from the other occurs in the presence of multipath fading, where a signal using two different paths results in signal cancellation at one antenna but not at the other. Another example is when two different signals received by the same antenna cause interference. Another reason for selecting one of the two antennas is due to environmental changes, such as when a remote station 120c is brought from the third region 115c to the first or second region 115a, 115b as indicated by arrow 125.

Fig. 1B is a block diagram of a subset of the network 100 shown in fig. 1A, in which the directional antenna lobes 130a-130i of an access point 110B are depicted in greater detail, employing the principles of the present invention. The directional antenna lobes 130a-130i will also be generally referred to by reference numeral 130. The ap 110b sequentially passes through the antenna lobes 130 during an environmental scan to determine a preferred antenna direction.

In a scanning process, the access point 110B scans for the RF signals transmitted by the remote station 120B using a directional antenna as described in more detail in fig. 2A and 2B. In each scan direction (i.e., angle or antenna pattern), the access point 110b measures a signal or probe response and calculates a corresponding metric for the scan angle. Examples of metrics include received signal strength Readings (RSSI), carrier-to-interference ratio (C/I), energy-to-bit ratio (Eb/No), or other suitable metrics of the quality of the received signal or signal environment, such as signal-to-noise ratio (SNR). A combination of these measurements may also be used to determine the best or preferred antenna arrangement, as will be readily understood by those of ordinary skill in the art. Based on these measured signal quality metrics, the access point 110b determines a preferred antenna angle or direction for communicating with the remote station 120 b.

Such scans may occur before or after the remote station 110b has been authenticated and combined with the patching system 105. Thus, the initial antenna scan may be done within the MAC layer. Alternatively, the initial antenna scan may be done outside the MAC layer. Similarly, the scanning that occurs after the remote station 110b has authenticated and combined with the patching system 105 can be done within the MAC layer or by procedures that occur outside of the MAC layer.

Fig. 2A is a diagram of an access point 110 using an external directional antenna array 200 a. Directional antenna array 200a includes five monopole passive antenna elements 205a, 205b, 205c, 205d, and 205e and one monopole active antenna element 206. The passive antenna components 205a, 205b, 205c, 205d, and 205e are generally referred to below by reference numeral 205. The directional antenna array 200a is connected to the access point 110 via a Universal Serial Bus (USB) port 215. Other types of connections are also accepted between the directional antenna array 200a and the access point 110.

The passive antenna elements 205 in the directional antenna array 200a are parasitically coupled to the active antenna elements 206 to allow scanning. In this description, "scanning" means that at least one antenna beam of directional antenna array 200a can be rotated in increments related to the number of passive antenna elements 205, which can be rotated 360 degrees as desired.

A detailed description of the directional antenna array 200a is provided in U.S. patent publication No. 2002/0008672 entitled "adaptive antenna for wireless communication system" which is published 2002, 1, 24, which is incorporated herein by reference and which has been assigned to the present assignee of the present invention. Exemplary methods for optimizing antenna direction based on signals received or transmitted by directional antenna array 200a are also disclosed.

Directional antenna array 200a may also be used in an omni-directional mode to provide an omni-directional antenna pattern. The ap 110 may transmit or receive using an omni-directional pattern. The access point 110 can also use the selected directional antenna when transmitting to or receiving from the remote station 120.

Fig. 2B is an isometric view of an access point 110 with an internal directional antenna 220B. In this embodiment, the directional antenna array 200b is located on a PCMCIA card 220. The PCMCIA card 220 is carried by the access point 110 and is connected to a processor (not shown). Directional antenna array 200b provides the same functionality as directional antenna array 200a shown in fig. 2A.

It should be understood that there are numerous other forms of directional antenna arrays available. Examples include U.S. patent No. 6515635 entitled "adaptive antenna for wireless communication system" as taught on 4/1/2003 and U.S. patent publication No. 2002/0036586 entitled "adaptive antenna for wireless communication system" as published on 28/3/2002, both of which are incorporated herein by reference and which have been assigned to the present assignee of the present invention.

Fig. 3A is a detailed view of a directional antenna array 200a including passive antenna elements 205 and active antenna elements 206 as previously described. The directional antenna array 200a also includes a ground plane 330 electrically coupled to the passive antenna elements, as described below with reference to fig. 3B.

Still referring to fig. 3A, the directional antenna array 200a provides a directional antenna lobe 300 that is angled away from the antenna elements 205a and 205 e. 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 elements 206 and the passive antenna elements 205 allows the directional antenna array 200a to scan the directional antenna lobe 300, which in this case is directed as shown due to the intended mode of the passive elements 205. As will be appreciated by those skilled in the art, different modes of the passive antenna element 205 combine to result in different antenna lobe 300 patterns and angles.

Fig. 3B is a diagram of an exemplary circuit that can be used to set the passive antenna elements 205 to either the reflective mode or the transmissive mode. The reflective mode is indicated by a representative long dashed line 305 and the transmissive mode is indicated by a short dashed line 310. The representative modes 305 and 310 are caused by coupling an inductive element 320 or a capacitive element 325 to a ground plane 330, respectively. The coupling of the passive antenna element 205a through the inductive element 320 or the capacitive element 325 is via a switch 315. The switch 315 may be a mechanical switch or an electrical switch that couples the passive antenna element 205a to the ground plane 330. The switch 315 is set via a control signal 335.

Coupled to the ground plane 330 via the inductor 320 is a passive antenna element 205a, which is effectively lengthened as shown by the longer representative dashed line 305. This may be considered as providing a "backplane" for an RF signal coupled to the passive antenna element 205a via the mutual coupling of the passive antenna element 205a and the active antenna element 206. In the 3A field example, both passive antenna elements 205a and 205e are connected to the ground plane 330 via respective inductive elements 320. Meanwhile, in the example of fig. 3A, other passive antenna elements 205b, 205c, and 205d are electrically connected to the ground plane 330 via corresponding capacitor elements 325.

The capacitive coupling effect effectively shortens the passive antenna element as shown by the shorter representative dashed line 310. The capacitive coupling of all passive elements 325 effectively makes directional antenna array 200a an omni-directional antenna. It should be understood that alternative coupling techniques may also be used between the passive antenna element 205 and the ground plane 330, such as delay lines and aggregate impedance.

Turning to fig. 9, a top view is provided that enables an access point 110b to generate an omni-directional antenna pattern 905 and a directional antenna pattern 910 by employing directional antenna arrays 200a or 200 b. The access point 110b communicates with a plurality of stations 120a-120 d. Since the access point 110 is typically remotely mounted without obstructions or moving reflectors in its vicinity (e.g., mounted high on a wall or ceiling), the selection of the preferred antenna pattern orientation will likely not change throughout the connection with a given remote station 120.

The illustrated access point 110b may transmit frames of downlink data to a selected remote station 120c using a directional antenna 200 a. For most broadcast and control frames, the access point may use the omnidirectional antenna pattern 905 and the lowest available data transmission rate to ensure reception by all remote stations 120. Directional antenna 200a may not increase the coverage area of network 100 but may increase the data transmission rate of data frames sent to remote station 120. The increased downlink transmission rate is advantageous because a significant portion of the data transferred over the network 100 is significantly downlinked (e.g., web page accesses, file transfers). One option is to employ switched spatial diversity when the access point 110b is required to receive in omni-directional mode. For example, a possible increased link margin of 5dB provides a 300% increase in throughput.

The uplink data frame sent by the selected remote station 120c to the access point 110b in the Contention Period (CP) is received using an omni-directional antenna pattern since any remote station may transmit the frame. For large frames, network configurations may require a remote station to employ a request-to-send/clear-to-send (RTS/CTS) mechanism to subscribe to the wireless medium. In this case, the ap 110b may receive in a directional mode to increase the data transmission rate of the uplink. This is somewhat dependent on the data transmission rate selection algorithm employed at remote station 120 c.

In downlink transmissions, the ap 110b may decide to transmit small packets using an omni-directional pattern and a lower data transmission rate during the contention period. The reason for this is that a remote station on the "other" side of the convergence zone, such as remote station 120e, cannot hear access point transmissions emanating from the directional antenna pattern pointed away from it. This is a familiar "hidden node" problem, where two remote stations 120 do not hear each other and end transmitting at the same time. In this case, the two remote stations are 120c and 120 e. One way to avoid this problem, particularly effective for large data frames, is described below with reference to FIG. 7.

A directional antenna pattern at the access point 110 provides higher data transmission rates for downlink and uplink data frames exchanged with remote stations 120 that are the subject of network traffic. Network connectivity is maintained at the nominal gain of the omni-directional antenna of access point 110. That is, the remote station 120 can associate with the access point 110 and maintain connectivity without using the directional antenna 200 a.

The set of rules as provided in table 1 can be defined to take advantage of the omnidirectional and directional characteristics of directional antenna 200 a. Table 1 includes the address of the remote station 120 currently associated with the access point 110 and its current antenna direction selection. Table 1 may describe an example antenna direction selection based on a sequence of frames in accordance with the 802.11 standard within which tables 21 and 22 are presented. In table 1, "Dir" refers to direction, "UL" refers to uplink, and "DL" refers to downlink.

Table 1-example antenna selection rules

Sequence of	Dir	Antenna selection
				Beacon	DL	Omnidirectional radio
Data of	DL	Orientation	See FIG. 5A
				RTS-CTS data	UL	Omnidirectional/directional	See FIG. 5B

A process may be described by a rule set that determines when to select an omni pattern and when to select a directional pattern. For example, the ap 110 may select a directional pattern during the time interval when transmitting or receiving to a single remote station 120.

A block diagram illustrating the interface of the access point 110 is shown in fig. 4. The icon access point 110 includes a plurality of subsystems and layers. An antenna subsystem 405 may include the directional antenna 220b and the supporting circuitry, buses, and software used to operate the directional antenna. The antenna subsystem 405 interfaces with the physical layer 410 and provides RF signals 412 to the latter.

The phy layer 410 processes the RF signal 412 and determines signal quality measurements for an antenna steering process 420. Physical layer 410 transmits the processed signal to MAC layer 415 based on RF signal 412. The MAC layer 415 generates timing control messages 422 that are also sent to the antenna steering process 420 to switch the antenna to either the full-direction mode or the directional mode, if desired.

The MAC layer 415 also sends the data frame 429 to other processes (not shown). The phy layer 410, MAC layer 415, and antenna steering process 420 may be present in a controller 400. The antenna steering process 420 can be stored in a memory, such as a separate memory or an embedded memory within a processor, for example.

The antenna steering process 420 maintains an "antenna table or database" or "direction table or database" of a function of the received signal quality measurements 417 made during the antenna scan of each remote station 120. For example, direction table 425 may store a station ID and a corresponding antenna direction for directional communication with remote station 120 (A, B, C). Once the antenna direction within the direction table 425 is determined, directional antenna control 427 is provided to the antenna subsystem 405 using the antenna steering process 420. If the signal quality measurement 417 is above a predetermined threshold indicating that a higher data transmission rate can be supported in the omni-directional mode, the antenna direction may be maintained in the omni-directional (O) mode.

Various techniques are described below for determining a preferred direction to point a directional antenna 220b from an access point 110 to a remote station 120 in accordance with the present invention. The first technique employs a spatial diversity selection mechanism. A second technique utilizes sounding reference signal sequences exchanged between the access point 110 and the remote station 120. A third technique uses control messages (e.g., ACK or CTS) to make signal quality measurements in the ap 110 for the receive antenna direction. The third technique is applicable to both forward and reverse links.

The first technique assumes that existing 802.11 devices incorporate antenna switched diversity scanning/control and that future 802.11 devices such as 802.11a/802.11g/802.11n will also support switched diversity. The first technique is available after a remote station 120 has been authenticated and combined with a network. It is assumed that the initial antenna scan is done within the MAC/network layer protocol. Using a directional or multi-element antenna 220a, this first technique can keep the antenna position/selection updated using diversity protocols.

Referring now to fig. 6, the first technique functions as follows. The illustrated access point 110 ' includes a controller 600 ' coupled to the antenna subsystem 405 '. The controller 600 'includes a physical layer 410' that is given access to antenna control signals and a MAC layer (fig. 4). The MAC layer writes the antenna selection into buffer a 605a 'and buffer B605B'. Buffer a 605a 'holds the selected antenna position and buffer B605B' holds a candidate antenna position. Physical layer 410 'is also in communication with a multiplexer 610'. Physical layer 410 ' sends a diversity selection switch control signal 607 ' to multiplexer 610 ' in a conventional diversity selection control manner, but in this case uses a diversity selection switch control signal that controls the contents of buffer a 605a ' or buffer B605B '.

The selected antenna location is initially selected during a network authentication/association protocol. The candidate antenna position is any other antenna position (including an omni-directional pattern). The candidate antenna positions are changed in a predetermined order after a valid packet has been received or after a predetermined period of time without receiving any packet.

After a packet is successfully received, the physical layer 410' sends the MAC layer received signal quality metrics (signal strength, signal-to-noise ratio, multipath/equalizer metrics, etc.) for the two antenna locations. During packet reception, the phy layer 410' operates as it is currently used for 802.11; that is, the antenna position best for packet reception is switched between the two antenna positions and used. After the physical layer 410' receives a valid packet, the signal quality metrics for the two antenna locations are sent to the MAC layer. The MAC layer updates the selected antenna position and the candidate antenna positions. The selected antenna location is changed to the best location based on the data received from the physical layer 410'. Filtering/hysteresis may be used to avoid "ping-pong" between two antenna locations.

As previously mentioned, this technique takes advantage of the existing 802.11 antenna switched diversity approach. It should be understood that this first technique may comprise hardware, software/firmware, or a combination thereof.

Referring now to fig. 10, a flow chart of the aforementioned method of operating an access point 110 in a WLAN 100 based on spatial diversity will be described. From the start (block 1000), the method includes communicating with the remote station 120 using a current angle of the directional antenna 220b, as per block 1010. Proceeding at block 1020, multiple alternative angles are swept through directional antenna 220b for communication with remote station 120 during the preamble. What is done at block 1030 is measuring the corresponding signal received from the remote station 120 via the current angle and a plurality of alternate angles. At block 1040, the current angle or one of the plurality of alternate angles is selected as a preferred angle for continuing communication with the remote station 120 based on the measured signal during the preamble. The method ends at block 105.

The second technique is based on the transmission by the access point 110 of an RTS message to the remote station 120 and the reception by the remote station of a CTS message in response to the transmission by the access point. The 802.11 standard also defines a probe request/probe response exchange that is typically used by remote stations 120 to determine the quality of the link to other stations 120.

When used by the access point 110 to determine a preferred pointing direction for a selected remote station 120 (as shown in fig. 8), the access point 110 transmits a probe request signal in an omni-directional pattern and in each possible directional pattern 130, and measures the signal quality of the probe response signal 810 returned from the remote station 110 while operating in the corresponding pattern.

The measurement of these response frames 810 makes them a more reliable technique than the diversity selection technique described above. This second technique is preferably used at least immediately after a remote station 120 has associated with the access point 110. Although the use of additional probe request/probe response signals may have an impact on network efficiency, such exchanges may occur infrequently.

Referring now to fig. 11, a flow chart illustrating the aforementioned method of operating an ap 110 in a WLAN 100 based on sounding signals will be described. Starting at a start point (block 1100), the method includes selecting a remote station 120 at block 1110, transmitting a first sounding signal to the selected remote station via an omni-directional angle of directional antenna 220b at block 1120, and measuring a first sounding response signal received from the selected remote station via the omni-directional angle in response to the first sounding signal at block 1130.

A respective second sounding signal is transmitted to the selected remote station 120 via each of a plurality of directional angles of directional antenna 200b at block 1140, and a second sounding response signal received from the selected remote station via each directional angle in response to the respective second sounding signal is measured at block 1150. At block 1160, the measured first probe response signal and the corresponding measured second probe response signal from the selected remote station 120 are stored in an antenna database.

A preferred orientation angle is selected for the selected remote station 120 based on the measured second probe response signals at block 1170. At block 1180, the measured first probe response signal from the omni-directional angle is compared to the measured second probe response signal from the preferred directional angle. The first probe signal includes a Request To Send (RTS) message and the first probe response signal includes a Clear To Send (CTS) message. Similarly, the second probe signal includes an RTS message and the second probe response signal includes a CTS message. At block 1190, the omni angle or the preferred directional angle is selected based on the comparison to continue communication with the selected remote station 120. The method ends at block 1195.

A third technique uses control frames that are used for normal data exchange between the ap 110 and the remote station 120. This technique may be used in both forward link communications as well as reverse link communications. Since Clear To Send (CTS) and Acknowledgement (ACK) messages are sent at a lower data transmission rate, the ap 110 can use these messages to compare the omni pattern 905 with the currently selected directional pattern 130. This is illustrated in fig. 5A, where there is a dashed line above the antenna selection timing. Which may be used as a method for determining whether the currently selected direction 130 retains its advantages over the omni-directional pattern 905. This advantage is typically based on a predetermined threshold to avoid frequent switching between two antenna patterns with similar signal quality metrics.

For example, during the CTS message, the message may be received in omni-directional mode to calculate a first signal quality measurement. During the ACK message, a second signal quality measurement may be calculated by receiving the message in a test antenna direction. A comparison of the first and second signal quality measurements is made and a determination is made as to whether the test antenna direction should be stored. That is, whether the directional mode provides a higher gain than the omni-directional mode. A comparison between two different directional antenna directions can also be made.

The direction table 425 of fig. 4 may be added with signal quality measurements of the omni-directional and selected directional antenna patterns from the previously described process. If the dominance falls below a predetermined threshold, the access point 110 reverts to omni-directional selection and performs antenna searching using one of the first two techniques described above.

In case the remote station 120 enters a power saving mode or an idle period without data transfer for a long time, the ap 110 reverts to omni-directional pattern selection. When the remote station 120 becomes active again, the access point 110 can perform another antenna search.

Referring now to fig. 12 and 13, flow charts of methods for operating an access point 120 in a WLAN 100 based on control frames in the forward and reverse links, respectively, are illustrated. From the start (block 1200), the method includes receiving a first control frame from the remote station 120 via a first antenna pattern of the directional antenna 220b in a forward link at block 1210, transmitting a first data frame to the remote station at block 1220, and receiving a second control frame from the remote station via a second antenna pattern of the directional antenna at block 1230. The signal quality of a first control frame received via the first antenna pattern and the signal quality of a second control frame received via the second antenna pattern are measured at block 1240. The respective measured signal qualities associated with the first and second antenna patterns are compared at block 1250. If the measured signal quality associated with the second antenna pattern exceeds the measured signal quality associated with the first antenna pattern by a predetermined threshold, the second antenna pattern is selected for transmission of a second data frame to the remote station 120 at block 1260. The received first control frame includes a clear-to-send message and the received second control frame includes an acknowledgement message. The method ends at block 1270.

The method for operating an access point 120 in a WLAN 100 based on control frames in the reverse link includes, starting at a starting point (block 1300), receiving a first control frame from the remote station via a first antenna pattern of a directional antenna 220b at block 1310, transmitting a second control frame to the remote station at block 1320, and receiving a first data frame from the remote station via a second antenna pattern of the directional antenna at block 1330. The signal quality of a first control frame received via the first antenna pattern and the signal quality of a first data frame received via the second antenna pattern are measured at block 1340. The corresponding measured signal qualities associated with the first and second antenna patterns are compared at block 1350. If the measured signal quality associated with the second antenna pattern exceeds the measured signal quality associated with the first antenna pattern by a predetermined threshold, the second antenna pattern is selected at block 1360 to allow the access point 110 to transmit a second data frame to the remote station 120. The received first control frame includes a request-to-send message and the transmitted second control frame includes a clear-to-send message. The method ends at block 1370.

A fourth technique is a hidden node protection technique that provides a protection mechanism to reduce or eliminate the occurrence of hidden nodes when the access point 110 uses a directional antenna 220 b. Hidden nodes occur when not all remote stations 120 in network 100 hear communications between access point 110 and a selected remote station 120, and therefore, un-heard remote stations may transmit while the medium is in use. This can cause collisions, particularly at the access point 110.

When the ap 110 has data to transmit to a remote station 120, the control process sets the selected antenna direction to determine if there are potential hidden nodes in a manner that scans the direction table 425 of fig. 4. For example, the ap 110 may look for the remote station 120 in a direction opposite to the selected antenna direction.

Referring to the timing diagram of fig. 7, if the controlling software determines that there is a possibility of hidden nodes, the access point 110 first transmits a CTS message to a known unused MAC address using the omni-directional pattern of antenna 220 a. This procedure is used to inform all remote stations 120 in the network that a switch is to occur and will not transmit until the switch is complete. The access point 110 then switches to the selected antenna direction of the intended remote station 120 and communicates. Another solution to prevent the hidden node problem is to perform a four-way frame exchange protocol (RTS, CTS, data and ACK) with a desired remote station 120.

If the control software determines that a hidden node is not possible, the access point 110 does not send a CTS message and can begin communication immediately when the access point 110 antenna is set to the correct direction. If the network protocol requires, an RTS message may be sent to the intended receiver, resulting in a CTS message back to the ap 110 as an acknowledgement message, as shown in fig. 5A.

It is noted that in the procedure described with reference to fig. 7, performance is improved because RTS messages are not transmitted by the access point 110, since CTS messages are only needed to cause the remote station 120 to cease transmission. The remote station 120 indicated in the ID field of the standard 802.11 protocol header ensures that the designated remote station receives the data frame.

Referring now to fig. 14, a method for operating an access point 120 in a WLAN 100 based on hidden node identification is illustrated. From a start (block 1400), the method includes creating 1410 an antenna database by associating respective measured signal qualities corresponding to a plurality of antenna patterns between the access point 110 and each remote station 120. The respective measured signal qualities are determined by the access point 110 based on its communications with each remote station 120. A preferred antenna pattern for each remote station 120 is determined based on the antenna database at block 1420 and a remote station and corresponding preferred antenna pattern are selected for communication at block 1430. At block 1440, a determination is made based on the antenna database and prior to communicating with the selected remote station as to whether any unselected remote stations may not be known at the time such communications actually occurred. This is determined by comparing the measured signal quality associated with the preferred antenna pattern for the selected remote station with the corresponding signal quality associated with non-selected remote stations using the same preferred antenna pattern.

If there may be a hidden node, a message is broadcast at block 1450 indicating that the access point 110 and the selected remote station 120 are communicating with each other. As previously described, this broadcast may be in the form of an active clear to send message sent to remote station 120 via the omni-directional antenna pattern. The CTS has an unused address that does not correspond to any remote station 120. Alternatively, a four-way frame exchange protocol (RTS, CTS, data, and ACK) is performed with the selected remote station 120 to prevent hidden node problems. The method ends at block 1460.

While the present invention has been particularly shown and described with reference 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. For example, an access point is not limited to the IEEE 802.11 standard. The ap antenna algorithm described above can be readily applied to other types of local area networks, such as those defined by the IEEE 802.16 standard, as will be readily understood by those skilled in the art.

Claims (36)

1. A method of operating an access point in a Wireless Local Area Network (WLAN), the access point including a directional antenna for communicating with at least one remote station in a forward link on a packet data exchange basis including a plurality of control frames and a data frame, the directional antenna including a plurality of antenna patterns, the method comprising:

receiving a first control frame from the at least one remote station via a first antenna pattern of the directional antenna;

transmitting a first data frame to the at least one remote station;

receiving a second control frame from the at least one remote station via a second antenna pattern of the directional antenna;

measuring a signal quality of the first control frame received via the first antenna pattern and a signal quality of the second control frame received via the second antenna pattern; and

the respective measured signal qualities associated with the first and second antenna patterns are compared.

2. The method of claim 1, wherein: the method also includes selecting the second antenna pattern to transmit a second data frame to the at least one remote station when the measured signal quality associated with the second antenna pattern exceeds the measured signal quality associated with the first antenna pattern by a predetermined threshold.

3. The method of claim 1, wherein: transmitting the first data frame using a preferred antenna pattern of the directional antenna; and wherein the comparing step further comprises comparing the respective measured signal qualities associated with the first and second antenna patterns with a signal quality associated with the preferred antenna pattern.

4. The method of claim 3, wherein: further comprising selecting the second antenna pattern for transmitting a second frame of data to the at least one remote station when the measured signal quality associated with the second antenna pattern exceeds the measured signal quality associated with the first and preferred antenna patterns by a predetermined threshold.

5. The method of claim 1, wherein: the first antenna pattern includes an omnidirectional angle and the second antenna pattern includes a directional angle.

6. The method of claim 1, wherein the first antenna pattern comprises a first directional angle and the second antenna pattern comprises a second directional angle.

7. The method of claim 1, wherein: the received first control frame includes a clear-to-send message and the received second control frame includes an acknowledgement message.

8. The method of claim 1, wherein: also included is generating an antenna database containing corresponding measured signal qualities associated with the first and second antenna patterns.

9. The method of claim 1, wherein: further comprising selecting the first antenna pattern and the second antenna pattern, and wherein the selecting is performed at a Medium Access Control (MAC) layer of the ap.

10. The method of claim 1, wherein: the step of measuring the corresponding signal quality includes determining at least one of a received signal strength reading, a carrier-to-interference ratio, an energy-to-bit ratio, and a signal-to-noise ratio.

11. The method of claim 1, wherein: the access point operates based on at least one of an IEEE 802.11 standard and an IEEE 802.16 standard.

12. The method of claim 1, wherein the directional antenna comprises at least one active element and a plurality of passive elements.

13. A method of operating an access point in a Wireless Local Area Network (WLAN), the access point including a directional antenna for communicating with at least one remote station in a reverse link on a packet data exchange basis including a plurality of control frames and a data frame, the directional antenna including a plurality of antenna patterns, the method comprising:

receiving a first control frame from the at least one remote station via a first antenna pattern of the directional antenna;

transmitting a second control frame to the at least one remote station;

receiving a first data frame from the at least one remote station via a second antenna pattern of the directional antenna;

measuring a signal quality of the first control frame received via the first antenna pattern and a signal quality of the first data frame received via the second antenna pattern;

the respective measured signal qualities associated with the first and second antenna patterns are compared.

14. The method of claim 13, wherein: further comprising selecting the second antenna pattern for transmitting a second data frame to the at least one remote station via the access point when the measured signal quality associated with the second antenna pattern exceeds the measured signal quality associated with the first antenna pattern by a predetermined threshold.

15. The method of claim 13, wherein: the first antenna pattern includes an omnidirectional angle and the second antenna pattern includes a directional angle.

16. The method of claim 13, wherein: the first antenna pattern includes a first orientation angle and the second antenna pattern includes a second orientation angle.

17. The method of claim 13, wherein: the received first control frame includes a request-to-send message and the transmitted second control frame includes a clear-to-send message.

18. The method of claim 13, wherein: also included is generating an antenna database containing corresponding measured signal qualities associated with the first and second antenna patterns.

19. The method of claim 13, wherein: the directional antenna includes at least one active element and a plurality of passive elements.

20. An access point for a Wireless Local Area Network (WLAN), comprising:

a directional antenna having a plurality of antenna patterns; and

a controller connected to the directional antenna for control, the controller communicating with at least one remote station in a forward link based on a packet data exchange including a plurality of control frames and a data frame, the controller

Receiving a first control frame from the at least one remote station via a first antenna pattern of the directional antenna;

transmitting a first data frame to the at least one remote station;

receiving a second control frame from the at least one remote station via a second antenna pattern of the directional antenna;

measuring a signal quality of the first control frame received via the first antenna pattern and a signal quality of the second control frame received via the second antenna pattern; and

the respective measured signal qualities associated with the first and second antenna patterns are compared.

21. The access point of claim 20, wherein: the directional antenna comprises at least one active component and a plurality of passive components.

22. The access point of claim 20, wherein: the controller selects the second antenna pattern for transmitting a next data frame to the at least one remote station when the measured signal quality associated with the second antenna pattern exceeds the measured signal quality associated with the first antenna pattern by a predetermined threshold.

23. The access point of claim 20, wherein: transmitting the data frame by the controller using a preferred antenna pattern of the directional antenna; and wherein the comparing step by the controller further comprises comparing respective measured signal qualities associated with the first and second antenna patterns with a signal quality associated with the preferred antenna pattern.

24. The access point of claim 23, wherein: the controller selects the second antenna pattern for transmitting a second data frame to the at least one remote station when the measured signal quality associated with the second antenna pattern exceeds the measured signal quality associated with the first and preferred antenna patterns by a predetermined threshold.

25. The access point of claim 20, wherein: the first antenna pattern includes an omnidirectional angle and the second antenna pattern includes a directional angle.

26. The access point of claim 20, wherein: the first antenna pattern includes a first orientation angle and the second antenna pattern includes a second orientation angle.

27. The access point of claim 20, wherein: the received first control frame includes a clear to send message and the transmitted second control frame includes an acknowledgement message.

28. The access point of claim 20, wherein: the controller generates an antenna database containing respective measured signal qualities associated with the first and second antenna patterns and stores the antenna database.

29. The access point of claim 20, wherein: the controller includes a physical layer and a Medium Access Control (MAC) layer, and the selecting of the first antenna pattern and the second antenna pattern is performed at the MAC layer.

30. The access point of claim 20, wherein: the controller measures a corresponding signal quality based on determining at least one of a received signal strength reading, a carrier-to-interference ratio, an energy-to-bit ratio, and a signal-to-noise ratio.

31. An access point for a Wireless Local Area Network (WLAN), comprising:

a directional antenna having a plurality of antenna patterns; and

a controller connected to the directional antenna for control, the controller communicating with at least one remote station in a reverse link based on a packet data exchange including a plurality of control frames and a data frame, the controller

Receiving a first control frame from the at least one remote station via a first antenna pattern of the directional antenna;

transmitting a second control frame to the at least one remote station;

receiving a first data frame from the at least one remote station via a second antenna pattern of the directional antenna;

measuring a signal quality of the first control frame received via the first antenna pattern and a signal quality of the first data frame received via the second antenna pattern; and

the respective measured signal qualities associated with the first and second antenna patterns are compared.

32. The ap of claim 31, wherein: the directional antenna comprises at least one active component and a plurality of passive components.

33. The ap of claim 31, wherein: the controller selects the second antenna pattern for transmitting a second data frame to the at least one remote station when the measured signal quality associated with the second antenna pattern exceeds the measured signal quality associated with the first antenna pattern by a predetermined threshold.

34. The ap of claim 31, wherein: the first antenna pattern includes an omnidirectional angle and the second antenna pattern includes a directional angle.

35. The ap of claim 31, wherein: the first antenna pattern includes a first orientation angle and the second antenna pattern includes a second orientation angle.

36. The ap of claim 31, wherein: the first control frame received includes a request-to-send message and the second control frame transmitted includes a clear-to-send message.