HK1090191B - An access point for a wireless local area network and the operating method thereof - Google Patents

Description

Access point of wireless local area network and operation method thereof

Technical Field

The present invention relates to the field of wireless local area networks, and more particularly, to antenna steering algorithms for aps 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 to an Access Point (AP) that is connected to a wired network via Radio Frequency (RF) transmissions. 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-order 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 comes 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 element, substantially in a horizontal plane. Monopole antennas are prone to degradation in the quality of communications between a remote station and an access point, 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 counteracting 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 through the antenna diversity switch. There remains a need to address the degradation of RF signals between a remote station 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 through 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 an access point and a remote station (i.e., wireless user device). 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 signal quality metrics that may be provided by the physical layer upon receiving signals from the remote station.

In accordance with the principles of the present invention, a preferred direction of the steered access point antenna is determined during a process 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 at the access point. The ap antenna control software/firmware may create a database containing the identities of remote stations and antenna orientations 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 access point may utilize the signaling to cause the remote station to transmit a probe response signal, wherein the access point 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 pattern to the metric corresponding to the signal received from the remote station in an omni-directional antenna pattern to determine whether a new antenna scanning operation 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 benefit 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 a 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 3-5dB gain benefit over omni-directional antennas traditionally 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-coverage band. The directional antenna steering procedure resides within the access point to support operation with the remote station.

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 a plurality of remote stations. The directional antenna comprises an omnidirectional angle and a plurality of directional angles. The method includes selecting a remote station from among the plurality of remote stations, transmitting a first probe signal to the selected remote station through the omni-directional angle of the directional antenna, and measuring a first probe response signal received from the selected remote station through the omni-directional angle in response to the first probe signal.

The method further includes transmitting a respective second probe signal to the selected remote station through each of a plurality of directional angles of the directional antenna, and measuring a second probe response signal received from the selected remote station through each directional angle in response to the respective second probe signal. The measured first probe response signal and the corresponding measured second probe response signal from the selected remote station are stored in an antenna database.

The method may further include selecting a preferred directional angle for the selected remote station based on the measured second probe response signal and comparing the measured first probe response signal from the omni-directional angle with the measured second probe response signal from the preferred directional angle. The omni-directional angle or preferred directional angle may be selected for continued communication with the selected remote station based on the comparison. The preferred orientation angle may be selected if the measured signal associated with the preferred orientation angle exceeds the measured signal associated with the omnidirectional angle by a predetermined threshold.

The method may further include the steps of selecting a next remote station from the plurality of remote stations, repeating the steps of transmitting the first and second probe signals for the next selected remote station, and measuring the first and second probe response signals received from the next selected remote station. The measured first probe response signal and the corresponding measured second probe response signal received from the next selected remote station are stored in an antenna database. The selecting, transmitting and storing steps are repeated for each of the remaining remote stations.

The first probe signal may comprise a Request To Send (RTS) message and the first probe response signal may comprise a Clear To Send (CTS) message. Similarly, the second probe signal may comprise an RTS message and the second probe response signal may comprise a CTS message.

The measuring step may include 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. The selection of the omni angle and the scanning of the plurality of directional angles may be performed at a Media Access Control (MAC) layer of the access point.

The method can further include updating an antenna database for the selected remote station when there is no communication between the access point and the selected remote station for a specified period of time. The updating may include repeating the steps of transmitting first and second probe signals to the selected remote station and measuring first and second probe response signals received from the selected remote station.

The access point may be based on the IEEE 802.11 standard or the IEEE 802.16 standard. The directional antenna may include at least one active element and a plurality of passive elements. Another aspect of the invention is directed to an access point including a directional antenna having an omni-directional angle and a plurality of directional angles and a controller connected to the directional antenna for control.

The controller selects a remote station from among a plurality of remote stations, transmits a first probe signal to the selected remote station through the omni angle of the directional antenna, and measures a first probe response signal received from the selected remote station through the omni angle in response to the first probe signal. The controller further transmits a respective second probe signal to the selected remote station through each of a plurality of directional angles of the directional antenna, measures a second probe response signal received from the selected remote station through each directional angle in response to the respective second probe signal, and stores the measured first probe response signal and the respective measured second probe response signal from the selected remote station in an antenna database.

Drawings

The above and other objects, features and advantages of the present invention will become apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.

FIG. 1A is a simplified diagram of a Wireless Local Area Network (WLAN) utilizing the principles of the present invention;

FIG. 1B is a diagram of an access point performing an antenna scan within the WLAN of FIG. 1A;

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

FIG. 3B is a simplified 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 the access point of fig. 1A employing 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 needed;

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

fig. 7 is a simplified signal diagram employing a hidden node technique as may be required for 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 probe signals in accordance with the present invention;

fig. 12 and 13 are flow diagrams of methods of operating an access point of 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 patching system 105 through a wired connection, such as a wired data network connection. Each access point 110a, 110b, and 110c has a respective area 115a, 115b, 115c capable of communicating with 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 may be used when referring to access points, remote stations, and areas, respectively.

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.

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

During a scan, the access point 110B searches for the RF signal transmitted by the remote station 120B using a directional antenna scan as depicted 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 ratios (C/I), energy-to-bit ratios (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 skilled in the art. Based on these measured signal quality metrics, the access point 110b determines a preferred antenna angle or direction for communication 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 through a Universal Serial Bus (USB) port 215. Other types of connections are also accepted between directional antenna array 200a and 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.

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

Fig. 2B is an isometric perspective view of an access point 110 having an internal directional antenna 220B. In this embodiment, directional antenna array 200b is located on a Personal Computer Memory Card International Association (PCMCIA) Card 220. PCMCIA card 220 is carried by ap 110 and 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 are combined resulting in different antenna lobe 300 patterns and angles.

Fig. 3B is a simplified diagram of an exemplary circuit that can be used to set the passive antenna elements 205 to either a reflective mode or a 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 patterns 305 and 310 are caused by an inductive element 320 or a capacitive element 325 coupled 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 through 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 by a control signal 335.

Coupled to the ground plane 330 through the inductor 320 is a passive antenna element 205a, which is effectively lengthened as shown by the longer representative dashed line 305. This can be viewed as providing a "backplane" for an RF signal coupled to the passive antenna element 205a by the mutual coupling of the passive antenna element 205a and the active antenna element 206. In the case of fig. 3A, both passive antenna elements 205a and 205e are connected to the ground plane 330 through 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 through respective capacitive 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.

Jumping to fig. 9, a top view is provided that allows the access point 110b to generate an omnidirectional antenna pattern 905 and a directional antenna pattern 910 by using the directional antenna arrays 200a or 200 b. Access point 110b communicates with multiple 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 direction is likely not to change throughout the connection with a given remote station 120.

The illustrated access point 110b may transmit downlink data frames to a selected remote station 120c using a directional antenna 200 a. For most broadcast and control frames, the access point may use the omni-directional 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 in the downlink (e.g., web page access, file transfers). One option is to employ switched spatial diversity when 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.

Uplink data frames sent by the selected remote station 120c to the access point 110b in the Contention Period (CP) are received using an omni-directional antenna pattern since any remote station may transmit the frame. For large frames, network configurations may require remote stations to employ a request-to-send/clear-to-send (RTS/CTS) mechanism to subscribe to the wireless medium. In this case, the access point 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 the 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.

The use of a directional antenna pattern at the access point 110 can provide higher data transmission rates for downlink and uplink data frames exchanged with remote stations 120, which 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, remote station 120 can associate with access point 110 and remain online without using 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 contains the address of the remote station 120 currently associated with the ap 110 and its current antenna direction selection. Table 1 may describe example antenna direction selection based on frame sequences in accordance with the 802.11 standard (within which tables 21 and 22 are included). 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 set of rules that determine when to select an omni-directional pattern and when to select a directional pattern. For example, the access point 110 may choose a directional pattern in 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 depicted in fig. 4. Icon access point 110 contains multiple 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 a "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 (0) mode.

Various techniques 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 are described below. The first technique employs a spatial diversity selection mechanism. A second technique utilizes probe 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 at 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 a605a 'and buffer B605B'. Buffer a605a '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 select switch control signal 607 ' to multiplexer 610 ' in a conventional diversity select control manner, but in this case uses the diversity select switch control signal that controls the contents of register A605a ' or register 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 diagram of a method for 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 performed at block 1030 is measuring the corresponding signal received from the remote station 120 over 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 for the remote station 120 and the reception by the remote station of a CTS message in response to the access point transmission. 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 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 the 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 engaged 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 flowchart illustrating the aforementioned method of operating an ap 110 in a WLAN 100 based on probe 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 probe signal to the selected remote station through an omni angle of directional antenna 220b at block 1120, and measuring a first probe response signal received from the selected remote station through the omni angle in response to the first probe signal at block 1130.

A respective second probe signal is transmitted to the selected remote station 120 at block 1140 through each of a plurality of directional angles of directional antenna 200b, and a second probe response signal received from the selected remote station at each directional angle in response to the respective second probe 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 exchanges between the ap 110 and the sta 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 lower data transmission rates, the access point 110 may use these messages to compare the omni pattern 905 with the now 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 a 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 with 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 from the omni-directional and selected-directional antenna patterns from the above-described process. If the dominance falls below a predetermined threshold, the ap 110 reverts to omni-directional selection and performs antenna searching using one of the first two techniques.

In case remote station 120 enters a power saving mode or an idle period with no data transfer for a long time, ap 110 reverts to omni-directional pattern selection. When remote station 120 becomes active again, access point 110 may perform another antenna search.

Referring now to fig. 12 and 13, flow diagrams 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 over a first antenna pattern of the directional antenna 220b in a forward link at block 1210, and transmitting a first data frame to the remote station at block 1220, and receiving a second control frame from the remote station over 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 corresponding 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 on the basis of control frames in the reverse link includes, starting at a starting point (block 1300), receiving a first control frame from the remote station through 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 through 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 within network 100 hear communications between access point 110 and a selected remote station 120, and therefore, un-heard remote stations may transmit while the media is in use. This can cause collisions, particularly at access point 110.

When the ap 110 has data to transmit to a remote station 120, the control program 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 access point 110 may look for the remote station 120 in a direction opposite the selected antenna direction.

Referring to the timing diagram of fig. 7, if the control 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 operation is to occur and that no transmissions are to be made until the switch is completed. 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 four-way frame exchange protocols (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 start 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 process described with reference to fig. 7, performance is enhanced because RTS messages are not transmitted by access point 110, since CTS messages are only needed to cause 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 of operating an access point 120 in a WLAN 100 based on hidden node identification is illustrated. Starting at a start point (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 communication 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 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 an omnidirectional 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, the 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 lans as those skilled in the art will appreciate, such as lans defined by the IEEE 802.16 standard.

Claims (16)

1. A method of operating an access point in a wireless local area network, the access point including a directional antenna for communicating with a plurality of remote stations, the directional antenna including an omnidirectional angle and a plurality of directional angles, the method comprising:

selecting a remote station from the plurality of remote stations;

transmitting a first probe signal to the selected remote station through the omni-directional angle of the directional antenna;

measuring a first probe response signal received from the selected remote station through the omni angle in response to the first probe signal;

transmitting a respective second probe signal to the selected remote station through each of a plurality of directional angles of the directional antenna;

measuring a second probe response signal received from the selected remote station through each orientation angle in response to the corresponding second probe signal; storing the measured first probe response signal and the corresponding measured second probe response signal from the selected remote station in an antenna database;

selecting a preferred orientation angle for the selected remote station based on the measured second probe response signal;

comparing the measured first probe response signal from the omni-directional angle with the measured second probe response signal from the preferred directional angle; and

the omni-directional angle or the preferred directional angle is selected based on the comparison for continued communication with the selected remote station.

2. The method of claim 1 wherein the preferred directional angle is selected when the measurement signal associated with the preferred directional angle exceeds the measurement signal associated with the omni-directional angle by a predetermined threshold.

3. The method of claim 1, further comprising:

selecting a next remote station from the plurality of remote stations;

repeating the steps of transmitting first and second probe signals and measuring first and second probe response signals received from the next selected remote station for the next selected remote station;

storing the measured first probe response signal and the corresponding measured second probe response signal from the next selected remote station in the antenna database; and

the selecting, transmitting and storing steps are repeated for each of the remaining remote stations.

4. The method of claim 1 wherein the first probe signal includes a request to send message and the first probe response signal includes a clear to send message; and wherein the second probe signal comprises a message and the second probe response signal comprises a message.

5. The method of claim 1 wherein the measuring 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.

6. The method of claim 1 wherein the selection of the omni angle and the scanning of the plurality of directional angles are performed at a MAC layer of the AP.

7. The method of claim 1 further comprising updating an antenna database for the selected remote station when there is idle time for communication between the access point and the selected remote station, the updating comprising repeating the steps of transmitting first and second probe signals to the selected remote station and measuring first and second probe response signals received from the selected remote station.

8. The method of claim 1 wherein the ap operates based on at least one of an IEEE 802.11 standard and an IEEE 802.16 standard.

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

10. An access point of a wireless local area network, comprising:

a directional antenna comprising an omnidirectional angle and a plurality of directional angles; and

a controller connected to the directional antenna for controlling the directional antenna

Selecting a remote station from a plurality of remote stations,

transmitting a first probe signal to the selected remote station through the omni-directional angle of the directional antenna

Measuring a first probe response signal received from the selected remote station through the omni angle, in response to the first probe signal,

transmitting a respective second probe signal to the selected remote station through each of a plurality of directional angles of the directional antenna,

measuring a second probe response signal received from the selected remote station through each orientation angle in response to the corresponding second probe signal,

storing the measured first probe response signal and the corresponding measured second probe response signal from the selected remote station in an antenna database;

selecting a preferred orientation angle for the selected remote station based on the measured second probe response signals;

comparing the measured first probe response signal from the omni-directional angle with the measured second probe response signal from the preferred directional angle; and

based on the comparison, the omni-directional angle or preferred directional angle is selected for continued communication with the selected remote station.

11. The ap of claim 10 wherein the directional antenna comprises at least one active element and a plurality of passive elements.

12. The access point of claim 10 wherein the controller includes a physical layer and a medium access control layer, and wherein the selection of the omni angle and the scanning of the plurality of directional angles are performed at the layer.

13. The ap of claim 10 wherein the preferred directional angle is selected when the measurement signal associated with the preferred directional angle exceeds the measurement signal associated with the omni-directional angle by a predetermined threshold.

14. The ap of claim 10 wherein the controller further performs the steps of:

selecting a next remote station from the plurality of remote stations;

repeating the steps of transmitting first and second probe signals and measuring first and second probe response signals received from the next selected remote station for the next selected remote station;

storing the measured first probe response signal and the corresponding measured second probe response signal received from the next selected remote station in the antenna database; and

the selecting, transmitting and storing steps are repeated for each of the remaining remote stations.

15. The access point of claim 10 wherein the first probe signal includes a request to send message and the first probe response signal includes a clear to send message; and wherein the second probe signal comprises a message and the second probe response signal comprises a message.

16. The access point of claim 10 wherein the controller updates the antenna database for the selected remote station when there is an idle period of time between the access point and the selected remote station without communication, the updating including repeating the steps of transmitting first and second probe signals to the selected remote station and measuring first and second probe response signals received from the selected remote station.