Abstract:
A method for implementing a smart antenna in establishing association between a station (STA) and an access point (AP) in a wireless local area network begins by transmitting a beacon frame by the AP on one antenna beam. The beacon frame is received at the STA, which measures the signal quality of the beacon frame. The AP switches to a different antenna beam and repeats the method until the beacon frame has been transmitted on all antenna beams. The STA associates to the AP that transmits the beacon frame with the highest signal quality on one of its antenna beams. A similar method may be used in which the STA sends a probe request frame to the AP, which then responds with probe response frames sent on multiple antenna beams.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application claims the benefit of U.S. Provisional Application No. 60/608,758, filed Sep. 10, 2004, which is incorporated by reference as if fully set forth herein. 

   FIELD OF INVENTION 
   The present invention generally relates to wireless local area networks, and more particularly, to a method and apparatus for implementing a smart antenna in establishing an association between an access point (AP) and a station (STA). 
   BACKGROUND 
   In a wireless local area network (WLAN) operating in infrastructure mode, a STA typically performs scanning in order to estimate which AP is the best candidate to serve it. Scanning performed by the STA can be passive or active. In passive scanning, the STA listens for beacon frames sent by the APs. In active scanning, the STA sends out probe requests and the APs respond by sending a probe response to the STA. 
   In order to enhance coverage and increase throughput, APs can be equipped with advanced antenna structures, which allow them to change the radiation pattern (the beam) they use. This is a function of the position of the STA to which the AP sends packets and/or of the time-varying channel. Because the beacon frames are not aimed at a particular STA, but rather at a multitude of STAs, they tend to be sent uniformly across all directions (i.e., with an omni-directional beam). Similarly, because it is desirable for an AP to hear the packets from all its associated STAs, the AP typically listens to the channel using an omni-directional beam. This type of beam does not necessarily allow the AP to determine which beam is the best to serve a STA, even after it has received a probe request from the STA. Therefore, a probe response also tends to be transmitted using an omni-directional beam. 
   A STA uses the beacons (passive scanning) and the probe requests (active scanning) to estimate, among other things, the quality of the radio links (e.g., signal to noise ratio (SNR)) that it could obtain from the different APs. Beacons and probe responses are typically sent by the APs using an omni-directional beam for the reasons described above. This creates a situation where a STA may estimate, based on the received signal quality it perceives from the beacons and/or probe responses, that a given AP is the best candidate, when in fact this AP will perform worse than another AP for the transmission of traffic frames which can be sent with directional beams. 
     FIG. 1  shows an exemplary WLAN  100  including a STA  102  and two APs, AP_A  110  and AP_B  120 . AP_A  110  can utilize an omni-directional beam  112  and a plurality of directional beams  114 ,  116 ,  118 . AP_B  120  can utilize an omni-directional beam  122  and a plurality of directional beams  124 ,  126 ,  128 . 
   The STA  102  measures the received power of the various beams as shown in Table 1. 
   
     
       
             
           
             
             
             
             
           
         
             
               TABLE 1 
             
           
           
             
                 
             
             
               Measured Received Signal Power at STA 102 
             
           
        
         
             
                 
                 
                 
               Received power at 
             
             
               Signal from 
               On beam number 
               Beam type 
               STA 102 
             
             
                 
             
             
               AP_A 
               112 
               omni 
               −75 dBm 
             
             
               AP_A 
               116 
               directional 
               −85 dBm 
             
             
               AP_A 
               118 
               directional 
               −85 dBm 
             
             
               AP_B 
               122 
               omni 
               −80 dBm 
             
             
               AP_B 
               124 
               directional 
               −70 dBm 
             
             
                 
             
           
        
       
     
   
   The STA  102  can receive stronger beacons and/or probe responses via omni-directional beam  112  from AP_A  110  than via omni-directional beam  122  from AP_B  120 . However, AP_B  120  would be a better candidate than AP_A  110  for transmitting traffic frames, since AP_B  120  is in a better position to make use of its advanced antenna structure and focus the energy towards the STA  102  (via directional beam  124 ). 
   SUMMARY 
   A method for implementing a smart antenna in establishing association between a STA and an AP in a WLAN begins by transmitting a beacon frame by the AP on one antenna beam. The beacon frame is received at the STA, which measures the signal quality of the beacon frame. The AP switches to a different antenna beam and repeats the method until the beacon frame has been transmitted on all antenna beams. The STA associates to the AP that transmits the beacon frame with the highest signal quality on one of its antenna beams. A similar method may be used in which the STA sends a probe request frame to the AP, which then responds with probe response frames sent on multiple antenna beams. 
   A system for implementing a smart antenna in establishing association between a STA and an AP in a WLAN includes a beacon frame sent from the AP to the STA. The beacon frame includes a field to identify a total number of antenna beams on which a transmission will be made and a field to identify the beam that is currently being transmitted on. 
   Another system for implementing a smart antenna in establishing association between a STA and an AP in a WLAN includes a probe request frame sent from the STA to the AP and a probe response frame sent from the AP to the STA. The probe request frame includes an indication whether the STA wants to scan multiple antenna beams from the AP. The probe response frame includes a field to identify a total number of antenna beams on which a transmission will be made and a field to identify the beam that is currently being transmitted on. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more detailed understanding of the invention may be had from the following description of a preferred embodiment, given by way of example, and to be understood in conjunction with the accompanying drawings, wherein: 
       FIG. 1  is a block diagram of a WLAN showing omni-directional and directional antenna beam patterns; 
       FIG. 2  is a diagram of a beacon frame format in accordance with the present invention; 
       FIG. 3  is a timing diagram for passive scanning in accordance with the present invention; 
       FIG. 4  is a flowchart of a method for transmitting a beacon frame in passive scanning in accordance with the present invention; 
       FIG. 5  is a diagram of a probe request frame format in accordance with the present invention; 
       FIG. 6  is a diagram of a probe response frame format in accordance with the present invention; 
       FIG. 7  is a timing diagram for active scanning in accordance with the present invention; and 
       FIG. 8  is a flowchart of a method for transmitting a probe response frame in active scanning in accordance with the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Hereafter, the term “station” (STA) includes, but is not limited to, a wireless transmit/receive unit, a user equipment, a fixed or mobile subscriber unit, a pager, or any other type of device capable of operating in a wireless environment. When referred to hereafter, the term “access point” (AP) includes, but is not limited to, a base station, a Node B, a site controller, or any other type of interfacing device in a wireless environment. 
   The present invention solves the above problem for both passive scanning and active scanning. 
   Passive Scanning 
   The present invention provides the signaling and support for an AP to send its beacon on multiple beams. This can be done by adding two fields to the WLAN beacon management frame, as shown in  FIG. 2 . The resulting management frame is referred to as an Advanced Antenna (AA) beacon frame  200 . Many of the fields of the frame  200  are in the existing beacon frame as defined by the 802.11 standards. These fields include frame control  202 , duration  204 , destination address (DA)  206 , source address (SA)  208 , basic service set (BSS) identification (BSSID)  210 , sequence control  212 , timestamp  214 , beacon interval  216 , capability information  218 , SSID information element (IE)  220 , supported rates IE  222 , frequency hop (FH)/distribution system (DS) parameter set IE  224 , contention free (CF) parameter set IE  226 , independent BSS (IBSS) parameter set IE  228 , and traffic indication map (TIM) IE  230 . 
   The first new field  232  of the frame  200  indicates to the STA that the beacon frames are sent N times within an AA beacon interval, where N corresponds to the number of beams on which the AP will send the beacon. The second new field  234  identifies the beam that was used to transmit the beacon; i.e., the beam identifier. When receiving any of the multiple AA beacon frames sent by an AP, the STA is able to identify the number of beams ( 232 ) included in the beam sweep performed by the AP and it is also able to distinguish the different versions it receives by looking at the beam identifier ( 234 ). 
   It is noted that the AA beacon interval  216  can be set to the same value or different value than the non-AA beacon interval. For the system to be able to offer service to users, the AA beacon interval has to be larger than the beam scan duration, so that there is time left to transmit traffic frames. This could be enforced, for example, as a configuration management rule that would prevent a user from configuring the AP such that the beam scan would take a large portion of the AA beacon interval time. 
   In a preferred embodiment, the N AA beacon frames  200  are sent successively in time. This is achieved by having the AP use a delay (X) shorter than the DIFS (Distributed Inter-Frame Space) but larger than the SIFS (Short Inter-Frame Space) when trying to access the wireless medium between the transmission of two beacon frames. Any value of X between SIFS and DIFS can be used. It should be noted that the AP would still need to wait for one entire DIFS when accessing the wireless medium to transmit the first of the N beacon frames. This places an upper bound on the maximum amount of time a STA would need to scan the medium once it detected one of the N beacons advertised by an AP; whereby:
 
Upper bound=( N− 1)×(beacon_duration+ X )  Equation 1
 
In other words, this prevents a STA from scanning a channel for an undetermined amount of time without ever knowing if all N beacons have been transmitted by the AP.
 
   The timing diagram for the passive scanning is shown in  FIG. 3 . Before the AP begins the AA beacon interval  300 , it waits for one DIFS  302 . At the start of the AA beacon interval  300 , the AP transmits the first beacon frame  304 . In between beacon frames  304 , the AP waits for the interval X ( 306 ), where X is shorter than the DIFS and larger than the SIFS. 
   A method  400  for transmitting an AA beacon frame in passive scanning is shown in  FIG. 4 . The method  400  begins with the AP transmitting an AA beacon frame  200  on one of N antenna beams, with the current beam identifier  234  set to the current beam (step  402 ). A determination is made if the AA beacon frame has been transmitted on all N beams (step  404 ). If the AA beacon frame has been transmitted on all N beams, then the method terminates (step  406 ). If the AA beacon frame has not been transmitted on all N beams (step  404 ), then the method waits for the interval X (step  408 ). The antenna system then switches to the next beam (step  408 ) and transmits the AA beacon frame  200  on the current beam, with the current beam identifier  234  set to the current beam (step  410 ) and the method continues with step  404 . It is noted that the step of switching to the next beam (step  408 ) can be performed either before or after waiting for the interval X. 
   The determination used by the STA whether to associate to the AP is implementation specific. One method is to use the power level or SNR perceived on the beacons to select the AP to associate to. The present invention allows a STA to use this method while fully capitalizing on the fact that the AP is equipped with an advanced antenna system. 
   Active Scanning 
   The present invention also solves the problem for active scanning by allowing a STA to request the AP to send probe responses on multiple beams. This can be done by adding a new field to the WLAN probe request frame. The resulting frame is shown in  FIG. 5  and is referred to as an AA probe request frame  500 . Many of the fields of the frame  500  are in the existing probe request frame as defined by the 802.11 standards. These fields include frame control  502 , duration  504 , DA  506 , SA  508 , BSSID  510 , sequence control  512 , SSID IE  514 , and supported rates IE  516 . The new field  518  of the frame  500  provides an indication to the AP (as a yes or no value) that the STA wants to scan all beams of the AP. 
   In addition, two new fields are added to the WLAN probe response frame. The resulting frame is shown in  FIG. 6  and is referred to as an AA probe response frame  600 . Many of the fields of the frame  600  are in the existing probe response frame as defined by the 802.11 standards. Fields  602 - 628  of the frame  600  are the same as fields  202 - 228  of the frame  200 . 
   The first new field  630  of the frame  600  indicates to the STA that the AA probe responses are sent N times within an AA beacon interval, where N corresponds to the number of beams on which the AP will send the probe response. The second new field  632  identifies the beam that was used to transmit the AA probe response; this is the beam identifier. An AP equipped with an AA system responds to the AA probe request by sending multiple (N) AA probe responses to the STA. 
   In a preferred embodiment, the N AA probe responses are sent successively in time. This is achieved by having the AP use a delay (X) shorter than the DIFS but larger than the SIFS when trying to access the wireless medium between the transmission of two AA probe responses. It should be noted that the AP would still need to wait for one entire DIFS when accessing the wireless medium to transmit the first of the N probe responses. This places an upper bound on the maximum amount of time a STA would need to wait once it received one of the N AA probe responses sent by an AP; whereby:
 
Upper bound=( N− 1)×(probe response duration+ X )  Equation 2
 
   The timing diagram for the active scanning is shown in  FIG. 7 . After the STA sends the AA probe request frame  700 , the AP waits for one DIFS  702  before sending the first AA probe response frame  704 . In between probe response frames  704 , the AP waits for the interval X ( 706 ), where X is shorter than the DIFS and larger than the SIFS. 
   A method  800  for transmitting an AA probe response frame in active scanning is shown in  FIG. 8 . The method  800  begins with the STA sending an AA probe request frame  500 , including setting the indicator  518  to scan all beams of the AP (step  802 ). The AP receives the AA probe request frame and waits for the DIFS period (step  804 ). The AP transmits an AA probe response frame  600  on one of N antenna beams, with the current beam identifier  632  set to the current beam (step  806 ). A determination is made if the AA probe response frame has been transmitted on all N beams (step  808 ). If the AA probe response frame has been transmitted on all N beams, then the method terminates (step  810 ). If the AA probe response frame has not been transmitted on all N beams (step  808 ), then the method waits for the interval X (step  812 ). The antenna system then switches to the next beam (step  812 ) and transmits the AA probe response frame  600  on the current beam, with the current beam identifier  632  set to the current beam (step  814 ) and the method continues with step  808 . It is noted that the step of switching to the next beam (step  812 ) can be performed either before or after waiting for the interval X. 
   In prior art systems, the decision made by a STA as to which AP to associate to could not take into account the radio link gain obtained from the advanced antenna structure at the AP. This meant that the data collected from scanning the RF environment, either passively or actively, where the beacon frames and probe responses are sent by APs in an omni-directional fashion, could lead a STA to associate to an AP that would offer worse performance than another AP, had the gains from the AA systems been taken into account. 
   With the present invention, the data collected by a STA when scanning the RF environment allows it to estimate which AP will be able to provide the best radio link, taking into consideration the gains that the AA systems will offer when traffic frames are transmitted. 
   Although the features and elements of the present invention are described in the preferred embodiments in particular combinations, each feature or element can be used alone (without the other features and elements of the preferred embodiments) or in various combinations with or without other features and elements of the present invention.