Abstract:
A method for use in an access point includes receiving a request for a reselection message from a wireless transmit/receive unit (WTRU). The reselection message is generated in response to the request and includes: at least one first identifier of one or more candidate basic service sets (BSSs) that may be considered for reselection; a preference indicator associated with each candidate BSS, to indicate the BSS selection preference; an expiration time indicator associated with each candidate BSS, to indicate a time period that the BSS may be considered for reselection; an indication of a measurement to be taken on a BSS and used in a target BSS reselection determination; and a margin by which the measurement must exceed a corresponding value for a currently selected BSS. The reselection message is transmitted to the WTRU. A reselection response message is received from the WTRU, indicating the target BSS reselection determination.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 11/319,811, filed Dec. 28, 2005, which claims the benefit of U.S. Provisional Patent Application No. 60/679,834, filed May 11, 2005, which are incorporated by reference as if fully set forth herein. 
     
    
     FIELD OF INVENTION 
       [0002]    The present invention is related to wireless communication systems. More particularly, the present invention is related to reselecting an access point (AP) in a wireless communication system including a plurality of APs and a wireless transmit/receive unit (WTRU). 
       BACKGROUND 
       [0003]    A typical wireless local area network (WLAN) comprises one or more basic service set (BSS). The BSS comprises an AP with at least one WTRU associated to the AP. Typically, in a system with multiple BSSs, (and thus multiple APs), there can be great differences between the quality of the radio connection a WTRU experiences based on which AP the WTRU associates with. Similarly, the radio resources available to a WTRU depend highly on which AP to which the WTRU is associated with. The system capacity can be optimized by managing which AP each WTRU associates and reassociates with during the system operation. 
         [0004]    The association of the WTRU and the AP is performed at the instant the WTRU joins the WLAN, (i.e., initial association). AP reselection can take place after the initial association. There are typically two reasons for the AP reselection, First, the WTRU may have moved such that the WNW can now be better served by another AP. Second, it may be desirable for the WTRU to be served by another AP because the radio resources used by the currently serving AP are strained, (i.e., the channel used by the serving AP is congested), while a neighboring AP has spare radio resources, (i.e., its channel is less congested). It should be understood that the Load Balancing is therefore the management of appropriate points of attachment for WTRUs while roaming inside the WLAN network. 
         [0005]    The AP reselection may be driven by either the WTRU or the network. In the WTRU-driven AP reselection, each WTRU is responsible for determining what criteria the WTRU would use for AP reselection. Because different WTRUs may use different policies, and these policies may be geared toward maximizing the quality perceived by only one WTRU rather than the quality perceived by the whole system, WTRU-driven AP reselection is not considered to perform optimally for load balancing. However, because it is the WTRU that first experiences the quality of the radio connection from the candidate APs, WTRU-driven AP reselection allows for minimal signaling and high reaction speed which is useful in cases that the WTRU is moving rapidly or that the conditions of the radio channel vary rapidly. 
         [0006]    In the network-driven AP reselection, the AP reselection process is controlled by the AP. This allows for consistent load balancing policies across all WTRUs in the network and theoretically offers the biggest performance gains from load balancing. However, the network-driven AP reselection may require a great amount of signaling overhead and may impair the ability of the WTRU to perform quick AP reselection when needed. 
       SUMMARY 
       [0007]    A list of candidate BSSs or APs is included in a medium access control (MAC) message received at a WTRU. The list includes an identity of each candidate BSS or AP and a preference indicator which indicates an extent that the candidate BSS or AP is preferred for selection and whether the candidate BSS or AP may not be considered for selection. A BSS or AP is selected by the WTRU using the list. 
         [0008]    A method for use in an access point includes receiving a request for a reselection message from a wireless transmit/receive unit (WTRU). The reselection message is generated in response to the request and includes: at least one first identifier of one or more candidate basic service sets (BSSs) that may be considered for reselection; a preference indicator associated with each candidate BSS, to indicate an extent to which the associated candidate BSS may be preferred for selection; an expiration time indicator associated with each candidate BSS, to indicate a period of time that the associated candidate BSS may be considered for reselection; an indication of a measurement to be taken on a candidate BSS and used as a part of a target BSS reselection determination; and a margin by which a value of the measurement of the candidate BSS must exceed a corresponding value for a currently selected BSS. The reselection message is transmitted to the WTRU. A reselection response message is received from the WTRU, indicating the target BSS reselection determination of the WTRU. 
         [0009]    A method for use in an AP includes receiving a request for a reselection message from a WTRU. The reselection message is generated in response to the request, and includes: at least one first identifier of one or more candidate APs that may be considered for reselection; a preference indicator associated with each candidate AP, wherein the preference indicator indicate an extent to which the associated candidate AP may be preferred for selection; an expiration time indicator associated with each candidate AP, wherein the expiration time indicator indicates a period of time that the associated candidate AP may be considered for reselection; an indication of a measurement to be taken on a candidate AP and used as a part of a target AP reselection determination; and a margin by which a value of the measurement of the candidate AP must exceed a corresponding value for a currently selected AP. The reselection message is transmitted to the WTRU. A reselection response message is received from the WTRU, indicating the target AP reselection determination of the WTRU. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    A more detailed understanding of the invention may be had from the following description, given by way of example and to be understood in conjunction with the accompanying drawings wherein: 
           [0011]      FIG. 1  shows a wireless communication system in accordance with the present invention; 
           [0012]      FIG. 2  is a block diagram of a WTRU used in the system of  FIG. 1 ; 
           [0013]      FIG. 3  is a block diagram of an AP used in the system of  FIG. 1 ; 
           [0014]      FIG. 4  is a flow diagram of a process for reselecting an AP in accordance with the present invention; 
           [0015]      FIG. 5  is a flow diagram of a handover process in accordance with the present invention; 
           [0016]      FIG. 6  is a simplified block diagram of a WTRU  600  utilizing MAC messaging; and 
           [0017]      FIG. 7  is a simplified block diagram of an AP  700  utilizing MAC messaging. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0018]    Hereafter, the terminology “WTRU” includes but is not limited to a user equipment (UE), a mobile station, 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 terminology “AP” includes but is not limited to a Node-B, a base station, a site controller or any other type of interfacing device in a wireless environment. 
         [0019]    The features of the present invention may be incorporated into an integrated circuit (IC) or be configured in a circuit comprising a multitude of interconnecting components. 
         [0020]      FIG. 1  shows a wireless communication system  100  in accordance with the present invention. The wireless communication system  100  includes a plurality of BSSs  102   a - 102   n . Each BSS  102   a - 102   n  comprises a respective AP  104   a - 104   n . An association of a WTRU  106  and an AP, such as AP  104   a , is performed when the WTRU  106  joins the BSS  102   a  covered by the AP  104   a . Thereafter, AP reselection may take place as the WTRU  106  moves away from the currently associated AP  104   a  or the radio condition in the currently associated AP  104   a  changes. The coverage of the different APs  104  overlap. 
         [0021]    In accordance with the present invention, an AP reselection triggering function is located in the WTRU  106  and an AP reselection policy is set by the AP  104   a - 104   n .  FIG. 2  is a block diagram of the WTRU  106  used in the wireless communication system  100  of  FIG. 1 . The WTRU  106  of  FIG. 2  includes a measurement unit  202  and an AP selection controller  204 . The measurement unit  202  performs measurements in accordance with the AP reselection policy, and the AP selection controller  204  determines whether the measurement results satisfies a condition defined by the AP reselection policy for reselecting a new AP and, if so, the WTRU performs reselection to the new AP. 
         [0022]      FIG. 3  is a block diagram of the APs  104   a - 104   n  used in the wireless communication system of  FIG. 1 . The AP  104  of  FIG. 3  includes an AP reselection policy controller  302  for generating and sending at least one AP reselection policy and/or a set of state transition triggers to the WTRU  106 , and an optional measurement unit  304  for controlling the state of the WTRU  106 , (i.e., an open state or a closed state). 
         [0023]    When the WTRU  106  considers reselecting a new AP, the decision is based on at least one AP reselection policy generated by the AP  104   a - 104   n . In the case where multiple reselection policies are sent from the AP  104  to the WTRU  106 , the AP  104  preferably signals to the WTRU  106  which policy, (or set of policies), should be used in reselecting a new AP. 
         [0024]      FIG. 5  is a flow diagram of a handover process that is implemented in a wireless communication system including a WTRU  505 , an old AP  510  and a new AP  515  in accordance with the present invention. 
         [0025]    In an optional step  520  of  FIG. 5 , the WTRU  505  requests the old AP  510  for an AP-reselection policy, triggers and configuration parameters. In step  525 , the old AP  525  sends an AP-selection policy, triggers and configuration parameters to the WTRU  505 . In step  530 , the WTRU  505  evaluates AP-reselection triggers. In step  535 , a trigger occurs. In optional step  540 , the WTRU  505  notifies the old AP  510  of the occurrence of a trigger condition and selected new AP  515 . In step  545 , the WTRU  505  initiates a re-association procedure from the old AP  510  to the new AP  515 . 
         [0026]    The AP reselection policy is defined by one or a combination of the following information: 
         [0027]    1) the measurement(s) (or quantity derived from the measurements) the WTRU has to use as part of the AP-selection procedure; 
         [0028]    2) a margin by which the value of a measurement, (or the quantity derived from the measurement) performed on a candidate AP must exceed the corresponding value for the currently selected AP; 
         [0029]    3) a duration for which the margin has to be exceeded before AP reselection takes place, (one duration per policy); 
         [0030]    4) a list of candidate APs or a list of frequency channels that the WTRUs has to consider in the AP reselection process; or reversely, a list of candidate APs or list of frequency channels that the WTRUs must not consider, or a list of APs plus a associated preference values as recommended by the AP, (i.e., a list of associated side information per AP or group of APs like load or the like). 
         [0031]    5) optionally, if more than one policy is specified, an indication whether the AP reselection takes place when any, or all, of the policies have its conditions satisfied; 
         [0032]    6) optionally, a validity time for the AP reselection policy, (i.e. expiry time period for the AP reselection policy); and 
         [0033]    7) optionally, an indication whether the AP reselection policy applies when a WTRU is in an associated state, in an authenticated but not associated state, or in an unauthenticated and unassociated state. 
         [0034]    The measurements to be taken by the WTRU  106  include at least one of, but are not limited to, a load metric indicating the amount of radio resources that are used, received signal quality, (such as a signal-to-noise ratio (SNR), a signal-to-interference and noise ratio (SINR), a received signal power level, or the like), an expected instantaneous data rate achievable for transmitting or receiving packets to/from the AP based on the received signal quality, an expected average throughput (EAT) achievable for transmitting or receiving packets to/from the AP based on channel utilization (CU) and the received signal quality. The CU is defined as the fraction of time the wireless medium is perceived as “busy” on the frequency channel. The EAT may be estimated by the following formula: EAT=data rate.times.(1-CU). 
         [0035]    The load metric may be a channel-based load metric or an AP-based load metric. The channel-based load metric includes the fraction of the time during which the channel is free on the frequency channel that the AP operates on. The channel-based load metric is equivalent to (1-CU). The AP-based load metric includes, but is not limited to, allocated time units and the number of queued packets. 
         [0036]    The margin, (i.e., hysteresis), is defined to avoid prematurely reselecting a new AP and subsequently reverting to the original AP leading to frequent switching back and forth between APs  104   a - 104   n.    
         [0037]    When a WTRU  106  is already associated to an AP, such as AP  104   a , the WTRU  106  may or may not be considering reselecting another AP  104   b - 104   n . In accordance with the present invention, two states are defined for the WTRU  106 : a closed state and an open state. In the closed state, the WTRU  106  is not applying any AP reselection policy and does not consider reselecting an AP. In the open state, the WTRU  106  applies the AP reselection policy and reselects an AP when the condition stated in the AP reselection policy is met. 
         [0038]    In accordance with one embodiment of the present invention, the WTRU  106  is always in the open state and, therefore, always applies the AP reselection policy. Alternatively, the WTRU  106  switches from the closed state to the open state, or vice versa, if a certain state transition triggering condition is met. The state transition triggering condition is met if a measurement result exceeds, (above or below depending on the nature of the measurement), a corresponding threshold for a pre-specified period of time. Alternatively, the AP  104   a  can switch the state of the WTRU  106 , (or group of WTRUs in its BSS), between the open state and the closed state by explicit signaling if a state transition triggering condition as observed by the AP  104   a  for the WTRU  106  is met. 
         [0039]    The state transition triggering condition may be monitored by the WTRU  106 , the AP  104   a - 104   n  or both. If the WTRU  106  monitors the state transition triggering condition, the WTRU  106  autonomously switches the state, once the state transition triggering condition is met. If the AP  104   a - 104   n  monitors the state transition triggering condition, the AP  104   a - 104   n  sends a signal to the WTRU  106  to change the state by means of layer 2 (L2) or layer 1 (L1) signaling once the state transition triggering condition is met. 
         [0040]    The state transition triggering condition for switching the state is met if one of the following events, but not limited to, occur. For the conditions “above” and “below” in the following events, the condition in the parenthesis applies to change the state from the open state to the closed state, and the condition not in the parenthesis applies to change the state from the closed state to the open state. Some of these triggers could be measured at either the AP or STA side. For example, the data rate of transmitted packets could be monitored by the AP  104  (downlink). Based on this, the AP  104  could determine to put the WTRU  106  in an open state. When the measurement can be performed from the AP side, then this applies for the case where the AP  104  sets the state of the WTRU, whereas when the measurement is performed from the WTRU side, then it applies for the case where the WTRU sets its state itself. 
         [0041]    The above-mentioned state transition triggering conditions may include: 
         [0042]    1) an error rate of transmitted or received packets above (below) a threshold for a predetermined period of time; 
         [0043]    2) a medium access delay, (observed either on the contention free access portion or observed for a particular IEEE 802.11e/wireless multimedia (WMM) access category), above (below) a threshold for a period of time; 
         [0044]    3) a data rate of transmitted packets below (above) a threshold for a period of time; 
         [0045]    4) a data rate of received packets below (above) a threshold for a period of time; 
         [0046]    5) a contention window size above (below) a threshold for a period of time; 
         [0047]    6) channel occupancy above (below) a threshold for a period of time; 
         [0048]    7) a rate of failed packet receptions above (below) a threshold for a period of time; 
         [0049]    8) a received signal quality, (such as a received signal strength indicator (RSSI), a received channel power indicator (RCPI), an SNR, an SINR or any other relevant measurement), of the current association below (above) a threshold for a period of time; 
         [0050]    9) timeout of a pre-configured or dynamically re-configured association evaluation timer value; 
         [0051]    10) reception of a signaling bit flag, information field or information element part of an existing or new medium access control (MAC) frame, preferably of a management type; and 
         [0052]    11) any combination of the above. 
         [0053]    The thresholds may be relative or absolute values and the time period may be defined by absolute values or relative values to the occurrence of certain events, such as beacon frame transmission. 
         [0054]    The AP  104   a - 104   n  sends unicast or multicast/broadcast-type of MAC signaling frames to the WTRU  106  to send the AP reselection related information, such as AP reselection policy(ies) and for the state transition triggering conditions. This information may be sent at regular intervals, when polled, (i.e., when the WTRU  106  explicitly requested this information from the AP  104   a - 104   n ), in an unsolicited manner or any combination thereof. 
         [0055]    The MAC signaling frame containing AP reselection relevant information can be sent to the WTRU  106  in either an associated state, when it is in an authenticated but unassociated state or when it is in an unauthenticated and unassociated state. Different AP reselection policies and state transition triggers may be selected depending on the current status of the WTRU  106 . Alternatively, the information may be sent to the WTRU  106  as part of an existing MAC frame with or without modification, (management, control or data frame). The AP reselection relevant information may be included as an information field or information element in either the header or the frame body of the MAC frame. For example, the state transition flag for switching between the closed state and the open state may be sent by the AP  104   a - 104   n  using one of the currently unused and reserved bits or combination values in any part of the existing IEEE 802.11 MAC preamble, header or frame body of the MAC frames. Although most of the examples provided above related to L2 frames, one of ordinary skill in the art may implement the present invention using any other type of frames to support higher layer protocol exchanges between a WTRU and any network entity, such as those used to simple manage network protocol (SNMP) via an AP, management information bases (MIBs), or the like. Rather that explicitly signaling frames, the same can be achieved implicitly by protocol design. For example, an LB frame specified as being part of a class 3 frame may only apply to associated and authenticated states. 
         [0056]      FIG. 4  is a flow diagram of a process  400  for reselecting an AP in accordance with the present invention. A first AP currently associated with a WTRU  106 , generates and sends at least one AP reselection policy and/or a set of state transition triggers to the WTRU  106  (step  402 ). In step  404 , the WTRU determines whether it is in a closed state or an open state. If the WTRU is determined to be in an open state at step  404 , the WTRU  106  then performs measurements in accordance with the AP reselection policy currently in force (step  406 ). The AP reselection policy defines required measurements and related parameters as stated above. The WTRU  106  then determines whether the measurement results satisfy a condition defined by the AP reselection policy (step  408 ). The condition is satisfied if the measurement results, (or a value derived from the measurement results), for a new AP exceeds, (above or below depending on the nature of the measurement), a corresponding threshold by the margin for the duration defined in the AP reselection policy. If the condition is satisfied, the WTRU  106  performs reselection to a second AP (step  410 ). If not, the currently selected AP is maintained and the process  400  returns to step  404 . 
         [0057]    In case that more than one candidate AP meets the condition defined by the policy, the WTRU  106  may select an AP whose measurement value exceeds that of the currently selected AP by the largest amount. 
         [0058]    The AP  104   a - 104   n  may define multiple policies and the WTRU  106  may optionally use a combination of policies to determine which AP to reselect to. In this case, the WTRU  106  may elect to reselect a new AP if any one of the policies in the combination has its conditions satisfied, or alternatively if all policies in the combination have their conditions satisfied. Preferably, the AP  104   a - 104   n  signals to the WTRU  106  which policy, (or set of policies), should be used in reselecting a new AP. 
         [0059]      FIG. 6  is a simplified block diagram of a WTRU  600  utilizing MAC messaging. A MAC  601  has a MAC message generator  602  which produces MAC messages and a MAC message receiver  603  which receives MAC messages. A transceiver  604  transmits the produced MAC messages and receives MAC messages utilizing an antenna or antenna array  605 . 
         [0060]      FIG. 7  is a simplified block diagram of an AP  700  utilizing MAC messaging. A MAC  701  has a MAC message generator  702  which produces MAC messages and a MAC message receiver  703  which receives MAC messages. A transceiver  704  transmits the produced MAC messages and receives MAC messages using an antenna or antenna array  705 . 
         [0061]    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.