Abstract:
During a contention-free period, or silent measurement period, of a first wireless communications channel, signals received on a second wireless communications channel are analyzed. The results of the analysis are used to perform advanced radio resource management functions, such as frequency selection, load balancing, or power management. Each of a plurality of wireless channels may be measured in order to perform the advanced radio resource management functions. During the analysis, an energy detect threshold may be lowered to a minimal value.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of non-provisional U.S. patent application Ser. No. 11/009,821, filed Dec. 10, 2004 which claims priority from U.S. provisional application No. 60/535,020 filed on Jan. 8, 2004, both of which are incorporated by reference as if fully set forth. 
     
    
     FIELD OF INVENTION 
       [0002]    The present invention relates wireless communications. 
       BACKGROUND 
       [0003]    In a wireless communication system comprising a plurality of basic service sets (BSSs), an access point (AP) of a BSS is at times required to listen to data packets transmitted from neighboring BSSs. In addition, an AP may be required to measure the level of external interference in different channels for the purpose of supporting advanced RRM functions such as frequency selection, load balancing or power management. However, an AP must also support transmission and reception of packets to and from wireless transmit/receive units (WTRUs) in its own BSS. The frequency channel used by an AP to communicate with WTRUs in its own BSS may be different from any of the frequency channels used by neighboring BSSs, or from channels on which external interference is measured. 
         [0004]    Due to cost limitations, many APs include only one receiver. As a result, it is not possible to simultaneously measure external interference or listen to packets transmitted from other BSSs on one frequency channel and communicate with WTRUs in the AP&#39;s own BSS when it is operating on a different frequency channel. Even if an AP&#39;s own BSS operates on the same frequency channel on which measurements are to be made, these measurements may be difficult to perform if the traffic in the AP&#39;s BSS is heavy. 
         [0005]    Accordingly, it is desirable to have a mechanism by which an AP could measure external interference and packets transmitted from neighboring BSSs, including those operating on other frequency channels, with minimal disruption of the AP&#39;s communications with WTRUs in its own BSS. 
       SUMMARY 
       [0006]    During a contention-free period, or silent measurement period, of a first wireless communications channel, signals received on a second wireless communications channel are analyzed. The results of the analysis are used to perform advanced radio resource management functions, such as frequency selection, load balancing, or power management. Each of a plurality of wireless channels may be measured in order to perform the advanced radio resource management functions. During the analysis, an energy detect threshold may be lowered to a minimal value. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    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: 
           [0008]      FIG. 1  shows a wireless communication system including a plurality of WTRUs and an AP which uses SMPs to support advanced RRM functions in accordance with a preferred embodiment of the present invention; 
           [0009]      FIG. 2  is a flow diagram of a process implemented in the system of  FIG. 1  for changing channels and EDT values during and after the AP transmits an SMP after a communication medium is determined to be available; 
           [0010]      FIG. 3  is a flow diagram of a process implemented in the system of  FIG. 1  for managing radio resources using SMPs for a plurality of channels in an ACS; and 
           [0011]      FIG. 4  is a timing diagram of the process of  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0012]    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. 
         [0013]    When referred to hereafter, the terminology “AP” includes but is not limited to an access point, a Node-B, a site controller, a base station or any other type of interfacing device in a wireless environment. 
         [0014]    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. 
         [0015]      FIG. 1  is a wireless communication system which includes an AP  105  in selective communication with a plurality of WTRUs  110 . The AP  105  uses SMPs to support advanced RRM functions. The AP  105  includes a transceiver  115 , an RRM unit  120  and a memory  125 . It should be noted that each of the blocks shown in  FIG. 1  is representative of a particular function. Many of these functions may be combined into a single or multiple units. Accordingly, the present invention is not dependent upon the segmentation of functions as shown. Additionally, not all of the functions shown in  FIG. 1  are required to implement the present invention, as these functions are illustrated for clarity. 
         [0016]    The present invention uses the clear channel assessment (CCA) function, which is specified in the IEEE 802.11 standards. The role of the CCA function of an IEEE 802.11 device, (either an AP or a WTRU), is to indicate when the communication medium is “busy”. An IEEE 802.11 device refrains from transmitting when the communication medium is busy. In addition, an idle receiver will attempt to decode an incoming packet when the CCA function starts to indicate that the communication medium is busy. 
         [0017]    In the AP  105 , the transceiver  115  transmits communications to, and receives communications from, other entities of a wireless communications system, such as the WTRUs  110 . Characteristics of signals sent through a radio channel are measured, such as a received signal strength indicator (RSSI) of a received packet, a duration over which the CCA function indicates that the communication medium is busy, or the level of external interference. 
         [0018]    In the AP  105 , the RRM unit  120  receives information about the contents of decoded packets, such as the source and destination address, the type of packet, or the like. As will be explained in detail hereinafter, the RRM unit  120  provides overall timing control of the operations of the AP  105 , including those associated with the SMP. The RRM unit  120  supports all of the typical RRM functions of a wireless device, such as an IEEE 802.11 device, (either as an AP or a WTRU). For example, RRM functions include functions to determine the most appropriate frequency channel to operate on, and the optimal transmission power level and data rate for different packets. RRM functions may also include functions to prevent or manage congestion due to excessive traffic load. The RRM unit  120  is also coupled to the memory  125  which stores the parameters related to the operation of the AP  105 , including the ACS and the values used in the calculation of the timers (Table 1). 
         [0019]    In the AP  105 , the memory  125  includes the ACS, which is defined as the set of channels on which an AP in the Extended Service Set (ESS) is allowed to operate. These channels should be the same for all APs of an ESS. The channels in the ACS will preferably be non-overlapping, but this is not a necessary condition. In general, the ACS could be the set of all possible channels in the frequency band used by the AP. However, all APs preferably have the same ACS, and the ACS will be only a subset of the possible channels of the band. 
         [0020]    As explained above, the role of the CCA function is to indicate whether the communication medium is idle or busy. The communication medium is deemed busy if certain conditions, depending on the CCA mode chosen, are met. For example, in one commonly used CCA mode for IEEE 802.11 systems, the communication medium is deemed busy if the received power is above an energy detect threshold (EDT) and if a signal is detected to be generated by an IEEE 802.11 device. 
         [0021]    Table 1 summarizes parameters involved in the set up of SMPs as will be explained with reference to  FIG. 2 . As those of skill in the art would realize, other parameters may be used in addition to, or in place of, these parameters and values. 
         [0000]    
       
         
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Symbol 
                 Description 
               
               
                   
                   
               
             
             
               
                   
                 ACS 
                 Allowable Channel Set 
               
               
                   
                 T mpp     —     fix   
                 Fixed component of the Silent Measurement Period 
               
               
                   
                   
                 generation interval 
               
               
                   
                 T mpp     —     var   
                 Amplitude of the variable component of the Silent 
               
               
                   
                   
                 Measurement Period generation interval 
               
               
                   
                 T mp     —     dur   
                 Duration of a Silent Measurement Period 
               
               
                   
                   
               
             
          
         
       
     
         [0022]      FIG. 2  is a flow diagram of a process  200  including method steps for changing channels and EDT values during and after the AP  105  transmits an SMP after a communication medium is determined to be available. The process  200  is activated in response to an SMP being triggered, as will be explained hereinafter with reference to  FIG. 3 . 
         [0023]    Referring to  FIG. 2 , the AP  105  waits until it is determined that the communication medium becomes available (step  205 ). Once the communication medium is available, then the AP  105  waits for a short interval of time to expire (step  210 ), (e.g., a priority interframe space (PIFS)). The short interval may be pre-determined or may be variable or random. The reason for waiting for the short interval to expire in step  210  is to avoid collisions with another packet when transmitting the SMP notification packet. If the communication medium is determined to still be available after the short interval expires (step  215 ), the process  200  proceeds to step  220 . Otherwise, the process  200  returns to step  205  to wait until it is determined that the communication medium is available again. In step  220 , the AP  105  transmits an SMP notification packet (step  220 ). The role of this notification packet is to reduce the probability that a WTRU  110  in the BSS served by the AP  105  transmits a packet during the SMP. 
         [0024]    There are several alternatives for implementing the SMP notification packet. In an alternative for implementation in an IEEE 802.11 system, the SMP notification packet may simply consist of a request-to-send (RTS) or a clear-to-send (CTS) packet with a “dummy” destination address, such as the address of the AP  105  itself, reserving the communication medium for a duration T mp     —       ur   . WTRUs  110  receiving the RTS or CTS packet will refrain from transmitting for the duration specified in the packet. 
         [0025]    Alternatively, the AP  105  may initiate a contention-free period by transmitting a beacon packet indicating the beginning of a contention-free period of duration T mp     —       dur   . WTRUs  110  receiving this beacon packet will refrain from transmitting during the indicated duration of the contention-free period. 
         [0026]    The transmission power of the SMP notification packet implemented using either method is preferably set to a value sufficiently low to avoid WTRUs  110  in neighboring BSSs utilizing the same operating channel as the AP  105  from receiving the SMP notification packet. The reception by such WTRUs  110  would prevent the WTRUs from transmitting, which could impair the ability of the SMP to estimate the channel utilization in neighboring BSSs on the same channel. 
         [0027]    Referring still to  FIG. 2 , the AP  105  switches from a normal operating channel to a newly selected channel (step  225 ), (if necessary). The newly selected channel is a channel on which the data packets from other BSSs will be sent. In step  230 , the AP  105  changes the value of an EDT from a normal value to a minimum value. This ensures that the AP  105  can listen to packets received at power levels as low as possible. 
         [0028]    The AP  105  measures and analyzes received signals on the newly selected channel (step  235 ). Measurements may include, but are not limited to: the duration of received packets; the RSSI of received packets; the duration over which the CCA function indicates that the communication medium is available or busy; external interference levels; or the like. While measuring the characteristics of received packets, the AP  105  determines whether the duration of the SMP (T mp     —       dur   ) has expired (step  240 ). If the duration of the SMP (T mp     —       dur   ) has not expired, the AP  105  continues to measure and analyze received signals on the newly selected channel in step  235 . If the AP  105  determines in step  240  that the period of the SMP (T mp     —       dur   ) has expired, the AP  105  switches back to the normal operating channel (if different), and the value of the EDT is changed back to the normal value, (i.e., the value prior to the SMP) (step  245 ). 
         [0029]    Scheduling SMPs for all channels in the ACS will now be explained with reference to both  FIGS. 3 and 4 .  FIG. 3  is a flow diagram of a process  300  including method steps for efficiently managing radio resources using SMPs for a plurality of channels in the ACS according to the present invention.  FIG. 4  is a timing diagram associated with the process  300 . 
         [0030]    As described while referring to  FIG. 2 , it should be noted that the AP  105  operates on a particular (normal) operating channel, switches to another channel during the SMP (if necessary), and then switches back to the particular operating channel after the SMP (if different). For the purposes of the present example, the operating channel of the AP  105  will be illustrated as channel  0 , (CHAN  0 ), although just an arbitrary sending channel could be utilized. 
         [0031]    Referring to  FIGS. 3 and 4 , in step  305 , the AP  105  operates on a normal operating channel  0  of an ACS. In step  310 , the AP  105  selects one of channels  1 -N, (CHAN  1 -CHAN N), in the ACS and initiates an SMP during a duration (T mp     —       dur   ) when a communication medium is available, as shown in  FIG. 4  by T mp     —       dur     1  of SMP  1 . In step  315 , the AP  105  determines whether the duration of the SMP (T mp     —       dur   ) has expired and, if so, the AP  105  switches back to the normal operating channel  0 , (CHAN  0 ), in step  320 . If, in step  325 , the AP  105  determines that there is another channel in the ACS that requires SMP scheduling, the AP  105  sets an interval (T mpp ) between the end of the previous SMP (SMP 1 ) and the beginning another SMP (SMP 2 ) of the next channel in the ACS, (CHAN  2 ), as shown in  FIG. 4  by T mpp    1  and SMP  2 . 
         [0032]    The interval T mpp  set by the AP  105  may be a constant period, but is preferably a variable period as determined by Equation 1: 
         [0000]      T mpp =T mpp   —     fix   +C×T mpp   —     var     Equation (1) 
         [0000]    where T mpp     —       fix    and T mpp     —       var    are fixed values, and C is a variable between −1 and 1. T mpp     —       fix    is the approximate “periodicity” of the SMP occurrences. T mpp     —       var    is the amplitude of a random “jitter” that prevents the SMP occurrences from happening regularly. Preferably, T mpp     —       var    is much smaller than T mpp     —       fix   . This is not required since the SMP may be periodic. However, implementing it in a non-periodic manner reduces the risk that the measurements made during an SMP are in-sync with some traffic patterns in the measured BSSs, which could result in some estimation bias. Thus, the interval between SMPs can be different according to the value of T mpp . As shown in  FIG. 4 , T mpp    1  is smaller than T mpp    2 , and T mpp  N, (for the Nth SMP), is larger than T mpp   2 . The length of the interval T mpp  varies as the periods change in accordance with Equation 1. 
         [0033]    Because the AP  105  cannot serve its own WTRUs  110  during an SMP, the overall fraction of time used for the SMP, (that is Σ T mpp     dur    N/(Σ T mpp  N  30  ΣT m     —       dur    N)), should be very small. The overall fraction of time for the SMP is a tradeoff between limiting the loss of time for an AP&#39;s own transmissions and allocating sufficient time for measurements on other BSS&#39;s transmissions. 
         [0034]    In setting the fraction, support for voice loads are considered since voice communications are very sensitive to delay jitter. This imposes a maximum limit on the duration of a single SMP (T mp     —       dur   ). On the other hand, a single SMP must be of a long enough duration to allow the reception of full packets from neighboring BSSs with a reasonable probability of success. 
         [0035]    In step  335 , the AP  105  determines whether the interval (T mpp ) expired and, if it did, the process returns to step  310  where another channel is selected from the ACS, (CHAN  2 ). Steps  330 ,  335 ,  310 ,  315  and  320  are continually repeated, as shown in  FIG. 4  by T mp     —       dur      2  of SMP  2  . . . , T mp     —       dur   N of SMP N, and T mmp    2 , . . . , T mmp  N, until it is determined in step  325  that there are no further channels in the ACS that require SMP scheduling. 
         [0036]    By performing this process, all SMPs are set for all channels in the ACS. As shown in  FIG. 4 , SMPs may be set up for all channels in the ACS in a cyclical manner. Thus, after the AP  100  operates SMPN for the Nth channel in the ACS, the AP  100  starts another new cycle of SMP operation from the 1st channel in the ACS. It should be noted that the operating channel of the AP is normally part of the ACS, so the AP will schedule an SMP for its own operating channel from time to time. Additionally, the SMPs may be set up for all channels in the ACS in a random order. 
         [0037]    It should also be noted that the actual duration between SMPs may not be exactly T mpp  N because the communication medium might be busy at the expiration of the timer, forcing the AP  105  to wait a little longer before transmitting the SMP notification packet as described above. 
         [0038]    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. 
         [0039]    While this invention has been particularly shown and described with reference to a preferred embodiment, 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 as described hereinabove.