Patent Publication Number: US-9848443-B2

Title: Method and apparatus of accessing channel in wireless communication system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is continuation of U.S. patent application Ser. No. 15/134,253, filed on Apr. 20, 2016, now U.S. Pat. No. 9,516,676, which is a continuation of U.S. patent application Ser. No. 14/878,758, filed on Oct. 8, 2015, now U.S. Pat. No. 9,351,316, which is a continuation of U.S. patent application Ser. No. 14/329,242, filed on Jul. 11, 2014, now U.S. Pat. No. 9,191,928, which is a continuation of U.S. patent application Ser. No. 13/002,301, filed on Dec. 30, 2010, now U.S. Pat. No. 8,811,312, which is the National Stage filing under 35 U.S.C. 371 of International Application No. PCT/KR2009/003572, filed on Jul. 1, 2009, which claims the benefit of earlier filing date and right of priority to Korean Application No. 10-2008-0126486, filed on Dec. 12, 2008, and also claims the benefit of U.S. Provisional Application Nos. 61/159,791, filed on Mar. 12, 2009, and 61/077,864, filed on Jul. 2, 2008, the contents of which are all hereby incorporated by reference herein in their entirety. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a wireless communication, and more particularly, to a method and apparatus of accessing a channel in a wireless communication system. 
     BACKGROUND ART 
     With the advancement of information communication technologies, various wireless communication technologies have recently been developed. A wireless local access network (WLAN) is a technology whereby super high-speed internet access is possible in a region providing a specific service by using a portable terminal such as a personal digital assistant (PDA), a laptop computer, a portable multimedia player (PMP), etc. 
     Ever since the institute of electrical and electronics engineers (IEEE) 802, i.e., a standardization organization for WLAN technologies, was established in February 1980, many standardization works have been conducted. Initially, WLAN used a frequency of 2.4 GHz to support a data rate of 1 to 2 Mbps by using frequency hopping, spread spectrum, infrared ray communication, etc. Recently, the WLAN can support a data rate of up to 54 Mbps by using orthogonal frequency division multiplexing (OFDM). In addition, the IEEE 802.11 is developing or commercializing standards of various technologies such as quality of service (QoS) improvement, access point (AP) protocol compatibility, security enhancement, radio resource measurement, wireless access in vehicular environments, fast roaming, mesh networks, inter-working with external networks, wireless network management, etc. 
     The IEEE 802.11b standard supports a data rate of up to 11 Mbps (bits per second) by using a frequency band of 2.4 GHz. The IEEE 802.11a standard uses a frequency band of 5 GHz instead of the frequency band of 2.4 GHz and thus significantly reduces influence of interference. The IEEE 802.11a standard has improved the data rate to up to 54 Mbps by using the OFDM technology. The IEEE 802.11n standard provides increased network speed and reliability, extended coverage. 
     Basic access mechanism of an IEEE 802.11 is a carrier sense multiple access with collision avoidance (CSMA/CA) combined with binary exponential backoff. The CSMA/CA mechanism is also referred to as a distributed coordinate function (DCF) and basically employs a “listen before talk” access mechanism. A station (STA) listens a wireless medium before starting transmission. As a result of listening, if it is sensed that the wireless medium is not in use, the listening STA starts its transmission. Otherwise, if it is sensed that the wireless medium is in use, the STA does not start its transmission but enters a delay duration determined by the binary exponential backoff algorithm. The CSMA/CA channel access mechanism is not so efficient since throughput at the MAC layer provides only 50 to 60% of throughput at the physical layer. 
     IEEE 802.11 VHT (Very High Throughput) is one of WLAN systems which have been recently proposed to support throughput of higher than 1 Gbps. Two kinds of VHT system are independently progressed: one is IEEE 802.11ac below 6 GHz band and another is IEEE 802.11ac for 60 GHz band. 
     The VHT system is expected to use bandwidths broader than at least 60 MHz. A AP simultaneously transmit data at different frequencies to multiple STAs to increase the overall throughput. 
     A technique for efficiently operating a wideband VHT system is required. 
     DISCLOSURE OF INVENTION 
     Technical Problem 
     The present invention provides a method and apparatus of accessing a channel to support wideband in a WLAN system. 
     Technical Solution 
     In an aspect, a method of accessing a channel in a wireless communication system is provided. The method includes receiving a first frame including configuration information on a channel allocated from a bandwidth including a primary channel, a secondary channel and an extension channel from an access point (AP), and transmitting a second frame to the AP by using the allocated channel, wherein the primary channel and the secondary channel have bandwidths used by legacy stations and the configuration information includes an extension channel offset element field that sets the extension channel as the offset of the primary channel. 
     The configuration information may be an operation element and the first frame may correspond to one of a beacon frame, a probe response frame and an association response frame. The first frame may be a channel switch announcement frame. 
     In another aspect, a method of accessing a channel in a wireless communication system is provided. The method includes selecting a first channel from a plurality of narrowband channels, starting a backoff timer if the selected first channel is idle, confirming whether an unselected second channel is idle if the backoff timer is expired, and transmitting a frame through the first and second channels if the second channel is idle. 
     In still another aspect, a station for wireless communication includes a radio frequency (RF) unit to transmitting a radio signal, and a processor coupled with the RF unit and configured to select a first channel from a plurality of narrowband channels, start a backoff timer if the selected first channel is idle, confirm whether an unselected second channel is idle if the backoff timer is expired, and transmit a frame through the first and second channels if the second channel is idle. 
     Advantageous Effects 
     A wideband channel is managed and used through a plurality of narrowband channels, and thus the efficiency of radio resources can be improved and throughput can be increased. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic view showing an exemplary structure of a wireless local access network (WLAN) system to implement an embodiment of the present invention. 
         FIG. 2  illustrates exemplary channel management that combines three subchannels to support a bandwidth of 60 MHz. 
         FIG. 3  illustrates exemplary channel management that combines four subchannels to support a bandwidth of 80 MHz. 
         FIG. 4  illustrates an exemplary format of an operation element for setting up multiple channels. 
         FIG. 5  illustrates an exemplary channel switch announcement frame according to an embodiment of the present invention; 
         FIG. 6  illustrates an exemplary extended channel switch announcement frame according to an embodiment of the present invention; 
         FIG. 7  illustrates exemplary channel allocation for hierarchical channel access; 
         FIG. 8  illustrates an exemplary operation element; 
         FIG. 9  is a flowchart showing a hierarchical channel access mechanism according to an embodiment of the present invention; 
         FIG. 10  illustrates an example of channel allocation for hierarchical channel access: 
         FIG. 11  illustrates another example of channel allocation for hierarchical channel access; and 
         FIG. 12  is a block diagram of a wireless communication system to implement an embodiment of the present invention. 
     
    
    
     MODE FOR THE INVENTION 
       FIG. 1  is a schematic view showing an exemplary structure of a wireless local access network (WLAN) system to implement an embodiment of the present invention. The WLAN system includes one or more basis service sets (BSSs). The BSS is a set of stations (STAs) which are successfully synchronized to communicate with one another. The BSS can be classified into an infrastructure BSS and an independent BSS (IBSS). The infrastructure BSSs (BSS 1  and BSS 2 ) shown in  FIG. 1  include STAs  10 ,  30  and  40 , access points (APs)  20  and  50 . The AP is a STA providing a distribution service. The APs  20  and  50  are connected by means of a distribution system (DS). The IBSS operates as Ad-hoc mode and does not include any AP. The IBSS constitutes a self-contained network since connection to the DS is not allowed. A plurality of infrastructure BSSs can be interconnected by the use of the DS. An extended service set (ESS) is a plurality of BSSs connected by the use of the DS. In the same ESS, a non-AP STA can move from on BSS to another BSS while performing seamless communication. 
     The STA is an arbitrary functional medium including a medium access control (MAC) and wireless-medium physical layer (PHY) interface conforming to the institute of electrical and electronics engineers (IEEE) 802.11 standard. The STA may be an AP or a non-AP STA. A non-AP STA may be a portable terminal operated by a user. The non-AP STA may be simply referred to as an STA. The non-AP STA may be referred to as a wireless transmit/receive unit (WTRU), a user equipment (UE), a mobile station (MS), a mobile terminal, a mobile subscriber unit, etc. The AP is a functional entity for providing connection to the DS through a wireless medium for an associated STA. Although communication between non-AP STAs in an infrastructure BSS including the AP is performed via the AP in principle, the non-AP STAs can perform direct communication when a direct link is set up. The AP may be referred to as a centralized controller, a base station (BS), a node-B, a base transceiver system (BTS), a site controller, etc. 
     A VHT (Very High Throughput) WLAN system aims to be reflected on one of IMT (International Mobile Telecommunication)-Advanced technologies corresponding to fourth-generation telecommunication standard. Accordingly, the VHT WLAN system is required to operate in IMT-Advanced candidate bands and the existing 2.4 GHz and 5 GHz bands. 
     The VHT WLAN system is required to use a bandwidth wider than at least 60 MHz in order to secure throughput higher than 1 Gbps. To segment a wideband into a plurality of narrowbands and use the narrowbands is efficient more than to use the wideband as a single channel in terms of backward compatibility and resource efficiency. 
     Hereinafter, a narrowband channel having a bandwidth of 20 MHz is referred to as a subchannel. A technique of binding three or four subchannels to support a bandwidth of 60 MHz or 80 MHz will now be described. The number of subchannels, the bandwidth of the subchannel and the overall bandwidth are exemplary purpose only. 
       FIG. 2  illustrates exemplary channel management that combines three subchannels to support a bandwidth of 60 MHz. The three subchannels include a primary channel and two extension channels. The primary channel is set in order to secure backward compatibility with STAs (hereinafter referred to as legacy STAs) supporting standards of lower than IEEE 802.11n using a bandwidth of 20 MHz. Legacy STAs supporting 20 MHz use the primary channel preferentially. STAs supporting 40 MHz or 60 MHz can use the primary channel, two extension channels and/or a combination thereof. 
     Subfigure (a) of  FIG. 2  shows that the extension channels using 20 MHz are located in a frequency band lower than the primary channel. Here, the two 20 MHz extension channels can be set as a single 40 MHz extension channel. Subfigure (b) of  FIG. 2  shows that the two extension channels are located in a frequency band higher than the primary channel. The two 20 MHz extension channels can be set as a single 40 MHz extension channel. Subfigure (c) of  FIG. 2  shows that the primary channel is located between the two extension channels. 
     In (a) and (b) of  FIG. 2 , the multiple channels can be administrated by using the primary channel and the single 40 MHz extension channel contiguous to the primary channel. In (c) of  FIG. 2 , the multiple channels can be administrated by the primary channel and two 20 MHz extension channels contiguous to the primary channel. 
     An AP can set a subchannel among overall available channels as a primary channel and use the primary channel as a common channel for control signal. The AP can set a bandwidth of an extension channel to one of 20 MHz, 40 MHz and 60 MHz according to channel availability if a STA supports the extension channel. 
       FIG. 3  illustrates exemplary channel management that combines four subchannels to support a bandwidth of 80 MHz. The four subchannels include a primary channel, a secondary channel and two extension channels. The primary channel and the secondary channel are set in order to secure backward compatibility with legacy STAs supporting standards of lower than IEEE 802.11n using 40 MHz. Legacy STAs supporting 20 MHz preferentially use the primary channel and use the secondary channel when the primary channel is being used. Legacy STAs supporting 40 MHz preferentially use the primary channel and the secondary channel. STAs supporting bandwidths higher than 60 MHz can use the primary channel, the secondary channel, the extension channels and/or a combination thereof. 
     Subfigure (a) of  FIG. 3  shows that the extension channels having 20 MHz are located in a frequency band lower than the primary channel and the secondary channel. The primary channel is located in the highest band and the secondary channel is located below the primary channel. Here, the two 20 MHz extension channels can be set as a single 40 MHz extension channel. Subfigure (b) of  FIG. 3  shows that the 20 MHz extension channels are located in a frequency band higher than the primary channel and the secondary channel. The primary channel is located in the lowest band and the secondary channel is located above the primary channel. Here, the two 20 MHz extension channels can be set as a single 40 MHz extension channel. Subfigure (c) of  FIG. 3  shows that the primary channel and the secondary channel are located between the two extension channels. The primary channel is located in a band higher than the secondary channel. Subfigure (d) of  FIG. 3  shows that the primary channel and the secondary channel are located between the two extension channels. The primary channel is located in a band lower than the secondary channel. 
     In (a) and (b) of  FIG. 3 , the multiple channels can be administrated by using the primary channel, the secondary channel and a extension channel having 40 MHz. In (c) and (d) of  FIG. 3 , the multiple channels can be administrated by using the primary channel, the secondary channel and two extension channels each having 20 MHz. 
     A method of setting and changing multiple channels will now be described. 
       FIG. 4  illustrates an exemplary format of an operation element for setting up multiple channels. An operation element  400  for setting multiple channels includes an element ID  410 , a primary channel field  420 , a secondary channel offset field  430 , an extension channel offset field  440  and a channel width field  450 . The element ID  410  is an identifier for identifying the operation element  400 . The primary channel field  420  indicates the position of a primary channel in the available bandwidth of a system and can be represented as a channel number. The channel width field  450  represents a channel bandwidth supported by a STA or a channel bandwidth used for transmission. The STA can support at least one of bandwidths 20 MHz, 40 MHz, 60 MHz and 80 MHz. 
     The secondary channel offset field  430  indicates the offset of the secondary channel relative to the primary channel. The secondary channel offset field  430  can be configured as represented by Table 1. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Value 
                 Name 
                 Description 
               
               
                   
               
             
            
               
                 0 
                 SCN (No Secondary 
                 No secondary channel is present. 
               
               
                   
                 Channel) 
               
               
                 1 
                 SCA (Secondary Channel 
                 The secondary channel is above the 
               
               
                   
                 Above) 
                 primary channel. 
               
               
                 2 
                 SCB (Secondary Channel 
                 The secondary channel is below the 
               
               
                   
                 Below) 
                 primary channel. 
               
               
                   
               
            
           
         
       
     
     The extension field offset field  440  indicates the position of an extension channel relative to the primary channel and/or the secondary channel. The value of the extension channel offset field  440  can be set based on the primary channel, as represented by Table 2. 
     
       
         
           
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                 Value 
                 Name 
                 Description 
               
               
                   
               
             
            
               
                 0 
                 ECN (No Extension 
                 No extension channel is present. 
               
               
                   
                 Channel) 
               
               
                 1 
                 ECA (Extension Channel 
                 The extension channel is above the 
               
               
                   
                 Above) 
                 primary channel. 
               
               
                 2 
                 ECB (Extension Channel 
                 The extension channel is below the 
               
               
                   
                 Below) 
                 primary channel. 
               
               
                 3 
                 ECC (Extension Channel 
                 The primary channel is in between the 
               
               
                   
                 Cross) 
                 extension channels. 
               
               
                   
               
            
           
         
       
     
     Though Table 2 shows that the extension channel offset field  440  indicates the position of the extension channel based on the primary channel, the extension channel offset field  440  may indicate the position of the extension channel based on the secondary channel or based on a combination of the primary channel and the secondary channel. 
     The terms and values represented in Tables 1 and 2 are exemplary and those who skilled in the art can easily change the terms and values. 
     All the aforementioned fields are not included in the operation element  400 . Some of the fields may be omitted or other fields may be added. For example, the operation element  400  may not include the secondary channel offset field  430  if the secondary channel is not used. 
     The operation element  400  may be included in at least one of a beacon frame, a probe response frame and an association response frame and transmitted from an AP to a STA. Above frames may be referred to section 7.2.3 of IEEE standard P802.11-REVma/D9.0 “Wireless LAN Medium Access Control (MAC) and physical layer (PHY) specifications” which is hereby incorporated by reference. 
     An AP that classifies 60 MHz, 80 MHz or wider bandwidth as a primary channel, a secondary channel and an extension channel and manages the channels is required to change a channel previously allocated thereto in consideration of channel status. For example, if considerable noise is generated or/and interference with other signals occurs in a subchannel used as a primary channel in the overall channel bandwidth, another subchannel is updated to the primary channel to efficiently manage a WLAN system. 
       FIG. 5  illustrates an exemplary channel switch announcement frame according to an embodiment of the present invention. A channel switch announcement frame  500  is used by an AP in a BSS or a STA in an IBSS to advertise when it is changing to a new channel. The channel switch announcement frame  500  includes a category field  510 , an action value field  520 , a channel switch announcement element field  530 , a secondary channel offset element field  540 , and an extension channel offset element field  550 . The category field  510  may represent spectrum management. The action value field  520  may represent a channel switch announcement frame. 
     The channel switch announcement element field  530  includes an element ID  531 , a length field  532 , a channel switch mode field  533 , a new channel number field  535  and a channel switch count field  536 . The channel switch mode field  533  indicates any restrictions on transmission until a channel switch. An AP in a BSS or a STA in an IBSS may set the channel switch mode field  533  to either 0 or 1 on transmission. The channel switch mode field  533  set to 1 means that the STA in a BSS to which the frame containing the element is addressed transmits to further frames within the BSS until the scheduled channel switch. The channel switch mode field  533  set to 0 does not impose any requirement on the receiving STA. The new channel number field  535  is set to the number of the channel to which the STA is moving. The channel switch count field  536  either is set to the number of target beacon transmission times (TBTTs) until the STA sending the channel switch announcement element switches to the new channel or it set to 0. A value of 1 indicates that the switch shall occur immediately before the next TBTT. A value of 0 indicates that the switch occurs at any time after the frame containing the element is transmitted. 
     The secondary channel offset element field  540  includes an element ID  541 , a length field  542  and a secondary channel offset field  543 . The secondary channel offset element field  540  represents information on a new secondary channel when the secondary channel is changed to the new secondary channel. The secondary channel offset field  543  may be set as represented by Table 1. 
     The extension channel offset element field  550  includes an element ID  551 , a length field  552  and an extension channel offset field  553  and represents information on a new extension channel when the extension channel is changed to the new extension channel. The extension channel offset field  553  may be set as represented by Table 2. 
       FIG. 6  illustrates an exemplary extended channel switch announcement frame according to an embodiment of the present invention. An extended channel switch announcement frame  600  is used by an AP in a BSS or a STA in an IBSS to advertise when it is changing to a new channel or a new channel in a new regulatory class. The extended channel switch announcement frame  600  includes a category field  610 , an action value field  620 , an extended channel switch announcement element field  630 , a secondary channel offset element field  640  and an extension channel offset element field  650 . 
     Compared with the channel switch announcement frame  500  in  FIG. 5 , the extended channel switch announcement frame  600  includes the extended channel switch announcement element field  630  which further includes a new regulatory class field  634 . The new regulatory class field  634  is set to the number of the regulatory class after the channel switch. A new channel number field  635  is set to the number of the channel after the channel switch. The channel number is a channel from the STA&#39;s new regulatory class. 
     The secondary channel offset element field  640  and the extension channel offset element field  650  can be set in the same manner in which the secondary channel offset element field  540  and the extension channel offset element field  550  of the channel switch announcement frame  500  shown in  FIG. 5  are set. 
     A hierarchical channel access mechanism in a system including multiple subchannels will now be described. Hierarchical channel access represents an attempt to perform channel access according to multiple classes. 
       FIG. 7  illustrates exemplary channel allocation for hierarchical channel access. The overall bandwidth of 80 MHz is divided into two bonding channels, that is, a primary bonding channel  710  and a secondary bonding channel  720 . The primary bonding channel  710  and the secondary bonding channel  720  respectively have a bandwidth of 40 MHz and include two 20 MHz subchannels. The primary bonding channel  710  includes a primary channel  711  and a secondary channel  712  and the secondary bonding channel  720  includes a primary channel  721  and a secondary channel  722 . The primary channels  711  and  721  and the secondary channels  712  and  722  are exemplary and their positions may be changed. 
     If a STA connected to a VHT system supports only a channel bandwidth of 20 MHz, channels are allocated such that the STA operates in a primary channel. The primary channel  711  of the primary bonding channel  710  or the primary channel  712  of the secondary bonding channel  720  may be allocated. Otherwise, the primary channel  711  of the primary bonding channel  710  may be given priority. 
     If the STA connected to the VHT system supports a channel bandwidth of 40 MHz, the channels are allocated such that STA operates in the primary bonding channel  710  or the secondary bonding channel  720 . Otherwise, the primary bonding channel  710  may be given priority. 
     An AP sets channels that will be used by STAs based on channel bandwidths supported by the STAs. Channel set-up may be processed through an association response frame and channel switch may be processed through a channel switch announcement frame. For example, when a STA supporting a channel bandwidth of 80 MHz is connected to the AP, the AP may use the primary channel field  420  of the operation element  400  included in the association response frame for allocation of the primary bonding channel  710  and use the secondary channel offset field  430  for allocation of the secondary bonding channel  720 . It is possible to add a new field to the operation element or change the existing fields in order to allocate the primary bonding channel  710  and the secondary bonding channel  720 . Furthermore, the AP may send the channel switch announcement frame  500  or the extended channel switch announcement frame  600  to switch the primary bonding channel  710  and the secondary bonding channel  720  to each other. Accordingly, the AP can achieve load balancing for STAs on the channel bandwidth of 80 MHz. 
       FIG. 8  illustrates an exemplary operation element. An operation element  800  includes a primary channel field  810  of a primary bonding channel, a secondary channel field  830  of the primary bonding channel, a primary channel field  840  of a secondary bonding channel and a secondary channel field  850  of the secondary bonding channel. An AP may inform STAs which subchannel of which bonding channel is allocated through the operation element  800 . 
     A STA supporting a channel bandwidth of 80 MHz (more specifically, a non-AP STA supporting VHT) may use four 20 MHz channels, two 40 MHz channels or a single 80 MHz channel. That is, both the primary bonding channel  710  and the secondary bonding channel  720  are allocated to the STA, and the STA may use the primary bonding channel  710  and the secondary bonding channel  720  as a 80 MHz channel or use one of the primary bonding channel  710  and the secondary bonding channel  702  as a 40 MHz channel. The STA may use channels based on CCA (Clear Channel Assessment) information. 
       FIG. 9  is a flowchart showing a hierarchical channel access mechanism according to an embodiment of the present invention. The hierarchical channel access mechanism may be performed by a STA. A STA supporting a channel bandwidth of 80 MHz selects a primary bonding channel or a secondary bonding channel (S 910 ). The STA confirms whether the selected bonding channel is idle (S 920 ). When the selected bonding channel is idle, the STA performs backoff (S 930 ). After a backoff timer is started, the STA confirms whether the unselected bonding channel is idle when the backoff timer is expired (S 940 ). When the unselected bonding channel is idle, the STA transmits frames by using the two bonding channels, that is, a channel having 80 MHz (S 940 ). When the unselected bonding channel is not idle, the STA transmits frame by using the selected bonding channel (S 950 ). 
     If the STA supporting the channel bandwidth of 80 MHz wants to use a 40 MHz channel, the STA may confirm whether the channel is used as follows. The STA randomly selects one of the primary bonding channel and the secondary bonding channel. When the selected bonding channel is idle, the STA performs backoff. After the backoff timer is started, the STA confirms whether the secondary channel is idle when the backoff timer is expired. When the secondary channel is idle, the STA transmits frames by using the selected bonding channel. When the secondary channel is not idle, the STA transmits frames by using the primary channel. 
       FIG. 10  illustrates an example of channel allocation for hierarchical channel access. A bandwidth of 60 MHz is segmented into two bonding channels, that is, a primary bonding channel  1010  and a secondary bonding channel  1020 . The primary bonding channel  1010  and the secondary bonding channel  1020  respectively have a bandwidth of 40 MHz and include two 20 MHz subchannels. The primary bonding channel  1010  includes a primary channel  1011  and a secondary channel  1012  and the secondary bonding channel  1020  includes a primary channel  1021  and a secondary channel  1022 . The secondary channel  1012  of the primary bonding channel  1010  and the secondary channel  1022  of the secondary bonding channel  1020  overlap each other. 
     If a STA connected to a VHT system supports only a channel bandwidth of 20 MHz, a channel is allocated to the STA such that the STA operates in a primary channel. The primary channel  1011  of the primary bonding channel  1010  or the primary channel  1012  of the secondary bonding channel  1020  may be allocated to the STA. Otherwise, the primary channel  1011  of the primary bonding channel  1010  may be given priority. 
     If the STA connected to the VHT system supports a channel bandwidth of 40 MHz, a channel is allocated to the STA such that the STA operates in the primary bonding channel  1010  or the secondary bonding channel  1020 . Otherwise, the primary bonding channel may be given priority. The primary bonding channel  1010  and the secondary bonding channel  1020  cannot be simultaneously used because the primary bonding channel  1010  and the secondary bonding channel  1020  overlap each other. 
     An AP sets channels that will be used by STAs based on channel bandwidths supported by the STAs. Channel set-up may be processed through an association response frame and channel switch may be processed through a channel switch announcement frame. 
     A STA supporting a channel bandwidth of 60 MHz may select and use one of a 20 MHz subchannel, a 40 MHz channel and a 60 MHz channel based on CCA information. The STA supporting the channel bandwidth of 60 MHz selects a primary bonding channel or a secondary bonding channel. The STA confirms whether the selected bonding channel is idle. When the selected bonding channel is idle, the STA performs backoff. After a backoff timer is started, the STA confirms whether the unselected bonding channel is idle when the backoff timer is ended. When the unselected bonding channel is also idle, the STA transmits frames by using the two bonding channels, that is, a channel having a bandwidth of 60 MHz. When the unselected bonding channel is not idle, the STA transmits frames by using the selected 40 MHz bonding channel. 
     If the STA supporting the channel bandwidth of 60 MHz wants to use a 40 MHz channel, the STA confirms whether the channel is used as follows. The STA selects one of the primary bonding channel and the secondary bonding channel. When the primary channel of the selected bonding channel is idle, the STA executes backoff. After the backoff timer is started, the STA confirms the secondary channel of the selected bonding channel is idle when the backoff timer is ended. If the secondary channel is idle, the STA transmits frames by using the selected bonding channel. If the secondary channel is not idle, the STA transmits frame by using the primary channel. 
     A first channel is randomly selected from a plurality of narrowband channels. Alternatively, the first channel may be selected based on configuration information from an AP. When the first channel is idle, it is confirmed whether a second channel is idle after the lapse of backoff time. When the second channel is idle, frames are transmitted using the first and second channels. The second channel may be contiguous to the first channel, and thus a wideband channel can be supported by narrowband channels. 
       FIG. 11  illustrates another example of channel allocation for hierarchical channel access. A bandwidth of 60 MHz is segmented into two bonding channels, that is, a primary bonding channel  1110  and a secondary bonding channel  1120 . The primary bonding channel  1110  and the secondary bonding channel  1120  respectively have a bandwidth of 40 MHz and include two 20 MHz subchannels. The primary bonding channel  1110  includes a primary channel  1111  and a secondary channel  1112  and the secondary bonding channel  1120  includes a primary channel  1121  and a secondary channel  1122 . Compared with the embodiment shown in  FIG. 10 , the primary channel  1111  of the primary bonding channel  1110  and the secondary channel  1122  of the secondary bonding channel  1120  overlap each other. The aforementioned channel access mechanism may be used without being changed. 
       FIG. 12  is a block diagram of a wireless communication system to implement an embodiment of the present invention is implemented. An AP  150  includes a processor  151 , a memory  152  and an RF (Radio Frequency) unit  153 . The processor  151  implements a proposed function, process and/or method. Channel allocation and channel switch can be performed by the processor  151 . The memory  152  is operatively connected to the processor  151  and stores information for operating the processor  151 . The RF unit  153  is operatively connected to the processor  151  and transmits and/or receives RF signals. A STA  160  includes a processor  161 , a memory  162  and an RF unit  163 . The processor  161  implements a proposed function, process and/or method. The aforementioned channel access method can be implemented by the processor  161 . The memory  162  is operatively connected to the processor  161  and stores information for operating the processor  161 . The RF unit  163  is operatively connected to the processor  161  and transmits and/or receives RF signals. 
     The processors  151 ,  161  may include application-specific integrated circuit (ASIC), other chipset, logic circuit and/or data processing device. The memories  152 ,  162  may include read-only memory (ROM), random access memory (RAM), flash memory, memory card, storage medium and/or other storage device. The RF units  153 ,  163  may include baseband circuitry to process radio frequency signals. When the embodiments are implemented in software, the techniques described herein can be implemented with modules (e.g., procedures, functions, and so on) that perform the functions described herein. The modules can be stored in memories  152 ,  162  and executed by processors  151 ,  161 . The memories  152 ,  162  can be implemented within the processors  151 ,  161  or external to the processors  151 ,  161  in which case those can be communicatively coupled to the processors  151 ,  161  via various means as is known in the art. 
     In view of the exemplary systems described herein, methodologies that may be implemented in accordance with the disclosed subject matter have been described with reference to several flow diagrams. While for purposed of simplicity, the methodologies are shown and described as a series of steps or blocks, it is to be understood and appreciated that the claimed subject matter is not limited by the order of the steps or blocks, as some steps may occur in different orders or concurrently with other steps from what is depicted and described herein. Moreover, one skilled in the art would understand that the steps illustrated in the flow diagram are not exclusive and other steps may be included or one or more of the steps in the example flow diagram may be deleted without affecting the scope and spirit of the present disclosure. 
     What has been described above includes examples of the various aspects. It is, of course, not possible to describe every conceivable combination of components or methodologies for purposes of describing the various aspects, but one of ordinary skill in the art may recognize that many further combinations and permutations are possible. Accordingly, the subject specification is intended to embrace all such alternations, modifications and variations that fall within the spirit and scope of the appended claims.