Abstract:
A method and apparatus controls power consumption of stations having a hierarchical structure when the stations transmit and receive a wireless signal to and from one another on a CSMA/CA wireless LAN. The controlling involves extracting information on frame transmission speed and transmission period information on first and second layers of the hierarchical structure from the wireless signal; determining a power-controlled period for each of the first and second layers based on the extracted information; and reducing the power consumption of the first and second layers by switching a current mode of the first and second layers to a predetermined mode for the power-controlled period if a reception address included in the extracted information is not identical to an address of the station.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a method and apparatus for controlling power consumption of stations on a carrier sense multiple access/collision avoidance (CSMA/CA)-based wireless local area network (LAN). More particularly, the present invention relates to a method and apparatus for controlling the power consumption of stations in a CSMA/CA-based wireless LAN system when the stations are in a standby mode for receiving a wireless signal from each other.  
         [0003]     2. Description of the Related Art  
         [0004]     Wireless local area network (LAN) systems based on the IEEE 802.11 standard employ carrier sense multiple access/collision avoidance (CSMA/CA) to share a wireless medium with one another.  FIG. 1  illustrates a schematic diagram of a wireless LAN system based on the IEEE 802.11 standard. Referring to  FIG. 1 , the wireless LAN system includes a basic service set (BSS)  10 , the Internet  11 , and a content server  12 .  
         [0005]     The BSS  10  includes a wireless terminal  101  and an access point (AP)  102 . The wireless terminal  101 , which is a type of subscriber terminal requesting wireless LAN services, is called a station. The AP  102  is connected to a wired network, such as the Internet  11 , or is wired, via a distribution system (DS), to another AP (not shown) that constitutes another BSS, allowing the AP  102  to bridge the Internet  11  and another network.  
         [0006]     The station, i.e., the wireless terminal  101 , has a hierarchical structure shown in  FIG. 2 . Referring to  FIG. 2 , the station includes a radio frequency (RF) layer  20 , a baseband layer  21 , and a media access control (MAC) layer  22 . Here, the RF layer  20  and the baseband layer  21  are called physical layers.  
         [0007]     The MAC layer  22  receives carrier sense information from the physical layers, i.e., the RF layer  20  and the baseband layer  21 , and determines, based on the received carrier sense information, whether a wireless medium is currently being used before transmitting a frame to a receiving station. If the wireless medium is in an idle state, a sending station transmits the frame to the receiving station. All stations within a propagation range of the sending station can determine whether the wireless medium is being used when the frame is transmitted thereto by referring to a duration field value of the frame. Writing the time of use of the wireless medium into a frame and transmitting the frame is called virtual carrier sensing.  
         [0008]     This process is illustrated in  FIG. 3A . As can be seen in  FIG. 3A , a receiving station A receives a data frame from a sending station B and transmits an acknowledgement (ACK) frame to the sending station B.  
         [0009]     Since wireless LAN systems are mainly employed in portable or mobile devices, battery power is an important issue to be considered. Current wireless LAN systems consume about 80% or more of their energy while performing operations on the physical layers.  
         [0010]     In order to minimize power consumption, stations have adopted a power management program allowing them to operate with reduced power consumption. The power management program is used in an infrastructure network using the AP  102 . Power management of the stations can be carried out every predetermined number of beacon cycles using the power management program. Each beacon cycle lasts for an average of 100 msec. During the exchange of power management frames in a network, the AP  102  needs to buffer the power management frames, which makes it impossible to control power of the station at less than every 100 msec. Therefore, if a station consumes too much power during a real-time bi-directional service, e.g., a voice/video call, it cannot properly provide the real-time bi-directional service.  
         [0011]      FIG. 3B  is a timing diagram illustrating a virtual carrier sense operation performed by stations in a wireless LAN environment. Referring to  FIG. 3B , first through third stations STA- 1 , STA- 2 , and STA- 3  and other stations periodically perform virtual carrier sense operations to share a wireless medium. For example, when the first station STA- 1  transmits ( 30 ) a data frame to the third station STA- 3 , the second station STA- 2  and the other stations, which are not the target stations for the data frame, perform a virtual carrier sense operation ( 31 ). This virtual carrier sense operation ( 31 ) results in the second station STA- 2  and the other stations consuming as much power as the third station STA- 3 . In response to the receipt of the data frame, the third station STA- 3  transmits ( 32 ) an ACK frame, which indicates that no error has occurred in the received data frame, to the first station STA- 1 . The second station STA- 2  and the other stations also perform a virtual carrier sense operation ( 33 ). Therefore, each of the second station STA- 2  and the other stations unnecessarily consumes as much power as the first station STA- 1  does.  
         [0012]     Stations constituting a wireless LAN system consume power when transmitting and receiving frames to and from one another or carrying out a virtual carrier sense operation. Each of the stations consumes 30-50% more power when transmitting a frame than when receiving a frame. In addition, each of the stations consumes power only for transmitting a frame after performing a carrier sense operation. When performing the carrier sense operation, each of the stations consumes as much power as they consume for receiving a frame. When a plurality of stations shares a wireless medium, however, each station consumes more power when performing a virtual carrier sense operation than when transmitting a frame. For example, if twenty or more stations share a single wireless medium for a predetermined amount of time, they consume at least ten times more power when performing a virtual carrier sense operation than when transmitting a frame. Therefore, in a wireless network structure, such as a hot spot, in which a plurality of stations is served by one AP, the plurality of stations consume most of their power performing a virtual carrier sense operation. Consequently, it is necessary to reduce power consumption of the plurality of stations consumed when the plurality of stations perform a virtual carrier sense operation.  
       SUMMARY OF THE INVENTION  
       [0013]     The present invention is therefore directed to a method and an apparatus, which substantially overcomes one or more of the problems due to the limitations and disadvantages of the related art.  
         [0014]     It is a feature of an embodiment of the present invention to provide a method and an apparatus that reduce power consumed by stations when carrying out a virtual carrier sense operation.  
         [0015]     It is another feature of an embodiment of the present invention to provide a method and an apparatus that power down all elements of each of the stations, other than those elements needed for stabilizing each of the stations, after power is applied to each of the stations.  
         [0016]     It is yet another feature of an embodiment of the present invention to provide a method and an apparatus controlling the power consumption of a station when it receives a wireless signal using a hierarchical structure.  
         [0017]     At least one of the above and other features and advantages of the present invention may be realized by providing a method of controlling power consumption of a station when it receives a wireless signal using a hierarchical structure having first and second layers. The method includes extracting information on frame transmission speed and transmission period information on the first and second layers from the wireless signal, determining a power-controlled period for each of the first and second layers based on the extracted information, and reducing power consumption of the first and second layers by switching an original mode of the first and second layers to a predetermined mode for the power-controlled period if a reception address included in the extracted information is not identical to an address of the station.  
         [0018]     In the predetermined mode, selected elements of the first and second layers, which require less time to be stabilized after being powered than other elements of the first and second layers, may be powered down. The power-controlled period for each of the first and second layers may be counted down, the first layer may be returned to its original mode when the power-controlled period for the first layer is over; and the second layer may be returned to its original mode when the power-controlled period for the second layer is over. After the power-controlled period is over, the first layer may be returned to its original mode before the second layer is returned to its original mode. If the reception address included in the excluded information is identical to the address of the station, the station may receive the wireless signal via the first and second layers.  
         [0019]     At least one of the above and other features of the present invention may be realized by providing an apparatus for controlling the power consumption of a station when it receives a wireless signal using a hierarchical structure. The apparatus includes a first layer of the hierarchical structure for converting the wireless signal into a baseband signal, a second layer of the hierarchical structure for restoring an original signal from the baseband signal, and a received power controller, which reduces the power consumed by the first and second layers by switching an original mode of the first and second layers to a predetermined mode when a reception address included in the restored original signal is not identical to an address of the station.  
         [0020]     The first layer may include a signal processor, which converts the wireless signal into the baseband signal, a reference frequency provider, which provides the signal processor with a reference frequency, and a first register, which stores data necessary for the operation of the signal processor. The received power controller may supply power only to the reference frequency provider and the first register in the predetermined mode.  
         [0021]     The second layer may include an analog-to-digital converter, which converts the baseband signal into a digital signal, a baseband signal processor, which restores the original signal from the digital signal, and a second register, which stores data necessary for the operation of the analog-to-digital converter or the baseband signal processor. The received power controller may supply power only to the second register in the predetermined mode.  
         [0022]     The received power controller may determine a power-controlled period for each of the first and second layers based on the restored original signal and may switch the original mode of the first and second layers to the predetermined mode during the power-controlled period. The first layer may be restored to its original mode before the second layer is restored to its original mode 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]     The above and other features and advantages of the present invention will become more apparent to those of ordinary skill in the art by describing in detail exemplary embodiments thereof with reference to the attached drawings in which:  
         [0024]      FIG. 1  illustrates a typical wireless LAN system;  
         [0025]      FIG. 2  illustrates a diagram of a hierarchical structure of a station of  FIG. 1 ;  
         [0026]      FIG. 3A  illustrates transmission of data and an acknowledgement (ACK) frame between two stations;  
         [0027]      FIG. 3B  is a timing diagram illustrating a carrier sense operation performed by stations in a wireless LAN environment;  
         [0028]      FIG. 4  illustrates a block diagram of a hierarchical structure of a station according to an embodiment of the present invention;  
         [0029]      FIG. 5  illustrates a flow of a method of controlling power of stations according to an embodiment of the present invention;  
         [0030]      FIG. 6  illustrates a format of a physical layer convergence procedure (PLCP) protocol data unit;  
         [0031]      FIGS. 7A through 7C  illustrate diagrams of formats of media access control (MAC) frames; and  
         [0032]      FIG. 8  illustrates power consumption of stations when performing a carrier sense operation using a method of controlling power consumption of stations according to an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0033]     Korean Patent Application No. 2003-52132, filed on Jul. 28, 2003, in the Korean Intellectual Property Office, and entitled “Method and Apparatus for Controlling Power Consumption of Stations on CSMA/CA-Based Wireless LAN,” is incorporated by reference herein in its entirety.  
         [0034]     The present invention will now be described more fully with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. The invention may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like reference numerals refer to like elements throughout.  
         [0035]      FIG. 4  illustrates a block diagram illustrating a hierarchical structure of a station according to an embodiment of the present invention. Referring to  FIG. 4 , the hierarchical structure of the station includes a radio frequency (RF) layer  41 , a baseband layer  42 , and a media access control (MAC) layer  43 . The RF layer  41  includes an RF/IF signal processor  411 , a voltage controlled oscillator-phase locked loop (VCO-PLL)  412 , and a first register  413 . The RF/IF signal processor  411  receives an RF signal via an antenna  40 , converts the received RF signal into an intermediate frequency (IF) signal, and converts the IF signal into a baseband signal. The VCO-PLL  412  provides a reference frequency to the RF/IF signal processor  411 . The first register  413  stores data necessary for the operation of the RF/IF signal processor  411  and provides the data to the RF/IF signal processor  411 .  
         [0036]     The baseband layer  42  includes an analog-to-digital converter (ADC)  421 , a baseband signal processor  422 , and a second register  423 . The ADC  421  converts the baseband signal output from the RF/IF signal processor  411  into a digital signal. The baseband signal processor  422  demodulates the digital signal output from the ADC  421  using a demodulation method, e.g., a frequency-shift keying (FSK) method, thereby recovering the original signal. The second register  423  stores data necessary for the operation of the baseband signal processor  422 , and provides the data to the baseband signal processor  422 .  
         [0037]     The MAC layer  43  includes a MAC service access point (SAP)  431 , a received power controller  433 , and a frame processor  432 . The MAC SAP  431  receives frames from the baseband signal processor  422 . The received power controller  433  controls power of the station based on frames received from the MAC SAP  431 . The frame processor  432  communicates with a host by performing protocol control or direct memory access.  
         [0038]     When receiving a signal, power of the station may be controlled in the following manner.  FIG. 5  is a diagram illustrating a method of controlling power according to an embodiment of the present invention. More specifically,  FIG. 5  illustrates the operation of the received power controller  433 .  
         [0039]     Referring to  FIG. 5 , the received power controller  433  receives a physical layer conversion protocol (PLCP) frame and a MAC frame via the MAC SAP  431  in step  50 . The PLCP frame and the MAC frame include various information, including header information and MAC frame transmission speed information.  FIG. 6  illustrates the structure of a PLCP data unit (PPDU). Referring to  FIG. 6 , the PPDU includes a PLCP preamble, a PLCP header, and a MAC sublayer protocol data unit (MPDU). In  FIG. 6 , a synchronization field (sync) is used for obtaining and synchronizing incoming signals, and a start-of-frame-delimiter field (SFD) includes information indicating a beginning point in the PPDU. A signal field (SIGNAL) indicates an adopted modulation scheme, and a service field (SERVICE) is a reserved field. A length field (LENGTH) indicates an amount of time required to transmit the MPDU, and a cyclical redundancy check (CRC) field has a frame check sequence (FCS) value calculated by a transmitting station. The PLCP preamble and the PLCP header can be transmitted at a speed of 1 Mbps, and the MPDU can be transmitted at a speed of 1 or 2 Mbps.  
         [0040]     In step  51 , the received power controller  433  extracts transmission speed information and frame transmission period information from the information received in step  50 . In step  52 , the received power controller  433  determines a power-controlled period by interpreting the extracted information. The frame transmission period information can be obtained from a duration field of each MAC frame.  
         [0041]      FIGS. 7A through 7C  illustrate different examples of MAC frames. More specifically,  FIG. 7A  illustrates a structure of a request-to-send (RTS) frame,  FIG. 7B  illustrates a structure of a clear-to-send (CTS) frame, and  FIG. 7C  illustrates a structure of a data frame. In  FIGS. 7A through 7C , RA represents a reception address, and TA represents a transmission address. In  FIG. 7C , ADDRESSES  1  through  4  are address fields, which can be respectively classified as one of a transmission address, a reception address, and a group address.  
         [0042]     A power-controlled period for the baseband layer  42  is calculated independently of a power-controlled period for the RF layer  41  because the time taken for the baseband layer  42  to stabilize differs from the time taken for the RF layer  41  to stabilize. For example, currently, it takes the baseband layer  42  no more than five microseconds to stabilize, while it takes the RF layer  41  no more than ten microseconds to stabilize.  
         [0043]     Once the power-controlled period of the baseband layer  42  and the RF layer  42  is determined, the received power controller  433  compares its address with a reception address of the received MAC frame in step  53 . If the reception address of the received MAC frame is identical to the address of the received power controller  433 , the method returns to step  50 , a PLCP is continuously received, and the frame processor  432  performs an operation. If the reception address of the received MAC frame is different from the address of the received power controller  433  in step  53 , the received power controller  433  switches current modes of the baseband layer  42  and the RF layer  41  to low power modes in step  54  for the duration of the power-controlled period determined in step  52 .  
         [0044]     When the baseband layer  42  is in a low power mode, all the elements thereof except for the second register  423  are powered down and clocks provided thereto are all blocked. When the RF layer  41  is in the low power mode, all the elements thereof, except for the VCO-PLL  412  and the first register  413 , are powered down. The first and second registers  413  and  423  and the VCO-PLL  412  are preferably continuously supplied with power because they require more time to stabilize after they are powered up than the rest of the baseband layer  42  and the RF layer  41 .  
         [0045]     When power is cut off, the received power controller  433  counts the power-controlled period in step  55 . When the power-controlled period of the RF layer  41  is finished, the received power controller  433  applies power to the RF layer  41  so that the RF layer  41  can be switched from the low power mode to a normal operational mode in step  56 . Once the RF layer  41  is powered up, it stabilizes within ten microseconds.  
         [0046]     When the RF layer  41  returns to the normal operational mode and the power-controlled period of the baseband layer  42  is over, the received power controller  433  applies power to the baseband layer  42  so that the baseband layer  42  can be switched from the low power mode to the normal operational mode in step  57 . Once the baseband layer  42  is powered up, it stabilizes within five microseconds.  
         [0047]     Thereafter, the method returns to step  50 , in which the received power controller  433  continuously receives a PLCP, and the receiving station performs a virtual carrier sense operation.  
         [0048]      FIG. 8  illustrates power consumption of stations when performing a carrier sense operation using a method of controlling power consumption of stations according to an embodiment of the present invention. Referring to  FIG. 8 , when a first station STA- 1  transmits ( 80 ) a MAC frame (DATA) to a third station STA- 3 , a second station STA- 2  and other stations, in addition to the third station STA- 3 , perform a carrier sense operation ( 81 ) so that they consume as much power as the third station STA- 3 . However, once the second station STA- 2  and the other stations determine that the MAC frame is not directed thereto, each layer therein is shifted to a low power mode ( 82 ). When the third station STA- 3  transmits ( 83 ) an ACK frame to the first station STA- 1 , the second station STA- 2  and the other stations also perform a virtual carrier sense ( 84 ). However, once the second station STA- 2  and the other stations determine that the ACK frame is not directed thereto, each layer therein is again shifted to a low power mode ( 85 ).  
         [0049]     The method of controlling the power consumption of stations according to an embodiment of the present invention was simulated under a circumstance where two stations and an AP were used in an Internet environment with some traffic. Simulation results indicate that the power consumption of stations in the present invention can be reduced to 30-50% of the power consumption of conventional stations.  
         [0050]     According to the present invention, it is possible to control the power consumption of stations in a hardware manner while the stations perform a carrier sense operation. Thus, it is possible to minimize the power consumption of the stations in a bi-directional real-time communication service. The present invention complies with the IEEE 802.11 standard, and thus it can be readily applied to IEEE 802.11 wireless LAN systems. If the present invention is applied to a structure, into which a MAC layer, a baseband layer, and an RF layer are integrated in a silicon-on-chip manner, power conservation can be maximized. In addition, the present invention can be easily applied to field programmable gate arrays (FPGAs) or digital signal processors (DSPs).  
         [0051]     Exemplary embodiments of the present invention have been disclosed herein and, although specific terms are employed, they are used and are to be interpreted in a generic and descriptive sense only and not for purpose of limitation. Accordingly, it will be understood by those of ordinary skill in the art that various changes in form and details may be made without departing from the spirit and scope of the present invention as set forth in the following claims