Abstract:
A method for controlling sleep mode of a base station and a mobile station in wireless communication networks, including: determining N of the N-ary exponential sleep mode to decide a length of sleep duration including a sleep interval; measuring downlink traffic addressed to the mobile station at the beginning of a listening interval right after the sleep interval; and when there exist downlink traffic, confirming whether the measured downlink traffic satisfies a mode transition condition. The method further includes: conducting a sleep interval of the next sleep duration of which the length is determined by multiplying the length of the current sleep duration by N unless the downlink traffic satisfies the mode transition condition; and transmitting the downlink traffic to the mobile station when m times the additional consecutive sleep duration is expired or when the measured amount of the downlink traffic satisfies the mode transition condition.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a method and an apparatus for controlling sleep mode in wireless communication networks; and, more particularly, to a method and an apparatus for maximizing power saving by lengthening sleep duration through adoption of the N-ary exponential sleep mode of a base station and a mobile station in the wireless communication networks. 
       BACKGROUND OF THE INVENTION 
       [0002]    In general, a communication system has been developed to provide high quality service capable of transmitting and receiving massive data at high speeds. In particular, demands for wireless communication network environments have recently increased. Due to a limited battery life of a mobile station (hereinafter, referred to as “MS”), power consumption of the MS is one of the factors significantly affecting overall performance of the wireless communication systems. 
         [0003]    Sleep mode of a base station (hereinafter, referred to as “ES”) and an MS can be employed to efficiently reduce power consumption of the MS. For example, the IEEE 802.16e standard for communication systems supports sleep mode, which is mandatory for the BS. Further, implementation of sleep mode is optional for the MS according to the IEEE 802.16e standard, however, the MS not supporting sleep mode keeps monitoring the downlink all the time even when it does not receive data from the BS or transmit data to the BS, resulting in high power consumption. 
         [0004]    Sleep mode is composed of repetition of a sleep interval and a listening interval. The sleep interval is the time duration when the MS does not receive downlink data for power saving, whereas the listening interval is the time duration when the MS receives instruction for the existence of downlink traffic addressed to the MS. During the sleep interval, the MS may not supply power to some of the physical components and it does not communicate with the BS. 
         [0005]      FIG. 1  illustrates the operation for controlling sleep mode in a communication system. 
         [0006]    As shown in  FIG. 1 , an MS  100  transmits a sleep request (MOB_SLP-REQ) message to a BS  120  to switch from active mode to sleep mode in step S 141 . On receiving this MOB_SLP-REQ message, the BS  120  determines whether to approve the switching request of the MS  100  to sleep mode or not, and transmits a sleep response message (MOB_SLP-RSP) depending on the determined result to the MS  100  in step S 143 . For example, according to the IEEE 802.16e standard, this MOB_SLP-RSP message contains several parameters such as initial-sleep window indicating the length of an initial sleep interval; listening window indicating the length of a listening interval; final-sleep window base indicating a base for a final sleep interval and final-sleep window exponent indicating an exponent for the final sleep interval, which are necessary to determine the maximum length of a sleep interval; and start_frame_number indicating the number of the starting frame of the initial sleep interval. In step S 143 , the BS  120  may request the MS  100  to start sleep mode by sending a sleep response (MOB_SLP-RSP) message without receiving a sleep request message from the MS  100 , which is called an unsolicited manner. Upon reception of the MOB_SLP-RSP message, the MS  100  goes to sleep mode at the beginning frame M of the initial sleep interval, and the sleep mode lasts for the length of the initial sleep interval N 1 . After the sleep interval, the MS  100  enters a listening interval with a length of L. 
         [0007]    During the listening interval, the BS  120  transmits a message instructing the MS  100  to switch to access mode if there is any downlink data destined for the MS  100 , whereas the BS  120  transmits a message instructing to remain in sleep mode to the MS  100  if there is no downlink data. 
         [0008]    Subsequently, during the listening interval right after the initial sleep interval, the BS  120  transmits a traffic indication (MOB_TRF-IND) message with negative indication for the MS  100  since the BS  120  has decided there is no downlink data for the MS  100  in step S 145 . This MOB_TRF-IND message with negative indication does not require the identification of the MS  100  and the MS  100  having received this message continues its sleep mode. The length of the next sleep interval of the MS  100  is 2×N 1 , which is a double of the length of the previous sleep interval. That is, if a MOB_TRF-IND message contains negative indication for the MS  100 , the length of the next sleep interval of the MS  100  doubles the length of the previous sleep interval until it reaches up to a maximum length N 2  of the sleep interval. After the sleep interval has ended, the MS  100  enters a listening interval with a length of L. For example, the sleep interval and the listening interval as described above are defined by Power Saving Class of type I in the IEEE 802.16e standard. 
         [0009]    Thereafter, if provided with a protocol data unit (PDU) for the MS  100 , that is, if the BS  120  determines that there is downlink data intended for the MS  100 , it transmits a traffic indication (MOB_TRF-IND) message with positive indication for the MS  100  in step S 147 . This MOB_TRF-IND message with positive indication has the identification of the MS  100 . The MS  100  having received this message switches to access mode, thereby receiving the downlink data. 
         [0010]    In the wireless communication networks, while the BS and the MS perform normal operations in access mode for data transmission or reception, they may enter sleep mode by minimizing the data transmission or reception in order to save power, thereby reducing power consumption of the MS. 
         [0011]    The method for controlling sleep mode described above by referring to  FIG. 1  is called the binary exponential algorithm. This algorithm is known to be adequate for a packet-by-packet service, which stores data as a unit of packet in a buffer of the BS and transmits packets to mobile subscribers. 
         [0012]      FIG. 2A  shows a configuration of sleep mode when the IEEE 802.16e communication system employs the binary exponential algorithm, whereas  FIG. 25  shows a configuration of sleep mode when the IEEE 802.16m communication system employs the binary exponential algorithm. In the IEEE 802.16e communication system as shown  FIG. 2A , the length of each listening interval L is identical, whereas if there is no downlink data intended for the MS  100 , then the length of a sleep interval So is doubled to S 1  and S 1  is doubled to S 2 , and so on. In the IEEE 802.16m communication system as shown  FIG. 2B , an initial sleep cycle C 0  consisting of a sleep interval is extended twice to the next sleep cycle C 1  consisting of a listening interval L and a sleep interval S. If there is no downlink data intended for the MS  100 , then C 1  is extended to C 2 , and so on. If there is any downlink data intended for the MS  100 , then during the listening interval, the data can be delivered to the MS  100 . 
         [0013]    In recent years, the communication systems have been developed to provide a high data rate service and have operated a scheduler to transmit multiple (bulk) packets or variable length packets to an MS at a time. 
         [0014]    However, if the conventional binary exponential algorithm is applied to these latest communication systems, power efficiency achieved by the sleep mode is reduced since transitions between access mode and sleep mode are unnecessarily frequent, it may degrade overall performance of the wireless communication systems. 
       SUMMARY OF THE INVENTION 
       [0015]    The present invention has been conceived to solve the problems described above and it is, therefore, an object of the present invention to provide a method and an apparatus for maximizing power saving by lengthening sleep duration through adoption of the N-ary exponential sleep mode of a BS and an MS in the wireless communication network, where N is set to be equal to or greater than 2, and in particular, if N=2 then it corresponds to the conventional binary exponential sleep mode. 
         [0016]    In accordance with an aspect of the prevent invention, there is provided a method for controlling sleep mode of a base station and a mobile station in wireless communication networks. The method includes: determining N of the N-ary exponential sleep mode to decide a length of sleep duration including a sleep interval, where N is equal to or greater than 2; measuring the amount of downlink traffic addressed to the mobile station at the beginning of a listening interval right after the sleep interval; when there exist downlink traffic but m times additional consecutive sleep duration is not expired, confirming whether the measured amount of the downlink traffic satisfies a mode transition condition; conducting a sleep interval of the next sleep duration of which the length is determined by multiplying the length of the current sleep duration by N unless the measured amount of the downlink traffic satisfies the mode transition condition; and transmitting the downlink traffic to the mobile station when m times the additional consecutive sleep duration is expired or when the measured amount of the downlink traffic satisfies the mode transition condition even when m times the additional consecutive sleep duration is not expired. 
         [0017]    In accordance with another aspect of the present invention, there is provided an apparatus for controlling sleep mode of a mobile station in a wireless communication network, including: an N-value decision unit for determining N of the N-ary exponential sleep mode to decide a length of sleep duration including a sleep interval, where N is equal to or greater than 2; a traffic measuring unit for measuring an amount of downlink traffic for the mobile station stored in a buffer at the beginning of a listening interval right after the sleep interval; a mode transition determination unit for determining the mode transition by confirming whether the measured amount of the downlink traffic satisfies a mode transition condition when there exist downlink traffic but m times additional consecutive sleep duration is not expired; a sleep duration managing unit for conducting a sleep interval of the next sleep duration, the length of the next sleep duration being determined by multiplying the length of the current sleep duration by N based on the determination of the mode transition determination unit; and a packet control unit for transmitting the downlink traffic to the mobile station when m times the additional consecutive sleep duration is expired or according to the determination of the mode transition determination unit even when m times the additional consecutive sleep duration is not expired. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    The above and other objects and features of the present invention will become apparent from the following description of embodiments given in conjunction with the accompanying drawings, in which: 
           [0019]      FIG. 1  is a signal flow chart for illustrating the operation for controlling sleep mode in a communication system; 
           [0020]      FIG. 2A  presents a configuration of sleep mode when the binary exponential algorithm is applied to a communication system based on the IEEE 802.16e standard; 
           [0021]      FIG. 2B  shows a configuration of sleep mode when the binary exponential algorithm is applied to a communication system based on the IEEE 802.16m standard; 
           [0022]      FIG. 3  is a flow chart describing a method for controlling sleep mode in accordance with the first embodiment of the present invention; 
           [0023]      FIG. 4  shows a flow chart describing a method for controlling sleep mode in accordance with the second embodiment of the present invention; 
           [0024]      FIG. 5  is a flow chart describing a method for controlling sleep mode in accordance with the third embodiment of the present invention; and 
           [0025]      FIG. 6  illustrates a block diagram of a sleep mode controlling apparatus provided in a base station for performing methods for controlling sleep mode in accordance with the embodiments of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0026]    Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings which form a part hereof. 
         [0027]    The present invention provides a method and an apparatus for controlling sleep mode of a BS and an MS in a wireless communication network. Although the present invention is described with a wireless communication system based on the IEEE 802.16e standard and a wireless system based on the IEEE 802.16m standard, the method and the apparatus of the present invention can be applied to other communication systems. Furthermore, although the present invention is described with a BS and a single MS, the method and the apparatus of the present invention can be applied to multiple MSs as well. 
         [0028]    Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. 
         [0029]      FIG. 3  is a flow chart describing a method for controlling sleep mode in a wireless communication network in accordance with the first embodiment of the present invention. 
         [0030]    First, in step S 201 , a BS and an MS  700  determine various parameters necessary for the mode transition operation. For example, a length of initial sleep duration T 0 , the length of the maximum sleep duration T max , the number of additional consecutive sleep duration m upon arrival of packets, the maximum N-ary exponent I max , the length of a time-out for retransmission R, the maximum number of allowed retransmissions N max , the required transmission delay constraint D* and the size of multiple packets M can be determined. Here, the length of the sleep duration corresponds to the length of each sleep interval S 0 , S 1  or S 2  in the wireless communication system based on the IEEE 802.16e standard shown in  FIG. 2A , whereas it corresponds to the length of each sleep cycle C 0 , C 1  or C 2  in the wireless communication system based on the IEEE 802.16m standard shown in  FIG. 2B . 
         [0031]    That is, the sleep duration means sleep mode with a variable length such as a sleep interval or a sleep cycle in wireless communication systems based on various standards. The above parameters can be determined by a sleep request (MOB_SLP-REQ) message and a sleep response (MOB_SLP-RSP) message exchanged between the BS and the MS. For example, if the MS transmits a sleep request MOB_SLP-REQ message to the BS for the mode transition operation, the BS determines whether to approve transition to sleep mode by using variables contained in the MOB_SLP-REQ message and then transmits a sleep response (MOB_SLP-RSP) message with the decision to the MS, or not. Otherwise, the BS transmits a sleep response (MOB_SLP-RSP) message to the MS in an unsolicited manner. This transmitted message contains both parameters of step S 201  and N determined as follows. 
         [0032]    After that, in step S 203 , N of the N-ary exponent for the sleep duration is determined using the various parameters. N is used to decide the length of the next sleep duration by multiplying the current sleep duration by N. 
         [0033]    The value of N needs to be determined to minimize power consumption as well as to satisfy the quality of service for transmission delay constraint. For this, N satisfying the following Equation 1 is chosen. 
         [0000]      ( m×N   I     max     T   0 )+( N   max   ×R )≦ D*   Eq. 1
 
         [0034]    In Equation 1, m is the number of the additional consecutive sleep duration upon arrival of packets, I max  is the maximum N-ary exponent, T 0  is the length of the initial sleep duration, N max  is the maximum number of allowed retransmissions, R is the length of a time-out for retransmission, and D* is the transmission delay constraint. Here, by collecting packets which arrived during m times the maximum sleep duration determined under the worst-case delay constraint and sending the multiple packets to the MS in downlink, the MS continues its sleep mode for m-times longer sleep duration, thereby saving extra power. 
         [0035]    Next, in step S 205 , the consecutive sleep duration parameter K is initialized to 1. 
         [0036]    The sleep duration with a length determined above is proceeded in step S 207 . Initially, the sleep duration is set to T 0 . During the sleep duration, the ES does not send downlink data to the MS. If the wireless communication network provides any protocol data unit for the MS during the sleep duration, the BS stores downlink traffic for the MS in a buffer. 
         [0037]    Thereafter, in step S 209 , whether the current sleep duration has expired or not is determined. When the sleep duration with a length determined above is not expired, then the sleep duration continues and the process described above is repeated. 
         [0038]    When the sleep duration has expired, then a listening interval begins in step S 211 . 
         [0039]    Next, n step S 213 , the existence of downlink traffic for the MS stored in the buffer is checked when the listening interval starts. When there is no traffic, the method goes to step S 215  where the ES transmits the MS a traffic indication message with negative indication for the MS to conduct the sleep duration. 
         [0040]    Subsequently, in step S 217 , the length of the next sleep duration is determined right after the current listening interval. The length of the next sleep duration is decided by using N determined as in step S 203 . When the maximum exponent is greater than I max , it is fixed to I max . As shown in Equation 2, the length of the next sleep duration T next  is determined by multiplying the length of the current sleep duration T cur  by N. Here, the length of the next sleep duration T next  is limited to the length of the maximum sleep duration T max . 
         [0000]        T   next   =N×T   cur , where  T   next   ≦T   max   Eq. 2
 
         [0041]    After the length of the next sleep duration T next  is decided in step S 217 , the method returns to step S 207  in which the next sleep duration T next  is proceeded. 
         [0042]    On the other hand, if the BS has decided that there is traffic in step S 213 , then it is checked, in step S 219 , whether or not the consecutive sleep duration parameter K reaches to the number of additional consecutive sleep duration m. 
         [0043]    If K does not reach to m, in step S 221 , whether or not the amount of the measured downlink traffic satisfies the mode transition condition is decided. Here, either whether the number of downlink data packets satisfies the mode transition condition or whether the amount of downlink data satisfies the mode transition condition can be adopted. 
         [0044]    As for whether the number of downlink data packets satisfies the mode transition condition or not, the following Equation 3 is used for decision making. 
         [0000]        N   pkt   ≧M   Eq. 3
 
         [0045]    In Equation 3, N pkt  is the number of the packets stored in the buffer and M is the threshold value representing the maximum number of packets allowed for one-time downlink transmission. 
         [0046]    As for whether the amount of downlink data satisfies the mode transition condition or not, the following Equation 4 is used for decision making. 
         [0000]        N   bit   ≧N   th   Eq. 4
 
         [0047]    In Equation 4, N b i t  is the number of the transmitted bits and N th  is a threshold value representing the maximum allowable number of the transmitted bits in a given time. 
         [0048]    When the BS determines that the mode transition condition is not satisfied in step S 221 , the BS increases K by 1 in step S 223  and then transmits the MS a traffic indication message with negative indication for the MS to conduct the sleep duration in step S 215 . 
         [0049]    On the other hand, if K reaches to m in step S 219 , the BS transmits the MS a traffic indication message with positive indication for the MS for the mode transition and transmits the downlink data stored in the buffer to the MS in step S 225  and then it switches to initial sleep mode I step S 227 . 
         [0050]    Although not shown in  FIG. 3 , if there is uplink data from the MS after the BS transmits the downlink data stored in the buffer to the MS, the BS may receive the uplink data from MS. When the BS transmits the MS the downlink data stored in the buffer, the parameter M or the threshold value Nth of the maximum allowable number of transmitted bits may be fixed, varied or unlimited. In case of the fixed or varied value, data conforming to the amount of data bits or the number of data packets assigned to the MS is transmitted. In case of the unlimited value, all of the stored downlink traffic is transmitted. 
         [0051]    Further, although not shown in  FIG. 3 , a time-out can follow after the data transmission or reception is completed. 
         [0052]      FIG. 4  is a flow chart describing a method for controlling sleep mode in a wireless communication network in accordance with the second embodiment of the present invention when a wireless communication system based on the IEEE 802.16e standard is employed. 
         [0053]    First, in step S 301 , a BS and an MS determine various parameters required for the mode transition operation. For example, the length of an initial sleep interval S 0 , the length of the maximum sleep interval S max , the number of additional consecutive sleep intervals m upon arrival of packets, the maximum N-ary exponent I max , the length of a time-out for retransmissions R, the maximum number of allowed retransmissions N max , the required transmission delay constraint D and the threshold value M for the maximum number of packets allowed for one-time downlink transmission can be determined. 
         [0054]    The above parameters can be determined by a sleep request (MOB_SLP-REQ) message and a sleep response (MOB_SLP-RSP) message exchanged between the BS and the MS. For example, if the MS transmits a sleep request MOB_SLP-REQ message to the BS for the mode transition operation, the BS determines whether to approve transition to sleep mode by using variables contained in the MOB_SLP-REQ message and then transmits a sleep response (MOB_SLP-RSP) message with the decision to the MS. Otherwise, the BS transmits a sleep response (MOB_SLP-RSP) message to the MS in an unsolicited manner. This transmitted message contains both parameters determined in step S 301  and N determined in step S 303  as follows. 
         [0055]    In step S 303 , N of the N-ary exponent for the sleep interval is determined using the various parameters. N is used to determine the length of the next sleep interval by multiplying_the current sleep interval by N. N needs to be determined to minimize power consumption as well as to satisfy the quality of service for a given transmission delay constraint. For this, N satisfying the following Equation 5 is chosen. 
         [0000]      ( m×N   I     max     S   0 )+( N   max   ×R )≦ D*   Eq. 5
 
         [0056]    In Equation 5, m is the number of the additional consecutive sleep interval upon arrival of packets, I max  is the maximum N-ary exponent, So is the length of an initial sleep interval, N max  is the maximum number of allowed retransmissions, R is the length of a time-out for retransmission, and D* is the required transmission delay constraint. 
         [0057]    Next, the consecutive sleep interval parameter K is initialized to 1 in step S 305  and then the sleep interval with a length determined above begins in step S 307 . 
         [0058]    Initially, the sleep interval is set with So. During the sleep interval, the BS does not send downlink data to the MS. If the wireless communication network provides any protocol data unit for the MS during the sleep interval, the BS stores downlink traffic for the MS in a buffer. 
         [0059]    Thereafter, in step S 309 , whether the current sleep interval has expired or not is determined. When the frames do not reach up to the sleep interval with a length determined above, then the method returns to step S 307  to continue the sleep interval and repeat the process described above. 
         [0060]    However, when the current sleep duration has expired, then a listening interval starts in step S 311 . 
         [0061]    Next, in step S 313 , the existence of downlink traffic for the MS stored in the buffer is checked when the listening interval begins. If there is no traffic, in step S 315 , the BS transmits the MS a traffic indication message with negative indication for the MS to conduct the sleep interval. 
         [0062]    The length of the next sleep interval is determined following right after the current listening interval. The length of the next sleep interval is decided by using N determined in step S 303 . When the maximum exponent is greater than I max , it is fixed to I max . As shown in Equation 6, the length of the next sleep interval S next  is determined by multiplying the length of the current sleep interval S cur  by N. Here, the length of the next sleep interval S next  is limited to the length of the maximum sleep interval S max . 
         [0000]        S   next   =N×S   cur , where S next   ≦S   max   Eq. 6
 
         [0063]    After that, in step S 317 , the length of the next sleep interval S next  is decided, and the method returns to step S 307  to start the next sleep interval S next . 
         [0064]    On the other hand, however, if the ES has decided that there is traffic in step S 313 , then it checks whether or not the consecutive sleep interval parameter K reaches to the number of additional consecutive sleep intervals m in step S 319 . 
         [0065]    When K does not reach to m, whether or not the amount of the measured downlink traffic satisfies the access mode transition condition is decided in step S 321 . Here, either whether the number of downlink data packets satisfies the access mode transition condition or whether the amount of downlink data satisfies the access mode transition condition can be adopted. 
         [0066]    As for whether the number of downlink data packets satisfies the access mode transition condition, the following Equation 7 is used for decision making. 
         [0000]        N   pkt   ≧M   Eq. 7
 
         [0067]    In the Equation 7, N pkt  is the number of the packets stored in the buffer and M is the threshold value requesting the maximum number of packets allowed for one-time downlink transmission. 
         [0068]    As for whether the amount of downlink data satisfies the access mode transition condition, the following Equation 8 is used for decision making. 
         [0000]        N   bit   ≧N   th   Eq. 8
 
         [0069]    In the Equation 8, N bit  is the number of the transmitted bits and N th  is a threshold value for the maximum allowable number of transmitted bits in a given time. 
         [0070]    When the BS determines that the access mode transition condition is not satisfied in step S 321 , the BS increases K by 1 in step S 323  and then transmits the MS a traffic indication message with negative indication for the MS to conduct the sleep interval in step S 315 . 
         [0071]    On the other hand, when K reaches to m in step S 319 , the BS transmits the MS a traffic indication message with positive indication for the MS for the mode transition to access mode and transmits the downlink data stored in the buffer to the MS in step S 325 . 
         [0072]    After completing the transmission of the downlink data, the BS switches to initial sleep mode in step S 327  and goes back to step S 305 . 
         [0073]    Although not shown in  FIG. 4 , if there is uplink data from the MS after the BS transmits the downlink data stored in the buffer to the MS, the BS may receive the uplink data from MS. When the BS transmits the MS the downlink data stored in the buffer, the parameter value of M or the threshold value Nth for the maximum allowable number of transmitted bits can be fixed, varied or unlimited. In case of the fixed or varied value, data conforming to the amount of data bits or the number of packets assigned to the MS is transmitted. In case of the unlimited value, all of the measured downlink traffic is transmitted. 
         [0074]    Although not shown in  FIG. 4 , a time-out can follow after the transmission and reception of data is completed. 
         [0075]      FIG. 5  is a flow chart describing a method for controlling sleep mode a wireless communication network in accordance with the third embodiment of the present invention when a wireless communication system based on the IEEE 802.16m standard is employed. 
         [0076]    First, in step S 401 , a BS and an MS determine various parameters required for the mode transition operation. For example, the length of an initial sleep cycle Co, the length of the maximum sleep cycle C max , the number of additional consecutive sleep cycles m upon arrival of packets, the maximum Nary exponent I max , the length of a time-out for retransmission R, the maximum number of allowed retransmissions N max , the required transmission delay constraint D* and the threshold value M for the maximum number of packets allowed for one-time downlink transmission can be determined. 
         [0077]    The above parameters can be determined by a sleep request (MOB_SLP-REQ) message and a sleep response (MOB_SLP-RSP) message exchanged between the BS and the MS. For example, if the MS transmits a sleep request MOB_SLP-REQ message to the BS for the mode transition operation to sleep mode, the BS determines whether to approve transition to sleep mode by using variables contained in the MOB_SIP-REQ message and then transmits a sleep response (MOB_SLP-RSP) message with the decision to the MS. Otherwise, the BS transmits a sleep response (MOB_SLP-RSP) message to the MS in an unsolicited manner. This transmitted message contains both parameters of step S 401  and N determined as follows. 
         [0078]    In step S 403 , N of the N-ary exponent for the sleep cycle is determined by using the various parameters. N is used to decide the length of the next sleep cycle by multiplying the current sleep cycle by N. The value of N needs to be determined to minimize power consumption as well as to satisfy the quality of service for q given transmission delay constraint. For this, N satisfying the following Equation 9 is chosen (S 403 ). 
         [0000]      ( m×N   T     max     C   0 )+( N   max   ×R )≦ D*   Eq. 9
 
         [0079]    In Equation 9, m is the number of the additional consecutive sleep cycle upon arrival of packets, I max  is the maximum N-ary exponent, Co is the length of initial sleep cycle, N max  is the maximum number of allowed retransmissions, R is the length of a time-out for retransmission, and D* is the required transmission delay constraint. 
         [0080]    Next, the consecutive sleep cycle parameter K is initialized to 1 in step S 405  and then whether or not the previously determined sleep cycle is the initial sleep cycle is checked in step S 407 . Initially, the sleep cycle is set with C 0 . 
         [0081]    When the sleep cycle is the initial sleep cycle, in step S 409 , the sleep mode lasts until the sleep cycle is expired. During the sleep mode, downlink data is not sent to the MS from the BS. When the wireless communication network provides any protocol data unit for the MS during the sleep mode, the BS stores downlink traffic for the MS in a buffer. 
         [0082]    Thereafter, the length of the next sleep cycle is determined. The length of the next sleep cycle is decided by using N determined as in step S 403 . When the maximum exponent is greater than I max , it is fixed to I max . As shown in the Equation 10, the length of the next sleep cycle C next  is determined by multiplying the length of the current sleep cycle C cur  by N. Here, the length of the next sleep cycle C next  is limited to the length of the maximum sleep cycle C max  as follows. 
         [0000]        C   next   =N×C   cur , where C next   ≦C   max   Eq. 10
 
         [0083]    Accordingly, the length of the next sleep cycle C next  is determined in step S 411 . 
         [0084]    Meanwhile, when the sleep cycle is not the initial sleep cycle, in step S 413 , a listening interval of the sleep cycle begins. In this step, a single frame is used for the listening interval. 
         [0085]    Next, in step S 415 , the existence of downlink traffic for the MS stored in the buffer is checked when the listening interval starts. When there is no traffic, the method returns to step S 409  where the BS transmits the MS a traffic indication message with negative indication for the MS to conduct the sleep duration. 
         [0086]    On the other hand, however, when the BS has detected that there is traffic in step S 415 , then it checks whether or not the consecutive sleep cycle parameter K reaches to the number of additional consecutive sleep cycle m in step S 417 . 
         [0087]    When K does not reach to m, in step S 419 , whether or not the amount of the measured downlink traffic satisfies the access mode transition condition is decided. Here, either whether the number of downlink data packets satisfies the access mode transition condition or whether the amount of downlink data satisfies the access mode transition condition can be adopted. 
         [0088]    As for whether the number of downlink data packets satisfies the access mode transition condition, the following Equation 11 is used for decision making. 
         [0000]        N   pkt   ≧M   Eq. 11
 
         [0089]    In Equation 11, N pkt  is the number of the packets stored in the buffer and M is the threshold value requesting the maximum number of packets allowed for one-time downlink transmission. 
         [0090]    As for whether the amount of downlink data satisfies the access mode transition condition, the following Equation 12 is used for decision making. 
         [0000]        N   bit   ≧N   th   Eq. 12
 
         [0091]    In Equation 12, N b j t  is the number of the transmitted bits and N th  is a threshold value for the maximum allowable number of the transmitted bits in a given time. 
         [0092]    When the BS determines that the access mode transition condition is not satisfied in step S 419 , the BS increases K by 1 in step S 421  and then transmits the MS a traffic indication message with negative indication for the MS to conduct the sleep mode in step S 409 . 
         [0093]    On the other hand, when K reaches to m in step S 417  or if the mode transition condition is satisfied in step S 419 , the method advances to step S 423 . In step S 423 , the BS transmits the MS a traffic indication message with positive indication for the MS for traffic transmission. Then, the BS sets the length of the current sleep cycle C cur  to the length of the initial sleep cycle C 0  and then initializes the consecutive sleep cycle parameter to 1. Next, in step S 425 , while keeping the listening interval, the BS transmits the downlink data stored in the buffer to the MS. When the BS transmits the MS the downlink data stored in the buffer, the parameter value of M or the threshold value Nth for the maximum allowable number of transmitted bits can be fixed, varied or unlimited. In case of the fixed or varied value, data conforming to the amount of data bits or the number of packets assigned to the MS is transmitted. In case of the unlimited value, all of the measured downlink traffic is transmitted. 
         [0094]    Then, in step S 427 , whether or not the current sleep cycle including the listening interval of step S 425  is expired is decided. 
         [0095]    When the current sleep cycle has expired in step S 427 , then the method returns to step S 411  where the length of the next sleep cycle is determined. When the current sleep cycle has not expired, whether or not the buffer is empty is then checked in step S 429 . The listening interval of the current sleep cycle lasts until the buffer is completely emptied. When the buffer is emptied, the sleep mode continues until the current sleep cycle ends in step S 431 . 
         [0096]      FIG. 6  as a block diagram of a sleep mode control apparatus  500  provided in a BS for implementing methods for controlling sleep mode in accordance with the embodiments of the present invention. 
         [0097]    As shown in  FIG. 6 , a sleep mode controlling unit  500  includes a sleep duration managing unit  510 , a traffic measuring unit  520 , a mode transition determination unit  530 , an N-value decision unit  540  and a packet control unit  550 . 
         [0098]    The sleep duration managing unit  510  determines the length of sleep duration in sleep mode to enter and then conducts a sleep interval of the sleep duration. The sleep duration managing unit  510  stores downlink traffic for an MS  700  in the buffer if a protocol data unit for the MS  700  is provided from a wireless communication network. The sleep duration managing unit  510  conducts a listening interval when the current sleep interval has expired. Here, the sleep duration managing unit  510  determines the length of the next sleep duration by multiplying the length of the current sleep duration by N provided from the N-value decision unit  540 . 
         [0099]    The traffic measuring unit  520  measures the amount of the downlink traffic for the MS  700  stored in the buffer at the beginning of the listening interval. As described above, the amount of the downlink traffic for the MS  700  can be measured in terms of the number of downlink data packets or the amount of downlink data bits, e.g., the number of data bits, addressed to the MS  700  stored in the buffer at the beginning of the listening interval. Upon arrival of packets, extending the current sleep duration by up to m times allows more packets to arrive. 
         [0100]    The mode transition determination unit  530  determines whether or not to send a traffic indication message with positive indication or to send a traffic indication message with negative indication based on either the measured amount of the downlink traffic or reaching the number of the additional consecutive sleep duration m. For example, when the measured amount of the downlink traffic does not satisfy the mode transition condition, the mode transition determination unit  530  transmits the MS  700  a traffic indication message with negative indication for the MS  700  during a listening interval so that the MS  700  can enter a sleep interval. On the other hand, when the measured amount of the downlink traffic satisfies the mode transition condition or when the number of the additional consecutive sleep duration m has been reached, the mode transition determination unit  530  transmits the MS  700  a traffic indication message with positive indication for the MS  700  during a listening interval to manage mode transition such as packet transmission. 
         [0101]    The N-value decision unit  540  determines N of the sleep duration by using various parameters required for the mode transition operation and provides it to the sleep duration managing unit  510 . Here, N can be sent to the sleep duration managing unit  510  either directly or via the mode transition determination unit  530 . N is used to determine the length of the next sleep duration by multiplying the length of the current sleep duration by N and it needs to minimize power consumption as well as to satisfy the quality of service for a given transmission delay constraint. For this, the length of the initial sleep duration, the number of the additional consecutive sleep duration, the maximum N-ary exponent, the maximum allowable number of retransmissions, the length of a time-out for retransmission and the transmission delay constraint are considered to determine N. Here, these various parameters requi 8 red for the mode transition operation can be determined by a sleep request message and a sleep response message exchanged between the BS and the MS  700 . 
         [0102]    The packet control unit  550  performs the following operation in sleep mode or access mode. The packet control unit  550  controls the BS to transmit the downlink data stored in the buffer to the MS  700 . When there is uplink data from the MS  700 , the packet control unit  550  controls the BS to receive the uplink data from the MS  700  and when the transmission and reception of the data is completed, it conducts a time-out as described above. 
         [0103]    The sleep mode controlling apparatus  500  performs the methods for controlling sleep mode described with respect to the embodiments described by referring to  FIGS. 3 to 5 . A detailed description thereof will be omitted to avoid redundancy since it can be readily implemented by those skilled in the art by using the description referring to  FIGS. 3 to 5 . 
         [0104]    In accordance with the embodiments of the present invention, power consumption due to too frequent mode transition caused when the wireless communication network supports sleep mode of the BS and MS can be reduced. Power saving can be significantly increased by lengthening a sleep cycle through adoption of the N-ary exponential sleep mode considering a multiple packet transmission technique as well as satisfying the required delay constraint for each service flow, where N is set to be equal to or greater than 2. In particular, if N=2 then it corresponds to the conventional binary exponential sleep mode. 
         [0105]    Furthermore, power consumption of the MS can be reduced without hardware modification in broadband wireless access communication systems based on the IEEE 802.16e and 802.16m standards. 
         [0106]    While the invention has been shown and described with respect to the embodiments, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the following claims.