Abstract:
A base station includes a deciding part configured to decide a period of performing data transmission/reception and a period of not performing data transmission/reception when performing intermittent communications, a generating part configured to generate a predetermined signal to a mobile station, and an adjusting part configured to adjust the period of performing data transmission/reception and/or the period of not performing data transmission/reception according to a response signal transmitted from the mobile station in response to the predetermined signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2008-174851 filed on Jul. 3, 2008, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The embodiments discussed herein are related to a wideband wireless access system. For example, a base station, a mobile station, and a method used for the wideband wireless access system. 
     BACKGROUND 
     Description of the Related Art 
     A mobile station has a limited battery capacity. Therefore, it is desired for a mobile station to reduce consumption of power while maintaining connection with a base station. The mobile station includes, for example, a mobile phone or a portable information terminal (e.g., PDA: Personal Digital Assistant). 
     In response to such desire, WiMAX (Worldwide Interoperability for Microwave Access) described in IEEE 802.16e-2005 introduces, for example, an operation mode referred to as a Sleep mode. With the sleep mode, there is a period of performing transmission/reception of data (Listening window) and a period of not performing transmission/reception of data (Sleep window) while maintaining a logically connected state between a mobile station (MS) and a base station (BS). 
     The mobile station and the base station negotiate the period of performing transmission/reception of data and the period of not performing transmission/reception of data. By the negotiation, the mobile station does not need to transmit/receive radio waves during the Sleep window. Accordingly, the mobile station may temporarily turn off the power of an unused circuit or temporarily turn off its clock. Thereby, consumption of power can be reduced. 
     The process of the Sleep mode is described in detail with reference to  FIG. 1 . 
     In  FIG. 1 , a mobile station (MS) transmits a sleep request message (MOB_SLP-REQ) to a base station (BS) for requesting a Power Saving Class (PSC) indicating a group of connections having the same intermittent communication schedules as the mobile station (Step S 102 ). 
     The BS transmits a sleep response message (MOB_SLP-RSP) together with parameters designating the size of the Sleep window and the Listening window (Step S 104 ). 
     The MS calculates the Sleep window and the Listening window according to the received parameters. 
     In  FIG. 1 , “SleepWin” indicates the time of the sleep window, “prevSleepWin” indicates a previous sleep window, “finalSlpWinBase” indicates a value that is finally assigned as a sleep interval, “finalSlpWinExp” is the exponent of 2 where 2^ finalslpWinExp is multiplied with the “finalSleepWinBase” for calculating the final sleep window. For example, the final sleep window is represented with the following formula.
 
final-sleep window=final−sleep window base×2^(final-sleep window exponent).
 
     Three types of behavior of intermittent communications are prepared according to situations such as a case where there is no communication traffic or a case where intermittent transmission is performed for saving power even though there is communications traffic. 
     The behavior of the Sleep mode is referred to as a power saving class (PSC). The power saving class includes the method of the schedule of the Listening window and the Sleep window for intermittent communications, and conditions for ending the intermittent communications. 
     The power saving class may be mapped to a connection ID between the MS and the BS. 
     The PSC type  1  is an intermittent communications type optimum for best effort communications of a WEB application. In the best effort type communications, real time communications are not required. The PSC type  1  has a schedule having a Sleep window for intermittent communications that grows larger. Further, the PSC type  1  can cancel the Sleep mode together with the generation of traffic. 
     The PSC type  2  is an intermittent communications type suited for applications in which real time communications are required and data flows constantly. In the PSC type  2 , the Sleep window and the Listening window are fixed. The PSC type  2  is used in situations where constant data communications are desired but at the same time reduction of power consumption of terminals is desired. 
     The PSC type  3  is a schedule that automatically cancels the Sleep mode at the end of the Sleep window corresponding to a predetermined time intervals. The PSC type  3  is used for transmitting/receiving, for example, control data flowing between the MS, BS at predetermined intervals. 
     In a case where a MS in a sleep mode is under poor radio wave reception conditions, there is a possibility that the Listening window anticipated by the BS may overlap with a time when the MS cannot receive radio waves. 
     The time when the MS cannot receive radio waves includes a time when the MS cannot synchronize with the BS. For example, this may apply to a case where the MS is located in-between buildings. For example, as illustrated in  FIG. 2 , the time of the Listening window of the BS may overlap with the time when the MS cannot receive radio waves. By the negotiation between the BS and the MS, the Listening window and the Sleep window are determined. The BS and the MS can communicate with each other during a period of the Listening window. However, the radio wave reception conditions of the MS may change. For example, in a case of unsatisfactory radio wave reception conditions, the MS and the BS may be unable to synchronize and communicate with each other. Therefore, the Listening window of the BS may overlap with the period in which the MS cannot synchronize with the Listening window. 
     Under such circumstance where the MS is in unsatisfactory radio wave reception conditions, the period in which the MS and the BS can communicate is an AND period between a communication-able period of the MS and a communication-able period of the BS. The amount of time in which data can be transmitted/received decreases in this AND period. In other words, the period in which the MS and the BS can communicate is a period where a synchronizable period of the MS and a listening window of the BS overlap. Therefore, data can be transmitted/received for a considerably short time. 
     In a case where the data from an upper layer becomes an amount that cannot be stored by a buffer of the BS, the data from the upper layer may be discarded. Further, a keep alive signal is transmitted between the MS and the BS. In a case where the transmission of the keep alive signal fails repeatedly, the logical connection between the MS and the BS is disconnected. In the case where the logical connection between the MS and the BS is disconnected, the MS cancels the Sleep mode, hands over (HO) the connection to a neighboring BS, and redefines the Sleep mode if necessary. In this case, the period in which data communications cannot be performed between the MS and the BS becomes long. Further, control traffic transmitted/received between the MS and the BS increases. 
     SUMMARY 
     According to an aspect of the invention, there is provided a base station including a deciding part configured to decide a period of performing data transmission/reception and a period of not performing data transmission/reception when performing intermittent communications, a generating part configured to generate a predetermined signal to a mobile station, and an adjusting part configured to adjust the period of performing data transmission/reception and/or the period of not performing data transmission/reception according to a response signal transmitted from the mobile station in response to the predetermined signal. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing generation description and the followed detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic diagram for describing a flow of an operation by a base station and a mobile station for power saving class  1 ; 
         FIG. 2  is a schematic diagram for describing intermittent communication; 
         FIG. 3  is a block diagram illustrating a base station and a mobile station according to an embodiment of the present invention; 
         FIG. 4  is a schematic diagram for describing a communication system according to an embodiment of the present invention; 
         FIG. 5  is a schematic diagram illustrating a block diagram of a base station according to an embodiment of the present invention; 
         FIG. 6  is a block diagram illustrating mobile station according to an embodiment of the present invention; 
         FIG. 7  is a schematic diagram for describing a flow of an operation of a communication system according to an embodiment of the present invention; 
         FIG. 8  is a table for describing a sleep request message according to an embodiment of the present invention; 
         FIG. 9  is a table for describing a cycle of intermittent communication by a communication system according to an embodiment of the present invention; 
         FIG. 10  is a table for describing a sleep request response according to an embodiment of the present invention; 
         FIG. 11  is a table for describing a feedback message according to an embodiment of the present invention; 
         FIG. 12  is a table for describing a feedback response according to an embodiment of the present invention; 
         FIG. 13  is a schematic diagram for describing a flow of an operation of a communication system according to an embodiment of the present invention; 
         FIG. 14  is a schematic diagram for describing a flow of an operation of a communication system according to an embodiment of the present invention; and 
         FIG. 15  is a schematic diagram for describing a flow of an operation of a communication system according to an embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     In this embodiment, a communications system includes a wireless communications apparatus. The wireless communications apparatus may include a mobile station (MS). The wireless communications apparatus may also include a base station (BS). 
     In this embodiment, the base station dynamically changes an intermittent cycle according to a radio wave reception condition of the base station from a standpoint of the mobile station. Furthermore, the base station dynamically changes the intermittent cycle depending on traffic (hereinafter referred to as “dynamic cycle mode”). In this embodiment, “intermittent cycle” is a period combining a period of transmitting/receiving data and a period of not transmitting/receiving data while maintaining a logically connected state between the MS and the BS in a case of performing intermittent communication. The period of transmitting/receiving data may be referred to as a Listening window. The period of not transmitting/receiving data may be referred to as a Sleep window. The period of transmitting/receiving data and/or the period of not transmitting/receiving data may be changed. Further, the period combining the period of transmitting/receiving data and the period of not transmitting/receiving data (so-called “cycle”) may be changed. Further, both the cycle and the period of transmitting/receiving data, and/or the period of not transmitting/receiving data may be changed. 
     For example, the following processes are performed. 
     The mobile station  100  and the base station  200  that perform intermittent communications exchange feedback signals with each other during a communication period of the intermittent cycle. The feedback signals may include information indicating whether standby traffic will occur in the next communication period. 
     In a case where the mobile station  100  determines that communication with the base station cannot be established, the mobile station  100  stops transmitting feedback signals indicating the radio wave reception status of the mobile station  100 . Alternatively, in this case, the mobile station  100  and the base station  200  may switch to a dynamic cycle mode negotiated beforehand. 
     Further, in a case where there is traffic for a base station, the mobile station  100  changes the cycle so that the period in which feedback from the base station  200  can be received (communication-able period of intermittent cycle) becomes longer than the communication-able period of a previous intermittent cycle. In this case, the mobile station  100  reports to the base station  200  that there is standby traffic destined to the base station  200 . For example, the mobile station  100  may report the standby traffic by using a control channel. The mobile station  100  may determine whether traffic will occur based on information of previously transmitted/received feedback signals indicating whether standby traffic will occur. 
     In a case where there are no feedback signals indicating occurrence of standby traffic from the mobile station  100 , the base station  200  changes the cycle of intermittent operations of the base station  200  itself depending on whether there is standby traffic of the base station  200  itself or the mobile station  100 . For example, the base station  200  may change the cycle of intermittent operations by shortening the cycle of intermittent operations during a period where no feedback signals indicating occurrence of standby traffic are received depending on whether there has been previous standby traffic of the base station  200  or previous standby traffic of the mobile station  100 . 
     Further, the mobile station  100  may determine that communications between the base station  200  cannot be established when no feedback signals are received from the base station  200 . 
     Alternatively, the mobile station  100  may determine that communications with the base station  200  cannot be established according to the radio waves measured from the base station  200 . 
     Further, when a transmission buffer becomes unoccupied, the mobile station  100  and the base station  200  may end the dynamic cycle mode and return to the regular cycle. For example, in a case where transmission of the data stored in the transmission buffers included in the mobile station  100  and the base station  200  is completed, the mobile station  100  and the base station  200  may end the dynamic cycle mode. Then, the mobile station  100  and the base station  200  may perform communications according to the regular intermittent cycle. 
     The mobile station  100  according to an embodiment of the present invention is described with reference to  FIG. 3 . 
     The mobile station  100  according to an embodiment of the present invention includes a transmission/reception part  102 . The transmission/reception part  102  receives feedback signals transmitted from the base station  200  during the period of performing data reception/transmission in the intermittent cycle. The feedback signals may include information indicating whether traffic exists in the next period of performing data reception/transmission. 
     The mobile station  100  according to an embodiment of the present invention includes a communication determining part  104 . The communication determining part  104  determines whether the mobile station  100  can communicate (communication-able/communications-not able) with the base station  200 . In a case where the communication determining part  104  determines that the mobile station  100  cannot communicate with the base station  200 , the communication determining part  104  reports to the below-described communication control part  108  that the mobile station  100  cannot communicate with the base station  200 . 
     The mobile station  100  according to an embodiment of the present invention also includes a traffic detecting part  106 . The traffic detecting part  106  detects traffic. The traffic detecting part  106  inputs information indicating existence of traffic to the communication control part  108 . 
     Further, the mobile station  100  according to an embodiment of the present invention includes the communication control part  108 . In a case where the communication determining part  104  determines that the mobile station  100  cannot communicate with the base station  200 , the communication control part  108  performs control for stopping the feedback to the base station  200 . Further, in a case where the communication determining part  104  determines that the mobile station  100  cannot communicate and detects traffic in the communications-not able period, the communication control part  108  changes the communications-able period in the intermittent cycle by extending the communications-able period in the intermittent cycle. 
     Further, the communication control part  108  may change a previous cycle of the intermittent operation where feedback signals are not transmitted by shortening the previous cycle of the intermittent operation according to traffic of the base station  200 . Further, the communication control part  108  may change the cycle of the intermittent operation where feedback signals are not transmitted by shortening the cycle of the intermittent operation according to existence of traffic in the mobile station  100 . 
     Further, the communication determining part  104  may determine that the mobile station  100  cannot communicate with the base station  200  where the mobile station  100  cannot receive feedback signals from the base station  200 . Further, the communication determining part  104  may determine that the mobile station  100  cannot communicate with the base station  200  depending on the strength of the radio waves received from the base station  200 . 
     Further, in a case where traffic has been eliminated as a result of communicating by changing the intermittent cycle, the communication control part  108  may end the dynamic cycle mode and return to the original intermittent cycle. 
     The base station  200  according to an embodiment of the present invention is described with reference to  FIG. 3 . 
     The base station  200  according to an embodiment of the present invention includes a transmission/reception part  202 . The transmission/reception part  202  receives feedback signals (feedback responses) transmitted from the mobile station  100  during the period of performing data reception/transmission in the intermittent cycle. The feedback signals may include information indicating whether traffic exists in the next period of performing data reception/transmission. 
     The base station  200  also includes a communication determining part  204 . The communication determining part  204  determines whether the base station  200  can communicate with the mobile station  100 . In a case where the communication determining part  204  determines that the base station  200  cannot communicate with the mobile station  100 , the communication determining part  204  reports to the below-described communication control part  208  that the base station  200  cannot communicate with the mobile station  100 . 
     The base station  200  according to an embodiment of the present invention also includes a traffic detecting part  206 . The traffic detecting part  206  detects traffic destined to the mobile station  100 . The traffic detecting part  206  inputs information indicating existence of traffic to the communication control part  208 . 
     Further, the base station  200  according to an embodiment of the present invention includes the communication control part  208 . In a case where the existence of traffic is reported from the traffic detecting part  206 , the communication control part  208  changes a feedback signal transmitting period (period of receiving a feedback response from the mobile station  100 ) in the communications-able period in the intermittent cycle by extending the communications-able period in the intermittent cycle. Further, the communication control part  208  may change a previous cycle of the intermittent operation where feedback signals are not transmitted by shortening the previous cycle of the intermittent operation according to standby traffic of the base station  200 . Further, the communication control part  208  may change the cycle of the intermittent operation where feedback signals are not transmitted by shortening the cycle of the intermittent operation according to the standby traffic of the mobile station  100 . 
     Further, the communication determining part  204  may determine that the base station  200  cannot communicate with the mobile station  100  where the base station  200  cannot receive feedback signals from the mobile station  100 . 
     Further, the communication determining part  204  may determine that the base station  200  cannot communicate with the mobile station  100  depending on the strength of the radio waves received from the mobile station  100 . 
     Further, in a case where traffic has been eliminated as a result of communicating by changing the intermittent cycle, the communication control part  208  may end the dynamic cycle mode and return back to the original intermittent cycle. 
     (Configuration of Communication System) 
     A communication system according to an embodiment of the present invention is described with reference to  FIG. 4 . 
     In the communication system  1000 , the mobile station  100  and the base station  200  negotiate a period(s) for performing data reception/transmission and a period(s) for not performing data reception/transmission while maintaining a logical connection. A communication system that performs communications under this condition includes, for example, WiMAX (Worldwide Interoperability for Microwave Access). Thus, in the following, WiMAX is used to describe the communication system  1000  according to an embodiment of the present invention. The communication system according to an embodiment of the present invention may be applied to other communication systems besides WiMAX as long as communications are performed under the above-described condition. With WiMAX, the base station  200  and the mobile station  100  may communicate based on, for example, IEEE 802.16e-2005 or its succeeding standards. With WiMAX, the period for performing transmission/reception of data may be referred to as a “listening window”, and the period for not performing transmission/reception of data may be referred to as a “sleep window”. With WiMAX, the mode in which such operations between the mobile station  100  and the base station  200  may be referred to as a “sleep mode”. 
     With WiMAX, a TDD (Time Division Duplex) method is used. With the TDD method, full duplex communications are performed by transmitting uplink signals and downlink signals with the same frequency bandwidth and switching uplink and downlink at high speed. The transmission frame of the TDD method includes a downlink subframe for transmitting downlink signals and an uplink subframe for transmitting uplink signals. Further, OFDM (Orthogonal Frequency Division Multiplexing) and OFDMA (Orthogonal Frequency Division Multiple Access) are applied to the communication system  1000  according to an embodiment of the present invention. 
     The communication system  1000  according to an embodiment of the present invention includes the base station  200 . Further, the communication system  1000  according to an embodiment of the present invention includes the mobile station  100 . The base station  200  and the mobile station  100  perform wireless communications with the TDD method. The transmission frame of the TDD method includes a downlink subframe (DL Subframe) and an uplink subframe (UL Subframe). A single frame is composed of a pair of the downlink subframe and the uplink subframe. The downlink subframe includes a preamble (Preamble), a frame control header (FCH: Frame Control Header), a DL-MAP, a UL-MAP, and a downlink burst (DL burst). The downlink burst may be classified (divided) into plural regions. The uplink subframe includes a ranging region and an uplink burst (UL burst). The uplink burst may be classified (divided) into plural regions. 
     The mobile station  100  transmits an initial ranging code by using a ranging region assigned beforehand in the downlink subframe. The initial ranging code may be referred to as a ranging request (RNG-REQ: Ranging Request). 
     Although a single mobile station  100  is illustrated in  FIG. 4 , there may be plural mobile stations  100 . Further, although a single base station  200  is illustrated in  FIG. 4 , there may be plural base stations  200 . 
     The mobile station  100  according to an embodiment of the present invention communicates with the base station  200  using an operation mode referred to as “sleep mode”. In this embodiment, an example where the mobile station  100  passes through a group of buildings is used to describe a case where radio wave reception conditions between the base station  200  and the mobile station  100  change. The below-described method according to an embodiment of the present invention may be applied to other cases where radio wave reception conditions between the base station  200  and the mobile station  100  change. 
     In a case where the mobile station  100  becomes hidden behind a building from the standpoint of the base station  200 , radio waves from the base station  200  may not reach the mobile station  100 . 
     In  FIG. 4 , the mobile station  100  moves from the left side to the right side of the drawing. In a case where the mobile station  100  is located in section A, C, or E of  FIG. 4 , radio waves from the base station  200  can reach the mobile station  100 . Accordingly, communications between the mobile station  100  and the base station  200  can be performed. In a case where the mobile station  100  is located in section B or D, the mobile station  100  is hidden behind a building from the standpoint of the base station  200 . Accordingly, radio waves from the base station  200  may not be able to reach the mobile station  100 . Thus, communications might not be performed between the mobile station  100  and the base station  200 . 
     The base station  200  according to an embodiment of the present invention is described with reference to  FIG. 5 . 
     The base station  200  according to an embodiment of the present invention includes an air section transmission/reception processing part  252 . The air section transmission/reception processing part  252  converts various messages received from the below-described various processing parts into radio waves that are to be transmitted to the mobile station  100 . Then, the air section transmission/reception processing part  252  transmits the radio waves containing the various messages to the mobile station  100 . Then, the air section transmission/reception processing part  252  extracts the messages included in the radio waves received from the mobile station  100 . Then, the air section transmission/reception processing part  252  inputs the extracted messages to each of the processing parts. 
     The base station  200  according to an embodiment of the present invention includes a network connection processing part  254 . The network connection processing part  254  executes a registration procedure protocol between the base station  200  and the mobile station  100 . The network connection processing part  254  inputs messages to the air section transmission/reception processing part  252  for exchanging control messages with the mobile station  100 . Further, the network connection processing part  254  receives control messages from the mobile station  100 . Even after completing the registration procedure, the network connection processing part  254  periodically exchanges messages with the mobile station  100  for confirming the presence of the mobile station  100 . 
     The base station  200  according to an embodiment of the present invention includes a backbone transmission/reception part  256 . The backbone transmission/reception part  256  performs reception/transmission of data for exchanging messages with other base stations via a backbone network. 
     The base station  200  according to an embodiment of the present invention includes a sleep protocol processing part  258 . The sleep protocol processing part  258  exchanges protocol messages with the mobile station  100  so that the base station  200  and the mobile station  100  can synchronize and perform intermittent communications with each other. The protocol message exchanged with the mobile station  100  may include a message for performing negotiations for intermittent communications. The message for performing negotiations for intermittent communications may include a combined period having both a period of performing data transmission/reception and a period of not performing data transmission/reception where the dynamic cycle mode is not used. Further, the message for performing negotiations for intermittent communications may include a period of performing data transmission/reception and/or a period of not performing data transmission/reception where the dynamic cycle mode is not used. Further, the protocol message may also include a start frame for performing negotiations for intermittent communications. 
     The base station  200  according to an embodiment of the present invention includes a message transmission/reception part  260 . The message transmission/reception part  260  receives user data destined to the mobile station from the backbone network and transmits the received user data on the air. For example, the message transmission/reception part  260  receives user data destined to the mobile station  100  from the backbone transmission/reception part  256  and inputs the user data to the air section transmission/reception processing part  252 . Further, the message transmission/reception part  260  receives user data from the mobile station  100  via the air section and transfers the user data to the backbone network. For example, the message transmission/reception part  260  receives user data from the mobile station  100  from the air section transmission/reception part  252  and inputs the user data to the backbone transmission/reception part  256 . Further, the message transmission/reception part  260  determines the existence of traffic destined for the mobile station  100 . 
     The base station  200  according to an embodiment of the present invention includes a radio wave strength measuring part  262 . The radio wave strength measuring part  262  measures the strength of the radio waves with respect to the location of the mobile station  100  transmitting signals in a frame. Further, the radio wave strength measuring part  262  identifies the radio wave strength of the uplink transmitted by the mobile station  100 . 
     The base station  200  according to an embodiment of the present invention includes a feedback transmission/reception part  264 . The feedback transmission/reception part  264  receives feedback signals transmitted from the mobile station  100  during the period of data transmission/reception. Further, the feedback transmission/reception part  264  instructs the below-described communication control part  266  to change the intermittent cycle depending on whether there are any feedback signals (existence of feedback signals). 
     The base station  200  according to an embodiment of the present invention includes a communication control part  266 . The communication control part  266  dynamically changes the intermittent cycle. For example, the communication control part  266  may dynamically change the intermittent cycle according to the existence of traffic. For example, the communication control part  266  may dynamically change the intermittent cycle depending on whether there are any data destined for the mobile station  100 . In a case where the communication control part  266  dynamically changes the intermittent cycle, the period of performing data transmission/reception and/or the period of not performing data transmission/reception may be changed. Further, in a case where the communication control part  266  dynamically changes the intermittent cycle, the combined period having both the period of performing data transmission/reception and the period of not performing data transmission/reception (i.e. cycle) may be changed. Further, in a case where the communication control part  266  dynamically changes the intermittent cycle, the combined period (i.e. cycle) may be changed together with changing the period of performing data transmission/reception and/or the period of not performing data transmission/reception. For example, in a case where the message transmission/reception part  260  reports the existence of traffic to the communication control part  266 , the communication control part  266  may change the intermittent cycle so that the period of performing data transmission/reception for receiving the feedback signals (feedback response) from the mobile station  100  is extended. Further, in a case where there is no standby traffic destined for the mobile station  100 , the communication control part  266  changes the intermittent cycle by shortening the period where no feedback is received. 
     Further, the communication control part  266  may shorten the intermittent cycle during the period of receiving no feedback according to the existence of standby traffic of a previous period. 
     Further, in a case where a feedback response cannot be received from the mobile station  100 , the communication control part  266  may determine that the base station  200  cannot communicate with the mobile station  100 . 
     Further, the communication control part  266  may determine that the base station  200  cannot communicate with the mobile station  100  according to the results of measuring the strength of the radio waves received from the mobile station  100 . 
     Further, in a case where the traffic is eliminated (i.e. transmission of standby traffic is completed) as a result of communicating with the changed cycle, the communication control part  266  may terminate the dynamic cycle mode and return to its initial intermittent cycle. For example, the communication control part  208  may return to its regular cycle. 
     The base station  200  according to an embodiment of the present invention includes a cycle information table  268 . The cycle information table  268  contains information of the combined period (cycle) having both the period of performing data transmission/reception and the period of not performing data transmission/reception in a case where the dynamic cycle mode is not used in the communication between the base station  200  and the mobile station  100 . The combined period may be plural. The information of the combined period (cycle) may include information indicating the period of performing data transmission/reception. The information indicating the period of performing data transmission/reception may be plural. Further, the information of the combined period (cycle) may include information indicating the period of not performing data transmission/reception. The information indicating the period of not performing data transmission/reception may be plural. 
     Next, the mobile station  100  according to an embodiment of the present invention is described with reference to  FIG. 6 . 
     The mobile station  100  according to an embodiment of the present invention includes an air section message transmission/reception processing part  152 . The air section message transmission/reception processing part  152  synchronizes with the frames from the base station  200  and performs transmission/reception of frames in accordance with conditions set for connecting with the base station  200 . 
     The mobile station  100  according to an embodiment of the present invention includes a radio wave strength measuring part  154 . The radio wave strength measuring part  154  measures the strength of radio waves based on synchronizing frames. 
     The mobile station  100  according to an embodiment of the present invention includes a network connection processing part  156 . The network connection processing part  156  processes protocol messages used for enabling the mobile station  100  to connect to a network and exchanges the protocol messages with the base station  200 . 
     The mobile station  100  according to an embodiment of the present invention includes a sleep protocol processing part  158 . The sleep protocol processing part  158  exchanges protocol messages with the base station  200  so that the mobile station  100  and the base station  200  can synchronize and perform intermittent communications with each other. The protocol message exchanged with the mobile station  100  may include a message for performing negotiations for intermittent communications. Further, the protocol message may also include a start frame for performing negotiations for intermittent communications. 
     The mobile station  100  according to an embodiment of the present invention includes a communication control part  160 . The communication control part  160  permits transmission/reception of messages in a communication-able period according to the intermittent cycle negotiated between the sleep protocol processing part  158  and the base station  200 . Further, the communication control part  160  stops transmission/reception of messages in an communication-not able period according to the intermittent cycle. Further, the communication control part  160  controls the power and/or the clock of each processing part of the mobile station  100  together with stopping the transmission/reception of messages. 
     The mobile station  100  according to an embodiment of the present invention includes a feedback transmission/reception part  162 . The feedback transmission/reception part  162  receives feedback signals transmitted from the base station  200  during the communication-able period. The feedback transmission/reception part  162  instructs the below-described communication control part  160  to change the intermittent cycle depending on whether there is any reception of feedback signals (existence of feedback signals). 
     The mobile station  100  according to an embodiment of the present invention includes a cycle information table  164 . The cycle information table  164  contains information of the combined period (cycle) having both the period of performing data transmission/reception and the period of not performing data transmission/reception in a case where the dynamic cycle mode is not used in the communication between the base station  200  and the mobile station  100 . The combined period may be plural. The information of the combined period (cycle) may include information indicating the period of performing data transmission/reception. The information indicating the period of performing data transmission/reception may be plural. Further, the information of the combined period (cycle) may include information indicating the period of not performing data transmission/reception. The information indicating the period of not performing data transmission/reception may be plural. Further, the cycle information table  164  contains information of the combined period (cycle) having combined the period of performing data transmission/reception and the period of not performing data transmission/reception in a case where the dynamic cycle mode is used in the communications between the base station  200  and the mobile station  100 . The combined period may be plural. The information of the combined period (cycle) may include information indicating the period of performing data transmission/reception. The information indicating the period of performing data transmission/reception may be plural. Further, the information of the combined period (cycle) may include information indicating the period of not performing data transmission/reception. The information indicating the period of not performing data transmission/reception may be plural. 
     The mobile terminal  100  according to an embodiment of the present invention includes a message transmission/reception part  166 . The message transmission/reception part  166  transmits user data destined for other mobile stations over the air. For example, the message transmission/reception part  166  inputs user data destined for other mobile stations to the air section transmission/reception processing part  152 . Further, the message transmission/reception part  166  receives user data of other mobile stations received from the base station  200  via the air section  152 . For example, the message transmission/reception part  166  inputs the user data of other mobile stations received from the air section transmission/reception processing part  152 . Further, the message transmission/reception part  166  determines whether there is traffic destined for other mobile stations. 
     Next, an operation of the communication system  1000  according to an embodiment of the present invention is described. 
     In the following, there are described examples where the mobile station  100  moves from section A to section E as illustrated in  FIG. 4 . That is, exemplary cases where the mobile station  100  is located in section A, section B, section C, and section E are separately described below. A case where the mobile station  100  is located in section D is substantially the same as a case where the mobile station  100  is located in section B. 
     The case where the mobile station  100  is located in section A is referred to as a “first phase”. In the first phase, negotiations for changing the dynamic intermittent communication mode between the base station  200  and the mobile station  100  are performed. 
     The case where the mobile station  100  is located in section B is referred to as a “second phase”. In the second phase, radio waves become unstable during communications between the base station  200  and the mobile station  100 . In this second phase, it is determined that communications cannot be performed. In the second phase, the intermittent cycle is changed. 
     The case where the mobile station  100  is located in section C is referred to as a “third phase”. In the third phase, radio waves are also unstable during communications between the base station  200  and the mobile station  100 . However, in this third phase, it is determined that communications can be performed. In the third phase, the intermittent cycle changed in the second phase is maintained. 
     The case where the mobile station  100  is located in section E is referred to as a “fourth phase”. In the fourth phase, radio waves become stable during communications between the base station  200  and the mobile station  100 . In this fourth phase, a predetermined dynamic cycle mode is used for the communications between the base station  200  and the mobile station  100 . 
     (First Phase) 
     The first phase is described with reference to  FIG. 7 . 
     In the first phase, negotiations between the mobile station  100  and the base station  200  are performed for starting an intermittent communication operation. As described above, the mobile station  100  performs intermittent communication with the base station  200  at an area (section) in which regular communications can be performed. For example, negotiations for starting an intermittent communication operation are performed between the communication control part  160  of the mobile station  100  and the communication control part  266  of the base station  200 . The negotiations for starting the intermittent communication operation include negotiation of the dynamic cycle mode. 
     The mobile station  100  starts the negotiation of the dynamic cycle mode (Step S 702 ). For example, the communication control part  160  starts the negotiation of the dynamic cycle mode with a predetermined trigger. The predetermined trigger may include initiation of an application which does not require a significantly wide bandwidth in an upper layer but has a degrading of sound quality unless messages are received at predetermined intervals. The application may be, for example, VoIP (Voice over Internet Protocol). In this case, the communication control part  160  starts the dynamic cycle mode. The application may be associated with the dynamic cycle mode. The upper layer may include a process that is executed by a personal computer (PC). 
     Further, the communication control part  160  may determine whether to start the negotiation of the dynamic cycle mode according to the strength of the radio waves measured by the radio wave strength measuring part  154 . For example, in a case where the communication control part  160  determines the strength of the radio waves is not so weak as to disconnect communications with the base station  200  according to the value of the measured radio wave strength but detects that downlink signals from the base station  200  cannot be received for a number of times, the communication control part  160  may determine that the mobile station  100  is located in an area in which radio wave reception is unstable. The downlink signal may include, for example, a DL-MAP. In a case where the communication control part  160  determines that the mobile station  100  is located in an area in which radio wave reception is unstable, the communication control part  160  may determine to change communication with the base station  200  to the dynamic cycle mode. In this case, the communication control part  160  may start the dynamic cycle mode. 
     Further, the communication control part  160  may start the dynamic cycle mode according to an explicit instruction from the user of the mobile station  100 . The user of the mobile station  100  may determine whether the location of the mobile station  100  is difficult for radio waves to reach, for example, by observing the surroundings of the user. In a case where the user desires to continue communication over the application which is currently being used by the user at higher quality, the user can explicitly instruct such a desire to the mobile station  100 . The application may include, for example, an internet television or an internet radio using IP communications. 
     In a case where the mobile station  100  determines to start the dynamic cycle mode, the mobile station  100  transmits a sleep request message (MOB_SLP-REQ) to the base station  200  (Step S 704 ). The sleep request message may include a frame number for starting the dynamic cycle mode. The sleep request message may include information of the intermittent cycle to be dynamically changed. The power saving class to be used when performing intermittent communication can be designated. For example, the power saving class to be used may be selected from plural types of power saving classes. For example, in addition to the above-described three types of power saving classes (PSC types  1 - 3 ) which exhibit different behaviors (processes), a fourth type of power saving class may be prepared. Thus, the four types of power saving classes may be referred to as power saving class types  1 - 4  (PSC types  1 - 4 ). 
     Accordingly, the mobile station  100  selects a power saving class depending on the intermittent communication desired to be performed. Further, the mobile station  100  sets the intermittent cycle. Further, the mobile station  100  generates a definition(s) of the intermittent cycle. The definition of the intermittent cycle may be referred to as a power saving class (PSC) instance. The mobile station  100  can define different power saving class instances for the power saving classes even where the power saving classes are the same. For example, the mobile station may define different VoIP sessions in correspondence with plural PSCs having different intermittent cycles. 
     In IEEE 802.16e, a connection in communications is distinguished according to identifiers. The identifier may be referred to as a connection ID (CID, connection identifier). The mobile terminal  100  maps the CID on the PSC instance. Accordingly, the mobile station  100  can negotiate the correspondence between CIDs and the behavior defined in each PSC with the base station  200 . In other words, the mobile station  100  can negotiate with the base station  200  in terms of the power saving class to be applied to the connection corresponding to the CID. Further, the mobile station  200  can negotiate the intermittent cycle to be used in communication with the base station  200  based on which CID is mapped to the intermittent cycle defined in the PSC instance. In other words, the mobile station  100  can negotiate with the base station  200  in terms of the intermittent cycle to be applied to the connection corresponding to the CID. 
     As one example of the information included in the sleep request, the fields of the MOB_SLP-REQ of IEEE 802.16e are described with reference to  FIG. 8 . 
     The MOB_SLP-REQ message may include a management message type (“Management Message Type”). The management message type is a field for indicating that the message type is a MOB_SLP-REQ message. For example, the management message type is defined as 0x50 according to IEEE 802.16e-2005. 
     The MOB_SLP-REQ message may include number of classes (“Number of Classes”). The number of classes indicates the number of PSC instances defined in the MOB_SLP-REQ message. 
     The MOB_SLP-REQ message may include a definition (“Definition”). The definition indicates whether a definition of a PSC instance is included in the MOB_SLP-REQ message. For example, in a case where the definition is indicated as “1”, a definition is included in the MOB_SLP-REQ message. In a case where the definition is indicated as “0”, a definition is not included in the MOB_SLP-REQ message. For example, the definition may be used to activate (start intermittent communication) or deactivate (temporarily stop intermittent communication) a PSC instance that is already defined in a case where the definition is indicated as 0. 
     The MOB_SLP-REQ message may include an operation (“Operation”). The operation is for instructing to temporarily stop intermittent communication or to start intermittent communication. For example, in a case where the operation is indicated as “0”, deactivation of intermittent communication is instructed. In a case where the operation is indicated as “1”, activation of intermittent communication is instructed. 
     The MOB_SLP-REQ message may include a power saving class ID (“Power_Saving_Class_ID”). The power saving class ID is a number for identifying a unique PSC instance inside the mobile station  100 . 
     The MOB_SLP-REQ message may include a start frame number (“Start Frame Number”). The start frame number is a number for indicating the starting frame of the first sleep window. 
     The MOB_SLP-REQ message may include a power saving class type (“Power Saving Class Type”). The power saving class type may indicate the above-described types of power saving classes (e.g., 0b01=Type I, 0b10=Type II, 0x11=Type III). Further, the power saving class type may also indicate a newly defined power saving class type (e.g., 0b100=Type IV). 
     The MOB_SLP-REQ message may include direction (“Direction”). The direction indicates the direction of the CID included in a PSC. For example, “0b00” indicates no designation of direction and complies to the direction of the CID assigned to the CID when the CID is generated. The direction may also indicate an uplink (UL) direction, a downlink (DL) direction, or both directions. For example, “0b01” may indicate designation of the DL direction, and “0b10”, may indicate designation of the UL direction. 
     The MOB_SLP-REQ message may include a traffic indication request (“TRF-IND_required”). The traffic indication request is a bit for designating transmission of a MOB_TRF-IND message in a case where the base station  200  finds traffic destined to the mobile station  100  in PSC Type  1 . In a case where the traffic indication request is indicated as “1”, traffic indication is requested. Although an example of the PSC Type  2  is described in this embodiment, other types of power saving classes may be used. 
     The MOB_SLP-REQ message may include a traffic triggered wakening flag (“Traffic Triggered Wakening flag”). In a case where traffic destined to the mobile station  100  is generated in the PSC Type  1 , the traffic triggered wakening flag designates whether to deactivate the PSC instance in the middle of performing intermittent communication. For example, in a case where the traffic triggered wakening flag is indicated as “0”, the PSC instance may not deactivated, and in a case where the traffic triggered wakening flag is indicated as “1”, the PSC instance may be deactivated. Although an example of the PSC Type  2  is described in this embodiment, other types of power saving classes may be used. 
     The MOB_SLP-REQ message may include an initial sleep window (“Initial Sleep Window”). The initial sleep window designates the length of an initial sleep window (period of performing no data transmission/reception, communication impossible period) during intermittent communication in frame units. 
     The MOB_SLP-REQ message may include a listening window (“Listening Window”). The listening window designates the length of a listening window (period of performing data transmission/reception, communication possible period) during intermittent communication in frame units. 
     The MOB_SLP-REQ message may include a final sleep window base (“Final sleep window base”). In the final sleep window base, the maximum value of the Sleep window of the PSC Type  1  and the PSC Type  3  is expressed as final-sleep window=final-sleep window base×2^(final-sleep window exponent). In this embodiment, since an example of the PSC Type  2  is described, the value of the field “Final sleep window base” is designated as “0”. 
     The MOB_SLP-REQ message may include a final sleep window exponent (“Final sleep window exponent”). In the final sleep window exponent, the maximum value of the Sleep window of the PSC Type  1  and the PSC Type  3  is expressed as final-sleep window=final-sleep window base×2^(final-sleep window exponent). In this embodiment, since an example of the PSC Type  2  is described, the value of the field “Final sleep window exponent” is designated as “0”. 
     The MOB_SLP-REQ message may include an unstable unavailable listening window (Unstable unavailable listening window). In this embodiment, the unstable unavailable listening window indicates the value of the listening window to be changed when communication becomes unstable during the dynamic cycle mode. That is, the unstable unavailable listening window indicates the value of the listening window in a case where radio wave reception is unstable and communication cannot be performed during the dynamic cycle mode. 
     The MOB_SLP-REQ message may include unstable unavailable sleep window (Unstable unavailable sleep window). In this embodiment, the unstable unavailable sleep window indicates the value of the sleep window to be changed when communication becomes unstable during the dynamic cycle mode. That is, the unstable unavailable sleep window indicates the value of the sleep window in a case where radio wave reception is unstable and communication cannot be performed during the dynamic cycle mode. 
     The MOB_SLP-REQ message may include unstable available listening window (Stable available listening window). In this embodiment, the unstable available listening window indicates the value of the listening window to be changed when communication temporarily becomes stable during the dynamic cycle mode. That is, the unstable available listening window indicates the value of the listening window in a case where radio wave reception is unstable while communication can be performed during the dynamic cycle mode. The value of the listening window in this case may include a value indicating a case where there is standby traffic for the mobile station  100  and/or a value indicating a case where there is no standby traffic for the mobile station  100 . 
     The MOB_SLP-REQ message may include an unstable available sleep window (Stable available sleep window). In this embodiment, the unstable available sleep window indicates the value of the sleep window to be changed when communication temporarily becomes unstable during the dynamic cycle mode. That is, the unstable available sleep window indicates the value of the sleep window in a case where radio wave reception is unstable while communication can be performed during the dynamic cycle mode. The value of the sleep window in this case may include a value indicating a case where there is standby traffic for the mobile station  100  and/or a value indicating a case where there is no standby traffic for the mobile station  100 . 
     The MOB_SLP-REQ message may include the number of connection identifiers (Number of CIDs). The number of CIDs indicates the number of CIDs mapped by the PSC instance. The number of CIDs designated may be plural. 
     The MOB_SLP-REQ message may include a connection ID (CID). The connection ID indicates the value of the CID mapped by the PSC instance. The CIDs designated may be plural. 
     Among the information included in the sleep request, the listening window, the final sleep window base, the unstable unavailable listening window, the unstable unavailable sleep window, the unstable available listening window, and the unstable available sleep window may be selected from the information stored in the cycle information table  164 . For example, the cycle information table  164  includes a power saving class ID (PSC ID) and listening and sleep windows corresponding to each power saving class ID that are decided according to communication conditions. The communication conditions include, for example, a case where communications can be performed normally (regular communication). Further, the communication conditions include, for example, a case where communications can be performed in a condition where reception of radio waves is unstable (radio wave unstable communication possible). Further, the communication conditions include, for example, a case where communications cannot be performed in a condition where reception of radio waves is unstable (radio wave unstable communication impossible). Further, in the case where communication condition is “radio wave unstable communication possible”, the listening window and the sleep window may be divided depending on a case where there is traffic destined to other mobile stations and another case where there is no traffic destined to other mobile stations. 
     The base station  200  receiving the MOB_SLP-REQ determines whether the PSC instance requested by the mobile station  100  can be approved. In a case where the PSC instance can be approved, the base station  200  registers the parameters of the PSC instance requested by the mobile station  100  in the cycle information table  268  (Step S 706 ). For example, the PSC ID of the mobile station  100  and the intermittent communication cycle corresponding to the communication condition of the mobile station  100  may be registered for each mobile station  100  in the cycle information table  268 . For example, the communication condition may include a case where communication can be performed normally (regular communication). Further, the communication condition may include a case where reception of radio waves is unstable but communication can be performed (radio wave unstable communication possible). Further, the communication condition may include a case where reception of radio waves is unstable and communication cannot be performed (radio wave unstable communication impossible). 
     Further, periods of the listening window and the sleep window during the intermittent reception cycle may be registered in correspondence with the communication conditions. Further, different listening windows and sleep windows may be registered according to a case where there is traffic transmitted from the mobile station  100  to the base station  200  and a case where there is no traffic transmitted from the mobile station  100  to the base station  200 . 
     Then, the base station  200  transmits a sleep response message (MOB_SLP-RSP) to the mobile station  100  (Step S 708 ). 
     As one example of the information included in the sleep response request, the fields of the MOB_SLP-RSP of IEEE 802.16e are described with reference to  FIG. 10 . In the following, information elements different from those included in the above-described MOB_SLP-REQ (see  FIG. 8 ) are described. With respect to the information items which are the same as those included in the above-described MOB_SLP-REQ, the values the same as those of the MOB_SLP-REQ may be stored in the sleep response message and transmitted to the mobile station  100 . 
     The MOB_SLP-RSP message may include the length of data (Length of Data). The length of data may include the total size of the PSC definition parameters written inside the “for” loop. 
     The MOB_SLP-RSP message may include sleep approval (Sleep Approved). The sleep approval indicates whether the PSC instance requested by the mobile station  100  has been approved by the base station  200 . For example, “1” of the sleep approval indicates that the PSC instance requested by the mobile station  100  is approved by the base station  200 , and “0” of the sleep approval indicates that the PSC instance requested by the mobile station  100  is not approved by the base station  200 . 
     Then, the mobile station  100  and the base station  200  start intermittent communication in accordance with the sleep response message MOB_SLP-RSP. For example, the listening window is started (Step S 710 , S 712 ). Alternatively, the intermittent communication may be started from the initial sleep window. Whether to start from the initial listening window or the initial sleep window is decided by the negotiation between the mobile station  100  and the base station  200 . This embodiment describes a case where the intermittent communication is started from the start of the listening window. For example, the mobile station  100  and the base station  200  may start the initial listening window from a frame matching the Start Frame Number included in the sleep response message. In the example of  FIG. 7 , the Start Frame Number is frame #0x85. The base station  200  starts a period Ta 0  that monitors the duration of the listening window. 
     In  FIGS. 7 ,  13 ,  14 , and  15 , the reference number “Ta 0 ” represents the same timer. The hatching area illustrated in  FIGS. 7 ,  13 ,  14 , and  15  represent the listening window. 
     Further, in the sleep response message, “Frame Offset” indicates that the offset between the frame in which the mobile station  100  receives a feedback signal from the base station  200  and a frame in which the mobile station  100  transmits a feedback response to the base station  200 . 
     Further, in the sleep response message, “OFDMA symbol offset”, “Subchannel offset”, “No. OFDMA symbols”, and “No. subchannel” indicate feedback information transmission location of the frame in which the feedback response is transmitted. 
     Then, the base station  200  generates a feedback signal to be transmitted to the mobile station  100  (Step S 714 ). 
     For example, as illustrated in  FIG. 11 , the feedback signal may include a format (FMT) used for a traffic indication message as illustrated in  FIG. 11 . The FMT indicates switching between instructing deactivation of a PSC with a SLPID bitmap and instructing deactivation of a PSC by directly designating a SLPID. 
     The feedback signal may include a sleep ID group indication bitmap (SLPID-Group Indication Bitmap). In this embodiment, the SLPIDs are 0 through 1023. The SLPIDs are divided into 32 SLPID groups. The SLPID indicate the existence of traffic with respect to each of the groups. For example, among 32 bits, the most significant bit (MSB) may be indicated as “group 0” and the least significant bit (LSB) may be indicated as “group 32”. The mobile station  100  including a PSC of a given group is to refer to the next traffic indication bitmap in a case where “1” is set as the bit of the group. 
     The feedback signal may include a traffic indication bitmap (Traffic Indication Bitmap). The traffic indication bitmap indicates traffic with respect to each PSC included in the SLPID group indicated as having traffic by the SLPID. Each group includes 32 PSCs. The traffic indication bitmap indicates the existence of traffic with respect to each of the PSCs with a field of 32 bits. The 32 bit field is indicated in correspondence with the number of SLPID groups set as “1”. 
     The feedback signal may include “Num_pos”. The “Num_pos” indicates the number of SLPIDs indicating as having traffic. 
     The feedback signal may include “SLPIDs”. The “SLPIDs” indicates the IDs of the PSCs indicated as having traffic. For example, the SLPIDs are arranged in a number corresponding to the number indicated by the “Num_pos”. 
     In this embodiment, the feedback signal may include “Num_dynamic_PSC”. The “Num_dynamic_PSC” indicates the number of PSCs to which the PSC Type  4  is set as the PSC type. 
     In this embodiment, the feedback signal may include next traffic prediction (“Next Traffic Prediction”). The next traffic prediction indicates whether there is traffic with respect to a PSC of a SLPID. For example, “1” indicates the existence of traffic, and “0” indicates no existence of traffic. 
     In this embodiment, an arbitrary value is assigned to the information elements included in the fields except for the fields “Num_dynamic_PSC”, “for” “SLPID”, and “Next traffic prediction” according to circumstance. 
     Then, the base station  200  broadcasts the feedback signal to the mobile station  100  in the listening window (Step S 716 ). The listening window may also be referred to as “listening interval”. 
     Then, the mobile station  200  determines whether it has received a feedback signal from the base station  200  (Step S 718 ). In this embodiment, since the mobile station  100  can receive the feedback signal (Yes in Step S 718 ), no change of intermittent cycle is performed (Step S 720 ). Thus, the mobile station  100  maintains the current intermittent cycle. 
     Then, the mobile station  100  generates a feedback response to be transmitted to the base station  200  in response to the feedback signal. For example, the mobile station  100  may generate and transmit the feedback response according to the MOB_SLP-RSP described above with reference to  FIG. 10 . For example, the mobile station  100  may transmit a feedback channel at a frame location defined in the frame designated by the MOB_SLP-RSP message. The feedback channel may include a feedback code(s). As illustrated in  FIG. 12 , the feedback code may include a code transmitted when no feedback signal is received, a code transmitted when there is no uplink traffic, and a code transmitted when there is uplink traffic. 
     Then, the mobile station  100  transmits the feedback response to the base station  200  (Step S 722 ). 
     The base station  200  determines whether it has received a feedback response from the mobile station  100  (Step S 724 ). In this step, since the base station  200  can receive the feedback response (Yes in Step S 724 ), no change of intermittent cycle is performed (Step S 726 ). That is, the base station  200  determines that the same cycle as the current intermittent cycle is to be applied to the next intermittent cycle. Thus, the base station  200  maintains the current intermittent cycle. The base station  200 , regardless of the meaning of the code included in the feedback response, determines that the mobile station  100  can receive radio waves transmitted from the base station  200  by the reception of the feedback response. 
     The mobile station  100  and the base station  200  start the Sleep window in period Tu 0  according to the intermittent cycle decided between the mobile station  100  and the base station  200 . In this embodiment, the intermittent cycle includes the listening window of the period Ta 0  and the sleep window of the period Tu 0 . 
     The base station  200  and the mobile station  100  each start the listening window when the period Tu 0  elapses. 
     The base station  200  generates a feedback signal again (Step S 728 ) and transmits the feedback signal to the mobile station  100  (Step S 730 ). 
     In a case where the mobile station  100  receives the feedback signal from the base station  200 , the mobile station  100  determines that the same cycle as the current intermittent cycle is to be applied to the next intermittent cycle (Yes in Step  734 ). 
     Then, the mobile station  100  transmits a feedback response to the base station  200  (Step S 736 ). 
     The base station  200  determines whether it has received a feedback response from the mobile station  100  (Step S 738 ). In this step, since the base station  200  can receive the feedback response (Yes in Step S 738 ), no change of intermittent cycle is performed (Step S 740 ). That is, the base station  200  determines that the same cycle as the current intermittent cycle is to be applied to the next intermittent cycle. Thus, the base station  200  maintains the current intermittent cycle. The base station  200 , regardless of the meaning of the code included in the feedback response, determines that the mobile station  100  can receive radio waves transmitted from the base station  200  by the reception of the feedback response. 
     (Second Phase) 
     The second phase is described with reference to  FIG. 13 . 
     In the second phase, the mobile station  100  moves from section A to section B described above with reference to  FIG. 4 . 
     The base station  200  and the mobile station  100  each start the listening window when the period Tu 0  of the sleep period elapses (Step S 1302 ). For example, in a case where the mobile station  100  is located in section A, the base station  200  continues the sleep window for a period Tu 0 . Then, the base station  200  broadcasts a feedback signal (feedback message) to the mobile station  100  in the listening window when the period Tu 0  elapses (Step S 1304 ). 
     Then, the mobile station  100  determines whether it has received a feedback signal from the base station  200  (Step S 1306 ). In a case where the mobile station  100  moves to section B, the feedback signal transmitted from the base station  200  cannot reach the mobile station  100 . That is, since the mobile station  100  becomes hidden behind a building from the standpoint of the base station  200 , the mobile station  100  cannot receive the feedback signal transmitted from the base station  200 . Since the mobile station  100  cannot receive the feedback signal from the base station  200  (No in Step S 1306 ), the mobile station  100  determines that the mobile station  100  is located in an area in which radio wave reception is unstable. Accordingly, the mobile station  100  refrains from transmitting a feedback response to the base station  200  (Step S 1308 ). The mobile station  100  starts an operation dedicated to a case where the mobile station  100  is located in an area in which radio wave reception is unstable. More specifically, the mobile station  100  sets the intermittent cycle to listening window Ta 1  and sleep window Tu 1  of the communication condition “radio wave unstable communication impossible” (described above with reference to  FIG. 9 ). Further, in a case where the radio wave reception condition temporarily recovers during the listening window to allow frames to be transmitted in the UL direction, the mobile station  100  may transmit a feedback channel (feedback response) to the base station  200 . The feedback channel may include a code indicating that there is no feedback signal such as the code “0b0000” described above with reference to  FIG. 12 . 
     On the other hand, the base station  200  determines whether it has received a feedback response from the mobile station  100  (Step S 1310 ). Since the base station  200  receives no feedback response from the mobile station  100 , the base station  200  determines that the mobile station  100  is unable to stably receive radio waves from the base station  200  (No in Step S 1310 ). In this case, the base station  200  decides to change the intermittent cycle (Step S 1312 ). For example, the base station  200  sets the intermittent cycle to a cycle where the communication condition is radio wave unstable and communication impossible with respect to the mobile station  100 . More specifically, the base station  200  sets the intermittent cycle to listening window Ta 1  and listening window Tu 1  of the communication condition “radio wave unstable communication impossible” (described above with reference to  FIG. 9 ). 
     The base station  200  starts a sleep window (Tu 1 ) according to the newly set cycle after the current listening window ends. In the cycle where radio wave reception is unstable and communication is impossible, the listening window is shorter compared to the cycle during section A. However, since the listening window becomes shorter, the listening window is set to appear more often (appears more frequently). Accordingly, the opportunity for the base station  200  and the mobile station  100  to find each other&#39;s radio waves can be increased. 
     Then, the base station  200  and the mobile station  100  start the sleep window. 
     Then, the base station  200  and the mobile station  100  start the listening window when the period Tu 1  of the sleep window of the communication condition “radio wave unstable communication impossible” elapses (Step S 1314 ). The base station  200  activates a timer. The timer measures the period Ta 1  of the listening window. 
     The base station  200  broadcasts the feedback signal (feedback message) to the mobile station  100  in the listening window (listening interval) (Step S 1316 ). 
     Then, the mobile station  100  determines whether it has received a feedback signal from the base station  200  (Step S 1318 ). Since the mobile station  100  is located in section B, the feedback signal transmitted from the base station  200  cannot reach the mobile station  100 . That is, since the mobile station  100  is hidden behind a building from the standpoint of the base station  200 , the mobile station  100  cannot receive the feedback signal transmitted from the base station  200 . Since the mobile station  100  cannot receive the feedback signal from the base station  200  (No in Step S 1318 ), the mobile station  100  determines that the mobile station  100  is located in an area in which radio wave reception is unstable. Accordingly, the mobile station  100  refrains from transmitting a feedback response to the base station  200  (Step S 1320 ). The mobile station  100  continues the operation dedicated to a case where the mobile station  100  is located in an area in which radio wave reception is unstable. In a case where the radio wave reception condition temporarily recovers during the listening window to allow frames to be transmitted in the UL direction, the mobile station  100  may transmit a feedback channel (feedback response) to the base station  200 . The feedback channel may include a code indicating that there is no feedback signal such as the code “0b0000” described above with reference to  FIG. 12 . 
     On the other hand, the base station  200  determines whether it has received a feedback response from the mobile station  100  (Step S 1322 ). Since the base station  200  receives no feedback response from the mobile station  100 , the base station  200  determines that the mobile station  100  is still unable to stably receive radio waves from the base station  200  (No in Step S 1322 ). In this case, the base station  200  determines to maintain the current intermittent cycle (Step S 1324 ). 
     The base station  200  and the mobile station  100  start a sleep window after the current listening window ends. 
     Then, the base station  200  and the mobile station  100  start the listening window when the period Tu 1  of the sleep window of the communication condition “radio wave unstable communication impossible” elapses (Step S 1326 ). The base station  200  activates the timer. The timer measures the period Ta 1  of the listening window. 
     Then, the base station  200  broadcasts the feedback signal (feedback message) to the mobile station  100  in the listening window (listening interval) (Step S 1328 ). 
     Then, the mobile station  100  determines whether it has received a feedback signal from the base station  200  (Step S 1330 ). Since the mobile station  100  is located in section B, the feedback signal transmitted from the base station  200  cannot reach the mobile station  100 . That is, since the mobile station  100  is hidden behind a building from the standpoint of the base station  200 , the mobile station  100  cannot receive the feedback signal transmitted from the base station  200 . Since the mobile station  100  cannot receive the feedback signal from the base station  200  (No in Step S 1330 ), the mobile station  100  determines that the mobile station  100  is located in an area in which radio wave reception is unstable. Accordingly, the mobile station  100  refrains from transmitting a feedback response to the base station  200  (Step S 1332 ). The mobile station  100  continues the operation dedicated to a case where the mobile station  100  is located in an area in which radio wave reception is unstable. In a case where the radio wave reception condition temporarily recovers during the listening window to allow frames to be transmitted in the UL direction, the mobile station  100  may transmit a feedback channel (feedback response) to the base station  200 . The feedback channel may include a code indicating that there is no feedback signal such as the code “0b0000” described above with reference to  FIG. 12 . 
     On the other hand, the base station  200  determines whether it has received a feedback response from the mobile station  100  (Step S 1334 ). Since the base station  200  receives no feedback response from the mobile station  100 , the base station  200  determines that the mobile station  100  is still unable to stably receive radio waves from the base station  200  (No in Step S 1334 ). In this case, the base station  200  determines to maintain the current intermittent cycle (Step S 1336 ). 
     (Third Phase) 
     The third phase is described with reference to  FIG. 14 . 
     In the third phase, the mobile station  100  moves from section B to section C described above with reference to  FIG. 4 . The mobile station  100  and the base station  200  can temporarily communicate with each other when the mobile station  100  moves from section B to section C. This embodiment describes a case where there is traffic from the mobile station  100  to the base station  200  in an uplink direction. 
     The operation of the mobile station  100  and the base station  200  in a case where the mobile station  100  is located in section B is the same as that described above with reference to  FIG. 13 . That is, the processes performed in Steps S 1402 -S 1404  and Steps S 1408 -S 1416  are the same as those of Steps S 1314 -S 1324 . However, in this embodiment, traffic in an uplink direction is generated where the mobile station  100  is located in section B (Step S 1406 ). 
     The base station  200  detects the elapse of the period Tu 1  of the sleep window (Step S 1418 ). Then, the base station  200  and the mobile station  100  start the listening window. 
     The base station  200  broadcasts a feedback signal (feedback message) to the mobile station  100  in the listening window (listening interval) (Step S 1420 ). 
     The mobile station  100  determines whether it has received a feedback signal from the base station  200  (Step S 1422 ). In this step, the mobile station is able to receive the feedback signal from the base station (Yes in Step S 1422 ). 
     The mobile station  100  generates a feedback response to be transmitted to the base station  200  in response to the feedback signal. For example, the mobile station  100  may generate and transmit the feedback response according to the MOB_SLP-RSP described above with reference to  FIG. 10 . For example, the mobile station  100  may transmit a feedback channel at a frame location defined in the frame designated by the MOB_SLP-RSP message. The feedback channel may include a feedback code(s). As illustrated in  FIG. 12 , the feedback code may include a code transmitted when no feedback signal is received, a code transmitted when there is no uplink traffic, and a code transmitted when there is uplink traffic. In this embodiment, the feedback code includes a code transmitted when there is uplink traffic. 
     Then, the mobile station  100  transmits the feedback response to the base station  200  (Step S 1426 ). 
     The base station  200  determines whether it has received a feedback response from the mobile station  100  (Step S 1428 ). In this step, the base station  200  is able to receive the feedback response (Yes in Step S 1428 ). Accordingly, the base station  200  determines that the mobile station  100  is in a condition of transmitting/receiving radio waves with the base station  200 . Further, the base station  200  determines that uplink traffic is stored in the mobile station  100  according to the feedback response received from the mobile station  100 . 
     Accordingly, the base station  200  decides to change the subsequent intermittent cycle (Step S 1430 ). For example, the base station  200  sets the period of the initial listening window and the initial sleep window so that the listening window of the subsequent intermittent cycle can be broader. This increases the opportunity for the mobile station  100  to transmit data and enables more uplink data to be transmitted. 
     For example, the base station  200  may use the period Ta 2  of the listening window and the period Tu 2  of the sleep window which are applied to case where there is traffic and communication is possible although radio wave reception is unstable. 
     Then, after receiving a feedback signal from the base station  200 , the mobile station  100  changes the subsequent intermittent cycle (Step S 1432 ). For example, the mobile station  100  sets the period of the initial listening window and the initial sleep window so that the listening window of the subsequent intermittent cycle can be broader. This increases the opportunity for the mobile station  100  to transmit data and enables more uplink data to be transmitted. 
     For example, the mobile station  100  may use the period Ta 2  of the listening window and the period Tu 2  of the sleep window which are applied to case where there is traffic and communication is possible although radio wave reception is unstable. 
     Then, the base station  200  and the mobile station  100  start the sleep window. 
     Then, the base station  200  detects the elapse of the period Tu 2  of the sleep window (Step S 1434 ). 
     Then, the base station  200  and the mobile station  100  start the listening window. 
     The base station  200  broadcasts a feedback signal (feedback message) to the mobile station  100  in the listening window (listening interval) (Step S 1436 ). 
     The mobile station  100  transmits the accumulated traffic as much as possible in the period Ta 2  of the listening window longer than the period Ta 0  of the initial listening window. 
     The mobile station  100  determines whether it has received a feedback signal from the base station  200  (Step S 1438 ). In this step, the mobile station is able to receive the feedback signal from the base station (Yes in Step S 1438 ). 
     The mobile station  100  generates a feedback response to be transmitted to the base station  200  in response to the feedback signal (S 1440 ). For example, the mobile station  100  may generate and transmit the feedback response according to the MOB_SLP-RSP described above with reference to  FIG. 10 . For example, the mobile station  100  may transmit a feedback channel at a frame location defined in the frame designated by the MOB_SLP-RSP message. In a case where traffic, which cannot be transmitted within the time span of the current listening window, is remaining in the buffer of the mobile station  100 , the mobile station  100  transmits a feedback channel including a feedback code which is transmitted when there is uplink traffic. Then, the mobile station  100  transmits the feedback response (feedback channel) to the base station  200  (Step S 1442 ). 
     The base station  200  determines whether it has received a feedback response from the mobile station  100  (Step S 1444 ). In this step, the base station  200  is able to receive the feedback response (Yes in Step S 1444 ). The base station  200  confirms the code included in the feedback response. In a case where the confirmed code is the code transmitted when there is uplink traffic, the base station  200  does not change the current intermittent cycle (Step S 1446 ). Since the intermittent cycle remains unchanged, the listening window having a long period can be maintained. 
     The mobile station  100  also does not change the intermittent cycle (Step S 1448 ). Since the intermittent cycle remains unchanged, the listening window having a long period can be maintained. 
     (Fourth Phase) 
     The fourth phase is described with reference to  FIG. 15 . 
     In the fourth phase, the mobile station  100  moves to section E (described above with reference to  FIG. 4 ). In section E, the mobile station  100  and the base station  200  can stably communicate with each other. This embodiment describes a case where the mobile station  100  has no uplink traffic to be transmitted to the base station  200 . 
     Then, the base station  200  and the mobile station  100  start the listening window when the period Tu 2  of the sleep window of the communication condition “radio wave unstable communication possible” elapses (Step S 1502 ). The base station  200  activates the timer. The timer measures the period Ta 2  of the listening window. 
     The base station  200  broadcasts the feedback signal (feedback message) to the mobile station  100  in the listening window (listening interval) (Step S 1504 ). 
     Then, the mobile station  100  determines whether it has received a feedback signal from the base station  200  (Step S 1506 ). Since the mobile station  100  is able to receive the feedback signal transmitted from the base station  200  (Yes in Step S 1506 ), the mobile station  100  generates a feedback response (S 1508 ) in response to the received feedback signal. Then, the mobile station  100  transmits the feedback response to the base station (Step S 1510 ). Since no uplink traffic accumulated in the buffer of the mobile station  100  has been transmitted, the feedback response includes a code indicating no there is no uplink traffic (0b0010). 
     The base station  200  determines whether it has received a feedback response from the mobile station  100  (Step S 1512 ). In this step, the base station  200  is able to receive the feedback response (Yes in Step S 1512 ). The base station  200  confirms the code included in the feedback response. In a case where the confirmed code is the code transmitted when there is no uplink traffic, the base station  200  determines that the mobile station  100  is located in an area where radio wave reception is stable. The base station  200  may also determine that the mobile station  100  is located in an area where radio wave reception is stable according to the number of times receiving the feedback response including the code indicating that there is no uplink traffic. 
     In a case where the base station  200  determines that the mobile station  100  is located in an area where radio wave reception is stable, the base station  200  changes the intermittent cycle (Step S 1514 ). For example, the base station  200  may return the intermittent cycle to its initial intermittent cycle. The initial intermittent cycle includes a listening window having a period Ta 0  and a sleep window having a period Tu 0 . 
     Likewise, the mobile station  200  may return the intermittent cycle to its initial intermittent cycle in a case where the feedback response including the code indicating no uplink traffic is transmitted for a predetermined threshold consecutive number of times N (N being an integer, N&gt;0) (Step S 1516 ). The threshold N may be the number of times of determining that there is no uplink traffic. 
     Then, the base station  200  and the mobile station  100  start the sleep window. 
     The base station  200  detects the elapse of the period Tu 0  of the sleep window (Step S 1518 ). 
     Then, the base station  200  and the mobile station  100  start the listening window. 
     The base station  200  broadcasts a feedback signal (feedback message) to the mobile station  100  in the listening window (listening interval) (Step S 1520 ). 
     The mobile station  100  determines whether it has received a feedback signal from the base station  200  (Step S 1522 ). In this step, the mobile station is able to receive the feedback signal from the base station (Yes in Step S 1522 ). 
     The mobile station  100  generates a feedback response to be transmitted to the base station  200  in response to the feedback signal (Step S 1524 ). For example, the mobile station  100  may generate and transmit the feedback response according to the MOB_SLP-RSP described above with reference to  FIG. 10 . For example, the mobile station  100  may transmit a feedback channel at a frame location defined in the frame designated by the MOB_SLP-RSP message. The feedback channel may include a feedback code which is transmitted when there is no uplink traffic. 
     Then, the mobile station  100  transmits the feedback response to the base station  200  (Step S 1526 ). 
     The base station  200  determines whether it has received a feedback response from the mobile station  100  (Step S 1528 ). In this step, the base station  200  is able to receive the feedback response (Yes in Step S 1528 ). The base station  200  confirms the code included in the feedback response. Since the code is the code transmitted when there is no uplink traffic, the base station  200  does not change the intermittent cycle (Step S 1530 ). 
     The mobile station  100  also does not change the intermittent cycle (Step S 1532 ). 
     Whenever the mobile station  100  becomes unable to receive feedback signals from the base station  200  again during the listening window, the base station  200  and the mobile station  100  may start the above-described second phase. 
     Hence, with the above-described embodiments of the present invention, the communication-able period of the intermittent cycle can be shortened in a case where the radio wave reception condition is unstable and communication cannot be performed whereas the communication-able period of the intermittent cycle can be extended in a case where the radio wave reception condition is stable. 
     Further, with the above-described embodiments of the present invention, the period corresponding to the listening window can be extended in a case where there is uplink traffic. Thereby, a mobile station and/or a base station, having data accumulated in its buffer, can promptly transmit the data. Accordingly, the amount of buffer of the mobile station and/or the base station can be reduced. Further, with the above-described embodiments of the present invention, the opportunities in performing communications can be improved while maintaining low power consumption. 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.