Abstract:
A wireless communication system, method and apparatus cooperate to use a wireless coordination signal from a control station in a first frequency band to establish direct links in a second frequency band between different terminals. Once established, the direct links in the second frequency band operate without further needing intervention by the control station. The first frequency band is 1 GHz or higher, and the second frequency band is at a higher frequency than the first frequency band.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of and is based upon and claims the benefit of priority under 35 U.S.C. §120 for U.S. Ser. No. 13/071,547, filed Mar. 25, 2011 and claims the benefit of priority under 35 U.S.C. §119 from Japanese Patent Application No. 2010-087485, filed Apr. 6, 2010. The entire contents of which are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to communication apparatuses, communication methods, and communication systems using millimeter waves to perform wireless communication and, more particularly, to a communication apparatus, a communication method, and a communication system capable of securing appropriate coordination with multiple communication partners performing millimeter-wave communication. 
         [0004]    2. Description of the Related Art 
         [0005]    Very high-speed data communication can be realized by wireless communication using millimeter waves, which uses a wavelength of 10 mm to 1 mm and a frequency of 30 GHz to 300 GHz and which can allocate channels in units of GHz. For example, in Institute of Electrical and Electronics Engineers (IEEE) 802.11 ad, standardization of wireless local area network (LAN) standard using a 60 GHz band is being advanced. Since the millimeter waves have shorter wavelengths and higher linearity, compared with microwaves being in widespread use in, for example, wireless LAN technologies, a very large amount of information can be transmitted by using the millimeter waves. However, since reflection of the millimeter waves is accompanied by strong attenuation, direct waves and waves reflected no more than once are mainly transmitted on the wireless communication path. In addition, since the millimeter waves have higher transmission loss, the millimeter waves have the property that radio signals are not transmitted to a distance. 
         [0006]    In order to resolve such a communication distance problem of the millimeter waves, methods are considered in which directivity is given to the antennas of transmitters-receivers and the transmission beams and the reception beams of the transmitters-receivers are directed to directions where communication partners are positioned to increase the communication distance. However, the methods have drawbacks in that signals do not reach neighboring stations at directions other than the directions of the communication partners although the transmission-reception power is increased at the directions of the communication partners. Accordingly, the directional communication is not appropriate for coordination between multiple communication stations. 
         [0007]    In IEEE 802.11n, directional communication using microwaves having a frequency band of 2.4/5 GHz is defined. In this directional communication, packets for the coordination are transmitted at a rate lower than that of data packets to allow scheduling information to be transmitted to neighboring stations within a wider range. However, a sufficient amount of scheduling information may not reach the neighboring stations at a frequency band of 60 GHz even if the packets are transmitted at a lower rate. 
         [0008]    In addition, wireless communication apparatuses are proposed (for example, refer to Japanese Unexamined Patent Application Publication No. 2009-188925), in which search tones are transmitted by using the millimeter waves while the radiation directions of directional antennas are reciprocated and rotated at a constant angular velocity in 360 degrees on the horizontal plane and communication nodes are searched for on the basis of response tones returned from the communication nodes that have received the search tones. However, it takes a time to reciprocate and rotate the search tones to search for the communication nodes in 360 degrees. 
       SUMMARY OF THE INVENTION 
       [0009]    It is desirable to provide an excellent communication apparatus, communication method, and communication system capable of appropriate coordination with multiple communication partners performing millimeter wave communication. 
         [0010]    In one embodiment, a wireless communication station includes a wireless transceiver including
       a first frequency receiver configured to receive a wireless coordination signal in a first frequency band from a control station, the wireless coordination signal including scheduling information that manages direct communications between the wireless communication station and another wireless communication station, and   a second frequency transceiver configured to perform wireless communications in a second frequency band with the another wireless communication station in accordance with the scheduling information, a lowest frequency of the first frequency band being 1 GHz or higher.       
 
         [0013]    In the exemplary systems, methods and apparatuses, a center frequency ratio of the second frequency band to the first frequency band may be at least 12:1. 
         [0014]    In the exemplary systems, methods and apparatuses, the first frequency band may be centered at approximately 5 GHz, and the second frequency band may be centered at approximately 60 GHz. 
         [0015]    The wireless transceiver may include a controller that processes the wireless coordination signal to extract the scheduling information, and establishes a direct communication link with the another communication station in accordance with the scheduling information, the scheduling information specifying a priority use period for the wireless transceiver and another priority use period for the another communication station. 
         [0016]    A first frequency transceiver may be included that includes the first frequency receiver, the first frequency transceiver being configured to transmit a data transmission request to the control station to initiate the wireless coordination signal from the control station. 
         [0017]    Optionally, the second frequency transceiver performs wireless communication with the another wireless communication station in the second frequency band in accordance with the scheduling information while a third communication station performs wireless communication with a fourth communication station in the second frequency band also in accordance with the scheduling information. 
         [0018]    Optionally, the second frequency transceiver performs wireless communication with the another wireless communication station in the second frequency band in accordance with the scheduling information and also performs wireless communications in the second frequency band with a third communication station also in accordance with the scheduling information, although the scheduling information prohibits direct link communication between the another communication station and the third communication station. 
         [0019]    Another embodiment is directed to a control station, or apparatus, for controlling wireless communications, including: 
         [0020]    a transmitter configured to transmit a wireless coordination signal in a first frequency band to a wireless communication station and another wireless control station, the wireless coordination signal including scheduling information that manages direct communications in a second frequency band between the wireless communication station and another wireless communication station; and 
         [0021]    a processor that assigns communication resources for the direct communications in the second frequency band, a lowest frequency of the first frequency band being 1 GHz or higher. 
         [0022]    Another exemplary embodiment includes 
         [0023]    receiving a wireless coordination signal in a first frequency band from a control station, the wireless coordination signal including scheduling information that manages direct communications in a second frequency band between a wireless communication station and another wireless communication station; and 
         [0024]    establishing the direct wireless communications between the wireless communication station and the another wireless communication station in accordance with the scheduling information, a lowest frequency of the first frequency band being 1 GHz or higher. 
         [0025]    Yet another exemplary embodiment includes assigning with a processor communication resources for direct communications in a second frequency band between wireless communication stations; and 
         [0026]    transmitting a wireless coordination signal in a first frequency band to a wireless communication station and another wireless control station, the wireless coordination signal including scheduling information that manages an establishment of the direct communications between the wireless communication station and another wireless communication station, wherein a lowest frequency of the first frequency band being 1 GHz or higher. 
         [0027]    An exemplary wireless communication system includes 
         [0028]    a control station that has
       a transmitter configured to transmit a wireless coordination signal in a first frequency band to a wireless communication station and another wireless control station, the wireless coordination signal including scheduling information that manages direct communications in a second frequency band between the wireless communication station and another wireless communication station, and   a processor that assigns communication resources for the direct communications in the second frequency band, a lowest frequency of the first frequency band being 1 GHz or higher; and       
 
         [0031]    the wireless communications station that includes
       a wireless transceiver having a first frequency receiver and a second frequency transceiver,
           the first frequency receiver being configured to receive the wireless coordination signal in the first frequency band, and   the second frequency transceiver being configured to perform direct wireless communications in a second frequency band with the another wireless communication station in accordance with the scheduling information.   
               
 
         [0035]    Another exemplary method is directed to assigning with a processor of a control station communication resources for direct communications between wireless communication stations in a second frequency band; and 
         [0036]    transmitting a wireless coordination signal in a first frequency band to a wireless communication station and another wireless control station, the wireless coordination signal including scheduling information that manages establishment of direct communications between the wireless communication station and another wireless communication station, wherein a lowest frequency of the first frequency band being 1 GHz or higher; 
         [0037]    receiving at the wireless communication station the wireless coordination signal; and 
         [0038]    establishing the direct wireless communications in the second frequency band in accordance with the scheduling information. 
         [0039]    The “system” means a logical collection of multiple apparatuses (or functional modules realizing certain functions) irrespective of whether the apparatuses or the functional modules are provided in a single casing. 
         [0040]    According to the present invention, it is possible to provide an excellent communication apparatus, communication method, and communication system capable of appropriate coordination with multiple communication partners performing millimeter wave communication. 
         [0041]    According to the present invention, it is possible to subsidiarily use microwave communication at a frequency band of 2.4/5 GHz to secure appropriate coordination with multiple communication partners in order to realize desired data transmission using millimeter-wave communication at a frequency band of 60 GHz. 
         [0042]    Further features and advantages of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0043]      FIG. 1  schematically illustrates an example of the arrangement of a millimeter-wave wireless communication system according to an embodiment of the present invention; 
           [0044]      FIG. 2  is a block diagram showing an example of the configuration of a wireless communication apparatus applicable to the present invention; 
           [0045]      FIG. 3  is a block diagram showing an example of the internal configuration of a first digital part in  FIG. 2 ; 
           [0046]      FIG. 4  is a block diagram showing an example of the internal configuration of a second digital part in  FIG. 2 ; 
           [0047]      FIG. 5  schematically illustrates an example of the arrangement of a communication system; 
           [0048]      FIG. 6  shows an exemplary communication sequence between a control station and terminal stations in the use environment shown in  FIG. 5 ; 
           [0049]      FIG. 7  schematically illustrates an example of the arrangement of another communication system; 
           [0050]      FIG. 8  shows an exemplary communication sequence between a control station and terminal stations in the use environment shown in  FIG. 7 ; 
           [0051]      FIG. 9  shows an exemplary communication sequence in which the control station schedules data exchange in accordance with a communication enabled state of each terminal station in the use environment shown in  FIG. 7 ; 
           [0052]      FIG. 10  schematically illustrates an example of the arrangement of another communication system; 
           [0053]      FIG. 11  shows an exemplary communication sequence between a control station and terminal stations in the use environment shown in  FIG. 10 ; 
           [0054]      FIG. 12  is a flowchart showing an example of a process in which the wireless communication apparatus in  FIG. 2  operates as a terminal station in any of the use environments shown in  FIGS. 5 ,  7 , and  10 ; and 
           [0055]      FIG. 13  is a flowchart showing an example of a process in which the wireless communication apparatus in  FIG. 2  operates as a control station in any of the use environments shown in  FIGS. 5 ,  7 , and  10 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0056]    Embodiments of the present invention will herein be described in detail with reference to the attached drawings. 
         [0057]    As described above in “Description of the Related Art”, the wireless communication systems using the millimeter waves can use multiple transmission-reception antennas to form sharp antenna directivity (that is, beam-shape antenna directivity) in order to extend the communication range. However, the directional communication is not appropriate for transmission of control frames and, thus, the coordination may not be secured although the communication distance can be increased by directing the beams to the directions where communication partners are positioned. 
         [0058]    For example, in directional communication systems using the microwaves (at a frequency band of 5-GHz) defined in IEEE 802.11, packets for the coordination are transmitted at a rate lower than that in data transmission to widely transmit scheduling information, etc. during a frame period to neighboring stations in order to secure the coordination. However, a sufficient amount of signal may not reach the neighboring stations over the communication using the millimeter waves even at a lower communication rate. 
         [0059]    Wireless communication systems according to the embodiments of the present invention use both the wireless communication at a frequency band of 60 GHz and the wireless communication at a frequency band of 5 GHz. The wireless communication systems subsidiarily use the 5-GHz band for transmission of control information, such as beacons, used for the coordination in the 60-GHz band communication to allow a sufficient amount of control information to reach the neighboring stations. While particular values are provided above as examples, the frequency band for the direct links (between the terminal stations) have a center frequency that is at least 12 times that of a center frequency used in the coordination link. Thus a ratio of center frequencies is at least 12:1, with the ratio being exactly 12:1 when the direct links are centered at 60 GHz, and the coordination link is centered at 5 GHz. In the embodiments described herein, 5 GHz is used as the most common example, but the center frequency for the coordination link can be 1 GHz or higher in frequency. 
         [0060]    A communication method using microwaves (electromagnetic waves at a frequency band of 5 GHz) used in IEEE 802.11a/b/g in widespread use as a wireless LAN standard is hereinafter referred to as a first communication method, and a communication method using millimeter waves (electromagnetic waves at a frequency band of 60 GHz) used in Very High Throughput (VHT) standard is hereinafter referred to as a second communication method. However, the first communication method and the second communication method are not necessarily limited to specific frequency bands. 
         [0061]      FIG. 1  schematically illustrates an example of the arrangement of a millimeter-wave wireless communication system according to an embodiment of the present invention. The wireless communication system in  FIG. 1  includes one control station, called an access point (AP) in IEEE 802.11, and at least one terminal station (two terminal stations STA1 and STA2 in the example in  FIG. 1 ). 
         [0062]    The control station (AP) establishes a coordination link according to the first communication method using the microwaves with each of the terminal stations (STA1 and STA2) in the own cell to notify the terminal stations of the control information including the scheduling information in the cell. This scheduling information may optionally include the assignment of communication resources (e.g., bandwidth, frequencies, and/or time) for use in the direct links. 
         [0063]    Although the data communication between the terminal stations STA1 and STA2 may be via the control station (AP), a direct link that directly connects the terminal stations is mainly established to perform direct data communication between the terminal stations STA1 and STA2 in the present embodiment. The second communication method using the millimeter waves is applied to the direct link. Accordingly, the terminal stations STA1 and STA2 scheduled by the control station (AP) form the directivity for each other to realize the improvement in communication quality. Optionally, the AP may include in the scheduling information an arrangement by which one or more terminal stations serve as a relay station for another terminal station. This approach allows for range extension between different terminal stations, to broaden a reach of “direct links” between stations using one or more intermediate stations. 
         [0064]      FIG. 2  is a block diagram showing an example of the configuration of a wireless communication apparatus  100  capable of operating as a terminal station (STA) in the wireless communication system according to the present embodiment. Although the control station (AP) performs only the coordination according to the first communication method in the example in  FIG. 1 , the wireless communication apparatus  100  is similarly configured also when the control station (AP) performs the data communication according to the second communication method. 
         [0065]    Referring to  FIG. 2 , the wireless communication apparatus  100  includes an antenna  110 , a first wireless communication unit  120 , a storage unit  150 , multiple antennas  160   a  to  160   n , and a second wireless communication unit  170 . The first wireless communication unit  120  includes a first analog part  122 , an analog-to-digital (AD) conversion part  124 , a digital-to-analog (DA) conversion part  126 , a first digital part  130 , and a control part  140 . The second wireless communication unit  170  includes a second analog part  172 , an AD conversion part  174 , a DA conversion part  176 , a second digital part  180 , and a control part  190 . 
         [0066]    The antenna  110  is used in the wireless communication according to the first communication method. The antenna  110  transmits, for example, a control signal for securing the coordination in the second communication method according to the first communication method using the microwaves. In addition, the antenna  110  receives a control signal for securing the coordination in the second communication method according to the first communication method to supply the received control signal to the first analog part  122 . 
         [0067]    The first analog part  122  typically corresponds to a radio-frequency (RF) circuit for transmitting and receiving radio signals according to the first communication method. This feature may be referred to as a transmitter (for transmitting), a receiver (for receiving), or a transceiver (for transmitting and receiving). Specifically, the first analog part  122  low-noise amplifies and down-converts an RF reception signal received by the antenna  110  to supply the RF reception signal to the downstream AD conversion part  124 . In addition, the first analog part  122  up-converts an analog transmission signal converted by the DA conversion part  126  into an RF-band transmission signal and power-amplifies the RF-band transmission signal to supply the transmission signal subjected to the up-conversion and the power amplification to the antenna  110 . 
         [0068]    The AD conversion part  124  converts the analog reception signal supplied from the first analog part  122  into a digital signal to supply the digital signal to the downstream first digital part  130 . The DA conversion part  126  converts a digital transmission signal supplied from the first digital part  130  into an analog signal to supply the analog signal to the first analog part  122 . 
         [0069]      FIG. 3  is a block diagram showing an example of the internal configuration of the first digital part  130 . Referring to  FIG. 3 , the first digital part  130  includes a synchronizer  131 , a demodulator-decoder  132 , and an encoder-modulator  133 . The synchronizer  131  synchronizes the timing to start reception processing of a reception signal received by the antenna  110 , for example, with detection of the preamble at the beginning of the packet in the first communication method. The demodulator-decoder  132  demodulates and decodes the reception signal according to an arbitrary modulation method and an arbitrary encoding method used in the first communication method to acquire a data signal and supplies the acquired data signal to the control part  140 . The encoder-modulator  133  encodes and modulates a data signal supplied from the control part  140  according to the arbitrary encoding method and the arbitrary modulation method used in the first communication method to generate a transmission signal and supplies the generated transmission signal to the DA conversion part  126 . 
         [0070]    Referring back to  FIG. 2 , the description of the configuration of the wireless communication apparatus  100  will be continued. 
         [0071]    The control part  140  is composed of an arithmetic unit, such as a micro processor, and controls the entire operation of the first wireless communication unit  120 . For example, the control part  140  supplies a control signal for securing the coordination in the second communication method to the first digital part  130  in response to a request from a certain application (for example, a higher layer program of the corresponding communication protocol). In addition, the control part  140  receives a decoded control signal from the first digital part  130  to acquire, for example, information about the coordination in the second communication method, described in the control signal, and appropriately stores the information in the storage unit  150 . 
         [0072]    The storage unit  150  is composed of a writable recording medium, such as a semiconductor memory, and is used as a working memory in which a program to execute the communication processing by the wireless communication apparatus  100  is loaded and various parameters are stored. In addition, the storage unit  150  stores the values of parameters for identifying optimal transmission and reception beam patterns in the wireless communication according to the second communication method by the second wireless communication unit  170 . 
         [0073]    The antennas  160   a  to  160   n  are used for the wireless communication according to the second communication method. Specifically, the antennas  160   a  to  160   n  each transmit a radio signal weighted with a certain weight factor by using the millimeter waves. In addition, the antennas  160   a  to  160   n  each receives a millimeter-wave radio signal and supplies the received millimeter-wave radio signal to the second analog part  172 . 
         [0074]    The second analog part  172  typically corresponds to an RF circuit for transmitting and receiving radio signals according to the second communication method. Specifically, the second analog part  172  low-noise amplifies and down-converts multiple reception signals received by the antennas  160   a  to  160   n  to supply the reception signals to the downstream AD conversion part  174 . In addition, the second analog part  172  up-converts multiple analog transmission signals converted by the DA conversion part  176  into RF-band transmission signals and power-amplifies the RF-band transmission signals to supply the RF-band transmission signals subjected to the up-conversion and the power amplification to the antennas  160   a  to  160   n.    
         [0075]    The AD conversion part  174  converts the multiple analog reception signals supplied from the second analog part  172  into digital signals to supply the digital signals to the downstream second digital part  180 . The DA conversion part  176  converts multiple digital transmission signals supplied from the second digital part  180  into analog signals to supply the analog signals to the second analog part  172 . 
         [0076]    The second digital part  180  is typically composed of a circuit for demodulating and decoding a reception signal according to the second communication method and a circuit for encoding and modulating a transmission signal according to the second communication method. 
         [0077]      FIG. 4  is a block diagram showing an example of the internal configuration of the second digital part  180 . Referring to  FIG. 4 , the second digital part  180  includes a synchronizer  181 , a reception beam processor  182 , an electric power calculator  183 , a determiner  184 , a demodulator-decoder  185 , an encoder-modulator  186 , and a transmission beam processor  187 . 
         [0078]    The synchronizer  181  synchronizes the timing to start reception processing of multiple reception signals received by the multiple antennas  160   a  to  160   n , for example, with detection of the preambles at the beginning of the packets and supplies the reception signals to the reception beam processor  182 . 
         [0079]    The reception beam processor  182  weights the multiple reception signals supplied from the synchronizer  181  according to, for example, uniform distribution or Taylor distribution to control the directivity of the reception beams. Then, the reception beam processor  182  supplies the weighted reception signals to the electric power calculator  183  and the demodulator-decoder  185 . 
         [0080]    In learning of the optimal transmission and reception beam patterns, the electric power calculator  183  calculates the value of reception power of the reception signal transmitted and received in each of the transmission and reception beam patterns and sequentially supplies the calculated value of reception power to the determiner  184 . The determiner  184  determines the values of parameters for identifying the optimal transmission beam pattern and reception beam pattern on the basis of the value of reception power supplied from the electric power calculator  183 . The optimal beam pattern typically means a beam pattern having a maximum value of reception power, among a series of values of reception power supplied from the electric power calculator  183  for one beam learning signal. 
         [0081]    The demodulator-decoder  185  demodulates and decodes the reception signal weighted by the reception beam processor  182  according to an arbitrary modulation method and an arbitrary encoding method used in the second communication method to acquire a data signal. Then, the demodulator-decoder  185  supplies the acquired data signal to the control part  190 . 
         [0082]    The encoder-modulator  186  encodes and modulates a data signal supplied from the control part  190  according to the arbitrary encoding method and the arbitrary modulation method used in the second communication method to generate a transmission signal. Then, the encoder-modulator  186  supplies the generated transmission signal to the transmission beam processor  187 . 
         [0083]    The transmission beam processor  187  generates multiple transmission signals weighted according to, for example, the uniform distribution or the Taylor distribution from the transmission signal supplied from the encoder-modulator  186  to control the directivity of the transmission beams. The weight value used by the transmission beam processor  187  is specified by, for example, a directivity control signal supplied from the control part  190 . The multiple transmission signals weighted by the transmission beam processor  187  are supplied to the DA conversion part  176 . 
         [0084]    Referring back to  FIG. 2 , the description of the configuration of the wireless communication apparatus  100  will be continued. 
         [0085]    The control part  190  is composed of an arithmetic unit, such as a micro processor, and controls the entire operation of the second wireless communication unit  170 . In addition, the control part  190  acquires the value of a parameter for identifying an optimal transmission beam pattern from the storage unit  150  and supplies the directivity control signal used for forming the optimal transmission beam pattern identified on the basis of the parameter value to the transmission beam processor  187  in the second digital part  180 . As a result, the transmission beam in the wireless communication according to the second communication method by the wireless communication apparatus  100  is directed to the direction where a communication partner is positioned. 
         [0086]      FIG. 5  schematically illustrates an example of the arrangement of a communication system. It is assumed that the communication system in  FIG. 5  is used in an environment in which the millimeter-wave communication according to the second communication method is enabled on the direct link between terminal stations (STA1 to STA3) while the millimeter-wave communication according to the second communication method is disabled between a control station (AP) and each of the terminal stations (STA1, STA2, and STA3) and only the microwave communication according to the first communication method is enabled therebetween. The communication according to the second communication method is disabled between the control station (AP) and each of the terminal stations (STA1, STA2, and STA3) because, for example, the communication distance is long or the communication is performed over a wall. 
         [0087]    The control station (AP) establishes the coordination link according to the first communication method using the microwaves with each of the terminal stations (STA1 to STA3) in the own cell to notify the terminal stations of the control information including the scheduling information in the cell. 
         [0088]    The direct link is established between the terminal stations STA1 and STA2 in accordance with the scheduling information notified from the control station (AP) via the coordination link, and the terminal stations STA1 and STA2 perform the direct data communication over the millimeter-wave communication according to the second communication method. Similarly, the direct link is established between the terminal stations STA1 and STA3, and the terminal stations STA1 and STA3 perform the direct data communication over the millimeter-wave communication according to the second communication method. The terminal stations STA1 and STA2 and the terminal stations STA1 and STA3 form the directivity for each other to realize the improvement in communication quality. 
         [0089]      FIG. 6  shows an exemplary communication sequence between the control station (AP) and the terminal stations (STA1 and STA2) in the use environment shown in  FIG. 5 . 
         [0090]    The control station (AP) allocates a priority use period #1 (as a communications resource) and a priority use period #2 during which the terminal stations (STA1 and STA2) have priority over other terminal stations in the millimeter-wave communication to the terminal stations (STA1 and STA2), respectively, in the own cell and notifies the terminal stations (STA1 and STA2) of the priority use periods #1 and #2, respectively, by using a scheduling frame (Schedule) in which the scheduling information including the priority use periods #1 and #2 is described. In other words, the control station (AP) notifies the terminal stations of the scheduling information to manage the data exchange in the own cell. The scheduling frame is transmitted over the coordination link established in accordance with the first communication method using the microwaves (5 GHz). 
         [0091]    The control station (AP) performs the scheduling, that is, the allocation of the corresponding priority use period in response to, for example, a data transmission request from the terminal station (STA1 or STA2). An arbitrary frame may be used as the scheduling frame. For example, a beacon frame may also be used as the scheduling frame. 
         [0092]    In the example in  FIG. 6 , only the millimeter-wave communication (60 GHz) according to the second communication method is performed during the priority use periods #1 and #2. Each of the terminal stations (STA1 and STA2) applies a Request to Send (RTS)/Clear to Send (CTS) procedure during the priority use period allocated to the own station. 
         [0093]    During the priority use period #1, the terminal station STA  1  transmits an RTS frame to the terminal station STA2 to which data is to be transmitted. Then, the terminal station STA1 transmits a data frame to the terminal station STA2 upon reception of a CTS frame from the terminal station STA2. When the reception of the data frame is terminated, the terminal station STA2 returns an acknowledgement (ACK) frame. 
         [0094]    During the subsequent priority use period #2, the terminal station STA2 transmits an RTS frame to the terminal station STA1 to which data is to be transmitted. Then, the terminal station STA2 transmits a data frame to the terminal station STA1 upon reception of a CTS frame from the terminal station STA1. When the reception of the data frame is terminated, the terminal station STA1 returns an ACK frame. 
         [0095]    The terminal stations may not necessarily use the RTS/CTS procedure and may transmit the data frame without the RTS/CTS procedure during the priority use periods. 
         [0096]    The use environment shown in  FIGS. 5 and 6  is exemplified by a case in which one control station (AP) is installed at home, the communication according to the first communication method using the microwaves is established with each terminal station installed in rooms different from the room of the control station (AP), and the high-speed communication according to the second communication method using the millimeter waves is established between the terminal stations (STA1 and STA2) in the same room. 
         [0097]      FIG. 7  schematically illustrates an example of the arrangement of another communication system. It is assumed that the communication system in  FIG. 7  is used in the following environment: 
         [0098]    (1) The millimeter-wave communication according to the second communication method is disabled between the control station (AP) and each of the terminal stations (STA1, STA2, STA3, and STA4) and only the microwave communication according to the first communication method is enabled therebetween. 
         [0099]    (2) The millimeter-wave communication according to the second communication method is enabled on the direct links between the terminal stations STA1 and STA2 and between the terminal stations STA3 and STA4 while the millimeter-wave communication according to the second communication method is disabled between the terminal stations STA1 and STA3, the terminal stations STA1 and STA4, the terminal stations STA2 and STA3, and the terminal stations STA2 and STA4. 
         [0100]    The millimeter-wave communication according to the second communication method is disabled because, for example, the communication distance is long or the communication is performed over a wall. For example, it is assumed that the terminal stations STA1 and STA2 are in one room and the terminal stations STA3 and STA4 are in another room. In this case, the millimeter-wave communication between the terminal stations STA1 and STA2 and the millimeter-wave communication between the terminal stations STA3 and STA4 are not affected by the transmission loss over the walls. Accordingly, it is possible to simultaneously schedule the data exchange over the millimeter-wave communication in each room. 
         [0101]      FIG. 8  shows an exemplary communication sequence between the control station (AP) and the terminal stations (STA1, STA2, STA3, and STA4) in the use environment shown in  FIG. 7 . 
         [0102]    The control station (AP) accommodates the terminal stations STA1 and STA2 in one room and the terminal stations STA3 and STA4 in another room in the own cell over the microwave communication according to the first communication method. 
         [0103]    The control station (AP) notifies the terminal stations of the scheduling information to manage the data exchange in the own cell. In the example in  FIG. 8 , the control station (AP) allocates a priority use period #1, a priority use period #2, and a priority use period #3 during which the millimeter-wave communication can be used to the terminal stations STA1, STA2, and STA3, respectively, and notifies the terminal stations STA1, STA2, and STA3 of the priority use periods #1, #2, and #3, respectively, by using a scheduling frame (Schedule) in which the scheduling information including the priority use periods #1, #2, and #3 is described. The scheduling frame is transmitted over the coordination link established in accordance with the first communication method using the microwaves (5 GHz). 
         [0104]    In the use environment in  FIG. 7 , it is possible to simultaneously schedule the data exchange over the millimeter-wave communication in each room. Accordingly, the control station (AP) can perform simultaneous scheduling, that is, duplicate scheduling of the priority use periods #1 and #2 allocated to one room and the priority use period #3 allocated to the other room. 
         [0105]    In the example in  FIG. 8 , during each of the priority use periods #1, #2, and #3, only the millimeter-wave communication (60 GHz) according to the second communication method is enabled. Each of the terminal stations STA1, STA2, and STA3 applies the RTS/CTS procedure during the priority use period allocated to the own station. 
         [0106]    In one room, during the priority use period #1, the terminal station STA1 transmits an RTS frame to the terminal station STA2 to which data is to be transmitted. Then, the terminal station STA1 transmits a data frame to the terminal station STA2 upon reception of a CTS frame from the terminal station STA2. When the reception of the data frame is terminated, the terminal station STA2 returns an ACK frame. During the subsequent priority use period #2, the terminal station STA2 transmits an RTS frame to the terminal station STA1 to which data is to be transmitted. Then, the terminal station STA2 transmits a data frame to the terminal station STA1 upon reception of a CTS frame from the terminal station STA1. When the reception of the data frame is terminated, the terminal station STA1 returns an ACK frame. 
         [0107]    In the other room, during the priority use period #3, the terminal station STA3 transmits an RTS frame to the terminal station STA4 to which data is to be transmitted. Then, the terminal station STA3 transmits a data frame to the terminal station STA4 upon reception of a CTS frame from the terminal station STA4. When the reception of the data frame is terminated, the terminal station STA4 returns an ACK frame. 
         [0108]    The terminal stations may not necessarily use the RTS/CTS procedure and may transmit the data frame without the RTS/CTS procedure during the priority use periods. 
         [0109]    As shown in  FIGS. 7 and 8 , in order to realize the simultaneous scheduling of the data exchange over the millimeter-wave communication, it is necessary for the control station (AP) to determine which terminal station each terminal station in the own cell is in a communication enabled state with. It is also important to acquire information indicating whether the terminal stations can establish the microwave communication according to the first communication method or the millimeter-wave communication according to the second communication method. 
         [0110]    The control station (AP) determines the communication enabled state of each terminal station by, for example, a method of causing the terminal station to report information about the communication enabled state. Alternatively, a method in which the terminal station uses a beacon signal to acquire the communication enabled state may be adopted. Specifically, each terminal station uses the microwave communication according to the first communication method and the millimeter-wave communication according to the second communication method to periodically transmit the beacon signal. The terminal station determines that the terminal station is in the communication enabled state with the neighboring station from which the beacon signal is transmitted and the communication method by which the communication is enabled on the basis of reception of the beacon signal or reception of the beacon signal of a signal strength higher than a certain value. Information about the communication enabled state of each terminal station can be used to flexibly switch between the data communication using the millimeter waves and the data communication using the microwaves. 
         [0111]      FIG. 9  shows an exemplary communication sequence in which the control station (AP) schedules the data exchange in accordance with the communication enabled state of each of the terminal stations (STA1 and STA2). The use environment shown in  FIG. 7  is assumed here. 
         [0112]    The terminal station STA1 transmits a report frame (Report) including the communication enabled state in which the millimeter-wave communication according to the second communication method (and the microwave communication according to the first communication method) is enabled with the terminal station STA2 but only the microwave communication according to the first communication method is enabled with the control station (AP) to the control station (AP). The terminal station STA2 transmits a report frame (Report) including the communication enabled state in which the millimeter-wave communication according to the second communication method (and the microwave communication according to the first communication method) is enabled with the terminal station STA1 but only the microwave communication according to the first communication method is enabled with the control station (AP) and the terminal station STA3 to the control station (AP). The report frame is transmitted over the coordination link established in accordance with the first communication method using the microwaves (5 GHz). 
         [0113]    The control station (AP) performs the scheduling for every communication method (frequency band) on the basis of the communication enabled state of each of the terminal stations STA1 and STA2 and notifies the terminal stations STA1 and STA2 of the scheduling information to manage the data exchange in the own cell. In the example in  FIG. 9 , the control station (AP) allocates a priority use period during which the millimeter-wave communication can be used to the terminal station STA1 and notifies the terminal station STA1 of the priority use period by using a scheduling frame (Schedule) in which the scheduling information including the priority use period is described. The scheduling frame is transmitted over the coordination link established in accordance with the first communication method using the microwaves (5 GHz). 
         [0114]    In the example in  FIG. 9 , the terminal station STA1 applies the RTS/CTS procedure during the priority use period allocated to the own station. During the priority use period, the direct data communication is performed according to the second communication method using the millimeter waves (60 GHz). The terminal station STA1 transmits an RTS frame to the terminal station STA2 to which data is to be transmitted. Then, the terminal station STA1 transmits a data frame to the terminal station STA2 upon reception of a CTS frame from the terminal station STA2. When the reception of the data frame is terminated, the terminal station STA2 returns an ACK frame. 
         [0115]    How the control station (AP) performs the scheduling of the data exchange in accordance with the communication enabled state of each terminal station will now be described, taking a use environment shown in  FIG. 10  as an example. 
         [0116]    In the use environment in  FIG. 10 , the millimeter-wave communication according to the second communication method is disabled between the control station (AP) and each of the terminal stations (STA1, STA2, and STA3) and only the microwave communication according to the first communication method is enabled therebetween. The following communication enabled states are assumed between the terminal stations in the example in  FIG. 10 . The millimeter-wave communication according to the second communication method (and the microwave communication according to the first communication method) is enabled on the direct link between the terminal stations STA1 and STA2, only the microwave communication according to the first communication method is enabled on the direct link between the terminal stations STA2 and STA3, and the communication is disabled between the terminal stations STA1 and STA3 by any communication method and at any frequency band. 
         [0117]    The millimeter-wave communication according to the second communication method is disabled between the terminal stations STA2 and STA3 because, for example, the communication distance is long or the communication is performed over a wall. The communication is disabled between the terminal stations STA1 and STA3 by any communication method and at any frequency band because, for example, the communication distance is longer or an obstacle blocking even the microwaves exists between the terminal stations STA1 and STA3. 
         [0118]    The control station (AP) establishes the coordination link according to the first communication method using the microwaves with each of the terminal stations (STA1 to STA3) in the own cell. The terminal stations (STA1 to STA3) each notify the control station (AP) of the communication enabled state of the own station over the coordination link. The control station (AP) performs the scheduling in the cell on the basis of the communication enabled state of each of the terminal stations (STA1 to STA3) and notifies the terminal stations of the control information including the scheduling information over the coordination link. 
         [0119]    The direct link is established between the terminal stations STA1 and STA2 in accordance with the scheduling information notified from the control station (AP) over the coordination link and the direct data communication is performed between the terminal stations STA1 and STA2 over the millimeter-wave communication according to the second communication method. The terminal stations STA1 and STA2 form the directivity for each other to realize the improvement in communication quality. 
         [0120]    In contrast, the direct link is established between the terminal stations STA2 and STA3 in accordance with the scheduling information notified from the control station (AP) over the coordination link. However, the direct data communication is performed between the terminal stations STA2 and STA3 over the microwave communication according to the first communication method. 
         [0121]      FIG. 11  shows an exemplary communication sequence between the control station (AP) and the terminal stations (STA1, STA2, and STA3) in the use environment shown in  FIG. 10 . It is assumed in the example in  FIG. 11  that each of the terminal stations (STA1, STA2, and STA3) has acquired the communication enabled state of own station. 
         [0122]    The terminal station STA1 transmits a report frame (Report) including the communication enabled state in which the millimeter-wave communication according to the second communication method (and the microwave communication according to the first communication method) is enabled with the terminal station STA2 but only the microwave communication according to the first communication method is enabled with the control station (AP) to the control station (AP). The terminal station STA2 transmits a report frame (Report) including the communication enabled state in which the millimeter-wave communication according to the second communication method (and the microwave communication according to the first communication method) is enabled with the terminal station STA1 but only the microwave communication according to the first communication method is enabled with the control station (AP) and the terminal station STA3 to the control station (AP). The report frame is transmitted over the coordination link established in accordance with the first communication method using the microwaves (5 GHz). 
         [0123]    The control station (AP) performs the scheduling for every communication method (every frequency band) on the basis of the communication enabled state of each of the terminal stations STA1 and STA2 and notifies the terminal stations STA1 and STA2 of the scheduling information to manage the data exchange in the own cell. In the example in  FIG. 11 , the control station (AP) allocates a priority use period #1 during which the millimeter-wave communication can be used to the terminal station STA1 and allocates a priority use period #2 during which the microwave communication can be used to the terminal station STA2. The control station (AP) notifies the terminal stations STA1 and STA2 of the priority use periods #1 and #2, respectively, by using a scheduling frame (Schedule) in which the scheduling information including the priority use periods #1 and #2 is described. The scheduling frame is transmitted over the coordination link established in accordance with the first communication method using the microwaves (5 GHz). 
         [0124]    In the example in  FIG. 11 , each of the terminal stations STA1 and STA2 applies the RTS/CTS procedure during the priority use period allocated to the own station. 
         [0125]    During the priority use period #1, the direct data communication according to the second communication method using the millimeter waves (60 GHz) is performed. The terminal station STA1 transmits an RTS frame to the terminal station STA2 to which data is to be transmitted. Then, the terminal station STA1 transmits a data frame to the terminal station STA2 upon reception of a CTS frame from the terminal station STA2. When the reception of the data frame is terminated, the terminal station STA2 returns an ACK frame. 
         [0126]    During the subsequent priority use period #2, the direct data communication according to the first communication method using the microwaves (5 GHz) is performed. The terminal station STA2 transmits an RTS frame to the terminal station STA3 to which data is to be transmitted. Then, the terminal station STA2 transmits a data frame to the terminal station STA3 upon reception of a CTS frame from the terminal station STA3. When the reception of the data frame is terminated, the terminal station STA3 returns an ACK frame. 
         [0127]    The terminal stations may not necessarily use the RTS/CTS procedure and may transmit the data frame without the RTS/CTS procedure during the priority use periods. 
         [0128]    Although the data communication capable of flexibly switching between the millimeter-wave communication and the microwave communication on the basis of the communication enabled state is applied to the direct data communication between the terminal stations in the examples in  FIGS. 10 and 11 , the above data communication may be applied to the data communication between the control station (AP) and each terminal station. 
         [0129]      FIG. 12  is a flowchart showing an example of a process in which the wireless communication apparatus  100  in  FIG. 2  operates as a terminal station (STA) in any of the use environments shown in  FIGS. 5 ,  7 , and  10 . 
         [0130]    Referring to  FIG. 12 , in Step S 1201 , the terminal station transmits a signal for acquiring the communication enabled state at each neighboring station over the microwave communication (5-GHz band) at predetermined timing. For example, a beacon signal can be used as the signal. 
         [0131]    In Step S 1202 , the terminal station transmits a signal for acquiring the communication enabled state at each neighboring station over the millimeter-wave communication (60-GHz band) at another predetermined timing. For example, a beacon signal can be used as the signal. 
         [0132]    In Step S 1203 , the terminal station acquires the communication enabled state with each neighboring station at the 5-GHz band upon reception of the signal from the neighboring station over the microwave communication (5-GHz band) at another predetermined timing. When the reception signal includes training information, the beam pattern in the first wireless communication unit  120  can be learned or updated. 
         [0133]    In Step S 1204 , the terminal station acquires the communication enabled state with each neighboring station at the 60-GHz band upon reception of the signal from the neighboring station over the millimeter-wave communication (60-GHz band) at another predetermined timing. When the reception signal includes training information, the beam pattern in the second wireless communication unit  170  can be learned or updated. 
         [0134]    In Step S 1205 , the terminal station determines whether the acquisition of the communication enabled state with each neighboring station is completed. If the terminal station determines that the acquisition of the communication enabled state with each neighboring station is not completed (NO in Step S 1205 ), the terminal station repeats the above steps S 1201  to S 1204 . 
         [0135]    If the terminal station determines that the acquisition of the communication enabled state with each neighboring station is completed (YES in Step S 1205 ), in Step S 1206 , the terminal station transmits a notification including information about the acquired communication enabled state to the control station. The information about the communication enabled state is described, for example, in a report frame and is notified over the coordination link established in accordance with the first communication method using the microwaves (5 GHz). The terminal station may transmit a request for data transmission over the direct communication with each neighboring station. 
         [0136]    In Step S 1207 , the terminal station receives a scheduling frame including the scheduling information concerning the allocation of the communication opportunity for the direct communication from the control station. The scheduling frame is transmitted over the coordination link established in accordance with the first communication method using the microwaves (5 GHz). When the reception signal includes training information, the beam pattern in the first wireless communication unit  120  can be learned or updated. 
         [0137]    The scheduling information may include information specifying transmission and reception beam patterns in the direct communication. 
         [0138]    In Step S 1208 , the terminal station analyzes the scheduling information in the scheduling frame to determine whether an opportunity for the direct transmission to the neighboring station specified in the scheduling information is given. If the terminal station determines that an opportunity for the direct transmission to the neighboring station specified in the scheduling information is given (YES in Step S 1208 ), in Step S 1209 , the terminal station directly transmits a data frame to the neighboring station during the specified communication period and by using the specified frequency band. When a beam pattern is specified in the scheduling information, the specified beam pattern is used to perform the transmission operation. 
         [0139]    In Step S 1210 , the terminal station analyzes the scheduling information in the scheduling frame to determine whether an opportunity for the direct reception from the neighboring station specified in the scheduling information is given. If the terminal station determines that an opportunity for the direct reception from the neighboring station specified in the scheduling information is given (YES in Step S 1210 ), in Step S 1211 , the terminal station directly receives a data frame from the neighboring station during the specified communication period and by using the specified frequency band. When a beam pattern is specified in the scheduling information, the specified beam pattern is used to perform the reception operation. 
         [0140]    The transmission operation in Steps S 1208  to S 1209  may be performed after the reception operation in Steps S 1210  to S 1211 . 
         [0141]      FIG. 13  is a flowchart showing an example of a process in which the wireless communication apparatus  100  in  FIG. 2  operates as a control station (AP) in any of the use environments shown in  FIGS. 5 ,  7 , and  10 . 
         [0142]    Referring to  FIG. 13 , in Step S 1301 , the control station transmits a signal for acquiring the communication enabled state at each neighboring station over the microwave communication (5-GHz band) at predetermined timing. For example, a beacon signal can be used as the signal. 
         [0143]    In Step S 1302 , the control station transmits a signal for acquiring the communication enabled state at each neighboring station over the millimeter-wave communication 60-GHz band) at another predetermined timing. For example, a beacon signal can be used as the signal. 
         [0144]    In Step S 1303 , the control station acquires the communication enabled state with each neighboring station at the 5-GHz band upon reception of the signal from the neighboring station over the microwave communication (5-GHz band) at another predetermined timing. When the reception signal includes training information, the beam pattern in the first wireless communication unit  120  can be learned or updated. 
         [0145]    In Step S 1304 , the control station acquires the communication enabled state with each neighboring station at the 60-GHz band upon reception of the signal from the neighboring station over the millimeter-wave communication (60-GHz band) at another predetermined timing. When the reception signal includes training information, the beam pattern in the second wireless communication unit  170  can be learned or updated. 
         [0146]    In Step S 1305 , the control station determines whether the acquisition of the communication enabled state with each neighboring station is completed. If the control station determines that the acquisition of the communication enabled state with each neighboring station is not completed (NO in Step S 1305 ), the control station repeats the above steps S 1301  to S 1304 . 
         [0147]    If the control station determines that the acquisition of the communication enabled state with each neighboring station is completed (YES in Step S 1305 ), in Step S 1306 , the control station receives a notification of information about the communication enabled state from each neighboring station. The information about the communication enabled state is described, for example, in a report frame and is notified over the coordination link established in accordance with the first communication method using the microwaves (5 GHz). The control station may receive a request for data transmission over the direct communication between the terminal stations from each neighboring station. 
         [0148]    In Step S 1307 , the control station performs scheduling for every communication method (frequency band) on the basis of the data transmission request and the communication enabled state from each terminal station. In Step S 1308 , the control station transmits the scheduling information to manage the data exchange in the own cell. 
         [0149]    The scheduling frame is transmitted over the coordination link established in accordance with the first communication method using the microwaves (5 GHz). When the reception signal includes training information, the beam pattern in the first wireless communication unit  120  can be learned or updated. The scheduling information may include information specifying transmission and reception beam patterns in the direct communication. 
         [0150]    In Step S 1309 , the control station determines whether a transmission opportunity is given to the own station. If the control station determines that a transmission opportunity is given to the own station (YES in Step S 1309 ), in Step S 1310 , the control station transmits a data frame to a neighboring station which the control station specifies during the specified communication period and by using the specified frequency band. When the control station specifies a beam pattern, the specified beam pattern is used to perform the transmission operation. 
         [0151]    In Step S 1311 , the control station determines whether a reception opportunity is given to the own station. If the control station determines that a reception opportunity is given to the own station (YES in Step S 1311 ), in Step S 1312 , the control station receives a data frame from a neighboring station which the control station specifies during the specified communication period and by using the specified frequency band. When the control station specifies a beam pattern, the specified beam pattern is used to perform the reception operation. 
         [0152]    The transmission operation in Steps S 1309  to S 1310  may be performed after the reception operation in Steps S 1311  to S 1312 . 
         [0153]    The present application contains subject matter related to that disclosed in Japanese Priority Patent Application JP 2010-087485 filed in the Japan Patent Office on Apr. 6, 2010, the entire contents of which are hereby incorporated by reference. 
         [0154]    It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.