Abstract:
A wireless communication system that includes a base station and one or more terminals carries out wireless data communication by use of a first frequency band. The wireless communication system includes a wireless access system that employs a CSMA/CA and/or TDMA/TDD system as the wireless access system of the wireless communication system and a control signal at the first frequency band is periodically broadcasts from the base station, where the control signal includes a control information configured to manage wireless data transmission by the one or more terminals. In addition to the first frequency band, one or more frequency bands different from the first frequency band for the wireless data transmission by the one or more terminals can be allocate by the base station, where the control signal at the first frequency band indicate the location of the one or more frequency bands which can be used.

Description:
[0001]    This is a Continuation Application of co-pending application Ser. No. 13/722,636, filed on Dec. 20, 2012, which is a Continuation Application of application Ser. No. 13/584,462, filed on Aug. 13, 2012, issued as U.S. Pat. No. 8,369,309 on Feb. 5, 2013, which is a Continuation Application of application Ser. No. 13/117,462 filed on May 27, 2011, issued as U.S. Pat. No. 8,270,390 on Sep. 18, 2012, which is a Divisional Application of application Ser. No. 10/432,994 filed on May 28, 2003 (abandoned), which is a National Phase of PCT International Application No. PCT/JP01/10498 filed on Nov. 30, 2001, which claims the benefit of Japanese Patent Application No. 2000-376278, filed on Dec. 11, 2000. The entire contents of all of the above applications are hereby incorporated by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to a wireless communication system comprised of a base station and mobile stations, particularly to an improvement relating to inter-mobile station direct communication in a wireless communication system based on a TDMA (Time Division Multiple Access) system. 
       BACKGROUND 
       [0003]    A wireless communication system is comprised of a base station for giving notices of allocations of bands (time regions for communications) and a plurality of mobile stations based on the TDMA system (in which different time regions are allocated to the mobile stations at the same frequency for carrying out communication). For such a system, a PHS inter-mobile station direct communication system in accordance with RCR STD-28 (second-generation cordless telephone system standard, version 1) of ARIB (Association of Radio Industries and Businesses) has been proposed as a method of carrying out inter-mobile station direct communication at a frequency other than that used by the base station. Improvements of the system are disclosed in JP Patent Publication (Kokai) Nos. 11-122663 A1(1999) and 11-341564 A1(1999), for example. 
         [0004]      FIG. 13  shows an example of the frame structure in the TDMA wireless communication system. 
         [0005]    As shown in  FIG. 13(   a ), a length of time in which the same frequency is used for communication is divided into frames  1  at predetermined time intervals. In each frame (at predetermined periods), the base station transmits control information  2  to all of the mobile stations in a wireless LAN area created by the base station. The control information  2  includes information allowing the multiple mobile stations to be synchronized with the base station, and information indicating the band (time region) within the frame allocated to each mobile station. 
         [0006]    The frame is divided into the time regions of DOWN-LINK  3  for the transmission of data from the base station to the mobile stations and UP-LINK  4  for the transmission of data from the mobile stations to the base station. In each time region, each mobile station is allocated a band  5  (band A) and a band  6  (band B), as shown in  FIG. 13(   b ), such that the base station and the mobile stations can communicate with one another bi-directionally. 
         [0007]    The control information  2  will be hereafter described by referring to  FIG. 13(   c ). 
         [0008]    Generally, in the physical layer of wireless systems, a digital signal comprised of a preamble  200  for broadcast and a data payload  201  (data payload  1 , . . . , data payload N), which is information data for transmission, is converted into an electric signal. The preamble  200  located at the head identifies the signal received from a wireless interface. Particularly, the preamble  200  for broadcast that is attached when the base station transmits broadcast information into a wireless cell functions as a synchronization signal with which a mobile station in the wireless cell attempts to achieve synchronization with the base station. Further, the control information  2  includes such control information as frame structure information and band allocation information. 
         [0009]      FIG. 14  shows a block diagram of an example of the configuration of a wireless LAN based on the above-described wireless communication system. 
         [0010]    In  FIG. 14 , numeral  7  designates a WAN (Wide Area Network), and numeral  8  designates a wireless LAN (Local Area Network). The wireless LAN  8  is comprised of a plurality of mobile stations  11 - 13  and a base station  10 . The base station  10  is either connected to a central control unit  9  for the central control of band allocations, or is equipped with the central control unit  9  inside. The wireless LAN  8  is connected to the WAN  7  via the base station  10 . 
         [0011]    The mobile stations  11  to  13  carry out a base station-mobile station communication according to the band allocation information notified by the base station  10 . The communication paths are indicated as communication paths  14 - 16 . The communication path for direct communication between a mobile station  11  (mobile station &lt; 1 &gt;) and a mobile station  12  (mobile station &lt; 2 &gt;) is indicated by a communication path  17 . 
         [0012]    When the mobile station  11  (mobile station &lt; 1 &gt;) transmits data to the mobile station  12  (mobile station &lt; 2 &gt;), if the normal base station-mobile station communication is employed, the same data would have to be transmitted twice via the communication paths  14  and  15  shown in  FIG. 14 , which is inefficient. Thus, it is necessary to use a method for carrying out inter-mobile station direct communication by which data is directly transmitted between the mobile stations via the communication path  17  without the intervention of the base station  10 . 
         [0013]      FIG. 15  shows an example of the frame structure for inter-mobile station direct communication in a conventional wireless communication system. 
         [0014]    In the example of  FIG. 15 , inter-mobile station direct communication is conducted at a frequency different from the one used by the base station  10 . One of the mobile stations that carry out inter-mobile station direct communication transmits control information  19  at the frequency for inter-mobile station direct communication. This frequency is different from the frequency at which frame synchronization is achieved and communications are carried out according to the control information  18  transmitted by the base station. The mobile stations that carry out the inter-mobile station direct communication are provided with a base-station function for synchronizing their frames and allocating bands. 
         [0015]    Other examples of conventional techniques for carrying out inter-mobile station direct communication include: a wireless LAN system and a PDC (Personal Digital Cellular) portable telephone employing the CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) system according to the ISO/IEC (International Organization for Standardization/International Electrotechnical Commission) 8802-11 or the IEEE (Institute of Electrical and Electronics Engineers) 802.11 standards; HiperLAN according to the ETSI (European Telecommunications Standards Institute), which is a 3.5-generation system transitioning into a 4 th -generation system to be standardized around the year 2010 following PHS (Personal Handy-phone System) and the W-CDMA (Wideband Code Division Multiple Access); and the MMAC (Multimedia Mobile Access Communication System) wireless system according to the ARIB (Association of Radio Industries and Businesses). 
         [0016]    The IEEE802.11 and the HiperLAN will be briefly described below, although they do not have direct relevance to the present invention in that the former is not a centrally controlled wireless system comprising a central control unit for centrally controlling band allocations, and that the latter is a system that employs the same frequency as that for base station-mobile station communication. 
         [0017]      FIG. 16  shows an example of communication according to the IEEE802.11 system. 
         [0018]    In  FIG. 16 , among mobile stations  1  to  4  in a wireless LAN area  20 , when a mobile station &lt; 1 &gt; wishes to send information to a mobile station &lt; 2 &gt;, the mobile station &lt; 1 &gt; broadcasts an RTS (Request to Send) signal  21  and  24 . Each mobile station analyzes the RTS signal. When it is not an information transmission request directed to a particular mobile station, the mobile station stands by. When the RTS signal is directed to a particular mobile station, the mobile station (mobile station &lt; 2 &gt;) transmits a CTS (Clear to Send) signal  22  and  25  to indicate that it is ready to receive. In response, the mobile station &lt; 1 &gt; transmits information via a MAC signal  23  and  26  to the mobile station &lt; 2 &gt;. During these processes, the other mobile stations refrain from transmission for a certain time period to avoid collision of transmission signals. 
         [0019]      FIG. 17  shows an example of the frame structure in the HiperLAN system. Parts or elements similar to those shown in  FIG. 13  are designated by similar references. 
         [0020]    In the HiperLAN system, a band  28  is provided in a TDMA frame  27  for carrying out inter-mobile station direct communication. In this band  28 , the base station ceases transmission to allow the mobile stations to transmit to each other, thus allowing base station-mobile station communication and inter-mobile station direct communication to take place on a single frequency. 
         [0021]    In a conventional wireless communication system, the frames for inter-mobile station direct communication and those for base station-mobile station communication are not synchronized, as shown in  FIG. 15 . Thus, in order to obtain control information for synchronizing the respective frames, the communication modes are switched. This is a process in which a mobile station terminates its connection with the base station for base station-mobile station communication and then sets up a connection with the mobile station that has the base-station function for carrying out inter-mobile station direct communication. An improved method of obtaining broadcast information from the base station is proposed by JP Patent Publication (Kokai) No. 11-122663 A1(1999), for example, in which the switching is carried out intermittently. 
         [0022]    However, the apparatus disclosed in the above publication has the problem that the mobile station  11  (mobile station &lt; 1 &gt;) cannot carry out data communication with the mobile station  12  (mobile station &lt; 2 &gt;) and the base station  10  simultaneously, as shown in  FIG. 14 . In recent communication systems, there is an ongoing shift from voice communication by telephone to data communication. Thus, the above problem means that, in the context of building a wireless LAN, the mobile stations are intermittently cut off from the network, creating further problems. For example, the base station may not be able to transmit data received from a connected WAN (such as the Internet) to a destination mobile station in the wireless LAN, or the base station may not even recognize the presence of the mobile station due to the absence of connection therewith. 
         [0023]    The above discussion is based on the assumption of creating, for example, a household wireless LAN (home network) comprised of a gateway unit including a base station connected to the Internet outside, and mobile stations including household information appliances (such as a refrigerator, microwave oven, television, video server, or set-top box, for example). In this case, it would be problematic if the external network could not send video information to the video server when the video server is wirelessly transmitting video information to the television, or if, when an air conditioner should be externally operated, the presence of the air conditioner on the network could not be confirmed due to the termination of its connection to the base station. 
         [0024]    Further, in order to realize inter-mobile station direct communication, there is the additional problem of having to add the base-station function to one of the mobile stations that is to act as the base station. 
         [0025]    It is an object of the invention to provide an efficient wireless communication system that allows for base station-mobile station communication even when inter-mobile station direct communication is carried out at a frequency other than that of the base station, which can reduce the burden on the mobile stations by having the base station control inter-mobile station direct communication, and which allows a plurality of frequencies to be simultaneously used by a single base station. 
       DISCLOSURE OF THE INVENTION 
       [0026]    A wireless communication system that is comprised of a base station and mobile stations wirelessly connected to the base station, wherein the mobile stations can communicate with one another without the intervention of the base station, the system further comprising a synchronizing means, wherein 
         [0027]    when a second frequency is used for an inter-mobile station direct communication which is different from a first frequency used for a base station-mobile station communication, the synchronizing means synchronizes a frame in the base station-mobile station communication with that in the inter-mobile station direct communication based on control information used in the base station-mobile station communication. 
         [0028]    A mobile station is provided that is wirelessly connected to a base station and capable of a base station-mobile station communication at a first frequency and an inter-mobile station direct communication at a second frequency that is different from the first frequency without the intervention of the base station, the mobile station comprising a synchronizing means for synchronizing a frame in a base station-mobile station communication with that in an inter-mobile station direct communication based on control information used in the base station-mobile station communication. 
         [0029]    A base station in a wireless communication system is provided that is comprised of the base station and mobile stations wirelessly connected to the base station, the base station comprising an allocation means for allocating a second frequency for an inter-mobile station direct communication which is different from a first frequency used for a base station-mobile station communication in response to a request from a mobile station. 
         [0030]    A base station in a wireless communication system is provided that is comprised of the base station and mobile stations wirelessly connected to the base station, the system capable of conducting a base station-mobile station communication at a first frequency and an inter-mobile station direct communication at a second frequency different from the first frequency without the intervention of the base station, the base station comprising: 
         [0031]    an allocation means for allocating bands in a frame for an inter-mobile station direct communication based on a request from a mobile station. 
         [0032]    The base station may comprise a carrying means for performing carrier sensing at a plurality of available frequencies and for carrying available frequencies other than a frequency used by the base station. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0033]      FIG. 1  shows block diagrams of examples of a base station and a mobile station in the wireless communication system. 
           [0034]      FIG. 2  shows an example of the frame structure in an inter-mobile station direct communication in the wireless communication system. 
           [0035]      FIG. 3  shows a block diagram of an example of a wireless LAN based on the wireless communication system. 
           [0036]      FIG. 4  shows a control sequence chart illustrating the flow of data in an example of communication in the wireless communication system. 
           [0037]      FIG. 5  shows a flowchart of communication processes performed in the base station in the wireless communication system. 
           [0038]      FIG. 6  shows a flowchart of communication processes performed in the mobile station in the wireless communication system. 
           [0039]      FIG. 7  shows a control sequence chart illustrating the flow of data in the wireless communication system. 
           [0040]      FIG. 8  shows a block diagram of band allocations in the wireless communication system. 
           [0041]      FIG. 9  shows a block diagram of frame allocations in the wireless communication system. 
           [0042]      FIG. 10  shows an example of a state of communication between the base station and the mobile stations in the wireless communication system. 
           [0043]      FIG. 11  shows a block diagram of frame allocation in the wireless communication system. 
           [0044]      FIG. 12  shows an example of a state of communication between the base station and the mobile stations in the wireless communication system. 
           [0045]      FIG. 13  shows an example of the frame structure in a conventional TDMA wireless communication system. 
           [0046]      FIG. 14  shows a block diagram of an example of a wireless LAN based on the conventional wireless communication system. 
           [0047]      FIG. 15  shows an example of the frame structure for an inter-mobile station direct communication in the conventional wireless communication system. 
           [0048]      FIG. 16  shows an example of communication according to the IEEE802.11 system. 
           [0049]      FIG. 17  shows an example of the frame structure according to the HiperLAN system. 
       
    
    
     DETAILED DESCRIPTION 
       [0050]    The invention will be hereafter described in detail by referring to the attached drawings. 
       First Embodiment 
       [0051]      FIG. 1  shows a block diagram of a base station and a mobile station in a wireless communication system according to the first embodiment of the invention. 
         [0052]    Referring to  FIG. 1 , numeral  30  designates a base station in the wireless communication system, and numeral  40  designates a mobile station that communicates wirelessly with the base station  30 . The base station  30  includes a central control unit  31 . 
         [0053]    The base station  30  comprises a central control unit  31  for allocating bands in a frame of a communication frequency in response to a band allocation request; an antenna  32  for wireless transmission and reception; a wireless processing means  33  for detecting available frequencies; a frame processing means  34  for carrying out frame processing by which control information such as allocated band allocation information is converted into transmission data; a wireless control means  35  for creating control information such as a frame synchronizing signal and band allocation information concerning allocations by the central control unit  31 ; and a line processing means  36  for sending external transmission data to the frame processing means  34  in a certain format in response to an instruction from the wireless control means  35 . The central control unit  31  carries out calculations for time allocation, for example, and it may be either included in the base station  30 , as according to the present embodiment, or located outside the base station  30 . 
         [0054]    The mobile station  40  comprises an antenna  42  for wireless transmission and reception; a wireless processing means  43  for receiving communication data and sending it to the frame processing means  44 ; a frame processing means  44  for achieving frame synchronization based on a control signal received from the base station  30  and separating the communication data into control information and reception data; a wireless control means  45  for decoding the band allocation information and the like and preserving it as control information for use in the communication data transmission/reception processing; and a line processing means  46  for sending external transmission data to the frame processing means  44  in a certain format in accordance with an instruction from the wireless control means  45 . 
         [0055]    The wireless control processing means  45  notifies the wireless processing means  43  of the start of a transmission/reception operation upon encountering an allocated band (time) in a frame. In the case of inter-mobile station direct communication, the wireless control processing means  45  notifies the wireless processing means  43  of a change of frequency to that used for inter-mobile station direct communication. 
         [0056]    In accordance with the present embodiment, the wireless communication system comprises a synchronizing means for synchronizing, when a second frequency is to be used for inter-mobile station direct communication that is different from a first frequency used for base station-mobile station communication, frames between the base station-mobile station communication and the inter-mobile station direct communication, on the basis of control information used in the base station-mobile station communication. The wireless communication system also comprises an allocation means for allocating bands and a frequency for inter-mobile station direct communication in accordance with an allocation request from the base station  30  or mobile station  40 . The base station  30  and mobile station  40  each comprise a transmission means and a reception means for transmitting and receiving allocation requests and allocation notifying information between them. The mobile station  40  further comprises a control means for changing the wireless frequency in accordance with an allocation. Thus, a base station-mobile station communication can be carried out simultaneously with an inter-mobile station direct communication. The base station  30  also carries out carrier sensing at a plurality of available frequencies and comprises a means for carrying available frequencies other than that used by the base station  30 . 
         [0057]      FIG. 2  shows an example of frame structure in inter-mobile station direct communication according to the present wireless communication system, showing examples of frame structure and band allocation. The frame structures correspond to those of the conventional example shown in  FIG. 15 . 
         [0058]    Referring to  FIGS. 2(   a ) and ( b ), the frame structure is divided into time regions consisting of control information  50 , a communication band DOWN-LINK  51 , and a communication band UP-LINK  52 . The control information  50  includes information for a plurality of mobile stations  40  to achieve synchronization with a base station  30  in each frame (at predetermined periods), and information indicating a band (time region) in a frame allocated to each mobile station. The DOWN-LINK  51  is for sending data from the base station  30  to the mobile stations  40 . The UP-LINK  52  is for sending data from the mobile stations  40  to the base station  30 . The control information  50  includes a preamble  200  for broadcast and a data payload  201 , as described with reference to  FIG. 13(   c ). The preamble  200  for broadcast includes a synchronizing signal for the mobile stations in a wireless cell to achieve synchronization with the base station, and control information such as frame structure information and band allocation information. 
         [0059]    Referring to  FIG. 2(   b ), a mobile station carries out inter-mobile station direct communication at an inter-mobile station direct communication frequency which is different from the frequency at which frame synchronization is achieved and communication is carried out based on the control information  50  transmitted by the base station  30 . The mobile station synchronizes frames between base station-mobile station communication and inter-mobile station direct communication by utilizing the base station-mobile station communication control information  50  instead of the control information  53  for the conventional inter-mobile station direct communication. Numeral  54  designates a direct communication band. 
         [0060]      FIG. 2(   c ) shows an example of band allocation in base station-mobile station communication. Numeral  55  designates control information where the control information  50  is allocated. Numeral  56  designates a band D 1  where the DOWN-LINK  51  is allocated. Numeral  57  designates a band U 1  where the UP-LINK  52  is allocated. 
         [0061]      FIG. 2(   d ) shows an example of band allocation in inter-mobile station direct communication. As described above, the control information (control information  53 ) for the conventional inter-mobile station direct communication is not used, because the base station-mobile station communication control information  50  is also used for the inter-mobile station direct communication. Numeral  58  designates a band Di 1  where the direct communication band  54  is allocated. 
         [0062]    Thus, by synchronizing frames between base station-mobile station communication and inter-mobile station direct communication, it becomes possible to process in parallel the communication band DOWN-LINK  51  from the base station  30  to the mobile stations  40 , the communication band UP-LINK  52  from the mobile stations  40  to the base station  30 , and the direct communication band  54  from one mobile station  40  to another mobile station  40 . 
         [0063]    Hereafter, the operation of the wireless communication system as configured above will be described. 
         [0064]      FIG. 3  shows a block diagram of an example of a wireless LAN based on the present wireless communication system. Parts or elements similar to those shown in  FIG. 14  are designated by similar references. 
         [0065]    In  FIG. 3 , a wireless LAN  8  is comprised of a plurality of mobile stations  40 A to  40 C, and a base station  30  with a central control unit  31  built inside. The wireless LAN  8  is connected to a WAN  7  via the base station  40 . 
         [0066]    The mobile stations  40 A to  40 C are similar in structure to those mobile stations  40  shown in  FIG. 1 , and they carry out base station-mobile station communication in accordance with band allocation information notified from the base station  40 . The communication paths are indicated by communication paths  14  to  16 . A communication path for inter-mobile station direct communication between the mobile station  40 A (mobile station &lt; 1 &gt;) and the mobile station  40 B (mobile station &lt; 2 &gt;) is indicated by a communication path  17 . 
         [0067]    An example of band allocation for the base station  30  and the mobile station  40 A (mobile station &lt; 1 &gt;) that wants to communicate with the mobile station  40 B (mobile station &lt; 2 &gt;) will be described by referring to  FIG. 2 . 
         [0068]    In  FIGS. 2 and 3 , the mobile station  40 A (mobile station &lt; 1 &gt;) receives frame synchronization and band allocation based on the control information  55  from the base station  30 . A band  56  (band D 1 ) is a communication band from the base station  30  to the mobile station  40 A (mobile station &lt; 1 &gt;). A band  57  (band U 1 ) is a communication band from the mobile station  40 A (mobile station &lt; 1 &gt;) to the base station  30 . A band  58  (band Di 1 ) is a communication band from the mobile station  40 A (mobile station &lt; 1 &gt;) to the mobile station  40 B (mobile station &lt; 2 &gt;). By carrying out communication according to these allocations, the mobile station  40 A (mobile station &lt; 1 )) can perform data communication simultaneously with the mobile station  40 B (mobile station &lt; 2 &gt;) and the base station  30  without interrupting its connection with the base station  30 . 
         [0069]      FIG. 4  shows a control sequence chart indicating the flow of data in the above example of communication. The numbers in the drawing show the individual processing steps. 
         [0070]    In  FIG. 4 , it is assumed that the mobile station  40 A (mobile station &lt; 1 &gt;) and the mobile station  40 B (mobile station &lt; 2 &gt;) have completed connection with the base station  30  and have started communication therewith (60). 
         [0071]    Initially, the mobile station  40 A (mobile station &lt; 1 &gt;) transmits a request ( 61 ) and another request ( 62 ) to the base station  30  in an allocated band of the UP-LINK  52 . The request ( 61 ) calls for the allocation of a band for inter-mobile station direct communication for transmitting data to the mobile station  40 B (mobile station &lt; 2 &gt;). The request ( 62 ) calls for the allocation of a band for normal base station-mobile station communication. 
         [0072]    The mobile station  40 B (mobile station &lt; 2 &gt;) also transmits a request ( 63 ) to the base station  30  in an allocated band of the UP-LINK  52 , calling for the allocation of a band for normal base station-mobile station communication. 
         [0073]    The base station  30  then transmits an allocation request ( 64 ) to the central control unit  31  which bundles the band allocation request information from each mobile station and the band allocation request from the base station  30  to the mobile station. 
         [0074]    The central control unit  31 , in response to the band allocation request ( 64 ), notifies the base station  30  of information about the allocations of bands in the next frame of the base station-mobile station communication frequency and the inter-mobile station direct communication frequency ( 65 ). 
         [0075]    The base station  30  transmits, in the control information band in the next frame, information about the band and frequency allocation for direct communication from the mobile station  40 A (mobile station &lt; 1 &gt;) to the mobile station  40 B (mobile station &lt; 2 &gt;) ( 66 ), and base station-mobile station communication band allocation information ( 67 ,  68 ) to the mobile station  40 A (mobile station &lt; 1 &gt;) and the mobile station  40 B (mobile station &lt; 2 )). Each mobile station carries out reception processing on the control information band. 
         [0076]    The base station  30 , in an allocated band in the DOWN-LINK  51  at the base station-mobile station communication frequency, transmits base station-mobile station communication data ( 69 ,  70 ) to the mobile station  40 A (mobile station &lt; 1 &gt;) and the mobile station  40 B (mobile station &lt; 2 &gt;). Each mobile station carries out reception processing on an allocated band. 
         [0077]    The mobile station  40 A (mobile station &lt; 1 &gt;) transmits inter-mobile station direct communication data ( 71 ) to the mobile station  40 B (mobile station &lt; 2 &gt;) in an allocated band for direct communication of the inter-mobile station direct communication frequency. The mobile station  40 B (mobile station &lt; 2 &gt;) carries out a reception processing on an allocated band. 
         [0078]    The mobile station  40 A (mobile station &lt; 1 &gt;) and the mobile station  40 B (mobile station &lt; 2 &gt;) transmit base station-mobile station communication data ( 72 ,  73 ) to the base station  30  on an allocated band in the UP-LINK  52 . The base station  30  carries out a reception processing on an allocated band. 
         [0079]      FIG. 5  shows a flowchart of the communication processing in the base station  30 . S indicates the steps thereof. 
         [0080]    Initially, in a carrier sensing process in step S 11 , available frequencies are detected by the wireless processing means  33  ( FIG. 1 ) of the base station  30 . Of the frequencies that are detected, one that is used by the base station  30  is selected, and preparations are made for transmission and reception. At the same time, notification of frequencies available for inter-mobile station direct communication is given to the central control unit  31 . Simultaneously, notification of available frequencies other than those used by the base station  30  is given to the central control unit  31 . 
         [0081]    In the process of transmitting a control signal in step S 12 , the wireless control means  35  creates control information including a signal for frame synchronization and band allocation information allocated by the central control unit  31 . The control information is converted into transmission data by the frame processing means  34 , and then transmitted via the wireless processing means  33  and antenna  32 . 
         [0082]    In the process of transmitting the communication data in step S 13 , the line processing means  36  transmits external transmission data to the frame processing means  34  in accordance with instructions from the wireless control means  35 . The data is converted into transmission data by the frame processing means  34  and then transmitted via the wireless processing means  33  and antenna  32 . 
         [0083]    In the process of receiving the communication data in step S 14 , the wireless signal is received by the antenna  32  and the wireless processing means  33 . The received data is separated into control information and reception data by the frame processing means  34 . The control information such as a band allocation request is processed by the wireless control means  35 , and necessary information is transmitted to the central control unit  31 , where it is processed. The reception data is transmitted to the outside by the line processing means  36  after being set in an external format. 
         [0084]    In step S 15 , it is determined whether or not communication is finished. If not, the process resumes from step S 11  and the above-described steps are repeated. If the communication is finished, the present process comes to an end. 
         [0085]      FIG. 6  shows a flowchart of the communication processing performed in the mobile station. 
         [0086]    In a carrier sensing step of step S 21 , available frequencies are detected by the wireless processing means  43 . 
         [0087]    In the process of receiving the control signal in step S 22 , frame synchronization is carried out based on the control signal transmitted from the base station  30  and received by the antenna  42  and the wireless processing means  43 . The control signal is converted into control information by the frame processing means  44 . The wireless control processing means  45  decodes band allocation information and the like from the control information and stores it as control information for use during the process of transmitting and receiving communication data. 
         [0088]    Upon reaching an allocated band (time) in a frame, the wireless control processing means  45  notifies the wireless processing means  43  of the start of a transmission/reception operation. In step S 23 , it is determined whether or not the communication is inter-mobile station direct communication. If so, the wireless processing means  43  is notified of a change of frequency to that used for inter-mobile station direct communication in step S 24 . The transmission/reception frequency is then changed and the routine progresses to step S 25 . If the communication is not inter-mobile station direct communication, the routine progresses to step S 25 . 
         [0089]    In the process of transmitting and receiving communication data in step S 25 , the wireless processing means  43  receives the communication data and sends it to the frame processing means  34 , where the data is separated into control information and reception data. The control information is processed by the wireless control means  35 . The reception data is sent to the outside by the line processing means  36  after being set in an external format. External transmission data is sent by the line processing means  36  to the frame processing means  34  in accordance with an instruction from the wireless control means  35 , where the data is converted into transmission data and then transmitted by the wireless processing means  43  and antenna  42 . 
         [0090]    In step S 26 , it is determined whether or not there is a band. If there is a band, the routine returns to step S 23  to repeat the above-described processes. If not, it is determined in step S 27  whether or not communication is finished. If not, the routine returns to step S 21  to repeat the relevant steps. If the communication is finished, the present process comes to an end. 
         [0091]    Thus, in accordance with the present embodiment, the wireless communication system comprises a synchronization means for, when the central control unit  31  uses a second frequency for inter-mobile station direct communication which is different from the first frequency used for base station-mobile that communication, synchronizing frames between base station-mobile station communication and inter-mobile station direct communication based on the control information used in base station-mobile station communication. The system also comprises an allocation means for allocating a band and frequency for the inter-mobile station direct communication in accordance with an allocation request from the base station  30  or the mobile stations  40 . Because the control information  55  for base station-mobile station communication is utilized instead of the control information for the normal inter-mobile station direct communication in synchronizing frames between base station-mobile station communication and inter-mobile station direct communication, it is possible to process in parallel the band DOWN-LINK  51  for communication from the base station  30  to the mobile stations  40 , the band UP-LINK  52  for communication from the mobile stations  40  to the base station  30 , and the band  54  for communication from one mobile station  40  to another mobile station  40 . 
         [0092]    Thus, an inter-mobile station direct communication and a base station-mobile station communication can be simultaneously conducted, or the inter-mobile station direct communication can be conducted while maintaining connection with the base station  30 , even when the inter-mobile station direct communication is conducted at a frequency other than that of the base station  30 . 
         [0093]    Further, because the base station controls the inter-mobile station direct communication, the burden on the mobile stations can be reduced. 
       Second Embodiment 
       [0094]    In the second embodiment, the manner in which communication bands are allocated is changed. 
         [0095]      FIG. 7  shows a control sequence illustrating the flow of data in the wireless communication system according to the second embodiment of the invention.  FIG. 8  shows a block diagram of band allocation in the present wireless communication system, which corresponds to  FIG. 2 . 
         [0096]    A wireless LAN can be constructed by using the present wireless communication system in the same manner as described in  FIG. 3 . 
         [0097]    Referring to  FIG. 7 , it is assumed that the base station  30 , mobile station  40 A (mobile station &lt; 1 &gt;), mobile station  40 B (mobile station &lt; 2 &gt;), and mobile station  40 C (mobile station &lt; 3 &gt;) all have their communication lines open ( 80 ). 
         [0098]    The mobile station  40 C (mobile station &lt; 3 &gt;) transmits a communication band request ( 81 ) to the base station. The mobile station  40 A (mobile station &lt; 1 &gt;) transmits a communication band request ( 82 ) to the mobile station  40 B (mobile station &lt; 2 &gt;). 
         [0099]    In response to these band requests, the central control unit  31  allocates a band  92  (band D 1 ) and a band  93  (band U 1 ) as communication bands between the base station  30  and the mobile station  40 C (mobile station &lt; 3 &gt;) ( 83 ). The central control unit  31  also allocates a band  94  (band D 2 ) and a band  95  (band U 2 ) as communication bands between the mobile station  40 A (mobile station &lt; 1 &gt;) and the mobile station  40 B (mobile station &lt; 2 &gt;) ( 84 ). Numeral  91  designates control information. According to these allocations, the base station  30  and the mobile station  40 C (mobile station &lt; 3 &gt;) communicate with each other ( 85 ,  87 ), while the mobile station  40 A (mobile station &lt; 1 &gt;) and the mobile station  40 B (mobile station &lt; 2 &gt;) communicate with each other ( 86 ,  88 ). 
         [0100]    In accordance with the wireless communication system of the present embodiment, the base station  30  instructs that the inter-mobile station direct communication between the mobile station  40 A (mobile station &lt; 1 &gt;) and the mobile station  40 B (mobile station &lt; 2 &gt;) be conducted at a frequency other than that of the base station  30 . Thus, the base station  30  can communicate with the mobile station  40 C (mobile station &lt; 3 &gt;). 
         [0101]    When the direct communication between the mobile station  40 A (mobile station &lt; 1 &gt;) and the mobile station  40 B (mobile station &lt; 2 &gt;) is conducted at the same frequency as that of the base station  30 , the base station  30  cannot transmit or receive data on that band. However, in accordance with the present embodiment, the base station  30  can transmit and receive data on all of the bands (times) in the frame because the direct communication between the mobile stations  40 A and  40 B is conducted at a different frequency. 
         [0102]    Thus, an efficient wireless communication system can be realized in which a single base station can use a plurality of frequencies for communication. 
       Third Embodiment 
       [0103]    This is an example where the manner in which communication frame bands are allocated is changed. 
         [0104]      FIG. 9  shows a block diagram of frame allocation in the wireless communication system according to the third embodiment of the invention, indicating examples of band allocations in frames for the base station-mobile station communication frequency and the inter-mobile station direct communication frequency. 
         [0105]    A wireless LAN can be constructed using the present wireless communication system in the same manner as described in  FIG. 3 . 
         [0106]    Referring to  FIGS. 3 and 9 , the base station  30  broadcasts frame synchronization and band allocations in control information  100 . Down-link phases D 1  ( 101 ), D 2  ( 102 ), D 3  ( 104 ), and D 4  ( 103 ) are allocated bands for transmission from the base station  30  to the mobile station  40 A (mobile station &lt; 1 &gt;), mobile station  40 B (mobile station &lt; 2 &gt;), mobile station  40 C (mobile station &lt; 3 &gt;), and mobile station  40 D (mobile station &lt; 4 &gt;) (not shown in  FIG. 3 ), respectively. Similarly, Up-link phases U 1  ( 107 ), U 2  ( 108 ), U 3  ( 106 ), and U 4  ( 105 ) are allocated bands for transmission from the mobile station  40 A (mobile station &lt; 1 &gt;), mobile station  40 B (mobile station &lt; 2 &gt;), mobile station  40 C (mobile station &lt; 3 &gt;), and mobile station  40 D (mobile station &lt; 4 &gt;), respectively, to the base station  30 . A 1  ( 109 ) is an allocation band for a direct communication between the mobile station  40 A (mobile station &lt; 1 &gt;) and the mobile station  40 B (mobile station &lt; 2 &gt;). While in this example the number of the direct communication frequency is one, there may be more than one such frequency as long as they can be used in the wireless communication system. 
         [0107]      FIG. 10  shows an example of communication between the base station and the mobile stations in the present wireless communication system, illustrating the state of communication at each communication terminal allocated in  FIG. 9 . 
         [0108]    The base station  30  broadcasts the control information (including band allocation information) for frame synchronization, transmits information to the individual mobile stations in D 1 , D 2 , D 3 , and D 4 , and receives information from the individual mobile stations in U 1 , U 2 , U 3 , and U 4 . In this example, the states of communication in about four frames are illustrated on the assumption that there is no change in allocations. 
         [0109]    The mobile station  40 A (mobile station &lt; 1 &gt;) receives the control information from the base station  30 , receives information from the base station  30  in D 1 , and transmits information to the base station  30  in U 1 . The mobile station  40 A (mobile station &lt; 1 &gt;) carries out a direct communication with the mobile station  40 B (mobile station &lt; 2 &gt;) in an allocation A 1  at a frequency different from that for the base station-mobile station communication, as shown in the second and third frames from the head. 
         [0110]    Thus, the mobile station  40 A (mobile station &lt; 1 &gt;) can directly communicate with the mobile station  40 B (mobile station &lt; 2 &gt;) while at the same time communicating with the base station  30 . Further, because the direct communication is carried out at a separate frequency, the base station  30  can communicate with the mobile station  40 C (mobile station &lt; 3 &gt;) and mobile station  40 D (mobile station &lt; 4 &gt;), even while the direct communication is taking place. 
       Fourth Embodiment 
       [0111]    This is an example where the manner in which bands in a communication frame are allocated is changed. 
         [0112]      FIG. 11  shows a block diagram of frame allocation in the wireless communication system according to the fourth embodiment of the invention, illustrating examples of band allocations in frames for base station-mobile station communication and inter-mobile station direct communication frequencies. 
         [0113]    A wireless LAN can be constructed by using the present wireless communication system in the same manner as described in  FIG. 3 . 
         [0114]    Referring to  FIGS. 3 and 11 , for the base station-mobile station communication frequency, control information ( 110 ) for frame synchronization, Down-link phases D 1  ( 111 ), D 2  ( 112 ), D 3  ( 114 ), and D 4  ( 113 ) are allocated bands for transmission from the base station to the mobile station  40 A (mobile station &lt; 1 &gt;), mobile station  40 B (mobile station &lt; 2 &gt;), mobile station  40 C (mobile station &lt; 3 &gt;), and mobile station  40 D (mobile station &lt; 4 &gt;) (not shown in  FIG. 3 ), respectively. Up-link phases U 1  ( 117 ), U 2  ( 118 ), U 3  ( 116 ), and U 4  ( 115 ) are allocated bands for transmission from the mobile station  40 A (mobile station &lt; 1 &gt;), mobile station  40 B (mobile station &lt; 2 &gt;), mobile station  40 C (mobile station &lt; 3 &gt;), and mobile station  40 D (mobile station &lt; 4 &gt;), respectively, to the base station. A 1  ( 121 ), A 2  ( 120 ), A 3  ( 123 ), A 4  ( 119 ), and A 5  ( 122 ) are allocations for direct communication between the mobile stations &lt; 2 &gt; and &lt; 3 &gt;, the mobile stations &lt; 1 &gt; and &lt; 3 &gt;, the mobile stations &lt; 1 &gt; and &lt; 2 &gt;, the mobile stations &lt; 1 &gt; and &lt; 4 &gt;, and the mobile stations &lt; 3 &gt; and &lt; 4 &gt;. 
         [0115]      FIG. 12  shows an example of communication between the base station and the mobile stations in the present wireless communication system, illustrating the state of communication at each communication terminal allocated in  FIG. 11 . 
         [0116]    For example, in D 1  where the mobile station  40 A (mobile station &lt; 1 &gt;) is communicating with the base station, the mobile station  40 A (mobile station &lt; 1 &gt;) cannot communicate with any other mobile station unless the mobile station  40 A (mobile station &lt; 1 &gt;) is provided with a plurality of wireless transmission/reception units. 
         [0117]    Thus, the central control unit  31  ( FIG. 3 ) provides allocations for direct communication in bands other than D 1 , as shown in  FIG. 12 . 
         [0118]    In the present embodiment, the central control unit  31  calculates combinations for carrying out base station-mobile station communication and inter-mobile station direct communication based on the band allocation requests from each mobile station and the base station, provides efficient allocations that maximally utilize the wireless bands, as shown in  FIG. 11 , and notifies the base station  30  as well as broadcasts to each mobile station about their allocations. By such a centralized manner of control, frequencies can be completely utilized without the stand-by times for avoiding collision of signals that are required in autonomous decentralized systems such as according to IEEE802.11. Furthermore, the central management of the requests for inter-mobile station direct communication and base station-mobile station communication by the single central control unit  31  allows transmission and reception to occur without collision of allocations. 
         [0119]    In the above-described embodiments, the invention is applied to a wireless communication system comprising the base station  30 , the mobile stations  40  wirelessly connected to the base station  30 , and the central control unit  31  provided within the base station  30  for allocating bands in the frames of communication frequencies in response to band allocation requests. However, the invention can be applied to any system as long as it is a TDMA-based wireless communication system. For example, the invention can be applied to cordless telephone systems in which a master unit is connected to a plurality of slave units via wireless communication, PDAs (Personal Digital Assistants) with wireless communication capabilities, and small-sized notebook personal computers. 
         [0120]    The wireless communication system may also be comprised of a plurality of identical wireless terminals having functions both as base stations, namely masters that manage communications, and as mobile stations, namely slaves that follow the master&#39;s instructions, each wireless terminal carrying out both base station and mobile station operations. 
       INDUSTRIAL FIELD OF APPLICABILITY 
       [0121]    Thus, the wireless communication system according to the invention allows inter-mobile station direct communication and base station-mobile station communication to take place simultaneously, or inter-mobile station direct communication to take place while maintaining connection with the base station, when inter-mobile station direct communication is carried out at a frequency other than that of the base station (see  FIGS. 9 and 10 ). 
         [0122]    Further, because the base station controls inter-mobile station direct communication, the burden on the mobile stations can be reduced. 
         [0123]    Further, because inter-mobile station direct communication can be carried out at a second frequency that is different from the first frequency used in base station-mobile station communication, the base station can carry out transmission or reception of data even in time regions where inter-mobile station direct communication is taking place, for example. Thus, an efficient wireless communication system can be realized in which the base station can utilize a plurality of frequencies for communication.