Abstract:
Wireless communication is carried out between devices. A highest frequency band is selected. When the selected frequency band includes an unused channel in which no disturbing wave is present, a maximum transmission rate at which a received field strength value exceeds a threshold value is determined. When the selected frequency band does not include an unused channel or one in which no disturbing wave is present or there is no transmission rate associated with the selected frequency band at which the received field strength value exceeds the threshold value, the next highest frequency band is successively selected and the above is repeated. When the maximum transmission rate is successfully determined, communication is initiated using the unused channel of the selected frequency band at the maximum transmission rate as a communication channel.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a national stage application under 35 U.S.C. §371 of International Application No. PCT/JP03/05107, filed Apr. 22, 2003, which claims priority from Japanese Application No. P2002-120518, filed Apr. 23, 2002, the disclosures of which are incorporated by reference herein. 
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates to a wireless communication method for use with a wireless communication system constituted by a plurality of wireless communication apparatuses and to wireless communication apparatuses that composes the wireless communication system. 
     2. Background Art 
     A system constituted of a base apparatus and a display terminal has been developed. The base apparatus functions as an information source or as an access point in which or to which a television broadcast receiving tuner is built or connected and that is connected to a telephone line through a modem as one type of a wireless LAN system that is structured in a limited area of a residence, an office, or the like. The display terminal executes functions for issuing a command to the base apparatus through a wireless communication with the base apparatus that includes receiving a picture of the television from the base apparatus, receiving information from the Internet, displaying the picture and information on a display, and transmitting and receiving electronic mail through the base apparatus. 
     As radio frequency bands that can be used for the wireless communication system, the IEEE 802.11a standard has defined a 5.8 GHz band (in U.S. a 5.2 GHz band, these bands are generally called 5 GHz band), whereas the IEEE 802.11b standard has defined a 2.4 GHz band. 
     When a wireless communication system deals with both the 5.2 GHz band and the 2.4 GHz band, it can perform a communication over a radio channel (radio frequency) properly selected as a communication channel from the 5.2 GHz band and 2.4 GHz band. 
     However, in a communicable area of the above-described wireless communication system, another wireless communication system of the same type as the present system or of a different type, such as Bluetooth (a registered trademark), that uses the same frequency bands as the present system might coexist. 
     In addition, if another system coexists with the present system, a communication radio wave of the other system may become a disturbing wave that causes data streams to be broken, moving pictures streams to be stopped, and images to be disturbed with respect to the data transmission in the present system. 
     Also, besides those wireless communication systems that interfere with the present system, there may be, for example, microwave ovens and so forth in the vicinity of the present system. When such a device radiates a radio wave of the radio frequency band that the present system uses, the radio wave may act as a disturbing wave and adversely affect the present system. 
     Furthermore, when a large volume of data, such as picture data from the television and moving picture data of the Internet, are transmitted, it would be desirable to increase the data transmission rate. 
     However, in the 5.2 GHz band defined in the IEEE 802.11a standard, the maximum transmission rate can be increased up to at most 54 M bps (mega bits/second). In contrast, in the 2.4 GHz band defined in the IEEE 802.11b standard, the transmission rate can be increased up to at most 11 Mbps. 
     If the transmission rate at which a large volume of data, such as picture data and moving picture data, is transmitted is low, it might be difficult to securely and smoothly transmit the data in a real time basis. 
     SUMMARY OF THE INVENTION 
     Therefore, the present invention provides a wireless communication system that deals with a plurality of communicable frequency bands having different transmission rates to allow a large volume of data to be securely and smoothly transmitted in real time without a disturbance from another wireless communication system and so forth and without abnormalities such as the stopping of a moving picture and disturbance of a still image. 
     A wireless communication method of the present invention is for use with a wireless communication system for performing a communication on a radio channel as a communication channel in a frequency band selected from a plurality of communicable frequency bands having different transmission rates, the wireless communication method comprising the steps of: 
     detecting radio channels that are not used in the system and that are free of a disturbing wave transmitted from the outside of the system from the frequency bands in decreasing order from relatively higher transmission rates; 
     detecting whether or not received field strengths at transmission rates of the detected radio channels exceed a predetermined threshold value in decreasing order from the relatively higher transmission rates; and 
     starting a communication on a channel having a transmission rate at which the received field strength reaches or exceeds the predetermined threshold value in a manner that a communicable frequency band having a relatively higher transmission rate is prioritized and that an unused channel that is free of a disturbing wave and that has a relatively higher transmission rate in one of the frequency bands is prioritized as a communication channel in accordance with the results of the first and second detecting steps. 
     In the wireless communication method according to the present invention of the above-described method, a frequency band communicable at a high transmission rate is preferentially selected. A communication is started on a communication channel that is free of a disturbing wave preferentially at a high transmission rate. Therefore, the wireless communication method according to the present invention is capable of securely and smoothly transmitting a large volume of data on real time basis without a disturbance of another wireless communication system and abnormalities of stop of a moving picture and a disturbance of an image. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing aspects, features and advantages of the present invention will be further appreciated when considered with reference to the following detailed description and accompanying drawings. 
         FIG. 1  is a schematic diagram showing an external structure of an example of a display terminal used as a wireless communication apparatus. 
         FIG. 2  is a schematic diagram showing an external structure of an example of a base apparatus used as the wireless communication apparatus. 
         FIG. 3  is a schematic diagram showing a raised state of the display terminal. 
         FIG. 4  is a functional block diagram of the example of the structure of the base apparatus as the wireless communication apparatus shown in  FIG. 2 . 
         FIG. 5  is a functional block diagram of the example of the structure of the display terminal as the wireless communication apparatus shown in  FIG. 1 . 
         FIG. 6  is a schematic diagram showing a channel structure of a 5.2 GHz band. 
         FIG. 7  is a schematic diagram showing a channel structure of a 2.4 GHz band. 
         FIG. 8  is a schematic diagram showing a transmission rate and modulation of the 5.2 GHz band. 
         FIG. 9  is a schematic diagram showing a transmission rate and modulation of the 2.4 GHz band. 
         FIG. 10  is a flow diagram showing a first part of an example of a setting process performed upon startup of communication. 
         FIG. 11  is a flow diagram showing a second part of the setting process shown in  FIG. 10 . 
         FIG. 12  is a flow diagram showing a first part of an example of a mode setting process for the 5.2 GHz band. 
         FIG. 13  is a flow diagram showing a second part of the mode setting process shown in  FIG. 12 . 
         FIG. 14  is a flow diagram showing an example of a mode setting process for the 2.4 GHz band. 
         FIG. 15  is a flow diagram showing a first part of an example of a changing process performed during communication in the 5.2 GHz band. 
         FIG. 16  is a flow diagram showing a second part of the changing process shown in  FIG. 15 . 
         FIG. 17  is a flow diagram showing a first half part of an example of a changing process performed during communication in the 2.4 GHz band. 
         FIG. 18  is a flow diagram showing a second part of the changing process shown in  FIG. 17 . 
         FIG. 19  is a flow diagram showing an example of a mode changing process performed during communication in the 5.2 GHz band when a transmission rate is increased. 
         FIG. 20  is a flow diagram showing a first part of an example of a mode changing process performed during communication in the 2.4 GHz band when a transmission rate is decreased. 
         FIG. 21  is a flow diagram showing a second part of the mode changing process shown in  FIG. 20 . 
     
    
    
     DETAILED DESCRIPTION 
     Best Mode for Carrying Out the Invention 
     Next, exemplifying the case in which the present invention is applied to the wireless communication system, which is constituted by the above-described base apparatus and display terminal, an embodiment of the present invention will be described. 
     External Structures of Display Terminal and Base Apparatus:  FIG. 1  to  FIG. 3 : 
       FIG. 1  shows an external structure of an example of the display terminal, and  FIG. 2  shows an external structure of an example of the base apparatus. 
     As shown in  FIG. 1 , an Liquid Crystal Display (LCD)  51  is disposed at the front of a display terminal  50 . A touch panel  53  is disposed on a display screen of the LCD  51 . Speakers  55  are disposed at an upper left position and an upper right position of the LCD  51 . Plain antennas  89   a ,  89   b  for performing wireless communication with the base apparatus  10 , which will be described later, are disposed at a lower left position and a lower right position of the LCD  51 . 
     The antenna  89   a  is used for a frequency band Ba (5.2 GHz band), whereas the antenna  89   b  is used for a frequency band Bb (a 2.4 GHz band). The left side antenna forms a semi-spherical surface radiation pattern in the forward direction of the display terminal  50 . The right side antenna forms a semi-spherical surface radiation pattern in the backward direction of the display terminal  50 . One of the antennas is selected to transmit and receive a radio wave based on reception level information of the left side antenna and the right side antenna. The combination of the left side antenna and the right side antenna forms an antenna that has an all spherical surface radiation pattern. Regardless of the relation of positions of the display terminal  50  and the base apparatus  10 , wireless communication between the display terminal  50  and the base apparatus  10  is securely performed. 
     Below the speaker  55  on the right side of the front of the display terminal  50 , an index button  57   a , a jump button  57   b , and channel buttons  57   c ,  57   d  are disposed. 
     Pressing the index button  57   a  causes an index screen, as shown in the drawing, to be displayed on the LCD  51 . By touching any item of the menu on the index screen with a touch pen or user&#39;s finger, the user can select, for example, a channel of the television, operate an external device that is connected to the base apparatus  10 , access the Internet, create and transmit e-mail, and/or create and display an album. 
     Pressing the jump button  57   b , it allows an immediately preceding television channel to be received. By pressing the channel button  57   c , the current mode displayed on the LCD  51  is switched in the order from television→external device→Internet→mail→album→television. By pressing the channel button  57   d , the operation screen displayed on the LCD  51  is switched in the reverse order. 
     An album is a picture or the like that is captured by a digital camera and recorded on a the memory card  77  and which is then displayed on the LCD  51 , processed on the LCD  51 , and attached to e-mail created on the LCD  51 . Alternatively, an album is a picture that is stored in the display terminal  50  or in the memory card  77 , a television image captured as a still picture, a picture received by e-mail, a picture obtained from the Internet, or the like. 
     On an upper surface of the display terminal  50 , a groove portion  69  and so forth are formed. The groove portion  69  accommodates a touch pen  59 . On the left side, a knob  91  and so forth are disposed. The knob  91  adjusts the brightness of the LCD  51 . On the right side, a slot  79  and so forth are formed. In the slot  79 , the memory card  77  is attached. At the bottom, charging terminals  94 ,  96  are disposed. 
     At a the back surface of the display terminal  50 , a U-shaped stand  99  that allows the display terminal  50  to be raised is extensively and contractively disposed. A battery accommodating portion (not shown) is disposed at a portion surrounded by the stand  99 . A battery is accommodated in the battery accommodating portion. 
     As shown in  FIG. 2 , the base apparatus  10  is constituted of a front portion  12  and a rear portion  14  that are integrally connected. At left and right positions of the front portion  12 , plain antennas  49   a ,  49   b  for performing wireless communication with the display terminal  50  are disposed. 
     Like the antennas  89   a ,  89   b  of the display terminal  50 , the antenna  49   a  is used for the frequency band Ba (5.2 GHz band) whereas the antenna  49   b  is used for the frequency band Bb (2.4 GHz band). The left side antenna forms a semi-spherical surface radiation pattern in the forward direction of the base apparatus  10 . The right side antenna forms a semi-spherical surface radiation pattern in the backward direction of the base apparatus  10 . In accordance with reception level information of the left side antenna and the right side antenna, one of the antennas is selected to transmit and receive a radio wave. The combination of the right side antenna and left side antenna forms an antenna that has a half-spherical surface radiation pattern. Regardless of the relation of the positions of the base apparatus  10  and the display terminal  50 , wireless communication can be securely performed between the base apparatus  10  and the display terminal  50 . 
     The front portion  12  is slanted backwards and, in a lower center portion thereof, a supporting member  16  is disposed that causes the display terminal  50  to be inclined against the base apparatus  10 . Charging terminals  24 ,  26  are disposed in the supporting member  16 . In addition, at a lower portion on the rear surface of the rear portion  14 , various types of terminals, such as an antenna terminal and a line terminal that will be described later, are disposed. 
     With respect to the above-described base apparatus  10  and display terminal  50 , the user can place the base apparatus  10  at a fixed position and carry the display terminal  50  to any place inside a communicable area. The user can execute functions for receiving a television broadcast, accessing the Internet, and transmitting and receiving electronic mail with the display terminal  50  that the user is holding at any place. 
     The user can operate the display terminal  50  by hand or, alternatively, with the stand  99  extended, as shown in  FIG. 3 , so that the display terminal  50  can be raised on a proper surface at a properly inclined angle. 
     In addition, the display terminal  50  may be inclined against the front portion  12  of the base apparatus  10 . In this case, the charging terminals  94 ,  96  of the display terminal  50  are brought into contact with the charging terminals  24 ,  26  of the base apparatus  10  and are connected. As a result, the battery accommodated in the display terminal  50  can be charged by the base apparatus  10 . 
     Functional Blocks of Structures of Base Apparatus and Display Terminal:  FIG. 4  and  FIG. 5 : 
       FIG. 4  shows the functional blocks of an example of a structure of the base apparatus  10 . A controlling portion  30  comprises a CPU  31 . The CPU  31  is connected to a bus  33 . A program executed by the CPU  31 , fixed data, and so forth are written to a memory  35  in advance. The memory  35  also functions as a work area and so forth of the CPU  31 . The memory  35  is connected to the bus  33 . 
     An antenna  1  for receiving a television broadcast is connected to an antenna terminal  11 . A television broadcast signal is received by the antenna  1  is channel-selected and demodulated by a tuner  21 . The channel-selected and demodulated signal is then compressed and further converted into video data and audio data. The video data and audio data are then sent to the bus  33 . 
     A telephone line  3  is connected to a line terminal  13 . The line terminal  13  is connected to the bus  33  through a modem  23 . 
     In addition, an Ethernet (registered trademark) terminal  15 , for connecting an ADSL modem, a CATV modem, or the like, is connected to the bus  33  through an interface  25 . 
     A DVD player, a hard disk recorder, a digital CS tuner, or the like is connected as external device  7  to a terminal  17 . Video data and audio data from the external device  7  are sent to the bus  33  through an interface  27 . 
     In addition, an AV mouse  9  is connected to a terminal  19 . The terminal  19  is connected to the bus  33  through an interface  29 . An infrared ray remote control signal is emitted by a light emitting portion of the AV mouse  9 , in accordance with a command signal that is outputted from the controlling portion  30 , and is received by a light detecting portion disposed in the external device  7 . As a result, using the infrared ray remote control signal, the external device  7  is operated. 
     Base Band Processors (BBP)  41   a ,  41   b  for the frequency bands Ba, Bb are connected, respectively, to the bus  33 . Transmitting and receiving portions  45   a ,  45   b  for the frequency bands Ba, Bb are connected to the BBPs  41   a ,  41   b , respectively. The above-described antennas  49   a ,  49   b  are connected to the transmitting and receiving portions  45   a ,  45   b , respectively. 
     Also, disturbing wave detecting portions  43   a ,  43   b  are connected between the BBPs  41   a ,  41   b  and the bus  33 , respectively. Received field strength detecting portions  47   a ,  47   b  are connected between the transmitting and receiving portions  45   a ,  45   b  and the bus  33 , respectively. The disturbing wave detecting portions  43   a ,  43   b  detect whether or not a disturbing wave exists on a radio channel selected from the frequency bands Ba, Bb, respectively, by a method that will be described later. The received field strength detecting portions  47   a ,  47   b  detect the received field strengths of the signals received by the transmitting and receiving portions  45   a ,  45   b  in accordance with control levels of an Automatic Gain Control (AGC) against the signals received by the transmitting and receiving portions  45   a ,  45   b , respectively. 
     A signal transmitted from the base apparatus  10  to the display terminal  50  is processed for a baseband by the BBPs  41   a ,  41   b  and is then modulated by the transmitting and receiving portions  45   a ,  45   b , respectively. The modulated signal is next converted into a signal of a radio channel selected from the frequency bands Ba, Bb. Thereafter, the radio channel signal is transmitted from the transmitting and receiving portions  45   a ,  45   b  to the display terminal  50  through the antennas  49   a ,  49   b , respectively. 
     In addition, a signal of a radio channel selected from the frequency bands Ba and Bb and transmitted from the display terminal  50  to base apparatus  10  is received by the transmitting and receiving portions  45   a ,  45   b  through the antennas  49   a ,  49   b , respectively. The received signal is frequency converted by the transmitting and receiving portions  45   a ,  45   b , respectively. Thereafter, the frequency converted signal is processed for a baseband by the BBPs  41   a ,  41   b  and then received by the bus  33 . 
       FIG. 5  shows functional blocks of an example of a structure of the display terminal  50 . A controlling portion  70  comprises a CPU  71 . The CPU  71  is connected to a bus  73 . 
     A program executed by the CPU  71 , fixed data, and so forth are written to a memory  75  in advance. The memory  75  also functions as a work area and so forth of the CPU  71 . The memory  75  is connected to the bus  73 . 
     The LCD  51  is connected to the bus  73  through a display controlling portion  61 . A speaker  55  is connected to the bus  73  through a D/A converter (DAC)  65  and an audio amplifying circuit  66 . In addition, the touch panel  53  is connected to the bus  73  through a coordinate detecting portion  63 . Moreover, a key operation portion  57  including the index button  57   a  shown in  FIG. 1 , is connected to the bus  73  through an interface  67 . 
     When the memory card  77  is attached to the slot  79 , shown in  FIG. 1 , the memory card  77  is connected to the bus  73 . 
     In addition, BBPs  81   a ,  81   b  for the frequency bands Ba, Bb, respectively, are connected to the bus  73 . Transmitting and receiving portions  85   a ,  85   b  for the frequency bands Ba, Bb are connected to the BBPs  81   a ,  81   b , respectively. The above-described antennas  89   a ,  89   b  are connected to the transmitting and receiving portions  85   a ,  85   b , respectively. 
     In addition, disturbing wave detecting portions  83   a ,  83   b  are connected between the BBPs  81   a ,  81   b  and the bus  73 , respectively. Received field strength detecting portions  87   a ,  87   b  are connected between the transmitting and receiving portions  85   a ,  85   b  and the bus  73 , respectively. The disturbing wave detecting portions  83   a ,  83   b  detect whether a disturbing wave is present in a radio channel selected from the frequency bands Ba, Bb, respectively, by a method that will be described later. The received field strength detecting portions  87   a ,  87   b  detect the received field strengths of signals received by the transmitting and receiving portions  85   a ,  85   b  in accordance with control levels of the AGC against signals received by the transmitting and receiving portions  85   a ,  85   b , respectively. 
     A signal transmitted from the display terminal  50  to base apparatus  10  is processed for a baseband by the BBPs  81   a ,  81   b  and is then modulated by the transmitting and receiving portions  85   a ,  85   b , respectively. Thereafter, the modulated signal is converted into a signal of a radio channel selected from the frequency bands Ba, Bb. The signal of the radio channel is then transmitted from the transmitting and receiving portions  85   a ,  85   b  to the base apparatus  10  through the antennas  89   a ,  89   b , respectively. 
     In addition, a signal of a radio channel selected from the frequency bands Ba and Bb is transmitted from the base apparatus  10  to the display terminal  50 . The signal is received by the transmitting and receiving portions  85   a ,  85   b  through the antennas  89   a ,  89   b , respectively. The received signal is frequency converted and demodulated by the transmitting and receiving portions  85   a ,  85   b , respectively. Thereafter, the demodulated signal is processed for a baseband by the BBPs  81   a ,  81   b  and is then received by the bus  73 . 
     Radio Frequency Bands, Radio Channels, and Transmission Rates:  FIG. 6  to  FIG. 9 : 
     The above-described wireless communication system uses the 2.4 GHz band and the 5.2 GHz band, as defined in the IEEE 802.11a standard and the IEEE 802.11b, standard as the frequency bands Ba and Bb, respectively. 
     It has been determined that when a plurality of radio channels are set in the 5.2 GHz band and 2.4 GHz at the same time and in the same area, as shown in  FIG. 6  and  FIG. 7 , the frequency intervals between adjacent radio channels should be apart by 20 MHz or more and by 25 MHz or more to prevent a signal from one radio channel from becoming a disturbing wave to a signal of the other radio channel, respectively. 
     Therefore, the number of radio channels that can be set at the same time in the 5.2 GHz band is a maximum of four channels C 1 , C 2 , C 3 , and C 4 , shown in  FIG. 6 . In the 2.4 GHz band is a maximum of three channels C 5 , C 6 , and C 7  can be set, as shown in  FIG. 7 . 
     The transmission rates and modulation systems in the 5.2 GHz can be set in eight modes A 1  to A 8 , shown in  FIG. 8 , and those in the 2.4 GHz band in four modes B 1  to B 4 , shown in  FIG. 9 . The terms “modes A 1  to A 8 ” and “modes B 1  to B 4 ” are not defined in the IEEE 802.11a and IEEE 802.11b standards, but are defined in this specification for convenience. 
     Modulation systems BPSK, QPSK, QAM, and CCK, shown in  FIGS. 8 and 9 , are initials for the following: BPSK: Binary Phase Shift Keying, QPSK: Quadrature Phase Shift Keying, QAM: Quadrature Amplitude Modulation, and CCK: Complementary Code Keying. 
     The modulation systems shown in  FIG. 8  and  FIG. 9  are multi-value digital modulation (primary modulation) systems for the BBPs  41   a  and  41   b  of the base apparatus  10  and for the BBPs  81   a  and  81   b  of the display terminal  50 , respectively. Orthogonal Frequency Division Multiplexing (OFDM) is used as the radio frequency modulation for the frequency band Ba of the transmitting and receiving portion  45   a  of the base apparatus  10  and for the transmitting and receiving portion  85   a  of the display terminal  50 . Direct Sequencing (DS) is used as the radio frequency modulation for the frequency band Bb of the transmitting and receiving portion  45   b  of the base apparatus  10  and of the transmitting and receiving portion  85   b  of the display terminal  50 . 
     The transmission rate of the mode B 4  in the 2.4 GHz band can be higher than that of each of the modes A 1  and A 2  in the 5.2 GHz band as shown in the  FIG. 8  and  FIG. 9 . Generally, the transmission rate in the 5.2 GHz band can be higher than that in the 2.4 GHz band. 
     Setting Process Performed Upon Startup of Communication:  FIG. 10  and  FIG. 11 : 
     In the state that the power of the base apparatus  10  has been turned on in the above-described wireless communication system, when the user turns on the power of the display terminal  50  and performs an operation for receiving a television broadcast or performs an operation for accessing the Internet using the display terminal  50 , a connection request and a command are transmitted from the display terminal  50  to the base apparatus  10  as signals of a predetermined radio channel of a predetermined frequency band. 
     After the connection request and command have been received by the base apparatus  10  and the operation for receiving a television broadcast or for accessing the Internet has been performed, a communication with the display terminal  50  is started. Picture and audio data of the television, information of the Internet, and so forth are transmitted from the base apparatus  10  to the display terminal  50 . 
       FIG. 10  and  FIG. 11  show an example of a setting process for setting a communication frequency band, a communication channel, and a transmission rate that the controlling portion  30  (CPU  31 ) of the base apparatus  10  executes. 
     First, in step  101  of the setting process  100 , the controlling portion  30  determines whether an unused channel exists in the frequency band Ba (5.2 GHz band) in which a high transmission rate can be set. 
     While communication between the base apparatus  10  and a display terminal of the same type as that of the display terminal  50  is performed on a radio channel C 1  to C 4  as a communication channel in the frequency band Ba, the radio channel is not an unused channel. An unused channel is a radio channel that is not used as a communication channel by the present system. 
     When the controlling portion  30  determines that an unused channel exists in the frequency band Ba in the step  101 , the flow advances to step  102  wherein the controlling portion  30  determines whether a disturbing wave exists on the unused channel based on the detected result of the disturbing wave detecting portion  43   a  for the frequency band Ba. 
     A disturbing wave is a communication radio wave transmitted from a wireless communication system that is of the same type as or of a different type than the present system. Alternatively, the disturbing wave is a radio wave transmitted from a non-wireless communication apparatus, such as a microwave oven. 
     When determining whether a signal received by the transmitting and receiving portion  45   a  is a disturbing wave, the disturbing wave detecting portion  43   a  and the controlling portion  30  detect whether transmission destination address information is contained in a received signal that has been processed in the BBP  41   a . When the transmission destination address information is contained therein, the controlling portion  30  determines whether the transmission destination address information matches an apparatus address of the base apparatus  10 . 
     When transmission destination address information is contained in the received signal and matches the apparatus address (identification information that identifies an apparatus) of the base apparatus  10 , the controlling portion  30  determines that the received signal is not a disturbing signal but is instead a signal transmitted from the display terminal  50  to the base apparatus  10 . When the received signal is a radio wave of other than a communication radio wave of another wireless communication system and transmission destination address information is not contained in the received signal or when the received signal is a communication radio wave of another wireless communication system and transmission destination address information contained in the received signal, the controlling portion  30  determines that the received signal is a disturbing wave. 
     However, the system may be structured in a manner that, when the controlling portion  30  has determined that a received signal is a disturbing wave and the received field strength detecting portion  47   a  determines that the received field strength is so low that it can be ignored, as shown in step  102 , the controlling portion  30  determines that a disturbing wave does not exist on the unused channel. 
     When the controlling portion  30  has determined that a disturbing wave exists on the unused channel in step  102 , the flow advances to step  103  in which the controlling portion  30  determines whether another unused channel exists. When another unused channel exists, the flow returns to step  102  where, in the same manner as described above, the controlling portion  30  determines whether a disturbing wave exists on the unused channel. 
     When the controlling portion  30  has determined that a disturbing wave does not exist on the unused channel in step  102 , the flow advances to step  104 . After the controlling portion  30  has set the unused channel as a communication channel in step  104 , the flow advances to process routine  200 . In process routine  200 , the controlling portion  30  executes a mode setting process for the frequency band Ba. 
     In the mode setting process  200  for this frequency band Ba, as will be described later with reference to  FIG. 12  and  FIG. 13 , the controlling portion  30  detects received field strengths at transmission rates in decreasing order starting from higher transmission rates on the communication channel that has been set in step  104  of process routine  100 . The controlling portion  30  sets as a mode the highest transmission rate at which the received field strength reaches or exceeds a predetermined threshold value. 
     After the controlling portion  30  has executed the mode setting process  200 , the flow advances to step  105  where the controlling portion  30  determines whether communication should start in the frequency band Ba. When it is determined that the communication should start in the frequency band Ba, the controlling portion  30  completes the setting process. The controlling portion  30  starts the communication in the mode (transmission rate) that was set in the process  200  on the communication channel that was set in step  104 . 
     When the controlling portion  30  has determined that an unused channel does not exist in the frequency band Ba in step  101 , has determined that an unused channel free of a disturbing wave does not exist in the frequency band Ba) in step  103 , or has determined that an unused channel free of a disturbing wave exists in the frequency band Ba but the received field strengths at all the transmission rates do not exceed the threshold value) in step  105 , the flow advances to step  111  wherein the controlling portion  30  determines whether an unused channel exists in the frequency band Bb (2.4 GHz band). 
     When the controlling portion  30  has determined that an unused channel exists in the frequency band Bb, the flow advances from step  111  to step  112 , and the controlling portion  30  determines whether a disturbing wave exists on the unused channel in accordance with the detected result of the disturbing wave detecting portion  43   b  for the frequency band Bb. 
     In this case, the controlling portion  30  determines whether the signal received by the transmitting and receiving portion  45   b  is a disturbing wave and whether a disturbing wave exists on an unused channel in the same manner as set out in step  102 . 
     When the controlling portion  30  has determined that a disturbing wave exists on the unused channel in step  112 , the flow advances to step  113  wherein the controlling portion  30  determines whether another unused channel exists. When the controlling portion  30  has determined that another unused channel exists, the flow returns to step  112  where, in the same manner as described above, the controlling portion  30  determines whether a disturbing wave exists on the unused channel. 
     When the controlling portion  30  has determined that a disturbing wave does not exist on the unused channel in step  112 , the flow advances to step  114  wherein the controlling portion  30  sets the unused channel as a communication channel. Thereafter, the flow advances to process routine  300  in which the controlling portion  30  executes a mode setting process for the frequency band Bb. 
     In the mode setting process  300  for the frequency band Bb, as will be described later with reference to  FIG. 14 , the controlling portion  30  detects a received field strength at the highest transmission rate on the communication channel that was set in step  114  of the setting process  100 . When the received field strength reaches or exceeds the threshold value, the controlling portion  30  sets the transmission rate as a mode. When the received field strength does not reach the threshold value, the controlling portion  30  sets the next highest transmission rate as a mode. 
     When the controlling portion  30  has determined that an unused channel does not exist in the frequency band Bb in step  111  or has determined that an unused channel free of a disturbing wave does not exist in the frequency band Bb, the flow advances to step  115 . As step  115  shows, the controlling portion  30  sets a predetermined radio channel in a predetermined frequency band as a communication channel and sets a predetermined mode (transmission rate). For example, the controlling portion  30  sets a particular radio channel in the frequency band Ba (5.2 GHz band) as a communication channel and sets mode A 8  (transmission rate: 54 Mbps) as a mode of the transmission rate. Thereafter, the controlling portion  30  completes the setting process performed upon startup of communication and starts the communication. 
     Alternatively, in place of step  115  of process  100 , the controlling portion  30  transmits a message indicating that data cannot be transmitted due to an improper communication environment between the base apparatus  10  and the display terminal  50  and then causes the message to be displayed on the LCD  51  of the display terminal  50  or to be outputted as audio data from the speaker  55  to inform the user. 
     Mode Setting Process for Frequency Band Ba:  FIG. 12  and  FIG. 13 : 
       FIGS. 12 and 13  show an example of a process routine  200  of a mode setting process for the frequency band Ba (5.2 GHz band). 
     When the controlling portion  30  starts communication in mode setting process e  200 , the controlling portion  30  has already set an unused channel free of a disturbing wave in the frequency band Ba as a communication channel in setting process  100 . Thereafter, in step  211 , the controlling portion  30  transmits a setup signal in mode A 8  (transmission rate: 54 Mbps), which has the highest transmission rate in the frequency band Ba, from the base apparatus  10  to the display terminal  50 . 
     Thereafter, the flow advances to step  212  wherein the controlling portion  30  determines whether the received field strength at the time reaches or exceeds the threshold value. 
     As an example, the following method for detecting and determining the received field strength may be used. The transmitting and receiving portion  85   a  of the display terminal  50  receives a signal transmitted from the base apparatus  10 . The received field strength detecting portion  87   a  of the display terminal  50  detects the received field strength. The controlling portion  70  of the display terminal  50  determines whether the received field strength reaches or exceeds the threshold value and transmits the result from the display terminal  50  to the base apparatus  10 . The controlling portion  30  of the base apparatus  10  then determines whether the received field strength reaches or exceeds the threshold value. 
     Alternatively, when the display terminal  50  has received a signal from the base apparatus  10 , the display terminal  50  transmits an acknowledge signal to the base apparatus  10  that notifies the base apparatus  10  that the display terminal  50  has received the signal. The transmitting and receiving portion  45   a  of the base apparatus  10  receives the acknowledge signal. The received field strength detecting portion  47   a  of the base apparatus  10  then detects the received field strength. The controlling portion  30  of the base apparatus  10  next determines whether the received field strength reaches or exceeds the threshold value. 
     When the controlling portion  30  has determined that the received field strength in mode A 8  reaches or exceeds the threshold value in step  212 , the controlling portion  30  completes the mode setting process for the frequency band Ba. When the controlling portion  30  starts the communication, the flow advances to step  105  of the setting process  100 . In step  105 , the controlling portion  30  determines that the communication should start in the frequency band Ba and then starts communication in mode A 8  on the communication channel that was set in step  104 . 
     When the controlling portion  30  has determined that the received field strength in mode A 8  does not reach the threshold value in step  212 , the flow advances to step  221 . Here, the controlling portion  30  transmits a setup signal in mode A 7  (transmission rate: 48 Mbps), which has the second highest transmission rate in the frequency band Ba, from the base apparatus  10  to the display terminal  50 . Thereafter, the flow advances to step  222  wherein the controlling portion  30  determines whether the received field strength at the time reaches or exceeds the threshold value in the same manner as described above. 
     Thereafter, when the controlling portion  30  has determined that the received field strength in mode A 7  reaches or exceeds the threshold value in step  222 , the controlling portion  30  completes the mode setting process for the frequency band Ba. When the controlling portion  30  starts communication, the flow advances to step  105  of the setting process  100  wherein the controlling portion  30  determines that the communication should start in the frequency band Ba and starts the communication in mode A 7  on the communication channel that was set in step  104 . 
     When the controlling portion  30  has determined that the received field strength in mode A 7  does not reach the threshold value in step  222 , the flow advances to step  231 . Here, the controlling portion  30  transmits a setup signal in mode A 6  (transmission rate: 36 Mbps) from the base apparatus  10  to the display terminal  50 . Thereafter, the flow advances to step  232  wherein the controlling portion  30  determines whether the received field strength at the time reaches or exceeds the threshold value in the manner described above. 
     When the controlling portion  30  has determined that the received field strength in mode A 6  reaches or exceeds the threshold value in step  232 , the controlling portion  30  completes the mode setting process for the frequency band Ba. When the controlling portion  30  starts communication in the setting process  100 , the controlling portion  30  starts the communication in mode A 6  in the same manner as described above. 
     When the controlling portion  30  has determined that the received field strength in mode A 6  does not reach the threshold value in step  232 , the flow advances to step  241 . Here, the controlling portion  30  transmits a setup signal in mode A 5  (transmission rate: 24 Mbps) from the base apparatus  10  to the display terminal  50 . Thereafter, the flow advances to step  242 . The controlling portion  30  determines whether the received field strength at the time reaches or exceeds the threshold value in the above-described manner. 
     When the controlling portion  30  has determined that the received field strength in mode A 5  reaches or exceeds the threshold value in step  242 , the controlling portion  30  completes the mode setting process for the frequency band Ba. When the controlling portion  30  then starts communication in the setting process  100 , the controlling portion  30  starts the communication in mode A 5  in the same manner as described above. 
     When the controlling portion  30  has determined that the received field strength in mode A 5  does not reach the threshold value in step  242 , the flow advances to step  251 . Now, the controlling portion  30  transmits a setup signal in mode A 4  (transmission rate: 18 Mbps) from the base apparatus  10  to the display terminal  50 . Thereafter, the flow advances to step  252  in which the controlling portion  30  determines whether the received field strength at the time reaches or exceeds the threshold value as described above. 
     When the controlling portion  30  has determined that the received field strength in mode A 4  reaches or exceeds the threshold value in step  252 , the controlling portion  30  completes the mode setting process for the frequency band Ba. When the controlling portion  30  next starts communication in the setting process  100 , the controlling portion  30  starts the communication in mode A 4  in the manner as described above. 
     When the controlling portion  30  has determined that the received field strength in mode A 4  does not reach the threshold value in step  252 , the flow advances to step  261 . Wherein, the controlling portion  30  transmits a setup signal in mode A 3  (transmission rate: 12 Mbps) from the base apparatus  10  to the display terminal  50 . Thereafter, the flow advances to step  262  in which the controlling portion  30  determines whether the received field strength at the time reaches or exceeds the threshold value in the same method as described above. 
     When the controlling portion  30  has determined that the received field strength in mode A 3  reaches or exceeds the threshold value in step  262 , the controlling portion  30  completes the mode setting process for the frequency band Ba. When the controlling portion  30  thereafter starts communication in process routine  100 , the controlling portion  30  starts the communication in mode A 3  in the manner described above. 
     When the controlling portion  30  has determined that the received field strength in mode A 3  does not reach the threshold value in step  262 , the flow advances to step  271 . Here, the controlling portion  30  transmits a setup signal in mode A 2  (transmission rate: 9 Mbps) from the base apparatus  10  to the display terminal  50 . Thereafter, the flow advances to step  272 . The controlling portion  30  determines whether the received field strength at the time reaches or exceeds the threshold value in the same method as described above. 
     When the controlling portion  30  has determined that the received field strength in mode A 2  reaches or exceeds the threshold value in step  272 , the controlling portion  30  completes the mode setting process for the frequency band Ba. When the controlling portion  30  subsequently starts a communication in the process  100 , the controlling portion  30  starts the communication in mode A 2  in the same manner as described above. 
     When the controlling portion  30  has determined that the received field strength in mode A 2  does not reach the threshold value in step  272 , the flow advances to step  281 . Where, the controlling portion  30  transmits a setup signal in mode A 1  (transmission rate: 6 Mbps), which has the lowest transmission rate in the frequency band Ba, from the base apparatus  10  to the display terminal  50 . Thereafter, the flow advances to step  282  in which the controlling portion  30  determines whether the received field strength at the time reaches or exceeds the threshold value as described above. 
     When the controlling portion  30  has determined that the received field strength in mode A 1  reaches or exceeds the threshold value in step  282 , the controlling portion  30  completes the mode setting process for the frequency band Ba. When the controlling portion  30  next starts a communication in the process  100 , the controlling portion  30  starts the communication in mode A 1  in the same manner as described above. 
     When the controlling portion  30  has determined that the received field strength in mode A 1  does not reach the threshold value in step  282 , the flow advances to step  291 . In step  291 , the controlling portion  30  has determined that no mode should be set in the frequency band Ba and completes the mode setting process for the frequency band Ba. When the controlling portion  30  starts communication, the flow advances to step  105  of the setting process  100  wherein the controlling portion  30  has determined that a communication should not start in the frequency band Ba. Here, as when the controlling portion  30  has determined that an unused channel does not exist in the frequency band Ba in step  101  or  30  has determined that an unused channel free of a disturbing wave does not exist in the frequency band Ba in step  103 , the flow advances to step  111  as described above. 
     When the communication environment does not vary, the received sensitivity point, namely the received field strength of which the bit error rate of the received data does not reach a predetermined value, becomes higher, as the transmission rate is increased. Therefore, the threshold values at the above-described steps  212 ,  222 ,  232 ,  242 ,  252 ,  262 ,  272 , and  282  are increased as the transmission rate increases. 
     Mode Setting Process for Frequency Band Bb:  FIG. 14 : 
       FIG. 14  shows an example of a mode setting process  300  for the frequency band Bb (2.4 GHz band). 
     When the controlling portion  30  starts communication in the process  300 , the flow first returns to step  114  of the process  100 . In step  114 , the controlling portion  30  sets an unused channel free of a disturbing wave in the frequency band Bb as a communication channel. Thereafter, the flow advances to step  311  where the controlling portion  30  transmits a setup signal in mode B 4 , which has the highest transmission rate in the frequency band Bb (transmission rate: 11 Mbps), from the base apparatus  10  to the display terminal  50 . 
     Thereafter, the flow advances to step  312  in which the controlling portion  30  determines whether the received field strength at the time reaches or exceeds the threshold value. 
     As an example, the following method for detecting and determining the received field strength may be used. The transmitting and receiving portion  85   b  of the display terminal  50  receives a signal transmitted from the base apparatus  10 . The received field strength detecting portion  87   b  of the display terminal  50  then detects the received field strength. The controlling portion  70  of the display terminal  50  then determines whether the received field strength reaches or exceeds the threshold value and transmits the result from the display terminal  50  to the base apparatus  10 . The controlling portion  30  of the base apparatus  10  thereafter determines whether the received field strength reaches or exceeds the threshold value. 
     Alternatively, when the display terminal  50  has received a signal transmitted from the base apparatus  10 , the display terminal  50  transmits an acknowledge signal to the base apparatus  10  that notifies the base apparatus that the display terminal  50  has received the signal. The transmitting and receiving portion  45   b  of the base apparatus  10  next receives the acknowledge signal. The received field strength detecting portion  47   b  of the base apparatus  10  then detects the received field strength. The controlling portion  30  of the base apparatus  10  subsequently determines whether the received field strength reaches or exceeds the threshold value. 
     When the controlling portion  30  has determined that the received field strength in mode B 4  reaches or exceeds the threshold value in step  312 , the controlling portion  30  completes the mode setting process for the frequency band Bb and starts the communication in mode B 4  on the communication channel that has been set in step  114  of the process  100 . 
     When the controlling portion  30  has determined that the received field strength in mode B 4  does not reach the threshold value in step  312 , the flow advances to step  313 . Here, the controlling portion  30  sets mode B 3  (transmission rate: 5.5 Mbps), which has the second highest transmission rate in the frequency band Bb, completes the mode setting process for the frequency band Bb and then starts the communication in mode B 3  on the communication channel that has been set in step  114  of the setting process  100 . 
     When the received field strength in mode B 4  does not reach the threshold value, the controlling portion  30  sets mode B 3  without determining whether the received field strength in mode B 3  reaches or exceeds the threshold value because if the received field strength in mode B 3  did not exceed the threshold value and the controlling portion  30  were to set mode B 2  (transmission rate: 2 Mbps) or mode B 1  (transmission rate: 1 Mbps), the resulting transmission rate would become too low. 
     Alternatively, the mode setting process  300  may be structured in the following manner. When the controlling portion  30  has determined that the received field strength in mode  34  does not reach the threshold value in step  312 , the controlling portion  30  transmits a setup signal in mode B 3  to the display terminal  50  and then determines whether the received field strength in mode B 3  reaches or exceeds the threshold value. When the received field strength reaches or exceeds the threshold value, the controlling portion  30  sets mode B 3 . When the received field strength does not reach the threshold value, the controlling portion  30  determines that no mode is to be set in the frequency band Bb. 
     When the controlling portion  30  has determined that no mode is to be set in the frequency band Bb in the process  300 , in the same manner that the controlling portion  30  determines that an unused channel exists in the frequency band Bb in step  111  or determines that an unused channel free of a disturbing wave does not exist in the frequency band Bb in step  113 , the flow advances to step  115 . In step  115 , the controlling portion  30  sets a predetermined radio channel of a predetermined frequency band as a communication channel, sets a predetermined mode (transmission rate), and starts the communication. 
     Changing Process that is Performed During Communication:  FIG. 15  to  FIG. 21 : 
     Changing Process that is Performed During Normal Communication:  FIG. 15  to  FIG. 18 : 
     When the controlling portion  30  starts communication at a high transmission rate in the frequency band Ba, if a disturbing wave is present in a communication channel, it is desirable to change the communication channel. Moreover, when the controlling portion  30  starts communication at a low transmission rate in the frequency band Bb and an unused channel now exists in the frequency band Ba, it is desirable to change the communication channel to the unused channel in the frequency band Ba to increase the transmission rate. 
     Therefore, the above-described wireless communication system is structured such that while the base apparatus  10  is communicating with the display terminal  50 , the controlling portion  30  of the base apparatus  10  executes a changing process. 
     Changing Process Performed During Communication in Frequency Band Ba:  FIG. 15  and  FIG. 16 : 
       FIG. 15  and  FIG. 16  show an example of a changing process  120  performed during communication in the frequency band Ba. 
     While the controlling portion  30  is communicating in the frequency band Bb, in step  129  of process routine  120 , the controlling portion  30  periodically determines whether a disturbing wave exists on a communication radio channel in the frequency band Ba in accordance with the detected result of the disturbing wave detecting portion  43   a.    
     In this case, as in the setting process  100  performed upon startup of communication, when the controlling portion  30  determines whether a signal received by the transmitting and receiving portion  45   a  is a disturbing wave, the above-described method for detecting/identifying transmission destination address information is used. 
     When the controlling portion  30  has determined that a disturbing wave exists on the communication radio channel in step  129 , the controlling portion  30 , in step  121 , determines whether an unused channel exists in the frequency band Ba. When it is determined that an unused channel exists, the flow advances to step  122  wherein the controlling portion  30  determines whether a disturbing wave exists on the unused channel. When such a disturbing wave is found to exist, the flow advances to step  123  where the controlling portion  30  determines whether or not another unused channel exists. When it is determined that an unused channel exists, the flow returns to step  122  in which the controlling portion  30  determines whether a disturbing wave exists on the unused channel. 
     When the controlling portion  30  has determined that a disturbing wave does not exist on the unused channel in step  122 , the flow advances to step  124  wherein the controlling portion  30  sets the unused channel as a communication channel. Thereafter, the controlling portion  30  executes the mode setting process  200  for the frequency band Ba. Thereafter, the flow advances to step  125  in which the controlling portion  30  determines whether the communication should be continued in the frequency band Ba in accordance with the result of the execution of the process  200 . When the controlling portion  30  has determined that the communication should be continued in the frequency band Ba, the controlling portion  30  restores the communicating state in the frequency band Ba. 
     When the controlling portion  30  has determined that an unused channel does not exist in the frequency band Ba in step  121 , has determined that an unused channel free of a disturbing wave does not exist in the frequency band Ba in step  123 , or has determined that an unused channel free of a disturbing wave exists in the frequency band Ba but the received field strengths at all the transmission rates in the frequency band Ba do not exceed the threshold value in step  125 , the flow advances to step  131 . Namely, the controlling portion  30  determines whether an unused channel exists in the frequency band Bb. 
     When the controlling portion  30  has determined that an unused channel exists in the frequency band Bb, the flow advances from step  131  to step  132  wherein the controlling portion  30  determines whether a disturbing wave exists on the unused channel. When it is determined that a disturbing wave exists, the flow advances to step  133  in which the controlling portion  30  determines whether another unused channel exists. When another unused channel exists, the flow returns to step  132 . Namely, the controlling portion  30  determines whether or not a disturbing wave exists on the unused channel. 
     When the controlling portion  30  has determined that a disturbing wave does not exist on the unused channel in step  132 , the flow advances to step  134  wherein the controlling portion  30  sets the unused channel as a communication channel. Thereafter, the controlling portion  30  executes the mode setting process  300  for the frequency band Bb. The controlling portion  30  completes the changing process performed during communication in the frequency band Ba and enters a communicating state in the frequency band Bb. 
     When the controlling portion  30  has determined that an unused channel does not exist in the frequency band Bb in step  131  or has determined that an unused channel free of a disturbing wave does not exist in the frequency band Bb in step  133 , the flow advances to step  135 . Namely, the controlling portion  30  continues the current communication for a predetermined time period and restores the communicating state in the frequency band Ba. 
     Changing Process Performed During Communication in Frequency Band Bb:  FIG. 17  and  FIG. 18 : 
       FIG. 17  and  FIG. 18  show an example of a changing process performed during communication in the frequency band Bb. 
     In changing process routine  140 , while the controlling portion  30  is communicating, in step  147 , the controlling portion  30  periodically determines whether a disturbing wave exists on a communication radio channel in accordance with the detected result of the disturbing wave detecting portion  43   b.    
     In this case, as in the setting process routine  100  performed upon startup of communication, when the controlling portion  30  determines whether a signal received by the transmitting and receiving portion  45   b  is a disturbing wave, the above-described method for detecting/identifying transmission destination address information is used. 
     When the controlling portion  30  has determined that a disturbing wave exists on the communication radio channel in step  147 , the flow directly advances from step  147  to step  141 . When the controlling portion  30  determines that the disturbing wave does not exist on the communication radio channel, the flow advances from step  147  to step  149  wherein the controlling portion  30  continues the current communication for a predetermined time period. Thereafter, the flow advances to step  141 . 
     In step  141 , the controlling portion  30  determines whether an unused channel exists in the frequency band Ba, and when an unused channel exists, the flow advances to step  142 . Namely, the controlling portion  30  determines whether or not a disturbing wave exists on the unused channel and when a disturbing wave exists, the flow advances to step  143  wherein the controlling portion  30  determines whether another unused channel exists. When it is determined that another unused channel exists, the flow returns to step  142  so that the controlling portion  30  determines whether a disturbing wave exists on the unused channel. 
     When the controlling portion  30  has determined that a disturbing wave does not exist on the unused channel in step  142 , the flow advances to step  144  wherein the controlling portion  30  sets the unused channel as a communication channel. Thereafter, the controlling portion  30  executes a mode setting process  200  for the frequency band Ba. The flow then advances to step  145  in which the controlling portion  30  determines whether the communication should be continued in the frequency band Ba. When it is determined that the communication should be continued in the frequency band Ba, the controlling portion  30  completes the changing process performed during communication in the frequency band Ba. Thereafter, the controlling portion  30  enters a communicating state in the frequency band Ba. 
     When the controlling portion  30  has determined that an unused channel does not exist in the frequency band Ba in step  141 , has determined that an unused channel free of a disturbing wave does not exist in the frequency band Ba) in step  143 , or has determined that an unused channel free of a disturbing wave exists in the frequency band Ba but the received field strengths at all the transmission rates in the frequency band Ba do not exceed the threshold value) in step  145 , the flow advances to step  151 . Namely, the controlling portion  30  determines whether an unused channel exists in the frequency band Bb. 
     When the controlling portion  30  has determined that an unused channel exists in the frequency band Bb, the flow advances from step  151  to step  152  wherein the controlling portion  30  determines whether a disturbing wave exists on the unused channel. When it is determined that a disturbing wave exists, the flow advances to step  153  in which the controlling portion  30  determines whether another unused channel exists. When the controlling portion  30  has determined that another unused channel exists, the flow returns to step  152 . Namely, the controlling portion  30  determines whether or not a disturbing wave exists on the unused channel. 
     When the controlling portion  30  has determined that a disturbing wave does not exist on the unused channel in step  152 , the flow advances to step  154  in which the controlling portion  30  sets the unused channel as a communication channel. Thereafter, the controlling portion  30  executes the mode setting process  300  for the frequency band Bb and restores the communicating state in the frequency band Bb. 
     When the controlling portion  30  has determined that an unused channel does not exist in the frequency band Bb in step  151  or has determined that an unused channel free of a disturbing wave does not exist in the frequency band Bb in step  153 , the flow advances to step  155 . Namely, the controlling portion  30  sets, for example, a predetermined radio channel of a predetermined frequency band as a communication channel, sets a predetermined mode (transmission rate), and completes the changing process performed during communication in the frequency band Bb. 
     Change of Transmission Rate:  FIG. 19  to  FIG. 21 : 
     The Case in which Transmission Rate is Increased:  FIG. 19 : 
     When the controlling portion  30  starts a communication in the frequency band Ba, even if the transmission rate cannot be increased because the electric field is weak, if the environment of the electric field varies, the transmission rate may be increased. Thus, the system is structured so that in that case the transmission rate can be increased. 
       FIG. 19  shows an example of a mode changing process  160  that the controlling portion  30  of the base apparatus  10  executes in such a case. 
     In the mode changing process  160 , while the controlling portion  30  is communicating in the frequency band Ba, it periodically determines whether a mode having a higher transmission rate than the current mode exists. When such a mode does not exist, namely, while the controlling portion  30  is communicating in mode A 8  (transmission rate: 54 Mbps), the flow advances to step  162  wherein the controlling portion  30  continues the communication in the current mode (transmission rate). 
     When a mode that has a higher transmission rate than the current mode exists, namely, during communication in a mode lower than mode A 7 , the flow advances from step  161  to step  163  wherein the controlling portion  30  changes the current mode to a mode having a higher transmission rate. Thereafter, in step  164 , the controlling portion  30  determines whether the received field strength at the changed transmission rate reaches or exceeds the threshold value. 
     When the received field strength at the changed transmission rate does not reach the threshold value, the flow advances from step  164  to step  165  so that the controlling portion  30  restores the preceding mode (transmission rate) from which the transmission rate was changed in step  163  and continues the communication. When the received field strength at the changed transmission rate reaches or exceeds the threshold value, the flow advances from step  164  to step  166  wherein the controlling portion  30  determines whether a mode having a higher transmission rate than the current mode exists. When such a mode exists, the controlling portion  30  executes steps  163  and the steps after step  163 . When a mode having a higher transmission rate than the current mode does not exist, the flow advances to step  167 . Namely, the controlling portion  30  continues the communication in the mode (transmission rate) that was changed in step  163 . 
     For example, while the controlling portion  30  is communicating in mode A 4 , if the received field strength reaches or exceeds the threshold value in mode A 5 , but not in mode A 6 , the controlling portion  30  successively executes steps  161 ,  163 ,  164 ,  166 ,  163 ,  164 , and  165 . As a result, the controlling portion  30  changes mode A 4  to mode A 4 . 
     In contrast, while the controlling portion  30  is communicating in mode A 7 , if the received field strength reaches or exceeds the threshold value in mode A 8 , the controlling portion  30  successively executes steps  161 ,  163 ,  164 ,  166 , and  167 . As a result, the controlling portion  30  changes mode A 7  to mode A 8 . 
     The Case in which Transmission Rate is Decreased:  FIG. 20  and  FIG. 21 : 
     In a good environment in which a radio wave is free of a disturbing wave while the controlling portion  30  is communicating at a high transmission rate in the frequency band Ba, if the user having the display terminal  50  goes away from the base apparatus  10 , the electric field will vary because the distance between the base apparatus  10  and the display terminal  50  becomes large. In such case, the received field strength becomes lower than the received sensitivity point and as a result, the bit error rate of the received data becomes large and communication cannot be securely performed. Thus, the system is structured for the case where the transmission rate is decreased and the received field strength becomes larger than the received sensitivity point. 
       FIGS. 20 and 21  show an example of a mode changing process  180  that the controlling portion  30  of the base apparatus  10  executes in such a case. 
     In the mode changing process  180 , while the controlling portion  30  is communicating in the frequency band Ba, it periodically determines whether the received field strength at the current transmission rate reaches or exceeds the received sensitivity point in step  181 . When the received field strength does reach or exceed the received sensitivity point, the flow advances to step  182  wherein the controlling portion  30  continues the communication in the current mode (transmission rate). 
     When the received field strength at that transmission rate does not reach the received sensitivity point, the flow advances from step  181  to step  183 . Namely, the controlling portion  30  determines whether a mode having a lower transmission rate than the current mode exists. When such a mode is determined to exist, the flow advances from step  183  to step  184  wherein the controlling portion  30  changes the current mode to a mode having a lower transmission rate by one level. Thereafter, the flow advances to step  185  in which the controlling portion  30  determines whether the received field strength at the changed transmission rate reaches or exceeds the received sensitivity point. 
     When the received field strength at the changed transmission rate reaches or exceeds the received sensitivity point, the flow advances from step  185  to step  186 . Namely, the controlling portion  30  continues the communication in the mode (transmission rate) changed in step  184 . When the received field strength at the changed transmission rate does not reach the received sensitivity point, the flow advances from step  185  to step  187  wherein the controlling portion  30  determines whether a mode having a lower transmission rate than the current mode exists. When such a mode exists, the controlling portion  30  repeats the steps starting from step  184 . 
     For example, while the controlling portion  30  is communicating in mode A 4 , and if the received field strength does not reach the received sensitivity point but does reach or exceed the received sensitivity point in mode A 3 , the controlling portion  30  successively executes steps  181 ,  183 ,  184 ,  185 , and  186 . As a result, the controlling portion  30  changes from mode A 4  to mode A 3 . 
     In contrast, when the controlling portion  30  has determined that a mode having a lower transmission rate than the current mode does not exist in step  183 , namely, while the controlling portion  30  is communicating in mode A 1 , if the received field strength does not reach the received sensitivity point, the flow advances to stop  191 . Also, when the controlling portion  30  has determined that a mode having a lower transmission rate than the current mode does not exist in step  187 , namely even if the controlling portion  30  decreases the transmission rate to mode A 1  but the received field strength does not reach the received sensitivity point, the flow advances to step  191 . In step  191 , the controlling portion  30  determines whether or not an unused channel exists in the frequency band Bb. 
     When the controlling portion  30  has determined that an unused channel exists in the frequency band Bb, the flow advances from step  191  to step  192  wherein the controlling portion  30  determines whether or not a disturbing wave exists on the unused channel. When it is determined that a disturbing wave exists, the flow advances to step  193 . Namely, the controlling portion  30  determines whether another unused channel exists. When another unused channel exists, the flow returns to step  192  in which the controlling portion  30  determines whether a disturbing wave exists on the unused channel. 
     When the controlling portion  30  has determined that a disturbing wave does not exist on the unused channel in step  192 , the flow advances to step  194 , namely, the controlling portion  30  sets the unused channel as a communication channel. Thereafter, the controlling portion  30  executes the mode setting process  300  for the frequency band Bb. Thereafter, the controlling portion  30  enters a communicating state in the frequency band Bb. 
     When the controlling portion  30  has determined that an unused channel does not exist in the frequency band Bb, in step  191 , or has determined that an unused channel free of a disturbing wave does not exist in the frequency band Bb, in step  193 , the flow advances to step  195 . Here, the controlling portion  30  continues the communication in mode A 1  having the lowest transmission rate on the original communication channel in the frequency band Ba, thus providing the highest possibility that the received field strength reaches or exceeds the received sensitivity point. 
     Other Embodiments 
     Frequency bands are currently defined in the IEEE standard and domestic standard only at 5.2 GHz (5 GHz band) and at 2.4 GHz. However, it is possible to use other frequency bands as the radio frequency bands of the invention. Other frequency bands may be defined in future. Thus the two frequency bands of the invention are not limited to 5.2 GHz (5 GHz band) and 2.4 GHz. In addition, the present invention can be applied to the case in which three or more frequency bands are used. 
     In addition, the wireless communication apparatuses that compose the wireless communication system are not limited to the above-described base apparatus and display terminal. 
     INDUSTRIAL APPLICABILITY 
     As described above, according to the present invention, in a wireless communication system that deals with a plurality of communicable frequency bands having different transmission rates, a large volume of data can be securely and smoothly transmitted in real time without a disturbance from another wireless communication system and so forth, and abnormalities such as the stopping of a moving picture or a disturbance of a still picture are avoided. 
     Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.