Abstract:
A radio communicator including a first antenna configured to emit radio signal as a first linear polarized wave; a second antenna arranged perpendicular to the first antenna and configured to emit radio signal as a second linear polarized wave; a receiver configured to monitor a radio wave condition in the air; a transmitter configured to convert transmission data to a first radio signal and a second radio signal with a phase orthogonal to the phase of the first radio signal, and to transmit the first radio signal and the second radio signal through the first antenna and the second antenna; and a controller configured to make judgment of the radio wave condition monitored by the receiver, to direct the transmitter to transmit the first radio signal through the first antenna and to transmit the second radio signal through the second antenna when the result of the judgment is first condition, and to direct the transmitter to transmit an addition signal which is an addition of the first radio signal and the second radio signal through the first antenna when the result of the judgment is the second condition.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is based upon and claims a benefit of priority under 35 U.S.C. § 119 from prior Japanese Patent Application P2006-30661 filed on Feb. 8, 2006, the entire contents of which are incorporated by reference herein.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     Exemplary embodiments of the invention relate to a radio communicator that has a plurality of polarization functions.  
         [0004]     2. Discussion of the Background  
         [0005]     Recently, radio communications use high frequency waves, so called “Millimeter-waves” including the 60 GHz band, which can be transmitted and received with small antennas.  
         [0006]     Jeon et al. suggest to develop such small antenna and circuits on an IC (see, Jeon et al., “Millimeter wave direct quadrature converter integrated with antenna for broad-band wireless communications, “Microwave Symposium Digest, 2002 IEEE MTT-S International Volume 2, 2-7 Jun. 2002 Page(s): 1277-1280). The text describes a radio communicator which modulates a baseband signal to an in-phase component and a quadrature component in QPSK (Quadrature Phase Shift Keying) by a quadrature modulator, converts the in-phase component and the quadrature component to a radio frequency signal by a frequency converter, and radiates the radio frequency signal from an antenna as linear polarized wave.  
         [0007]     However, there is a problem that linear polarized wave is easily delayed by environmental delay factors such as a wall. Although the absolute amount of such delay is very small, nevertheless it is large for a transmission data rate used with the millimeter-wave.  
         [0008]     On the other hand, a circular polarized wave hardly experiences interference by such environmental delay factors. JP-A-2004-320583 discloses a combined use of the linear polarized wave and the circular polarized wave in a radio communicator, where the linear polarized wave is converted to circular polarized wave when the radio communicator is in an environment where wave reflection is likely to happen. But the structure for such a conversion is too large to implement on an IC for a millimeter-wave radio communicator.  
       SUMMARY OF THE INVENTION  
       [0009]     According to one aspect of the present invention there is provided a radio communicator, including a first antenna configured to emit a radio signal as a first linear polarized wave; a second antenna arranged perpendicular to the first antenna and configured to emit a radio signal as a second linear polarized wave; a receiver configured to monitor a radio wave condition in the air; a transmitter configured to convert transmission data to a first radio signal and to a second radio signal with a phase orthogonal to the phase of the first radio signal, and to transmit the first radio signal and the second radio signal through the first antenna and the second antenna; and a controller configured to make a judgment of the radio wave condition monitored by the receiver, to direct the transmitter to transmit the first radio signal through the first antenna and to transmit the second radio signal through the second antenna when the result of the judgment is a first condition, and to direct the transmitter to transmit an addition signal which is an addition of the first radio signal and the second radio signal through the first antenna when the result of the judgment is a second condition. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:  
         [0011]      FIG. 1  is a block diagram illustrating an example of a radio communicator according to a first exemplary embodiment;  
         [0012]      FIG. 2  is a block diagram illustrating a diagram of an example of a transmitter of a radio communicator according to a first exemplary embodiment;  
         [0013]      FIG. 3  is a functional block diagram illustrating a first partial diagram of a transmitter of a radio communicator according to a first exemplary embodiment;  
         [0014]      FIG. 4  is a functional block diagram illustrating a second partial diagram of a transmitter of a radio communicator according to a first exemplary embodiment;  
         [0015]      FIG. 5  is a functional block diagram illustrating a third partial diagram of a transmitter of a radio communicator according to a first exemplary embodiment;  
         [0016]      FIG. 6  is a block diagram illustrating a diagram of an example of a transmitter of a radio communicator according to a second exemplary embodiment;  
         [0017]      FIG. 7  is a functional block diagram illustrating operation of a mixer of a transmitter of a radio communicator according to a second exemplary embodiment;  
         [0018]      FIG. 8  is a block diagram illustrating a partial diagram of a transmitter of a radio communicator according to a second exemplary embodiment;  
         [0019]      FIG. 9  is a flow chart illustrating a first exemplary operation of a controller of a radio communicator according to a third exemplary embodiment;  
         [0020]      FIG. 10  is a flow chart illustrating a second exemplary operation of a controller of a radio communicator according to a third exemplary embodiment;  
         [0021]      FIG. 11  is a flow chart illustrating a third exemplary operation of a controller of a radio communicator according to a third exemplary embodiment; and  
         [0022]      FIG. 12  is a flow chart illustrating a fourth exemplary operation of a controller of a radio communicator according to a third exemplary embodiment. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]     Referring now to the drawings in which like reference numerals designate identical or corresponding parts throughout the several views, various exemplary embodiments and operation of the present invention are next described.  
       First Exemplary Embodiment  
       [0024]      FIG. 1  illustrates a diagram of an example of a first exemplary embodiment of a radio communicator. In  FIG. 1 , the radio communicator includes a transmitter  1 , receiver  2  and controller  3 . The transmitter  1  includes a transmission signal processor  11 , a radio transmission circuit  12 , and a transmission antenna section  13 . The receiver  2  monitors a radio wave condition in the air, and includes a reception signal processor  21 , a radio reception circuit  22 , and a reception antenna section  23 . The controller  3  controls the transmitter  1  and the receiver  2 .  
         [0025]      FIG. 2  illustrates further details of the radio communicator of  FIG. 1 . As shown in  FIG. 2 , the transmission signal processor  11  has three output terminals  11 - 1  to  11 - 3  connected to a switch set  121  included in the radio transmission circuit  12 . The transmission signal processor  11  modulates transmission data to a baseband signal using either a QPSK modulation or a FSK modulation as a digital modulation.  
         [0026]     Under control of the controller  3  which directs the modulation scheme, the baseband signal modulated using the FSK modulation is provided to the radio transmission circuit  12  from the output terminal  11 - 2 . The baseband signal modulated using the QPSK modulation is provided to the radio transmission circuit  12  from the output terminals  11 - 1  and  11 - 3 .  
         [0027]     The radio transmission circuit  12  includes the switch set  121  including switches  121 - 1  to  121 - 3 , a first local oscillator  122 , a second local oscillator  222 , a first mixer  124 , a second mixer  125 , a first phase shifter  126 , a second phase shifter  226 , and an adder  127 .  
         [0028]     The first local oscillator  122  generates a sine wave, and provides it to the first phase shifter  126 .  
         [0029]     The second local oscillator  222  is connected to the output terminal  11 - 2  of the transmission signal processor  11  through the switch  121 - 2 . When the switch  121 - 2  shorts, the baseband signal is provided from the transmission signal processor  11  to the second local oscillator  222 . Then, the second local oscillator  222  outputs a modulated sine wave to the second phase shifter  226 .  
         [0030]     The first mixer  124  has two input terminals  124 - 1  and  124 - 2 , and an RF output terminal  124 - 3 . The input terminal  124 - 1  connects to the output terminal  11 - 1  of the transmission signal processor  11  through the switch  121 - 1 . The input terminal  124 - 2  connects to the first phase shifter  126 . Signals input from input terminals  124 - 1  and  124 - 2  are converted to first RF signal by frequency mixing. The RF output terminal  124 - 3  provides the first RF signal to the adder  127 .  
         [0031]     The second mixer  125  has two input terminals  125 - 1  and  125 - 2 , and RF output terminal  125 - 3 . The input terminal  125 - 1  connects to the output terminal  11 - 3  of the transmission signal processor  11  through the switch  121 - 3 . The input terminal  125 - 2  connects to the first phase shifter  126 . Signals input from input terminals  125 - 1  and  125 - 2  are converted to a second RF signal by frequency mixing. The RF output terminal  125 - 3  provides the second RF signal to the adder  127 .  
         [0032]     The first phase shifter  126  generates a first shift signal and a second shift signal whose phase is orthogonal to that of the first shift signal. In this embodiment, the phase of the first shift signal shifts 0 degrees from the phase of the sine wave provided from the first local oscillator  122 , and the phase of the second shift signal shifts 90 degrees from the phase of the sine wave provided from the first local oscillator  122 . The first phase shifter  126  provides the first shift signal to the first mixer  124 , and provides the second shift signal to the second mixer  125 .  
         [0033]     The adder  127  generates an addition signal that is an addition of the first RF signal provided form the first mixer  124  and the second RF signal provided from the second mixer  125 . The adder  127  provides the addition signal to the first monopole antenna  131  of the transmission antenna section  13  and to the second phase shifter  226 .  
         [0034]     The first local oscillator  122 , the first mixer  124 , the second mixer  125 , the first phase shifter  126 , and the adder  127  configure a quadrature modulator.  
         [0035]     The second phase shifter  226  generates a third shift signal, the phase of which shifts 0 degrees from phase of the modulated sine wave provided from the second local oscillator  222 . The second phase shifter  226  also generates a fourth shift signal, the phase of which shifts 90 degrees from the phase of the modulated sine wave provided from the second local oscillator  222 . The second phase shifter  226  further generates a fifth shift signal, the phase of which shifts 90 degrees from the phase of the addition signal provided from the adder  127 . The second phase shifter  226  provides the third shift signal to the first monopole antenna  131 , and provides the fourth shift signal to the second monopole antenna  132 . Additionally, the second phase shifter  226  provides the fifth shift signal to the second monopole antenna  132 .  
         [0036]     The transmission antenna section  13  includes the first monopole antenna  131  and the second monopole antenna  132 , as described above. The first monopole antenna  131  is connected to the adder  127 . The second monopole antenna  132  is connected to the second phase shifter  226  and is physically perpendicular to the first monopole antenna  131 .  
         [0037]     The first monopole antenna  131  and the second monopole antenna may be another type antenna that can emit a linear polarized wave.  
         [0038]     Operation of the radio communicator in this embodiment is described below referring to FIGS.  3  to  5 .  
         [0039]     The controller  3  judges the radio wave condition in the air based on a reception signal received by the receiver  2 . The controller  3  classifies the radio signal condition in the air into three classes, including case  1  which indicates the radio signal condition in the air is bad, case  3  which indicates the radio signal condition in the air is good, and a case  2  which indicates the radio signal condition in the air is medium between the case  1  and the case  3 .  
         [0040]     When the controller  3  determines the actual radio signal condition to be case  1 , the controller  3  directs the transmission signal processor  11  to modulate transmission data to the baseband signal using the FSK modulation which is robust over fluctuation of signal level and noise, and to send the FSK modulated signal using circular polarization which is robust over fading.  
         [0041]     When the controller  3  determines the actual radio signal condition to be case  2 , the controller  3  directs the transmission signal processor  11  to modulate transmission data to the baseband signal using the QPSK modulation which enables high transmission efficiency, and to send the QPSK modulated signal using the circular polarization.  
         [0042]     When the controller  3  determines the actual radio signal condition to be case  3 , the controller  3  directs the transmission signal processor  11  to modulate transmission data to the baseband signal using the QPSK modulation, and to send the QPSK modulated signal using linear polarization.  
         [0043]     Details of each case are described below, respectively.  
         [0000]     (Case  1 )  
         [0044]      FIG. 3  illustrates a partial diagram of the transmitter  1  of the radio communicator when the controller  3  determines actual radio signal condition to be case  1 .  
         [0045]     The controller  3  directs the transmission signal processor  11  to modulate transmission data to the baseband signal using FSK modulation, and to output the FSK modulating signal from the output terminal  11 - 2 . The controller  3  lets the switch  121 - 2  short, and the controller  3  lets switches  121 - 1  and  121 - 3  open.  
         [0046]     The second local oscillator  222  outputs a modulated sine wave to the second phase shifter  226 . The frequency of the modulated sine signal depends on the FSK modulating signal obtained through the switch  121 - 2 . The second local oscillator  222  provides the modulated sine wave to the second phase shifter  226 .  
         [0047]     The second phase shifter  226  generates a third shift signal, the phase of which is shifted 0 degrees from the phase of the modulated sine wave provided from the second local oscillator  222 . The second phase shifter  226  also generates a fourth shift signal, the phase of which is shifted 90 degrees from phase of the modulated sine wave provided from the second local oscillator  222 . The second phase shifter  226  provides the third shift signal to the first monopole antenna  131 , and provides the fourth shift signal to the second monopole antenna  132 .  
         [0048]     The first monopole antenna  131  emits the third shift signal as a linear polarized wave. The second monopole antenna  132  emits the fourth shift as a linear polarized wave.  
         [0049]     Since phase difference between the third shift signal and the fourth shift signal is 90 degrees, those signals emitted from the first monopole antenna  131  and the second monopole antenna  132  are combined into a circular polarized wave.  
         [0000]     (Case  2 )  
         [0050]      FIG. 4  illustrates a partial diagram of the transmitter  1  of the radio communicator when the controller  3  determines the actual radio signal condition to be case  2 .  
         [0051]     The controller  3  then directs the transmission signal processor  11  to modulate transmission data to the baseband signal using the QPSK modulation, and directs to output the QPSK modulated signal from the output terminals  11 - 1  and  11 - 3 . The controller  3  lets switches  121 - 1  and  121 - 3  short, and the controller  3  lets the switch  121 - 2  open.  
         [0052]     The second phase shifter  226  generates the fifth shift signal, the phase of which is shifted 90 degrees from phase of the addition signal provided from the adder  127 . The second phase shifter  226  provides the fifth shift signal to the second monopole antenna  132 .  
         [0053]     The transmission signal processor  11  provides the I channel component of the QPSK modulated signal from the output terminal  11 - 1  to the first mixer  124  through the switch  121 - 1 , and provides the Q channel component of the QPSK modulated signal from the output terminal  11 - 3  to the second mixer  125  through the switch  121 - 3 .  
         [0054]     The first mixer  124  converts the I channel component to the first RF signal using the first shift signal provided from the first phase shifter  126 . The first mixer  124  provides the first RF signal from the RF output terminal  124 - 3  to the adder  127 .  
         [0055]     The second mixer  125  converts the Q channel component to the second RF signal using the second shift signal provided from the first phase shifter  126 . The second mixer  125  provides the second RF signal from the RF output terminal  125 - 3  to the adder  127 .  
         [0056]     The adder  127  generates the addition signal that is an addition of the first RF signal and the second RF signal. The adder  127  provides the addition signal to the first monopole antenna  131  of the transmission antenna section  13 , and to the second phase shifter  226 .  
         [0057]     The second phase shifter  226  generates the fifth shift signal, the phase of which is shifted 90 degrees from phase of the addition signal provided from the adder  127 . The second phase shifter  226  provides the fifth shift signal to the second monopole antenna  132 .  
         [0058]     The first monopole antenna  131  emits the addition signal. The second monopole antenna  132  emits the fifth shift signal. Then, the addition signal and the fifth shift signal are linear polarized, respectively.  
         [0059]     Since the phase difference between the addition signal and the fifth shift signal is 90 degrees, those signals emitted from the first monopole antenna  131  and the second monopole antenna  132  are combined into a circular polarized wave.  
         [0000]     (Case  3 )  
         [0060]      FIG. 5  illustrates a partial diagram of the transmitter  1  of the radio communicator when the controller  3  determines the actual radio signal condition to be case  3 .  
         [0061]     The controller  3  directs the transmission signal processor  11  to modulate transmission data to the baseband signal using the QPSK modulation, and directs to output the QPSK modulated signal from the output terminals  11 - 1  and  11 - 3 . The controller  3  lets switches  121 - 1  and  121 - 3  short, and the controller  3  lets the switch  121 - 2  open.  
         [0062]     The transmission signal processor  11  provides the I channel component of the QPSK modulated signal from the output terminal  11 - 1  to the first mixer  124  through the switch  121 - 1 , and provides the Q channel component of the QPSK modulated signal from the output terminal  11 - 3  to the second mixer  125  through the switch  121 - 3 .  
         [0063]     The first mixer  124  converts the I channel component to the first RF signal using the first shift signal provided from the first phase shifter  126 . The first mixer  124  provides the first RF signal to the adder  127  through the RF output terminal  124 - 3 .  
         [0064]     The second mixer  125  converts the Q channel component to the second RF signal using the second shift signal provided from the first phase shifter  126 . The second mixer  125  provides the second RF signal to the adder  127  through from the RF output terminal  125 - 3 .  
         [0065]     The adder  127  generates the addition signal that is an addition of the first RF signal and the second RF signal. The adder  127  provides the addition signal to the first monopole antenna  131  of the transmission antenna section  13 .  
         [0066]     The first monopole antenna  131  emits the addition signal. Then, the addition signal is linear polarized.  
         [0067]     As described above, when the actual radio signal condition is bad, data is converted to two sine wave signals orthogonal to each other. One of two sine wave signals is emitted from the first monopole antenna  131  as a linear polarized wave, and the other of two sine wave signals is emitted from the second monopole antenna  132  perpendicular to the first monopole antenna  131  as linear polarized wave. Those two linear polarized waves perpendicular to each other combine into a circular polarized wave.  
         [0068]     Therefore this invention eliminates the need for a special circuit for converting polarization type.  
         [0069]     Frequency shift keying in the broad sense of the term, including GMSK and GFSK, can be used as substitutes for the FSK. Phase shift keying in the broad sense of the term, including 8PSK and quadrature amplitude modulation schemes such as 16QAM, can be used as substitutes for the QPSK.  
       Second Exemplary Embodiment  
       [0070]     A second exemplary embodiment of a radio communicator is described below referring to FIGS.  6  to  8 .  
         [0071]     In this embodiment, a radio transmission circuit  1012  is different from radio transmission circuit  12  in the first exemplary embodiment.  
         [0072]      FIG. 6  illustrates a block diagram of another example of the transmitter  1  including an example of a radio transmission circuit  1012  and a controller  1003 .  
         [0073]     The radio transmission circuit  1012  includes a switch set  1121  including switches  1121 - 1  to  1121 - 3 , a first local oscillator  1122 , a first mixer  1124 , a second mixer  1125 , a phase shifter  1126 , and an adder  1127 .  
         [0074]     Compared to the radio transmission circuit  12  in the first exemplary embodiment, the radio transmission circuit  1012  does not include components corresponding to the second local oscillator  222  and second phase shifter  226 . The first mixer  1124  has two input terminals  1124 - 1  and  1124 - 2 , and two RF output terminals  1124 - 3  and  1124 - 4 . The input terminal  1124 - 1  connects to the output terminal  11 - 1  of the transmission signal processor  11  through the switch  1121 - 1 . The input terminal  1124 - 2  connects to the phase shifter  1126 . Signals input from input terminals  1124 - 1  and  1124 - 2  are converted to a first RF signal by frequency mixing. The first mixer  1124  has two operation modes including mixer mode and local leak mode. In the mixer mode, the RF output terminal  1124 - 3  provides the first RF signal to the adder  1127  and the RF output terminal  1124 - 4  provides nothing, or the RF output terminal  1124 - 3  provides nothing and the RF output terminal  1124 - 4  provides the first RF signal to the first monopole antenna  131 . In the local leak mode, the RF output terminals  1124 - 3  and  1124 - 4  output a first shift signal provided from the phase shifter  1126  without frequency change.  
         [0075]     The second mixer  1125  has two input terminals  1125 - 1  and  1125 - 2 , and two RF output terminals  1125 - 3  and  1125 - 4 . The input terminal  1125 - 1  connects to the output terminal  11 - 3  of the transmission signal processor  11  through the switch  1121 - 3 . The input terminal  1125 - 2  connects to the phase shifter  1126 . Signals input from input terminals  1125 - 1  and  1125 - 2  are converted to a second RF signal by frequency mixing. The second mixer  1125  also has two operation modes including mixer mode and local leak mode. In the mixer mode, the RF output terminal  1125 - 3  provides the second RF signal to the adder  1127  and the RF output terminal  1125 - 4  provides nothing, or the RF output terminal  1125 - 3  provides nothing and the RF output terminal  1125 - 4  provides the second RF signal to the second monopole antenna  132 . In the local leak mode, the RF output terminals  1125 - 3  and  1125 - 4  output a second shift signal provided from the phase shifter  1126  without frequency change.  
         [0076]     The local oscillator  1122  is connected to the output terminal  11 - 2  of the transmission signal processor  11  through the switch  1121 - 2 . The local oscillator  1122  generates a sine wave, and provides the sine wave to the phase shifter  1126 .  
         [0077]     When the switch  1121 - 2  shorts, the baseband signal is provided from the transmission signal processor  11  to the local oscillator  1122 . Then, the local oscillator  1122  outputs a modulated sine wave to the phase shifter  1126 .  
         [0078]      FIG. 7  illustrates a functional block diagram of an example of the first mixer  1124 . The second mixer  1125  may be similar configuration.  
         [0079]     The first mixer  1124  includes a V-I converter  301 , a first switch  302 , a second switch  303 , a first local buffer amplifier  304 , and a second local buffer amplifier  305 .  
         [0080]     The V-I converter  301  receives modulated signal provided from the output terminal  11 - 1  of the transmission signal processor  11  through the switch  1121 - 1 . The V-I converter  301  converts the modulated signal, which is a kind of voltage signal, into a current signal. The V-I converter  301  provides the current signal to the first switch  302  and the second switch  303 .  
         [0081]     The local buffer amplifier  304  amplifies the first shift signal provided from the phase shifter  1126  to generate a first amplified local signal. The local buffer amplifier  305  amplifies the first shift signal provided from the phase shifter  1126  to generate a second amplified local signal.  
         [0082]     The first switch  302  mixes frequencies of the current signal and the first amplified local signal to generate a first mixed signal. The second switch  303  mixes frequencies of the current signal and the second amplified local signal to generate a second mixed signal.  
         [0083]     The controller  1003  controls the local buffer amplifier  304 , the local buffer amplifier  305 , and the digital signal processor  11 .  
         [0084]     In the mixer mode, the controller  1003  enables only one of two switches. The controller  1003  enables the first local buffer amplifier  304  to output the first amplified local signal, and disables the second local buffer amplifier  305  to output the second amplified local signal. Then, the first amplified local signal is provided to the first switch  302 , but the second amplified local signal is not provided to the second switch  303 . That is, the controller  1003  enables only the first switch  302  between two switches. Or, the controller  1003  enables the second local buffer amplifier  305  to output the second amplified local signal, and disables the first local buffer amplifier  304  to output the first amplified local signal. Then, the second amplified local signal is provided to the second switch  303 , but the first amplified local signal is not provided to the first switch  302 . That is, the controller  1003  enables only the second switch  303  between two switches. In the local leak mode, the controller  1003  directs the digital signal processor  11  to output stationary signal from the output terminal  11 - 1 . The stationary signal is provided to the V-I converter  301  through the switch  1121 - 1 . Then, since frequency of the current signal become zero, the first switch  302  outputs the first amplified local signal as the first mixed signal, and the second switch  303  outputs the second amplified local signal as the second mixed signal.  
         [0085]     An operation of the transmitter  1  in this embodiment is described below.  
         [0086]     First, the controller  1003  judges the radio signal condition in the air based on a reception signal received by the receiver  2 .  
         [0087]     When the controller  1003  determines the actual radio signal condition to be case  1  which indicates bad condition, the controller  1003  directs the transmission signal processor  11  to modulate transmission data to the baseband signal using the FSK modulation and to send the FSK modulated signal using circular polarization.  
         [0088]     When the controller  3  determines the actual radio signal condition to be case  3  which indicates good condition, the controller  3  directs the transmission signal processor  11  to modulate transmission data to the baseband signal using the QPSK modulation, and to send the QPSK modulated signal using linear polarization.  
         [0000]     (Case  1 )  
         [0089]     The operation of the transmitter  1  when the controller  1003  determines the actual radio signal condition to be case  1  is described below using  FIG. 6 .  
         [0090]     The controller  1003  directs the transmission signal processor  11  to modulate transmission data to the baseband signal using the FSK modulation, and directs to output the FSK modulated signal from the output terminal  11 - 2 . The controller  1003  directs the transmission signal processor  11  to output the stationary signal from output terminals  11 - 1  and  11 - 3 , also.  
         [0091]     The controller  1003  lets switches  1121 - 1  to  1121 - 3  short, lets the first mixer  1124  output the first RF signal from the RF output terminal  1124 - 4 , and lets the second mixer  1125  output the second RF signal from the RF output terminal  1125 - 4 .  
         [0092]     The FSK modulated signal is provided to the local oscillator  1122  through the switch  1121 - 2 . The local oscillator  1122  outputs modulated sine wave to the first phase shifter  1126 .  
         [0093]     The phase shifter  1126  generates the first shift signal, the phase of which shifts 0 degrees from phase of the modulated sine wave provided from the local oscillator  1122 . The phase shifter  1126  generates the second shift signal also, the phase of which shifts 90 degrees from phase of the modulated sine wave provided from the local oscillator  1122 . The phase shifter  1126  provides the first shift signal to the first mixer  1124 , and provides the second shift signal to the second mixer  1125 .  
         [0094]     The first mixer  1124  receives the stationary signal from the output terminal  11 - 1  of the digital signal processor  11  through the switch  1121 - 1 , and then the first mixer  1124  operates under the local leak mode. The second mixer  1125  receives the stationary signal from the output terminal  11 - 3  of the digital signal processor  11  through the switch  1121 - 3 , and then the second mixer  1125  operates under the local leak mode. That is, the first mixer  1124  provides the first shift signal to the first monopole antenna  131  without change, and the second mixer  1125  provides the second shift signal to the second monopole antenna  132  without change.  
         [0095]     The first monopole antenna  131  and the second monopole antenna  132  emit the first shift signal and the second shift signal as linear polarized waves, respectively.  
         [0096]     Since phase difference between these sine waves is 90 degrees, those signals emitted from the first monopole antenna  131  and the second monopole antenna  132  are combined into a circular polarized wave.  
         [0000]     (Case  3 )  
         [0097]      FIG. 8  illustrates a functional block diagram of the transmitter  1  of the radio communicator when the controller  1003  determines the actual radio signal condition to be case  3 .  
         [0098]     The controller  1003  directs the transmission signal processor  11  to modulate transmission data to the baseband signal using the QPSK modulation, and directs to output an I channel component of the QPSK modulated signal from the output terminal  11 - 1  and a Q channel component of the QPSK modulated signal from the output terminal  11 - 3 .  
         [0099]     The controller  1003  lets switches  1121 - 1  to  1121 - 3  short, lets the switch  1121 - 2  open, lets the first mixer  1124  output the first RF signal from the RF output terminal  1124 - 3 , and lets the second mixer  1125  output the second RF signal from the RF output terminal  1125 - 3 .  
         [0100]     The I channel component and the Q channel component are provided to the first mixer  1124  and the second mixer  1125 , respectively.  
         [0101]     The first mixer  1124  also receives the first shift signal and converts the I channel component and the first shift signal to the first RF signal by frequency mixing. The RF output terminal  1124 - 3  provides the first RF signal to the adder  1127 .  
         [0102]     The second mixer  1125  also receives the second shift signal and converts the Q channel component and the second shift signal to the second RF signal by frequency mixing. The RF output terminal  1125 - 3  provides the second RF signal to the adder  1127 .  
         [0103]     The adder  1127  generates an addition signal from addition of the first RF signal provided from the first mixer  1124  and the second RF signal provided from the second mixer  1125 , and provides the addition signal to the first monopole antenna  131 .  
         [0104]     The first monopole antenna  131  emits the addition signal as a linear polarized wave.  
         [0105]     As described above, the local leak mode of the first mixer  1124  and the second mixer  1125  enables to omit components corresponding to the second local oscillator  222  and second phase shifter  226  from the radio transmission circuit  1012 .  
       Third Exemplary Embodiment  
       [0106]     A third exemplary embodiment of a radio communicator is described below by reference to FIGS.  9  to  12 .  
         [0107]     The physical configuration of the radio communicator in the third embodiment may be the same as in the first or the second exemplary embodiment. The physical configuration of the first exemplary embodiment is employed to explain the characteristic operation of a controller in this embodiment.  
         [0108]     The radio communicator in this embodiment transmits a data packet as the transmission data to a radio receiver. The radio receiver can send back some response such as an ACK packet, which is sent as an acknowledgement of normal reception of the data packet, to the radio communicator.  
         [0109]      FIGS. 9 and 10  illustrate flowcharts of the operation of the controller  3  with judgment of radio signal condition in the air based on a retransmission counter.  
         [0110]     The controller  3  has a memory to store a modulation scheme and polarization type. The memory stores which modulation scheme and polarization type are finally chosen at transmission of the previous packet.  
         [0111]      FIG. 9  illustrates an operation of the controller  3  if the modulation scheme and polarization type finally employed at transmission of the previous packet are the QPSK modulation and the linear polarization.  
         [0112]     At the first transmission of the data packet, the controller  3  initializes a retransmission counter N as 0 (step S 101 ).  
         [0113]     The controller  3  compares the counter N with a predetermined threshold number TH 1  (step S 102 ).  
         [0114]     The threshold TH 1  is greater than zero. Therefore, the counter N is smaller than the threshold TH 1  at the first time of transmission to operate the step S 102 .  
         [0115]     If the counter N is smaller than the threshold TH 1  (“Yes” of the step S 102 ), the controller  3  judges the radio signal condition in the air as still good, and the controller  3  directs the transmission signal processor  11  to modulate the data packet to the baseband signal using the QPSK modulation and to send the QPSK modulated signal using linear polarization (step S 103 ), to increase the counter N by 1 (step S 104 ), and to wait the ACK packet from the radio receiver.  
         [0116]     The controller  3  checks whether the receiver  2  has received the ACK packet (step S 105 ).  
         [0117]     If the controller  3  determines the receiver  2  has received the ACK packet (“Yes” of the step S 105 ), the controller  3  directs the transmission signal processor  11  to terminate the transmission of the data packet.  
         [0118]     If the controller  3  determines the receiver  2  has not received the ACK packet (“No” of the step S 105 ), the controller  3  directs the transmission signal processor  11  to retry the step S 102 .  
         [0119]     If the counter N equal or is greater than the threshold TH 1  (“No” of the step S 102 ), the controller  3  judges the radio signal condition in the air as bad, and the controller  3  directs the transmission signal processor  11  to modulate the data packet to the baseband signal using the FSK modulation and to send the FSK modulated signal using circular polarization (step S 106 ), and to terminate the transmission of the data packet.  
         [0120]      FIG. 10  illustrates an operation of the controller  3  if the modulation scheme and polarization type finally employed at transmission of the previous packet are the FSK modulation and the circular polarization such as the step S 106  in the  FIG. 9 .  
         [0121]     At first transmission of the data packet, the controller  3  initializes a retransmission counter N as zero (step S 201 ).  
         [0122]     The controller  3  directs the transmission signal processor  11  to modulate the data packet to the baseband signal using the FSK modulation and to send the FSK modulated signal using circular polarization (step S 202 ), the increase the counter N by 1 (step S 203 ), and to wait the ACK packet from the radio receiver.  
         [0123]     The controller  3  checks whether the receiver  2  has received the ACK packet (step S 204 ).  
         [0124]     If the controller  3  determines the receiver  2  has not received the ACK packet (“No” of the step S 204 ), the controller  3  directs the transmission signal processor  11  to retry the step S 202 .  
         [0125]     If the controller  3  determines the receiver  2  has received the ACK packet (“Yes” of the step S 204 ), the controller  3  compares the counter N with a predetermined threshold number TH 2  (step S 205 ).  
         [0126]     If the counter N equals the threshold TH 2  or less (“No” of the step S 205 ), the controller  3  judges the radio signal condition in the air as improved, and the controller  3  determines the modulation scheme and the polarization type for a data packet following the data packet transmitted in the step S 202  as the QPSK modulation and linear polarization (step S 206 ).  
         [0127]     If the counter N is greater than the threshold TH 2  (“Yes” of the step S 205 ), the controller  3  judges the radio signal condition in the air as still bad, and the controller  3  determines the modulation scheme and the polarization type for a data packet following the data packet transmitted in the step S 202  as the FSK modulation and circular polarization (step S 207 ).  
         [0128]      FIG. 11  illustrates a flowchart of the operation of the controller  3  using judgment of radio signal condition in the air based on a BER (bit error rate).  
         [0129]     At first, the radio communicator transmits a predetermined data packet. The controller  3  directs the transmission signal processor  11  to modulate the predetermined data packet to the baseband signal using the same modulation scheme as the previous packet transmission, and to send the modulated signal using the same polarization type as the previous packet transmission (step S 301 ). The radio receiver sends back a BER calculated by comparing a received packet with a predetermined data pattern stored in the radio receiver.  
         [0130]     On receiving the BER from the radio receiver, the controller  3  compares the BER with a predetermined threshold value TH 3  (step S 302 ).  
         [0131]     If the BER equals the threshold TH 3  or less (“Yes” of the step S 302 ), the controller  3  judges the radio signal condition in the air as good, and the controller  3  determines the modulation scheme and the polarization type for a data packet following the predetermined data packet as the QPSK modulation and linear polarization (step S 303 ).  
         [0132]     If the BER is greater than the threshold TH 3  (“No” of the step S 302 ), the controller  3  judges the radio signal condition in the air as bad, and the controller  3  determines the modulation scheme and the polarization type for a data packet following the predetermined data packet as the FSK modulation and circular polarization (step S 304 ).  
         [0133]      FIG. 12  illustrates a flowchart of the operation of the controller  3  with judgment of radio signal condition in the air based on signal electric field strength.  
         [0134]     At first transmission of the data packet, the controller  3  directs the transmission signal processor  11  to modulate the data packet to the baseband signal using the same modulation scheme as the previous packet transmission, and to send the modulated signal using the same polarization type as the previous packet transmission (step S 401 ). The radio receiver sends back signal electric field strength measured with a received packet.  
         [0135]     On receiving the signal electric field strength from the radio receiver, the controller  3  compares the signal electric field strength with a predetermined threshold value TH 4  (step S 402 ).  
         [0136]     If the signal electric field strength equals the threshold TH 4  or less (“Yes” of the step S 402 ), the controller  3  judges the radio signal condition in the air as bad, and the controller  3  determines the modulation scheme and the polarization type for the following the data packet as the FSK modulation and circular polarization (step S 403 ).  
         [0137]     If the signal electric field strength is bigger than the threshold TH 4  (“No” of the step S 402 ), the controller  3  judges the radio signal condition in the air as good, and the controller  3  determines the modulation scheme and the polarization type for the following data packet as the QPSK modulation and linear polarization (step S 404 ).  
         [0138]     If the radio receiver also judges a radio signal condition in the air for transmission of the radio receiver itself, according to reciprocity theorem, the radio receiver can use the signal electric field strength measured by the radio receiver itself for the judgment.  
         [0139]     In above explanation, the controller  3  judges the radio signal condition in the air into two levels such as good or bad, but three or more level judgment can be realized by employing two or more threshold numbers.  
         [0140]     Numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.