Patent Publication Number: US-7720172-B2

Title: Transmitting apparatus receiving apparatus, radio communication method and radio communication system

Description:
This application is a U.S. National Phase application of PCT International Application PCT/JP03/11688. 
     TECHNICAL FIELD 
     The present invention relates to a transmitting apparatus, receiving apparatus, radio communication system and radio communication method for sending confidential information between particular radio stations. 
     BACKGROUND ART 
     Drastic improvements in transmission speed and transmission quality in recent years have led digital radio communications to occupy an important place in the communication field. On the other hand, since radio communications use radio space that is a public asset, there is a basic defect, that a third party may be able to receive, from the standpoint of confidentiality. Namely, there is always a constant risk of communication contents being intercepted by a third party and information leaked out. 
     Thus, in conventional radio communications, techniques such as encryption of confidential information are used to prevent a third party from understanding the contents of the hidden information even if transmitting data is intercepted by the third party. Encryption technique has been studied and applied in various fields. This is because encryption has the advantage of enabling constant security to be assured without changing a radio communication system. 
     However, through the process of information encryption, there is such a problem that information can be decrypted comparatively easily if the code and/or procedure for encryption are known. With the current state prevalent of high-speed computers, in particular, security can no longer be assured without performing rather complicated encryption processing. 
     Against such a problem accompanying the encryption technique, there is an invention disclosed in, for example, JP-A-2002152191 and so forth as a radio communication method that pays attention to a physical feature of the propagation environment thereof.  FIG. 23  shows the conventional radio communication system as described in the publication. 
     In  FIG. 23 , a propagation environment estimator  2311  in a transmitting station  2310  estimates the state information of a radio propagation channel  2330  shared only between the transmitting station  2310  and a receiving station  2320  which is the destination of transmitting data including confidential information. Then the transmitting station  2310  transmits the data including confidential information in view of this radio propagation environment. Due to this, because other radio stations having different radio propagation path environments cannot receive or reconstruct confidential information, the transmitting station can transmit confidential information with a high security. 
     However, in a broadband radio communication in general, due to its enhanced transmission rate, propagation parameters characterizing the propagation path, directivity, and polarization of antennas come to have frequency characteristic. Consequently, such a radio communication method as disclosed in the patent publication, wherein the transmitting station controls the propagation parameters by using a plurality of antennas, is available on the premise that the propagation parameters should be controlled within a particular frequency band, that is to say, within the range where the frequency characteristic of the antennas and the propagation paths are deemed to be uniform. 
     What is the problem to be solved is that, in the case of a broadband radio communication, characteristics of propagation paths and antennas can not be effectively utilized enough. 
     DISCLOSURE OF THE INVENTION 
     The present invention addresses the problems discussed above, and aims to provide a transmitting apparatus, receiving apparatus, radio communication method and radio communication system with a highly-advanced security in which the characteristic itself of propagation parameters and antennas that have the frequency characteristic can be utilized as an information for identifying the transmitting signals in a broadband radio communication. 
     The transmitting apparatus according to the present invention comprises: an array antenna configured by M (M is an integer of 2 or more) pieces of antenna elements for receiving carrier modulation signals of a known symbol that is transmitting from a radio station; a reference symbol generation means for generating a reference symbol which is equal to the known symbol and which is to provide a phase reference; and a propagation channel estimation means for generating M pieces of receiving symbols, which are the estimate values for the complex propagation channels between the transmitting antenna and the array antennas, from the baseband signals received at the antenna elements based on the reference symbol, wherein the receiving symbols are estimate values for a complex propagation channel between a transmitting antenna and the array antenna. 
     Due to this configuration, in a complicated propagation environment for mobile communication, it is made possible to characterize the propagation channel characteristic shared only with the radio station that is the destination of transmitting data including confidential information, by a channel estimate value obtained by signals received at a plurality of antennas. That means the data including confidential information is transmitted based on the correlation for the channel estimates between antennas. Due to this other radio stations with different radio propagation environments cannot receive or reconstruct confidential information. As a result, confidential information can be transmitted with high security thanks to the feature of mobile communication, where relative physical relationship between transmission/reception devices is always changing. 
     Also, the transmitting apparatus according to the present invention, further comprises a carrier separation means for separating the received baseband signals received at M pieces of antenna elements into N (N is an integer of 2 or more) pieces of sub-carriers, wherein the carrier modulation signal is configured by multiple carriers, and the carrier separation means, after separating the received baseband signals into N (N is an integer of 2 or more) pieces of sub-carriers, generates “M×N” pieces of receiving symbols that are estimate values of the complex propagation channel based on the reference symbols. 
     Due to this configuration, in a complicated mobile communication propagation environment, the propagation channel characteristic, shared only with the radio station that is the destination of transmitting data including confidential information, can be characterized by channel estimate values that can be obtained from the receiving signals for each sub-carrier. As a result, this mechanism enables to transmit the amount of data equal to the maximum pieces of sub-carriers simultaneously in parallel, making it possible to transmit confidential information with high security in a short period. 
     Also, the propagation channel estimation means in the transmitting apparatus according to the present invention is, after applying a reverse spread separation process to the baseband signal received at the M (herein, M is an integer of 2 or more) pieces of antenna elements with N (N is an integer of 2 or more) pieces of spread codes, generates “M×N” pieces of receiving symbols that are estimate values of a complex propagation channel based on the reference symbol. 
     Due to this configuration, in a complicated mobile communication propagation environment, the propagation channel characteristic, shared only with the radio station that is the destination of transmitting data including confidential information, can be characterized by channel estimate values that can be obtained from the receiving signals for each spread code. As a result, this mechanism allows it to transmit the amount of data equal to the maximum pieces of spread codes simultaneously in parallel, making it possible to transmit confidential information with high security in a short period. 
     Also, M pieces of antenna elements configuring the array antenna in the transmitting apparatus according to the present invention have a mutually-different directional pattern, or a mutually-different polarization. 
     Due to this, the propagation channel characteristics shared only with the radio station that is the destination will vary depending on the directional patterns of the antenna elements configuring the array antenna. Therefore, in order to receive and reconstruct confidential information by other radio stations, it is necessary to consider the propagation channel characteristics including the antenna directional patterns. That means it is made more difficult for a third party to reconstruct confidential information, following that confidential information can be transmitted with high security. Further, if the number of the antenna elements is the same, the change of polarization can make it possible to downsize the array antenna as compared with the change of directional patterns, following that the whole apparatus can be downsized. 
     Also, the transmitting apparatus according to the present invention comprises a transmitting symbol calculation means that calculates a plurality of sets of transmitting symbol vectors from M pieces of receiving symbols so that each transmitting symbol vector is configured by M pieces of transmitting symbols and then generates a reference table configured by the plural sets of transmitting symbol vectors, a symbol mapping means that generates M pieces of transmitting symbols by selecting one of transmitting symbol vector from the reference table based on a transmitting data, and a single carrier modulation means that generates baseband signals from M pieces of transmitting symbols. 
     Also, the transmitting apparatus according to the present invention comprises a transmitting symbol calculation means that calculates a plurality of sets of transmitting symbol vectors from the “M×N” pieces of receiving symbols for each of N pieces of sub-carriers so that each vector is configured by M pieces of transmitting symbols; and then generate reference tables configured by the plural sets of transmitting symbols vector, a symbol mapping means for generating “M×N” pieces of transmitting symbols by selecting one set of transmitting symbol vector from each of N pieces of reference tables that correspond to the N pieces of sub-carriers based on transmitting data, and a single carrier modulation means for generating transmitting baseband signals from “M×N” pieces of transmitting symbols with N pieces of sub-carrier elements. 
     Due to this configuration, the propagation channel characteristics shared only with the radio station that is the destination of the transmitting data including confidential information can be characterized by the channel estimate values obtained from the receiving signals of a plurality of sub-carrier elements that configure multiple carriers over a plurality of antennas. By this means, the transmitting data including confidential information is transmitted depending on the correlation for the channel estimate values between antennas and so on. Namely, other radio stations with a different radio propagation environment cannot receive or reconstruct confidential information. As a result, in a mobile communication system, where the relative physical relationship between the transmitting/reception apparatus constantly changes and so does the frequency characteristic of the propagation channel accordingly, confidential information can be transmitted with even higher security. 
     Also, the transmitting apparatus according to the present invention comprises a transmitting symbol calculation means for calculating plural sets of transmitting symbol vectors from “M×N” pieces of the receiving symbols for each of N pieces of spread codes so that each transmitting symbol vector is configured by M pieces of transmitting symbols, and then generating reference tables configured by the plural sets of symbol vectors, a symbol mapping means for generating “M×N” pieces of transmitting symbols by selecting one set of transmitting symbol vector from each of the N pieces of reference tables based on the transmitting data including confidential information; and a single carrier modulation means for generating transmitting baseband signals from the “M×N” pieces of transmitting symbols by spread process with N pieces of reverse spread codes. 
     Due to this configuration, the propagation channel characteristic, which is shared only with the radio station that is the destination of transmitting data including confidential information, can be characterized by the channel estimate values estimated from the signals received at the a plurality of antennas depending on each of the a plurality of spread codes. By this, the transmitting data including confidential information is transmitted based on correlation for the channel estimate values between antennas. As a result, it is made impossible for other radio stations with a difference propagation environment to receive or reconstruct confidential information. Thus, thanks to the feature of mobile communication system where the relative physical relationship between transmitting/reception apparatus constantly changes and so does the propagation channel characteristic accordingly, it is able to utilize not only the confidentiality of spread codes but also the random characteristic of propagation parameters. As a result, it is made possible to assure even higher degree of security. 
     Also a transmitting symbol calculation means in the transmitting apparatus according to the present invention generates the plural sets of symbol vectors in order to control any one of the receiving power and the phase of the radio station. 
     Due to this configuration, because only receiving power has to be detected by the radio station, it is made possible for a wireless application to be very simply configured. As a result, the transmitting data with high security can be realized at low cost. Also, because the phase rotation of a receiving signal, which is caused in accordance with the move of a radio station in a multi-path propagation environment, is 360 degrees in length virtually equal to the wavelength of a carrier, it is made impossible for a third party to reconstruct the transmitting data including confidential information based on the phase information, especially by a mobile phone or wireless LAN having wavelength from dozens-cm to several-cm. As a result, confidential information can be transmitted with even higher degree of security as compared with the case of symbol determination using a receiving power. 
     The receiving apparatus according to the present invention comprises a propagation parameter estimation means for estimating propagation parameters from receiving signals and a symbol determination means for reconstructing the transmitting data based on the propagation parameters. 
     Also the receiving apparatus according to the present invention, further comprises a carrier separation means for separating the receiving signal, in which a receiving signal is configured by multiple carriers, into a plurality of sub-carriers, wherein the propagation parameter estimation means estimates a propagation parameter for each of the sub-carriers and the symbol determination means reconstructs the transmitting data from the receiving signal for each of the sub-carriers. 
     Also, the receiving apparatus according to the present invention has sub-carriers that are any one of an OFDM signal that is so configured as to be mutually-orthogonal in a frequency space and a CDMA signal that is so configured as to be mutually-orthogonal in a code space. 
     Also, the receiving apparatus according to the present invention comprises an array antenna that is configured by at least one antenna element, wherein the propagation parameter estimation means estimates the propagation parameter for each of the antenna. 
     Also, the receiving apparatus according to the present invention comprises a propagation parameter estimation means for generating a receiving symbol that is a complex symbol by applying orthogonal detection to a received baseband signal; and a symbol determination means for reconstructing the transmitting data from the receiving symbols based on predetermined criteria. 
     Due to this configuration, the data transmitting including confidential information is transmitted based on correlation for the channel estimates between antennas, which are the predetermined criteria. By this means can be made a symbol determination for receiving signals in the radio station, therefore other radio stations with a different propagation environment can not receive or reconstruct confidential information. As a result, thanks to the feature of mobile communication system where relative physical relationship between transmitting/reception apparatus constantly changes, confidential information can be transmitted with high security. 
     Also, the receiving apparatus according to the present invention, further comprises a carrier separation means for separating the baseband signal, which is configured by a multiple carriers, into N (N is an integer of 2 or more) pieces of sub-carrier elements, wherein the propagation parameter estimation means generates the receiving symbols for each of the sub-carriers after the carrier separation means separates the baseband signal into the sub-carriers. 
     Due to this configuration, the transmitting data including confidential information can be transmitted based on correlation for the channel estimates between antennas, which are the predetermined criteria. By this means can be made a symbol determination for receiving signals in the radio station, thus it is impossible for other radio stations with a different radio propagation environment to receive or reconstruct confidential information. As a result, thanks to the feature of mobile communication system where relative physical relationship between transmitting/reception apparatus constantly changes and so does the frequency characteristics of the propagation channel accordingly, it is made possible to transmit confidential information with even higher degree of security. 
     Also, a symbol determination means in the receiving apparatus according to the present invention reconstructs the transmitting data based on predetermined criteria after the propagation parameter estimation means applies a reverse spread process to the baseband signal with N (N is an integer of 2 or more) pieces of spread codes. 
     Due to this configuration, the transmitting data including confidential information is transmitted based on correlation for the channel estimates between antennas, which are predetermined criteria. By this means can be made a symbol determination for receiving signals by the radio station, thus it is impossible for other radio stations with a different radio propagation environment to receive or reconstruct confidential information. As a result of that, thanks to the feature of mobile communication system where relative physical relationship between transmitting/reception apparatus constantly changes and so does the characteristic of the propagation channel accordingly, it can utilize not only confidentiality of spread codes but also the random characteristic of propagation parameters. Therefore even higher degree of security can be assured. 
     Also, the symbol determination means of the receiving apparatus according to the present invention determines a symbol based on the receiving power of the antenna. 
     The radio communication method according to the present invention is the one of transmitting a data on a single carrier from the first radio station to a second radio station, comprising the steps of transmitting an information known by both radio stations from the second radio station to the first radio station, estimating a propagation parameter, which is a parameter of propagation channel shared only between the first radio station and the second radio station, based on the known information and the received information transmitted from the second radio station by the first radio station; transmitting the data from the first radio station to the second radio station by superimposing the transmitting data including a confidential information on the estimated propagation parameter, and calculating a plurality of propagation parameters that are obtained from receiving signals of a plurality of antennas in the second radio station; and reconstructing the transmitting data based on a plurality of propagation parameters calculated by the second radio station. 
     Due to this method, other radio stations having a different propagation channel with the first radio station can not reconstruct the above confidential information. This is because, in a multi-path propagation environment for mobile communications, there is shown a difference between the characteristics of propagation channels if observed at a different point. Namely, the propagation parameter that configures the propagation channel can become a particular information shared only between the first and the second radio stations. Further, the transmitting data is identified based on a plurality of propagation parameters obtained from the signals received at a plurality of antennas. Namely, the receiving signals in the particular antennas can be used as criteria for determining propagation parameters. That allows the modulation method to be more sophisticated, following that even higher degree of security is assured as a result. 
     Also, the radio communication method according to the present invention is the one of transmitting a data on a multiple carriers from the first radio station to the second radio station, comprising the steps of transmitting a information known by both radio stations from the second radio station to the first radio station, estimating the propagation parameter, which is a parameter of the propagation channel shared only between the first radio station and the second radio station, based on the known information and the received information transmitted from the second radio station by the first radio station, transmitting the data from the first radio station to the second radio station by superimposing the transmitting data on the estimated propagation parameter; calculating a plurality of propagation parameters obtained from receiving signals of a plurality of antennas in the second radio stations; and reconstructing the transmitting data based on the a plurality of propagation parameters calculated in the second radio station. 
     Also, in the radio communication method according to the present invention, the second radio station reconstructs transmitting data based on the propagation parameter estimated from the receiving signal for each carrier configuring the multiple carriers. 
     Also, in the radio communication method according to the present invention, each carrier configuring a multiple carriers is any one of OFDM signal that is so configured as to be mutually-orthogonal in a frequency space and a CDMA signal that is so configured as to be mutually-orthogonal in a code space. 
     The radio communication system according to the present invention is the one of transmitting a data by a single carrier modulation method from a first radio station to a second radio station. And the system has the first radio station that comprises a propagation channel estimation means for estimating the propagation channel parameter shared only between the first radio station and the second radio stations when the first radio station transmits a data including a confidential information to a second radio station; and a transmitting means for transmitting the data from the first radio station to the second radio station by superimposing the transmitting signal on the estimated propagation channel parameter, and the second radio station comprising a propagation parameter estimation means for calculating a plurality of propagation parameters obtained from receiving signals of a plurality of antennas and a symbol determination means for reconstructing a transmitting data from the first radio station based on a plurality of the calculated propagation parameters wherein the data transmitting from the first radio station to the second radio station includes confidential information. 
     As described above, the present invention can realize a transmitting apparatus, receiving apparatus, radio communication system and radio communication method that are capable of transmitting confidential information with high security. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a figure showing a configuration of general mobile communication system. 
         FIGS. 1B and 1C  are figures showing a frequency spectrum that configures a propagation channel between a transmitting antenna and a receiving antenna. 
         FIG. 2A  is a block diagram showing a configuration of a radio communication system according to Embodiment 1 of the present invention. 
         FIGS. 2B and 2C  are figures showing a frequency spectrum that configure a propagation channel between a transmitting antenna and a receiving antenna. 
         FIG. 3  is a block diagram showing a configuration of a transmitting station according to Embodiment 1 of the present invention. 
         FIG. 4  is a block diagram showing a configuration of a receiving station according to Embodiment 1 of the present invention. 
         FIG. 5  is a block diagram showing a configuration of a symbol mapping section in the transmitting station according to Embodiment 1 of the present invention. 
         FIG. 6A  is a block diagram showing a configuration of a radio communication system according to Embodiment 2 of the present invention. 
         FIGS. 6B and 6C  are figures showing a frequency spectrum that configures a propagation channel between a transmitting antenna and a receiving antenna. 
         FIG. 7  is a block diagram showing a configuration of a receiving station according to Embodiment 2 of the present invention. 
         FIG. 8A  is a block diagram showing a configuration of a radio communication system according to Embodiment 3 of the present invention. 
         FIG. 8B  is a figure showing eight sub-carrier elements that configure a multiple carriers. 
         FIGS. 8C and 8D  are figures showing a frequency spectrum that configures a propagation channel between a transmitting antenna and a receiving antenna. 
         FIG. 9  is a block diagram showing a configuration of a transmitting station according to Embodiment 3 of the present invention. 
         FIG. 10  is a block diagram showing a configuration of a receiving station according to Embodiment 3 of the present invention. 
         FIG. 11  is a block diagram showing a symbol mapping station of the transmitting station according to Embodiment 3 of the present invention. 
         FIG. 12A  is a block diagram showing a configuration of a radio communication system according to Embodiment 4 of the present invention. 
         FIGS. 12B and 12C  are figures showing a frequency spectrum that configures a propagation channel between a transmitting antenna and a receiving antenna. 
         FIG. 13  is a block diagram showing a configuration of a receiving station according to Embodiment 4 of the present invention. 
         FIG. 14  is a figure showing a way of symbol determination according to Embodiment 3. 
         FIG. 15  is a figure showing a way of symbol determination according to Embodiment 4. 
         FIG. 16  is a block diagram showing a configuration of a transmitting station according to Embodiment 2 of the present invention. 
         FIG. 17  is a block diagram showing a receiving station according to Embodiment 4 of the present invention. 
         FIGS. 18A and 18B  are block diagrams showing a reference table of the transmitting station according to Embodiment 1 of the present invention. 
         FIG. 19  is a block diagram showing a reference table of the transmitting station according to Embodiment 2 of the present invention. 
         FIGS. 20A ,  20 B and  20 C are figures showing away of allocating transmission times for a known symbol according to Embodiment 2 of the present invention. 
         FIG. 21  is a block diagram showing a configuration of a transmitting station according to Embodiment 5 of the present invention. 
         FIG. 22  is a block diagram showing a configuration of a receiving station according to Embodiment 5 of the present invention. 
         FIG. 23  is a block diagram showing a configuration of a conventional radio communication system. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Exemplary embodiments of the present invention are demonstrated in detail hereinafter with reference to the accompanying drawings. 
     Embodiment 1 
       FIG. 1A  is a schematic diagram showing a general mobile communication system  100 .  FIGS. 1B and 1C  show examples of a frequency spectrum, as one of examples of propagation parameters which configure the propagation channels between an transmitting antenna and two receiving antennas. 
     In  FIG. 1A , the mobile communication system  100  comprises a transmitting antenna  101  and receiving antennas  102   a  and  102   b , configuring a propagation channel  103   a  between the transmitting antenna  101  and the receiving antenna  102   a  and configuring a propagation channel  103   b  between the transmitting antenna  101  and the receiving antenna  102   b .  FIG. 1B  shows a frequency spectrum  104   a  of receiving signals observed by the receiving antenna  102   a .  FIG. 1C  shows a frequency spectrum  104   b  of receiving signals observed by the receiving antenna  102   b.    
     Assuming a radio wave propagation environment of cellular phone, wireless LAN and so forth as a general mobile communication system  100 , relative position between the transmitting/reception sides changes according to the move of terminals or peripheral objects. As a result, there is a variation occurred in the propagation channels  103   a  and  103   b  and that leads to the change of the frequency spectrums  104   a  and  104   b.    
     This is because a plurality of arrival waves generated through what is called a multi-path propagation are synthesized in frequency-dependent amplitudes and phase differences. Therefore, when the propagation channel  103   a  varies, the frequency spectrum  104   a  also varies accordingly. 
     Meanwhile, in the case both antennas  102   a  and  102   b  receive simultaneously, there is a difference between the two receiving antennas in arrival waves, amplitudes and phase differences thereof depending on their antenna parameters and propagation parameters. Therefore, there is a difference occurred between the propagation channels  103   a  and  103   b , following that the frequency spectrums  104   a  and  104   b  show a different characteristic each other. 
     Incidentally, in the present invention, “a propagation parameter” is defined to include: a complex channel coefficient which is expressed by amplitudes and phases of receiving signals against amplitudes and phases of reference signals including transmitting signals, from-station-signals and so on; radiation direction from a transmitting antenna; propagation time and propagation distance; incoming direction to a receiving antenna; an attenuation coefficient due to propagation; and further polarization for indicating the direction of electric field, all of which characteristics are dependent on the space propagation mechanism of radio waves. Further, it is defined that “antenna parameters” include all of design parameters such as directional pattern, polarization and matched impedance concerning general antenna designs. 
     Incidentally, in the case where the propagation channels on a same frequency do not vary due to time, the propagation path is allowed to keep its reciprocity between transmitting/reception. Therefore, even if the configuration of transmitting/reception in  FIG. 1  is made reversed, characteristics of frequency spectrums  104   a  and  104   b  can be retained. 
     Details will be demonstrated hereinafter on the radio communication system where such a propagation channel characteristics in the mobile communication can be utilized by superimposing a signal to be transmitted on the propagation parameters. 
       FIG. 2A  shows a radio communication system according to Embodiment 1 of the present invention. 
     In  FIG. 2A , a radio communication system  200  includes a transmitting station  201  and a receiving station  202 , performing a single-carrier radio communication in particular frequency bands. Here, as regards the transmitting station  201  and receiving station  202 , the side that transmits confidential information is simply called the transmitting station  201 , and the side that receives confidential information is simply called the receiving station  202 , both of them having both of transmitting/reception functions each other. 
     The transmitting station  201  includes a transmitting station antennas  203   a  and  203   b , and the receiving station  202  includes a receiving station antenna  204   a .  FIG. 2B  shows a single-carrier power spectrum  206   a  of a propagation channel  205   a  between the transmitting station antenna  203   a  and the receiving station antenna  204   a .  FIG. 2C  shows a single-carrier power spectrum  206   b  of a propagation channel  205   b  between the transmitting station antenna  203   b  and the receiving station antenna  204   a.    
     As described above, the power spectrums  206   a  and  206   b  have a different characteristic each other. Further, it is quite natural that other frequency spectrums such as estimated by other radio stations with a different propagation path will show different characteristics. 
     Next,  FIG. 3  shows a particular configuration of the transmitting station  201  while  FIG. 4  shows a particular configuration of the receiving station  202 . 
     In  FIG. 4 , a known symbol generation means  400  generates a known symbol  401  that is shared between the transmitting station  201  and the receiving station  202 . A single carrier modulation means  402  modulates the known symbol  401  to a baseband signal  403  to be transmitted. A frequency conversion means  404  modulates the baseband signal  403  to a RF signal  405  to be transmitted as well as modulates a RF signal that is received at the antenna  204   a  to a baseband signal  408   a . Also, a propagation parameter estimation means  409  generates a receiving symbol  410   a , which is a complex symbol, from the baseband signal  408   a  by orthogonal detection. A symbol determination means  411  performs a process of determining the receiving symbols  410   a  based on predetermined criteria. The antenna  204   a  transmits/sends out the RF signal  405  as a single carrier modulation signal  406   a.    
     In  FIG. 3 , the transmitting antennas  203   a  and  203   b  receives/transmits RF signals. The frequency conversion means  301  converts received RF signals  300   a  and  300   b  into received baseband signals  302   a  and  302   b  respectively, while to convert transmitting baseband signals  317   a  and  317   b  into transmitting RF signals  318   a  and  318   b.    
     Meanwhile, a reference symbol generation means  303  generates a reference symbol  304  that is equal to the known symbol  401  and that has a function to give a phase reference for the received baseband signals  302   a  and  302   b . A propagation channel estimation means  305  accepts inputs of the received baseband signals  302   a  and  302   b  and generates receiving symbols  306  and  307  respectively based on the reference symbol  304 , wherein the receiving symbol  306  is an estimate value for the complex propagation channel between the receiving station antenna  204   a  and the transmitting station antenna  203   a  while the receiving symbol  307  is an estimate value for the complex propagation channel between the receiving station antenna  204   a  and the transmitting station antenna  203   b.    
     A transmitting symbol calculation means  308  inputs the receiving symbols  306  and  307 , calculating a plurality of pairs of transmitting symbol vectors wherein each pair of vector is made of two transmitting symbols that correspond to the transmitting station antennas  203   a  and  203   b  respectively. And the calculated a plurality of pairs of transmitting symbol vectors configure a reference table  309 . Hereinafter, details will be demonstrated on how to generate the transmitting vectors and reference table  309 . 
     First, explanation is made on how to calculate a plurality of pairs of transmitting symbol vectors (each pair thereof is made of two transmitting symbols corresponding to the transmitting station antennas  203   a  and  203   b  respectively) for controlling the power of the receiving symbol  410   a  in the receiving station  202 . 
     Herein, letting the receiving symbols  306  and  307  be denoted by h 1  and h 2  respectively, a channel matrix h that denotes the propagation characteristics between the transmitting station antennas  203   a / 203   b  and the receiving station antenna  204   a  is defined as in the following equation (1):
 
h=[h1h2]  (1)
 
     Herein, letting the vector h be processed in Singular Value Decomposition, h can be given by the equation (2) as follows:
 
 h=U·Λ·V   (2)
 
     This is based on the fact that a random matrix can be reproduced as a product of three new matrixes by performing a Singular Value Decomposition process. In the case of the equation (2), let h be thought of as a one-row/two-columns matrix, U can be thought of as a one-row/one-column matrix. This case comes to ‘1’. Meanwhile, Λ is a one-row/two-columns matrix, and V is a two-rows/two-columns matrix wherein v1 and v2, which are the column vector elements of V, are the particular vector of h. Those are respectively given by the following equations (3):
 
Λ=[s0], V=[v1v2]  (3)
 
where s denotes a scalar and each of v1 and v2 is a two-rows/one-column vector.
 
     Herein, suppose that v1 or v2 are the transmitting symbol vectors to be selected or multiplexed for transmitting depending on the data transmitting and that the transmitting station  201  transmits them from the transmitting station antenna  203   a  and  203   b  to the receiving station  202 . 
     In cases where only v1 is transmitting or where v1 and v2 are vector-multiplexed and transmitting simultaneously, the receiving signal is given by the equation (4) as follows. The power of the receiving symbol  410   a  is virtually equal to |s| 2 , where y denotes the receiving symbol  410   a, n  denotes some noise component mainly due to thermal noise from receiving devices, C 1  denotes a symbol selection vector to be multiplied by V for selecting transmitting symbol vectors in this process. 
     
       
         
           
             
               
                 
                   
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     Similarly, in cases where only v2 is transmitting or where neither v1 nor v2 are transmitting, the receiving signal is given by mathematical formula (5) as follows, where the power of the receiving symbol  410   a  is nearly equal to zero. However, process is virtually similar except that C 0  is substituted for C 1  as a symbol selection vector. 
     
       
         
           
             
               
                 
                   
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                         [ 
                         
                           
                             
                               0 
                             
                           
                           
                             
                               0 
                             
                           
                         
                         ] 
                       
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     Based on the above, it will be possible to control the power of the receiving symbol  410   a  in the receiving station antenna  204   a  by calculating the transmitting symbol vector V·C with the symbol selection vector C (C 1  or C 0 ) and transmitting the transmitting symbol vector V·C as a transmitting symbol transmitting from the transmitting station antennas  203   a  and  203   b.    
     For example, in a case where transmitting information is to be denoted by binary one-bit values, that is, ‘1’ and ‘0’, the transmitting station  201  is to select V·C 1  when the transmitting information is ‘1’, and select V·C 0  when the transmitting information is ‘0’ for transmitting. Due to this, it is made possible for the receiving station to determine the bit characteristic based on the power of the receiving symbol  410   a.    
     Consequently the configuration of a reference table  309  generated by the transmitting symbol calculation means  308  is shown in  FIG. 18A . 
     Incidentally, as for the case wherein the number of the transmitting station antennas is three, it can be processed similarly to the case of two antennas thereof just by considering the fact that the channel matrix h becomes one-row/three-columns (from one-row/two-columns). In this case, v1 and v2 will become a 3 dimensional vector. Further, in the reference table  309  as shown in  FIG. 18B , there are increasing the set pattern for the symbol selection vector C in proportion to the increase in the number of antennas. 
     As described above, the transmitting symbol calculation means  308  is to calculate a plurality of pairs of complex symbols each symbol corresponding to the transmitting station antennas  203   a  and  203   b  respectively in order to control the power of the receiving symbol  410   a  in the receiving station  202 , thus generating a reference table  309  that is made of the calculated transmitting symbol vectors. 
     A symbol mapping section  311  is to calculate the transmitting symbols  314  and  315  depending on the data transmitting  310  so that the power of the receiving symbol  410   a  will become equal to or more than the particular threshold value, or below. Now configuration and operation as for the symbol mapping section  311  will be demonstrated hereinafter. 
       FIG. 5  is a block diagram showing a configuration of the symbol mapping section  311 . As shown in  FIG. 5 , the symbol mapping section  311 , recognizing the data transmitting  310  as input data, is configured by a table memory means  312  for storing the reference table  309  and a symbol selection means  313 . 
     The symbol selection means  313  is to select the transmitting symbols  314  and  315  that correspond to the transmitting station antennas  203   a  and  203   b  respectively, by referring to the table memory means  312  depending on the data transmitting  310 . 
     Next, a single carrier modulation means  316  is to generate baseband signals transmitting  317   a  and  317   b  by inputting the transmitting symbols  314  and  315  respectively. 
     In the following examples are made an explanation on the radio communication method performed between the transmitting station  201  and the receiving station  202  as configured above. 
     First, a known symbol  401 , which is generated by the known symbol generation means  400  in the receiving station  202 , is to be modulated to a baseband signal transmitting  403  by the single carrier modulation means  402 . 
     Next, the modulated baseband signal transmitting  403  is to be converted into a RF signal transmitting  405  by the frequency conversion means  404 , then being transmitting from the antenna  204   a  as a single carrier modulation signal  406   a.    
     Next, this single carrier modulation signal  406   a  that is modulated from the known symbol  401  and transmitting from the receiving station  202  is to be received simultaneously at the antennas  203   a  and  203   b  in the transmitting station  201 , then being converted into received baseband signals  302   a  and  302   b  respectively through the frequency conversion means  301 . 
     Next, this baseband signals  302   a  and  302   b  are processed in the propagation channel estimation means  305  based on a reference symbol  304  that is generated by the reference symbol generation means  303 . Due to this, there are generated receiving symbols  306  and  307 , which denote the estimate values of complex propagation channels between the receiving station antenna  204   a  and each of the respective transmitting station antennas  203   a  and  203   b.    
     Next, those receiving symbols  306  and  307  are processed in the transmitting symbol calculation means  308 , calculating a transmitting symbol vector made of two elements corresponding to the transmitting station antenna  203   a  and  203   b  respectively. Consequently there is generated a reference table  309  configured by the a plurality of sets of those symbol vectors. 
     As described above, both of the transmitting station  201  and the receiving station  202  are to calculate the propagation parameters therebetween with the respective known symbols in advance, thus storing the calculated results as a reference table in the transmitting station  201 . 
     Next, data transmitting  310  is worked out in the symbol mapping section  311  as a set of a transmitting symbols  314  and  315  with the aforementioned reference table, so that the power variation of the receiving symbol  410   a  in the receiving station  202  can be made equal to the data sequence of the data transmitting  310 . 
     Next, the transmitting symbols  314  and  315  are processed in the single carrier modulation means  316  to generate baseband signals  317   a  and  317   b  transmitting. 
     Next, the baseband signals  317   a  and  317   b  transmitting are simultaneously converted into RF signals  318   a  and  318   b  transmitting through the frequency conversion means  301 , then being transmitting respectively from the transmitting station antennas  203   a  and  203   b  to the receiving station  202 . 
     Next, the RF signals  318   a  and  318   b  transmitting from the transmitting station  201  are synthesized at the receiving station antenna  204   a  and received, then being converted into a received baseband signal  408   a  through the frequency conversion means  404 . 
     Next, this baseband signal  408   a  is processed in the propagation parameters estimation means  409  to generate a receiving symbol  410   a  by orthogonal detection. 
     Next, the receiving symbol  410   a  is determined in the symbol determination means  411  based on predetermined threshold value of the power, to obtain a received data  412 . 
     In doing so as described above, the transmitting data  310  including confidential information that are transmitting from the transmitting station  201  are reconstructed. 
     In the following examples are described aforementioned operations in detail. 
     For example, suppose a case where the data transmitting  310  is a binary data sequence [10001101] and this data sequence is transmitting in time-series for transmitting 8 bits of information. 
     First, in the symbol mapping section  311  in the transmitting station  201 , the symbol selection means  313  will, when the data transmitting  310  is ‘1’ for example, select the set of transmitting symbols  314  and  315  from the table memory means  312  so that the power of the receiving symbol  410   a  in the receiving station  202  can be equal or above the particular threshold value. On the other hand, when the data transmitting  310  is ‘0’, the symbol selection means  313  will select the set of transmitting symbols  314  and  315  from the table memory means  312  so that the power of the receiving symbol  410   a  can be less than the particular threshold value. 
     Next, the selected transmitting symbols will be modulated and then transmitting from the antennas  203   a  and  203   b  respectively. 
     Next, in the receiving station  202  that received the above, the symbol determination means  411  will, when the power of the receiving symbol  410   a  is equal or above the particular threshold, determine the received data as ‘1’; when the power of the receiving symbol  410   a  is less than the particular threshold, the symbol determination means  411  will determine the received data as ‘0’. Due to this, data will be demodulated. Then compare the demodulated sequence for the power of the receiving symbol  410   a  that was determined in time series with the transmitting data sequence [10001101]. If they correspond each other, now it is proved that the data has been appropriately transmitting. 
     Such controlling system can be available because, in a situation where the propagation parameter is stable, power and phase difference of arrival paths vary at the receiving antenna side in accordance with the change of directional pattern of the transmitting antenna, and power of the receiving signal also changes accordingly. 
     Namely, variation in amplitudes and phases of transmitting symbols  314  and  315  (which are complex symbols) will change the synthesized directional patterns generated by both of the transmitting station antennas  203   a  and  203   b . As a result, the signal power of the receiving symbol  410   a  that are received at the receiving station antenna  204  will also change. 
     Further, power spectrums  206   a  and  206   b , depending on the propagation space configured between the transmitting station and the receiving station, are thought to characterize the physical relationship between transmitting/receiving stations. Therefore it can be observed that the same transmitting signals from the same transmitting station  201  have a different frequency spectrum if received at a different receiving station other than the receiving station  202 . 
     For this reason, in a radio communication system where demodulation of transmitting data  310  is made based on the power of the receiving signals under the above-described configuration, it is quite difficult for a third party to demodulate or reconstruct the transmitting data  310  including confidential information at another receiving station. Consequently it is possible to transmit confidential information with a high security. 
     In the above descriptions are demonstrated the way of modulation where symbol information of data transmitting is superimposed on the power (an amplitude) of a single carrier as the propagation parameter. However, it is also possible to superimpose the symbol information on a phase. 
     Namely, the transmitting symbol calculation means  308  can also be configured so as to generate such transmitting symbols for controlling the phase of the receiving symbol  410   a  in the receiving station  202 , wherein each of the transmitting symbol  314  corresponding to the transmitting station antenna  203   a  and the transmitting symbol  315  corresponding to the transmitting station antenna  203   b  is a complex symbol. 
     Then, the propagation parameter estimation means  409  estimates the receiving symbol  410   a  as a complex symbol. Therefore, when the symbol determination means  411  uses a phase as a determination criterion, symbol determination will be made by, for example, dividing the complex plane for mapping the receiving symbol  410   a  into a right and a left half in order to see which area the receiving symbol  410   a  belongs to. 
     Namely, letting the imaginary axis on the complex plane become the borderline for the phase-basis determination in advance, symbol determination will be made as follows: when the receiving symbol  410   a  belongs to, for example, the right-half on the complex plane, it will be determined as ‘1’; when the receiving symbol  410   a  belongs to the left-half, it will be determined as ‘0’. 
     Up to now, there are demonstrated the modulation system where symbol information of data transmitting is superimposed on an amplitude or phase of a single carrier as the propagation parameter. On the other hand, however, it is also possible to superimpose the symbol information on the difference value of amplitudes or phases between a plurality of single carriers. In this case, two methods are possible: one is to predetermine a particular single carrier to be used as the criteria for symbol determination; another is to providing a multiple carriers subset that comprises a plurality of single carriers. 
     As for the first method to predetermine a particular single carrier to be used as the criteria for symbol determination, the transmitting station  201  transmits the transmitting symbol information as amplitude or phase information for receiving signals received at the antenna of the receiving station  202 . The receiving station  202  calculates the difference value of amplitudes or phases between the particular single carrier that is predetermined as the symbol determination criteria and other single carriers, and can demodulate the transmitting information by, for example, determining the bit characteristics with the calculation results. 
     As for the second method to provide a multiple carriers subset comprising a plurality of single carriers, on the other hand, the transmitting station  201  transmits the transmitting symbol information to the receiving station  202  as a relative amplitude or phase information between a plurality of single carriers that configure the predetermined multiple carriers subset. The receiving station  202  calculates each difference value between amplitudes or phases of a plurality of single carriers that configure a multiple carriers subset, on a subset-by-subset basis. Consequently, the demodulation of transmitting information is made possible by, for example, determining the bit characteristics with the calculation results. 
     Incidentally, in a radio propagation environment where the propagation channel  205   a  and the propagation channel  205   b  in the radio communication system  200  are virtually constant respectively, the reference table  309  for transmitting symbols can be generated with estimation values for the propagation channels  205   a  and  205   b  obtained in advance. As a result, there is no necessity for the propagation channel estimation means  305  as shown in  FIG. 3 , enabling the configuration of the transmitting station  201  to be more simplified. 
     Meanwhile, if the transmitting station  201  increases the number of antenna to three or more, there will be several patterns of antenna set available. Due to this, it will become furthermore difficult for a third party to demodulate or reconstruct the transmitting data  310  including confidential information. Further, if the transmitting station antennas  203   a  and  203   b  have a different directional pattern or polarization each other, it will become more difficult for a third party to estimate the power spectrums  206   a  and  206   b , assuring a higher degree of security. 
     Incidentally, the way to get information on the downlink channel condition is as follows: in TDD, which makes use of the same frequency carrier both in uplink and downlink, it is possible for the transmitting station to estimate/measure the channel conditions with the uplink line from the receiving station, thanks to the channel&#39;s reciprocity characteristic. Embodiment 1 of the present invention is similar to this. 
     On the other hand, however, this invention is not exclusively applied to the TDD-radio communication system. This is because even in FDD, which makes use of a different frequency carrier between uplink and downlink, it is possible for the transmitting station to get correct information on the downlink channel condition only if the downlink channel condition is to be estimated/measured at the receiving station and notified to the transmitting station. 
     Embodiment 2 
     In the following examples will be demonstrated Embodiment 2 with reference to drawings. 
       FIG. 6A  shows a radio communication system  600  according to Embodiment 2 of the present invention, having virtually the same configuration with the radio communication system  200  according to Embodiment 1, except that the receiving station  601  comprises a receiving station antenna  204   b  in addition to a receiving station antenna  204   a.    
       FIG. 6B  shows a single carrier power spectrum  206   c  for a propagation channel  205   c  between a transmitting station antenna  203   a  and a receiving station antenna  204   b .  FIG. 6C  shows a single carrier power spectrum  206   d  for a propagation channel  205   d  between a transmitting station antenna  203   b  and the receiving station antenna  204   b.    
       FIG. 7  is a block diagram showing a particular configuration of the receiving station  601 . In  FIG. 7 , a known symbol generation means  400  is to generate a known symbol  401  as well as a reference clock signal  700  for determining the time-slot timing. 
     A frequency conversion means  404  is to switch between the receiving station antenna  204   a  and the receiving station antenna  204   b  in synchronization with time slots T 1  and T 2  respectively. Due to this, at Time  1  for example, the RF signal transmitting will be transmitting as a single carrier modulation signal  406   a  from the receiving station antenna  204   a . On the other hand, at T 2 , the same RF signal transmitting will be transmitting as a single carrier modulation signal  406   b  from the receiving station antenna  204   b.    
       FIG. 16  is a block diagram showing a configuration of the transmitting station  201  according to the present Embodiment. 
     The transmitting station  201  is different from that of Embodiment 1 in that a reference symbol generation means  303  is to generate a reference clock signal  701  for determining the time-slot timings T 1  and T 2  and generate two kinds of reference symbols  304  at each of the timings T 1  and T 2 , and that a propagation channel estimation means  305  is to generate receiving symbols from baseband signals respectively at each of the timings T 1  and T 2 . 
     In the following examples are demonstrated the radio communication method that is performed between the transmitting station  201  and the receiving station  601  as configured above. 
     First, in the receiving station  601 , the known symbol  401  generated by the known symbol generation means  400  is modulated into a baseband signal transmitting  403  by a single carrier modulation means  402 . 
     Next, the modulated baseband signals transmitting  403  are converted into RF signals transmitting  407   a  and  407   b  at the timings determined by the reference clock signals  700  that are generated by the known symbol generation means  400 . Then a single carrier modulation signal  406   a  is transmitting from the antenna  204   a  at time slot T 1  while a single carrier modulation signal  406   b  is transmitting from the antenna  204   b  at time slot T 2  respectively. 
     As a next stage in the transmitting station  201 , the single carrier modulation signal  406   a  transmitting from the receiving station antenna  204   a  and the single carrier modulation signal  406   b  transmitting from the receiving station antenna  204   b  are received at transmitting station antennas  203   a  and  203   b.    
     Then, a frequency conversion means  301  separates the received single carrier modulation signal  406   a  and the received single carrier modulation signal  406   b , from received RF signals  300   a  and  300   b . By this means, received baseband signals  302   a  corresponding to the transmitting station antenna  203   a  and received baseband signals  302   b  corresponding to the transmitting station antenna  203   b  can be generated for each of the time slots T 1  and T 2 , then being outputted to the propagation channel estimation means  305 . 
     Next, at time slot T 1 , the propagation channel estimation means  305  processes these baseband signals  302   a  and  302   b  based on the reference symbol  304  that is generated by the reference symbol generation means  303 , generating receiving symbols  306   a  and  307   a , wherein the receiving symbol  306   a  is an estimate value for the complex propagation channel between the receiving station antenna  204   a  and the transmitting station antenna  203   a  and the receiving symbol  307   a  is an estimate value for the complex propagation channel between the receiving station antenna  204   a  and the transmitting station antenna  203   b . In a similar way, at time slot T 2  are generated receiving symbols  306   b  and  307   b  from the received baseband signals  302   a  and  302   b  based on the reference symbol  304 , wherein the receiving symbol  306   b  is an estimate value for the complex propagation channel between the receiving station antenna  204   b  and the transmitting station antenna  203   a  while the receiving symbol  307   b  is an estimate value for the complex propagation channel between the receiving station antenna  204   b  and the transmitting station antenna  203   b.    
     Next, a transmitting symbol calculation means  308  processes the receiving symbols  306   a  and  307   a  that are estimated from the receiving station antenna  204   a  and the receiving symbols  306   b  and  307   b  that are estimated from the receiving station antenna  204   b , to calculate a plurality of pairs of transmitting symbol vectors, wherein each pair consists of two transmitting symbols respectively corresponding to the transmitting station antennas  203   a  and  203   b , similarly to Embodiment 1. A reference table  309  comprises those a plurality of pairs of transmitting symbol vectors. 
     Explanation will be made in detail hereinafter on how to make a reference table  309  calculated by the transmitting symbol calculation means  308  in the transmitting station  201  in accordance with the symbol information for assumed data transmitting  310 . 
     Two examples will be described hereinafter on how to calculate a transmitting symbol by a transmitting symbol calculation means  308 : one is the case of using MMSE (Minimum Mean Square Error), a method more commonly utilized for calculating weighting factors for adaptive array antennas [B. Widrow, P. E. Mantey, L. J. Griffiths, and B. B. Goode, “Adaptive Antenna Systems”, Proc. IEEE, vol. 55, no. 12, pp. 2143-2158, December 1967.]; and the other is case of using Zero-Forcing method [J. G. Proakis, Digital Communications, 3rd Edition, McGraw-Hill, New York, 1995.] 
     As for MMSE, weighting factors for the transmitting station antennas  203   a  and  203   b  should be calculated, for example, by assuming the receiving station antenna  204   b  to be the source of interference signals. When the calculated weighting factor is directly used as a transmitting symbol, it is made possible for the receiving station  601  to control the power of signals received at the receiving station antenna  204   a  to the maximum. 
     As for Zero-forcing method, on the other hand, weighting factors for the transmitting station antennas  203   a  and  203   b  should be calculated by assuming the receiving station antenna  204   a  to be the source of interference signals in reverse. When the calculated weighting factor is directly used as a transmitting symbol, it is made possible for the receiving station  601  to control the power of signals received at the receiving station antenna  204   b  to the minimum. 
     In the following are explained in detail how to calculate transmitting symbols and how to generate a reference table  309 , with Zero-forcing method. 
     First is described how to calculate a plurality of pairs of transmitting symbol vectors, wherein each pair thereof consists of two transmitting symbols respectively corresponding to the transmitting station antenna  203   a  and transmitting station antenna  203   b , for controlling the power of the receiving symbol  410   a  and the receiving symbol  410   b  in the receiving station  601 . 
     Herein, letting the receiving symbols  306   a  and  307   a  be “h 11 ” and “h 12 ” respectively, and the receiving symbols  306   b  and  307   b  be “h 21 ” and “h 22 ” respectively, where channel matrix H, representing the propagation channel characteristics between the transmitting station antenna  203   a / 203   b  and the receiving station antenna  204   a , is to be given by the following equation (6): 
     
       
         
           
             
               
                 
                   H 
                   = 
                   
                     [ 
                     
                       
                         
                           
                             h 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             11 
                           
                         
                         
                           
                             h 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             12 
                           
                         
                       
                       
                         
                           
                             h 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             21 
                           
                         
                         
                           
                             h 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             22 
                           
                         
                       
                     
                     ] 
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
     Next, letting the pseudo-inverse matrix (Moore-Penrose matrix) against the matrix H be “H + ”, there is shown such a feature as given by the following equation (7): 
                     H   ·     H   +       =     J   =     [           s   ⁢           ⁢   1         0           0         s   ⁢           ⁢   2           ]               (   7   )               
where “H + ” is a 2-rows/2 columns matrix, J being a unit matrix, where its diagonal elements are comprising “s1” and “s2” while the rest of all elements are comprising zero.
 
     However, in a case there exists an inverse matrix against the matrix H, “s1” and “s2” denote ‘1’ respectively. Further, let the column vectors that configure the matrix H +  be denoted by “w1” and “w2”, then H +  is given by the equation (8) as follows:
 
 H   +   =[w 1 w 2]  (8)
 
     Herein, suppose the case that, in the transmitting station  201 , “w1” and “w2” are the transmitting symbol vector to be selected or multiplexed depending on the data transmitting, being transmitting from the transmitting station antennas  203   a  and  203   b  toward the receiving station  601 . 
     Due to above equations (7) and (8), a receiving signal can be given by the following formula (9) in a case where only “w1” is used for transmitting. In this case, the power of the receiving symbol  410   a  is virtually equal to |s1| 2  and the power of the receiving symbol  410   b  is virtually equal to zero. 
     
       
         
           
             
               
                 
                   
                     y 
                     = 
                     
                       
                         [ 
                         
                           
                             
                               
                                 y 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 1 
                               
                             
                           
                           
                             
                               
                                 y 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 2 
                               
                             
                           
                         
                         ] 
                       
                       = 
                       
                         
                           
                             H 
                             · 
                             
                               ( 
                               
                                 
                                   
                                     H 
                                     + 
                                   
                                   · 
                                   C 
                                 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 10 
                               
                               ) 
                             
                           
                           + 
                           n 
                         
                         = 
                         
                           
                             [ 
                             
                               
                                 
                                   
                                     s 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     1 
                                   
                                 
                               
                               
                                 
                                   0 
                                 
                               
                             
                             ] 
                           
                           + 
                           n 
                         
                       
                     
                   
                   , 
                   
                       
                   
                   ⁢ 
                   
                     
                       C 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       10 
                     
                     = 
                     
                       [ 
                       
                         
                           
                             1 
                           
                         
                         
                           
                             0 
                           
                         
                       
                       ] 
                     
                   
                 
               
               
                 
                   ( 
                   9 
                   ) 
                 
               
             
           
         
       
         
         
           
             where y 1  denotes the receiving symbol  410   a , y 2  denotes the receiving symbol  410   b, n  denotes a noise component vector mainly due to thermal noise of receiving devices, and C 10  denotes a symbol selection vector to be multiplied by H for selecting a transmitting symbol vector through the process. 
           
         
       
    
     Also, in a case where only “w2” is used for transmitting, a receiving signal can be given by the following formula (10). In this case, the power of the receiving symbol  410   a  is virtually equal to zero and the power of the receiving symbol  410   b  is virtually equal to |s2| 2 . 
     
       
         
           
             
               
                 
                   
                     y 
                     = 
                     
                       
                         [ 
                         
                           
                             
                               
                                 y 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 1 
                               
                             
                           
                           
                             
                               
                                 y 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 2 
                               
                             
                           
                         
                         ] 
                       
                       = 
                       
                         
                           
                             H 
                             · 
                             
                               ( 
                               
                                 
                                   
                                     H 
                                     + 
                                   
                                   · 
                                   C 
                                 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 01 
                               
                               ) 
                             
                           
                           + 
                           n 
                         
                         = 
                         
                           
                             [ 
                             
                               
                                 
                                   0 
                                 
                               
                               
                                 
                                   
                                     s 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     2 
                                   
                                 
                               
                             
                             ] 
                           
                           + 
                           n 
                         
                       
                     
                   
                   , 
                   
                       
                   
                   ⁢ 
                   
                     
                       C 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       01 
                     
                     = 
                     
                       [ 
                       
                         
                           
                             0 
                           
                         
                         
                           
                             1 
                           
                         
                       
                       ] 
                     
                   
                 
               
               
                 
                   ( 
                   10 
                   ) 
                 
               
             
           
         
       
         
         
           
             where C 01  is a symbol selection vector to be multiplied by H for selecting a transmitting symbol vector through the process. 
           
         
       
    
     Further, in a case where “w1” and “w2” are vector-multiplexed for transmitting, a receiving signal can be given by the following formula (11). In this case, the power of the receiving symbol  410   a  is virtually equal to |s1| 2  and the power of the receiving symbol  410   b  is virtually equal to |s2| 2 . 
     
       
         
           
             
               
                 
                   
                     y 
                     = 
                     
                       
                         [ 
                         
                           
                             
                               
                                 y 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 1 
                               
                             
                           
                           
                             
                               
                                 y 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 2 
                               
                             
                           
                         
                         ] 
                       
                       = 
                       
                         
                           
                             H 
                             · 
                             
                               ( 
                               
                                 
                                   
                                     H 
                                     + 
                                   
                                   · 
                                   C 
                                 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 11 
                               
                               ) 
                             
                           
                           + 
                           n 
                         
                         = 
                         
                           
                             [ 
                             
                               
                                 
                                   
                                     s 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     1 
                                   
                                 
                               
                               
                                 
                                   
                                     s 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     2 
                                   
                                 
                               
                             
                             ] 
                           
                           + 
                           n 
                         
                       
                     
                   
                   , 
                   
                       
                   
                   ⁢ 
                   
                     
                       C 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       11 
                     
                     = 
                     
                       [ 
                       
                         
                           
                             1 
                           
                         
                         
                           
                             1 
                           
                         
                       
                       ] 
                     
                   
                 
               
               
                 
                   ( 
                   11 
                   ) 
                 
               
             
           
         
       
         
         
           
             where C 11  denotes a symbol selection vector to be multiplied by H for selecting a transmitting symbol vector through the process. 
           
         
       
    
     Meanwhile, in a case where neither “w1” nor “w2” is used for transmitting, a receiving signal can be given by the following formula (12). In this case, it is natural that both of the power of the receiving symbols  410   a  and  410   b  are virtually equal to zero. 
     
       
         
           
             
               
                 
                   
                     y 
                     = 
                     
                       
                         [ 
                         
                           
                             
                               
                                 y 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 1 
                               
                             
                           
                           
                             
                               
                                 y 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 2 
                               
                             
                           
                         
                         ] 
                       
                       = 
                       
                         
                           
                             H 
                             · 
                             
                               ( 
                               
                                 
                                   
                                     H 
                                     + 
                                   
                                   · 
                                   C 
                                 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 00 
                               
                               ) 
                             
                           
                           + 
                           n 
                         
                         = 
                         
                           
                             [ 
                             
                               
                                 
                                   0 
                                 
                               
                               
                                 
                                   0 
                                 
                               
                             
                             ] 
                           
                           + 
                           n 
                         
                       
                     
                   
                   , 
                   
                       
                   
                   ⁢ 
                   
                     
                       C 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       00 
                     
                     = 
                     
                       [ 
                       
                         
                           
                             0 
                           
                         
                         
                           
                             0 
                           
                         
                       
                       ] 
                     
                   
                 
               
               
                 
                   ( 
                   12 
                   ) 
                 
               
             
           
         
       
         
         
           
             where C 00  is a symbol selection vector to be multiplied by H for selecting a transmitting symbol vector through the process. Based on the above, it is made possible to control the power of the receiving symbol  410   a  in the receiving station antenna  204   a  by calculating the transmitting symbol vector H + ·C with the aforementioned symbol selection vector C (C 10 , C 01 , C 11 , C 00 ), and by transmitting this transmitting symbol vector H + ·C as a transmitting symbol from the transmitting station antennas  203   a  and  203   b.    
           
         
       
    
     For example, let the transmitting information be denoted by four 2-bit values like [10, 01, 11, 00], the transmitting station  201  will select H + ·C 0  for transmitting when the transmitting bit is ‘1’, and will select H + ·C 1  for transmitting when the transmitting bit is ‘0’. Due to this, it is made possible for the receiving station to perform the bit determination based on the power of the receiving symbol  410   a . Therefore, the configuration of the reference table  309  generated by the transmitting symbol calculation means  308  is shown by  FIG. 19 . 
     Incidentally, in a case where the number of antennas in the transmitting station is three, the same process can be applied to as in the case of two antennas thereof if only considering the fact that the channel matrix H becomes 2-rows/3-columns matrix. In this case, because H +  becomes 2-rows/3-columns matrix, each of “w1” and “w2” will become a 3 dimensional vector. 
     As described above, the transmitting station  201  and the receiving station  601  cooperatively calculate the propagation parameter therebetween in advance with known symbols, and store it as a reference table. 
     Next, data transmitting  310  is calculated in the symbol mapping section  311  as a set of transmitting symbols  314  and  315  with the aforementioned reference table, so that the power variation of the receiving symbol  410   a  in the receiving station  601  can correspond to the data sequence of the data transmitting  310 . 
     Then, the transmitting symbols  314  and  315  are processed in the single carrier modulation means  316  to generate a baseband signal transmitting  317   a  and a baseband signal transmitting  317   b.    
     Next, the baseband signals transmitting  317   a  and  317   b  are simultaneously converted into RF signals transmitting  318   a  and  318   b  by the frequency conversion means  301 , then being transmitting from the transmitting station antennas  203   a  and  203   b  toward the receiving station  601 . 
     Then, the RF signals  318   a  and  318   b  transmitting from the transmitting station  201  are synthesized and received by the receiving station antenna  204   a , converted into a received baseband  408   a  through the frequency conversion means  404 . On the other hand, at the receiving station antenna  204   b , the RF signals  318   a  and  318   b  are also synthesized and received, converted into a received baseband  408   b  through the frequency conversion means  404  in a similar way. 
     Next, orthogonal detection is made to this baseband signal  408   a  by a propagation parameter estimation means  409  to generate the receiving symbol  410   a  as a complex symbol. In the same manner, orthogonal detection is made also to the baseband signal  408   b  by the propagation parameter estimation means  409  to generate the receiving symbol  410   b  as a complex symbol. 
     Next, the difference of powers between the receiving symbols  410   a  and  410   b  is calculated by a symbol determination means  411 . Then the calculated value of the power-difference is determined based on predetermined particular thresholds. Namely, whether the symbol should be ‘1’ or ‘0’ will be determined depending on whether the power-difference is no fewer than the threshold or no more than that. The results will be outputted as a received data  412 . 
     In this manner, the transmitting data  310  including confidential information that are transmitting from the transmitting station  201  will be demodulated. 
     Therefore, in a radio communication system as described above where demodulation of the transmitting data  310  is made based on the relative power-difference between the receiving station antennas  204   a  and  204   b , a third party in another receiving station has to specify all the four propagation channels configured by the two antennas in the receiving station  601  and the two antenna in the transmitting station  201  in order to demodulate or reconstruct the transmitting data  310  including confidential information. For this reason, it is made possible for the present embodiment to transmit confidential information with even a higher degree of security. 
     Incidentally, in this embodiment, the receiving station  601  is so configured that the single carrier modulation signals  406  modulated from the known symbol  401  are separately transmitting from the receiving station antennas  204   a  and  204   b  at different time slots T 1  and T 2 . However, it is not limited to this configuration. It is also configured in such a way that two known symbols P 1  and P 2 , with mutually-perpendicular codes, will be transmitting at a same time slot, wherein the known symbol P 1  will be transmitting from the receiving station antenna  204   a  and the known symbol P 2  will be transmitting from the receiving station antenna  204   b.    
     In this case, in the transmitting station  201 , the reference symbol generation means  303  generates a reference symbol  304   a  as the same symbol with the known symbol P 1  and a reference symbol  304   b  as the same symbol with the known symbol P 2 . Then, the propagation channel estimation means  305 , inputting the received baseband signals  302   a  and  302   b , will generate the receiving symbols  306   a  and  307   a  based on the reference symbol  304   a , wherein the receiving symbol  306   a  is an estimate value for the complex propagation channel between the receiving station antenna  204   a  and the transmitting station antenna  203   a  and the receiving symbol  307   a  is an estimate value for the complex propagation channel between the receiving station antenna  204   a  and the transmitting station antenna  203   b . In the same manner as this, the propagation channel estimation means  305 , inputting the received baseband signals  302   a  and  302   b , will generate the receiving symbols  306   b  and  307   b  based on the reference symbol  304   b , wherein the receiving symbol  306   b  is an estimate value for the complex propagation channel between the receiving station antenna  204   b  and the transmitting station antenna  203   a  and the receiving symbol  307   b  is an estimate value for the complex propagation channel between the receiving station antenna  204   b  and the transmitting station antenna  203   b.    
       FIGS. 20A to 20C  show how to allocate the transmitting time to the known symbol  401  and the known symbol P 1  or P 2 .  FIG. 20A  shows an example where the known symbol  401  is transmitting from the two receiving station antennas  204   a  and  204   b  in time-division. For example, the known symbol  401  is transmitting from the receiving station antenna  204   a  within the time length T 1 , and transmitting from the receiving station antenna  204   b  within the time length T 2 . In this case, let the time length for transmitting the known symbol  401  from the two antennas be denoted by TR. 
     Meanwhile,  FIG. 20B  shows an example where the known symbols P 1  and P 2  with mutually-perpendicular codes are multiplexed and transmitting simultaneously from each of the receiving station antennas  204   a  and  204   b  within the time length TR. 
     Further, in a radio communication system exemplified by cellular TDMA (Time Division Multiple Access) method as represented by cellular mobile phone and Frequency Detection Access (Carrier Sense Access) method for WLAN, wherein a plurality of communication channels share the time for access each other in order to assure access, explanation will be made hereinafter on how to allocate time TR that is necessary for the aforementioned known symbols transmitting with reference to  FIG. 20C . 
     In  FIG. 20C , TD 1  and TD 2  denote the time respectively allocated to different communication channels. In general, it is believed that TD 1  and TD 2  will vary depending on the length of transmitting data sequences. Further it is not always necessary that the times occupied by TD 1  and TD 2  should be allocated periodically. Therefore, the receiving station  601  can allocate TR at an appropriate timing within the time not occupied by TD 1  and TD 2  in order to transmit known symbols, by defining in advance that TR, the time for transmitting the known symbols, should utilize the time slot that is not occupied by TD 1  and TD 2 . 
     Incidentally, if the number of antennas used in the receiving station  601  increases to three or more, the more kinds of antenna-sets will be available. As a result it will become even more difficult for a third party to demodulate or reconstruct the data transmitting  310  including confidential information by other receiving stations, thus assuring more high security. 
     Embodiment 3 
       FIG. 8A  shows a radio communication system  800  according to the present Embodiment. In  FIG. 8A , the radio communication system  800  comprises a transmitting station  801  and a receiving station  802  and is different from Embodiment 1 in that it performs a multiple carriers radio communication as represented by OFDM (orthogonal frequency division multiplexing) and so forth. 
       FIG. 8B  shows eight pieces of sub-carrier elements  803   a  to  803   h  configuring the multiple carriers,  FIG. 8C  shows a multiple carriers power spectrum  804   a  for a propagation channel  205   a  between a transmitting station antenna  203   a  and a receiving station antenna  204   a , and  FIG. 8D  shows a multiple carriers power spectrum  804   b  for a propagation channel  205   b  between a transmitting station antenna  203   b  and the receiving station antenna  204   a . The power spectrums  804   a  and  804   b , being calculated from the respective propagation channel estimate values for each of the eight sub-carrier elements, are to configure the multiple carriers frequency spectrum in total. The number of sub-carriers, however, is not limited to eight. It is just configured by a eight sub-carriers herein as a matter of convenience for demonstrating the present Embodiment. 
     As already described in Embodiment 1, the multiple carriers power spectrums  804   a  and  804   b  show a different characteristic each other. Further, it is quite natural that a multiple carriers frequency spectrum, which is estimated by another radio station with a different propagation path, should exhibit another different characteristic. 
     Hereinafter,  FIGS. 9 and 11  show a particular configuration of the transmitting station  801  and  FIG. 10  shows a particular configuration of the receiving station  802 . 
     In  FIG. 10 , a known symbol generation means  1000  is to generate known symbols  1001  shared between the transmitting station  801  and the receiving station  802  for each of the sub-carrier elements  803   a  to  803   h . A multiple carriers modulation means  1002  is to modulate the known symbol  1001  to a baseband signal transmitting  1003  with the sub-carrier elements  803   a  to  803   h . A frequency conversion means  1004  is to convert the baseband signal transmitting  1003  into a RF signal  1005  transmitting, or to convert RF signals received at the antenna  204   a  into a received baseband signal  1008   a . A propagation parameter estimation means  1009  is to generate receiving symbols  1010   a  to  1010   h  (complex symbols) from the received baseband signals  1008   a  by means of orthogonal detection. A symbol determination means  1011  is to perform determination of each receiving symbol  1010   a  to  1010   h  based on predetermined criteria. The antenna  204   a  is to send the RF signal  1005  as a multiple carriers modulation signal  1006   a.    
     In  FIG. 9 , a transmitting station antenna  203   a  and a transmitting station antenna  203   b  in the transmitting station  801  is to receive the RF signals sent from the receiving station  802  simultaneously, or transmit. A frequency conversion means  901  is to convert received RF signals  900   a  and  900   b  into received baseband signals  902   a  and  902   b  respectively. 
     On the other hand, a reference symbol generation means  903  is to generate a reference symbol  904 , which is the same symbol with the known symbol  1001 , for giving a phase reference to the received baseband signals  902   a  and  902   b.    
     A carrier separation means  920  is to separate the received baseband signals  902   a  and  902   b  into eight pieces of sub-carrier elements  803   a  to  803   h  by means of FFT (Fast Fourier Transform) or band-limiting filtering process. A propagation channel estimation means  905  is to generate eight pieces of receiving symbols  906   a  to  906   h  and also eight pieces of receiving symbols  907   a  to  907   h  based on the reference symbol  904 , wherein the receiving symbols  906   a  to  906   h  are the estimate values for the complex propagation channel between the receiving station antenna  204   a  and the transmitting station antenna  203   a  while the receiving symbols  907   a  to  907   h  are the estimate values for the complex propagation channel between the receiving station antenna  204   a  and the transmitting station antenna  203   b.    
     Transmitting symbol calculation means  908   a  to  908   h  is to correspond to the eight pieces of sub-carrier elements  803   a  to  803   h  respectively 
     This transmitting symbol calculation means  908   a  to  908   h  separately calculate a plurality of pairs of transmitting symbol vectors, wherein each pair is made of two transmitting symbols that are respectively corresponding to the transmitting station antenna  203   a  and the transmitting station antenna  203   b . Then, reference tables  909   a  to  909   h  (eight, in total) are generated for each of the sub-carrier elements  803   a  to  803   h , wherein each reference table is made of the calculated a plurality of pairs of transmitting symbol vectors. For example, the transmitting symbol calculation means  908   a  that are corresponding to the sub-carrier element  803   a , calculates a plurality of pairs of complex symbol vectors from the receiving symbols  906   a  and  907   a  that are corresponding to sub-carrier element  803   a , in order to control the power of the receiving symbol  1010   a  in the receiving station  802  as in the same manner with Embodiment 1. A reference table  909   a  is made of those calculated a plurality of pairs of complex symbol vectors. The aforementioned process is to be repeated for all of the sub-carrier elements in a similar way, thus generating eight transmitting symbol reference tables  909   a  to  909   h.    
     A serial-parallel conversion means  911  is to apply a parallel conversion to the data sequence transmitting  910  for each of the sub-carrier elements. 
     A symbol mapping section  913  is to calculate set between transmitting symbols  916   a  to  916   h  and transmitting symbols  917   a  to  917   h  from data transmitting  912   a  to  912   h  so that the power of the receiving symbols  1010   a  to  1010   h  in the receiving station  802  can be set equal to or more than the particular threshold, or below. Configuration of the symbol mapping section  913  will be demonstrated hereinafter. 
       FIG. 11  is a block diagram showing a configuration of the symbol mapping section  913 . 
     In  FIG. 11 , the symbol mapping section  913  comprises table memory means  914   a  to  914   h  for storing the reference tables  909   a  to  909   h  and symbol selection means  915   a  to  915   h.    
     The symbol selection means  915   a  to  915   h  are to select a transmitting symbol from the group  916   a  to  916   h  (that is a group corresponding to the transmitting station antenna  203   a ) and select another from the group  917   a  to  917   h  (a group corresponding to the transmitting station antenna  203   b ) for each of the sub-carrier elements  803   a  to  803   h , based on the data transmitting  912   a  to  912   h , referring to the table memory means  914   a  to  914   h.    
     Next, a multiple carriers modulation means  918  is to generate a baseband signal transmitting  919   a  from the input transmitting symbols  916   a  to  916   h  with the eight sub-carrier elements  803   a  to  803   h , and to generate a baseband signal transmitting  919   b  from the input transmitting symbols  917   a  to  917   h  with the eight sub-carrier elements  803   a  to  803   h.    
     In the following examples are demonstrated the way of radio communication to be performed between the transmitting station  801  and the receiving station  802  as configured above. 
     First, the known symbols  1001 , which are respectively generated for each of the sub-carrier elements  803   a  to  803   h  by the known symbol generation means  1000  in the receiving station  802 , are modulated to the baseband signals transmitting  1003  by the multiple carriers modulation means  1002 . 
     Next, the modulated baseband signals transmitting  1003  are converted into the RF signals transmitting  1005  by the frequency conversion means  1004 , being transmitting from the antenna  204   a  as multiple carriers modulation signals  1006   a.    
     Next, the multiple carriers modulation signals  1006   a  transmitting from the receiving station  802  are simultaneously received at the antennas  203   a  and  203   b  in the transmitting station  801 , to be converted into the received baseband signals  902   a  and  902   b  respectively by the frequency conversion means  901 . 
     Next, each of the baseband signals  902   a  and  902   b  is separated into eight sub-carrier elements  803   a  to  803   h  by the carrier separation means  920 . Then, in the propagation channel estimation means  905 , they are processed based on the reference symbol  904  that is generated by the reference symbol generation means  903 , generating eight receiving symbols  906   a  to  906   h  and another eight receiving symbols  907   a  to  907   h  respectively; wherein the former are the estimate values for the complex propagation channel between the receiving station antenna  204   a  and the transmitting station antenna  203   a , while the latter are the estimate values for the complex propagation channel between the receiving station antenna  204   a  and the transmitting station antenna  203   b.    
     Then, these receiving symbols  906   a  to  906   h  and  907   a  to  907   h  are processed in the transmitting symbol calculation means  908   a  to  908   h , to be calculated as a plurality of pairs of transmitting symbol vectors wherein the two elements included in one pair (of transmitting vector) is corresponding to the transmitting station antenna  203   a  and  203   b  respectively. Thus eight kinds of reference tables  909   a  to  909   h  are made, each of the reference tables comprising the calculated a plurality of pairs of transmitting symbol vectors. 
     In this manner, both of the transmitting station  801  and the receiving station  802  calculate in advance the propagation parameters therebetween with the known symbols to store the results as a reference table. 
     In such a condition, the confidential data transmitting  910  is parallel-converted by the serial-parallel conversion means  911  and then inputted to the symbol mapping section  913 . 
     Next, the signals transmitting  912   a  to  912   h , divided into eight signals, are processed by the symbol mapping section  913  with the reference tables. As a result, there are calculated eight pairs of transmitting symbols from the pair symbols  916   a  and  917   a  to the pair symbols  916   h  and  917   h  so that the power variation of the receiving symbols  1010   a  to  1010   h  in the receiving station  802  can be equal to the data sequence transmitting  910 . 
     Then, those transmitting symbols  916   a  to  916   h  and  917   a  to  917   h  are processed in the multiple carriers modulation means  918  to generate baseband signals transmitting  919   a  and  919   b.    
     Next, the baseband signal transmitting  919   a  is converted into the RF signal transmitting  900   a  by the frequency conversion means  901 , then being transmitting from the transmitting station antenna  203   a  to the receiving station  802 . At the same time, the baseband signal transmitting  919   b  is converted into the RF signal transmitting  900   b  by the frequency conversion means  901 , then being transmitting from the transmitting station antenna  203   b  to the receiving station  802 . 
     Then, in the receiving station  802 , the RF signal  900   a  transmitting from the transmitting station antenna  203   a  in the transmitting station  801  and the RF signal  900   b  transmitting from the transmitting station antenna  203   b  thereof are synthesized and received at the receiving station antenna  204   a . This receiving signal, denoted by RF signal  1005 , are converted into the received baseband signal  1008   a  by the frequency conversion means  1004 . 
     The baseband signal  1008   a  is separated into eight sub-carrier elements  803   a  to  803   h  in a carrier separation means  1020  by means of orthogonal detection after FFT (Fast Fourier Transform) or band-limiting filtering process 
     Then, from the signals of the separated sub-carrier elements  1021   a  to  1021   h , there are detected the receiving symbols  1010   a  to  1010   h  as a complex symbol by the propagation parameter estimation means  1009 . 
     Next, in the symbol determination means  1011 , symbol determination is performed for the generated receiving symbols  1010   a  to  1010   h  based on predetermined criteria, thus generating received data  1012   a  to  1012   h.    
     Then, these received data  1012   a  to  1012   h  are converted into a received data sequence  1014  (a serial data sequence) by a parallel-serial conversion means  1013 , thus reconstructing the data sequence  910  including confidential information that were transmitting from the transmitting station  801 . 
     In the following examples are particularly demonstrated the above operation. 
     For example, supposing that the data sequence transmitting  910  is a binary data sequence “10001101” and is transmitting by allocating the sequence to each of the sub-carrier elements sequentially for transmitting the 8 bits of information. 
     First, in the symbol mapping section  913  of the transmitting station  801 , the symbol selection means  915   a  selects the set of transmitting symbols  916   a  and  917   a  from the table memory means  914   a  so that, when the data transmitting  912   a  is ‘1’ for example, the power of the receiving symbol  1010   a  in the receiving station  802  can be equal or more than the particular threshold. On the other hand, when the data transmitting  912   a  is ‘0’, the symbol selection means  915   a  selects the set of transmitting symbols  916   a  and  917   a  from the table memory means  914   a  so that the power of the receiving symbol  1010   a  is less than the particular power threshold  1401 . 
     Then, the selected transmitting symbols are modulated and transmitting from the antenna  203   a  and antenna  203   b.    
     In the next place, in the symbol determination means  1011  of the receiving station  802  that received the above signals, it is determined as ‘1’ when the powers of the respective receiving symbols  1010   a  to  1010   h  (that are separated from the received baseband signal  1008   a  into eight sub-carrier elements  803   a  to  803   h ) is equal to or more than the particular power threshold  1400 , and determined as ‘0’ when the powers thereof is less than the threshold. Data is to be reconstructed in this manner. If determination of the powers of the receiving symbols  1010   a  to  1010   h  is resulted to be equal to 10001101 corresponding to the data sequence transmitting  910  (that is, “10001101”), it is recognized that data has been accurately transmitting. 
     Such way of controlling is made possible because a power, phase difference and so forth of the arrival path at the receiving antenna side will be changed due to the change of the directional pattern on the transmitting antenna side. Due to this, the multiple carriers power spectrums of receiving signals will also be changed. 
     Namely, the change of amplitudes and phases of the transmitting symbols  916   a  to  916   h  and the transmitting symbols  917   a  to  917   h  that are all complex symbols will cause the change of synthesized directional pattern made at the transmitting station antennas  203   a  and  203   b . Consequently it is true that the signal power of the receiving symbols  1010   a  to  1010   h  received at the receiving station antenna  204   a  will be changed. 
     Further, the multiple carriers power spectrums  804   a  and  804   b  depend on the propagation spaces configured between the transmitting station and the receiving station, characterizing their physical relationship. For this reason, in a receiving station other than the receiving station  802 , even the same signal that are transmitting from the same transmitting station  801  will have a frequency spectrum different from the multiple carriers power spectrums  804   a  and  804   b  observed for the receiving station  802 . 
     Therefore, according to the present Embodiment, it is difficult for other receiving stations to demodulate or reconstruct the data sequence transmitting  910  including confidential information. 
     Meanwhile in the transmitting symbol calculation means  908   a  to  908   h , the transmitting symbols  916   a  to  916   h  corresponding to the transmitting station antenna  203   a  and the transmitting symbols  917   a  to  917   h  corresponding to the transmitting station antenna  203   b  are respectively complex symbols. Therefore in the above Embodiment is described the case of varying the amplitude of transmitting symbol so that the power of the receiving symbol  1010   a  to  1010   h  in the receiving station  802  can be controlled. But it is not limited to the case. It is also possible to configure such transmitting symbol calculation means  908   a  to  908   h  whereby transmitting symbols for controlling the phase of the receiving symbols  1010   a  to  1010   h  will be made. 
     In this case, the propagation parameter estimation means  1009  estimates the receiving symbols  1010   a  to  1010   h  as a complex symbol respectively. Therefore, in the symbol determination means  1011 , let the receiving symbols  1010   a  to  1010   h  be mapped as a phase difference against the reference symbol over the complex plane and this complex plane be separated into a right and a left half for example, symbol determination can be made depending on which area (right half/left half) the receiving symbols  1010   a  to  1010   h  belong to. 
     Namely, letting the imaginary axis on the complex plane be the borderline for the phase-basis determination in advance, symbol determination can be made as follows: when the receiving symbol  1010   a  to  1010   h  belong to, for example, the right half of the complex plane, it will be determined as ‘1’; when the receiving symbol  1010   a  to  1010   h  belong to the left half, it will be determined as ‘0’. 
     In the radio communication system  800  according to the present Embodiment, if a third party intends to specify the data sequence transmitting  910 , it is necessary for the third party to accurately estimate the propagation channel between the transmitting station  801  and the receiving station  802  in all of the a plurality of sub-carrier elements. As a result, even higher degree of security is made possible for data transmitting as compared with the single-carrier-basis radio communication system. 
     Incidentally, in the above description hitherto are demonstrated the radio communication system configured based on frequency multiplexing system as represented by OFDM (orthogonal frequency division multiplexing). In addition, however, it can be also applied to CDMA (Code Division Multiple Access) with the radio communication system configured in the same way as the present Embodiment, if only sub-carrier elements of OFDM are corresponded to the spread codes of CDMA. 
     On the other hand, suppose CDMA using spread spectrum modulation method, the sub-carrier elements  803   a  to  803   h  in the radio communication system according to the present Embodiment are to be replaced by spread codes C 1  to C 8  respectively. In the followings are demonstrated the operation in this case. 
     First, in the receiving station  802 , the multiple carriers modulation means  1002  spreads the known symbol  1001  by the spread codes C 1  to C 8  to generate the baseband signal transmitting  1003 , the signal being transmitting at the receiving station antenna  204   a.    
     As the next stage, in the transmitting station  801 , the propagation channel estimation means  905  applies reverse spread process to the received baseband signals  902   a  and  902   b  with eight spread codes C 1  to C 8 , generating eight receiving symbols  906   a  to  906   h  and eight receiving symbols  907   a  to  907   h  based on the reference symbol  904 , wherein the receiving symbols  906   a  to  906   h  are the estimate values for the complex propagation channel between the receiving station antenna  204   a  and the transmitting station antenna  203   a , while the receiving symbols  907   a  to  907   h  are the estimate values for the complex propagation channel between the receiving station antenna  204   a  and the transmitting station antenna  203   b.    
     Then, the transmitting symbol calculation means  908   a  to  908   h  separately calculate a plurality of pairs of transmitting symbol vectors from the receiving symbols  906   a  to  906   h  and  907   a  to  907   h , wherein each pair of transmitting symbol vector consists of two transmitting symbols each of which is corresponding to the transmitting station antennas  203   a  and  203   b  respectively. And then eight reference tables  909   a  to  909   h  are made of the calculated a plurality of pairs of transmitting symbol vectors depending on each of the reference codes C 1  to C 8 . 
     As described above, both of the transmitting station  801  and the receiving station  802  calculate the propagation parameters between them in advance with the symbols known by both sides, then storing the result as reference tables. 
     Next, when the number of spread codes for the data transmitting  910  is eight, the serial-parallel conversion means  911  does apply parallel-conversion to it, the data sequence  910  being buffered on a 8data-by-8data basis. The data transmitting  912   a  to  912   h  are outputted in parallel to the symbol mapping section  913 . 
     Then, in the symbol mapping section  913 , the data transmitting  912   a  to  912   h  are calculated to become the pairs of transmitting symbols  916   a  and  917   a ,  916   b  and  917   b , . . . to  916   h  and  917   h  with the reference tables so that the change of the powers of the receiving symbols  1010   a  to  1010   h  in the receiving station  802  can be equal to the data sequence transmitting  910 . 
     Next, the multiple carriers modulation means  918  performs spread process to generate the baseband signal transmitting  919   a  from the transmitting symbols  916   a  to  916   h  with eight spread codes C 1  to C 8 , then transmitting it from the transmitting station antenna  203   a . In the same manner is generated the baseband signal transmitting  919   b  from the transmitting symbols  917   a  to  917   h  by spread process with eight spread codes C 1  to C 8 , then being transmitting from the transmitting station antenna  203   b.    
     Then, in the receiving station  802 , the propagation parameter estimation means  1009  applies reverse spread process to the baseband signals  1008  received at the antenna  204   a  with eight spread codes C 1  to C 8 . Trough the process are separated the signals into eight receiving symbols (all are complex symbols)  1010   a  to  1010   h  and detected by orthogonal detection with eight spread codes. 
     After that, the symbol determination means  1011  reconstructs the data sequence transmitting  910  including confidential information from the receiving symbols  1010   a  to  1010   h.    
     In such CDMA-basis radio communication system as described above, even higher degree of security can be assured not only thanks to the CDMA-basis confidentiality but also by using the modulation method that utilizes the random-characteristics of propagation parameters. 
     Incidentally, in a radio propagation environment of the radio communication system  800  wherein the propagation channels  205   a  and  205   b  are deemed to be virtually constant, the reference table  909   a  and  909   h  can be generated by using the pre-obtained estimate values for the propagation channels  205   a  and  205   b . In this case, the propagation channel estimation means  905  as shown in  FIG. 9  is made unnecessary, realizing a simple configuration of the transmitting station  801 . 
     Meanwhile, if the number of antennas in the transmitting station  801  increases to three or more, that allows wider variation of antenna-set available. That leads it more difficult for a third party to demodulate or reconstruct the data sequence transmitting  910  including confidential information by other receiving stations. Further, if the transmitting station antennas  203   a  and  203   b  are made to have a different directional pattern each other, it will become furthermore difficult for a third party to estimate the power spectrums  804   a ( 206   a ?) and  804   b ( 206   b ?), assuring even higher degree of security. 
     Embodiment 4 
       FIG. 12A  shows a radio communication system  1200  according to the present Embodiment 4, having virtually the same configuration with Embodiment 3 except that the receiving station  1201  comprises a receiving station antenna  204   b  in addition to a receiving station antenna  204   a .  FIG. 12B  shows a multiple carriers power spectrum  804   c  for a propagation channel  205   c  between a transmitting station antenna  203   a  and the receiving station antenna  204   b .  FIG. 12C  shows a multiple carriers power spectrum  804   d  for a propagation channel  205   d  between a transmitting station antenna  203   b  and the receiving station antenna  204   b . The eight sub-carrier elements  803   a  to  803   h  configuring multiple carriers are the same as shown in  FIG. 8B . 
       FIG. 13  shows a particular configuration of the receiving station  1201 . In  FIG. 13 , a known symbol generation means  1000  is to generate known symbols  1001  for each of the sub-carrier elements  803   a  to  803   h . Multi-carrier modulation signals  1006   a  and  1006   b  derived from the known symbols  1001  are transmitting from the receiving station antennas  204   a  and  204   b  respectively using the different time slots T 1  and T 2 , just as Embodiment 2. A reference clock signal  1300  for determining the respective time-slot timings are to be generated by the known symbol generation means  1000 . 
       FIG. 17  is a block diagram showing the configuration of the transmitting station  801  according to the present Embodiment. In  FIG. 17 , the transmitting station thereof is different from that of Embodiment 3 in that a reference symbol generation means  903  is to generate a reference clock signal  1301  that has a function to determine the timings of time-slots T 1  and T 2 . 
     In the following examples are demonstrated the radio communication methods, which are performed between the transmitting station  801  and the receiving station  1201  configured as above. 
     First, in the receiving station  1201 , the known symbols  1001  generated by the known symbol generation means  1000  for each of the sub-carrier elements  803   a  to  803   h  are modulated to baseband signals transmitting  1003  by a multiple carriers modulation means  1002 . 
     Next, as for the modulated baseband signals  1003 , a frequency conversion means  1004  switches between the receiving station antennas  204   a  and  204   b  in synchronization with the time slots. By this means, at time slot T 1  for example, the RF signal transmitting  1005   a  is transmitting as a multiple carriers modulation signal  1006   a  from the receiving station antenna  204   a . Meanwhile, at time slot T 2 , the RF signal transmitting  1005   b  is transmitting as a multiple carriers modulation signal  1006   b  from the receiving station antenna  204   b  in the same manner. 
     As a next stage in the transmitting station  801 , the transmitting station antennas  203   a  and  203   b  receive the multiple carriers modulation signals  1006   a  that are transmitting from the receiving station antenna  204   a  and the multiple carriers modulation signals  1006   b  that are transmitting from the receiving station antenna  204   b.    
     Next, a frequency conversion means  901  separates these receiving signals into the multiple carriers modulation signal  1006   a  and the multiple carriers modulation signal  1006   b . And then there are generated received baseband signals  902   a  and  902   b  for each of the time slots (T 1 , T 2 ), wherein the baseband signals  902   a  correspond to the transmitting station antenna  203   a  and the baseband signals  902   b  correspond to the transmitting station antenna  203   b . Then, at time slot T 1 , a carrier separation means  920  separates the received baseband signals  902   a  and  902   b  into eight sub-carrier elements  803   a  to  803   h , that is to say, sub-carrier signals  921   a  to  921   h  and  922   a  to  922   h  respectively by fast Fourier Transform (FFT) process or band-limiting filtering process. 
     Then, a propagation channel estimation means  905  generates receiving symbols  906   a  to  906   h  and receiving symbols  907   a  to  907   h  from the sub-carrier signals  921   a  to  921   h  and  922   a  to  922   h  respectively based on a reference symbol  904 , wherein the receiving symbols  906   a  to  906   h  are the estimate values for the complex propagation channel between the receiving station antenna  204   a  and the transmitting station antenna  203   a  while the receiving symbols  907   a  to  907   h  are the estimate values for the complex propagation channel between the receiving station antenna  204   a  and the transmitting station antenna  203   b.    
     Similarly at time slot T 2 , the inputted received baseband signals  902   a  and  902   b  are respectively separated into eight sub-carrier elements  803   a  to  803   h , that is to say, sub-carrier signals  921   i  to  921   p  and  922   i  to  922   p . Then, there are generated receiving symbols  906   i  to  906   p  that are the estimate values for the complex propagation channel between the receiving station antenna  204   b  and the transmitting station antenna  203   a , and receiving symbols  907   i  to  907   p  that are the estimate values for the complex propagation channel between the receiving station antenna  204   b  and the transmitting station antenna  203   b , based on the reference symbol  904 . 
     Next, transmitting symbol calculation means  908   a  to  908   h  process the receiving symbols  906   a  to  906   h  and  907   a  to  907   h  that are estimated from the receiving signals from the receiving station antenna  204   a , and the receiving symbols  906   i  to  906   p  and  907   i  to  907   p  that are estimated by the receiving signals transmitting from the receiving station antenna  204   b . Through that process, one transmitting symbol calculation means (for example,  908   a ) calculates a plurality of pairs of transmitting symbol vectors, wherein each pair is made of two transmitting symbols corresponding to the transmitting station antennas  203   a  and  203   b  respectively. Then there are generated eight reference tables  909   a  to  909   h , each reference table being configured by the a plurality of pairs of transmitting symbol vectors that are calculated for each of the sub-carrier elements  803   a  to  803   h.    
     As described above, both of the transmitting station  801  and the receiving station  1201  calculate the propagation parameters between them in advance with the known symbols, and store the calculation results as a reference table. 
     In such situation like this, confidential data transmitting  910  is parallel-converted first by a serial-parallel conversion means  911 , being inputted to a symbol mapping section  913 . 
     Then, the symbol mapping section  913  processes the eight separated signals transmitting  912   a  to  912   h  with the reference tables, generating the transmitting symbol pairs  916   a  and  917   a ,  916   b  and  917   b , . . . to  916   h  and  917   h  and the transmitting symbol pairs  916   i  and  917   i ,  916   j  and  917   j , . . . to  916   p  and  917   p  (sixteen pairs in total) so that the change of powers of the receiving symbols  1010   a  to  1010   h  and  1010   i  to  1010   p  is equal to the data sequence transmitting  910 . 
     Next, a multiple carriers modulation means  918  processes the transmitting symbols  916   a  to  916   h ,  917   a  to  917   h ,  916   i  to  916   p ,  917   i  to  917   p , thus generating baseband signals transmitting  919   a  and  919   b.    
     Next, the frequency conversion means  901  converts the baseband signal transmitting  919   a  into a RF signal transmitting  900   a , then transmitting it from the transmitting station antenna  203   a  to the receiving station  1201 . Similarly the frequency conversion means  901  converts the baseband signal transmitting  919   b  into a RF signal transmitting  900   b , then transmitting it from the transmitting station antenna  203   b  to the receiving station  1201 . 
     Then, in the receiving station  1201 , the receiving station antenna  204   a  synthesizes and receives the RF signal  900   a  transmitting from the transmitting station antenna  203   a  in the transmitting station  801  and the RF signal  900   b  transmitting from the transmitting station antenna  203   b  thereof, thus generating a RF signal  1005   a . The RF signal  1005   a  is converted into a received baseband signal  1008   a  by the frequency conversion means  1004 . 
     In the same manner, the receiving station antenna  204   b  synthesizes and receives the RF signals transmitting  900   a  and  900   b , generating a RF signal  1005   b . The RF signal  1005   b  is then converted into a received baseband signal  1008   b  by the frequency conversion means  1004 . 
     Next, a carrier separation means  1020  processes the received baseband signal  1008   a  by Fast Fourier Transform (FFT) or band-limiting filtering process. 
     After that, a propagation parameter estimation means  1009  separates the baseband signal  1008   a  into the eight complex sub-carrier elements and detected by orthogonal detection, thus generating receiving symbols  1010   a  to  1010   h . In the same manner, the carrier separation means  1020  processes the received baseband signal  1008   b  by FFT or by band-limiting filtering process, thus generating eight receiving symbols  1010   i  to  1010   p , which are the complex symbols separated into the eight sub-carrier elements and detected by orthogonal detection. 
     Next, a symbol determination means  1011  calculates the power differences between the receiving symbols  1010   a  to  1010   h  and  1010   i  to  1010   p . Then symbol determination process is performed based on predetermined criteria, thus generating received data  1012   a  to  1012   h.    
     Finally a parallel-serial conversion means  1013  converts these received data  1012   a  to  1012   h  into a received data sequence  1014  (a serial data sequence), thus reconstructing the transmitting data sequence  910  including confidential information which is transmitting from the transmitting station  801 . 
       FIG. 15  shows the operation of symbol determination, wherein symbol determination is made between ‘1’ or ‘0’ based on a particular symbol-determination-criterion by calculating the power differences between the receiving symbols  1010   a  to  1010   h  and  1010   i  to  1010   p  for each of the sub-carrier elements  803   a  to  803   h  in advance. In  FIG. 15 , it is determined as ‘1’ when the power difference is positive while it is determined as ‘0’ when the power difference is negative. 
     Namely, as shown in  FIG. 15 , in a case where the power of each of the receiving symbols  1010   a  to  1010   h  is higher, the symbol value should be determined as ‘1’ while in a case where the power of each of the receiving symbols  1010   i  to  1010   p  is higher, it should be determined as ‘0’ for each of the sub-carrier elements  803   a  to  803   h.    
     As described above, in such a radio communication system where the transmitting data sequence  910  is to be demodulated based on relative receiving power differences between the receiving station antennas  204   a  and  204   b , if a third party intends to demodulate or reconstruct the transmitting data  910  including confidential information by other receiving stations, it is necessary for the third party to specify all the four propagation channels made between the two antennas of the receiving station  1201  and the two antennas of the transmitting station. As a result of this, it is made possible to transmit confidential information with even higher degree of security. 
     Incidentally, in the receiving station  1201 , the configuration is such that the multiple carriers modulation signals derived from the known symbol  1001  are separately transmitting from the receiving station antennas  204   a  and  204   b  at the different time slots T 1  and T 2 . But it is not limited to this configuration. It is also possible to use mutually-orthogonal known symbols P 1  and P 2  instead of different time slots, transmitting the known symbol P 1  from the receiving station antenna  204   a  and the known symbol P 2  from the receiving station antenna  204   b  at the same time slot after performing multiple carriers modulations. 
     In this case, in the transmitting station  801 , the reference symbol generation means  903  is to generate reference symbols R 1  and R 2 , wherein R 1  is equal to the known symbol P 1  and R 2  is equal to the known symbol P 2 . And then, after the carrier separation means  920  separates the received baseband signals  902   a  and  902   b  into eight sub-carrier elements  803   a  to  803   h  by Fast Fourier Transform (FFT) or band-limiting filtering process, the propagation channel estimation means  905  generates the receiving symbols  906   a  to  906   h  and the receiving symbols  907   a  to  907   h  respectively based on the reference symbol R 1 , wherein the receiving symbols  906   a  to  906   h  are the estimate values for the complex propagation channel between the receiving station antenna  204   a  and the transmitting station antenna  203   a  while the receiving symbols  907   a  to  907   h  are the estimate values for the complex propagation channel between the receiving station antenna  204   a  and the transmitting station antenna  203   b . In the same way, the propagation channel estimation means  905 , inputting the received baseband signals  902   a  and  902   b , generates the receiving symbols  906   i  to  906   p  and the receiving symbols  907   i  to  907   p  respectively based on the reference symbol R 2 , wherein the receiving symbols  906   i  to  906   p  are the estimate values for the complex propagation channel between the receiving station antenna  204   b  and the transmitting station antenna  203   a  while the receiving symbols  907   i  to  907   p  are the estimate values for the complex propagation channel between the receiving station antenna  204   b  and the transmitting station antenna  203   b.    
     Incidentally in the above examples are demonstrated the radio system configuration assuming the frequency multiplexing system as represented by OFDM. However, this radio communication system, configured similarly to the present Embodiment, can be also applied to CDMA by corresponding to the sub-carrier elements of OFDM to spread codes of CDMA. 
     Herein, in the following example is demonstrated the case where CDMA with spread spectrum modulation method is assumed. In this case, spread codes C 1  to C 8  are to be substituted for the sub-carrier elements  803   a  to  803   h  in the radio communication according to the present Embodiment. 
     First, in the receiving station  1201 , spread spectrum modulation signals, derived from the known symbols  1001  that are corresponding to each of the spread codes C 1  to C 8 , are transmitting from the receiving station antennas  204   a  and  204   b  separately at the different time slots T 1  and T 2 . 
     Next, the propagation channel estimation means  905  of the transmitting station  801  applies reverse spread process to the received baseband signals  902   a  and  902   b  with eight spread codes C 1  to C 8  at time slot T 1 , generating the receiving symbols  906   a  to  906   h  and the receiving symbols  907   a  to  907   h  from the baseband signals  902   a  and  902   b  respectively based on the reference symbol  904 , wherein the receiving symbols  906   a  to  906   h  are the estimate values for the complex propagation channel between the receiving station antenna  204   a  and the transmitting station antenna  203   a  while the receiving symbols  907   a  to  907   h  are the estimate values for the complex propagation channel between the receiving station antenna  204   a  and the transmitting station antenna  203   b . Similarly at time slot T 2 , the propagation channel estimation means  905  inputs the received baseband signals  902   a  and  902   b  and generates therefrom the receiving symbols  906   i  to  906   p  and the receiving symbols  907   i  to  907   p  based on the reference symbol  904 , wherein the receiving symbols  906   i  to  906   p  are the estimate values for the complex propagation channel between the receiving station antenna  204   b  and the transmitting station antenna  203   a  while the receiving symbols  907   i  to  907   p  are the estimate values for the complex propagation channel between the receiving station antenna  204   b  and the transmitting station antenna  203   b.    
     Then, the transmitting symbol calculation means  908  calculates a plurality of pairs of transmitting symbol vectors from the receiving symbols  906   a  to  906   h  and  907   a  to  907   h  that are estimated from the receiving station antenna  204   a  and the receiving symbols  906   i  to  906   p  and  907   i  to  907   p  that are estimated from the receiving station antenna  204   b , wherein each pair (of transmitting symbol vector) consists of two transmitting symbols corresponding respectively to each of the transmitting station antennas  203   a  and  203   b . In this means are calculated these a plurality of pairs of transmitting symbol vectors for each of the spread codes C 1  to C 8  (8 times in total), thus generating eight reference tables  909   a  to  909   h  where each one of reference tables consists of the a plurality of pairs of transmitting symbol vectors. 
     As described above, both the transmitting station  801  and the receiving station  1201  calculate the propagation parameters between them in advance with the known symbols, then storing them as a reference table. 
     Next, in the same way as the case of aforementioned OFDM, the data transmitting  910  are converted into sets of transmitting symbols with the reference tables, thus being transmitting from the transmitting station antennas  203   a  and  203   b.    
     Then, as for the signals received at receiving station  1201 ( 1202 ?), the propagation parameter estimation means  1009  applies reverse spread process to the received basedband signal  1008   a  with the eight spread codes C 1  to C 8 . Through the process, it is separated depending on the eight spread codes C 1  to C 8  and detected by orthogonal detection, thus generating receiving symbols  1010   a  to  1010   h , which are complex symbols. 
     In a similar way, reverse spread process is provided to the received baseband signal  1008   b  with the eight spread codes C 1  to C 8 . And through that process, it is separated depending on the eight spread codes C 1  to C 8  and detected by orthogonal detection, thus generating receiving symbols  1010   i  to  1010   p , which are complex symbols. 
     Finally the symbol determination means  1011  reconstructs the transmitting data sequence  910  including confidential information that is transmitting from the transmitting station  801 , based on the receiving symbols  1010   a  to  1010   h  and the receiving symbols  1010   i  to  1010   p.    
     In such a radio communication system with CDMA as described above, it is made possible to assure even higher degree of security not only thanks to the confidentiality of spread codes utility but also by utilizing the modulation method based on the random characteristics of propagation parameter. 
     Incidentally, if the number of antennas set in the receiving station  1201  increases to three or more, more variety can be expected in the antenna set to be used. Consequently it is made much more difficult for a third party to demodulate or reconstruct confidential information by other receiving stations, assuring even higher degree of security. 
     As described, the radio communication system according to the present invention makes it possible to assure high security in the physical layer of communication. Further, because those processes are basically independent of the conventional arithmetic way of encryption/demodulation and can be used at the same time, it is made possible to expect even higher degree of security by using the present invention in set with the prior arts. 
     Embodiment 5 
       FIG. 21  is a block diagram showing the configuration of an array-antenna transmitting station according to the present Embodiment. In  FIG. 21 , amplitude/phase control sections  2102   a  to  2102   n  are to control the amplitude and phase from respective antennas to form directional beams. Any other configuration blocks provided in respective branches are the same as those of Embodiment 3. There are not figured the propagation channel estimation means that generates a reference table after receiving known symbols from a receiving station, reference symbol generation means or transmitting symbol calculation means herein, but they are all provided in the respective branches in the same way as Embodiment 3. 
       FIG. 22  is a block diagram showing the configuration of an array antenna receiving station according to the present Embodiment. In  FIG. 22 , it is different from Embodiment 3 in that the symbols generated by the known symbol generation means  1000  are, after modulated by the multiple carriers modulation means  1002 , to be generated as directional beams by the respective amplitude/phase control sections  2202   a  to  2202   n  for each of the array antennas. Any other configuration blocks are the same as that of Embodiment 3. 
     By that configuration as described above, the transmitting station can control the receiving power of the antennas in the receiving station  802  by generating a plurality of directional beams and appropriately combining those beams. 
     Such control can be realized because, in a situation where the propagation parameter is deemed to be fixed, the power or phase differences of the arrival paths at the side of a receiving antenna will vary in accordance with the change of directional pattern at the side of a transmitting antenna. 
     Meanwhile, other than array antennas, it is possible for the transmitting station  801  to transmit a bit information transmitting by controlling the position of the single carrier elements one-by-one on the frequency-axis, wherein the single carrier element is to be detected from the multiple carriers receiving signal in the receiving station  1201 . 
     To be more precise, the transmitting station controls the transmitting antennas one-by-one for changing the directional patterns thereof based on the propagation parameter particularly shared between the transmitting station and the receiving station. By this means is controlled the receiving power of each of the single carrier elements at the receiving antennas. 
     In this case, each position of the respective single-carriers on the frequency-axis (these sing-carriers in total configure a multiple carriers signal received at the receiving station) is to correspond to the bit-information-to-be-transmitting respectively. For example, if a multiple carriers transmitting signal comprises eight single carriers, it is made in advance that each 3-bit information transmitting “000” to “111” should correspond to each of the single carriers f 1  to f 8  respectively on the frequency-axis. When the bit information transmitting is “010”, the transmitting station changes the directional patterns of the transmitting antennas for controlling the single carrier f 3  in the receiving station so that it can receive the maximum powered signal as compared with other single carrier elements. Then the receiving station calculates the frequency spectrum for the receiving signal and if it is estimated that the power of the single carrier f 3  is the maximum, now the bit information transmitting can be determined as “010”. 
     Further, in a way where the receiving station determines bit-information-to-be-transmitting based on the results obtained by carrier detection, the transmitting station will control the transmitting power of each single carrier, for example, where a plurality of single carriers configure a multiple carriers. In this way there is occurred neither big drop in the receiving power under the multi-path fading environment nor a bit-error. Also a third party can hardly estimate the bit-information-to-be-transmitting by other radio stations. 
     Namely, according to the present invention, the transmitting station can change the transmitting antennas&#39; directional patterns based on the propagation parameter particularly shared between the transmitting station and the receiving station, thus controlling the receiving power of each of the single carriers in the receiving antennas. In addition, bit error caused by multi-path fading can be compensated for. And furthermore, it is made possible to prevent the transmitting information from being leaked to a third party that is characterized by a different propagation parameter. 
     INDUSTRIAL APPLICABILITY 
     As described above, the present invention is useful for a communication method where a broadband radio communication is performed between particular radio stations, being applicable for transmitting confidential information with high security. 
     LIST OF DRAWING REFERENCE NUMBERS 
     
         
           100  mobile communication system 
           200 , 600 , 800 , 1200  radio communication system 
           101  transmitting antenna 
           102   a , 102   b  receiving antenna 
           103   a , 103   b , 205   a , 205   b , 205   c , 205   d  propagation channel 
           104   a , 104   b  frequency spectrum 
           201 , 801  transmitting station 
           202 , 601 , 802 , 1201  receiving station 
           203   a , 203   b  transmitting station antenna 
           204   a , 204   b  receiving station antenna 
           206   a , 206   b , 206   c , 206   d  single-carrier power spectrum 
           300   a , 300   b , 407   a , 407   b , 900   a , 900   b , 1007   a , 1007   b  received RF signal 
           301 , 404 , 901 , 1004  frequency conversion means 
           302   a , 302   b , 408   a , 408   b , 902   a , 902   b , 1008   a , 1008   b  received baseband signal 
           303 , 903  reference symbol generation means 
           304 , 904  reference symbol 
           305 , 905  propagation channel estimation means 
           306 , 307 , 410   a , 410   b , 906   a - 906   p , 907   a - 907   p , 1010   a - 1010   h  receiving symbol 
           308 ,  908   a - 908   h  transmitting symbol calculation means 
           309 , 909   a - 909   h  reference table 
           310 , 912   a - 912   h  data transmitting 
           311 , 913  symbol mapping section 
           312 , 914   a - 914   h  table memory means 
           313 , 915   a - 915   h  symbol selection means 
           314 , 315 , 916   a - 916   h , 917   a - 917   h  transmitting symbol 
           316 , 402  single carrier modulation means 
           317   a , 317   b , 403 , 919   a , 919   b , 1003  baseband signal transmitting 
           318   a ,  318   b ,  4041   920   a ,  920   b ,  1005  RF signal 
           400 , 1000  known symbol generation means 
           401 , 1001  known symbol 
           402  single carrier modulation means 
           406   a , 406   b  single carrier modulation signal 
           409 , 1009  propagation parameters estimation means 
           411 , 1011  symbol determination means 
           412 , 1012   a - 1012   h  received data 
           700 , 701 , 1300 , 1301  reference clock signal 
           803   a - 803   h  sub-carrier element 
           804   a , 804   b , 804   c , 804   d  multiple carriers power spectrum 
           910  data sequence transmitting 
           911  serial-parallel conversion means 
           918 , 1002  multiple carriers modulation means 
           920  carrier separation means 
           1006   a , 1006   b  multiple carriers modulation signal 
           1013  parallel-serial conversion means 
           1014  received data sequence 
           1400  particular power threshold 
           1020  carrier separation means 
           2102   a - 2102   n , 2202   a - 2202   n  amplitude/phase control section 
           2101 , 2201  array antenna 
           2310  transmitting station 
           2311  propagation environment estimator 
           2320  receiving station 
           2330  radio propagation channel