Abstract:
A wireless communication device which includes: a sensing antenna for sensing communication by a primary user; a first transmission antenna; a second transmission antenna; first transmitting unit that generates a transmission signal to be transmitted from the first transmission antenna; second transmitting unit that generates a transmission signal to be transmitted from the second transmission antenna and that cancels, in the sensing antenna, a transmission signal from the first transmission antenna; digital signal processing unit that suppresses the transmission signal from the first transmission antenna, from a received signal of the sensing antenna; and sensing unit that detects communication by a primary user on the basis of the received signal of the sensing antenna that has been processed by the digital signal processing unit. While performing communication as a secondary user, the device detects communication by a primary user. Hence, constant sensing is enabled in a cognitive radio communication device.

Description:
TECHNICAL FIELD 
     The present invention relates to a wireless communication technology in a cognitive radio system, and more particularly relates to a sensing technology for enabling a secondary user to detect communication by a primary user. 
     BACKGROUND ART 
     Active research and development of cognitive radio systems that rely on detection of unused frequencies have been ongoing in recent years. Secondary utilization of frequencies requires that a primary user be not interfered with. To that end, spectrum sensing is performed, such that secondary utilization of a frequency must be discontinued immediately once communication by a primary user is detected. 
     When implementing cognitive radio in a wireless communication device that moves at high-speed, for instance in an in-vehicle wireless device, the usable frequencies vary, both temporarily and spatially, at any given time. Preferably, therefore, sensing is performed constantly, in order to protect the primary user reliably. 
     In a case where communication by a secondary user is being performed, a problem arises in that communication by the primary user cannot be detected on account of the near-far problem. Therefore, a method has been proposed that involves providing quiet periods (QP) at which none of the secondary users communicates, as illustrated in  FIG. 4 , such that sensing is performed in those periods. Such a method is problematic in that the appearance of a primary user can only be detected in the quiet periods, and also problematic in that communication throughput as a secondary user drops due to presence of the quiet periods. 
     Methods have also been proposed in which communication as a secondary user and detection of a primary user are performed simultaneously. In Non-patent literature 1, part of a signal of a primary user is held as known information, and secondary utilization is discontinued upon reception of the known information in a primary signal, during communication by a secondary user. In this method, the primary signal must be known beforehand as known information, and hence the method is problematic in that, as a result, the scope of application of the method is limited. Further, the method does not address elimination of the near-far problem derived from transmission signals from an own node in a sensing antenna. 
     CITATION LIST 
     Non Patent Literature 
     NPL 1: Yamada Takayuki et al., “A study on a primary signal detection with known signal while receiving a secondary signal for cognitive radio”, IEICE technical report SR2008-18, pp. 1-5 (July 2008). 
     SUMMARY OF INVENTION 
     Technical Problem 
     It is an object of the present invention to provide a sensing technology that allows detecting communication by a primary user while performing communication as a secondary user. 
     Solution to Problem 
     In order to attain the above goal, a self-signal suppression process is performed, in the present invention, in two domains, i.e. the analog domain and the digital domain. As a suppression scheme in the analog domain, specifically, an anti-phase signal is transmitted that cancels a self-signal. Further, digital signal processing is resorted to as suppression in the digital domain. By combining these suppression processes, it becomes possible to detect communication by a primary user while during communication as a secondary user. 
     More specifically, the wireless communication device of the present invention comprises: a sensing antenna for sensing communication by a primary user; a first transmission antenna; a second transmission antenna; first transmitting unit that generates a transmission signal to be transmitted from the first transmission antenna; second transmitting unit that generates a transmission signal to be transmitted from the second transmission antenna, and that cancels, in the sensing antenna, a transmission signal from the first transmission antenna; digital signal processing unit that suppresses the transmission signal from the first transmission antenna, from a received signal of the sensing antenna; and sensing unit that detects communication by a primary user, on the basis of the received signal of the sensing antenna that has been processed by the digital signal processing unit. 
     Although the influence of a self-signal cannot be completely eliminated just as a result of the suppression process, in the analog domain, by a cancel signal that is transmitted from the second antenna, it is nevertheless possible to suppress the self-signal at least to such a degree that the problem of dynamic range in the sensing antenna can be eliminated. Further, communication by the primary user can be detected by performing self-signal suppression process by digital signal processing. Therefore, it becomes possible to simultaneously sense the primary user, using the sensing antenna, while performing wireless communication using the first transmission antenna. Sensing is enabled while communication is in progress, and, therefore, constant sensing can be performed without incurring drops in communication throughput. 
     In the present invention, preferably, the distance between the first transmission antenna and the sensing antenna is set to be greater than the distance between the second transmission antenna and the sensing antenna. Preferably, the first transmission antenna and the sensing antenna are disposed according to a positional relationship such that communication between the two antennas is non-line-of-sight communication. Preferably, the second transmission antenna is imparted with directivity towards the sensing antenna. 
     By increasing the distance between the first transmission antenna and the sensing antenna, and by setting a non-line-of-sight communication, it becomes possible to reduce the self-signal that arrives at the sensing antenna, due to the influence of inter-antenna distance attenuation. The second transmission antenna is close to the sensing antenna, and hence there can be reduced the transmission intensity of a wireless signal for self-signal cancelling. As a result, it becomes possible to reduce the influence on the directivity of the signal transmitted from the first transmission antenna, and to suppress adverse effects on communication as a secondary user. Adverse effects on the directivity of the first transmission antenna can be further reduced by imparting directivity to the second transmission antenna. 
     Preferably, the digital signal processing unit of the present invention suppresses the transmission signal from the first transmission antenna by minimum mean square error estimation in which a recursive least squares algorithm is used. The amount of computation is reduced as a result, and hence real-time sensing is enabled even in a wireless communication device that is equipped with only few computational resources. 
     The present invention can also be regarded as a wireless communication device having at least some of the above means. The present invention can also be regarded as a wireless communication method or sensing method comprising at least some of the above processes, and as a computer program for executing the above methods in a computer. The present invention can be configured by mutually combining, as far as possible, the above means and processes. 
     Advantageous Effects of Invention 
     The present invention allows detecting communication by a primary user while performing communication as a secondary user. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram illustrating the configuration of a wireless communication device according to the embodiment; 
         FIG. 2  is a diagram for explaining a suppression process of a secondary signal in which a cancellation signal is used; 
         FIG. 3  is a diagram for explaining a suppression process of a secondary signal by channel estimation; and 
         FIG. 4  is a diagram for explaining a sensing method according to conventional art. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Preferred embodiments of the present invention will be exemplarily explained next in detail with reference to accompanying drawings. 
     &lt;Overview&gt; 
     A system according to an embodiment of the present invention is a cognitive radio communication system for performing communication using a frequency not being utilized by a user (primary user) that has obtained a frequency utilization license, from among frequencies allocated to that primary user. Frequencies having been allocated to the primary user but not being used in practice are referred to as white space. In the present embodiment, the frequency utilization efficiency is enhanced by performing communication using such white space. 
     In the present description, communication by the primary user and communication by the secondary user will be referred to as primary communication and secondary communication, respectively. Wireless signals in primary communication and secondary communication will be referred to as primary signals and secondary signals, respectively. 
     A node (wireless communication device) that constitutes the cognitive radio system of the present embodiment is a movable wireless communication device. In the present embodiment, in particular, a cognitive radio system will be explained that is made up of a wireless communication device installed in a vehicle. To simplify the explanation below, no distinction will be made between an onboard (in-vehicle) communication device and a vehicle, and for instance the feature whereby onboard communication devices communicate with each other will be described as vehicles communicating with each other. 
     In the present embodiment, the onboard communication device performs sensing of detecting communication by a primary user, at the same time that the onboard communication device, as a secondary user, performs wireless communication using the white space. As a result, interference with the primary user is avoided through immediate discontinuation of communication as a secondary user when the primary user initiates communication. 
     As illustrated in  FIG. 1 , the wireless communication device of the present embodiment performs communication using three antennas  11 ,  21 ,  31 . A secondary communication antenna  11  is an antenna for secondary communication, and a sensing antenna  31  is an antenna for detecting primary communication. That is, secondary communication is performed using the secondary communication antenna  11 , and primary communication is detected using the sensing antenna  31 . The purpose of this is to discontinue the utilization of a white space frequency, so as not to interfere with a primary user, in a case where a primary user appears in the midst of communication in which the white space is utilized. 
     A signal, which is a superposition of a transmission signal of secondary communication and a primary transmission signal, arrives at the sensing antenna  31  in a case where the wireless communication device is performing secondary communication. The sensing antenna  31  is disposed closer to the secondary communication antenna  11  than the primary user, and, accordingly, the power component of the secondary signal in the signal received at the sensing antenna  31  is very large. In a case where the received signal is amplified, the primary signal may fail to be identified, due to the occurrence of a signal amplification plateau caused by saturation phenomena. In particular, a hardware-reliant receiver having not that high a performance cannot identify the primary signal. 
     The power component of the self-signal in the sensing antenna  31  can be expected to be reduced, through the influence of distance attenuation, by setting the distance between the secondary communication antenna  11  and the sensing antenna  31  to be of several meters. In ordinary automobiles, this distance between antennas can be set to range from about 2 meters to about 5 meters. The distance between antennas may be increased in the case of larger vehicles. The influence of the self-signal can be further suppressed if the two antennas are disposed according to a positional relationship such that communication between the antennas is non-line-of-sight communication. 
     However, eliminating the influence of the self-signal by relying only on distance attenuation between antennas is practically impossible. A process is therefore carried out, for mitigating the influence of a self-signal, in two domains, i.e. the analog domain and the digital domain. This enables as a result detection of the primary user, unaffected by the self-signal, even in the midst of secondary communication. 
     As a self-signal suppression process in the analog domain there is provided a cancellation antenna  21 , as illustrated in  FIG. 2 , that transmits from the cancellation antenna  21   a  cancellation signal such that the self-signal is cancelled at the point of installation of the sensing antenna  31 . The cancellation signal is a signal of amplitude identical to, but phase opposite to, those of the signal that is transmitted from the secondary communication antenna  11 , and the point of installation of the sensing antenna  31 . This way, it becomes possible to mitigate the influence of a self-signal in the sensing antenna  31 . 
     Preferably, the distance between the cancellation antenna  21  and the sensing antenna  31  is set to be smaller than the distance between the secondary communication antenna  11  and the sensing antenna  31 . For instance, the distance between the cancellation antenna  21  and the sensing antenna  31  can be set to range from several centimeters to several tens of centimeters. As a result, it becomes possible to reduce the transmission intensity of the cancellation signal that is transmitted from the cancellation antenna  21 , without significantly compromising the directivity of the secondary transmission signal that is transmitted from the secondary communication antenna  11 . 
     Preferably, the cancellation antenna  21  is imparted with directivity in the direction of the sensing antenna  31 . As a result, it becomes possible to further reduce adverse effects on the directivity of the secondary transmission signal. 
     It is difficult to eliminate completely the influence of the self-signal using the cancellation signal. Therefore, digital signal processing is performed on the signal received through the sensing antenna  31 , to further suppress thereby the influence of the self-signal. Specifically, the self-signal is suppressed, from a superimposed signal, through estimation of the self-signal by minimum mean square error (MMSE) estimation. In this case, the self-signal can be suppressed in real-time, through recursive processing, by resorting to a recursive least squares (RLS) algorithm, on the basis of past stored information. 
     The primary user can be detected on the basis of the signal received through the sensing antenna  31 , even in the midst of secondary communication, by performing thus the self-signal suppression process in two domains, i.e. the analog domain and the digital domain. 
     &lt;Device Configuration&gt; 
       FIG. 1  is a diagram illustrating the configuration of a wireless communication device  1  of the present embodiment. The wireless communication device  1  comprises the secondary communication antenna  11 , a secondary transmitter  12 , the cancellation antenna  21 , a cancellation transmitter  22 , the sensing antenna  31 , a sensing amplifier  32 , a digital cancellation circuit  33  and a primary communication detector  34 . The secondary communication antenna  11  corresponds to a first transmission antenna, the secondary transmitter  12  to a first transmitting unit, the cancellation antenna  21  to a second transmission antenna, the cancellation transmitter  22  to a second transmitting unit, the sensing antenna  31  to a sensing antenna, the digital cancellation circuit  33  to a digital signal processing unit and the primary communication detector  34  to a sensing unit. 
     The secondary communication antenna  11  is an antenna for performing secondary communication. Wireless signals for secondary communication are transmitted from the wireless communication device  1 , while wireless signals for secondary communication are received from another wireless communication device, using the secondary communication antenna  11 . Preferably, the secondary communication antenna  11  is disposed at a comparatively large distance from the sensing antenna  31 , for instance a distance of about 2 meters. Preferably, the antennas are disposed in such a manner that communication between the secondary communication antenna  11  and the sensing antenna  31  is non-line-of-sight communication. 
     The secondary transmitter  12  is a functional unit that generates, on the basis of information inputted from an information source, a transmission signal for secondary communication that is transmitted from the secondary communication antenna  11 . The secondary transmitter  12  outputs the transmission signal having been generated on the basis of the information inputted from the information source, also to the cancellation transmitter  22  and the digital cancellation circuit  33 . Although the focus herein is laid only on transmission, in fact transmission-reception functionality is required in order for transmission and reception to be performed via the secondary communication antenna  11 . 
     The cancellation antenna  21  is an antenna for transmitting a signal (cancellation signal) such that the signal transmitted from the secondary communication antenna  11  is cancelled at the sensing antenna  31 . Preferably, the cancellation antenna  21  is disposed at a short distance from the sensing antenna  31 , for instance about 10 centimeters. Preferably, the cancellation antenna  21  is set to have directivity towards the sensing antenna  31 . 
     The cancellation transmitter  22  is a functional unit that generates, a cancellation signal that is transmitted from the cancellation antenna  21 , on the basis of the transmission signal for secondary communication generated by the secondary transmitter  12 . The cancellation signal is a wireless signal such that the secondary signal that is transmitted from the secondary communication antenna  11  is canceled at the point of installation of the sensing antenna  31 . The cancellation transmitter  22  can generate the cancellation signal on the basis of the transmission signal obtained from the secondary transmitter  12 , since the positional relationship between the three antennas is known. Preferably, the cancellation signal is generated on the basis of a signal obtained from the sensing antenna  31 , in order to enhance cancellation precision. 
     The sensing antenna  31  is an antenna for detecting communication by the primary user. The sensing amplifier  32  is a functional unit that amplifies a wireless signal received by the sensing antenna  31 . The digital cancellation circuit  33  (digital signal processing unit) performs a process of estimating the self-signal, and suppressing the self-signal from the signal received through the sensing antenna  31 . Although the signal of secondary communication (self-signal) in the sensing antenna  31  is reduced by using the cancellation signal, the influence of the self-signal is difficult to eliminate completely. Therefore, the transmission signal from the secondary transmitter  12  is acquired, the self-signal is worked out by performing channel estimation between the secondary communication antenna  11  and the sensing antenna  13 , and the self-signal is removed from the received signal of the sensing antenna  31 . 
     As illustrated in  FIG. 3(A) , a superimposed signal r that arrives at the sensing antenna  31  is described as follows. 
     
       
         
           
             
               
                 
                   
                     
                       
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     Herein, r s  is a secondary incoming signal (after cancellation), r p  is a primary incoming signal, S s  is a secondary transmission signal, S p  is a primary transmission signal, h s  is a channel coefficient between the sensing antenna  31  and the secondary communication antenna  11 , h p  is a channel coefficient between the sensing antenna  31  and the primary user, and w is noise. 
     The primary incoming signal r p  must be detected accurately in order to sense primary communication. The secondary incoming signal r s , which is an unwanted component, must be removed in order to emphasize the primary incoming signal. To that end, a signal (≅h s S s ) that simulates a signal having passed through the channel is generated, in the digital cancellation circuit  33 , on the basis of the secondary transmission signal, and is subtracted from the received signal. 
     Modeling according to a linear system, such as the one illustrated in  FIG. 3(B) , is resorted to herein in order to estimate the secondary transmission signal. Herein, the input S s  is a secondary signal, and the output R s  is a sensing received signal, the propagation path is the free space between the secondary communication antenna  11  and the sensing antenna  31 , and the noise is signal attenuation and/or fading variation. 
     Rewritten according to the MMSE model, the input-output relationship is as follows.
 
 R   s,t   =z   T θ t   +w   t  
 
 z   t   T   =[−R   s,t−1   , . . . , −R   s,t−n   , S   s,t−1   , . . . , S   s,t−n ]
 
θ t   =[a   1   , . . . , a   n   , b   1   , . . . , b   n ]  [Math. 2]
 
     Herein, z t  is sensing information and self-signal information in a state where no primary communication exists, such that past information of the sensing information and self-signal information is held in the digital cancellation circuit  33 . 
     To estimate the system parameters, it is necessary to design a weight coefficient θ in such a manner that the difference between the current state and a state estimated from the past is minimal. At that rate, however, such design involves an enormous amount of computation, and is accordingly not suitable for real-time processing. 
     Therefore, a recursive least squares algorithm is resorted to, so that it becomes possible as a result to estimate the self-signal (secondary incoming signal) that arrives at the sensing antenna  31  on the basis of the recursively computable expressions below. Herein, ρ is a forgetting factor. 
     
       
         
           
             
               
                 
                   
                     
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     The secondary communication signal that arrives at the sensing antenna  31  can be estimated thus, and hence the influence of the self-signal can be suppressed through removal of the signal having been estimated from a signal received at the sensing antenna  31 . As a result, only the primary signal (if present) and noise are comprised in the signal that is outputted by the digital cancellation circuit  33 . 
     The primary communication detector  34  (sensing unit) is a functional unit that detects the presence or absence of a primary signal on the basis of the signal outputted by the digital cancellation circuit  33 . For instance, an energy detection method can be resorted to herein as the detection algorithm. Primary communication can also be detected in accordance with, for instance, a matched filter method or feature method, in cases where the primary communication is known. 
     In the present embodiment, secondary communication from the secondary communication antenna  11  is carried out constantly, without quiet periods being particularly provided. Primary communication can be detected as a result of the self-signal suppression process, even if secondary communication is being performed constantly. In the present embodiment, therefore, secondary communication and primary detection are performed simultaneously and constantly. Upon detection of primary communication, the secondary transmitter  12  discontinues immediately wireless communication in which the current frequency is used. 
     Workings and Effect of the Present Embodiment 
     In the present embodiment, two self-signal suppression processes, in the analog domain and the digital domain, are combined so that, as a result, it becomes possible to remove, substantially completely, the influence of a self-signal in the sensing antenna. Therefore, primary communication can be detected using the sensing antenna  31 , also in cases where a secondary signal is being transmitted from the secondary communication antenna  11 . Specifically, primary communication can be detected without providing quiet periods for primary communication detection, such as the one illustrated in the related art of  FIG. 4 . Drops in the throughput of the secondary communication can be therefore suppressed. Further, primary detection can be carried out constantly, and hence the primary user can be detected immediately. 
     &lt;Variations&gt; 
     In the explanation of the above embodiment, an example has been described of a cognitive radio system that is made up of a wireless communication device installed in a vehicle. However, the present invention can be configured in the form of any movable wireless communication device other than an automotive wireless communication device. The term movable wireless communication device applies herein, for instance, to a wireless communication device with which a vehicle, aircraft of vessel is equipped, to a wireless communication device that is brought into a vehicle, aircraft or vessel and that moves accompanying the motion of the foregoing, and to a personal portable wireless communication device. 
     In the explanation of the embodiments above, the digital cancellation circuit  33  and the primary communication detector  34  are envisaged to be realized in the form of dedicated hardware circuits (integrated circuits), such as an ASIC or the like. However, the functional units above may be realized as software, by combining a program with a CPU (Central Processing Unit), MPU (Micro Processing Unit) or FGPA (Field-Programmable Gate Array). The functional units may also be realized in the form of combinations of both hardware and software. 
     REFERENCE SIGNS LIST 
       11  secondary communication antenna 
       12  secondary transmitter 
       21  cancellation antenna 
       22  cancellation transmitter 
       31  sensing antenna 
       33  digital cancellation circuit 
       34  primary communication detector