Abstract:
A repeating station, a communication apparatus, and an antenna directivity control method are disclosed. The repeating station includes
       a first antenna group including at least one antenna element,   a second antenna group that is different from the first antenna group, the second antenna group including two or more antenna elements adjusted so that radiation in the direction of the first antenna group is reduced, wherein the second antenna group is adjusted such that radiated power in the direction of the first antenna group is reduced.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a repeating station (repeater), a communication apparatus, and a directivity control method for relaying and amplifying a radio signal in mobile communications. 
     2. Description of the Related Art 
     In mobile communication systems using an electric wave such as a cellular phone system, there is a problem in that communications are not available out of radio coverage, e.g., in a tunnel. In order to cope with such situations, a radio repeating station, often called a repeater or a booster, is employed. Repeaters are used not only as a measure to expand radio coverage, but also in order to provide a multi-path propagation environment such that the channel capacity of MIMO (Multiple Input Multiple Output) communications is increased, the MIMO communications providing high frequency efficiency in a non-line-of-sight propagation environment. 
     The configuration of the repeater is described with reference to  FIGS. 1A and 1B . 
       FIG. 1A  shows the configuration of a direct relay method wherein a signal received by a receiving antenna is amplified by a low noise amplifier (LNA) and a high power amplifier (HPA), and the amplified signal is transmitted from a transmitting antenna. According to the direct relay method, the frequency of transmission is the same as that of reception; for this reason, its frequency efficiency is high, and it has an advantage in that the configuration is simple. However, a loop interference may take place by the signal feeding back from the transmitting antenna to the receiving antenna, which is a problem in that the gain of the amplifiers cannot be made great. 
       FIG. 1B  shows the configuration of a heterodyne relay method wherein a first signal frequency is received by the receiving antenna, the signal is converted to an intermediate frequency, amplified, and is then converted to a second frequency for transmission. According to this method, the frequency of transmission and the frequency of reception differ; for this reason, there is no feedback from the transmission antenna to the receiving antenna, and sufficient amplification can be provided. However, since two frequencies are used, the frequency efficiency is degraded. 
     Next, a method of increasing the channel capacity of MIMO communications in a line-of-sight environment by a repeater is described (for example, Patent Reference 1 refers). The MIMO communications employ spatial multiplexing, wherein different data streams are transmitted from i antennas, and are simultaneously received by j antennas (i and j are positive integers). Accordingly, under a multi-path propagation environment wherein two or more independent propagation paths are available between transmission and reception, the channel capacity is increased. However, in a propagation environment where the transmitting station and the receiving station are in line-of-sight, independent propagation paths are difficult to obtain, and the channel capacity is degraded. 
       FIG. 2  shows a radio communications system that includes repeaters for artificially arranging a multi-path propagation environment in order to increase the channel capacity in the line-of-sight environment. 
     The radio communications system includes a first radio station (transmitting station), two or more second radio stations (repeaters), and a third radio station (receiving station). 
     A signal is transmitted from the transmission antennas of the first radio station (transmitting station), and received by the second radio stations (repeaters). Each of the repeaters stores the received signal in a buffer, and transmits the signal after a delay of a predetermined period. Consequently, the signals stored in the buffers are simultaneously transmitted from the repeaters. The signals transmitted from the repeaters are then received by the third radio station (receiving station). The receiving station restores the signal transmitted by performing signal separation of the received signals. In this manner, a multi-path propagation is obtained in the line-of-sight propagation environment, and the channel capacity is increased. However, according to this method, the repeaters transmit by time division, i.e., at a timing that is different from the transmitting station. Consequently, the amount of signals that can be transmitted is approximately halved in comparison with the case where no time division is used, and a transmission delay is inevitable. 
     In mobile communications, transmission speed keeps increasing, and frequency efficiency is required. Further, information to be transmitted is of a real time nature, i.e., requires the least possible delay. In other words, repeaters to be applied to future mobile communications are required to provide high frequency efficiency, and the smallest possible delay. From this viewpoint, the direct relay method shown in  FIG. 1A  is suitable as the relaying method; however, there is the problem of the loop interference between the antennas if great amplification is required. 
     According to the conventional direct relay method, the directivity of an antenna is set in the direction of a base station, and the directivity of an antenna on the other side is set in the direction of a terminal, such that the directional gain between the antennas is reduced. Further, feedback between the antennas is prevented by providing a sufficient distance between the antennas such that propagation loss increases. However, the position of the terminal, which is often in motion, is unknown; therefore, the antenna directivity of the repeater on the side of the terminal is desired to be omni-directional in the horizontal plane. 
     In the case wherein the repeaters are used in order to realize a multi-path environment for MIMO communications, it is further desirable that the antenna provided on the base station side of the repeater be omni-directional so that the number of propagation paths between the base station and the repeater is increased. When the channel capacity is to be increased in the MIMO communications under line-of-sight conditions, it is seldom necessary to provide a great amplification factor. Nevertheless, in the case of the repeater using the direct relay method, the omni-directional antennas provided to both transmission and reception cause the transmission signal to be fed back to the receiving antenna, i.e., a loop interference takes place. 
     [Patent Reference 1] JPA 2003-198442 
     [Problem(s) to be Solved by the Invention] 
     As described above, problems in the background technology are as follows. 
     As for the repeater for mobile communications, the direct relay method is suitable from the viewpoint of the frequency efficiency and the transmission delay. Further, when the repeater is to be applied to the MIMO communications for providing a multi-path propagation environment, the omni-directional antennas for transmission and reception are desirable. 
     However, when using the omni-directional antennas for transmission and reception with the direct relay method, the loop interference takes place from the transmission antenna to the reception antenna, causing an oscillation. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention aims at offering a repeating station (repeater), a communication apparatus, and a directivity control method that substantially obviate one or more of the problems caused by the limitations and disadvantages of the related art. 
     Specifically, the present invention aims at offering a repeating station (repeater), a communication apparatus, and a directivity control method whereby a signal is transmitted by an antenna having an almost-omni-directional pattern while feedback of the signal from the transmitting antenna to the receiving antenna is reduced. 
     Features and advantages of the present invention are set forth in the description that follows, and in part will become apparent from the description and the accompanying drawings, or may be learned by practice of the invention according to the teachings provided in the description. Objects as well as other features and advantages of the present invention will be realized and attained by a repeating station (repeater), a communication apparatus, and a directivity control method particularly pointed out in the specification in such full, clear, concise, and exact terms as to enable a person having ordinary skill in the art to practice the invention. 
     To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides as follows. 
     [Means for Solving the Problem] 
     In order to solve the above-mentioned problems, the repeating station of the present invention includes 
     a first antenna group including at least one antenna element, 
     a second antenna group different from the first antenna group including two or more antenna elements adjusted so that the radiation output in the direction of the first antenna group is reduced, wherein the second antenna group is adjusted so that gain in the direction of the first antenna group is reduced. 
     By arranging the repeater as described above, the feedback from the second antenna group to the first antenna group is reduced, and a transmitting antenna pattern that is almost omni-directional is obtained as a total antenna pattern. 
     According to another aspect of the present invention, the repeater includes 
     the first antenna group including at least one antenna element, 
     the second antenna group including two or more adaptive array antennas containing two or more antenna elements, 
     a received-power measuring unit for measuring power of a signal received by the first antenna group, the signal being transmitted from the second antenna group, and 
     a weight calculating unit for calculating transmission weights that are multiplied by corresponding signals to be transmitted such that the output of each adaptive array antenna in the radiation direction causing the feedback to the first antenna group is reduced, and the output in other directions is made uniform based on the measured received power. 
     By structuring the repeater in this way, the feedback from the second antenna group to the first antenna group is reduced, and the total antenna pattern that is almost omni-directional is obtained. 
     The communication apparatus according to the present invention includes a received signal property calculating unit for calculating the property of a received signal, and a determining unit for determining whether to start or stop transmission of a signal from the repeater based on the property of the received signal. 
     In this way, the signal property at the receiving station is improved. 
     The directivity control method of the present invention includes 
     a step of transmitting a directivity adjustment signal from each antenna element of the second antenna group that includes two or more antenna elements, 
     a step of measuring power of the directivity adjustment signal received by the first antenna group that includes at least one antenna element, and 
     a step of controlling the directivity of each antenna element that constitutes the second antenna group based on the measured received power. 
     In this way, the directivity of the transmitting antenna can be adaptively adjusted. 
     [Effect of the Invention] 
     According to the embodiments of the present invention, the repeating station, the communication apparatus, and the directivity control method are realized, whereby the loop interference from the transmitting antenna to the receiving antenna is reduced, and an almost omni-directional transmission pattern of the antenna is obtained. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  and  FIG. 1B  are block diagrams showing configurations of a repeater; 
         FIG. 2  is a block diagram explaining a relay transmission system of MIMO communications; 
         FIG. 3  is a block diagram showing the configuration of the repeater according to an embodiment of the present invention; 
         FIG. 4  is a block diagram showing the configuration of the repeater according to the embodiment of the present invention; 
         FIG. 5  is a block diagram showing the configuration of the repeater according to the embodiment of the present invention; 
         FIG. 6  is a flowchart showing operations of the repeater according to the embodiment of the present invention; 
         FIG. 7  is a block diagram showing the configuration of the repeater according to the embodiment of the present invention; 
         FIG. 8  is a block diagram showing the configuration of the repeater according to the embodiment of the present invention; 
         FIG. 9  is a flowchart showing operations of the repeater according to the embodiment of the present invention; 
         FIG. 10  is a block diagram showing the configuration of the repeater according to the embodiment of the present invention; 
         FIG. 11  is a block diagram showing the configuration of the repeater according to the embodiment of the present invention; 
         FIG. 12  is a block diagram showing the configuration of the repeater according to the embodiment of the present invention; 
         FIG. 13  is a block diagram showing the configuration of the repeater according to the embodiment of the present invention; 
         FIG. 14  is a flowchart showing operations of the repeater according to the embodiment of the present invention; 
         FIG. 15  is a block diagram showing the configuration of a transmitting station according to the embodiment of the present invention; 
         FIG. 16  is a block diagram showing the configuration of a receiving station according to the embodiment of the present invention; 
         FIG. 17  is a block diagram showing the configuration of the repeater according to the embodiment of the present invention; 
         FIG. 18  is a sequence diagram showing operations of a radio communications system according to the embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following, embodiments of the present invention are described with reference to the accompanying drawings, Here, as for the drawings for describing the embodiments, the same reference numbers are given to the same functional units, and descriptions thereof are not repeated. 
     A repeater  100  according to the first embodiment of the present invention is described with reference to  FIG. 3 . 
     The repeater  100  has two transmitting antennas. 
     The repeater  100  includes a receiving antenna  101 , an LNA  102  connected to the receiving antenna  101 , an HPA  103  connected to the LNA  102 , a splitter  104  connected to the HPA  103 , and transmitting antennas  105  and  106  connected to the splitter  104 . 
     A signal is received by the receiving antenna  101  that is omni-directional, is amplified by the LNA  102 , is further amplified by the SPA  103 , and then is split into two signals by the splitter  104 . The split signals are transmitted from the corresponding transmitting antennas  105  and  106 . 
     In the present embodiment, the transmitting antennas  105  and  106  are designed such that the radiation output in the direction of the receiving antenna  101  is reduced, and the radiation output in other directions is made uniform. The directivity as described can be realized by using, for example, a corner reflector antenna. By configuring in this way, the loop interference from the transmitting antennas to the receiving antenna is reduced. 
     Further, the transmitting antennas  105  and  106  are installed on the repeater  100  such that the total antenna pattern formed by the transmitting antennas  105  and  106  provides an almost omni-directional transmitting antenna pattern. For example, the direction from the receiving antenna  101  to the transmitting antenna  105  is made approximately orthogonal to the direction from the receiving antenna  101  to the transmitting antenna  106 . In this way, the transmitting antenna pattern that is almost omni-direction is synthetically formed by the transmitting antennas  105  and  106 . 
     Although the embodiment is described for the case where there are two transmitting antennas, the number of the transmitting antennas may be greater than two that are similarly arranged, whereby the loop interference can be reduced, and the repeater with the almost omni-directional transmission and reception characteristics can be realized. 
     Next, a repeater according to the second embodiment of the present invention is described with reference to  FIG. 4 . 
     The repeater  100  according to the second embodiment has two receiving antennas and two transmitting antennas. 
     The repeater  100  according to the second embodiment includes the receiving antenna  101 , a receiving antenna  107 , the LNA  102  and an LNA  108  connected to the receiving antennas  101  and  107 , respectively, a coupler  109  connected to the LNA  102  and the LNA  108 , the HPA  103  connected to the coupler  109 , the splitter  104  connected to the HPA  103 , and the transmitting antennas  105  and  106  connected to the splitter  104 . 
     The signals received by the receiving antennas  101  and  107  are amplified by the LNA  102  and  108 , respectively, and are compounded by the coupler  109 . The compounded signal is amplified by the HPA  103 , split into two signals by the splitter  104 , and then transmitted from the corresponding transmitting antennas  105  and  106 . 
     The transmitting antennas  105  and  106  are designed such that the radiation output in the direction to the receiving antennas  101  and  107  is reduced, and the radiation output in other directions is made uniform. Such a directivity pattern is realizable by using, for example, a corner reflector antenna. By constituting the repeater in this way, the loop interference from the transmitting antennas to the receiving antennas is reduced. Further, the transmitting antennas  105  and  106  are arranged so that they form a synthetic antenna pattern that is almost omni-directional. 
     Further, the gain (amplification factor) of the receiving antennas  101  and  107  for a signal received from the direction of the transmitting antennas is reduced, and the gain for a signal received from other directions is made uniform as much as possible. The receiving antennas are realizable by using, e.g., corner reflector antennas. By constituting the repeater in this way, the loop interference from the transmitting antennas is reduced. Further, the receiving antennas  101  and  107  are arranged so that they form a total antenna pattern that is almost omni-directional. 
     By using the above configuration, the influence of loop interference is reduced, and a repeater with transmitting antennas and receiving antennas having almost-omni-directional total patterns is realized. 
     Although the second embodiment is described for the case where the repeater includes two transmitting antennas and two receiving antennas, the same configuration is applicable to the case where the numbers of the transmitting antennas and receiving antennas are greater than 2, wherein the loop interference is reduced and a repeater with the almost-omni-directional transmission and reception characteristics can be realized. 
     Next, a repeater according to the third embodiment of the present invention is described with reference to  FIG. 5 . 
     The repeater  100  according to the third embodiment includes the receiving antenna  101 , the LNA  102  connected to the receiving antenna  101 , the HPA  103  connected to the LNA  102 , the splitter  104  and a received-power measuring unit  110  connected to the HPA  103 , a directivity control unit  111  connected to the received-power measuring unit  110 , the transmitting antennas  105  and  106  connected to the directivity control unit  111 , directivity adjustment signal generating units  112  and  113 , and switches  114  and  115 . Here, the switch  114  is connected to the transmitting antenna  105 , and switches between the splitter  104  and the directivity adjustment signal generating unit  112 ; and the switch  115  is connected to the transmitting antenna  106 , and switches between the splitter  104  and the directivity adjustment signal generating unit  113 . 
     As described above, the repeater  100  according to the third embodiment includes the directivity adjustment signal generating units  112  and  113 , the received-power measuring unit  110 , the directivity control unit  111 , and the switches  114  and  115  in addition to the repeater described with reference to  FIG. 3 . 
     The directivity of the transmitting antennas  105  and  106  is controlled based on the signal acquired from the directivity control unit  111 . 
     The directivity adjustment signal generating units  112  and  113  are connectable to the transmitting antennas  105  and  106 , respectively. The directivity adjustment signal generating units  112  and  113  generate directivity adjustment signals for adjusting the respective transmitting antennas  105  and  106 . The received power of the directivity adjustment signals at the receiving antenna  101  is measured by the received-power measuring unit  110 . The directivity control unit  111  controls the directivity of each transmitting antenna according to the measured received power. For example, the directivity control unit  111  controls the directivity of the transmitting antennas  105  and  106  so that the received power of the directivity adjustment signals measured by the received-power measuring unit  110  becomes small. 
     By constituting the repeater in this way, the output of the radiation in a direction that causes the loop interference from the transmitting antennas to the receiving antennas is reduced. 
     Next, operations in the case of controlling the directivity of the transmitting antenna of the repeater  100  according to the present embodiment are described with reference to  FIG. 6 . 
     The directivity adjustment signal generating unit  112  generates the directivity adjustment signal, which is then transmitted through the transmitting antenna  105  (Step S 602 ). The received-power measuring unit  110  measures the received power of the directivity adjustment signal at the receiving antenna  101  (Step S 604 ). Next, the directivity control unit  111  controls the directivity of the transmitting antenna  105  based on the measured received power (Step S 606 ). For example, the directivity control unit  111  controls the directivity of the transmitting antenna  105  so that the received power measured by the received-power measuring unit  110  becomes small. 
     Next, it is determined whether directivity control has been completed for all the transmitting antennas (Step S 608 ). The process is finished if the directivity control is completed for all the transmitting antennas (YES at Step S 608 ). Otherwise, if the directivity control is not completed for all the transmitting antennas (NO at Step S 608 ), the process returns to Step S 602 . Then, the directivity control of the transmitting antenna  106 , for example, is performed. 
     The repeater  100  performs directivity control of each transmitting antenna using the directivity adjustment signal when the antenna is installed. Further, the control may be periodically or otherwise performed when the transmission from the transmitting station is stopped. 
     The direction of a wave that is transmitted from each transmitting antenna, and that causes the loop interference to the receiving antenna  101  changes with the installation location of the repeater  100 , and the environment around the repeater  100 . By the configuration as described above, the directivity of the transmitting antenna(s) is adaptively adjusted, and the loop interference is reduced when the environment changes. 
     Although the third embodiment is described for the case where the repeater has two transmitting antennas, the present invention is applicable to the case where more than two transmitting antennas are arranged, wherein the directivity of the transmitting antenna is adaptively adjusted, and the loop interference is reduced according to the environment change. 
     Next, a repeater according to the fourth embodiment of the present invention is described with reference to  FIG. 7 . 
     The repeater  100  according to the fourth embodiment includes the receiving antenna  101  the LNA  102  connected to the receiving antenna  101 , the HPA  103  connected to the LNA  102 , a loop interference canceller  116  connected to the HPA  103 , the splitter  104  connected to the loop interference canceller  116 , and the transmitting antennas  105  and  106  connected to the splitter  104 . 
     The repeater  100  according to the fourth embodiment includes the loop interference canceller  116  in addition to the configuration described with reference to  FIG. 3 . The directivity of the transmitting antennas of the repeater  100  is adjusted so that the radiation output in the direction to the receiving antenna is reduced as described in the first embodiment. Further, the repeater  100  according to the fourth embodiment uses the loop interference canceller  116 . In this way, the loop interference due to the feedback is mitigated. 
     The radiation direction from the transmitting antenna causing the loop interference to the receiving antenna changes with the surrounding environment. By using the loop interference canceller  116 , the influence of the loop interference that cannot be reduced by only the directivity of the antenna can be reduced. 
     Although the fourth embodiment is described for the case where the repeater has two transmitting antennas, the present invention is also applicable to the case where the number of transmitting antennas is greater than 2, in which case also, the influence of the loop interference that cannot be reduced by only the directivity of the antennas can be reduced. 
     Next, a repeater  200  according to the fifth embodiment of the present invention is described with reference to  FIG. 8 . 
     The repeater  200  according to the fifth embodiment includes 
     a receiving antenna  201 , 
     an LNA  202  connected to the receiving antenna  201 , 
     a received-power measuring unit  203  and multipliers  205  through  208  connected to the LNA  202 , 
     an antenna weight updating unit  204  connected to the received-power measuring unit  203  and the multipliers  205  through  208 , 
     HPAs  209  through  212  respectively connected to the multipliers  205  through  208 , and 
     transmitting antennas  213  through  216  respectively connected to the HPAs  209  through  212 . 
     The repeater  200  according to the fifth embodiment controls the directivity of each transmitting antenna using a transmitting adaptive array antenna. 
     The transmitting adaptive array antenna is constituted by two or more antennas, and its directivity is controlled by multiplying a weighting factor called a transmitting antenna weight by a signal to be transmitted from each transmitting antenna. In the following description of the fifth embodiment, two transmitting adaptive array antennas are used, each transmitting adaptive array antenna consisting of two transmitting antennas. 
     The first transmitting adaptive array antenna is constituted by the transmitting antennas  213  and  214 , to which transmitting antenna weights W 213  and W 214 , respectively, are applied such that the directivity is controlled. Similarly, the second transmitting adaptive array antenna is constituted by the transmitting antennas  215  and  216 , to which transmitting antenna weights W 215  and W 216 , respectively, are applied such that the directivity is controlled. 
     Each transmitting adaptive array antenna is controlled such that the output in the radiation direction causing the loop interference is reduced, and that the output in other directions is as uniform as possible, 
     By constituting the repeater in this way, an antenna pattern that is almost omni-directional is formed as the total transmitting antenna pattern by the first and second transmitting adaptive array antennas. 
     Further, by updating the transmitting antenna weights by the antenna weight updating unit  204 , the directivity can be adaptively controlled. When the radiation direction from the transmitting antenna causing the loop interference changes with changes to the surrounding environment and propagation environment over time, the adaptive control can mitigate the influence of the loop interference by controlling the directivity according to the change. 
     Updating of the transmitting antenna weight is described with reference to  FIG. 9 . The received power of the signal received by the receiving antenna  201  is measured by the received-power measuring unit  203  (Step S 902 ). The antenna weight updating unit  204  updates the value of transmitting antenna weight so that the received power becomes small (Step S 904 ). In this way, the output in the radiation direction causing the feedback can be reduced. 
     Here, in order to avoid all transmitting antenna weights becoming zero, it is necessary to provide a constraint condition. Methods to provide the constraint condition include setting a square sum of all transmitting antenna weights at a constant value, fixing a certain transmitting antenna weight at a predetermined value, and fixing a transmitting antenna weight of one antenna of each transmitting adaptive array antenna at a predetermined value. By using the constraint conditions as described above, the directivity that mitigates the influence of the loop interference is attained without all transmitting antenna weights becoming zero. 
     Further, the transmitting antenna weight can be updated using the signal transmitted by a transmitting station. Further, when transmission of the signal from the transmitting station is stopped, the transmitting antenna weight can be updated by transmitting the directivity adjustment signal from the repeater, as described in the third embodiment. 
     A repeater that includes the signal generating unit for directivity adjustment is described with reference to  FIG. 10 . 
     The repeater  200  shown in  FIG. 10  includes a signal generating unit  217  for generating the directivity adjustment signal, and a switch  218  in addition to the repeater  200  described with reference to  FIG. 8 . The switch is connected to the multipliers  205  through  208 , and is switched between the LNA  202  and the signal generating unit  217 . 
     The signal generating unit  217  transmits the directivity adjustment signal through the transmitting antennas  213  through  216 . The received power of the directivity adjustment signal at the receiving antenna  201  is measured by the received-power measuring unit  203 . The antenna weight updating unit  204  updates each transmitting antenna weight according to the measured received power. For example, the antenna weight updating unit  204  updates the transmitting antenna weight so that received power may become small. 
     By constituting the repeater in this way, the directivity adjustment signal can be transmitted from the repeater, and the transmitting antenna weight is updated based on the directivity adjustment signal. 
     Although a repeater using two transmitting adaptive array antennas constituted by two transmitting antennas is described, the control is applicable to the case where the number of transmitting antennas that constitute a transmitting adaptive array antenna is greater than 2. In the case where the number of the transmitting adaptive array antennas is greater than 2, adaptive directivity control is also possible, and the influence of the loop interference, which changes over time, can be mitigated 
     Next, a repeater according to the sixth embodiment of the present invention is described with reference to  FIG. 11 . 
     The repeater  200  according to the embodiment includes 
     receiving antennas  201 ,  218 ,  220 , and  222 , 
     LNAs  202 ,  219 ,  221 , and  223  connected to the respective receiving antennas  201 ,  218 ,  220 , and  222 , 
     multipliers  224 ,  225 ,  226 , and  227  connected to the respective LNAs  202 ,  219 ,  221 , and  223 , 
     an adder  228  connected to the multipliers  224  and  225 , 
     an adder  229  connected to the multipliers  226  and  227 , 
     an adder  230  connected to the adders  228  and  229 , 
     the received-power measuring unit  203  connected to the adder  230 , 
     the multipliers  205 ,  206 ,  207  and  208 , 
     the antenna weight updating unit  204  connected to the received-power measuring unit  203  and the multipliers  224 ,  225 ,  226 ,  227 ,  205 ,  206 ,  207 , and  208 , 
     HPAs  209 ,  210 ,  211 , and  212  connected to the respective multipliers  205 ,  206 ,  207 , and  208 , and 
     the transmitting antennas  213 ,  214 ,  215 , and  216  connected to the respective HPAs  209 ,  210 ,  211 , and  212 . 
     The receiving antennas of the repeater  200  according to the sixth embodiment are constituted by two or more adaptive array antennas, which is the difference from the repeater  200  described with reference to  FIG. 8 . The signal is received by the receiving adaptive array antennas, compounded, and transmitted from each transmitting adaptive array antenna. Like the fifth embodiment, the loop interference is reduced by the transmitting antennas being constituted by the adaptive array antennas, whereby the output in the direction of the receiving antennas is reduced. 
     The receiving antennas of the repeater  200  according to the sixth embodiment are constituted by the first and second receiving adaptive array antennas. The first receiving adaptive array antenna is constituted by receiving antennas  201  and  218 , and the directivity is controlled by multiplying the received signals by receiving antenna weights X 201  and X 218 , respectively. Similarly, the second receiving adaptive array antenna is constituted by receiving antennas  220  and  222 , and the directivity is controlled by multiplying the received signals by receiving-antenna weights X 220  and X 222 , respectively. 
     The directivity of each receiving adaptive array antenna is controlled such that the gain in the arrival direction of the loop interference is reduced, and the gain in other directions is made as uniform as possible. 
     By structuring the repeater in this way, the total antenna pattern formed by the first and the second receiving adaptive array antennas can be made almost omni-directional. 
     Further, the transmitting antenna weights and the receiving antenna weights are updated so that the power levels of the received signals as compounded become small, the signals being received by the receiving adaptive array antenna. At this juncture, in order to avoid all the transmission and receiving antenna weights being set to zero, it is necessary to use a constraint condition. As described in the fifth embodiment, methods to provide the constraint condition include setting the square sum of all transmitting antenna weights to a constant value, fixing a certain transmitting antenna weight at a predetermined value, and fixing a transmitting antenna weight of one antenna of each transmitting adaptive array antenna at a predetermined value. 
     In this way, the influence of the loop interference, which changes over time, can be reduced, and the repeater that provides the almost-omni-directional transmission and reception patterns can be realized. 
     Although the sixth embodiment is described for a repeater including two transmission antennas and the receiving adaptive array antennas, each of which is constituted by two antennas, the present invention is applicable to the case where the number of antennas that constitute each of the transmission and receiving adaptive array antennas is greater than 2, and the case where the number of each of the transmission and receiving adaptive array antennas is greater than 2. In such cases also, the influence of the feedback, which changes over time, is mitigated, and a repeater that provides almost-omni-directional transmission and reception patterns can be realized. 
     Next, the repeater according to the seventh embodiment of the present invention is described with reference to  FIG. 12 . 
     The repeater  200  according to the seventh embodiment includes a loop interference canceller  231  in addition to the repeater described with reference to FIG. B. The loop interference canceller  231  is connected to the LNA  202 , the received-power measuring unit  203 , and the multipliers  205 ,  206 ,  207 , and  208 . 
     The output of the loop interference canceller  231  is provided to the received-power measuring unit  203 . The received-power measuring unit  203  measures the received power of the provided signal, and provides a measuring result to the antenna weight updating unit  204 . The antenna weight updating unit  204  updates the transmitting antenna weight based on the provided result. 
     By constituting the repeater in this way, the influence of the loop interference, which cannot be mitigated by only directivity control of the transmitting adaptive array antenna, can be reduced. 
     Although the seventh embodiment is described for the case where the output of the loop interference canceller  231  is measured by the received-power measuring unit  203 , and the transmitting antenna weight is updated by the antenna weight updating unit  204 , the configuration can be such that the received-power measuring unit  203  measures the input to the loop interference canceller  231 , and updates the transmitting antenna weight. 
     Next, a repeater  300  according to the eighth embodiment of the present invention is described with reference to  FIG. 13 . 
     The repeater  300  according to the eighth embodiment includes 
     a receiving antenna  301 , 
     an LNA  302  connected to the receiving antenna  301 , 
     an HPA  307  and a switch  303  connected to the LNA  302 , 
     a transmitting antenna  308  connected to the HPA  307 , 
     an HPA  305  and a transmitting antenna quantity change signal receiving unit  304  connected to the switch  303 , and 
     a transmitting antenna  306  connected to the HPA  305 . 
     The repeater  300  according to the eighth embodiment includes the transmitting antenna quantity change signal receiving unit  304 , and the switch  303  for switching between transmitting and halting the transmission of the transmitting antenna  306 . The transmitting antenna quantity change signal receiving unit  304  causes the switch  303  to switch between transmission and transmission halt of the signal from each transmitting antenna, for example in  FIG. 13 , the transmitting antenna  306  based on a notice signal of a transmitting antenna quantity change. 
     According to the repeater  300  described above, a receiving station receives signals through a number of paths (multi-path signals) regardless of the position of the receiving station, given that two or more transmitting antennas are used, and the omni-directional characteristic of the synthetic antenna pattern. Nevertheless, depending on the position of the receiving station in relation to the repeater, multi-path signals may be received by the receiving station even when the repeater uses only one transmitting antenna. In this case, the repeater stops transmission from other antennas. 
     In this way, interference at other receiving stations using the same frequency can be reduced, and operations of the HPAs connected to the non-operational transmitting antennas can be stopped, which reduces power consumption of the repeater. 
     Further, according to the embodiment, when the transmission from the transmitting antenna  306  is stopped, and when a receiving station moves and necessary communication quality is no longer available at the receiving station, the receiving station can still improve the receiving quality by sending the notice signal of the transmitting antenna quantity change to the repeater, and by causing the transmission from the transmitting antenna  306  to start. 
     Next, operations of the repeater in a radio network according to the eighth embodiment are described with reference to  FIG. 14 . 
     The transmitting antenna quantity change signal receiving unit  304  receives the notice signal of the transmitting antenna quantity change. (Step S 1402 ). 
     The transmitting antenna quantity change signal receiving unit  304  determines whether the received notice signal of the transmitting antenna quantity change is a signal that requires transmission from the transmitting antenna  306  (Step S 1404 ). If affirmative (YES at Step S 1404 ), the transmitting antenna quantity change signal receiving unit  304  determines whether the transmitting antenna  306  is performing transmission. 
     If transmission from the transmitting antenna  306  is not being performed (NO at Step S 1406 ), the transmitting antenna quantity change signal receiving unit  304  causes the switch  303  to switch to the contact for performing transmission from the transmitting antenna  306 , and the transmission from the transmitting antenna  306  is started (Step S 1408 ). Otherwise, i.e., if the transmission from the transmitting antenna  306  is being performed (YES at Step S 1406 ), no action is required. 
     In a case where the received notice signal of the transmitting antenna quantity change does not require transmission from the transmitting antenna  306  (i.e., when it is a signal that requires a transmission halt of the transmitting antenna  306 ) (NO at Step S 1404 ), the transmitting antenna quantity change signal receiving unit  304  determines whether the transmission from the transmitting antenna  306  is being performed (Step S 1410 ). 
     If affirmative (YES at Step S 1410 ), the transmitting antenna quantity change signal receiving unit  304  stops the transmission from the transmitting antenna  306  by causing the switch  303  to turn to the contact for stopping the transmission from the transmitting antenna  306  (Step S 1412 ). On the other hand, nothing is carried out if the transmission from the transmitting antenna  306  is not being performed (NO at Step S 1410 ). 
     As described above, according to the present embodiment, transmission and transmitting halt of the transmitting antenna  306  are switched based on the notice signal of the transmitting antenna quantity change acquired by the transmitting antenna quantity change signal receiving unit  304 . 
     A receiving station measures the received signal power and correlation between antennas of the received signal, and transmits the notice signal of the transmitting antenna quantity change that shows whether the number of transmission antennas to be used by the repeater is to be increased or decreased. The notice signal of the transmitting antenna quantity change is provided to the repeater directly in the radio network, or is once provided to the transmitting station using the radio network, and then provided to the repeater through the radio network or a wired network from the transmitting station. The repeater changes the number of antennas to be used based on the provided notice signal of the transmitting antenna quantity change. 
     In this way, when the signal is received with satisfactory signal properties, such as the power and the correlation, at the receiving station, the number of antennas used by the repeater is decreased, which reduces interference at other receiving stations using the same frequency. Further, the power consumption of the repeater can be reduced. Conversely, when the received signal property is not satisfactory at the receiving station, the signal property can be improved by increasing the number of the transmission antennas used by the repeater. 
     In the embodiment, although the repeater including two transmitting antennas is described, the present invention can apply to the case where the number of transmitting antennas is greater than 2, wherein transmission/transmission-halt of corresponding transmitting antennas is controlled, and the number of the transmitting antennas to be used can be adjusted. Consequently, when the signal is received with satisfactory signal properties, such as power and correlation, at the receiving station, the interference to other receiving stations using the same frequency can be reduced by reducing the number of antennas to be used by the repeater. Further, the power consumption of the repeater can be reduced. Further, when the signal is not received with a satisfactory property at the receiving station, the signal property at the receiving station can be improved by increasing the number of antennas to be used by the repeater. 
     Next, a radio communications system according to the ninth embodiment of the present invention is described with reference to  FIGS. 15 through 18 . 
     The radio communications system according to the embodiment includes a transmitting station  400 , a receiving station  500 , and a repeater  600 . 
     First, the transmitting station  400  according to the embodiment is described with reference to  FIG. 15 . 
     The transmitting station  400  according to the embodiment includes transmitting signal generating units  450   1  through  450   M  (M is a positive integer), and a guard interval length control unit  405  to which a notice signal is provided. The notice signal is for a new repeater to become operational and for a working repeater to stop. The guard interval length control unit  405  is connected to the transmitting signal generating units  450   1  through  450   M . 
     The transmitting signal generating unit  450   1  includes a serial/parallel converting unit  401  to which transmit data are input, an inverse Fourier transforming unit  402  connected to the serial/parallel converting unit  401 , a parallel/serial converting unit  403  connected to the inverse Fourier transforming unit  402 , and a guard interval inserting unit  404  connected to the parallel/serial converting unit  403  and the guard interval length control unit  405 , the guard interval inserting unit  404  outputting a transmitting signal. The transmitting signal generating units  450   2  through  450   M  have the same configuration as the transmitting signal generating unit  450   1 . 
     Next, the receiving station  500  according to the embodiment is described with reference to  FIG. 16 . 
     The receiving station  500  according to the embodiment includes 
     received signal processing units  550   1  through  550   N  (N is a positive integer), 
     a received signal property calculating unit  505  to which a received signal is input, the received signal property calculating unit  505  being connected to the received signal processing units  550   1  through  550   N , 
     a repeater operation determining unit  506  for providing the transmitting station and the repeater with the notice signal for a new repeater to become operational and for a working repeater to stop, the repeater operation determining unit S 06  being connected to the received signal property calculating unit  505 , and 
     a guard interval length control unit  507  connected to the repeater operation determining unit  506  and the received signal processing units  550   1  through  550   N . 
     The received signal processing unit  550   1  includes 
     a guard interval removing unit  501  to which the received signal is input, the guard interval removing unit  501  being connected to the guard interval length control unit  507 , 
     a serial/parallel converting unit  502  connected to the guard interval removing unit  501 , 
     a Fourier transforming unit  503  connected to the serial/parallel converting unit  502 , and 
     a parallel/serial converting unit  504  for outputting received data, the parallel/serial converting unit  504  being connected to the Fourier transforming unit  503 . The received signal processing units  550   2  through  550   N  have the same configuration as the received signal processing unit  550   1 . 
     Next, the repeater  600  according to the embodiment is described with reference to  FIG. 17 . 
     The repeater  600  according to the embodiment includes 
     a receiving antenna  601 , 
     a switch  607  to which the notice signal is provided, and that controls transmission/transmission-halt of the repeater, the notice signal being for a new repeater to become operational and for a working repeater to stop, the switch  607  being connected to the receiving antenna  601 , 
     an LNA  602  connected to the switch  607 , 
     an HPA  603  connected to the LNA  602 , 
     a splitter  604  connected to the HPA  603 , and 
     transmitting antennas  605  and  606  connected to the splitter  604 . 
     The radio communications system according to the embodiment is a radio communications system using an OFDM (Orthogonal Frequency Division Multiplexing) method. 
     In addition to the directivity control of the transmitting antennas performed by the repeater  100  described with reference to  FIG. 3 , the repeater  600  according to the ninth embodiment controls transmission/transmission-halt of the repeater itself. The transmitting station  400  and the receiving station  500  control the guard interval length based on control of the repeater  600 . 
     The transmitting station  400  according to the embodiment includes M transmitting antennas. For each of the transmitting antennas, serial/parallel conversion of the transmitting data is carried out by the serial/parallel converting unit  401 , and an inverse Fourier transform is performed by the inverse Fourier transforming unit  402 . Then, the transmitting data are converted into serial data by the parallel/serial converting unit  403 , and a guard interval is added by the guard interval inserting unit  404 . Here, the guard interval length is controlled by the guard interval length control unit  405  based on the notice signal for a new repeater to become operational and for a working repeater to stop. 
     The receiving station  500  according to the embodiment includes N receiving antennas. The portion that corresponds to the guard interval of a received signal is removed by the guard interval removing unit  501 , and the signal received by each receiving antenna is converted into parallel signals by the serial/parallel converting unit  502 . After Fourier transformation is performed by the Fourier transforming unit  503  and the parallel signals are converted into a serial signal by the parallel/serial converting unit  504 , signal detection such as signal separation is performed. 
     Further, as for the receiving station  500 , signal properties, such as received power and correlation between antennas, are calculated by the received signal property calculating unit  505  using the received signal by each receiving antenna. Based on the calculated signal property, the repeater operation determining unit  506  determines transmission or transmission-halt of the signal from the repeater  600 , and signals the determination (i.e., the notice signal for a new repeater to become operational or for a working repeater to stop) to the transmitting station  400  and the repeater  600 . Further, the receiving station  500  controls the guard interval length by the guard interval length control unit  507  based on the notice signal. 
     The repeater  600  according to the embodiment switches between transmission and transmission-halt of the repeater itself using the switch  607  based on the notice signal. 
     Next, an operations process of the radio communications system according to the embodiment is described with reference to  FIG. 18 . 
     The transmitting station  400  and the receiving station  500  communicate through the repeater  600  (Step S 1802 ). 
     The receiving station  500  calculates signal properties, such as received power and correlation between the antennas, by the received signal property calculating unit  505  (Step S 1804 ), and provides a result to the repeater operation determining unit  506 . The repeater operation determining unit  506  determines transmission or transmission-halt of the signal of the repeater  600  based on the signal property of the input signal (Step S 1806 ). 
     For example, the repeater operation determining unit  506  determines to start transmission from a repeater that is not operational, when received power is low, or when the correlation between the antennas is high. On the other hand, the repeater operation determining unit  506  determines to stop transmission from a repeater that is operating when received power is high, or when the correlation between the antennas is small. 
     Next, the notice signal for a new repeater to become operational and for a working repeater to stop is provided to the transmitting station  400  and the repeater  600  through the radio network from the receiving station  500  (Step S 1808  and Step S 1810 ). 
     Then, the repeater operation determining unit  506  provides the notice signal for a new repeater to become operational and for a working repeater to stop to the guard interval length control unit  507 . The guard interval length control unit  507  controls the guard interval length based on the notice signal (Step S 1812 ). For example, the guard interval length control unit  507  sets the guard interval length to the same length as the guard interval length used by the transmitting station  400 . 
     At the transmitting station  400 , after the notice signal is received, the guard interval length is controlled by the guard interval length control unit  405  (Step S 1814 ). When performing transmission from the repeater  600 , since the signal transmitted from the repeater  600  is received after a delay in comparison with the signal transmitted from the transmitting station  400 , the guard interval length control unit  405  extends the guard interval length. The delay is caused by the process in the repeater  600 . If the guard interval length is set up without consideration of the delay, the received signal property at the receiving station  500  is sharply degraded. 
     On the other hand, when stopping transmission from the repeater  600  that is transmitting, the guard interval length control unit  405  controls such that the guard interval length is shortened. Shortening the guard interval length is for raising transmission efficiency when the signal with the delay from the repeater  600  is not received by the receiving station  500 . 
     The repeater  600  switches between transmission and transmission-halt of the signal from the repeater based on the notice signal (Step S 1816 ). When the signal is received by the receiving station  500  with a satisfactory receiving property, the transmission from the repeater  600  is stopped such that the power consumption of the repeater  600  is reduced, and the interference at other receiving stations using the same frequency is reduced. 
     The transmitting station  400  transmits a transmitting signal by the updated guard interval length (Step S 1818 ). 
     Although the embodiment is described for the case wherein the receiving station  500  determines transmission or transmission-halt of the repeater  600  based on the received signal property, a configuration can be such that one of the transmitting station  400  and the repeater  600  determines transmission or transmission-halt of the repeater  600 . 
     When the transmitting station  400  makes the determination, the signal property at the receiving station  500  is determined to be poor based on the number of resending requests for the same packet in a packet communication using ARQ (Automatic Repeat Request), etc. In this case, the notice signal is provided through the radio network to the receiving station  500 , and is provided to the repeater  600  through a wired network or the radio network. 
     In the case of the repeater  600  determining transmission or transmission-halt, the determination is based on the received power of the signal received at the repeater. Here, the control is such that if the received power is high, transmission is stopped, and if the received power is low, transmission is started. In this case, the notice signal is provided to the receiving station through the radio network, and is provided to the transmitting station through the radio network or a wired network. 
     As described above, according to the present embodiment, the transmission directivity and transmission/transmission-halt of the repeater  600  are controlled, based on which the guard interval lengths of the transmitting station  400  and the receiving station  500  are controlled. In this manner, when performing transmission from the repeater  600 , the influence of the loop interference at the repeater can be reduced, and the control of the guard interval length can protect against degradation of the receiving quality at the receiving station. 
     Further, when stopping the transmission from the repeater, transmission efficiency can be improved by shortening the guard interval length, and interference at other receiving stations using the same frequency can be reduced. 
     As described above, according to the embodiment, the influence of the loop interference at the repeater is reduced, and an almost-omni-directional antenna pattern is obtained. 
     ADAPTABILITY TO INDUSTRY 
     The repeating station, the communication apparatus, and the directivity control method according to the present invention are applicable to a mobile communications system. 
     Further, the present invention is not limited to these embodiments, but variations and modifications may be made without departing from the scope of the present invention. 
     The present application is based on Japanese Priority Application No. 2004-198049 filed on Jul. 5, 2004 with the Japanese Patent Office, the entire contents of which are hereby incorporated by reference.