Abstract:
An apparatus comprises first and second radios, an antenna, a four-port combiner connected between each of the first and second radios and the antenna, a complex vector modulator for producing a correction signal in response to a sample signal and an error signal, a first switch for routing the sample signal to the complex vector modulator and for routing the correction signal to one of the first and second radios, and a second switch for routing the error signal to the complex vector modulator.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to radio systems, and more particularly, to such systems in which multiple radios are connected to a common antenna. 
       BACKGROUND OF THE INVENTION 
       [0002]    In communications systems that include multiple transceivers, it is desirable to reduce interference resulting from signals produced by the collocated transceivers. 
         [0003]    In a typical interference canceling system that might be used when two radios are located in near proximity and with limited radio frequency (RF) isolation, each radio operates independently as either a receiver or transmitter without dependency on the operating mode of the other radio. The system should be designed such that operation of one radio does not affect operation of the other. 
         [0004]    When two radios with limited RF isolation are operated such that one radio is transmitting and the other is receiving, an interference canceling system may be employed to cancel the transmitter&#39;s RF signal at the receiver. A sample of the transmitted signal can be routed to an interference canceller via a transfer switch. Then the interference canceller adjusts the amplitude and phase of the sample signal to produce a correction signal. The correction signal is routed to the receiver via the transfer switch. A downstream error signal is extracted and fed back to the interference canceller and used to control the correction signal. The interference canceller adjusts the correction signal to minimize the error signal by causing the correction signal to sum destructively with the portion of the transmitted signal that is received via the receiver system antenna. 
         [0005]    To reduce the size of a multiple transceiver system, it is desirable to connect the transceivers to a common antenna. Thus there is a need for an interference cancellation device in a multiple transceiver system that uses a common antenna. 
       SUMMARY OF THE INVENTION 
       [0006]    The invention provides an apparatus comprising first and second radios, an antenna, a four-port combiner connected between each of the first and second radios and the antenna, a complex vector modulator for producing a correction signal in response to a sample signal and an error signal, a first switch for routing the sample signal to the complex vector modulator and for routing the correction signal to one of the first and second radios, and a second switch for routing the error signal to the complex vector modulator. 
         [0007]    In another aspect, the invention provides an apparatus comprising first and second radios, an antenna, a four-port combiner connected between each of the first and second radios and the antenna, wherein the four-port combiner produces a sample signal, a complex vector modulator for producing a correction signal in response to the sample signal and an error signal, a first switch for routing the correction signal to one of the first and second radios, and a second switch for routing the error signal to the complex vector modulator. 
         [0008]    In yet another aspect, the invention provides an apparatus comprising first and second radios, an antenna, a four-port combiner connected between each of the first and second radios and the antenna, wherein the four-port combiner produces a sample signal, a complex vector modulator for producing a correction signal in response to the sample signal and an error signal, a circulator for routing the correction signal to the four-port combiner, and a switch for routing the error signal to the complex vector modulator. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a schematic diagram of a typical interference canceling system. 
           [0010]      FIG. 2  is a schematic diagram of an interference canceling system constructed in accordance with one embodiment of the invention. 
           [0011]      FIG. 3  is a schematic diagram of an interference canceling system constructed in accordance with another embodiment of the invention. 
           [0012]      FIG. 4  is a schematic diagram of an interference canceling system constructed in accordance with yet another embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0013]      FIG. 1  is a schematic diagram of a typical interference canceling system  10  that might be used when two radios are located in near proximity and with limited radio frequency (RF) isolation. The system includes a first radio  12  and a second radio  14 . The first radio is connected to a first antenna  16  and the second radio is connected to a second antenna  18 . Each radio operates independently as either a receiver or a transmitter without dependency on the operating mode of the other radio. 
         [0014]    In  FIG. 1 , when radio  12  is transmitting and radio  14  is receiving, a directional coupler  20  provides a sample of the transmitted RF signal to the interference canceller  22  via a transfer switch  24 . The directional coupler  20  is limited in coupling value to keep the through line loss to an acceptable value; for example, a 10 dB coupler will have approximately 1 dB through line loss. The interference canceller adjusts the amplitude and phase of the sample signal on line  26  to produce a correction signal on line  28 . The correction signal is routed to the receiver system via the transfer switch and a directional coupler  30 . A downstream error signal is extracted by directional coupler  32  and fed back to the interference canceller through single pole double throw (SPDT) switch  34  to be used to control the correction signal. The closed-loop interference canceller adjusts the correction signal to minimize the error signal by causing the correction signal to sum destructively with the portion of the transmitted signal that is received via the receiver system antenna  18 . When radio  12  is receiving and radio  14  is transmitting, the positions of the SPDT switch and the transfer switch are changed, and directional coupler  36  extracts the error signal that is fed to the interference canceller. 
         [0015]      FIG. 2  is a schematic diagram of an interference canceling system  50  constructed in accordance with one embodiment of the invention.  FIG. 2  shows a system in which the two radios  52 ,  54  are coupled to a common antenna  56  through a four-port combiner  58 . The four-port combiner can be implemented using a hybrid coupler. 
         [0016]    When radio  52  is transmitting and radio  54  is receiving, a directional coupler  60  provides a sample of the transmitted RF signal to a complex vector modulator  62  via a transfer switch  64 . The directional coupler  60  is limited in coupling value to keep the through line loss to an acceptable value; for example, a 10 dB coupler will have approximately 1 dB through line loss. The complex vector modulator adjusts the amplitude and phase of the sample signal on line  66  to produce a correction signal on line  68 . The correction signal is routed to the receiver system via the transfer switch and a directional coupler  70 . A downstream error signal is extracted by directional coupler  72  and fed back to the complex vector modulator through single pole double throw (SPDT) switch  74  to be used to control the correction signal. 
         [0017]    The closed-loop complex vector modulator adjusts the correction signal to minimize the error signal by causing the correction signal to sum destructively with the portion of the transmitted signal that is received via the antenna  56 . The complex vector modulator can be an application-specific integrated circuit complex vector modulator. After decomposing the error signal into complex in-phase and quadrature-phase (I&amp;Q) error components, these components can be used by a closed-loop control system to adjust the amplitudes of the I&amp;Q components of the sample signal to create a correction signal that can minimize the error signal. 
         [0018]    When radio  52  is receiving and radio  54  is transmitting, the positions of the SPDT switch and the transfer switch are changed, and directional coupler  76  extracts the error signal that is fed to the complex vector modulator. Most radios include a push-to-talk (PTT) or ready-to-send (RTS) input signal that causes the radio to go into a transmit mode. These signals could be used to control the positions of the SPDT switch and the transfer switch. If this signal is not available in a particular system, then detection of transmitted power (involving additional equipment) can provide this same information. 
         [0019]    Generally the system is not limited to two radios. But some system-level advantages disappear with greater numbers. For example, if there are three radios (two transmit, one receive) in some combiner arrangements, a single error signal has the potential to have samples of two transmitters on the same port. To use interference cancellation, the two transmitting signals would have to be separated and applied to an interference canceller independently of each. 
         [0020]    The four-port combiner is a four-port device that includes ports  80 ,  82 ,  84  and  86 . The termination (shown as resistor  78 ) on port  86  of the four-port combiner absorbs half of the power of each of the transmitted signals. Various types of hybrid couplers can be used as the four-port combiner, such as 0°, 90° or 180° couplers. Other system factors may drive the choice. 
         [0021]    The coupler can be a commercially available device. Port names vary by manufacturer. Furthermore, the port names also vary by the type of coupler (0° vs. 90° vs. 180°). However, there are generally two classes of terminals. Terminals  80  and  82  can be generically designated as inputs, and terminals  84  and  86  can be designated as outputs. However, one skilled in the art will know that four-port devices are symmetrical. Therefore, outputs can be inputs, and inputs can be outputs. 
         [0022]    The interference canceling system of the invention is used to minimize the error signal by causing the correction signal to sum destructively with the portion of the transmitted signal that is received via the receiver system antenna. In the configuration of  FIG. 2 , the interfering signal comes from the fact that the antenna reflects a portion of the transmitted signal. 
         [0023]      FIG. 3  is a schematic diagram of an interference canceling system  90  constructed in accordance with another embodiment of the invention.  FIG. 3  shows an improvement to the system in  FIG. 2 . In the system of  FIG. 3  two radios,  92  and  94 , are coupled to a common antenna  96  through a four-port combiner  98 . When radio  92  is transmitting and radio  94  is receiving, a sample of the transmitted RF is extracted at a port or terminal  100  of the four-port combiner and fed to the complex vector modulator  102  on line  104 . The complex vector modulator adjusts the amplitude and phase of the sample signal on line  104  to produce a correction signal on line  106 . The correction signal is routed to the receiver system, via an SPDT switch  108 , and a directional coupler  110 . A downstream error signal is extracted by directional coupler  112  and fed back to the complex vector modulator through single pole double throw (SPDT) switch  114  to be used to control the correction signal. The closed-loop complex vector modulator adjusts the correction signal to minimize the error signal by causing the correction signal to sum destructively with the portion of the transmitted signal that is reflected at the antenna  96 . When radio  92  is receiving and radio  94  is transmitting, the positions of the SPDT switches are changed, the directional coupler  116  extracts the error signal that is fed to the complex vector modulator, and the directional coupler  118  is used to feed the correction signal to the radio  92 . 
         [0024]    The four-port combiner is a four-port device that includes ports  120 ,  122 ,  124  and  100 . In the configuration of  FIG. 3 , the sample signal is taken from port  100  of the four-port combiner. This allows the transmit sample to be approximately equal to the transmitted signal at the antenna, which is far better than that available using the directional coupler. In any system in which two radios are combined on a single antenna, it is inescapable that only half of the transmitted power arrives at the antenna and the other half is generally dissipated in a termination port of the coupler as shown in  FIG. 2 . Additionally the system of  FIG. 3  is somewhat reduced in complexity by swapping the transfer switch for a second SPDT switch. 
         [0025]      FIG. 4  is a schematic diagram of an interference canceling system  130  constructed in accordance with another embodiment of the invention.  FIG. 4  shows a further improvement to the system in  FIG. 3 .  FIG. 4  shows a system in which the two radios  132 ,  134  are coupled to a common antenna  136  through a four-port combiner  138 . When radio  132  is transmitting and radio  134  is receiving, a sample of the transmitted RF is extracted at the fourth terminal  140  of the four-port combiner and fed to the complex vector modulator  142  on line  144  (through circulator  148 ). The complex vector modulator adjusts the amplitude and phase of the sample signal on line  156  to produce a correction signal on line  146 . The correction signal is fed back to the four-port combiner via a circulator  148 . A downstream error signal is extracted by directional coupler  150  and fed back to the complex vector modulator through single pole double throw (SPDT) switch  152  to be used to control the correction signal. The closed-loop complex vector modulator adjusts the correction signal to minimize the error signal by causing the correction signal to sum destructively with the portion of the transmitted signal that is reflected at the antenna  136 . When radio  132  is receiving and radio  134  is transmitting, the position of the SPDT switch is changed, and the directional coupler  154  extracts the error signal that is fed to the complex vector modulator. The circulator directs the sample signal from line  144  to line  156  and directs the correction signal from line  146  to line  144 . 
         [0026]    The four-port combiner is a four-port device that includes ports  160 ,  162 ,  164  and  140 . In the configuration of  FIG. 3 , the sample signal is taken from port  100  of the four-port combiner. In  FIG. 4 , a circulator is used to feed the correction signal back into the four-port combiner. The complexity of the system is further reduced by eliminating two directional couplers (and the attendant through line loss) and one RF switch, but adding only the circulator.  FIG. 4  additionally exploits the reciprocal nature of the four-port coupler to use it as a means of injecting the correction signal, further eliminating two directional couplers. 
         [0027]    In addition to the reduced number of components, the systems in  FIGS. 3 and 4  utilize RF power that is generally thrown away as heat for the benefit of having more sample power to use for signal cancellation and, as in  FIG. 4 , eliminating the sample couplers. 
         [0028]    The objects in the dashed box  158  in  FIG. 4  create a useful device in the form of an active RF termination. Using an error/control signal developed externally, the active RF termination is able to produce a load with a reflection coefficient at any phase. This active RF termination can be used, as in this case, to minimize a reflected signal or to cause the reflected signal to have specific amplitude and phase characteristics. 
         [0029]    While the invention has been described in terms of several embodiments, it will be apparent to those skilled in the art that various changes can be made to the described embodiments without departing from the scope of the invention as set forth in the following claims.