Abstract:
An active cancellation unit is disclosed for improving the noise cancellation between a transmitter and a receiver which are connected to an antenna using a duplexer, the unit comprising a coupler sampling a signal to transmit provided by the transmitter, a cancellation duplexer having characteristics similar to the duplexer and receiving the sampled signal to provide a simulated signal and an active component receiving the simulated signal and providing an amplified signal having a phase 180 degree shifted with respect to the simulated signal; and a coupler for injecting the simulated signal at the receiver.

Description:
CROSS-REFERENCE AND RELATED APPLICATIONS  
       [0001]     This patent application claims priority of U.S. Provisional Patent Application No. 60/671,100 entitled “Balanced Active and Passive Duplexers” that was filed Apr. 14, 2005, the specification of which is hereby incorporated by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The invention relates to telecommunications. In particular, the invention pertains to an active balanced duplexer.  
       BACKGROUND OF THE ART  
       [0003]      FIG. 1  shows a prior art active signal cancellation loop  8  added to a filter-based duplexer  12  to improve isolation between a transmitter TX and a receiver RX. More precisely, the cancellation loop  8  comprises a first coupler  10 , a unit  14  comprising a variable attenuator and a phase shifter, and a second coupler  16 .  
         [0004]     The first coupler  10  samples a signal generated by the transmitter TX. The variable attenuator controls the amplitude of the sampled signal generated to provide a signal which is then shifted in phase using the phase shifter. The resulting signal shifted in phase is then injected at the receiver using the second coupler  16 . The skilled addressee will appreciate that as the phase is shifted by 180 degrees, the second coupler  16  acts as a subtraction unit which removes a signal leaking from the transmitter TX to the receiver RX.  
         [0005]     The skilled addressee will appreciate that in order to cancel the signal leaking from the transmitter TX to the receiver RX, it is necessary that the resulting signal shifted in phase and injected by the second coupler  16  has an amplitude equal to the amplitude of the signal leaking and a phase shifted by 180 degrees. Unfortunately, the skilled addressee will appreciate that such condition is only achieved for a given frequency with the apparatus disclosed in  FIG. 1 . This topology is therefore inherently narrow band and is highly sensitive to the phase response of the duplexer path. The phase response is particularly problematic in the case of SAW filters. Therefore, if the topology of  FIG. 1  is used, the transmitter TX receiver RX isolation is expected to be improved over a very narrow band. Such improvement may be significant but would have to be adjusted dynamically through the control of the variable attenuator and the phase shifter by the cell phone to place the cancellation null at the user&#39;s exact frequency in the case where the transmitter and the receiver are embedded in a cell phone. Such type of improvement has already been demonstrated with other filters and is reproduced here using a commercial duplexer (part number 856356 from SawTek). It has also been contemplated that if the antenna to which the transmitter and the receiver are connected is not well adapted, the cancellation loop  8  has only a limited effect on the signal leaking from the transmitter to the receiver.  
         [0006]     The results are shown in  FIGS. 9   a ,  9   b ,  10   a  and  10   b . As illustrated,  FIGS. 9   a ,  9   b ,  10   a  and  10   b  show that a high level of isolation may be achieved over a narrow bandwidth, which is typically around 1-2 MHz. It will be appreciated that this cancellation may be tuned to the desired frequency by adjusting mainly the phase shifter. While this topology is simple, the access to active tuning and the adaptive control of the tuning require the interface to the digital processing power of the receiver.  
         [0007]     Referring to  FIG. 2 , there is shown another embodiment of a prior art active signal cancellation loop  17  added to a circulator-based duplexer  24  to improve isolation between a transmitter TX and a receiver RX. More precisely, the cancellation loop  17  comprises a first coupler  18 , a unit  17  comprising a variable attenuator and a phase shifter, and a second coupler  22 .  
         [0008]     The first coupler  18  samples a signal generated by the transmitter TX. The variable attenuator controls the amplitude of the sampled signal generated to provide a signal which is shifted in phase using the phase shifter. The resulting signal shifted in phase is injected at the receiver using the second coupler  22 . The skilled addressee will appreciate that as the phase is shifted by 180 degrees, the second coupler  22  acts as a subtraction unit which removes a signal leaking from the transmitter TX to the receiver RX. The skilled addressee will appreciate that the circulator-based duplexer  24  provides a less important transmitter receiver isolation which therefore cause a greater signal leaking. The cancellation loop  17  therefore brings an improvement while still suffering from the same drawbacks outlined in the embodiment disclosed in  FIG. 1 .  
         [0009]     There is a need for a method and apparatus that will overcome at least one of the above-identified drawbacks.  
         [0010]     Features of the invention will be apparent from review of the disclosure, drawings and description of the invention below.  
       DISCLOSURE OF THE INVENTION  
       [0011]     According to a first aspect of the invention, there is provided an active cancellation unit for improving the noise cancellation between a transmitter and a receiver which are connected to an antenna using a duplexer, the unit comprising a coupler sampling a signal to transmit provided by the transmitter, a cancellation duplexer having characteristics similar to the duplexer and receiving the sampled signal to provide a simulated signal, an active component receiving the simulated signal and providing an amplified signal having a phase 180 degree shifted with respect to the simulated signal and a coupler for injecting said simulated signal at the receiver.  
         [0012]     According to a second aspect of the invention, there is provided a new architecture for purely passive signal splitting and combining for improving the noise cancellation between a transmitter and a receiver, the unit comprising a 90° hybrid coupler to equally divide the transmit signal, the divided transmit signal is fed to two duplexers having similar characteristics, the divided transmit signals emerging from the antenna ports of the two duplexers are summed through a 90° hybrid coupler reconstructing the original transmit signal and fed to the antenna, the divided transmit signals leaking through the two duplexers to the receiver are subtracted through a 90° hybrid coupler canceling the said leaking signal at the receiver.  
         [0013]     According to a further aspect of the invention, there is provided a balanced passive duplexer 
     
    
     DESCRIPTION OF TEE DRAWINGS  
       [0014]     In order that the invention may be readily understood, embodiments of the invention are illustrated by way of example in the accompanying drawings.  
         [0015]      FIG. 1  is an electrical schematic which shows a first prior art embodiment of a signal cancellation loop added to a filter-based duplexer to improve TX-RX isolation.  
         [0016]      FIG. 2  is an electrical schematic which shows a second prior art embodiment of a signal cancellation loop added to a circulator-based duplexer to improve TX-RX isolation.  
         [0017]      FIG. 3  is an electrical schematic which shows a signal cancellation loop added to a filter-based duplexer to improve TX-RX isolation according to a first embodiment of the invention.  
         [0018]      FIG. 4  is an electrical schematic which shows a signal cancellation loop added to a circulator-based duplexer to improve TX-RX isolation according to a second embodiment of the invention.  
         [0019]      FIG. 5  is an electrical schematic which shows a signal cancellation loop added to a filter-based duplexer to improve TX-RX isolation according to a third embodiment of the invention.  
         [0020]      FIG. 6  is an electrical schematic which shows a signal cancellation loop added to a circulator-based duplexer to improve TX-RX isolation according to a fourth embodiment of the invention.  
         [0021]      FIG. 7  is an electrical schematic which shows a signal cancellation loop added to a filter-based duplexer to improve TX-RX isolation according to a fifth embodiment of the invention.  
         [0022]      FIG. 8  is a block diagram which shows a signal cancellation architecture used with filter-based or circulator based duplexers to improve TX-RX isolation according to a sixth embodiment of the invention.  
         [0023]      FIG. 9   a  is a graph which shows the level TX-RX isolation improvement that can be obtained using prior art on a real circuit having a commercial part as the filter-based duplexer. This figure shows the improvement when the cancellation loop controls are optimized to minimize the noise of the transmitter in the receiver&#39;s band. It also shows the limited bandwidth over which improvement can be made. The grey areas in this figure identify the transmit and receive bandwidths considering cellular telephony frequencies.  
         [0024]      FIG. 9   b  is a graph which shows the same results as those of  FIG. 9   a  in a zoomed-in view on the RX bandwidth. The grey areas in this figure identify the transmit and receive bandwidths considering cellular telephony frequencies.  
         [0025]      FIG. 10   a  is a graph which shows the level of TX-RX isolation improvement that can be obtained using prior art. This figure shows the improvement when the cancellation loop controls are optimized to minimize the noise of the transmitter in the its own band. It also shows the limited bandwidth over which improvement can be made. The grey areas in this figure identify the transmit and receive bandwidths considering cellular telephony frequencies.  
         [0026]      FIG. 10   b  is a graph which shows the same results as those of  FIG. 10   a  in a zoomed-in view on the TX bandwidth. The grey areas in this figure identify the transmit and receive bandwidths considering cellular telephony frequencies.  
         [0027]      FIG. 11   a  is a graph which shows the level TX-RX isolation improvement that can be obtained using a circuit that implements the first embodiment of the invention. This figure shows the improvement when the cancellation loop controls are optimized to minimize the noise of the transmitter in the receiver&#39;s band. It also shows the wide bandwidth over which improvement can be made. The grey areas in this figure identify the transmit and receive bandwidths considering cellular telephony frequencies.  
         [0028]      FIG. 11   b  is a graph which shows the same results as those of  FIG. 11   a  in a zoomed-in view on the X bandwidth. The grey areas in this figure identify the transmit and receive bandwidths considering cellular telephony frequencies.  
         [0029]      FIG. 12a  is a graph which shows the level TX-RX isolation improvement that can be obtained using a circuit that implements the first embodiment of the invention. This figure shows the improvement when the cancellation loop controls are optimized to minimize the noise of the transmitter in its own band. It also shows the wide bandwidth over which improvement can be made. The grey areas in this figure identify the transmit and receive bandwidths considering cellular telephony frequencies.  
         [0030]      FIG. 12   b  is a graph which shows the same results as those of  FIG. 12   a  in a zoomed-in view on the TX bandwidth. The grey areas in this figure identify the transmit and receive bandwidths considering cellular telephony frequencies.  
         [0031]      FIGS. 13   a ,  13   b  and  13   c  are graphs which show the level of TX-RX isolation improvement that can be obtained by using a circuit that implements the sixth embodiment of the invention. These figures show that the improvement is achieved for the TX and RX bands simultaneously without any adjusted controls.  
         [0032]     Further details of the invention and its advantages will be apparent from the detailed description included below.  
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0033]     In the following description of the embodiments, references to the accompanying drawings are by way of illustration of an example by which the invention may be practiced. It will be understood that other embodiments may be made without departing from the scope of the invention disclosed.  
         [0034]     In order to combat the narrow band nature of the cancellation obtained with the prior art configurations shown in  FIG. 1  and  FIG. 2 , new balanced topologies are proposed.  
         [0035]     Referring to  FIG. 3 , there is shown a first embodiment of a cancellation loop  29 .  
         [0036]     The cancellation loop  29  comprises a first coupler  30 , a filter-based duplexer  34 , an active component  36  and a second coupler  38 .  
         [0037]     The active component  36  comprises a variable attenuator, a variable phase shifter and an amplifier.  
         [0038]     In this embodiment, the same filter-based duplexer is used in both branches. It will be appreciated that while not exactly identical, the two filter-based duplexers should preferably have comparable phase responses in order to make the cancellation loops more wideband. It will be appreciated that the use of the filter-based duplexer  34  in the cancellation loop  29  implies that an amplifier is preferably required to compensate for all losses in the cancellation loop  29 .  
         [0039]     It will be appreciated that such amplifier should have a low P1 dB (i.e. Output power at 1 dB gain compression). For the configuration used in the tests, the amplifier gain is set to 30 dB and the required P1 dB is less than 0 dBm.  
         [0040]     It will be noted that if an all pass saw filter could be designed that would produce the same phase response as the filter-based duplexer, the cancellation bandwidth could be increased without the need for an amplifier in the cancellation loop  29 .  
         [0041]      FIG. 3  shows the topology of the balanced cancellation loop  29  using the filter-based duplexer  34 . A prototype based on this topology was assembled. The prototype was fully characterized and the results in each case are compared to those of the duplexer-only. These results are shown in  FIGS. 11   a ,  11   b ,  12   a  and  12   b . The skilled addressee will appreciate that these results show an improved isolation over a much larger bandwidth at a cost of a slight increase in insertion loss.  
         [0042]     It will be appreciated that the performance improvements obtained have still been limited by the fact that the filter-based duplexers are not identical and that the design is in MIC technology. Removing these limitations may further enable a greater performance improvement. Furthermore, no effort was made to compensate the additional delay due to the amplifier. In light of these results, a controlled SAW/MMIC, MMIC only or MHMIC-only design should certainly yield a filter-based duplexer with substantially improved isolation. The insertion losses will increase but this may be minimized by proper choice and design of the couplers  30  and  38 . The skilled addressee will appreciate that the topology disclosed in  FIG. 3  is referred to as a “balanced” cancellation loop as the cancellation loop  29  comprises a duplexer.  
         [0043]     Now referring to  FIG. 4 , there is shown a balanced cancellation loop  39  according to a second embodiment of the invention. The balanced cancellation loop  39  is used for canceling the signal leaking from a transmitter to a receiver. The transmitter is connected to the receiver and the antenna using a circulator-based duplexer  42 .  
         [0044]     The balanced cancellation loop  39  comprises a first coupler  40 , a circulator-based duplexer  44 , an active unit  46  and a second coupler  48 .  
         [0045]     The active unit  46  comprises a variable attenuator, a variable phase shifter and an amplifier.  
         [0046]     Now referring to  FIG. 5 , there is shown a balanced cancellation loop  49  according to a third embodiment of the invention. The balanced cancellation loop  49  is used for canceling the signal leaking from a transmitter to a receiver. The transmitter is connected to the receiver and the antenna using a filter-based duplexer  52 .  
         [0047]     The balanced cancellation loop  49  comprises a first coupler  50 , a filter-based duplexer  54 , an active unit  56  and a second coupler  58 . The filter-based duplexer  54  is further connected to an impedance tuner  55 . The impedance tuner  55  simulates the behavior of the antenna to which the filter-based duplexer  52  is connected. Having the impedance tuner  55  enables to address the problem raised when the antenna is not properly impedance matched.  
         [0048]     The active unit  56  comprises a variable attenuator, a variable phase shifter and an amplifier.  
         [0049]     Now referring to  FIG. 6 , there is shown a balanced cancellation loop  59  according to a fourth embodiment of the invention. The balanced cancellation loop  59  is used for canceling the signal leaking from a transmitter to a receiver. The transmitter is connected to the receiver and the antenna using a circulator-based duplexer  62 .  
         [0050]     The balanced cancellation loop  59  comprises a first coupler  60 , a circulator-based duplexer  64 , an active unit  66  and a second coupler  68 . The circulator-based duplexer  64  is further connected to an impedance tuner  65 . The impedance tuner  65  simulates the behavior of the antenna to which the circulator-based duplexer  62  is connected.  
         [0051]     The active unit  66  comprises a variable attenuator, a variable phase shifter and an amplifier.  
         [0052]     Now referring to  FIG. 7 , there is shown a balanced cancellation loop  69  according to a fifth embodiment of the invention. The balanced cancellation loop  69  is used for canceling the signal leaking from a transmitter to a receiver. The transmitter is connected to the receiver and the antenna using a filter-based duplexer  72 .  
         [0053]     The balanced cancellation loop  69  comprises a first coupler  70 , a circulator-based duplexer  74 , an active unit  78  and a second coupler  80 . The circulator-based duplexer  74  is further connected to an impedance tuner  75 . The impedance tuner  75  simulates the behavior of the antenna to which the filter-based duplexer  72  is connected.  
         [0054]     The active unit  78  comprises a variable attenuator, a variable phase shifter and an amplifier.  
         [0055]     Referring to  FIG. 8 , there is shown another embodiment of a balanced duplexer.  
         [0056]     In this embodiment, a signal to transmit is generated by the transmitter  86 . The generated signal is then received by a divider  88 . The divider  88  is a 90° hybrid couplers that divides by 2 an incoming signal. It will be appreciated that one of the divided signal outputted by the divider  88  is further shifted in phase by an amount of 90 degrees with respect to the other divided signal.  
         [0057]     One of the divided signals outputted by the divider  88  is provided to a first duplexer  90  while the other divided signal outputted by the divider  88  is provided to a second duplexer  92 . It will be appreciated that the first duplexer  90  and the second duplexer  92  are preferably the same. It will be appreciated that optional filters may be provided after the duplexers  90  and  92  as well as before  88  and after  96 .  
         [0058]     A first combiner/divider  94  is another 90° hybrid coupler, which is used to combine, i.e. adds, the signals outputted by both duplexers  90  and  92  to be fed to the antenna  98 .  
         [0059]     A second combiner  96 , a third 90° hybrid coupler, also receives signals provided by the first and the second duplexers  90  and  92 . The second combiner  96  is adapted to subtract both signals canceling any leaking signal.  
         [0060]     In the case of a signal received by the antenna  98 , the 90° hybrid coupler  94  separates the received signal into two signals, one of which is provided to the first duplexer  90  while the other is provided to the second duplexer  92 . It will be appreciated that one of the divided received signal outputted by the hybrid coupler  94  is phased shifted an amount of 90 degrees with respect to the other divided signal. The combiner  96  then adds both signals outputted by the first duplexer  90  and the second duplexer  92  and feeds the summed received signal to the receiver.  
         [0061]     It will be appreciated that in the case where both duplexers are circulator-based duplexers, it is possible to add filters in order to enhance performance.  
         [0062]     It will be appreciated that using hybrid couplers leads to improved impedance matching at the three ports (TX, RX and ANT), It will be further appreciated by those skilled in the field that the use of the hybrid coupler in the transmitter path ( 88 ) coupled with the use of a dummy load at its forth port can replace the use of an isolator component at the output of the transmitter to protect the transmitter&#39;s power amplifier, thus resulting in dual function (isolator/duplexer) and system cost savings.  
         [0063]     It will be appreciated that the topology of the present embodiment of the invention enables that desired signals are summed, i.e. added constructively, while undesired leakage signals are subtracted, i.e. added destructively, without the need for any control elements. This topology also insures virtually no degradation on insertion loss in transmitter to antenna and antenna to receiver paths.  
         [0064]     While illustrated in the block diagrams as groups of discrete components communicating with each other via distinct data signal connections, it will be understood by those skilled in the art that the preferred embodiments are provided by a combination of hardware and software components, with some components being implemented by a given function or operation of a hardware or software system, and many of the data paths illustrated being implemented by data communication within a computer application or operating system. The structure illustrated is thus provided for efficiency of teaching the present preferred embodiment.  
         [0065]     It should be noted that the present invention can be carried out as a method, can be embodied in a system, a computer readable medium or an electrical or electro-magnetical signal.  
         [0066]     Although the above description relates to a specific preferred embodiment as presently contemplated by the inventors, it will be understood that the invention in its broad aspect includes mechanical and functional equivalents of the elements described herein.