Patent Publication Number: US-11025287-B1

Title: Interference cancellation system

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS 
     This application is a divisional of U.S. application Ser. No. 16/130,344, filed Sep. 13, 2018, which is hereby incorporated by reference herein in its entirety. 
    
    
     GOVERNMENT LICENSE RIGHTS 
     This invention was made with government support under DOTC-16-01/W15QKN-14-9-1001 and DOTC-16-01-INIT0266-RC-01 awarded by the Department of Defense Ordnance Technology Consortium. The government has certain rights in the invention. 
    
    
     BACKGROUND 
     Radio frequency (RF) communication systems can be utilized in various environments. Interferences may occur in such RF systems and may interrupt, obstruct, or otherwise degrade or limit the effective performance of the communication. Various interferences can occur in an RF communication system such as, for example, co-site interferences, self-interferences, self-network interferences, and intentional interferences (or sometimes referred to as jammer interferences). 
     SUMMARY 
     In one aspect, embodiments of the inventive concepts disclosed herein are directed to a system for cancelling interference. The system may include a first antenna and a second antenna spatially separated from the first antenna. The system may include a first time delay unit, coupled to the first antenna, and configured to apply a first time delay and first power gain on a first signal received by the first antenna. The system may include a control circuit, coupled to the first time delay unit, and configured to determine the first time delay and first power gain to cause a modified version of the first signal and a second signal, received by the second antenna, to be aligned in time and power levels. 
     In another aspect, embodiments of the inventive concepts disclosed herein are directed to a system for automatically cancelling interference. The system may include a first antenna and a second antenna spatially separated from the first antenna. The system may include a first time delay unit configured to apply a first time delay and first power gain on a first signal received by the first antenna to provide a modified version of the first signal. The system may include a second time delay unit configured to apply a second time delay and second power gain on a second signal received by the second antenna to provide a modified version of the second signal. The system may include a first subtractor configured to subtract respective power levels of the modified version of the first signal and the modified version of the second signal to provide a first output signal. The system may include a first control circuit configured to determine the first and second time delays and first and second power gains based on a power level of the first output signal to cause the modified version of the first signal and the modified version of the second signal to be aligned in time and power levels. 
     In yet another aspect, embodiments of the inventive concepts disclosed herein are directed to a system for automatically cancelling interference. The system may include a first antenna and a second antenna spatially separated from the first antenna. The system may include a first time delay unit configured to apply a first time delay and first power gain on a first signal received by the first antenna to provide a first modified version of the first signal. The system may include a second time delay unit configured to apply a second time delay and second power gain on a second signal received by the second antenna to provide a first modified version of the second signal. The system may include a third time delay unit configured to apply a third time delay and third power gain on the first signal received by the first antenna to provide a second modified version of the first signal. The system may include a fourth time delay unit configured to apply a fourth time delay and fourth power gain on the second signal received by the second antenna to provide a second modified version of the second signal. The system may include a first subtractor configured to subtract respective power levels of the first modified version of the first signal and the first modified version of the second signal to provide a first output signal. The system may include a first control circuit configured to determine the first and second time delays and first and second power gains based on a power level of the first output signal to cause the first modified version of the first signal and the first modified version of the second signal to be aligned in time and power levels. The system may include a second subtractor configured to subtract respective power levels of the second modified version of the first signal and the second modified version of the second signal to provide a second output signal. The system may include a second control circuit configured to determine the third and fourth time delays and third and fourth power gains based on a power level of the second output signal to cause the second modified version of the first signal and the second modified version of the second signal to be aligned in time and power levels. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Implementations of the inventive concepts disclosed herein may be better understood when consideration is given to the following detailed description thereof. Such description makes reference to the included drawings, which are not necessarily to scale, and in which some features may be exaggerated and some features may be omitted or may be represented schematically in the interest of clarity. Like reference numerals in the drawings may represent and refer to the same or similar element, feature, or function. In the drawings: 
         FIG. 1A  is a block diagram of an RF communication system, in accordance with some embodiments of the inventive concepts disclosed herein; 
         FIG. 1B  is a block diagram of an RF communication system, in accordance with some embodiments of the inventive concepts disclosed herein; 
         FIG. 1C  is a block diagram of an RF communication system, in accordance with some embodiments of the inventive concepts disclosed herein; 
         FIG. 1D  is a block diagram of a mutual coupling mitigation circuit of the RF communication system of  FIG. 1C , in accordance with some embodiments of the inventive concepts disclosed herein. 
         FIG. 2  shows a block diagram of an RF communication system, in accordance with some embodiments of the inventive concepts disclosed herein; 
         FIG. 3  shows a block diagram of an RF communication system, in accordance with some embodiments of the inventive concepts disclosed herein; 
         FIG. 4  shows a block diagram of an RF communication system, in accordance with some embodiments of the inventive concepts disclosed herein; 
         FIGS. 5A and 5B  collectively show a block diagram of an RF communication system, in accordance with some embodiments of the inventive concepts disclosed herein; 
         FIG. 6  shows a flow chart of an exemplary method to operate the RF communication systems of  FIGS. 1A-D , in accordance with some embodiments of the inventive concepts disclosed herein; 
         FIG. 7  shows a flow chart of an exemplary method to operate the RF communication systems of  FIG. 2 , in accordance with some embodiments of the inventive concepts disclosed herein; 
         FIG. 8  shows a flow chart of an exemplary method to operate the RF communication systems of  FIG. 3 , in accordance with some embodiments of the inventive concepts disclosed herein; 
         FIG. 9A  shows a symbolic diagram of nulls, in accordance with some embodiments of the inventive concepts disclosed herein; 
         FIG. 9B  shows an antenna polar plot corresponding to the symbolic diagram of  FIG. 9A , in accordance with some embodiments of the inventive concepts disclosed herein; 
         FIG. 10  shows various symbolic diagrams and corresponding antenna polar plots of nulls and/or aliased nulls, in accordance with some embodiments of the inventive concepts disclosed herein; and 
         FIGS. 11A and 11B  show symbolic diagrams of overlapped and non-overlapped nulls, respectively, in accordance with some embodiments of the inventive concepts disclosed herein 
     
    
    
     DETAILED DESCRIPTION 
     Before describing in detail embodiments of the inventive concepts disclosed herein, it should be observed that the inventive concepts disclosed herein include, but are not limited to a novel structural combination of components and circuits, and not to the particular detailed configurations thereof. Accordingly, the structure, methods, functions, control and arrangement of components and circuits have, for the most part, been illustrated in the drawings by readily understandable block representations and schematic diagrams, in order not to obscure the disclosure with structural details which will be readily apparent to those skilled in the art, having the benefit of the description herein. Further, the inventive concepts disclosed herein are not limited to the particular embodiments depicted in the schematic diagrams, but should be construed in accordance with the language in the claims. 
     The present disclosure provides various embodiments of systems and methods to cancel the above-mentioned interferences. In some embodiments, an RF communication system and a method to operate the same are disclosed. The RF communication system can include a pair of antennas. At least a first one of the pair of antennas can be coupled by a time delay unit that can apply dynamically configurable, adjustable, or determined time delays and power gains on a signal received by the first antenna. A second one of the pair of antennas can be coupled by no such a time delay, another time delay unit that can also apply dynamically configurable, adjustable, or determined time delays and power gains on a signal received by the second antenna, or yet another time delay unit that can apply a fixed time delay and power gain on the signal received by the second antenna. In some embodiments, the dynamically configurable, adjustable, or determined time delays and power gains, respectively applied on the signals received by first and second antennas, can be determined by a control circuit through monitoring or measuring whether such modified (e.g., delayed in time and amplified in power) signals can be aligned in time and power levels. In response to determining that the two modified signals are aligned in time and power levels, the RF communication system can provide one or more nulls against an interference based on the time delays and power gains respectively applied on the signals received by the first and second antennas. 
     For purposes of reading the description of the various embodiments below, the following descriptions of the sections of the specification and their respective contents may be helpful: 
     Section A describes various embodiments of an RF communication system. 
     Section B describes exemplary methods to respectively operate the RF communication systems described in Section A. 
     Section C describes nulls and/or aliased nulls generated by the RF communications systems described in Section A. 
     A. RF Communication System 
     Referring to  FIG. 1A , depicted is a functional block diagram of an RF communication system  100 . As shown in  FIG. 1A , the RF communication system  100  includes a first antenna  102 , a second antenna  104 , a first time delay unit (TDU)  106 , a second TDU  106 , a combiner  110 , a control circuit  112 , and an RF front end  114 . The components  102 - 114  shown in the illustrated embodiment of  FIG. 1A  may constitute a portion of the RF communication system  100 , which can further include any of various other RF communication components such as, for example, one or more digital baseband processing subsystems, one or more digital-to-analog processing subsystems, one or more transmitting subsystems, etc., while remaining within the scope of the present disclosure. Such subsystems shall be discussed with respect to  FIGS. 5A-B . 
     In some embodiments, the TDU  106  can be coupled to the antenna  102 , and the TDU  108  can be coupled to the antenna  104 . The antennas  102  and  104 , physically separated apart from each other by a distance  115 , can receive one or more RF signals e.g., signal  116 . Depending on the directions along which the antennas  102  and  104  receive the one or more RF signals  116 , signals  118  and  120  respectively received through or by the antennas  102  and  104  (sometimes respectively referred to as “received signal  118 ” and “received signal  120 ”) can be different, e.g., presenting a phase difference therebetween. Such a phase difference may be associated with a propagation delay between the antennas  102  and  104 . The TDU  106  can apply dynamically configurable, adjustable, or determined time delays and power gains on the received signal  118  to provide a modified version of the received signals  118  (sometimes respectively referred to as “modified signal  122 ”); and the TDU  108  can apply dynamically configurable, adjustable, or determined time delays and power gains on the received signal  120  to provide a modified version of the received signals  120  (sometimes respectively referred to as “modified signal  122 ”). 
     Each of the TDUs  106  and  108  can include one or more components to apply the time delay and power gain on the received signal. In some embodiments, the TDU can include one or more true time delay (TTD) devices/units, at least one of which can provide an adjustable, a variable, or programmable time delay, and one or more amplifiers or attenuators, at least one of which can provide an adjustable, a variable, or programmable power gain, or gain. As shall be discussed below, respective values of such adjustable time delays and power gains may be determined by the control circuit  112 . 
     The combiner  110  is coupled to the TDUs  106  and  108  to receive the modified signals  122  and  124 . In some embodiments, the combiner  110  can be a 180° hybrid combiner, which can perform a subtraction function on the modified signals  122  and  124 . In response to receiving the modified signals  122  and  124 , the combiner  110  can perform a subtraction function on the modified signals  122  and  124  to provide an output signal  126  to the control circuit  112  and RF front end  114 . In the case where the modified signals  122  and  124  are aligned in time and power levels (by the TDUs  106  and  108 , respectively), the combiner  110  can provide the output signal  126  as one or more nulls, which may be used to minimize or eliminate interferences that can be included within the one or more RF signals  116 . Such a null shall be discussed in further detail below with respect to  FIGS. 9A-B . 
     The control circuit  112  can use the output signal  126 , received from the combiner  110 , to determine the time delay and power gain that the TDU  106  applies on the received signal  118 , and provide the determined time delay and power gain to the TDU  106 . Similarly, the control circuit  112  can use the output signal  126 , received from the combiner  110 , to determine the time delay and power gain that the TDU  108  applies on the received signal  118 , and provide the determined time delay and power gain to the TDU  108 . 
     In some embodiments, the control circuit  112  can determine a power level of the output signal  126 , and based on the power level of the output signal  126  to determine the time delays and power gains respectively used by the TDUs  106  and  108 . For example, the control circuit  112  can iteratively update the time delays and/or power gains that the TDUs  106  and  108  respectively use on the received signals  106  and  108  based on the power level of the output signal  126 . In some embodiments, the control circuit  112  can determine the respective time delays and power gains respectively used by the TDUs  106  and  108  to cause the combiner  110  to provide the output signal  126  as one or more nulls, responsive to determining that the corresponding power level of the output signal  126  is a minimum over plural iterations of updating the time delays and/or power gains (used by the TDUs  106  and  108 , respectively). Such a null shall be discussed in further detail below with respect to  FIGS. 9A-B . 
     The RF front end  114  can process the output signal  126  to generate one or more digital signals to be further processed by one or more other subsystems of the RF communication system  100 . In some embodiments, the RF front end  114  can include one or more components to collectively perform at least a function to convert the output signal from an analog domain to a digital domain. As such, the RF front end  114  can be referred to as an analog-to-digital portion, or RF-to-baseband portion, of the RF communication system  100 . For example, the RF front end  114  can include one or more filters, one or more detectors, one or more amplifiers, one or more local oscillators, one or more analog-to-digital converters, etc., which shall be discussed with respect to  FIGS. 5A-B . 
     In some embodiments, the control circuit  112  can automatically perform the above-discussed iterations, which is sometimes referred to as an auto-nulling function or nulling function, until at least a null is determined (e.g., a direction of the null is determined). Such a nulling function can be selectively enabled by the RF communication system  100  based on various criteria such as, for example, responsive to the RF communication system  100  determining that the power consumption of the RF front end  114  has exceeded a predefined threshold. By enabling the nulling function, the RF communication system  100  can minimize or eliminate one or more interferences received by the RF communication system  100 , even though sources, locations, and/or directions of such interferences are unknown. In some other embodiments, the RF communication system  100  can calibrate the TDUs  106  and  108  to determine, log, or manage the respective directions of one or more nulls that the RF communication system  100  may provide. As such, if the direction of an interference is known, the RF communication system  100  can readily use the calibrated settings of TDUs  106  and  108  to eliminate or minimize the known interference. 
     Referring to  FIG. 1B , depicted is a functional block diagram of an RF communication system  130 . As shown in  FIG. 1B , the RF communication system  130  includes a first antenna  132 , a second antenna  134 , a time delay unit (TDU)  136 , a combiner  138 , a control circuit  140 , and an RF front end  142 . The components  132 - 142  shown in the illustrated embodiment of  FIG. 1B  may constitute a portion of the RF communication system  130 , which can further include any of various other RF communication components such as, for example, one or more digital baseband processing subsystems, one or more digital-to-analog processing subsystems, one or more transmitting subsystems, etc., while remaining within the scope of the present disclosure. Such other subsystems shall be discussed with respect to  FIGS. 5A-B . 
     The RF communication system  130  is substantially similar as the RF communication system  100  of  FIG. 1A  except that in the RF communication system  130 , only one of the antennas  132  and  134  is coupled with a TDU. Accordingly, the similar components (e.g.,  136 - 142 ) may be briefly discussed below. 
     In the illustrated embodiment of  FIG. 1B , the antennas  132  and  134 , physically separated apart from each other by a distance  135 , can receive one or more RF signals e.g., signal  146 . Depending on the directions along which the antennas  132  and  134  receive the one or more RF signals  146 , signals  148  and  150  respectively received through or by the antennas  132  and  134  (sometimes respectively referred to as “received signal  148 ” and “received signal  150 ”) can be different, e.g., presenting a phase difference therebetween. 
     In some embodiments, the antenna  132  may be directly coupled to the combiner  138 , and the antenna  134  may be coupled to the combiner via the TDU  136 . The TDU  136  can apply dynamically configurable, adjustable, or determined time delays and power gains on the received signal  150  to generate a modified signal  152  (e.g., delayed in time by the time delay and amplified in power by the power gain), while the received signal  148  may not be delayed in time or amplified in power. The combiner  138  (e.g., a subtractor) can receive the received signal  148  and modified signal  152  as inputs and perform a subtraction function on the signals  148  and  152  to provide an output signal  154 . The control circuit  140  and RF front end  142  can receive the output signal  154  as respective inputs. 
     Similar as the control circuit  112  of  FIG. 1A , the control circuit  140  can determine a power level of the output signal  154 , and based on the power level of the output signal  154  to determine the time delay and power gain used by the TDU  136 . In some embodiments, the control circuit  140  can iteratively update the time delays and/or power gains that the TDU  136  uses on the received signals  150  based on the power level of the output signal  154 . In some embodiments, the control circuit  140  can determine the time delay and power gain used by the TDU  136  to cause the combiner  138  to provide the output signal  154  as one or more nulls, responsive to determining a minimum power level of the output signal  154  over a number of iterations. In other words, the received signal  148 , without being delayed and amplified, and the modified signal  152 , with being delayed by the time delay and amplified by the power gain determined by the control circuit  140  in response to determining that the corresponding power level of the output signal  154  is a minimum over plural iterations of updating the time delays and/or power gains (used by the TDU  136 ). Such a null may be used to minimize or eliminate interferences that can be included within the one or more RF signals  146 . The null, which shall be discussed in further detail below with respect to  FIGS. 9A-B , can be provided to the RF front end  142  for further processing. The RF front end  142  is substantially similar to the RF front end  114  of  FIG. 1A  so that the discussions are not repeated. 
     The combiner  138  may receive the signal  148 , without being delayed by dynamically adjustable time delays and amplified by dynamically adjustable power gains, as one of the inputs, as discussed above. In some other embodiments, the combiner  138  may receive the signal  148 , which can be delayed in time by a predefined or fixed amount of time delay and amplified in power by a predefined or fixed amount of power gain. The fixed amount of time delay may be greater than the maximum value of a propagation delay between the antennas  132  and  134 . Such a predefined amount of time delay and predefined amount of power gain may be applied on the signal  148  by one or more components such as, for example, a transmission line, a phase shifter, an amplifier, etc. 
     Referring to  FIG. 1C , depicted is a functional block diagram of an RF communication system  160 . As shown in  FIG. 1C , the RF communication system  160  includes a first antenna  162 , a second antenna  164 , a mutual coupling (MC) mitigation circuit  166 , a first time delay unit (TDU)  168 , a second TDU  170 , a combiner  172 , a control circuit  174 , and an RF front end  176 . The components  162 - 176  shown in the illustrated embodiment of  FIG. 1C  may constitute a portion of the RF communication system  160 , which can further include any of various other RF communication components such as, for example, one or more digital baseband processing subsystems, one or more digital-to-analog processing subsystems, one or more transmitting subsystems, etc., while remaining within the scope of the present disclosure. Such subsystems shall be discussed with respect to  FIGS. 5A-B . 
     The RF communication system  160  is substantially similar as the RF communication system  100  of  FIG. 1A  except that the RF communication system  160  can further include an MC mitigation circuit (e.g.,  166 ). Accordingly, the similar components (e.g.,  168 - 174 ) may be briefly discussed below. 
     In the illustrated embodiment of  FIG. 1C , the antennas  162  and  164 , physically separated apart from each other by a distance  177 , can receive one or more RF signals e.g., signal  178 . Depending on the directions along which the antennas  162  and  164  receive the one or more RF signals  178 , signals  180  and  182  respectively received through or by the antennas  162  and  164  (sometimes respectively referred to as “received signal  162 ” and “received signal  164 ”) can be different, e.g., presenting a phase difference therebetween. In some instances, when the antennas  162  and  164  receive the RF signal  178 , at least one of the antennas  162  and  164  may re-radiate, backscatter, or scatter the RF signal  178 , which can cause a mutual coupling between the antennas  162  and  164 . Such a mutual coupling effect may be present in one or both of the received signals  180  and  182 . 
     In some embodiments, the MC mitigation circuit  166 , coupled to the antennas  162  and  164 , can mitigate, minimize, or eliminate the mutual coupling effect present in one or both of the received signals  180  and  182  by measuring a power level of a signal re-radiated or scattered from one of the antennas  162  and  164  and, if needed, compensating the signal received by the other of the antennas  162  and  164 .  FIG. 1D  illustrates an example of an embodiment of the MC mitigation circuit  166 . As shown, the MC mitigation circuit  166  can include branches 1 and 2, each of which can include a splitter, a delay circuit (e.g., a true time delay (TTD)), an attenuation circuit (ATTN), and a subtractor. For example, branch 1 can include a splitter  193 - 1 , a TTD  195 - 1 , an ATTN  196 - 1 , and a subtractor  197 - 1 ; and branch 2 can include a splitter  193 - 2 , a TTD  195 - 2 , an ATTN  196 - 2 , and a subtractor  197 - 2 . The splitter  193 - 1  of branch 1 can route the received signal  180  to the subtractor  197 - 1  of the same branch and to the TTD  195 - 2  and ATTN  196 - 2  of branch 2. Similarly, the splitter  193 - 2  of branch 2 can route the received signal  182  to the subtractor  197 - 2  of the same branch and to the TTD  195 - 1  and ATTN  196 - 1  of branch 1. As such, a first signal containing the mutual coupling effect, received by one of the antennas, can be mitigated by subtracting a second signal, received by the other of the antennas and then adjusted by the TTD and ATTN, from the first signal. In response to mitigating the mutual coupling effect, the MC mitigation circuit  166  can provide signals  184  and  186  to the TUDs  168  and  170 , respectively. The signals  184  and  186  may be sometimes referred to as “mitigated signal  184 ” and “mitigated signal  186 ,” respectively. 
     In response to receiving the mitigated signals  184  and  186 , the TDU  168  can apply dynamically configurable, adjustable, or determined time delays and power gains on the mitigated signal  184  to generate a modified signal  188  (e.g., delayed in time by the time delay and amplified in power by the power gain); and the TDU  170  can apply dynamically configurable, adjustable, or determined time delays and power gains on the mitigated signal  186  to generate a modified signal  190  (e.g., delayed in time by the time delay and amplified in power by the power gain). Similarly, such modified signals  188  and  190  can be combined by the combiner  172  to generate an output signal  192 . The control circuit  174  can receive the output signal  192  to determine the respective time delays and power gains that the TDUs  168  and  170  use. The combiner  174  can generate the output signal  192  as one or more nulls to eliminate or minimize interferences that can be included in RF signal  178 . The RF front end  176  can receive the output signal  192  to further process the signal  192 . Since the RF front end  176  can be substantially similar to the above-discussed RF front end, the discussions as to the RF front end  176  are not repeated. 
     In some other embodiments, the RF communication system  160  may optionally include one of the TDUs  168  and  170 . The RF communication system  160  can be substantially similar to the RF communication system  130  except for including the MC mitigation circuit  166 . In such embodiments, one of the mitigated signals  184  or  186  may be delayed in time by a dynamically configurable, adjustable, or determined time delay and/or amplified in power by a dynamically configurable, adjustable, or determined power gain, while the other of the mitigated signals  184  or  186  may or may not be delayed in time by a fixed time delay and/or amplified in power by a fixed power gain. 
     The above-discussed RF communication systems  100 ,  130 , and  160  may be used to receive an RF signal with a single band, in accordance with some embodiments of the present disclosure. Embodiments of the RF communication systems of the present disclosure, however, are not limited to being used in a single band environment. According to some embodiments, the RF communication systems  200 ,  300 , and  400 , respectively illustrated in  FIGS. 2, 3, and 4 , can each be used in a multi-band environment. For example, each of the RF communication systems  200 ,  300 , and  400  can receive a number of RF signals, which can reside in respective different bands, and provide one or more nulls at each of the different bands. 
     Referring to  FIG. 2 , a functional block diagram of the RF communication system  200  is depicted. As shown in  FIG. 2 , the RF communication system  200  can include a first antenna portion  210 , a second antenna portion  230 , a combiner  250 , and an RF front end  252 . Each of the antenna portions  210  and  230  may include one or more components, which shall be discussed below. The components shown in the illustrated embodiment of  FIG. 2  may constitute a portion of the RF communication system  200 , which can further include any of various other RF communication components such as, for example, one or more digital baseband processing subsystems, one or more digital-to-analog processing subsystems, one or more transmitting subsystems, etc., while remaining within the scope of the present disclosure. Such other subsystems shall be discussed with respect to  FIGS. 5A-B . 
     In some embodiments, the first antenna portion  210  may provide one or more nulls at a first band; and the second antenna portion  230  may provide one or more nulls at a second band, wherein the second band is different from the first band. Although two antenna portions are shown in the illustrated embodiment of  FIG. 2 , the RF communication system  200  can include any desired number of antenna portions while remaining within the scope of the present disclosure. As such, the RF communication system  200  can provide nulls at respective different bands that are more than 2. 
     The antenna portion  210  can include a first antenna  212 , a second antenna  214 , a first filter  216 , a second filter  218 , a first time delay unit (TDU)  220 , a second TDU  222 , a control circuit  224 , and a combiner  226 . The antennas  212  and  214  may be physically apart from each other by a distance  227 . Similarly, the antenna portion  230  can include a first antenna  232 , a second antenna  234 , a first filter  236 , a second filter  238 , a first TDU  240 , a second TDU  242 , a control circuit  244 , and a combiner  246 . The antennas  232  and  234  may be physically apart from each other by a distance  247 . The antenna portions  210  and  230  can be each substantially similar as a portion of the RF communication system  100  (e.g.,  102 - 110  of  FIG. 1A ) except that the antenna portions  210  and  230  can each include a pair of filters coupled between respective antennas and TDUs. Thus, the discussions of similar components are repeated again. 
     In the antenna portion  210 , the filter  216  can be coupled between the antenna  212  and TDU  220 ; and the filter  218  can be coupled between the antenna  214  and TDU  222 . In some embodiments, each of the filters  216  and  218  can include a band-pass filter that can allow the portion of a signal within a certain frequency range to pass therethrough and reject, or attenuate, the portion of the signal outside the frequency range. Such a frequency range used by the filters  216  and  218  may correspond to a first band (hereinafter “band 1”) in which the antenna portion  210  generates nulls. 
     For example, the RF communication system  200  can receive one or more RF signals  254  with a frequency ranging from about 2 GHz to 6 GHz. In response to the antenna portion  210  receiving the RF signal  254 , the filters  216  and  218  may filter received signals  256  and  258 , respectively received through or by the antennas  212  and  214 . The filter  216  may pass the portion of the signal  256  inside band 1 (e.g., about 2˜3 GHz) and reject the portion of the signal  256  outside band 1. The filter  218  may pass the portion of the signal  258  inside band 1 (e.g., about 2˜3 GHz) and reject the portion of the signal  258  outside band 1. As such, in response to receiving such filtered signals  260  and  262 , the TDUs  220  and  222 , control circuit  224 , and combiner  226  of the antenna portion  210  can follow the similar operations, as discussed above, to generate an output signal  264  as one or more nulls at band 1. 
     Similarly, in the antenna portion  230 , the filter  236  can be coupled between the antenna  232  and TDU  240 ; and the filter  238  can be coupled between the antenna  234  and TDU  242 . In some embodiments, the filters  236  and  238  of the antenna portion  230  may use another different frequency range (e.g., about 3˜4 GHz), which may correspond to a second band (hereinafter “band 2”), to filter the respective signals received through or by the antennas  232  and  234 . Accordingly, the TDUs  240  and  242 , control circuit  244 , and combiner  246  of the antenna portion  230  can follow the similar operations, as discussed above, to generate an output signal  266  as one or more nulls at band 2. 
     In some embodiments, the combiner  250  can combine the respective output signals generated by the antenna portions, e.g.,  210  and  230 , and provide a single combined signal  268  to the RF front end  252  for further processing, as discussed above. Continuing with the above example, the combiner  250  can combine (e.g., add) the output signals  264  and  266 , each of which can include one or more nulls at the respective band, and provide such the signal  268  to the RF front end  252  for further processing. 
     Referring to  FIG. 3 , a functional block diagram of the RF communication system  300  is depicted. As shown in  FIG. 3 , the RF communication system  300  can include a first antenna  302 , a second antenna  304 , a first splitter  306 , a second splitter  308 , a first nulling portion  310 , a second nulling portion  330 , a combiner  344 , and an RF front end  346 . Each of the nulling portions  310  and  330  may include one or more components, which shall be discussed below. The components shown in the illustrated embodiment of  FIG. 3  may constitute a portion of the RF communication system  300 , which can further include any of various other RF communication components such as, for example, one or more digital baseband processing subsystems, one or more digital-to-analog processing subsystems, one or more transmitting subsystems, etc., while remaining within the scope of the present disclosure. Such other subsystems shall be discussed with respect to  FIGS. 5A-B . 
     The antennas  302  and  304  of the RF communication system  300  may be physically apart from each other by a distance  303 . The RF communication system  300  can receive one or more RF signals  350  using the antennas  302  and  304 . In response to receiving the RF signals  350 , the splitters  306  and  308 , respectively coupled to the antennas  302  and  304 , can each split, divide, or route the respective received signal into a number of routed signals to respective nulling portions, according to some embodiments. Each of such a number of routed signals may have an equal amplitude, 0° phase difference between one and anther, and/or an equal power level. For example, the splitter  306  can split received signal  352  (the signal  350  received by the antenna  302 ) into routed signals  352 - 1 ,  352 - 2 , etc.; and the splitter  308  can split signal  354  (the signal  350  received by the antenna  304 ) into routed signals  354 - 1 ,  354 - 2 , etc. The routed signals  352 - 1  and  354 - 1  can be provided to the first nulling portion  310 ; and the routed signals  352 - 2  and  354 - 2  can be provided to the nulling portion  330 . 
     In some embodiments, the first nulling portion  310  may provide one or more nulls at a first band; and the second nulling portion  330  may provide one or more nulls at a second band, wherein the second band is different from the first band. Although two nulling portions are shown in the illustrated embodiment of  FIG. 3 , the RF communication system  300  can include any desired number of nulling portions while remaining within the scope of the present disclosure. As such, the RF communication system  300  can provide nulls at respective different bands that are more than 2. 
     The nulling portion  310  can include a first filer  312 , a second filter  314 , a first time delay unit (TDU)  316 , a second TDU  318 , a control circuit  320 , and a combiner  322 . The nulling portion  330  can include a first filer  332 , a second filter  334 , a first TDU  336 , a second TDU  338 , a control circuit  340 , and a combiner  342 . In some embodiments, each of the nulling portions  310  and  330  of the RF communication system  300  can be substantially similar to the antenna portion of the RF communication system  200  (e.g.,  210  and  230 ) except that the nulling portions may not include an antenna, and each of the TDUs of the nulling portion may receive a routed signal (e.g.,  352 - 1 ,  354 - 1 , etc.) as input. Thus, the nulling portions  310  and  330  of the RF communication system  300  may be briefly discussed below. 
     According to some embodiments, each of the filters  312 ,  314 ,  332 , and  334  can include a band-pass filter that can allow the portion of a signal within a certain frequency range to pass therethrough and reject, or attenuate, the portion of the signal outside the frequency range. The frequency range used by the filters  312  and  314  of the first nulling portion  310  may correspond to a first band (hereinafter “band 1”) in which the nulling portion  310  generates nulls; and the frequency range used by the filters  312  and  314  of the second nulling portion  330  may correspond to a second band (hereinafter “band 2”) in which the nulling portion  330  generates nulls. 
     For example, the RF communication system  300  can receive one or more RF signals  350  with a frequency ranging from about 2 GHz to 6 GHz. In response to the antenna  302  and  304  receiving the RF signal  350  as the received signals  352  and  354 , respectively, the splitter  306 , coupled to the antenna  302 , can split the received signal  352  into routed signals  352 - 1  and  352 - 2 ; and the splitter  308 , coupled to the antenna  304 , can split the received signal  354  into routed signals  354 - 1  and  354 - 2 . Each of the routed signals  352 - 1 - 2  and  354 - 1 - 2  may remain to have the frequency, as received (e.g., substantially similar as the respective received signal), according to some embodiments. Each of the respective different nulling portions  310  and  330  can receive a number of routed signals respectively split from different splitters. 
     In response, the filters  312  and  314  of the nulling portion  310  may filter routed signals  352 - 1  and  354 - 1 , respectively. The filter  312  may pass the portion of the signal  352 - 1  inside band 1 (e.g., about 2˜3 GHz) and reject the portion of the signal  352 - 1  outside band 1. The filter  314  may pass the portion of the signal  354 - 1  inside band 1 (e.g., about 2˜3 GHz) and reject the portion of the signal  354 - 1  outside band 1. Similar as the operation discussed above with respect to the antenna portions  210  and  230  of  FIG. 2 , responsive to receiving the signals filtered by the filters  312  and  314 , the TDUs  316  and  318 , control circuit  320 , and combiner  322  can generate an output signal  356  as one or more nulls at band 1. 
     Similarly, in response to receiving the routed signals  352 - 2  and  354 - 2 , the filters  332  and  334  of the nulling portion  330  may filter routed signals  352 - 2  and  354 - 2 , respectively. The filter  332  may pass the portion of the signal  352 - 2  inside band 2 (e.g., about 3˜4 GHz) and reject the portion of the signal  352 - 2  outside band 2. The filter  334  may pass the portion of the signal  354 - 2  inside band 2 (e.g., about 3˜4 GHz) and reject the portion of the signal  354 - 2  outside band 2. And responsive to receiving the signals filtered by the filters  332  and  334 , the TDUs  336  and  338 , control circuit  340 , and combiner  342  can generate an output signal  358  as one or more nulls at band 2. 
     In some embodiments, the combiner  344  can combine the respective output signals generated by the nulling portions, e.g.,  310  and  330 , and provide a single combined signal  360  to the RF front end  346  for further processing, as discussed above. Continuing with the above example, the combiner  344  can combine (e.g., add) the output signals  356  and  358 , each of which can include one or more nulls at the respective band, and provide the signal  360  to the RF front end  346  for further processing. 
     Referring to  FIG. 4 , a functional block diagram of the RF communication system  400  is depicted. As shown in  FIG. 4 , the RF communication system  400  can include a first antenna array portion  402 , a second antenna array portion  452 , a first beamforming circuit  454 , a second beamforming circuit  456 , a combiner  458 , and an RF front end  460 . Each of the antenna array portions  402  and  452  may include one or more components, which shall be discussed below. The components shown in the illustrated embodiment of  FIG. 4  may constitute a portion of the RF communication system  400 , which can further include any of various other RF communication components such as, for example, one or more digital baseband processing subsystems, one or more digital-to-analog processing subsystems, one or more transmitting subsystems, etc., while remaining within the scope of the present disclosure. Such other subsystems shall be discussed with respect to  FIGS. 5A-B . 
     In some embodiments, each of the antenna array portions (e.g.,  402  and  452 ) of the RF communication system  400  can be substantially similar as a combination of the pair of antennas  302  and  304 , the pair of splitters  306  and  308 , and the plural nulling portions  310  and  330  of  FIG. 3 . Therefore, the antenna array portions of the RF communication system  400  may be briefly described as follows. 
     As shown in the illustrated embodiment of  FIG. 4 , the first antenna array portion  402  can include a first antenna  404 , a second antenna  406  physically separated from the first antenna  404  by a distance  405 , a first splitter  408 , a second splitter  410 , a first nulling portion  411 , and a second nulling portion  431 . Similar as the nulling portions discussed with respect to  FIG. 3 , the first nulling portion  411  can include filters (e.g., band-pass filters)  412  and  414 , TDUs  416  and  418 , a control circuit  420 , and a combiner (e.g., a subtractor)  422 ; and the second nulling portion  431  can include filters (e.g., band-pass filters)  432  and  434 , TDUs  436  and  438 , a control circuit  440 , and a combiner (e.g., a subtractor)  442 . The first nulling portion  411  can provide an output signal  423  as one or more nulls at a first band (hereinafter “band 1”); and the second nulling portion  431  can provide an output signal  445  as one or more nulls at a second band (hereinafter “band 2”), wherein the second band is different from the first band. 
     Similarly, the second antenna array portion  452  can include a first antenna  454 , a second antenna  456  physically separated from the first antenna  454  by a distance  455 , a first splitter  458 , a second splitter  460 , a first nulling portion  461 , and a second nulling portion  481 . Similar as the nulling portions discussed with respect to  FIG. 3 , the first nulling portion  461  can include filters (e.g., band-pass filters)  462  and  464 , TDUs  466  and  468 , a control circuit  470 , and a combiner (e.g., a subtractor)  472 ; and the second nulling portion  481  can include filters (e.g., band-pass filters)  482  and  484 , TDUs  486  and  488 , a control circuit  490 , and a combiner (e.g., a subtractor)  492 . The first nulling portion  461  can provide an output signal  473  as one or more nulls at the band 1; and the second nulling portion  481  can provide an output signal  493  as one or more nulls at the second band. 
     According to some embodiments, the nulling portions of respective different antenna array portions of the RF communication system  400  can provide output signals as one or more nulls at a same band to a beamforming circuit as inputs. For example, the nulling portion  411  of the antenna array portion  402  can provide the output signal  423  as one or more nulls at the band 1 to the beamforming circuit  454 , and the nulling portion  461  of the antenna array portion  452  can provide the output signal  473  as one or more nulls at the band 1 to the beamforming circuit  454 . The nulling portion  431  of the antenna array portion  402  can provide the output signal  445  as one or more nulls at the band 2 to the beamforming circuit  456 , and the nulling portion  481  of the antenna array portion  452  can provide the output signal  493  as one or more nulls at the band 2 to the beamforming circuit  456 . 
     Although two antenna array portions are shown in the illustrated embodiment of  FIG. 4 , the RF communication system  400  can include any desired number of antenna array portions while remaining within the scope of the present disclosure. Although two nulling portions are shown in each of the antenna array portions of the RF communication system  400 , the RF communication system  400  can include any desired number of nulling portions in each of the antenna array portions while remaining within the scope of the present disclosure. For example, the RF communication system  400  can include more than 2 antenna array portions, and/or each of the antenna array portions can include more than 2 nulling portions. 
     In response to receiving the nulls at a certain band from the respective nulling portions of different antenna array portions, the beamforming circuit of the RF communication system  400  can use one or more beamforming techniques (e.g., digital beamforming techniques, analog beamforming techniques, hybrid beamforming techniques, and/or adaptive beamforming techniques) to further minimize the nulls and/or generate one or more additional nulls. In some embodiments, such additional nulls may be each pointed along a direction different from ones of the nulls provided by the nulling portions. Such beamforming techniques may be known in the art, so that the beamforming circuits  454  and  456  shall be briefly discussed as follows. In some embodiments, the beamforming circuit  454  may combine the output signals  423  and  473  to generate signal  494  at band 1 in a way (e.g., respective different weights) where an expected pattern or radiation is preferentially observed; and the beamforming circuit  454  may combine the output signals  445  and  493  to generate signal  495  at band 2 in a way (e.g., respective different weights) where an expected pattern or radiation is preferentially observed. 
     In response to the beamforming circuits  454  and  456  generating the signals, respectively, in some embodiments, the combiner  458  can combine the signals  494  and  495 , and provide a single combined signal  496  to the RF front end  460  for further processing, as discussed above. Continuing with the above example, the combiner  458  can combine (e.g., add) the output signals  494  and  495 , each of which can include one or more nulls at the respective band, and provide the signal  496  to the RF front end  460  for further processing. 
     Referring to  FIGS. 5A-B , a functional block diagram of the RF communication system  500  is depicted. In some embodiments, the RF communication system  500  may selectively switch between a receiving mode and transmitting mode, which shall be discussed below. As shown in the illustrated embodiment of  FIGS. 5A-B , the RF communication system  500  can include a first RF head  502 , a second RF head  542 , a digital processing subsystem  578 , a digital-to-RF subsystem  584 , and an RF front end  594 . 
     In some embodiments, the RF heads  502  and  542 , each of which includes a pair of antennas and components that can perform the above-described nulling function, can receive an RF signal to generate one or more digital signals. Such one or more digital signals can include one or more nulls, generated by performing the nulling function. The RF communication system  500  can use the one or more nulls to eliminate or minimize interferences that can be included in the received RF signals. The digital processing subsystem  578  can digitally process (e.g., channelize, beamforming, etc.) the digital signals. The digital-to-RF subsystem  584  can convert the digitally processed signal back to one or more RF (e.g., analog) signals, and provide the one or more RF signals to the RF front end  594  for further processing. 
     As shown in the illustrated embodiment of  FIGS. 5A-B , the RF head  502  can include a first antenna  504 , a second antenna  506  separated from the first antenna  504  by a distance  505 , a transmitting/receiving (T/R) switch  508 , a bypass switch  510 , filters (e.g., band-pass filters)  512  and  514 , time delay units (TDUs)  516  and  518 , a subtractor  520 , a control circuit  522 , band-pass filters  524  and  528  with an amplifier  526  coupled therebetween, an amplifier  530 , and an analog-to-digital converter  532 . The RF head  542  can include a first antenna  544 , a second antenna  546  separated from the first antenna  544  by a distance  545 , a transmitting/receiving (T/R) switch  548 , a bypass switch  550 , filters (e.g., band-pass filters)  552 ,  554 , and  556 , time delay units (TDUs)  558  and  560 , a subtractor  562 , a control circuit  564 , band-pass filters  566  and  570  with an amplifier  568  coupled therebetween, an amplifier  572 , and an analog-to-digital converter  574 . In some embodiments, the first and second RF heads  502  and  542  can be substantially similar to each other. Thus, the first RF head  502  is selected as a representative example in the following discussions. 
     The filters  512  and  514 , TDUs  516  and  518 , subtractor  520 , and control circuit  522  are substantially similar to the components discussed of  FIG. 1A-4 , respectively, such that the discussions are not repeated. In some embodiments, the T/R switch  508  may switch the RF head  502  between the receiving mode and transmitting mode based on a control signal. For example, when the control signal causes the T/R switch  508  to switch the RF head  502  to the receiving mode, the T/R switch  508  may allow a signal received by the antenna  504  to be received by the following subsystems or circuits such as, for example, the RF front end  580 . When the control signal causes the T/R switch  508  to switch the RF head  502  to the transmitting mode, the T/R switch  508  may allow a signal directly received from the RF front end  580  to be transmitted by the antenna  508 . 
     In some embodiments, the bypass switch  510  can allow the signal received by the antenna  508  to bypass the following components of the RF head  502 , the digital processing subsystem  544 , and a portion of the digital-to-RF subsystem  550 , and to be directly received by the RF front end  580 . In some embodiments, the bypass switch  510  may be controlled by a control signal to determine whether to allow the signal received by the antenna  508  to bypass the following components of the RF head  502  and the rest, as discussed above. In some embodiments, in response to allowing the bypass, the RF head  502  may not perform a nulling function on the signal received by the antenna  508 . 
     In response to the bypass not being allowed, the filters  512  and  514 , TDUs  516  and  518 , subtractor  520 , and control circuit  522  can perform a nulling function on the signals received by the antennas  506  and  508  to generate an output signal  523 , which may or may not include a null. The components  524 - 532  can “filter” and “sample” the signal  523 , as known in the art, such that the components  524 - 532  may be briefly described as follows. In some embodiments, each of the filters  524  and  528  may include a band-pass filter, wherein the filer  524  can allow a signal with a very high frequency to pass and the filter  528  can reject image and/or be anti-aliasing. The amplifier  526  may include a non-linear amplifier configured to avoid saturation. The amplifier  530  can avoid clipping. The analog-to-digital converter  532 , which can receive a clock signal, can convert the signal  523 , through the respective operations of the components  524 - 530 , into a digital signal  533 . Similarly, the RF head  542  can provide a digital signal  575  by using the included components to perform respective functions. 
     In some embodiments, the digital processing subsystem  578  can include a first channelization circuit  579 , a second channelization circuit  580 , a beamforming circuit  581 , and a mixer  582 . The first channelization circuit  579  can channelize the digital signal  533 , received from the first RF head  502 , into a number of different (frequency) channels. The second channelization circuit  580  can channelize the digital signal  575 , received from the second RF head  542 , into a number of different (frequency) channels. The beamforming circuit  581  can use at least one of the above-mentioned beamforming techniques to process the signals at each of the channels. The mixer  582  can then mix (e.g., convert) the signals at different channels to a single processed signal  583 . 
     The digital-to-RF subsystem  584  can include a digital-to-analog converter  585 , a mixer  586 , a filter  587 , an amplifier  588 , a first bypass filter  589 , a second bypass filter  590 , a first T/R switch  591 , and a second T/R switch  582 . The digital-to-analog converter  585  can convert the signal  583  in the digital domain into the analog domain, and such a converted analog signal can be further processed by the mixer  586 , filter  587 , and amplifier  588 . The bypass switch  589  may be controlled by the same control signal that controls the bypass switch  510  of the RF head  502 , and the bypass switch  590  may be controlled by the same control signal that controls the bypass switch  550  of the RF head  542 . As such, when the bypass switch  510  allows the signal received by the antenna  508  to bypass the RF head  502 , the digital processing subsystem  578 , and a portion of the digital-to-RF subsystem  584 , the bypass switch  589  can route the signal to be directly received by the RF front end  594  through the T/R switch  591 . The bypass switch  590  and the bypass switch  550  of the RF head  542  may operate similarly. 
     The T/R switch  591  may be controlled by the same control signal that controls the T/R switch  508  of the RF head  502 , and the T/R switch  592  may be controlled by the same control signal that controls the T/R switch  548  of the RF head  542 . As such, when the T/R switch  508  switch switches the RF head  502  to the receiving mode, the T/R switch  591  can allow the signal received from the bypass switch  589  to be received by the RF front end  594 . When the T/R switch  508  switch switches the RF head  502  to the transmitting mode, the T/R switch  591  can allow the signal received from the RF front end  594  to be transmitted through the antenna  504  directly. The T/R switch  592  and the T/R switch  548  of the RF head  542  may operate similarly. 
     B. Methods to Operate RF Communication Systems 
       FIG. 6  illustrates a flow chart of an exemplary method  600  to operate an RF communication system. In accordance with some embodiments of the present disclosure, the method  600  may be performed by the respective components of the RF communication systems  100 ,  130 , and  160  discussed with respect to  FIGS. 1A-C . For purposes of discussion, the following embodiment of the method  600  will be described in conjunction with  FIGS. 1A-D . The illustrated embodiment of the method  600  is merely an example. Therefore, it should be understood that any of a variety of operations may be omitted, re-sequenced, and/or added while remaining within the scope of the present disclosure. 
     In brief overview, a pair of antennas can receive an RF signal at operation  602 . At operation  604 , one or more time delay units (TDUs) can modify a time delay and/or power gain of at least one of received signals. At operation  606 , a subtractor can combine modified signals. At operation  608 , a control circuit can determine whether the modified signals are aligned. If not, the method  600  may proceed again to operation  604 ; but if so, the method  600  proceeds to operation  610  in which the subtractor can generate a plurality of nulls. At operation  612 , the subtractor can provide an output signal, which can contain the plurality of nulls, to a receiver. 
     Referring still to  FIG. 6 , and in greater detail, the pair of antennas can receive an RF signal at operation  602 . As a representative example, in  FIG. 1A , the pair of antennas  102  and  104  can receive the RF signal  116 . In some embodiments, the pair of antennas  102  and  104  may be physically separated apart from each other by distance  115 . The distance  115  can be associated with one or more characteristics of the RF signal  116 . For example, the distance  115  may be either greater or less than a half of the wavelength (λ/2) of the RF signal  116 . As the separation distance between the pair of antenna becomes larger than λ/2, one or more aliased nulls can be generated by the RF communication system, which shall be discussed in further detail below with respect to  FIG. 10 . 
     At operation  604 , one or more time delay units (TDUs) can modify a time delay and/or power gain of at least one of received signals. According to some embodiments, in response to the pair of antennas receiving the RF signal, a first TDU, coupled to one of the pair of antennas, can modify a corresponding signal received by the one of the pair of antennas by applying a dynamically configurable time delay and/or power gain on the received signal. Simultaneously or subsequently, a second TDU, coupled to the other of the pair of antennas, can modify a corresponding signal received by the other of the pair of antennas by applying another dynamically configurable time delay and/or power gain on the received signal. In another embodiment, a phase shifter and/or transmission line, coupled to the other of the pair of antennas, can modify the signal received by the other of the pair of antennas by applying a fixed time delay and/or power gain on the received signal. 
     Continuing with the example of  FIG. 1A , the TDUs  106  and  108  can apply dynamically configurable time delays and/or power gains on the signals  118  and  120  that are respectively received by the antennas  102  and  104 . In the example of  FIG. 1B , one TDU  136  can apply a dynamically configurable time delay and/or power gain on the signal  150  that is received by the antenna  134 , while the signal  148 , received by the antenna  132 , may be applied with no time delay or power gain. 
     At operation  606 , the subtractor combines the modified signals. According to some embodiments, in response to the received signals being modified, the subtractor, coupled to the TDU(s), combines (e.g., subtract) the modified signals to generate an output signal. 
     At operation  608 , the control circuit can determine whether the modified signals are aligned. According to some embodiments, in response to the subtractor combining the modified signals, the control circuit can determine whether the modified signals are aligned in time and power levels by monitoring or detecting whether a power level of the output signal has reach a minimum. In some embodiments, the control circuit can iteratively adjust the time delay(s) and/or power gain(s) that the TDU(s) apply on the respective received signal(s) until the control circuit has detected a minimum of the power levels of the output signal over a number of iterations. 
     If the control circuit cannot determine a minimum of the power level at a current iteration, the control circuit can update the time delay(s) and/or power gain(s) that the TDU(s) apply on the received signal(s) (operation  604 ). If the control circuit can determine a minimum of the power level at the current iteration, the subtractor can provide the output signal, generated by combining the modified signals, as a plurality of nulls (operation  610 ). In some embodiments, in response to the control circuit determining the minimum power level of the output signal, the control circuit can record, manage, or store the time delay(s) and/or power gain(s) that the TDU(s) apply on the received signal(s). 
     At operation  612 , the subtractor can provide the output signal, which can contain the plurality of nulls, to the receiver. In the case where the receiver receives an output signal containing nulls, the receiver may receive an RF signal through the antennas that is aligned with respective directions of the nulls. In some embodiments, the subtractor may provide an output signal to the receiver, which can be an RF front end, during each time of the iterations. As the control circuit can iteratively update the time delay(s) and/or power gain(s) that the TDU(s) apply on the received signal(s), at least one of the output signals respectively provided over the plural iterations can include a null. The RF front end can use such a null to eliminate or minimize a known or an unknown interference. 
       FIG. 7  illustrates a flow chart of an exemplary method  700  to operate an RF communication system. In accordance with some embodiments of the present disclosure, the method  700  may be performed by the respective components of the RF communication system  200  discussed with respect to  FIG. 2 . For purposes of discussion, the following embodiment of the method  700  will be described in conjunction with  FIG. 2 . The illustrated embodiment of the method  700  is merely an example. Therefore, it should be understood that any of a variety of operations may be omitted, re-sequenced, and/or added while remaining within the scope of the present disclosure. 
     In brief overview, multiple pairs of antennas can receive an RF signal at operation  702 . At operation  704 , one or more filters, coupled to a respective pair of antennas, may filter received signals. At operation  706 , one or more time delay units (TDUs), coupled to the respective pair of antennas, can modify a time delay and/or power gain of at least one of the filtered signals. At operation  708 , a subtractor, coupled to the respective pair of antennas, can combine modified signals. At operation  710 , a control circuit, coupled to the respective pair of antennas, can determine whether the modified signals are aligned. If not, the method  700  may proceed again to operation  706 ; but if so, the method  700  proceeds to operation  712  in which the subtractor can generate a plurality of (spatial) nulls at a respective band. At operation  714 , a combiner can combine output signals from respective subtractors. At operation  716 , the combiner can provide a combined signal, which can contain the plurality of nulls at respective bands, to a receiver. 
     Referring still to  FIG. 7 , and in greater detail, the multiple pairs of antennas can receive the RF signal at operation  702 . Using the RF communication system  200  of  FIG. 2  as an example, the pair of antennas  212  and  214  and the pair of antennas  232  and  234  can respectively receive the RF signal  254 . The pair of antennas  212  and  214  may be physically separated apart from each other by distance  227 ; and the pair of antennas  232  and  234  may be physically separated apart from each other by distance  247 . The distances  227  and  247  can be each associated with one or more characteristics (e.g., a wavelength) of the RF signal  254 . In some embodiments, the antennas  212  and  214  may be referred to as a part of the antenna portion  210 ; and the antennas  232  and  234  may be referred to as a part of the antenna portion  230 . 
     At operation  704 , the one or more filters, coupled to the respective pair of antennas, may filter the received signals. In response to receiving the RF signal, the one or more filters may each use a respective frequency range (or band) to allow a portion of the signal received by the respective pair of antennas to pass therethrough. The one or more filters can be referred to as frequency filters. Continuing with the example of  FIG. 2 , the filters  216  and  218 , respectively coupled to the antennas  212  and  214 , can filter the signals respectively received by the antennas  212  and  214 ; and the filters  236  and  238 , respectively coupled to the antennas  232  and  234 , can filter the signals respectively received by the antennas  232  and  234 . In some embodiments, the filters  216  and  218  may filter out the portions of the received signals (e.g.,  256  and  258 ) that are outside the band 1, and leave the portions of the received signals that are within the band 1; and filters  236  and  238  may filter out the portions of the received signals that are outside the band 2, and leave the portions of the received signals that are within the band 2. In some embodiments, the bands 1 and 2 may be referred to respective different frequency ranges. 
     At operation  706 , the one or more time delay units (TDUs), coupled to the respective pair of antennas, can modify the time delay and/or power gain of at least one of the filtered signals. In response to the one or more filters filtering the respective received signals, one or more TDUs can modify the filtered signals by applying dynamically configurable time delays and/or power gains on the filtered signals. Using the example of  FIG. 2  again, the TDUs  220  and  222  can apply dynamically configurable time delays and/or power gains on the signals that are respectively filtered (e.g., allowed to pass) by the filters  216  and  218 ; and the TDUs  240  and  242  can apply dynamically configurable time delays and/or power gains on the signals that are respectively filtered (e.g., allowed to pass) by the filters  236  and  238 . 
     At operation  708 , the subtractor, coupled to the respective pair of antennas, can combine modified signals. In response to the one or more TDUs modifying the received signals, the subtractor can combine (e.g., subtract) the modified signals to generate an output signal. Using the example of  FIG. 2  again, the subtractor  226  can combine (e.g., subtract) the signals respectively modified by the TDUs  220  and  222  to generate an output signal; and the subtractor  246  can combine (e.g., subtract) the signals respectively modified by the TDUs  240  and  242  to generate an output signal. 
     At operation  710 , the control circuit, coupled to the respective pair of antennas, can determine whether the modified signals are aligned. In response to the generation of the output signal, the control circuit can determine whether the modified signals are aligned in time and power levels by monitoring or detecting whether a power level of the output signal has reach a minimum. In some embodiments, the control circuit can iteratively adjust the time delays and/or power gains that the one or more TDUs apply on the respective filtered signals until the control circuit has detected a minimum of the power levels of the output signal over a number of iterations. 
     Continuing with the example of  FIG. 2 , the control circuit  224  can iteratively adjust the time delays and/or power gains that the TDUs  220  and  222  apply on the respective signals, filtered by the filters  216  and  218 , until the control circuit  224  has detected a minimum of the power levels of the output signal  264  over a number of iterations; and the control circuit  244  can iteratively adjust the time delays and/or power gains that the TDUs  240  and  242  apply on the respective signals, filtered by the filters  236  and  238 , until the control circuit  244  has detected a minimum of the power levels of the output signal  266  over a number of iterations. 
     If the control circuit cannot determine a minimum of the power level at a current iteration, the control circuit can update the time delay(s) and/or power gain(s) that the one or more TDUs apply on the filtered signals (operation  706 ). If the control circuit can determine a minimum of the power level at the current iteration, the subtractor can provide the output signal, generated by combining the modified signals, as a plurality of nulls at the respective band (operation  712 ). 
     At operation  714 , the combiner can combine the output signals from the respective subtractors. Using the above example again, the combiner  250  can combine the output signals  264  and  266  provided by the subtractors  226  and  246 , respectively. At operation  716 , the combiner can provide the one or more nulls at respective bands to the receiver. Continuing with the example of  FIG. 2 , the combiner  250  may provide the output signal  268  to the receiver, which can be an RF front end  252 , during each time of the iterations. As the control circuits  224  and  244  can iteratively update the time delays and/or power gains that the TDUs apply on the filtered signals, the output signals  264  and  266  respectively provided over the plural iterations can include a plurality of nulls at respective bands. In other words, a first plurality of nulls at a first band can be included in the output signal  264 ; and a second plurality of nulls at a second band can be included in the output signal  266 . Upon receiving the output signals  264  and  266 , the RF front end can use such nulls, included in the output signals  264  and  266 , to eliminate or minimize known or unknown interferences at respective bands. 
       FIG. 8  illustrates a flow chart of an exemplary method  800  to operate an RF communication system. In accordance with some embodiments of the present disclosure, the method  800  may be performed by the respective components of the RF communication system  300  discussed with respect to  FIG. 3 . For purposes of discussion, the following embodiment of the method  800  will be described in conjunction with  FIG. 3 . The illustrated embodiment of the method  800  is merely an example. Therefore, it should be understood that any of a variety of operations may be omitted, re-sequenced, and/or added while remaining within the scope of the present disclosure. 
     In brief overview, a pair of antennas can receive an RF signal at operation  802 . At operation  804 , a pair of splitters can route received signals. At operation  806 , one or more filters can filter respective routed signals. At operation  808 , one or more time delay units (TDUs) can modify a time delay and/or power gain of at least one of the filtered signals. At operation  810 , a subtractor, coupled to respective filters, can combine modified signals. At operation  812 , a control circuit, coupled to the respective filters, can determine whether the modified signals are aligned. If not, the method  800  may proceed again to operation  808 ; but if so, the method  800  proceeds to operation  814  in which the subtractor can generate a plurality of nulls at a respective band. At operation  816 , a combiner can combine output signals from respective subtractors. At operation  818 , the combiner can provide a combined signal, which can contain the plurality of nulls at respective bands, to a receiver. 
     Referring still to  FIG. 8 , and in greater detail, the pair of antennas can receive an RF signal at operation  802 . Using the RF communication system  300  of  FIG. 3  as an example, the pair of antennas  302  and  304  can respectively receive the RF signal  350 . The pair of antennas  302  and  304  may be physically separated apart from each other by distance  303 . The distance  303  can be each associated with one or more characteristics (e.g., a wavelength) of the RF signal  350 . 
     At operation  804 , the pair of splitters can route received signals. Using the above example of  FIG. 3  again, the splitter  306  can route the signal  352  received by the antenna  302  by dividing the signal  352  into routed signals  352 - 1  and  352 - 2  and forward the signals  352 - 1  and  352 - 2  to the nulling portions  310  and  330 , respectively; and the splitter  308  can route the signal  354  received by the antenna  304  by dividing the signal  354  into routed signals  354 - 1  and  354 - 2  and forward the signals  354 - 1  and  354 - 2  to the nulling portions  310  and  330 , respectively. 
     At operation  806 , the one or more filters can filter the respective routed signals. In response to receiving the routed signal, the one or more filters at each nulling portion may each use a respective frequency range (or band) to allow a portion of the routed signal to pass therethrough. Continuing with the example of  FIG. 3 , the filters  312  and  314  at the nulling portion  310  can filter the signals respectively routed by the splitters  306  and  308 ; and the filters  332  and  334  at the nulling portion  330  can filter the signals respectively routed by the splitters  306  and  308 . In some embodiments, the filters  312  and  314  may filter out the portions of the routed signals (e.g.,  352 - 1  and  354 - 1 ) that are outside the band 1, and leave the portions of the routed signals that are within the band 1; and filters  332  and  334  may filter out the portions of the routed signals (e.g.,  652 - 2  and  354 - 2 ) that are outside the band 2, and leave the portions of the received signals that are within the band 2. In some embodiments, the bands 1 and 2 may be referred to respective different frequency ranges. 
     At operation  808 , the one or more time delay units (TDUs) can modify the time delay and/or power gain of at least one of the filtered signals. In response to the one or more filters filtering the respective received signals, one or more TDUs can modify the filtered signals by applying dynamically configurable time delays and/or power gains on the filtered signals. Using the example of  FIG. 3  again, the TDUs  316  and  318  can apply dynamically configurable time delays and/or power gains on the signals that are respectively filtered (e.g., allowed to pass) by the filters  310  and  314 ; and the TDUs  336  and  338  can apply dynamically configurable time delays and/or power gains on the signals that are respectively filtered (e.g., allowed to pass) by the filters  332  and  334 . 
     At operation  810 , the subtractor, coupled to the respective filters, can combine modified signals. In response to the one or more TDUs modifying the received signals, the subtractor can combine (e.g., subtract) the modified signals to generate an output signal. Using the example of  FIG. 3  again, the subtractor  322  can combine (e.g., subtract) the signals respectively modified by the TDUs  316  and  318  to generate an output signal; and the subtractor  342  can combine (e.g., subtract) the signals respectively modified by the TDUs  336  and  338  to generate an output signal. 
     At operation  812 , the control circuit, coupled to the respective filters, can determine whether the modified signals are aligned. In response to the generation of the output signal, the control circuit can determine whether the modified signals are aligned in time and power levels by monitoring or detecting whether a power level of the output signal has reach a minimum. In some embodiments, the control circuit can iteratively adjust the time delays and/or power gains that the one or more TDUs apply on the respective filtered signals until the control circuit has detected a minimum of the power levels of the output signal over a number of iterations. 
     Continuing with the example of  FIG. 3 , the control circuit  320  can iteratively adjust the time delays and/or power gains that the TDUs  316  and  318  apply on the respective signals, filtered by the filters  312  and  314 , until the control circuit  320  has detected a minimum of the power levels of the output signal  356  over a number of iterations; and the control circuit  340  can iteratively adjust the time delays and/or power gains that the TDUs  336  and  338  apply on the respective signals, filtered by the filters  332  and  334 , until the control circuit  340  has detected a minimum of the power levels of the output signal  266  over a number of iterations. 
     If the control circuit cannot determine a minimum of the power level at a current iteration, the control circuit can update the time delay(s) and/or power gain(s) that the one or more TDUs apply on the filtered signals (operation  808 ). If the control circuit can determine a minimum of the power level at the current iteration, the subtractor can provide the output signal, generated by combining the modified signals, as a plurality of nulls at the respective band (operation  814 ). 
     At operation  816 , the combiner can combine the output signals from the respective subtractors. Using the above example again, the combiner  344  can combine the output signals  356  and  358  provided by the subtractors  322  and  342 , respectively. At operation  818 , the combiner can provide the plurality of nulls at respective bands to the receiver. Continuing with the example of  FIG. 3 , the combiner  344  may provide the output signal  360  to the receiver, which can be an RF front end  346 , during each time of the iterations. As the control circuits  320  and  340  can iteratively update the time delays and/or power gains that the TDUs apply on the filtered signals, at least one of the output signals  356  and  358  respectively provided over the plural iterations can include a null at a respective band. The RF front end can use such a null to eliminate or minimize a known or an unknown interference. 
     C. Nulls Generated by RF Communication Systems 
     Referring to  FIGS. 9A and 9B , a symbolic diagram  900  of exemplary nulls and a corresponding antenna polar plot  920  of the exemplary nulls are depicted, respectively. As shown in the illustrated embodiment of  FIG. 9A , a pair of antennas  902  and  904  can be arranged along an axis  905 . The pair of antennas  902  and  904  can be a part of one of the above-discussed RF communication systems (e.g.,  100 ,  130 ,  160 ,  200 ,  300 ,  400 , and  500 ) that are configured to receive one or more RF signals. In some embodiments, each of the antennas  902  and  904  can be an omnidirectional antenna. 
     By performing the nulling function discussed above, a null  910 , and a null  912  symmetrically mirrored from the null  910  over the axis  905  can be both generated, in accordance with some embodiments of the present disclosure. For example, the null  910  can be tilted from an axis  907 , substantially perpendicular to the axis  905 , by an angle θ, while the null  912  can be tilted from the axis  907  by the same angle θ. In some embodiments, the null  912  can be referred to as a symmetric or sympathetic null with respect to the null  910 . 
     Referring to  FIG. 9B , the antenna polar plot  920 , corresponding to the symbolic diagram  900  of  FIG. 9A , is illustrated. As shown in  FIG. 9B , the respective power levels (in the unit of dB) of null  910  and symbolic null  912  can each present a minimum on the antenna polar plot  920 , and each of the minimum power levels can be aligned along a certain direction with respect to the antennas  902  and  904 . As discussed above, each of the disclosed RF communication systems can utilize the nulls to eliminate or minimize interferences, for example, adjusting the direction along which the null is aligned to be aligned with a source of the interference. 
     In some embodiments, the spacing (e.g.,  115 ,  135 ,  177 ,  227 ,  247 ,  303 ,  505 , and  545 ) between a pair of antennas of the disclosed RF communication system can be adjusted to be greater than a half of the wavelength of the RF signal received by the antennas. As such, one or more aliased nulls, as mentioned above, can be generated by the disclosed RF communication systems. 
       FIG. 10  illustrates a number of exemplary symbolic diagrams of nulls and/or aliased nulls with respect to a pair of antennas  1001 A and  1001 B, and corresponding antenna polar plots. In some embodiments, the antennas  1001 A and  1001 B may be separated from each other by a fixed distance. The distance can be associated with the wavelength of the RF signal received by the antenna  1001 A and  1001 B (e.g., quantized by the wavelength). When the frequency of the RF signal (reciprocal to the wavelength) varies, quantized weights of the distance may vary. When the quantized weight of the distance becomes greater than ½ (i.e., the distance between antennas  1001 A and  1001 B, when expressed in wavelength, is greater than ½ wavelength), in some embodiments, the one or more aliased nulls can be generated by the disclosed RF communication systems. In some embodiments, the disclosed RF communication system can perform the nulling function to adjust the time delays that the respective TDUs use, thereby causing a certain range of time delays to include more RF cycles. In response to including more RF cycles, the RF communication system can generate more aliased nulls accordingly. 
     For example, when the frequency of the RF signal is about 2 GHz, a pair of nulls  1003 A and  1003 B can be generated by a currently disclosed RF communication system that includes the antennas  1001 A and  1001 B, which can be seen in the symbolic diagram  1002  and corresponding antenna polar plot  1004 . As the frequency of the RF signal becomes about 3 GHz, a pair of aliased nulls  1007 A and  1007 B can be generated, which can be seen in the symbolic diagram  1006  and corresponding antenna polar plot  1008 . As the frequency of the RF signal becomes about 4 GHz, a pair of aliased nulls  1011 A and  1011 B (along with different directions than the aliased nulls  1007 A-B) can be generated, which can be seen in the symbolic diagram  1010  and corresponding antenna polar plot  1012 . As the frequency of the RF signal becomes about 5 GHz, two pairs of aliased nulls  1015 A and  1015 B and  1015 C and  1015 D can be generated, which can be seen in the symbolic diagram  1014  and corresponding antenna polar plot  1016 . As the frequency of the RF signal becomes about 6 GHz, two pairs of aliased nulls  1019 A and  1019 B and  1019 C and  1019 D can be generated, which can be seen in the symbolic diagram  1018  and corresponding antenna polar plot  1020 . 
       FIGS. 11A and 11B  respectively illustrates examples in which the directions of nulls generated with respect to two different pairs of antennas can be adjusted, in accordance with some embodiments of the present disclosure. Referring first to  FIG. 11A , an RF communication system  1100  can include a first pair of antennas  1104 A-B and a second pair of antennas  1114 A- 1114 B. The RF communication system  1100  can be deployed on a device (e.g., a vehicle), which may move along a direction  1101  in the illustrated embodiment of  FIG. 11A . On or in the device, the first pair of antennas  1102 A-B can be deployed along an axis  1103 , and the second pair of antennas  1112 A-B can be deployed along an axis  1113 . The axis  1103  can be aligned along a direction either different from or similar to the direction along which the axis  1113  extends. 
     By performing the above-discussed nulling function, the RF communication system  1100  can generate a pair of nulls  1104 A-B with respect to the antennas  1102 A-B, and a pair of nulls  1114 A-B with respect to the antennas  1112 A-B. In a case where an interference source  1120  is located along the direction  1101 , the RF communication system can align or overlap the directions of null  1104 A and  1114 A to eliminate or minimize the interference by performing the nulling function and/or adjusting the directions of axis  1103  and axis  1113 . 
       FIG. 11B  illustrates another example in which an interference source  1122  is located away from the direction  1101 . To eliminate or minimize the interference, the RF communication system  1100  can generate another pair of nulls  1124 A-B with respect to the antennas  1102 A-B by performing the nulling function and/or adjusting the direction of axis  1103 , and another pair of nulls  1134 A-B with respect to the antennas  1112 A-B by performing the nulling function and/or adjusting the direction of axis  1113 . As such, at least two of the nulls (e.g.,  1124 A and  1134 A in  FIG. 11B ) can be combined to be aligned with the interference source  1122 , even though no nulls are overlapped. 
     The construction and arrangement of the systems and methods as shown in the various exemplary embodiments are illustrative only. Although only a few embodiments have been described in detail in this disclosure, many modifications are possible (e.g., variations in sizes, dimensions, structures, shapes and proportions of the various elements, values of parameters, mounting arrangements, use of materials, colors, orientations). For example, the position of elements may be reversed or otherwise varied and the nature or number of discrete elements or positions may be altered or varied. Accordingly, all such modifications are intended to be included within the scope of the inventive concepts disclosed herein. The order or sequence of any operational flow or method operations may be varied or re-sequenced according to alternative embodiments. Other substitutions, modifications, changes, and omissions may be made in the design, operating conditions and arrangement of the exemplary embodiments without departing from the broad scope of the inventive concepts disclosed herein. 
     The inventive concepts disclosed herein contemplate methods, systems and program products on any machine-readable media for accomplishing various operations. Embodiments of the inventive concepts disclosed herein may be implemented using existing computer operational flows, or by a special purpose computer operational flows for an appropriate system, incorporated for this or another purpose, or by a hardwired system. Embodiments within the scope of the inventive concepts disclosed herein include program products comprising machine-readable media for carrying or having machine-executable instructions or data structures stored thereon. Such machine-readable media can be any available media that can be accessed by a special purpose computer or other machine with an operational flow. By way of example, such machine-readable media can comprise RAM, ROM, EPROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other medium which can be used to carry or store desired program code in the form of machine-executable instructions or data structures and which can be accessed by a general purpose or special purpose computer or other machine with an operational flow. When information is transferred or provided over a network or another communications connection (either hardwired, wireless, or a combination of hardwired or wireless) to a machine, the machine properly views the connection as a machine-readable medium. Thus, any such connection is properly termed a machine-readable medium. Combinations of the above are also included within the scope of machine-readable media. Machine-executable instructions include, for example, instructions and data which cause a special purpose computer, or special purpose operational flowing machines to perform a certain function or group of functions.