METHOD

FIELD OF THE INVENTION

The present invention pertains to the field of communications, and in particular, to an active interference cancellation system and method.

BACKGROUND

In many communication systems, signal interference can be a relatively common problem which, depending on the system and the nature of the interference, can be reasonably mitigated to achieve a satisfactory end signal reception quality. Of particular interest in the present context are solutions derived to mitigate interference generated between distinct communication systems commonly operated within a same device or with respect to a same application operating combined, parallel and/or overlapping communicating resources, such that outgoing signals transmitted by one of these systems may in fact, interfere in the reception of incoming signals by another of these systems. For example, one such application may include the co-implementation of distinct wireless communication systems by a common communication device, such as a laptop or cellular telephone, or again the co-implementation of one or more wireless communication systems and a global positioning system (GPS) in GPS-enabled communication devices. In the former example, interference is generally experienced due to substantial overlap between the communication bands of respective communication services, whereas in the later example, relatively strong transmissions in a given wireless communication band may effectively drown out or saturate a GPS receiver configured for the reception of relatively weak GPS satellite signals.

Depending on the application, different solutions have been proposed to mitigate such inter-system interference. Some examples include time separation, wherein transmissions by an otherwise interfering system are limited to periods between active receptions by a co-implemented system; frequency separation/filtering wherein appropriate guard bands are implemented between the transmission and reception bands of otherwise interfering systems so to allow for effective filtering; and passive interference cancellation, wherein a portion of an interfering transmission signal is effectively subtracted from a susceptible received signal. These and other such solutions, while potentially effective in certain scenarios, are not readily amenable to all systems. For example, time separation is only generally applicable where control of each communication system is centrally controlled, and wherein communications with respect to at least one of these systems can be delayed without unduly affecting the intended purpose or application associated with this delayed system. As for frequency separation, in general, communication bands associated with respective systems are generally preset and, where a sufficiently broad guard band is not allocated between respective communication systems, the intentional introduction of such guard bands, for example by reducing the operational bandwidth of a given system, would result in a reduction in the operational characteristics and throughput of such systems, which may not be of particular interest. Finally, while passive interference cancellation can provide a solution that overcomes some of the above disadvantages, such solution cannot generally accommodate for variations in the operation of co-implemented systems, and therefore, may not be sufficient in providing a useful effect.

An alternative to the above solutions was proposed in the following publications: U.S. Patent Application Publication No. 2008/0219377, U.S. Patent Nos. 6,961,019 and International Application Publication No. WO 01/15329, wherein the amplitude and phase associated with an interference cancellation signal can be adjusted as a function of a measured effectiveness of such cancellation. While these solutions do provide some improved flexibility in interference reduction, their applicability is generally geared to the co-implementation within a same static communication device of distinct Wi-Fi communication standards, or again of a cellular communication system and GPS.

Therefore there is a need for an active interference cancellation system and method that overcomes some of the drawbacks of known solutions, or at least, provides the public with a useful alternative. This background information is provided to reveal information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the invention.

SUMMARY

An object of the invention is to provide an active interference cancellation system and method. In accordance with an aspect of the invention, there is provided a system for mitigating interference from an interfering communication system having a transmitting antenna to a susceptible communication system having a receiving antenna, wherein the interfering system is distinct from the susceptible system, and wherein at least one of said transmitting antenna and said receiving antenna comprises a directional antenna, the system comprising: a cancellation signal generation module for generating a cancellation signal at least in part as a function of an interference signal from the transmitting antenna; a signal combination module for combining said cancellation signal with a received signal from the receiving antenna to provide an interference compensation therefor and generate a feedback signal; and a control signal generation module for generating a control signal at least in part as a function of said interference compensation and said feedback signal; wherein said cancellation signal is generated at least in part in response to said control signal.

In accordance with another aspect of the invention, there is provided a method for mitigating interference from an interfering communication system having a transmitting antenna to a susceptible communication system having a receiving antenna, wherein the interfering system is distinct from the susceptible system, and wherein at least one of said transmitting antenna and said receiving antenna comprises a directional antenna, the method comprising the steps of: generating a cancellation signal at least in part as a function of an interference signal from the transmitting antenna; combining said cancellation signal with a received signal from the receiving antenna to provide an interference compensation therefor and generate a feedback signal; and generating a control signal at least in part as a function of said interference compensation and said feedback signal; wherein said cancellation signal is generated at least in part in response to said control signal.

In accordance with another aspect of the invention, there is provided a kit for use in mitigating interference from an interfering communication system having a transmitting antenna to a susceptible communication system having a receiving antenna, wherein the interfering system is distinct from the susceptible system, and wherein at least one of said transmitting antenna and said receiving antenna comprises a directional antenna, the kit comprising: a cancellation signal generation module for generating a cancellation signal at least in part as a function of an interference signal from the transmitting antenna; a signal combination module for combining said cancellation signal with a received signal to provide an interference compensation therefor and generate a feedback signal; and a control signal generation module for generating a control signal at least in part as a function of said feedback signal; wherein said cancellation signal is generated at least in part in response to said control signal.

Other aims, objects, advantages and features of the invention will become more apparent upon reading of the following non-restrictive description of specific embodiments thereof, given by way of example only with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE FIGURES

Figure 1 is a high-level diagrammatic representation of an active interference cancellation system, in accordance with one embodiment of the invention.

Figure 2 is a diagrammatic representation of an active interference cancellation system, in accordance with one embodiment of the invention. Figure 3 is a diagrammatic representation of an equalizer of an active interference cancellation system, in accordance with one embodiment of the invention. Figure 4 is a diagrammatic representation of an optional radio frequency (RF) cross- correlator of an active interference cancellation system, in accordance with one embodiment of the invention.

Figure 5 is a diagrammatic representation of an optional radio frequency (RF) cross- correlator of an active interference cancellation system, in accordance with another embodiment of the invention.

Figure 6 is a diagrammatic representation of an optional intermediate frequency (IF) cross-correlator of an active interference cancellation system, in accordance with one embodiment of the invention. Figure 7 is a diagrammatic representation of an optional intermediate frequency (IF) cross-correlator of an active interference cancellation system, in accordance with another embodiment of the invention.

Figure 8 is a diagrammatic representation of an active interference cancellation system, in accordance with another embodiment of the invention. Figure 9 is a plot of measured characteristics of an exemplary coupling path between antennas of distinct communication systems, in accordance with one embodiment of the invention, showing frequency-dependent log magnitude and residual phase variations generated thereby for an exemplary aircraft communication systems configuration.

Figure 10 is a simplified plot of characteristics of an interfering signal transmitted by an interfering communication system, relative to a distinct signal intended for reception by a distinct communication system susceptible to the interfering signal, in accordance with one embodiment of the invention.

DETAILED DESCRIPTION

Different embodiments of a system and method are described for mitigating interference between distinct communication systems. The distinct communication systems comprise an interfering system comprising a transmitter and transmitting antenna, and a susceptible system comprising a receiver and receiving antenna, wherein the transmissions of the interfering system interfere with the receiver and receiving antenna, and wherein at least one of the transmitting antenna and the receiving antenna comprises a directional antenna dynamically reoriented in operation. In general, the system and method dynamically process interference compensation and external system and configuration data to adaptively control the cancellation of interference signals from the received signals.

Referring to Figure 1 , and in accordance with one embodiment of the invention, such a system, generally referred to using the numeral 100, will now be described. The system 100 is generally configured to mitigate interference between distinct communication systems, an interfering one of which comprising a transmitter and transmitting antenna (i.e. interfering antenna 102) whose transmissions interfere with a susceptible receiver 104 and receiving antenna 106 of the other. For example, interference detected at the receiving antenna 106 may be characterized, at least in part, as a function of the interfering transmissions and a particular interference path coupling the respective antennas of the distinct communication systems, depicted herein as coupling function 108.

In order to mitigate interference between the two distinct communication systems, the system 100 generally comprises a cancellation signal generation module 110 receiving as input an interference signal 112 at least partially representative of the interfering transmissions (e.g. a sample of these transmissions), for generating a cancellation signal 114 as a function thereof in response to a control signal 116, for example provided by control module 118. A signal combination module (e.g. combiner 120), communicatively coupled to the cancellation signal generation module 110, enables combination of the generated cancellation signal 114 with a received signal 122 from the receiving antenna 106, to provide interference compensation therefor. For example, in one embodiment, the cancellation signal 114 will comprise at least one of a time- shifted, a phase-shifted and an amplitude shifted component of the interference signal (i.e. represented by an interference cancellation function ^CANCEL), namely representative of the particular interference path coupling the communication systems' antennas. For example, in one embodiment, the cancellation signal generation module 110 adaptively operates on the interference signal 112 to effectively mimic the characteristic path between antennas (e.g. F _CO U _P L _I N _G ), and apply an inverse function thereto for applying appropriate interference cancellation (e.g. F _C ANCEL should be roughly equal to the inverse of F _CO U _PL ING)- Upon combining the cancellation signal 114 with the received signal 122, for example by summing these signals, at least partial compensation can be achieved, an effectiveness of which being at least partially dictated by the system's ability to adequately mimic the coupling function F _CO U _P LING between antennas 102 and 106.

In order to account for variations in the coupling characteristics between the interfering and receiving antennas, the cancellation signal generation module 110 is dynamically implemented in response to the control signal 116 generated by the control module 118, which is at least in part configured to assess an effectiveness of the interference compensation and adjust the control signal accordingly, namely effectively adjust F _C A _N CEL- For example, in one embodiment, the system comprises a sampling module or coupler 124 for sampling the compensated signal 129, e.g. generating feedback signal 126 for input to the control module 118, which is further configured to assess a quality thereof, e.g. assess an effectiveness of the interference cancellation applied thereto. The control module 118, may further receive as input external data, either selected based on preset operational characteristics and/or dynamically updated in response to operational characteristic variations, for example. For instance, in one embodiment, the control module 118 receives two kinds of input: open-loop inputs (e.g. external input(s) 125 provided directly or via selection module 127) defining the current state of the system, generally with values of stored parameters or the like; and feedback values obtained by sampling the compensated signal 129 at the input to the receiver 104. In one embodiment the open-loop inputs may assist with initial optimization and/or tracking rapid changes, whereas the feedback values may indicate how effective the current settings are in providing adequate compensation. The cancellation signal is thus continuously updated in an attempt to minimize the impact of interference (e.g. noise) in the sampled feedback signal.

For example, in one embodiment, at least one of the transmitting and receiving antennas comprises a directional antenna that is dynamically reoriented during operation, for example in tracking or selectively transmitting a signal in a particular direction. Examples of directional antennae may include, but are not limited to, different types of antennae or antenna arrays configured to radiate greater power in one or more directions, wherein a general spatial orientation of a beam generated or selectively received thereby can be reoriented by mechanical and/or electrical means (e.g. phased array). As this antenna is reoriented, so will the effective coupling path between antennas be modified, thereby requiring responsive adjustment of the cancellation signal to maintain sufficient interference suppression. Accordingly, the system 100 is configured to generate the control signal both as a function of a measure of interference compensation effectiveness and variations therein affected, for example, by reorientation of the directional antenna, thereby providing adaptive control of the cancellation signal responsive to such reorientations. Depending on the implementation of the system 100, such adaptive control may be provided solely on the basis of the feedback signal, or further on the basis of external data 125 provided to the control module 118 in providing real-time antenna positional/directional data, which data can be used to accelerate response to such variations in the cancellation signal. For example, where external data is provided, the control signal may be automatically adjusted as a function of preset antenna orientation-specific compensation parameters providing increased system responsiveness to antenna reorientations as compared to that available solely through the feedback loop. The feedback loop could then provide for further fine tuning of the control signal parameters.

One example for which the above system may be considered consists of the co- implementation of two satellite communication systems suitable for use with airborne systems, for example as offered by Inmarsat™ PLC and Iridium™ Communications Inc. While these services are somewhat complementary, it is sometimes desirable to provide both services on the same aircraft. However, the frequency allocations for the two services make this problematic, i.e. there is no guard band between the upper edge of the Iridium™ band and the lower edge of the Inmarsat™ uplink band, the boundary being located at 1626.5 MHz. As a consequence, mutual interference may occur, particularly in the Inmarsat to Iridium direction due to the considerably higher effective isotropic radiated power (EIRP) used by the Inmarsat™ transceiver. This is generally unacceptable for users of the Iridium™ service as such interference may present serious concerns when using the Iridium™ system for safety services. According to one embodiment, the system can be used to provide active interference cancellation to remove the unwanted Inmarsat™ signal from the signals received by the Iridium™ transceiver. Namely, the system can address interference arising due to operation of two different services, each operating in one of two contiguous non overlapping bands, with both transceiver and antenna sets collocated on one aircraft, for example. The system may further be adjusted to address both broadband noise (e.g. appearing in the pass band of the Iridium™ receiver and originating from the Inmarsat™ system transmitter) and specific modulated carriers (e.g. originating from the onboard Inmarsat™ system transmitter and appearing at the onboard Iridium™ receiver as out of band interference), both of which are unwanted jammers and should thus be cancelled, as much as possible, at the Iridium™ receiver. Furthermore, as will be appreciated by the person of ordinary skill in the art, the co- implementation of distinct communication systems on a same aircraft, or generally on a same vehicle or device, may present additional challenges not readily addressed by known solutions. For instance, in the present example, the Inmarsat™ system is generally provided with a directional antenna whose beam orientation is continuously adjusted in operation to maintain reliable satellite communications. Namely, the Inmarsat™ antenna will generally seek to maintain a fix on the position of an associated communication satellite, and thus, its orientation will be adjusted as a function of the aircraft's relative position and orientation (e.g. longitude, latitude, altitude, pitch, roll and yaw). Clearly, while the physical positioning of Inmarsat™' s directional antenna relative to the Iridium™ antenna, which generally consists of an omnidirectional antenna in most implementations, may be fixed by the aircraft structure, the reorientation of the Inmarsat™ antenna in operation may have noticeable consequences on the extent and characteristics of interference generated by Inmarsat™ transmissions in Iridium™ signals. To follow from the generic embodiment depicted in Figure 1 , reorientation of the Inmarsat™ antenna, in this example representing the interfering antenna 102, may have a non-negligible impact on FCOUPLING, thereby requiring appropriate adjustment of FCANCEL if appropriate interference compensation is to be provided.

While the following makes regular reference to the above-suggested application of the herein described system and method in the context of the co-implementation of Iridium™ and Inmarsat™ communication systems on a same aircraft, it will be appreciated by the person of ordinary skill in the art that these examples may be readily adjusted for implementation with respect to other communication systems wherein active interference cancellation may be desired or required. Namely, the challenges overcome by the design and development of the solutions provided herein may be similarly identified in other communication systems. For instance, co-implementation of distinct communication systems operatively mounted on other vehicular devices (trains, buses, ships, trucks, etc.) can be considered as well as other applications where at least one of the systems of interest is configured to dynamically reorient its communication antenna, thereby dynamically altering a coupling path between respective system antennas. Other examples may also include coupling between a directional communication system and a co-implemented global positioning system (GPS), simultaneously reorientable communication systems, and the like, as will be readily appreciated by the person of ordinary skill in the art. Referring now to Figure 2, and in accordance with one embodiment of the invention, a system, generally referred to using the numeral 200, will be described, wherein active interference cancellation is applied between Inmarsat™ and Iridium™ transceivers mounted and operated on a same aircraft or vehicle. In this example, an Inmarsat™ Satellite Data Unit (SDU) 230 generates a signal to be transmitted at low power, which is amplified by a High Power Amplifier (HP A) 232 and fed to a directional Inmarsat antenna 202 (e.g. the interfering antenna) via a Diplexer Low Noise Amplifier (DLNA) 234. Only the transmit signal path is shown for clarity. On the other hand, an omnidirectional Iridium™ antenna 206 is mounted at a distance from the Inmarsat™ antenna 202 (e.g. commonly defining an intricate coupling path between the two antennas resulting in multiple signal reflections/paths, the combined effect of which effectively represented by FCOUPLING), ^an ^ communicates received signals to an Iridium™ Transceiver 204 (e.g. the susceptible receiver), which received signals originally comprise a combination of the desired Iridium™ signal and an Inmarsat™ interference component.

In this embodiment, a sample of the Inmarsat™ transmitter signal (e.g. interference signal 212) is processed by a cancellation signal generation module, provided herein by adjustable equalizer 210, configured to apply a cancellation function (e.g. FCANCEL) to this signal that attempts to mimic the characteristics of the signal path between the antennas (e.g. FCOUPLING), and thereby generate a cancellation signal appropriate for interference compensation, namely where FCANCEL substantially represents the inverse of FCOUPLING- Accordingly, the output of the equalizer 210 is applied to a combiner (coupler) 220, which sums it with the signal from the receiving antenna 206 to provide interference compensation resulting, if the equalizer is adequately adjusted, in improving reception of the wanted Iridium™ signal.

In this particular embodiment, given that the relative positioning of the antennas on the aircraft is known, the propagation delay between the antennas (e.g. path loss 236), which can generally yield a steep slope in the phase characteristics of the interfering transmissions, may be compensated for by interposing a substantially equivalent delay 238 in the communication path of the interference signal 212. Accordingly, the equalizer 210 may then be configured to provide a match for residual deviations from linear phase.

As will be appreciated by the person of skill in the art, successful interference compensation is dependent on proper adjustment of the equalizer 210, which in this embodiment, is performed by a control module, depicted herein as adaptation controller 218. As in the control module 118 of Figure 1 , the adaptation controller 218 is configured to receive as input feedback values sampled from the compensated Iridium™ signal (e.g. via coupler 224) so to provide equalizer adjustments as a function of a measure of the effectiveness of interference compensation. Namely, a measure as to the effectiveness of interference compensation can be implemented in real-time, wherein parameters guiding various aspects of the generated cancellation signal (e.g. phase, amplitude, delay, etc.) can be modified accordingly so to improve or maximize the interference compensation provided by the system. Various approaches to implementing such adjustments will be described in greater detail below.

In addition, the adaptation controller 218 can optionally be further configured to receive as input external data 225 provided to improve the system's responsiveness and/or efficiency, which may include, but is not limited to information such as a beam pointing direction of the interfering antenna (e.g. the Inmarsat™ antenna in this example). It will be appreciated that should the receiving antenna alternatively or additionally comprise a directional and/or reorientable antenna, information related to the orientation of this receiving antenna may also be provided in adapting the active cancellation system to current conditions. Furthermore, optional signal quality data 240, in this example provided by the Iridium™ transceiver, may also be taken into account in producing an adequate control signal for controlling the equalizer function.

In one embodiment (not shown), the feedback signal 226 is processed directly by the adaptation controller. In the embodiment of Figure 2, however, the feedback signal 226 may rather first be cross-correlated by cross-correlator 242 with the interference signal 212 in order to improve the sensitivity of the feedback path. In either case the controller 218 attempts to reduce or minimize the power of the feedback signal. Since there is no significant wanted signal contained in the injected cancellation signal 214, this action generally results in reducing or minimizing the interference level at the Iridium™ receiver input. Depending on the application at hand, different approaches may be implemented in manipulating the feedback signal such that, upon minimization, interference compensation is directed to the most significant component(s) of the interfering signal. For example, by filtering the feedback signal for frequencies having a greater impact on the quality of the susceptible signal, interference compensation may be maximized for such signal frequencies, rather than maximized for potentially larger interference contributions at frequencies of lesser significance or having a reduced impact on the overall performance of the susceptible system.

For instance, a particular constraint on the design of the adaptive control system for a particular application can arise from the nature of the interfering signal in this application. For example, a transmitter typically produces a strong communication signal that occupies an assigned channel along with a broad spectrum of noise at a much lower level, as shown for example by the interfering signal 800 of Figure 10. In the application being considered in this example, however, both the communication signal (e.g. peak transmitted signal) and the associated noise spectrum are problematic, albeit at differing levels. For example, the case may be that the transmitter noise floor (e.g. noise sidebands 820) extends into the adjacent receiver band of the susceptible receiver (e.g. as depicted by susceptible signal 850) and interferes directly, while the transmitted communication signal 800 causes compression and cross-modulation in the front-end of the susceptible receiver, which can in fact be the case in the example of the Inmarsat™ and Iridium™ transceivers. In this situation, the more serious mode of interference may arise from the broadband noise floor, which, while of lower amplitude, may in fact be more relevant to interference cancellation than the stronger communication signal peak. Accordingly, the system may be further configured to provide greater suppression of the noise floor, possibly at the expense of reduced suppression of the actual transmitter signal peak. For example, if implemented as depicted in Figure 2, the adaptive controller will naturally respond to the strongest components of the feedback signal, which in some embodiments, may lead to the most significant interference cancellation. However, where greater suppression would be beneficially applied to weaker, albeit more significant components of the interference signal, the feedback signal may be filtered to attenuate stronger and less significant components to allow the noise floor to dominate the adaptation process, for example. Namely, in the present example, a bandpass filter (not shown) centered on the Iridium band may be placed in the feedback path to favourably modify the response of the adaptation controller. While this approach may be less effective in the event that the dominant interfering signal is very close to the edge of the Iridium™ band, the frequency separation in this situation would be sufficiently small that adequate suppression could be attained even when the transmitted communication signal peak dominates the adaptation process.

Other constraints relevant in the design of embodiments specific to a particular application may also include the type and characteristics of the particular transceivers and/or communication systems considered. For example, the embodiment of Figure 2 allows certain constraints particular to this example to be overcome, which constraints may also be applicable in a variety of other applications. Namely, while the implementation of interference cancellation and adaptation can, in some applications, be more easily implemented in the digital domain, as in the embodiment described below with reference to Figure 8, the present example does not provide ready access to the inner workings of the Iridium™ system, and therefore, some if not all processes involved in interference cancellation may be more readily accessible and/or implemented in the RF domain (e.g. analog signal processing), which implementation can be readily achieved using the system design depicted in Figure 2. For example, in the present and similar examples where the susceptible receiver is externally supplied and operated as a "black box", cancellation may be more readily performed in the RF domain, with the resulting compensated RF signal applied directly to the receiver. In particular, the _C ANCEL or equalizer function can be implemented as an analog equalizer, typically with voltage-controlled elements to implement the required amplitude and phase characteristics, for example, thereby generating an analog interference cancellation signal that can be coupled in the analog domain with a received signal prior to processing by the susceptible receiver.

As will be appreciated by the person of ordinary skill in the art, the complexity of the interference coupling path and the level of compensation generally required to yield a compensated signal of sufficient quality, will generally dictate the complexity required in the equalizer. For example, Figure 9 shows the measured frequency-dependent characteristics of an unwanted coupling path between a typical pair of antennas (following the above example), wherein the upper trace 700 is the log magnitude of the unwanted signal and the lower trace 750 is its phase after removal of the slope due to propagation delay. Clearly, sufficient complexity in the equalizer is required in order to sufficiently mimic these characteristics.

Figure 3 provides an example of an equalizer, generally referred to using the numeral 300, and in accordance with one embodiment of the invention, that is usable in the present context and whose level of complexity can be designed to adequately reflect the level of complexity observed in the interference coupling path and the level of compensation accuracy/efficiency required by the application at hand. In this embodiment, the interference signal 312 is divided by an n-way power divider 350 and directed to a bank of bandpass filters 310, these filters having center frequencies disposed throughout the band of interest. Each filter has an associated variable attenuator 320 and variable phase-shifter 330, thereby providing control over its amplitude and phase contribution to the overall frequency characteristic. These adjusted signal components are then recombined by another n-way power divider 360 and output as cancellation signal 314. As will be appreciated by the person of skill in the art, while a dedicated inversion function may be implemented independently, it may also be achieved by selecting suitable settings for the phase-shifters 330. By choice of an appropriate number of filters with suitable bandwidths, the interference path may be modeled to the desired degree of accuracy. While a similar parametric equalizer could be implemented using filters having variable center frequencies and/or bandwidths, such implementation is not as easily achieved at microwave frequencies. Such implementations may nonetheless be considered herein and are therefor not meant to depart from the general scope and nature of the present disclosure.

Figure 4 provides an example of an optional RF cross-correlator, generally referred to using the numeral 400, and in accordance with one embodiment of the invention, that may be used in preconditioning the sample or feedback signal 426 (e.g. signal 226 of Figure 2) for processing by the control module. For instance, the provision of a cross- correlator may yield a less noisy feedback signal for the adaptation controller than is available by simply using the total power of the feedback signal sampled from the compensated signal. While the latter may be more easily implemented, in applications where a limited signal-to-noise ratio is available, direct feedback signal processing may not permit sufficiently rapid adaptation, particularly for a system configured to adapt to dynamically changing interference conditions, such as provided in the event of antenna reorientation. In this embodiment, both the sample or feedback signal 426 and interference signal 412 are split and provided to respective analog multipliers 460, wherein the interference signal 412 is divided by quadrature phase shift network 462 thereby phase-shifting one of the resulting interference signals by 90 degrees relative to the other. Each signal output from the respective multipliers 460 is then processed through a low pass filter 464 to derive an error magnitude output signal in complex form comprising an in-phase (I) output component and a quadrature (Q) output component. The derived complex signal generally retains the phase information as well as the magnitude of the input signals, and may form the basis for further deriving an error magnitude value for processing by the control module.

Figure 5 provides another embodiment of the optional RF cross-correlator. In this embodiment, sample/feedback and interference signals are processed in a manner similar to what has been described above in reference to Figure 4. However, each signal output from the respective multipliers 460 is processed through a low pass filter 464 and squaring function 466 before being summed by summer 468 and processed by a square root function 470, thereby providing an error magnitude value for processing by the control module (not shown).

Figure 6 provides an example of an optional IF cross-correlator, generally referred to using the numeral 500, and in accordance with another embodiment of the invention. In this embodiment, the interference (512) and sample or feedback (526) signals are first down-converted to an intermediate frequency (e.g. via local oscillator 572 and mixers 574, and IF filters 576) before reaching the analog multipliers 560, thereby bringing the multiplier operating frequency down to a range that is more readily manageable. This approach also provides the opportunity to perform the 90 degree split on the interference signal on a fixed frequency rather than the entire RF band, which can be relatively simpler to implement. The process then proceeds through low pass filters 564, respective analog to digital converters 565 to derive an error magnitude output signal in complex form comprising an in-phase (I) output component and a quadrature (Q) output component. The derived complex signal generally retains the phase information as well as the magnitude of the input signals, and may form the basis for further deriving an error magnitude value for processing by the control module.

Figure 7 provides another embodiment of the optional IF cross-correlator. In this embodiment, sample/feedback and interference signals are processed in a manner similar to what has been described above in reference to Figure 6. However, the signals proceed through low pass filters 564, respective analog to digital converters 565 and squaring functions 566, before being summed by summer 568 and processed by square root function 570 to provide an error magnitude value for processing by the control module (not shown). As will be appreciated by the person of ordinary skill in the art, different cross-correlator implementations, whether analog or at least partially digitally implemented, may be considered herein without departing from the general scope and nature of the present disclosure.

Referring now to Figure 8, and in accordance with another embodiment of the invention, a system, generally referred to using the numeral 600, will now be described. Again, this embodiment is presented in the context of the co-implementation of Inmarsat™ and Iridium™ systems, however, the person of skill in the art will appreciate that other applications may be readily considered herein without departing from the general scope and nature of the present disclosure. As in Figure 2, an Inmarsat™ Satellite Data Unit (SDU) 630 generates a signal to be transmitted at low power, which is amplified by a High Power Amplifier (HPA) 632 and fed to a directional Inmarsat™ antenna 602 (e.g. the interfering antenna) via a Diplexer/Low Noise Amplifier (DLNA) 634. Only the transmit signal path is shown for clarity. On the other hand, an omnidirectional Iridium™ antenna 606 is mounted at a distance from the Inmarsat antenna 602, and communicates received signals to an Iridium™ Transceiver 604 (e.g. the susceptible receiver), which received signals originally comprise a combination of the desired Iridium signal and an Inmarsat interference component. A digitized sample of the Inmarsat™ transmitter signal (e.g. interference signal 612 processed by analog-to-digital converter 680) is processed by a cancellation signal generation module, provided herein by a finite impulse response (FIR) filter 610, configured to apply a cancellation function (e.g. FCANCEL) to this signal that attempts to mimic the characteristics of the signal path between the antennas (e.g. FCOUPLING), and thereby generate a cancellation signal appropriate for interference compensation, namely where FCANCEL substantially represents the inverse of FCOUPLING- Accordingly, the digital output of the FIR filter 610 is converted back to analog by digital-to-analog converter 682, which analog cancellation signal 614 is then applied to a combiner (coupler) 620, which sums it with the signal from the receiving antenna 606 to provide interference compensation resulting, if the FIR filter is adequately adjusted, in improving reception of the wanted Iridium™ signal.

Again, successful interference compensation is dependent on proper adjustment of the FIR filter 610, which in this embodiment, is performed by adaptation controller 618. As in the control module 118 of Figure 1 , the adaptation controller 618 is configured to receive as input feedback values sampled from the compensated Iridium™ signal (e.g. via coupler 624 and analog-to-digital converter 684) so to provide filter adjustments as a function of a measure of the effectiveness of interference compensation. For example, samples of the compensated signal 626 may be fed to the adaptation controller 618, which can be configured to run a least mean square (LMS) algorithm or the like and thereby, as a result, adjust the tap weights at each sample interval in an attempt to improve the cancellation. External data (not shown) may also be used in adjusting control parameters.

In practice, the implementation of the foregoing scheme directly at microwave frequencies can be relatively more difficult and expensive than its analog counterpart (depicted for example at Figure 2), however, this and other such digital implementations may nonetheless be considered herein without departing from the general scope and nature of the present disclosure. For example, a design implementing a sampling rate sufficient only to track changes in the antenna coupling path may provide greater efficiency while still benefiting from at least some of the benefits of digital signal processing.

As will be appreciated by the person of skill in the art, the herein described active interference cancellation systems and methods may provide, according to different embodiments of the invention, a number of operational benefits and/or advantages. For example, the system can be self calibrating, thereby allowing for the automatic response to different interference levels generated in a variety of situations. Furthermore, once initiated, little to no user intervention is generally required, as the system can adaptively adjust itself to changing interference conditions. Further, preset configuration parameters may provide for increased responsiveness to known or predictable communication system variations (e.g. antenna reorientation, environmental conditions, etc.). Also, the system may be configured to self-adjust based on historical performance values or recalibrations, and thereby further improve the system's responsiveness and performance. This self calibration may further allow a system or system design to be adapted to different applications and/or installations without significant system redesign.

Also, the system may be implemented independently of the two communication systems at hand, wherein appropriate input and output signals may be operatively coupled to each system in providing adaptive interference cancellation without accessing the inner workings of either system. This feature may be particularly useful in embodiments where access to the inner workings of one or both communication systems is prohibited (e.g. as is the case for the Iridium™ system). Clearly, should access to the inner workings of the susceptible system be readily available, the adaptive interference cancellation system may be configured to leverage some of the functionality of this system and/or provide for greater integration of the compensation system within such inner workings, for example. It is apparent that the foregoing embodiments of the invention are exemplary and can be varied in many ways. Such present or future variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.