The present invention is related to interference cancellation in communication systems. More particularly, the present invention is related to the implementation of an adaptive variable-bandwidth integrated interference cancellation system that minimizes the effects of undesired signals on receiver performance.
A receiver can be subjected to undesired signals that are present over an operating bandwidth. Interfering signals can degrade the performance of wideband communication receivers. The undesired signals could be intentionally generated in order to jam or disrupt the receiver performance, or simply exist as a part of the surrounding signal environment. The interfering signals are classed as cosite or remote interferers. A cosite interferer is physically collocated with the receiver permitting a physical circuit connection from the interference generator to the receiver. A remote interferer is located far enough from the receiver to preclude a physical circuit connection. It is desirable to null interfering signals for improved performance. The design of the receiving antenna connected to the receiver and the physical separation between the interferer and the receiver antenna significantly affects the choice of the interference suppression system.
Often, adaptive antenna null-pattern generators are applied in order to minimize the effect of the unwanted signals on receiver performance. In other cases, the pattern of the antenna is not or cannot be adjusted. Instead, a sample of the interfering signal that is generated at a known location and having specific signal characteristics is obtained from an auxiliary antenna for the case of remote interference, or directly coupled from an interfering transmitter for the case of cosite interference. The system requires an auxiliary antenna, directional coupler, or multi-horn antenna to extract the interfering signal. The auxiliary antenna can be one of the horns in the main aperture of a multihorn array. However, some antenna systems have limited capabilities that adjust the antenna characteristics for obtaining a sample of the interfering signal through a known input location of the antenna system. One method of removing the interference is when the received signal is digitized and digital signal processing circuits can be used to filter out the undesired signals. Some receiver systems have limited processing capabilities for applying digital filtering techniques to a digitized version of the received signal. Instead all signals in the operating bandwidth are received, and adaptive filtering techniques are applied to these received signals to minimize the amplitude of any received but undesired signal. In this case, the undesired signal must first be detected, according to predefined criteria, and then isolated from the desired signals.
The general nulling function is well known and has been used in existing antenna systems. For Example, U.S. Pat. No. 5,729,829 discloses an interference mitigation method and apparatus for multiple collocated transceivers for band filtering of unwanted signals. Usually, a reference signal consisting of a non-coherent but correlated version of the undesired signal is obtained. The amplitude of the reference signal is equal to the amplitude of the interfering signal. The phase of the reference signal is set to 180xc2x0 different from the interfering signal so that when the reference signal is reinjected back into the received signal, the undesired signal is cancelled in order to create a transmission null at the location of the undesired signal. When the receiver is collocated with the interference, a portion of the interference signal can be coupled from the transmission path by a directional coupler or another physical connection. This sampled signal is phase-shifted by 180xc2x0 and vector-summed with the received signal. The 180xc2x0 phase shift is produced by a vector modulation circuit. This vector sum is adaptively adjusted to produce a null at the frequency of the interfering signal.
One method uses transversal filters and mixers to generate the canceling signal. Another method uses a personal computer and a computation intensive routine to control a programmable transversal filter that detects the undesired signal. In these cases, the reference signals are obtained by coupling through additional antennas or by special connections to the interference source. Antenna arrays are used in communications systems. The signals from the array elements are vector summed together to produce the received signal. With adaptive control, the array can adjust the antenna pattern to minimize the effect of remote interference. The adaptive adjustment of the phase and amplitude weights of the array elements generates an antenna pattern null in the direction of the interfering signal. In other cases, a main antenna is combined with auxiliary antenna elements as a sidelobe canceller. In this case, the interfering signal is sampled by the broadbeam auxiliary antennas placed near the main antenna. The vector sum of the auxiliary antenna signals and the main antenna signal is adaptively processed to null the interference. The success of these adaptive antenna techniques depends on an ability to resolve the locations of the desired and interfering signals, and provide equalization to achieve effective interference over the required bandwidth. In many cases, sufficient space is unavailable to implement an array large enough to resolve the desired signals and remote interference. A wideband communication applications might preclude channelizing the operating bandwidth by a fixed channelization scheme or by a tunable bandpass filter, or by a lack of sufficient dynamic range to process large signal amplitudes. The above nulling systems use only relative signal power to determine whether a received signal is to be nulled. Adaptive filtering techniques could be applied to the unknown signals, but these techniques require initial conditions in the filter that depend on the characteristics of the received signals.
Usually prior cancellation methods require adjustment of the antenna pattern to create nulls for cancellation of unwanted signals, or external feeds containing unwanted signals that are then cancelled. In both cases, apriori knowledge is required. These prior methods typically use a narrowband tunable bandpass filter as a preselector at the front end of the receiver. The front-end preselector has a disadvantage in a wideband communications receiver. The narrowband preselector would filter out most of the desired signal along with an interfering signal. Series tunable band-notch filters could be placed before the receiver. The bandpass and bandnotch filtering methods are serial in-line processes that reduce the reliability of the receiver. When the tuning mechanism in the preselector fails, the filter may lock at one center frequency, other signals cannot be received. The disabled filter would then significantly and permanently degrade receiver performance in part of the passband. These and other disadvantages are solved or reduced using the invention.
An object of the invention is to provide cancellation of unwanted received signals received by a communication receiver
Another object of the invention is to provide cancellation of unwanted signals having predetermined frequency, amplitude and modulation criteria.
Yet another object of the invention is to provide scanning by searching selected frequencies for unwanted signals having predetermined frequency, amplitude and modulation criteria and to cancel the located unwanted signals to result in desired received signals.
Still another object of the invention is to provide an adaptive variable bandwidth cancellation system for isolating and canceling unwanted signals having predetermined frequency, amplitude and modulation criteria.
The present invention is directed to a microcontroller based adaptive variable-bandwidth cancellation system for use in a wideband communication receiver system. The cancellation system is placed in parallel with and becomes part of a receiver. The use of a microcontroller allows for flexibility in defining the characteristics of the interfering signal. The preferred cancellation system provides narrowband and wideband cancellation nulls for canceling unwanted interfering signals. The limitation on null depth is caused by the finite resolution of the phase-shift transmission lines and attenuation steps. The signals within the scanned frequency bandwidth are detected in a detection path and parameterized according to frequency, amplitude or modulation, such as pulse-width modulation or continuous wave modulation. These characteristics are then compared against the definition of an undesired signal that is stored in the microcontroller. When an undesirable signal is detected, a tunable reference path is set so as to cancel the undesirable signal from the received signal and so as to reduce the undesirable signal detected signal. Iterations of detection and cancellation achieve desired cancellation of the unwanted signal using adaptive cancellation. The detection path is tunable for scanning across step bandwidths for detecting unwanted signals of interest. Once an undesirable signal is located at a particular frequency location, the tunable reference path is tuned to that particular frequency location to isolate the undesired signal that is then inverted and added to the composite receive signal to cancel the unwanted signal from the composite receive signal to provide only desired received signal with the detected unwanted signal canceled. The reference path serves to isolate an undesired signal from the desired signals, and then serves to amplify and shift the phase of the undesired signal for nulling summation with the original received signal that is delayed for coherent nulling. Once an undesired signal has been detected, the microcontroller sets the values of the reference path components according to a predetermined look up table. The undesired received signal is continuously fed into the detection path for monitoring the effect of the cancellation and when further cancellation is needed, the reference path is appropriately tuned to remove the undesired signals. The microcontroller adaptively continuously scans the receiver bandwidth and monitors the detected signal from the detection path searching for unwanted interfering signals, and characterizes the detection signals, and then tunes both reference path and detection path circuit parameters to maximize detection of unwanted signals to minimize the amplitude of an undesired signal in the surviving received signal. The microcontroller-based system is preferred for signal detection and evaluation. Undesired signals are detected and isolated at a location internal to the cancellation circuitry. Tunable bandwidth bandpass filters in the detection path and reference path are used to generate wideband or narrowband nulls depending on the signal characteristics. The controller continuously monitors the cancellation result by sensing the detection signal from the detection path, and adaptively minimizes any residual of the interfering signal.
The use of a programmable microcontroller allows for flexibility in the detection and classification of interfering signals. In the preferred form, the microcontroller searches for either narrowband or wideband signals. Detection threshold amplitudes can also be varied as a function of frequency. The cancellation system can be used in space or airborne applications wherein weight, size, and power are prime considerations. The system also has applications in the commercial sector where receivers, such as GPS receivers, are used near emitters at the same frequency or at multiple harmonics of television or radio station frequencies. The microcontroller can be efficiently programmed without floating-point mathematics, matrix inversion, or other higher mathematical functions. The controller can be programmed so that different classes of signals are cancelled depending on signal parameters, such as frequency, pulse-width, and amplitude a stand alone configuration with external controls. In the event of system failure, the cancellation function can be disabled without affecting reception of the received signal by the operating receiver.
The microcontroller enables flexible programming and adaptive control allowing for compensation of component performance drift over lifetime and environments in which the system cannot be reached for manual repair or replacement of parts. Flexible control of the signal detection system allows for a relatively large detection range of over 60 dB using amplitude detection techniques. The null also can be located within a wide bandwidth according to a relatively coarse calibration table. The microcontroller can then measure the null efficiency and adjust the cancellation performance to improve the null. In an exemplar form, the system can generate 5 MHz wideband nulls with 15 dB in cancellation or narrowband bandwidth nulls with 30 dB in cancellation over a 100 MHz to 160 Mhz operating bandwidth. These and other advantages will become more apparent from the following detailed description of the preferred embodiment.