U.S. patent application number 10/234434 was filed with the patent office on 2004-03-04 for method and apparatus to provide communication protection technology for satellite earth stations.
This patent application is currently assigned to Electro-Radiation Incorporated. Invention is credited to Casabona, Mario M., Paulson, Paul H. III, Rosen, Murray W..
Application Number | 20040042569 10/234434 |
Document ID | / |
Family ID | 31977412 |
Filed Date | 2004-03-04 |
United States Patent
Application |
20040042569 |
Kind Code |
A1 |
Casabona, Mario M. ; et
al. |
March 4, 2004 |
Method and apparatus to provide communication protection technology
for satellite earth stations
Abstract
An interference signal canceling system for communications
protection is described for canceling interference signals from
earth station received signals using a two input adaptive
cancellation network. The present invention addresses terrestrial,
co-channel and adjacent satellite interference sources resulting
from emitting, isolated, frequency re-use and polarized sources.
Main and auxiliary signals are derived directly or by coherent
conversion to intermediate frequency using an auxiliary antenna,
cross-polarized feed, or auxiliary squinted feed. Filters set
receive and canceling bands. Cancellation combines the main signal
with a phase, amplitude and time modulated sample of the auxiliary
signal. A receiver correlates the auxiliary signal with a sample of
the combined output to minimize interference. A control drives the
modulator from internal measurements, or satellite receiver
measurements, i.e., BER, C/No, SNR, etc., or both. The control
searches the modulation space to locate interference nulls followed
by null acquisition and tracking to maximize cancellation effects.
The present invention is also configurable for multiple
interferences using series and parallel arrangements.
Inventors: |
Casabona, Mario M.; (Cedar
Grove, NJ) ; Rosen, Murray W.; (Parsippany, NJ)
; Paulson, Paul H. III; (West Milford, NJ) |
Correspondence
Address: |
WOLFF & SAMSON, P.C.
ONE BOLAND DRIVE
WEST ORANGE
NJ
07052
US
|
Assignee: |
Electro-Radiation
Incorporated
Fairfield
NJ
|
Family ID: |
31977412 |
Appl. No.: |
10/234434 |
Filed: |
September 3, 2002 |
Current U.S.
Class: |
375/346 |
Current CPC
Class: |
H04B 1/109 20130101;
H04B 7/18513 20130101 |
Class at
Publication: |
375/346 |
International
Class: |
H04L 001/00; H03D
001/04; H03D 001/06; H04B 001/10; H03K 006/04; H04L 025/08 |
Claims
We claim:
1. A system for communication protection that reduces interference
signals in L-band, C-band or Ku-band satellite down links resulting
from terrestrial, co-channel or adjacent channel satellite
interference signals resulting from a variety of sources, the
system comprising: Antenna or interface means for receiving the
main satellite down link signal directly or by down conversion to a
convenient intermediate frequency, An auxiliary reference input
port means for receiving and discriminating in band interference or
jamming signals using either a separate auxiliary antenna, antenna
feed or transmission line reference signal, used either directly or
by down conversion to the same intermediate frequency, Coherent and
synchronized down conversion means for both main and auxiliary
signals effectively using the same reference local oscillator from
a common source for injection into the main conversion path and to
the auxiliary conversion path, phase locking or injection locking
the local oscillator of the auxiliary conversion path to the local
oscillator of the main conversion path, or providing a common
reference clock signal from a common source to all conversion paths
using a separate phase lock loop (PLL) local oscillator circuit for
each conversion path, etc., A group delay equalization network in
the main and/or auxiliary paths to match the cumulative delays in
the main and auxiliary signals so that the interference signal
components substantially correspond in the frequency range or
bandwidth of interest, An auxiliary channel filter in the auxiliary
signal path to select a segment of the channel or band and to
discriminate against out-of-channel or out-of-band such that the
signal output corresponds to the interference signal of interest,
An automatic gain control (AGC) in the auxiliary signal path to set
the dynamic range of the auxiliary signal path to substantially
match the signal levels of interference on the two signals to the
gain and dynamic range of the signal modulator. A reference coupler
connected to the auxiliary input port following delay and gain
control with a portion of the reference signal being provided for
modulation and substantially representing the interference signal,
the remaining portion of the reference signal being provided as a
reference local oscillator for correlation with a sample of the
system output error, A summing coupler connected to the main input
port following delay control with the input port coupled to the
main received signal, the second input port coupled to a modulated
sample of the auxiliary input signal, and the output port being the
sum of the two signals providing the cancellation signal, A channel
filter in the post-cancellation error signal path to establish the
bandwidth of the main channel and interference signal and the
bandwidth of the cancellation process either coupled to the output
port of the summing coupler after cancellation and whose output is
provided to the input to the output coupler, or coupled to the
error monitor port of the output coupler and whose output is
provided to the correlating receiver as the error signal, An output
coupler coupled to the output of the summing coupler or the output
of the channel filter receiving a signal corresponding to the sum
of the main receive signals and the modulated auxiliary signal, and
corresponding to the cancellation or error signal, A correlating
receiver having a first input coupled to the cancellation or error
signal and a second input coupled to the auxiliary input reference
coupler and responsive to these two signals where the reference
signal serves as the local oscillator, the correlating receiver
determining the phase and amplitude relationship between the
reference and error signals, and providing a two port output
conveying a measure of the error signal correlated to the reference
signal in a defined bandwidth, either as quadrature (I/Q) signals,
or as magnitude and phase signals, A system control having two
inputs corresponding to the two output ports of the correlating
receiver and responsive to error or cancellation signal magnitude
and phase, applying the interference detection and suppression
algorithm by providing two output ports to a signal modulator to
modulate the auxiliary reference signal to cancel the interference,
an output port to optimize the delay mismatch between the main
received and the auxiliary reference signals, an output port to set
the gain and dynamic range of the auxiliary reference signal into
the process, and an output port consisting of a system data bus to
communicate between control functions in other channels or bands
and in an integrated manner with the satellite receiver to receive
performance measurements of the channel, A signal modulator with
input port connected to the reference coupler and output port
connected to the second input port of the summing. coupler,
electrically coupled to the system control, and responsive to the
amplitude and phase controls for adaptive adjustment of the
auxiliary signal to generate the canceling signal that is equal in
level and opposite in phase with respect to the interference in the
main channel.
2. The system of claim 1, wherein the system controller and signal
modulator are responsive to align the signal modulator to suppress
interference using the outputs of the correlating receiver.
3. The system of claim 1, wherein the system controller and signal
modulator are responsive to align the signal modulator to suppress
interference using the information on satellite signal processing
performance provided by the satellite receiver.
4. The system of claim 2, wherein the system controller, signal
modulator, delay equalizer and AGC are responsive to align
modulators and equalizers to suppress interference from moving and
varying interference sources resulting in dynamically adaptive
operation.
5. The system of claim 1, wherein the system is responsive to
terrestrial interference.
6. The system of claim 1, wherein the system is responsive to
co-channel interference.
7. The system of claim 1, wherein the system is responsive to
adjacent satellite interference.
8. The system of claim 1, wherein the system is responsive to
frequency re-use interference.
9. An interference cancellation system composed of a number of
systems of claim 1 configured in a serial arrangement wherein the
leading system defines the cancellation channel bandwidth and
configured to cancel multiple uncorrelated interference signals
each defines separately by an auxiliary reference signal, and
providing interference cancellation in a common channel of
operation for a satellite receiver.
10. An interference cancellation system composed of a number of
systems of claim 1 configured in a parallel arrangement wherein
each system defines a separate and distinct channel bandwidth and
configured to cancel multiple correlated or uncorrelated
interference signals defines separately by multiple auxiliary
reference signals or a reference signal in multiple channels, and
providing interference cancellation for multiple satellite
receivers.
11. An interference cancellation system composed of a number of
systems of claim 1 configured in a combination of the serial and
parallel arrangements of claims 9 and 10, wherein each system leg
defines a separate and distinct channel bandwidth, each system
element within each leg defines a separate interference component
within the channel bandwidth, each system element is defined
separately by auxiliary reference signals, and providing
interference cancellation for multiple satellite receivers.
12. A method for canceling an interfering signal using the system
in claim 1 and providing an output from a main received signal and
an auxiliary reference signal, the method comprising: A sample of
an interference signal from an auxiliary input used as a reference
signal; A sample of a main receive signal composed of the desired
receive signal and interference signal combined with a modulated
variant of the reference signal, and providing an error signal
corresponding to the interference; A correlation between the
reference and error signals to generate a set of control signals
for operation of modulators and equalizers; Adjusting the auxiliary
signal gain and level to match the dynamic range of the
interference in both the main and the reference signals; Adjusting
the delay mismatch between the main and reference signals to match
the delay or time of the interference in both the main and
reference signals; Adjusting the amplitude and phase of the
modulated reference signal over the control space of the modulators
using a coarse or sparse scan of both dimensions to detect
cancellation or null settings; Adjusting the amplitude and phase of
the modulated reference signal using varying control resolution and
following a down hill gradient to optimize the cancellation of the
interference.
13. A method for canceling an interfering signal using the system
in claim 1 and providing an output from a main received signal and
an auxiliary reference signal, the method operates in conjunction
with the method in claim 12 and comprising: A sample of the signal
processing performance measurement of the satellite receiver as a
secondary error signal; Adjusting the delay, amplitude and phase of
the modulated reference signal using varying control resolution and
following a down hill gradient to further optimize the cancellation
of the interference.
Description
DESCRIPTION OF THE INVENTION
[0001] 1. FIELD OF THE INVENTION
[0002] Interference in satellite down links arises from several
sources: terrestrial telecommunication sources; cross-polarization
sources from channel frequency reuse; and adjacent satellite
sources. These interference sources can occur independently or in
combination to limit the performance of L-Band, C-Band and Ku-band
satellite downlinks in many locations. The present invention
relates to an adaptive signal canceling system and a system and
method that can be configured for the cancellation of one or more
interference signals to permit a communication satellite down link
signal lying in the same band or channel(s) to be received and
processed. The procedure exploits the ability to resolve each
source of interference using an auxiliary sense antenna or
auxiliary feed separate from the earth station main or primary
antenna feed, coherently correlate this (these) sample(s) with the
interference component of the received signal, and adaptively
suppress the interference in an intermediate band going to the
satellite receiver.
[0003] The typical satellite earth station down link operates in
L-Band, C-Band or Ku-Band with interference entering into the main
satellite receiver at the antenna. The typical extended C-Band
transponder down link operates in the 3,400 to 4,200 MHz (or the
conventional C-Band being 3,700 to 4,200 MHz), and the down link is
generally converted to an intermediate frequency at 70 MHz, 140 MHz
or block conversion to L-Band 950-1,750 MHz (or 950-1,450 MHz) at
the antenna using a low noise amplifier and block converter (LNB)
for local distribution from the antenna to the receiver at the
earth station. Interference to C-band satellite downlink reception
commonly arising from several sources including terrestrial
interference, cross-polarized channel interference, and interfering
signals from adjacent satellites enters the process at the antenna
via main lobe, side lobe or back lobe coupling, or via anomalies in
the satellite or earth station antenna or feed. In essence, the
received input signal can contain both the interference signal(s)
and the desired communication signal in the same frequency band or
channel, where these signals can share common modulation properties
and bandwidths, and can have an arbitrary relative amplitude
relation that impacts the signal processing capability of the
receiver.
[0004] The need exists for a canceling system that permits a
satellite communication signal to be received and processed in the
presence of interference from one or more sources in the same band
or channel. Such a canceling system is applied by the present
invention to the down link signal processing for satellite earth
stations. It is the object of the invention to provide a signal
canceling system for suppressing interference from a received input
signal, and to be configurable in multiple channels of operation to
cancel multiple interferences from a received input signal in the
same channel or in different channels. The object of the invention
is to use a sample of the interference derived via an auxiliary
antenna or feed to produce a canceling signal from the source of
interference that is combined with the received signal to suppress
the interference signal in the output. The object of the invention
is to adaptively cancel the interference using measurement
techniques that correlate the auxiliary interference signal with
the level of interference in the output signal, and/or correlate
the auxiliary interference signal with a measurement of signal
processing performance in the victim satellite receiver. The object
of the invention for multiple instances of interference is to
provide cumulative cancellation of one or more interference sources
using separate auxiliary signals and separate cancellation channels
configured in series and/or parallel arrangements. SUMMARY OF THE
INVENTION
[0005] One object of the present invention is to provide an
interference suppression system for satellite down link
communication which exploits the common mode aspects of man-made
interference observed via two paths to cancel in band interference
present on the main receive signal and available on an auxiliary
signal. The forms of satellite receive interference addressed by
the invention include terrestrial interference, cross-polarization
or co-channel interference from frequency re-use, and adjacent
satellite interference. The present invention cancels both
narrowband and wideband interference signals and noise.
[0006] It is a further object of the present invention to provide
an antenna and signal preprocessing system that coherently
processes main and auxiliary received signals to adaptively cancel
common in band components.
[0007] Another object of the present invention is to receive the
interference signal using one port of an adaptive microwave network
and to sample the interference signal so as to modulate the
combined interference signals and satellite signal and to null out
the interference signal in the one port to the satellite
receiver.
[0008] Still further, a general objective of the present invention
is to coherently detect and modulate the (high-level) interference
signal in a correlating receiver in the canceling system without
the need to directly process the satellite signal.
[0009] Another general objective of the present invention is to use
the processing capability of the satellite receiver of interest to
provide a monitor of the impact of (low-level) interference and
effects on portions of the recovered/processed signal band of
interest, under favorable signal-to-noise situations, to optimize
the quality of the received signal.
[0010] Yet another general objective of the present invention is to
adaptively cancel interference without incurring significant losses
or changes to the main signal.
[0011] Another general objective of the present invention is to
partition the main and auxiliary antenna circuitry such that the
adaptive cancellation system may be located near the satellite
receiver and coherent band conversions may be remotely located and
powered.
[0012] Another general objective of the present invention is to use
multiple implementations of the adaptive cancellation configuration
and system modularity to address multiple instances of independent
interference in a channel or band, or in adjacent or non-adjacent
channels or bands. The object being to use serial and/or parallel
implementations of the invention with proper filters and control to
independently address interference sources.
[0013] According to these and other objects of the present
invention, there is provided sets of coherently operated receive
channels that provide main and auxiliary signals that allow for
adaptive cancellation of interference signals common to main and
auxiliary cannels. The main and auxiliary signals are filtered,
amplified, and transmitted from antenna conversion to the adaptive
cancellation system using separate cables. The main signal is
essentially controlled in delay with little variation in amplitude
and phase, except to amplify the signal. The auxiliary signal is
controlled in relative amplitude, phase and delay, and combined
with the main signal to cancel common interference signals.
Cancellation is accomplished by combining the auxiliary signal with
the main signal in approximately equal delay, equal amplitude and
1800 relative phase with regard to the common interference signal.
Control of the replica of the auxiliary signal in amplitude and
phase used in this process is derived from the coherent detection
of the interference at the output of the process, or input to the
satellite receiver, using the auxiliary signal as the reference or
local oscillator. A control circuit minimizes the relative delay
between main and auxiliary signals, and sets the gain of the
auxiliary channels to match the relative amplitude ranges of the
channels. The control circuit sets the modulated auxiliary signal
using a search of the modulator phase and amplitude control space
to locate interference nulls in the monitored signal. The control
implements acquisition and tracking of the detected null to
optimize suppression of the interference using an energy
minimization technique, satellite receiver performance optimization
criteria, or a combination of the two techniques. Under a no
interference condition, the adaptive cancellation system reduces
the contribution of the auxiliary channel in the combined output by
attenuation or switching. The present invention also addresses
multiple interference sources and multiple channels of operation by
linking a cascade or series arrangement of the invention, and/or a
cascode or parallel arrangement to suppress multiple, independent,
channel interference signals.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 is a simplified model of the interference environment
showing how the desired receive signal is summed with interference
and noise.
[0015] FIG. 2 is a simplified model of the communications
protection system processing solution to cancel interference by
modulating a sample of the interference signal to be equivalent in
amplitude and 180.degree. out of phase, and summing the signal with
the received signal so only the desired signal remains.
[0016] FIG. 3 is a high-level block diagram of the implementation
of the communications protection system showing a main and
auxiliary channel coherent frequency translation to an intermediate
frequency for the communication interference suppression unit
(CISU) embodiment and interface with a satellite receiver.
[0017] FIG. 4 is a functional block diagram of a serial embodiment
of multiple interference cancellers configured to suppress multiple
interferences in a common main band of interest according to the
present invention.
[0018] FIG. 5 is a functional block diagram of a parallel
embodiment of multiple interference cancellers configured to
suppress multiple interferences in separate auxiliary bands of
interest according to the present invention.
[0019] FIG. 6 is a functional block diagram of a serial/parallel
embodiment of multiple interference cancellers configured to act in
concert against interferences in a mixture of multiple in band and
multiple separate bands of interest according to the present
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0020] The interference cancellation system of the invention
operates directly at RF or at an intermediate frequency (IF)
developed by coherent frequency translation or conversion at the
antenna to facilitate signal distribution to the earth station
satellite receiver. The cancellation system can act on a band or on
a channel in a band as determined by filters in the system. The
system can be implemented directly at RF or at IF using analog
modulation techniques or digital signal processing (DSP).
[0021] Interference encountered by satellite earth stations results
from terrestrial sources, cross-polarization or co-channel
interference, or adjacent satellites. The model in FIG. 1
characterizes the interference phenomenon showing the summation of
the desired signal, interference signal and noise. Terrestrial
interference can result from microwave communications sources
operating in or near satellite downlink bands or channels and
generally within direct line-of-sight. Multipath and reflections
can result in strong interference. Interference combines with the
downlink signal entering the system directly at RF in the skirts of
the main lobe, side lobe or back lobe of the main antenna, or via
coupling into the LNA, LNB conversion blocks, or transmission
lines. Interference sources can be stationary or moving, with
levels substantially higher than the satellite signal.
[0022] Co-channel interference can result from frequency re-use of
the alternate polarization of the channel. Interference can result
from poor isolation or misalignment between polarizations at the
satellite, earth station antenna, or because of satellite viewing
orientation and sharp angles close to the east/west horizons.
Co-channel interference levels may be equivalent to the satellite
signal of interest. Adjacent satellite interference occurs when
closely spaced satellites with common channels drop into view of
the earth station antenna due to broad main lobe beamwidth, poor
pointing between satellites or high adjacent radiated signal
levels. Adjacent satellite signal levels can match or exceed the
desired signal. Co-channel and adjacent channel interference may
vary dynamically with channel programming.
[0023] Getting a sample of the interference signal using an
auxiliary antenna, feed or a reference source and combining the
signals adaptively to null the interference can accomplish
cancellation of these forms of interference. The model in FIG. 2
illustrates the adaptive cancellation process of the present
invention using a detection and suppression algorithm, a sample of
the interference signal, and modulating the sample and combining it
with the received signal to cancel the interference component in
the received signal.
[0024] Theoretical cancellation or suppression in a wide bandwidth
adaptive cancellation system is limited by the degree of mismatch
and control between main and auxiliary channels. Cancellation ratio
(CR) is an established metric used to specify how well two channels
in an adaptive cancellation system are matched. The following
describes the system requirements to meet a desired objective CR.
Ultimately CR may be bounded by signal-to-noise factors. In
general, cancellation performance must address amplitude, phase,
frequency and time matching error sources and control resolutions.
The following equation characterizes the CR in dB as a function of
these errors and resolutions:
CR(dB).congruent.10Log(1+.alpha..sup.2-2.alpha. cos
(2.pi..function.T+.phi.))
[0025] where, signal amplitude error ratio, .alpha., is defined for
.alpha.>1, phase error, .phi., is defined for differential phase
relative to anti-phase (i.e., 180.degree. or .pi.), frequency
error, .function. is defined as the frequency offset or one-half
the bandwidth, and time/delay mismatch, T, is defined as the
difference or error in apparent group delay between the signals at
the frequency offset.
[0026] FIG. 3 is a block diagram of the implementation of a single
cancellation channel of the communications protection system. The
diagram shows a main and auxiliary channel coherently translated to
an intermediate frequency for the Communication Interference
Suppression Unit (CISU) embodiment shown, and interface to the
satellite receiver. The intermediate frequency interface
facilitates the physical separation of antenna, interference
preprocessing and satellite receiver locations and the use of
coaxial transmission lines. The auxiliary signal is generally the
output of a secondary feed or antenna, or a reference signal. For
the case of terrestrial interference, an antenna directed at the
terrestrial source can provide the auxiliary signal. Such an
antenna can provide a degree of spatial discrimination and gain.
Care must be taken in the placement and selection of auxiliary
antenna properties to match the interference source while
discriminating against other interference signals in the antenna
beam. The auxiliary path can provide a way for interference to
enter the system. If the source of interference were moving, the
auxiliary antenna would broader in beam to cover the expected field
of view of the interference. When the terrestrial source is
collocated with or near the earth station, a hard reference signal
line may be available. The system would equalize or compensate for
the transmission line delay. For the case of co-channel
interference, a cross-polarized antenna feed of the main antenna
can provide the auxiliary signal. For the case of adjacent
satellite interference, a squinted auxiliary feed for the main
antenna can provide the auxiliary signal. The squinted feed
polarization may have to be optimized to provide the best reference
signal for cancellation.
[0027] As shown in FIG. 3, the main and auxiliary antenna outputs
or feed signals are coherently converted to the operating
intermediate frequency of the cancellation system. The main signal
path is composed of the desired signal and interference signal. The
auxiliary signal path is composed of the interference signal with
the common interference signal dominant. Both paths are essentially
linear and operated in small signal mode. In most cases, the
waveguide interface between the conversion and antenna feed
provides a degree of signal filtering. Additional filtering can be
placed in this path to increase out-of-band rejection and to
attenuate out-of-band signals. A low noise amplifier (LNA) is
generally used to define the front-end noise figure of the system
for each signal with sufficient gain to overcome losses in later
stages of processing and cables. RF conversion to IF is generally
accomplished in low noise block converters (LNB). Coherent
conversion can be accomplished by several means: providing a
reference local oscillator signal from a common source for
injection into the main conversion path and to all auxiliary
conversion paths; phase locking or injection locking the local
oscillator of the auxiliary conversion path(s) to the local
oscillator of the main conversion path; providing a common
reference clock signal (e.g., 10 MHz) from a common source to all
conversion paths using a separate phase lock loop (PLL) local
oscillator circuit for each conversion path; etc. The main signal
and auxiliary signal are provided to the CISU at RF or at IF. The
main signal path includes a programmable delay equalizer to balance
the group delay between the two signal paths. Delay equalization
maximizes the cancellation bandwidth of the process for broadband
interference. The interference suppression algorithm includes a
procedure to optimize delay mismatch and to set amplitude and phase
for interference suppression.
[0028] The main signal is received through the antenna main lobe
and interference enters the satellite receiver through the
satellite antenna main lobe, side lobe, or back lobe. An auxiliary
antenna or feed senses the common interference signal at a
different amplitude and phase. The two signal paths are sampled and
combined using the coupler arrangement shown in FIG. 3 and
subsequently processed by a correlating receiver. The combined
signal at the first summing coupler output consists of the desired
signal and the interference accompanying the signal, and a
modulated replica of the interference signal in the auxiliary
input. The auxiliary input signal is filtered using an auxiliary
band pass filter (BPF) to define the interference signal spectrum
and to attenuate signals outside the band or channel. A DC bias tee
may be used in the auxiliary input path to provide remote de power
via the coaxial cable to the auxiliary LNA/LNB and reference
oscillator. A DC continuity path is also be provided through the
main signal path to route any DC bias from the satellite receiver
to the main LNA/LNB.
[0029] The level of the auxiliary input sense signal can vary
widely between terrestrial, co-channel and adjacent satellite
interference conditions. Automatic Gain Control (AGC) is used to
set the dynamic range of the auxiliary signal path and match the
signal levels of interference on the two signals to the gain and
dynamic range of the signal modulator. The auxiliary signal is
divided and modulated for cancellation, and used as the
interference reference local oscillator for the correlating
receiver. The relative amplitude and phase of the interference
signal in the auxiliary signal cancellation path are applied using
amplitude (.kappa.) and phase (.phi.) modulation controls as shown.
Signal modulation can be implemented in several ways: vector
modulator using bi-phase modulators, PIN modulators, varactor phase
shifters, etc. The arrangement provides adaptive adjustment of the
amplitude (gain/attenuation) and the phase of the interference
signal to generate a canceling signal that is equal in level and
opposite in phase with respect to the interference in the main
path. Proper adjustment of amplitude and phase results in recovery
of the desired communication signal with common interference
suppressed.
[0030] The restored output signal is sampled in the second coupler
for the correlating receiver. The restored signal path is filtered
using a main filter to define the signal band or channel of
interest. The main filter can be placed between the couplers to
define the main channel, or in the arm of the coupler to the
correlating receiver to define the interference band. The
correlating receiver produces a measure of the interference
residual in the restored signal path using the sample of the output
signal to the satellite receiver and mixing it with the amplified
interference sense signal in the auxiliary path acting as the local
oscillator.
[0031] Quadrature mixing and complex processing in the receiver
supports correlation and adaptive control of amplitude and relative
phase sense. The quadrature mixing outputs indicate the phase and
amplitude difference of the two interference signals entering the
correlator since the two frequencies are the same. Filtering these
outputs with a low-pass filter insures that only signals that are
close in frequency produce an error output. The two error signals
are the in-phase (I) and quadrature-phase (Q) components of the
correlation of the microwave signals over a time period equal to
the inverse bandwidth of the filter. If the two signals do not
correlate, then both error signals are zero. That is, the
interfering signal has been successfully cancelled at the receiver.
The two error signals I and Q drive the attenuator and phase
shifter controls of the signal modulator in a null seeking mode to
insure a null at the receiver.
[0032] To generate control signals for adaptive cancellation and
control the attenuation and phase shift in the modulator, we obtain
the measure of interference remaining in the restored signal path
and develop proportional controls. The control function digitally
encodes the error signal from the correlating receiver and develops
the error magnitude and sense. This error signal is processed to
generate analog/digital controls to the modulators, delay equalizer
and AGC functions. The control shown consists of analog-to-digital
conversion (ADC) of the error signal, system processing in a
microprocessor to produce control signals, and digital-to-analog
conversion (DAC) of the control signals to drive analog RF
modulators. Digital look-up and calibration tables may be used to
linearize the analog components over temperature and frequency.
[0033] The control function implements the control and cancellation
algorithms for interference detection, auxiliary signal AGC, main
signal delay control, and for interference suppression search,
acquisition and track. Interference detection identifies
interference conditions by measuring the interference level at the
receiver interface against defined thresholds. Gain control sets
the dynamic range of the auxiliary path to establish the proper
level for correlation and cancellation. The search algorithm
coarsely scans the control space of the signal modulator to rapidly
determine candidate nulling regions in the control space as
measured in the correlating receiver. The search of modulator
control space covers over a full cycle of phase and the equivalent
in amplitude using a coarse resolution sparse search or linear
stepped routine to locate drops in post-cancellation interference.
An alternate modulator control space can use I/Q control.
[0034] The acquisition algorithm selects the best null candidate
and maximizes the interference null using a variable resolution
control of the modulator control space based on the
post-cancellation error signal. The variable resolution controls
include changes to modulator step size, stepping rate, etc. The
track algorithm maintains the maximum interference null. By
adjusting the amplitude and phase of the added auxiliary signal, we
can arrange that it cancel or suppress the interfering signal in
the receive channel. Since the relative amplitude and phase of the
interfering signal may vary due to relative motion, vibration,
frequency changes, fading, environmental factors, multipath, etc.,
the system continuously adjusts the phase, amplitude and relative
delay of the canceling signal to maintain the nulled condition at
the receiver.
[0035] The control function operates automatically and includes a
system interface bus and receiver interface that allows control to
monitor the performance of the external satellite receiver for
complementary control capability. The control can examine receiver
performance parameters, i.e., Bit Error Rate (BER),
Carrier-to-Noise ratio (C/No), Signal-to-Noise Ratio (SNR), etc.
When the receiver interface is available, the control can use the
CISU internal correlating receiver to suppress the interference
level to the satellite receiver, then use the satellite receiver
performance measure to further optimize operation. When using the
internal correlating receiver, the control tracks the gradient of
the measured correlated interference level in a down hill manner to
the noise sensitivity of the CISU system in the processing
bandwidth. When using the external satellite receiver performance
measure, the control tracks the gradient of the appropriate
detection parameter, i.e., to minimize BER, maximize C/No, maximize
SNR, etc. This secondary tracking capability can improve
suppression performance of the cancellation system below the noise
sensitivity of the CISU system.
[0036] An alternate implementation of the modulation and control
processing can use software radio concepts and Digital Signal
Processing (DSP) techniques whereby the main and auxiliary signals
from the LNA/LNB converters are filtered, coherently down converted
to a convenient IF for digital processing, encoded to digital
format by ADCs, and digitally down converted (DDC) to a base band
or zero IF, decimated and filtered for complex processing, or
processed as real signals. Group delay equalization can be
performed using digital delay and/or digitally controlled analog
delay techniques. Conversion and filtering can match the satellite
receiver channel or band constraints for processing. The control
and interference cancellation algorithms are implemented in digital
processing, and the digital output of can be provided to the
satellite receiver. Processing may utilize a variety of
technologies including: microprocessor, DSP, FPGA (Field
Programmable Gate Arrays), CPLD (Complex Programmable Logic
Devices), ASIC (Application Specific Integrated Circuit) devices,
etc. Cancellation and control are implemented numerically. An
analog IF output can be generated by digital up conversion (DUC),
filtering, DAC, and up conversion to the satellite receiver
interface frequency.
[0037] Several configurations of the present invention can be
implemented to suppress instances of multiple interference sources
when they occur as separable interferers in the same channel or
band, separable interferers in separate channels or bands, and
combinations of multiple separable interferers in the same and
different channels or bands. These configurations can combine
terrestrial, co-channel and adjacent satellite cancellation. Filter
specification and placement in these combinational configurations
have to address the interaction of multiple serial filters on
matched group delay between the main signal and the auxiliary
signals, and the channel bandwidth. Serial/parallel configurations
will also impact the cumulative noise figure of the configuration
as it appears to the satellite receiver and can degrade the
signal-to-noise ratio and BER of channels. In addition, the
implications of multiple combined paths increase the possibilities
of sneak paths whereby signals and/or noise in the auxiliary paths
can be added to main line signals. Coordination of multiple
interferers requires the control functions of the CISU's to
synchronize and harmonize operations in the collective system. For
this purpose, the control functions assume a master/slave
relationship, whereby the master control, whether it is one of the
CISU control functions or a separate controller, assigns
responsibilities to each CISU in hierarchal fashion to rank the
response, reduce interaction and maximize combined effectiveness.
The present invention, implemented in modular fashion, uses a
common system interface bus that supports configuration detection
and master/slave determination. In configurations servicing
multiple satellite receivers, the present invention provides
separate RF output interfaces, and separate receiver data
interfaces.
[0038] FIG. 4 shows a block diagram of a preferred serial or
cascade arrangement of the present invention when used to cancel
multiple interference signals in a common channel or band. The
arrangement shown places multiple instances of the CISU channel in
series operation using different auxiliary feeds or reference
signals. The auxiliary signals should be reasonably uncorrelated in
the same channel. A common reference oscillator provides a single
LO signal for all LNA/LNB converters. The serial arrangement of
CISU channels uses a common main channel filter for the
communication channel or band of interest. Each auxiliary path uses
a band pass filter (BPF) selected to isolate the interference
signal and attenuate out-of-band signals that can enter the chain.
The main line filter arrangement can use a single filter in the
leading CISU of the serial arrangement to define the main channel
or band. Successive CISU's can delete the main filter to better
match the group delay between main and auxiliary channels. Each of
the auxiliary antennas or feeds is selected to suppress a different
interference component with the ability to mix terrestrial,
co-channel and adjacent satellite cancellation requirements in any
combination, e.g., the system can cancel two or more terrestrial
interferers, or cancel a terrestrial interferer and a co-channel
interferer and an adjacent satellite interferer, et al. Each CISU
functional block acts on the correlated interference in the main
line as defined by the auxiliary reference signal. The control
functions of the separate CISU's elements shown interface using a
common system interface bus structure to coordinate CISU operation
between elements and the external satellite receiver.
[0039] FIG. 5 shows a block diagram of a preferred parallel or
cascode arrangement of the present invention when used to cancel
multiple interference sources in different channels or bands. The
arrangement shown places multiple CISU channels in parallel
operation using different auxiliary feeds or reference signals. A
common reference oscillator provides the LO signal for each
LNA/converter. The main antenna feed is split between the parallel
channels. The parallel arrangement of CISU channels can use
different main channel filters for the different communication
channels and bands of interest. Each auxiliary path uses a filter
selected to isolate the interference signal and attenuate
out-of-band signals that may enter the chain. Each of the auxiliary
antennas or feeds is selected to suppress a different interference
with the capability to share the main antenna for multiple channel
operation with independent interference cancellation in each
channel. The types of interference cancellation implemented can be
mixed and include combinations of terrestrial, co-channel or
adjacent channels. A lone parallel filter channel is shown in the
figure that indicates a channel that may not have an interference
condition or require cancellation. All channels can be combined
onto a single output to drive a single or multiple satellite
receivers, or provided separately. Separate satellite receiver data
interfaces would be used when separate RF outs are provided.
[0040] FIG. 6 shows a block diagram of a preferred series-parallel
arrangement of the present invention when used to cancel multiple
interference sources in different channels or bands where a channel
may have one or more interference cancellation needs. The
arrangement shown places multiple CISU channels in series-parallel
configurations. The parallel channels operate over different
frequency ranges as defined by main line and auxiliary filters. The
serial channels operate in the same frequency range against
different interference frequencies as defined by the auxiliary
filters and reference signals. The arrangement can implement any
combination of interference cancellation and channel needs that can
be separately defined by appropriate auxiliary refernce signals. As
with other arrangements, a common reference oscillator provides the
LO signal for the LNA/LNB converter, and the main antenna feed
which is split between parallel channels. All channels can be
combined onto a single output to drive a single or multiple
satellite receivers, or provided separately. Separate satellite
receiver data interfaces would be used when separate RF outs are
provided.
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