Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02288633 1999-11-OS
An Interference Canceller for the Protection of Direct-Sequence Spread-
Spectrum
Communications from High-Power Narrowband Interference
Field of the Invention
The present invention relates to apparatus and methods of canceling
narrowband interference present in the bandwidth of a wideband direct-sequence
spread-spectrum received signal. High-power interference is understood to mean
an
interfering signal whose total power, as measured over the signal bandwidth of
the
narrowband interference, is greater than the total power of the wideband DSSS,
as
measured over the signal bandwidth of the narrowband interference. Wideband is
understood to mean a frequency domain bandwidth, which is much greater than
the
minimum bandwidth required to transmit the information.
Background of the Invention
The need for a communication service which is convenient, affordable, and
reliable has been fueled by market demand. The convenience requirement is
covered by providing the customer with portable wireless communication
devices.
To date, wireless portable communications devices have primarily consisted of
wireless portable telephones (cellular) for wireless voice communications.
Recent
market trends are gravitating towards providing integrated wireless voice and
data
communications (fax, e-mail, etc.) and making it available "on the move/on
demand." Affordability is achieved through the economies of scale from the
ever
increasing number of users of these technologies and the resulting decrease in
the
manufacturing costs of integrated electronics. Lastly, the issue of
reliability of
wireless communications is the topic of the present invention; more
specifically, the
ability of wireless communications receiver systems to better reject
interfering noise
in wireless communications.
In referring to noise in what follows, one understands its definition in the
context of electrical noise with respect to an electrical system under
consideration.
Electrical noise thus defined can take a variety of forms, including: radio
frequency
(RF) noise, thermally induced electrical noise and signal distortions
introduced by
electrical components in performing their intended function. Thermally induced
electrical noise and signal distortion introduced by electrical components can
be
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characterized and establishes a lower threshold for the transmission and
detection of
radio frequency signals.
RF noise comprises RF signals transmitted by electrical equipment as a side
effect of their intended function, RF signals of a natural source which create
a
background level, and RF signals transmitted by other electrical equipment as
a
direct effect of its intended function but which interferes with the operation
of a
system under consideration. The first form of RF noise can be handled by
careful
equipment design and proper shielding and is usually mandated through
government regulation. The second form of RF noise can be characterized and
further degrades the lower threshold for transmission and detection of RF
signals of
interest. The last form of RF noise should not normally constitute a problem
due to
careful government control and allocation of the scarce radio spectrum. This
last
form of RF noise encompasses signal jamming, whether it is intentional or
otherwise. It is this last type of RF noise that is characterized as RF
interference.
The present invention attempts to isolate RF interference and to alleviate its
effects.
In wireless communications there are many levels at which noise can disturb
a wireless link and therefore introduce signal degradation. RF transmissions
for
example, are subject to signal distortion due to the severe nature of the
propagation
medium. There are many types of signal distortions and generally they are
characterized in order to design wireless communications systems that are
immune
to their effects. One such signal distortion is a type of self interference
that arises
from the reception of multiple reflections of the same signal. These multiple
reflections cause attenuation of the received signal and in a digital system
induce a
delay spread that tends to smear the bits comprising the digitized voice. This
is
known as multipath fading. Although multipath fading is a frequency dependant
phenomenon that places severe constraints on the ability to provide a reliable
wireless link, it can nevertheless be characterized and its affects minimized
through
conservative design (i.e., providing adequate fade margin) and by employing
such
techniques as spatial diversity.
One of the most recent techniques to be employed in commercial wireless
applications, one which is inherently resilient to multipath fading, employs a
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modulation technique which encodes the digital sequence of bits to be
transmitted in
such a way that the resulting signal spectrum is spread over a much wider
range of
frequencies than is necessary to transmit the information. This technique is
referred
to as direct-sequence spread-spectrum or DSSS .
The Code Division Multiple Access (CDMA) digital cellular standard,
which is based upon DSSS techniques, is arguably one of the most robust and
potentially highest capacity systems yet deployed. However, even with its
theoretical ability to suppress interference due to its "processing gain" (a
function of
the ratio of the RF bandwidth of the DSSS signal to the information signal
bandwidth) it is nevertheless still susceptible to and is fundamentally an
interference
limited system.
RF interference which is characterized as being high-power (i.e., strong
amplitude) and narrow bandwidth, is a form of intra-system interference in
that it is
produced by other concurrent users, either legal or illegal, who are
transmitting in
the same spectrum allocated for the CDMA service. This can lead to severe
degradation of the DSSS-based CDMA system in terms of capacity (i.e., number
of
users), voice quality, etc. In some cases it can result in the complete loss
of wireless
communications. Under such conditions, this narrowband interference or NBI is
said to overwhelm the DSSS receiver.
The present invention is concerned with a system for the detection, isolation
and cancellation of NBI that falls within the bandwidth of a DSSS signal for
the
purpose of restoring the DSSS receiver to an acceptable operational state and
thereby render it immune to NBI.
Description of the Prior Art
It is known to provide NBI cancellation. U.S. patent 4,991,165 Cronyn,
issued Feb. 5, 1991, describes an interference canceller that requires a copy
of the
interfering RF signal from a nearby transmitter on a separate input port and
then
uses that signal to perform the cancellation. U.S. patent 5,629,929 Blanchard
issued
May 13, 1997, describes a receiver design employing Fast Fourier Transform
(FFT)
techniques for characterizing the input power spectrum which allows the
receiver to
isolate NBI and cancel it. U.S. patent 5,596,600 Dimos, issued Jan. 21, 1997,
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suppresses NBI by digitally filtering the received signal prior to despreading
by
employing an adaptive transversal filter (ATF) whereby the spread spectrum
signal
is converted from RF to baseband for digital processing by the ATF followed by
conversion back to RF before being fed into the receiver.
The above mentioned inventions generally take the form of either a
completely new receiver implementation or an in-line signal pre-processor. In
the
case of a new receiver implementation, this would require already deployed
receivers to be replaced and/or modified which would represent a costly
solution.
The in-line signal pre-processor, which by definition is installed in the
received
signal path, has a twofold drawback in that a malfunction would result in
complete
system failure in addition to the fact that in-line signal processing, which
is usually
performed at baseband, can cause serious degradation to the desired received
signal
due to amplitude and/or phase distortion that necessarily results from the
signal
processing performed to cancel the NBI. Yet another drawback of the above
mentioned inventions is the fact that their implementation relies heavily on
specific
information contained in the received signal. Loss of ability to extract such
information leads to failure.
Summary of the Invention
A superior NBI canceller, as envisioned in the present invention, would take
the form of add-on hardware which is compatible with present and future
wireless
communications systems employing DSSS techniques. This would, among other
things, provide for a comparatively less costly solution for combating NBI as
it
could be deployed on an "as required basis" and without the need to replace
and/or
modify the existing receiver equipment. A superior NBI canceller, as
envisioned in
the present invention, would not require any auxiliary inputs such as a
separate
sample of the interference. A superior NBI canceller, as envisioned in the
present
invention, would process the incoming signal so as to remove NBI at RF before
it
enters the receiver and do so without performing any in-line signal processing
functions such as down- or up-conversion, analog-to-digital conversion, etc.
and
thereby minimize any potential distortion of the received wideband DSSS
signal. In
addition, the present invention would allow the receiver, which is downstream
from
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the interference canceller, to remain operational in the event of failure of
the NBI
canceller, albeit without the benefit of interference cancellation. A superior
NBI
canceller would have the ability to detect the presence of, isolate,
characterize and
suppress one or more sources of NBI in an incoming wideband DSSS signal
without
having to make use of any signal dependent characteristics such as the
modulation
structure of the DSSS signal for example.
It is an aim of the present invention to provide cancellation of high-power
NBI in a wideband DSSS signal.
It is another aim of the present invention to provide cancellation of NBI for
a
wideband DSSS receiver employing spatial diversity techniques, wherein the
interference cancellation is done in parallel for each diversity
antenna/receiver.
It is another aim of the present invention to provide cancellation of NBI for
a
wideband DSSS receiver without the requirement for any external auxiliary
inputs
such as a sample of the NBI.
It is another aim of the present invention to provide cancellation of NBI for
a
wideband DSSS receiver by using add-on equipment and without having to replace
and/or modify the signal processing system of the existing receiver and
without
performing any in-line signal processing functions such as up- and/or down-
conversion, analog-to-digital conversion, etc. in the received signal path.
It is another aim of the present invention to provide cancellation of NBI for
a
wideband DSSS receiver without requiring a priori knowledge of the modulation
methods employed by either the wideband DSSS signal or the narrowband
interfering signal(s).
It is another aim of the present invention to provide cancellation of NBI for
a
wideband DSSS receiver without requiring a priori knowledge of the direction
of
arrival of the interfering signal(s).
It is another aim of the present invention to provide adaptive cancellation of
variable NBI for a wideband DSSS receiver, in which the amplitude and/or
center
frequency of the NBI is neither a priori known nor time invariant.
It is another aim of the present invention to provide cancellation of NBI for
a
wideband DSSS receiver wherein the cancellation bandwidth is limited to the
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bandwidth of the NBI so as to minimize the effects on the wideband DSSS
receive
signal.
It is another aim of the present invention to provide constant cancellation
performance of the NBI over the input power dynamic range of the NBI
signal(s). It
is another aim of the present invention to provide adaptive cancellation of
the
varying NBI for a wideband DSSS receiver whereby optimum cancellation
performance is adaptively maintained through the action of closed-loop
feedback
techniques.
It is another aim of the present invention to provide cancellation of the
frequency variable NBI for a wideband DSSS receiver whereby the presence of
valid NBI is automatically determined by employing a "decision-aided" system
that
uses a software based search algorithm in an embedded controller.
It is another aim of the present invention to provide cancellation of NBI for
a
wideband DSSS receiver whereby the NBI is first isolated, then characterized
as a
valid interferer in terms of amplitude and then cancelled.
It is another aim of the present invention to provide cancellation of NBI for
a
wideband DSSS receiver whereby the cancellation performance is independent of
the fading environment.
It is another aim of the present invention to provide cancellation of NBI for
a
DSSS wideband receiver whereby each narrowband interferer is channelized and
wherein channelization is understood to mean frequency isolation of the NBI
and
frequency-selective correlation over the NBI signal bandwidth so as to limit
the
cancellation of NBI to the signal bandwidth of the NBI.
It is another aim of the present invention to provide cancellation of NBI for
a
wideband DSSS receiver whereby a frequency-isolated and selectively correlated
copy of the NBI signal is complex weighted (i.e. the amplitude and phase is
adjusted) and used as a counterinterference for vector cancellation under
closed-
loop control.
It is another aim of the present invention to provide cancellation of NBI for
a
wideband DSSS receiver whereby the signal processing function of complex
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weighting and the action of vector cancellation of the NBI is performed at RF
thereby minimizing the deleterious effects of NBI at the earliest possible
stage.
It is another aim of the present invention to provide a design of an
interference canceling system that is scalable in terms of the number of
simultaneous NBIs to be processed and the degree of cancellation required.
It is another aim of the present invention to provide a design of an
interference canceling system that is fault tolerant in that the failure of
any
interference processor does not result in complete failure of the canceller
but rather
a graceful degradation in terms of a reduction in the number of simultaneous
NBI's
that can be cancelled.
It is another aim of the present invention to provide a method of interference
canceling that is fault tolerant in that the failure of the interference
canceling system
itself does not completely hinder the operation of the receiver but rather
leaves the
receiver in an operational state corresponding to the case in which the
interference
canceller was not present at all.
It is another aim of the present invention to provide an interference
canceling
system comprising a plurality of cascaded interference cancellers which
provide for
scalability and fault tolerance.
According to a first broad aspect of the invention, there is provided an
interference cancellation system that mitigates deleterious effects of one or
more
narrowband interference (NBI) signals against a wideband signal of interest in
a
received signal in a received signal path. The system comprises auxiliary
sample
means for extracting a copy of the received signal to obtain a wideband
auxiliary
signal; feedback sample means for extracting from the received signal path a
wideband error feedback signal downstream from the auxiliary sample means, and
for processing the error feedback signal to isolate a narrowband feedback
signal in
response to a variable frequency control signal determining a central
frequency of
the narrowband feedback signal; means for generating a narrowband reference
signal from the auxiliary signal in response to a variable frequency control
signal
determining a central frequency of the narrowband reference signal, the
narrowband
reference signal containing one of the one or more NBI signals and having a
strong
CA 02288633 1999-11-OS
correlation with respect to the narrowband feedback signal as a result of the
NBI
and not as a result of the signal of interest; controller means for sweeping
through at
least a portion of a frequency range of the wideband signal of interest by
setting the
variable frequency control signal, for determining whether the wideband signal
of
interest is contaminated by NBI by examining the narrowband reference signal,
and
for generating in response to detection of NBI an NBI cancellation control
signal;
means for correlating the narrowband reference signal and the narrowband
feedback
signal and for generating a complex correlation signal; means providing an
injection signal containing the one of the NBI signals; complex weighting
means for
adjusting an amplitude and a phase of the injection signal in response to the
correlation signal, and for generating, subject to the NBI cancellation
control signal,
an NBI cancellation signal; and means for injecting the NBI cancellation
signal into
the received signal path downstream of the auxiliary sample means and upstream
of
the feedback sample means. In this way, the contaminating NBI is cancelled
from
the wideband signal of interest in the received signal path.
To avoid correlation and canceling of the signal-of interest, the reference
signal can be delayed with respect to the feedback signal, and/or the signal
strength
of the signal-of interest in the one or both of the reference and feedback
signals can
be attenuated. Preferably, the means for generating a narrowband reference
signal
comprise a channelization network suppressing signal strength and introducing
a
delay at frequencies away from the central frequency. The channelization
network
may comprise a first filter receiving the auxiliary signal passing a bandwidth
of the
NBI, a second filter receiving the auxiliary signal passing a bandwidth wider
than
the NBI, and summing means for summing an output of the first and second
filters.
According to a second broad aspect of the invention, there is provided a
method of canceling NBI in a wideband signal. The method comprises: extracting
a
reference signal and a feedback signal in a receiver signal path; scanning the
reference signal for contaminating NBI to determine the central frequency of
the
contaminating NBI; isolating narrowband copies about the central frequency
from
the reference signal and the feedback signal, wherein the narrowband copy of
the
reference signal is strongly correlated with the narrowband copy of the
feedback
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CA 02288633 1999-11-OS
signal as a result of the NBI and not as a result of a signal of interest;
correlating the
narrowband copies; complex weighting an injection copy of the wideband
reference
signal using a result of the correlation; and injecting the weighted injection
copy
into the receiver signal path at a point between a point of extraction of the
reference
signal and of the feedback signal. In this way, an equivalent of a notch
filter is
applied to the wideband signal of interest at the frequency of the
contaminating
NBI.
The invention also provides a method of canceling contaminating NBI in a
wideband signal in a receiver signal path, the method comprising: providing at
least
two cancellation devices of the type which extract a copy of the wideband
signal
from the receiver signal path and inject a cancellation signal into the
receiver signal
path prior to entering a receiver; and coupling the cancellation devices in a
cascade
arrangement to a receiver antenna. In this way, fault tolerance and
scalability is
achieved. Preferably, the method further comprises steps of: detecting NBI
within
the wideband signal prior to cancellation by the cancellation devices; and
indicating
to the cancellation devices at which frequency NBI to be cancelled is found.
Also,
preferably, the method further comprises steps of: detecting a quality of NBI
cancellation in the receiver signal path after the cancellation devices; and
adjusting,
if necessary, the indicating to optimize NBI cancellation.
Brief Description of the Drawings
The present invention will be better understood by way of the following
detailed description of embodiments as shown in the appended diagrams in
which:
Figure 1 schematically illustrates a wideband DSSS signal with NBI
superimposed;
Figure 2 schematically illustrates the same wideband DSSS signal with the
NBI suppressed by an interference canceller according to an embodiment of the
present invention;
Figure 3 schematically illustrates an embodiment of the present invention as
it relates to an interference canceller comprising one interference processor
in its
preferred embodiment;
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Figure 4 is a functional diagram of a channelization and frequency selective
correlation network according to the present invention;
Figure 5 is a functional diagram of an embodiment of a decision-aided
canceller using an embedded controller running a software based search
algorithm
according to the present invention;
Figure 6 is a functional diagram of an embodiment of the present invention
as it relates to an interference canceller having a plurality of interference
processors
connected in parallel;
Figure 7 is a functional diagram of a preferred embodiment of an
interference canceller according to the present invention in which a plurality
of
cascaded interference processors canceling NBI are substantially equally
divided
between the two antennae of a diversity receiver; and
Figure 8 is a connection diagram illustrating according to the present
invention the scalability and fault tolerant set-up of two interference
cancellers with
interference processors connected in parallel.
Detailed Description of the Invention
An NBI-contaminated wideband DSSS signal 10, is received at an antenna
12 of a DSSS receiver 14 (shown in Fig. 3). The structure of the NBI-
contaminated
wideband DSSS signal 10 is characterized by the presence of narrowband
interference 16 with peaks extending above the DSSS signal strength level 18
and,
as shown in Fig. 1, having an NBI signal strength that may vary from only
slightly
stronger than the DSSS signal strength to many orders of magnitude stronger.
In
addition, the NBI peaks 16 may be present at any frequency 20 or frequencies
20
within the bandwidth 22 of the DSSS signal 10 corresponding to the frequency
channels) of the NBI. The presence of contaminating NBI 16 of sufficient
strength,
number and duration in the DSSS signal bandwidth 10, leads to a degradation in
the
operation of a DSSS receiver in terms of its ability to receive and process a
DSSS
signal and subsequently recover/demodulate the information. If the signal
strength
of the NBI is sufficiently strong, a complete loss of the communication
function
would result. In such cases wherein the inherent processing gain of a DSSS
receiver
is insufficient to overcome a NBI whose peak amplitude is a few orders of
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magnitude greater than that of the DSSS signal, the DSSS receiver is said to
be
overwhelmed.
Fig. 2 shows the frequency domain notch effect 28 due to interference
cancellation by interference processors 26, described according to the present
invention, and its effect on the DSSS received signal. As shown, the output
frequency spectrum of the DSSS signal 30, as processed by a narrowband
interference canceller, contains narrowband notches at frequencies
corresponding to
the center frequencies 20 of the contaminating NBI 16. The effect of notching
out
the NBI is to restore the DSSS receiver's 14 ability to perform its intended
function,
even in the presence of high-power narrowband interference.
Figure 3 shows a schematic diagram of an interference canceller 11
according to an embodiment of the present invention comprising an interference
processor 26. The preferred embodiment of a interference processor 26 is shown
in
Figure 3. The interference processor 26 searches for, isolates, characterizes
(in
terms of being a valid NBI) and finally cancels one narrowband interferer 16.
According to the preferred embodiment of the interference processor 26 as
related to the present invention, a copy 40 of the received NBI-contaminated
wideband DSSS received signal 10 is extracted from the received signal path 42
using a directional coupler 44 and represents an auxiliary input (AUX) to the
interference processor 26.
The extracted copy 40 of the received wideband signal 10 is preprocessed by
a programmable input attenuator 43 according to a input attenuator control
signal
41. The input attenuator 43 changes the signal strength of the AUX input
signal 40
so that it matches the dynamic range of a channelization network 46 to which
it is
fed.
The AUX copy of the DSSS signal 40 is processed by the channelization
network 46 that provides frequency isolation of the NBI and enables frequency-
selective correlation over the NBI signal bandwidth.
Figure 4 shows the AUX input being down-converted 48, channelized 50,
and then upconverted 52. The down-converter 48 and upconverter 52 perform
their
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function in accordance with a local oscillator frequency input 54 and
effectively
render the channelization function independent of input frequency.
The channelization is performed by band-pass filters 47 and 49. The band-
pass filter 47 takes a bandwidth slice around the NBI covering more than the
bandwidth of the NBI and the band-pass filter 49 takes a bandwidth slice
around the
NBI center frequency covering the bandwidth of the NBI. The two slice widths
are
a few orders of magnitude apart. A side effect of filtering is that the two
signals are
delayed in time. The magnitude of the delay of the NBI bandwidth is larger. A
signal attenuator 51 is used for its side effect of decreasing the magnitude
of the
time delay incurred by the NBI bandwidth and is adjusted, at set-up, so that
the two
delays match in magnitude. A signal combiner 53 takes the two signals and in
performing its function outputs a signal of the same bandwidth at allowed by
band-
pass filter 47 with the NBI bandwidth undelayed but the rest of it delayed.
The signal as processed by the channelization network 46 is a reference
(REF) signal 56 for the interference processor 26.
Referring to Figure 3, another copy 60 of the wideband DSSS signal is
extracted by the directional coupler 45 from the received signal path 42
downstream
of where the AUX copy 40 is extracted. This second copy becomes the feedback
(FBK) signal input 60 to the interference processor 26 as required in a
feedback
closed-loop system.
The REF signal 56 and FBK signal 60 are independently passed through
LNA stages 61 and 63, respectively. The LNA stages are activated by LNA
control
signal 65 as more sensitivity is required to process the NBI 16.
The interference processor 26 sweeps through the DSSS signal bandwidth 22
according to a software based search algorithm in the controller to detect the
presence of NBI 16, by programming a frequency generator 58 to provide the
local
oscillator output signal 54 corresponding to the NBI frequency channels
spanning
the DSSS signal bandwidth.
The REF signal 56 and the FBK signal 60 are filtered separately by
narrowband signal extractors 62 and 64. The reference narrowband signal
extractor
62 takes as another input the current scanning frequency of the local
oscillator 54
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and outputs a narrowband intermediate frequency (IF) reference signal 66
centered
around the current scanning NBI channel frequency. The feedback narrowband
signal extractor 64 takes as another input the current scanning frequency of
the local
oscillator 54 and outputs a narrowband IF feedback signal 68 centered around
the
current scanning NBI channel frequency.
The narrowband IF REF signal 66 and the narrowband IF FBK signal 68 are
then fed into an automatic gain controller (AGC) stage 70. The AGC stage 70 is
configured as a master/slave AGC for the narrowband IF REF signal 66 and the
narrowband IF FBK signal 68 respectively. This master/slave AGC stage 70
provides a constant amplitude/phase reference input to the reference input
side of
the correlator 100 as well as ensuring constant cancellation performance over
the
NBI input signal dynamic due to "slaving" of the FBK AGC to the master or REF
AGC.
The outputs of the automatic gain control 70 consists of two pairs of signals.
The first pair, referred to as REF LVL signal 72 and FBK_LVL signal 74,
respectively, provide information to a controller 80 regarding the signal
power level
of the NBI. The second pair of output signals of the AGC stage 70 is the
normalized narrowband IF reference signal (REF AGC) 76 and the normalized
narrowband IF feedback signal (FBK AGC) 78.
The control signals REF LVL 72 and FBK LVL 74 are fed into the
controller 80 whose functional diagram is shown in Figure 5.
Copies of the control signals 72 and 74 are fed into a sensitivity and input
signal attenuation subcontroller 73. Under dynamic software control,
subcontroller
73 optimizes the input dynamic range of the AUX input 40 through input
attenuator
control signal 41 depending upon the nature of the interfering environment
(i.e.
predominantly strong or predominantly weak interferers). Strong signals which
might overload the front end are attenuated and weak signals that require
maximum
sensitivity are not attenuated. Under dynamic software control, subcontroller
73
further optimizes input dynamic range of the REF input 56 and the FBK input 60
through the LNA control signal 65 depending upon the nature of the interfering
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environment (i.e. predominantly strong or predominantly weak interferers).
Weak
signals that require maximum sensitivity are amplified and strong signals are
not.
A copy of the REF LVL signal 72 is fed into a narrowband interference
discriminator 82. The NBI discriminator 82 when not detecting the presence of
NBI
outputs a logic high signal 84 which effectively puts the interference
processor 26 in
sweep mode. The sweep mode signal 84 activates a software search algorithm 86
which determines the next frequency to be scanned for the presence of NBI. The
next frequency to be scanned is output as a local oscillator program signal
(LO PROG) 88.
In the case in which the NBI discriminator detects the presence of NBI in the
REF LVL signal 82, the sweep mode signal 84 is logic low. A copy of the sweep
mode signal is inverted by inverter 90 into an interference canceller mode
enable
signal 92. A copy of the interference canceller mode enable signal 92 termed
as
complex weighter enable signal (WT_EN) 94 is output from the controller 80 and
controls the NBI canceling function by enabling or disabling the complex
weighter
104. A data acquisition module 98 tabulates the REF LVL 72, FBK LVL 74 and
LO PROG 88 signals.
Returning to Figure 3, the REF AGC 76 and FBK AGC 78 signals are fed
into a correlator 100 which outputs a complex signal 102 and which after
suitable
filtering and amplification by a stage 101, is used to control the complex
weighter
104. The complex weighter 104 is enabled by the controller 80 when a valid NBI
is
detected, in terms of signal strength, and takes as input the phase and
amplitude of
the baseband correlation control signal 102 and a copy of the channelized
reference
signal 56. The complex weighter 104 then outputs an NBI cancellation signal
106
or counterinterference which is summed into the received signal path 42 using
a
directional signal coupler 108 placed between couplers 44, 45.
A feedback programmable attenuator 79 in the signal path of the FBK AGC
signal 78 is used in combination with the stage 101 to determine the overall
cancellation loop gain and response-time time-constant of the feedback loop.
The
combination of the feedback programmable attenuator 79 and stage 101 is used
to
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provide maximum flexibility for all types of installations and interference
environments, and is largely adjusted at system set-up.
According to another embodiment of the present invention, an interference
canceller 11 shown in Figure 6, is able to provide simultaneous interference
cancellation of a plurality of narrowband interferers by employing a plurality
of
interference processors 26. For this purpose the AUX copy 40 of the DSSS
signal
coming from the directional coupler 44 is split up in a plurality of signals
by an N-
way divider 110 with each of the splits going to an interference processor 26.
The
FBK copy 60 of the DSSS signal coming from the directional coupler 45 is split
up
into a plurality of signals by an N-way divider 112 with each of the splits
going to
an interference processor 26. Similarly, the NBI cancellation signals 106 from
each
of the interference processors 26 are passed through an N-way combiner 114 to
provide a combined NBI cancellation signal 116 to be coupled in the receive
signal
path through directional coupler 108. Similarly on the controller 80 side, all
its
inputs and outputs are multiple.
Although the NBI canceller is based upon an innovative analog
implementation of an LMS-based correlation type interference canceller,
significant
improvements in the canceller's overall performance have been achieved through
the use of an embedded controller running a software based algorithm. The use
of
such a software algorithm transforms the relatively limited capabilities of a
hardware only implementation to the much more flexible and powerful "decision-
aided" system. Apart from the obvious benefits of increased flexibility and
ease of
use, the ability to automatically search for interferers, determine if they
are valid or
not, based upon some predefined criteria such as being within a certain power
level
window, imparts intelligence to what would otherwise be a dumb canceller. In
addition, combining an innovative hardware design with the software based
capabilities such as only attacking the strongest interferers, overcomes
certain
hardware limitations such as what to do when the number of interferers exceeds
the
number of available interference processors in the interference canceller. In
effect
maximizing the use of limited hardware resources. As well, the "intelligence"
imparted by the embedded controller/software algorithm enables the
interference
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CA 02288633 1999-11-OS
canceller to monitor and optimize its own performance by, for example,
deciding
that a particular interference processor assigned to cancel some interference
signal is
not doing an adequate job and reassigning it to some other source of
interference. In
essence, the canceller can be viewed as a layered algorithm consisting of a
low-level
hardware algorithm embedded in a high-level software algorithm.
According to the present invention, a preferred embodiment of an
interference canceller, shown in Figure 7, is able to cancel a plurality of
contaminating NBIs 16 for a diversity receiver. Under control of a software
based
search algorithm 86 in the controller 80; the search algorithm 86 uses all
interference processors 26 to scan for NBI. On finding NBI 16 present in at
least
one of the signals from the two antennas 12, a pair of interference processors
26,
one for each side in the diversity configuration, is assigned to cancel the
NBI 16.
According to this preferred embodiment of the present invention, an
interference
canceller 11 shown in Figure 7, is able to provide simultaneous interference
cancellation of a plurality of narrowband interferers 16 by employing a
plurality of
interference processors 26. For this purpose the AUX copy 40 of the DSSS
signal
for each of the plurality of interference processors comes from a dedicated
directional coupler 44 for each interference processor 26. Likewise the FBK
copy
60 of the DSSS signal for each of the plurality of interference processors
comes
from a dedicated directional coupler 45 for each interference processor 26.
Similarly, the NBI cancellation or counterinterference signals 106 from each
of the
of interference processors 26 are summed individually into the received signal
path
through dedicated directional couplers 108. Similarly on the controller 80
side, all
its inputs and outputs are multiple.
This configuration of the interference processors in the interference
canceller is referred to as a cascade configuration. An added benefit of the
cascade
configuration is that there is an inherent capability to provide even greater
cancellation than a single interference processor could provide by allocating
two
interference processors to the same NBI and hence thereby achieving twice the
cancellation. This would be useful in cases where such a strong source of NBI
was
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CA 02288633 1999-11-OS
present that assigning two interference processors would be worthwhile even
though
this would leave a fourth but weaker interference uncancelled.
The design of the interference canceller 11 allows for scalability and fault
tolerance. One such embodiment is presented in Figure 8 in which, two
interference
cancellers 11 a and 11 b are connected to the received signal path 42 of a
receiver 14.
In the illustrated configuration the interference canceller llb is downstream
with
respect to interference canceller 11 a or in cascade.
During normal operation, if there are fewer NBIs 16 present in a received
DSSS signal 10 than pairs of interference processors 26 in interference
canceller
11 a, then interference canceller 11 b remains operationally idle,
continuously
scanning for NBI. Sometimes interference canceller l 1b finds interference
that was
too strong for canceller 11 a to cancel. In that case, canceller 11 b will
provide
further canceling.
If a situation arises in which during normal operation, there are more NBIs
16 present in the received DSSS signal 10 than pairs of interference
processors 26 in
interference canceller 11 a, then interference canceller 11 b is able to find
and
suppress the spillover NBIs 16. This illustrates the scalability of the
interference
cancellers 11.
If a situation arises in which there are NBIs 16 in a received DSSS signal 10
and interference canceller 11 a fails, then interference canceller 11 b
processors
automatically takes over by design. This illustrates the fault tolerance of
the design.
Special mention is given to the fact that in the case of a fault in
interference
canceller 11 a no information need be sent to interference canceller 11 b to
take over.
In all embodiments, the embedded controller 80 has an interface means 118
including a communications port allowing it to be connected to an external
personal
computer. If a modem 120 is connected to the communications port of the
interface
118, then remote access to the interference canceller 11 is possible. Not only
can
the interference canceller 11 be remotely programmed but monitored as well.
It will be appreciated that variant embodiments of the invention are possible.
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One such variant would be the use of a channel analyzer to detect the
presence or NBI and its center frequency. This would replace the sweep mode of
the interference processors 26 of an interference canceller 11.
Yet another variant of the preferred embodiment would be the separation of
functions inside interference canceller 11. By way of this example,
interference
processors 26 would only be interference canceller mode enabled and activated
as
presence of NBI is detected by dedicated processors whose sole function would
be
to sweep the wide bandwidth of the signal continuously. A method of deployment
can be envisioned in which interference canceller mode enabled interference
processors and NBI sweepers form part of separate systems interconnected by a
communication protocol whereby the NBI sweepers need not necessarily be
installed at the receiver site.
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