Note: Descriptions are shown in the official language in which they were submitted.
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METHOD AND APPARATUS FOR FILTERING
INTERFERENCE AND NONLINEAR DISTORTIONS
The present invention relates to electronic
communications systems, and more particularly to a
method and apparatus for filtering interference and
nonlinear distortions in a signal communicated from a
transmitter to a receiver via a communication path.
The invention is particularly suited for use in
connection with a television distribution system, such
as a hybrid fiber/coax (HFC) network, in which a
subscriber terminal such as a set-top box or cable
modem receives television and/or data signals from the
distribution system "headend" via a "downstream"
communication path and sends information back to the
headend on an "upstream" return path. In such an
environment, the interference filtered by the present
invention is often referred to as "ingress" or
"ingress noise" and the nonlinear distortions of
concern comprise composite second order (CSO) and
composite triple beat (CTB) distortions.
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Hybrid fiber/coax networks, which are based on
branch and tree architecture, provide a cost-effective
means for delivering downstream broadband services
such as analog/digital video and high-speed data. In
addition, they provide subscribers with high-speed
data upstream transmission, for example, in the 5-42
MHz portion of the RF spectrum. Cumulative ingress
noise is the main impairment in the return-path
portion of HFC networks. The types of ingress noise,
which appear on the return-path, can be classified as
follows:
A. Narrowband short-wave signals, originating
from radio stations and other sources, coupled
to the return-path cable plant at the subscriber
location or in the distribution plant.
B. Common mode distortion originating from non-
linearities in the cable plant.
C. Location specific interference generated by
an electrical device at the subscriber location.
See, e.g., C. A. Eldering, N. Himayat, and F. M.
Gardner, "CATV Return Path Characterization for
Reliable Communications", IEEE Communications 8, 62-68
(1995). The amount of cumulative ingress noise in the
return-path network is essentially the limiting factor
in determining the maximum number of simultaneous
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users and the maximum data transmission rate that can
be achieved.
Video signals sent to set-top boxes are also
often subject to "burst/impulse noise" originating
from peak Composite Second Order (CSO) and/or
Composite Triple Beat (CTB) distortions. These
distortions are generally present at known
frequencies, which depend upon the television
frequency plan used for the analog video signals. In
cable television systems, such frequency plans include
the integrally-related-carrier (IRC) plan and the
harmonically-related-carrier (HRC) plan. CSO and CTB
distortions can lead to video blocking and visually
degraded areas in a television picture. One method
for addressing the CSO/CTB distortion problem in
multi-channel AM-VSB (amplitude modulated vestigial
sideband)/QAM (quadrature amplitude modulation) video
transmission systems and the like is disclosed in co-
mmonly asigned United States Patent No. 6,546,557
entitled "Method
and System for Enhancing Digital Video Transmission To
A Set-Top Box." In the system disclosed in that
patent application, the performance of hybrid analog
and digital video transmission systems is improved by
determining the relative magnitude and frequency
locations of nonlinear distortions, identifying the
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analog channel frequency plan, and then selecting a
digital channel map based thereon.
It would be advantageous to have a robust and
cost-effective method and apparatus for filtering
interference (such as ingress noise or other
interference types) and nonlinear distortions in a
signal communicated from a transmitter to a receiver
via a communication path, such as a return path signal
from a set-top box or the like. It would be further
advantageous to provide such a method and apparatus
that operate adaptively, so that interference (e.g.,
ingress) is efficiently filtered even when the
frequency of the interference peaks changes over time.
Such a method and apparatus should enable a plurality
of interference peaks to be filtered, and should
automatically adapt to changing conditions in the,
interference.
It would be still further advantageous to provide
a method and apparatus for filtering nonlinear
distortion in a signal communicated from a transmitter
to a receiver via a communication path. Such a method
and apparatus would be particularly useful in the
downstream channel of an HFC cable television
distribution system, where either an IRC or HRC
frequency plan is used.
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The present invention provides methods and
apparatus enjoying the aforementioned and other
advantages.
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In accordance with the present invention, a
method and apparatus are provided for filtering
interference in a signal communicated from a
transmitter to a receiver via a communication path.
The transmitter is momentarily disrupted from
transmitting over the communication path, e.g., by
placing it in an idle state. During the momentary
disruption, the receiver analyzes interference on the
communication path to determine the frequency of at
least one noise peak of the interference. Information
is communicated from the receiver to the transmitter
identifying the frequency of the at least one
interference noise peak. Based on this information,
the transmitter pre-distorts the signal to accentuate
the signal magnitude at the identified frequency or
frequencies of the interference peak(s). The pre-
distorted signal is then transmitted by the
transmitter to the receiver, which filters the pre-
distorted signal to attenuate the signal magnitude at
the identified frequency or frequencies.
In an illustrated embodiment, the receiver
performs a real or complex signal frequency analysis
on the interference to determine the frequency peak(s)
thereof. The filtering at the receiver can use, for
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example, a transfer function that is the inverse of
the transfer function used to pre-distort the signal
at the transmitter. In one possible implementation,
the filtering at the receiver uses the Z-transform
transfer function:
H(z) - 1 + 2Re(a)z + z-2
1-2Re(a)R=z-' +R2 =z-2
where a= exp(2j#), ~ is the normalized center
frequency of the filter, and R is a constant. The
pre-distortion at the transmitter can implement the
inverse transfer function H(z)-1.
A power threshold detection can be used during
the analysis to identify the frequency location(s) of
the interference peak(s). For example, only peaks
exceeding a predefined power threshold level might be
identified for pre-distortion at the transmitter and
subsequent filtering at the receiver.
In an adaptive method, the transmitter is
periodically disrupted from transmitting over the
communication path. Interference on the communication
path is analyzed at the receiver during the periodic
disruptions, and information is communicated from the
receiver to the transmitter identifying changes in the
interference peak(s) determined during the periodic
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disruptions. The transmitter then pre-distorts the
signal to accentuate the signal magnitude in
accordance with the changes of the interference peaks.
Also disclosed are a method and apparatus for
filtering nonlinear distortion in a signal
communicated from a transmitter to a receiver via a
communication path. The signal is pre-distorted at
the transmitter to accentuate the signal magnitude at
a fixed frequency where the nonlinear distortion takes
place. The pre-distorted signal is transmitted to the
receiver, which provides filtering to attenuate the
signal magnitude at said fixed frequency.
If the signal is, for example, an integrally-
related carrier (IRC) television channel signal having
composite second order (CSO) and composite triple beat
(CTB) distortions present at different fixed
frequencies, the CSO and CTB distortions are filtered
by pre-distorting the signal at the transmitter to
accentuate the signal magnitude at a first fixed
frequency where the CSO distortion resides, and at a
second fixed frequency where the CTB distortion
resides. The pre-distorted signal is then filtered at
the receiver at the first and second fixed
frequencies.
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If the signal is, for example, a harmonically
related carrier (HRC) television channel signal having
composite second order (CSO) and composite triple beat
(CTB) distortions present at a common fixed frequency,
the CSO and CTB distortions are filtered by pre-
distorting the signal at the transmitter to accentuate
the signal magnitude at the common fixed frequency.
The pre-distorted signal is then filtered at the
receiver at the common fixed frequency.
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Figure 1 is a block diagram of illustrating a
cable television headend or cable modem termination
system (CMTS) with a return path receiver;
5 Figure 2 is a more detailed block diagram of the
return path receiver component of Figure 1;
Figure 3 is a detailed block diagram of one
embodiment of a burst receiver that can be used in the
receiver of Figure 2 in accordance with the invention;
10 Figure 4 is a detailed block diagram of an
alternate embodiment of a burst receiver that can be
used in the receiver of Figure 2 in accordance with
the invention;
Figure 5 is a block diagram of an example second-
order notch filter structure that can be used in a
receiver in accordance with the invention;
Figure 6 is a graph illustrating a simulated
baseband 256-QAM spectrum with a narrowband
interference peak located at 0.5-MHz off the center of
the channel with C/(N+I)=OdB;
Figure 7 is a graph illustrating a simulated
baseband 256-QAM spectrum of a pre-distorted signal to
be transmitted in accordance with the invention;
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Figure 8 is a graph illustrating the simulated
256-QAM baseband spectrum of the signal of Figure 7
after the interference has been removed by filtering
at the receiver in accordance with the invention;
Figure 9 is a graph illustrating the amplitude
response of a notch filter that can be used at the
receiver for filtering the pre-distorted signal;
Figure 10 is a graph illustrating the phase
response of a notch filter that can be used at the
receiver for filtering the pre-distorted signal;
Figure 11 is an illustration of a simulated
quadrant of a 256-QAM I/Q constellation with
narrowband interference that has not been filtered in
accordance with the invention; and
Figure 12 is an illustration of a simulated
quadrant of a 256-QAM I/Q constellation with
narrowband interference that has been filtered in
accordance with the invention.
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The present invention provides techniques for
filtering both interference and nonlinear distortions
in a communication system. For example, the invention
is useful to filter ingress noise which may include,
for example, narrowband interference, common mode
distortion and location specific interference in an
upstream channel (e.g., return path) of a cable
=television system. The invention may also be used to
filter nonlinear distortions such as CSO and CTB
distortions, which may, for example, be present in the
downstream path of a cable television system.
Additional uses of the invention for other
interference and distortion types will be apparent to
those skilled in the art.
Figure 1 illustrates, in simplified block diagram
form, a cable modem termination system (CMTS) at a
cable television headend. The CMTS is controlled by a
computer processor (CPU) 10 which communicates with
the other CMTS components over a bi-directional bus
coupled to a media access controller (MAC) 12. The
MAC 12 controls the physical layer of the
communication signals and coordinates various aspects
of the data carried by the signals such as the time
data is sent, the time data is received, etc. The MAC
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12 also receives signals from a return path receiver
20 which, in turn, receives signals from a remote
subscriber terminal (e.g., a cable modem or set-top
box). The data signals to be transmitted are provided
by the MAC to a QAM modulator 14 for modulation in a
conventional manner. An upconverter 16 converts the
output of the QAM modulator to a suitable radio
frequency (RF) for transmission over, e.g., a bi-
directional HFC network 22. The CMTS is coupled to
the HFC network via a diplexer 18 in a.conventional
manner.
The front-end of the return-path receiver 20
converts a bursty analog signal received from the
subscriber terminal to a sampled digital signal, which
can be fed to an off-line (i.e., non-real time)
Digital Signal Processing (DSP) chip 38 in the
receiver, as illustrated in Figure 2. In order to
filter interference (e.g., narrowband interference
such as ingress), the interference must first be
detected. To do this, an initialization process is
commenced wherein the digital transmitter (e.g., QPSK
or QAM) in the cable modem (CM) or at the set-top box
is placed in an idle mode, so that only the cumulative
interference will be received by the return path
receiver 20 at the CMTS. The off-line DSP 38 provided
within the return path receiver 20 analyzes the
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received noise. This analysis can be performed using
a complex signal frequency analyzer which uses, for
example, a Discrete Fourier Transform (DFT) algorithm.
The spectral power density of the interference is
significantly higher than that of the white Gaussian
noise (WGN), and the noise peaks can easily be
identified using a threshold power detector. The
threshold level can be set, for example, to be 10-dB
higher than the WGN floor to assure that only large
interference peaks are being identified. To achieve,
for example, 10-kHz interference peak resolution in a
3.2-MHz data channel (worst-case), 640 sampling points
are required for the DFT.
Once the frequencies of the interference peaks
have been determined, information identifying these
specific frequencies is transmitted to the subscriber
terminal via the downstream QAM modulator 14,
upconverter 16 and diplexer 18 as shown in Fig. 1. It
is noted that although a QAM modulator is shown for
purposes of illustration, any suitable form of digital
modulation can be used to transmit the interference
peak identification information to the subscriber
terminal. As an alternative, an alternate channel
(which can be analog or digital) could be used to pass
this information to the subscriber terminal.
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Relevant portions of one possible implementation
of the return path receiver 20 are illustrated in
block diagram form in Figure 2. It is noted that the
illustrated portions are provided as an example only,
5 and that other implementations will be apparent to
those skilled in the art. In a cable television
implementation, the receiver portions illustrated can
be provided in a CMTS or other headend embodiment.
The HFC network 22 is coupled to a tuner 30
10 provided in the receiver. A desired upstream signal
received from the HFC network, such as a data signal,
is tuned using tuner 30 and passed to an analog to
digital converter (A/D) 32. The digitized signal from
A/D 32 is passed to an appropriate receiver, such as
15 burst receiver 36 and a DSP 38. The DSP performs a
real or complex signal frequency analysis of the
return path signa.ls to determine the frequency of each
of the interference peaks. The DSP sends information
indicative of the frequency of each peak (e.g., the
filters' coefficient data) to the microprocessor 40 to
set up notch filters in the burst receiver 36. This
information is also communicated to the subscriber's
set-top box or cable modem for use in setting up
complementary pre-distortion filters. The purpose of
the pre-distortion filters in the subscriber's cable
modem or set-top box is to accentuate the transmitted
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return path (upstream) signal at the frequencies where
the interference is expected to occur at the receiver.
Corresponding notch filters at the CMTS will the.n
attenuate the same frequencies at the return path
receiver, thereby effectively filtering out the effect
of the interference. The attenuation at the burst
receiver 36 not only filters out the interference; it
also returns the signal level to its proper magnitude
at the interference peak frequencies.
Figures 3 and 4 illustrate further detail for two
different embodiments of a return path receiver using
notch filters in accordance with the present
invention. The receivers illustrated in these figures
can be used to provide the functions of return path
receiver 20 shown in Figure 1. In particular, the
intermediate frequency (IF) return path signal from
the tuner 30 of Figure 2 is input to an A/D converter
32 (Figure 2), which digitizes the signal and passes
it to I and Q phase quadrature mixers 52, 54
respectively (Figure 3 or 4). In the embodiment of
Figure 3, square-root Nyquist filters 56, 58 filter
the I and Q signals, and pass them to respective notch
filters (NF) combined with =4 decimation filters 60,
62, thereby providing down-sampled I and Q signals
that have been attenuated back to normal levels at the
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interference frequency peaks. In the embodiment of
Figure 4, notch filters 51 are provided immediately
prior to quadrature mixers 52, 54 instead of being
combined with the decimator filters 61, 63.
The filtered and down-sampled I and Q signals are
passed to a feed forward equalizer (FFE) 64 and
decision feedback equalizer (DFE) 66 for conventional
equalization. A forward error correction (FEC)
decoder 68 then processes the equalized I and Q
signals in a conventional manner. Acquisition and
tracking loop 70 enables the receiver to properly
acquire and track the received signal, as well known
in the art.
The following difference equation describes a
second-order digital notch filter design that can be
used to implement the invention:
y(n) = bo = x(n) + b, = x(n -1) + b2 - x(n - 2) - a, = y(n -1) - a2 = y(n - 2)
where:
x(n) and y(n) are the time-discrete input and
output sequences,
b2 = bo = 1, bl = 2Re (a) ,
al = -2R2,Re (a) , a2 = R2, and
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a= exp(2jn~)
~= the normalized center frequency the notch,
and
R is a constant related to the notch magnitude.
Thus, only two coefficients are needed for this
filter.
The second-order notch filter structure, which is
shown in Fig. 5, is known as a direct form II
structure. The transfer function or the Z-transform
of the notch filter 80 is described by the following
equation:
H(z) = 1 + 2 Re(a)z' + -z2
1-2Re(a)R=z' +R2 z 2
The pre-distortion filter at the subscriber
terminal transmitter has exactly the inverse transfer
function as the second-order notch filter shown above
[H(z)-']. The pre-distortion filters can, for example,
be inserted after the symbol mapper for a QPSK
(quadrature phase shift keyed) or QAM (quadrature
amplitude modulation) implementation, and before a
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programmable transmitter pre-equalizer that is used to
cancel the effects of intersymbol interference (ISI).
It is noted that although a notch filter is described
herein for use with the return path receiver, other
types of filtering may be used instead to provide the
same or similar results.
Figure 6 illustrates a simulated baseband 256-QAM
spectrum 82 with a narrowband interference peak 83
located 0.5-MHz off the center of the channel and with
C/(N+I)=OdB.
Figure 7 illustrates the spectrum 84 of the
simulated baseband 256-QAM spectrum with the pre-
distortion 85 added at the transmitter.
Figure 8 shows the simulated baseband 256-QAM
spectrum 86 at the upstream receiver after the narrow
interference peak 83 and pre-distortion 85 have been
removed.
Figures 9 and 10 illustrate the amplitude and
phase responses 88, 90 respectively, of the second
order notch filter of Figure 5, as a function of the
normalized frequency.
Figure 11 shows a simulated single quadrant 92 of
the 256-QAM constellation when the interference peak
illustrated in Figure 6 is present. It is clear from
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this figure that the received signal contains many
errors.
Figure 12 shows a simulated single quadrant 94 of
the 256-QAM constellation in the presence of the
5 interference peak, but after the application of the
interference filtering technique of the present
invention. The simulation results indicate an error-
free reception for the demodulated 256-QAM signal.
In order to further refine the present invention,
10 the filtering of interference such as ingress noise
can be made adaptive. In particular, between upstream
burst intervals, (e.g., once every second) the
subscriber terminal transmitter may be idled to
"quiet" the return path. This will enable the CMTS to
15 monitor the ingress peaks on the quieted return path.
The off-line DSP chip 38 at the return-path receiver
in the CMTS can determine if the previously detected
ingress peaks are still present or if there are new
ingress peaks. The updated information (parameter (x
20 in the transfer function for H(z)) on the frequency of
the current interference peak(s) is then sent to the
subscriber terminal (e.g., set-top or cable modem)
transmitter via the downstream modulator to enable
and/or disable appropriate pre-distortion filters at
the return-path transmitter. At the same time,
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appropriate notch filters at the return path receiver
are enabled and inappropriate notch filters are
disabled.
A variation of the techniques described above can
be applied to downstream signal (e.g., digital video)
transmission to subscriber terminals to overcome the
effect of nonlinear (CSO/CTB) distortions. The
location of CSO and CTB distortions in an HFC network
depends on the cable TV frequency plan used for the
analog video signals. The two most widely used cable
TV frequency plans are the integrally-related-carrier
(IRC) and harmonically-related-carrier (HRC) plans.
In the IRC plan the first picture carrier frequency is
located at 55.2625-MHz with successive picture
carriers located six MHz apart up to 1-GHz. In the
HRC frequency plan, the picture carrier frequencies
are downshifted 1.25-MHz compared with the
corresponding picture carriers in IRC plan. The
advantage of the HRC plan is that the CSO and CTB
distortion products fall on the picture carrier, and
thus their effect becomes almost invisible. In the
IRC plan, the CSO distortions are located 1.25-MHz
from the corresponding picture carrier frequency, and
thus can become visible.
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The following Table 1 shows the various options
for the IRC and HRC frequency plans:
Table 1 - The location of CSO and CTB distortions
relative to the QAM channel center frequency.
Frequency CSO Distortions CTB Distortions
Plan
0.5-MHz relative to 1.75-MSz relative
IRC Plan
QAM channel center to QAM channel
frequency. center frequency.
1.75-MHz relative to 1.75-MHz relative
HRC Plan
QAM channel center to QAM channel
frequency. center frequency.
To overcome the impact of CSO/CTB distortions
using the IRC plan at the QAM receiver, two pre-
distortion filters (one for CSO and one for CTB) with
H(z)-1 frequency response are enabled in the QAM
modulator, which is located at the cable headend. Two
notch filters with H(z) frequency response and with
the same coefficients are also enabled in the QAM
receiver in the set-top box. No adaptation is
required here since the frequencies of the nonlinear
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distortions are always known in hybrid analog/digital
HFC networks. For the HRC plan, only one pre-
distortion filter and one corresponding notch filter
are required, since CSO and CTB occur at the same
frequency.
It should now be appreciated that the present
invention provides methods and apparatus for filtering
interference and nonlinear distortions in
communications systems. Although the invention has
been described in connection with cable television
systems, wherein the downstream path may suffer from
nonlinear distortions (CSO/CTB) and the upstream path
may suffer from ingress noise, the invention is not
limited to use in such systems or to these types of
interference and distortion. The novel techniques for
interference detection and reduction and for filtering
nonlinear distortion are applicable to any
communication system where such interference resides
at detectable or already known frequencies. By pre-
distorting the transmitted signal and filtering the
pre-distorted signal at the receiver, effective
reduction or elimination of interference and nonlinear
distortion is obtained. Accordingly, various
adaptations and modifications may be made to the
invention without departing from the scope thereof as
set forth in the claims.
