U.S. patent application number 09/864765 was filed with the patent office on 2001-10-11 for digital radio frequency interference canceller.
Invention is credited to Bingham, John A.C., Wiese, Brian R..
Application Number | 20010028692 09/864765 |
Document ID | / |
Family ID | 27360530 |
Filed Date | 2001-10-11 |
United States Patent
Application |
20010028692 |
Kind Code |
A1 |
Wiese, Brian R. ; et
al. |
October 11, 2001 |
Digital radio frequency interference canceller
Abstract
Disclosed are radio frequency (RF) interference cancellation
techniques that effectively estimate RF interference to the data
signals being received using a frequency domain model, and then
remove the estimated RF interference from the received data
signals. Improved techniques for digitally filtering multicarrier
modulation samples to reduce sidelobe interference due to the RF
interference are also disclosed.
Inventors: |
Wiese, Brian R.; (San
Carlos, CA) ; Bingham, John A.C.; (Palo Alto,
CA) |
Correspondence
Address: |
TEXAS INSTRUMENTS INCORPORATED
P O BOX 655474, M/S 3999
DALLAS
TX
75265
|
Family ID: |
27360530 |
Appl. No.: |
09/864765 |
Filed: |
May 24, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09864765 |
May 24, 2001 |
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08834503 |
Apr 4, 1997 |
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6014412 |
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60016251 |
Apr 19, 1996 |
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60016252 |
Apr 19, 1996 |
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Current U.S.
Class: |
375/346 ;
375/296 |
Current CPC
Class: |
H04L 27/2647 20130101;
H04L 5/0007 20130101; H04L 5/0044 20130101; H04B 1/126 20130101;
H04B 1/123 20130101; H04B 2001/1063 20130101; H04B 1/1027 20130101;
H04B 1/1036 20130101; H04L 5/0062 20130101; H04L 27/2602
20130101 |
Class at
Publication: |
375/346 ;
375/296 |
International
Class: |
H04L 001/00 |
Claims
1. A method for mitigating radio frequency (RF) interference in a
multicarrier modulation system, said method comprising the
operations of: (a) obtaining frequency domain data associated with
a frequency band; (b) identifying a restricted frequency sub-band
within the frequency band; (c) estimating a frequency of the RF
interference within the restricted frequency sub-band; (d)
estimating the RF interference in accordance with a frequency
domain model for the RF interference and the estimated frequency of
the RF interference; and (e) removing the estimated RF interference
from the frequency domain data.
2. A method as recited in claim 1, wherein the frequency domain
data is provided in a plurality of frequency tones within the
frequency band, and wherein the frequency domain model is produced
in accordance with the following equation: 11 RFI n + m = [ k = 1
MO + 1 A k ( m - ) k ] where RFI.sub.n+m is the RF interference at
a frequency tone n+m due to a radio interferer at frequency
(n+.delta.), .delta. is an offset amount, MO is a model order for
the frequency domain model, and A.sub.k is a complex number that is
determined for each frequency tone m.
3. A method as recited in claim 1, wherein the RF interference is
due to radio transmissions by an amateur radio operator.
4. A method as recited in claim 1, wherein the frequency restricted
sub-band is approximately one of: 1.8 to 2.0 MHz; 3.5 to 4.0 MHz;
7.0 to 7.3 MHz and 10.1 to 10.15 MHz.
5. A method as recited in claim 1, wherein the frequency domain
data contains a plurality of frequency domain data samples, and
wherein said estimating of the frequency of the RF interference
comprises the operations of: determining a largest data sample of
the frequency domain data samples within the restricted frequency
sub-band, and determining a largest adjacent data sample that is
adjacent to the largest data sample; and determining the frequency
of the RF interference within the restricted frequency sub-band
based on the largest data sample and the largest adjacent data
sample.
6. A method as recited in claim 1, wherein the frequency domain
model is based on a time domain model for the RF interference in
which the RF interference is modeled as a windowed, modulated
sinusoid.
7. A method as recited in claim 6, wherein the sinusoid is
modulated by a windowed, modulation envelope.
8. A method as recited in claim 6, wherein the sinusoid is
modulated by a linearly-varying, windowed, modulation envelope.
9. A method as recited in claim 6, wherein the sinusoid is
modulated by a no order polynomial modulation envelope.
10. A method as recited in claim 1, wherein the frequency domain
data contains a plurality of frequency domain data samples, wherein
said estimating the RF interference estimates the RF interference
for at least a portion of the frequency domain data samples, and
wherein said removing of the estimated RF interference from the
frequency domain data comprises, for each of the frequency domain
data samples in the portion, the operation of subtracting from the
frequency domain data sample the estimated RF interference on that
frequency domain data sample.
11. A method as recited in claim 10, wherein the frequency domain
data contains a plurality of frequency domain data samples, and
wherein said estimating of the frequency of the RF interference
comprises the operations of: determining a largest data sample of
the frequency domain data samples within the restricted frequency
sub-band, and determining a largest adjacent data sample that is
adjacent to the largest data sample; and determining the frequency
of the RF interference within the restricted frequency sub-band
based on the largest data sample and the largest adjacent data
sample.
12. A method as recited in claim 11, wherein the frequency domain
model is based on a time domain model for the RF interference in
which the RF interference is modeled as a modulated sinusoid.
13. A method as recited in claim 12, wherein the RF interference is
due to radio transmissions by an amateur radio operator.
14. A method as recited in claim 13, wherein the frequency domain
data is provided in a plurality of frequency tones within the
frequency band, and wherein the frequency domain model is produced
in accordance with the following equation: 12 RFI n + m = [ A m - +
B ( m - ) 2 ] W m . where RFI.sub.n+m is the RF interference at a
frequency tone n+m due to a radio interferer at frequency
(n+.delta.), .delta. is an offset amount, W.sub.m is an attenuation
factor due to time domain windowing and varies with each of the
frequency tones, and A and B are complex numbers.
15. A method as recited in claim 14, wherein A and B are model
parameters and are determined by the following equation: 13 [ A B ]
= [ - 1 1 1 - ] [ 2 X n W 0 ( 1 - ) 2 W n + 1 W 1 ] where the
complex parameters A and B are determined once for each symbol, and
the offset amount .delta. is computed once per symbol for each RF
interferer being modeled.
16. A method as recited in claim 1, wherein the frequency domain
data contains a plurality of frequency domain data samples, wherein
said method further comprises the operation of comparing the
frequency domain data samples within the restricted frequency band
with a threshold amount, and wherein, for the restricted frequency
band, at least one of said estimating (d) and said removing (e) are
bypassed when said comparing determines that the frequency domain
data samples are less than the threshold amount.
17. A method as recited in claim 1, wherein no data is transmitted
in the restricted frequency sub-band.
18. A method as recited in claim 1, wherein said obtaining (a) of
the frequency domain data is initially received as time domain
data, the time domain data undergoes a time domain windowing
operation, and thereafter the windowed time domain data is
converted to the frequency domain.
19. A method for mitigating radio frequency interference in a
multicarrier modulation system, said comprising the operations of:
prior to data transmission, identifying AM radio interference in
the multicarrier modulation system; estimating a frequency of the
AM radio interference; disabling certain frequency tones of the
multicarrier modulation system adjacent to the estimated frequency
of the AM radio interference from carrying frequency domain data
during the data transmission; thereafter, during or following data
reception, estimating the AM radio interference in accordance with
a frequency domain model for the AM radio interference and the
estimated frequency of the AM radio interference; and removing the
estimated AM radio interference from the frequency domain data.
20. A method as recited in claim 19, wherein said identifying of
the AM radio interference is performed during an initialization
period of the multicarrier modulation system that occurs prior to
data transmission.
21. A method as recited in claim 19, wherein the frequency domain
data contains a plurality of frequency domain data samples, and
wherein the frequency domain data is initially received as time
domain data, the time domain data undergoes a time domain windowing
operation, and thereafter the windowed time domain data is
converted to the frequency domain.
22. A method as recited in claim 19, wherein the AM radio
interference resides within a AM radio band, wherein the frequency
domain data contains a plurality of frequency domain data samples,
and wherein said estimating of the frequency of the AM radio
interference comprises the operations of: determining a largest
data sample of the frequency domain data samples within a frequency
range, and determining a largest adjacent data sample that is
adjacent to the largest data sample; and determining the frequency
of the AM radio interference within the frequency range based on
the largest data sample and the largest adjacent data sample in a
portion of the radio band.
23. A method as recited in claim 22, wherein the frequency domain
model is based on a time domain model for the RF interference in
which the RF interference is modeled as a windowed, modulated
sinusoid.
24. A method as recited in claim 23, wherein the sinusoid is
modulated by a windowed, modulation envelope.
25. A method as recited in claim 23, wherein the modulated is
modulated by a linearly-varying, windowed, modulation envelope.
26. A method as recited in claim 23, wherein the sinusoid is
modulated by an no order polynomial modulation envelope.
27. A method as recited in claim 19, wherein the frequency domain
data contains a plurality of frequency domain data samples, wherein
said estimating the AM radio interference estimates the AM radio
interference for at least a portion of the frequency domain data
samples, and wherein said removing of the estimated AM radio
interference from the frequency domain data comprises, for each of
the frequency domain data samples in the portion, the operation of
subtracting from the frequency domain data sample the estimated AM
radio interference on that frequency domain data sample.
28. A method as recited in claim 27, wherein the AM radio
interference resides within a AM radio band, wherein the frequency
domain data contains a plurality of frequency domain data samples,
and wherein said estimating of the frequency of the AM radio
interference comprises the operations of: determining first and
second largest data samples of the frequency domain data samples
within the portion of the frequency domain data samples; and
determining the frequency of the AM radio interference based on the
first and second largest data samples in a portion of the radio
band.
29. A method as recited in claim 28, wherein the frequency domain
model is based on a time domain model for the AM radio interference
in which the AM radio interference is modeled as a modulated
sinusoid.
30. A method as recited in claim 29, wherein the AM radio
interference is due to radio broadcasts by radio stations.
31. A method as recited in claim 30, wherein the frequency domain
data is provided in a plurality of frequency tones, and wherein the
frequency domain model is produced in accordance with the following
equation: 14 RFI n + m = [ A m - + B ( m - ) 2 ] W m . where
RFI.sub.n+m is the RF interference at a frequency tone n+m due to a
radio interferer at frequency (n+.delta.), .delta. is an offset
amount, W.sub.m is an attenuation factor due to time domain
windowing and varies with each of the frequency tones, and A and B
are complex numbers.
32. A method as recited in claim 31, wherein A and B are model
parameters and are determined by the following equation: 15 [ A B ]
= [ - 1 1 1 - ] [ 2 X n W 0 ( 1 - ) 2 W n + 1 W 1 ] where the
complex parameters A and B are determined once for each symbol, and
the offset amount .delta. is computed once per symbol for each RF
interferer being modeled.
33. A method as recited in claim 19, wherein the frequency domain
data contains a plurality of frequency domain data samples, wherein
said method further comprises the operation of comparing the
frequency domain data samples with a threshold amount, and wherein
at least one of said estimating the AM radio interference and said
removing of the estimated AM radio interference are bypassed when
said comparing determines that the frequency domain data samples
are less than the threshold amount.
34. A method as recited in claim 19, wherein said estimating of the
AM radio interference further being in accordance with the
frequency domain data on the certain frequency tones on which no
data, just AM radio interference, is present.
35. A method as recited in claim 19, wherein said estimating of the
frequency of the AM radio interference is performed while data is
not being transmitted.
36. A method as recited in claim 19, wherein the frequency domain
data is provided in a plurality of frequency tones, and wherein the
frequency domain model is produced in accordance with the following
equation: 16 RFI n + m = [ k = 1 MO + 1 A k ( m - ) k ] where
RFI.sub.n+m is the RF interference at a frequency tone n+m due to a
radio interferer at frequency (n+.delta.), .delta. is an offset
amount, MO is a model order for the frequency domain model, and
A.sub.k is a complex number.
37. A method for digitally filtering multicarrier modulation
samples to reduce sidelobe interference from a radio frequency (RF)
interferer, the multicarrier modulation samples occur at
predetermined frequency tones and form a multicarrier modulation
symbol, said method comprising the operations of: receiving x
samples of a multicarrier modulation symbol and y samples of a
cyclic prefix associated with the multicarrier modulation symbol,
the y samples of the cyclic prefix preceding the x samples of the
multicarrier modulation symbol; discarding an initial portion of
the y samples of the cyclic prefix associated with the multicarrier
modulation symbol; storing a remaining portion of the y samples of
the cyclic prefix associated with the multicarrier modulation
symbol; retaining a first portion of the x samples of the
multicarrier modulation symbol without modification; and modifying
a second portion of the x samples of the multicarrier modulation
symbol in accordance with the stored samples of the remaining
portion of the y samples of the cyclic prefix and predetermined
multiplication coefficients.
38. A method as recited in claim 37, wherein said receiving of the
x samples of a multicarrier modulation symbol and y samples of a
cyclic prefix associated with the multicarrier modulation symbol is
a stream of data received over a transmission media from a
transmitter of a multicarrier modulation system.
39. A method as recited in claim 38, wherein the transmission media
is a subscriber line.
40. A method as recited in claim 37, wherein for each x samples of
the multicarrier modulation symbol, said method uses j multiply
operations and 2j addition operations for performing said
modifying, where j is an integer representing the number of samples
in the remaining portion of the y samples of the cyclic prefix.
41. A method as recited in claim 40, wherein the predetermined
multiplication coefficients are associated with a raised cosine
function.
42. A method as recited in claim 37, wherein said modifying of the
second portion of the x samples of the multicarrier modulation
symbol comprises: retrieving an appropriate one of the
predetermined multiplication coefficients; determining a difference
amount between corresponding pair of samples of the remaining
portion of the y samples of the cyclic prefix and the second
portion of the x samples of the multicarrier modulation system;
multiplying the difference amount with the appropriate one of the
predetermined multiplication coefficients to produce an adjustment
amount; and adding the adjustment amount to the sample of the
second portion of the x samples of the corresponding pair.
43. A method for digitally filtering DMT samples to reduce sidelobe
interference from a radio frequency (RF) interferer to frequency
tones of a DMT symbol, said method comprising: receiving X samples
of a DMT symbol and Y samples of a cyclic prefix associated with
the DMT symbol; discarding an initial portion of the Y samples of
the cyclic prefix; storing a remaining portion of the Y samples of
the cyclic prefix; retaining a first portion of the X samples of
the DMT symbol without modification; and modifying a second portion
of the X samples of the DMT symbol in accordance with the stored
samples of the remaining portion of the Y samples of the cyclic
prefix and predetermined multiplication coefficients.
44. A method as recited in claim 43, wherein said modifying
operates to attenuate sidelobe interference from a radio frequency
(RF) interferer at a rate faster than would occur without said
modifying.
45. A method as recited in claim 43, wherein said method reduces
the number of the frequency tones of the DMT symbol that are
closest to the frequency of the RF interferer than are seriously
impacted by the RF interference.
46. A receiver for a multicarrier modulation system, comprising: an
analog-to-digital (AND) converter, said AID converter receives
analog signals that have been transmitted to said receiver over a
transmission media and converts the analog signals to digital time
domain signals; a multicarrier demodulator operatively connected to
said A/D converter, said multicarrier modulator receives the
digital time domain signals and converts the digital time domain
signals into digital frequency domain data; and a digital RF
interference canceller operatively coupled to said multicarrier
demodulator, said digital RF interference canceller mitigates the
effect of RF interference on the digital frequency domain data by
modeling the RF interference in accordance with a frequency domain
model.
47. A receiver as recited in claim 46, wherein said digital RF
interference canceller mitigates the effect of RF interference on
the digital frequency domain data by estimating a frequency of the
RF interference, estimating the RF interference in accordance with
the frequency domain model for the RF interference and the
estimated frequency of the RF interference, and removing the
estimated RF interference from the digital frequency domain
data.
48. A receiver as recited in claim 46, wherein the digital
frequency domain data is provided on a plurality of frequency tones
used by the multicarrier modulation system, and wherein the
frequency domain model is produced in accordance with the following
equation: 17 RFI n + m = [ k = 1 MO + 1 A k ( m - ) k ] where
RFI.sub.n+m is the RF interference at a frequency tone n+m due to a
radio interferer at frequency (n+.delta.), .delta. is an offset
amount, MO is a model order for the frequency domain model, and
A.sub.k is a complex number.
49. A receiver as recited in claim 46, wherein the digital time
domain signals include a plurality of multicarrier modulation
symbols carrying data, each of the symbols having a cyclic prefix,
wherein said receiver further comprises: a cyclic prefix removal
and windowing processor operatively connected between said A/D
converter and said multicarrier demodulator, said processor
performs a time domain windowing operation on the symbols, the time
domain windowing includes, for each symbol, adding a portion of the
cyclic prefix multiplied by a predetermined coefficient to a rear
portion of the symbol.
50. A receiver as recited in claim 49, wherein the digital
frequency domain data is provided on a plurality of frequency tones
used by the multicarrier modulation system, and wherein the
frequency domain model is produced in accordance with the following
equation: 18 RFI n + m = [ k = 1 MO + 1 A k ( m - ) k ] W m where
RFI.sub.n+m is the RF interference at a frequency tone n+m due to a
radio interferer at frequency (n+.delta.), .delta. is an offset
amount, A.sub.k is a complex number, MO is a model order for the
frequency domain model, and W.sub.m is an attenuation factor
associated with the time domain windowing operation.
51. A receiver as recited in claim 49, wherein said receiver
further comprises: an analog RF canceller operatively connected to
reduce RF interference from the analog signals prior to their being
supplied to said A/D converter.
52. A receiver as recited in claim 49, wherein the time domain
windowing is extended windowing, wherein, for each symbol, the
window extends beyond the boundaries of the symbol into the cyclic
prefix.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of Provisional Patent
Application No. 60/016,251, filed Apr. 19, 1996, and Provisional
Patent Application No. 60/016,252, filed Apr. 19, 1996, and both of
which are hereby incorporated by reference. Further, this
application is related to U.S. Application Ser. No. ______ by
Cioffi et al., filed Apr. 4, 1997, entitled "Radio Frequency Noise
Canceller", and U.S. Application Ser. No. ______ by Bingham et al.,
filed Apr. 4, 1997, entitled "Mitigating Radio Frequency
Interference in Discrete Multicarrier Transmissions Systems", both
of which are hereby incorporated by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to radio frequency (RF)
interference cancellation, and more particularly, to RF
interference cancellation in multicarrier transmission systems.
[0004] 2. Description of the Related Art
[0005] Bi-directional digital data transmission systems are
presently being developed for high-speed data communication. One
standard for high-speed data communications over twisted-pair phone
lines that has developed is known as Asymmetric Digital Subscriber
Lines (ADSL). Another standard for high-speed data communications
over twisted-pair phone lines that is presently proposed is known
as Very High Speed Digital Subscriber Lines (VDSL).
[0006] The Alliance For Telecommunications Information Solutions
(ATIS), which is a group accredited by the ANSI (American National
Standard Institute) Standard Group, has finalized a discrete multi
tone based approach for the transmission of digital data over ADSL.
The standard is intended primarily for transmitting video data and
fast Internet access over ordinary telephone lines, although it may
be used in a variety of other applications as well. The North
American Standard is referred to as the ANSI T1.413 ADSL Standard
(hereinafter ADSL standard). Transmission rates under the ADSL
standard are intended to facilitate the transmission of information
at rates of up to 8 million bits per second (Mbits/s) over
twisted-pair phone lines. The standardized system defines the use
of a discrete multi tone (DMT) system that uses 256 "tones" or
"sub-channels" that are each 4.3125 kHz wide in the forward
(downstream) direction. In the context of a phone system, the
downstream direction is defined as transmissions from the central
office (typically owned by the telephone company) to a remote
location that may be an end-user (i.e., a residence or business
user). In other systems, the number of tones used may be widely
varied. However when modulation is performed efficiently using an
inverse fast Fourier transform (IFF), typical values for the number
of available sub-channels (tones) are integer powers of two, as for
example, 128, 256, 512, 1024 or 2048 sub-channels.
[0007] The ADSL standard also defines the use of a reverse signal
at a data rate in the range of 16 to 800 Kbit/s. The reverse signal
corresponds to transmission in an upstream direction, as for
example, from the remote location to the central office. Thus, the
term ADSL comes from the fact that the data transmission rate is
substantially higher in the downstream direction than in the
upstream direction. This is particularly useful in systems that are
intended to transmit video programming or video conferencing
information to a remote location over telephone lines.
[0008] Because both downstream and upstream signals travel on the
same pair of wires (that is, they are duplexed) they must be
separated from each other in some way. The method of duplexing used
in the ADSL standard is Frequency Division Duplexing (FDD) or echo
canceling. In frequency division duplexed systems, the upstream and
downstream signals occupy different frequency bands and are
separated at the transmitters and receivers by filters. In echo
cancel systems, the upstream and downstream signals occupy the same
frequency bands and are separated by signal processing.
[0009] ANSI is producing another standard for subscriber line based
transmission system, which is referred to as the VDSL standard. The
VDSL standard is intended to facilitate transmission rates of at
least 12.98 Mbit/s and up to 51.92 Mbit/s or greater in the
downstream direction. To achieve these rates, the transmission
distance over twisted-pair phone lines must Generally be shorter
than the lengths permitted using ADSL. Simultaneously, the Digital,
Audio and Video Council (DAVIC) is working on a similar system,
which is referred to as Fiber To The Curb (FTTC). The transmission
medium from the "curb" to the customer premise is standard
unshielded twisted-pair (UTP) telephone lines.
[0010] A number of modulation schemes have been proposed for use in
the VDSL and FTTC standards (hereinafter VDSL/FITC). Most of the
proposed VDSL/FITC modulation schemes utilize frequency division
duplexing of the upstream and downstream signals. Another promising
proposed VDSL/FITC modulation scheme uses periodic synchronized
upstream and downstream communication periods that do not overlap
with one another. That is, the upstream and downstream
communication periods for all of the wires that share a binder are
synchronized. With this arrangement, all the very high speed
transmissions within the same binder are synchronized and time
division duplexed such that downstream communications are not
transmitted at times that overlap with the transmission of upstream
communications. This is also referred to as a (i.e. "ping pong")
based data transmission scheme. Quiet periods, during which no data
is transmitted in either direction, separate the upstream and
downstream communication periods. For example, with a 20-symbol
superframe, two of the DMT symbols in the superframe are silent
(i.e., quite period) for the purpose of facilitating the reversal
of transmission direction on the phone line. In such a case,
reversals in transmission direction will occur at a rate of about
4000 per second. For example, quiet periods of about 10-25 .mu.s
have been proposed. The synchronized approach can be used a wide
variety of modulation schemes, including multi-carrier transmission
schemes such as Discrete Multi-Tone modulation (DMT) or Discrete
Wavelet Multi-Tone modulation (DNVMT), as well as single carrier
transmission schemes such as Quadrature Amplitude Modulation (QAM),
Carrierless Amplitude and Phase modulation (CAP), Quadrature Phase
Shift Keying (QPSK), or vestigial sideband modulation. When the
synchronized time division duplexed approach is used with DMT it is
referred to as synchronized DMT (SDMT).
[0011] A common feature of the above-mentioned transmission systems
is that twisted-pair phone lines are used as at least a part of the
transmission medium that connects a central office (e.g., telephone
company) to users (e.g., residence). It is difficult to avoid
twisted-pair wiring from all parts of the interconnecting
transmission medium. Even though fiber optics may be available from
a central office to the curb near a user's residence, twisted-pair
phone lines are used to bring in the signals from the curb into the
user's home or business.
[0012] Although the twisting of the twisted-pair phone lines
provides some protection against external radio interference, some
radio interference is still present. As the frequency of
transmission increases, the radio interference that is not
mitigated by the twisting becomes substantial. As a result, the
data signals being transmitted over the twisted-pair phone lines at
high speeds can be significantly degraded by the radio
interference. As the speed of the data transmission increases, the
problem worsens. For example, in the case of VDSL signals being
transmitted over the twisted-pair phone lines, the radio
interference can cause significant degradation of the VDSL signals.
This problematic radio interference is also referred to as radio
frequency noise.
[0013] The undesired radio interference can come from a variety of
sources. One particular source of radio interference is amateur (or
ham) radio operators. Amateur radios broadcast over a wide range of
frequencies with significant amount of power. The amateur radio
operators also tend to change their broadcast frequency quite
often, for example, about every two minutes. Another source of
radio interference is AM radio transmissions by radio stations
which broadcast over a wide range of frequencies. With high speed
data transmission, the radio interference (noise) produced by
various sources can significantly degrade the desired data signals
being transmitted over twisted-pair phone lines.
[0014] Consequently, the problem with using twisted-pair phone
lines with high frequency data transmission rates, such as
available with ADSL and VDSL, is that radio interference becomes a
substantial impediment to a receiver being able to be properly
receive transmitted data signals. Thus, there is a need to provide
techniques to eliminate or compensate for radio interference.
SUMMARY OF THE INVENTION
[0015] Broadly speaking, the invention pertains to radio frequency
(RF) interference cancellation techniques that effectively estimate
RF interference to transmitted data signals being received using a
frequency domain model for the RF interference, and then remove the
estimated RF interference from the received data signals. The
invention also pertains to improved techniques for digitally
filtering multicarrier modulation samples to reduce sidelobe
interference due to the RF interference.
[0016] The invention can be implemented in numerous ways, including
as an apparatus, system, method, or computer readable media.
Several embodiments of the invention are discussed below.
[0017] As a method for mitigating radio frequency (RF) interference
in a multicarrier modulation system, one embodiment of the
invention includes the operations of: obtaining frequency domain
data associated with a frequency band; identifying a restricted
frequency sub-band within the frequency band; estimating a
frequency of the RF interference within the restricted frequency
sub-band; estimating the RF interference in accordance with a
frequency domain model for the RF interference and the estimated
frequency of the RF interference; and thereafter removing the
estimated RF interference from the frequency domain data.
[0018] As a method for mitigating radio frequency interference in a
multicarrier modulation system, another embodiment of the invention
includes the operations of: identifying AM radio interference to
the multicarrier modulation system, estimating a frequency of the
AM radio interference, and disabling certain frequency tones of the
multicarrier modulation system adjacent to the estimated frequency
of the AM radio interference from carrying data during the data
transmission, these operations occur prior to data transmission.
Thereafter, during or following data reception, the invention also
includes the operations of estimating the AM radio interference in
accordance with a frequency domain model for the AM radio
interference and the estimated frequency of the AM radio
interference, and removing the estimated AIM radio interference
from the frequency domain data on those of the frequency tones of
the multicarrier modulation system that carry data.
[0019] As a method for digitally filtering multicarrier modulation
samples to reduce sidelobe interference from a radio frequency (RF)
interferer, the multicarrier modulation samples occur at
predetermined frequency tones and form a multicarrier modulation
symbol, an embodiment of the invention includes the operations of:
receiving x samples of a multicarrier modulation symbol and y
samples of a cyclic prefix associated with the multicarrier
modulation symbol, the y samples of the cyclic prefix preceding the
x samples of the multicarrier modulation symbol; discarding an
initial portion of the y samples of the cyclic prefix associated
with the multicarrier modulation symbol; storing a remaining
portion of the y samples of the cyclic prefix associated with the
multicarrier modulation symbol; retaining a first portion of the x
samples of the multicarrier modulation symbol without modification;
and modifying a second portion of the x samples of the multicarrier
modulation symbol in accordance with the stored samples of the
remaining portion of the y samples of the cyclic prefix and
predetermined multiplication coefficients.
[0020] As a receiver for a multicarrier modulation system, an
embodiment of the invention includes: an analog-to-digital (AID)
converter, a multicarrier demodulator operatively connected to the
AID converter, and a digital RF interference canceller operatively
coupled to the multicarrier demodulator. The A/D converter receives
analog signals that have been transmitted to the receiver over a
transmission media and converts the analog signals to digital time
domain signals. The multicarrier demodulator receives the digital
time domain signals and converts the digital time domain signals
into digital frequency domain data. The digital RF interference
canceller mitigates the effect of RF interference on the digital
frequency domain data by modeling the RF interference in accordance
with a frequency domain model. Preferably, the digital time domain
signals include a plurality of multicarrier modulation symbols
carrying data, with each of the symbols also including a guard
band, and the receiver further includes a cyclic prefix removal and
windowing processor operatively connected between the A/D converter
and the multicarrier demodulator. The cyclic prefix removal and
windowing processor performs a time domain windowing operation on
the symbols.
[0021] Other aspects and advantages of the invention will become
apparent from the following detailed description, taken in
conjunction with the accompanying drawings, illustrating by way of
example the principles of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] The present invention will be readily understood by the
following detailed description in conjunction with the accompanying
drawings, wherein like reference numerals designate like structural
elements, and in which:
[0023] FIG. 1 is a block diagram of a representative
telecommunications system suitable for using the invention;
[0024] FIG. 2 is a graph illustrating a proposed transmission power
spectral density for VDSL/FTTC upstream communications;
[0025] FIG. 3 is a graph illustrating the magnitude of the maximum
in-tone received power at a remote unit as a function of
transmission frequency in a typical VDSL application over
twisted-pair transmission lines;
[0026] FIG. 4 is a graph illustrating the magnitude of the maximum
in-tone received power at a remote unit as illustrated in FIG. 3
further taking into consideration the effects turning off the tones
in restricted frequency bands;
[0027] FIG. 5 is a diagram illustrating frequency tones of a
multicarrier modulation system having radio interference in a
restricted frequency band;
[0028] FIG. 6 is a diagram illustrating the amount of radio
interference induced by a radio interferer on various frequency
tones of a multicarrier modulation system;
[0029] FIG. 7 is a block diagram of a receiver for a multicarrier
modulation system according to an embodiment of the invention;
[0030] FIGS. 8A-8C are diagrams illustrating various time domain
models that modulate a sinusoid;
[0031] FIG. 9 is a diagram of basic radio frequency (RF)
cancellation processing according to a basic embodiment of the
invention;
[0032] FIGS. 11A and 10B are flow diagrams of digital RF
cancellation processing according to an embodiment of the
invention;
[0033] FIG. 11 is a flow diagram of AM radio frequency (RF)
cancellation processing according to an embodiment of the
invention;
[0034] FIG. 12 is a flow diagram of prefix removal and windowing
processing according to an embodiment of the invention; and
[0035] FIG. 13 is a diagram illustrating a 512 sample DMT symbol
1300 with a 40 sample prefix 1302, and a non-rectangular, extended
window.
DETAILED DESCRIPTION OF THE INVENTION
[0036] In a multicarrier modulation system using wideband
multicarrier modulation, radio frequency (RF) interference can
often prevent proper reception of data transmitted by the
multicarrier modulation system. The invention provides improved
techniques for cancelling RF interference, particularly from
narrowband interferers, from the data transmitted by the
multicarrier modulation system. More particularly, the invention
pertains to radio frequency (RF) interference cancellation
techniques that effectively estimate RF interference to transmitted
data signals being received using a frequency domain model, and
then remove the estimated RF interference from the received data
signals. The invention also pertains to improved techniques for
digitally filtering multicarrier modulation samples to reduce
sidelobe interference due to the RF interference.
[0037] Embodiments of the invention are discussed below with
reference to FIGS. 1-12. However, those skilled in the art will
readily appreciate that the detailed description given herein with
respect to these figures is for explanatory purposes as the
invention extends beyond these limited embodiments.
[0038] FIG. 1 is a block diagram of a representative
telecommunications system 2 suitable for using the invention. The
telecommunications system 2 represents portions of a typical wired
telecommunications system that is suitable for the VDSL and FTTC
(hereinafter VDSL/FTTC) applications. The system 2 includes a
central office 10 that services a plurality of distribution posts
which may take the form of optical network units (ONUs) 1. Each
distribution post communicates with the central office 10 over one
or more high speed, multiplexed transmission lines 12 (e.g., a
fiber optic line). The ONU 11 typically serves a multiplicity of
discrete subscriber lines 15. Each subscriber line 15 typically
services a single end user that is located within 1.5 kilometers of
the ONU 11. The end user would have a remote unit 18 suitable for
communicating with the ONU 11 at very high data rates. The remote
unit 18 includes a modem but may take the form of a variety of
different devices, as for example, a telephone, a television, a
monitor, a computer, a conferencing unit, etc. Of course, it is
possible that the end user may have a plurality of phones or other
remote units 18 wired to a single line. The subscriber lines 15
serviced by a single ONU typically leave the ONU 11 in a shielded
binder 21. The shielding in the binder generally serves as a good
insulator against the emission (egress) and reception (ingress) of
RF noise. However, the last segment of this subscriber line,
commonly referred to as a "drop" 23 branches off from the binder
and is coupled directly or indirectly to the end user's remote unit
18. This "drop" 23 portion of the subscriber line 15 between the
remote unit 18 and the binder 21 is typically unshielded. In most
applications the length of the "drop" is not more than about 30
meters. However, the unshielded wire of the "drop" 23 effectively
acts as an antenna that both emits and receives RF signals.
Additionally, there is some concern that the connection 25 between
the ONU 11 and the twisted-pair subscriber lines 15 may also serve
both as an RF energy emission source and as the receptor of RF
energy.
[0039] The amount of energy that a particular communication system
may transmit is regulated by both governmental and practical
considerations. As indicated above, in discrete multi-tone systems
suitable for use in the VDSL/FTTC applications, frequency bands on
the order 12 MHz are being contemplated. Within that 12 MHz
frequency range, there are several narrow bands that are allocated
to amateur radio users. Thus, one proposed transmission power
spectral density for VDSL/FTTC upstream communications is
illustrated in FIG. 2. In this embodiment the transmit power mask
permits a maximum of -60 dBm/Hz throughout the majority of the
frequency band. However, in selected frequency bands where amateur
radio RF interference is expected (i.e., 1.8 to 2.0 MHz, 3.5 to 4.0
MHz, 7.0 to 7.3 MHz, and 10.1 to 10.15 MHz) transmissions are
limited to significantly lower levels. The permissible output power
level in these restricted frequency bands varies somewhat between
proposals. However, most parties to the VDSL/FTTC standardization
process have proposed maximum power densities in the range of
approximately -70 dBm/Hz to -85 dBm/Hz. Regardless of the actual
transmission power that is eventually agreed upon, it is clear that
a conscious effort needs to be made to minimize emissions in the
prohibited ranges.
[0040] A number of multi-carrier modulation schemes have been
proposed for use in the VDSL and FTTC standards (hereinafter
VDSL/FTTC). One proposed multi-carrier solution utilizes discrete
multi-tone signals in a system that is similar in nature to the
ADSL standard. Other proposed modulation schemes include
carrierless amplitude and phase modulated (CAP) signals and
discrete wavelet multi-tone modulation (DWMT). In order to achieve
the data rates required by VDSL/FTTC, the transmission bandwidth
must be significantly broader than the bandwidth contemplated by
the ADSL. By way of example, the discrete multi-tone system adopted
for ADSL applications utilizes a transmission bandwidth on the
order of 1.1 MHz, while bandwidths on the order of 12 MHz are being
contemplated for VDSL/FTTC applications. In one proposed DMT system
for VDSL/FTTC applications, the use of 256 "tones" or
"sub-channels" that are each 43.125 kHz wide is contemplated.
[0041] As will be appreciated by those skilled in the art, high
frequency multi-carrier signals transmitted over twisted-pair
transmission lines experience significant attenuation when they are
transmitted a relatively long distance over the twisted-pair lines.
FIG. 3 is a graph illustrating the magnitude of the maximum in-tone
received power at a remote unit (e.g., receiver) as a function of
transmission frequency in a typical VDSL application over
twisted-pair transmission lines. By way of example, referring to
FIG. 3, when the transmit power is on the order of -60 dBm/Hz
throughout the transmission bandwidth of a DNT based VDSL
modulation scheme, the receive power at a typical remote user may
be on the order of -70 dBm/Hz at the lower end of the frequency
spectrum, but may drop to as low as -125 dBm/Hz at the higher end
of the frequency spectrum. Thus, in situations where the "drop" 23
is located relatively far from the source, the downstream signals
may be attenuated enough by the time they reach the "drop" 23 that
they are already below the permissible power spectral density. FIG.
4 is a graph illustrating the magnitude of the maximum in-tone
received power at a remote unit as illustrated in FIG. 3 further
taking into consideration the effects of turning off the tones in
the restricted frequency bands.
[0042] In any event, in multi-carrier transmissions schemes such as
DMT, there will naturally be a number of subcarriers (tones) that
fall within the restricted frequency bands. Accordingly, a first
step in reducing transmissions in the restricted frequency bands is
to turn off those particular subcarriers. This has the advantage of
both reducing the emissions in the prohibited frequency range as
well as reducing the adverse impacts associated with ingress
(receipt) of the radio signals. However, as will be appreciated by
those skilled in the art, it is difficult to contain the amount of
power emitted for a particular tone tightly around a desired
frequency center (f.sub.c). Emissions associated with a particular
tone typically include a relatively high power emission centered
about the frequency center (f.sub.c) and a number of side lobes of
decreasing intensity extending on either side thereon.
[0043] The magnitude and phase of the sidelobe power can make it
difficult to limit the power spectral density in a narrow range
within the DMT transmission band by simply turning off the tones
within the restricted frequency band. By way of example, consider a
system which uses tones that are 43.125 kHz wide. If an attempt is
made to form a 200 kHz wide notch in the 1.8 to 2.0 MHz range by
simply turning off the tones within the 200 kHz wide prohibited
range, the emission power at the center of the prohibited range
would only be reduced from -60 dBm/Hz to on the order of -73
dBm/Hz. Obviously, this might result in emissions above the desired
range of -70 or -85 dBm/Hz even in the center of the prohibited
frequency range. Of course, the emission power at frequencies
closer to the boundaries of the prohibited frequency range would be
significantly higher. Thus, if an attempt is made to reduce the
emissions simply by turning off a range of tones in the
multi-carrier transmissions system, the number of tones that need
to be turned off would be significantly higher than the number of
tones associated with a prohibited frequency range. Although the
discrete multi-tone system is very flexible in its ability to pick
and choose the subcarrier frequencies, the requirement of turning
off such large frequency bands to avoid amateur radio interference
is undesirable and may reduce system performance. Improved
techniques for reducing RF emissions in restricted frequency bands
are described in U.S. application Ser. No. ______, filed Apr. 4,
1997, entitled "Mitigating Radio Frequency Interference in Discrete
Multicarrier Transmissions Systems," which has been incorporated by
reference.
[0044] The invention primarily concerns the ingress of RF
interference (RF energy) into twisted-pair transmission lines
(e.g., "drop" 23). The RF interference may be from a variety of
different RF interference sources, including an amateur radio
operator and AM radio stations. According to the invention, the RF
interference is able to be located, estimated and cancelled from
data signals being received.
[0045] FIG. 5 is a diagram 500 illustrating frequency tones of a
multicarrier modulation system having radio interference in a
restricted frequency band. As an example, the multicarrier
modulation system may be a Discrete Multi-Tone (DMT) modulation
system. The diagram 500 is a frequency domain illustration of a
plurality of tones 502 on predetermined frequencies of the
multicarrier modulation system. Data information is transmitted on
the tones 502. However, the frequencies over which the tones 502
are able to be transmitted often include one or more restricted
frequency bands in which data should not be transmitted 504.
However, radio interference is often produced in the restricted
frequency band 504 because of radio transmissions by others. As an
example, in the restricted frequency band 504 illustrated in FIG.
5, a radio interferer 506 transmits within the restricted frequency
band 504. The radio interferer 506, for example, could be an
amateur radio operator and the restricted frequency band 504 could
be associated with one of the amateur radio bands previously
described with respect to FIG. 4.
[0046] The multicarrier modulation system does not utilize the
frequencies in the restricted frequency band 504. Hence, as
illustrated in FIG. 5, the frequencies within the restricted
frequency band 504 are not illustrated as carrying data as are the
frequency tones 502 outside of the restricted frequency band 504.
However, the presence of the radio interferer 506, even though
within the restricted frequency band 504, has a detrimental effect
on the frequency tones outside the restricted frequency band 504
that are carrying data. Consequently, due to the radio interferer
506, the signals on the frequency tones 502 that are carrying data
are corrupted by radio interference. The amount of corruption will
vary depending upon the transmitting power of the radio interferer
506 and how close the particular frequency of the tone is to the
carrier frequency of the radio interferer 506.
[0047] In the example illustrated in FIG. 5, the radio interferer
506 transmits at a frequency that is contained within the
restricted frequency band of the larger frequency range over which
the multicarrier modulation system operates. The radio interferer
could also be adjacent to the frequency range of the multicarrier
modulation system. Still further, as discussed with reference to
FIG. 11, the radio interferer could occur in the frequency range of
the multicarrier modulation system but without regard to a
restricted frequency band.
[0048] FIG. 6 is a diagram 600 illustrating the amount of radio
interference induced by the radio interferer 506 referenced in FIG.
6 on various frequency tones of a multicarrier modulation system.
In this illustration, the height of the arrows on the frequency
tones 602 indicate the magnitude of the radio interference induced
on that frequency tone by the radio interferer 506. As can be seen
from FIG. 6, the magnitude of the radio interference induced on the
frequency tones 602 decreases as the frequency becomes further
removed from the carrier frequency of the radio interferer 506. In
order to perform radio interference cancellation, the frequency
tones outside of the restricted frequency band 504 need to be
corrected for the radio interference. In other words, to cancel the
radio interference, the radio interference induced on the frequency
tones 602 outside of the restricted frequency band 504 needs to be
estimated and then subtracted from the data received on the
frequency tones 602. The number of the frequency tones that are
removed in frequency from the carrier frequency of the radio
interferer 506 which must be corrected (to mitigate the radio
interference from the radio interferer 506 on those tones carrying
data) depends upon the processing techniques utilized and the
degree of radio frequency mitigation desired.
[0049] FIG. 7 is a block diagram of a receiver 700 for a
multicarrier modulation system according to an embodiment of the
invention. The receiver 700 receives radio signals 701 that have
been transmitted by a multicarrier modulation system. The receiver
700 operates to process the received radio signals 701 to recover
data that was transmitted by a transmitter of the multicarrier
modulation system. The transmitter operates to transmit the data in
blocks of data (e.g., DMIT symbols). The cyclic prefix is added by
the transmitter to provide a guard space to minimize inter-symbol
interference and normally consists of a repetition of data from the
end of a given data block.
[0050] The radio signals 701 are received by an analog radio
frequency interference (RFI) canceller 702. The analog RFI
canceller 702 operates to mitigate radio interference in the analog
domain, and then outputs radio frequency (RF) corrected radio
signals 704. One suitable analog RFI canceller is described in U.S.
application Ser. No. ______, filed Apr. 4, 1997, entitled "Radio
Frequency Noise Canceller", by Cioffi et al., which adaptively
estimates radio interference noise during data transmissions using
information obtained when no data is actually being transmitted.
The RF corrected radio signals 704 are supplied to an
analog-to-digital converter 706. The correction to the radio signal
701 also ensures that the power level of the RF interference is
below the saturation level for the analog-to-digital converter 706.
The analog-to-digital converter 706 converts the RF corrected radio
signals 704 to digital signals 708 which are output to a time
domain equalization (TEQ) circuit 710. The time domain equalization
circuit 710 produces time equalized digital signals 712. The time
equalized digital signals 712 are then supplied to a cyclic prefix
removal and windowing processor 714. The cyclic prefix removal and
windowing processor 714 produces modified digital signals 716 which
are supplied to a multicarrier demodulator 718. The processing
performed by the cyclic prefix removal and windowing processor 714
is described in detail below with reference to FIG. 12. In one
embodiment, the multicarrier demodulator 718 may be a Fast Fourier
Transform (FFT). The TEQ circuit 710 limits the inter-symbol
interference by reducing the length of the channel impulse
response.
[0051] The multicarrier demodulator 718 outputs digital frequency
signals 720 to a digital RH canceller 722. Although the received
radio signals 701 have already undergone RF interference
cancellation by the analog RFI canceller 702, additional RF
interference cancellation is often needed. For example, additional
RF interference cancellation is particularly needed when a radio
interferer (e.g., an amateur radio operator) is transmitting in a
restricted frequency band within a frequency range of a
multicarrier transmission system transmission or when AM radio
broadcasting is nearby. The digital RFI canceller 722 outputs RF
corrected digital signals 724 to a frequency-domain equalizer (FEQ)
circuit 726. The FEQ circuit 726 outputs received digital signals
728 from which the transmitted data are obtained. The FEQ circuit
726 operates on each carrier (subchannel) and adaptively adjusts
for the attenuation and phase delay of each carrier.
[0052] Radio interference is initially modeled as a modulated,
windowed sinusoid in the time domain. FIGS. 8A-8C are
representative diagrams illustrating examples of modulated
sinusoids used to model radio frequency (RF) interference. The
modulation of the sinusoid can take many forms as illustrated in
FIGS. 8A-8C. In particular, in FIG. 8A, a time domain model
modulates a sinusoid 800 using a rectangular envelope 802. In FIG.
8B, the time domain model modulates a sinusoid 804 with a
linearly-varying envelope 806. In FIG. 8C, the time domain model
modulates a sinusoid 808 with a (second-order)
quadratically-modulated envelope 810. In general, the modulated
sinusoid is modulated by an no order polynomial modulation
envelope.
[0053] According to one aspect of the invention, the frequency
domain model for RF interference that is utilized is derived and
verified by the following discussion. For this discussion, the time
domain model illustrated in FIG. 8B is used as the exemplary
embodiment. The RF interference is modeled in the time domain as a
sinusoid multiplied by a linearly-modulated rectangular window.
More precisely, Equation (1) which follows provides the time domain
model.
RFI(t)=rect(t)(1+at)cos[2.pi.(f.sub.ot)+.phi.] (1)
[0054] where rect(t) is a rectangular window, f.sub.o is a carrier
frequency of the radio interference, a is a small positive
constant, and .phi. is a phase offset. This time domain model is
equivalent to fitting a first-order polynomial to the modulation
envelope of the RF interference within the time duration of a data
block (e.g., DMT symbol). The time domain model is suitable so long
as the bandwidth of the radio interference (i.e., radio interferer)
is much less than the symbol rate of the transmission system. For
example, in the case of a amateur radio operator as the radio
interferer and DMT as the transmission system, the bandwidth of the
radio interferer is about 2.4 MHz which is substantially less than
the symbol rate of the transmission system which is about 40
MHz.
[0055] Next, this time domain model is converted into the frequency
domain for RF interference cancellation in the frequency domain. A
Fourier Transform of the time domain model is performed to achieve
the conversion. Equation (2) which follows details the conversion
to the frequency domain. 1 F { rect ( t ) ( 1 + at ) } = { sin f f
+ j a 2 [ cos f f - sin f ( f ) 2 ] } ( 2 )
[0056] The Fourier Transform of the cosine function of Equation (1)
is a Dirac delta function at +f and -f. The negative frequency
delta function is ignored because its contribution at the positive
frequencies is minimal, particularly when non-rectangular windowing
as discussed below is used. However, the positive component could
be used if the data transmission system does not utilize
non-rectangular windowing.
[0057] As illustrated in Equation (2), there are two terms that
drop off as 1/f and one term that drops off as 1/f.sup.2. Let
f.sub.0=n+.delta., where n is a frequency tone number, and .delta.
(0<.delta.<1) being an offset amount of the carrier frequency
of the RF interference from the frequency tone n.
[0058] The resulting frequency domain model is as defined in
Equation (3) which follows. 2 RFI n - m = [ A m - + B ( m - ) 2 ] (
3 )
[0059] where RFI.sub.n+m is the RF interference to frequency tone m
due to RF interference at a frequency n+.delta., where A and B are
complex numbers that must be determined for each symbol.
[0060] Further, when non-rectangular windowing is also used with
the frequency domain model, the effect of the windowing can be
approximated by multiplication by a single complex number W.sub.m
for each value of m, where W.sub.m represents the phase rotation
and additional attenuation (over that of rectangular windowing) due
to the non-rectangular windowing operation. The complex number
W.sub.m is determined from the following Equation (4). 3 W m = F {
win ( t ) } f = m F { rect ( t ) } f = m = F { win ( t ) } f = m
sin c ( m ) ( 4 )
[0061] where win(t) is the effective window used. Therefore, the
resulting frequency domain model from Equation (3) now with
non-rectangular windowing becomes as shown in Equation (5). 4 RFI n
+ m = [ A m - + B ( m - ) 2 ] W m ( 5 )
[0062] where RFI.sub.n+m is the RF interference to frequency tone m
due to RF interference at a frequency n+.delta., where A and B are
complex numbers. Note that the frequency domain model requires that
no data be transmitted on the frequency tones to either side of the
frequency of the carrier frequency of the RF interference, namely
frequency tones n and n+1, because these tones are used to
determine values for A and B and .delta..
[0063] Instead of using three frequency tones to precisely
determine A and B and .delta., the offset amount .delta. can be
approximated by the following Equation (6). Equation (6) is precise
when the RF interference is a pure sinusoid. 5 = Re { X n + 1 W 1 }
+ Im { X n + 1 W 1 } Re { X n + 1 W 1 } + Im { X n + 1 W 1 } + Re {
X n W 0 } + Im { X n W 0 } ( 6 )
[0064] where X.sub.i represents the samples values for the
frequency domain tones. The offset amount .delta. is thus
approximately equal to
.vertline.X.sub.n-1.vertline./{.vertline.X.sub.n.vertline.+.vertline.X.su-
b.n+1.vertline.}, which is accurate enough for estimating RF
interference from an amateur radio operator. The frequency domain
model has shown to be rather insensitive to small errors in the
offset amount .delta..
[0065] Then, using Equation (5) for tones n and n+1, two equations
(Equations 7 and 8) can be written. 6 X n W 0 = - A + B 2 ( 7 ) X n
+ 1 W 1 = A 1 - + B ( 1 - ) 2 ( 8 )
[0066] Simultaneously solving these two equations provides a
technique to determine the complex parameters A and B of the
frequency domain model. The complex parameters A and B are thus
determined by the following equation. 7 [ A B ] = [ - 1 1 1 - ] [ 2
X n W 0 ( 1 - ) 2 X n + 1 W 1 ] ( 9 )
[0067] The complex parameters A and B are determined, at each
symbol, for each RF interferer, the W.sub.m is a function of the
windowing and varies with each of the frequency tones, and the
offset amount .delta. is computed once per symbol for each RF
interferer being modeled. More generally, as noted above, .delta.,
A and B could be determined by simultaneously solving three
equations obtained from Equation (5) for three different tones
(e.g., n, n+1 and n+2), provided data is not transmitted on these
tones. Alternatively, the system could determine .delta. as given
by Equation (6) when the RF interference is first detected, and
then again use Equation (6) to average over many symbols to provide
an estimate that becomes more accurate as the number of symbols
averaged over increases.
[0068] In one embodiment, the frequency domain model provides
sufficiently accurate modeling of the RF interference that only the
model parameter A, as computed in Equation (9), is used for
cancellation, while the model parameter B is assumed to be zero.
With this simplification to the frequency domain model, the
complexity is reduced, yet the frequency domain model still
provides sufficient accuracy in modeling the RF interference in
many cases. As an example, for RF interference caused by amateur
radio operators, this simplification has shown to still provide
sufficiently accurate modeling (such as in a VDSL system). In other
cases, the simplification may not be appropriate and the model
parameter B should also be utilized, such as with higher bandwidth
signals like AM radio signals.
[0069] Furthermore, higher order models could be likewise used to
provide an even more accurate model for the RF interference.
However, the higher the order of the models used, the greater the
processing requirements to compute the parameters for the model.
Hence, more generally, the frequency domain model of Equation (3)
according to the invention is in accordance with the following
equation: 8 RFI n + m = [ k = 1 MO + 1 A k ( m - ) k ] ( 10 )
[0070] where RFI.sub.n+m is the RF interference at a frequency tone
m due to a radio interferer at frequency n, .delta. is an offset
amount, MO is a model order for the frequency domain model, and
{A.sub.k} are complex numbers that are determined at each symbol
for each interferer. Hence, the frequency domain model derived
above and defined by Equation (3) is a first order model (MO=1). Of
course, when non-rectangular windowing is also used with the
frequency domain model, the effect of the windowing can be
approximated by multiplication by a single complex number W.sub.m
for each value of m, as was done in Equation (5).
[0071] FIG. 9 is a diagram of basic radio frequency (RF)
cancellation processing 900 according to a basic embodiment of the
invention. The RF cancellation processing 900 is preferably
performed by a receiver or receiver portion of a transceiver of a
multicarrier modulation system.
[0072] The RF cancellation processing 900 initially receives 902
frequency domain data. The frequency domain data is data that has
been transmitted by a transmitter of the multicarrier modulation
system over a transmission media to a receiver. Next, a restricted
frequency band having radio frequency (RE) interference is
identified 904. Then, assuming that a restricted frequency band has
been identified as containing RF interference, the frequency of the
RF interference within the restricted frequency band is estimated
906. After estimating the frequency for the RF interference, the RF
interference is estimated 908 in accordance with the estimated
frequency and a frequency domain model for the RF interference.
Thereafter, the estimated RF interference is removed 910 from the
frequency domain data. Following block 910, the RB cancellation
processing 900 is complete and ends.
[0073] FIGS. 10A and 10B are flow diagrams of digital RF
cancellation processing 1000 according to an embodiment of the
invention. It should be noted that the digital RF cancellation
processing 1000 is associated with the processing performed by a
receiver or receiver portion of a transceiver of a multicarrier
modulation system upon receiving each symbol of a multicarrier
transmission system.
[0074] The digital RF cancellation processing 1000 initially
receives 1002 data vectors X.sub.i for a symbol. The data vectors
X.sub.i are typically complex numbers for each of the frequency
tones within a symbol. For example, in a 256-carrier DMT system, a
data point X.sub.i would be received for each of 256 frequency
tones.
[0075] Next, a restricted frequency band for RF cancellation
processing is selected 1004. When there are multiple restricted
frequency bands within the transmission frequency range of the
multicarrier transmission system, the processing described below is
repeated for each of the restricted frequency bands. In any event,
one of the restricted frequency bands is selected for RF
cancellation processing in which RF interference produced in the
restricted frequency band is cancelled from the received data
vectors X.sub.i. The RF cancellation processing 100 is described
assuming at most one RF interferer is present in each of the
restricted frequency bands.
[0076] Within the restricted frequency band that has been selected
1004, the largest data vector .vertline.X.sub.i.vertline..sub.L
within the restricted frequency band is determined 1006. Next, a
decision block 1008 determines whether the largest data vector
.vertline.X.sub.i.vertline..su- b.L within the restricted frequency
band is greater than a threshold. The value of the threshold will
vary with system design but is normally set to a level such that a
data vector .vertline.X.sub.i.vertline. in the restricted frequency
band that is about 20 dB above the noise floor will exceed the
threshold. When the largest data vector
.vertline.X.sub.i.vertline..sub.L is greater than the threshold,
then the processing for the selected restricted frequency band
continues.
[0077] Next, a largest adjacent data vector
.vertline.X.sub.i.vertline..su- b.LA is determined 1010. Then, data
vectors X.sub.n and X.sub.n+1 are selected 1012 from the largest
data vector .vertline.X.sub.i.vertline..su- b.L and the largest
adjacent data vector .vertline.X.sub.i.vertline..sub.L- A. The
value of n provides an indication of an estimated frequency of the
RF interference within the restricted frequency band because the
received data vectors for the frequencies within the restricted
frequency band do not carry information. In effect, at this point
in the digital RF cancellation processing 1000, the carrier
frequency of the RF interference is generally estimated to be
between frequencies associated with n and n+1.
[0078] Next, an offset amount .delta. is determined 1014 from the
selected data vectors X.sub.n and X.sub.n+1. For example, the
offset amount .delta. can be determined with Equation (6) with
W.sub.0.apprxeq.1 and W.sub.1 pre-stored in memory. Then, for the
frequency domain model for the RF interference that has been
selected (e.g., Equation (3)), model parameters A and B are
computed 1016. As an example, Equation (9) can be used to determine
the model parameters A and B. Once .delta., A and B have been
determined, the frequency domain model for the RF interference is
completed and may be used to cancel the RF interference from the
received data vectors.
[0079] A frequency tone is selected 1018 to receive cancellation.
As previously noted, a predetermined number of the frequency tones
that are adjacent to the restricted frequency band having the RF
interference are selected such that they may be processed to cancel
out the RF interference. Although the canceling could be performed
on all the frequency tones, it is computationally advantageous to
perform cancellation only on a predetermined number of adjacent
frequency tones. In any event, the selection 1018 of the frequency
tone to receive cancellation operates to select one of these
adjacent frequency tones. Then, for the selected frequency tone,
the RF interference is estimated 1020 using the frequency domain
model. Next, the estimated RF interference is subtracted 1022 from
the data vector for the selected frequency tone. The subtraction
performs the cancellation as illustrated in the following
equation:
X.sub.n+m(cancelled)=X.sub.n+m(uncancelled)-RFI.sub.n+m
[0080] where RFI.sub.n+m is obtained from Equation (10).
[0081] A decision block 1024 then determines whether cancellation
of the RF interference has been completed. The decision block 1024
determines whether all of the frequency tones adjacent to the
restricted frequency band having the RF interference that require
cancellation (e.g., the predetermined number) have received the
necessary cancellation processing. Hence, if the cancellation has
not been completed for all of the frequency tones to receive
cancellation, the digital RF cancellation processing 1000 operates
to select 1026 another frequency tone to receive cancellation.
Following block 1026, the digital RF cancellation processing 1000
returns to repeat block 1020 and subsequent blocks for the newly
selected frequency tone. Note that for the newly selected frequency
tone, the RF interference is again estimated for this newly
selected frequency tone.
[0082] On the other hand, when a decision block 1024 determines
that the cancellation for the frequency tones has been completed, a
decision block 1028 determines whether all of the restricted
frequency bands have been processed. When all of the restricted
frequency bands have not been processed, the next restricted
frequency band is selected 1030 for RF cancellation processing.
Following block 1030, the digital RF cancellation processing 1000
returns to repeat block 1006 and subsequent blocks so as to cancel
RF interference in other restricted frequency bands. Alternatively,
when the decision block 1028 determines that all of the restricted
frequency bands have been processed, the digital RF cancellation
processing 1000 is complete and ends.
[0083] Further, when the decision block 1008 determines that the
largest data vector .vertline.X.sub.i.vertline..sub.L does not
exceed the threshold, then the processing for canceling RF
interference within the particular restricted frequency band is
bypassed, and therefore not performed. In this case, the decision
block 1008 causes the digital RF cancellation processing 1000 to
jump to the decision block 1028 and thus bypass blocks 1010 through
1026.
[0084] In one implementation of the digital RF cancellation
processing 1000, for a VDSL system, the processing is implemented
by a digital ASIC coupled to or integrated with random access
memory (RANM) and read only memory (ROM). The predetermined number
of adjacent tones to receive RF interference cancellation is 31
tones on each side of the RF interferer (neglecting tones n and
n+1), though the RF interference on the tones within the restricted
frequency band need not be cancelled. In the case where the model
order (MO) is one and B is assumed equal to zero, the 1/(m-.delta.)
term in the frequency domain model for the RF interference can be
computed using a first order polynomial approximation to avoid
having to perform time-consuming divide operations. The
coefficients a.sub.0 and a.sub.1 for the polynomial approximation
are stored in memory for each value of m (a set for
0<.delta.<0.5 and a set for 0.5<.delta.<1) and can thus
be rapidly retrieved. The complex number W.sub.m is also preferably
24-bits and stored in RAM. The data vectors for the frequency tones
undergoing RF interference cancellation are preferably frequency
domain data samples output from a FFT. Each of the restricted
frequency bands can have its own threshold value.
[0085] Preferably, the computations for estimating the RF
interference can be performed as follows. The largest element in
the frequency band, and the largest element to either side of the
largest element, are X.sub.n and X.sub.n+1. Next, intermediate
values .alpha., and .beta. are computed as follows. 9 = 1 2 X n + 1
( 1 W 1 ) = 1 2 X n
[0086] where 1/W.sub.1 is held in RAM, and where W.sub.0.apprxeq.1.
Then, intermediate values a and b are computed as follows. 10 a =
Re { a } 4 + Im { a } 4 b = a + Re { } 4 + Im { } 4
[0087] The scaling down by a factor of 2 is done to prevent
overflow during addition. The numbers a and b are then shifted such
that 0.5<b <1. Newton's method with eight iterations (I=0:7)
is then used to find .delta.=a/b. Set .delta..sub.0=0.5, and
then
.delta..sub.i+1+.delta..sub.i-(.delta..sub.ib-a)
[0088] The model parameter A (as scaled by a factor of 2) is then
determined by the following equation.
{fraction (A/2)}=-.delta..sup.2.beta.+(1-.delta.).sup.2.alpha.
[0089] The estimate of the RF interference for tone m is computed
by forming
r1=.delta.a.sub.0+a.sub.1
r2=AW.sub.m
[0090] and thus the estimate of the RF interference becomes
RFI.sub.n+m=2(r1)(r2).
[0091] The estimated RF interference is computed is then subtracted
from the data for the predetermined number of adjacent tones of the
symbol (e.g., m=-31:32).
[0092] FIG. 11 is a flow diagram of AM radio frequency (RF)
cancellation processing 1100 according to an embodiment of the
invention. AM radio transmissions also cause RF interference to
radio transmissions by a multicarrier modulation system. Unlike RF
interference due to amateur radio operators, the AM RF interference
is typically steadily present as AM radio stations tend to transmit
24 hours a day. The AM RF cancellation processing 100 is preferably
performed by a receiver or receiver portion of a transceiver of a
multicarrier modulation system. The modeling of the RF interference
described above equally applies to AM RF interference. For example,
a first order model for the frequency domain model (e.g., Equation
(5)) is also suitable for modeling AM RF interference at VDSL
rates.
[0093] The AM RF cancellation processing 1100 initially identifies
1102 AM RF interference during an initialization period in which no
data is being transmitted. Then, the frequency of the AM RF
interference is estimated 1104. For example, by measuring the data
signals received during the initialization period in which no data
is being transmitted (as often the case with multicarrier
modulation systems), the magnitude of the AM RF interference
measured at different frequencies is found. Then, in this example,
the areas in which the magnitude is maximized indicates a general
location of the carrier frequency for the AM RF interference.
Thereafter, in this example, the system can average the determined
carrier frequencies over a period of time (e.g., many data blocks)
to accurately determine the carrier frequency for the AM RF
interference. By averaging the results of
.vertline.X.sub.n+1.vertline.+/{.vertline.X.sub.-
n.vertline.+.vertline.X.sub.n+1.vertline.} (or using Equation (6))
during the initialization period, the offset amount .delta. is able
to be accurately determined and thus identifies the carrier
frequency for the AM RF interference. Once the carrier frequency
for the AM RF interference is estimated 1104, the initialization is
complete for this portion the AM RF cancellation processing 1100.
Generally, the AM cancellation assumes that larger AM interferers
are not close together in the AM frequency band.
[0094] Thereafter when data is subsequently being transmitted or
received, the AM RF cancellation processing 1100 further operates
to cancel the AM RB interference from the data signals being
received. In the case of data transmission, the frequency tones
adjacent to the estimated frequency of the AM RF interference are
disabled 1106 so that no data is transmitted thereon. Here, at
least the two frequency tones adjacent to the estimated frequency
of the AM RF interference are disabled 1106 because the RF model
uses these tones in modeling the RF interference.
[0095] The cancellation of the AM RF interference by the AM RF
cancellation processing 1100 is then as follows. The AM RF
interference is estimated 1108 in accordance with the estimated
frequency and a frequency domain model for the AM RF interference.
Thereafter, the estimated AM RF interference is removed 1110 from
the frequency domain data. Following block 1110, the RB
cancellation processing 900 is complete and ends.
[0096] Non-rectangular windowing is generally known to reduce
sidelobe levels in multicarrier modulation systems. See, e.g.,
Spruyt, Reusens and Braet, "Performance of improved DMT transceiver
for VDSL, Alcatel Telecom T1E1.4 Submission, Apr. 22-25, 1996. The
non-rectangular windowing described by Spruyt et al. extends beyond
the boundary of a symbol into a cyclic prefix and a cyclic suffix
of the symbol.
[0097] The frequency domain model discussed above preferably uses
extended, non-rectangular windowing to cause sidelobes to attenuate
faster so that the RB interference affects less frequency tones.
The particular type of non-rectangular windowing used can vary.
FIG. 12 describes a possibly preferred type of non-rectangular
windowing that is also another aspect of the invention that is
useful not only with the RF cancellation techniques described
herein but also by itself for mitigating intercarrier interference
in general.
[0098] FIG. 12 is a flow diagram of prefix removal and windowing
processing 1200 according to an embodiment of the invention. Here,
the windowing preferably performed is non-rectangular, extended
windowing. The non-rectangular windowing acts to cause the
sidelobes of the frequency tones to attenuate faster than
rectangular windowing. The extended windowing means that the window
width extends beyond the data symbol itself into a cyclic prefix.
The cyclic prefix normally consists of a repetition of data from
the end of the corresponding data symbol. The cyclic prefix is a
guard band that provides a guard time to reduce the intersymbol
interference caused because channel responses are not ideal. As one
example, in VDSL, the data symbol might have 512 samples and 40
samples of cyclic prefix. The prefix removal and windowing
processing 1200 is preferably performed by the cyclic prefix
removal and windowing processor 714 illustrated in FIG. 7.
[0099] The prefix removal and windowing processing 1200 initially
receives 1202 X-samples of a DMT symbol and Y-samples of its cyclic
prefix. For example, 512 samples of a DMT symbol and 40 samples of
a cyclic prefix may make up a DMT symbol. FIG. 13 is a diagram
illustrating a 512 sample DMT symbol 1300 (samples 40-551) with a
40 sample prefix 1302 (samples 0-39), and a non-rectangular,
extended window. In FIG. 13, the non-rectangular, extended
windowing extends from sample 20 to sample 551, with samples 20-39
being that portion that extends into the cyclic prefix. The
processing of the X-samples of the DMT symbol and the Y-samples of
the cyclic prefix are processed as follows.
[0100] An initial portion of the Y-samples of the cyclic prefix are
dropped 1204 because they are no longer needed. A remaining portion
of the Y-samples are retained 1206 for later retrieval. The size of
the remaining portion of the Y-samples depends on the amount of
extended windowing being used. For example, with a 40 sample cyclic
prefix, the size of the remaining portion of the 40-samples could
by any whole number between 0 and 40. Next, a first portion of the
X-samples of the DMT symbol are retained 1208. Then, a second
portion of the X-samples of the DMT symbol are modified 1210 in
accordance with the retained samples of the remaining portion of
the cyclic prefix and predetermined multiplication coefficients.
Following block 1210, the prefix removing and windowing processing
1200 is complete and ends.
[0101] According to the prefix removal and windowing processing
1200, the DMT symbol and its prefix have been processed such that
the resulting samples have been filtered such that an initial group
of samples of the prefix are removed and then extended
non-rectangular windowing processing is performed on the remaining
samples. The extended non-rectangular windowing operates to
multiply the samples of the remaining portion of the cyclic prefix
by a raised-cosine-function (or other smoothing function)
representing the non-rectangular portion of the window, and then
combines the resulting value into the samples of the second portion
of the X-samples. The advantage of the extended non-rectangular
windowing operation is that the effective sidelobe levels data
attenuate faster which is generally advantageous in a multicarrier
modulation system. In the case where the extended non-rectangular
windowing is used with the RF cancellation techniques according to
the invention, the advantage of the extended non-rectangular
windowing is that RF cancellation can be performed on fewer
adjacent frequency tones which reduces processing needed to
compensate for the RF interference. The saved processing time which
can be important in high-speed systems such as multicarrier
modulation systems (e.g., VDSL). The extended non-rectangular
windowing according to the invention further reduces the
computational burden required to implement the extended,
non-rectangular windowing. The following examples helps to explain
the additional computational savings offered by this aspect of the
invention.
[0102] An example of the prefix removal and windowing processing
1200 is explain for a case where 32-sample extended windowing is
utilized with 512 DMT frequency tones and 40-samples of cyclic
prefix. The values x.sub.0 through x.sub.551 represent a single DMT
symbol with its cyclic prefix, and the values w.sub.0 through
W.sub.31 are window taps that are preferably stored in RAM. In this
example, the prefix removal and windowing processing 1200 is as
follows:
[0103] Discard x.sub.0 through x.sub.7
[0104] Store x.sub.i for i=8 to 39
[0105] x.sub.i=x.sub.i for i=40 to 519
[0106] Form x.sub.520+i=x.sub.520+i+(x.sub.8+i-x.sub.520+i)w.sub.i,
for i=0 to 31.
[0107] Note that
x.sub.520+i=(1-w.sub.i)x.sub.520+i+w.sub.ix.sub.8+i=x.sub-
.520+i+(x.sub.8+i-x.sub.520+i)w.sub.i, and the implementation
requires 32 real multiply operations and 64 addition operations per
DMT symbol. In contrast, the conventional approach would utilize 64
real multiply operations and 32 or 64 addition operations per DMT
symbol. Given that the computational burden to perform a multiply
operation is significantly greater than the computational burden
for an addition operation, the ability of the invention to save 32
multiply operations is noteworthy.
[0108] It should be understood that the present invention may be
embodied in many forms and modulation schemes (e.g., Discrete
Wavelet Multi-tone Modulation (DWE)) at both the central and remote
station locations without departing from the spirit or scope of the
invention. For instance, although the specification has primarily
described the invention in the context of subscriber line based
high speed data transmission systems, the invention may be used in
other systems which experience significant narrow band interference
or have restricted frequency bands of RF interference within their
designated transmission bands.
[0109] The many features and advantages of the present invention
are apparent from the written description, and thus, it is intended
by the appended claims to cover all such features and advantages of
the invention. Further, since numerous modifications and changes
will readily occur to those skilled in the art, it is not desired
to limit the invention to the exact construction and operation as
illustrated and described. Hence, all suitable modifications and
equivalents may be resorted to as falling within the scope of the
invention.
[0110] What is claimed is:
* * * * *