U.S. patent application number 11/522839 was filed with the patent office on 2008-11-27 for cancellation of interference in a communication system with application to s-cdma.
This patent application is currently assigned to Broadcom Corporation, a California Corporation. Invention is credited to Bruce J. Currivan, Thomas J. Kolze, Gottfried Ungerboeck, Nabil R. Yousef.
Application Number | 20080291980 11/522839 |
Document ID | / |
Family ID | 28456670 |
Filed Date | 2008-11-27 |
United States Patent
Application |
20080291980 |
Kind Code |
A1 |
Currivan; Bruce J. ; et
al. |
November 27, 2008 |
CANCELLATION OF INTERFERENCE IN A COMMUNICATION SYSTEM WITH
APPLICATION TO S-CDMA
Abstract
Cancellation of interference in a communication system with
application to S-CDMA. A relatively straight-forward implemented
and computationally efficient approach of selecting a predetermined
number of unused codes is used to perform weighted linear
combination selectively with each of the input spread signals in a
multiple access communication system. If desired, the predetermined
number of unused codes is always the same in each implementation.
Alternatively, the predetermined number of unused codes is selected
from within a reordered code matrix using knowledge that is shared
between the two ends of a communication system, such as between the
CMs and a CMTS. While the context of an S-CDMA communication system
having CMs and a CMTS is used, the solution is generally applicable
to any communication system that seeks to cancel narrowband
interference. Several embodiments are also described that show the
generic applicability of the solution across a wide variety of
systems.
Inventors: |
Currivan; Bruce J.; (Dove
Canyon, CA) ; Kolze; Thomas J.; (Phoenix, AZ)
; Ungerboeck; Gottfried; (Langnau, CH) ; Yousef;
Nabil R.; (Foothill Ranch, CA) |
Correspondence
Address: |
GARLICK HARRISON & MARKISON
P.O. BOX 160727
AUSTIN
TX
78716-0727
US
|
Assignee: |
Broadcom Corporation, a California
Corporation
Irvine
CA
|
Family ID: |
28456670 |
Appl. No.: |
11/522839 |
Filed: |
September 18, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10142189 |
May 8, 2002 |
7110434 |
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11522839 |
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09652721 |
Aug 31, 2000 |
6778611 |
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10142189 |
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60367564 |
Mar 26, 2002 |
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60151680 |
Aug 31, 1999 |
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Current U.S.
Class: |
375/148 ;
375/E1.021; 375/E1.029 |
Current CPC
Class: |
H04B 1/71 20130101; H04B
7/216 20130101; H04J 13/0003 20130101; H04J 13/16 20130101; H04B
1/7105 20130101; H04B 1/7107 20130101; H04B 1/7102 20130101 |
Class at
Publication: |
375/148 |
International
Class: |
H04B 1/707 20060101
H04B001/707 |
Claims
1. An apparatus, comprising: an equalizer that is operable to:
receive a signal from a communication channel, wherein the signal
includes additive white noise and a frequency response of the
communication channel includes a null at a frequency; and equalize
the signal using an inverse, zero forcing response that includes a
peak at approximately the frequency of the null within the
frequency response of the communication channel thereby
transforming white noise within the signal to become colored noise
located at approximately the frequency; and a canceller that is
operable to perform interference cancellation on the colored noise
located at approximately the frequency.
2. The apparatus of claim 1, wherein: the signal received from the
communication channel is a spread signal; the canceller includes a
vector de-spreader, a weight computation block, and a linear
combiner; the vector de-spreader is operable to employ a first
plurality of codes and a second plurality of codes to de-spread the
spread signal that is received from the equalizer; the weight
computation block is operable to determine a plurality of
complex-valued combining weights using at least one portion of the
de-spread signal and the colored noise located at approximately the
frequency; and the linear combiner is operable to: scale a
plurality of de-spread signal portions, that is de-spread using the
first plurality of codes, using the plurality of complex-valued
combining weights; and combine the scaled, plurality of de-spread
signal portions with one signal portion that is de-spread using one
code of the second plurality of codes thereby performing the
interference cancellation on the colored noise located at
approximately the frequency.
3. The apparatus of claim 2, wherein: the weight computation block
is operable to calculate one complex-valued combining weight of the
plurality of complex-valued combining weights using the one signal
portion that is de-spread using one code of the second plurality of
codes and adjacent signal portions that are de-spread using codes
adjacent to the one code of the second plurality of codes.
4. The apparatus of claim 2, wherein: the weight computation block
is operable to calculate one complex-valued combining weight of the
plurality of complex-valued combining weights using the plurality
of de-spread signal portions, that is de-spread using the first
plurality of codes, and at least one additional plurality of
de-spread signal portions, that is de-spread using the second
plurality of codes.
5. The apparatus of claim 2, wherein: at least one complex-valued
combining weight of the plurality of complex-valued combining
weights is a predetermined complex-valued combining weight.
6. The apparatus of claim 1, wherein: the signal received from the
communication channel is a spread signal; the canceller includes a
vector de-spreader; the vector de-spreader is operable to employ a
first plurality of codes and a second plurality of codes to
de-spread the spread signal that is received from the equalizer;
and of first de-spread signal portions that are de-spread using the
first plurality of codes and second de-spread signal portions that
are de-spread using the second plurality of codes, only the first
de-spread signal portions that are de-spread using the first
plurality of codes are employed by the canceller to perform
interference cancellation on the colored noise located at
approximately the frequency.
7. The apparatus of claim 1, wherein: the canceller includes a
vector de-spreader and a plurality of feed-forward equalizers; the
vector de-spreader is operable to employ a first plurality of codes
and a second plurality of codes to de-spread the spread signal that
is received from the equalizer; each feed-forward equalizer of the
plurality of feed-forward equalizers includes a tapped delay line
and a plurality of combining weights; each feed-forward equalizer
of the plurality of feed-forward equalizers is operable to sum its
corresponding de-spread signal portion with delayed and scaled
versions of its corresponding de-spread signal portion using the
tapped delay line and the plurality of combining weights; the
linear combiner is operable to sum outputs from each feed-forward
equalizer of the plurality of feed-forward equalizers and the one
signal portion that is de-spread using one code of the second
plurality of codes; the plurality of feed-forward equalizers
includes a first number of feed-forward equalizers; the first
plurality of codes and the second plurality of codes together
include a second number of codes; and the first number is less than
the second number.
8. The apparatus of claim 1, wherein: the canceller includes a
plurality of feed-forward equalizers; each feed-forward equalizer
of the plurality of feed-forward equalizers includes a
corresponding plurality of equalizer taps; and each equalizer tap
within each corresponding plurality of equalizer taps has a
frequency selective response.
9. The apparatus of claim 1, wherein: the apparatus is operable to
be implemented within an S-CDMA (Synchronous Code Division Multiple
Access) communication system.
10. The apparatus of claim 1, wherein: the apparatus is operable to
be implemented within a cellular communication system that employs
at least one type of S-CDMA (Synchronous Code Division Multiple
Access) communications.
11. An apparatus, comprising: an equalizer that is operable to:
receive a spread signal from a communication channel, wherein the
spread signal includes additive white noise and a frequency
response of the communication channel includes a null at a
frequency; and equalize the signal using an inverse, zero forcing
response that includes a peak at approximately the frequency of the
null within the frequency response of the communication channel
thereby transforming white noise within the signal to become
colored noise located at approximately the frequency; and a
canceller, that includes a vector de-spreader, a weight computation
block, and a linear combiner, that is operable to perform
interference cancellation on the colored noise located at
approximately the frequency; and wherein: the vector de-spreader is
operable to employ a first plurality of codes and a second
plurality of codes to de-spread the spread signal that is received
from the equalizer thereby generating a first plurality of
de-spread signal portions and a second plurality of de-spread
signal portions; the weight computation block is operable to
determine a plurality of complex-valued combining weights using at
least one portion of the de-spread signal and the colored noise
located at approximately the frequency; and the linear combiner is
operable to: scale the first plurality of de-spread signal portions
using the plurality of complex-valued combining weights; and
combine the scaled, plurality of de-spread signal portions with one
signal portion that is de-spread using one code of the second
plurality of codes thereby performing the interference cancellation
on the one signal portion that is de-spread using one code of the
second plurality of codes that corresponds to the colored noise
located at approximately the frequency; and the apparatus is
operable to be implemented within an S-CDMA (Synchronous Code
Division Multiple Access) communication system.
12. The apparatus of claim 11, wherein: the weight computation
block is operable to calculate one complex-valued combining weight
of the plurality of complex-valued combining weights using the one
signal portion that is de-spread using one code of the second
plurality of codes and adjacent signal portions that are de-spread
using codes adjacent to the one code of the second plurality of
codes.
13. The apparatus of claim 11, wherein: at least one complex-valued
combining weight of the plurality of complex-valued combining
weights is a predetermined complex-valued combining weight.
14. The apparatus of claim 11, wherein: of the first de-spread
signal portions and the second de-spread signal portions, only the
first de-spread signal portions are employed by the canceller to
perform interference cancellation on the colored noise located at
approximately the frequency.
15. The apparatus of claim 11, wherein: a plurality of numerically
values symbols is modulated using the first plurality of codes by
at least one additional apparatus when generating the spread the
signal; and the spread signal is launched into the communication
channel by the at least one additional apparatus.
16. The apparatus of claim 11, wherein: the linear combiner
includes a plurality of feed-forward equalizers; each feed-forward
equalizer of the plurality of feed-forward equalizers includes a
corresponding plurality of equalizer taps; and each equalizer tap
within each corresponding plurality of equalizer taps has a
frequency selective response.
17. A method, comprising: receiving a spread signal from an S-CDMA
(Synchronous Code Division Multiple Access) communication channel,
wherein the spread signal includes additive white noise and a
frequency response of the communication channel includes a null at
a frequency; equalizing the signal using an inverse, zero forcing
response that includes a peak at approximately the frequency of the
null within the frequency response of the S-CDMA communication
channel thereby transforming white noise within the signal to
become colored noise located at approximately the frequency; and
performing interference cancellation on the colored noise located
at approximately the frequency.
18. The method of claim 17, further comprising: employing a first
plurality of codes and a second plurality of codes to de-spread the
spread signal; determining a plurality of complex-valued combining
weights using at least one portion of the de-spread signal and the
colored noise located at approximately the frequency; scaling a
plurality of de-spread signal portions, that is de-spread using the
first plurality of codes, using the plurality of complex-valued
combining weights; and combining the scaled, plurality of de-spread
signal portions with one signal portion that is de-spread using one
code of the second plurality of codes thereby performing the
interference cancellation on the one signal portion that is
de-spread using one code of the second plurality of codes that
corresponds to the colored noise located at approximately the
frequency.
19. The method of claim 17, further comprising: calculating at
least one complex-valued combining weight of the plurality of
complex-valued combining weights; or selecting at least one
complex-valued combining weight of the plurality of complex-valued
combining weights from a plurality of predetermined complex-valued
combining weights.
20. The method of claim 17, wherein: the method is performed within
an apparatus implemented within a cellular communication system
that employs S-CDMA communications.
Description
CROSS REFERENCE TO RELATED PATENTS/PATENT APPLICATIONS
Continuation Priority Claim, 35 U.S.C. .sctn. 120
[0001] The present U.S. Utility Patent Application claims priority
pursuant to 35 U.S.C. .sctn. 120, as a continuation, to the
following U.S. Utility Patent Application which is hereby
incorporated herein by reference in its entirety and made part of
the present U.S. Utility Patent Application for all purposes:
[0002] 1. U.S. Utility application Ser. No. 10/142,189, entitled
"Cancellation of interference in a communication system with
application to S-CDMA," (Attorney Docket No. BP2285), filed May 8,
2002, pending, which claims priority pursuant to 35 U.S.C. .sctn.
119(e) to the following U.S. Provisional Patent Application which
is hereby incorporated herein by reference in its entirety and made
part of the present U.S. Utility Patent Application for all
purposes: [0003] 1. U.S. Provisional Application Ser. No.
60/367,564, entitled "Cancellation of interference in a
communication system with application to S-CDMA," (Attorney Docket
No. BP2285), filed Mar. 26, 2002.
Parent's Continuation-in-Part (CIP) Priority Claim, 35 U.S.C.
.sctn. 120
[0004] The U.S. Utility application Ser. No. 10/142,189, from which
the present U.S. Utility Patent Application is a continuation, also
claims priority pursuant to 35 U.S.C. .sctn. 120, as a
continuation-in-part (CIP), to the following U.S. Utility Patent
Application which is hereby incorporated herein by reference in its
entirety and made part of the present U.S. Utility Patent
Application for all purposes:
[0005] 1. U.S. Utility application Ser. No. 09/652,721, entitled
"Subdimensional single carrier modulation," (Attorney Docket No. BP
1140), filed Aug. 31, 2000, now U.S. Pat. No. 6,778,611 B1, issued
on Aug. 17, 2004, which claims priority pursuant to 35 U.S.C.
.sctn. 119(e) to the following U.S. Provisional Patent Application
which is hereby incorporated herein by reference in its entirety
and made part of the present U.S. Utility Patent Application for
all purposes: [0006] 1. U.S. Provisional Application Ser. No.
60/151,680, entitled "Subdimensional single carrier modulation,"
(Attorney Docket No. BP2285), filed Aug. 31, 1999.
BACKGROUND OF THE INVENTION
[0007] 1. Technical Field of the Invention
[0008] The invention relates generally to communication systems;
and, more particularly, it relates to communication systems that
may be affected by undesirable interference.
[0009] 2. Description of Related Art
[0010] Signal processing within communication systems having a
communication channel, in an effort to improve the quality of
signals passing through the communication channel, has been under
development for many years. In the past several years, emphasis has
moved largely to the domain of digital communication systems that
modulate bit streams into an analog signal for transmission over a
communication channel. This channel can be a variety of channel
types. Many different approaches are employed in the prior art to
try to minimize or substantially reduce the effects of interference
that may be introduced into a signal that is transmitted across a
communication channel. In particular, the prior art approaches that
seek to perform cancellation of interference that occupies a small
number of signal dimensions in a signal are typically deficient for
a number of reasons as is briefly referenced below. One particular
type of interference that these prior art schemes seek to minimize
is the narrowband interference that is sometimes referred to as
ingress interference. Another type of narrowband interference that
may be problematic is the interference of impulse/burst noise. Yet
another type of interference that may be problematic is within the
code division multiple access (CDMA) context when the interference
is on a small number of codes.
[0011] One of the main methods employed in the prior art to
eliminate this interference is the use of a notch filter. This
solution is sufficient in some applications, but the notch filter
itself oftentimes causes distortion of the desired signal. In the
CDMA context, this distortion is called inter-code interference
(ICI). Then, another means must oftentimes be included to remove
the very ICI that has been introduced by the notch filter. One way
to do this is to de-spread the signal. Then, hard decisions are
made using the de-spread signal. The hard decisions are then
respread and passed through the notch filter and to subtract from
the original signal to remove the estimated distortion. In some
instances, this process is repeated numerous times to try to
achieve an adequate result.
[0012] These prior art approaches described above are deficient in
that they suffer the effect of the error propagation. The decision
circuit is prone to make incorrect decisions, requiring many
iterations before the process converges, if it ever converges at
all.
[0013] Further limitations and disadvantages of conventional and
traditional systems will become apparent through comparison of such
systems with the invention as set forth in the remainder of the
present application with reference to the drawings.
BRIEF SUMMARY OF THE INVENTION
[0014] Various aspects of the invention can be found in a
communication receiver that supports interference cancellation
functionality. The present invention uses a linear combination of
the unused dimensions (for example, the unused codes) to cancel the
interference. This may be done after the de-spreader, or,
equivalently, as part of the de-spreading process. There is no
appreciable inter-code interference introduced and no decision
errors are made as part of this process. The allocation of unused
codes reduces the capacity of the system by a small amount. In one
embodiment, the reduction of capacity of the system will be to
120/128 of the original capacity for a system of 128 codes where 8
unused codes are employed. Clearly, other numbers of available
codes may also be employed without departing from the scope and
spirit of the invention.
[0015] It is noted that the present invention may be extended
across a wide variety of application contexts. The present
technique can be applied to cancel not only narrowband
interference, but any interference that occupies a small number of
dimensions in the signal space. The narrowband interference
includes just one of the many types of interferences that may be
substantially cancelled according to the present invention. A
narrowband signal occupies a small number of DFT (discrete Fourier
transform) bins, showing that it occupies a small number of
dimensions in signal space. An extremely simple example is a CW
(Continuous Wave) signal whose frequency is an integer multiple of
the de-spread symbol rate; this CW signal occupies only a single
bin in the DFT, or only one dimension in the signal space, where
each dimension is, in this case, one DFT bin. Another example is a
short burst of noise (impulse or burst noise). A short burst signal
occupies a small number of time samples, again showing that it too
occupies a small number of dimensions, where each dimension is, in
this case, a time sample.
[0016] Other types of signals may be constructed, without limit,
that satisfy the property that they occupy a small number of
dimensions in their respective signal space. All such signals can
be canceled by the present technique. It is noted that the DFT is
just one example of an orthonormal expansion. A second example is
the code matrix in DOCSIS 2.0 S-CDMA. A third example may be the
identity matrix. Innumerable other orthonormal transforms also
exist that are used to transform a signal into a finite signal
space. If a signal occupies a small number of dimensions in any
orthonormal transformation, the interference cancellation performed
according to the present invention may be used for canceling it. It
is noted here that orthogonal transformations may also be used
without departing from the scope and spirit of the invention in any
way within embodiments where normalization of not of concern or of
low priority.
[0017] A number of specific embodiments are illustrated to show the
versatility and wide applicability of the present invention across
a variety of communication systems contexts. However, it is
generally noted that the present invention may be practiced within
any communication system that seeks to perform interference
cancellation when the communication system employs signaling that
occupies a small number of dimensions in the communication signal's
space.
[0018] The present invention is directed to apparatus and methods
of operation that are further described in the following Brief
Description of the Several Views of the Drawings, the Detailed
Description of the Invention, and the claims. Other features and
advantages of the present invention will become apparent from the
following detailed description of the invention made with reference
to the accompanying drawings.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0019] A better understanding of the invention can be obtained when
the following detailed description of various exemplary embodiments
is considered in conjunction with the following drawings.
[0020] FIG. 1 is a system diagram illustrating an embodiment of a
cable modem (CM) communication system that is built according to
the present invention.
[0021] FIG. 2 is a system diagram illustrating another embodiment
of a CM communication system that is built according to the present
invention.
[0022] FIG. 3A is a system diagram illustrating an embodiment of a
cellular communication system that is built according to the
present invention.
[0023] FIG. 3B is a system diagram illustrating another embodiment
of a cellular communication system that is built according to the
present invention.
[0024] FIG. 4 is a system diagram illustrating an embodiment of a
satellite communication system that is built according to the
present invention.
[0025] FIG. 5A is a system diagram illustrating an embodiment of a
microwave communication system that is built according to the
present invention.
[0026] FIG. 5B is a system diagram illustrating an embodiment of a
point-to-point radio communication system that is built according
to the present invention.
[0027] FIG. 6 is a system diagram illustrating an embodiment of a
high definition (HDTV) communication system that is built according
to the present invention.
[0028] FIG. 7 is a system diagram illustrating an embodiment of a
communication system that is built according to the present
invention.
[0029] FIG. 8 is a system diagram illustrating another embodiment
of a communication system that is built according to the present
invention.
[0030] FIG. 9 is a system diagram illustrating an embodiment of a
cable modem termination system (CMTS) system that is built
according to the present invention.
[0031] FIG. 10 is a system diagram illustrating an embodiment of a
burst receiver system that is built according to the present
invention.
[0032] FIG. 11 is a system diagram illustrating an embodiment of a
single chip DOCSIS/EuroDOCSIS CM system that is built according to
the present invention.
[0033] FIG. 12 is a system diagram illustrating another embodiment
of a single chip DOCSIS/EuroDOCSIS CM system that is built
according to the present invention.
[0034] FIG. 13 is a system diagram illustrating an embodiment of a
single chip wireless modem system that is built according to the
present invention.
[0035] FIG. 14 is a system diagram illustrating another embodiment
of a single chip wireless modem system that is built according to
the present invention.
[0036] FIG. 15 is a diagram illustrating an embodiment of a vector
de-spreader that is built according to the present invention.
[0037] FIG. 16 is a diagram illustrating an embodiment of an
interference canceller that is built according to the present
invention.
[0038] FIG. 17 is a diagram illustrating another embodiment of an
interference canceller that is built according to the present
invention.
[0039] FIG. 18 is a diagram illustrating another embodiment of an
interference canceller that is built according to the present
invention.
[0040] FIG. 19 is a diagram illustrating an embodiment of an
interference canceller with memory that is built according to the
present invention.
[0041] FIG. 20 is a diagram illustrating an embodiment of
equalization with canceller that is arranged according to the
present invention.
[0042] FIG. 21 is a diagram illustrating an embodiment of Least
Means Square (LMS) training of an interference canceller according
to the present invention.
[0043] FIG. 22A is a diagram illustrating an embodiment of signal
transformation according to the present invention.
[0044] FIG. 22B is a diagram illustrating another embodiment of
signal transformation according to the present invention.
[0045] FIG. 23 is an operational flow diagram illustrating an
embodiment of an interference cancellation method that is performed
according to the present invention.
[0046] FIG. 24 is an operational flow diagram illustrating another
embodiment of an interference cancellation method that is performed
according to the present invention.
[0047] FIG. 25 is an operational flow diagram illustrating an
embodiment of an unused code selection method that is performed
according to the present invention.
[0048] FIG. 26 is an operational flow diagram illustrating an
embodiment of an S-CDMA interference cancellation method that is
performed according to the present invention.
[0049] FIG. 27 is an operational flow diagram illustrating another
embodiment of an interference cancellation method that is performed
according to the present invention.
[0050] FIG. 28 is a diagram illustrating an embodiment of a
spectrum of narrowband interference that may be addressed and
overcome when practicing the present invention.
[0051] FIG. 29 is a diagram illustrating an embodiment of a
spectrum of an adapted code showing a null at a location of
interference that may be achieved when practicing the present
invention.
[0052] FIG. 30A is a diagram illustrating an embodiment of a
received constellation before interference has been cancelled when
practicing the present invention.
[0053] FIG. 30B is a diagram illustrating an embodiment of a
received constellation after interference has been cancelled when
practicing the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0054] The present invention provides a solution for interference
cancellation for communication systems (where a medium is used by
one user or if shared among many users). More specifically, the
present invention is applicable within code-division multiple
access (CDMA) communication systems, as well as synchronous
code-division multiple access (S-CDMA) communication systems. One
particular type of S-CDMA communication system that may benefit
from the present invention is the Data Over Cable Service Interface
Specifications (DOCSIS) version 2.0 S-CDMA that is operable for
communication systems. The present invention presents a solution
that provides for cancellation of interference in any such
communication systems. The interference cancellation may be viewed
as being directed primarily towards the type of interference that
occupies a small number of signal dimensions. Examples of such
types of interference include narrowband interference (herein also
referred to as ingress) or impulse/burst noise. The present
invention also provides a solution where it may operate in the
presence of simultaneous narrowband interference and impulse/burst
noise and to substantially eliminate them both.
[0055] The present invention provides an approach for interference
cancellation that provides a number of benefits including a
completely linear method of canceling ingress that employed no DFE
(Decision Feedback Equalizer) or SIC (Successive Ingress
Cancellation). This approach can cancel wideband ingress and may be
implemented in a relatively simple and efficient structure.
Moreover, the present invention may be combined relatively easily,
given its simple and efficient structure, with other methods and
systems that may assist in the interference cancellation.
[0056] The present invention uses a linear combination of the
unused dimensions (for example, the unused codes) to cancel the
interference. This may be done after the de-spreader, or,
equivalently, as part of the de-spreading process. There is no
appreciable inter-code interference introduced and no decision
errors are made as part of this process. The allocation of unused
codes reduces the capacity of the system by a small amount; for
example, to 120/128 of the original capacity for a system of 128
codes where 8 unused codes are employed.
[0057] It is noted here that the specific examples of 120 active
codes, and 8 unused codes in a system having 128 available codes is
exemplary. Clearly, other embodiments may be employed (having
different numbers of codes--both different numbers of used and
unused) without departing from the scope and spirit of the
invention.
[0058] From certain perspectives, the present invention may be
viewed as being an extension of sub-dimensional modulation that is
employed for TDMA (Time Division Multiple Access), and it may then
be extended to S-CDMA (Synchronous Code Division Multiple Access).
To begin this discussion, we consider an example case where 120
active codes are used and narrowband ingress (narrowband
interference) is present. The remaining 8 codes are then
transmitted as true zero symbols. At the receiver, the unused 8
codes are received with samples of the narrowband ingress. This
information may then be used to cancel the ingress in the same
manner that is used in sub-dimensional modulation. The 8 unused
codes are considered as extra dimensions and the receiver is
trained based on this information.
[0059] We continue on with 128 code example (8 of the codes being
unused). By employing the 8 unused spreading codes, linear
combinations of these 8 codes may be added to any other code that
is transmitted from a transmitter to a receiver, and the resulting
composite waveform can be used for de-spreading that one
transmitted code. Since, ideally, no signal is present on any of
these 8 codes (being zero codes), then little or no inter-code
interference from the transmitters will be induced. In AWGN
(Additive White Gaussian Noise), the inclusion of these additional
de-spreading codes serves to merely increase the noise at the
decision slicer for the code of interest. However, in the presence
of interference, such as narrowband interference, some combination
of these codes in the de-spreading process is operable to reduce
the ingress enough to overcome the increased AWGN and make it
worthwhile. Heuristically, the unused codes may correlate somewhat
with the interference, and if so, may be used to "subtract" some
component of the interference at the decision slicer, as a type of
noise canceller.
[0060] The similarity of this approach is somewhat analogous to the
functionality of an ICF (Ingress Cancellation Filter). In certain
embodiments, a TDMA-only, CDMA-only, and/or a TDMA/CDMA burst
receiver may be leveraged for this S-CDMA application, including
the use of the Trench method or derivative that is mentioned
below.
[0061] It is noted that the approach to reducing the interference
power at the decision point using only the unused codes may not be
optimal in certain embodiments, since using codes with data carried
on them may offer some interference rejection overcoming the
introduction of the transmitter inter-code interference. It is also
noted that including "in use" codes would actually provide benefit
in practical situations.
[0062] It is also noted that using a strict LMS (Least Mean Square)
type of approach to converging 128 taps to the desired de-spreading
code may prove laborious (in terms of requiring many iterations).
This approach does not focus on just the unused codes, and has far
more degrees of freedom than just finding 8 coefficients for the
weightings of the 8 unused codes.
[0063] Analysis has shown that indeed there is great similarity in
the formulation and solution of the optimal taps in the monic
filter of the ICF for ingress cancellation, and in this application
of unused spreading codes in ingress cancellation for S-CDMA. Both
can be formulated in a LS (Least Squares) type of problem, with the
ensuing typical solution taking form.
[0064] One advantage that may arise for the TDMA ICF case is when
the matrix to be inverted is Toeplitz (a Toeplitz matrix in
addition a matrix in which all the elements are the same along any
diagonal that slopes from northwest to southeast), and thus admits
significant computational advantages, such as discovered by William
Trench.
[0065] However, in the S-CDMA formulation with unused codes, the
matrix to be inverted is not Toeplitz, and the Trench approach does
not apply. There still may be some computational advantages due to
the underlying construction of the matrix to be inverted, but it
may be that only the traditional numerical methods such as Cholesky
decomposition may introduce simplification. The matrix to be
inverted is at least of the form C*RC, where C=128.times.8, with
each column orthogonal with the others (a sub-matrix of a Unitary
matrix), and R=128.times.128 and is Toeplitz, and * stands for
complex conjugate.
[0066] One implementation of the present invention may be described
as shown below.
[0067] Let R=R.sub.m,n=E{r*(m) r(n)}, where r(n) are noise and
ingress samples, containing little or no signal. R is
128.times.128, where the samples correspond to the noise in the
chips of a spreading interval. The symbol * denotes complex
conjugate. This is the noise (or noise plus interference)
covariance matrix.
[0068] Let C=[c1 c2 c3 c4 c5 c6 c7 c8], where ci=column of 128
chips of i.sup.th unused spreading code. C is 128 rows by 8
columns.
[0069] Let:
[0070] dsc.sub.opt=optimal de-spreading code for code-of-interest
c.sub.k, (written alternatively below)
dsc.sub.opt=c.sub.k+w.sub.1c1+w.sub.2c2+ . . . +w.sub.8c8,
[0071] where dsc.sub.opt is a column vector with 128 components,
and w.sub.i are scalar weighting coefficients.
[0072] Thus, dsc.sub.opt=[1 w.sub.1 w.sub.2 . . . w.sub.8][c.sub.k
c1 c2 . . . c8], and we can see that the optimal solution for the
S-CDMA case indeed has a form similar to the monic filter in the
TDMA ICF solution.
[0073] Let w.sub.opt=[w.sub.1 w.sub.2 . . . w.sub.8] which provides
the optimal de-spreading for signaling with the k.sup.th spreading
code.
[0074] It can be shown that
w.sub.opt=-[C.sup.TRC].sup.-1[C.sup.TR]c.sub.k, where all the
vectors, matrices, and notation are as defined above.
[0075] It is noted that with the ICF in TDMA,
w opt = - [ R ] - 1 [ r 1 r 2 r 3 r 16 ] T , where ##EQU00001## R =
[ r 0 r 1 r 2 r 16 r 1 * r 0 r 1 r 15 r 16 * r 15 * r 1 * r 0 ]
##EQU00001.2##
[0076] Another characteristic of the present invention is that the
complexity of the method with the LS approach is high for large
matrices compared to the ICF and Trench method with TDMA. However,
this complexity is mitigated by the reduced matrix size resulting
from the sub-space projection approach described here and below.
From certain perspectives, the S-CDMA approach with unused codes
may be viewed as not lending itself to the computational
efficiencies of Trench approach or its derivatives.
[0077] However, using an LMS tap update approach has been found to
work. The weights of the unused spreading codes, wi, are updated in
LMS fashion. In this method, the de-spreader for the code of
interest is input to a decision slicer. The resulting complex error
is computed. Similarly, each unused spreading code has its
corresponding de-spreading operating. The result of the de-spreader
corresponding to an unused spreading code is "signal," just as the
"signal" rests in the various shift registers in the conventional
FIR (Finite Impulse Response) filter in the normal LMS. The
de-spread outputs for each of the unused codes is multiplied by the
error vector, and then multiplied by a step size factor "-mu" and
added to the existing tap weight, to update the tap weight.
[0078] With 8 unused spreading codes, there is a set of 8 tap
weights for each used spreading code. Thus, with the 8 unused
spreading codes, there are 8.times.120 tap weights to iterate.
[0079] This approach is much less computationally intensive than
using an LS-based. The S-CDMA LMS approach introduces little
additional computation when compared to operating the de-spreading
codes themselves. This approach is beneficial for a number of
reasons. It is computationally acceptable, in that, it limits the
number of taps to only 8 (in our continuing example of 8 unused
spreading codes), which almost certainly provides much more rapid
convergence and less "tap noise." In addition, it eliminates from
the search for optimal coefficients in the de-spreader those codes
that are known to correspond to used codes.
[0080] FIG. 1 is a system diagram illustrating an embodiment of a
CM communication system 100 that is built according to the present
invention. The CM communication system includes a number of CMs
(shown as a CM user #1 111, a CM user #2 115, . . . , and a CM user
#n 121) and a CMTS 130. The CMTS 130 is a component that exchanges
digital signals with CMs on a cable network.
[0081] Each of a number of CM users, shown as the CM user #1 111,
the CM user #2 115, . . . , and the CM user #n 121, is able to
communicatively couple to a CM network segment 199. A number of
elements may be included within the CM network segment 199. For
example, routers, splitters, couplers, relays, and amplifiers may
be contained within the CM network segment 199 without departing
from the scope and spirit of the invention.
[0082] The CM network segment 199 allows communicative coupling
between a CM user and a cable headend transmitter 120 and/or a CMTS
130. In some embodiments, a cable CMTS is in fact contained within
a headend transmitter. In other embodiments, the functionality of
the cable CMTS and the headend transmitter are represented as two
distinct functional blocks so that their respective contribution
may be more easily appreciated and understood. This viewpoint is
shown in the situation where the CMTS 130 is pictorially shown as
being located externally to a cable headend transmitter 120. In the
more common representation and implementation, a CMTS 135 is
located within the cable headend transmitter 120. The combination
of a CMTS and a cable headend transmitter may be referred to as
being the "cable headend transmitter;" it then being understood
that the cable headend transmitter supports the CMTS functionality.
The CMTS 130 may be located at a local office of a cable television
company or at another location within a CM communication system. In
the following description, the CMTS 130 is used for illustration;
yet, the same functionality and capability as described for the
CMTS 130 may equally apply to embodiments that alternatively employ
the CMTS 135. The cable headend transmitter 120 is able to provide
a number of services including those of audio, video, telephony,
local access channels, as well as any other service known in the
art of cable systems. Each of these services may be provided to the
one or more CM users 111, 115, . . . , and 121.
[0083] In addition, through the CMTS 130, the CM users 111, 115, .
. . , 121 are able to transmit and receive data from the Internet,
. . . , and/or any other network to which the CMTS 130 is
communicatively coupled. The operation of a CMTS, at the
cable-provider's head-end, may be viewed as providing many of the
same functions provided by a digital subscriber line access
multiplexor (DSLAM) within a digital subscriber line (DSL) system.
The CMTS 130 takes the traffic coming in from a group of customers
on a single channel and routes it to an Internet Service Provider
(ISP) for connection to the Internet, as shown via the Internet
access. At the head-end, the cable providers will have, or lease
space for a third-party ISP to have, servers for accounting and
logging, dynamic host configuration protocol (DHCP) for assigning
and administering the Internet protocol (IP) addresses of all the
cable system's users, and typically control servers for a protocol
called Data Over Cable Service Interface Specifications (DOCSIS),
the major standard used by U.S. cable systems in providing Internet
access to users.
[0084] The downstream information flows to all of the connected CM
users 111, 115, . . . , 121; this may be viewed to be in a manner
that is similar to that manner within an Ethernet network. The
individual network connection, within the CM network segment 199,
decides whether a particular block of data is intended for it or
not. On the upstream side, information is sent from the CM users
111, 115, . . . , 121 to the CMTS 130; on this upstream
transmission, the users within the CM users 111, 115, . . . , 121
to whom the data is not intended do not see that data at all. As an
example of the capabilities provided by a CMTS, the CMTS will
enable as many as 1,000 users to connect to the Internet through a
single 6 MHz channel. Since a single channel is capable of 30-40
megabits per second of total throughput, this means that users may
see far better performance than is available with standard dial-up
modems. Embodiments implementing the present invention are
described below and in the various Figures that show the data
handling and control within one or both of a CM and a CMTS within a
CM system that operates by employing S-CDMA (Synchronous Code
Division Multiple Access).
[0085] The CMs of the CM users 111, 115, . . . , 121 and the CMTS
130 communicate synchronization information to one another to
ensure proper alignment of transmission from the CM users 111, 115,
. . . , 121 to the CMTS 130. This is where the synchronization of
the S-CDMA communication systems is extremely important. When a
number of the CMs all transmit their signals at a same time such
that these signals are received at the CMTS 130 on the same
frequency and at the same time, they must all be able to be
properly de-spread and decoded for proper signal processing.
[0086] Each of the CMs users 111, 115, . . . , 121 is located a
respective transmit distance from the CMTS 130. In order to achieve
optimum spreading diversity and orthogonality for the CMs users
111, 115, . . . , 121 to transmission of the CMTS 130, each of the
CM transmissions must be synchronized so that it arrives, from the
perspective of the CMTS 130, synchronous with other CM
transmissions. In order to achieve this goal, for a particular
transmission cycle, each of the CMs 111, 15, . . . , 121 will
typically transmit to the CMTS 130 at a respective transmission
time, which will likely differ from the transmission times of other
CMs. These differing transmission times will be based upon the
relative transmission distance between the CM and the CMTS 130.
These operations may be supported by the determination of the round
trip delays (RTPs) between the CMTS 130 and each supported CM. With
these RTPs determined, the CMs may then determine at what point to
transmit their S-CDMA data so that all CM transmissions will arrive
synchronously at the CMTS 130.
[0087] The present invention enables interference cancellation
within the CMTS 130, as shown in a functional block 131. The
present invention may also be implemented to support interference
cancellation within any one of the CMs 111, 115, . . . , 121; the
particular implementation of interference cancellation is shown as
a functional block 122 within the CM 122, yet it is understood that
the interference cancellation functionality may also be supported
within the other CMs as well. The CMTS 130 receives an input spread
signal and is operable to perform dispreading and interference
cancellation according to the present invention. The CMTS 130 is
operable to employ a linear combiner, that uses as inputs complex
valued combining weights to the particular codes that are
selectively used to assist in the interference cancellation of one
of the de-spread signals that is de-spread from the input spread
signal that the CMTS 130 receives. Alternatively, the present
invention may be viewed as employing at least one of an adapted
code and an adapted code matrix to perform interference
cancellation according to the present invention.
[0088] FIG. 2 is a system diagram illustrating another embodiment
of a CM communication system 200 that is built according to the
present invention. From certain perspectives, the FIG. 2 may be
viewed as a communication system allowing bi-directional
communication between a customer premise equipment (CPE) 240 and a
network. In some embodiments, the CPE 240 is a personal computer or
some other device allowing a user to access an external network.
The network may be a wide area network (WAN) 280, or alternatively,
the Internet 290 itself. For example, the CM communication system
200 is operable to allow Internet protocol (IP) traffic to achieve
transparent bi-directional transfer between a CMTS-network side
interface (CMTS-NSI: viewed as being between the CMTS 230 and the
Internet 290) and a CM to CPE interface (CMCI: viewed as being
between the CM 210 and the CPE 240).
[0089] The WAN 280, and/or the Internet 290, is/are communicatively
coupled to the CMTS 230 via a CMTS-NSI. The CMTS 230 is operable to
support the external network termination, for one or both of the
WAN 280 and the Internet 290. The CMTS 230 includes a modulator and
a demodulator to support transmitter and receiver functionality to
and from a CM network segment 299. The receiver functionality
within the CMTS 230 is operable to support interference
cancellation functionality 231 according to the present invention.
It is also noted that there may be embodiment where the CM 210 is
also operable to support interference cancellation functionality
using the present invention, as shown by a functional block 211.
Implementing interference cancellation in the transmitter prevents
noise enhancement that occurs when interference cancellation is
performed in the receiver.
[0090] A number of elements may be included within the CM network
segment 299. For example, routers, splitters, couplers, relays, and
amplifiers may be contained within the CM network segment 299
without departing from the scope and spirit of the invention. The
CM network segment 299 allows communicative coupling between a CM
user and the CMTS 230. The FIG. 2 shows just one of many
embodiments where the interference cancellation, performed
according to the present invention, may be performed to provide for
improved operation within a communication system.
[0091] FIG. 3A is a system diagram illustrating an embodiment of a
cellular communication system 300A that is built according to the
present invention. A mobile transmitter 310 has a local antenna
311. The mobile transmitter 310 may be any number of types of
transmitters including a cellular telephone, a wireless pager unit,
a mobile computer having transmit functionality, or any other type
of mobile transmitter. The mobile transmitter 310 transmits a
signal, using its local antenna 311, to a base station receiver 340
via a wireless communication channel. The base station receiver 340
is communicatively coupled to a receiving wireless tower 349 to be
able to receive transmission from the local antenna 311 of the
mobile transmitter 310 that have been communicated via the wireless
communication channel. The receiving wireless tower 349
communicatively couples the received signal to the base station
receiver 340.
[0092] The base station receiver 340 is then able to support
interference cancellation functionality according to the present
invention, as shown in a functional block 341, on the received
signal. The FIG. 3A shows yet another of the many embodiments where
the interference cancellation, performed according to the present
invention, may be performed to provide for improved operation
within a communication system.
[0093] FIG. 3B is a system diagram illustrating another embodiment
of a cellular communication system that is built according to the
present invention. From certain perspectives, the FIG. 3B may be
viewed as being the reverse transmission operation of the cellular
communication system 300B of the FIG. 3A. A base station
transmitter 320 is communicatively coupled to a transmitting
wireless tower 321. The base station transmitter 320, using its
transmitting wireless tower 321, transmits a signal to a local
antenna 339 via a wireless communication channel. The local antenna
339 is communicatively coupled to a mobile receiver 330 so that the
mobile receiver 330 is able to receive transmission from the
transmitting wireless tower 321 of the base station transmitter 320
that have been communicated via the wireless communication channel.
The local antenna 339 communicatively couples the received signal
to the mobile receiver 330. It is noted that the mobile receiver
330 may be any number of types of transmitters including a cellular
telephone, a wireless pager unit, a mobile computer having transmit
functionality, or any other type of mobile transmitter.
[0094] The mobile receiver 330 is then able to support interference
cancellation functionality according to the present invention, as
shown in a functional block 331, on the received signal. The FIG.
3B shows yet another of the many embodiments where the interference
cancellation functionality, performed according to the present
invention, may be performed to provide for improved operation
within a communication system.
[0095] It is also noted that the embodiments described above within
the FIGS. 3A and 3B may operate in conjunction within a single
communication system. That is to say, a mobile unit (that supports
both transmit and receive functionality) may be implemented to
support interference cancellation functionality during receipt of
signals while the base station device (that supports both transmit
and receive functionality) may also be implemented to support
interference cancellation functionality during receipt of signals.
This way, both devices are operable to support the interference
cancellation functionality according to the present invention at
both ends of the communication link. This dual-end interference
cancellation functionality is also true within other of the various
embodiments described herein that illustrate both ends of a
communication link.
[0096] It is further noted that the embodiments described above
within the FIGS. 3A and 3B may operate in conjunction within a
single communication system from yet another perspective. A mobile
transmitter may be implemented to support interference cancellation
functionality during signal processing and transmission of its
signals to a base station receiver. Similarly, a base station
transmitter may be implemented to support interference cancellation
functionality during signal processing and transmission of its
signals to a mobile unit receiver. This may be performed, at least
in part, by adjusting a transmitted spectrum to meet a desired
spectral mask. It may be desirable to attenuate certain portions of
the spectrum using the subspace canceller. In these applications
the canceller is predominantly located in the transmitter. Further
detail of this interference cancellation within a transmitter
device is presented below. This adjusting of a transmitted spectrum
to meet a desired spectral mask may be performed within any of the
various embodiments that include a transmitter that transmits a
signal to a receiver according to the present invention.
[0097] FIG. 4 is a system diagram illustrating an embodiment of a
satellite communication system 400 that is built according to the
present invention. A transmitter 420 is communicatively coupled to
a wired network 410. The wired network 410 may include any number
of networks including the Internet, proprietary networks, and other
wired networks. The transmitter 420 includes a satellite earth
station 451 that is able to communicate to a satellite 453 via a
wireless communication channel. The satellite 453 is able to
communicate with a receiver 430. The receiver 430 is also located
on the earth. Here, the communication to and from the satellite 453
may cooperatively be viewed as being a wireless communication
channel, or each of the communication to and from the satellite 453
may be viewed as being two distinct wireless communication
channels.
[0098] For example, the wireless communication "channel" may be
viewed as not including multiple wireless hops in one embodiment.
In other embodiments, the satellite 453 receives a signal received
from the satellite earth station 451, amplifies it, and relays it
to the receiver 430; the receiver 430 may include terrestrial
receivers such as satellite receivers, satellite based telephones,
and satellite based Internet receivers, among other receiver types.
In the case where the satellite 453 receives a signal received from
the satellite earth station 451, amplifies it, and relays it, the
satellite 453 may be viewed as being a "transponder." In addition,
other satellites may exist that perform both receiver and
transmitter operations. In this case, each leg of an up-down
transmission via the wireless communication channel would be
considered separately. The wireless communication channel between
the satellite 453 and a fixed earth station would likely be less
time-varying than the wireless communication channel between the
satellite 453 and a mobile station.
[0099] In whichever embodiment, the satellite 453 communicates with
the receiver 430. The receiver 430 may be viewed as being a mobile
unit in certain embodiments (employing a local antenna 412);
alternatively, the receiver 430 may be viewed as being a satellite
earth station 452 that may be communicatively coupled to a wired
network in a similar manner that the satellite earth station 451,
within the transmitter 420, communicatively couples to a wired
network. In both situations, the receiver 430 is able to support
interference cancellation functionality, as shown in a functional
block 431, according to the present invention. For example, the
receiver 430 is able to perform interference cancellation, as shown
in a functional block 431, on the signal received from the
satellite 453. The FIG. 4 shows yet another of the many embodiments
where the interference cancellation, performed according to the
present invention, may be performed to provide for improved
receiver performance.
[0100] FIG. 5A is a system diagram illustrating an embodiment of a
microwave communication system 500A that is built according to the
present invention. A tower transmitter 511 includes a wireless
tower 515. The tower transmitter 511, using its wireless tower 515,
transmits a signal to a tower receiver 512 via a wireless
communication channel. The tower receiver 512 includes a wireless
tower 516. The wireless tower 516 is able to receive transmissions
from the wireless tower 515 that have been communicated via the
wireless communication channel. The tower receiver 512 is then able
to support interference cancellation functionality, as shown in a
functional block 533. The FIG. 5A shows yet another of the many
embodiments where interference cancellation, performed according to
the present invention, may be performed to provide for improved
receiver performance.
[0101] FIG. 5B is a system diagram illustrating an embodiment of a
point-to-point radio communication system 500B that is built
according to the present invention. A mobile unit 551 includes a
local antenna 555. The mobile unit 551, using its local antenna
555, transmits a signal to a local antenna 556 via a wireless
communication channel. The local antenna 556 is included within a
mobile unit 552. The mobile unit 552 is able to receive
transmissions from the mobile unit 551 that have been communicated
via the wireless communication channel. The mobile unit 552 is then
able to support interference cancellation functionality, as shown
in a functional block 553, on the received signal. The FIG. 5B
shows just yet another of the many embodiments where interference
cancellation, performed according to the present invention, may be
performed to provide for improved receiver performance.
[0102] FIG. 6 is a system diagram illustrating an embodiment of a
high definition (HDTV) communication system 600 that is built
according to the present invention. An HDTV transmitter 610
includes a wireless tower 611. The HDTV transmitter 610, using its
wireless tower 611, transmits a signal to an HDTV set top box
receiver 620 via a wireless communication channel. The HDTV set top
box receiver 620 includes the functionality to receive the wireless
transmitted signal. The HDTV set top box receiver 620 is also
communicatively coupled to an HDTV display 630 that is able to
display the demodulated and decoded wireless transmitted signals
received by the HDTV set top box receiver 620.
[0103] The HDTV set top box receiver 620 is then able to support
interference cancellation functionality, as shown in a functional
block 623 to provide for improved receiver performance. The FIG. 6
shows yet another of the many embodiments where interference
cancellation, performed according to the present invention, may be
performed to provide for improved receiver performance.
[0104] FIG. 7 is a system diagram illustrating an embodiment of a
communication system that is built according to the present
invention. The FIG. 7 shows communicative coupling, via a
communication channel 799, between two transceivers, namely, a
transceiver 701 and a transceiver 702. The communication channel
799 may be a wireline communication channel or a wireless
communication channel.
[0105] Each of the transceivers 701 and 702 includes a transmitter
and a receiver. For example, the transceiver 701 includes a
transmitter 749 and a receiver 740; the transceiver 702 includes a
transmitter 759 and a receiver 730. The receivers 740 and 730,
within the transceivers 701 and 702, respectively, are each
operable to support interference cancellation functionality
according to the present invention. This will allow improved signal
processing for both of the transceivers 701 and 702. For example,
the receiver 740, within the transceiver 701, is able to support
interference cancellation functionality, as shown in a functional
block 741, on a signal received from the transmitter 759 of the
transceiver 702. Similarly, the receiver 730, within the
transceiver 702, is able to support interference cancellation
functionality, as shown in a functional block 731, on a signal
received from the transmitter 749 of the transceiver 701.
[0106] If desired in certain embodiments, the transmitters 749 and
759, within the transceivers 701 and 702, respectively, are each
operable to support interference cancellation functionality
according to the present invention. This will also allow improved
signal processing for both of the transceivers 701 and 702. For
example, the transmitter 749, within the transceiver 701, is able
to support interference cancellation functionality, as shown in a
functional block 748, on a signal that is to be transmitted from
the transmitter 759 of the transceiver 702. Similarly, the
transmitter 759, within the transceiver 702, is able to support
interference cancellation functionality, as shown in a functional
block 758, on a signal that is to be transmitted from the
transmitter 759 of the transceiver 702.
[0107] This interference cancellation functionality, within the
transmitters 749 and 759, respectively, may be performed, at least
in part, by adjusting a transmitted spectrum to meet a desired
spectral mask according to the present invention. The FIG. 7 shows
yet another of the many embodiments where interference
cancellation, performed according to the present invention, may be
performed to provide for improved performance.
[0108] FIG. 8 is a system diagram illustrating another embodiment
of a communication system 800 that is built according to the
present invention. The FIG. 8 shows communicative coupling, via a
communication channel 899, between a transmitter 849 and a receiver
830. The communication channel 899 may be a wireline communication
channel or a wireless communication channel. The receiver 830 is
operable to support interference cancellation, as shown in a
functional block 831, according to the present invention. The FIG.
8 shows yet another of the many embodiments where interference
cancellation, performed according to the present invention, may be
performed to provide for improved performance.
[0109] In certain embodiments, the transmitter 849 is also operable
to support interference cancellation, as shown in a functional
block 848, according to the present invention. This interference
cancellation functionality, within the transmitter 849, may be
performed, at least in part, by adjusting a transmitted spectrum to
meet a desired spectral mask according to the present invention.
For example, the interference cancellation functionality of the
present invention may be located at the transmitter 849, the
receiver 830, or partly in each (as shown by the functional blocks
848 and 831). In many of the various embodiments described herein,
the interference cancellation functionality has been located in a
receiver of a communication system. The following describes one
embodiment of how the interference cancellation functionality may
be located at the transmitter. The unused codes, instead of being
modulated at the transmitter with zero symbols, are modulated with
a linear combination of the desired signals from the used codes. In
the case of narrowband interference, the resulting transmitted
signal will have a null on the interferer.
[0110] FIG. 9 is a system diagram illustrating an embodiment of a
CMTS system 900 that is built according to the present invention.
The CMTS system 900 includes a CMTS medium access controller (MAC)
930 that operates with a number of other devices to perform
communication from one or more CMs to a WAN 980. The CMTS MAC 930
may be viewed as providing the hardware support for MAC-layer
per-packet functions including fragmentation, concatenation, and
payload header suppression that all are able to offload the
processing required by a system central processing unit (CPU) 972.
This will provide for higher overall system performance. In
addition, the CMTS MAC 930 is able to provide support for carrier
class redundancy via timestamp synchronization across a number of
receivers, shown as a receiver 911, a receiver 911, and a receiver
913 that are each operable to receive upstream analog inputs. In
certain embodiments, each of the receivers 911, 912, and 913 are
dual universal advanced TDMA/CDMA (Time Division Multiple
Access/Code Division Multiple Access) PHY-layer burst receivers.
That is top say, each of the receivers 911, 912, and 913 includes
at least one TDMA receive channel and at least one CDMA receive
channel; in this case, each of the receivers 911, 912, and 913 may
be viewed as being multi-channel receivers.
[0111] In addition, the CMTS MAC 930 may be operated remotely with
a routing/classification engine 979 that is located externally to
the CMTS MAC 930 for distributed CMTS applications including mini
fiber node applications. Moreover, a Standard Programming Interface
(SPI) master port may be employed to control the interface to the
receivers 911, 912, and 913 as well as to a downstream modulator
920.
[0112] The CMTS MAC 930 may be viewed as being a highly integrated
CMTS MAC integrated circuit (IC) for use within the various DOCSIS
and advanced TDMA/CDMA physical layer (PHY-layer) CMTS products.
The CMTS MAC 930 may employ hardware engines for upstream and
downstream paths. The upstream processor design is segmented and
uses two banks of Synchronous Dynamic Random Access Memory (SDRAM)
to minimize latency on internal buses. The two banks of SDRAM used
by the upstream processor are shown as upstream SDRAM 975 (operable
to support keys and reassembly) and SDRAM 976 (operable to support
Packaging, Handling, and Storage (PHS) and output queues). The
upstream processor performs Data Encryption Standard (DES)
decryption, fragment reassembly, de-concatenation, payload packet
expansion, packet acceleration, upstream Management Information
Base (MIB) statistic gathering, and priority queuing for the
resultant packets. Each output queue can be independently
configured to output packets to either a Personal Computer
Interface (PCI) or a Gigabit Media Independent Interface (GMII).
DOCSIS MAC management messages and bandwidth requests are extracted
and queued separately from data packets so that they are readily
available to the system controller.
[0113] The downstream processor accepts packets from priority
queues and performs payload header suppression, DOCSIS header
creation, DES encryption, Cyclic Redundancy Check (CRC) and Header
Check Sequence (of the DOCSIS specification), Moving Pictures
Experts Group (MPEG) encapsulation and multiplexing, and timestamp
generation on the in-band data. The CMTS MAC 930 includes an
out-of-band generator and TDMA PHY-layer (and/or CDMA PHY-layer)
interface so that it may communicate with a CM device's out-of-band
receiver for control of power management functions. The downstream
processor will also use SDRAM 977 (operable to support PHS and
output queues). The CMTS MAC 930 may be configured and managed
externally via a PCI interface and a PCI bus 971.
[0114] Each of the receivers 911, 912, and 913 is operable to
support interference cancellation functionality. For example, the
receiver 911 is operable to support interference cancellation
functionality, as shown in a functional block 991; the receiver 912
is operable to support interference cancellation functionality, as
shown in a functional block 992; and the receiver 913 is operable
to support interference cancellation functionality, as shown in a
functional block 993. The FIG. 9 shows yet another embodiment in
which interference cancellation may be performed according to the
present invention. Any of the functionality and operations
described in the other embodiments may be performed within the
context of the CMTS system 900 without departing from the scope and
spirit of the invention.
[0115] FIG. 10 is a system diagram illustrating an embodiment of a
burst receiver system 1000 that is built according to the present
invention. The burst receiver system 1000 includes at least one
multi-channel receiver 1010. The multi-channel receiver 1010 is
operable to receive a number of upstream analog inputs that are
transmitted from CMs. The upstream analog inputs may be in the form
of either TDMA (Time Division Multiple Access) and/or CDMA (Code
Division Multiple Access) format. A number of receive channels may
be included within the multi-channel receiver 1010.
[0116] For example, the multi-channel receiver 1010 is operable to
support a number of TDMA receive channels 1020 (shown as TDMA
signal 1 and TDMA signal 2) and to support interference
cancellation functionality, as shown in a functional block 1021,
for those received TDMA signals. The multi-channel receiver 1010 is
operable to support a number of TDMA receive channels 1030 (shown
as CDMA signal 1 and CDMA signal 2) and to support interference
cancellation functionality, as shown in a functional block 1031,
for those received CDMA signals. Generically speaking, the
multi-channel receiver 1010 is operable to support a number of
receive channels 1040 (shown as received signal 1 and received
signal 2) and to support interference cancellation functionality,
as shown in a functional block 1041, for those received signals.
The multi-channel receiver 1010 of the FIG. 10 is operable to
interface with a CMTS MAC. The burst receiver system 1000 may
include a number of multi-channel receivers that are each operable
to interface with the CMTS MAC.
[0117] In certain embodiments, the multi-channel receiver 1010
provides a number of various functionalities. The multi-channel
receiver 1010 may be a universal headend advanced TDMA PHY-layer
QPSK/QAM (Quadrature Phase Shift Keying/Quadrature Amplitude
Modulation) burst receiver; the multi-channel receiver 1010 also
include functionality to be a universal headend advanced CDMA
PHY-layer QPSK/QAM burst receiver; or the multi-channel receiver
1010 also include functionality to be a universal headend advanced
TDMA/CDMA PHY-layer QPSK/QAM burst receiver offering both TDMA/CDMA
functionality. The multi-channel receiver 1010 may be
DOCSIS/EuroDOCSIS based, IEEE 802.14 compliant. The multi-channel
receiver 1010 may be adaptable to numerous programmable
demodulation including BPSK (Binary Phase Shift Keying), and/or
QPSK, 8/16/32/64/128/256/516/1024 QAM. The multi-channel receiver
1010 is adaptable to support variable symbols rates as well. Other
functionality may likewise be included to the multi-channel
receiver 1010 without departing from the scope and spirit of the
invention. Such variations and modifications may be made to the
communication receiver.
[0118] FIG. 11 is a system diagram illustrating an embodiment of a
single chip DOCSIS/EuroDOCSIS CM system 1100 that is built
according to the present invention. The single chip
DOCSIS/EuroDOCSIS CM system 1100 includes a single integration and
offering a very high level of performance. A coaxial cable in input
to a DiPlexer to provide CM access to the single chip
DOCSIS/EuroDOCSIS CM system 600. The DiPlexer communicatively
couples to a CMOS (Complementary Metal Oxide Semiconductor) tuner.
The CMOS tuner may be implemented with a companion part that
includes a low noise amplifier (LNA) and performs radio frequency
(RF) automatic gain control (AGC). This two part solution is
operable to support 64 and 256 QAM. These two parts operate
cooperatively with the single chip DOCSIS/EuroDOCSIS CM 1110. The
CMOS tuner may be operable to support an intermediate frequency
(IF) output frequency range of 36-44 MHz, and specifically support
the 36.125 and 43.75 MHz center frequencies for the Phase
Alteration Line (PAL) and National Television System Committee
(NTSC) standards. Also, the CMOS tuner and the LNA and RF AGC are
DOCSIS and EuroDOCSIS standard supportable.
[0119] However, it is also noted that the CMOS tuner is operable to
perform direct RF to baseband (BB) frequency transformation without
requiring the IF transformation. The received signal from the
DiPlexer. An external bandpass Surface Acoustic Wave (SAW) filter
removes the channels distant from the desired signal.
[0120] The output from the SAW filter is then passed to the single
chip DOCSIS/EuroDOCSIS CM 1110. The single chip DOCSIS/EuroDOCSIS
CM 1110 is supported by Synchronous Dynamic Random Access Memory
(SDRAM) and Flash. In addition, the single chip DOCSIS/EuroDOCSIS
CM 1110 supports both Ethernet and USB interfacing to any other
devices that may exist within the single chip DOCSIS/EuroDOCSIS CM
system 1100. The FIG. 11 shows yet another embodiment in which
interference cancellation may be performed according to the present
invention. The interference cancellation functionality may be
supported directly within the single chip DOCSIS/EuroDOCSIS CM
1110. The single chip DOCSIS/EuroDOCSIS CM system 1100 shows an
application context of yet another implementation of a device that
may perform the present invention.
[0121] FIG. 12 is a system diagram illustrating another embodiment
of a single chip DOCSIS/EuroDOCSIS CM system 1200 that is built
according to the present chip DOCSIS/EuroDOCSIS CM 1210 that
combines an RF receiver with an advanced QAM demodulator, an
advanced QAM and S-CDMA modulator/transmitter, a complete DOCSIS
2.0 Media Access Controller (MAC), a 200 MHz MIPS32 Communication
Processor, a 16 bit, 100 MHz SDRAM interface, 10/100 Ethernet MAC
with integrated transceiver and Media Independent Interface (MII),
and a USB 1.1 controller with integrated transceiver.
[0122] The QAM receiver directly samples a tuner output (such as
the CMOS tuner of the FIG. 6) with an 11 bit analog to digital
converter (ADC) and input AGC amplifier. The receiver digitally
re-samples and demodulates the signal with recovered clock and
carrier timing, filters and equalizes the data, and passes soft
decisions to an ITU-T J.83 Annex A/B/C compatible decoder. The
receiver supports variable symbol rate 4/16/32/64/128/256/1024 QAM
Forward Error Correction (FEC) decoding. The final received data
stream is delivered in a serial MPEG-2 transport format. All gain,
clock, and carrier, acquisition and tracking loops are integrated
in the QAM receiver.
[0123] The upstream transmitter takes burst or continuous data,
provides FEC encoding and pre-equalization for DOCSIS applications,
filters and applies 2/4/8/16/64/256 QAM or S-CDMA modulation to the
data stream, amplifies the signal through the integrated upstream
power amplifier and provides a direct 0-65 MHz analog output.
[0124] The MAC of the single chip DOCSIS/EuroDOCSIS CM 1210
includes all features required for full DOCSIS 1.0, 1.1, and 2.0
compliance, including full support for baseline privacy (BPI+)
encryption and decryption. Single-user support includes four SIDS
(Standard Interoperable Datalink System) in downstream, four DA
perfect match filters, a 256 entry CAM for multicast/unicast hash
filter and four independent upstream queues for simultaneous
support of Quality of Service (QoS) and BE traffic. To enhance
operational support, the MAC of the MAC of the single chip
DOCSIS/EuroDOCSIS CM 1210 provides extended Network Management
MIB/Diagnostic features, as well as immediate UCC (on the fly)
using independent resets for downstream and upstream queues and
both individual queue reset/flush for upstream queues. The MAC of
the single chip DOCSIS/EuroDOCSIS CM 1210 uses advance PROPANE.TM.
techniques to provide packet acceleration to significantly improve
upstream channel utilization.
[0125] With the incorporation of an upstream power amplifier, the
MAC of the single chip DOCSIS/EuroDOCSIS CM 1210 allows a complete
CM to be assembled with a minimal set of external components. When
used with a CMOS tuner, such as the CMOS tuner of the FIG. 11, a
very low cost solution for a high performance, single user DOCSIS
2.0 CM is provided. The MAC of the single chip DOCSIS/EuroDOCSIS CM
1210 of the FIG. 12 is operable to support all digital reference
frequency locking functionality according to the present invention.
The FIG. 12 shows yet another embodiment where interference
cancellation functionality may be supported according to the
present invention. The interference cancellation functionality may
be viewed as being supported and performed within the DOCSIS 2.0
MAC of the single chip DOCSIS/EuroDOCSIS CM 1210 of the FIG.
12.
[0126] FIG. 13 is a system diagram illustrating an embodiment of a
single chip wireless modem system 1300 that is built according to
the present invention. The single chip wireless modem system 1300
includes a single chip wireless modem 1310 that is operable to
support a variety of functionalities. The single chip wireless
modem system 1300 is operable to perform wireless LAN operation
using an 802.11 radio that is operable to communicatively couple to
an external device that is wireless capable (example shown as the
pen computer having wireless functionality). The single chip
wireless modem 1310 of the single chip wireless modem system 1300
employs a 10/100 Ethernet PHY and an HPNA (Home Phoneline Network
Alliance) analog front end (AFE) that is operable to interface with
the HPNA 2.0 network. The single chip wireless modem 1310 of the
single chip wireless modem system 1300 also supports capability to
communicate with an external device via a USB 1.1 interface.
[0127] The single chip wireless modem 1310 of the single chip
wireless modem system 1300 is compatible with existing cable modem
application code. In addition, the single chip wireless modem 1310
supports advanced QAMLink.RTM. modulation/demodulation TP provide
for higher throughputs and performance in noisy plant environments.
The 802.11b MAC and baseband allow for wireless connectivity as
mentioned above. In addition, the integrated HPNA 2.0 MAC supports
high-speed multimedia services over phone lines. The integrated
10/100 Ethernet and USB 1.1 with integrated transceiver provide for
a low cost CPE (Customer Premises Equipment), and the MPI
interfaces provide for great flexibility through additional
connectivity options. The single chip wireless modem 1310 is a part
of a comprehensive solution that is operable to support certifiable
DOCSIS/EuroDOCSIS 1.1 software as well as supporting residential
gateway software including Firewall, NAT and DHCP. The FIG. 13
shows yet another embodiment where interference cancellation
functionality may be supported according to the present invention.
The interference cancellation functionality may be viewed as being
supported and performed within the single chip wireless modem 1310
of the single chip wireless modem system 1300.
[0128] FIG. 14 is a system diagram illustrating another embodiment
of a single chip wireless modem system 1400 that is built according
to the present invention. The single chip wireless modem system
1400 includes a single chip wireless modem that is operable to
support a variety of functionalities. The single chip wireless
modem of the single chip wireless modem system 1300 integrates the
DOCSIS/EuroDOCSIS 2.0 cable based modem with a
2/416/32/64/128/256/1024 QAM downstream receiver with Annex A, B, C
FEC support. In addition, the single chip wireless modem integrates
the DOCSIS/EuroDOCSIS 2.0 cable based modem with
2/4/8/16/32/64/128/256 QAM FA-TDMA ad S-CDMA. The 802.11b wireless
MAC and baseband are also integrated on the single chip wireless
modem. A number of other functional blocks are also integrated
thereon, including, a 300 MHz MIPS32 CPU, a 32 bit 100 MHz
SDRAM/DDR controller, an integrated upstream amplifier, an
integrated IP SEC engine, an integrated advance PROPANE.TM. packet
accelerator, a 12 Mbps USB 1.1 slave port with integrated
transceiver, a 10/100 Ethernet MAC/PHY with MII interface, an MPI
expansion bus (that supports PCI, Cardbus, and PCMCIA interfaces),
a single 28 MHz reference crystal, and ability to operate using
voltages of 1.8 V and/or 3.3 V.
[0129] The advanced QAMLink.RTM. technology of the single chip
wireless modem, compliant with DOCSIS 2.0, supports up to 1024 QAM
downstream modulation formats and both FA-TDMA and S-CDMA, with 256
QAM upstream modulation formats. This advanced technology provides
a higher throughput and superior performance in noise plant
environments, paving the way for symmetrical services, such as
video conferencing.
[0130] The single chip wireless modem integrates both wireless and
wireline networking functions for distributing broadband content
throughout the home. An 802.11b solution is provided for wireless
connectivity, while both 10/100 Ethernet and 32 Mbps HPNA 2.0
solutions provide wired connectivity. HPNA 2.0 allows multimedia
services to be streamed across existing home phone lines.
[0131] The PROPANE.TM. technology provides bandwidth and
performance enhancements to existing cable plants allowing up to
twice as many subscribers per node, thereby minimizing the need for
node splits. The FIG. 14 shows yet another embodiment where
interference cancellation functionality may be supported according
to the present invention. The interference cancellation
functionality may be viewed as being supported and performed within
the single chip wireless modem of the single chip wireless modem
system 1400.
[0132] FIG. 15 is a diagram illustrating an embodiment of a vector
de-spreader 1500 that is built according to the present invention.
The following description of embodiments of the present invention
using the FIGS. 15, 16, 17, and 18 are made within the context of
the DOCSIS 2.0 system. This system uses S-CDMA modulation for the
upstream with 128 orthogonal codes. In the example there are 120
active (data-carrying) codes, with 8 unused codes. This example is
for illustrative purposes only, and should by no means limit the
scope of the invention. Again, it is noted that the specific
examples of 120 active codes, and 8 unused codes in a system having
128 available codes is exemplary. Clearly, other embodiments may be
employed (having different numbers of codes--both different numbers
of used and unused) without departing from the scope and spirit of
the invention.
[0133] The FIG. 15 depicts a vector de-spreader, arranged according
to the present invention, consisting of 128 individual
de-spreaders. De-spreading is the process of multiplying by a given
code sequence and summing (or integrating) over the chips of a
spreading sequence, in this case the length of the code, 128 chips.
Each scalar de-spreader performs the function of de-spreading the
received signal (input spread signal to be de-spread) using a
single de-spreading code (c.sub.1, . . . , c.sub.128). There are
128 orthogonal de-spreading codes in the present example.
[0134] FIG. 16 is a diagram illustrating an embodiment of an
interference canceller 1200 that is built according to the present
invention. The spread input signal x, consisting of the sum of
multiple spreading codes modulating multiple data streams, enters
the diagram at the left. The undesired interference n is added to
the signal. The signal is applied to the vector de-spreader, which
de-spreads each of the 128 codes. The upper 8 codes are not used
for data transmission and are modulated with numerically
zero-valued symbols instead of data. Clearly, there may be
embodiments where other numerically constant-valued symbols may be
employed instead of data as well. Further, the symbol may contain
data represented as a reduced constellation, such as BPSK or QPSK,
on the "unused" codes.
[0135] One of the 120 data-carrying codes, code d.sub.s, is
identified for illustration in the FIG. 16. In order to cancel the
interference, the de-spreader output d.sub.s is processed in a
linear combiner, where it is summed with a linear combination of
the 8 de-spreader outputs from the zero-modulation codes
d.sub.1-d.sub.8. The complex-valued combining weights applied to
these codes are w.sub.1-w.sub.8, respectively. These weights are
computed in a weight computation method as shown in the lower right
hand corner of the FIG. 16 using the weight computation
functionality.
[0136] The weight computation functionality may employ a method
that utilizes the input spread signal plus interference, and may
utilize some system outputs if an iterative method is used. Weight
computation methods that have been found valuable are the LMS
(least mean square) method and the LS (least squares) method. The
result of the linear combination is the output {circumflex over
(d)}.sub.s, which is the data stream d.sub.s with the interference
largely removed. Although not shown in the figure, the same linear
combiner structure is applied to the other 119 codes as well (all
of the other active codes besides the code d.sub.s). In each case,
the desired code (one of the 120 "active" or data-carrying codes)
is applied to a linear combiner to cancel the interference from
that code. For each data-carrying code, the same 8 zero-modulation
codes are summed with the desired code, but for each active code
the weights w.sub.1-w.sub.8 are in general unique.
[0137] An alternative viewpoint is to define the adapted code
as
c a ( n ) = c s ( n ) + k = 1 N u w k c k ( n ) ##EQU00002##
[0138] that is, the desired code plus the linear combination of the
weights times the unused ("inactive") codes. In this view, the
adapted code is a modified code with complex coefficients, which is
used instead of the code c.sub.s to de-spread a single desired
signal from a single modulated code, while simultaneously canceling
the interference.
[0139] This approach can be extended to matrix notation by defining
the adapted code matrix as
C.sub.adapted=C.sub.used+WC.sub.unused
[0140] where:
[0141] C.sub.adapted=adapted code matrix, dimension
(N.sub.c-N.sub.u).times.128, for example, 120.times.128
[0142] C.sub.used=matrix whose rows are the used codes in the
original code matrix, dimension (N.sub.c-N.sub.u).times.N.sub.c,
for example, 120.times.128
[0143] W=matrix whose rows are the adaptive weight vectors for each
unused code, dimension (N.sub.c-N.sub.u).times.N.sub.u, for
example, 120.times.8
[0144] C.sub.unused=matrix whose rows are the unused codes in the
original code matrix, dimension N.sub.u.times.N.sub.c, for example,
8.times.128
[0145] N.sub.c=number of total codes=number of chips in each code,
for example, 128
[0146] N.sub.u=number of unused codes, for example, 8
[0147] In this view, the adapted code matrix is a modified code
matrix with complex coefficients, which is used instead of the code
matrix C to de-spread the desired signals from all used codes,
while simultaneously canceling the interference on all used
codes.
[0148] It is noted that that the unused codes may be de-spread as
well, and this side information, though not data-carrying, is of
use in characterizing the interference environment.
[0149] It is also noted that the weight computation functionality
may be performed offline, and these pre-computed complex-valued
combining weights, w.sub.1-w.sub.8, may then be stored in memory
and/or a look up table (LUT) that may be used to provide the
complex-valued combining weights, w.sub.1-w.sub.8. The appropriate
set of weights may be selected after analyzing the interference
environment.
[0150] FIG. 17 is a diagram illustrating another embodiment of an
interference canceller 1700 that is built according to the present
invention. The FIG. 17 may be viewed as being somewhat similar to
the interference canceller 1600 of the FIG. 16 with some exceptions
relating to the specific codes that are used to perform the linear
combination in an effort to perform the interference cancellation
according to the present invention.
[0151] The FIG. 17 shows an embodiment where all of the codes are
included in the linear combiner. This includes both the used codes
and the unused codes, instead of only the unused codes. This will
be useful if there is inter-code interference (ICI), since in that
case the desired signal will appear on all codes. Conversely, the
signals modulated onto all codes will appear on the desired
de-spreader output, and can be subtracted from the desired
de-spreader output.
[0152] In yet another alternative embodiment, to add to the number
of effective unused codes, we may use codes bearing preamble
symbols in addition to the codes carrying zero-valued symbols. The
preamble symbols are known and can be subtracted once their
amplitude and phase have been measured, for example using a
preamble correlator. Thus the preamble-bearing codes can also be
used as inputs to the linear combiner in order to better cancel the
interference.
[0153] There are some other embodiments that may be employed as
well. For example, the selection of the inactive codes may be
performed as follows: (1) use codes 0, 1, 2, 3, . . . (adjacent
codes, as done in DOCSIS 2.0 spec) in which the codes are adjacent
and the lower codes used in the coding and/or (2) spacing the codes
maximally apart. For example, using DOCSIS 2.0 S-CDMA code set, the
8 unused codes out of 128 total codes might be code numbers {15 31
47 63 79 95 111 127} when seeking to perform the maximally spaced
apart embodiment. Moreover, the selection of the unused codes may
be performed according to an optimality criterion. Examples of some
potential optimality criteria include: (1) select unused codes that
have maximal correlation with the interference, (2) minimize
enhancement of white noise resulting from cancellation process, and
(3) minimize residual interference power after cancellation.
[0154] It is also noted that the particular codes that are selected
as the unused codes may change over time during the processing of
received signals. Moreover, the particular selection of the codes
may vary from one iteration to the next. For example, in one
situation, adjacent codes may be selected as the unused codes. In
another situation, the maximally spaced codes may be selected as
the unused codes.
[0155] The selection of the codes that are to be designated the
unused codes may be performed using a variety of approaches
including: (1) employing code matrix reordering, (2) employing null
grant periods, (2) zero padding data, and/or (4) employing some
optimality criterion (or criteria).
[0156] Similar to the embodiment of the FIG. 16, it is also noted
that the weight computation functionality may be performed offline,
and these pre-computed complex-valued combining weights that are
used here in the FIG. 17 may similarly be stored in memory and/or a
look up table (LUT) that may be used to provide the complex-valued
combining weights. The appropriate set of weights may be selected
after analyzing the interference environment. This may similarly be
performed in the embodiments of the FIGS. 18 and 19 described in
further detail below where these pre-computed complex-valued
combining weights may also be stored in memory and/or a look up
table (LUT).
[0157] FIG. 18 is a diagram illustrating another embodiment of an
interference canceller 1800 that is built according to the present
invention. The FIG. 18 may be viewed as being a variant of the FIG.
17 that has access to any of the codes (including both used and
unused codes). The FIG. 18 includes a subset of the codes for use
in the linear combiner, instead of all the codes or only the unused
codes. For example, in DOCSIS 2.0 S-CDMA, adjacent codes are nearly
shifts of each other. When a timing offset occurs, the codes lose
orthogonality and ICI occurs. However, the ICI is predominant on
the adjacent codes. For example, in the presence of a timing
offset, code 35 will be interfered with predominantly by codes 34
and 36, with lesser effects coming from codes 33 and 37, even
lesser effects from codes 32 and 38, and so on. Hence including as
inputs to the linear combiner the data-bearing codes 33, 34, 36 and
37, plus the unused codes, but not the remaining data-bearing
codes, will reduce the number of weights that have to be solved for
compared to the more general case above in which all codes are
included in the linear combination. Another embodiment would
involve including as inputs to the linear combiner the data-bearing
codes 34, and 36, plus the unused codes, but not the remaining
data-bearing codes, in an effort to try to reduce the number of
weights that have to be solved for compared to the more general
case above in which all codes are included in the linear
combination.
[0158] FIG. 19 is a diagram illustrating an embodiment of an
interference canceller with memory 1900 that is built according to
the present invention. The FIG. 19 shows an interference canceller
with memory, that is, it uses the history of previous samples in
computing the output. The weight w.sub.1 has been replaced with
"feed-forward equalizer 1" (FF Eqer. 1), a tapped delay line or FIR
filter with L weights. The other adaptive weights have similarly
been replaced with FF equalizers 2-8. It is noted that both the
current and past soft de-spread symbols are included in the linear
combination. Moreover, future soft de-spread symbols may also be
included in the linear combination; these future soft de-spread
symbols are "future" relative to the symbol currently being
estimated. This permits each tap to have a frequency selective
response.
[0159] FIG. 20 is a diagram illustrating an embodiment of
equalization with canceller 2000 that is arranged according to the
present invention. The FIG. 20 may be viewed as being a
representation of equalization of channel response, and
cancellation of resulting colored noise. The canceller structure
can also be used to help with equalization. The FIG. 20 considers a
communications system with a transmitter 2010, a channel 2020
having a response Ht, and a receiver 2025. Let the channel response
be H(f). We assume, as an example, that H(f) exhibits a null at
some frequency in the signaling band of interest. Assume AWGN
(additive white Gaussian noise) is added in the channel after H(f).
We may use a standard adaptive equalizer 2030 at the receiver to
provide the inverse (zero forcing) response, 1/H(f), which will
have a narrow peak at the frequency location where the null exists
in H(f). This peak will cause the white noise to be colored and to
have a peak as well. This narrowband colored noise can be canceled
(using the canceller 2040) by the present technique in exactly the
same manner that other narrowband interference is canceled. The
FIG. 20 shows yet another embodiment of how interference
cancellation, according to the present invention, may be
performed.
[0160] FIG. 21 is a diagram illustrating an embodiment of Least
Means Square (LMS) training of an interference canceller 2100
according to the present invention. The FIG. 21 may be viewed as
being one embodiment that is operable to perform adaptation of an
interference canceller using iterative methods. The present
interference canceller 2100 can be adapted using iterative methods
such as LMS or RLS. The FIG. 21 illustrates how the LMS method may
be used to adapt the canceller weights. The output of the
de-spreader for the desired code (containing soft decisions) is
sliced to produce hard symbol decisions. If known training symbols
are available, they replace the hard decisions, which may contain
symbol errors, especially upon startup. The difference between the
hard (or known) and soft decisions gives the LMS error sample. The
error is correlated with the outputs of the unused code
de-spreaders and used to update the adaptive weights w.sub.i.
[0161] Within the FIG. 21, the slicer, the MUX, and the LMS error
(and LMS step-size scaling .mu.) that are used to update the
adaptive weights w.sub.i may be viewed as being just one embodiment
of an iterative, error determining approach. Clearly, other error
determining approaches (besides LMS) may be employed without
departing from the scope and spirit of the invention. The error
calculation and correlation with the outputs of the unused code
de-spreaders that are used to update the adaptive weights w.sub.i
may be viewed as being an iterative adaptive weight functionality
that may be viewed as being provided in an implementation via an
iterative adaptive weight functional block that communicatively
couples to each of the outputs of the unused code de-spreaders.
[0162] FIG. 22A is a diagram illustrating an embodiment of signal
transformation according to the present invention. The FIG. 22A
includes the pre-processing of an input signal, and unused inputs,
via an orthogonal transformation 2210 to generate a representation
of the input signal within a finite signal space. The orthogonal
transformation 2210 may be an orthonormal transformation in certain
embodiments. Now that the input signal is represented in the finite
signal space, the signal is then passed through a communication
channel 2230 after which it is provided to an interference
cancellation functional block 2220 that is operable to perform any
of the various embodiments of interference cancellation described
herein. The communication channel 2230 may introduce interference.
It is noted that the present invention is operable to perform
cancellation of interference of a variety of types including (1)
narrowband interference in general, (2) Ham radio, CB radio and HF
radio, (3) adjacent channel interference (spillover from desired
signals in neighboring channels), (4) CDMA on a small number of
codes, and (5) impulse/burst note. The present invention envisions
any orthogonal transformation 2210 that is operable to transform an
input signal into a representation of a finite number of elements
within a finite signal space so as to facilitate the interference
cancellation according to the present invention.
[0163] FIG. 22B is a diagram illustrating another embodiment of
signal transformation according to the present invention. The FIG.
22B includes the pre-processing of an input signal, and unused
inputs, via an identity matrix transformation 2215 to generate a
representation of the input signal within a finite signal space.
Again, the orthogonal transformation 2215 may be an orthonormal
transformation in certain embodiments. Now that the input signal is
represented in the finite signal space, the signal is then passed
through a communication channel 2235 after which it is provided to
an interference cancellation functional block 2225 that is operable
to perform any of the various embodiments of interference
cancellation described herein. The communication channel 2235 may
introduce interference. It is again noted that the present
invention is operable to perform cancellation of interference of a
variety of types including (1) narrowband interference in general,
(2) Ham radio, CB radio and HF radio, (3) adjacent channel
interference (spillover from desired signals in neighboring
channels), (4) CDMA on a small number of codes, and (5)
impulse/burst note. Further details are described below with
respect to impulse noise cancellation.
[0164] Impulse noise is nearly zero most of the time, and large
during a few samples. For the purpose of analysis only, we consider
the rows of the N.times.N identity matrix as the basis set, where N
is the number of samples per frame (or chips per spreading
interval) under consideration. In this basis set, each time sample
represents one dimension. Hence we see that the impulse noise only
occupies a small number of dimensions. Thus it can be canceled by
this technique, using an arbitrary basis set, such as the S-CDMA
codes. The adapted de-spreading code has zeros (or nearly zeros) at
the chips corresponding to the time location of the impulse noise.
However, impulse noise occurs at a random, unpredictable location
in each frame. If we know where it is, we can solve the equations
for the weights. But the next frame it will be in a different
place. This means re-doing the computations every frame, resulting
in high complexity.
[0165] For low-level impulse noise, it may be difficult to locate
the chips that are affected by impulse noise. We may use one or
more "indicator codes" for this purpose, as follows. As an example,
say we have 128 total codes--for example in the DOCSIS 2.0
situation. We designate 119 of these codes as used, or
data-carrying codes. We designate 9 of the codes as unused codes,
on which numerically-zero-valued symbols are transmitted. Of these
9 unused codes, 8 codes participate in the linear combiner for
noise cancellation, and there is 1 extra or "indicator" code. The
indicator code is de-spread as if it were a used code, that is, it
is given the benefit of the linear combiner canceller. We expect to
get a zero symbol at its de-spreader output; if we see noise
instead of zero that provides an indication of the amount of noise
that has not been canceled. We then proceed as follows to locate
the impulse noise. Assume for example that there is one occurrence
of impulse noise in a given symbol, and that the impulse noise
affects 8 or fewer chips. We begin with a set of weights w that
null chips 1 through 8 in the time domain. We use w to de-spread
the indicator code, and observe the output y. We then modify w to
null chips 2-9, and again observe y. In a similar manner, we scan w
across the entire symbol, measuring y at each time offset. We
believe that the power |y|.sup.2 will exhibit a minimum for the
weight set w that corresponds to the time location of the impulse
noise. In this manner the location of the impulse noise can be
determined. Once located, it can be canceled.
[0166] FIG. 23 is an operational flow diagram illustrating an
embodiment of an interference cancellation method 2300 that is
performed according to the present invention. In a block 2310, a
spread signal is received that contains interference. Then, the
received spread signal is de-spread into a number of codes in a
block 2320. Each of the codes is selectively processed using linear
combination processing as shown in a block 2330. There are a
variety of ways in which the linear combination processing may be
performed according to the present invention including using a
number of unused codes, using all of the available codes, and/or
using selected adjacent codes in addition to the unused codes.
Ultimately, the interference cancelled de-spread codes are output
as shown in a block 2340.
[0167] FIG. 24 is an operational flow diagram illustrating another
embodiment of an interference cancellation method 2400 that is
performed according to the present invention. Initially, in some
embodiments, the method involves selecting those codes that are to
be used as the unused codes as shown in a block 2402. As shown
within the FIG. 24, there are three different ways in which this
may be performed. They include code matrix reordering, employing
null grant periods, and/or zero padding data. Even other ways are
described when referring to the other Figures as well. These will
be the codes that are used to perform the linear combining to
effectuate the interference cancellation according to the present
invention. In even other embodiments as shown in a block 2404, the
unused codes (N.sub.u) are modulated with numerically zero-valued
symbols. Alternatively, the unused codes (N.sub.u) may be modulated
with numerically constant-valued symbols that are non-zero without
departing from the scope and spirit of the invention.
[0168] In a block 2410, a spread signal is received that contains
interference. Then, the received spread signal is de-spread into a
number of codes (N.sub.c) as shown in a block 2420. in a block
2430, each of the number of unused codes (N.sub.u) is selectively
de-spread. The method then will continue to the block 2440 in most
instances.
[0169] However, in certain embodiments, the method will continue
from the block 2430 to the block 2432 in which each of the number
of preamble codes is selectively de-spread. The preamble symbols
are known and can be subtracted once their amplitude and phase have
been measured, as shown in a block 2434, for example using a
preamble correlator. Thus the preamble-bearing codes can also be
used as inputs to the linear combiner in order to better cancel the
interference.
[0170] As shown in the block 2440, complex-valued weights for
linear combination processing of the unused codes (N.sub.u) are
selectively calculated. This processing in the block 2440 may be
performed by inputting the spread signal, interference, and/or
outputs as shown in a block 2442. In the embodiments where the
blocks 2432 and 2434 are performed, the preamble-bearing codes may
be input as shown in a block 2444 when performing the processing in
the block 2440. The processing in the block 2440 may be performed
be employing LMS processing as shown in a block 2446 and/or LS
processing as shown in a block 2448.
[0171] Then, in a block 2450, the complex value weights are
selectively applied to scale the unused codes (N.sub.u). In a block
2460, the now scaled unused codes (N.sub.u) are selectively summed
with the desired code. Ultimately, the interference cancelled
de-spread codes are output as shown in a block 2440.
[0172] FIG. 25 is an operational flow diagram illustrating an
embodiment of an unused code selection method 2500 that is
performed according to the present invention. The question arises
whether any subset of the codes is a good choice for the unused
codes. We consider the example of narrowband interference
cancellation in a DOCSIS 2.0 S-CDMA system. For efficient
narrowband interference canceling capability, the unused codes have
to be chosen such that it is possible to combine them in the
adapted codes (in the linear combiner) to form one or more notches
in the frequency domain. Thus, for optimal performance, one might
need to designate specific codes as unused. The current DOCSIS 2.0
draft specification does not permit the selection of which codes
are unused. It has been found that successive, or "adjacent",
DOCSIS 2.0 codes are not a good choice. This is because each code
is approximately a shift of the previous code. This implies that
adjacent or nearly adjacent codes have nearly the same frequency
response. Some techniques that could be used to "force" unused
codes at specific rows of the code matrix are the following:
[0173] The code matrix may be reordered. In this technique, both
the CM and CMTS re-order the code matrix as shown in a block 2510,
prior to spreading or de-spreading. This may be performed such that
desired unused codes are grouped together (say at the lower part of
the rearranged code matrix) as shown in a block 2520. Similarly,
the desired used codes should be grouped as well (say at the upper
part of the rearranged code matrix) as shown in a block 2530. Using
such technique requires the knowledge of the reordering pattern at
the CMTS as well as all CMs; this may be ensured as shown in a
block 2540.
[0174] Alternatively, the selection of unused codes may be
performed using null grant periods. In this technique, the CMTS
instructs all CMs to be silent during a specific grant as shown in
a block 2505 (i.e., the desired unused codes). This technique has
the advantage that the CMs need not have prior knowledge of the
unused codes and just follow the CMTS grants. However, it may be
viewed as causing inefficiencies to the CMTS scheduling process
that may prohibit this approach in some implementations.
[0175] Alternatively, the selection of unused codes may be
performed by zero-padding the data. In this technique, the CMTS
grants the CM a longer grant period that what is needed to transmit
the grant data as shown in a block 2555. If desired when performing
the operation of the block 2555, the grant sizes are chosen by the
CMTS in a way such that the CM zero-padding occurs at the desired
unused codes as shown in a block 2557. The CMTS also instructs the
CM to append the transmitted data with zero-symbols as shown in a
block 2565.
[0176] FIG. 26 is an operational flow diagram illustrating an
embodiment of an S-CDMA interference cancellation method 2600 that
is performed according to the present invention. In a block 2610, a
set of used codes is selected. Then, in a block 2620, a set of
unused codes is selected. A signal is transmitted using the used
codes as shown in a block 2630. In certain embodiments, as shown in
a block 2632, we transmit one or more zeroes on the inactive/unused
codes. Alternatively, as shown in a block 2634, we transmit a known
sequence (training or pilot symbols) on the inactive/unused codes.
Alternatively, we may transmit lower order modulation on "inactive"
codes.
[0177] Then, in a block 2640, the received signal is processed
using the received signal's projection on the active (used) codes
and the inactive (unused) codes thereby canceling interference. We
process the received signal using both its projection onto a
desired (active) code, and its projection onto the inactive codes,
in order to cancel interference on the desired code. From certain
perspectives, in the context of a system and method that employ
vector de-spreading to a spread signal, the projection may be
viewed as being the vector de-spreader output. However, in other
contexts, a projection may be viewed as being the representation of
the received signal across its finite signal space. This
understanding of projection may be used to describe the
representation of the signal across a finite signal space.
[0178] The FIG. 26 is performed within the context of an S-CDMA
communications system in the presence of interference. There are a
number of types of S-CDMA systems that may support the method of
the FIG. 26. For example, some types of S-CDMA systems include
DOCSIS 2.0 set of codes and Walsh-Hadamard codes. The selection of
the inactive/unused codes may be performed as illustrated and
described above within some of the embodiments shown in a block
2621 including use codes 0, 1, 2, 3, . . . (adjacent codes, as done
in DOCSIS 2.0 spec), spacing the inactive/unused codes maximally
apart as shown in a block 2622, and selecting the codes according
to an optimality criterion as shown in a block 2623.
[0179] For example, when spacing the inactive/unused codes
maximally apart within DOCSIS 2.0 S-CDMA code set, the 8 unused
codes out of 128 total codes might be code numbers {15 31 47 63 79
95 111 127}. When selecting a different number of unused codes
within the DOCSIS 2.0 S-CDMA code set, they may be similarly
maximally spaced apart.
[0180] In addition, the inactive/unused codes may be selected
according to the optimality criterion of the block 2623. Examples
of an optimality criteria would be to select unused codes include:
(1) selecting inactive/unused codes that have maximal correlation
with the interference as shown in a block 2624, (2) minimizing
enhancement of white noise resulting from cancellation process as
shown in a block 2625, and (3) minimizing residual interference
power after cancellation as shown in a block 2626.
[0181] FIG. 27 is an operational flow diagram illustrating another
embodiment of an interference cancellation method 2700 that is
performed according to the present invention. In a block 2710, a
set of active/used basis waveforms is selected. These basis
waveforms may include orthogonal (or nearly orthogonal) waveforms;
these waveforms may be viewed as being substantially orthogonal.
There are a number of types of sets of orthogonal waveforms may be
employed. Some specific examples of sets of orthogonal waveforms
include: (1) S-CDMA codes, including DOCSIS 2.0 and Walsh-Hadamard,
(2) an orthogonal set of binary spreading codes, (3) any orthogonal
set of quaternary spreading codes, and (4) the rows of the identity
matrix.
[0182] Then, in a block 2720, a set of inactive/unused basis
waveforms is selected. In certain embodiments as shown in a block
2722, we may assume that the number of inactive/unused basis
waveforms is less than number of active/used basis waveforms. A
signal is transmitted using the active/used basis waveforms as
shown in a block 2730. In certain embodiments, as shown in a block
2732, we transmit one or more zero valued symbols on the
inactive/unused basis waveforms. Alternatively, as shown in a block
2734, we transmit a known sequence (training or pilot symbols) on
the inactive/unused basis waveforms.
[0183] Then, in a block 2740, the received signal is processed
using the received signal's projection on the active (used) basis
waveforms and the inactive (unused) basis waveforms thereby
canceling interference. We process the received signal using both
its projection onto a desired (active) waveform, and its projection
onto the inactive waveforms, in order to cancel interference on the
desired waveform.
[0184] Alternatively, in a block 2742, the received signal is
processed using the received signal's projection on the active
(used) basis waveforms thereby canceling interference. We process
the received signal using its projection onto a desired (active)
waveform in order to cancel interference on the desired
waveform.
[0185] In even alternative embodiments, in a block 2744, we compute
the projection of the interference on the inactive/unused basis
waveforms, and subtract it from the projection on the active/used
basis waveforms of the received signal including interference
thereby canceling interference. It is noted here that we can reduce
the computational complexity by computing the null-space projection
(the projection of the interference on the inactive basis
waveforms) and subtracting it from the overall projection (the
projection of the signal+interference on the active basis
waveforms). As an example, if we have 120 active codes and 8
inactive codes, in the present method we only need to invert an
8.times.8 matrix. In the standard least-squares approach, we would
have to invert a 120.times.120 matrix.
[0186] The selection of the inactive/unused basis waveforms may be
performed as illustrated and described above within some of the
embodiments selecting adjacent basis waveforms, spacing the
inactive/unused basis waveforms maximally apart, and selecting the
basis waveforms according to an optimality criterion. Examples of
an optimality criteria would be to select unused basis waveforms
include: (1) selecting inactive/unused basis waveforms that have
maximal correlation with the interference, (2) minimizing
enhancement of white noise resulting from cancellation process, and
(3) minimizing residual interference power after cancellation.
These parameters that may be used to perform the selection of the
inactive/unused basis waveforms is analogous to the selection of
the inactive/unused codes that is performed above with respect to
the FIG. 26, except here, the selection is with respect to the
basis waveforms of the signal space.
[0187] FIG. 28 is a diagram illustrating an embodiment of a
spectrum of narrowband interference 2600 that may be addressed and
overcome when practicing the present invention. The FIG. 28 shows
the spectrum of narrowband interference (for example, the signal n
in the FIGS. 16, 17, and/or 18) that may be present at the input of
a communications receiver. The desired signal is not present in
this Figure. In this example, the 3-dB bandwidth of the
interference is 1/32 of the symbol rate of the desired signal. Its
power is equal to the desired signal (0 dBc), when the desired
signal is present. The SNR (Signal to Noise) of the desired signal,
when present, is 35 dB in the example.
[0188] FIG. 29 is a diagram illustrating an embodiment of a
spectrum of an adapted code showing a null at a location of
interference 2900 that may be achieved when practicing the present
invention. The FIG. 29 shows the spectrum of the adapted code. The
adapted code is seen to have a null corresponding to the narrowband
interference. Hence, the adapted code cancels the narrowband
interference.
[0189] FIG. 30A is a diagram illustrating an embodiment of a
received constellation before interference has been cancelled 3000
when practicing the present invention. The FIG. 30A shows the
output d.sub.s of the vector de-spreader before interference
cancellation is enabled using the linear combiner and weight
computation functionality. This may be viewed as being the output
d.sub.s within any of the FIGS. 16, 17, and/or 18. That is, all the
adaptive weights w.sub.i are zero, and only the normal de-spreading
code c.sub.s is used to de-spread the desired signal. We see that
the signal constellation is unrecognizable due to the large amount
of interference, which has not yet been canceled.
[0190] FIG. 30B a diagram illustrating an embodiment of a received
constellation after interference has been cancelled 3005 when
practicing the present invention. The FIG. 30B shows the output
d.sub.s of the de-spreader after the interference cancellation of
the present invention is enabled using the linear combiner and
weight computation functionality. Now the adaptive weights w.sub.i
have adapted and are nonzero, and as a result the adapted
de-spreading code c.sub.a is used to de-spread the desired signal.
We see that the 64 QAM signal constellation, plus the QPSK
constellation used for the preamble, is now clearly recognizable
and the interference has been effectively canceled.
[0191] Another variant embodiment of the present invention may be
performed by applying the linear combiner at the chip level instead
of the de-spread symbol level. In this approach, the 128 chips
(again using the 128 code embodiment example) used in the
de-spreader for the desired code are adapted (for example, using
the LMS method or LS method) until they converge to the
near-optimal adapted de-spreading code. This adapted de-spreading
code will be complex-valued and will be a linear combination of all
128 de-spreading codes. In this approach there are 128 adaptive
complex weights that need to be trained, so convergence is slower
than for the baseline approach in which, for example, only 8
weights need to be trained for the 8 unused codes.
[0192] The general applicability of the present invention across a
wide variety of contexts is to be understood. The present invention
is operable to cancel not only narrowband interference, but any
interference that occupies a small number of dimensions in the
signal space. The first example, given above, is narrowband
interference. However, other types of interference may be
substantially eliminated according to the present invention as
well. A narrowband signal occupies a small number of DFT bins (DFT:
discrete Fourier transform--one example of an orthonormal
expansion) bins, showing that it occupies a small number of
dimensions in signal space.
[0193] An extremely simple example is a CW (Continuous Wave) signal
whose frequency is an integer multiple of the de-spread symbol
rate; this CW signal occupies only a single bin in the DFT, or only
one dimension in the signal space, where each dimension is, in this
case, one DFT bin. Another example is a short burst of noise
(impulse or burst noise). A short burst signal occupies a small
number of time samples, again showing that it too occupies a small
number of dimensions, where each dimension is, in this case, a time
sample. Other types of signals can be constructed without limit
that satisfy the property that they occupy a small number of
dimensions. All such signals can be canceled by the present
technique.
[0194] Again, the DFT is just one example of an orthonormal
expansion. A second example is the code matrix in DOCSIS 2.0
S-CDMA. Innumerable other orthonormal transforms exist. If a signal
occupies a small number of dimensions in any orthonormal transform,
the present invention's technique will help cancel it.
[0195] The present invention may also be implemented to use unused
spatial dimensions to cancel interference according to the present
invention. For example, we can generalize the technique to spatial
dimensions as well. One such interpretation of spatial dimensions
is with respect to MIMO (multi-input multi-output) systems. We may
designate certain transmit antennas to transmit zeros at certain
times. At the receiver, we may also utilize the samples from extra
receive antennas to cancel interference.
[0196] In any of the embodiments described herein, the present
invention is operable to store pre-computed weights using any
number of various storage techniques, such as a look up table
(LUT), memory, or some other storage technique. This may be
beneficial in some cases where it may be impractical to compute the
weights fast enough, so we may want to pre-compute some "canned"
sets of weights. As an example, consider very wideband interference
that occupies 1/2 the bandwidth of the desired signal. Assuming an
S-CDMA system, we will need to have 64 unused codes out of 128
total codes. Each desired code now has 64 adaptive weights that
need to be solved for. This implies a very large matrix to invert,
which is very complex to implement in real time. However, we note
that in our favor, there are very few notches of this size (half
the bandwidth) to go across the band. If, for example, we
pre-compute the weights for several wideband notches and store the
weight sets, then we can simply select the notch that most closely
matches the interference when it occurs.
[0197] The present invention is also operable to support adjusting
and tracking of pre-computed weights. As an extension to the above
concept of storing pre-computed weights, we may store a single
prototype set of weights, and modify the weights to move the notch
around. For example, we can adjust the frequency of the notch
without having to completely re-compute the weights. We can also
adjust the depth of the notch. We can weight and superimpose
pre-computed notches to build up a more complex notch structure. We
can build a tracking loop that automatically adjusts the frequency
(or other parameter) of the notch as the interference changes. Say
there is narrowband interference, and we have applied a notch that
cancels it. Now let the interference slew its frequency. We can
implement a tracking loop that automatically slews the frequency
location of the notch to track the frequency of the interference.
There are many ways to implement such a tracking loop. One way is
by taking an FFT of the interference, and tracking the energy in
the peak corresponding to the interference. Another way is to
dither the location of the notch, and measure the power or SNR at
the output. We then move the notch in the direction of increasing
SNR or decreasing interference or total power. Many other tracking
methods can be devised.
[0198] The present invention may also be implemented to perform
adjacent channel interference (ACI) cancellation when performing
interference cancellation. For example, the canceller can also be
used to cancel ACI. Consider a desired signal with signals present
in the upper and lower adjacent channels. If the adjacent channel
signals overlap slightly with the desired-signal band, ACI results.
ACI is in general colored and can therefore be canceled using the
present canceller technique.
[0199] In performing the processing, the present invention may
employ a sliding window. The processing has thus far been described
as being done on a block basis, where a block is typically a symbol
of 128 chips in the DOCSIS 2.0 implementation. However, the present
invention is also operable when employing a sliding block approach
as well. This may have the greatest benefit when the code matrix is
the identity matrix, where no spreading occurs.
[0200] Code hopping is defined in the DOCSIS 2.0 spec as a process
whereby the code matrix is modified by a cyclic rotation of the
rows of the entire code matrix (except possibly the all -1's code)
on each spreading interval. This causes the set of unused codes to
be different on each spreading interval, requiring the
re-computation of the adaptive weights in the canceller on each
spreading interval. It would be better to hop over the set of codes
excluding the unused codes, so that the unused codes will be the
same on each spreading interval. This would obviate the need to
re-computation of the adaptive weights on each spreading interval,
reducing processing complexity. Or, we can just turn off code
hopping when the canceller is used. However, this latter approach
removes the benefits of code hopping, which include fairness
(equality of average performance) for all users.
[0201] In the case of the DOCSIS 2.0 S-CDMA code matrix, the rows,
or basis functions, are nearly cyclic shifts of each other, with
the exception of code 0. This property relates adjacent codes to
shifts in time by one chip. This in turn relates the linear
combiner used on the de-spreader outputs to a FIR filter in the
time domain. This implies that the adaptive weights for the unused
codes are similar to the weights that are produced by the time
domain interference cancellation structure used for TDMA. The
latter weights can be computed efficiently by the Trench method.
Hence we might use the Trench method to compute an initial set of
weights for the subspace-based canceller, and iterate the weights
to a more exact solution using the LMS or similar tracking
method.
[0202] In addition, a decision feedback equalizer (DFE)-like
structure may also be performed, as follows. The canceller begins
by using the de-spreader outputs of codes 1-8 (for example) in the
linear combiner to estimate the output of the code 9 de-spreader. A
hard decision is made on the symbol from code 9. Now that the
symbol on code 9 is available, it can be used to estimate the
symbol on code 10. Once the symbol on code 10 is available, it can
be used to estimate the symbol on code 11, and so on. A DFE-like
structure can be constructed that can run in both directions.
[0203] As mentioned and described above, the subspace canceller can
be applied to an arbitrary code matrix (basis set). One such basis
set is the discrete Fourier transform (DFT). In this context one
should also mention the fast DFT methods collectively referred to
as the fast Fourier transform (FFT). The DFT is not well suited for
narrowband interference cancellation in the receiver without
modifications to the cancellation approach. This is because each
basis function, the complex tone e.sup.jn.omega. (or alternatively
written as exp(jn.omega.)), is designed to have minimal support in
the frequency domain. The basis functions cannot cancel a
narrowband interferer without very large weights. Instead, locating
the canceller predominantly in the transmitter is a better choice.
One simply does not transmit those tones that overlap with the
interference, designating them as unused "codes" or tones. This
approach has been known in the industry for some time in
conjunction with discrete multi-tone (DMT) or orthogonal frequency
division multiplexing (OFDM) systems.
[0204] Moreover, the subspace canceller can be integrated with FEC
coding. In one approach, a Reed-Solomon code can provide parity
symbols that are transmitted on the unused spreading codes instead
of zero symbols. In another approach, different SNRs may exist on
different spreading codes (for example, when spreading code hopping
is turned off). In this case unequal transmitted power may be sent
on each spreading code, or unequal numbers of bits per symbol of
modulation. Or, unequal coding strength can be used on each
spreading code: for example, rate 7/8 on one spreading code and
rate 3/4 on another spreading code. An important point is that
these approaches will give a better result than code hopping. In
code hopping, performance is averaged over all spreading codes. It
is better to use our prior knowledge of which spreading codes are
disadvantaged, and give them more processing power or transmit
power.
[0205] The present invention is also operable, when performing
interference cancellation, to perform adjustment of transmitted
spectrum to meet a desired spectral mask. It may be desirable to
attenuate certain portions of the spectrum using the subspace
canceller. In these applications the canceller is predominantly
located in the transmitter. One example is in a wireless local area
network (LAN) applications. Here one has to place a notch in the
transmit spectrum at the spectral location of the XM satellite
radio service. The subspace canceller has a great advantage over a
notch filter. A notch filter can notch out a designated spectral
region, but in doing so, it distorts the desired signal. The
subspace canceller can create the notch without distorting the
desired signal. The subspace canceller may also have the following
advantage over a notch filter. A notch filter can cause large
excursions in the transmitted power, whereas the subspace canceller
does not.
[0206] The interference cancellation according to the present
invention may also be integrated with pre-coding. The subspace
canceller can be integrated with pre-coding, such as
Tomlinson-Harashima pre-coding. We note that the subspace canceller
can be used for narrowband interference cancellation and also for
equalization of a deep notch in the channel. These are two
applications of pre-coding. Hence, if subspace cancellation and
pre-coding were combined, would we get some of the advantages of
both.
[0207] In view of the above detailed description of the invention
and associated drawings, other modifications and variations will
now become apparent. It should also be apparent that such other
modifications and variations may be effected without departing from
the spirit and scope of the invention.
* * * * *