U.S. patent application number 13/004337 was filed with the patent office on 2011-07-28 for ofdm channel estimation system and method components.
Invention is credited to Wei An, HAIM PRIMO, Yosef Stein.
Application Number | 20110182374 13/004337 |
Document ID | / |
Family ID | 39314653 |
Filed Date | 2011-07-28 |
United States Patent
Application |
20110182374 |
Kind Code |
A1 |
PRIMO; HAIM ; et
al. |
July 28, 2011 |
OFDM CHANNEL ESTIMATION SYSTEM AND METHOD COMPONENTS
Abstract
Channel estimation for high mobility OFDM channels is achieved
by identifying a set of channel path delays from an OFDM symbol
stream including carrier data, inter-channel interference noise and
channel noise; determining the average channel impulse response for
the identified set of channel path delays in each symbol;
generating a path delay curvature for each channel path delay in
each symbol based on stored average channel impulse responses for
the identified channel path delays; estimating the carrier data in
the symbols in the OFDM symbol stream in the presence of
inter-channel interference noise and channel noise from the OFDM
symbol steam and the average impulse responses for the identified
channel path delays; reconstructing the inter-channel interference
noise in response to the path delay curvature, the identified set
of channel path delays and estimated carrier data to produce a
symbol stream of carrier data and channel noise with suppressed
inter-channel interference noise.
Inventors: |
PRIMO; HAIM; (Gane-Tikwa,
IL) ; Stein; Yosef; (Sharon, MA) ; An;
Wei; (Auburndale, MA) |
Family ID: |
39314653 |
Appl. No.: |
13/004337 |
Filed: |
January 11, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11978841 |
Oct 30, 2007 |
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13004337 |
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11789180 |
Apr 24, 2007 |
7830994 |
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11978841 |
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60852607 |
Oct 18, 2006 |
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Current U.S.
Class: |
375/260 |
Current CPC
Class: |
H04B 17/345 20150115;
H04L 25/03038 20130101; H04L 25/03159 20130101; H04L 25/0228
20130101; H04L 27/2647 20130101; H04L 2025/03783 20130101; H04B
17/364 20150115; H04L 2025/03414 20130101; H04L 25/0218 20130101;
H04L 25/025 20130101 |
Class at
Publication: |
375/260 |
International
Class: |
H04L 27/28 20060101
H04L027/28 |
Claims
1-13. (canceled)
14. A regenerator circuit, responsive to a curve generator, path
delay estimator circuit and carrier data estimation circuit, for
reconstructing inter-channel interference noise comprising: a local
OFDM symbol generator, responsive to estimated carrier data to
generate locally OFDM symbol replicas; and an ICI distortion
generator for shifting an OFDM symbol replica by each associated
channel path delay, multiplying it by the associated path delay
curvature and summing the shifted, multiplied symbol replicas to
produce local inter-channel interference noise.
15-27. (canceled)
28. A method of reconstructing inter-channel interference noise in
response to path delay curvature, an identified set of channel path
delays; and estimated carrier data, the method comprising:
generating locally OFDM symbol replicas from said estimated carrier
data; and shifting an OFDM symbol replica by each associated
channel path delay, multiplying it by the associated path delay
curvature, and summing the shifted, multiplied symbol replicas to
produce local inter-channel interference noise.
29-30. (canceled)
31. The regenerator circuit of claim 14 further comprising a
carrier data estimator circuit for providing the estimated carrier
data, the carrier data estimator circuit comprising (i) a vector
generating circuit for creating a vector with zeros and inserting
average path gains in associated delay locations and (ii) an
equalization circuit for calculating equalization coefficients.
32. The regenerator circuit of claim 14 further comprising a curve
generator circuit for providing the path delay curvature, the curve
generator circuit comprising (i) a selection circuit for selecting
from storage average channel gains of neighboring OFDM symbols,
(ii) a rate determining circuit for determining a rate of change of
the neighboring average channel gains, and (iii) a model selection
circuit for identifying a best fit average free curve for the
stored channel impulse responses.
33. The regenerator circuit of claim 14 further comprising a path
delay estimator circuit for providing the channel path delays, the
path delay estimator circuit comprising (i) a threshold setting
circuit for setting a local predetermined threshold for sets of
channel path delays in each said window in accordance with their
energy levels, and (ii) a threshold circuit for selecting channel
path delays in each said window meeting their local predetermined
threshold and combining the selected channel path delays, from all
said windows, to determine the total channel path delays.
34. The method of claim 28 wherein estimating the carrier data
comprises (i) creating a vector with zeros and inserting average
path gains in associated delay locations and (ii) calculating
equalization coefficients in response to a Fourier transform and
applying them to an associated symbol.
35. The method of claim 28 further comprising generating the path
delay curvature by (i) averaging channel gains of neighboring OFDM
symbols, (ii) determining a rate of change of the neighboring
average channel gains, and (iii) identifying a best fit average
free curve for stored channel impulse responses.
36. The method of claim 28 further comprising identifying the
channel path delay by (i) setting a threshold for groups of channel
path delays in accordance with their energy levels and (ii)
selecting channel path delays meeting a predetermined threshold.
Description
RELATED APPLICATIONS
[0001] This application is a continuation-in-part of U.S. patent
application Ser. No. 11/789,180 filed Apr. 24, 2007 which claims
benefit of and priority to U.S. Provisional Application Ser. No.
60/852,607 filed Oct. 18, 2006 each of which are incorporated
herein by this reference.
FIELD OF THE INVENTION
[0002] This invention relates to channel estimator system and
method components for high mobility OFDM channels.
BACKGROUND OF THE INVENTION
[0003] Binary phase shift keying (BPSK) is a conventional data
modulation scheme that conveys data by changing, the phase of a
reference carrier signal, for example, during each BPSK symbol
period carrier data in the form of either a positive or negative
sine wave is transmitted. A positive sine wave represents a data
"1", a negative sine wave a data "0". When the symbol stream
arrives at the receiver it is decoded by multiplying with a
positive sine wave. The multiplying of it by another positive sine
wave produces a average positive level; if the symbol period
contains a negative sine wave the multiplexing by a positive sine
wave produces an average negative level. Orthogonal Frequency
Division Multiplexing (OFDM) employs the same idea but instead of
one carrier wave per bit, the bit stream to be transmitted is split
into several parallel low-rate bit streams, two, ten or any number;
presently over 8 k (8192). Each low-rate bit stream is transmitted
over one sub-channel by modulating a sub-carrier using a standard
modulation scheme, for example BPSK. The sub-carrier frequencies
are chosen so that the modulated data streams are orthogonal to
each other. The demodulation at the receiver is done in the same
way with the symbol period sine waves being multiplied selectively
by a positive sine wave of each of the frequencies transmitted. By
virtue of orthogonality it is possible to distinguish between the
various carrier sine waves. OFDM is thus a much higher density data
encoding technique. OFDM has shortcomings but works well especially
where the transmitter and received are fixed or not moving fast
with respect to each other and so the transmitter channel between
them remains constant or fairly constant. That is, the amplitude
and phase of the various sine waves transmitted over that channel
within a symbol period do not vary significantly over the symbol
period. However in high mobility situations where the channel does
change over the time of a symbol period, e.g. video streaming to a
receiver on a moving vehicle or train, different sine waves can
experience different channel paths resulting in variations in their
phase and/or amplitude. Such variations referred to as
inter-carrier or inter-channel interference (ICI) noise interferes
with the orthogonality of the sine waves and can cause errors in
the data decoding causing "1"s to appear to be "0"s and "0"s to
appear as "1"s. This ICI noise accompanies but is different then
the conventional channel noise that accompanies the carrier
data.
BRIEF SUMMARY OF THE INVENTION
[0004] It is therefore an object of this invention to provide
improved OFDM estimator system and method components for high
mobility OFDM channels.
[0005] It is a further object of this invention to provide such
improved OFDM estimator system and method components which make
efficient use of memory and power.
[0006] It is a further object of this invention to provide such
improved OFDM estimator system and method components which are
power adaptive to channel conditions.
[0007] The invention results from the realization that a channel
estimation for high mobility OFDM channels can be achieved with
improved system and method components for identifying a set of
channel path delays from an OFDM symbol stream including carrier
data, inter-channel interference noise and channel noise; for
determining the average channel impulse response for the identified
set of channel path delays in each symbol; for generating a path
delay curvature for each channel path delay in each symbol from the
stored average channel impulse responses for the identified channel
path delays; for estimating the carrier data in the symbols in the
OFDM symbol stream in the presence of inter-channel interference
noise from the OFDM symbol stream and said average impulse
responses for the identified channel path delays; for
reconstructing the inter-channel interference noise in response to
the identified set of channel path delays and estimated carrier
data to produce a symbol stream of carrier data and channel noise
with suppressed inter-channel interference noise.
[0008] The subject invention, however, in other embodiments, need
not achieve all these objectives and the claims hereof should not
be limited to structures or methods capable of achieving these
objectives.
[0009] This invention features a path delay estimator circuit
responsive to an OFDM symbol stream including carrier data,
inter-channel interference noise and channel noise for identifying
a set of channel path delays in a group of non-overlapping windows
which are above a predetermined energy threshold including a
threshold setting circuit for setting a local predetermined
threshold for sets of channel path delays in each window in
accordance with their energy levels and a threshold circuit for
selecting channel path delays in each window meeting their local
predetermined threshold and combining the selected channel path
delays, from all the windows, to determine the total channel path
delays.
[0010] In a preferred embodiment the path delay estimator circuit
may include a Fourier transform circuit for performing Fourier
transform on an OFDM symbol. The path delay estimator circuit may
include a normalizing circuit for extracting the channel frequency
response for known carriers and inserting zeros for unknown
carriers. The path delay estimator circuit may include an inverse
Fourier transform for performing inverse Fourier transform on the
channel frequency response. The path delay estimator circuit may
include a noise estimator circuit for determining the channel noise
level.
[0011] This invention also features an average channel estimator
circuit, responsive to the OFDM symbol stream and an identified set
of channel path delays, for determining the average channel impulse
response for the identified set of channel path delays in each
symbol including an estimator circuit for determining average path
gains based on least squares and known noise.
[0012] In a preferred embodiment the channel estimator circuit may
include a normalizing circuit for extracting the channel frequency
response for known carriers. The channel estimator circuit may
include a Fourier transform circuit for performing a Fourier
transform on an OFDM symbol
[0013] This invention also features a curve generator circuit,
responsive to stored average impulse responses, for generating a
path delay curvature for required channel path delay in each
symbol. There is a selection circuit for selecting from storage the
average channel gains of neighboring OFDM symbols, a rate
determining circuit for determining the rate of change of the
neighboring average channel gains and a model selection circuit for
identifying a best fit average free curve for the stored channel
impulse responses.
[0014] This invention also features a carrier data estimator
circuit, responsive to an OFDM symbol stream and average impulse
responses from an average channel estimator circuit, for estimating
the carrier data in the symbols in the OFDM symbol stream in the
presence of inter-channel interference and channel noise including
a vector generating circuit for creating a vector with zeros and
inserting average path gains in associated delay locations and an
equalization circuit for calculating equalization coefficients.
[0015] In a preferred embodiment the carrier data estimator circuit
may include a Fourier transform circuit for performing a Fourier
transform on the vector. The carrier data estimator circuit may
include an averaging circuit for calculating noise level. The
carrier data estimator circuit may include a slicer circuit for
matching the equalized symbols to a predefined grid of levels.
[0016] This invention also features a regenerator circuit,
responsive to a curve generator, path delay estimator circuit and
carrier data estimation circuit, for reconstructing inter-channel
interference noise including a local OFDM symbol generator,
responsive to estimated carrier data to generate locally OFDM
symbol replicas and an ICI distortion generator for shifting an
OFDM symbol replica by each associated channel path delay,
multiplying it by the associated path delay curvature and summing
the shifted, multiplied symbol replicas to produce local
inter-channel interference noise.
[0017] This invention also features a method for identifying a set
of channel path delays from an OFDM symbol stream including carrier
data, inter-channel interference noise and channel noise including
setting a threshold for groups of channel path delays in accordance
with their energy levels and selecting channel path delays meeting
a predetermined threshold.
[0018] In a preferred embodiment the method may include performing
a Fourier transform on an OFDM symbol. The method may include
extracting the channel frequency response for known carriers and
inserting zeros for unknown carriers. The method may include
performing IFT on the channel frequency response. The method may
include determining the channel noise level.
[0019] This invention also features a method for determining the
average channel impulse response for an identified set of channel
path delays in each symbol including determining average path gains
based on least squares and known noise.
[0020] In a preferred embodiment the method may include extracting
the channel frequency response for known carriers. The method may
include performing a FT on an OFDM symbol.
[0021] This invention also features a method for generating a path
delay curvature for each channel path delay in each symbol based on
stored average channel impulse responses for the identified channel
path delays including averaging the channel gains of neighboring
OFDM symbols, determining the rate of change of the neighboring
average channel gains, and identifying a best fit average free
curve for the stored channel impulse responses.
[0022] This invention also features a method for estimating the
carrier data in the symbols in the OFDM symbol stream in the
presence of inter-channel interference noise and channel noise from
the OFDM symbol stream and average impulse responses for the
identified channel path delays including creating a vector with
zeros and inserting average path gains in associated delay
locations and calculating equalization coefficients in response to
an FT and applying them to the associated symbol.
[0023] In a preferred embodiment the method may include performing
FT on the vector. The method may include calculating noise level.
The method may include matching the equalized symbols to a
predefined grid of levels.
[0024] This invention also features a method of reconstructing the
inter-channel interference noise in response to the path delay
curvature, the identified set of channel path delays and estimated
carrier data including generating locally OFDM symbol replicas from
the estimated carrier data and shifting an OFDM symbol replica by
each associated channel path delay, multiplying it by the
associated path delay curvature and summing the shifted, multiplied
symbol replicas to produce local inter-channel interference
noise.
[0025] This invention also features a system for identifying a set
of channel path delays from an OFDM symbol stream including carrier
data, inter-channel interference noise and channel noise for
identifying a set of channel path delays in a group of
non-overlapping windows which are above a predetermined energy
threshold including a threshold setting circuit for setting a local
predetermined threshold for sets of channel path delays in each
window in accordance with their energy levels and a threshold
circuit for selecting channel path delays in each window meeting
their local predetermined threshold and combining the selected
channel path delays, from all windows, to determine the total
channel path delays.
This invention also features a method for identifying a set of
channel path delays from an OFDM symbol stream including carrier
data, inter-channel interference noise and channel noise for
identifying a set of channel path delays in a group of
non-overlapping windows which are above a predetermined energy
threshold including setting a local predetermined threshold for
sets of channel path delays in each window in accordance with their
energy levels and selecting channel path delays in each window
meeting their local predetermined threshold and combining the
selected channel path delays, from all windows, to determine the
total channel path delays.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0026] Other objects, features and advantages will occur to those
skilled in the art from the following description of a preferred
embodiment and the accompanying drawings, in which:
[0027] FIG. 1 is a schematic, time domain, representation of two
OFDM symbols;
[0028] FIG. 2 is a schematic, frequency domain, representation of
the OFDM symbols of FIG. 1;
[0029] FIG. 3 is a schematic diagram showing an example of multiple
paths occurring in a channel between a transmitter and
receiver;
[0030] FIG. 3A is a graphical illustration of the gain and delay
associated with each path in FIG. 3;
[0031] FIG. 4 is a schematic block diagram of one embodiment of a
channel estimator system according to this invention;
[0032] FIG. 5 is a diagram of a flow chart of the path delay
estimator circuit of FIG. 4;
[0033] FIG. 5A is a graphical illustration of the forcing of zeros
in the unknown data carriers, referred to in FIG. 5;
[0034] FIG. 5B is a graphical illustration of the windowing and
thresholding of the channel impulse responses, referred to in FIG.
5;
[0035] FIG. 6 is a diagram of a flow chart of the average channel
estimator circuit of FIG. 4;
[0036] FIG. 7 is a diagram of a flow chart of the carrier data
estimator circuit of FIG. 4;
[0037] FIG. 7A is a graphical illustration of the insertion of
average path gains and zeros for unknown carriers in an N size
vector, referred to in FIG. 5;
[0038] FIG. 7B is a graphical illustration of the slicing of
equalized data to set thresholds, referred to in FIG. 7;
[0039] FIG. 8 is a diagram of a flow chart of the curve generator
estimator circuit of FIG. 4;
[0040] FIG. 8A is a graphical illustration of curve modeling and
filtering operation, referred to in FIG. 8;
[0041] FIG. 9 is a diagram of a flow chart of the regenerator ICI
circuit of FIG. 4;
[0042] FIG. 9A a graphical illustration of the building of an N
size vector and insertion of carrier data estimation, pilots and
zeros, referred to in FIG. 9; and
[0043] FIG. 9B is a graphical illustration of the distortion or
adjusting of an OFDM symbol according to the associated delay and
gain referred to in FIG. 9.
DETAILED DESCRIPTION OF THE INVENTION
[0044] Aside from the preferred embodiment or embodiments disclosed
below, this invention is capable of other embodiments and of being
practiced or being carried out in various ways. Thus, it is to be
understood that the invention is not limited in its application to
the details of construction and the arrangements of components set
forth in the following description or illustrated in the drawings.
If only one embodiment is described herein, the claims hereof are
not to be limited to that embodiment. Moreover, the claims hereof
are not to be read restrictively unless there is clear and
convincing evidence manifesting a certain exclusion, restriction,
or disclaimer.
[0045] There is shown in FIG. 1 an OFDM symbol stream 10 including
two symbols 12 and 14 each of which includes a cyclical prefix
section 16 and carrier data section 18. Each carrier data section
18, FIG. 2, includes a plurality of carrier data a.sub.0, a.sub.1,
a.sub.2, a.sub.3 - - - a.sub.n-1, a.sub.n where the filled circles
represent pilot carrier data whose amplitude and phase are known
and the empty circles represent unknown carrier data. The OFDM
symbol stream is typically propagated along a channel from a
transmitter 20, FIG. 3, to a receiver 22. Because of reflection
from objects 24 in the area the channel may have multiple paths,
the most direct path 28 with a phase of m.sub.0 and additional
paths 30, 32, and 34 having phases of m.sub.1, m.sub.2, m.sub.3,
respectively. Each path has its own gain or attenuation as shown in
FIG. 3A, where each path has associated it with it a gain or
amplitude h.sub.0, h.sub.1, h.sub.2, h.sub.3, and an associated
phase shift m.sub.0, m.sub.1, m.sub.2, m.sub.3. If the transmitter
20 and 22 move relatively fast with respect to one another,
inter-channel interference (ICI) noise develops due to the loss of
orthogonality because the carrier data sine wave arrives at the
receiver 22 along four paths with different phases and different
amplitudes. This can result in inaccuracies in determining the
nature of the data, possibly reading ones as zeros and zeros as
ones.
[0046] In accordance with this invention the inter-channel
interference (ICI) noise is suppressed by generating a replica ICI
noise function and subtracting it from the signal in channel noise:
thus where the incoming signal is represented by S+f(S)+n where S
is the OFDM carrier data, f(S) is the ICI noise and n is the
general channel noise this invention contemplates the generation of
a replica ICI noise f' (S) and subtracting it from the incoming
signal S+f(S)+n resulting in an output of simply S+n
[0047] One embodiment of a channel estimation system 36 having
improved components: average channel estimation circuit 44, curve
generation circuit 48, carrier data estimation circuit 50, path
delays estimation circuit 40 and regenerator ICI circuit 42
according to this invention is shown in FIG. 4. Path delay
estimator circuit 40 which responds to OFDM symbol stream 38 and
estimates the path delays m.sub.0-m.sub.n; the certain identified
ones of the estimated path delays are delivered both to ICI
regenerator circuit 42 and average channel estimator circuit 44.
Average channel estimator circuit 44 responds to the identified set
of channel path delays from path delay estimator circuit 40 and the
OFDM symbol stream on line 38 and determines the average channel
impulse response h.sub.0, h.sub.1, . . . h.sub.n for the identified
set of channel path delays in each symbol. Those average channel
impulse responses for the identified channel path delays are stored
in storage circuit 46 and then used by curve generator circuit 48
to generate a path delay curvature for each channel path delay in
each symbol. Carrier data estimator circuit 50 also responds to the
average impulse responses from the average channel estimator
circuit and the OFDM symbol stream on input line 38 to locally
estimate the carrier data (a0, a1, . . . an) in the OFDM symbol
stream in the presence of inter-channel interference and channel
noise. Regenerator ICI circuit 42 responds to the locally produced
estimated carrier data from carrier data estimator circuit 50 and
the path delay curvature for each channel path delay for curve
generator circuit 48 and adjusts their phase in accordance with the
path delay estimator circuit output 40 to reconstruct a replica ICI
noise. This replica ICI noise on line 52 is then subtracted from
the incoming OFDM symbol stream on line 38 in subtraction circuit
54 resulting in a symbol stream of carrier data and channel noise
with suppressed inter-channel interference noise.
[0048] Channel estimator system 36 in one embodiment may be
constructed using a programmable device such as a Digital Signal
Processor (DSP) programmed to operate as indicated in FIGS.
5-9.
[0049] Path delay estimator circuit 40. FIG. 5, first extracts the
next OFDM symbol 60 and a Fourier Transform (FT) 62 (typically an
FFT) is performed. The results are then normalized in a normalizing
circuit using the known carriers. Thus, where, for example, a known
carrier data a.sub.0 is known and its frequency response H.sub.0
can be determined, the carrier can be normalized by dividing
a.sub.0H.sub.0 by the known a.sub.0 to obtain the channel frequency
response H.sub.0 alone 64. Zero's are now forced in positions of
all the unknown carriers 66 as shown graphically in FIG. 5A; the
known or pilot carriers are shown as filled circles 70; the empty
circles 72 represent the unknown carriers in which the zeros are
forced, and the inverse Fourier transform (IFT) 68 (typically an
inverse FFT or IFFT) is performed. This is done for a number of
iterations, K, over a number of symbols to obtain an average
H.sub.0 and successively an average H.sub.1, H.sub.2, H.sub.3. The
noise level is then estimated in a noise estimator circuit 78 to
determine the channel noise level. After the Kth iteration, 76, the
noise level 78 is estimated and then a window including a group of
channel impulse responses are monitored to determine their energy
level and accordingly a local threshold is set for the particular
group 80 of that window. Then those channel impulse responses above
the threshold level are identified and become the identified set of
channel path delays 82. This is shown more graphically in FIG. 5B
where, for example, channel impulse responses 90, 92, 94 and 96 are
viewed in window 98 to determine the energy level of that group of
impulse responses 90-96. Based on that energy level a first local
threshold level 100 is set. The noise level is shown at 102.
Anything above threshold 100 is then selected as the identified
channel path delays and the delays m.sub.0, m.sub.1, m.sub.2,
m.sub.3 can be determined. In a second group 104, 106, 108, 110,
viewed through a second window 112, a lower energy is detected
resulting in a second lower local threshold 114 being set.
[0050] Average channel estimator 44, FIG. 6, begins by extracting
the OFDM symbol 120 and then performing FFT on it, 122. The results
are normalized by known carriers, step 124, in the same way as
previously, where the known carrier, a.sub.0, accompanied by the
frequency response, H.sub.0, is normalized by being divided by
a.sub.0 to obtain the frequency response H.sub.0. The average path
gains such as 90-96 shown in FIG. 5B are then estimated 126 using
the Least Squares (LS) model and the known noise. Carrier data
estimator circuit 50, FIG. 7, may be implemented by performing an
FFT 130 on a received OFDM signal, then building a vector size N
with zeros 132 and average path gains 134 inserted in the proper
delay locations. This is shown in greater detail in FIG. 7A where
the average path gains are shown at 138 and the unknown carriers
which receive the zero insertions are shown at 140. Following the
insertion of the average path gains FFT is performed 136 to obtain
the channel frequency response H.sub.0, H.sub.1 . . . . The noise
level is again calculated 138 using an averaging circuit based on
H.sub.0, H.sub.1, H.sub.2 . . . and the pilot carriers. After this
the equalization coefficients
1 H 0 , 1 H 1 , 1 H n ##EQU00001##
are calculated using an equalization circuit and equalization is
performed 140. This can be done using the minimum mean square error
(MMSE) method which is well known in the art. After this, slicing
is performed 142 to match the equalized values to a predefined grid
of level. For example, as shown in FIG. 7B, there are a grid of
levels +1, +2, +3, -1, -2, -3, and the equalized data 144 are
assigned to thresholds consistent with their levels: equalized data
144a is assigned level three, while equalized data 144b is assigned
level 1, equalized data 144c is assigned level -2.
[0051] Curve generator circuit 48 may be implemented as shown in
FIG. 8. Initially the average channel gains of the selected symbol
P and neighboring symbols P+1, P+2. P-1, P-2 . . . are retrieved,
selected using a selection or addressing circuit 170 from storage
46. The curvature model is then determined using an FFT operation
172 and an estimation model is built 174 to estimate the tap
function parameters. For example, if the best estimate is a line
the model would be ax+b, if it were a parabola it would be
ax.sup.2+bx+c, a third order curve it would be
ax.sup.3+bx.sup.2+cx+d. After the estimation the system returns to
inquire whether the last path delay in the set has been processed
176. If it has the routine is finished. If not it returns to
retrieve average channel gain symbols 170 from storage 46. A
selection circuit performs the retrieving of the average channel
gains in 170 and the FFT operation 172 functions as a rate
determining circuit for determining the rate of change of the
neighboring average channel gains. Model selection is accomplished
by building the estimation model 174. The operation is shown
graphically in FIG. 8A where the instant symbol P has average
channel response h.sub.0 along with the neighboring symbols P+1,
P+2, P+3 . . . P-1, P-2 . . . in order to obtain an indication of
the best fit average free curve 180. In this case a first order or
straight line best fit is indicated. In FIG. 8B, however, the curve
180b changes at a much higher rate and so it requires a higher
order best fit average free curve, for example, a parabolic shape
182 whose average should be equal to the average channel response
of the symbol P. The order of the best fit curve thus depends upon
the rate of change of the average channel gain as determined by the
FFT operation 172.
[0052] Regenerator ICI circuit 42 may be implemented, FIG. 9, by
building a vector size
[0053] N with zeros 190 and then inserting carrier data a.sub.0
estimation 192 and inserting the pilot data 194. This is shown
graphically in FIG. 9A where the inserted carrier data estimation
and pilots are shown at 198 along with carrier data labeled
a.sub.0-a.sub.n-3 and null carriers 200 indicated by zeros. After
this FIG. 9, FFT is performed 202 and then ICI distortion is
accomplished 204 and the results are summed 206. The ICI distortion
is accomplished by a local OFDM symbol replica generator 209 as
shown in FIG. 9B. OFDM symbol 210 represented as OFDM symbol sine
wave 212 is multiplied by the ICI average free gain curve 214
associated path delay curvature. Each of the phases m.sub.0 through
m.sub.3 is shifted. The shifted forms of OFDM symbol are multiplied
212 by each of the ICI average free gains h.sub.0, h.sub.1,
h.sub.2, h.sub.3, represented as one curve at 214. The
multiplication occurs in multiplier 212 and each of the waves,
phase shifted by their phase m.sub.0-m.sub.3 is presented at 210a,
210b, 210c, 210d, respectively. These are then summed 216 to
generate the ICI replica 218.
[0054] Although the preferred embodiment herein is shown with the
Fourier transform operation being fast Fourier transforms (FFT's)
or IFFT's, Fourier transforms (FT) of any type e.g., DFT, IDFT
could be used.
[0055] Although specific features of the invention are shown in
some drawings and not in others, this is for convenience only as
each feature may be combined with any or all of the other features
in accordance with the invention. The words "including",
"comprising", "having", and "with" as used herein are to be
interpreted broadly and comprehensively and are not limited to any
physical interconnection. Moreover, any embodiments disclosed in
the subject application are not to be taken as the only possible
embodiments.
[0056] In addition, any amendment presented during the prosecution
of the patent application for this patent is not a disclaimer of
any claim element presented in the application as filed: those
skilled in the art cannot reasonably be expected to draft a claim
that would literally encompass all possible equivalents, many
equivalents will be unforeseeable at the time of the amendment and
are beyond a fair interpretation of what is to be surrendered (if
anything), the rationale underlying the amendment may bear no more
than a tangential relation to many equivalents, and/or there are
many other reasons the applicant can not be expected to describe
certain insubstantial substitutes for any claim element
amended.
[0057] Other embodiments will occur to those skilled in the art and
are within the following claims.
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