U.S. patent application number 14/605903 was filed with the patent office on 2015-09-24 for pilot assisted channel estimation.
The applicant listed for this patent is SiTune Corporation. Invention is credited to Mahdi Khoshgard, Marzieh Veyseh.
Application Number | 20150270989 14/605903 |
Document ID | / |
Family ID | 52350756 |
Filed Date | 2015-09-24 |
United States Patent
Application |
20150270989 |
Kind Code |
A1 |
Veyseh; Marzieh ; et
al. |
September 24, 2015 |
PILOT ASSISTED CHANNEL ESTIMATION
Abstract
Systems and methods are described for the implementation of a
receiver that includes a channel estimation block that uses known
pilots to estimate the value of channel gain and phase at data
subcarrier indexes. Time interpolation as well as an auto
regression filter can be to estimate the channel gain and phase at
the "missing" pilot indexes as well as frequency interpolation to
estimate the value of the channel at data subcarrier indexes.
Inventors: |
Veyseh; Marzieh; (Los Altos,
CA) ; Khoshgard; Mahdi; (Los Gatos, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SiTune Corporation |
San Jose |
CA |
US |
|
|
Family ID: |
52350756 |
Appl. No.: |
14/605903 |
Filed: |
January 26, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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14088145 |
Nov 22, 2013 |
8942303 |
|
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14605903 |
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Current U.S.
Class: |
375/260 |
Current CPC
Class: |
H04L 25/0204 20130101;
H04L 27/2647 20130101; H04L 27/265 20130101; H04L 25/0232
20130101 |
International
Class: |
H04L 25/02 20060101
H04L025/02; H04L 27/26 20060101 H04L027/26 |
Claims
1. A method, comprising: receiving an Orthogonal Frequency Division
Multiplexing (OFDM) transmission that includes a plurality of OFDM
symbols, each one of the plurality of OFDM symbols including data
subcarriers and pilot subcarriers; determining a channel value at
pilot subcarrier indexes for a current OFDM symbol of the plurality
of OFDM symbols; updating, based at least in part on the channel
value at the pilot subcarrier indexes of the current OFDM symbol,
the channel value of pilot subcarriers indexes for a first grouping
of OFDM symbols of the plurality of OFDM symbols and a second
grouping of OFDM symbols of the plurality of OFDM symbols;
determining a channel value at pilot subcarrier indexes for a
reference OFDM symbol by computing a weighted average of the
channel value of the pilot subcarrier indexes for the first
grouping of OFDM symbols and the channel value of the pilot
subcarrier indexes for the second grouping of OFDM symbols; time
filtering the channel value for the pilot subcarrier indexes of the
reference symbol; estimating, using at least one frequency
interpolation function, the channel value at data subcarrier
indexes for the reference symbol based at least in part on the
channel value for the pilot subcarrier indexes of the reference
symbol; and dividing at least a portion of the data subcarriers in
the reference OFDM symbol by the channel value of the data
subcarrier indexes to equalize the OFDM transmission and reduce a
portion of channel distortion.
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 14/088,145, filed on Nov. 22, 2013
(20400.0011.NPUS00) and titled "PILOT ASSISTED CHANNEL ESTIMATION",
of which the full disclosure of this application is incorporated
herein by reference for all purposes.
COPYRIGHT NOTICE
[0002] A portion of the disclosure of this patent document contains
material which is subject to copyright protection. The copyright
owner has no objection to the facsimile reproduction by anyone of
the patent document or the patent disclosure, as it appears in the
Patent and Trademark Office patent file or records, but otherwise
reserves all copyright rights whatsoever.
FIELD OF THE INVENTION
[0003] This invention relates generally to the field of
communications, and more specifically to pilot-aided channel
estimation for orthogonal frequency division multiplexed (OFDM)
transmission.
BACKGROUND
[0004] In the age of rapid innovations in the field of
telecommunications, the requirements for communication devices that
enable faster, cheaper and more reliable data transfer is
escalating. Orthogonal frequency-division multiplexing (OFDM) has
developed as a method for reliable, high-volume data transfer in
both wired and wireless mediums to transfer data and compensate for
the effects of distortion at the receiver side. Wideband digital
applications such as digital television, audio broadcasting,
wireless networking, and broadband internet have become popular
applications for OFDM transmission. When a signal travels through a
transmission medium, such as a cable or air, the signal is affected
and distorted due to multipath effects. This distortion is
generally considered as the "channel". Several approaches have been
proposed to estimate the channel. In one such approach, cross-talk
between subchannels can be eliminated by selecting subcarrier
frequencies such that the subcarrier frequencies are orthogonal to
each other. If the channel is accurately estimated, its effects can
be compensated and the transmitted signal can be recovered more
accurately. However, a solution is needed for the estimation of
channel when "pilots" of pre-defined amplitude and phase are
inserted into the signal at regular intervals in both time and
frequency, where the pilots can be used by the receiver to estimate
changes in channel response in both time and frequency
dimensions.
BRIEF DESCRIPTION OF THE DRAWINGS
[0005] Various embodiments in accordance with the present
disclosure will be described with reference to the drawings, in
which:
[0006] FIG. 1 illustrates an example of an OFDM receiver with a
channel estimation component and a channel equalization component,
in accordance with various embodiments.
[0007] FIG. 2 illustrates an example of a channel estimation
component and a channel equalization component, in accordance with
various embodiments;
[0008] FIG. 3 illustrates an example block diagram for channel
estimation and equalization in accordance with various
embodiments;
[0009] FIG. 4 illustrates a sample pilot symbol grid used for
channel estimation in accordance with various embodiments;
[0010] FIG. 5 illustrates a sample pilot symbol grid used for
channel estimation in accordance with an alternate embodiment;
[0011] FIG. 6 illustrates a sample pilot symbol grid used for
channel estimation after temporal interpolation takes a weighted
average of the channel at assigned pilots;
[0012] FIG. 7 illustrates an example block diagram of an auto
regression filter in accordance with various embodiments; and
[0013] FIG. 8 illustrates an example process for pilot assisted
channel estimation in accordance with various embodiments.
DETAILED DESCRIPTION
[0014] In the following description, various embodiments will be
illustrated by way of example and not by way of limitation in the
figures of the accompanying drawings. References to various
embodiments in this disclosure are not necessarily to the same
embodiment, and such references mean at least one. While specific
implementations and other details are discussed, it is to be
understood that this is done for illustrative purposes only. A
person skilled in the relevant art will recognize that other
components and configurations may be used without departing from
the scope and spirit of the claimed subject matter.
[0015] Systems and methods in accordance with various embodiments
of the present disclosure may overcome one or more of the foregoing
or other deficiencies experienced in conventional approaches for
wireless and/or wired communication. In particular, various
embodiments describe systems and methods for pilot-aided channel
estimation for orthogonal frequency division multiplexed (OFDM)
communication systems, such as digital video broadcasting (DVB),
audio broadcasting, and cable transmissions, among other. In
general, a receiver including a channel estimation system can use
known pilot subcarriers to estimate the channel (i.e., channel gain
and channel phase) at data subcarrier indexes. By estimating the
channel and applying an inverse function of the channel to a
received OFDM transmission, an original transmitted OFDM
transmission can be equalized (i.e., recovered) from an affected
received OFDM transmission.
[0016] In accordance with various embodiments, a channel estimation
system can utilize, for example, at least a time interpolation
component as well as an auto regression filter to estimate the
channel of "missing" pilot indexes as well as a frequency
interpolation component to estimate the value of channel at data
subcarrier indexes for an OFDM symbol. For example, at every OFDM
symbol in an OFDM transmission, there can exist some indexes that
carry data subcarriers, while in previous or next OFDM symbols of
the OFDM transmission, the same indexes can carry pilot
subcarriers. Time interpolation can be used to estimate the channel
at these pilot subcarriers from pilot subcarriers of previous OFDM
symbols and future OFDM symbols relative to a reference OFDM symbol
and frequency interpolation can be used to estimate the value of
channel at data subcarrier indexes for an OFDM symbol. In
accordance with various embodiments, an OFDM symbol corresponds to
a symbol rate (also known as baud or modulation rate), which is the
number of symbol changes (waveform changes or signalling events)
made to the transmission medium per second using a digitally
modulated signal or a line code. The Symbol rate can be measured in
baud (Bd) or symbols/second. In the case of a line code, the symbol
rate is the pulse rate in pulses/second. Each symbol can represent
or convey one or several bits of data.
[0017] For example, the channel estimation system can receive an
OFDM transmission that includes a plurality of OFDM symbols, where
each of the OFDM symbols can include data subcarriers and pilot
subcarriers. In this example, the plurality of OFDM symbols can be
stored in one or more memory components upon being received. The
number of memory components can depend on a repetition period of
the pilot subcarrier patterns of the OFDM transmission. In various
embodiments, the repetition period can be set by a transmitter of
the OFDM transmission. For example, the transmitter can determine
which one of a plurality of pilot patterns is suitable for a
specific transmission and can store information indicated of the
pilot pattern in a header portion of the OFDM transmission. The
receiver, upon receiving the OFDM transmission, can determine the
repetition period by the selected pattern. In this way, if pilot
pattern number three was selected, the receiver can index an
appropriate interpolation filter (as well as weights for the
interpolation filter) for the channel estimation system.
[0018] In various embodiments, upon receiving the OFDM transmission
at the receiver, a Fast Fourier Transmission (FFT) of the
transmission is computed. As described, the OFDM transmission
includes a plurality of OFDM symbols. Accordingly, a FFT of the
OFDM transmission includes computing a FFT of each OFDM symbol in
the transmission. Upon computing the FFT of an OFDM symbol, the FFT
of the symbol is stored to one of the memory components. For
example, for an OFDM transmission that includes at least a first, a
second, and a third OFDM symbol, upon computing a FFT of the first
OFDM symbol, the output can be stored to one of the memory
component. In this example, the output of the FFT for the first
OFDM symbol can be stored in the N-d.sub.th memory component, where
d is the repetition period and N is the number of the current OFDM
symbol received in the OFDM transmission (e.g., the tenth OFDM
symbol of the OFDM transmission). The output of the FFT for the
second OFDM symbol can be stored in the N-1.sub.th memory component
and the output of the FFT for the first OFDM can be stored in the
N.sub.th memory component. An equalizer can use a de-multiplexer to
extract data subcarriers from the output of the N-d.sub.th memory
component and can use the result of a channel estimation block that
includes information from pilot subcarriers of the N.sub.th until
N-d.sub.th OFDM symbol, as well as information from N-d-1.sub.th
OFDM symbol until N-2d.sub.th OFDM symbol. In accordance with
various embodiments, determining the result of the channel
estimation block can include estimating a channel value at pilot
subcarrier indexes for a current symbol "N" of the plurality of
OFDM symbols, estimating a first channel value at pilot subcarrier
indexes for a first grouping of OFDM symbols (i.e., n.sub.th until
n-d) and a second channel value at pilot subcarrier indexes for a
second grouping of OFDM symbols (n-d-1.sub.th until n-2d) based at
least in part on the estimate of a channel value of the pilot
subcarrier indexes for the current symbol, estimating a channel
value at pilot subcarrier indexes for a reference symbol (n-d) by
taking a weighted average of the first grouping of OFDM symbols and
the second grouping of OFDM symbols, and estimating, using at least
one frequency interpolation function, a channel value at data
subcarrier indexes for the reference symbol based at least in part
on the estimated channel at pilot indexes of the reference
symbol.
[0019] Various other applications, processes, and uses are
presented below with respect to the various embodiments.
[0020] OFDM is a method of encoding digital data on multiple
carrier frequencies where a large number of closely spaced
orthogonal subcarrier signals are used to carry data on several
parallel data streams or channels. Each subcarrier signal is
modulated with a conventional modulation scheme (such as quadrature
amplitude modulation or phase-shift keying, etc.) at a low symbol
rate, maintaining total data rates similar to conventional
single-carrier modulation schemes in the same bandwidth. As
described, a solution is needed for channel estimation when OFDM is
in use and known pilots of pre-defined amplitude and phase are
inserted into the signal at regular intervals in both time and
frequency directions. Accurate channel estimation in an OFDM
receiver is important for the recovery of the transmitted
information data at the receiver. If the receiver makes a
significant error in its channel estimation, the original
modulation symbol can be decoded in error because each subcarrier
in the OFDM symbol is multiplied by fading coefficients that have
different amplitudes and phases. Further, a solution is needed for
efficient estimation of channel when different pilot spacing may be
in place based at least in part on the guard interval to provide a
range of efficiency and to increase capacity by increasing the
pilot spacing for small channel delay spreads. Further still, the
solution should consider efficient channel estimation by using
pilot subcarriers from previous and next OFDM symbols with minimum
memory requirements when channel is not heavily suffered from the
Doppler Effect. Accordingly, in accordance with various
embodiments, a channel estimation block at a receiver can use known
pilots to estimate the value of channel gain and phase at data
subcarrier indexes. A time interpolation component as well as an
auto regression filter or other similar filter can be used to
estimate the channel gain and phase of the "missing" pilot indexes
and a frequency interpolation component can be used to estimate the
value of the channel at data subcarrier indexes. By estimating the
value of the channel at the data subcarrier indexes and applying an
inverse function of the channel to a received OFDM transmission, an
original transmitted OFDM transmission can be equalized from an
affected received OFDM transmission.
[0021] FIG. 1 illustrates an example of an OFDM receiver with a
channel estimation component and a channel equalization component,
in accordance with various embodiments. In this example, a signal
(e.g., an OFDM transmission) can be received through an antenna
100. The signal can be provided to a tuner 102, where the signal
can be amplified, filtered, and/or down-converted, for example to a
baseband or intermediate frequency (IF). After the tuner, the
signal can be provided to an analog to digital converter (ADC) 104
for analog to digital conversion. The signal can then be provided
to a filtering and synchronization component 106 for further
filtering and synchronization and then to a Fast Fourier Transform
(FFT) component 108. The FFT component 108 can output OFDM symbols.
The OFDM symbols can be provided to a channel estimation component
110, where the channel can be estimated based on the pilot signals.
OFDM symbols, the estimated channel, as well as noise power data
from a noise estimator component 116 can be provided to the channel
equalization component 112, where equalized data values can be
produced. The equalized data values can then be provided to a data
demapper 114 and to a decoder 118 for further processing.
[0022] FIG. 2 illustrates an example of a channel estimation
component and a channel equalization component, in accordance with
various embodiments. In this example, OFDM symbols can be provided
from the FFT component 208. Pilot subcarrier values can be provided
to a channel estimation component 210 and data subcarrier values
can be provided to a channel equalization component 212. The
channel estimation component 210 can estimate the channel in data
subcarriers based on the pilot subcarrier values, as will be
described in greater detail below. In various embodiments, a
channel in every subcarrier, in every OFDM symbol can be estimated.
The estimated channel values from the channel estimation component
210, data subcarrier values from the FFT component 208, and noise
power data per OFDM symbol from a noise power estimation component
216 can be provided to the channel equalization component 212. The
channel equalization component 212 can adjust data subcarrier
values (i.e., equalize the values), based on the estimated channel
in each subcarrier value and the noise power data. The equalized
data values can be provided to a data demapper for further
processing.
[0023] In accordance with various embodiments, a channel estimation
block at a receiver can use known pilot subcarriers to estimate the
value of channel gain and phase at data subcarrier indexes. Time
interpolation as well as an auto regression filter or other similar
filter can be used to estimate the channel gain and phase of the
"missing" pilot indexes as well as frequency interpolation to
estimate the value of the channel at data subcarrier indexes. To
increase capacity when channel delay spread is small, known pilot
subcarriers can have different patterns in time and frequency. The
pilot patterns can be repeated in time where the repetition period
can depend on the channel characteristic and its variation in
period with time. At every OFDM symbol, there can exist some
indexes that carry data subcarriers, while at the previous or next
OFDM symbols, the same indexes can carry pilot subcarriers. The
channel gain and phase at these pilot indexes can be estimated
using pilot values from previous and future/next OFDM symbol by
means of time interpolation. The channel in the remaining,
non-pilot subcarriers can be estimated based on the estimated
channel in the pilot subcarriers. Once the channel is estimated,
channel equalization can be performed to compensate the data
signals for the channel to recover the original transmitted
data.
[0024] For example, FIG. 3 illustrates an example block diagram for
channel estimation and equalization in a receiver in accordance
with various embodiments. In an OFDM transmission, each transmitted
subcarrier of the OFDM transmission can be affected by the
environment and the medium through which the signal travels as the
signal is being transmitted. In accordance with various
embodiments, channel estimation can be performed to determine the
channel in an OFDM transmission by estimating the channel in the
subcarriers. The channel in subcarriers in an OFDM transmission can
be estimated based on pilot subcarriers. The channel in pilot
subcarriers can be estimated at the receiver based on a measured
subcarrier value and an expected, known subcarrier value. The
channel in the remaining, non-pilot subcarriers can be estimated
based on the estimated channel in pilot subcarriers. Accordingly,
the channel can be estimated in every subcarrier of every OFDM
symbol and once the channel is estimated, channel equalization can
be performed to compensate the data signals for the channel to
recover the original transmitted data.
[0025] As shown in example 300 of FIG. 3, a receiver, such as an
OFDM receiver or other receiver, can include, for example, a memory
block 302 and a channel estimation block 304. The memory block 302
can include a series of memory components, e.g., fast Fourier
Transform (FFT) output memory components 306, 308, 310. The channel
estimation block 304 can include, for example, a pilot sign
correction component 312, an update channel estimation component
314, a temporal interpolation component 316, an auto regression
(AR) filtering component 318, a frequency interpolation component
320, and an equalizer 322. It should be noted that the receiver can
include any device that can convert a signal from a modulated radio
wave into usable information. Example receivers can include
consumer audio receivers (e.g., hi-fi/home theater, portable radios
(e.g., transistor radios that can receive AM, FM, or short wave
broadcast bands), etc.
[0026] An input OFDM symbol is received at a FFT unit 324. The FFT
unit computes the Fourier Transform of the input OFDM symbol and
the output of the FFT unit is provided to the series of memory
components, e.g., FFT output memory components (306, 308, 310). In
accordance with various embodiments, the number of memory
components, d, can depend on a repetition period of the pilot
patterns in time. The N-d.sub.th previous OFDM symbol is stored in
the last memory component (i.e., the n-d.sub.th memory unit). The
equalizer can use a de-multiplexer to extract data subcarriers from
the output of the last memory component and can use the result of
the channel estimation block that had information from pilots of
the N.sub.th until N-d.sub.th OFDM symbol, as well as information
from N-d-1.sub.th until N-2d.sub.th OFDM symbol.
[0027] For example, de-multiplexer 326 can extract data subcarriers
from the output of memory component 310 and the data subcarriers
can be provided to the equalizer. De-multiplexer 328 can extract
pilot subcarriers from the output of N until N-d.sub.th memory
components. The equalizer can use the result of the channel
estimation block (e.g., components 312-320) that had information
from pilot subcarriers of the N.sub.th until N-d.sub.th OFDM
symbol, as well as data subcarriers from N-d-1.sub.th until
N-2d.sub.th OFDM symbol to estimate the channel. The equalized data
symbol can be provided to the forward error correction (FEC) 330 or
channel coding component to decode the input data and control
errors in data transmission over unreliable or noisy communication
channels. Accordingly, embodiments provide a solution to perform
interpolation in time by considering pilot subcarrier information
from the previous d OFDM symbols as well as the next or future d
OFDM symbols. The equalized data symbol enters the FEC or channel
coding component to decode the input data and control errors in
data transmission over unreliable or noisy communication
channels.
[0028] As described, the grouping of previous OFDM symbols and the
grouping of next/future OFDM symbols can be used to interpolate the
value of channel estimates for pilot subcarriers over time to
determine the value of channel for data subcarriers. FIG. 4
illustrates an example of updating a channel block for grouping of
previous OFDM symbols and FIG. 5 illustrates an example of updating
a channel block for grouping of future/next OFDM symbols.
[0029] FIG. 4 illustrates an example 400 of updating a channel
estimate block for N.sub.th previous pilot estimations upon
receiving N number of pilot estimates, in accordance with an
embodiment. In this example, the number of memory components "d" is
three. Each horizontal row of circles represents an OFDM symbol and
each vertical row of circles represents a subchannel. The
horizontal axis represents frequency and the vertical axis
represents time. The frequency of the subchannels can increase from
left to right. The solid circles are pilot subcarriers and the
other circles are data subcarriers. The distance in time, or the
repetition factor, between the pilot subcarriers in this example is
three. It should be noted that various types of OFDM signals in
various standards may contain different numbers and configurations
of pilot subcarriers, data subcarriers, than illustrated in FIG. 4.
As one skilled in the art would appreciate, this disclosure is not
limited to any particular configuration or type of OFDM signal.
[0030] In accordance with various embodiments, as OFDM symbols are
processed, the channel estimation system can update the channel
value of the pilot subcarriers. As described, channel estimation
can be performed to measure the channel in an OFDM transmission.
The channel in an OFDM transmission can be estimated by estimating
the channel in the subcarriers. In various embodiments, the channel
in subcarriers in an OFDM transmission can be estimated based at
least in part on pilot subcarriers. The channel in pilot
subcarriers can be estimated at the receiver based at least in part
on a measured subcarrier value and an expected, known subcarrier
value. For example, the channel in the pilot subcarriers can be
based on expected values, for example, by dividing the measured
value of the pilot subcarriers by the expected value of the pilot
subcarrier. The channel in the remaining, non-pilot subcarriers can
be estimated using methods of interpolation based on the estimated
channel in pilot subcarriers. Accordingly, the channel can be
estimated in every subcarrier of every OFDM symbol. Once the
channel is estimated, channel equalization can be performed to
compensate the data subcarriers for the channel to recover the
original transmitted data.
[0031] In this example, a pilot sign correction component (e.g.,
component 312 of FIG. 3) is responsible for dividing the pilot
subcarriers with pre assigned gain and sign to estimate the value
of the channel at the pilot indexes for the current OFDM symbol.
For example, as shown in FIG. 4, upon receiving a new OFDM symbol,
the channel estimation system can update the pilot subcarriers for
previous pilot subcarrier estimates and future/next pilot
subcarrier estimates, where the updated channel value for each
pilot is determined by dividing the received pilot value by an
expected channel value known at the receiver.
[0032] FIG. 5 illustrates an example 500 of updating a channel
estimate block for N-d future pilot estimations upon receiving N
number of pilot estimates, in accordance with an embodiment. As
described, the channel value estimates for pilot subcarriers from
previous and next OFDM symbols can be interpolated over time to
determine the value of channel in data subcarriers. In this
example, the updated channel estimation component (e.g., component
314 of FIG. 3) is responsible for using the new channel estimates
at the current pilot indexes to update the value of the channel at
all pilot indexes. For example, the update channel estimation
component can pass pilot estimates from the N.sub.th until
N-d+1.sub.th OFDM symbol (i.e., next/future OFDM symbols) as well
as channel estimates for all pilots based on the N-d-1.sub.th until
N-2d.sub.th OFDM symbol for the N-d.sub.th (previous OFDM symbols)
OFDM symbol. For example, as shown in FIG. 5, upon receiving a new
OFDM symbol, the channel estimation system can update the pilot
subcarriers for future pilot subcarrier estimates, where the
updated channel value for each pilot can be determined by dividing
the received pilot value by an expected channel value known at the
receiver.
[0033] As described, the previous OFDM symbols and the next/future
OFDM symbols can be used to interpolate the value of channel
estimates on pilots over time to determine the value of channel of
data subcarriers. Upon updating the channel block for previous OFDM
symbols and updating the channel block for future/next OFDM
symbols, a weighted average of the channel block can be taken. For
example, FIG. 6 illustrates an update of the channel estimate block
after the temporal interpolation component computes a weighted
average of the channel at assigned pilots from three previous and
three next OFDM symbols when the number of memory components is
three (d=3). In this example, the temporal interpolation component
(e.g., component 316 of FIG. 3) can use the value of channel from
the N.sub.th until N-d+1.sub.th OFDM symbol as well as the updated
value of the channel at pilot indexes from N-d-1.sub.th until
N-2d.sub.th OFDM symbols. For example, the temporal interpolation
block can compute a weighted average of the pilot values at each
given index. As shown in FIG. 6, as a result of this averaging, all
pilot indexes at the N-d.sub.th OFDM symbol have an estimated
channel value that is averaged in time using d previous and d next
OFDM symbols. In various embodiments, subcarrier pilot values for
an OFDM symbol that was received earlier in time are weighted
higher than subcarrier pilot values of an OFDM symbol that was
received later in time.
[0034] FIG. 7 illustrates a block diagram of an auto regression
filter 700, in accordance with an embodiment. As shown in FIG. 7,
the auto regression filter (e.g., component 318 of FIG. 3) can
compute a weighted average of the estimated channel values at the
N-d.sub.th OFDM symbol (i.e., the current estimation of the channel
block) 702 with the result of the channel estimation at all pilot
indexes after time interpolation for N-d-1.sub.th OFDM symbol
(i.e., the result of the value of the channel estimation block
previous to the current value of the channel estimation block) 704.
In accordance with various embodiments, the value of the weight 706
can be adjusted, where the higher the value, previous or past
channel estimates are weighted more heavily, and the lower the
value, current channel estimates are weighted more heavily. In
accordance with various embodiments, a frequency interpolation
component (e.g., component 320 of FIG. 3) can interpolate the value
of the channel at data subcarrier indices based on the estimated
channel at pilot subcarrier indices. Since capacity is increased by
increasing pilot spacing for small channel delay spreads, different
pilot spacing's may be in place depending on the guard interval as
to achieve a range of efficiency. The frequency interpolation
component can use different filter coefficients for different pilot
spacing. The output of the frequency interpolation can contain an
estimation of the channel at all subcarriers. The equalizer can
compensate the effects of the channel on the signal through
division or other techniques. The equalized data symbol enters the
forward error correction (FEC) or channel coding component to
decode the input data and control errors in data transmission over
unreliable or noisy communication channels. In various embodiments,
the frequency interpolation component contains a spline filter.
Different filter coefficients can be used when different pilot
patterns (different d, pilot pattern repetition period in time.
[0035] FIG. 8 illustrates an example process 800 for pilot assisted
channel estimation in accordance with various embodiments. It
should be understood that, for any process discussed herein, there
can be additional, fewer, or alternative steps performed in similar
or alternative orders, or in parallel, within the scope of the
various embodiments unless otherwise stated. An Orthogonal
Frequency Division Multiplexing (OFDM) transmission that includes a
plurality of OFDM symbols is received 802, each one of the
plurality of OFDM symbols including data subcarriers and pilot
subcarriers. A channel estimation block including a plurality of
components (e.g., multiple FFT output memories, a pilot sign
correction component, an update channel estimation component, a
temporal interpolation component, a time filtering component, a
frequency interpolation component, and an equalizer) can buffer the
OFDM symbols up to a pre-defined number (e.g. d) after the Fast
Fourier Transform was applied. A channel value at pilot subcarrier
indexes for a current OFDM symbol of the plurality of OFDM symbols
can be determined 804. The channel value at pilot subcarrier
indexes for a first grouping of OFDM symbols of the plurality of
OFDM symbols and a second grouping of OFDM symbols of the plurality
of OFDM symbols can be updated 806 based at least in part on the
channel value at the pilot subcarrier indexes of the current OFDM
symbol (i.e., the d.sub.th received OFDM symbol). In accordance
with various embodiments, the first grouping of OFDM symbols can
correspond to a predetermined number of OFDM symbols received prior
to the current OFDM symbol and the second grouping of OFDM symbols
can correspond to OFDM symbols received after the reference OFDM
symbol.
[0036] The channel value can be determined 808 at pilot subcarrier
indexes for a reference OFDM symbol (i.e., the n-d.sub.th received
OFDM symbol) by computing a weighted average of the channel value
of the pilot subcarrier indexes for the first grouping of OFDM
symbols and the channel value of the pilot subcarrier indexes for
the second grouping of OFDM symbols. In various embodiments, the
reference OFDM symbol is received prior to the current OFDM symbol,
the reference OFDM symbol being received an amount corresponding to
the predetermined number of OFDM symbols. In various embodiments,
the predetermined number of OFDM symbols (i.e., "d") can correspond
to a repetition period. As described, the number of memory
components can depend on a repetition period of the pilot
subcarrier patterns of the OFDM transmission. In various
embodiments, the repetition period can be set by a transmitter of
the OFDM transmission. For example, the transmitter can determine
which one of a plurality of pilot patterns is suitable for a
specific transmission and can store information indicated of the
pilot pattern in a header portion of the OFDM transmission.
[0037] The channel value for the pilot subcarrier indexes of the
reference symbol is time filtered 810. Using at least one frequency
interpolation function, the channel value at data subcarrier
indexes for the reference symbol can be estimated 812 based at
least in part on the channel value for the pilot subcarrier indexes
of the reference symbol. Thereafter, at least a portion of the data
subcarriers in the reference OFDM symbol is divided 814 by the
channel value of the data subcarrier indexes to equalize 816 the
OFDM transmission and reduce a portion of channel distortion.
[0038] Various embodiments discussed or suggested herein may be
conveniently implemented using a conventional general purpose or a
specialized digital computer or microprocessor programmed according
to the teachings of the present disclosure. These devices also can
include other electronic devices, such as dummy terminals,
thin-clients, gaming systems, and other devices capable of
communicating via a network. Appropriate software coding can
readily be prepared by skilled programmers based on the teachings
of the present disclosure, as will be apparent to those skilled in
the software art.
[0039] Such devices also can include a computer-readable storage
media having instructions stored thereon/in which can be used to
program a computer to perform any of the processes of the present
invention. The storage medium can include, but is not limited to,
any type of disk including floppy disks, optical discs, DVD,
CD-ROMs, microdrive, and magneto-optical disks, ROMs, RAMs, EPROMs,
EEPROMs, DRAMs, VRAMs, flash memory devices, magnetic or optical
cards, nanosystems (including molecular memory ICs), or any type of
media or device suitable for storing instructions and/or data. The
computer-readable storage media can be connected with, or
configured to receive, a computer-readable storage medium,
representing remote, local, fixed, and/or removable storage devices
as well as storage media for temporarily and/or more permanently
containing, storing, transmitting, and retrieving computer-readable
information. The system and various devices also typically will
include a number of software applications, modules, services, or
other elements located within at least one working memory device.
It should be appreciated that alternate embodiments may have
numerous variations from that described above. For example,
customized hardware might also be used and/or particular elements
might be implemented in hardware, software (including portable
software, such as applets), or both. Further, connection to other
computing devices such as network input/output devices may be
employed. Based on the disclosure and teachings provided herein, a
person of ordinary skill in the art will appreciate other ways
and/or methods to implement the various embodiments.
[0040] The foregoing description of embodiments of the present
invention has been provided for the purposes of illustration and
description. It is not intended to be exhaustive or to limit the
invention to the precise forms disclosed. Many modifications and
variations will be apparent to one of ordinary skill in the
relevant arts. For example, steps preformed in the embodiments of
the invention disclosed can be performed in alternate orders,
certain steps can be omitted, and additional steps can be added.
The embodiments were chosen and described in order to best explain
the principles of the invention and its practical application,
thereby enabling others skilled in the art to understand the
invention for various embodiments and with various modifications
that are suited to the particular used contemplated. It is intended
that the scope of the invention be defined by the claims and their
equivalents.
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