Title of Invention

METHOD AND APPARATUS FOR FAST FOURIER TRANSFORMER WINDOW POSITION RECOVERY IN AN OFDM RECEIVER

Abstract This invention relates to a method of Fast Fourier Transform (FFT) window position recovery in an Orthogonal Frequency Division Multiplexing (OFDM) receiver to reduce errors in the user data portion of a received OFDM symbol, the method comprising : processing a received OFDM singla to produce a Fast Fourier Transformed and equalized OFDM signal; extracting a ; a pilot from the Fast Fourier Transformed and equalized OFDM signal; processing the extracted pilot to derive an FFT window adjustment factor (65); and an associated equalizer tap adjustment value (67); and utilizing the adjustment values (65,67) in the preprocessing step.
Full Text The present invention relates to processing orthogonal frequency division
multiplexed (OFDM) signals.
A wireless LAN (WLAN) is a flexible data communications system
implemented as an extension to, or as an alternative for, a wired LAN within a
building or campus. Using electromagnetic waves, WLANs transmit and receive
data over the air, minimizing the need for wired connections. Thus, WLANs
combine data connectivity with user mobility, and, through simplified
configuration, enable movable LANs. Some industries that have benefited from
the productivity gains of using portable terminals (e.g., notebook computers) to
transmit and receive real-time information are the digital home networking,
health-care, retail, manufacturing, and warehousing industries.
Manufacturers of WLANs have a range of transmission technologies to
choose from when designing a WLAN. Some exemplary technologies are
multicarrier systems, spread spectrum systems, narrowband systems, and
infrared systems. Although each system has its own benefits and detriments,
one particular type of multicarrier transmission system, orthogonal frequency
division multiplexing (OFDM), has proven to be exceptionally useful for WLAN
communications.
OFDM is a robust technique for efficiently transmitting data over a
channel. The technique uses a plurality of sub-carrier frequencies (sub-carriers)
within a channel bandwidth to transmit data. These sub-carriers are arranged for
optimal bandwidth efficiency compared to conventional frequency division
multiplexing (FDM) which can waste portions of the channel bandwidth in order
to separate and isolate the sub-carrier frequency spectra and thereby avoid inter-
carrier interference (ICI). By contrast, although the frequency spectra of OFDM
sub-carriers overlap significantly within the OFDM channel bandwidth, OFDM
nonetheless allows resolution and recovery of the information that has been
modulated onto each sub-carrier.
The transmission of data through a channel via OFDM signals also
provides several other advantages over more conventional transmission
techniques. Some of these advantages are a tolerance to multipath delay spread
and frequency selective fading, efficient spectrum usage simplified sub-channel
equalization, and good interference properties.
Referring now to FIG. 1, an OFDM signal 10 is transmitted as blocks of
user data 12 separated by guard intervals known as cyclic prefixes 14. A cyclic
prefix 14 is a copy of a portion of an adjacent block of user data 12 and is used
to reduce Inter-Symbol Interference {ISO caused by multipath fading. More
particularly, only cyclic prefixes 14, as opposed to user data 12, are effected by
ISI, as is known by those skilled in the art. Thus the removal of cyclic prefixes
14 by an OFDM receiver removes the effects of ISI from the received OFDM
signal.
At the OFDM receiver a received OFDM signal 10 is digitized or sampled
to convert the OFDM signal from an analog to a digital signal. Afterwards, the
OFDM receiver applies Fast Fourier Transform (FFT) windows to the OFDM
signal to remove the cyclic prefixes from a received OFDM signal. Ideally, an
OFDM window 16 only passes user data 12 to an FFT unit 18 and discards
cyclic prefixes 14. However, if there is a sampling frequency offset between the
OFDM transmitter and the OFDM receiver, FFT window 16 may drift beyond the
boundaries of user data 12. If this drift occurs, as shown in FIG. 2, a portion or
sample 20 of cyclic prefix 14 may be passed to FFT unit 18 and a portion or
sample 22 of user data 12 may be lost. As a result, the window drifting effect
may result in the presence of ISI in a received OFDM signal. Furthermore, an
offset of FFT window 16 will result in a phase rotation in the output of FFT unit
18. The rotation occurs because a time shift in the time domain results in a
phase rotation in the frequency domain. The phase rotation may generate errors
in the user data recovered by the OFDM receiver.
One way to correct for the drifting effect is to lock the frequency of the
receiver"s sampler or ADC to the transmitter sampling frequency using a phase-
locked loop. Turning to FIG. 3, an exemplary phase-locked loop configuration 24
includes an ADC 26 that samples a received OFDM signal. An FFT window unit
28 receives the OFDM samples, removes cyclic prefixes, and passes user data to
a FFT unit 30, as discussed above. A pilot extractor 32 extracts pilots imbedded
in the user data and passes the pilots to a phase difference calculator 32. A
pilot is a reference signal (having a known phase) that is embedded in an OFDM
symbol on a predetermined subcarrier. Phase difference calculator 32 calculates
the phase difference between the pilots within the OFDM symbols and passes
the calculated difference to a sampling offset detector 36. Sampling offset
detector 36 detects a sampling offset between the transmitter and receiver using
the calculated difference and outputs the sampling offset to a digital phase-
locked loop 38. Digital phase-locked loop 38 controls the sampling clocks of
ADC 26 and ensures consistent FFT window positioning throughout the
reception of the transmission once digital phase-locked loop 38 has locked.
Although PLL configuration 24 ensures consistent FFT window positioning
once digital phase-locked loop 38 has locked, PLL configuration 24 has several
drawbacks. One drawback is that PLL configuration 24 may not correctly
position the FFT window due to noise and channel effects. The incorrect
positioning (i.e., window offset) may cause a phase rotation in the output of FFT
unit 30 that, in turn, may cause errors in the user data recovered by the OFDM
receiver. Another drawback is that digital phase-locked loop 38 of PLL
configuration 24 is costly to implement.
If the local sampling clock of the OFDM receiver has a small offset with
respect to the transmitter sampling frequency it may be advantageous (e.g., to
reduce costs) to remove the digital phase-locked loop and utilize a free-running
local clock. However, by utilizing a free-running clock without a phase-locked
loop, a small sampling offset, over time, can accumulate and shift the FFT
window beyond the user data boundaries. As noted above, the FFT window
shift may introduce errors, such as ISI, into the user data portion of a received
OFDM symbol. The present invention is directed to the correction of this
problem.
An Orthogonal Frequency Division Multiplexing (OFDM) receiver that
extracts pilots from a fast Fourier transformed and equalized OFDM signal, and
processes the extracted pilots to derive an FFT window adjustment factor and an
associated equalizer tap adjustment value. The OFDM receiver simultaneously
controls the position of an FFT window and the phase of equalizer taps using the
FFT adjustment factor and equalizer tap adjustment value.
BRIEF DESCRIPTION OF THE ACCOMOANYING DRAWINGS
The aforementioned advantages of the invention, as well as additional
advantages thereof, will be more fully understood as a result of a detailed
description of the preferred embodiment when taken in conjunction with the
accompanying drawings in which:
FIG. 1 is a diagram of an OFDM signal having user data and cyclic prefix
portions, and associated processors;
FIG. 2 is diagram illustrating the presence of FFT window drift;
FIG. 3 is a block diagram of a phase lock loop configuration for a
conventional OFDM receiver;
FIG. 4 is a diagram illustrating the placement of a training sequence, user
data, and pilot signals within an OFDM symbol frame according to the present
invention;
FIG. 5 is a block diagram illustrating a window shift correction
arrangement for an OFDM receiver according to the present invention: and
FIG. 6 is a flowchart illustrating an FFT window correction algorithm of
the present invention.
The characteristics and advantages of the present invention will become
more apparent from the following description, given by way of example.
Turning to FIG. 4, an exemplary OFDM symbol frame 40 of the present
invention is shown. Symbol frame 40 includes a training sequence 44
containing known transmission values for each subcarrier in the OFDM carrier,
and a predetermined number of cyclic prefix 42 and user data 46 pairs. User
data 46 has a predetermined number of pilots 48, also containing known
transmission values, embedded on predetermined subcarriers. For example, the
proposed ETSI-BRAN HIPERLAN/2 (Europe) and IEEE 802.11a (USA) wireless
LAN standards, herein incorporated by reference, have four pilots located at bins
or subcarriers ±7 and ±21.
Referring now to FIG. 5, an FFT window synchronization network or
system 50 of the present invention is shown. It should be noted that system 50
may be embodied in software, hardware, or some combination thereof. For
example, system 50 may be part of an WLAN adapter that is implemented as a
PC card for a notebook or palmtop computer, as a card in a desktop computer,
or integrated within a hand-held computer. System 50 is coupled to a source 52
of OFDM time-domain samples (e.g., the output of an ADC driven by a free
running clock that is not controlled by a PLL) that has a small sampling frequency
offset with respect to the sampling frequency of an OFDM transmitter. As noted
above, such an offset could cause an FFT window drift which, in turn, may
result in a phase rotation in the output of an FFT unit and ISI. System 50
includes a coarse FFT window synch unit 54 coupled to source 52 and an FFT
unit 56. Coarse FFT window synch unit 54 obtains an initial estimate of the FFT
window position and triggers FFT unit 56 when the samples from source 52 fall
within the estimated window position. Coarse window synch unit 54 may use
known window synch techniques such as detection of cross-correlation peaks or
autocorrelation peaks of a known training sequence (e.g., training sequence 44
of FIG. 4). Coarse window synch unit 54 obtains an approximate (within several
samples of the correct window position) initial estimate of the window position.
Afterwards, the window position is finely adjusted, as described in further detail
below.
An equalizer 58 is coupled to the output of FFT unit 56. Equalizer 58
reduces the multi-path distortion effects of the channel that the OFDM signal is
transmitted through. Equalizer 58 is initialized using a training sequence (e.g.,
training sequence 44 of FIG. 4) stored in a memory 60 to set the equalizer tap
settings. As discussed above, the training sequence contains known
transmission values on all of the subcarriers of the OFDM carrier. A conventional
technique for computing an initial tap value for each subcarrier is to set the tap
for the subcarrier equal to the known transmission value of the subcarrier (as
stored in memory 60) divided by the output on the subcarrier received from FFT
unit 56. The initialization of equalizer 58 not only reduces the effect of the
channel but also cancels out a phase rotation generated by an incorrect FFT
window position. However, according to a feature of the present invention, the
initialization only cancels out the phase rotation of the pilot subcarriers at the
time of initialization, and equalizer 58 does not track the continuous phase
rotations of the pilot subcarriers caused by a drifting window position.
After the taps of equalizer 58 are initially set, equalizer 58 adapts the
equalizer taps for the data subcarriers but does not adapt the taps for the pilot
subcarriers (e.g., pilots 48 of FIG. 4). Equalizer 58 does not adapt the pilot taps
so the phase rotation generated by incorrect FFT window position is passed on
the pilot subcarriers to a pilot extraction unit 62 and a fine FFT window synch
unit 64, as discussed in further detail below.
Pilot extraction unit 62 is coupled to an output of equalizer 58 and an
input of fine FFT window synch unit 64. Pilot extraction unit 62 extracts pilots
(e.g., pilots 48 of FIG. 4) embedded in the user data (e.g., user data 46 of FIG.
4) sent to downstream processing 66 (e.g., demodulation, decoding, and the
like) and passes the pilots to a fine FFT window synch unit 64. Fine FFT
window synch unit 64 is coupled to an input of FFT unit 56 to finely adjust FFT
window position. Fine FFT window synch unit 64 is also coupled to an input of
equalizer 58 to adjust the phase rotation of the equalizer taps. More specifically,
fine FFT window synch unit 64 executes an algorithm that provides two outputs.
The first output is a fine window adjustment factor 65 that is passed to FFT unit
56 to shift an FFT window in one sample increments. The second output is a
phase compensation value 67 that is passed to equalizer 58 in order to rotate
the equalizer taps in an equal but opposite direction from the phase rotation that
is induced by the fine adjustment of the FFT window. The rotation of the
equalizer taps is necessary to avoid a discrete phase jump that could disturb the
tracking ability of equalizer 58.
Referring now to FIG. 6, a flowchart 70 illustrating the algorithm of the
present invention is shown. Initially, at step 72, fine FFT window synch unit 64
acquires the phase of a pilot embedded in the user data. Next, at step 74, fine
FFT window synch unit 64 compares the absolute value of the acquired phase to
the absolute value of O. O is defined as:
Wherein k is the subcarrier or bin location of the pilot (e.g., ±7 or ±21). Ts is
the phase shift of the lowest positive frequency subcarrier that results from an
FFT window offset of 1 sample (e.g., the phase shift that would occur at
subcarrier or bin location +1). Thus, for k-th carrier, the corresponding phase
shift is kTs (e.g., at the 7th subcarrier the phase shift is 7Ts). ? is a safety margin
or buffer value that is added to kTs in order to prevent a false window
adjustment due to noise. The sgn( ) denotes a signum function that generates a
+ 1 or a -1 depending on the sign of the subcarrier location (e.g., at the +7 bin
location the sgn( ) function generates a +1 and at the -7 bin location the sgn( )
function generates a -1).
If the absolute value of the phase does not exceed the absolute value of
O, the fine FFT window synch unit 64 resets or zeroes the positive phase shift
(M+) and negative phase shift (M-) counters and, returning to step 72, acquires
the phase of the next pilot. It should be noted that there are positive and
negative phase shift counters (M+ and M-) for each pilot subcarrier. For example,
if pilots are located at the ±7 and ±21 bins, there will be eight counters
allocated for tracking positive and negative phase shifts on the ±7 and ±21
bins.
If the absolute value of the phase exceeds the absolute value of O, the
fine FFT window synch unit 64, at step 78, determines if the phase shift is
positive. If the phase shift is positive, the fine FFT window synch unit 64, at
step 82, increments the positive phase shift counter (M+) associated with the
pilot by 1. If the phase shift is not positive, fine FFT window synch unit 64, at
step 80, increments the negative phase shift counter (M-) associated with the
pilot by 1.
After incrementing a counter (either M+ or M-) for a given pilot, fine FFT
window synch unit 64, at step 84, determines if a majority of the counters (M +
or M-) for all the pilots have reached a threshold or predetermined value. For
example, if the pilots are located at ±7 and ±21, fine FFT window synch unit
64 acquires the count of the eight counters (four M+ counters and four M-
counters). Afterwards, fine FFT window synch unit 64 determines if a majority
of the M+ or M- counters have reached a predetermined value (e.g., a value of
5). If the predetermined value has not been reached, the fine FFT window synch
unit 64 returns to step 72 and acquires the phase of the next pilot. When the
predetermined value is reached the FFT window is adjusted by at least 1 sample.
The direction of adjustment is selected based upon which counters (either M+ or
M-) have reached the predetermined value. Thus, if the predetermined value has
been reached, fine FFT window synch unit 64, at step 86, adjust the window
position of FFT 56 and the phase of the equalizer taps of equalizer 58, as
discussed in further detail below. It should be noted that the threshold or
predetermined value is used to reduce the effects of noise on the detection of a
window offset. For example, an increase in noise may cause the absolute value
of a detected pilot phase to exceed O. once. However, only a phase rotation
induced by a window shift would cause multiple successive occurrences of a
pilot phase exceeding O.
Adjusting the FFT window position by a sample creates a discontinuous
jump in phase for the frequency-domain data. To avoid the discontinuous jump
in phase, the phase of each equalizer data tap is adjusted by kTs radians where
the direction of the rotation adjustment is opposite the direction of the rotation
that would have otherwise been induced by the FFT window shift. However, it
should be noted that the phases of the equalizer pilot taps are not adjusted so
fine FFT window synch unit 64 can track the phase changes of the pilot
subcarriers caused by FFT window drift.
Thus according to the principle of the present invention, an OFDM receiver
extracts pilots from a Fast Fourier Transformed and equalized OFDM signal, and
processes the extracted pilots to derive an FFT window adjustment factor and an
associated equalizer tap adjustment value. The OFDM receiver simultaneously
controls the position of an FFT window and the phase of equalizer taps using the
FFT adjustment factor and equalizer tap adjustment value.
While the present invention has been described with reference to the
preferred embodiments, it is apparent that that various changes may be made in
the embodiments without departing from the spirit and the scope of the
invention, as defined by the appended claims.
WE CLAIM
1. A method of Fast Fourier Transform (FFT) window position recovery in an
Orthogonal Frequency Division Multiplexing (OFDM) receiver to reduce
errors in the user data portion of a received OFDM symbol, the method
comprising the steps of:
processing a received OFDM signal to produce a Fast Fourier Transformed
and equalized OFDM signal;
extracting a plot from the Fast Fourier Transformed and equalized OFDM
signal;
processing the extracted plot to derive an FFT window adjustment factor
(65) and an associated equalizer tap adjustment value (67); and
utilizing the adjustment values (65,67) In the preprocessing step.
2. The method as claimed in claim 1, wherein the OFDM receiver is
implemented in a wireless LAN adapter.
3. The method as claimed in claim 1, wheren the OFDM receiver is
integrated within one of a portable or desktop computer.
4. The method as claimed in claim 1, wherein the step of utilizing the
adjustment values in the preprocessing step comprises simultaneously
controlling a position of an FFT window and a phase of an equalizer tap
using the FFT window adjustment value and the associated equator top
adjustment value.
5. The method as claimed in claim 1, wherein the window adjustment value
represents a window drift correction.
6. The method as claimed in claim 5, wherein the equafeer top adjustment
value represents a phase correction that negates the effect of the window
drift correction on an equateer tap.
7. The method as claimed in claim 6, wherein the equalizer tap is an
equalizer data tap.
8. The method as claimed in claim 1, wherein the step of processing
comprises:
comparing a phase of the extracted pilot to a predetermined value;
incrementing a counter if the phase exceeds the predetermined
value; and
generating the FFT window adjustment value and the associated
equalizer tap adjustment value if the counter exceeds a threshold
value.
9. The method as claimed in claim 8, wherein the predetermined value
represents a phase rotation due to an FFT window offset.
10.The method as claimed in claim 9, wherein the predetermined value
comprises a noise buffer value.
11.The method as claimed in claim 8, wherein the threshold vaiue is selected
to reduce the effects of noise-on FFT window position recovery.
12.An apparatus for Fast Fourier Transformer (FFT) window position recovery
in an Orthogonal Frequency Division Multiplexing (OFDM) receiver, the
apparatus comprising:
an FFT window module (54) for receiving an OFDM signal and
removing a cyclic prefix from the OFDM signal;
an FFT module (56) for Fast Fourier Transforming the windowed
OFDM signal;
an equalizer module (58) for removing a channel distortion from
the transformed OFDM signal; and
a window adjustment module (62) for extracting a pilot from the
transformed OFDM signal and controlling the operation of the FFT
window module (54) and the equalizer module (58) in response to
a phase of the extracted pilot.
13.The apparatus as claimed in claim 12, wherein the window adjustment
module (62) compares the phase of the extracted pilot with a
predetermined value, and alters the operation of the FFT window module
(54) and the equalizer module (58) if the phase of the extracted pilot
exceeds the predetermined value a predetermined number of times.
14.The apparatus as claimed in claim 13, wherein the predetermined value
represents a phase rotation due to an FFT window offset.
15.The apparatus as claimed in claim 14, wherein the predetermined value
comprises a noise buffer value.
16.The apparatus as claimed in claim 14, wherein the FFT window offset is
an offset of one sample.
17.The apparatus as claimed in claim 12, wherein the window adjustment
module (62) simultaneously controls a window position of the FFT window
module (54) and a phase of an equalizer data tap of the equalizer module.
18.The apparatus as claimed in claim 17, wherein the phase of the equalizer
data tap is adjusted to cancel out a phase rotation induced by a shifting of
the window position.
19.The apparatus as claimed in claim 12, wherein the OFDM receiver is
implemented in a wireless LAN adapter.
20.The apparatus as claimed in claim 12, wherein the OFDM receiver is
integrated within one of a portable or desktop computer.
This invention relates to a method of Fast Fourier Transform (FFT) window
position recovery in an Orthogonal Frequency Division Multiplexing (OFDM)
receiver to reduce errors in the user data portion of a received OFDM symbol,
the method comprising: processing a received OFDM signal to produce a Fast
Fourier Transformed and equalized OFDM signal; extracting a pilot from the Fast
Fourier Transformed and equalized OFDM signal; processing the extracted pilot
to derive an FFT window adjustment factor (65) and an associated equator tap
adjustment value (67); and utilizing the adjustment values (65,67) In the
preprocessing step.

Documents:

85-cal-2001-granted-abstract.pdf

85-cal-2001-granted-claims.pdf

85-cal-2001-granted-correspondence.pdf

85-cal-2001-granted-description (complete).pdf

85-cal-2001-granted-drawings.pdf

85-cal-2001-granted-examination report.pdf

85-cal-2001-granted-form 1.pdf

85-cal-2001-granted-form 18.pdf

85-cal-2001-granted-form 2.pdf

85-cal-2001-granted-form 3.pdf

85-cal-2001-granted-form 5.pdf

85-cal-2001-granted-gpa.pdf

85-cal-2001-granted-letter patent.pdf

85-cal-2001-granted-reply to examination report.pdf

85-cal-2001-granted-specification.pdf

85-cal-2001-granted-translated copy of priority document.pdf


Patent Number 214649
Indian Patent Application Number 85/CAL/2001
PG Journal Number 07/2008
Publication Date 15-Feb-2008
Grant Date 13-Feb-2008
Date of Filing 14-Feb-2001
Name of Patentee THOMSON LICENSING, S.A.
Applicant Address 46, QUAI A. LE GALLO, 92648 BOULOGNE CEDEX
Inventors:
# Inventor's Name Inventor's Address
1 LOUIS ROBERT LITWIN, JR. 126 PINEVIEW DRIVE #8, CARMEL, INDIANA 46032
2 MAXIM B. BELOTSERKOVSKY 9108 BRYANT LANE 3A, INDIANAPOLIS, INDIANA 46250
PCT International Classification Number H 04 L 27/26
PCT International Application Number N/A
PCT International Filing date
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 09/511,185 2000-02-22 U.S.A.