U.S. patent application number 11/049004 was filed with the patent office on 2005-07-07 for synchronization in ofdm systems.
Invention is credited to Jones, Vincent K. IV, Pollack, Michael, Raleigh, Gregory G..
Application Number | 20050147026 11/049004 |
Document ID | / |
Family ID | 34380577 |
Filed Date | 2005-07-07 |
United States Patent
Application |
20050147026 |
Kind Code |
A1 |
Jones, Vincent K. IV ; et
al. |
July 7, 2005 |
Synchronization in OFDM systems
Abstract
Highly effective systems and methods for synchronizing OFDM
receiver parameters to an OFDM transmitter are provided. These
parameters may include carrier frequency, burst timing, and cyclic
prefix length. These systems and methods incorporate special
structural features into the OFDM signal to facilitate
synchronization. In one embodiment, a supplemental cyclic prefix is
added to an OFDM signal to facilitate synchronization. In an
alternative embodiment, a synchronization burst with a periodic
structure is used to facilitate synchronization. According to the
present invention, synchronization may be maintained even if low
cost analog oscillator components are used.
Inventors: |
Jones, Vincent K. IV;
(Redwood Shores, CA) ; Pollack, Michael;
(Cupertino, CA) ; Raleigh, Gregory G.; (El
Granada, CA) |
Correspondence
Address: |
DOV ROSENFELD
5507 COLLEGE AVE
SUITE 2
OAKLAND
CA
94618
|
Family ID: |
34380577 |
Appl. No.: |
11/049004 |
Filed: |
January 31, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11049004 |
Jan 31, 2005 |
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09245168 |
Feb 5, 1999 |
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6876675 |
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60074331 |
Feb 6, 1998 |
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Current U.S.
Class: |
370/208 |
Current CPC
Class: |
H04L 27/2662 20130101;
H04L 27/266 20130101; H04L 27/2659 20130101; H04L 27/2678 20130101;
H04L 27/2675 20130101; H04L 27/2666 20130101 |
Class at
Publication: |
370/208 |
International
Class: |
H04J 011/00 |
Claims
1. (canceled)
2. A method for transmitting an OFDM signal to facilitate receiver
synchronization comprising: forming a synchronization burst from a
set of frequency domain symbols, the set of frequency domain
symbols including periodically spaced frequency domain symbols that
can take on respective non-zero values and that are spaced at least
four frequency domain symbols apart, and only zero energy frequency
domain symbols between the periodically spaced frequency domain
symbols, such that all the frequency domain symbols of the
synchronization burst that can take on non-zero values are
periodically spaced in the frequency domain; and transmitting the
synchronization burst.
3. A method for synchronizing a receiver to an OFDM signal
comprising: receiving at least one synchronization OFDM burst, the
synchronization OFDM burst formed of a set of frequency domain
symbols, the set of frequency domain symbols including periodically
spaced frequency domain symbols that can take on respective
non-zero values and that are spaced at least two frequency domain
symbols apart, and only zero energy frequency domain symbols
between the periodically spaced frequency domain symbols, such that
all the frequency domain symbols of the synchronization burst that
can take on non-zero values are periodically spaced in the
frequency domain; evaluating a cost function that varies depending
on burst timing alignment, the cost function measuring time domain
periodicity of the synchronization OFDM burst; and setting the
burst timing alignment to optimize the cost function.
4. A method as recited in claim 3, further comprising: computing a
receiver frequency offset based on the burst timing alignment that
is set to optimize the cost function in the step of setting.
5. A method for synchronizing a receiver to an OFDM signal
comprising: receiving at least one OFDM synchronization burst, the
synchronization OFDM burst formed of a set of frequency domain
symbols, the set of frequency domain symbols including periodically
spaced frequency domain symbols that can take on respective
non-zero values and that are spaced at least two frequency domain
symbols apart, and only zero energy frequency domain symbols
between the periodically spaced frequency domain symbols, such that
all the frequency domain symbols of the synchronization burst that
can take on non-zero values are periodically spaced in the
frequency domain; and determining burst timing alignment and
frequency offset responsive to optimization of at least one cost
function determined in response to content of the at least one OFDM
synchronization burst.
6. A system for transmitting an OFDM signal to facilitate receiver
synchronization comprising: a synchronization burst generation
stage to generate a frequency domain synchronization burst formed
of a set of frequency domain symbols including periodically spaced
frequency domain symbols that can take on respective non-zero
values and that are spaced at least four frequency domain symbols
apart, and only zero energy frequency domain symbols between the
periodically spaced frequency domain symbols, such that all the
frequency domain symbols of the synchronization burst that can take
on non-zero values are periodically spaced in the frequency domain;
and a transform processing stage that transforms the frequency
domain synchronization burst into a time domain synchronization
burst.
7. A system as recited in claim 6, wherein at least one of the
periodically spaced frequency domain symbols carries data.
8. A system for processing an OFDM signal comprising: an OFDM
receiver system that receives at least one synchronization OFDM
burst, the synchronization OFDM burst formed of a set of frequency
domain symbols, the set of frequency domain symbols including
periodically spaced frequency domain symbols that can take on
respective non-zero values and that are spaced at least two
frequency domain symbols apart, and only zero energy frequency
domain symbols between the periodically spaced frequency domain
symbols, such that all the frequency domain symbols of the
synchronization burst that can take on non-zero values are
periodically spaced in the frequency domain; and a synchronization
block that evaluates a cost function that varies depending on burst
timing alignment, the cost function measuring time domain
periodicity of the synchronization OFDM burst, wherein the
synchronization block is to set the burst timing alignment to
optimize the cost function.
9. A system as recited in claim 8, wherein the synchronization
system is further to compute a receiver frequency offset based on
the burst timng alignment that is set by the synchronization system
to optimize the cost function.
10. A system for processing an OFDM signal comprising: an OFDM
receiver system to receive at least one OFDM synchronization burst,
the synchronization OFDM burst formed of a set of frequency domain
symbols, the set of frequency domain symbols including periodically
spaced frequency domain symbols that can take on respective
non-zero values and that are spaced at least two frequency domain
symbols apart, and only zero energy frequency domain symbols
between the periodically spaced frequency domain symbols, such that
all the frequency domain symbols of the synchronization burst that
can take on non-zero values are periodically spaced in the
frequency domain; and a synchronization block that determines burst
timing alignment and frequency offset burst timing alignment and
frequency offset responsive to optimization of at least one cost
function determined in response to content of the at least one OFDM
synchronization burst.
11. A system as recited in claim 10, wherein at least one of the
periodically spaced frequency domain symbols carries data.
12. An apparatus for transmitting an OFDM signal to facilitate
receiver synchronization, the apparatus comprising: means for
forming a synchronization burst from a set of frequency domain
symbols, the set of frequency domain symbols including periodically
spaced frequency domain symbols that can take on respective
non-zero values and that ae spaced at least four frequency domain
symbols apart, and only zero energy frequency domain symbols
between the periodically spaced frequency domain symbols, such that
all the frequency domain symbols of the synchronization burst that
can take on non-zero values are periodically spaced in the
frequency domain; and means for transmitting the synchronization
burst.
13. An apparatus for synchronizing a receiver to an OFDM signal,
the apparatus comprising; means for receiving at least one
synchronization OFDM burst, the synchronization OFDM burst formed
of a set of frequency domain symbols, the set of frequency domain
symbols including periodically spaced frequency domain symbols that
can take on respective non-zero values and that are spaced at least
two frequency domain symbols apart, and only zero energy frequency
domain symbols between the perodically spaced frequency domain
symbols, such that all the frequency domain symbols of the
synchronization burst that can take on non-zero values are
periodically spaced in the frequency domain; means for evaluating a
cost function that varies depending on burst timing alignment, the
cost function measuring time domain periodicity of the
synchronization OFDM burst; and means for setting the burst timing
alignment to optimize the cost function.
Description
STATEMENT OF RELATED APPLICATIONS
[0001] The present application is a continuation of U.S. patent
application Ser. No. 09/245,168 titled SYNCHRONIZATION IN OFDM
SYSTEMS, filed Feb. 5, 1999.
[0002] U.S. patent application Ser. No. 09/245,168 and the present
invention are related to the subject matter of U.S. application
Ser. No. 09/244,754 (original Attorney Docket No. CISCP625),
entitled ENHANCED SYNCHRONIZATION BURST FOR OFDM SYSTEMS, co-filed
and co-assigned with U.S. patent application Ser. No. 09/245,168,
and now U.S. Pat. No. 6,549,592.
[0003] U.S. patent application Ser. No. 09/245,168 and the present
application are related to and claim the benefit of U.S.
Provisional Application No. 60/074,331, filed Feb. 6, 1998.
[0004] The contents of these three related application are herein
incorporated by reference for all purposes.
BACKGROUND OF THE INVENTION
[0005] The present invention relates to digital communication and
more particularly to synchronization of certain parameters between
a receiver and a transmitter.
[0006] In an OFDM (orthogonal frequency division multiplexing)
communication system, data is communicated in a series of time
domain bursts. To form each time domain burst, an IFFT is applied
to a group of frequency domain symbols and a cyclic prefix is added
to the transform result prior to transmission. Transmission may
involve conversion to an analog signal, conversion to an
intermediate frequency (IF), then upconversion to a desired
selectable carrier frequency prior to final amplification and
propagation across a transmission medium. Upconversion is typically
achieved by mixing the IF signal with a variable frequency
oscillator signal. The carrier frequency is varied by varying the
oscillator frequency.
[0007] On the receiver end, preamplification is followed by
downconversion to IF from the carrier frequency, again by mixing
with a variable frequency oscillator. The resulting IF signal is
typically converted to a baseband digital symbol sequence. The
cyclic prefix is removed and an FFT is applied to recover the
original frequency domain symbols.
[0008] For successful communications, certain parameters must be
synchronized between the transmitter and the receiver. For example,
the transmitter and receiver must have a common and precise shared
understanding of the transmission frequency. In the exemplary
system described above, this means that the variable frequency
oscillators at the transmitter and receiver should be locked to
each other. Imprecision with respect to the transmission frequency
will cause inaccurate recovery of the OFDM symbols. To maintain
system performance, it is desirable to always maintain frequency
offset between the transmitter and receiver to within 1% of the
spectral width occupied by a single frequency domain OFDM
symbol.
[0009] When the receiver initially acquires the transmitter, it is
desirable that the synchronization system tolerates and corrects a
very wide misalignment between the transmitter and receiver
oscillators. This allows the use of much lower cost analog
components.
[0010] Also since OFDM communication proceeds on a burst by burst
basis, the receiver and transmitter must agree on exactly when each
burst begins. Again, the consequence of missychronization will be
lost data.
[0011] Additionally, the length of the cyclic prefix used by the
transmitter will depend on the transmitter's understanding of the
duration of the transmission channel's impulse response. This
information may not necessarily be immediately available to the
receiver in systems where the transmitter is a hub of a point to
multipoint system. In these cases, the modulation parameters,
including the cyclic prefix, may be programmable at the hub and
each subscriber unit must "turn on" and acquire this OFDM
modulation parameter. Upon acquisition, the receiver therefore must
also determine the cyclic prefix length used by the
transmitter.
SUMMARY OF THE INVENTION
[0012] By virtue of the present invention, highly effective systems
and methods for synchronizing OFDM receiver parameters to an OFDM
transmitter are provided. These parameters may include carrier
frequency, burst timing, and cyclic prefix length. These systems
and methods incorporate special structural features into the OFDM
signal to facilitate synchronization. In one embodiment, a
supplemental cyclic prefix is added to an OFDM signal to facilitate
synchronization. In an alternative embodiment, a synchronization
burst with a periodic structure is used to facilitate
synchronization. According to the present invention,
synchronization may be maintained even if low cost analog
oscillator components are used.
[0013] A first aspect of the present invention provides a system
for transmitting an OFDM signal via a channel to facilitate
receiver synchronization. The system includes: a transforming stage
that transforms a series of frequency domain data symbols into a
burst of time domain symbols, and a cyclic prefix appending stage
that appends to a beginning of the time domain burst a cyclic
prefix duplicating a last segment of the time domain burst, wherein
the cyclic prefix includes a first portion having length .nu.
wherein .nu. is greater than or equal to an impulse response of the
channel, and further includes a second portion after the first
portion to facilitate receiver synchronization.
[0014] A second aspect of the present invention provides a system
for transmitting an OFDM signal to facilitate receiver
synchronization including: a synchronization burst generation stage
that develops a frequency domain burst wherein periodically spaced
frequency domain symbols of the burst have predetermined values and
frequency domain symbols between the periodically spaced frequency
domain symbols have null energy; and a transform processing stage
that transforms the frequency domain burst into a time domain
burst.
[0015] Other features and advantages of the invention will become
readily apparent upon review of the following detailed description
in association with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 depicts an OFDM transmitter system according to one
embodiment of the present invention.
[0017] FIG. 2 depicts an OFDM receiver system according to one
embodiment of the present invention.
[0018] FIG. 3 depicts a null tone pattern in a synchronization
burst according to one embodiment of the present invention.
[0019] FIG. 4 is a flowchart describing steps of acquiring burst
timing and frequency offset based on the synchronization burst of
FIG. 3 according to one embodiment of the present invention.
[0020] FIG. 5 is a diagram of an OFDM data burst having a
supplemental cyclic prefix appended according to one embodiment of
the present invention.
[0021] FIG. 6 is a flowchart describing steps of acquiring burst
timing and frequency offset timing based on the structure of the
OFDM data burst of FIG. 5 according to one embodiment of the
present invention.
[0022] FIG. 7 is a flowchart describing steps of tracking burst
timing and frequency offset timing based on the structure of the
OFDM data burst of FIG. 5 according to one embodiment of the
present invention.
[0023] FIG. 8 depicts a graph of a cost function for acquiring
burst timing according to one embodiment of the present
invention.
DESCRIPTION OF SPECIFIC EMBODIMENTS
[0024] OFDM System Overview
[0025] FIG. 1 depicts an OFDM transmitter system 100 according to
one embodiment of the present invention. A source 102 originates
data symbols to be transmitted in orthogonal frequency domain
subchannels. Any channel coding technique or combination of
techniques may be applied to the frequency domain data symbols. To
assist in estimating characteristics of the channel, training
symbols having known values may be interspersed with the data
symbols by a training symbol injection system 104. The output of
training symbol injection system 104 is a series of N-symbol
frequency domain bursts where N is a length of an IFFT procedure
applied by an IFFT processing block 106. IFFT processing block 106
generates time domain output bursts from frequency domain input
bursts.
[0026] According to the present invention, certain bursts may be
special synchronization bursts generated by a synchronization burst
generation system 108. As described below, these synchronization
bursts have special frequency domain characteristics to facilitate
receiver alignment to the transmitter's burst timing and carrier
frequency.
[0027] The time domain burst output by IFFT processing block 106 is
augmented with a cyclic prefix prior to transmission by a cyclic
prefix addition block 110. The cyclic prefix addition process can
be characterized by the expression:
[z(1) . . . z(N)].sup.T[z(N-.nu.+1) . . . z(N)z(1) . . .
z(N)].sup.T
[0028] The cyclic prefix has length .nu. where .nu. is greater than
or equal to a duration of the impulse response of the channel and
assures orthogonality of the frequency domain subchannels.
According to the present invention, a supplemental cyclic prefix
having length L may be added by cyclic prefix addition block 110 to
the end of the v-length cyclic prefix to facilitate receiver
synchronization to transmitter burst timing and/or transmitter
carrier frequency.
[0029] A digital filtering stage 112 rejects interpolation images
if interpolation is used and attenuates spectral regrowth due to
discontinuities at boundaries between OFDM bursts. A
digital-to-analog conversion system 114 converts the digital
baseband signal, whether complex or real, to analog. An IF
processing stage 116 converts the analog signal to an intermediate
frequency and performs further filtering. The IF signal is then
upconverted to an RF transmission frequency by the operation of a
mixer 118. The RF transmission frequency is set by a variable
frequency oscillator 120 feeding mixer 118. An RF processing system
122 includes further amplification and/or filtering prior to
transmission via a transmission antenna 124.
[0030] FIG. 2 depicts an OFDM receiver system 200 according to one
embodiment of the present invention. An RF antenna 202 receives an
RF carrier modulated by a baseband OFDM signal. An RF processing
stage 204 performs pre-amplification and filtering at the received
frequency prior to downconversion to an intermediate frequency by a
mixer 206. The carrier frequency received is determined by the
frequency of a variable frequency oscillator 208. The IF output of
mixer 206 is subject to further filtering and amplification within
an IF processing stage 210. An analog to digital conversion system
212 converts the analog IF signal to a complex baseband digital
signal. A digital filter 214 rejects any decimation images and
out-of-band interference.
[0031] The output of digital filter 214 is a series of time domain
symbol bursts. A cyclic prefix removal stage 216 determines the
boundaries between bursts and removes both the v-length cyclic
prefix and the supplemental cyclic prefix if one is used for
synchronization. The remaining symbols are converted to the
frequency domain by an FFT processing block 218. A training symbol
removal block 220 removes training symbols injected for channel
estimation purposes. The remaining data symbols are forward to a
data symbol sink 222.
[0032] A first synchronization system 224 may perform burst and/or
frequency synchronization based on the use of a supplemental cyclic
prefix. A second synchronization system 226 may perform burst
and/or frequency synchronization based on the use of special
synchronization bursts. Although both systems are depicted, a
particular implementation may use one or another or combine
functions of both to accomplish frequency and burst
synchronization.
[0033] Either of the two synchronization systems may provide input
to a frequency control block 228 which controls reception frequency
by varying the output frequency of variable frequency oscillator
208. In one embodiment, a frequency lock loop (FLL) exists whereby
receiver variable frequency oscillator 208 of a first transceiver
acts as a slave and locks to transmitter variable frequency
oscillator 118 of a second transceiver which acts as the master.
Also, the FLL may be used to lock the transmitter variable
frequency oscillator 118 of the first transceiver to the
transmitter variable frequency oscillator of the second
transceiver, allowing a single timing source to control both
transmission and reception at the second transceiver. Also, either
of the two synchronization systems may provide input to a burst
timing control system 230 which controls the determination of
inter-burst boundaries within cyclic prefix removal stage 216.
[0034] The above-described transmitter system is merely
representative. For example, selection of transmission frequency
may occur in some other way rather than the operation of
transmitter variable frequency oscillator 120 and receiver variable
frequency oscillator 208. Furthermore, the overall communication
system may be a wireline system rather than a wireless system so
some other transmission medium input and output may substitute for
the depicted antennas.
[0035] Also, the present invention may operate in the context of a
system that exploits multiple transmission antennas, multiple
reception antennas, spatial diversity, and/or spatial processing as
described in e.g., PCT Publication No. WO 98/09385, the contents of
which are herein incorporated by reference. Many of the techniques
presented herein will be presented in the context of a receiver
system employing multiple reception antennas where elements 202,
204, 206, 210, 212, 214, 216, 218, and 220 are duplicated for each
receiver antenna employed. First synchronization system 224, second
synchronization system 226, receiver variable frequency oscillator
208, frequency control block 228, and/or burst timing control
system 230 may be common to all of a set of multiple reception
antennas.
[0036] Synchronization Overview
[0037] According to the present invention, techniques are provided
for both frequency synchronization and burst synchronization. In
one embodiment, the synchronization functions operate without any
knowledge of the particular channel in which data is being
transmitted.
[0038] There are typically two synchronization modes: acquisition
and tracking. Acquisition mode refers to establishment of the link
between transmitter system 100 and receiver system 200. Both burst
timing and frequency offset are typically required in acquisition.
Furthermore, in one embodiment, the cyclic prefix parameter, .nu.
is also initially unknown at the receiver. After the burst timing
and frequency offset are acquired, they are maintained in tracking
mode. In tracking mode, correct burst timing and/or frequency
offset are periodically or continuously re-estimated.
[0039] It is desirable that there be a large frequency acquisition
range, e.g., 8 OFDM bins of frequency so that requirements for
oscillator accuracy are reduced. During tracking, however, the
offset error should be maintained to within +/-1% of an OFDM bin to
minimize errors in recovering symbols.
[0040] In the description that follows, the structure of a special
synchronization burst is described first. Then the use of a
synchronization burst to acquire burst timing and frequency offset
is described. Another synchronization technique depends on the use
of a supplemental cyclic prefix. The contents of this supplemental
cyclic prefix within a data burst will be presented followed by a
technique for using the supplemental cyclic prefix to acquire burst
timing and frequency offset. Then a technique for tracking burst
timing and frequency offset using the supplemental cyclic prefix
will be described. Finally a technique will be described for
refining the selection of optimal burst timing where the channel
impulse response duration may vary in length.
[0041] Synchronization Burst. Content and Use for Acquisition
[0042] Acquisitions may be performed through the use of
synchronization OFDM bursts. In one embodiment, one synchronization
bursts is employed in every transmission frame of 32 bursts.
[0043] FIG. 3 depicts a null tone pattern in a synchronization
burst 302 according to one embodiment of the present invention. The
pattern is depicted in the frequency domain. The synchronization
bursts include zero energy frequency domain symbols except for M
symbols, equally spaced, beginning in frequency bin n=0. The
synchronization tone spacing N/M is chosen to provide a
sufficiently large frequency offset acquisition range where N is
the length of the IFFT/FFT pair. Here N/M=4. This frequency domain
structure insures that received, time-domain samples repeat every M
samples within each OFDM burst, regardless of the channel. Note,
however, that the received data is typically not periodic within
other bursts. The non-zero symbols within the synchronization burst
typically have energy such that the synchronization burst has
average energy equal to a data burst.
[0044] Second synchronization system 226 determines burst timing
and frequency offset using the synchronization bursts. There are
two cost functions that are employed:
[0045] The frequency cost function is given by: 1 d ( ) = i M R k =
+ N - M - 1 x i ( k ) * x i ( k + M ) .
[0046] where .delta. represents a particular burst timing
alignment, i is an index to multiple receiver antennas, M.sub.R is
the number of receiver antennas, x.sub.i(k) is the time domain
received data for antenna i at time index k, N is the FFT/IFFT
length, and M is the number of frequency domain synchronization
symbols used in the burst. It should be noted that d may be found
recursively. Preferably, this cost function should be smoothed with
a forward/backward FIR filter given by: 2 d _ ( ) = k = - 1 + 1 d (
k ) .
[0047] Also, this cost function may be averaged every, e.g., 32
bursts.
[0048] The burst timing alignment cost function is given by: 3 c (
) = i M R k = + N - M - 1 x i ( k ) 2 + x i ( k + M ) 2 - 2 i M R k
= + N - M - 1 x i ( k ) * x i ( k + M )
[0049] This function can also be calculated recursively and should
preferably also be smoothed with a forward/backward FIR. The burst
timing alignment cost function may also be averaged every, e.g., 32
bursts.
[0050] FIG. 4 is a flowchart describing steps of acquiring burst
timing and frequency offset based on the synchronization burst of
FIG. 3 according to one embodiment of the present invention. The
burst timing cost function is recursively calculated for using the
time-domain received samples. Every, e.g., 32 bursts, a minimum is
found for the burst timing function. These minima identify the
beginning of a synchronization burst. The number of samples between
synchronization bursts will be equal to 32(N+.nu.), or 32(N+.nu.+L)
if a supplemental cyclic prefix is included. Since N and L are
typically fixed by system design, second synchronization system 226
can solve for .nu. at step 402. At step 404 second synchronization
system 226 then finds the optimal burst timing, .delta..sup.opt, to
be the one that gives the minimum value for the burst timing cost
function.
[0051] At step 406, the frequency offset is then found to be: 4 f
offset = 1 2 M tan - 1 Im d _ ( opt ) Re d _ ( opt )
[0052] The non-zero frequency domain symbols of the synchronization
bursts may carry training information or system configuration data.
If M>.nu. where .nu. is the impulse response duration of the
channel, .nu. of the non-zero frequency domain symbols will
preferably be used for training and the remaining frequency domain
symbols may be used for data. Training procedures are discussed in
U.S. application Ser. No. 09/234,929, (Attorney Docket No.
CISCP608) filed on Jan. 21, 1999, entitled IMPROVED OFDM CHANNEL
IDENTIFICATION, the contents of which are herein incorporated by
reference. If M<.nu., all non-zero frequency domain symbols are
used for training. The training symbols may, however, carry data in
the form of phase differences between corresponding training
symbols of successive bursts. In one embodiment, a constant phase
difference between sets of training symbols in successive bursts
may identify to the receiver the particular set of training symbols
that are now being used.
[0053] Supplemental Cyclic Prefix Structure
[0054] FIG. 5 is a diagram of an OFDM burst 500 according to one
embodiment of the present invention. OFDM burst 500, as depicted in
the time domain, includes a .nu. length cyclic prefix 502 and a
supplemental cyclic prefix 504 having length L. Together, .nu.
length cyclic prefix 502 and supplemental cyclic prefix 504
duplicate the last .nu.+L of N time domain symbols. The .nu. length
cyclic prefix 500 assures orthogonality of the frequency domain
subchannels as discussed above. Supplemental cyclic prefix 504 is
used for synchronization purposes and is positioned after the
.nu.-length cyclic prefix so as to be protected from the effects of
a time dispersive channel on the previous time domain burst. An
alternative burst structure using a supplemental cyclic prefix for
burst timing is described in the co-filed application entitled
ENHANCED SYNCHRONIZATION BURST FOR OFDM SYSTEMS.
[0055] Use of Supplemental Cyclic Prefix for Acquisition
[0056] FIG. 6 is a flowchart describing steps of acquiring burst
timing and frequency offset timing based on the structure of the
OFDM data burst of FIG. 5 according to one embodiment of the
present invention. At step 602, burst timing and the cyclic prefix
length .nu. are determined based on a burst timing cost function: 5
c ( ) = i M g k = - L + 1 x i ( k ) 2 + x i ( k + N ) 2 - 2 i M R k
= - L + 1 x i ( k ) * x i ( k + N ) .
[0057] This burst timing cost function may be computed
recursively:
cp(.delta.)=cp(.delta.-1)+p(.delta.)+p(.delta.+N)-p(.delta.-1)-p(.delta.+N-
-1)
c(.delta.)=cp(.delta.)-2.vertline.d(.delta.).vertline.
[0058] where d(.delta.) is the frequency offset function defined
below and where 6 p ( k ) = i M R x i ( k ) 2 ,
[0059] a power measurement also usable for automatic gain control
purposes.
[0060] The burst timing cost function is at a minimum where a block
of data of length L repeats after an interval N, indicating
repetition of the supplemental cyclic prefix. The cost function
will repeat every N+.nu.+L samples. Since L is known and the value
of N is known, this fact is used to determine the value of .nu..
For each possible value of the cyclic prefix .nu..sub.j, calculate
the burst-averaged timing cost function: 7 c _ j ( ) = k = 0 31 c (
+ k ( N + v j + L ) ) , [ 0 , N + v i + L - 1 ] .
[0061] Whichever cost value, {overscore (c)}.sub.j, has the
smallest minima determines the value of .nu..
[0062] The value of .delta. that corresponds to the minimum value
of {overscore (c)}.sub.j is the burst timing value,
.delta..sup.opt. Before finding .delta..sup.opt, however, the
burst-averaged timing cost function may be smoothed with
forward/backward FIR filter (.iota.=4), 8 c _ j ( ) = k = - 1 + 1 c
_ j ( k )
[0063] The frequency offset can be understood to be an integer
number of OFDM bins plus a fractional part of an OFDM bin:
f.sub.offset=f.sub.int+.DELTA.f.sub.offset
[0064] At step 604, the fractional part of the frequency offset is
determined with the frequency offset cost function: 9 d ( ) = i M R
k = - L + 1 x i ( k ) * x i ( k + N ) .
[0065] Note that d can be found recursively, 10 d ( ) = d ( - 1 ) +
i M R x i ( ) * x i ( + N ) - i M R x i ( - L ) * x i ( - L + N )
,
[0066] and is a component of the timing cost function.
[0067] As with the timing cost function, d repeats every N+.nu.+L
samples. The burst-averaged frequency offset cost must be
determined for the correct value of .nu.: 11 d _ ( ) = k = 0 31 d (
+ k ( N + v + L ) ) , [ 0 , N + v + L - 1 ] .
[0068] Then, the fractional frequency offset is given by 12 f
offset = 1 2 N tan - 1 Im d _ ( opt ) Re d _ ( opt ) .
[0069] where .delta..sup.opt is the minimizing value of the timing
cost function.
[0070] Once the .DELTA.f.sub.offset has been determined, variable
frequency oscillator 208 is driven to a new frequency. Continued
updates to frequency offset and burst timing are done with the
timing and frequency offset functions. After the FLL has converged,
OFDM bursts of data are converted to the frequency domain through
the N-point FFT.
[0071] Determination of the integer portion of frequency offset at
step 606 is based on the repetition of frequency domain training
symbols at the same positions in successive bursts. Let
X.sub.i(n,k) be the received frequency domain value at tone n,
burst k and antenna i. The integer component of the frequency
offset is determined by calculating 1+N/.nu. costs, d.sub.j.
[0072] This is done with
[0073] for all n, 13 Y ( n ) = i = 1 M R k = 1 K X i * ( n , k ) X
i ( n + k + 2 )
[0074] for 1+N/.nu. sets of frequencies, 14 d j = n J j Y ( n ) j {
- 4 , 4 }
[0075] where K may be as small as 1 and J.sub.j are a set of .nu.
frequency indices, equally spaced by N/.nu.: 15 J j = [ j j + N v
]
[0076] The values for d.sub.j are complex. The integer component of
the frequency offset is determined based on the largest d.sub.j: 16
f int = arg max j d j 2
[0077] as expressed in bin widths.
[0078] Use of Supplemental Prefix for Tracking
[0079] FIG. 7 is a flowchart describing steps of tracking burst
timing and frequency offset timing based on the structure of the
OFDM data burst of FIG. 5 according to one embodiment of the
present invention. In tracking mode, the supplemental cyclic
prefix, may be used for burst timing and frequency offset estimate
updates. In a preferred embodiment, it can be assumed that during
tracking mode the sample clock does not slide more than one sample
clock interval in a frame of successively transmitted OFDM bursts,
although this is not essential for tracking. Furthermore, it is
assumed that the frequency offset contained within plus-or-minus
1/2 OFDM bin. Hence, .function..sub.int=0 and
.function..sub.offset=.DELTA..f- unction..sub.offset. Given these
constraints, the timing and frequency cost values need only be
determined within a small interval around the current timing
estimate, .delta..epsilon..kappa.. The cost functions should at a
minimum be determined for a window that is slightly larger than the
cyclic prefix.
[0080] The tracking mode burst timing and frequency offset cost
functions are the same as the acquisition ones above that take
advantage of the cyclic prefix.
[0081] As in acquisition mode, both the timing and frequency offset
cost functions should preferably be averaged across a number of
OFDM bursts determined according the amount of noise in the system.
In one embodiment, the cost functions are averaged across 32
bursts. Hence, the average cost function over 32 burst period is
given by, where 17 c _ ( ) = k = 1 32 c ( k , ) ,
[0082] where c(k,.delta.) is the timing cost function for burst k.
In the same way, 18 d _ ( ) = k = 1 32 d ( k , ) ,
[0083] where d(k,.delta.) is the frequency offset cost function for
burst k.
[0084] As in acquisition, the timing cost function should be
forward/backward smoothed with an FIR. The minimum of {overscore
(c)}(.delta.) has a timing estimate .delta..sup.opt. This new
timing estimate may be exponentially averaged with past estimates
to produce a smoothly varying timing value to use for control of
burst timing at step 702. The frequency offset may be exponentially
averaged with past estimates to produce a smoothly varying timing
value to control frequency at step 704. The frequency offset is
again given by: 19 f offset = 1 2 N tan - 1 Im d _ ( opt ) Re d _ (
opt ) .
[0085] Refinement to Optimizing Burst Timing Cost Function Based on
Supplemental Cyclic Prefix
[0086] Where the burst timing cost function is based on the
supplemental cyclic prefix, the optimal burst timing may vary
within a range depending on the duration of the channel impulse
response. For channels having impulse responses shorter than .nu.,
there will be range of timings for which the cost function will be
at a minimum or near minimum. This is because symbols at the end of
the .nu.-length cyclic prefix will be as protected from corruption
from the previous burst as symbols with in the L-length
supplemental cyclic prefix and thus will correlate well with
symbols more than L before the end of the current burst. This range
of possible optimal burst timings need not be a problem because for
timings that determine the beginning of the N time domain symbols
too early, the symbols added at the beginning are uncorrupted and
identical to the ones lost at the end. The operation of OFDM will
be insensitive to this timing slip and thus each recovered
frequency domain symbol will remain independent of other
symbols.
[0087] It may however, be desirable to select the optimal burst
timing that would be found for a .nu.-length channel even if the
actual channel impulse response duration is shorter. For example,
in a mobile application, channel impulse response duration may vary
and even if the channel impulse response has a short duration at
one time, it may quickly lengthen making a burst timing that was
once optimal no longer acceptable. To address this problem, one
embodiment of the present invention provides a system and method
for determining the optimal burst timing that would be found for a
.nu.-length channel even if the current impulse response is
shorter.
[0088] FIG. 8 depicts a graph 800 of a cost function for burst
timing where the channel has an impulse response shorter than .nu.
and there are therefore a range of optimal burst timings. In
certain applications, it would be desirable to select the burst
timing that corresponds to a right edge 802 of the range of optimal
burst timings.
[0089] Edge 802 is defined to be the value of the cost function as
.delta. progresses from the optimal value at the beginning of the
N-interval into the N-interval. The shape of the expected cost
function is 20 Ec p ( ) = 2 l v 2 + 2 x 2 v a ( )
[0090] where a .sigma..sub..nu..sup.2 is the additive noise
variance, .sigma..sub.x.sup.2 is the variance of the noise-free
received samples, and 21 a ( ) = { 0 for < b k = 0 - b k for
b
[0091] Let 22 a = [ 0 1 3 6 k = 0 L - 1 k ] T c ( ) = [ c p ( ) c p
( + L - 1 ) ] T
[0092] To find the edge, .delta..sub.b is determined by 23 b = arg
min , x ; c ( ) - a x r; 2 2 = arg min c ( ) T c ( ) - c ( ) T a 2
a T a = arg max c ( ) T a 2 a 2 ; c ( ) r; 2
[0093] Note for the purposes of recursive implementations that:
.vertline.c(.delta.+1).vertline..sup.2=.vertline.c(.delta.).vertline..sup.-
2+c.sub.p(.delta.+L).sup.Tc.sub.p(.delta.+L)-c.sub.p(.delta.).sup.Tc.sub.p-
(.delta.)
Conclusion
[0094] While the above is a complete description of preferred
embodiments of the invention, there is alternatives, modifications,
and equivalents may be used. It should be evident that the
invention is equally applicable by making appropriate modifications
to the embodiments described above. For example, in a system that
employs both synchronization bursts and a supplemental cyclic
prefix, the receiver may utilize the synchronization bursts for the
frequency cost function and the supplemental cyclic prefix for the
burst timing cost function, or vice versa. Also, according to the
present invention, synchronization bursts may be used for tracking.
Therefore, the above description should not be taken as limiting
the scope of the invention that is defined by the metes and bounds
of the appended claims along with their full scope of
equivalents.
* * * * *