U.S. patent application number 09/795726 was filed with the patent office on 2002-10-10 for carrier frequency acquisition method and apparatus having improved reliability for detecting carrier acquisition or loss thereof.
Invention is credited to Sayeed, Zulfiquar.
Application Number | 20020145969 09/795726 |
Document ID | / |
Family ID | 25166284 |
Filed Date | 2002-10-10 |
United States Patent
Application |
20020145969 |
Kind Code |
A1 |
Sayeed, Zulfiquar |
October 10, 2002 |
Carrier frequency acquisition method and apparatus having improved
reliability for detecting carrier acquisition or loss thereof
Abstract
A carrier signal acquisition technique is disclosed that
improves the reliability of the generated carrier frequency offset.
An improved course carrier frequency offset algorithm is employed
in conjunction with a conventional fine carrier frequency offset
algorithm. The course carrier frequency offset algorithm associated
with the present invention estimates large offsets that are
multiples of the carrier spacing that may occur at system startup.
A spectral null is placed in the center of the transmit spectrum
and is thereafter located in a received signal. The position of the
spectral null provides an estimate of the local oscillator carrier
offset. A frequency finite state machine (FSM) is disclosed that
processes a number of metrics to ensure the reliability of the
course carrier frequency offset and of transitions between
acquisition and tracking modes. The frequency FSM will utilize the
frequency offset (modin) generated by a MODSC algorithm provided
one or more predefined thresholds are satisfied. A first metric
value, c2adj, is the normalized ratio of average power (excluding
null carrier power) of carriers to the null carrier power. A second
metric, p2av, is the normalized ratio of average power (excluding
minimum carrier power) of carriers to the minimum carrier power. If
the average difference of the ratios of the powers of the carriers
(p2av and c2adj) is less than a predefined threshold, then
frequency acquisition is completed. Similarly, while in tracking
mode, if the differences are consistently greater than the
threshold, then loss of track is achieved and the frequency FSM
process will switch operation to the acquisition mode.
Inventors: |
Sayeed, Zulfiquar; (East
Windsor, NJ) |
Correspondence
Address: |
Kevin M. Mason
Ryan, Mason & Lewis, LLP
Suite 205
1300 Post Rd.
Fairfield
CT
06430
US
|
Family ID: |
25166284 |
Appl. No.: |
09/795726 |
Filed: |
February 28, 2001 |
Current U.S.
Class: |
370/206 |
Current CPC
Class: |
H04L 27/2602 20130101;
H04L 27/2659 20130101; H04L 27/2676 20130101 |
Class at
Publication: |
370/206 |
International
Class: |
H04J 011/00; H04J
003/06 |
Claims
I claim:
1. A method for determining a local oscillator carrier offset
frequency in a communication system, said method comprising the
steps of: receiving a signal having at least one spectral null
inserted therein; locating a position of said spectral null in said
signal; and using said position of said spectral null to estimate
said local oscillator carrier offset frequency if one or more
predefined conditions are satisfied.
2. The method according to claim 1, wherein said signal is an OFDM
signal.
3. The method according to claim 1, wherein said signal is a DMT
signal.
4. The method according to claim 1, wherein said predefined
conditions ensure that a signal-to-noise ratio satisfies predefined
conditions.
5. The method according to claim 1, wherein said predefined
conditions evaluate a ratio of the powers of the carriers to
determine if their averaged difference is less than some predefined
threshold.
6. The method according to claim 5, wherein an acquisition mode is
complete if said averaged difference is less than said predefined
threshold.
7. The method according to claim 1, wherein said predefined
conditions evaluate a ratio of the powers of the carriers during a
tracking mode to determine if their averaged difference are
consistently greater than a threshold indicating a that a loss of
tracking has occurred.
8. The method according to claim 1, wherein said local oscillator
carrier offset frequency is determined by said position of said
spectral null in a tracking mode and is zero in an acquisition
mode.
9. The method according to claim 1, further comprising the steps
of: extracting a plurality of sub-carriers of said signal after a
fast fourier transform (FFT) operation; determining a minimum
energy sub-carrier of said plurality of sub-carriers; calculating
at least one average energy of said plurality of sub-carriers,
excluding an energy of said minimum energy sub-carrier; comparing a
ratio of said average energy and said energy of said minimum energy
sub-carrier to a predefined threshold; and multiplying an index of
said minimum energy sub-carrier by an inter-carrier spacing to
generate said local oscillator carrier offset frequency.
10. A system for determining a local oscillator carrier offset
frequency in a communication system, comprising: a memory that
stores computer-readable code and information relating to local
oscillator carrier offset calculations; and a processor operatively
coupled to said memory, said processor configured to implement said
computer-readable code, said computer-readable code configured to:
receive a signal having at least one spectral null inserted
therein; locate a position of said spectral null in said signal;
and use said position of said spectral null to estimate said local
oscillator carrier offset frequency if one or more predefined
conditions are satisfied.
11. The system according to claim 10, wherein said signal is an
OFDM signal.
12. The system according to claim 10, wherein said signal is a DMT
signal.
13. The system according to claim 10, wherein said predefined
conditions ensure that a signal-to-noise ratio satisfies predefined
conditions.
14. The system according to claim 10, wherein said predefined
conditions evaluate a ratio of the powers of the carriers to
determine if their averaged difference is less than some predefined
threshold.
15. The system according to claim 14, wherein an acquisition mode
is complete if said averaged difference is less than said
predefined threshold.
16. The system according to claim 10, wherein said predefined
conditions evaluate a ratio of the powers of the carriers during a
tracking mode to determine if their averaged difference are
consistently greater than a threshold indicating a that a loss of
tracking has occurred.
17. The system according to claim 10, wherein said local oscillator
carrier offset frequency is determined by said position of said
spectral null in a tracking mode and is zero in an acquisition
mode.
18. The system according to claim 10, wherein said processor is
further configured to: extract a plurality of sub-carriers of said
signal after a fast fourier transform (FFT) operation; determine a
minimum energy sub-carrier of said plurality of sub-carriers;
calculate at least one average energy of said plurality of
sub-carriers, excluding an energy of said minimum energy
sub-carrier; compare a ratio of said average energy and said energy
of said minimum energy sub-carrier to a predefined threshold; and
multiply an index of said minimum energy sub-carrier by an
inter-carrier spacing to generate said local oscillator carrier
offset frequency.
19. A communication system, comprising: a receiver for receiving a
signal having at least one spectral null inserted therein; means
for locating a position of said spectral null in said signal; and
means for using said position of said spectral null to estimate
said local oscillator carrier offset frequency if one or more
predefined conditions are satisfied.
20. The communication system according to claim 19, wherein said
signal is an OFDM signal.
21. The communication system according to claim 19, wherein said
signal is a DMT signal.
22. The communication system according to claim 19, wherein said
predefined conditions ensure that a signal-to-noise ratio satisfies
predefined conditions.
23. The communication system according to claim 19, wherein said
predefined conditions evaluate a ratio of the powers of the
carriers to determine if their averaged difference is less than
some predefined threshold.
24. The communication system according to claim 23, wherein an
acquisition mode is complete if said averaged difference is less
than said predefined threshold.
25. The communication system according to claim 19, wherein said
predefined conditions evaluate a ratio of the powers of the
carriers during a tracking mode to determine if their averaged
difference are consistently greater than a threshold indicating a
that a loss of tracking has occurred.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present invention is related to U.S. patent application
Ser. No. 09/382,847, filed Aug. 25, 1999, entitled "Orthogonal
Frequency Division Multiplexed (OFDM) Carrier Acquisition Method,"
(Attorney Docket Number Sayeed 9) and U.S. patent application Ser.
No. 09/398,502, filed Sep. 17, 1999, entitled "Method and Apparatus
for Performing Differential Modulation Over Frequency in an
Orthogonal Frequency Division Multiplexing (OFDM) Communication
System," (Attorney Docket Number Riazi 3-11-3), each assigned to
the assignee of the present invention and incorporated by reference
herein.
FIELD OF THE INVENTION
[0002] The present invention relates to wireless communication
systems, and more particularly, to methods and apparatus for
performing carrier acquisition in an orthogonal frequency division
multiplexing (OFDM) communication system or another communication
system.
BACKGROUND OF THE INVENTION
[0003] Satellite broadcasting systems for transmitting programming
content have become increasingly popular in many parts of the
world. Direct Broadcasting Satellite (DBS) systems transmit
television programming content, for example, to a geo-stationary
satellite, which broadcasts the content back to the customers. In
such a wireless broadcast environment, the transmitted programming
can be received by anyone with an appropriate receiver, such as an
antenna or a satellite dish.
[0004] In addition, a number of satellite broadcasting systems have
been proposed or suggested for broadcasting audio programming
content from geo-stationary satellites to customers in a large
coverage area, such as the continental United States. Proposed
systems for providing digital audio broadcasting (DAB), for
example, are expected to provide near CD-quality audio, data
services and more robust coverage than existing analog FM
transmissions. Satellite broadcasting systems for television and
radio content provide potentially national coverage areas, and thus
improve over conventional terrestrial television stations and AM/FM
radio stations that provide only regional coverage.
[0005] Satellite broadcasting systems transmit digital music and
other information from an uplink station to one or more mobile
receivers. Satellite broadcasting systems typically include a
plurality of satellites and terrestrial repeaters operating in a
broadcast mode. The satellites are typically geo-stationary, and
are located over a desired geographical coverage area. The
terrestrial repeaters typically operate in dense urban areas, where
the direct line of sight (LOS) between the satellites and the
mobile receiver can be blocked due to the angle of elevation and
shadowing by tall buildings.
[0006] Orthogonal frequency division multiplexing (OFDM) techniques
have also been proposed for use in such satellite broadcasting
systems and other wireless networks. In an OFDM communication
system, the digital signal is modulated to a plurality of small
sub-carrier frequencies that are then transmitted in parallel. It
has been found that OFDM communication systems do not require
complex equalizers, even at high data rates and under multipath
propagation conditions. Among other benefits, OFDM communication
systems provide a guard interval that absorbs the multipath
distortion into the guard interval duration. As long as the arrival
times of the multipath signals differ from one another by less than
the guard interval, an equalizer is not necessary.
[0007] OFDM communication systems are especially sensitive to
frequency offsets in the carrier signal. Typically, OFDM systems
use additional pilot and/or synchronization signals to perform the
initial acquisition of the carrier signal. One popular technique
utilizes training sequences over two consecutive OFDM symbols.
While this technique acquires the carrier signal in a satisfactory
manner, it suffers from a number of limitations, which if overcome,
could greatly expand the efficiency of OFDM communication systems.
More specifically, prior techniques relying on additional pilot
and/or synchronization signals reduce the effective OFDM capacity
for carrying data, which is unsatisfactory for capacity-limited
systems.
[0008] Another technique for acquiring the carrier signal, referred
to as the Guard Interval Based (GIB) algorithm, can only
distinguish to within one half of the inter-carrier spacing. Thus,
the GIB algorithm is insufficient for estimating frequency offsets
that are greater than one half of the inter-carrier spacing, which
are typical at system startup. For a more detailed discussion of
the GIB algorithm, see, for example, Jan-Jaap van de Beek et al.,
ML Estimation of Time and Frequency Offset in OFDM Systems, IEEE
Transactions on Signal Processing, Vol. 45, No 7, 1800-05 (July
1997) or Jan-Jaap van de Beek et al., "A Time and Frequency
Synchronization Scheme for Multiuser OFDM," IEEE J. on Selected
Areas in Communications, Vol. 17, No. 11, 1900-14, (November 1999),
each incorporated by reference herein.
[0009] U.S. patent application Ser. No. 09/382,847, filed Aug. 25,
1999, entitled "Orthogonal Frequency Division Multiplexed (OFDM)
Carrier Acquisition Method," (Attorney Docket Number Sayeed 9),
assigned to the assignee of the present invention and incorporated
by reference herein, discloses an improved technique, referred to
herein as the modulo-subcarrier (MODSC) algorithm, for acquiring
the carrier signal. The MODSC algorithm inserts a spectral null in
the transmitted OFDM signal at a predefined location, locates the
spectral null at the receiver and uses the shifting of the spectral
null to estimate the local oscillator carrier offset. The location
of the detected null estimates the carrier offset in units of the
number of intercarrier spacings. Among other benefits, the MODSC
algorithm does not require the use of additional pilot signals and
thus optimizes the bandwidth utilization.
[0010] While the MODSC algorithm accurately determines the carrier
offset without reducing the effective bandwidth utilization, a
further need exists for a method and apparatus that acquires the
carrier signal and also declares when the carrier signal has been
acquired or when a false-lock or out-of-lock condition has
occurred. Another need exists for a method and apparatus that
acquires the carrier signal and that provides for reliable
transitions between acquisition and tracking modes.
SUMMARY OF THE INVENTION
[0011] Generally, a method and apparatus are disclosed for
acquiring a carrier signal in a communication system and ensuring
the reliability of the carrier frequency offset. The present
invention employs an improved course carrier frequency offset
algorithm, as well as a conventional fine carrier frequency offset
algorithm. The course carrier frequency offset algorithm associated
with the present invention estimates large offsets that are
multiples of the carrier spacing that may occur at system startup
using the MODSC algorithm discussed above. The fine carrier
frequency offset algorithm identifies a local oscillator carrier
offset within one half the inter-carrier spacing.
[0012] According to one aspect of the invention, a frequency finite
state machine (FSM) is disclosed that processes a number of metrics
to ensure the reliability of the course carrier frequency offset
and of transitions between acquisition and tracking modes. A first
metric value, c2adj, is the normalized ratio of average power
(excluding null carrier power) of carriers to the null carrier
power. A second metric, p2av, is the normalized ratio of average
power (excluding minimum carrier power) of carriers to the minimum
carrier power. The frequency FSM will utilize the frequency offset
(modin) generated by a MODSC algorithm provided one or more
predefined thresholds are satisfied. Generally, if one of the
conditions fail, then the signal-to-noise ratio is insufficient to
reliably determine the frequency offset.
[0013] From a process point of view, a frequency FSM process
utilizes the ratios of the powers of the carriers (p2av and c2adj)
to determine if their averaged difference is less than some
predefined threshold. The averaging is done over a fixed number of
OFDM frames. If the average difference is less than the predefined
threshold, then frequency acquisition is completed. Similarly,
while in tracking mode, the difference of the ratios of powers is
compared. If the differences are consistently greater than the
threshold, then loss of track is achieved and the frequency FSM
process will switch operation to the acquisition mode. The
frequency offset generated by the frequency FSM (modout) is same as
modin in the tracking mode and is 0 in the acquisition mode.
[0014] A more complete understanding of the present invention, as
well as further features and advantages of the present invention,
will be obtained by reference to the following detailed description
and drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 is a schematic block diagram illustrating an
exemplary OFDM transmitter;
[0016] FIG. 2 is a schematic block diagram illustrating an
exemplary OFDM receiver in which the present invention can
operate;
[0017] FIG. 3 is a schematic block diagram illustrating a carrier
acquisition method in accordance with the present invention;
[0018] FIG. 4 is a schematic block diagram illustrating the MODSC
and frequency FSM blocks of FIG. 3 in further detail;
[0019] FIG. 5 illustrates the sub-carrier assignment utilized by
the MODSC algorithm as generated by the Fast Fourier Transform
(FFT) stage of FIG. 3; and
[0020] FIG. 6 is a flow chart describing a frequency FSM process
implemented by the frequency FSM shown in FIG. 3.
DETAILED DESCRIPTION
[0021] The carrier acquisition method and apparatus of the present
invention are illustrated in connection with an exemplary OFDM
communication system. Although described in connection with an
exemplary wireless OFDM communication system, it will be understood
that the present invention is equally applicable to a wired
discrete multi-tone (DMT) communication system. The functionality
performed by the transmitter and receiver sections of an exemplary
OFDM system are discussed below in conjunction with FIGS. 1 and 2,
respectively. The carrier acquisition techniques of the present
invention are then discussed below in conjunction with FIGS. 3 and
5.
OFDM Terminology and Signal Parameters
[0022] Generally, in an exemplary OFDM communication system, a
digital signal is transmitted as a plurality of parallel
sub-carries (also known as "bins"). Collectively, these
sub-carriers are referred to an OFDM "symbol." However, only some
of the sub-carriers actually contain information about the signal,
referred to as "active sub-carriers." The remaining sub-carriers
are nulled, i.e., filled with zeroes, and thus, are referred to as
"inactive sub-carriers." An inverse Fast Fourier Transform (IFFT)
of a significantly longer length than the number of active
sub-carriers is then used to encode the signal for transmission to
an OFDM receiver. In particular, the IFFT length is maintained at
twice the number of sub-carriers in order to properly reproduce the
highest frequency sub-carriers and avoid the effects of
aliasing.
[0023] In the illustrative embodiment, each OFDM symbol of duration
Ts will be composed of 2048 samples corresponding to the useful
symbol duration and 184 samples corresponding to the guard
interval, and the symbol represents up to 2048 sub-carriers each
spaced 4 kHz apart (.DELTA.f). The useful OFDM symbol duration, Tu,
illustratively equals 250 mu-sec and the guard interval duration or
cyclic prefix duration, Tg, illustratively equals 22.46 mu-sec. The
duration of the symbol, Ts, is 272.46 mu-sec, where Ts equals Tu
plus Tg. The inter-carrier spacing, .DELTA.f, of 4 KHz is equal to
the inverse of the useful symbol duration (l/Tu).
OFDM Carrier Acquisition
[0024] According to one feature of the present invention, the OFDM
receiver 200, discussed below in conjunction with FIG. 2, employs a
course carrier frequency offset algorithm, in accordance with the
present invention, as well as a conventional fine carrier frequency
offset algorithm. As discussed hereinafter, the course carrier
frequency offset algorithm which is the focus of the present
invention is embodied as the MODSC algorithm and can estimate large
offsets that are multiples of the carrier spacing (4 KHz) that may
occur at system startup. The fine carrier frequency offset
algorithm must identify a local oscillator carrier offset within
one half the inter-carrier spacing. Thus, the fine carrier
frequency offset algorithm can detect the carrier offset and
correct the local oscillator even when the spectral null moves to
the middle of the sub-carrier spacing.
[0025] Thus, if the local oscillator moves the spectrum by one half
of the inter-carrier spacing (2 KHz in the exemplary OFDM
communication system), the MODSC algorithm will not be able to
detect the null at the output of the FFT. Once the offset moves
sufficiently away from the middle of the sub-carrier separation,
the MODSC algorithm can detect the integral part of the offset and
remove the inherent ambiguity equal to the inter-carrier spacing (4
KHz in the exemplary embodiment) due to the 2.pi. periodicity of
the fine carrier tracking estimate. Therefore, the course and fine
carrier frequency offset algorithms work together in estimating the
local oscillator carrier offset more quickly and with greater
accuracy than the carrier tracking algorithm alone.
[0026] The fine carrier frequency offset algorithm may be embodied,
for example, as the well-known Guard Interval Based (GIB) algorithm
described in, e.g., Jan-Jaap van de Beek et al., ML Estimation of
Time and Frequency Offset in OFDM Systems, IEEE Transactions on
Signal Processing, Vol. 45, No 7, 1800-05 (July 1997) or Jan-Jaap
van de Beek et al., "A Time and Frequency Synchronization Scheme
for Multiuser OFDM," IEEE J. on Selected Areas in Communications,
Vol. 17, No. 11, 1900-14, (November 1999), each incorporated by
reference herein. In addition, the manner in which the MODSC course
carrier frequency offset algorithm and the fine carrier frequency
offset algorithm cooperate to estimate the local oscillator carrier
offset is described in U.S. patent application Ser. No. 09/382,847,
filed Aug. 25, 1999, entitled "Orthogonal Frequency Division
Multiplexed (OFDM) Carrier Acquisition Method," (Attorney Docket
Number Sayeed 9), assigned to the assignee of the present invention
and incorporated by reference herein. Since the fine carrier
frequency offset algorithm is not the focus of the present
invention, the reader is referred to these references for a more
detailed discussion.
[0027] In accordance with the carrier offset estimation technique
of the present invention, the OFDM transmitter, shown in FIG. 1,
places a spectral null in the center of the transmit spectrum. FIG.
1 is a schematic block diagram illustrating an exemplary OFDM
transmitter. As shown in FIG. 1, a random bit generator 100 is used
to generate a digital data stream (i.e., I and Q pairs). The I and
Q pairs are applied to a .pi./4 QPSK modulator 110 that generates
an active IS sub-carrier by mapping each I and Q pair to a QPSK
constellation, in a known manner. Active sub-carriers from the
modulator are then sent to a frequency interleaver 120 where they
are frequency interleaved over one OFDM symbol (i.e., N.sub.a
sub-carriers or, in the exemplary embodiment, 1002 sub-carriers).
In the exemplary embodiment, the frequency interleaver 120
comprises a matrix of R rows and C columns where R equals 61 and C
equals 16 to accommodate the 1002 active sub-carriers for each
symbol. The sub-carriers are written into the matrix row-by-row and
are read therefrom column-by-column to effect the interleaving. The
output of the frequency interleaver 120 (c[n], where, in the
exemplary embodiment, n=1 to 1002 sub-carriers) is then fed to a
differential modulator 130.
[0028] The output of the differential modulator 130 comprises
complex elements d[n]=c[n]d[n-1], where, in the exemplary
embodiment, n=1 to 1002 active sub-carriers. The output of the
differential modulator 130 for each symbol is then sent to and
stored in a buffer 140 of width equal to N*Os sub-carriers, where N
is the number of sub-carriers and Os is the over-sampling rate.
Thus, in the exemplary embodiment, the buffer 140 is 2048
sub-carriers wide. The spectral null is inserted at the center of
the transmit spectrum in accordance with the present invention by
loading a zero complex value in the first location in the buffer
140 (i.e., with z[0]=0). This null is used in the carrier
acquisition method described herein. The next Na/2 locations in
buffer 140 are loaded with the samples corresponding to the first
set of Na/2 active sub-carriers, where Na is the number of active
sub-carriers. The next (N*Os)-Na-1 locations in buffer 140 are then
padded with zeros corresponding to the inactive sub-carriers. As
will be discussed hereinafter, the insertion of zeros in these
locations of the buffer 140 will facilitate using a larger IFFT
size than the number of active sub-carriers thereby ensuring that
the highest frequency sub-carriers will be at least 2.times.
over-sampled to eliminate the adverse effects of aliasing. Finally,
the last Na/2 locations in the buffer 140 are loaded with samples
corresponding to the remaining or second set of Na/2 active
sub-carriers. The contents of buffer 140 represents the OFDM symbol
in the frequency domain. In radian frequency, the 0th sub-carrier
corresponds to 0 radians and the 2047.sup.th sub-carrier
corresponds to 2.pi. radians.
[0029] The contents of the buffer 140 are then transmitted to an
IFFT 150 for conversion from the frequency domain to the time
domain. The IFFT 150 is of length Na*Os. In the exemplary
embodiment, N and Os are 1024 and 2.times. over-sampling,
respectively. The output of the IFFT 150 is the time domain
representation of the symbol as defined by: 1 s [ k ] = n = Na / 2
Na / 2 z [ n ] exp ( j 2 n / N O s )
[0030] Thus, the output of the IFFT 150 will be time domain samples
for N*Os sub-carriers or, in the exemplary embodiment, samples for
2048 sub-carriers. As discussed above, however, d[n] will equal
zero for n=0 and for each of the (N*Os)-(Na-1) inactive
sub-carriers. Next, the N*Os samples are sent to GI Insertion
Module 160 where a guard interval of length L*Os is added to the
symbol. The guard interval is an exact duplicate of the last L*Os
samples of the OFDM symbol which gets prepended to the transmitted
symbol. In the exemplary embodiment, the length L of the guard
interval is 184 samples. Thus, the total length of the transmitted
symbol is (N+L)*Os samples or, in the exemplary embodiment, 2,232
samples.
[0031] Next, the symbol is sent to a transmitter 170 that includes
a digital-to-analog (D/A) converter operating at Fs=N*(F/Na)*Os Hz
(in the exemplary embodiment, 8.192 MHz) and is converted from
digital to analog. The D/A converter imposes a sin x/x spectrum
onto each sub-carrier. After D/A conversion, the 0th sub-carrier of
the symbol will be located at 0 Hz and extend to 4 KHz, and the
2047th sub-carrier will be located at 8.188 MHz and extend to 8.192
MHz. However, as is well known in the art, a digital signal
replicates itself every 2.pi. radians.
[0032] Thus, the sample and hold function of the D/A converter will
serve to filter out any replicas of the symbol and retain only that
portion of the symbol extending from -N*(F/Na) Hz to +N*(F/Na) Hz
(i.e., -4.096 MHz to +4.096:MHz in the exemplary OFDM system) for
transmission to the OFDM receiver (FIG. 2). However, the energy
content of the inactive sub-carriers is almost zero. Thus, the
active sub-carriers will be transmitted such that the 0 sub-carrier
will be located at 0 Hz; the first set of Na/2 sub-carriers will be
located from 0 Hz to F/2 Hz and the second set of Na/2 sub-carriers
will be located at -F/2 Hz to 0 Hz.
[0033] FIG. 2 is a block diagram illustrating an exemplary OFDM
receiver. It will be understood that, in a wireless system, the
receiver may be located in a handset, a base station or the like.
As shown in FIG. 2, samples corresponding to the sub-carriers of
the OFDM symbol are received by the OFDM receiver 200 and converted
from analog to digital. Next, the guard interval is removed by GI
Removal Module 210 to obtain N*Os sub-carriers. The symbol is then
applied to an FFT 220 of length L equal to N*Os for conversion from
the time domain to the frequency domain. As discussed above, for
purposes of carrier acquisition, a null will be present in the
center of the spectrum if the receiver is tuned to the transmit
carrier. After the FFT is performed, the inactive sub-carriers are
discarded by unused carrier removal module 225, and the complex
elements d[n] are sent to a differential demodulator 230.
Demodulation results in active sub-carriers c[n]=d[n]d*[n-1]. The
active sub-carriers are then sent to a frequency de-interleaver 240
and thereafter to a QPSK demodulator 250. After QPSK de-modulation,
the digital bits are sent to a data sink 260 for
application-specific processing.
[0034] FIG. 3 illustrates the OFDM carrier acquisition method of
the present invention. As discussed above, it is used to estimate
and correct the initial local oscillator carrier offset (course)
for the OFDM signal in the exemplary OFDM system. As shown in FIG.
3, the received complex baseband OFDM samples 305 are rotated by
the integrated phase due to the local oscillator carrier offset in
Symbol Rotator Module 310. These samples are then sent to the GI
Removal Module 210 where the guard interval is removed. The samples
are applied to the FFT operation 220, and the carrier acquisition
process in accordance with the present invention begins. As
previously indicated, the carrier acquisition process of the
present invention employs a course carrier frequency offset
algorithm (MODSC) 350 as well as a conventional fine carrier
frequency offset algorithm (GIB) 320. The MODSC algorithm estimates
large offsets that are multiples of the carrier spacing (4 KHz)
that may occur at system startup. The fine carrier frequency offset
algorithm must identify a local oscillator carrier offset within
one half the inter-carrier spacing.
[0035] As discussed further below in conjunction with FIG. 4, the
course frequency offset generated by the MODSC algorithm is further
processed by a frequency finite state machine (FSM) 360 to ensure
the reliability of the course carrier frequency offset and of
transitions between acquisition and tracking modes. The fine and
course frequency offsets generated by the GIB and MODSC algorithms
320, 350, respectively, are applied to the local oscillator 370 to
control the tracking of the carrier frequency.
[0036] FIG. 4 is a schematic block diagram illustrating the
operation of the MODSC block 350 and frequency FSM block 360 of
FIG. 3 in further detail. As shown in FIG. 4, the first step in the
MODSC algorithm 350 is the extraction of the appropriate number of
sub-carriers in the extract module 415. The extracted FFT
sub-carrier bins have integer indices with the central sub-carrier
having an index of 0. For example, in a system where 37
sub-carriers are extracted, the sub-carrier bins would have the
following indices: -18, -17, . . . -2, -1, 0, 1, 2, . . . 17, 18.
As shown in FIG. 5, the output of the FFT stage 220 (FIG. 3) is
2048 complex samples. Of these 2048 samples, 37 samples
(sub-carrier outputs) are extracted by the extract module 415, as
follows:
y(k)=fft_buffer(k) 0<=k<=18
y(-k)=fft_buffer(2048-k) 1<=k<=18
[0037] In other words, the desired 37 central bins are extracted
from sample positions 0 through 18 and 2030 through 2047.
[0038] Once the extraction operation is completed, the extracted
FFT bins can be observed to locate the null, which indicates the
MODSC offset estimate. Specifically, the MODSC algorithm 350
distinguishes whether the local oscillator carrier offset is an
integer, i.e., . . . -2, -1, 0, 1, 2. . . , sub-carriers away from
the central position, which is equivalent to . . . -8 KHz, -4 KHz,
0 KHz, 4 KHz, 8 KHz in the exemplary OFDM system. While a
per-sub-carrier based operation on all the available sub-carriers
can be performed, it is unnecessary. The number of sub-carriers
that are needed depends on the parts-per-million (ppm) accuracy of
the local oscillator 370 and the carrier frequency. In the
exemplary OFDM system, where the local oscillator accuracy is 8
ppm, the per-sub-carrier operations can be constrained to
sub-carriers: -4, -3, . . . 0, . . . 3, 4 because 2*M+1 sub-carrier
tracking can generally estimate +/-M*(.DELTA.f) (Hz).
[0039] The absolute values of the extracted FFT outputs are then
calculated on a per-sub-carrier basis in the absolute value module
430. Because the location of the null is rendered uncertain by
channel noise and fading, the extracted FFT outputs are preferably
filtered on a per-sub-carrier basis to remove or mitigate the
effects of the channel in the filter module 435, which may be
embodied as, e.g., 16-tap moving average filters. After the
filtering operation 435, four operations are performed in parallel.
More specifically, the average power of the 37 central sub-carriers
is computed at stage 450, excluding the power associated with the
null carrier (0). The null carrier is selected at stage 455. The
sub-carrier having the minimum power is selected during stage 460
(modin) indicating the offset in multiples of sub-carrier spacing.
In addition, the average power of the 37 central sub-carriers is
computed at stage 465, excluding the power associated with the
minimum power carrier (selected at state 460).
[0040] Following the various computations performed during stages
450-465 of the MODSC algorithm 350, the present invention computes
additional metrics that are utilized by the frequency FSM 360 to
improve the reliability of transitions between acquisition and
tracking modes. As shown in FIG. 4, a value, c2adj, is computed
during stage 470, as follows: 2 c2adj = [ F ave F 0 ] ,
[0041] where F.sub.0 is the null carrier power and F.sub.ave is the
average power of adjacent carriers. Thus, c2adj is the normalized
ratio of average power (excluding null carrier power) of carriers
to the null carrier power.
[0042] A value, p2av, is computed during stage 475, as follows: 3
p2av = [ F ave F min ] ,
[0043] where F.sub.min is the minimum carrier power of the 37
central carriers. Thus, p2av is the normalized ratio of average
power (excluding minimum carrier power) of carriers to the minimum
carrier power.
[0044] The present invention provides improved reliability of the
course frequency offset estimate and of the transitions between
acquisition and tracking modes using a number of thresholds. Thus,
as discussed further below in conjunction with FIG. 6, the metric
modin generated by the MODSC algorithm 350 is modified to
incorporate thresholds based on the metrics c2adj and p2av, as
follows:
mod in.sub.new=E.multidot.Min_index.multidot.4 KHz,
[0045] where Min_Index is index of the carrier with minimum power,
in the range -18 to 18, and
E=1 if [c2adj.ltoreq.threshlt1]& [p2av>threshgt1] are true 0
otherwise
[0046] Thus, if at least one of the conditions fail, the modin
expression will evaluate to zero and the frequency offset falls
back to the central carrier (zero frequency offset). Generally, if
one of the conditions fail, then the signal-to-noise ratio is
insufficient to reliably determine the frequency offset.
[0047] As shown in FIG. 4, the frequency FSM 360 also generates a
signal, FACQSTAT (frequency acquired status) that can be used by
other parts of the OFDM receiver to monitor the system state. The
output FACQSTAT is a binary value of one (1) in the tracking mode
and it is a binary value of zero (0) in the acquisition mode.
[0048] FIG. 6 is a flow chart describing a frequency FSM process
600 implemented by the frequency FSM 360 shown in FIGS. 3 and 4.
Generally, the frequency FSM process 600 provides reliable
transitions between acquisition and tracking modes. The frequency
FSM process 600 uses the ratios of the powers of the carriers (p2av
and c2adj) to determine if their averaged difference is less than
some predefined threshold. The averaging is done over a fixed
number of OFDM frames. If the average difference is less than the
predefined threshold, then frequency acquisition is completed.
Similarly, while in tracking mode, the difference of the ratios of
powers is compared. If the differences are consistently greater
than the threshold, then loss of track is achieved and the
frequency FSM process 600 will switch operation to the acquisition
mode. The output FACQSTAT is a binary value of one (1) in the
tracking mode and it is a binary value of zero (0) in the
acquisition mode. The modout is same as modin in the tracking mode
and it is 0 in the acquisition mode. Thus, as shown in FIG. 6, the
frequency FSM process 600 initially performs a test during step 605
to ensure that the metric values c2adj and p2av satisfy the
following condition:
[c2adj<threshlt1]& [p2av>threshgt1].
[0049] If it is determined during step 605 that the metric values
c2adj and p2av do not satisfy the specified condition, then the
modin value does not have sufficient reliability and program
control returns to step 605 until the modin value has sufficient
reliability.
[0050] If, however, it is determined during step 605 that the
metric values c2adj and p2av satisfy the specified condition, then
the modin value has sufficient reliability and program control
proceeds to step 610 where a test is performed to determine if the
frequency has been changed or if FACQSTAT equals one (indicating
that acquisition was previously achieved).
[0051] If it is determined during step 610 that the frequency has
not been changed and FACQSTAT does not equal one, then program
control proceeds to steps 620 and 630 to make a frequency change.
Thus, a further test is performed during step 620 to determine if
modin has been the same for a predefined number of frames. If it is
determined during step 620 that modin has not been the same for a
predefined number of frames, then program control returns to step
620 until this condition is satisfied.
[0052] If, however, it is determined during step 620 that modin has
been the same for a predefined number of frames, then the frequency
is changed during step 630 using modout, and modout is recorded as
oldmodout. Program control then returns to step 610.
[0053] If, however, it is determined during step 610 that the
frequency has been changed or that FACQSTAT equals one, then a
metdiff value is computed during step 640 over a predefined number
of frames as the absolute value of the difference between the
metric values c2adj and p2av. A test is performed during step 650
to determine if c2adj exceeds a threshold at least once during a
predefined number of frames. A more stringent threshold can
optionally be applied to c2adj during step 650 than was applied
during step 605 since satisfying the threshold will place the FSM
in a tracking mode.
[0054] If it is determined during step 650 that c2adj does exceed a
threshold at least once during a predefined number of frames, then
a flag is set during step 655. If, however, it is determined during
step 650 that c2adj does not exceed a threshold at least once
during a predefined number of frames, then the flag is not set. A
further test is performed during step 665 to determine if the
average metdiff (calculated during step 640) is greater than a
defined threshold and that the c2adj flag has been set. If it is
determined during step 665 that the average metdiff is not greater
than a defined threshold or the c2adj flag has not been set, then
program control returns to step 610 and continues in the manner
described above.
[0055] If, however, it is determined during step 665 that the
average metdiff is greater than a defined threshold and the c2adj
flag has been set, then acquisition is complete (null has moved to
center) and tracking may begin. The status flag FACQSTAT is set
during step 670. A further test is performed during step 675 to
again determine if the average metdiff (calculated during step 640)
is greater than a defined threshold and modin does not equal zero
for a predefined number of frames.
[0056] If it is determined during step 675 that the average metdiff
is not greater than the threshold or that modin equals zero for a
predefined number of frames, then program control returns to step
610 and continues in the manner described above. If, however, it is
determined during step 675 that the average metdiff is greater than
a defined threshold and modin does not equal zero for a predefined
number of frames, then acquisition is lost and the frequency change
that was made is undone during step 680, before program control
returns to step 610 and continues in the manner described
above.
[0057] While there are 37 sub-carriers tracked in the exemplary
modsc block, all absolute frequency offsets greater than 36000 are
ignored by the frequency FSM 360 in the acquisition mode. However,
the entire 2*72000 Hz offsets (all 37 sub-carriers) are used in
declaring false acquisitions during step 675.
[0058] It is to be understood that the embodiments and variations
shown and described herein are merely illustrative of the
principles of this invention and that various modifications may be
implemented by those skilled in the art without departing from the
scope and spirit of the invention.
* * * * *