United States Patent [191
`Schmidl et al.
`
`111111111111111111111111111 ~~11111111111111 m111111111 m11
`US005732113A
`5,732,113
`[11] Patent Number:
`[45] Date of Patent:
`Mar. 24, 1998
`
`[54] TIMING AND FREQUENCY
`SYNCHRONIZATION OF OFDM SIGNALS
`
`[75]
`
`Inventors: Timothy M. Schmidl; Donald C. Cox,
`both of Stanford, Calif.
`
`[73] Assignee: Stanford University, Stanford, Calif.
`
`[21] Appl. No.: 666,237
`
`Jun. 20, 1996
`
`[22] Filed:
`Int. CI.6
`........................................................ H04L7/00
`[51]
`[52] U.S. CI .............................................. 375/355; 375/354
`[58] Field of Search ..................................... 375/354, 355;
`370/206, 208
`
`[56]
`
`References Cited
`
`U.S. PATENf DOCUMENTS
`
`5,166,924
`5,228,025
`5,406,551
`5,444,697
`5,471,464
`5,506,836
`5,521,943
`.5,550,812
`5,555,268
`5,602,835
`
`11/1992 Moose .................................... 370/32.1
`7/1993 Le Floch et al .......................... 370/20
`4/1995 Saito et al ................................. 370/19
`8/1995 Leung et al ............................... 370/19
`1111995 Ikeda ......................................... 370/19
`4/1996 Ikeda et al. ............................... 370/19
`5/19% Dambacher ............................. 3751295
`8/1996 Phillips ..................................... 370/19
`9/1996 Fattouche et al ....................... 375/206
`2/1997 Seki et al ................................ 370/206
`
`Primary Examiner-Stephen Chin
`Assistant Examiner-Mohammad Ghayow
`Attorney, Agent, or Firm-Lumen Intellectual
`Services
`[57]
`
`ABSTRACT
`
`Property
`
`A method and apparatus achieves rapid timing
`synchronization, carrier frequency synchronization, and
`sampling rate synchronization of a receiver to an orthogonal
`frequency division multiplexed (OFDM) signal. The method
`uses two OFDM training symbols to obtain full synchroni(cid:173)
`zation in less than two data frames. A first OFDM training
`symbol has only even-numbered sub-carriers, and substan(cid:173)
`tially no odd-numbered sub-carriers, an arrangement that
`results in half-symbol symmetry. A second OFDM training
`symbol has even-numbered sub-carriers differentially modu(cid:173)
`lated relative to those of the first OFDM training symbol by
`a predetermined sequence. Synchronization is achieved by
`computing metrics which utilize the unique properties of
`these two OFDM training symbols. Timing synchronization
`is determined by computing a timing metric which recog(cid:173)
`nizes the half-symbol symmetry of the first OPDM training
`symbol. Carrier frequency offset estimation is performed in
`using the timing metric as well as a carrier frequency offset
`metric which peaks at the correct value of carrier frequency
`offset Sampling rate offset estimation is performed by
`evaluating the slope of the locus of points of phase rotation
`due to sampling rate offset as a function of sub-carrier
`frequency number.
`
`26 Claims, 15 Drawing Sheets
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`ERIC-1002
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`ERIC-1002 / Page 2 of 33
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`

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`U.S. Patent
`
`Mar. 24, 1998
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`ERIC-1002 / Page 4 of 33
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`U.S. Patent
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`Mar. 24, 1998
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`ERIC-1002 / Page 5 of 33
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`

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`U.S. Patent
`
`Mar. 24, 1998
`
`Sheet 5of15
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`Even-No. Freq.
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`Training
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`
`ERIC-1002 / Page 7 of 33
`
`

`
`U.S. Patent
`
`Mar. 24, 1998
`
`Sheet 7of15
`
`5,732,113
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`ERIC-1002 / Page 8 of 33
`
`

`
`U.S. Patent
`
`Mar. 24, 1998
`
`Sheet 8of15
`
`5,732,113
`
`Frequency
`Number, k
`
`-4
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`
`ERIC-1002 / Page 9 of 33
`
`

`
`U.S. Patent
`
`Mar. 24, 1998
`
`Sheet 9of15
`
`5,732,113
`
`Imaginary
`
`Real
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`
`FIG. 98
`
`Imaginary
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`
`ERIC-1002 / Page 10 of 33
`
`

`
`U.S. Patent
`
`Mar. 24, 1998
`
`Sheet 10 of 15
`
`5,732,113
`
`Best timing
`
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`
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`
`FIG. 10
`
`ERIC-1002 / Page 11 of 33
`
`

`
`U.S. Patent
`
`Mar. 24, 1998
`
`Sheet 11 of 15
`
`5,732,113
`
`1
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`
`ERIC-1002 / Page 12 of 33
`
`

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`
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`
`Data
`
`Transmitted
`
`Number
`Frequency
`
`ERIC-1002 / Page 13 of 33
`
`

`
`U.S. Patent
`
`Mar. 24, 1998
`
`Sheet 13 of 15
`
`5,732,113
`
`....-.....
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`
`ERIC-1002 / Page 14 of 33
`
`

`
`U.S. Patent
`
`Mar. 24, 1998
`
`Sheet 14 of 15
`
`5,732,113
`
`1
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`ERIC-1002 / Page 15 of 33
`
`

`
`U.S. Patent
`
`Mar. 24, 1998
`
`Sheet 15 of 15
`
`5,732,113
`
`Accumulated
`Phase
`Rotation in Time
`Ts+Tg Due to
`Sampling Rate Offset
`
`FIG. 15
`
`Sub-carrier
`Frequency
`Number
`
`ERIC-1002 / Page 16 of 33
`
`

`
`5,732,113
`
`1
`TIMING AND FREQUENCY
`SYNCHRONIZATION OF OFDM SIGNALS
`
`FIELD OF THE INVENTION
`
`The present invention relates to a method and apparatus
`for the reception of orthogonal frequency division multi(cid:173)
`plexed (OFDM) signals. More particularly, the invention
`concerns timing and frequency synchronization of an
`OFDM receiver to an OFDM signal to enable the OFDM
`receiver to accurately demodulate, decode, and recover data 10
`transmitted across an OFDM channel on the OFDM sub(cid:173)
`carriers of the OFDM signal.
`
`BACKGROUND OF THE INVENTION
`
`2
`sub-carrier is typically assigned c0=0. Encoder 14 then
`passes the sequence of sub-symbols, along with any addi(cid:173)
`tional zeroes that may be required for interpolation to
`simplify filtering, onto an inverse discrete Fourier trans-
`5 former (IDFT) or, preferably, an inverse fast Fourier trans(cid:173)
`former (IFFT) 16.
`Upon receiving the sequence of OFDM frequency-
`domain sub-symbols from encoder 14, IFFT 16 performs an
`inverse fast Fourier transform on the sequence of sub(cid:173)
`symbols. In other words, it uses each of the complex-valued
`sub-symbols, cko to modulate the phase and amplitude of a
`corresponding one of 2N+ 1 sub-carrier frequencies over a
`symbol interval Ts. The sub-carriers are given by e-21tffl!, and
`therefore, have baseband frequencies of f k=k!f s• where k is
`15 the frequency number and is an integer in the range
`-N~k~N. IFFT 16 thereby produces a digital time-domain
`OFDM symbol of duration Ts given by:
`
`1. General Description of Transmission Using OFDM
`Orthogonal frequency division multiplexing (OFDM) is a
`robust technique for efficiently transmitting data over a
`channel. The technique uses a plurality of sub-carrier fre(cid:173)
`quencies (sub-carriers) within a channel bandwidth to trans- 20
`mit the data. These sub-carriers are arranged for optimal
`bandwidth efficiency compared to more conventional trans(cid:173)
`mission approaches, such as frequency division multiplex(cid:173)
`ing (FDM). which waste large portions of the channel
`bandwidth in order to separate and isolate the sub-carrier 25
`frequency spectra and thereby avoid inter-carrier interfer(cid:173)
`ence (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 30
`onto each sub-carrier. Additionally, OFDM is much less
`susceptible to data loss due to multipath fading than other
`conventional approaches for data transmission because
`inter-symbol interference is prevented through the use of
`OFDM symbols that are long in comparison to the length of 35
`the channel impulse response. Also, the coding of data onto
`the OFDM sub-carriers can take advantage of frequency
`diversity to mitigate loss due to frequency-selective fading.
`The general principles of OFDM signal transmission can
`be described with reference to FIG. 1 which is a block 40
`diagram of a typical OFDM transmitter according to the
`prior art An OFDM transmitter 10 receives a stream of
`baseband data bits 12 as its input. These input data bits 12
`are immediately fed into an encoder 14, which takes these
`data bits 12 in segments of B bits every T g +Ts seconds, 45
`where Ts is an OFDM symbol interval and T g is a cyclic
`prefix or guard interval. Encoder 14 typically uses a block
`and/or convolutional coding scheme to introduce error(cid:173)
`correcting and/or error-detecting redundancy into the seg(cid:173)
`ment of B bits and then sub-divides the coded bits into 2N 50
`sub-segments of m bits. The integer m typically ranges from
`2 to 6.
`In a typical OFDM transmission system, there are 2N+ 1
`OFDM sub-carriers, including the zero frequency DC sub(cid:173)
`carrier which is not generally used to transmit data since it
`has no frequency and therefore no phase. Accordingly,
`encoder 14 then typically performs zm -ary quadrature ampli(cid:173)
`tude modulation (QAM) encoding of the 2N sub-segments
`of m bits in order to map the sub-segments of m bits to
`predetermined corresponding complex-valued points in a
`2m-ary constellation. Each complex-valued point in the
`constellation represents discrete values of phase and ampli(cid:173)
`tude. In this way, encoder 14 assigns to each of the 2N
`sub-segments of m bits a corresponding complex-valued
`2m-ary QAM sub-symbol ck=ak+jbk, where -N~k~N, in
`order to create a sequence of frequency-domain sub-symbols
`that encodes the B data bits. Also, the zero-frequency
`
`N
`u( t) = l: Ck exp(-2rtffit)
`k=-N
`
`0;;:;; t;;:;; T,
`
`(1)
`
`As a result of this discrete-valued modulation of the
`OFDM sub-carriers by frequency-domain sub-symbols over
`symbol intervals ofT s seconds, the OFDM sub-carriers each
`display a sine x=( sin x)/x spectrum in the frequency domain.
`By spacing each of the 2N+ 1 sub-carriers 1/f s apart in the
`frequency domain, the primary peak of each sub-carrier's
`sine x spectrum coincides with a null of the spectrum of
`every other sub-carrier. In this way, although the spectra of
`the sub-carriers overlap, they remain orthogonal to one
`another. FIG. 2 illustrates the arrangement of the OFDM
`sub-carriers as well as the envelope of their modulated
`spectra within an OFDM channel bandwidth, BW, centered
`around a carrier frequency, fer Note that the modulated
`sub-carriers fill the channel bandwidth very efficiently.
`Returning to FIG. 1, the digital time-domain OFDM
`symbols produced by IFFT 16 are then passed to a digital
`signal processor (DSP) 18. DSP 18 performs additional
`spectral shaping on the digital time-domain OFDM symbols
`and also adds a cyclic prefix or guard interval of length T g
`to each symbol. The cyclic prefix is generally just a repeti(cid:173)
`tion of part of the symbol. This cyclic prefix is typically
`longer than the OFDM channel impulse response and,
`therefore, acts to prevent inter-symbol interference (ISI)
`between consecutive symbols.
`The real. and imaginary-valued digital components that
`make up the cyclically extended, spectrally-shaped digital
`time-domain OFDM symbols are then passed to digital-to(cid:173)
`analog converters (DACs) 20 and 22, respectively. DACs 20
`and 22 convert the real and imaginary-valued digital com(cid:173)
`ponents of the time-domain OFDM symbols into in-phase
`and quadrature OFDM analog signals, respectively, at a
`55 conversion or sampling rate fc1u as determined by a clock
`circuit 24. The in-phase and quadrature OFDM signals are
`then passed to mixers 26 and 28, respectively.
`In mixers 26 and 28, the in-phase and quadrature OFDM
`signals from DACs 20 and 22 are used to modulate an
`60 in-phase intermediate frequency (IF) signal and a 90° phase(cid:173)
`shifted (quadrature) IF signal, respectively, in order to
`produce an in-phase IF OFDM signal and a quadrature IF
`OFDM signal, respectively. The in-phase IF signal that is fed
`to mixer 26 is produced directly by a local oscillator 30,
`65 while the 90° phase-shifted IF signal that is fed to mixer 28
`is produced by passing the in-phase IF signal produced by
`local oscillator 30 through a 90° phase-shifter 32 before
`
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`5,732,113
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`3
`feeding it to mixer 28. These two in-phase and quadrature IF
`OFDM signals are then combined in combiner 34 to form a
`composite IF OFDM signal. In some prior art transmitters,
`the IF mixing is performed in the digital domain using a
`digital synthesizer and digital mixers before the digital-to(cid:173)
`analog conversion is performed.
`This composite IF OFDM signal is then passed into radio
`frequency (RF) transmitter 40. Many variations of RF trans(cid:173)
`mitter 40 exist and are well known in the art, but typically,
`RF transmitter 40 includes an IF bandpass filter 42, an RF
`mixer 44, an RF carrier frequency local oscillator 46, an RF
`bandpass filter 48, an RF power amplifier 50, and an antenna
`52. RF transmitter 40 takes the IF OFDM signal from
`combiner 34 and uses it to modulate a transmit carrier of
`frequency fer generated by RF local oscillator 46, in order
`to produce an RP OFDM-modulated carrier that occupies a
`channel bandwidth. BM. Because the entire OFDM signal
`must fit within this channel bandwidth, the channel band(cid:173)
`width must be at least (l/fs)-(2N+l) Hz wide to accommo(cid:173)
`date all the modulated OFDM sub-carriers. The frequency(cid:173)
`domain characteristics of this RF OFDM-modulated carrier
`are illustrated in FIG. 2. This RF OFDM-modulated carrier
`is then transmitted from antenna 52 through a channel, to an
`OPDM receiver in a remote location. In alternative embodi(cid:173)
`ments of RF transmitter 40, the OFDM signal is used to
`modulate the transmit carrier using frequency modulation
`(FH), single-sideband modulation (SSB), or other modula(cid:173)
`tion techniques. Therefore, the resulting RP OFDM(cid:173)
`modulated carrier may not necessarily have the exact shape
`of the RP OPDM-modulated carrier illustrated in FIG. 2 (i.e.
`the RF OPDM-modulated carrier might not be centered
`around the transmit carrier, but instead may lie to either side
`of it).
`In order to receive the OFDM signal and to recover the
`baseband data bits that have been encoded into the OFDH 35
`sub-carriers at a remote location, an OFDM receiver must
`perform essentially the inverse of all the operations per(cid:173)
`formed by the OFDM transmitter described above. These
`operations can be described with reference to FIG. 3 which
`is a block diagram of a typical OPDM receiver according to
`the prior art.
`The first element of a typical OFDM receiver 60 is an RF
`receiver 70. Like RF transmitter 40, many variations of RF
`receiver 70 exist and are well known in the art, but typically, 45
`RF receiver 70 includes an antenna 72, a low noise amplifier
`(LNA) 74, an RF bandpass filter 76, an automatic gain
`control (AGC) circuit 77, an RF mixer 78, an RF carrier
`frequency local oscillator 80, and an IF bandpass filter 82.
`Through antenna 72, RF receiver 70 couples in the RF
`OFDM-modulated carrier after it passes through the chan(cid:173)
`nel. Then, by mixing it with a receive carrier of frequency
`fer generated by RF local oscillator 80, RF receiver 70
`downconverts the RF OFDM-modulated carrier to obtain a
`received IF OFDM signal. The frequency difference 55
`between the receive carrier and the transmit carrier contrib(cid:173)
`utes to the carrier frequency offset, Mc.
`This received IF OFDM signal then feeds into both mixer
`84 and mixer 86 to be mixed with an in-phase IF signal and
`a 90° phase-shifted (quadrature) IF signal, respectively, to
`produce in-phase and quadrature OFDM signals, respec(cid:173)
`tively. The in-phase IF signal that feeds into mixer 84 is
`produced by an IF local oscillator 88. The 90° phase-shifted
`IF signal that feeds into mixer 86 is derived from the
`in-phase IF signal of IF local oscillator 88 by passing the
`in-phase IF signal through a 90° phase shifter 90 before
`feeding it to mixer 86.
`
`4
`The in-phase and quadrature OFDM signals then pass into
`analog-to-digital converters (ADCs) 92 and 93, respectively,
`where they are digitized at a sampling rate fck_r as deter(cid:173)
`mined by a clock circuit 94. ADCs 92 and 93 produce digital
`5 samples that form an in-phase and a quadrature discrete-time
`OFDM signal, respectively. The difference between the
`sampling rates of the receiver and that of the transmitter is
`the sampling rate offset, Mck=fck_r-fc1c;_r
`The unfiltered in-phase and quadrature discrete-time
`10 OFDM signals from ADCs 92 and 93 then pass through
`digital low-pass filters 96 and 98, respectively. The output of
`lowpass digital filters 96 and 98 are filtered in-phase and
`quadrature samples, respectively, of the received OFDM
`signal. In this way, the received OFDM signal is converted
`into in-phase ( q;) and quadrature (p;) samples that represent
`15 the real and irnaginary-valued components, respectively, of
`the complex-valued OFDM signal, r,=q;+jp;. These in-phase
`and quadrature (real-valued and imaginary-valued) samples
`of the received OFDM signal are then delivered to DSP 100.
`Note that in some prior art implementations of receiver 60,
`20 the analog-to-digital conversion is done before the IF mixing
`process. In such an implementation, the mixing process
`involves the use of digital mixers and a digital frequency
`synthesizer. Also note that in many prior art implementa(cid:173)
`tions of receiver 60, the digital-to-analog conversion is
`25 performed after the filtering.
`DSP 100 performs a variety of operations on the in-phase
`and quadrature samples of the received OFDM signal. These
`operations may include: a).synchronizing receiver 60 to the
`timing of the symbols and data frames within the received
`30 OFDM signal, b) estimating and correcting for the carrier
`frequency offset Mc, of the received OFDM signal, c)
`removing the cyclic prefixes from the received OFDM
`signal, d) computing the discrete Fourier transform (DFT) or
`preferably the fast Fourier transform (FFT) of the received
`OFDM signal in order to recover the sequences of
`frequency-domain sub-symbols that were used to modulate
`the sub-carriers during each OFDM symbol interval, and e)
`performing any required channel equalization on the sub(cid:173)
`carriers. In some implementations, DSP 100 also estimates
`and corrects the sampling rate offset, Mele' Finally, DSP 100
`40 computes a sequence of frequency-domain sub-symbols, yk,
`from each symbol of the OFDM signal by demodulating the
`sub-carriers of the OFDM signal by means of the FFT
`calculation. DSP 100 then delivers these sequences of sub-
`symbols to a decoder 102.
`Decoder 102 recovers the transmitted data bits from the
`sequences of frequency-domain sub-symbols that are deliv(cid:173)
`ered to it from DSP 100. This recovery is performed by
`decoding the frequency-domain sub-symbols to obtain a
`stream of data bits 104 which should ideally match the
`50 stream of data bits 12 that were fed into the OFDM trans(cid:173)
`mitter 10. This decoding process can include soft Viterbi
`decoding and/or Reed-Solomon decoding, for example, to
`recover the data from the block and/or convolutionally
`encoded sub-symbols.
`In a typical OFDM data transmission system such as one
`for implementing digital television or a wireless local area
`network (WI.AN), data is transmitted in the OFDM signal in
`groups of symbols known as data frames. This prior art
`concept is shown in FIG. 4 where a data frame 100 includes
`60 M consecutive symbols 111.a, 112b, ... , 112M, each of
`which includes a guard interval, T g• as well as the OFDM
`symbol interval, T... Therefore, each symbol has a total
`duration of T g +Ts seconds. Depending on the application,
`data frames can be transmitted continuously, such as in the
`65 broadcast of digital TV, or data frames can be transmitted at
`random times in bursts, such as in the implementation of a
`WLAN.
`
`ERIC-1002 / Page 18 of 33
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`5,732,113
`
`6
`of a problem than that of determining symbol timing and
`correcting carrier frequency offset, uncorrected sampling
`frequency offset can contribute to increased BER.
`
`DESCRIPTION OF THE PRIOR ARf
`
`10
`
`5
`The transmission of data through a channel via an OFDM
`signals provides several advantages over more conventional
`transmission techniques. These advantages include:
`a) Tolerance to multipath delay spread. This tolerance is
`due to the relatively long symbol interval Ts compared 5
`to the typical time duration of the channel impulse
`response. These long symbol intervals prevent inter(cid:173)
`symbol interference (ISI).
`b) Tolerance to frequency selective fading. By including
`redundancy in the OFDM signal, data encoded onto
`fading sub-carriers can be reconstructed from the data
`recovered from the other sub-carriers.
`c) Efficient spectrum usage. Since OFDM sub-carriers are
`placed in very close proximity to one another without
`the need to leave unused frequency space between
`them, OFDM can efficiently fill a channel.
`d) Simplified sub-channel equalization. OFDM shifts
`channel equalization from the time domain (as in single
`carrier transmission systems) to the frequency domain
`where a bank of simple one-tap equalizers can indi(cid:173)
`vidually adjust for the phase and amplitude distortion
`of each sub-channel.
`e) Good interference properties. It is possible to modify
`the OFDM spectrum to account for the distribution of
`power of an interfering signal. Also, it is possible to
`reduce out-of-band interference by avoiding the use of
`OFDM sub-carriers near the channel bandwidth edges.
`Although OFDM exhibits these advantages, prior art
`implementations of OFDM also exhibit several difficulties
`and practical limitations. The most important difficulty with
`implementing OFDM transmission systems is that of
`achieving timing and frequency synchronization between
`the transmitter and the receiver. There are three aspects of
`synchronization that require careful attention for the proper
`reception of OFDM signals.
`First, in order to properly receive an OFDM signal that
`has been transmitted across a channel and demodulate the
`symbols from the received signal, an OFDM receiver must
`determine the exact timing of the beginning of each symbol
`within a data frame. If correct timing is not known, the
`receiver will not be able to reliably remove the cyclic
`prefixes and correctly isolate individual symbols before
`computing the FFT of their samples. In this case, sequences
`of sub-symbols demodulated from the OFDM signal will
`generally be incorrect, and the transmitted data bits will not
`be accurately recovered.
`Equally important but perhaps more difficult than achiev(cid:173)
`ing proper symbol timing is the issue of determining and
`correcting for carrier frequency offset, the second major
`aspect of OFDM synchronization. Ideally, the receive carrier
`frequency, fen should exactly match the transmit carrier
`frequency, fer If this condition is not met, however, the
`mis-match contributes to a non-zero carrier frequency offset,
`Mc, in the received OFDM signal. OFDM signals are very
`susceptible to such carrier frequency offset which causes a
`loss of orthogonality between the OFDM sub-carriers and
`results in inter-carrier interference (ICI) and a severe
`increase in the bit error rate (BER) of the recovered data at
`the receiver.
`The third synchronization issue of concern when imple(cid:173)
`menting an OFDM communication system is that of syn(cid:173)
`chronizing the transmitter's sample rate to the receiver's
`sample rate to eliminate sampling rate offset. Any mis-match
`between these two sampling rates results in a rotation of the
`2m-ary sub-symbol constellation from symbol to symbol in
`a frame. Although correcting for sampling rate offset is less
`
`15
`
`20
`
`In order to solve the above-mentioned synchronization
`problems associated with the proper reception OFDM
`signals, several synchronization and correction techniques
`have been previously suggested and developed.
`In U.S. Pat. No. 5,444,697, Leung et al. suggest a tech-
`nique for achieving timing synchronization of a receiver to
`an OFDM signal on a frame-by-frame basis. The method,
`however, requires that a plurality of the OFDM sub-carriers
`be reserved exclusively for data synchronization, thus reduc(cid:173)
`ing the number of sub-carriers used for encoding and
`transmitting data. Furthermore, Leung does not suggest a
`technique for correcting the carrier frequency offset or
`sampling rate offset. Finally, Leung's technique requires a
`loop-back to determine the phase and amplitude of each
`sub-channel, thereby rendering the technique unsuitable for
`broadcast applications such as digital TV.
`In U.S. Pat. No. 5,345,440, Gledhill et al. present a
`method for improved demodulation of OFDM signals in
`25 which the sub-carriers are modulated with values from a
`quadrature phase shift keying (QPSK) constellation.
`However, the disclosure does not teach a reliable way to
`estimate the symbol timing. Instead, assuming approximate
`tirning is already known, it suggests taking an FFT of the
`30 OFDM signal samples and measuring the spread of the
`resulting data points to suggest the degree of timing syn(cid:173)
`chronization. This technique, however, requires a very long
`time to synchronize to the OFDM signal since there is an
`FFT in the timing synchronization loop. Also, their method
`35 for correcting for carrier frequency offset assumes that
`timing synchronization is already known. Furthermore, the
`achievable carrier offset acquisition range is limited to half
`a sub-channel bandwidth. This very limited range for carrier
`offset correction is insufficient for applications such as
`40 digital television where carrier frequency offsets are likely to
`be as much as several tens of sub-carrier bandwidths.
`Finally, the disclosure does not teach a method for correcting
`for sampling rate offset.
`In U.S. Pat. No. 5,313,169, Fouche et al. suggest a method
`45 for estimating and correcting for the carrier frequency offset
`and s sampling rate offset of a receiver receiving an OFDM
`signal. The method requires the inclusion of two additional
`pilot frequencies within the channel bandwidth. The success
`of this method is limited because these pilot carriers are
`50 susceptible to multipath fading. Furthermore, Fouche et al.
`do not suggest a reliable method for determining symbol
`timing. They discuss subtracting the cyclic prefix from each
`symbol and then trying to find where there is a cancellation,
`but such a cancellation will not occur in the presence of
`55 carrier frequency offset. Also, because their synchronization
`loop includes a computationally complex FFT, synchroni(cid:173)
`zation takes a long time. Additionally, because the method
`does not correct for carrier frequency offset before taking the
`FFTs, the method will suffer from inter-carrier interference
`60 between the sub-carriers, thus limiting its performance.
`Finally, the method also has a limited acquisition range for
`the carrier frequency offset estimation.
`In "A Technique for Orthogonal Frequency Division
`Multiplexing Frequency Offset Correction," IEEE Transac-
`65 tions on Communications, Vol. 42, No. 10, October 1994,
`pp. 2908-14; and in "Synchronization Algorithms for an
`OFDM System for Mobile Communications." ITG-
`
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`5,732,113
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`8
`c) to provide a method for rapidly estimating and cor(cid:173)
`recting for the sampling rate offset of an OFDM
`receiver, thereby allowing reception and demodulation
`of the OFDM symbols in a burst frame with minimized
`BER;
`d) to provide a method for continuously tracking the
`symbol and frame timing of an OFDM signal consist(cid:173)
`ing of continuously transmitted data frames;
`e) to provide a method for continuously tracking and
`correcting for the carrier frequency offset of an OFDM
`receiver thereby allowing continuous reception of an
`OFDM signal without loss of orthogonality between
`the sub-carriers and an corresponding increase in BER;
`f) to provide a method for continuously tracking and
`correcting the sampling rate offset of an OFDM
`receiver;
`g) to provide a low-complexity method and apparatus for
`all the above that requires relatively little computation;
`h) to provide a robust method and apparatus for all the
`above that works well even in a fading channel by
`averaging over many or all of the sub-ca