(19) United States
`(12) Patent Application Publication (10) Pub. No.: US 2004/0228270 A1
`(43) Pub. Date:
`Nov. 18, 2004
`Chen et al.
`
`US 2004022827OA1
`
`(54) METHOD OF PROCESSING AN OFDM
`SIGNAL AND OFDM RECEIVER USING THE
`SAME
`(76) Inventors: Hou-Shin Chen, Taipei (TW);
`Wei-Hung He, Taipei (TW);
`Shiou-Hung Chen, Taipei (TW)
`Correspondence Address:
`BRCH STEWART KOLASCH & BRCH
`PO BOX 747
`FALLS CHURCH, VA 22040-0747 (US)
`(21) Appl. No.:
`10/436,138
`(22) Filed:
`May 13, 2003
`
`Publication Classification
`
`(51) Int. Cl." ...................................................... H04J 11/00
`(52) U.S. Cl. .............................................................. 370/210
`
`ABSTRACT
`(57)
`A method of processing an OFDM signal to determine its
`FFT mode or the number of carriers without regard to the
`pilot pattern. The OFDM receiver determines autocorrela
`tion functions corresponding to a plurality of possible FFT
`modes and variation-to-average ratioS of these autocorrela
`tion functions, respectively. The correct FFT mode is deter
`mined based on the variation-to-average ratios. In addition,
`a novel time and frequency Synchronization Scheme is
`performed after the correct FFT mode is detected.
`
`OO
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`

`

`Patent Application Publication Nov. 18, 2004 Sheet 1 of 11
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`US 2004/0228270 A1
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`Patent Application Publication Nov. 18, 2004 Sheet 2 of 11
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`US 2004/0228270 A1
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`US 2004/0228270 A1
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`Patent Application Publication Nov. 18, 2004 Sheet 6 of 11
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`US 2004/0228270 A1
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`Patent Application Publication Nov. 18, 2004 Sheet 7 of 11
`
`US 2004/0228270 A1
`
`Blind Mode Detection
`
`Convert the OFDM signal
`
`S10
`
`Determine autocorrelation
`functions corresponding
`to possible FFT modes
`
`Calculate variation-to
`average ratios
`
`
`
`Choose the correct FFT
`mode according to the
`largest variation-to
`average ratio
`
`
`
`S11
`
`S12
`
`S13
`
`Determine ideal waveforms
`of the autocorrelation
`function using different
`guard interval values
`
`S15
`
`Calculate cross-correlation
`functions
`
`S16
`
`
`
`
`
`Calculate the maximal
`samples of the cross
`correlation functions
`corresponding to different
`guard interval values
`
`Choose the correct guard
`interval
`
`FIG. 7
`
`S17
`
`S18
`
`8
`
`

`

`Patent Application Publication Nov. 18, 2004 Sheet 8 of 11
`
`US 2004/0228270 A1
`
`Time Synchronization
`
`Compute an average
`autocorrelation
`function corresponding
`to the correct FFT mode
`
`
`
`
`
`Designate an initial
`sample index of a first
`OFDM symbol
`
`
`
`
`
`
`
`FIG. 8
`
`S20
`
`S22
`
`9
`
`

`

`Patent Application Publication Nov. 18, 2004 Sheet 9 of 11
`
`US 2004/0228270 A1
`
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`

`

`Patent Application Publication Nov. 18, 2004 Sheet 10 of 11
`
`US 2004/0228270 A1
`
`Frequency Synchronization
`
`Assume Af(K+b)/T
`
`S30
`
`Calculate an estimate bo
`using Xno
`
`-->
`
`S32
`
`Determine an average
`correlation coefficient of
`continual pilot carriers
`
`S34
`
`Determine an estimate Ko
`
`S36
`
`Perform the frequency
`compensation using
`Af(Kobo)/TU
`
`S38
`
`FIG. 10
`
`
`
`
`
`
`
`
`
`11
`
`

`

`Patent Application Publication Nov. 18, 2004 Sheet 11 of 11
`
`US 2004/0228270 A1
`
`Modified Frequency Synchronization
`
`compensation usingbo/TU
`
`compensation using Ko/TU
`
`Perform the frequency
`compensation using b/T
`
`S52
`
`FIG 11
`
`12
`
`

`

`US 2004/022827O A1
`
`Nov. 18, 2004
`
`METHOD OF PROCESSING AN OFDM SIGNAL
`AND OFDM RECEIVER USING THE SAME
`
`BACKGROUND OF THE INVENTION
`0001) 1. Field of the Invention
`0002 The present invention relates to a Terrestrial Digital
`Video Broadcasting (DVB-T) system, and in particular to a
`broadband DVB-T receiver using concatenated-coded
`Orthogonal Frequency Division Multiplexing (OFDM) tech
`nology, which provides a System-independent mode detec
`tion Scheme and performs required time and frequency
`Synchronization.
`0003 2. Description of the Related Art
`0004 DVB-T is a next-generation standard for wireless
`broadcast of Motion Picture Experts Group 2 (MPEG-2)
`Video. In order to provide the high data rate required for
`video transmission, concatenated-coded OFDM have been
`adopted into the DVB-T standard.
`0005 OFDM is a multi-carrier communication scheme to
`deal with data transmission over multi-path channels. In
`OFDM, each carrier used for transmission has rectangular
`waveform, which can be easily formed by Inverse Fast
`Fourier Transform (IFFT) in a transmitter and reversed by
`FFT in a receiver. In addition, the carriers of an OFDM
`signal are orthogonal to each other. FIG. 1 (Prior Art)
`illustrates an example of Spectra of a multitude of carriers in
`an OFDM signal. As shown in FIG. 1, the frequency spacing
`1/Tu of these carriers is chosen in Such a way that at the
`frequency where one of the carriers is evaluated, Such as
`carrier 1, all other carriers are Zero. Thus, the information
`transmitted over the different carriers can be properly Sepa
`rated.
`0006 In order to reduce inter-symbol interference (ISI)
`due to multi-path channels, the OFDM symbols are
`extended by individually copying a tail portion of the
`OFDM symbol to precede the same one. FIG. 2 (Prior Art)
`is a diagram illustrating the structure of OFDM symbols in
`an OFDM signal. As shown in FIG. 2, a tail portion 10a of
`the symbol X is copied to be a head of the symbol X, which
`is called a guard interval 20a, where the duration of the
`usable part of a symbol is set to be T and the duration is of
`the guard interval 20a is set to be A. If the guard interval 20a
`is longer than the maximum channel delay, all reflections of
`previous OFDM symbols can be removed and the orthogo
`nality is preserved.
`0007. In order to cope with a multitude of propagation
`conditions encountered in the wireleSS broadcast channel,
`many parameters of OFDM for the DVB-T system can be
`dynamically changed according to channel conditions. In
`particular, the number of OFDM carriers, which can be
`either 2048 (2k mode) or 8196 (8k mode), and the guard
`interval value, which can be /4, /s, /16 or /32, should be
`properly determined So that the desired trade-off can be
`Struck between ISI mitigation capability and robustneSS
`against Doppler-Spread. As a result, a “mode detection' that
`detects both the number of carriers and the guard interval
`value in the transmitted OFDM symbol is required in a
`DVB-T receiver.
`0008. The carriers of the transmitted OFDM symbols in
`a DVB-T system can be categorized into four types, includ
`
`ing data carriers, Scattered pilot carriers, continual pilot
`carriers and Transmission Parameter Signaling (TPS) carri
`ers. FIG.3 (Prior Art) is a diagram illustrating the allocation
`of these kinds of carriers in the OFDM symbols of a DVB-T
`signal. It is noted that, in the DVB-T standard, only 1705
`carriers are active in the 2k mode and only 6817 carriers are
`valid in the 8k mode. As shown in FIG. 3, the data carriers
`are used for ordinary data transmission. The Scattered pilot
`carriers are allocated in different carriers in different Sym
`bols and the continual pilot carriers are allocated every 48
`carriers in these symbols. The TPS carriers convey various
`transmission parameters, Such as modulation information,
`hierarchical information, guard intervals, inner code rates,
`transmission modes (such as 2k or 8k modes) and So on,
`from the DVB-T transmitter to the DVB-T receiver. More
`details about the DVB-T standard can be found in the Digital
`Video Broadcasting Standards.
`0009. As described above, the mode detection that
`detects the correct FFT mode and the selected guard interval
`value of a transmitted DVB-T signal is essential since other
`processing operations must be performed after the correct
`number of carriers and the length of the guard interval or
`cyclic prefix have been both determined. The conventional
`Scheme for mode detection is carried out by detecting
`positions of pilot SubSymbols, which, however, requires the
`knowledge of the pilot pattern in advance and is therefore
`System-dependent. It is also noted that the mode information
`carried by the TPS carriers cannot be located and interpreted
`before the correct mode has been detected. Thus, a blind
`mode detection Scheme which is System-independent is
`necessary for a DVB-T receiver.
`0010. In addition to mode detection, time and frequency
`synchronization are also required in any OFDM transmis
`Sion System. Time Synchronization finds out the beginning
`of a received OFDM symbol, that is, an initial sample of a
`first OFDM symbol in an OFDM-based signal, to synchro
`nize the time Scales of the transmitter and the receiver.
`Frequency Synchronization eliminates the frequency devia
`tion between oscillators of the transmitter and the receiver.
`After the correct mode is detected, an efficient and accurate
`Scheme for time and frequency Synchronization has to be
`taken prior to Succeeding manipulation.
`
`SUMMARY OF THE INVENTION
`0011. Accordingly, an object of the present invention is to
`provide an OFDM receiver, especially a DVB-T receiver,
`which can initially detect the correct FFT mode and the
`guard interval value of a received OFDM signal without
`knowledge of the pilot pattern.
`0012 Another object of the present invention is to pro
`vide a method of processing an OFDM or DVB-T signal,
`which can initially detect the correct mode without knowl
`edge of the pilot pattern and efficiently perform time and
`frequency Synchronization.
`0013 In a preferred embodiment, the present invention
`provides a method of processing an OFDM signal transmit
`ted by an OFDM transmitter to determine the correct FFT
`mode and the guard interval length in an OFDM receiver.
`Preferably, the OFDM signal can be a DVB-T signal for
`video transmission. The OFDM signal is first converted to a
`digital received signal. There are several possible FFT
`modes and guard interval lengths defined in the OFDM
`
`13
`
`

`

`US 2004/022827O A1
`
`Nov. 18, 2004
`
`receiver. In the DVB-T case, the possible FFT modes
`include the 2k mode and the 8k mode. Next, the OFDM
`receiver determines autocorrelation functions of the digital
`received signal corresponding to these possible FFT modes,
`respectively. In addition, variation-to-average ratioS of these
`autocorrelation functions are calculated, respectively. Pref
`erably, the variation-average ratio is a ratio of a variance and
`an average of the corresponding autocorrelation function. If
`the proper FFT mode is chosen, its variation-to-average ratio
`is the highest due to the cyclic nature of OFDM symbols.
`Thus, one of the possible FFT modes with the largest
`variation-to-average ratio is chosen as the correct mode. In
`addition, the guard interval value defined in the OFDM
`Signal can be determined by the autocorrelation function.
`First, a plurality of ideal waveforms of the autocorrelation
`function using a plurality of possible guard interval values
`are respectively determined. Next, a plurality of croSS
`correlation functions of the ideal waveforms and the auto
`correlation function corresponding to the correct FFT mode
`are also calculated. Next, the maximal Samples of the
`cross-correlation functions corresponding to the possible
`guard interval values are estimated. If the proper guard
`interval value is chosen, the maximal Sample of its croSS
`correlation function is the highest due to the Similarity
`between the true autocorrelation function and the corre
`sponding ideal waveform. Thus, one of the possible guard
`interval values is chosen as the correct one according to the
`maximal Samples of their cross-correlation functions.
`0.014. Since such scheme does not utilize the pilot infor
`mation, the mode-detection method of the present invention
`is System-independent.
`0.015 Using the detected FFT mode and guard interval
`length, the OFDM receiver can further perform the time and
`frequency Synchronization. With respect to the time Syn
`chronization, an average autocorrelation function of the
`autocorrelation function corresponding to the correct FFT
`mode is computed over a length of observed OFDM sym
`bols of the digital received signal. Thus, an initial Sample
`index of a first OFDM symbol among the observed OFDM
`Symbols of the digital received signal can be designated
`according to the average autocorrelation function. With
`respect to the frequency Synchronization, the frequency
`offset between oscillators of the OFDM transmitter and the
`OFDM receiver is assumed as (K+b)/T, where K is an
`integer and -0.5s b<0.5, and 1/T represents a carrier spac
`ing of the digital received signal. An estimate for the
`parameter b can be determined using phase information of
`the average autocorrelation function with respect to the
`initial sample index of the first OFDM symbol. An estimate
`for the parameter K can be determined by the property of the
`continual pilot carriers in the DVB-T system. An average
`correlation coefficient of continual pilot carriers in two
`consecutive OFDM symbols of the digital received signal
`over a continual pilot carrier indeX is first determined. The
`estimate for the parameter K is determined by an index of
`the average correlation coefficient, which maximizes the
`average correlation coefficient. The frequency compensation
`for the digital received Signal is performed using the fre
`quency offset determined by the estimates for b and K.
`0016 A detailed description is given in the following
`embodiments with reference to the accompanying drawings.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`0017. The present invention can be more fully understood
`by reading the Subsequent detailed description and examples
`with references made to the accompanying drawings,
`wherein:
`0018 FIG. 1 illustrates an example of spectra of a
`multitude of carriers in an OFDM signal;
`0019 FIG. 2 is a diagram illustrating the structure of
`OFDM symbols in an OFDM signal;
`0020 FIG. 3 is a diagram illustrating the allocation of
`different kinds of carriers in the symbols of a DVB-T signal;
`0021 FIG. 4 is a block diagram of a DVB-T receiver in
`accordance with the preferred embodiment of the present
`invention;
`0022 FIG. 5 is a diagram showing the autocorrelation
`functions corresponding to the 2k mode and the 8k mode
`when a 2k-mode Signal is transmitted;
`0023 FIG. 6 is a diagram illustrating the waveforms of
`the autocorrelation functions in terms of four possible guard
`interval values of a DVB-T signal;
`0024 FIG. 7 is a flowchart showing the blind mode
`detection Scheme of the present invention;
`0025 FIG. 8 is a flowchart showing the time synchro
`nization Scheme of the present invention;
`0026 FIG. 9 is a diagram showing a frequency offset Af
`between transmitted and received signals in the present
`invention;
`0027 FIG. 10 is a flowchart showing the frequency
`Synchronization Scheme of the present invention; and
`0028 FIG. 11 is a flowchart showing the modified fre
`quency Synchronization Scheme of the present invention.
`
`DETAILED DESCRIPTION OF THE
`INVENTION
`0029. The present invention discloses a blind mode
`detection Scheme, which is System-independent, to deter
`mine the correct mode of an OFDM signal transmitted by a
`DVB-T transmitter or other OFDM transmitters. In the
`preferred embodiment, a DVB-T receiver is used as an
`example for explaining the mode detection Scheme and the
`time and frequency Synchronization Schemes in the present
`invention. The mode detection Scheme, however, is not
`limited in the DVB-T application and can be applied to other
`OFDM systems.
`0030 FIG. 4 is a block diagram of a DVB-T receiver in
`accordance with the preferred embodiment of the present
`invention. As shown in FIG. 4, the DVB-T receiver includes
`an antenna 70, a RF tuner 80, an analog-to-digital converter
`90, a mode detector 100, a time/frequency synchronizer 110,
`a frequency compensation circuit 120, a Cyclic Prefix (CP)
`remover 130, a serial-to parallel converter 140, a FFT unit
`150, a channel estimation circuit 160, a frequency domain
`equalizer 170 and decision circuit 180.
`0031 ADVB-T signal is first received by antenna 70 and
`processed by RF tuner 80. Then the received DVB-T signal
`is converted to a digital received signal rn by A/D con
`verter 90. The digital received signal rn is first fed to the
`
`14
`
`

`

`US 2004/022827O A1
`
`Nov. 18, 2004
`
`mode detector 100 and the time/frequency synchronizer 110
`to determine the FFT mode (2k or 8k mode), the guard
`interval value, optimal timing and carrier frequency offset.
`After the correct mode is detected, the frequency compen
`sation circuit 120 uses the frequency offset estimated by the
`time/frequency synchronizer 110 to deal with the digital
`received signal rn. In addition, the CP remover 130 uses
`the optimal timing information determined by the time/
`frequency synchronizer 110 to remove the CP (or called the
`guard interval) of the digital received signal rn. The
`resulted signal is then serial-to-parallel converted by S/P
`converter 140 and transformed into the frequency domain
`using FFT unit 150. Channel estimation circuit 160 esti
`mates the channel transfer function and Frequency domain
`equalizer 170 performs the frequency domain equalization.
`Finally, the decision circuit 180 recovers the transmitted
`Symbols of the digital received signal.
`0032 Mode detection performed by mode detector 100
`and time/frequency Synchronization performed by Synchro
`nizer 110 are main issues in the preferred embodiment,
`described in detail as follows.
`0033) Mode Detection
`0034.
`In the preferred embodiment, blind mode detec
`tion, which is performed in the absence of the pilot pattern
`and therefore System-independent, is adopted to determine
`the correct mode of the digital received signal rn. The
`mode detector 100 detects the correct number of carriers and
`the proper guard interval value by exploiting the cyclic
`nature of OFDM symbols. In the DVB-T system, two
`possible FFT modes, namely 2k and 8k, can be utilized for
`data transmission. In addition, four possible guard interval
`values, including/4, /s, /16 and /32, can be used to determine
`the length of the cyclic prefix. The goal of the mode detector
`100 is to choose the correct FFT mode from the two FFT
`modes and to determine the proper guard interval value.
`0035) Mathematically, the autocorrelation function of a
`Signal with finite energy gives a measure of Similarity or
`coherence between a signal and a delayed version of the
`Signal. The autocorrelation function of the digital received
`Signal rn can, therefore, exhibit the Similarity between
`OFDM symbols. In the preferred embodiment, the autocor
`relation functions of the digital received signal rn corre
`sponding to the possible FFT modes are defined as:
`
`0036) where the index i indicates the FFT mode,
`namely the 2k or 8k mode and Q is an integer. N is
`the number of carriers of the corresponding FFT
`mode. Thus, N, is 2048 for the 2k mode and 8192 for
`the 8k mode. Apparently, the autocorrelation func
`tion shown in equation (1) can indicate the Similarity
`of Subsymbols rn- and rn-j-N separated by N. It
`is noted that the format of the autocorrelation func
`tion shown in equation (1) is not intended to limit the
`Scope of the present invention.
`0037. Due to the cyclic nature of OFDM symbols, peri
`odic peaks can be observed in the autocorrelation function
`
`Xn only when the N is set to the correct value. The
`autocorrelation function appears as a noise-like waveform
`when an incorrect value of Ni is employed. FIG. 5 is a
`diagram showing the autocorrelation functions correspond
`ing to the 2k mode and the 8k mode when a 2k-mode Signal
`is transmitted, where numeral 30 indicates the 2k-mode case
`and numeral 40 indicates the 8k-mode case. As shown in
`FIG. 5, the autocorrelation function of the correct 2k mode
`shows periodic peaks, Such that those of the incorrect 8k
`mode look like noise.
`0038 According to FIG. 5, the relative dynamic ranges
`of the autocorrelation functions X-in and Xsin can be
`regarded as a measure of difference between correct and
`incorrect FFT modes. A variation-to-average ratio, which is
`a ratio of a variance and an average of the autocorrelation
`function in the preferred embodiment, is used to examine the
`relative dynamic range and can be expressed as:
`
`(1x (nl) - (x (n1)
`
`(2)
`
`0039 where the function <>denotes time-averaging
`over a number of samples. Obviously, the variation
`to-average ratio of the autocorrelation function cor
`responding to the correct FFT mode is the larger one.
`Thus, the transmission mode can then be detected as
`follows:
`
`(3)
`i=argmaxM.
`0040 where arg() denotes an argument function for
`determining the index (or mode) of the maximal
`variation-to-average ratio.
`0041 As well, the autocorrelation function determined in
`equation (1) can be applied to determine the proper guard
`interval value of the digital received signal. According to the
`property of the autocorrelation function, the waveform of the
`autocorrelation function is similar to the Square wave. In
`addition, the duty cycle and the duration of its waveform
`depend on the guard interval length of the digital received
`signal. FIG. 6 is a diagram illustrating the waveforms of the
`autocorrelation functions in terms of four possible guard
`interval values, namely, 4, /s, /16 and /32. The ideal
`waveform of the autocorrelation function, denoted by yik,
`is a periodic Square wave and can be expressed as:
`
`1
`- - - - - - Y -
`yik - an(l -- st) sk < 57.1 + an( -- shi). a e Z
`
`(4)
`
`O
`
`otherwise
`
`0042 where N is the number of carriers and index i1=1,
`2, 3 or 4 representing the guard interval value /4, /s, /16 or
`/32, respectively.
`0043. The straight-forward Scheme for detecting the
`guard interval value is to calculate a cross-correlation func
`tion of the autocorrelation function xn and different ideal
`autocorrelation functions as shown in FIG. 6. The cross
`correlation function can be expressed as:
`
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`US 2004/022827O A1
`
`Nov. 18, 2004
`
`c'N(1+shi)
`
`(5)
`
`0044) where c' is a positive integer and represents
`the observation length. Thus, the correct guard inter
`Val value (or the corresponding index i1) can be
`determined by the maximal value of the croSS
`correlation function Bin over the indices n and i1.
`0.045 According to the above description, the practical
`process of determining the correct guard interval value can
`be implemented as follows. At first, the cross-correlation
`function of the autocorrelation function X. and the ideal
`waveform yi
`is determined. In order to reduce the com
`puting complexity, equation (5) can be modified as:
`
`0046 where the parameter b is defined as “block
`Size' and Satisfies the condition:
`
`0047 Next, a sample position index “n” of the cross
`correlation function Bin for eachii, which has a maximum
`value, is determined. That is,
`
`nil = argmax B;
`
`(8)
`
`0048. Using the sample position index “n”, an estimate
`B for the similarity of the autocorrelation function and the
`ideal waveform Y. is determined as:
`
`N(1+1)
`Xiyi ni Xb - i.
`
`(9)
`
`0049 Finally, the index for the correct guard interval
`value can be determined since the estimate 6 with respect
`to the correct guard interval value is maximal.
`
`1 = argmax B;
`
`(10)
`
`More particularly, steps S11-S13 are used to determine the
`correct FFT mode and steps S15-S18 are used to determine
`the correct guard interval value. If there are plural possible
`FFT modes or carrier numbers employed in the OFDM
`signal known by the OFDM receiver in advance, then first,
`the OFDM signal is converted to a digital received signal
`rn for further processing (step S10). Next, a plurality of
`autocorrelation functions Xin of the digital received signal
`rn corresponding to a plurality of these possible FFT
`modes are determined using equation (1) or the like (Step
`S11), where the symbol “i” is an index of FFT modes. Next,
`a plurality of variation-to-average ratioS M of the autocor
`relation functions Xn are determined using equation (2) or
`the like, respectively (step S12). Thus, the correct FFT mode
`can be determined according to the largest variation-to
`average ratio among these ratioS M (step S13).
`0051. Some ideal waveforms of the autocorrelation func
`tion using different guard interval values are determined
`(step S15). In the DVB-T case, there are four possible guard
`interval values, including /4, /s, /16 and /32, available in the
`Standard Specification, and the corresponding ideal wave
`forms are shown in FIG. 6. Next, the cross-correlation
`functions of these ideal waveforms and the autocorrelation
`function corresponding to the correct FFT mode are respec
`tively calculated using equation (6) (Step S16). Using equa
`tions (8) and (9), the maximal Samples of the cross-corre
`lation functions corresponding to different guard interval
`values can be calculated and estimated (step S17). Finally,
`the correct guard interval value can be determined according
`to the largest sample among these maximal samples B, (step
`S18).
`0052 AS described above, the blind mode detection
`Scheme of the present invention does not utilize the pilot
`information of the received OFDM signal, which implies
`that Such Scheme is System-independent.
`0053) Time Synchronization
`0054. After the transmission mode is detected, time and
`frequency Synchronization is next performed by the time/
`frequency synchronizer 110. In the proposed DVB-T
`receiver, the boundary between successive OFDM symbols
`is acquired using the following time Synchronization algo
`rithm.
`0055 According to FIG. 5, the autocorrelation function
`Xin corresponding to the correct FFT mode for blind mode
`detection exhibits the periodic property based on the OFDM
`Symbols and can be exploited for time Synchronization. In
`the preferred embodiment, an average autocorrelation func
`tion of the autocorrelation function corresponding to the
`detected FFT mode in blind mode detection over a number
`of observed OFDM symbols is defined as:
`
`-l
`
`(11)
`
`0050. The blind mode detection scheme described above
`can also be applied to other OFDM system. FIG. 7 is a
`flowchart showing the blind mode detection Scheme in an
`OFDM receiver in accordance with the present invention.
`
`0056 where L is the number of the observed OFDM
`symbols and N' is the symbol duration with guard
`interval. The digital received signal has been frame
`Synchronized, the optimal timing no, which repre
`
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`US 2004/022827O A1
`
`Nov. 18, 2004
`
`sents an initial sample index of a first OFDM sym
`bol, is given by
`
`fio
`
`arg, as, xn - d.
`sn-3.N.
`
`(5)
`
`0057 where the integer 8 is a margin that is empiri
`cally determined to reduce the sensitivity of this
`method.
`0.058 FIG. 8 is a flowchart showing the time synchro
`nization scheme in an OFDM receiver in accordance with
`the present invention. First, an average autocorrelation func
`tion of the autocorrelation function corresponding to the
`correct FFT mode is computed (step S20). The average
`autocorrelation function is a time-averaging function over a
`fixed number of observed OFDM symbols. Thus, the initial
`sample index of a first OFDM symbol of the digital received
`Signal can be designated according to the average autocor
`relation function, especially the maximum of the average
`autocorrelation function (step S22). The proposed time
`Synchronization Scheme employs the autocorrelation func
`tion corresponding to the correct FFT mode determined in
`the blind mode scheme.
`0059 Frequency Synchronization
`0060. After the optimal timing is determined, the fre
`quency offset between oscillators of the transmitter and the
`receiver is next estimated by the time/frequency Synchro
`nizer 110 and compensated by the frequency compensation
`circuit 120.
`0061. In the proposed DVB-T receiver, the frequency
`offset Alf is first assumed as:
`
`0062) In equation (12), T is the duration of a usable part
`of an OFDM symbol and 1/T denotes the carrier spacing of
`the OFDM signal. The parameters K and b are the integer
`part and the fractional part, respectively, where -0.5s b<0.5.
`FIG. 9 is a diagram showing a frequency offset Af between
`transmitted and received Signals in the present invention,
`where the frequency offset Afis in the units of the carrier
`spacing 1/T. Determining estimates for the parameters K
`and b are shown as follows.
`0.063 With respect to the parameter b, it can be verified
`that the phase difference between a Sample in the guard
`interval (or called cyclic prefix) of a received OFDM symbol
`and a sample shifted by T. Seconds is roughly -2Jub. Thus,
`the parameter b can be estimated by:
`
`-1
`bo = Arg(x|no)
`
`(13)
`
`0064 where the function Arg(x) denotes the phase
`angle (modulo 2 L) of X. Equation (13) indicates that
`an estimate bo is determined by phase information of
`the average autocorrelation function Xno with
`respect to the initial sample index of the first OFDM
`Symbol determined in the time Synchronization
`Scheme.
`
`0065. In the DVB-T receiver of the preferred embodi
`ment, the parameter K is estimated by the use of the property
`of continual pilot carriers in the DVB-T signal. Basically, the
`active carriers of the DVB-T signal are shifted, ignoring the
`effect of the fractional part b, by K times the carrier spacing
`1/T. Since the continual pilot carriers of two adjacent
`Symbols in the same carrier indeX should be highly corre
`lated, an average correlation coefficient p(ko), which is
`expressed in equation (14), can be utilized to estimate the
`integer part K.
`
`(R(i+1, k + ko) R' (j, k + ko))
`
`(14)
`
`0066 where R(k) represents a received subsymbol
`of the j-th OFDM symbol at the k-th carrier, and the
`function <>represents an average over the continual
`pilot carrier index ki. It can be verified that the
`average correlation coefficient p(ko) is maximized
`when ko=K. Thus, an estimate Ko for the parameter
`K is given by:
`
`Ko = max p(ko)
`k0
`
`(15)
`
`0067. Therefore, the estimated frequency offset is given
`by (Kol-bo)/T.
`0068 FIG. 10 is a flowchart showing the frequency
`Synchronization Scheme of the present invention. First, the
`frequency offset between oscillators of the OFDM transmit
`ter and the OFDM receiver is set as Af=(K+b)/T (step S30),
`where K is an integer part and b is a fractional part. The
`estimate boof the fractional part b is calculated using phase
`information of the average autocorrelation function X.no
`with respect to the initial sample index no of the first OFDM
`symbol (step S32). In the determination of the estimate Koof
`the integer part K, an average correlation coefficient p(k) of
`continual pilot carriers in two consecutive OFDM symbols
`of the digital received signal Over the continual pilot carrier
`index k is first determined (step S34). When the average
`correlation coefficient p(ko) is maximized, the index kois
`regarded as an estimate Kofor the parameter K (step S36).
`Finally, the frequency compensation circuit 120 utilizes the
`estimated frequency offset Af=(Kol-bo)/T to perform a fre
`quency compensation for the digital received signal.
`0069 Preliminary simulat