WO1997041672A1 - Symbole de synchronisation d'un recepteur multiporteuse - Google Patents

Symbole de synchronisation d'un recepteur multiporteuse Download PDF

Info

Publication number
WO1997041672A1
WO1997041672A1 PCT/IB1997/000422 IB9700422W WO9741672A1 WO 1997041672 A1 WO1997041672 A1 WO 1997041672A1 IB 9700422 W IB9700422 W IB 9700422W WO 9741672 A1 WO9741672 A1 WO 9741672A1
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WO
WIPO (PCT)
Prior art keywords
frequency
average phase
phase variation
pilot signals
sample clock
Prior art date
Application number
PCT/IB1997/000422
Other languages
English (en)
Inventor
Nicolas Chauve
Jean-François NORMAND
Original Assignee
Philips Electronics N.V.
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Philips Electronics N.V. filed Critical Philips Electronics N.V.
Publication of WO1997041672A1 publication Critical patent/WO1997041672A1/fr

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Classifications

    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/26—Systems using multi-frequency codes
    • H04L27/2601—Multicarrier modulation systems
    • H04L27/2647—Arrangements specific to the receiver only
    • H04L27/2655—Synchronisation arrangements
    • H04L27/2662—Symbol synchronisation
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/26—Systems using multi-frequency codes
    • H04L27/2601—Multicarrier modulation systems
    • H04L27/2647—Arrangements specific to the receiver only
    • H04L27/2655—Synchronisation arrangements
    • H04L27/2668—Details of algorithms
    • H04L27/2673—Details of algorithms characterised by synchronisation parameters
    • H04L27/2675—Pilot or known symbols
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L7/00—Arrangements for synchronising receiver with transmitter
    • H04L7/04—Speed or phase control by synchronisation signals

Definitions

  • the invention relates to a digital transmission system comprising a transmitter of multicarrier signals transmitted in frames, a part of the multicarriers transmitting pilot signals which are distributed in the frame in a pre-established order, and a receiver comprising: sampling means for sampling the received baseband-converted multicarrier signals with a sample clock, and synchronization means for synchronizing the sample clock.
  • the invention also relates to a receiver utilized in such a system and a method of attaining the synchronization.
  • the system may be utilized, for example, for transmitting digital television signals by a radio channel, more particularly, a disturbed channel called Rayleigh channel.
  • the transmission channel can play an important role by producing intersymbol interference mainly due to multiple paths. This comes from echo phenomena the transmission channel is to process by complex equalizers in monocarrier transmission systems.
  • An interesting solution to solve these echo problems consists of the use of a multicarrier transmission which implements multiplexed distribution of orthogonal frequency division carriers known as OFDM modulation (Orthogonal Frequency-Division Multiplexing).
  • OFDM modulation Orthogonal Frequency-Division Multiplexing
  • An OFDM signal is organized in frames, one frame comprising a sequence of OFDM symbols, one OFDM symbol being capable of transmitting data on each carrier or being capable of transmitting service signals, such as signals used for the automatic gain control, signals used for the synchronization (of frame, timing, radio frequency carrier), or used as reference signals for the differential modulation, or used for other functions.
  • service signals such as signals used for the automatic gain control, signals used for the synchronization (of frame, timing, radio frequency carrier), or used as reference signals for the differential modulation, or used for other functions.
  • known data are inserted into each frame on specific carriers at given instants. These known data are notably pilot signals.
  • the pilot signals appearing for a moment at given instants form the set of the distributed pilot signals.
  • Certain carriers also transmit pilot signals which are continuously transmitted over the whole length of the frame. They form the set of pilot signals called "continuous" .
  • a correct decoding of the transmitted data at the transmitting end calls for a correct synchronization of the receiver.
  • various synchronizations are to be effected: - a synchronization of the radio frequency carrier which converts the received radio frequency signal into a baseband OFDM signal; a synchronization for adjusting the window during which a Fourier transform operation is performed to demultiplex the OFDM carriers; a synchronization of the sampling frequency used for digitizing the baseband OFDM signal so as to avoid an offset of the window which permits of performing the Fourier transform.
  • the invention relates to the latter synchronization.
  • This synchronization is mentioned in the document PCT WO 92/05646.
  • This document indicates the possibility of considering the carrier phases received during a frame, any sampling frequency error having repercussions on the phases of the received carriers. None the less, whereas the synchronization of the sampling frequency is not considered necessary for the system described in that document, the result is that the manner for actually implementing such a technique is not revealed therein.
  • SUMMARY OF THE INVENTION It is thus an object of the invention to describe means which permit of effecting a synchronization of the sampling frequency to effect the digitization of the OFDM signal in optimum fashion.
  • the synchronization means comprise a frequency detector which corrects frequency deviations of the sample clock by calculating a first average phase variation for the pilot signals appearing on the high-frequency carriers, and a second average phase variation for the pilot signals appearing on the low-frequency carriers, said frequency deviations being corrected in proportion to the difference of average phase variations which separates the first average phase variation from the second average phase variation in relation to a frequency gap separating the high frequencies from the low frequencies.
  • the invention also relates to a sample clock synchronization method which comprises the following steps: calculation of a first average phase variation for the pilot signals appearing on high-frequency carriers, calculation of a second average phase variation for the pilot signals appearing on low-frequency carriers, calculation of an error signal for synchronizing the sample clock, the error signal being formed by a difference of average phase variations which separates the first average phase variation from the second average phase variation in relation to a frequency gap separating the high frequencies from the low frequencies.
  • the invention thus provides a rapid synchronization of the sampling frequency.
  • the residual deviation of the sampling frequency resulting therefrom is reduced to a very small minimum depending on the characteristics of the loop used for the frequency alignment.
  • the system is very well adapted to operating with Rayleigh channels.
  • Fig. 1 shows a general diagram of a digital transmission system
  • Fig. 2 shows a diagram of a receiver comprising a sampling frequency detector according to the invention
  • Fig. 3 gives a representation of an example of a composition of certain
  • Fig. 4 shows a curve representing the average phase variations of the pilot signals plotted against the value of the carrier frequency
  • Fig. 5 shows a curve representing the standardized residual deviation of the sampling frequency plotted against the number of symbols used for attaining the synchronization.
  • the OFDM technique consists of frequency multiplexing various orthogonal carriers modulated by transmit signals. These signals are generally the result of coded digital modulations such as, for example, QAM, QPSK and other modulations.
  • An OFDM symbol s(t) may be written as:
  • R ⁇ . real part of a complex number
  • k index of the orthogonal carriers
  • T useful duration of an OFDM symbol
  • T G duration of the guard interval
  • ⁇ proportion of the guard interval in relation to the useful duration
  • N number of useful samples of an OFDM symbol and also maximum number of carriers.
  • N G number of samples of the guard interval.
  • j index of the OFDM symbol in the frame;
  • f 0 frequency of the first carrier.
  • N u is the number of useful carriers (N u ⁇ N).
  • N u is the number of useful carriers (N u ⁇ N).
  • QAM, QPSK or other modulation appears between the instants j.T' and (j + l)T ⁇
  • Each symbol x k modulates an orthogonal carrier with 0 ⁇ k ⁇ N-l .
  • the main role for the guard interval is to absorb the echoes of the previous symbol coming from the multipath channel and for which the delays are less than T G .
  • a signal is transmitted copied from a part of the useful signal.
  • symbol will be used to designate the actual OFDM symbols.
  • the symbols x k (coming from a QAM, QPSK or other modulation) which modulate a given carrier at a given instant will be designated by the term "cell", an OFDM frame thus being formed by OFDM cells frequency-divided over all the carriers and over the entire length of the frame.
  • Fig. 1 diagrammatically shows the means used at the transmitting end for effecting the OFDM modulation, that is to say, for forming the signal s(t) of the equation (1), and the means used at the receiving end.
  • a data source DATA 11 delivers the data x k of the QAM type or other types, to an OFDM modulator 13.
  • the modulator preferably comprises a calculating device 51 for calculating an inverse fast Fourier transform FFT 1 .
  • the device 51 is preceded by a multiplexer MUX 52 which also has an input 53 used to insert control signals intended for controlling the transmission (channel estimation, synchronization, service signals, etc.).
  • the multiplexer 52 is followed by a deserializer 55.
  • a serializer P/S 54 delivers a digitized OFDM signal s k formed by N G time-division samples which correspond to the guard interval, and formed by N time-division samples for the useful duration of each OFDM symbol.
  • the output of the modulator 13 is connected to a radio frequency modulator 15 which conveys the modulated signals s(t) to the transmission channel.
  • the received signal r(t) is transformed into a baseband signal r B (t) in a radio frequency demodulator RF TUNER 115.
  • the baseband signal is then processed by an OFDM demodulator 113.
  • the synchronization means are not shown in Fig. 1.
  • the signal r B (t) is first of all digitized in a digitizer 30.
  • the digitized signal r B is then transformed in parallel in a deserializer S/P 154, which eliminates the N G time-division samples in the guard interval and which deserializes the N time-division samples of the useful duration of the OFDM symbol.
  • the data then enter a device FFT 151 which performs a Fourier transformation.
  • the data then enter a deserializer P/S 155 and then a demultiplexer DEMUX 152 which removes from each OFDM symbol the control signals 153 which do not contain the actual data.
  • the symbols R k coming from the demodulator 113 are then decoded in a decoder DEC 111 for recovering the QAM (or QPSK or other modulation) symbols x k .
  • the symbols on the output of the modulator 13 are transmitted in frames.
  • one frame gathers various time-division multiplex OFDM symbols.
  • the cells of an OFDM symbol enclose for the greater part data, but certain cells contain particular signals, for example, pilot signals, or particular signals used for synchronization (of frame, timing, carrier) purposes, or used as a reference symbol of the differential modulation.
  • a carrier will be capable of, for example, transmitting the following data structure:
  • the duration of the frame is 20 ms.
  • the symbol PILOT is a known symbol for the receiver. The symbol is used for estimating the characteristics of the transmission channel so as to decode the data correctly.
  • certain carriers transmit a given pilot (PILOT) several times. Nevertheless, the multicarriers are provided for transmitting for the major part data x k (DATA).
  • Fig. 3 shows a frame part where an example of a composition of the OFDM symbols appears.
  • the sequence of carriers having index k In the vertical direction of the raster is indicated the sequence of carriers having index k, the carrier having the lowest frequency having index k,,,.,, and the carrier having the highest frequency having the index k ⁇ .
  • horizontal direction is indicated the number of the OFDM symbol in the frame.
  • An OFDM symbol is thus represented by a column of the raster.
  • a carrier is represented by a row of the raster.
  • a raster element is a cell that contains a data or a pilot signal or a service signal.
  • the pilot signals are, for example, pilot frequencies. For simplicity, only the data and the pilot signals are shown in Fig.
  • the black cells represent the pilot signals transmitted with much power.
  • the grey cells represent the pilot signals transmitted with reduced power.
  • the carrier having the lowest frequency and the carrier having the highest frequency continuously transmit only pilot signals.
  • the pilot signals are distributed regularly, for example, with one step in 4 in horizontal direction (time) and one step in 12 in vertical direction (frequency) .
  • the frequency detector according to the invention utilizes the phase variations of the pilot signals which are different depending on whether the carrier frequency is high or low. These phase variations are due to the drift of the FFT window in the course of time, notably when the sampling frequency is not synchronized correctly.
  • f k —.k, where k is a number comprised between 1
  • ⁇ d be the time-division drift of the FFT window between two symbols, then the drift is due to a sampling frequency error equal to ⁇ f s .
  • the drift ⁇ d is thus so that:
  • H n (f) be the transfer function of the channel at the time nT.
  • the transfer function H n (f) results from the following recurrent relation:
  • C p be the set of (n,k) cells belonging to the set of pilot frequency cells called continuous cells and let S p be the set of the (n,k) cells belonging to the set of pilot frequency cells called dispersed cells.
  • R ⁇ is the received data (symbol n on the carrier k):
  • An estimate of the sampling frequency error may be obtained by measuring the difference between, on the one hand, a first weighted average, measured according to the phase variations relating to the pilot frequencies appearing on the carriers which have high carrier frequencies and, on the other hand, a second weighted average, measured according to the phase variations relating to the pilot frequencies appearing on the carriers having the low carrier frequencies.
  • the sub-set S is a sub-set of S p corresponding to the high pilot frequencies which have an average index k + .
  • the sub-set S-- of S p corresponding to the low pilot frequencies which have an average index k " .
  • Fig. 4 shows the phase variations of the pilot frequencies plotted against the index k of the carriers.
  • Fig. 2 shows a diagram of the receiver according to the invention.
  • the baseband signal r B (t) is sampled in the digitizer 30.
  • the sampled signal r B enters a transforming device 31 which transforms the real data r B into analytic complex data (I,Q).
  • a deserializer S/P 154 eliminates the N G complex time-division data which correspond to the guard interval and produces the N complex data of the useful interval in parallel. They enter the Fourier transforming device 151 which produces the received R k symbols in parallel. The data then enter a serializer P/S 155 and then a demultiplexer 152 which removes the signals which are no data to deliver the symbols R k without control signals.
  • the receiver For synchronizing the sample clock H s , the receiver comprises a loop 1.
  • the demultiplexer 152 comprises a pilot frequency retriever 32 which determines the instant at which the pilot frequencies appear in the frame.
  • the pilot frequencies thus detected are applied (connection 153a) to a frequency detector 33 SAMPL DET which calculates the error ⁇ f signal — . Therefore, it performs the calculations which correspond to the equations 6 and
  • Each new estimate of the error signal is formed with the OFDM symbol frequency.
  • the frequency detector may be, for example, a signal processing device or a microcomputer. After being filtered in a loop filter 34, the error signal triggers the clock generator VCXO 35 which applies the sample clock H s to the converter 30.
  • the frequency detector 33 is a fine detector that is active when the offset ⁇ f of the carrier frequency occurring in the tuner 115 and also the sampling frequency errors are small. In practice, one establishes limits defined by, for example: ⁇ f.T 1 ⁇ 0,125 ⁇ bf s .2p ⁇ (k + -k-) — ⁇ 0.75.
  • two sub-sets S p and S/ closer to the center of the frame may be used, or two sub-sets C p and C p ' to have smaller phase variations.
  • the detector for detecting sampling frequency errors accelerates the speed of the synchronization with a low complexity addition.
  • the residual sampling frequency deviation depends on the loop filter used for synchronizing the sampling frequency and the noise level.

Abstract

La présente invention concerne un récepteur de signaux multiporteuse équipé d'un dispositif de synchronisation de l'horloge d'échantillonnage. Le procédé utilisé consiste à calculer la variation moyenne de déphasage des signaux pilotes concernant les sous-porteuses de l'extrémité supérieure du spectre. Le procédé consiste également à calculer la variation moyenne de déphasage des signaux pilotes concernant les sous-porteuses de l'extrémité inférieure du spectre. La différence entre ces deux moyennes, pondérée au moyen de l'écart de fréquence des fréquences pilote, constitue un signal d'erreur qui régit la fréquence de l'horloge d'échantillonnage.
PCT/IB1997/000422 1996-04-29 1997-04-21 Symbole de synchronisation d'un recepteur multiporteuse WO1997041672A1 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
FR9605364 1996-04-29
FR96/05364 1996-04-29

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WO1997041672A1 true WO1997041672A1 (fr) 1997-11-06

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Cited By (11)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2320871A (en) * 1996-12-28 1998-07-01 Daewoo Electronics Co Ltd Frame synchronization in digital communication systems utilizing OFDM
KR19990079604A (ko) * 1998-04-07 1999-11-05 구자홍 심볼 디인터리빙 장치
WO2000051301A1 (fr) * 1999-02-26 2000-08-31 Stmicroelectronics S.A. Recepteur de signaux multiporteuse a correction de defauts d'egalisation provoques par les deplacements de la fenetre trf
GB2370733A (en) * 2000-09-13 2002-07-03 Mitsubishi Electric Corp Clock regeneration in an Orthogonal Frequency Division Multiplex (OFDM) receiver
EP1330087A2 (fr) * 2002-01-17 2003-07-23 Samsung Electronics Co., Ltd. Récupération de l'horloge de symboles pour récepteurs multiporteuses
EP1363435A3 (fr) * 2002-05-13 2006-12-13 Texas Instruments Incorporated Estimation des décalages en fréquence porteuse et en fréquence d'échantillonage dans un récepteur multiporteuse
EP1953982A1 (fr) * 2007-02-05 2008-08-06 Sequans Communications Procédé et dispositif de synchronisation temporelle et d'analyse voisine pour les systèmes cellulaires OFDM
EP1988676A1 (fr) * 2007-05-03 2008-11-05 Telefonaktiebolaget LM Ericsson (publ) Détermination d'une erreur de fréquence dans un récepteur d'un système de communication sans fil
EP1933516A3 (fr) * 1997-05-02 2009-03-04 LSI Logic Corporation Démodulation de signaux de diffusion vidéo
US7583740B2 (en) 2005-01-24 2009-09-01 Realtek Semiconductor Corp. Apparatus and method for tracking sampling clock in multi-carrier communication system
US7583770B2 (en) 2002-10-11 2009-09-01 Ihp Gmbh-Innovations For High Performance Microelectronics/Institut Fur Innovative Mikroelektronik Multiplex signal error correction method and device

Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2145906A (en) * 1983-09-01 1985-04-03 Nec Corp Circuit for establishing accurate sample timing
EP0453203A2 (fr) * 1990-04-16 1991-10-23 Telebit Corporation Procédé et dispositif de correction du décalage de la fréquence d'horloge et de porteuse et de la gigue de phase dans des modems à porteuses multiples
WO1992005646A1 (fr) * 1990-09-14 1992-04-02 National Transcommunications Limited Reception de signaux multiplexes de division de frequence orthogonale (ofdm)
EP0656706A2 (fr) * 1993-11-12 1995-06-07 Kabushiki Kaisha Toshiba Synchronisation de signaux MDFO
WO1995019671A1 (fr) * 1994-01-18 1995-07-20 Telia Ab Procede et dispositif de synchronisation dans une modulation a multiplexage frequentiel orthogonal

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2145906A (en) * 1983-09-01 1985-04-03 Nec Corp Circuit for establishing accurate sample timing
EP0453203A2 (fr) * 1990-04-16 1991-10-23 Telebit Corporation Procédé et dispositif de correction du décalage de la fréquence d'horloge et de porteuse et de la gigue de phase dans des modems à porteuses multiples
WO1992005646A1 (fr) * 1990-09-14 1992-04-02 National Transcommunications Limited Reception de signaux multiplexes de division de frequence orthogonale (ofdm)
EP0656706A2 (fr) * 1993-11-12 1995-06-07 Kabushiki Kaisha Toshiba Synchronisation de signaux MDFO
WO1995019671A1 (fr) * 1994-01-18 1995-07-20 Telia Ab Procede et dispositif de synchronisation dans une modulation a multiplexage frequentiel orthogonal

Cited By (18)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2320871A (en) * 1996-12-28 1998-07-01 Daewoo Electronics Co Ltd Frame synchronization in digital communication systems utilizing OFDM
US6172993B1 (en) 1996-12-28 2001-01-09 Daewoo Electronics Co., Ltd. Frame synchronization method and apparatus for use in digital communication system utilizing OFDM method
GB2320871B (en) * 1996-12-28 2001-08-08 Daewoo Electronics Co Ltd Frame synchronization method and apparatus for use in digital communication system utilizing OFDM method
EP1933516A3 (fr) * 1997-05-02 2009-03-04 LSI Logic Corporation Démodulation de signaux de diffusion vidéo
KR19990079604A (ko) * 1998-04-07 1999-11-05 구자홍 심볼 디인터리빙 장치
WO2000051301A1 (fr) * 1999-02-26 2000-08-31 Stmicroelectronics S.A. Recepteur de signaux multiporteuse a correction de defauts d'egalisation provoques par les deplacements de la fenetre trf
FR2790344A1 (fr) * 1999-02-26 2000-09-01 St Microelectronics Sa Demodulateur cofdm avec compensation de deplacement de fenetre d'analyse fft
GB2370733B (en) * 2000-09-13 2004-07-14 Mitsubishi Electric Corp Clock signal regeneration system and receiver system and clock signal regeneration method and receiving method
GB2370733A (en) * 2000-09-13 2002-07-03 Mitsubishi Electric Corp Clock regeneration in an Orthogonal Frequency Division Multiplex (OFDM) receiver
EP1330087A2 (fr) * 2002-01-17 2003-07-23 Samsung Electronics Co., Ltd. Récupération de l'horloge de symboles pour récepteurs multiporteuses
EP1330087A3 (fr) * 2002-01-17 2006-11-02 Samsung Electronics Co., Ltd. Récupération de l'horloge de symboles pour récepteurs multiporteuses
EP1363435A3 (fr) * 2002-05-13 2006-12-13 Texas Instruments Incorporated Estimation des décalages en fréquence porteuse et en fréquence d'échantillonage dans un récepteur multiporteuse
US7224666B2 (en) 2002-05-13 2007-05-29 Texas Instruments Incorporated Estimating frequency offsets using pilot tones in an OFDM system
US7583770B2 (en) 2002-10-11 2009-09-01 Ihp Gmbh-Innovations For High Performance Microelectronics/Institut Fur Innovative Mikroelektronik Multiplex signal error correction method and device
US7583740B2 (en) 2005-01-24 2009-09-01 Realtek Semiconductor Corp. Apparatus and method for tracking sampling clock in multi-carrier communication system
EP1953982A1 (fr) * 2007-02-05 2008-08-06 Sequans Communications Procédé et dispositif de synchronisation temporelle et d'analyse voisine pour les systèmes cellulaires OFDM
EP1988676A1 (fr) * 2007-05-03 2008-11-05 Telefonaktiebolaget LM Ericsson (publ) Détermination d'une erreur de fréquence dans un récepteur d'un système de communication sans fil
WO2008135541A1 (fr) * 2007-05-03 2008-11-13 Telefonaktiebolaget Lm Ericsson (Publ) Détermination d'une erreur de fréquence dans un récepteur d'un système de communication sans fil

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