WO1996019056A1 - Procede de correction de frequence, de fenetre temporelle, d'horloge d'echantillonnage et de variations de phase finale dans une reception de signaux a multiplexage mrf orthogonal - Google Patents

Procede de correction de frequence, de fenetre temporelle, d'horloge d'echantillonnage et de variations de phase finale dans une reception de signaux a multiplexage mrf orthogonal Download PDF

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Publication number
WO1996019056A1
WO1996019056A1 PCT/SE1995/001413 SE9501413W WO9619056A1 WO 1996019056 A1 WO1996019056 A1 WO 1996019056A1 SE 9501413 W SE9501413 W SE 9501413W WO 9619056 A1 WO9619056 A1 WO 9619056A1
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WO
WIPO (PCT)
Prior art keywords
frequency
phase error
amplitude
reference symbols
carriers
Prior art date
Application number
PCT/SE1995/001413
Other languages
English (en)
Inventor
Staffan NYSTRÖM
Erik Stare
Mårten Rignell
Göran Roth
Brian LØNROTH
Vidar Ringset
Original Assignee
Hd Divine
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 Hd Divine filed Critical Hd Divine
Priority to AU42751/96A priority Critical patent/AU4275196A/en
Priority to JP8518667A priority patent/JPH10510958A/ja
Publication of WO1996019056A1 publication Critical patent/WO1996019056A1/fr

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Classifications

    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L5/00—Arrangements affording multiple use of the transmission path
    • H04L5/02—Channels characterised by the type of signal
    • H04L5/06—Channels characterised by the type of signal the signals being represented by different frequencies
    • 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
    • H04L27/2657—Carrier 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
    • H04L27/00—Modulated-carrier systems
    • H04L27/26—Systems using multi-frequency codes
    • H04L27/2601—Multicarrier modulation systems
    • H04L27/2602—Signal structure
    • H04L27/261—Details of reference signals
    • H04L27/2613—Structure of the reference signals

Definitions

  • the present invention relates to a method at digital system for correction of frequency, sampling clock and phase error which varies slowly with time, i.e. low- frequency phase noise.
  • the receiver is a so called OFDM- receiver which receives digital information in frames.
  • OFDM Orthogonal Frequency Division Multiplex
  • COFDM Coded Orthogonal Frequency Division Multiplex
  • the Patent document EP 448493 describes a system for transmission of TV digitally.
  • the picture information is transmitted to a mobile user and is divided into two parts; one is used to recreate a normal TV-picture and the other together with the first one to create a larger picture.
  • a receiver for digital radio signals The receiver utilizes a window method to minimize the intersymbol interference arising at multipath propagation.
  • the receiver will be equipped with a time window module which is used to extract usable samples from the received signal.
  • the American document US 5 228 025 describes a method to transmit digital data via radio, preferably to mobile receivers.
  • the method transmits a synchronization sequence in the form of at least one frequency which varies in one for the receiver known way.
  • the synchronization sequence is utilized for tuning the local oscillator.
  • the first prototype for transmission and reception of DAB Digital Audio Broadcasting
  • the first is called zero symbol and contains nothing but is used by the receiver on one hand for symbol synchronization, and on the other for estimation of interference in the channel.
  • the second symbol consists of a so called chirp or sine sweep signal which is a sine shaped signal, the frequency of which changes linearly with time and which sweeeps over the whole channel width. This is used by the receiver on one hand for adjustment of the location of the time window, i.e. division of the received signal in segments which are each processed by means of FFT, and on the other for estimation of the transmission function of the channel and estimation of deviations in the carrier frequency, if any.
  • TFPC-signal Time Frequency Phase Control
  • CAZAC-symbol which is used by the receiver both for timing, frequency adjustment and for estimation of the transmission function.
  • the present invention relates to a method for correction of frequency, time window, sampling clock and time variable phase error at OFDM-reception.
  • One from a transmitter transmitted signal is received by the OFDM- receiver.
  • the signal which is divided into consecutive frames, which each in its turn is divided into symbols, contains with certains intervals reference symbols with a predetermined content.
  • Each frame is divided into a number of symbols which regarding time follow each other.
  • Respective symbol is allotted a serial number, and the mentioned reference symbols are preferably transmitted in pairs.
  • the receiver analyzes the different reference symbols.
  • the signals in the reference symbols consist of so called chirp signals, i.e. so called sine sweeep signals which are sine signals the frequency of which is linearly changed with the time and which sweeps over the whole channel width.
  • One of the chirp signals goes from the highest frequency to the lowest in time, and the other chirp signal from the lowest frequency to the highest frequency.
  • the relation between the contents of the reference symbols in pair regarding time and frequency is utilized for adjustment of the frequency of the receiver.
  • the impulse response is calculated from the signals of the received reference frames at which correction of time window and sampling clock can be performed.
  • the main focus of the impulse response is determined, at which the real position of the time window can be determined.
  • the position of the time window is adjusted in relation to wished position by the sampling clock being adjusted in relation to the difference between the mentioned main focus and wished position.
  • Each symbol consists of a number of superimposed carriers with among themselves different frequencies.
  • each of the carriers further is arranged that its phase and amplitude, which can also be described by its real and imaginary part, is modulated by the data information which shall be transmitted.
  • the mentioned real and imaginary parts are allotted a definite position in a matrix system, in the matrix system real and imaginary parts are allowed to take different positions which are accepted.
  • the relation between the point which is indicated by received real and imaginary part and the ideal position in the matrix indicates an angle difference to the ideal position which is utilized for correction of the phase error at reception.
  • the indicated method gives a possibility to make accurate adjustments of the receiver in a way which has not previously been possible and which gives increased robustness at OFDM-reception.
  • the method further allows that the necessary adjustments in the receiver are possible to perform in a simple way.
  • the method thus allows that the program transmission can be performed with the high precision which is expected in these connections.
  • Figure 1 shows schematically how the receiver at first locates the chirp signals by means of a set of binary correlators and after that precision adjusts time window, carrier frequency and phase according to the invention.
  • Figure 2 shows how the OFDM-signal is created by means of IFFT (Invers Fast Fourier Transform). Each entry on the IFFT corresponds to a carrier.
  • IFFT Invers Fast Fourier Transform
  • Figure 3 shows the matrix for estimation of phase error with points according to the 16QAM-system and illustrates the angle relation between the real position of the received vector and the ideal position.
  • a signal sequence is transmitted from a transmitter and received by a receiver.
  • the signal sequence comprises a number of symbols which are arranged to a frame. At the beginning of each frame one or more synchronization symbols are arranged. At least two of the mentioned synchronization frames contain so called chirp signals.
  • a chirp signal is a sine signal the frequency of which is changed linearly with time.
  • a set of correlators are used in order to detect the position of the chirp signal .
  • the auto correlation for a chirp signal has a very acute maximum.
  • the symbol bit of the chirp signal is received and is compared with a stored signal. The signal is compared by being processed through a number of XNOR-gates. The number of equal bits, i.e.
  • the hamming weight of the resulting vector is obtained as an output signal from the correlator.
  • the synchronization signals which at that are obtained in 1 in Figure 1 are brought back to a frame structure generator, 2 in Figure 1, which controls division of the incoming signal in frames and symbols, and numbers the individual samples within each symbol.
  • the frame structure generator in this way controls the division of the signal in appropriate time windows to the FFT (Fast Fourier Transform) .
  • the symbols coming out from the FFT are after that forwarded to among other things a signal processor, 3 in Figure 1, where correction values for carrier frequency and the frequency of the sampling clocks is calculated.
  • the transmitted signal from the transmitter has been created by means of an IFFT (Inverse Fast Fourier Transform) according to Figure 2.
  • IFFT Inverse Fast Fourier Transform
  • each of the carriers has been given an amplitude and phase according to he formula e j ⁇ ⁇ / N ⁇ W here j ⁇ corresponds to the number of the carrier, and N the number of carriers.
  • a down-chirp signal is created in a similar way but with negative phase; consequently the formula becomes e -:, ⁇ /N.
  • the received signal is made subject to a Fast Fourier Transform (FFT) in the receiver.
  • FFT Fast Fourier Transform
  • the correction calculations which are performed in unit 3 in Figure 1 implies that a multiplication by the for the carrier allotted inverse angle is performed by received up-chirp being multiplied by an ideal down-chirp, and that received down-chirp is multiplied with an ideal up- chirp.
  • signals which we call de-rotated chirps, each constitutes an estimate of the transmission function of the channel. For an ideal channel these estimates become equal to 1 for all carriers. If a shift of the carrier frequency has occurred somewhere in the channel, the de-rotated chirps will have a remaining phase shift which is linearly depending on the serial number of the carrier. The changes get different symbols in the up- and the down- chirp.
  • the phase positions of the de-rotated chirps are also influenced by frequency selective fading and by wrong setting of the time window, but this influence has the same symbol in both chirps. Therefore the carrier error can be extracted by subtraction of the de-rotated phase positions of one of the chirps from the other.
  • the numerical value of the carrier error can in one in itself known way be estimated from the phase position's linear depending on the carrier number. The obtained number is after that utilized in an in itself known way to correct the frequency.
  • the impulse response of the channel is obtained by an IFFT-transform on the de-rotated chirps.
  • the impulse response is multiplied with a weighting function before the position of the main focus is calculated.
  • the difference between this position and a predetermined wished position constitutes a correction signal which after filtering controls the clock-frequency of the A/D-converter, at which the correction value successively will be adjusted towards zero. Consequently the time window will land up in wished position.
  • the impulse response of the channel can be estimated both from received up-chirp and down-chirp. It is to advantage to use the sum of these two estimates at the main focus calculation because an error in carrier frequency will influence the position of the main focus with different symbols depending on whether it is calculated on an up-chirp or on a down-chirp. The main focus position of the sum of the two impulse responses will therefore be insensible to errors in carrier frequency because the error will neutralize itself.
  • the received carriers are arranged in a matrix system with respect to their imaginary respective real part. These points are in the complex number plan allotted an area within which they are allowed to exist. A point in the complex number plan which occur within the mentioned area is regarded to symbolize a certain transmitted data sequence.
  • the relation of the point to the ideal position indicates an angle relation between the ideal position and the real position.
  • the mentioned angle difference indicates the phase error in the reception.
  • the average of the angle differences calculated from all the different carriers in the same symbol constitutes an estimate of the phase error.
  • the in this way obtained phase error can after that be combined with previously obtained phase errors and be utilized for phase correction of all carriers in the symbol in question, and for estimation of the expected phase error at the reception of next symbol. It can also be used for estimation of small frequency deviations because these give rise to phase errors with constant change from symbol to symbol.
  • the mentioned method for estimation of the phase error can be further developed in different ways. Two different improvements of the method have been identified. One or both improvements can be applied.
  • the first improvement is based on the fact that the amplitude of the received signal on one and the saute occasion can be of different strength for different carriers, frequencies. This is due to interference from reflections of the signal, so called multipath propagation, or interference from other transmitters which transmits the same signal in a so called single frequency network.
  • the carriers which are subject to destructive interference are weakened and get a worse signal/noise-relation than the other.
  • the frequency depending transmission function for the channel can be calculated in the receiver by analysis of the received chirps. At calculation of the average of the angle differences the values from the different carriers can be weighted with the calculated transmission function at which angle differences from carriers with high attenuation in the channel is given a lower weight than those with low attentuation.
  • the strong noise from the attenuated carriers can by that be made to have a minimal influence on the estimation of the phase error.
  • the second improvement is based on the fact that one and the same noise effect in the received signal gives different uncertainty in the estimation of the phase error for signals far from or close to the origin of coordinates.
  • the mentioned angle relations are weighed in relation to the distance to the origin of coordinates.

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  • Engineering & Computer Science (AREA)
  • Signal Processing (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Synchronisation In Digital Transmission Systems (AREA)
  • Television Systems (AREA)
  • Digital Transmission Methods That Use Modulated Carrier Waves (AREA)

Abstract

La présente invention porte sur un procédé de correction de la fréquence, de l'erreur de fenêtre temporelle, de l'horloge d'échantillonnage et de l'erreur de phase dans des récepteurs de signaux à multiplexage MRF orthogonal (MRFO). L'information transmise sous forme de signal comportant plusieurs porteuses numérisées, est constituée de trames consécutives comportant un certain nombre de symboles. Au moins une modulation d'impulsions vers le haut et une modulation d'impulsion vers le bas sont exécutées dans les trames respectives et la correction de la fréquence est réalisée à partir de l'analyse de ces modulations. On détermine, en outre, la focalisation principale de la réponse impulsionnelle pondérée et la position de cette focalisation sert à corriger l'erreur de fenêtre temporelle ainsi que l'horloge d'échantillonnage. On enregistre, de surcroît, les vecteurs provenant des emplacements des porteuses reçues en fonction de leur emplacement idéal dans une matrice. On détermine les angles constitués par les vecteurs reçus et le vecteur situé dans un emplacement idéal, que l'on pondère ensuite par rapport à l'amplitude de la fonction de transmission à la fréquence de la porteuse et à la distance séparant le vecteur de l'origine des coordonnées. On fait ensuite la moyenne des distances angulaires pondérées, la moyenne obtenue servant à corriger l'erreur de phase. Une fois connues, l'erreur de phase ainsi obtenue et celle des précédentes erreurs servent à calculer la prochaine erreur de phase dans la réception suivante.
PCT/SE1995/001413 1994-12-14 1995-11-27 Procede de correction de frequence, de fenetre temporelle, d'horloge d'echantillonnage et de variations de phase finale dans une reception de signaux a multiplexage mrf orthogonal WO1996019056A1 (fr)

Priority Applications (2)

Application Number Priority Date Filing Date Title
AU42751/96A AU4275196A (en) 1994-12-14 1995-11-27 Method at ofdm-reception for correction of frequency, time window, sampling clock and slow phase variations
JP8518667A JPH10510958A (ja) 1994-12-14 1995-11-27 Ｏｆｄｍ受信において、周波数、時間窓、サンプリングクロック及び緩やかな位相変動を補正する方法

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
SE9404356A SE504787C2 (sv) 1994-12-14 1994-12-14 Metod vid OFDM-mottagning för korrigering av frekvens, tidsfönster, samplingsklocka och långsamma fasvariationer
SE9404356-9 1994-12-14

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WO1996019056A1 true WO1996019056A1 (fr) 1996-06-20

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JP (1) JPH10510958A (fr)
KR (1) KR980700750A (fr)
CN (1) CN1170486A (fr)
AU (1) AU4275196A (fr)
SE (1) SE504787C2 (fr)
WO (1) WO1996019056A1 (fr)

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WO1997007620A1 (fr) * 1995-08-16 1997-02-27 Philips Electronics N.V. Systeme de transmission a traitement des symboles ameliore
EP0818904A2 (fr) * 1996-07-12 1998-01-14 Roke Manor Research Limited Récupération d'horloge pour récepteur "DAB"
EP0829988A2 (fr) * 1996-09-16 1998-03-18 NOKIA TECHNOLOGY GmbH Synchronisation par symbole et réglage de la fréquence d'échantillonnage dans un récepteur pour MDFO
EP0837582A2 (fr) * 1996-10-18 1998-04-22 Alpine Electronics, Inc. Synchronisation de symbole, dans un récepteur RAN
EP0859494A2 (fr) * 1997-02-17 1998-08-19 Matsushita Electric Industrial Co., Ltd. Synchronisation de l'oscillateur local, dans des systèmes multiporteurs
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Cited By (80)

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Publication number Priority date Publication date Assignee Title
WO1997007620A1 (fr) * 1995-08-16 1997-02-27 Philips Electronics N.V. Systeme de transmission a traitement des symboles ameliore
US5848107A (en) * 1995-08-16 1998-12-08 U. S. Philips Corporation Transmission system with improved symbol processing
US6156181A (en) * 1996-04-16 2000-12-05 Caliper Technologies, Corp. Controlled fluid transport microfabricated polymeric substrates
EP0818904A2 (fr) * 1996-07-12 1998-01-14 Roke Manor Research Limited Récupération d'horloge pour récepteur "DAB"
EP0818904A3 (fr) * 1996-07-12 2001-01-24 Roke Manor Research Limited Récupération d'horloge pour récepteur "DAB"
EP0829988A2 (fr) * 1996-09-16 1998-03-18 NOKIA TECHNOLOGY GmbH Synchronisation par symbole et réglage de la fréquence d'échantillonnage dans un récepteur pour MDFO
EP0829988A3 (fr) * 1996-09-16 1998-03-25 NOKIA TECHNOLOGY GmbH Synchronisation par symbole et réglage de la fréquence d'échantillonnage dans un récepteur pour MDFO
US6125124A (en) * 1996-09-16 2000-09-26 Nokia Technology Gmbh Synchronization and sampling frequency in an apparatus receiving OFDM modulated transmissions
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SE504787C2 (sv) 1997-04-28
KR980700750A (ko) 1998-03-30

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