Pulse or digital communications – Synchronizers – Frequency or phase control using synchronizing signal
Reexamination Certificate
1996-05-30
2001-05-01
Tse, Young T. (Department: 2634)
Pulse or digital communications
Synchronizers
Frequency or phase control using synchronizing signal
C375S343000, C370S210000
Reexamination Certificate
active
06226337
ABSTRACT:
The invention relates to a method for digital signal transmission in frames using a multiplicity of modulated carriers, to corresponding evaluation methods for this signal and to a corresponding device for decoding.
Prior Art
Various methods for terrestrial transmission of digital broadcast signals are known, such as OFDM, QPSK and QAM modulation. One of the main problems connected with such systems is synchronization when a receiver is switched on or tuned to another channel.
A synchronization method of this type for DAB (Digital Audio Broadcasting) is known, in which a complete zero symbol and a so-called TFPC (Time Frequency Phase Control) symbol are successively transmitted and are evaluated in a specific manner in the receiver. In the same way as the signal currents of the useful information to be transmitted, the TFPC symbol is assigned to the individual carriers or frequencies of the OFDM multi-carrier method. For the purpose of evaluation, the samples are transformed into a frequency representation and evaluated in that form, and the results are transformed back into the time domain. A COFDM modulation method of this type is described in DE-A-4128713.
Invention
The invention is based on the object of specifying a method in which only a single symbol is required for the synchronization and which permits the detection of appreciable deviations from the normal receiver oscillator frequency or of a deviation of the transmitter frequency from the given frequency pattern, and the correction of the oscillator frequency.
The invention is based on the further object of specifying an evaluation method for the signal transmitted according to the invention.
The invention is based on the further object of specifying a device for the application of the method according to the invention.
The transmitted signal contains a multiplicity of modulated carriers (OFDM modulation, which is described, for example, in “Data transmission by frequency division multiplexing using the discrete Fourier transform”, Weinstein, S. B. et al., IEEE Transactions on Communication Technology, Vol. COM-19, No. 15, October 1971 and in “An orthogonally multiplexed QAM system using the discrete Fourier transform”, Hirosaki, B., IEEE Transactions on Communication Technology, Vol. COM-29, No. 7, July 1981). QPSK and/or QAM modulation can be used, for example, for these carriers. A specific amount of the entire channel capacity is reserved for the synchronization and the channel estimation/correction data.
In the case of the solution according to the invention, the power of the signal is zero or virtually zero for part of the duration of the synchronization symbol. During a further time segment, it is possible to apply a modulation method which differs from the OFDM method used in the remaining part. The modulation of this part contains at least one sequence having optimum autocorrelation properties, for example an M sequence, i.e. PRN sequence of maximum length, or a specific number of so-called CAZAC sequences (constant amplitude zero autocorrelation). CAZAC sequences of this type are described in EP-A-0529421.
This bit sequence which is defined in temporal order and is modulated onto a centrally positioned carrier can be transmitted instead of an information sequence assigned to the frequencies, the spacing between the bits of the sequence or sequences corresponding to the time intervals used when (over)sampling the OFDM symbols, or to a multiple of these time intervals, or only every other carrier is used for half the effective symbol length.
The length of the part with zero power corresponds to approximately half the (OFDM) symbol duration, as a result of which one sequence has the length of approximately a quarter of the symbol. If, for example in the case of a TV transmission method, 1900 of 2048 possible carriers (length of the FFT/Fast Fourier Transform) are effectively utilized, then this yields 950 usable carriers for the signal part of the synchronization symbol. As a result, an M sequence having a length of 512-1 can be transmitted about 1.85 times. Each value of the sequence (for example 0 or 1) is assigned a carrier phase angle, for example 0° and 180° in the case of QPSK. Another carrier phase assignment is expediently selected for the component of 1.85−1=0.85 or 85% of the second sequence. For a higher-level QAM, for example 64 QAM in the case of TV transmission, only the basic values corresponding to a QPSK system are used for the signal components of the synchronization symbol, i.e. four phase angles which differ by 90° and have a constant amplitude.
In the case of QPSK modulation, the sequences can be transmitted only in a subchannel (I or Q) and the data sequence in the other subchannel is constant. In a different solution, the sequences are transmitted in both subchannels (I and Q), but with different signs (0 and 1). In the case of a higher-level QAM or a so-called multi-resolution QAM, the modulation of the synchronization signal then takes place at the lowest level, i.e. on a QPSK basis.
The proposed division of the synchronization symbol has the advantage that the zero component and the actual signal component of the sync symbol each take up about half the symbol duration. In the event of multipath reception, it is still possible to identify delay time differences up to the duration of the effective sequence, that is to say up to the length of the signal component and thus up to half the symbol duration. This duration is generally longer than the length of the guard interval used, thereby achieving comparable or even slightly improved protection from multipath propagation. During the zero component, it is possible additionally to transmit a reduced number of carriers for transmitter identification with a power which is in total so low that the detection of the zero component in the receiver is not significantly affected thereby. For optimum selection of the relationships between the frame length, the number of useful symbols per frame and the sampling sequence, the length of the synchronization symbol can be selected to deviate slightly from the duration of the OFDM symbols, the zero component being somewhat shortened or lengthened in the said synchronization symbol.
In order to achieve reduced computing complexity in the receiver and in order to permit a specific form of evaluation in the receiver, a significantly shorter sequence can be selected and transmitted correspondingly frequently.
The spacing between the bits of the sequence can correspond here to n times the value of the time intervals used during the oversampling of the OFDM symbols, designated below as n spacing.
CAZAC sequences having a length of 16 are preferably used, which means a 59.4-fold repetition in the case of the 950 usable carriers. Since the repetition of precisely identical arrangements would lead to ambiguities during correlation evaluation, the following sequences are modified by a different selection of the assignment to the modulation angles and by adding constant angular changes. Each newly obtained sequence is transmitted twice for reasons of unambiguity in correlation, thereby producing a total of 29 pairs and a single sequence in the case of the example chosen. The modulation can also be carried out differentially instead of directly via the carrier sequence. A more detailed description is contained in DE-A-4128713.
In a receiver for this method, rough synchronization takes place on the basis of the part-symbol with zero power in that, for example, synchronization pulses are derived by rectification and filtering of the received signal, which is converted into baseband, and are used for defining the frame start or symbol window. After this, the evaluation of the signal component of the received synchronization symbol takes place on the basis of the rough synchronization, and afterwards a more precise determination of the time occupied by the symbol or symbols is undertaken.
For this purpose, as in the case of the signal component of the useful information, the signal sampled in a temporal
Klank Otto
Laabs Jurgen
Deutsche Thomson-Brandt GmbH
Shedd Robert D.
Tripoli Joseph S.
Tse Young T.
Wein Frederick A.
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