Pulse or digital communications – Receivers – Automatic frequency control
Patent
1995-12-07
1997-11-25
Tse, Young T.
Pulse or digital communications
Receivers
Automatic frequency control
375362, 370210, 370510, H04L 704, H04L 2716, H04L 2722
Patent
active
056920164
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention concerns a method and an arrangement for adjusting the local oscillators of a receiver in a multi-channel transmission system with which a coarse and fine synchronization of the oscillators is effected by means of known synchronism symbols in the receiver.
Multi-channel transmission systems utilized, among other things, for the digital transmission of radio signals, are known in the art as DAB (Digital Audio Broadcast). The multi-channel transmission system thereby used exercises the COFDM method (Coded Orthogonal Frequency Division Multiplex). Furthermore, known in the art from the publication "Digitale Television Transmission System Concepts and Introduction Opportunities"! Technology!, No. 9/1992, pages 559-570, is the fact that multi-channel transmission systems based on the OFDM or COFDM method can also be used for realizing a digital terrestrial television system.
In a multi-channel transmission system for digital data transmission the data stream to be transmitted is divided into a plurality (several hundred to several thousand) of subcarriers which are adjacent to each other in frequency, whereby the spectra of the subcarriers can also overlap each other. By means of this procedure a data rate is transmitted for each subcarrier which is orders of magnitude lower than the original data rate, as a result of which, the symbol duration correspondingly increases. This is advantageous in the event that echoes occur on the transmission channel. By means of a suitable determination of the number of subcarriers it is always possible to configure a multi-channel transmission system in such a fashion that the symbol duration is large compared to the expected echo time.
The influence of echoes with certain echo times can be completely removed, in the event that the symbols are not transmitted in direct sequence rather when, in each case, a protection time is provided for between two sequential symbols, during which the echoes of the initially transmitted symbol die-off.
Additional disrupting influences in the transmission channel can be reduced in that the time and/or frequency sequence of the information to be transmitted is scrambled.
The modulation of the transmission signal in a multi-channel transmission system can advantageously be effected by means of an inverse fast Fourier transformation (IFFT), whereby the number of values to be transformed is given by the number of subcarriers. The multi-channel modulation signal is divided into time-frequency slots. The time slots are formed by the discrete symbol cycle and the frequency slots by the frequency separation of the subcarriers. The demodulation of the multi-channel modulation signal on the receiver side can also advantageously be effected by means of fast Fourier transformation (FFT).
A more detailed description of multi-channel transmission systems according to the OFDM or COFDM methods can be found, for example, in laid open publication WO 88/00417 or in the publication "Digital Sound Broadcasting to Mobile Receivers", IEEE Transactions on Consumer Electronics, Vol. 35, No. 3, August 1989, pages 493-503 the contents of which is hereby incorporated by reference.
Data transmission in multi-channel tranmission methods in accordance with the OFDM method is effected in a frame oriented fashion. Each frame begins with a zero-symbol which serves the purpose of identifying the beginning of the frame and for time synchronization. Synchronism symbols and the data symbols carrying the information then follow.
The OFDM method requires that the carrier frequency of the transmitter and receiver coincide very precisely. The tolerable deviation is determined by the frequency separation of the subcarriers and the complexity of the modulation of the individual subcarriers. The requirements increase rapidly when the modulation of the subcarriers is effected, for example, with a 64 QAM (quadrature amplitude modulation) instead of a 4 PSK (phase shift keying).
Subsequent to the receiver sided Fourier transformation and due to de
REFERENCES:
patent: 4599732 (1986-07-01), LeFever
patent: 5550812 (1996-08-01), Philips
patent: 5608764 (1997-03-01), Sugita et al.
"Digitale Fernsehubertragung" by Dr.-Ing. Rudiger Kays, in: Fernseh-Und K-Technik, vol. 46, No. 9, 1992, pp. 599-570.
"Digital Sound Broadcasting to Mobile Receivers" by Le Floch et al., in: IEEE Transactions on Consumer Electronics, vol. 35, No. 3, 1989, pp. 493-503.
"Analysis and Simulation of a Digital Mobile Channel Using Orthogonal Frequency Division Multiplexing" by L.J. Cimini, Jr. in: IEEE Transactions on Communications, vol.33,No.7, 1985, pp. 665-675.
"OFDM/FM Frame Synchronization for Mobile Radio Data Communication" by W.D. Warner and C.Leung, in: IEEE Transactions on Vehicular Technology, vol.42,No.3,1993,pp.302-313.
Grundig E.M.V. Elektromechanische Versuchsanstalt Max Grundig Gm
Tse Young T.
Vincent Paul J.
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