Multiplex communications – Communication techniques for information carried in plural... – Combining or distributing information via frequency channels
Reexamination Certificate
1998-05-21
2001-11-06
Olms, Douglas (Department: 2661)
Multiplex communications
Communication techniques for information carried in plural...
Combining or distributing information via frequency channels
C370S281000, C370S208000
Reexamination Certificate
active
06314113
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a method of determining a transmission format for complex symbols in a signal formatted in complex symbol blocks, which method can be applied to a transmission system, each block having a first occurrence of a group of complex symbols and a second occurrence of the same group of complex symbols, the two occurrences having a like duration, the method effecting a correlation between the first occurrence and the second occurrence for determining characteristic features of the transmission format.
The invention also relates to a system and a receiver implementing the method. The method is used for receiving signals transmitted, for example, in an orthogonal frequency-division multiplexing mode, notably transmitted by radio, by cable, by satellite, for example for digital television applications.
BACKGROUND OF THE INVENTION
The signals transmitted in the orthogonal frequency-division multiplexing mode will hereinafter be identified by the short name of OFDM (Orthogonal Frequency-Division Multiplexing). The OFDM technique consists of splitting up a transmit information signal by spreading it over a large number of digital low-rate channels. Thus a strongly selective wideband channel is transformed into a large number of non-selective digital channels. As they all form one wideband channel, it is not very likely that fadings simultaneously affect the whole channel in the course of the transmission. This technique also permits of reducing intersymbol interference. Thus, one frequency corresponds with each digital channel, while all the frequencies are symmetrically spread around one central carrier frequency. As it is hardly acceptable to use selective filters at the receiver end, it is preferred to tolerate an overlap of the spectrums, but while imposing conditions of orthogonality between the frequencies to eliminate the intersymbol interference at the sampling instants.
The OFDM signal is organized in frames formed by blocks of which certain blocks are service blocks and others are data blocks. To avoid intersymbol interference, each block contains redundancy data. A random block is formed by all the OFDM frequencies used by the system, the frequencies being modulated by digital transmit symbols caused by a coded modulation, for example, a digital PSK or QAM modulation. At the transmitter end, these symbols areas coded in accordance with a certain format and with a certain timing which is to be detected at the receiver end so that they can be decoded correctly. Thus, at the receiver end, the received signals are to be demodulated and then decoded to restore the original data. The invention proposes to retrieve the essential characteristic features of the transmission format used for the transmission.
European patent EP 608 024 which corresponds to U.S. Pat. No. 5,559,833, issued Sept. 24, 1996, assigned to U.S. Philips Corporation, describes a digital transmission system which has a timing recovery of OFDM signals. This document exploits the two occurrences of a like group of symbols which appear in an OFDM signal. A correlation is established between these two occurrences by delaying one of the occurrences by a constant delay. Indeed, this document considers this delay to be known, which is not the case in all the applications of this technique. The device thus described recovers the timing of the symbols on the basis of a local clock, on a frequency division and on a control by a phase-locked loop. Moreover, the correlation utilizes a technique of subtraction which necessitates the use of much memory capacity.
SUMMARY OF THE INVENTION
It is an object of the invention to permit the recovery of complex symbols notably organized in OFDM blocks working without reference, without knowing the transmission format a priori.
This object is achieved with a method of recovering complex symbols which method comprises the following steps:
storage of a first sequence of complex symbols taken from the received signal via an acquisition time window which has a longer duration than the duration of the occurrences of complex symbols,
correlation of the first sequence of complex symbols with a second sequence of complex symbols selected at another moment in the received signal, via a correlation time window which has a duration equal to that of the acquisition time window,
determination of characteristics of the transmission format by searching for maximum correlation between the first and the second sequences of complex symbols by synchronizing the acquisition window with the received signal and by positioning the windows relative to each other in time, the maximum correlation appearing when the first and second sequences of complex symbols include said first and second occurrences, respectively.
More particularly, while the signal is an orthogonal frequency division multiplexing signal called OFDM signal, the method determines a number of orthogonal frequencies included in at least one transmission format of the OFDM signal.
The invention likewise relates to a transmission system for transmitting an orthogonal frequency-division multiplexing signal called OFDM signal, the system comprising transmission means for transmitting said signal formatted in blocks of complex symbols in accordance with a transmission format, each block comprising a first occurrence of a group of complex symbols and a second occurrence of the same group of complex symbols, the two occurrences having a like duration, and receiving means comprising correlation means for determining a correlation between the first occurrence and the second occurrence and determining characteristic features of the transmission format, characterized in that the receiving means comprise:
means for selecting and storing a first sequence of complex symbols selected, in the received signal via an acquisition time window which has a duration that is longer than the duration of the occurrence of complex symbols,
means for selecting a second sequence of complex symbols which is selected at another moment in the received signal via a correlation time window which has a duration equal to that of the acquisition time window, the correlation means effecting the correlation between the first sequence and the second sequence of complex symbols,
means for determining the characteristic features of the transmission format by searching for maximum correlation between the first sequence and the second sequence of complex symbols by synchronizing the acquisition window with the received signal and by positioning the windows with respect to each other in time, the maximum correlation occurring when the first and the second sequence of complex symbols include said first and second occurrences, respectively.
More particularly, the receiving means comprises a plurality of parallel-arranged channels which permit determination of a plurality of transmission formats. Preferably, the channels work simultaneously. The invention also relates to the receiver used in such a system.
REFERENCES:
patent: 5559833 (1996-09-01), Hayet
patent: 5602833 (1997-02-01), Zehavi
patent: 5745484 (1998-04-01), Scott
patent: 5751761 (1998-05-01), Gilhousen
patent: 5903614 (1999-05-01), Suzuki et al.
patent: 5959980 (1999-09-01), Scott
patent: 6088406 (2000-07-01), Suzuki
patent: 6125124 (2000-09-01), Junell et al.
patent: 8257422 (1997-08-01), None
patent: WO9726742 (1997-07-01), None
patent: WO 97/26742 (1997-07-01), None
Olms Douglas
Pizarro Ricardo M.
Treacy David R.
U.S. Philips Corporation
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