1. Field of the Invention
The present invention relates to apparatuses and methods for OFDM demodulation, and more particularly to an apparatus and a method for demodulating a signal transmitted under an orthogonal frequency division multiplex (OFDM) technique.
2. Description of the Background Art
In recent years, a transmission mode applying the OFDM technique has been popular for digital terrestrial television broadcasting, mobile communications, and the like. The OFDM mode is included in a multicarrier modulation scheme, and carries out OFDM signal transmission from a transmitter to a receiver. The transmitter assigns transmission data to a large number of subcarriers having an orthogonality relation between any adjoining two, and the subcarriers are each modulated by the transmission data assigned thereto. Thereafter, the transmitter collectively subjects the modulated subcarriers to inverse Fourier transform to generate an OFDM signal. In such OFDM signal, the transmission data divided and assigned to the subcarriers may be prolonged in cycle, thereby the OFDM signal is characteristically not susceptible to a delay wave, such as multipath.
Such transmission under the OFDM mode is carried out on a transmission symbol basis. The transmission symbol is structured by a valid symbol period and a guard interval (GI). The valid symbol period is a period over which a signal (hereinafter, valid symbol) corresponding to the transmission data is transmitted, and is defined according to the inverse Fourier transform processing. The guard interval is a period over which a signal obtained by partially and cyclically repeating a waveform of the valid symbol is transmitted. The guard interval is for reducing the impact of the delay wave.
The receiver receives the transmission symbol, and extracts the valid symbol therefrom. The receiver then subjects the extracted valid symbol to Fourier transform to separate the valid symbol into the subcarriers. Thereafter, the receiver demodulates each of the separated subcarriers so as to reproduce the transmission data.
The above-described OFDM signal may take such waveform as random noise, therefore it is difficult to establish synchronization in frequency and symbol of the OFDM signal in the receiver. In the case that the OFDM signal is demodulated without frequency synchronization, the orthogonality relation among the subcarriers is lost and thus interference occurs. Further, if the receiver cannot establish symbol synchronization, that is, when the receiver cannot correctly extract the valid symbol from the transmission symbol, interference occurs between the symbols. Accordingly in either case, the receiver fails to correctly reproduce the transmission data.
To get around such problem, in the OFDM signal (see FIG. 16) for transmission under the OFDM mode, a symbol (hereinafter, synchronous symbol) being a reference to synchronization is provided at the head of a transmission frame. The transmission frame is structured by several predetermined transmission symbols.
The conventional OFDM receiver which establishes symbol synchronization by using such synchronous symbol is found in Japanese Patent Laying-Open No. 11-32025 (99-32025) titled “OFDM receiver and method for detecting synchronization therein”. The conventional technique applies a chirp symbol to the synchronous symbol. By calculating a correlation coefficient between the chirp symbol and a received signal, the conventional receiver detects symbol timing from a maximum value thereof, and establishes symbol synchronization therein.
In the case that the multipath occurs due to a transmission path characteristic, as shown in FIGS. 17A and 17B, the receiver may receive both a direct wave and a delay wave of a transmitting signal, i.e., a combined wave thereof. Herein, when a delay of the delay wave is within the guard interval, the receiver can extract the valid symbol located in a section having no adjacent-symbol interference (FIG. 17A) by following the timing of the valid symbol period of the direct wave. However, this is not applicable to a case where the delay of the delay wave is beyond the guard interval. It may result in extracting the valid symbol having been influenced by the adjacent-symbol interference (FIG. 17B). Therefore, if the adjacent-symbol interference occurs in the receiver, there is a need for setting a section from which the valid symbol is extracted so as to minimize the interference. Note that, the diagonally shaded area in FIGS. 17A and 17B shows a part where the adjacent-symbol interference is observed.
In the conventional receiver, however, the symbol timing is set according to the maximum value of the correlation coefficient between the received signal and the synchronous symbol. Consequently, when the delay of the delay wave is beyond the guard interval, the conventional receiver is incapable of setting the symbol timing in such a manner as to minimize the adjacent-symbol interference.
Further, in the case that the transmitting signal and the received signal have a shift in frequency (hereinafter, frequency shift) therebetween, the correlation coefficient between the received signal and the synchronous symbol becomes smaller. Consequently, the conventional receiver cannot satisfactorily detect the synchronous symbol therein.
Still further, a sampling frequency for sampling symbols may be shifted (hereinafter, sampling frequency shift) between the transmitter and the receiver. If this is the case, the symbol timing setting based on detection of the synchronous symbol as the conventional receiver is not sufficient. It leads to a shift of the symbol timing, i.e., a shift of the valid symbol period between the transmission symbols at the head and at the tail in the transmission frame.