An Orthogonal Frequency Division Multiplexing (OFDM) method has been adopted as a transmission method for digital terrestrial broadcasting, a wireless LAN (Local Area Network), and the like. With the OFDM method, a plurality of carriers are closely arrayed while remaining orthogonal to each other. Therefore, the OFDM method enables efficient use of frequency. Furthermore, as the OFDM method allows setting long symbol lengths, the OFDM method is robust against Inter-Symbol Interference caused by a plurality of incoming waves.
Also, a guard interval technique is commonly used in the OFDM method. According to the guard interval technique, a portion of the end of a useful symbol is inserted in front of the useful symbol as a guard interval. With this structure, the guard interval technique makes it possible to perform the Fast Fourier Transform (FFT) while avoiding interference components pertaining to Inter-Symbol Interference caused by a delayed wave in a case where the delayed wave is delayed from the dominant wave by a delay period not greater than the duration of the guard interval. As a result, the receiver becomes able to demodulate the received OFDM transmission signals without deterioration.
However, when the delay time of a delayed wave exceeds the duration of the guard interval, Inter-Symbol Interference (ISI) and Inter-Carrier Interference (ICI) occur, and as a result, the receiver is unlikely to be able to demodulate the received OFDM transmission signals accurately, causing deterioration in reception quality.
Single Frequency Network (SFN) has been adopted for a terrestrial digital broadcasting method, and transmission stations are installed such that the delay times of delayed waves are within the duration of the guard interval. However, in some environment, the delay times of delayed waves exceed the duration of the guard interval due to reflection off mountains, building, etc. In such an environment, it is difficult for the receiver to demodulate the received OFDM transmission signals accurately due to ISI and ICI, and the reception quality significantly deteriorates as a result.
Accordingly, accurate demodulation of the OFDM transmission signals in the above-described environment requires a technology to remove, from the OFDM transmission signals, interference components pertaining to ISI and ICI due to delayed waves. For example, Patent Literature 1 suggests the following method.
Demodulated data in a frequency domain is calculated by performing an FFT on the OFDM transmission signal in a time domain; channel response data in the frequency domain is estimated based on the demodulated data; and channel response signals in the time domain are calculated by performing an Inverse Fast Fourier Transform (IFFT) on the channel response data in the frequency domain. Waveform equalization is performed on the OFDM transmission signals in the time domain based on the channel response signals in the time domain; interference components pertaining to ISI and ICI are estimated; and the interference components are removed from the OFDM transmission signals.
Note that Patent Literature 2 discloses another method. According to this method, however, interference components pertaining to ISI and ICI due to preceding waves which arrive at the receiver earlier than the dominant wave cannot be estimated and removed.