1. Field of the Invention
The present invention relates to an orthogonal frequency division multiplex signal transmitting method in which a coded digital picture signal transformed into an orthogonal frequency division multiplex (OFDM) signal of a limited frequency band is transmitted and the digital picture signal is reproduced, an orthogonal frequency division multiplex signal transmitting apparatus in which the picture signal is transformed to the OFDM signal and is transmitted, and an orthogonal frequency division multiplex signal receiving apparatus in which the OFDM signal is demodulated and the picture signal is reproduced.
2. Description of the Related Art
As one method for transmitting a coded digital picture signal in a limited frequency band, an orthogonal frequency division multiplex (OFDM) method in which digital information processed in a multivalued modulation according to a 256 quadrature amplitude modulation (QAM) method is transmitted as an orthogonal frequency division multiplex signal by using a large number of carrier waves is well-known because an adverse influence of a multi-path transmission or a transmission disturbance is not comparatively exerted on the OFDM signal and a frequency utilization factor in the OFDM method is comparatively superior. In this OFDM method, a large number of carrier waves orthogonal to each other are prepared, and a large number of pieces of digital information are separately transmitted by using the carrier waves. In this case, a sentence "a first carrier wave is orthogonal to a second carrier wave" denotes that a spectrum of the second (or first) carrier wave is zero at a carrier frequency of the first (or second) carrier wave.
In a transmitting apparatus for transmitting the OFDM signal, a plurality of pairs of in-phase (I) signals and quadrature (Q) signals in a digital base band are obtained by performing an inverse discrete Fourier transformation (IDFT) for a piece of transmission data, the I signals and Q signals pass through a digital-to-analog (D/A) converter and a low pass filter (LPF) and are transformed and combined to an OFDM signal of an intermediate frequency (IF) in an orthogonal modulating unit, the OFDM signal of the intermediate frequency is frequency-changed to a radio frequency band in a frequency changing circuit, an unnecessary frequency component is removed from the OFDM signal of the radio frequency band in a band pass filter (BPF), an electric power of the OFDM signal of the radio frequency band is amplified in a transmission unit, and the OFDM signal of the radio frequency band is radiated from a transmission antenna.
In a receiving apparatus for receiving the OFDM signal, the OFDM signal of the radio frequency band is frequency-changed to an intermediate frequency in a frequency changing circuit and is amplified in an amplifier. Thereafter, an unnecessary frequency component is removed from the OFDM signal of the intermediate frequency in a band pass filter, and the OFDM signal of the intermediate frequency is demodulated to the I signals and the Q signals in an orthogonal demodulating unit. Thereafter, the I signals and Q signals pass through a low pass filter, an analog-to digital (A/D) converter, a discrete Fourier transforming circuit and a quadrature amplitude modulation decoding circuit, and the transmission data is reproduced.
In the above transmission of the OFDM signal in the transmitting and receiving apparatuses, in cases where a relative amplitude characteristic difference or a relative phase characteristic difference between a group of the I signals and a group of the Q signals exists or in cases where a pair of modulating waves or a pair of demodulating waves exactly having a 90 degrees phase difference are not fed to the orthogonal modulating unit or the orthogonal demodulating unit, a code error occurs in an I or Q signal in the transmitting or receiving apparatus. That is, amplitude and phase changes of a frequency of the I or Q signal and a crosstalk (or an image component) from the frequency of the I or Q signal to an symmetric opposite frequency occurs.
To prevent the occurrence of the code error, various methods for preventing the occurrence of a characteristic difference such as an amplitude characteristic difference or a phase characteristic difference between a group of the I signals and a group of the Q signals are proposed in Published Unexamined Japanese Patent Applications H6-350658 (1994), H3-76623 (1991), H5-227239 (1993), H5-110369 (1993), H3-53735 (1991), H6-188932 (1994) and H4-290337 (1992).
Also, a correction for setting the frequency amplitude characteristics and the frequency phase characteristics of the I signals and the Q signals to prescribed characteristics has been proposed. For example, a correction of the frequency amplitude characteristics and the frequency phase characteristics in the I signals and the Q signals, a correction of phases of the orthogonal modulating waves, a correction of the frequency amplitude characteristics and the frequency phase characteristics in the processing of the signal of the intermediate frequency band and/or a correction of the frequency amplitude characteristics and the frequency phase characteristics in the processing of the signal of the radio frequency band are proposed for the transmitting apparatus, and a correction of the frequency amplitude characteristics and the frequency phase characteristics in the I signals and the Q signals, a correction of phases of the orthogonal modulating waves, a correction of the frequency amplitude characteristics and the frequency phase characteristics in the processing of the signal of the intermediate frequency band, a correction of the frequency amplitude characteristics and the frequency phase characteristics in the processing of the signal of the radio frequency band and/or a correction of the frequency amplitude characteristics and the frequency phase characteristics in a multi-path circumstance for an electric wave transmission are proposed for the receiving apparatus (Published Unexamined Japanese Patent Applications H6-311134 (1994) and H5-219021 (1993)).
3. Problems to be Solved by the Invention
However, configurations of the above conventional transmitting and receiving apparatuses in which one of the above correcting methods is performed are complicated, so that a manufacturing cost of each apparatus is increased. Also, in cases where a plurality of code errors occur, the configurations of the above conventional transmitting and receiving apparatuses are moreover complicated to correct the all code errors. Also, the code errors cannot be corrected with a high precision even though one of the above correcting methods is performed. Also, all of the above correcting methods are not appropriate for the OFDM signal.
In addition, a difference in amplitude between a group of I signals and a group of Q signals is not compensated in a conventional correcting method disclosed in a Published Unexamined Japanese Patent Application H6-188932 (1994) in which a difference in an orthogonal characteristic between transmitting and receiving apparatuses is compensated. Also, in a conventional correcting method disclosed in a Published Unexamined Japanese Patent Application H4-290337 (1992), though a digital orthogonal modulating unit is arranged at a front stage of a D/A converter in a transmitting unit and an error occurring in the transmitting apparatus is prevented in principle, because an operational speed of the D/A converter and a bit width are limited, there is a drawback that a useful transmitting apparatus is limited.
Therefore, because each of the amplitude characteristic difference between a group of I signals and a group of Q signals, the phase characteristic difference between a group of I signals and a group of Q signals, a difference in the orthogonal characteristic between a group of I signals and a group of Q signals is a direct cause of a code error in a transmitted signal and a received signal, the code error is removed or compensated in conventional transmitting/receiving method and apparatus. However, the improvement of the correction of the code error is limited. In addition, because frequency characteristic difference between a group of I signals and a group of Q signals is not compensated in any conventional transmitting/receiving method and apparatus, any transmitting/receiving method and apparatus appropriate for the OFDM signal is not proposed.