The present invention relates to a transmission bandwidth changing method and a digital transmission apparatus for an orthogonal frequency division multiplexing modulation system. 
In recent years, in Europe, the United States of America and Japan, the digitalization of the television broadcasting is being studied, while the orthogonal frequency division multiplexing (OFDM) modulation system is promising as the modulation system of the digitalization. 
The OFDM modulation system is of a kind of multi-carrier modulation system and a multiplicity of digital modulation waves are added to one another to be transmitted.  The quadrature phase shift keying (QPSK) system, the quadrature amplitude modulation system or the like is used as the modulation system of carriers thereof. 
An OFDM signal is expressed by equations as follows:
First, when a QPSK signal of each carrier is xcex1k(t), the signal is expressed by the following equation (1):
xcex1k(t)=ak(t)xc3x97cos(2xcfx80kft)+bk(t)xc3x97sin(2xcfx80kft)xe2x80x83xe2x80x83(1)
where k represents a number of the carrier, and ak(t) and bk(t) represent data of the k-th carrier and have a value of [xe2x88x921] or [1]. 
Next, when the number of carriers are N, the OFDM signal is a composite signal of N carriers and when the composite signal is xcex2k(t), the signal can be expressed by the following equation (2):
xcex2k(t)=xc2x7xcexa3xcex1k(t)xe2x80x83xe2x80x83(2)
where k assumes 1 to N, that is, k=1 to N (which is an integer equal to or larger than 1). 
The OFDM signal is constituted by a time-axis base signal defined by the equation (2).  For example, 32 samples of guard interval data are added to 1024 effective samples of data to constitute one symbol of 1056 samples and 396 sets of symbols are added to 4 sets of synchronizing symbols to constitute a series of signals of 400 symbols in total.  The series of signals of 400 symbols is named a frame.  The OFDM signal is constituted by frames generated repeatedly. 
The digital communication technique using the OFDM system is described in, for example, a paper on xe2x80x9cA Study on Field Pickup Unit using OFDM Modulation Schemexe2x80x9d, Shigeki Moriyama et al.,  ITE Technical Report Vol.  19, No.  38, pp.  7-17, August 1955 and JP-A-7-30513.  In the conventional OFDM system, a frequency bandwidth of a signal to be transmitted is a fixed value previously assigned to a communication system and is not changed. 
According to the present invention, there is provided an OFDM system having a changeable frequency bandwidth for a transmission signal.  When the bandwidth is changed in the transmission side, a demodulation bandwidth for a received signal is changed to follow the changed bandwidth of the transmission signal in a receiving side.  The transmission frequency bandwidth in the OFDM system is expressed by a product of an interval of frequency between adjacent carriers and the number of total carriers.  According to an embodiment of the present invention, the frequency bandwidth is changed by changing the number of carriers or by changing the interval between carriers. 
In the digital communication system utilizing the present invention, the frequency bandwidth assigned for transmission of a digital signal can be employed effectively.  The bandwidth can be broadened in the communication requiring a wide bandwidth as an image signal and the bandwidth can be narrowed in the transmission of a signal having a relatively narrow bandwidth as an audio signal and a control signal.  Further, when the occupancy frequency bandwidth required for a signal to be transmitted is narrower than the assigned frequency bandwidth, the assigned bandwidth is divided into a plurality of channels each having a narrower bandwidth, so that different data can be transmitted by means of the plurality of channels.  Bidirectional communication can be attained by means of the plurality of channels.  In addition, occupied bandwidth of each channel can be changed or shifted in accordance with the amount of transmitted data on the way to thereby change the quality of the transmission signal.