OFDM is used as a transmission scheme for digital terrestrial broadcasting in Europe and Japan, wireless LAN, and the like.
The OFDM scheme is a method of performing modulation/demodulation while assigning data to a plurality of carriers orthogonal to each other. An Inverse Fast Fourier Transform (IFFT) process is performed in a transmitter, while a Fast Fourier Transform (FFT) process is performed in a receiver. Any modulation scheme can be applied to each carrier, and QPSK (Quaternary Phase Shift Keying), QAM (Quadrature Amplitude Modulation), and the like can be selected.
According to the transmission standards ISDB-T for digital terrestrial television broadcasting which are adopted in Japan, a transmission signal in digital terrestrial television broadcasting is composed of 13 OFDM segments (hereinafter simply referred to as “segments”). FIG. 3 is a spectral diagram of a transmission signal defined in ISDB-T. Each segment has a band width which is obtained by dividing a transmission band width of one television broadcasting channel into 14 equal parts, and is composed of a plurality of carriers.
One or a plurality of segments constitute one hierarchical layer, and for each hierarchical layer, transmission parameters, such as a modulation scheme (e.g., QPSK, 16 QAM, or 64 QAM), a coding rate (e.g., ½, ⅔, or ¾) of inner code, and the like, can be set. Note that the largest number of hierarchical layers which can be simultaneously transmitted is 3 (a hierarchical layer A, a hierarchical layer B, and a hierarchical layer C), and a partial reception portion described below is counted as one hierarchical layer. Further, in ISDB-T, in order to reduce an influence of interference on a transmission channel, interleaving on a frequency axis (frequency interleaving) and interleaving on a time axis (time interleaving) are assumed to be performed.
The frequency interleaving defined in ISDB-T is performed in ways which vary depending on the structure and contents of hierarchical layers constituting an OFDM signal. A specific example thereof will be described.
FIG. 25 is a schematic diagram illustrating the frequency interleaving of an OFDM signal when single-hierarchical layer transmission is performed. FIG. 25 illustrates the case where only the hierarchical layer A (modulation scheme: 64 QAM) is transmitted. As can be seen, in the frequency interleaving for single-hierarchical layer transmission, carriers belonging to all the 13 segments switch places (interleave) with each other across a whole transmission band.
On the other hand, FIG. 26 is a schematic diagram illustrating the frequency interleaving of an OFDM signal when two-hierarchical layer transmission for partial reception is performed. FIG. 26 illustrates the case where the hierarchical layer A for partial reception (modulation scheme: QPSK) and the hierarchical layer B not for partial reception (modulation scheme: 64 QAM) are transmitted. As illustrated in FIG. 26, in ISDB-T, the hierarchical layer A is composed only of a segment having a segment number 0 which is located at the center of a transmission band, and the frequency interleaving can be performed in the segment. In this case, a carrier belonging to the center segment does not switch places with the other segment carriers. Therefore, it is possible to receive a portion of a service using a reception apparatus which can receive only the center segment, i.e., “partial reception”. Hereinafter, a hierarchical layer for partial reception is referred to as a partial reception portion.
As described above, in ISDB-T, the process of the frequency interleaving significantly varies, depending on the configuration and contents of hierarchical layers and the presence or absence of the partial reception portion. Therefore, in order to perform frequency deinterleaving with respect to a received OFDM signal, it is necessary to change the process, depending on whether or not the partial reception portion is transmitted. Also, in order to perform an error correction with respect to the received OFDM signal, it is necessary to identify a modulation scheme for a demodulated carrier before soft decision (demapping) of transmitted data.
In the case of ISDB-T, information about transmission parameters, such as the presence or absence of partial reception portion, a modulation scheme and the like, are generally obtained by obtaining and employing transmission control information called TMCC (Transmission and Multiplexing Configuration Control) information inserted in an OFDM transmission signal.
The TMCC information is information which is required for demodulation and error correction processes in a reception apparatus, and includes transmission parameters for each hierarchical layer (e.g., a modulation scheme, a coding rate, a time interleaving length, the number of segments, etc.), a flag indicating the presence or absence of partial reception portion, and the like. The TMCC information is inserted in a predetermined carrier (hereinafter referred to as a TMCC carrier) of an OFDM signal with predetermined symbol timing, and since the position of the insertion is known, various kinds of information can be obtained in a receiver as required.
According to the transmission format of ISDB-T, one frame is composed of 204 symbols as one cycle. The TMCC information, which has 204 bits, is transmitted using a TMCC carrier at a rate of one bit per symbol. For example, information indicating the presence or absence of partial reception portion, i.e., a partial reception flag, is transmitted using a TMCC carrier at a symbol having symbol number 27. Regarding information about the modulation scheme for each hierarchical layer, the hierarchical layer A is transmitted using TMCC carriers at symbol numbers 28 to 30, the hierarchical layer B is transmitted using TMCC carriers at symbol numbers 41 to 43, and the hierarchical layer C is transmitted using TMCC carriers at symbol numbers 54 to 56.
In a reception apparatus which receives an OFDM signal having such a transmission format, a time required from the start of reception of an OFDM signal selected by a tuning operation or the like, followed by demodulation and error correction, until video, audio or the like is output, is discussed.
FIG. 27 is a diagram for explaining timing with which TMCC information is obtained. As illustrated in FIG. 27, it is assumed that a reception apparatus starts a reception operation at time t0, followed by input level adjustment, frequency synchronization or the like of a received signal, and obtaining of TMCC information is started at time t1, and all 204 bits of the TMCC information are completely obtained at time t2. After completing obtaining of a partial reception flag, the reception apparatus recognizes the segment structure of the OFDM signal based on the partial reception flag, successively followed by processes, such as frequency deinterleaving, time deinterleaving, demapping, error correction, and the like.
A time Δta from the start of obtaining of TMCC information to the completion of obtaining of a partial reception flag is maximum when the obtaining of TMCC information starts at symbol number 28 and the obtaining of a partial reception flag is completed at symbol number 27 of the next frame. In this case, the time Δta is a time required for transmission of 204 symbols. The time is as large as about 257 msec when a Mode and a guard ratio defined in ISDB-T are 3 and ¼, respectively.
On the other hand, the same is true of a time required to obtain information about a modulation scheme for each hierarchical layer. For example, a time from the start to completion of obtaining of TMCC information required for identification of a modulation scheme for the hierarchical layer A is maximum when the obtaining of TMCC information is started at symbol number 31 and is completed at symbol number 30. Thus, it takes as large as about 257 msec (Mode: 3; and guard ratio: ¼) to transmit 204 symbols (the same time is required for hierarchical layers B and C).
Therefore, an OFDM reception apparatus is known in which TMCC information (e.g., a partial reception flag, a carrier modulation scheme, a time interleaving length, etc.) required for demodulation and error correction processes is previously stored in a memory, the TMCC information stored in the memory is read out every time a reception operation is started, and based on the TMCC information thus read out, demodulation and error correction operations are performed. Such an OFDM reception apparatus is disclosed in, for example, Patent Document 1 below.    Patent Document 1: JP 2001-292121 A