1. Field of the Invention
The present invention relates to a receiving apparatus and a receiving method and, more particularly, to a receiving apparatus and a receiving method that are configured to early and surely detect that a signal based on the DVB-T2 (Digital Video Broadcasting—Terrestrial 2) standard is not transmitted by a channel in reception, for example.
2. Description of the Related Art
With terrestrial digital broadcasting and so on, OFDM (Orthogonal Frequency Division Multiplexing) is used for data modulating.
With OFDM, many orthogonal subcarriers are arranged in a transmission band and digital modulation, such as PSK (Phase Shift Keying) or QAM (Quadrature Amplitude Modulation), is performed to allocate data to the amplitude and phase of each of these subcarriers.
As described above, in OFDM, data is allocated to two or more subcarriers, so that the modulation can be performed by performing IFFT (Inverse Fast Fourier Transform) which performs inverse Fourier transform; while the demodulation of an OFDM signal obtained as a result of the modulation can be performed by FFT (Fast Fourier Transform) which performs Fourier transform.
Therefore, a transmission apparatus configured to transmit OFDM signals can be configured by use of an IFFT computation circuit and a receiving apparatus configured to receive OFDM signals can be configured by use of an FFT computation circuit.
Terrestrial digital broadcast standards that use OFDM having the above-mentioned features include DVB-T2 (the second-generation European terrestrial digital broadcasting standard). For DVB-T2, refer to so-called DVB BlueBook A122: “Frame structure channel coding and modulation for a second generation digital terrestrial television broadcasting system (DVB-T2),” DVB Document A122 June 2008.
Terrestrial digital broadcasting standards based on OFDM define a unit called a frame which is composed of plural OFDM symbols, in which data is transmitted on the frame basis. The DVB-T2 standard also defines a frame called a T2 frame. Data is transmitted in the unit of this T2 frame.
Now, referring to FIG. 1, there is shown a T2 frame format.
As shown in FIG. 1, each T2 frame has a P1 symbol, a P2 symbol, and data symbols in this order.
The P1 symbol is a symbol for transmitting P1 signalling. The P1 symbol includes S1 and S2 transmission parameters. The parameters S1 and S2 are indicative in which of the schemes, SISO (Single Input, Single Output (meaning one transmission and one receiving antenna)) and MISO (Multiple Input, Single Output (meaning multiple transmitting antennas but one receiving antenna)), the P2 symbol is transmitted, an FFT size (the number of samples (or symbols) subject to one session of FFT computation) for FFT computation of P2, and so on.
The P2 symbol is a symbol for transmitting L1 pre-signalling and L1 post-signalling.
The purposes of the P1 symbol intended by the DVB-T2 standard include the following:
(a) the receiving apparatus early determines that a signal being received is a signal based on the DVB-T2 standard;
(b) the receiving apparatus identifies a preamble signal itself as a preamble signal of a frame based on the DVB-T2 standard;
(c) a transmission parameter necessary for the starting of demodulation is transmitted; and
(d) the receiving apparatus is cable of performing frame positional detection and carrier error correction.
Referring to FIG. 2, there is shown a configuration of the P1 symbol.
As shown in FIG. 2, the P1 symbol has 1K (=1024) symbols as valid symbols. The P1 symbol has a structure in which signal C obtained by frequency-shifting a part of the beginning of valid symbol A by frequency fSH is copied to the front side of the valid symbol and signal B obtained by frequency-shifting the remaining part of valid symbol A by frequency fSH is copied to the rear of the valid symbol. Performing frequency shifting provides, on the basis of the standard, a mechanism in which it is difficult to erroneously detect an interference signal as the P1 symbol.
In the receiving apparatus, P1 symbol detection is performed by obtaining a correlation value for each section by use of the fact that the P1 symbol contains a copy of partial data thereof. For example, P1 symbol detection is performed in an initial scan for determining on which channel a DVB-T2 standard signal is transmitted. If the P1 symbol is detected, it is indicative that a DVB-T2 signal is being transmitted on the receiving channel; if the P1 symbol is not detected, it is indicative that a DVB-T2 signal is not being transmitted on the receiving channel. In what follows, a DVB-T2 signal is appropriately referred to as a T2 signal.
Exemplary Configuration of a Receiving Apparatus
Referring to FIG. 3, there is shown a block diagram illustrating a related-art receiving apparatus.
A receiving apparatus 1 has a P1 detection block 11, a delay block 12, a frequency correction block 13, an FFT computation block 14, a CDS (Carrier Distribution Sequence) correlation computation block 15, a decode section 16, a TS signal presence/absence determination block 17, and a control block 18. The P1 detection block 11 has a correlation value computation portion 11A. The decode section 16 has a coarse correction/descramble processing block 21, a DBPSK demodulation block 22, an S1 demodulation block 23, and an S2 demodulation block 24.
Frequency conversion, A/D conversion, and quadrature demodulation are performed on an RF signal supplied from an antenna. A resultant OFDM signal is supplied to the P1 detection block 11 and the delay block 12 as an input signal. This input signal is a complex signal containing an in-phase component (I component) and a quadrature-phase component (Q component) and an OFDM signal of time domain before FFT computation is performed.
The P1 detection block 11 computes a correlation value for each section of the input signal in the correlation value computation portion 11A, thereby detecting a P1 symbol. The maximum value of the correlation values computed by the correlation value computation portion 11A is supplied to the T2 signal presence/absence determination block 17. Details of the computation of correlation values performed by the correlation value computation portion 11A will be described later.
If the P1 symbol has been detected on the basis of the correlation value for each section, the P1 detection block 11 sets a start position of FFT computation with reference to the position of the detected P1 symbol, thereby outputting information indicative of the set position to the FFT computation block 14.
In addition, the P1 detection block 11 detects a frequency carrier frequency offset) in a carrier interval and outputs a fine offset value that is information indicative of the detected frequency offset to the frequency correction block 13. According to the DVB-T2 Implementation Guidelines (ETSI TR 102 831: IG), the P1 symbol allows the detection of a “fine” frequency offset having an accuracy of ±0.5× subcarrier interval.
The delay block 12 delays an OFDM signal supplied as an input signal by a period of time required for the P1 detection block 11 to detect the P1 symbol, for example, and supplies the delayed OFDM signal to the frequency correction block 13.
On the basis of the file correction value supplied from the P1 detection block 11, the frequency correction block 13 corrects the frequency offset of the OFDM signal supplied from the delay block 12 and outputs the corrected OFDM signal to the FFT computation block 14.
Using the position set by the P1 detection block 11 as the start position, the FFT computation block 14 performs FFT computation on the OFDM signal (a symbol of valid symbol length) supplied from the frequency correction block 13. The FFT computation provides the data transmitted by subcarrier, namely, an OFDM signal that is representative of a symbol on IQ constellation. The OFDM signal in the frequency domain obtained by the FFT computation is supplied to the CDS correlation computation block 15.
The CDS correlation computation block 15 computes a correlation value between a subcarrier sequence having a power of the OFDM signal supplied from the FFT computation block 14 and a known CDS. In the OFDM signal of the frequency domain obtained by performing FFT computation on the P1 symbol signal, the subcarriers having a power are allocated to only a frequency defined by the known sequence. Details of the known sequence will be described later:
The CDS correlation computation block 15 detects the P1 symbol on the basis of the detected correlation values and outputs the maximum correlation value to the T2 signal presence/absence determination block 17. For example, a section of subcarrier sequence having the maximum correlation value with the known sequence and having a power is detected as a P1 symbol section.
In what follows, the correlation value for each OFDM signal section in the time domain computed by the correlation value computation portion 11A of the P1 detection block 11 is referred to as a signal section correlation value and the correlation value computed by the CDS correlation computation block 15 is referred to as a CDS correlation value. The maximum value of the signal section correlation values is referred to as a signal section correlation peak value and the maximum value of the CDS correlation values is referred to as a CDS correlation peak value.
If the OFDM signal supplied from the FFT computation block 14 is a P1 symbol signal, the CDS correlation computation block 15 detects a coarse carrier frequency offset for each carrier. According to the Implementation Guidelines of the DVB-T2 standard (ETSI TR 102 831: IG), the detection of “coarse” frequency offset in units of subcarrier interval is enabled by use of the correlation with the known sequence of P1 symbols.
The CDS correlation computation block 15 outputs the FFT-computed OFDM signal and the coarse correction value that is information indicative of the detected frequency offset to the coarse correction/descramble processing block 21.
The coarse correction/descramble processing block 21 corrects the frequency offset of the OFDM signal supplied from the CDS correlation computation block 15 on the basis of the coarse correction value and outputs an OFDM signal obtained by performing descramble and so on to the DBPSK demodulation block 22.
The DBPSK demodulation block 22 performs DBPSK demodulation on the OFDM signal supplied from the coarse correction/descramble processing block 21. Of the signal point sequences obtained by the DBPSK demodulation, the DBPSK demodulation block 22 outputs the sequence of a S1 part contained in the P1 symbol to the S1 demodulation block 23 and outputs the sequence of a S2 part to the S2 demodulation block 24.
The S1 demodulation block 23 computes correlation values between the signal point sequence supplied from the DBPSK demodulation block 22 and eight types of known sequences corresponding to the 3-bit S1 defined by the DVB-T2 standard. Details of these known sequences will be described later. The S1 demodulation block 23 selects, as S1, a 3-bit value corresponding to a known sequence with the maximum correlation value obtained of the eight types and outputs the selected 3-bit value.
The S2 demodulation block 24 computes correlation values between the signal point sequence supplied from the DBPSK demodulation block 22 and 16 types of known sequences corresponding to the 4-bit S2 defined by the DVB-T2 standard. The S2 demodulation block 24 selects, as S2, a 4-bit value corresponding to the known sequence with the maximum correlation value obtained of the 16 types and outputs the selected 4-bit value.
On the basis of the S1 outputted from the S1 demodulation block 23 and the S2 outputted from the S2 demodulation block 24, various kinds of processing operations will be performed in subsequent circuits.
On the basis of the signal section correlation peak value supplied from the correlation value computation portion 11A and the CDS correlation peak value supplied from the CDS correlation computation block 15 at the time of the initial scan, the T2 signal presence/absence determination block 17 determines whether or not the T2 signal is being transmitted on the receiving channel. If the T2 signal is found not being transmitted on the receiving channel, then the T2 signal presence/absence determination block 17 outputs a T2 “absence” flag that is a signal indicative thereof.
The control block 18 controls the entire operation of the receiving apparatus 1 that includes the configuration shown in FIG. 3. For example, the receiving channel is controlled by the control block 18.
Flow of the Initial Scan
The following describes the processing to be performed by the receiving apparatus 1 at the time of the initial scan with reference to flowcharts shown in FIG. 4 and FIG. 5.
FIG. 4 and FIG. 5 partially show the processing to be performed at the initial scan described in index figure 74 of the Implementation Guidelines of the DVB-T2 standard (ETSI TR 102 831: IG). The initial scan is performed to check if there is a T2 signal in a tunable frequency band when the power supply is first turned on, for example.
In step S1, the control block 18 controls a tuner, not shown, to select the bandwidth of a channel to be received from among two or more bandwidths, such as 6 MHz, 7 MHz, and 8 MHz.
In step S2, the control block 18 sets the center frequency of the channel to be received. When the bandwidth of the channel is selected and the center frequency of the channel having the selected bandwidth is set, an OFDM signal is entered in the P1 detection block 11 and the delay block 12.
In step S3, the P1 detection block 11 computes a signal section correlation value for each section of an input signal in the correlation value computation portion 11A, thereby detecting a P1 symbol. A signal section correlation peak value computed by the correlation value computation portion 11A is supplied to the T2 signal presence/absence determination block 17.
In step S4, T2 signal presence/absence determination block 17 determines whether or not a P1 symbol has been detected. For example, if a signal section correlation peak value equal to or higher than a threshold value has been detected in a predetermined section, the T2 signal presence/absence determination block 17 determines that a P1 symbol has been detected.
If a P1 symbol is found detected in step S4, then the P1 detection block 11 sets the position at which the signal section correlation peak value has been detected to the beginning of the T2 frame in step S5. The P1 detection block 11 sets the start position of FFT computation with reference to the position (the beginning of the T2 frame) of the P1 symbol and outputs information indicative of the FFT computation start position to the FFT computation block 14. In addition, the P1 detection block 11 detects a frequency offset in the carrier interval and outputs a fine offset value to the frequency correction block 13.
The OFDM signal delayed by the delay block 12 and frequency-offset-corrected by the frequency correction block 13 based on the fine offset value is supplied to the FFT computation block 14.
In step S6, the FFT computation block 14 performs FFT computation on the OFDM signal of P1 signal supplied from the frequency correction block 13. The OFDM signal of the frequency domain obtained by the FFT computation is supplied to the CDS correlation computation block 15.
In step S7, the CDS correlation computation block 15 computes a CDS correlation value on the basis of the FFT-computed OFDM signal and a known sequence, thereby detecting a P1 symbol. A CDS correlation peak value computed by the CDS correlation computation block 15 is supplied to the T2 signal presence/absence determination block 17.
In step S8, the T2 signal presence/absence determination block 17 determines whether or not the CDS correlation peak value is equal to or higher than a threshold value and the P1 symbol has been detected by the CDS correlation computation block 15.
If the CDS correlation peak value is found to be below the threshold value in step S8 or, if a P1 symbol is found not detected in step S4, then the control block 18 determines whether or not a time-out has occurred in step S9.
If a time-out is found not encountered in step S9, then the procedure returns to step S3 to repeat the detection of a P1 symbol on the basis of a signal section correlation value. The period of time for one T2 frame is 250 ms at maximum. If a T2 signal is being transmitted on a channel in reception, a P1 symbol is detected every 250 ms. Therefore, here, if a time from the start of the detection of P1 symbol in step S3 has passed a predetermined time obtained by adding 250 ms to a margin, then a time-out is determined to have occurred; otherwise, no time-out is determined to have occurred.
If a time-out is determined to have occurred in step S9, then the control block 18 determines whether or not there remains any unset center frequency in step S10.
If an unset center frequency is found remaining in step S10, then the procedure returns to step S2, in which the control block 18 sets a new frequency as a center frequency, thereby repeating the processing mentioned above.
On the other hand, if an unset center frequency is found not remaining in step S10, then the control block 18 determines whether or not there remains any unselected bandwidth in step S11.
If an unselected bandwidth is found remaining in step S11, then the procedure returns to step S1, in which the control block 18 selects a new bandwidth, thereby repeating the processing mentioned above.
On the other hand, if an unselected bandwidth is found not remaining in step S11, then the control block 18 ends the above-mentioned initial scan processing.
If the CDS correlation peak value is higher than the threshold and a P1 symbol is found detected in step S8, then the CDS correlation computation block 15 detects a frequency offset for each carrier on the basis of the CDS correlation value in step S12. Further, the CDS correlation computation block 15 outputs the FFT-computed OFDM signal and the coarse correction value to the coarse correction/descramble processing block 21.
In step S13, the coarse correction/descramble processing block 21 corrects the frequency offset of the OFDM signal on the basis of the coarse correction value, thereby performing descramble processing and so on.
In sep S14, the decode section 16 decodes S1 and S2. To be more specific, the DBPSK demodulation block 22 executes DBPSK demodulation on the OFDM signal with the frequency offset correction and so on performed by the coarse correction/descramble processing block 21. The S1 demodulation block 23 and the S2 demodulation block 24 compute correlation values between a signal point sequence supplied from the DBPSK demodulation block 22 and known sequences.
In step S15, the S1 demodulation block 23 selects S1 on the basis of the computed correlation value and the S2 demodulation block 24 selects S2 on the basis of the computed correlation value. S1 selected by the S1 demodulation block 23 and S2 selected by the S2 demodulation block 24 are also supplied to the control block 18.
In step S16, the control block 18 determines whether or not S1 selected by the S1 demodulation block 23 is “00X” (X being 0 or 1).
In the DVB-T2 standard, 3 bits of S1 being “00X” is indicative that a frame including this S1 is a T2 frame. The 3 bits of S1 being other than “00X” is indicative of the frame including this S1 is not a T2 frame but an FEF (Future Extension Frame). The FEF is a frame for extension specified in the DVB-T2 standard.
If the S1 selected by the S1 demodulation block 23 is found to be not “00X” in step S16, then the control block 18 determines whether or not the S2 selected by the S2 demodulation block 24 is “XXX1” in step S17.
In the DVB-T2 standard, 4 bits of S2 being “XXX1” is indicative that a T2 frame and an FEF exist together on the channel being received.
If the S1 selected by the S1 demodulation block 23 is found to be “00X” in step S16 or the S2 selected by the S2 demodulation block 24 is found to be “XXX1” in step S17, then the processing is further continued.
For example, the S1 selected by the S1 demodulation block 23 is found to be “00X” in step S16, then it is determined whether or not the S2 is “XXX1” (not shown). If the S2 is found to be not “XXX1” in this determination, then the channel being received is determined to be a channel that is transmitting only T2 signals. If the S2 is found to be “XXX1,” then the channel being received is determined to be a channel on which both a T2 signal and an FEF exist together. Next, as information associated with the channel transmitting T2 signals, the center frequency and bandwidth of the channel being received are stored by the control block 18.
Meanwhile, if the S2 is found to be not “XXX1” in step S17, then the processing goes back to step S10 in FIG. 4 and the above-described processing operations are repeated.
FIG. 6 shows a flowchart mainly indicative of the processing to be executed by the T2 signal presence/absence determination block 17 in the processing to be executed at the time of the initial scan described with reference to FIG. 4 and FIG. 5.
In step S31, the T2 signal presence/absence determination block 17 waits until the input signal is stabilized. When the input signal is stabilized due to a normal operation of ABC for example, a signal stabilization flag indicative thereof is supplied to the T2 signal presence/absence determination block 17.
When a signal stabilization flag is supplied, then the T2 signal presence/absence determination block 17 waits for a period of time equivalent to 250+α (ms) in step S32. It should be noted here that 250 ms is indicative of a maximum length (time) of the T2 frame and α is indicative of a preset margin time. It should also be noted that, of the processing operations to be performed in step S31 and step S33, the processing of step S31 is to be performed before the process of step S3 in FIG. 4, the processing of step S32 corresponds to the processing to be executed during a period of time in which the control block 18 determines whether or not a time-out has occurred in step S9 shown in FIG. 4.
In step S33, the T2 signal presence/absence determination block 17 compares the signal section correlation peak value with the threshold value like the processing of step S4 shown in FIG. 4, thereby determining whether or not a P1 symbol has been detected.
If the signal section correlation peak value is found to be equal to or higher than the threshold value within a predetermined section and a P1 symbol is found detected in step S33, then the T2 signal presence/absence determination block 17 determines in step S34 whether or not the CDS correlation peak value is equal to or higher than the threshold value like the processing of step S8 shown in FIG. 4. It should be noted that, before the processing of step S34, the processing operations of step S5 through step S7 shown in FIG. 4 are performed.
If a P1 symbol is found not detected in step S33 or the CDS correlation peak value is found to be below the threshold value in step S34, then the processing to be performed when there is no T2 signal in the channel being received is performed in step S35. Namely, the processing to be performed when NO is determined in step S4 or step S8 shown in FIG. 4 is performed.
On the other hand, if the CDS correlation peak value is found to be equal to or higher than the threshold value in step S34, then the processing to be performed when it is possible that the channel contains a T2 signal is performed in step S36. Namely, the processing operations beginning with step S12 shown in FIG. 5 are performed.
As described above, the determination itself for checking whether or not there is a T2 signal by use of a P1 symbol is intended by the standard. In addition, in the initial scan, the determination for checking if there is a T2 signal (a P1 symbol) or not by use of a CDS correlation peak value is explicitly shown in the Implementation Guidelines (ETSI TR 102 831: IG) of the DVB-T2 standard.