JP2002171239A - Modulator - Google Patents

Abstract

(57) [Problem] To prevent characteristic deterioration caused by intermodulation,
Provides high quality transmission. SOLUTION: A symbol sequence setting means 13 sets a symbol sequence for a symbol sequence representing information such that the frequency of a distortion component generated by intermodulation is outside the transmission band of OFDM modulation. The inverse Fourier transform unit 14 performs an inverse Fourier transform by superimposing the symbol sequence on each carrier,
Outputs a signal corresponding to the phase axis. The combining unit 15 controls the combining of the signals and generates an OFDM modulated signal.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a modulator, a broadcast system, a broadcast transmitter, a transmission system, and a transmitter, and more particularly to an OFDM (Orthogonal Frequency Division Multip).
lexing (orthogonal frequency division multiplexing). A modulation device that performs modulation and transmits a signal.
(Moving Picture Experts Group), a broadcasting system for performing digital broadcasting, a broadcast transmitting apparatus for transmitting an OFDM-modulated signal in digital broadcasting, a transmission system for controlling signal transmission through a transmission line having non-linear transmission characteristics, and non-linear transmission The present invention relates to a transmission device for transmitting a signal through a transmission path having characteristics.

[0002]

2. Description of the Related Art In recent years, a modulation method called OFDM has been proposed as a digital signal transmission technique. In this OFDM, a number of orthogonal subcarriers (subcarriers) are provided in a transmission band, data is assigned to the amplitude and phase of each subcarrier, and PSK (Phase Shift
Keying) and QAM (Quadrature Amplitude Modulatio)
This is a method of performing digital modulation according to n).

[0003] In OFDM, a transmission band is divided by a large number of subcarriers. By narrowing the band per subcarrier, the symbol length of each subcarrier is lengthened and a guard interval section is added. This can eliminate the influence of the intersymbol interference due to the delayed wave.

Further, by simultaneously modulating all the subcarriers and maintaining the frequency orthogonal relationship, the interval between the subcarriers can be set to a minimum, so that the total transmission speed is not much different from that of the conventional modulation method. Can be configured.

Further, in OFDM, since data is allocated to a plurality of subcarriers, an IFFT (Inverse Fast Foued) for performing an inverse Fourier transform at the time of modulation.
A transmission / reception circuit using a FT (Fiber Fourier Transform) arithmetic circuit that performs a Fourier transform during demodulation is configured.

[0006] OFDM having the above characteristics has been widely studied for application to terrestrial digital broadcasting which is strongly affected by multipath interference.
T (Digital Video Broadcasting-Terrestrial) and IS
DB-T (Integrated Services Digital Broadcasting)
-Terrestrial) has been proposed.

[0007]

However, in the conventional OFDM transmission as described above, when the transmission path has nonlinearity, the intermodulation wave generated by the intermodulation of each carrier is superimposed on its own band. There is a problem that characteristics are deteriorated.

In order to avoid such a characteristic deterioration, conventionally, for example, the output stage has been designed so that the peak power output can be at least about 10 times the average output power, and the nonlinearity of the transmission line is reduced. However, in this case, there is a problem that the power consumption is increased and the output amplifier becomes expensive.

The present invention has been made in view of such a point, and prevents the characteristic deterioration caused by intermodulation, and
An object of the present invention is to provide a modulation device that performs high-quality transmission. Another object of the present invention is to provide a broadcasting system that performs high-quality broadcasting control while preventing characteristic degradation caused by intermodulation.

Another object of the present invention is to provide a broadcast transmitting apparatus for transmitting a high-quality broadcast signal while preventing characteristic degradation caused by intermodulation. Another object of the present invention is to provide a transmission system that performs high-quality communication control while preventing characteristic degradation caused by intermodulation.

Still another object of the present invention is to provide a transmitting apparatus for transmitting a high-quality signal while preventing characteristic degradation caused by intermodulation.

[0012]

According to the present invention, in order to solve the above-mentioned problems, in a modulator for transmitting a signal by performing OFDM modulation, a frequency of a distortion component caused by intermodulation is changed with respect to a symbol sequence representing information. A symbol sequence setting means for setting the symbol sequence so as to be out of the transmission band of the OFDM modulation, performing an inverse Fourier transform by superimposing the symbol sequence on each carrier, and outputting a signal corresponding to a phase axis. A modulation device is provided, comprising: an inverse Fourier transform unit; and a combining unit that controls the combining of the signals to generate an OFDM modulated signal.

Here, the symbol sequence setting means sets the symbol sequence such that the frequency of the distortion component generated by the intermodulation is outside the transmission band of the OFDM modulation for the symbol sequence representing the information. The inverse Fourier transform means performs an inverse Fourier transform by superimposing the symbol sequence on each carrier, and outputs a signal corresponding to the phase axis. The combining unit controls the combining of the signals to generate an OFDM modulated signal.

[0014]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a diagram illustrating the principle of the modulation device according to the present invention. The modulation device 10 transmits a signal by performing OFDM (Orthogonal Frequency Division Multiplexing) modulation.

The symbol sequence setting means 13 controls the symbol sequence representing the information so that the frequency of the distortion component caused by the intermodulation (hereinafter also referred to as an intermodulation wave) is out of the transmission band of the OFDM modulation. Set columns. Here, the intermodulation refers to a phenomenon that when a plurality of signals are input to a circuit having nonlinearity such as an amplifier, a new frequency is generated by a combination of input frequencies.

The inverse Fourier transform means 14 performs an inverse Fourier transform by superimposing a symbol sequence on each carrier, and performs I, Q
And outputs a signal corresponding to the phase axis. The synthesizing means 15
D / A (digital / analog) converters 15a, 15
b, including an adder 15c. The D / A converter 15a converts an I-channel signal into an analog signal, and
5b converts the Q channel signal into an analog signal.
The adder 15c adds these analog signals to generate an OF signal.
Output as a DM modulation signal. Although a circuit for adding a guard interval for ghost signal countermeasures is actually included, it is omitted in the figure. The detailed configuration of a specific device to which the modulation device 10 is applied will be described later.

Next, the characteristic degradation due to the intermodulation will be described in detail. When a multi-carrier signal such as OFDM is passed through a non-linear transmission path, a distortion component appears due to mutual modulation of carriers. For example, frequency fa and frequency f
When the signal of b passes through a non-linear path, the lower side of fa and f
A distortion component having the following frequency appears above b.

[0018]

Fab = 2 × fa−fb (1a) fba = 2 × fb−fa (1b) Equations (1a) and (1b) represent the highest value of the spectrum of the intermodulation wave. is there. In the conventional OFDM transmission method, the carriers are arranged without gaps. Therefore, when the distortion component falls within the band of the OFDM signal, the distortion component overlaps with the carrier having exactly the same frequency.

Further, in the OFDM receiving apparatus, since the distortion component of the same frequency overlapping the original carrier cannot be separated, this distortion component becomes an interference signal and causes an error in signal transmission.

As an example, a case of a four-wave carrier will be described. The frequency of each carrier is determined by setting the lowest subcarrier frequency to 1000 KHz and the subcarrier interval to 10 KHz without any gap. Here, f
1 = 1000 KHz, f2 = 1010 KHz, f3 = 1
020 KHz and f4 = 1030 KHz.

The frequency of the distortion component due to this signal is as follows.

[0022]

[Equation 2] f12 = 2 × f1-f2 = 990KHz, f13 = 2 × f1-f3 = 980KHz, f14 = 2 × f1-f4 = 970KHz, f21 = 2 × f2-f1 = 1020KHz, f23 = 2 × f2- f3 = 1000KHz, f24 = 2 × f2-f4 = 990KHz, f31 = 2 × f3-f1 = 1040KHz, f32 = 2 × f3-f2 = 1030KHz, f34 = 2 × f3-f4 = 1010KHz, f41 = 2 × f4- f1 = 1060KHz, f42 = 2 × f4-f2 = 1050KHz, f43 = 2 × f4-f3 = 1040KHz,... (2) FIG. 2 is a diagram showing an intermodulation wave. Among the generated distortion components, components f12, f13, f14, f24, f31, out of the range of the signal band from 1000 KHz to 1030 KHz.
f41, f42, f43 (f12, f13, f1 in the figure)
4 only) can be removed by filtering with an OFDM receiver, and does not actually become a disturbing signal.

However, the signal band of 1000 KHz to 10 KHz
Components f21, f23, f32, f in the range of 30 KHz
Numeral 34 completely overlaps with the original signal, and is an interference signal.

As described above, in the conventional OFDM transmission, if the transmission path has nonlinearity, the intermodulation wave generated by the intermodulation of each carrier is superimposed on its own band. Was.

Next, how to select a carrier so that the intermodulation wave is not superimposed in its own band will be described. Equation (1
a) The frequency of each carrier of OFDM is selected so that the distortion component represented by equation (1b) does not overlap the frequency of the original signal.

For example, in the case of four-wave carriers, the frequency of each carrier is defined as f1 = 1000 KHz, f2 = 1010
KHz, f3 = 1030KHz, f4 = 1040KHz
Decide. The frequency of the distortion component due to this signal is as follows.

[0027]

[Equation 3] f12 = 2 × f1-f2 = 990KHz, f13 = 2 × f1-f3 = 970KHz, f14 = 2 × f1-f4 = 960KHz, f21 = 2 × f2-f1 = 1020KHz, f23 = 2 × f2- f3 = 990KHz, f24 = 2 × f2-f4 = 980KHz, f31 = 2 × f3-f1 = 1060KHz, f32 = 2 × f3-f2 = 1050KHz, f34 = 2 × f3-f4 = 1020KHz, f41 = 2 × f4- f1 = 1080KHz, f42 = 2 × f4-f2 = 1070KHz, f43 = 2 × f4-f3 = 1040KHz,... (3) Since all of the frequencies of the distortion components do not overlap with the original signal frequencies, There is no interference with the signal. If each carrier is used in a specific arrangement as described above, the frequency band is used redundantly. However, even in a transmission system having a strong nonlinearity, a signal transmission system configuration that is resistant to multipath or the like using the OFDM technology can be realized. .

As other examples, 4 waves, 20 waves, 100 waves
3 to 6 show examples of assignment of 512 waves. Here, in the case of the 20 waves shown in FIG. 3, of the three examples, Example 3 is a combination having a narrower carrier bandwidth. As described above, by selecting a combination of carriers that makes the bandwidth narrower, the bandwidth can be effectively used.

It should be noted that any combination of carriers other than those described above in which the intermodulation wave is not superimposed in its own band can be used as the combination of carriers of the present invention. Here, in the modulation device 10 of the present invention, FIGS.
In order to select a carrier as indicated by a symbol sequence is set such that the frequency of the intermodulation wave is outside the transmission band. In order to generate a frequency array of carriers that do not generate an intermodulation wave, by giving a null signal of zero magnitude to the inverse Fourier transform processing, the frequency of the intermodulation wave becomes
The frequency is outside the transmission band of the OFDM modulation.

That is, since the value of the carrier frequency at which the intermodulation wave entering the transmission band is generated is known in advance, a null signal is given at the time of the inverse Fourier transform for the carrier at which the intermodulation wave is generated. Thus, the intermodulation wave is prevented from being generated in the transmission band (specific circuit configuration and operation will be described later with reference to FIG. 12 and later).

By performing such processing, the influence of intermodulation waves can be prevented. Therefore, even when it is unavoidable that the transmission path has nonlinearity, it is possible to use the OFDM transmission system to perform multipath transmission. It is possible to realize transmission without influence.

Next, a broadcast system for performing digital broadcast to which the modulation device 10 of the present invention is applied will be described. FIG. 7 is a diagram showing the configuration of the broadcasting system. The broadcast system 1 includes a broadcast transmission device 100 and a broadcast reception device 200, and performs OFDM modulation transmission to perform MPEG2-compliant digital broadcasting.

A transport stream (TS), which is a multiplexed signal of MPEG2, can handle a plurality of programs in an erroneous environment and is mainly used in the broadcasting field. For the broadcast transmitting apparatus 100, the encoding means 101
An encoded signal is generated for the G2 transport stream. Mapping means 102 generates a symbol sequence by allocating a code sequence of the coded signal to a mapping point according to a modulation format. Frame adaptation means 103 (corresponding to symbol string setting means 13 of the present invention)
Is set such that the frequency of the distortion component caused by the intermodulation is outside the transmission band of the OFDM modulation with respect to the symbol sequence.
Set the symbol string.

Inverse Fourier transform means (hereinafter, IFFT) 1
Reference numeral 04 performs inverse Fourier transform by superimposing a symbol sequence on each carrier, and outputs a signal corresponding to the phase axis. The guard interval adding means 105-1 combines the signals and adds a guard interval to generate a guard interval added signal.

The transmitting means 105-2 performs D / A conversion on the guard interval additional signal to obtain an analog signal, and up-converts the analog signal into a radio frequency band to perform O / O conversion.
An FDM modulated wave is generated and transmitted through an antenna (through a non-linear transmission path).

The broadcast receiving apparatus 200 receives an OFDM modulated wave, performs OFDM demodulation, generates a demodulated signal, and decodes the demodulated signal. FIG. 8 is a diagram showing a configuration of the broadcast transmitting apparatus 100. It should be noted that the encoding means 101 comprises a MUX adaptation / energy spreading means 101a to a bit &
The transmitting unit 105-2 includes components up to the symbol interleaving unit 101e, and the transmitting unit 105-2 includes components of the D / A 105-2a and the front end 105-2b.

First, video and audio signals are compressed and multiplexed,
The signal converted to the MPEG2 transport stream format is supplied to the MUX adaptation / energy spreading means 101a.

The MUX adaptation / energy diffusion means 101a includes a header 1 of the transport stream.
The byte data 47h is bit-inverted for each 8MPEG2 transport packet to obtain B8h. At this time, at the same time, the PRBS (Pseudo Random Bi
t Sequence) is initialized with a predetermined seed. The sequence of PRBS is X ¹⁵ + X ¹⁴ +1 and the seed is 009A
h. The energy spreading circuit performs an exclusive OR operation on the data of the transport stream excluding one byte of the header.

In short, the MUX adaptation / energy spreading means 101a spreads the input data by scrambling it with a pseudo-random sequence so that energy is not concentrated following "0" or "1", and erroneous synchronization is performed. (Pseudo synchronous lock).

Reed-Solomon Coding
The encoding unit 101b adds a parity of 16 bytes to each transport stream packet, and performs encoding for error correction. Convolutional interleaving (Convol
The solution interleaving means 101c interleaves data (for example, depth 12) as a countermeasure for burst error.

The convolution coding means 101d calculates G1 = 1
It has an encoder of 71 (Octal) and G2 = 133 (Octal), and performs 2-bit encoded output for an input of each bit. The coding rate is 1/2, 2/3, 3/4, 5/6,
You can select from 7/8. The convolutional code uses a technique called punctured. This is 1 /
If the same processing circuit as that of the second coding circuit is prepared, other coding rates (2
/ 3 to 7/8) can be automatically decoded.

Bit and symbol interleaving means 101
e performs interleaving of the frequency within the OFDM symbol and interleaving within the bit allocated to the mapping point. The mapping means 102 allocates a corresponding code sequence to a predetermined mapping point (for example, a code of 6 bits in the case of 64QAM) according to the modulation format. At this time, it becomes two-dimensional information of the I and Q components. This is supplied to the frame adaptation means 103.

The frame adaptation means 103
In addition to the mapped information, a predetermined pilot signal and a transmission path multiplexing control signal (TPS: Transmission Parameter S)
ignaling) and generates a null signal to provide the symbol sequence to the IFFT 104.

The IFFT 104 performs IFFT processing on 2048 sets of I and Q data collectively and supplies the data to the guard interval adding means 105-1. The guard interval adding means 105 copies the signal waveform of the latter half of the signal of the effective symbol output from the IFFT 104 and adds this to the beginning of the effective symbol.

The D / A 105-2a converts a digital signal into an analog signal and supplies it to the front end 105-2b. The front end 105-2b performs up-conversion to the RF band to radiate into the air, and transmits a signal through an antenna.

FIG. 9 shows a convolutional interleaving means 101c.
FIG. Convolutional interleaving means 10
1c has twelve branches, selects the same branch for input and output, and 0, 1, 2, 3,.
The branches are switched in the order of 4,..., 11, 0, 1,. Each branch has a delay element and outputs one byte for one byte input.

FIG. 10 is a diagram showing a transmission form of OFDM. A transmission signal by OFDM is transmitted in symbol units. The OFDM symbol includes an effective symbol, which is a signal period during which an IFFT operation is performed at the time of transmission, and a guard interval obtained by copying a part of the waveform of the latter half of the effective symbol as it is.

This guard interval is provided in the first half of the symbol. For example, the DVB-T standard (2K
Mode), the effective symbol includes 2048 subcarriers, and the subcarrier interval is 4.14 KHz.

Data is modulated on 1705 subcarriers out of the 2048 subcarriers in the effective symbol, and the length of the guard band interval is ４ of the effective symbol. Note that A in FIG.
Indicates the boundary of the OFDM symbol, and B indicates the end position of the guard band interval.

FIG. 11 is a diagram showing the configuration of the broadcast receiving apparatus 200. The signal wave transmitted from the broadcast transmitting apparatus 100 is received by an antenna and supplied to the tuner 201 as an RF signal.

The tuner 201 converts the frequency of a received signal into an IF signal. The A / D 202 A / D converts the IF signal and digitizes it. For example, the effective symbol of the OFDM time domain signal is sampled at 2048 samples and the guard band interval is sampled at 512 samples and digitized.

Digital orthogonal demodulation means 203 orthogonally demodulates the digitized IF signal using a carrier signal of a predetermined frequency (carrier frequency), and outputs a baseband OFDM signal. The baseband OFDM signal output from the digital orthogonal demodulation means 203 is a signal in the time domain before Fourier transform (FFT) operation.

From this, the baseband signal before the FFT operation after the digital quadrature demodulation is hereinafter referred to as OFD.
Called M time domain signal. This OFDM time domain signal is
Real axis component (I channel signal) as a result of quadrature demodulation
And an imaginary axis component (Q channel signal). The OFDM time domain signal output from the digital quadrature demodulation means 203 is supplied to the FFT 204 and the narrowband carrier frequency error calculation means 205.

The FFT 204 performs an FFT operation on the OFDM time domain signal, and extracts and outputs data orthogonally modulated on each subcarrier. This FFT 204
Are signals in the so-called frequency domain after the FFT operation. From this, the following FFT
The signal after the calculation is called an OFDM frequency domain signal.

The FFT 204 has a range from one OFDM symbol to an effective symbol length (for example, 2048 samples).
(That is, the guard band interval is excluded from one OFDM symbol), and the FFT operation is performed on the extracted 2048-sample OFDM time domain signal.

The calculation start position is any position from the boundary of the OFDM symbol (position A in FIG. 10) to the end position of the guard band interval (position B in FIG. 10). This calculation range is called an FFT window.

As described above, the OFDM frequency domain signal output from the FFT 204 has a real axis component (I channel signal) and an imaginary axis component (Q
Channel signal). O
The FDM frequency domain signal is supplied to the wideband carrier frequency error calculating means 206 and the equalizer 208.

On the other hand, narrow band carrier frequency error calculating means (FAFC) 205 calculates a carrier frequency error included in the OFDM time domain signal. Specifically, a narrow-band carrier frequency error with an accuracy of ± 1/2 or less of the subcarrier frequency interval (4.14 KHz) is calculated.

The carrier frequency error is an error of the center frequency position of the OFDM time domain signal caused by a shift of the reference frequency output from the local transmission of the tuner 201. If the error becomes large, the error rate of the output data becomes large. Increase. Narrowband carrier frequency error calculating means 205
Is supplied to the numerical control oscillating means 207.

The wideband carrier frequency error calculating means (WAFC) 206 calculates a carrier frequency error included in the OFDM time domain. Specifically, a wideband carrier frequency error with subcarrier frequency (e.g., 4.14 KHz) interval accuracy is calculated. The wideband carrier frequency error calculating means 206 refers to the continuous pilot signal (CP signal), calculates how much the CP signal is shifted from the original CP signal insertion position, and calculates the shift amount. I'm asking. The wideband carrier frequency error obtained by the wideband carrier frequency error calculation means 206 is supplied to the numerical control oscillation means 207.

Numerical control oscillation means (NCO) 20
7 is a narrow-band carrier frequency error of ± 1/2 accuracy of the sub-carrier frequency interval calculated by the narrow-band carrier frequency error calculation unit 205 and a sub-carrier frequency interval accuracy calculated by the wide band carrier frequency error calculation unit 206. A wideband carrier frequency error is added, and a carrier frequency error correction signal whose frequency increases or decreases according to the carrier frequency error obtained by the addition is output.

The carrier frequency error correction signal is a complex signal and is supplied to the digital quadrature demodulation means 203. The carrier frequency error correction signal performs digital quadrature demodulation while correcting the carrier frequency based on the carrier frequency error correction signal.

The equalizer 208 performs phase equalization and amplitude equalization of the OFDM frequency domain signal using the scattered pilot signal (SP signal). The OFDM frequency domain signal subjected to the phase equalization and the amplitude equalization is supplied to the TPS demodulation unit 209 and the demapping unit 210.

TPS (Transmission Parameter Signali)
ng) The demodulation means 209 separates a TPS signal assigned to a predetermined frequency component and demodulates information such as a coding rate, a modulation scheme, and a guard interval length from the signal.

The demapping means 210 is provided for the equalizer 2
08, the OFDM frequency domain signal subjected to amplitude equalization and phase equalization is subjected to demapping according to the modulation scheme to decode data, and is supplied to the bit & symbol deinterleaver 211.

Bit & Symbol Deinterleaving Means 21
1 performs the inverse operation of bit interleaving and symbol interleaving performed on the modulation side, and the processed signal is supplied to the Viterbi decoder 212.

Viterbi Decoding 21
2 performs maximum likelihood decoding using the Viterbi algorithm, and the processed signal is supplied to a convolutional deinterleaver 213.

Convolution deinterleaving means (Convolutio
n Decoding) 213 performs the reverse operation of the convolutional interleaving control performed on the modulation side, and the processed signal is supplied to the Reed-Solomon decoding means 214.

Reed-Solomon D
The encoding 214 decodes the Reed-Solomon code and corrects any errors, and the processed signal is supplied to the MUX adaptation / energy despreading means 215.

The MUX adaptation / energy despreading means 215 performs bit inversion when the first byte data of the header of the MPEG2 transport stream is 47h, and performs bit inversion when it is B8h. Perform logic. This PRBS is the same as the PRBS used in the energy diffusion circuit on the modulation side. Then, the signal is output as an MPEG2 transport stream.

Next, the frame adaptation means 103
Will be described in detail. FIG. 12 is a diagram showing the configuration of the frame adaptation means 103. The frame adaptation unit 103 includes a signal selection control circuit 103a.
And a signal switching circuit 103b.

The signal selection control circuit 103a includes a symbol counter 103a-1, a carrier address counter 103
a-2 and a flag generator 103a-3. The symbol counter 103a-1 receives the system clock and the frame reset, resets the symbol counter at the start position of the frame, and generates a scattered address having a period of, for example, four symbols.

Carrier address counter 103a-2
Receives the system clock and the symbol reset,
The carrier address counter is reset at the head position of the symbol to generate the carrier address of the carrier number.

The flag generator 103a-3 refers to a table prepared in advance based on these addresses, and
Continual pilot inserted at a fixed position for each symbol
And the position of the null signal for not generating carriers,
Scattered p inserted at different positions in 4 symbol periods
The position of the ilot is determined, and a flag for signal selection is generated at that timing.

The signal switching circuit 103b includes signal data generated by the mapping means 102, pilot signal (PLT: Pilot signal) data to be referred to when performing amplitude and phase waveform equalization at the time of demodulation, and at the time of transmission. Transmission line multiplexing control signal (TP
S: Transmission Parameter Signaling 3
Signal selection control circuit 10 as a kind of data and control signal
The flag is supplied from 3a.

Then, the signal switching circuit 103b switches between three types of data and zero which is a null signal in accordance with the instruction of the flag and supplies the data to the IFFT 104. Where P
As an example of the insertion position of the LT signal, ITU-, a digital broadcasting standard using OFM of the EBU (European Broadcasting Union)
The position of the PLT signal specified in the 2K mode of R121 / 11 will be described.

The PLT signal includes a scattered pilot inserted at a position slightly changed depending on the symbol, and a continuous pilot inserted at a fixed position for each symbol.

The scattered pilot carrier position K
Is defined by the following equation.

[0079]

K = Kmin + 3 × (I mod 4) + 12 × p (4) where Kmin is the minimum value of the carrier number (here, 0), I is the symbol number (0 to 67), and p is an integer. . It is also assumed that K does not exceed Kmax (1704 here) which is the maximum value of the carrier number.

There are 45 continuous pilots, which are determined as follows. 0, 48, 54, 87, 141, 156, 1
92,201,255,279,282,333,432,450,483,525,531,618,71
4, 759, 765, 780, 804, 873, 888, 918, 939, 942, 969, 984, 105
0, 1101, 1107, 1110, 1137, 1140, 1146, 1206, 1269, 1323, 137
7, 1491, 1683, 1704 Next, the operation of the IFFT 104 will be described. For simplicity, first consider the inverse operation, FFT. The FFT of a signal is broken down into spectra, as is well known.

A signal of a certain bandwidth is sampled by a clock of a fixed cycle to obtain a fixed amount of digital data.
This flows into the FFT circuit. Data is calculated in the FFT circuit. After the operation, the real part and the imaginary part of the data come out in order as a result of being decomposed into a spectrum from the FFT circuit.

Here, if there is no spectrum which is the first signal, the result of the FFT is the spectrum of the signal, so that the size of the place where the data exists should be zero. Since the operation results are sequentially output from the FFT circuit, the magnitude of the signal output at a certain timing is actually zero.

An operation in which a certain carrier is not output using IFFT 104 is to perform the above operation in the opposite direction. At a certain timing, zero-size data is IF
When the data is sent to the FT 104, the signal data from which the carrier at the corresponding position is missing is output from the IFFT 104.

Next, using the data array (symbol sequence) output from the signal switching circuit 103b, the IFFT 10
4 will be described. FIG. 13 is a diagram showing an example of an array of data output from the signal switching circuit 103b, and FIG. 14 is a diagram for explaining an operation modulated by the IFFT 104.

The signal (No. 0 to No. 0) input to the IFFT 104
No. 4: 2 bits each) sequentially modulates the amplitude of Δf to Δ5f. For example, the case of QPSK will be described. First,
The first one bit of the two bits of the signal of No. 0 is Δf
(Data = 0) or -1 (Data = 1). Next, the next one of the two bits of the signal of No. 0 multiplies the imaginary part of Δf by one (Data = 0) or −1 (Data =
1) Yes.

Next, the first bit of the signal of No. 1 is 2
Multiply the real part of Δf by 1 (Data = 0) or -1 (Data =
1) The next bit of the signal of No. 1 multiplies the imaginary part of 2Δf by one (Data = 0) or −1 (Data = 1).

In the same way, in the same way, The second one of the four signals
The calculation is performed until the bit modulates the imaginary part of 5Δf. Then, the calculated waveforms are added and synthesized by a real part and an imaginary part, respectively, and finally, the real part and the imaginary part are alternately used as data to obtain an output waveform. This output waveform has Δf ~
All carriers of 5Δf are included. In this figure, as can be seen from the shapes of the respective waveforms, No. 0 to N
o. This is a case where all the codes of data up to 4 are 0.

Here, for example, when not outputting the carrier of 3Δf, this means that the real part and the imaginary part of 3Δf need only be multiplied by zero (the inverse Fourier for the carrier that generates an intermodulation wave). A null signal is given at the timing of conversion), At the timing of modulation by 2, 3Δf is multiplied by zero. The output waveform synthesized in this manner does not include a carrier of 3Δf, as a matter of course.

As described above, according to the present invention, OFD
By setting each carrier of M to a specific arrangement, it is possible to prevent characteristic degradation due to unnecessary carrier interference caused by intermodulation, to reduce inter-symbol interference such as multipath, and to perform high-quality transmission control. Becomes possible.

Further, since deterioration due to intermodulation does not occur, it is not necessary to provide an extra margin for the amplification degree of the output stage, and power consumption can be reduced. further,
In the OFDM modulation, a large peak value occurs when the phases of the respective carriers are aligned. In this case, if the capacity of the output stage is designed to be small, the signal is clipped, and an intermodulation wave has conventionally been generated. On the other hand, in the present invention, even if clipping occurs and an intermodulation wave is generated, it is not superimposed in its own band, so that it is possible to prevent characteristic deterioration even when clipping (partial carrier level). It only needs to be lower).

In the above description, the modulation device 10 of the present invention is applied to the MPEG2 broadcasting system, but can be widely applied to other transmission systems having a transmission system with nonlinear transmission characteristics. It is. For example, LE
By applying the modulation device 10 of the present invention to a transmission system including a photoelectric conversion system using a D or a laser diode, or a transmission system including an electromagnetic conversion using a magnetic head, etc., Good transmission control can be performed while suppressing the occurrence of errors due to modulation.

[0092]

As described above, according to the modulation apparatus of the present invention, the frequency of the distortion component caused by the intermodulation is OFD.
The symbol sequence is set so as to be outside the transmission band of M modulation,
The configuration is such that an inverse Fourier transform is performed to generate an OFDM modulated signal. As a result, it is possible to perform high-quality transmission while preventing characteristic degradation caused by intermodulation.

Further, the broadcasting system of the present invention uses MPEG
The symbol stream is set so that the frequency of the distortion component generated by the intermodulation is out of the transmission band of the OFDM modulation with respect to the transport stream, and the OFDM modulation signal is generated by performing the inverse Fourier transform. This makes it possible to perform high-quality broadcasting while preventing characteristic degradation caused by intermodulation.

Further, the broadcast transmitting apparatus according to the present invention has an MPE
For the G transport stream, a symbol sequence is set such that the frequency of the distortion component generated by the intermodulation is outside the transmission band of the OFDM modulation, and an inverse Fourier transform is performed to generate an OFDM modulated signal. . This makes it possible to perform high-quality broadcasting while preventing characteristic degradation caused by intermodulation.

Further, the transmission system of the present invention is configured to generate a modulated signal by setting a symbol string so that the frequency of the distortion component generated by the intermodulation is outside the transmission band. This makes it possible to perform high-quality transmission control while preventing characteristic degradation caused by intermodulation.

Further, the transmitting apparatus of the present invention is configured to generate a modulated signal by setting a symbol sequence so that the frequency of the distortion component generated by the intermodulation is outside the transmission band. As a result, it is possible to perform high-quality transmission while preventing characteristic degradation caused by intermodulation.

[Brief description of the drawings]

FIG. 1 is a principle diagram of a modulation device according to the present invention.

FIG. 2 is a diagram showing an intermodulation wave.

FIG. 3 is a diagram illustrating an example of carrier assignment.

FIG. 4 is a diagram illustrating an example of carrier assignment.

FIG. 5 is a diagram illustrating an example of carrier assignment.

FIG. 6 is a diagram illustrating an example of carrier assignment.

FIG. 7 is a diagram showing a configuration of a broadcast system.

FIG. 8 is a diagram illustrating a configuration of a broadcast transmission device.

FIG. 9 is a diagram showing a configuration of a convolutional interleaver.

FIG. 10 is a diagram showing a transmission form of OFDM.

FIG. 11 is a diagram illustrating a configuration of a broadcast receiving device.

FIG. 12 is a diagram showing a configuration of a frame adaptation unit.

FIG. 13 is a diagram illustrating an example of an array of data output from a signal switching circuit.

FIG. 14 is a diagram for explaining an operation modulated by IFFT.

[Explanation of symbols]

10: Modulating device, 13: Symbol string setting means, 14: Inverse Fourier transform means, 15: Combining means, 15a, 15b ...
D / A converter, 15c ... adder

Claims (10)

[Claims] 1. A modulation apparatus for transmitting a signal by performing OFDM modulation, wherein a symbol string representing information has a symbol sequence such that a frequency of a distortion component caused by intermodulation is outside a transmission band of the OFDM modulation. Symbol sequence setting means for setting a sequence; inverse Fourier transform means for performing an inverse Fourier transform by superimposing the symbol sequence on each carrier; and outputting a signal corresponding to a phase axis; A modulating device, comprising: synthesizing means for generating a modulated signal. 2. The symbol sequence setting means sets the symbol sequence by performing a carrier combination such that the frequency of the distortion component does not overlap a carrier in its own band. The modulation device according to any one of the preceding claims. 3. The modulation apparatus according to claim 1, wherein the symbol sequence setting means sets the symbol sequence such that a combination of multi-carriers having a narrower carrier bandwidth is output. 4. The symbol sequence setting means provides a null signal having a magnitude of zero to the inverse Fourier transform means so that the frequency of the distortion component is out of the transmission band of the OFDM modulation. 2. The modulation device according to claim 1, wherein a frequency array is generated. 5. A broadcast system for performing MPEG digital broadcasting by performing OFDM modulation transmission, wherein: an encoding means for generating an encoded signal for an MPEG transport stream; and a code sequence of the encoded signal. Mapping means for generating a symbol sequence by allocating to a mapping point according to a modulation format, and for the symbol sequence, a frequency of a distortion component caused by intermodulation is out of a transmission band of the OFDM modulation. Frame adaptation means for setting a symbol sequence, inverse Fourier transform by superimposing the symbol sequence on each carrier, and performing inverse Fourier transform means for outputting a signal corresponding to a phase axis; and synthesizing the signals to form a guard interval. A guard interval addition means for generating a guard interval addition signal by adding The analog signal by a D / A converter to over de interval adding signal, OFDM upconverts the analog signal to a radio frequency band
A broadcast transmitting apparatus comprising: a transmitting unit that generates and transmits a modulated wave; and a broadcast receiving apparatus that receives the OFDM modulated wave, performs OFDM demodulation, generates a demodulated signal, and decodes the demodulated signal. A broadcasting system, comprising: 6. A broadcast transmitting apparatus for transmitting an OFDM-modulated signal in digital broadcasting, comprising: an encoding unit for generating an encoded signal for an MPEG transport stream; and a code sequence of the encoded signal. Mapping means for generating a symbol sequence by assigning to a mapping point in accordance with a modulation format; and the symbol sequence so that a frequency of a distortion component caused by intermodulation is out of the transmission band of the OFDM modulation for the symbol sequence. A frame adaptation unit for setting a column; an inverse Fourier transform unit for performing an inverse Fourier transform by superimposing the symbol sequence on each carrier and outputting a signal corresponding to a phase axis; and adding a guard interval by synthesizing the signal. A guard interval addition means for generating a guard interval addition signal Transmitting means for performing D / A conversion on the interval addition signal to obtain an analog signal, up-converting the analog signal into a radio frequency band to generate and transmit an OFDM modulated wave, . 7. The frame adaptation means,
The broadcast transmitting apparatus according to claim 6, wherein the symbol sequence is set such that a combination of multi-carriers having a narrower carrier bandwidth is output. 8. The frame adaptation means,
By providing a null signal having a magnitude of zero to the inverse Fourier transform means, a frequency array of carriers is generated such that the frequency of the distortion component is outside the transmission band of the OFDM modulation. Item 7. The broadcast transmitting device according to Item 6. 9. A transmission system for controlling signal transmission through a transmission line having non-linear transmission characteristics, wherein a frequency of a distortion component caused by intermodulation of a symbol sequence representing information is out of a transmission band. A transmitting device including: a symbol sequence setting unit that sets a symbol sequence; a modulating unit that modulates the symbol sequence to generate a modulation signal; and a receiving device that receives and demodulates the modulation signal. A transmission system, characterized in that: 10. A transmitting apparatus for transmitting a signal through a transmission path having a non-linear transmission characteristic, wherein a symbol sequence representing information is arranged such that a frequency of a distortion component caused by intermodulation is out of a transmission band. And a modulation means for modulating the symbol sequence to generate a modulation signal.