JPH1022973A - Ofdm transmission system and its transmission/reception device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To eliminate the need for transmitting plural kinds of synchronous symbols and to improve transmission efficiency by validating only every Nmax / Nmin carriers in Nmax -pieces of carriers on a reference symbol. SOLUTION: In the format of the frame of OFDM transmission data, every Nmax /Nmin =8 carriers are set to be effective carriers for the reference symbol when the number Nmax of the samples of a fixed reception is 8192 and the number Nmin of the samples of the symbol for movement reception is 1024. Only every eight carriers with DC as the center in whole carriers (5696 carriers) are used as the valid carriers. Thus, 1024 samples are extracted from the valid symbol period of the reference symbol and the reference symbol is demodulated by using the discrete Fourier transformation of 1024 points.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to an OFDM transmission system for transmitting a digital signal by employing an OFDM modulation method, and an OFDM transmitting / receiving apparatus used in the system.

[0002]

2. Description of the Related Art In recent years, in the transmission of audio signals and video signals, digital modulation systems have been actively developed. Among them, in digital terrestrial broadcasting or communication, orthogonal frequency division multiplexing (OFDM: Orthogonal Frequency Division Multiplexing), which is characterized by being resistant to multipath interference and having a high frequency utilization factor.
queney Division Multiplex) modulation schemes are receiving attention.

[0003] The OFDM modulation system is a system in which data is distributed and transmitted to a plurality of carriers that are orthogonal to each other, and a discrete Fourier transform is generally used for modulation and demodulation. That is, on the transmitting side, an OFDM is performed by performing an inverse discrete Fourier transform (IDFT) on a plurality of symbol data.
An effective symbol part of the signal is generated, and a guard period for reducing multipath interference is added before the effective symbol and transmitted. On the other hand, the receiving side demodulates the symbols by performing a discrete Fourier transform (DFT) on the effective symbol period of the received signal.

[0004] The number of OFDM carriers is D
Determined by the number of points in the FT circuit, the OFDM symbol length is equal to the reciprocal of the carrier interval. Therefore, when the sampling clock frequency is constant, the carrier interval decreases as the number of DFT circuit points increases,
The symbol length increases.

[0005] An OFDM transmission frame is formed by time-division multiplexing a plurality of OFDM symbols, and a synchronization symbol for reception synchronization is transmitted for each transmission frame. FIG. 12 illustrates an example of an OFDM transmission frame. In the figure, a no-signal period called a null symbol is transmitted at the head position of a frame. The null symbol is used on the receiving side for detection of frame synchronization and detection of coarse synchronization of symbol timing.

After the null symbol, a reference symbol for timing synchronization is transmitted. This reference symbol is used to detect precise symbol synchronization on the receiving side. An example of the reference symbol is a chirp (sine sweep) symbol. The chirp symbol has a waveform with a large autocorrelation, and the receiving side can detect precise symbol synchronization by detecting the correlation of the chirp symbol included in the received signal. Then, at a position after the reference symbol, a plurality of information symbols are time-division multiplexed and transmitted.

[0007]

By the way, recently, OFD
In terrestrial digital broadcasting or communication using M, it has been proposed that symbols for fixed receiving apparatuses and symbols for mobile receiving apparatuses are time-division multiplexed and transmitted in one transmission frame. The effective symbol length of OFDM is
As described above, it is determined by the number of points of the DFT circuit used for modulation and demodulation. Therefore, when only fixed reception is considered, if the effective symbol period is made longer, the guard period can be made longer accordingly, which is advantageous for multipath interference. However, in the case of mobile reception, if the effective symbol period is too long, the transmission characteristics deteriorate due to the temporal variation of the transmission path.

Therefore, when transmitting a symbol for the fixed receiving apparatus and a symbol for the mobile receiving apparatus in the same frame, a method of setting the symbol length for the mobile receiving apparatus to be shorter than the symbol length for the fixed receiving apparatus. It is considered.

However, in order to realize this transmission system, it is necessary for the receiving side to demodulate each OFDM symbol with a DFT circuit having the number of points corresponding to the symbol length. As for the type of the receiving device, either OFDM
In addition to a receiving device capable of demodulating symbols, a receiving device that demodulates either a symbol for a mobile receiving device or a symbol for a fixed receiving device may be considered.

In this case, as a transmission method of the synchronization symbol in the transmission frame, the transmission side inserts a synchronization symbol corresponding to each symbol length into the transmission frame and transmits the transmission symbol, and the reception side responds to the number of points of the DFT circuit. A method of demodulating either of them is conceivable. However, in this case, the number of synchronization symbols to be transmitted in the transmission frame is doubled, which causes a decrease in transmission efficiency, which is extremely undesirable.

The present invention has been made in view of the above circumstances. It is an object of the present invention to transmit a plurality of types of synchronization symbols even when multiplexing a plurality of types of symbol groups having different symbol lengths. It is an object of the present invention to provide an OFDM transmission system capable of improving transmission efficiency and a transmission / reception apparatus therefor.

[0012]

In order to achieve the above object, an OFDM transmission system according to the present invention is arranged such that, on the transmitting side, the maximum number of information symbol samples Ni is Nmax.
(Nmax / Ni is an integer), the minimum value is Nmin, and when transmitting using the maximum Nmax carriers, every Nmax / Nmin carriers among the Nmax carriers are used as effective carriers. Generates an OFDM symbol including pattern information defining the presence or absence of these effective carriers, multiplexes the OFDM symbol as a reference symbol at a predetermined position of the transmission frame, and transmits the OFDM symbol. An arbitrary Ni sample in a symbol is demodulated, and reception synchronization is achieved based on the demodulation result.

On the receiving side, the reference symbol is detected from the received signal, frame synchronization is detected based on the detection result, and the pattern information is detected from the demodulation result of the reference symbol. The frequency detuning of the reception signal is adjusted based on the information.

Therefore, according to the present invention, since only Nmax / Nmin carriers out of Nmax carriers are valid as reference symbols, an arbitrary sample within the entire symbol period is extracted and demodulated. Becomes possible. Therefore, in a system where a plurality of types of symbol groups having different numbers of samples in the effective symbol period are transmitted in a time-division multiplexed manner in one transmission frame,
Regardless of which of the plurality of types of symbol groups the receiving side intends to receive, it is possible to detect the reference symbol and synchronize the reception.

Further, if the pattern information defining the presence or absence of an effective carrier is generated based on a pseudo noise sequence,
On the receiving side, it is possible to detect a shift in carrier frequency at the carrier interval, whereby frequency detuning can be reliably corrected at the carrier interval.

Further, each effective carrier of the reference symbol is represented by P
By SK-modulating and making the phases of the effective carriers random with each other, it is possible to prevent the demodulation amplitude of the effective carriers from protruding more than others.

Further, by setting the amplitude of each effective carrier of the reference symbol to be larger than the amplitude of the carrier of another OFDM symbol, the detection of the reference symbol can be performed with higher accuracy.

On the transmitting side, pattern information defining the presence or absence of an effective carrier is inserted at each of a plurality of locations in the reference symbol, and then OFDM modulated and transmitted. On the receiving side, the plurality of locations in the reference symbol are OFDM demodulated. Then, by obtaining the average of the respective demodulation results and detecting the pattern information from the average of the demodulation results, it is possible to perform time diversity, thereby reducing the effects of fading and the like and further improving the detection of the reference symbol. It is possible to perform with high accuracy.

Further, the OFDM transmitting apparatus according to the present invention has Nmax
The maximum number Nmax / minimum value Nmin of the number of samples among the number of carriers is set as an effective carrier, and an OFDM symbol including pattern information defining the presence or absence of these effective carriers is generated as a reference symbol for reception synchronization. The reference symbol generated by the reference symbol generator is multiplexed with a plurality of types of information symbol groups by multiplexing means, and the reference symbol is arranged at a predetermined position in the frame. A transmission frame is formed, and each symbol of the formed transmission frame is OFDM-modulated by performing an inverse discrete Fourier transform according to the number of samples in the effective symbol period, and transmitted to a transmission line. .

An OFD using such an OFDM transmitting apparatus
By performing M transmission, even if the receiving side intends to demodulate any of a plurality of types of symbol groups having different numbers of samples, it is possible to reliably demodulate the reference symbol and achieve reception synchronization.

Further, the OFDM receiving apparatus of the present invention
By performing a discrete Fourier transform on the received signal in accordance with the number of samples Ni in the effective symbol period of the symbol group to be received, OFDM demodulating the reference symbol, and performing reception synchronization detection using the demodulated reference symbol, An arbitrary Ni sample in the reference symbol is demodulated, and reception synchronization is achieved based on the demodulation result.

Further, the OFDM receiving apparatus of the present invention detects a reference symbol from a received signal, detects frame synchronization based on the detection result, and detects the number of samples Ni in a valid symbol period of a symbol group to be received. OFDM demodulation of a reference symbol by subjecting the received signal to discrete Fourier transform in accordance with the following formula, detecting the pattern information from the demodulated reference symbol, and adjusting the frequency detuning of the received signal based on the pattern information. It is also characterized.

Therefore, not only coarse frame synchronization but also frequency detuning correction can be performed using the reference symbols, so that even if a frequency shift occurs due to frequency conversion in a transmission system, this frequency shift can be corrected. By removing the signal, more accurate reception synchronization can be achieved, and thereby highly reliable OFDM demodulation can be performed.

[0024]

BEST MODE FOR CARRYING OUT THE INVENTION

(First Embodiment) Hereinafter, a first embodiment of an OFDM transmission system according to the present invention will be described. In addition,
In the following description, a case will be described in which two types of OFDM symbols for fixed reception and mobile reception are time-division multiplexed and transmitted in one transmission frame using the transmission parameters shown in the table.

[0025]

[Table 1]

FIG. 1 shows a frame format of OFDM transmission data according to the first embodiment of the present invention. In this transmission frame, a reference symbol is arranged at the beginning, a chirp symbol is arranged following the reference symbol, and thereafter, a plurality of information symbols for mobile reception and a plurality of information symbols for fixed reception are time-divided. It is configured to be multiplexed and arranged. The effective symbol length and the guard interval length of the reference symbol and the chirp symbol are set equal to the symbol length of the fixed reception symbol. In the system of the present embodiment,
The receiving side uses the above reference symbol at the beginning of the frame,
In addition to detecting frame synchronization, frequency synchronization is also detected.

The carrier interval of the reference symbol is set to an interval corresponding to the ratio between the number Nmax of effective symbol samples of the fixed reception symbol and the number Nmin of effective symbol samples of the mobile reception symbol. For example, if the number of fixed reception symbol samples Nmax is 8192 and the number of mobile reception symbol samples Nmin is 1024, Nmax / Nmin = every eight carriers are effective carriers for reference symbols. The presence or absence of these effective carriers is defined by a predetermined pattern. The pattern defining the presence or absence of the effective carrier is generated based on, for example, a pseudo noise (PN) sequence.

FIG. 2 shows the effective carrier for the reference symbol. The horizontal axis shows the frequency represented by the carrier number, and the vertical axis shows the carrier amplitude. As shown in the figure, out of all carriers (5696), only every eighth carrier centering on DC is used as an effective carrier. Also, the pattern of the presence or absence of these effective carriers is defined by the PN sequence as described above. For example,
The carrier corresponding to the code of the PN sequence "1" is defined as an effective carrier, and the carrier corresponding to the code "0" is defined as an invalid carrier. In the example of FIG. 2, a solid line indicates an effective carrier, and a broken line indicates an invalid carrier.

As described above, the number of samples in the effective symbol period of the reference symbol is 8,192. However, since only every eighth carrier centering on DC is effective, 1024 samples are extracted from the effective symbol period. 1
It is also possible to demodulate the reference symbol by using the 024-point discrete Fourier transform. That is, a discrete Fourier transform of 8192 points and 102
The reference symbol can be demodulated by any of the four-point discrete Fourier transforms, and can be received by the fixed receiver or the mobile receiver.

Also, since the number of effective carriers of the reference symbol is 1/8 or less of other information symbols,
The amplitude of the time axis waveform is smaller than that of the FDM symbol. Therefore, it is possible to detect the reference symbol from the envelope of the received signal, thereby achieving frame synchronization.

Further, a reference symbol having a known carrier arrangement is transmitted from the transmitting side, and the receiving side detects a carrier pattern of the reference symbol and compares it with the known pattern, thereby detecting a frequency error per carrier interval. The frequency detuning of the carrier can be corrected using the frequency error information.

In particular, since the pattern information that defines the presence or absence of an effective carrier is generated based on the PN sequence, it is possible to detect the carrier frequency deviation at the carrier interval on the receiving side, thereby making frequency detuning possible. Correction can be made more reliably at the carrier interval.

Further, if the amplitude of the effective carrier of the reference symbol is set to be larger than the carrier amplitude of the other OFDM symbols, the reference symbol can be easily distinguished from other OFDM symbols, thereby improving the detection accuracy of the reference symbol. It becomes possible.

Next, an OFDM transmitting apparatus and an OFDM receiving apparatus used in the OFDM transmission system of the present embodiment will be described. First, the OFDM transmitting apparatus inputs hierarchically coded information of two systems, allocates high-importance data (HP data) to OFDM symbols for mobile reception, and assigns low-importance data (LP data) data. (HP data) is assigned to the OFDM symbol for fixed reception. FIG. 3 shows the OFDM of this embodiment.
FIG. 2 is a circuit block diagram illustrating a configuration of a transmission device.

That is, the HP data is error-correction-coded by the error-correction coding circuit 11,
2, after interleaving is performed, DQPSK (diff
erentially encoded quadrature phase shift keying)
Input to the encoding circuit 13. DQPSK encoding circuit 1
In 3, differential encoding is performed between data allocated to the same carrier, and then converted into a QPSK symbol, and this QPSK symbol is input to the multiplexer 14.

The LP data is sent to the error correction encoding circuit 1
5 is subjected to error correction coding, and further interleaved by an interleaver 16, after which 64QAM (quadratutu
re amplitude modulation) input to the encoding circuit 17. In the 64QAM encoding circuit 17, the input data is converted into 64QAM symbols, and 6
The 4QAM symbol is input to the multiplexer 14 like the QPSK symbol.

The reference symbol generation circuit 18 generates reference symbol data and supplies the reference symbol data to the multiplexer 14. For example, as shown in FIG. 4, first and second PN sequence generation circuits 181 and 182 and a BPSK symbol generation circuit It comprises a circuit 183 and a selector 184.

The first PN sequence generation circuit 181 has a BPS
This is for controlling the phase of the K symbol.
The N sequence is supplied to a BPSK symbol generation circuit 183.
The BPSK symbol generation circuit 183 outputs I and Q data whose phase becomes 0 ° when the PN sequence generated from the first PN sequence generation circuit 181 is “0”, and 180 phase when it is “1”. ° and I and Q data, respectively.

The second PN sequence generation circuit 182 is for determining the carrier arrangement of the reference symbol, and supplies the generated PN sequence to the selector 184 as a selection control signal. The selector 184 is connected to the second PN sequence generation circuit 18.
When the PN sequence generated from 2 is "0", 0 data is selected. When the PN sequence is "1", the I and Q data generated from the BPSK symbol generation circuit 183 are selected. Output to the multiplexer 14 as reference symbol data. Thus, the presence or absence of carriers is defined by the PN sequence, and the phase of each carrier is random BP
Reference symbol data modulated by the SK symbol data is generated.

The first PN sequence generation circuit 181
And a second PN sequence generating circuit 182
Are set to be different from each other or to have the same phase even in the same sequence. this is,
This is to prevent the carrier phases from matching.

By appropriately setting the amplitude of the BPSK symbol generated by the BPSK symbol generation circuit 183, the carrier amplitude of the reference symbol can be made larger than that of the other symbols.

The chirp symbol generation circuit 19 generates data on the frequency axis of the chirp symbol. The multiplexer 14 includes a QPSK symbol output from the DQPSK encoding circuit 13, a 64QAM symbol output from the 64QAM encoding circuit 17, the reference symbol data generated by the reference symbol generation circuit 18, and a chirp symbol generation circuit. The chirp symbol data generated in step 19 is time-division multiplexed in a predetermined order to form the transmission frame shown in FIG. The multiplexed symbol data is supplied to an inverse discrete Fourier transform (IDFT) circuit 21.

In the IDFT circuit 21, for each symbol of the multiplexed symbol data, an inverse Fourier transform process is performed using the number of points corresponding to the symbol length. For example, an inverse discrete Fourier transform of 1024 points is performed on symbol data for mobile reception, while an inverse discrete Fourier transform of 8192 points is performed on reference symbol, chirp symbol, and symbol data for fixed reception. Done.

The circuit configuration of the IDFT circuit 21 is as follows.
024-point IDFT circuit and 8192-point I
A DFT circuit may be provided, and the IDFT circuit may be selectively used to perform the inverse discrete Fourier transform of the above two types of symbol data. Further, as another circuit configuration, the maximum number of points is 8192.
It is also conceivable that one DFT circuit is provided and the number of points of the IDFT circuit is switched according to the symbol length of the symbol data, thereby performing the inverse discrete Fourier transform of the two types of symbol data.

OF output from the IDFT circuit 21
The DM symbol is subsequently input to the guard period adding circuit 22. In the guard period adding circuit 22, for each OFDM symbol output from the IDFT circuit 21, a part of the tail is copied before the symbol as a guard period. FIG. 6 shows the configuration of the OFDM symbol after the addition of the guard period.

The OFDM symbol data output from the guard period adding circuit 22 is then input to an orthogonal modulation circuit 23, where it is orthogonally modulated with a carrier signal of a predetermined frequency, and then digital / analog (D / A). The conversion circuit 24 converts the signal into an analog signal. And this D / A
The output signal of the conversion circuit 24 is frequency-converted by the frequency conversion circuit 25 into a radio high-frequency signal corresponding to a carrier frequency, and transmitted from an antenna (not shown).

Reference numeral 20 denotes a timing generation circuit.
A clock signal and a timing signal necessary for the operation of each of the above circuits are generated based on a reference clock signal, and supplied to each of the above circuits.

On the other hand, the OFDM receiver is configured as follows. This apparatus has a function of demodulating both OFDM symbols time-division multiplexed into a transmission frame. FIG. 5 is a circuit block diagram showing the configuration. A radio frequency signal received by an antenna (not shown) is frequency-converted to an intermediate frequency signal by a frequency conversion circuit 31 and then converted to an analog / digital (A / A / D) signal.
D) The digital signal is converted by the conversion circuit 32 and input to the quadrature detection circuit 33. The quadrature detection circuit 33 performs quadrature detection of the input signal with a reproduced carrier, and obtains a baseband OFDM modulated wave signal by a discrete Fourier transform (DFT) circuit 34, an envelope detection circuit 42, a chirp symbol detection circuit. 44 and a frequency error detection circuit 49.

The DFT circuit 34 performs, for each symbol of the input OFDM modulated wave signal, a discrete Fourier transform process on the effective symbol excluding the guard period with the number of points corresponding to the symbol length. For example, when a transmission frame having the configuration shown in FIG. 1 is received, a discrete Fourier transform of 1024 points is performed on a symbol for mobile reception, and a chirp symbol and each symbol for fixed reception are performed. Is subjected to a discrete Fourier transform process of 8192 points.

The reference symbol can be demodulated by either the 1024-point DFT or the 8192-point DFT. Here, the 8192-point DFT is used to cover the entire effective symbol period of the reference symbol as shown in FIG. Demodulate.

As the circuit configuration of the DFT circuit 34, a 1024-point DFT circuit and an 8192-point DFT circuit are provided, and by selectively using these DFT circuits, a discrete Fourier transform of the above two types of OFDM symbols is performed. Something to convert is possible. As another circuit configuration, a DFT having a maximum point number of 8192
It is also conceivable that one circuit is provided, and the number of points of the DFT circuit is switched according to the symbol length of the OFDM symbol, thereby performing the discrete Fourier transform of the two types of OFDM symbols.

The demodulated symbol data output from the DFT circuit 34 is input to a demultiplexer 35.
In the demultiplexer 35, the demodulated symbol data is separated for each symbol. Among them, the demodulated symbol data of the mobile reception symbol is sent to the delay detection circuit 36, and the demodulation symbol data of the fixed reception symbol is sent to the synchronous detection circuit 3.
7, and the demodulated data of the reference symbol is further input to the carrier pattern detection circuit 46.

In the delay detection circuit 36, the demodulated data of the mobile reception symbol is subjected to delay detection between data allocated to the same carrier, and the detected symbol data is deinterleaved by a deinterleave circuit 38. , The data is subjected to error correction decoding processing in an error correction decoding circuit 39, and is output as HP data. In the synchronous detection circuit 37, synchronous detection is performed on the demodulated data of the fixed reception symbol, and the detection output is subjected to deinterleaving by a deinterleave circuit 40 and then subjected to an error correction decoding process in an error correction decoding circuit 41. Output as LP data.

A circuit for detecting timing synchronization using a reference symbol is configured as follows. In the envelope detection circuit 42, the quadrature detection circuit 33
By detecting the envelope of the baseband OFDM modulated wave signal output from, the reference symbol at the head of the frame is detected, and the detection timing is supplied to the timing recovery circuit 43. Timing recovery circuit 43
Generates a clock and a timing signal in synchronization with the received signal. The detection timing resets the generation timing of the frame timing signal. That is, coarse synchronization of frame timing is performed.

The chirp symbol detection circuit 44 outputs the baseband OF signal output from the quadrature detection circuit 33.
By detecting the correlation between the DM modulated wave signal and the reference chirp symbol data generated from the chirp symbol generation circuit 45, a chirp symbol in the transmission frame is detected, and the detection timing is detected by the timing recovery circuit 4.
Supply 3 The timing generation circuit 43 performs precise synchronization control of the frame timing and synchronization control of the clock based on the detection timing of the chirp symbol. The timing signal and the clock signal output from the timing reproduction circuit 43 are supplied to each circuit that needs these signals.

On the other hand, a circuit unit for performing adjustment for frequency synchronization using the reference symbol is configured as follows. That is, the demodulated data of the reference symbol is input to the carrier pattern detection circuit 46. The carrier pattern detection circuit 46 determines the presence or absence of a carrier by calculating the amplitude of the reference symbol from the demodulated data and comparing the amplitude with a predetermined threshold value. Give to. Further, the reference pattern generation circuit 48 generates pattern information composed of a PN sequence defining the presence or absence of the carrier of the reference symbol shown in FIG. 2 and supplies the pattern information to the frequency error detection circuit 47.
The frequency error detection circuit 47 obtains a correlation between the carrier pattern information detected from the demodulated data of the reference symbol and the reference pattern information generated from the reference pattern generation circuit 48, thereby detecting a deviation of the received carrier pattern. . Then, the shift of the received carrier pattern is output as a frequency error signal in units of 1 kHz.

The frequency error detection circuit 49 uses the baseband OFDM modulated wave signal output from the quadrature detection circuit 33 to detect a frequency shift up to ± 0.5 kHz corresponding to ± 1/2 of the carrier interval. Is detected. Then, the frequency error signal detected by the frequency error detection circuit 49 and the frequency error signal output from the frequency error detection circuit 47 are added to each other by the adder 50 and then supplied to the quadrature detection circuit 33. Is done.

The quadrature detection circuit 33 controls the detection carrier frequency based on the frequency error signal supplied from the adder 50. Therefore, a frequency shift caused by frequency conversion or the like in transmission is removed.

The detailed configuration and operation of the frequency error detecting circuit 49 are described in the document "Study of a New Frequency Synchronization Method Using Guard Period in OFDM" (Technical Report of the Institute of Television Engineers of Japan, Vol. 19, No. 18,
P. 13-18).

As described above, in the OFDM transmission system of this embodiment, the reference symbol generation circuit 18 of the OFDM transmission device
, The number of valid period samples of the symbol for mobile reception is 1
024 and the number of valid period samples of the fixed reception symbol 81
In accordance with the ratio of N.92, a carrier for transmitting the reference symbol is determined every eight carriers, and carrier pattern information defining whether these carriers are valid or not is generated, and this carrier pattern information is included in the reference symbol. Sending. On the other hand, in the OFDM receiver, the O.D.
The position of the reference symbol is detected from the envelope of the FDM modulated wave signal, the frame synchronization is coarsely adjusted based on the detection timing, and the carrier pattern is detected from the demodulated data of the reference symbol. , A frequency error in the unit of a carrier interval is detected, and the detuning of the carrier frequency is corrected based on the frequency error.

Therefore, according to the present embodiment, the carrier used for the reference symbol transmission is determined every eight lines, so that the reference symbol data can be demodulated using either the 8192 point or 1024 point DFT circuit. Then, by comparing the demodulated carrier pattern information of the reference symbol with the known pattern information, a frequency error per carrier interval is detected, and the frequency detuning is corrected based on the detection result. For this reason,
Demodulation with less errors can be performed by removing the frequency shift.

(Second Embodiment) In this embodiment, an OF
The DM receiver is configured to receive only mobile reception symbols. FIG. 7 is a circuit block diagram illustrating a configuration of the OFDM receiver according to the present embodiment.
3, the same parts as those in FIG. 3 are denoted by the same reference numerals, and detailed description is omitted.

The OFDM receiving apparatus of this embodiment has a DFT circuit 61 having 1024 points as a DFT circuit for demodulating only mobile reception symbols. In the DFT circuit 61 having 1024 points, each symbol for mobile reception is demodulated and a reference symbol is demodulated. The demodulation of the reference symbol is performed, for example, by extracting the last 1024 samples of the area in the valid period of the reference symbol and demodulating it, as shown in FIG. The reason why the 1024 sample area can be extracted and demodulated is that the effective carriers for transmitting the reference symbols are set every eight in the OFDM transmission apparatus as shown in FIG.

The demodulation result of the reference symbol is separated by the demultiplexer 35 and input to the carrier pattern detection circuit 46. The carrier pattern detection circuit 46 detects carrier pattern information indicating the presence or absence of the carrier of the reference symbol from the demodulation result of the reference symbol, and supplies the carrier pattern information to the frequency error detection circuit 47. Reference pattern generation circuit 48
Generates a reference pattern indicating the presence / absence of a carrier only for every eight carriers centered at DC in the reference symbol shown in FIG.
7 The frequency error detection circuit 47 detects the deviation of the received carrier pattern by calculating the correlation between the two pieces of pattern information, and outputs this as a frequency deviation in units of 8 kHz.

On the other hand, the frequency error detection circuit 49 uses the mobile reception symbol from the baseband OFDM modulated wave signal output from the quadrature detection circuit 33 and uses ± 4 kHz.
The frequency deviation up to is detected. The adder 50 adds the frequency error signals output from the frequency error detection circuits 47 and 49 to each other, and supplies the added frequency error signal to the quadrature detection circuit 33. For this reason, in the quadrature detection circuit 33, the deviation of the reproduction carrier frequency is corrected according to the frequency error signal, thereby enabling high-precision quadrature detection.

As described above, according to the present embodiment, the OFDM
Since every 8 effective carriers for transmitting the reference symbol are set in the transmitting apparatus, the number of points is 10 points.
By using the 24 DFT circuits 61, it is possible to reliably demodulate not only the symbols for mobile reception but also the reference symbols.

Further, since pattern information indicating the validity of the carrier of the reference symbol is transmitted and the pattern information is compared with the reference pattern, a frequency error per carrier interval can be detected. The frequency detuning can be corrected using the frequency error signal.

In the above embodiment, as shown in FIG. 8, the last 102 in the valid period of the reference symbol is used.
Although an area for four samples is extracted and demodulated, 1024 samples of an arbitrary part in the reference symbol may be extracted and the carrier pattern information demodulated except for the guard period of the mobile reception symbol. .

(Third Embodiment) This embodiment shows another configuration of an OFDM receiver for demodulating only mobile reception symbols.
The carrier pattern information is detected from the average of the respective demodulated outputs and used for detecting a frequency error.

FIG. 9 is a circuit block diagram showing the configuration of the OFDM receiver according to the present embodiment. 7, the same parts as those in FIG. 7 are denoted by the same reference numerals, and detailed description is omitted.

In the 1024-point DFT circuit 63,
As shown in FIG. 10, 1024 samples are demodulated eight times over the entire effective symbol period of the reference symbol. Then, the demodulated data of the eight reference symbols is
After being separated by the demultiplexer 35, it is input to the averaging circuit 62. The averaging circuit 62 averages the demodulated data of the reference symbols obtained by the eight demodulations. At this time, the reference symbol is 1024
Since the same waveform is repeated in the sample period, it is possible to take an average.

In the present embodiment, the guard length and the effective symbol length of the mobile reception symbol and the fixed reception symbol both have a relationship of 1: 8.
As shown in FIG. 11, it is also possible to perform eight restorations in the reference symbol.

The average value of the reference symbol demodulated data is
The signal is input to a carrier pattern detection circuit 46, where carrier pattern information is detected. Then, a frequency error per carrier interval is detected by the frequency error detection circuit 47 from the correlation between the carrier pattern information and the reference pattern information, and the detuning of the carrier frequency is corrected based on the frequency error signal.

As described above, according to the present embodiment, the reference symbol is demodulated eight times, the average of the data obtained by the demodulation is obtained by the averaging circuit 62, and the carrier pattern is obtained from the averaged demodulated data. Since the frequency error is detected by detecting the information, it is possible to increase the C / N of the demodulated output of the reference symbol, thereby improving the accuracy of detecting the frequency error.

The present invention is not limited to the above embodiments. For example, in each of the above embodiments, an information symbol whose effective symbol period is 1024 samples,
A case has been described in which information symbols of 92 samples are transmitted in a time-division multiplexed manner within one transmission frame.
The present invention can be similarly applied to a case where only information symbols of 24 samples or only information symbols of 8192 samples are multiplexed and transmitted. Although two types of information symbols are multiplexed, the present invention can be applied to a case where three or more types of information symbols are multiplexed and transmitted as long as the condition of the number of samples is satisfied.

In the first embodiment, as shown in FIG. 1, a reference symbol is arranged at the head position of a transmission frame, a chirp symbol is arranged at the next position, and an information symbol is arranged thereafter. However, the multiplex transmission position of the reference symbol and the chirp symbol can be arbitrarily set.

Further, in the first embodiment, two systems of hierarchically encoded data are allocated to the mobile reception symbol and the fixed reception symbol, respectively, and transmitted. However, for example, audio data and video data, Different data such as data of different programs may be allocated to the mobile reception symbol and the fixed reception symbol, respectively, and multiplexed.

Further, in each of the above embodiments, the configuration of only the OFDM transmitting apparatus or only the OFDM receiving apparatus has been described. However, the present invention may be applied to an OFDM transmitting / receiving apparatus having both an OFDM transmitting function and an OFDM receiving function. it can.

Further, the timing synchronization detecting means does not need to detect the reference symbol from the envelope of the received signal, and the configuration of the frequency detuning correcting means may be variously modified without departing from the present invention. It is possible. In addition, the various transmission parameters shown in the table, the circuit configuration of the transmission device and the reception device, the format of the transmission frame, the configuration of the pattern that defines the validity of the carrier, and the like are variously modified without departing from the gist of the present invention. Of course, it can be implemented.

[0080]

As described above in detail, according to the present invention, on the transmitting side, the maximum value of the number of information symbol samples Ni is Nmax (Nmax / Ni is an integer), the minimum value is Nmin, and the maximum is Nmax. When transmission is performed using a carrier, every Nmax / Nmin of the Nmax carriers is set as an effective carrier, and pattern information including the pattern information for defining the presence or absence of these effective carriers is included.
An FDM symbol is generated, and the OFDM symbol is multiplexed at a predetermined position in the transmission frame as a reference symbol and transmitted. On the receiving side, the reference symbol is detected from a received signal and the detection result is used as a basis. The frame synchronization is detected at the same time, the pattern information is detected from the demodulation result of the reference symbol, and the frequency detuning of the received signal is adjusted based on the pattern information.

Therefore, according to the present invention, even when a plurality of symbol groups having different symbol lengths are multiplexed and transmitted,
This eliminates the need to transmit a plurality of types of synchronization symbols, thereby providing an OFDM transmission system that can improve transmission efficiency and a transmission / reception apparatus therefor.

[Brief description of the drawings]

FIG. 1 is a diagram showing a transmission frame format according to a first embodiment of the present invention.

FIG. 2 is a diagram showing a configuration of an effective carrier for a reference symbol according to the first embodiment of the present invention.

FIG. 3 is a circuit block diagram showing a configuration of an OFDM transmission device according to the first embodiment of the present invention.

FIG. 4 is a circuit block diagram showing a configuration of a basic symbol generation circuit in the OFDM transmitting apparatus shown in FIG.

FIG. 5 is a circuit block diagram showing a configuration of an OFDM receiver according to the first embodiment of the present invention.

FIG. 6 is a diagram for explaining a reference symbol demodulation operation in the receiving apparatus shown in FIG. 5;

FIG. 7 is a circuit block diagram showing a configuration of an OFDM receiver according to a second embodiment of the present invention.

FIG. 8 is a diagram for explaining a reference symbol demodulation operation in the receiving apparatus shown in FIG. 7;

FIG. 9 is a circuit block diagram showing a configuration of an OFDM receiver according to a third embodiment of the present invention.

FIG. 10 is a diagram for explaining a reference symbol demodulation operation in the receiving apparatus shown in FIG. 9;

11 is a diagram for explaining another demodulation operation of a reference symbol in the receiving apparatus shown in FIG.

FIG. 12 is a diagram showing an example of the configuration of a conventional OFDM transmission frame.

[Explanation of symbols]

 11, 15: error correction encoding circuit 12, 16, ... interleaver 13: DQPSK encoding circuit 14: multiplexer 17: 64QAM encoding circuit 18: reference symbol generation circuit 19: chirp symbol generation circuit 20: timing generation circuit 21: reverse Discrete Fourier Transform (IDFT) circuit 22 Guard period addition circuit 23 Quadrature modulation circuit 24 Digital / analog (D / A) conversion circuit 25 Frequency conversion circuit for transmission 31 Frequency conversion circuit for reception 32 Analog / Digital (A / D) conversion circuit 33: Quadrature detection circuit 34, 61, 63 Discrete Fourier transform (DFT) circuit 35: Demultiplexer 36: Delay detection circuit 37: Synchronous detection circuit 38, 40 ... Deinterleaver 39, 41 ... Error correction decoding circuit 42 ... Envelope detection circuit 43 ... Tie Regeneration circuit 44 Chirp symbol detection circuit 45 Chirp symbol generation circuit 46 Carrier pattern detection circuit 47, 49 Frequency error detection circuit 48 Reference pattern generation circuit 50 Adder 62 Average circuit 181 First PN sequence Generation circuit 182 Second PN sequence generation circuit 183 BPSK symbol generation circuit 184 Selector

Continued on the front page (72) Inventor Tatsuya Ishikawa 5-2-2 Akasaka, Minato-ku, Tokyo Inside the Next Generation Digital Television Broadcasting System Research Laboratories (72) Inventor Kenshi Sawada 5-2-2 Akasaka, Minato-ku, Tokyo No. Next Generation Digital Television Broadcasting System Laboratory Co., Ltd. (72) Inventor Masanori Saito 5-2-2-8 Akasaka, Minato-ku, Tokyo Inside Next Generation Digital Television Broadcasting System Laboratories Co., Ltd. (72) Inventor Tetsuomi Ikeda Tokyo 5-2-8, Akasaka, Minato-ku Next Generation Digital Television Broadcasting System Research Laboratories Co., Ltd. (72) Yasushi Sugita 8 Shinsugita-cho, Isogo-ku, Yokohama-shi, Kanagawa Pref. Toshiba Multimedia Technology Research Laboratories

Claims (14)

[Claims] 1. The number of samples Ni in a unit transmission frame having the same sampling frequency and an effective symbol period
In an OFDM transmission system in which a plurality of types of information symbols having different (i is an integer) are time-division multiplexed and transmitted at an arbitrary ratio, the transmission side sets the maximum value of the number of samples Ni to Nmax (N
x / Ni is an integer) and the minimum value is Nmin.
In addition, when transmission is performed using a maximum of Nmax carriers, every Nmax / Nmin carriers among the Nmax carriers are used as effective carriers, and a reference including pattern information defining the presence or absence of these effective carriers is used. A symbol is generated, the reference symbol is multiplexed at a predetermined position of the transmission frame and transmitted, and the receiving side demodulates an arbitrary Ni sample in the reference symbol and synchronizes reception based on the demodulation result. An OFDM transmission system characterized by achieving. 2. The transmitting side transmits N symbols in the reference symbol.
max / Nmin For every valid carrier, the presence or absence of carriers is arranged in a predetermined pattern and transmitted.
Detecting the reference symbol from the received signal, detecting frame synchronization based on the detection result, detecting the pattern information from the detection result of the reference symbol, and detecting the frequency of the received signal based on the detected pattern information. The OFDM transmission system according to claim 1, wherein detuning is adjusted. 3. The OFDM transmission system according to claim 1, wherein the transmitting side generates pattern information for defining the presence or absence of an effective carrier based on a pseudo noise sequence. 4. The OFDM transmission system according to claim 1, wherein the transmitting side PSK-modulates each effective carrier of the reference symbol to make the phase of each effective carrier random. 5. The OFDM transmission system according to claim 1, wherein the transmitting side sets the amplitude of each effective carrier of the reference symbol to be larger than the amplitude of the carrier of another OFDM symbol. 6. The receiving device according to claim 6, wherein Ni <
3. The demodulator according to claim 1, wherein when demodulating the Ni samples of Nmax, the Ni symbols at a plurality of locations in the reference symbol are demodulated, and the plurality of demodulation results are averaged to detect the reference symbol. OFDM transmission system. 7. The number of samples Ni in a unit transmission frame having the same sampling frequency and effective symbol period
In an OFDM transmission apparatus used in an OFDM transmission system for transmitting a plurality of types of information symbol groups having different (i is an integer) by time division multiplexing at an arbitrary ratio, the maximum value of the number of samples Ni is set to Nmax (Nmax / Ni Is an integer), the minimum value is Nmin, and the maximum N
When transmitting using max carriers, the Nma
a reference symbol generating means for generating, as a reference symbol for reception synchronization, a symbol in which every Nmax / Nmin carriers out of x carriers are effective carriers; and a reference symbol generated by the reference symbol generating means; Multiplexing means for multiplexing a plurality of types of symbol groups to form a transmission frame in which the reference symbols are arranged at predetermined positions in the frame; and each symbol of the transmission frame formed by the multiplexing means. Modulation means for performing an inverse discrete Fourier transform in accordance with the number of samples in the effective symbol period, and transmission means for transmitting an output signal of the OFDM modulation means to a transmission path. OFDM transmitter. 8. The method according to claim 7, wherein the reference symbol generating means includes a signal for generating the Nma.
The OFDM transmission apparatus according to claim 7, wherein a reference symbol in which presence / absence of carriers is arranged in a predetermined pattern is generated for every x / Nmin carriers. 9. The OFD according to claim 7, wherein the reference symbol generation means generates pattern information for defining the presence or absence of an effective carrier based on a pseudo noise sequence.
M transmitter. 10. The reference symbol generating means according to claim 7, wherein each effective carrier of the reference symbol is PSK-modulated to make the phase of each effective carrier random.
Or the OFDM transmission apparatus according to 8. 11. The OFDM transmitting apparatus according to claim 7, wherein the reference symbol generating means sets the amplitude of each effective carrier of the reference symbol to be larger than the amplitude of the carrier of another OFDM symbol. 12. The number Ni of samples in an effective symbol period having the same sampling frequency in a unit transmission frame.
A system for transmitting a plurality of symbol groups having different (i is an integer) by time division multiplexing at an arbitrary ratio, wherein the maximum value of the number of samples Ni is Nmax (Nmax / Ni is an integer) and the minimum value is Nmax. In the case of Nmin, an OFDM transmission system used in an OFDM transmission system in which the transmitting side multiplexes a reference symbol having every Nmax / Nmin carriers out of Nmax carriers as an effective carrier at a predetermined position in the transmission frame and transmits the multiplexed reference symbol. In a receiving apparatus, an OFDM demodulator for performing a discrete Fourier transform of a received signal in accordance with a sample number Ni of an effective symbol period of a symbol group to be received, and the reference symbol demodulated by the OFDM demodulator is used. Synchronization detection means for performing reception synchronization detection, and demodulates an arbitrary Ni sample in the reference symbol; OFDM receiving apparatus characterized by achieving reception synchronization based. 13. The number Ni of samples in an effective symbol period having the same sampling frequency in a unit transmission frame.
A system for transmitting a plurality of symbol groups having different (i is an integer) by time division multiplexing at an arbitrary ratio, wherein the maximum value of the number of samples Ni is Nmax (Nmax / Ni is an integer) and the minimum value is Nmax. In the case of Nmin, the transmitting side sets every Nmax / Nmin carriers out of the Nmax carriers as effective carriers, and sets a reference symbol defining the presence or absence of these effective carriers in a predetermined pattern at a predetermined position in the transmission frame. An OFDM receiver used in an OFDM transmission system that multiplexes and transmits a signal, a timing synchronization detecting unit that detects the reference symbol from a received signal and detects frame synchronization based on the detection result, An OFDM demodulator for performing a discrete Fourier transform on the received signal according to the number of samples Ni in an effective symbol period of a symbol group. And a frequency detuning correcting means for detecting the pattern information from the reference symbol demodulated by the OFDM demodulating means and adjusting the frequency detuning of the received signal based on the pattern information. An OFDM receiver comprising: detecting frame synchronization by a symbol; demodulating an arbitrary Ni sample in the reference symbol; and achieving frequency synchronization based on the demodulation result. 14. On the receiving side, when demodulating Ni samples in which Ni <Nmax in the reference symbol,
14. The OFD according to claim 12, wherein a plurality of Ni samples in a reference symbol are demodulated, and the plurality of demodulation results are averaged to detect the reference symbol.
M receiving device.