JPH10107758A - Orthogonal frequency division multiplex modulator-demodulator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To allow transmission and reception of an orthogonal frequency division multiplex(OFDM) modulation wave without provision of a special radio equipment by generating an OFDM modulation signal for a base band. SOLUTION: Transmission data fed to an input terminal 1 at a transmitter side are given to an S/P conversion circuit 26, where the data are S/P-converted and a conjugate complex data supply circuit 27 generates data of k=N/2, (N/2)+1,..., N-1. The data are fed to an IFFT(inverse fast Fourier transform) circuit 28, in which fast discrete Fourier inverse transform is conducted. As a result, since digital data of the orthogonal frequency division multiplex(OFDM) modulation wave are outputted to a real component modulation output of the IFFT circuit 28, the data are D/A-converted by a D/A converter circuit 29, after a harmonic component is eliminated by an LPF 30, an OFDM modulation wave for a base band is outputted from an output terminal 31.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a transmission symbol sequence as information to be carried on each sub-channel carrier,
The information is subjected to an inverse fast Fourier transform (IFFT) to generate an orthogonal frequency division multiplex (OFDM) modulated signal, and the OFDM modulated signal is subjected to a fast Fourier transform (FFT) to be loaded on each subcarrier. The present invention relates to an OFDM modulator / demodulator for extracting information and demodulating each symbol from each information, and in particular, for generating and transmitting an OFDM modulated signal in a baseband band to transmit and receive an OFDM modulated wave without changing a conventional radio. And an OFDM modem.

[0002]

2. Description of the Related Art Generally, orthogonal frequency division multiplexing (OFD)
M) Modulation is a method in which the phases of a plurality of carriers in the frequency multiplexing method are orthogonalized, and the spectra of the modulated waves corresponding to the respective carriers overlap, but overlap so that the orthogonal condition is satisfied. Therefore, it is possible to completely separate them on the receiving side. This orthogonal state is the same as the Nyquist condition without intersymbol interference in the time domain. That is, if the Nyquist condition is satisfied, although the response waveforms of the respective pulses overlap each other, by extracting the signal at an appropriate sampling timing, it is possible to completely receive the signals without interference between the pulses. What realizes the above orthogonal condition in the frequency domain is OFDM.
This is a modulation method (orthogonal frequency division multiplex modulation method). As a result, there is no need to provide a guard band for preventing spectral overlap between modulated waves as in conventional frequency division multiplexing, and high frequency use efficiency is achieved. Also,
The OFDM modulation utilizes multiple carriers, which must be in an orthogonal relationship. To realize this, a Fourier transform circuit is used for the OFDM modulation / demodulation circuit.

[0003] This Fourier transform circuit can generally be realized by an FFT (fast Fourier transform) circuit by digital signal processing, and the number of carriers can be set almost arbitrarily. That is, several hundreds to several thousand carriers can be used. For this reason, the rate of digital data allocated to each carrier is reduced from several hundredths to several thousandths, and each carrier is modulated at a very low rate. Therefore, for each carrier, the modulation symbol rate becomes very low and the symbol period becomes long, so that ordinary multipath can be regarded as almost close multipath, and the influence of multipath is greatly reduced. Can be reduced. On the other hand, in OFDM demodulation, F
An FT circuit (Fast Fourier Transform circuit) is used.
The circuit can obtain phase and amplitude information of each carrier by accurately taking in only one symbol period of the effective symbol period with a prescribed clock and performing a Fourier transform operation. That is, this is a demodulation operation.

A conventional orthogonal frequency division multiplexing (OFDM) modulator / demodulator for realizing such an OFDM modulation / demodulation operation is shown in FIG.
The configuration was as shown in FIG. That is, as shown in FIG. 3, the conventional OFDM modulator / demodulator includes a modulation part C on the transmission side and a demodulation part D on the reception side.
Is an S / P conversion circuit (serial / parallel conversion circuit) 2 connected to the input terminal 1, an IFFT circuit (fast Fourier inverse conversion circuit) 3 connected to the S / P conversion circuit 2,
The first and second D connected to the FFT circuit 3, respectively.
/ A conversion circuit (digital / analog conversion circuit) 4, 5
And first and second LPFs (low-pass filters) 6, 7 connected to the first and second D / A conversion circuits 4, 5, respectively, and first and second LPFs 6, 7, respectively. Connected first and second mixing circuits 8, 9
A 90 ° phase circuit 11 connected to the second mixing circuit 9, and the first mixing circuit 8 and the 90 ° phase circuit 1
1 and a summing circuit 12 connected to the first and second mixing circuits 8 and 9. The demodulation part D includes a BPF (bandpass filter) 14 connected to the input terminal 13 and the BPF
A third and fourth mixing circuit 15 connected to the PF 14,
16; a 90 ° phase circuit 18 connected to the fourth mixing circuit 16; a local oscillation circuit 17 connected to the third mixing circuit 15 and the 90 ° phase circuit 18;
And fourth LPFs (low-pass filters) 19, 2 connected to the third and fourth mixing circuits 15, 16, respectively.
0, third and fourth A / D conversion circuits (analog / digital conversion circuits) 21 and 22 connected to the third and fourth LPFs 19 and 20, and the third and fourth A
It has an FFT circuit (fast Fourier transform circuit) 23 connected to the / D conversion circuits 21 and 22, and a P / S conversion circuit (parallel / serial conversion circuit) 24 connected to the FFT circuit 23.

Next, the operation of the above-described conventional OFDM modulator / demodulator will be described with reference to the operation flowchart of FIG. First, on the transmission side, transmission data Ck = ak + jbk input to the input terminal 1 is subjected to S / P conversion (serial / parallel conversion) by the S / P conversion circuit 2 (FIG. 4).
(Step 100 in (a)), the S / P-converted transmission data Ck = ak + jbk is input to the IFFT circuit 3 and inverse fast Fourier-transformed (Step 101).
The I component output and the Q component output of the IFFT circuit 3 are
The signals are input to the first and second D / A conversion circuits 4 and 5, respectively, and D / A converted (digital / analog converted) (step 102). The outputs of the first and second D / A conversion circuits 4 and 5 are the first and second LPs, respectively.
The signals are input to F6 and F7, filtered, and output as a baseband signal I component and a baseband signal Q component (step 103). Here, the baseband signal I component is represented by X
_{Assuming that I} (t), X _I (t) is given by the following equation.

[0006]

(Equation 1) Here, fk is the k-th carrier frequency in the baseband. Then, assuming that the length of the effective symbol period is ts, the above fk is given by fk = k / ts. In addition, the baseband signal Q component is X _Q
Assuming (t), X _Q (t) is given by the following equation.

[0007]

(Equation 2) Next, the local oscillation signal output from the local oscillation circuit 10 and the local oscillation signal are output to the baseband signal I component and Q component by the orthogonal transformation circuit including the first and second mixing circuits 8 and 9. The signals obtained by phase-shifting the signals by 90 ° by the 90 ° phase circuit 11 are mixed (step 104), and the two signals are added by the adder circuit 12 to be frequency-converted into an IF (or RF) signal (step 105). ). The above IF (or RF)
Assuming that the signal is X _IF (t), X _IF (t) is given by the following equation.

[0008]

(Equation 3) Then, the IF signal is sent to a transmitter (not shown),
Sent. Next, on the receiving side, after the out-of-band noise is removed by the BPF 14 from the IF (or RF) band modulated signal received by the receiver (not shown) and supplied to the input terminal 13 (FIG. 4). Step 10 of (b)
6) The local oscillation signal output from the local oscillation circuit 17 and the local oscillation signal are subjected to a 90 ° phase shift by the 90 ° phase circuit 18 by the quadrature detection circuit including the third and fourth mixing circuits 15 and 16. The mixed signals are converted into a baseband band by mixing the signals (step 1).
07). The output of each of the mixing circuits 15 and 16 is converted into digital data by the third and fourth A / D conversion circuits 21 and 22, after the harmonic components are removed by the third and fourth LPFs 19 and 20, respectively. (Steps 108 and 109). The third and fourth A / Ds
The output data from the conversion circuits 21 and 22 are complex digital data, and the output data of the third A / D conversion circuit 21 becomes the baseband signal I component, and the fourth A / D
Output data of the conversion circuit 22 becomes a baseband signal Q component. These first and second A / D conversion circuits 21, 2
2 are supplied to the FFT circuit 23 and subjected to high-speed discrete Fourier transform (step 110), and ck = ak + jbk, which is transmission data, is output. The received data is obtained by performing P / S conversion in the S conversion circuit 24 (step 111).

[0009]

However, in the above-mentioned conventional system, OFDM is performed using a conventional radio.
When a DM signal is transmitted and received, first, in a conventional radio, there is usually only one input to a transmitter, and the frequency band of the input is usually a baseband band. Therefore, in the conventional OFDM modulated wave generation method, the IF
Since the signal becomes an OFDM modulated wave only after the band of (or RF) is reached, if the signal is directly input to the above-mentioned conventional wireless device, it will exceed the input range of the transmitter and cannot be transmitted. Therefore, in order to transmit, a modification has to be made to change the input frequency band of the conventional radio, or a dedicated transmitter has to be used. Next, the baseband signal I component and the baseband signal Q component from the first and second LPFs 6 and 7 shown in FIG. 3 are in the input frequency range of the conventional transmitter. Since the baseband signal is divided into two waves as described above, two waves cannot be simultaneously input to a conventional transmitter which usually has only one input, and transmission cannot be performed. Further, in order to prevent the output from the adder circuit 12 shown in FIG. 3 from changing the input frequency band of the conventional radio, if the frequency of the local oscillation circuit 10 is lowered, the output of the adder circuit 12 is used as it is. Can be sent as input. But this, after all,
As shown in FIG. 3, two D / A conversion circuits 4, 5 and an LPF
6, 7, the mixing circuits 8, 9, as well as one 90 ° phase shift circuit 11 and the local oscillator circuit 10 are required in the modulation part C. This problem also applies to the demodulation part D. As described above, the conventional method of transmitting an OFDM modulated wave has a problem in that it is not possible to transmit an OFDM modulated wave using a conventional wireless device as it is. Further, even if the frequency of the local oscillation circuit 10 shown in FIG. 3 is reduced so that the signal can be directly input to a conventional wireless device, there is also a problem that the circuit scale becomes large. SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and generates an OFDM modulated signal in a baseband so that ODM can be performed without changing a conventional radio.
An object of the present invention is to provide an OFDM modem capable of transmitting and receiving FDM modulated waves.

[0010]

In order to achieve the above object, the present invention provides a method of serially / parallel-converting a transmission symbol sequence, and converting the serial / parallel-converted symbol sequences into information to be carried on respective sub-channel carriers. An OFD that generates an orthogonal frequency division multiplexing (OFDM) modulated signal by performing an inverse fast Fourier transform (IFFT) on the information.
In the M modulator, the N-point IFFT input N / 2 to N
Means for performing IFFT by inputting conjugate complex information to information of 0 to N / 2-1 as inputs up to -1;
And a means for outputting only the real part of the calculation result to generate an OFDM modulated signal. Another feature of the present invention is to perform fast Fourier transform (FFT) on an orthogonal frequency division multiplexing (OFDM) modulated signal to extract information carried on each sub-channel carrier, and demodulate each symbol from each information. In the OFDM demodulator that converts the symbol sequence from parallel to serial and uses it as a received symbol of the OFDM demodulator, the OFDM modulation signal generated by the OFDM modulator is used as the real part input of the FFT, and 0 is input to the imaginary part input of the FFT. Enter and FFT
Is provided. That is, in the modulator according to the present invention, C _Nk = Ck ^* (k = 0, C _Nk = Ck ^*) as an input of an N-point IFFT for generating an OFDM modulated wave.
When the input is converted to satisfy the condition of (1,..., (N / 2) -1), the real part of the output of the IFFT is given by the following equation.

[0011]

(Equation 4) The imaginary part of the IFFT output is given by the following equation.

[0012]

(Equation 5) Therefore, the real part of the output of the IFFT becomes an OFDM modulated wave in the baseband, and can be directly input to a conventional radio and transmitted. Also, since the imaginary component of the IFFT output is 0, there is no need to output it,
Only one / D conversion circuit and one LPF are required, and a local oscillation circuit and a 90 ° phase shift circuit are not required. On the other hand, in the demodulator, the modulated wave generated in the modulator is OFF
The N-point FFT for demodulating the DM modulated wave is put in the real part of the input, the imaginary part of the input of the N-point FFT is put in 0,
By performing T, the OFDM modulated wave can be demodulated.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described based on illustrated embodiments. FIG. 1 is a configuration diagram showing an embodiment of an OFDM modem (orthogonal frequency division multiplexing modem) according to the present invention. As shown in FIG. 1, the OFDM modulator / demodulator is composed of a modulation part A on the transmission side and a demodulation part B on the reception side.
/ P conversion circuit (serial / parallel conversion circuit) 26 and the S /
Conjugate complex data supply circuit 2 connected to P conversion circuit 26
7 and I connected to the conjugate complex data supply circuit 27.
An FFT circuit (Fast Fourier inverse transform circuit) 28;
D / A connected to the real component output terminal of the FFT circuit 28
A conversion circuit (digital / analog conversion circuit) 29 and an L terminal connected to the D / A conversion circuit 29 and the output terminal 31
And a PF (low-pass filter) 30. The demodulation section B is connected to the LP terminal connected to the input terminal 32.
A (low-pass filter) 33 and an A / D conversion circuit (analog / digital conversion circuit) connected to the LPF 33
An FFT circuit (fast Fourier transform circuit) 35 whose real part input is connected to the A / D conversion circuit 34;
P connected to the FFT circuit 35 and the output terminal 37
/ S conversion circuit (parallel / serial conversion circuit) 36. Next, referring to the operation flowchart of FIG.
The operation of the FDM modulator / demodulator (orthogonal frequency division multiplexing modulator / demodulator) will be described.

First, on the transmission side, Ck = ak + jbk (k = 0, 1,..., (N
The transmission data in the (/ 2) -1) format is subjected to S / P conversion (serial / parallel conversion) by the S / P conversion circuit 26 (FIG. 2).
(Step 200 of (a)), the conjugate complex data supply circuit 27 converts the data of Ck (k = 0, 1,..., (N / 2) -1) into k = N
, (N / 2) +1,..., N−1 are generated (step 201). That is, step 201
Can be said to be a process of inserting conjugate complex information into information of 0 to N / 2-1 as inputs from N / 2 to N-1 of the input of the N-point IFFT. The conjugate complex data supply circuit 2
7, the data of Ck (k = 0, 1,..., N−1) is supplied to the IFFT circuit 28, and is subjected to high-speed discrete Fourier inverse transform (step 202).

As a result, digital data of an OFDM modulated wave is output to the real number component output of the IFFT circuit 28, and this data is output to the D / A conversion circuit 29 by the D / A conversion circuit 29.
After A-conversion (Step 203), and the harmonic components are removed by the LPF 30, the output terminal 31 outputs the signal as a baseband OFDM modulated wave (Step 2).
04). Then, the OFDM modulated wave is transmitted from a transmitter (not shown). On the demodulation side, the OFDM modulated wave frequency-converted to the baseband band by the modulator is supplied to an input terminal 32 via a receiver (not shown). The OFDM modulated wave supplied to the input terminal 32 is converted into digital data by the A / D conversion circuit 34 after the harmonic component is removed by the LPF 33 (step 205 in FIG. 2B). 20
6). The output data of the A / D conversion circuit 34 is
The input is input to the real part input of the FT circuit 35, and 0 is input to the imaginary part input of the FFT circuit 35. And the above F
The signal input to the real part of the FT circuit 35 is subjected to high-speed discrete Fourier transform by the FFT circuit 35 (step 2).
07), transmission data Ck (k = 0, 1,..., (N
/ 2) -1) is demodulated and demodulated transmission data Ck
(K = 0, 1,..., (N / 2) -1) are subjected to P / S conversion (parallel / serial conversion) by the P / S conversion circuit 36 (step 208), and demodulated data is output from the output terminal 37. Is output.

The present invention is not limited to the above embodiment, but can be modified as follows. That is, as a modified example, the conjugate complex data supply circuit 27 is a circuit that performs the conversion given by the following equation, and Ck = Ck ^* (k = 0, 1,..., (N / 2) −1) C = Ck (k = 0, 1,..., (N / 2) -1) Next, by replacing the IFFT circuit 28 with an FFT circuit (fast Fourier transform circuit), the same effect as in the above embodiment can be obtained. .

[0017]

According to the present invention, as described above, OFDM
As an input of an N-point IFFT for generating a modulated wave,
By converting to an input that satisfies the condition of _Nk = Ck ^* (k = 0, 1,..., (N / 2) -1), no special radio is prepared, and Can be used to transmit and receive the OFDM modulated wave using the conventional wireless device as it is. Further, compared with the conventional OFDM modulator / demodulator, the number of analog circuits is reduced,
This is effective in miniaturizing the circuit and improving the reliability.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing an embodiment of an OFDM modem (orthogonal frequency division multiplexing modem) according to the present invention.

FIG. 2 is an operation flowchart of the OFDM modulator / demodulator shown in FIG. 1;
(B) is a flowchart of the demodulation operation.

FIG. 3 is a configuration diagram of a conventional OFDM modem.

4 is an operation flowchart of the conventional OFDM modulator / demodulator shown in FIG. 3, (a) is a flowchart of a modulation operation, and (b) is a flowchart of a demodulation operation.

[Explanation of symbols]

1, 13, 32 ... input terminal, 2, 26 ...
S / P conversion circuit, 3, 28 IFFT circuit (fast Fourier inverse conversion circuit), 4, 5, 29 D / A conversion circuit (digital / analog conversion circuit), 6, 7, 19, 20, 3,
0, 33 ... LPF (low-pass filter), 8, 9, 1
5, 16: mixing circuit, 10, 17: local oscillation circuit, 11, 18: 90 ° phase shift circuit, 12
... Addition circuit, 14 ... BPF (bandpass filter), 2
1, 22, 34 ... A / D conversion circuit (analog / digital conversion circuit), 23, 35 ... FFT circuit (fast Fourier transform circuit), 24, 36 ... P / S conversion circuit,
25, 31, 37 ... output terminals, 27 ... conjugate complex data supply circuit,

Claims (2)

[Claims] 1. A transmission symbol sequence is subjected to serial-to-parallel conversion, and each of the serial-to-parallel-converted symbol sequences is used as information to be put on each sub-channel carrier, and the information is subjected to inverse fast Fourier transform (IFFT) to perform orthogonal transform. OFD for generating frequency division multiplexed (OFDM) modulated signal
An OFDM modulator / demodulator having an M modulator, comprising an N-point IF
FT input from N / 2 to N-1
It is characterized by comprising means for performing IFFT by putting conjugate complex information into N / 2-1 information, and means for outputting only the real part of the IFFT operation result to generate an OFDM modulated signal. Orthogonal frequency division multiplexing modem. 2. A fast Fourier transform (FFT) is performed on an OFDM modulated signal to extract information carried on each subchannel carrier, each symbol is demodulated from each information, and the symbol sequence is subjected to parallel-serial conversion. , OFDM as received symbol of OFDM demodulator
An OFDM modem having a demodulator, wherein the OFD
FFT the OFDM modulated signal generated by the M modulator
And input 0 to the imaginary part input of the FFT,
2. The apparatus according to claim 1, further comprising means for performing FFT.
4. The orthogonal frequency division multiplexing modem according to item 1.