JPH1032558A - Ofdm modulation and demodulation circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the error rate of the whole system by making the level of a specific carrier wave within a band relatively larger compared to the level of the other carrier waves on a transmission side and returning the level of the specific carrier wave to the level of the other carrier waves on a receiving side. SOLUTION: A pre-emphasis circuit 11 is provided between an orthogonal frequency multiplexing(OFDM) modulation circuit 4 and a frequency conversion circuit 5, and a de-emphasis circuit 12 is provided between a frequency conversion circuit 6 and an OFDM demodulation circuit 7. The circuit 11 emphasizes the central part of the band of a modulated wave obtained by OFDM modulation. On the other hand, on the receiving side, the circuit 12 de-emphasizes the modulated wave pre-emphasized by the transmission side with opposite characteristic to correct the level to the level of an original modulated wave. As a carrier at the center of the band is deteriorated in C/N to a large extent, the level of the carrier is made larger in advance by the circuit 11 to equalize C/N of each carrier of obtain a satisfactory characteristic.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to OFDM (orthog
More particularly, the present invention relates to an OFDM modulator / demodulator which is used in digital transmission lines such as digital televisions and which divides and multiplexes information by orthogonalizing a large number of carriers and transmits the information.

[0002]

2. Description of the Related Art OFDM modulation has an advantage that carrier waves are orthogonal to each other and that the frequency use efficiency can be maximized.

FIG. 4 is a schematic block diagram of a conventional OFDM modem. In FIG. 4, on the transmission side, information to be transmitted is encoded by an information source encoding circuit 1, error correction is performed by an error correction encoding circuit 2, trellis encoding is performed by a trellis encoding circuit 3, and an OFDM modulation circuit 4 makes an orthogonal Fourier transform (IFFT) process to orthogonalize a large number of carriers and divides and multiplexes the encoded information. Then, the divided and multiplexed code is up-converted by the frequency conversion circuit 5 and transmitted to the transmission path.

On the other hand, on the receiving side, the signal transmitted from the transmission path is down-converted, subjected to Fourier transform (FFT) processing by an OFDM demodulation circuit 7 and demodulated, and then trellis-decoded by a trellis decoding circuit 8 to obtain an error. The error correction decoding circuit 9 performs error correction decoding, and the information source decoding circuit 1
0 decrypts the original information.

[0005]

The conventional O shown in FIG.
The FDM transmission line has been studied in an ideal state, and so-called emphasis such as giving a frequency characteristic to a modulated signal itself has not been particularly considered.
For this reason, when there is nonlinear distortion in the transmission path, there is a phenomenon that noise concentrates on a specific portion in the band, and there is a problem that a specific carrier is easily affected by the noise.

If the transmission path has a non-linear characteristic as shown in FIG. 5, for example, the size of a component other than the desired data received by each carrier obtained by simulation is shown in FIG. This is generated by the intermodulation of each carrier and can be regarded as noise.
According to FIG. 6, it has been clarified by simulation that the C / N ratio of the carrier at the center of the band becomes worse.
This means that when the C / N ratio becomes worse, the error rate of a specific carrier in the center becomes large, thereby increasing the error rate of the entire system.

[0007] Therefore, a main object of the present invention is to provide an OFDM modulation / demodulation device capable of reducing the error rate of the entire system.

[0008]

The invention according to claim 1 is
In an OFDM modulation / demodulation apparatus for transmitting information by dividing and multiplexing a large number of carriers by orthogonally dividing a number of carriers, a pre-emphasis method in which the level of a specific carrier in a band is relatively increased as compared with the levels of other carriers on the transmitting side. The receiving side is provided with de-emphasis means for returning the level of a specific carrier to the level of another carrier.

According to the second aspect of the present invention, the pre-emphasis means further includes a modulating means, and the pre-emphasis means emphasizes the level of the carrier by using a filter having a frequency characteristic with respect to the output data of the modulating means.

According to a third aspect of the present invention, a modulating means for performing an inverse Fourier transform is provided, and the pre-emphasis means of the first aspect includes a means for multiplying all or a part of each frequency component data inputted to the modulating means by a coefficient. .

According to a fourth aspect of the present invention, there is further provided a demodulating means, and the de-emphasis means of the first aspect comprises a filter having a frequency characteristic with respect to input data of the demodulating means.

In the invention according to claim 5, the demodulation means for performing Fourier transform is further included, and the de-emphasis means according to claim 1 includes means for multiplying all or a part of each frequency component data inputted to the demodulation means by a coefficient. .

[0013]

FIG. 1 is a schematic block diagram showing an embodiment of the present invention. In FIG. 1, a pre-emphasis circuit 11 is provided between the OFDM modulation circuit 4 and the frequency conversion circuit 5 shown in FIG. 4, and a de-emphasis circuit 12 is provided between the frequency conversion circuit 6 and the OFDM demodulation circuit 7. Other than the above, it is the same as FIG. 4 described above. The pre-emphasis circuit 11 emphasizes the central part of the band of the modulated wave that has been subjected to the OFDM modulation. The pre-emphasis circuit 11 is basically composed of a filter, and an analog filter or a digital filter is used.

FIG. 2 is a diagram showing an example of a frequency spectrum output from the frequency conversion circuit 5 shown in FIG.
In the example shown in FIG. 2, the frequency is 770 MHz (62c
The band is set to 6 MHz in h), and the center portion of the band of the modulated wave is emphasized as compared with the conventional example.

On the other hand, on the receiving side, a de-emphasis circuit 1
2 de-emphasizes the modulated wave that has been pre-emphasized on the transmission side with the opposite characteristic, and corrects the level of the original modulated wave. This de-emphasis circuit 12 is also constituted by an analog filter or a digital filter.

As described above, the pre-emphasis circuit 11
And the provision of the de-emphasis circuit 12, the error rate of the entire system can be reduced. That is, an OFDM signal has a large ratio between an average value and a peak, and requires a dynamic range in a transmission path. Generally, under such conditions, nonlinear distortion due to an amplifier or the like through which a signal passes easily occurs. Due to the intermodulation due to this nonlinear distortion, C /
N deteriorates, and this effect differs depending on each carrier, and large noise is generated in the central portion of the band as shown in FIG. Therefore, the carrier located in the center of the band has a high C / N deterioration degree. If the carrier level is increased in advance by the pre-emphasis circuit 11, the C / N of each carrier becomes equal, Good characteristics are obtained. The amount of emphasis may be determined in consideration of the actual transmission path conditions.

FIG. 3 is a block diagram showing another embodiment of the present invention. In the embodiment shown in FIG.
A pre-emphasis circuit 11 is provided between the DM modulation circuit 4 and the frequency conversion circuit 5, and a de-emphasis circuit 12 is provided between the frequency conversion circuit 6 and the OFDM demodulation circuit 7.
In the embodiment shown in FIG. 3, a pre-emphasis circuit 11 is provided between the trellis coding circuit 3 and the OFDM modulation circuit 4.
And a de-emphasis circuit 12 is provided between the OFDM demodulation circuit 7 and the trellis decoding circuit 8. The pre-emphasis circuit 11 and the de-emphasis circuit 12 are constituted by circuits for multiplying all or a part of the corresponding frequency component data by a coefficient. The frequency component and the coefficient by which this coefficient is multiplied are selected according to the desired frequency characteristics. For example, if the center portion is emphasized, the coefficient may be increased so that a desired enhancement amount is obtained from both ends of the carrier number toward the center.

In the embodiment shown in FIG. 3, since digital processing is easy on both the transmitting side and the receiving side, it is more effective for implementing an LSI than the first embodiment. In addition, it is necessary to perform an operation opposite to that of the transmitting side on the receiving side, but it is obvious that this can be easily estimated from the configuration of the transmitting side.

Further, in the above-described embodiment, it is assumed that interference occurs at the center of the carrier. However, when interference is present at an arbitrary part of the carrier, the amplitude near the carrier receiving the interference is generally reduced. Increase the size in advance on the transmitting side and set C
It is clear that good characteristics can be obtained by increasing / N. Thus, the present invention is effective even when the carrier at any specific position is relatively emphasized.

[0020]

As described above, according to the present invention, the level of a specific carrier in the band is made relatively higher than that of the other carriers on the transmitting side, and the level of the specific carrier is made higher on the receiving side. Since the level is returned to the level of another carrier, the C / N of each carrier is not limited even in an actual transmission path where nonlinear distortion is present in the transmission path, and even when interference is present at an arbitrary position of the carrier. It is almost the same, and good characteristics can be obtained.

[Brief description of the drawings]

FIG. 1 is a schematic block diagram showing one embodiment of the present invention.

FIG. 2 is a diagram illustrating an example of a frequency spectrum output from the frequency conversion circuit illustrated in FIG. 1;

FIG. 3 is a schematic block diagram showing another embodiment of the present invention.

FIG. 4 is a schematic block diagram of a conventional OFDM modem.

FIG. 5 is a diagram illustrating nonlinear characteristics of a transmission path.

FIG. 6 is a diagram showing a noise distribution of a carrier of an OFDM modulation means.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 information source coding circuit 2 error correction coding circuit 3 trellis coding circuit 4 OFDM modulation circuit 5, 6 frequency conversion circuit 7 OFDM demodulation circuit 8 trellis decoding circuit 9 error correction decoding circuit 10 information source decoding circuit 11 pre-emphasis circuit 12 De-emphasis circuit

Claims (5)

[Claims] 1. An OFDM modulator / demodulator for transmitting information by dividing and multiplexing a large number of carriers by orthogonally multiplexing a plurality of carriers, wherein a transmitting side relatively compares a level of a specific carrier within a band with a level of another carrier. Including pre-emphasis means for increasing the level of the specific carrier on the receiving side, comprising de-emphasis means for returning the level of the specific carrier to the level of another carrier.
OFDM modem. 2. The apparatus according to claim 1, further comprising modulating means, wherein said pre-emphasis means emphasizes a level of said carrier by a filter having a frequency characteristic with respect to output data of said modulating means. 1
OFDM modem. 3. The apparatus according to claim 1, further comprising a modulating means for performing an inverse Fourier transform, wherein said pre-emphasis means includes a means for multiplying all or a part of each frequency component data inputted to said modulating means by a coefficient. OFDM modem. 4. The OFDM modulator / demodulator according to claim 1, further comprising demodulation means, wherein said de-emphasis means includes a filter having a frequency characteristic with respect to input data of said demodulation means. 5. The OFDM according to claim 1, further comprising demodulation means for performing Fourier transform, wherein said de-emphasis means includes means for multiplying all or a part of each frequency component data input to said demodulation means by a coefficient. Modem.