JPH09294151A - Digital modulator-demodulator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To recover a carrier and to correct a phase error with high stability and high precision by receiving a transmission signal with a pilot carrier inserted thereto not via a high pass filter, using an AFC circuit to obtain a frequency difference, and thereby controlling a transmitter. SOLUTION: Data at an input terminal 1 are given to a mapping section 2, in which mapping signals I, Q are obtained, and subject to band limit and waveform shaping b roll-off filters 3i, q and the result is given to a frequency conversion section 4. The results are fed to a prime filter 5 and a D/A converter, a DC component is added to the signal I by an adder 8 and the result is subject to quadrature modulation by a quadrature modulator consisting of a phase shifter 11 and an adder 12 or the like. Thus, a signal to which a pilot carrier is inserted is obtained not via a high pass filter. The signal is received at a reception terminal 14 and an AFC circuit 21 obtains a frequency difference by the pilot carrier at a transmission side to control a transmitter 19. Since the pilot carrier is inserted to detect a frequency error and a phase error, carrier recovery and correction of a phase error are attained with high stability and high precision.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital transmission device for transmitting image and audio information using digital data.

[0002]

2. Description of the Related Art As a conventional technique for transmitting an image or the like by digital data by adding a pilot carrier by giving a DC offset to a baseband signal, for example, a conference of the Television Society Annual Meeting (1993, 14
-6). This conventional example will be described below with reference to FIG.

The digital data input from the input terminal 1 is mapped by the mapping unit 2 into the in-phase signal I and the quadrature signal Q, and band-limited by the roll-off filters 3i and 3q, respectively. This signal is converted into an analog signal by the DA converters 6i and 6q, and then the high-pass filters 29i and 29q.
The component near the direct current is suppressed by q. Thereafter, the adder 8 adds the DC component from the DC power supply unit 7 to the I signal,
Quadrature modulation is performed by a quadrature modulator including multipliers 9i and 9q, an oscillator 10, a 90-degree phase shifter 11, and an adder 12 together with the Q signal. At this time, the DC component added to the I signal is
The carrier frequency is converted into the carrier frequency itself, which serves as a pilot carrier for carrier recovery on the receiving side.

In digital transmission, the frequency component of the modulated wave also exists near DC. Therefore, the modulated wave component near the direct current falls into the band of the filter for reproducing the received carrier, and the component becomes carrier jitter of the reproduced carrier. High pass filters 29i, 29q
Is inserted for the purpose of suppressing the carrier jitter after reproduction by suppressing the modulation component near the carrier.

The signal output from the carrier wave output terminal 13 is input from the reception input terminal 14, and the multipliers 15i, 15
q is input to the carrier recovery circuit 30. The carrier wave regenerating circuit 30 regenerates the carrier wave using the pilot carrier inserted on the transmitting side. This reproduced carrier wave is multiplied by the multiplier 1
The reproduced carrier wave is sent to 5i and 15q to perform I and Q separation. I
After the DC component of the signal is removed by the adder 18, the Q signal is directly fed from the multiplier 15q to the AD converters 20i and 20i, respectively.
It is input to q and converted into a digital signal. afterwards,
The waveform is shaped by the roll-off filters 24i and 24q, the decoding unit 27 performs code determination, parallel-serial conversion,
It is for obtaining the reproduction data.

[0006]

In the above-mentioned conventional example, the I and Q baseband signals are passed through a high pass filter for the purpose of suppressing the carrier jitter of the reproduced carrier, and the signal components to be transmitted are suppressed. It Therefore, equivalently C /
There is a problem that the N ratio is deteriorated.

That is, in the carrier reproduction on the receiving side, in order to obtain a stable reproduced carrier wave, it is not enough to superimpose the pilot carrier on the modulator, and modulation in the vicinity of the carrier (in the base band, it is near DC) Remove wave components,
Or it needs to be suppressed (because the modulation component is regarded as mere noise when focusing on only the phase of the pilot carrier).

On the other hand, in order not to deteriorate the transmission efficiency,
It is important not to deteriorate the baseband spectrum as much as possible, and in this respect, the removal or suppression of the above-mentioned modulated wave component is contradictory. In other words, the two are in a trade-off relationship.

On the receiving side, the carrier is reproduced in the high frequency band or the intermediate frequency band, and there is a problem that the phase error generated thereafter cannot be corrected.

An object of the present invention is to solve these problems.

That is, the first object of the present invention is to eliminate the high-pass filter, reduce the influence of the phase noise of the transmission local oscillator without removing or suppressing the signal component to be transmitted, and reduce the reproduction carrier wave. The purpose is to suppress carrier jitter.

A second object of the present invention is to correct the phase error as much as possible even for a phase error that cannot be followed by carrier reproduction performed in the high frequency band or the intermediate frequency band, and perform highly accurate and highly efficient digital transmission. The purpose is to

[0013]

In order to solve the above-mentioned problems, the present invention adds a function of digitally frequency-shifting in a transmission baseband band, and further removes a frequency error between transmission and reception by AFC. However, the remaining phase error can be corrected in the base band.

As a result, the required signal components are removed,
Stable carrier wave recovery and phase error correction are possible without suppression, and highly efficient digital data transmission is possible. According to the present invention, the modulation component (noise) in the vicinity of the carrier while maintaining the baseband spectrum (power)
The carrier jitter due to the noise of the reproduced carrier is suppressed by removing the.

However, in the structure of the demodulator described in the preceding paragraph, the frequency error and the phase error of the carrier are attempted to be corrected at the same time, and a phase correction residue occurs due to the trade-off with the synchronization high speed, and the judgment after the multiplier is made. There is no correction means for the phase error caused by the noise added up to the part.

A relatively low efficiency transmission scheme (eg, BPS
Even if the influence of these phase errors in (K, QPSK) is small, as 16PSK, 16QAM, 64QAM, etc., become more multi-valued and highly efficient, their influence cannot be ignored. In order to deal with this, in the high frequency part AF
Only the frequency error is removed by C, and in the baseband section,
A method for performing phase error correction by a digital signal station is required.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIG.

The input terminal 1 of the transmission unit is connected to the mapping unit 2, and the mapping unit 2 applies roll-off filters 3i and 3q, a frequency conversion unit 4, and an I signal and a Q signal, respectively.
And DA converters 6i and 6q via the complex filter 5. On the I signal side, the DA converter 6i is connected to the multiplier 9i via the adder 8, on the Q signal side, the DA converter 6q and the multiplier 9q are connected, and the multipliers 9i and 9q connect the adder 12 to each other. It is connected to the carrier wave output terminal 13 via. The DC power supply unit 7 is connected to the adder 8, and the oscillator 10 is connected to the multiplier 9i on the I signal side and the 90-degree phase shifter 11. 90
The phase shifter 11 is connected to the multiplier 9q on the Q signal side. The reception input terminal 14 of the reception unit is connected to the multipliers 15i and 15q on the I signal side and the Q signal side, respectively, and
It is connected to the phase correction circuit 26 via the FC circuit 21 and the phase error detection unit 25. The multiplier 15i on the I signal side is connected to the complex filter 22 via the adder 18 and the AD converter 20i, and the multiplier 15q on the Q signal side is the AD converter 20q.
Is connected to the complex filter 22 via. The complex filter 22 includes a frequency converter 23, a roll-off filter 24i,
24q and the phase correction circuit 26, and is connected to the decoding unit 27 and the reproduction data output terminal 28.

This operation will be described below. The data input from the input terminal 1 of the transmission unit is transferred to the mapping unit 2
After serial-parallel conversion is performed, the data is mapped according to a predetermined signal point arrangement. The mapped signal is I
Roll-off filters 3i and 3q for signal and Q signal, respectively
Then, the band is limited and the waveform is shaped and sent to the frequency converter 4. The frequency converter 4 shifts the frequency by ω1 (ω1 << ωb, ωb; baseband), and inputs it to the complex filter 5. The operations of the frequency conversion unit 4 and the complex filter 5 at this time will be described with reference to FIG. The spectrum of the signal after mapping and band limitation is as shown in FIG. By passing this signal through the frequency conversion unit 4, FIG.
As shown in (b), a signal whose frequency is shifted by ω1 from the center frequency θ is obtained. The frequency characteristic of the complex filter 5 is shown by the broken line in FIG. 2B, and the signal shown in FIG. 2C is obtained by passing this complex filter 5. This signal is converted into I / Q signals by DA converters 6i and 6q, respectively.
In the adder 8, a DC component given from the DC power supply unit 7 is added to the I signal side, and the oscillator 10,
90-degree phase shifter 11, multipliers 9i and 9q, and adder 12
Quadrature modulation is performed by the quadrature modulator configured by. As a result, as shown in FIG. 2D, a signal in which the pilot carrier is inserted is obtained without passing through the high pass filter.

On the receiving side, this signal is received by the receiving input terminal 14
And sends it to the multipliers 15i and 15q and the AFC circuit 21. The AFC circuit 21 uses the pilot carrier inserted on the transmission side to find the frequency difference between transmission and reception, and controls the oscillator 19 to correct this. Multiplier 15i,
In 15q, quadrature detection and I / Q separation are performed by the signal from the oscillator 19 or the 90-degree phase shifter 16. On the I signal side, the DC offset is removed by the adder 18, AD-converted by the AD converter 20i, and then input to the complex filter 22, and on the Q signal side, AD-converted and similarly input to the complex filter 22. The complex filter 22 and the frequency conversion unit 23 perform the reverse processing of the transmission unit to restore the signal spectrum to the original,
Waveform shaping is performed by the roll-off filters 24i and 24q.
The AFC 21 corrects only the frequency difference between transmission and reception, and the output signals of the roll-off filters 24i and 24q have phase distortion due to fading of the transmission path and phase distortion added in the analog system of the transceiver. ing. In order to correct this distortion, the phase error detection unit 25 finds the phase difference between the pilot carrier phase and the demodulated signal, and the phase correction circuit 2
After correcting the calculated phase error in 6, the decoding unit 27 performs code determination and parallel-serial conversion, and outputs the reproduction data from the reproduction data output terminal 28.

In this embodiment, it is indispensable to perform complex filter processing and frequency shift.

However, although the quasi-coherent detection method using the AFC circuit 21 and the phase correction circuit 26 has been shown, as shown in the conventional example, the synchronous detection method in which the carrier is reproduced in the high frequency band or the intermediate frequency band. You can also take

Next, an application example using this embodiment will be described with reference to FIG.

FIG. 3 shows an FPU (Field Pickup unit) which is a broadcast relay device using this embodiment.
It is a block diagram of. The transmission control unit 40 is connected to the antenna 43 via the coaxial cable 41 and the transmission high frequency unit 42, and the antenna 44 of the reception unit is connected to the reception control unit 47 via the reception high frequency unit 45 and the coaxial cable 46.

This operation will be described below. The transmission control unit 40 uses the input digital data to
Modulation is performed according to the operation described above to obtain a 130 MHz first intermediate frequency signal. The first intermediate frequency signal is superposed on a control signal for device control, a power source of the high frequency section, etc., and is input to the transmission high frequency section 42 by the coaxial cable 41. In the transmission high frequency section 42, the first intermediate frequency signal to the second intermediate frequency signal of 1500 MHz, the transmission frequency of 7 GHz band,
Alternatively, the frequency is converted to the 10 GHz band, the power is amplified, and the transmission wave is output from the antenna 43. The transmitted wave is received by the receiving antenna 44 and input to the receiving high frequency unit 45. In the reception high frequency section 45, 1500M from the reception wave
The frequency is converted sequentially to Hz and 130 MHz,
An intermediate frequency signal of MHz is input to the reception control unit 47 via the coaxial cable 46.

The reception control section 47 obtains a baseband signal and further reproduced data from this intermediate frequency signal, but at this time, the pilot carrier inserted on the transmitting side is used for synchronous detection or quasi-synchronous detection.

In the above embodiments, the demodulator side is provided with the AFC circuit and the phase correction circuit in the baseband section, but the means and algorithms for frequency error correction and phase error correction are limited to this. Not a thing. Further, although it is difficult in reality, it is not always necessary unless there is a frequency offset in the high frequency part and a phase error in the baseband.

[0028]

According to the present invention, since a pilot carrier for detecting a frequency error and a phase error is inserted without removing or suppressing a part of a signal to be transmitted, stable and highly accurate carrier wave reproduction can be achieved. Phase error correction becomes possible, and highly efficient and highly accurate digital transmission becomes possible.

As described above, the modulation component in the vicinity of the pilot carrier is regarded as noise when it is desired to focus only on the pilot carrier phase itself in the carrier reproducing section or the AFC circuit. The reproduced carrier jitter is caused by noise that falls in the band of the filter that extracts the carrier component in the carrier reproduction system.Therefore, on the transmission side, suppress the modulation component near the carrier by using a high-pass filter or frequency shift means. Eliminate and suppress playback carrier jitter.

[Brief description of drawings]

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

FIG. 2 is an explanatory diagram of signal frequency characteristics according to the present invention.

FIG. 3 is a block diagram showing an application example of the present invention.

FIG. 4 is a block diagram showing a conventional technique.

[Explanation of symbols]

1 input terminal, 2 mapping section 3i, 3q, 24
i, 24q, roll-off filter, 4, 23 frequency conversion unit, 5, 22 complex filter, 6i, 6q
DA converter, 7, 17 DC power supply section, 8, 12, 1
8 adder, 9i, 9q, 15i, 15q multiplier,
10, 19 oscillator, 11, 1690 degree phase shifter,
13 carrier wave output terminal, 14 reception input terminal, 20
i, 20q AD converter, 21 AFC circuit, 25
Phase error detection unit, 26 phase correction circuit, 27 decoding unit, 28 reproduction data output terminal, 29i, 29q
High pass filter, 30 carrier recovery circuit, 40
Transmission control unit, 41, 46 coaxial cable, 42
Transmission high frequency part, 43, 44 antenna, 45 reception high frequency part, 47 reception control part, ωb baseband band, ωs sampling frequency, ω1 frequency offset amount, ωc transmission wave center frequency

Claims (4)

[Claims] 1. A digital modulator / demodulator using an orthogonal modulation / demodulation method, wherein a modulation unit of the digital modulator / demodulator provides means for frequency-shifting a baseband signal, and a DC offset is given to the frequency-shifted baseband signal. Therefore, the demodulation section has means for adding a pilot carrier for carrier recovery, and means for removing the DC component given by the modulation section and frequency offset given by the frequency shift of the modulation section. Means to remove,
A digital modulator / demodulator having a function of reproducing a carrier using a pilot carrier. 2. A digital modulator / demodulator according to claim 1, wherein the demodulator has a function of removing a frequency error by using frequency control means, a phase error detection section, and a signal from the phase error detection section, thereby providing a baseband signal. A digital modulator / demodulator characterized by removing a phase error in a band. 3. A digital modulator / demodulator for transmitting an electric signal as digital data, comprising: mapping means for obtaining an I signal and a Q signal from the digital data, and respective bands of the I signal and the Q signal obtained by the mapping means. Limiting and waveform shaping roll-off filter means, frequency transforming means for frequency shifting the waveform-shaped baseband I and Q signals from the roll-off filter means, and band limiting of the frequency shifted I and Q signals. Complex filter means and I through the complex filter means
I signal D / A conversion means and Q signal D / A conversion means for converting respective signals of Q and Q signals into analog signals, addition means for giving a DC offset to the D / A converted I signal, and the DC offset Is applied to the I signal and the Q signal, and a quadrature modulation means for adding a quadrature modulation verse pilot carrier, and a signal transmitted from the digital modulation unit are received to quadrature the I signal and the Q signal. Quadrature demodulation means for demodulating, frequency control means for controlling the local oscillation frequency of the quadrature demodulation means by receiving the output of the modulation section, and means for removing a DC component given to the I signal by the digital modulation section, I signal and the demodulation Q
I signal A / D conversion means and Q signal A / D conversion means for converting a signal into a digital signal, and these two A / D
Complex filter means for band limiting the digital data from the converting means, frequency shift means for inverse frequency shifting the digital signal band limited by the complex filter, and roll off filter means for shaping the waveform of the inverse frequency shifted digital signal. A phase correction circuit of the next stage, a phase error detection circuit for giving a correction signal to the phase correction circuit from the pilot carrier signal from the frequency control means and the demodulation signal from the inverse frequency shift means,
A digital modulator / demodulator characterized by having a demodulation section composed of a decoding means for decoding the I and Q signal data passing through the phase correction circuit. 4. A broadcast relay device comprising the digital modulator / demodulator according to claim 1.