JPH06315040A - Digital transmitter-receiver - Google Patents

Abstract

PURPOSE:To provide the digital transmitter-receiver which can optimally control fixed degradation at the time of low CN or adjacent transmission. CONSTITUTION:This digital transmitter-receiver calculates a transmission error rate from the state of error correction at an error correcting circuit 8 by using a control part 10. An input C/N is calculated from that value, and either the cut-off frequency of base band filters 51 and 52 or the time constant of a control signal to an oscillator 44 for carrier reproduction is variably controlled so as to optimize the fixed degradation. Thus, the error rate at the low C/N is optimized and the fixed degradation is reduced so that a wide dynamic range can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION This invention is applicable to satellite broadcasting and C
The present invention relates to a receiver when digital image transmission is introduced in ATV broadcasting or the like and a digital transmission method of quadrature amplitude phase modulation is used.

[0002]

2. Description of the Related Art In conventional satellite broadcasting, digital transmission is performed by a modulation method such as QPSK, and its receiving device has a structure as shown in FIG. In FIG. 2, the antenna 1 is, for example, 11.7 to 12.2 GHz.
The satellite broadcast signal in the frequency band of is received. This satellite broadcast signal is group-converted into 950 to 1350 MHz by the frequency converter 2, and then further preamplifier 31, mixer 32,
It is converted into an intermediate frequency by the down converter 3 including the local oscillator 33 and the buffer amplifier 34. here,
It is selected by adjusting the oscillation frequency of the local oscillator 33, converted to an intermediate frequency (IF) signal in the 140 MHz band, and sent to the quadrature detection circuit 4.

The quadrature detection circuit 4 splits the channel selection signal input by the splitter 41 into two systems and sends them to the mixers 42 and 43. Carriers orthogonal to each other obtained by branching the oscillation output of a VCO (voltage controlled oscillator) 44 into two by a splitter 45 and shifting one of them by 90 ° by a 90 ° phase shifter 46 to the mixers 42, 43. A reproduction signal is supplied and is mixed with the channel selection signal by each of the mixers 42 and 43 to decompose the channel selection signal into baseband signals of I and Q axis components orthogonal to each other.

The decomposed I and Q baseband signals are waveform-shaped by baseband filters (LPF) 51 and 52, respectively, and then converted into digital signals by A / D (analog / digital) converters 61 and 62. Is sent to the digital processing circuit 7.

The digital processing circuit 7 demodulates the I and Q axis data strings from the digitized I and Q baseband components, reproduces a bit clock, and also generates a VCO control signal for carrier reproduction. The regenerated bit clock is supplied to the A / D converters 61 and 62 as a sampling clock and also to the FEC circuit 8 in the subsequent stage. Further, the VCO control signal is supplied to the VCO 44 of the quadrature detection circuit 4 and used for phase adjustment of the carrier reproduction signal.

The FEC circuit 8 uses the forward error correction (Fo
rward Error Correction) circuit. The FEC circuit 8 performs error correction on the digital signal that has become the I and Q axis data strings in the digital processing circuit 7, removes redundant bits, and is converted into a desired demodulated data string.

By the way, in the digital transmission / reception apparatus as described above, due to the recent development of error correction such as Viterbi decoding, it has become a system which can operate even with an error rate of about 10 ^-2 , and when the CN is low, The error rate is becoming an important item for performance.

However, in the conventional, fixed deterioration in the error rate of the order of 10 ^-5, that is, the system design is made so that the error rate is optimized by the difference between the CN value on the theory in the above 10 ^-2 It was not usually designed to have the optimum fixed deterioration of. Therefore, it is strongly desired that the fixed deterioration can be optimally controlled at about 10 ^-2 during low CN or adjacent transmission.

[0009]

As described above, the conventional digital transmission / reception device cannot optimally control the fixed deterioration during low CN or adjacent transmission. The present invention has been made to solve the above problems, and an object of the present invention is to provide a digital transmission / reception apparatus capable of optimally controlling fixed deterioration during low CN or adjacent transmission.

[0010]

In order to achieve the above object, the present invention provides a receiving means for receiving a transmission signal carried on a carrier by digital quadrature modulation of a data string, and a transmission signal received by this means. A quadrature for quadrature detection of baseband components of the I-axis and Q-axis from the received signal of the receiving means based on the carrier reproduction signal generated by the oscillator. A detection circuit, a baseband filter for shaping the I-axis and Q-axis baseband signals obtained by this means, a digital data demodulation from this filter output by digital processing, and based on the filter output. Control signal to the oscillator for carrier reproduction, and the phase of the carrier reproduction signal is changed by changing the oscillation frequency of the oscillator. A digital processing circuit that matches the phase of the carrier, and an error correction circuit that performs error correction of the data string demodulated by this circuit, and further determine the transmission error rate from the error correction state of the error correction circuit, The input C / N is calculated from the value, and the cutoff frequency of the baseband filter is variably controlled based on the calculated C / N value to optimize the fixed deterioration or the error of the error correction circuit. The transmission error rate is obtained from the corrected state, the input C / N is calculated from the value, and the calculated C / N
At least one of carrier reproduction time constant control means for variably controlling the time constant of the control signal sent from the digital processing circuit to the carrier reproduction oscillator based on the value to optimize fixed deterioration. Is characterized by.

[0011]

In the digital transmission / reception apparatus having the above-mentioned configuration, the transmission error rate is calculated from the error correction state in the error correction circuit, and the input C / N is calculated from the calculated value, so that the fixed deterioration is optimized. By variably controlling at least one of the cutoff frequency of the band filter and the time constant of the control signal to the oscillator for carrier regeneration, the error rate at low C / N is optimized, fixed degradation is reduced, and a wide range is thereby achieved. Get the input dynamic range.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described in detail below with reference to FIG. However, in FIG. 1, the same parts as those in FIG. 1 are denoted by the same reference numerals, and the different parts will be mainly described here.

In FIG. 1, the I and Q baseband signals obtained by the quadrature detection circuit 4 are LPFs (baseband filters) 51 ', 5 whose cutoff frequencies can be controlled.
After the waveform is shaped by 2 ', it is converted into a digital signal by the A / D converters 61 and 62, and further demodulated into I and Q data strings by the digital processing circuit 7.

At this time, the VCO control signal SC for carrier reproduction is output from the digital processing circuit 7 and sent to the VCO 44 of the quadrature detection circuit 4 via the variable time constant circuit 9. The signal demodulated by the digital processing circuit 7 into the I and Q data strings is error-corrected by the FEC circuit 8 and is guided to the subsequent data processing circuit.

The feature of the present invention lies in that the CPU 1
At 0, the transmission error rate is calculated from the error correction state in the FEC circuit 8, and the input C / N is calculated from that value,
The time constant of the variable time constant circuit 9 and the cutoff frequencies of the LPFs 51 'and 52' are controlled so that the fixed deterioration is optimized.

That is, in the conventional receiver, the VCO4
The control time constant of No. 4 and the cutoff frequencies of the LPFs 51 'and 52' for shaping the I and Q baseband signal waveforms are normally uniquely determined by the transmission rate and the roll-off characteristic on the transmission side. . On the other hand, in the present invention, the transmission error rate is calculated from the error correction state in the FEC circuit 8, and the input C / N is calculated from that value,
The control time constant of the VCO 44 and the cutoffs of the LPFs 51 'and 52' are controlled so that the fixed deterioration is optimized.

Specifically, the C / N ratio is high (9 to 10d).
At the time of B), the control time constant of the VCO 44 is increased to improve the pull-in accuracy. This improves fixed deterioration. When the C / N is low, the control time constant of the VCO 44 is lowered. As a result, the sensitivity to noise is reduced, the locked state is maintained for a long time, and stable operation can be performed even with an error rate of about 10 ^-2 .

Regarding the LPFs 51 'and 52',
By lowering the cutoff frequency when the C / N is low, the amount of equivalent noise is reduced, thereby improving the error rate.

Therefore, the digital transmission / reception apparatus having the above-mentioned configuration has the FEC circuit 8 performing strong error correction.
, The received C / N is calculated from the operating state of the FEC circuit, and the cutoff frequency of the LPF and the time constant of the carrier recovery loop are controlled. Therefore, the optimum error rate is suppressed and fixed at low C / N. It is possible to reduce the deterioration, and it is possible to operate in a wide input dynamic range.

In the above embodiment, the cutoff frequency of the LPF and the time constant of the carrier reproduction loop are both controlled, but the effect can be obtained with either one. Needless to say, various modifications can be made in the same manner without departing from the scope of the present invention.

[0021]

As described above, according to the present invention, low CN
It is possible to provide a digital transmission / reception device capable of optimally controlling fixed deterioration during time or adjacent transmission.

[Brief description of drawings]

FIG. 1 is a block circuit diagram showing the configuration of an embodiment of a digital transmission / reception device according to the present invention.

FIG. 2 is a block circuit diagram showing a configuration of a conventional digital transmission / reception device.

[Explanation of symbols]

1 ... Antenna, 2 ... Converter, 3 ... Down converter, 4 ... Quadrature detection circuit, 41, 45 ... Divider, 42, 4
3 ... Mixer, 44 ... VCO, 46 ... 90 ° phase shifter, 5
1, 52 ... LPF (base band filter), 51 ',
52 '... Variable LPF, 61, 62 ... A / D converter, 7 ...
Digital processing circuit, 8 ... FEC circuit, 9 ... Variable time constant circuit, 10 ... CPU, SC ... VCO control signal.

Claims (2)

[Claims] 1. A reception means for receiving a transmission signal carried on a carrier by digital quadrature modulation of a data string, and a carrier regeneration oscillator for generating a frequency signal having the same frequency as the carrier of the transmission signal received by this means. A quadrature detection circuit for quadrature detecting baseband components of the I-axis and Q-axis from a received signal of the receiving means based on a carrier reproduction signal generated by this oscillator; and I-axis and Q-axis obtained by this means And a baseband filter that shapes the waveform of each baseband signal, and demodulates the original data string from this filter output by digital processing, and sends a control signal to the carrier regeneration oscillator based on the filter output. A digital processing circuit that matches the phase of the carrier reproduction signal with the phase of the carrier of the transmission signal by varying the transmission frequency. An error correction circuit that performs error correction on the data string demodulated by the circuit, and a transmission error rate is obtained from the error correction state of this circuit, and the input C / N is calculated from that value, and the calculated C / N value is obtained. And a filter control means for variably controlling the cutoff frequency of the baseband filter based on the above to optimize fixed deterioration. 2. A receiving means for receiving a transmission signal carried on a carrier by digitally quadrature modulating a data string, and a carrier reproducing oscillator for generating a frequency signal having the same frequency as the carrier of the transmission signal received by this means. A quadrature detection circuit for quadrature detecting baseband components of the I-axis and Q-axis from a received signal of the receiving means based on a carrier reproduction signal generated by this oscillator; and I-axis and Q-axis obtained by this means And a baseband filter that shapes the waveform of each baseband signal, and demodulates the original data string from this filter output by digital processing, and sends a control signal to the carrier regeneration oscillator based on the filter output. A digital processing circuit that matches the phase of the carrier reproduction signal with the phase of the carrier of the transmission signal by varying the transmission frequency. An error correction circuit that performs error correction on the data string demodulated by the circuit, and a transmission error rate is obtained from the error correction state of this circuit, and the input C / N is calculated from that value, and the calculated C / N value is obtained. And a carrier reproduction time constant control means for variably controlling a time constant of a control signal sent from the digital processing circuit to the carrier reproduction oscillator based on the basis to optimize fixed deterioration. .