JPS62125742A - Transmission system for digital signal - Google Patents

Abstract

PURPOSE:To obtain a practical code error rate even when a transmission band width narrower than a required Nyquist band width by applying an error correction coding having a high gain and a 1/2 of coding rate to an input signal and limiting the band of a modulation output number band width using the said signal as a modulation signal to a band smaller than the required Nyquist band of the input signal so as to send the signal while an inter-code interference is given in advance. CONSTITUTION:An input signal of speed Ri is inputted to a convolution error correction coder 2 whose code rate is 1/2 and a quadruple phase modulation is applied to the output at the next stage. The modulation output passes through a narrow band filter 4 of a band width Bw for transmission band limit. The output signal of the filter 4 is subject to frequency conversion into a radio frequency by a transmission circuit 5. The demodulated output passes through a reception band limit filter 8 having a width Bw and is demodulated by a quadruple phase demodulator 9 and inputted to an error correction decoder 10. The decoder 10 is a decoder having the same logic as that of the coder 2, applies error correction to an output signal of the quadruple phase demodulator 9 and sends an output signal of the speed Ri to a terminal 11. The relation between the speed Ri of the input signal and the band width Bw of the transmission signal is selected to be Bw<Bn<=Ri, where Bn is the Nyquist band width.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention is suitable for use in large-capacity digital wireless communication systems. In particular, it is suitable as a method for increasing the frequency utilization efficiency of a satellite communication method.

[Conventional technology]

In order to increase frequency utilization efficiency, multi-level modulation methods such as a polyphase psK modulation method and 16QAM are known. Both methods require a transmission frequency bandwidth in proportion to the signal speed, and in general, if the transmission frequency bandwidth is set narrower than the required Nyquist bandwidth, the amount of code interference will increase rapidly, making it difficult to carry out useful signal transmission. It is not possible.

[Problem that the invention seeks to solve]

In polyphase modulation or multi-level modulation, the equipment or circuits become complicated, and there are still many problems to be solved when applied to nonlinear lines such as satellite communications.

An object of the present invention is to provide a digital signal transmission method that can obtain a practical bit error rate even if a transmission frequency bandwidth is set narrower than the required Nyquist bandwidth.

[Means for solving problems]

In the present invention, on the transmitting side, the rate Ri (bits/Se
The digital input signal in c) is subjected to error correction encoding using a convolutional code word with a coding rate of 2, and the frequency bandwidth BW (Hz) of the transmission signal is determined as Bw < Bn (where Bn is the above digital input signal). The transmission is limited to the required Nyquist bandwidth (Hz), Bn≦Ri), and on the receiving side, the demodulated output is subjected to error correction decoding using logic corresponding to the error correction encoding described above.

[Effect]

A digital input signal with a speed Ri (bits/Sec) is subjected to high gain error correction coding with a coding rate of 2, and the modulated output using this signal as a modulation signal is divided into a frequency bandwidth Bw (Bw<Bn). The digital signal is transmitted in a manner in which code amount interference is given in advance. In this case, the equivalent transmission efficiency of the signal is 'A < R
<%(Ri/Bw).

In a line with a small signal-to-noise ratio, errors caused by transmission path noise become dominant, and errors caused by code amount interference appear to be smaller. Even if the signal is subjected to error correction coding of the descendant gain and the transmission bandwidth is set to be narrower than the required Nyquist bandwidth, this wireless link with a relatively low signal-to-noise ratio, which is close to the actual satellite communication link, A transmission system with a practical bit error rate can be obtained.

The Nyquist bandwidth Bn may be equal to the speed R4 of the input signal or smaller than the speed Ri depending on the number of phases of modulation.

〔Example〕

FIG. 1 is a block diagram of a communication device embodying the present invention.

On the transmitting side, a digital input signal to be transmitted arrives at terminal 1. The speed of this digital input signal is Ri (
bits/Sec). This signal at terminal 1 is input to error correction encoder 2 . The error correction encoder 2 here is a convolutional encoder with a coding rate of 1/2. The output of this error correction encoder 2 is input to a four-phase phase modulator 3, where it is subjected to known four-phase phase modulation. The output of this four-phase phase modulator 3 passes through a narrowband filter 4 for limiting the transmission band.

The passband width of this filter 4 is BW (T-1z). The output signal of this filter 4 is frequency-converted into a radio frequency by a carrier wave in a transmission circuit 5 and is transmitted to a radio transmission path 6.

On the receiving side, the signal on the wireless transmission path 6 is received by a receiving circuit 7 and demodulated. This demodulated output passes through a reception band-limiting filter 8, is demodulated by a four-phase demodulator 9, and is input to an error correction decoder 10. This error correction decoder 10 is a decoder having the same logic as the error correction encoder 2 on the transmitting side, and performs error correction on the demodulated output signal of the four-phase phase demodulator 9.
Sends a digital output signal to 1. The speed of this output signal is R4, which is equal to the input signal.

Here, the feature of the present invention is that the relationship between the speed Ri of the digital signal to be transmitted and the bandwidth Bw of the transmitted signal is set to Bw < Bn ≦ R4, where the Nyquist bandwidth is Bn. There it is.

FIG. 2 is a diagram schematically showing the signal waveform and signal spectrum at each position in the digital transmission system of this embodiment. FIG. 2 Fat is an output signal waveform diagram of the error correction encoder 2 (
bl is an input signal waveform diagram (eye pattern) of the error correction decoder 10.
is the output signal spectrum of the four-phase phase modulator 3 (fd)
is the signal spectrum of each filter output.

In such a digital signal transmission system, as described above, even if the pass frequency bandwidth Bw of the filter 4 is set narrow, signal transmission with a practical bit error rate can be performed.

Next, the results of testing this will be explained.

The test used the system shown in Figure 1. A pseudo transmission line was used for the transmission line 6, and a separate noise generator [3] mixed noise into the transmission line 6 to create a state similar to a practical radio line. The amount of noise mixed in is variable.

The speed of the digital signal input to terminal 1 is Ri=6.
144 Mb/S, and the passband width of filter 4 (and 8) is BW=
8. 192 Mllz. As the error correction code and decoding method, a convolutional code/Viterbi decoding method with a coding rate of R=1/2 and a constraint length of K=7 was used. This error correction encoder and decoder can be easily obtained off-the-shelf using integrated circuits.

Figure 3 shows the test results in graph form. FIG. 3 is a diagram showing the characteristics of an example of the present invention and a comparative example, with the horizontal axis representing the signal-to-noise ratio and the vertical axis representing the code error rate.

The signal-to-noise ratio on the horizontal axis indicates the ratio of the noise power NO to the received power Eb per 1 bit of the signal in decibels.

Curve A shown in FIG. 3 is the test result of the example of the present invention.

Curves B, C, and D are actual measurement results of comparative examples. curve T
is the theoretical value without error correction.

That is, curve A is the result of measuring the bit error rate of terminal 1 to terminal 11 by transmitting and receiving digital signals according to the above parameters in the embodiment of the present invention and varying the amount of mixed noise. Curve B is an example of a practical conventional method using error correction, which has the same information transmission amount as the embodiment of the present invention shown in curve A, and is a BCH code (2 Bit error correction, 3 bit error detection (31, 2
This is an actual measurement result using 2). curve A
Comparing B and B, it can be seen that the signal-to-noise ratio required to obtain a bit error rate of 10-4 is reduced by about 1.7 dB in the method of the present invention compared to the conventional method.

Further, curves C and D are comparative examples of a system for transmitting a digital signal at the same signal speed Ri as in the embodiment of the present invention when error correction is not performed. Curve C is an example in which the passband width of the filter is set equal to the signal speed R1, and this results in a result that approximates the theoretical value T when no error correction is performed. Further, the curved line is an example in which the passband width of the filter is set to be the same as in the embodiment of the present invention, and the value is considerably worse than the theoretical value shown by the curve T.

As shown, the system of the present invention can realize a digital transmission system with a code error rate superior to the conventional system in a region where the signal-to-noise ratio is relatively small.

〔Effect of the invention〕

As described above, according to the present invention, a digital transmission line with a practical bit error rate can be obtained even if the bandwidth of the transmission signal is set narrower than the Nyquist bandwidth. The present invention is highly effective when applied to satellite communication systems in which it is desired to have a relatively low signal-to-noise ratio and extremely high frequency utilization efficiency.

[Brief explanation of drawings]

FIG. 1 is a block diagram of a communication system according to an embodiment of the present invention. FIG. 2 is a diagram showing signal waveforms and signal spectra of each part of the system according to the embodiment of the present invention. Figure 3 shows the embodiment method of the present invention and the comparative example method.
A diagram showing test results. ■... Digital signal to be transmitted is input ◇Moder 1, 2... Error correction encoder, 3... Four-phase phase modulator,
4... Transmission band limiting filter, 5... Transmission circuit, 6
...Wireless transmission path, 7...Reception circuit, 8...Reception:
1? range limiting filter, 9... four-phase phase demodulator, 10...
...Error correction decoder, 11...Terminal to which the received output signal is transmitted, 13...Noise generator.

Claims (1)

[Claims] (1) On the transmitting side, the digital input signal at a rate Ri (bits/Sec) is subjected to error correction encoding using a convolutional code word with a coding rate of 1/2, and furthermore, the frequency bandwidth Bw (Hz) of the transmitted signal is ) is transmitted with the band limited to Bw<Bn (where Bn is the required Nyquist bandwidth (Hz) of the above digital input signal, Bn≦Ri), and on the receiving side, the above error correction encoding is performed from the demodulated output. A digital signal transmission method characterized by error correction decoding using logic.