JPH04117833A - Transmitter and digital communication system - Google Patents

Abstract

PURPOSE:To reduce a code error due to deviation in a symbol clock by using a filter with a short impulse response as a band limit filter for the transmitter which superimposes a digital signal subject to band limit onto a carrier and sending the result as a transmission signal. CONSTITUTION:A short impulse response characteristic is provided to 1st and 2nd band limit filters 12a, 12b, which receive respectively 1st and 2nd digital signals 32a, 32b and limit the band for the signals 32a, 32b to output digital signals 33a, 33b subject to band limit. Even when an optimum symbol clock is recovered at a receiver side, inter-code interference takes place at a preceding and succeeding position by each symbol. However, when the optimum symbol clock is not recovered, the inter-code interference component to be caused is each preceding and succeeding symbol only. Thus, the deterioration in the code error characteristic is less when the optimum symbol clock is recovered.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention relates to a digital communication system that performs communication using a band-limited digital signal. The present invention relates to a transmitter that includes a band-limiting filter that outputs a digital signal, and transmits the band-limited digital signal on a carrier wave as a transmission signal.

[Prior Art] When performing digital communication, from the viewpoint of transmission path characteristics or effective frequency utilization, the band of the digital signal is usually limited by a band limiting filter provided on the transmitting side.

Conventionally, it has been common to use a so-called Nyquist filter as this band-limiting filter.

In particular, when phase modulation, amplitude modulation, or the like is used as a modulation method, a Nyquist filter is used in most cases. This Nyquist filter has an impulse response characteristic as shown in Figure 6, and when sampling is performed using the optimal symbol clock, what happens at sample points other than the corresponding symbol? ] It has the characteristic that the distance between the numbers is O. FIG. 6 shows the case where the optimal symbol clock is regenerated, 101 is the response waveform when the optimal symbol clock is regenerated, and 201 is the sample value and code amount interference component when the optimal symbol clock is regenerated. shows.

However, as can be seen from FIG. 6, the impulse response of this Nyquist filter is very long with respect to the time per symbol. Therefore, as shown in Figure 7, if the optimal symbol clock cannot be regenerated on the receiving side, or as shown in Figure 8, if the transmission line has multi-channel characteristics and the signal between the main wave and the interfering wave is If the delay of There was a drawback that it ended up happening. In addition, in FIG. 7, 102 shows the response waveform when the symbol clock is not optimal, 202 shows the sample value and code amount interference component when the symbol clock is not optimal, and in FIG. 8, 103 shows the response waveform when the symbol clock is not optimal. 104 is the component due to the preceding wave of the response wave, 105 is the component due to the delayed wave of the response wave, 203
shows the sample value and the inter-signal interference component when two waves overlap.

In addition, if the receiver uses an adaptive filter type equalizer for reception, the amount of code will interfere with the delay time to be compensated and will take a long time, so more taps are needed to obtain good equalization characteristics. This has the disadvantage of increasing circuit scale and power consumption.

[Problem to be solved by the invention] An object of the present invention is to eliminate the above-mentioned drawbacks and solve problems when the symbol clock reproduced by the receiver deviates from the optimum value,
Even when delayed waves occur due to multi-channel characteristics,
An object of the present invention is to provide a transmitter that causes only short-term code amount interference.

Another problem with this invention is that even if the symbol clock reproduced by the receiver deviates from the optimal value or if delayed waves occur due to multichannel characteristics, only short-term code amount interference occurs. The purpose is to provide a digital communication method.

[Means for Solving the Problems] According to the present invention, a band-limiting filter that receives an input digital signal, limits the band of the input digital signal, and outputs a band-limited digital signal is a A transmitter that transmits a band-limited digital signal on a carrier wave as a transmission signal is characterized in that the band-limiting filter is a filter with a short impulse response.

Further, according to the present invention, the filter includes a band-limiting filter that receives an input digital signal, limits the band of the input digital signal, and outputs a band-limited digital signal, and converts the band-limited digital signal into a carrier wave. an adaptive filter type equalizer that receives the transmitted signal as a receiver input signal, equalizes the receiver input signal, and outputs an equalized signal; and a receiver for demodulating the band-limited digital signal from an equalized signal. A communication method is obtained.

[Operation] In this invention, since a filter with a short impulse response is used as the transmitter's band-limiting filter, even if the symbol clock regenerated by the receiver is not the optimum value, the code amount interference will only occur for a few symbols at most. Since this does not occur, code errors due to symbol clock slippage can be reduced.

Further, since the code amount interference due to the preceding wave or the delayed wave is at most a few symbols, an adaptive filter type fixture can be constructed without requiring a larger number of taps than necessary for the delay time to be compensated.

[Example] Next, the present invention will be explained with reference to the drawings.

Referring to FIG. 1, a digital communication system according to an embodiment of the present invention includes a transmitter which is a modulator using a BPSK (phase shift keying using two phases of 0° and 180°) modulation system] H. This transmitter has a short impulse response band-limiting filter 12 that receives an input digital signal 1, limits the band of the input digital signal 11, and outputs a band-limited digital signal. The multiplier 13 transmits the band-limited digital signal on a carrier wave generated from the oscillator 14 as a transmission signal 15.

This digital communication system further includes a receiver 20.

This receiver 20 receives the transmitted signal 15 as the receiver input signal 2]
an adaptive filter type equalizer 22 which equalizes the receiver input signal 21 and outputs an equalized signal; The demodulator 23 demodulates the I-limited digital signal.

Referring to FIG. 2, a transmitter 30 that can be used in place of the transmitter 10 in the digital communication system of FIG. 1 is shown. This transmitter 30 uses QPSK (
This is a modulator that uses a phase shift keying (phase shift keying) modulation method that uses four phases of integral multiples of 90°. The input digital signal 11 consists of pairs of two symbols each. Assuming that the first symbol of each of these pairs is a and the second symbol is b, the serial/parallel converter 31 transforms the human input digital signal]1 into a first digital signal consisting of the first symbol a of the six pairs. 32a and a second digital signal 32b consisting of six pairs of subsequent symbols b.

The first band-limiting filter 1.2a has a short impulse response characteristic and receives the first digital signal 32a,
The band of the first digital signal 32a is limited, and a first band-limited digital signal 33a is output. Similarly, the second band-limiting filter 12b also has a short impulse response characteristic and receives the second digital signal 32b,
The second digital (by limiting the band of M number 32b,
A band-limited digital signal 33b is output.

The first multiplier 13a puts the first band-limited digital signal 33a on the carrier wave generated from the oscillator 4 to generate a first transmission signal 34a.

The phase shifter 35 shifts the carrier wave generated from the oscillator 14 to 90
° Phase-shifted carrier wave. The second multiplier 13b adds a second band-limited digital signal 33b to this 90° shifted carrier wave to produce a second transmission signal 34b.

The adder 36 adds the first transmission signal 34a and the second transmission signal 34b to obtain a transmission signal 15.

Band-limiting filter in Fig. 1] 2 and band-limiting filter in Fig. 2,
An example of the impulse response of filters 12a and 12b is shown in the third example.
As shown in the figure. Note that FIG. 3 shows a case where the optimal symbol clock is regenerated on the receiving side, 101 is a response waveform when the optimal symbol clock is regenerated, and 201 is a sample when the optimal symbol clock is regenerated. Value and? It shows the interference component between 0 signals. In this case as well, inter-n signal interference occurs at the positions of one symbol before and after each symbol. Therefore,
In such a case, the error characteristic deteriorates compared to the conventional example. However, as shown in Fig. 4, even if the optimal symbol clock is not regenerated, the code amount interference component that occurs is only one symbol each before and after the symbol clock. There is little deterioration in code error characteristics. In FIG. 4, reference numeral 102 indicates a response waveform when the symbol clock is not optimal, and 202 indicates a sample value and a code amount interference component when the symbol clock is not optimal. Moreover, in actual implementation, the optimum symbol clock is almost never recovered, so the code error characteristics are improved compared to the case where a conventional transmitting-side band-limiting filter is used. Furthermore, the symbol clock reproduction accuracy can be made more relaxed in order to obtain the same code error characteristics as compared to the conventional method.

Furthermore, if the transmission path has multiple propagation characteristics, the maximum code amount interference component caused by the interference wave will be within the time of the actual delay time plus one symbol, as shown in FIG. Even when the adaptive filter type equalizer 22 is used, it is possible to perform adaptive equalization processing with the number of taps equal to the allowable delay time plus one symbol, and good equalization characteristics can be obtained. In FIG. 5, ]03 is the response waveform when two waves overlap, 104 is the component due to the preceding wave of the response wave, and 1
05 indicates a component due to a delayed wave of the response wave, and 203 indicates a sample value and code amount interference component when two waves overlap.

[Effects of the Invention] As explained above, according to the digital communication system of the present invention, since the impulse response of the band-limiting filter of the transmitter is short, better code error characteristics can be obtained in practice compared to conventional systems. . Further, by providing the receiver with an adaptive filter type equalizer having a number of taps approximately equal to the allowable delay time, good equalization characteristics can be obtained.

Furthermore, according to the present invention, even when the symbol clock reproduced by the receiver deviates from the optimum value or when delayed waves occur due to the characteristics of multiple propagation paths, digital Communication method can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a digital communication system according to an embodiment of the present invention, and FIG. 2 is a block diagram of another transmitter that can be used in place of the transmitter in the digital communication system of FIG. Figures 3 to 8 show 1 at the receiver.
3 to 5 show waveforms in the digital communication system according to the above embodiment of the present invention, and FIGS. 6 to 8 show waveforms in the conventional digital communication system. 1]...Input digital signal, 12.12a, and 12
b...Short impulse response band-limiting filter, ]3.
13a1 and 1.2 b-... Multiplier, 14-... Oscillator, ]5... Transmission signal, 20 and 30... Receiver,
21... Receiver input signal, 22... Adaptive filter type equalizer, 23... Demodulator, 35... Phase shifter, 36... Adder 36° 101... Optimal symbol clock is regenerated 102...Response waveform when the symbol clock is not optimal, ]3...Response waveform when two waves overlap, 1[]4... Leading response wave Component due to waves, 105 - Component due to delayed wave of response wave, 20] - Sample value and code amount interference component when optimal symbol clock is regenerated, 202 - Sample value and code amount interference when symbol clock is not optimal The sample values and inter-signal interference components when the 203 and 2 waves overlap are shown.

Claims (1)

[Claims] 1. A band-limiting filter that receives an input digital signal, limits the band of the input digital signal, and outputs a band-limited digital signal, and transmits the band-limited digital signal to a carrier wave. What is claimed is: 1. A transmitter for transmitting a signal as a transmission signal by placing a signal on the transmitter, wherein the band-limiting filter is a filter with a short impulse response. 2. It has a band-limiting filter that receives an input digital signal, limits the band of the input digital signal, and outputs a band-limited digital signal, and carries the band-limited digital signal on a carrier wave as a transmission signal. a transmitter for transmitting; receiving the transmission signal as a receiver input signal;
a receiver comprising an adaptive filter type equalizer that equalizes the receiver input signal and outputs an equalized signal, and demodulates the band-limited digital signal from the equalized signal;
1. A digital communication system characterized in that the band-limiting filter of the transmitter is a filter with a short impulse response.