JPS6331134B2 - - Google Patents

Description

[Detailed description of the invention] The present invention relates to a narrowband digital transmission system.

Mobile wireless communication is a data transmission method with a narrow transmission band and less deterioration due to passing through nonlinear circuits.
It is desired in sanitary communications, etc.

Conventionally, this type of method uses MSK (Minimum Sift
Keying) TFM (Tamed Frequency)
Modulation) GMSK (FM with Gauss filter)
There were methods such as MSK) which band-limited the modulated data, and methods which band-limited the FM modulated data using a Nyquist filter. These methods suffer from spectrum spread, error rate characteristics, and equipment complexity (if synchronous detection is not performed, error rate characteristics deteriorate).
Both have their advantages and disadvantages. FIG. 1 shows an example of a conventional configuration, and FIGS. 2A and 2B show the spectrum spread and error rate characteristics in that case. In other words, the digital data (16K bits/second) output from the data generator 1 is input to the converter 2, which converts binary to 4-values, and becomes a 4-value digital signal (8K symbols/second), which is passed through the low-pass filter 3a (roll-off). It has a square Nyquist characteristic of 0.2, and the cutoff frequency is 4×1.2=
4.8kHz) and is input to the FM modulator 4 with a peak deviation of 4kHz.

The output of the FM modulator 4 propagates through the transmission line 4a, passes through a bandpass filter 5 (Gaussian characteristic with a 3B bandwidth of ±5 kHz) and a limiter 6, and is input to a discriminator 7 where it is demodulated into a baseband signal. The demodulated baseband signal passes through the same low-pass filter 3b as on the transmitting side and is demodulated into original data by a digital demodulator 8.

The characteristics of the above configuration are as shown by curve A in FIG. 2A and curve A in FIG. 2B. Second
The horizontal axis of Figure A is frequency at 5 kHz increments, and the vertical axis is power density at 10 dB increments. Also, in Figure 2B, the horizontal axis shows the signal energy per bit versus noise (EB/
NO), and the error rate (bit error BER) is plotted on the vertical axis. As is clear from FIGS. 2A and 2B, the spectrum spread at a level 50 dB below the peak is 25 kHz, and the BER at EB/NO 15 dB is 3×10 ⁻⁴ .

In order to narrow the FM spectrum, the present invention inserts a low-pass filter on the transmitting side to limit the band below the Nyquist frequency before the FM modulator.
Also, we focused on the fact that the output noise of the discriminator has a triangular characteristic (the noise level in the high frequency range is high), and used the discriminator for demodulation. By employing a discriminator, the device can be simplified and compatible with analog transmission types. Furthermore, the output of the discriminator is applied to the data reproducing device via a low-pass filter, and the overall characteristics of this filter and the filter described above are such that intersymbol interference is within a few symbols and the band is below the Nyquist frequency. do.

FIG. 3 is a block diagram showing an embodiment of the present invention. The digital data (16K bits/sec) output from the data generator 1 is input to the converter 2, which converts binary to 4-level signals, and becomes a 4-level digital signal of 8K symbols/sec, which prevents intersymbol interference during data demodulation. After being input to the precoder 10 to prevent the resulting errors from spreading to subsequent data, the band is divided by a filter 11a (having a square characteristic of dual-binary characteristics) at the cut-off or Nyquist frequency (4kHz). It is input to an FM modulator 4 with a limited peak deviation of 4kHz.
The output of the FM modulator 4 is input to a discriminator 7 via a transmission line 4a, a bandpass filter 5 (Gaussian characteristic with a 3 dB bandwidth of ±5 kHz) and a limiter 6, and demodulated into a baseband signal. The demodulated signal passes through the same filter 11b as on the transmitting side and is input to the digital demodulator 8, where the digital data is reproduced.

The spread of the spectrum of the FM wave and the error rate in this embodiment are as shown by the curves B in FIGS. 2A and 2B. As is clear from FIG. 2A, the spread of the spectrum becomes narrower due to strong band restriction on the transmitting side.

Moreover, since a discriminator is used for FM demodulation, the device can be easily configured and compatible with analog FM transmission. Furthermore, because the output noise has triangular characteristics, the degradation that occurs when using a dual-binary filter in the baseband (approximately 2 dB worse than when using a Nyquist filter) does not occur, and on the contrary, gain occurs. There is. That is, the deterioration in data transmission in the baseband is a value determined from the relationship between the reduction in the determination level due to intersymbol interference and the improvement in S/N due to the reduction in noise bandwidth. In this case, the noise is white (independent of frequency)
This is the evaluation. However, when the noise has triangular characteristics (the higher the frequency, the higher the level), the reduction rate in the high range is greater than in the case of white, that is, the S/
N gets better.

[Brief explanation of the drawing]

Figure 1 is a block diagram showing the conventional transmission method, Figures 2A and 2B are characteristic diagrams showing the spectrum and error rate of FM modulated waves obtained by the conventional method and the present method, and Figure 3 is the diagram of the present invention. FIG. 1 is a block diagram showing one embodiment of the invention. 1...Data generator, 2...Converter, 3a, 3
b...Nyquist filter, 4...FM modulator,
5... Band pass filter, 6... Limiter, 7
... Discriminator, 8 ... Digital demodulator, 10 ... Precoder, 11a, 11b ... Filter.

Claims (1)

[Claims] 1 A transmission system that FM-modulates digital data via a low-pass filter whose cut-off frequency is lower than the Nyquist frequency and sends it to the transmission path; and a transmission system that demodulates the transmitted data of the transmission system with a discriminator and whose cut-off frequency is the Nyquist frequency. An FM data transmission system consisting of a receiving system that outputs data via the following low-pass filter.