RU2454015C1 - Method for demodulation of frequency-manipulated absolute double-pulse signals used for information transfer via short-wave channel - Google Patents

Abstract

FIELD: information technology.

SUBSTANCE: invention relates to the domain of radio engineering and is designed for demodulation of the frequency-manipulated (FM) signals. Method for demodulation for frequency-manipulated absolute double-pulse signals with large frequency deviation, used to transmit information through the short-wave communication channel consists in that the signals are received as two independent frequency-spaced amplitude-manipulated variations, and for each element of the message received the assessment of signal and noise levels in the first and second halves of the message element is carried out, the (signal+noise)/noise ratio is determined and the decision is made, corresponding to the figure of one or zero, depending on the received estimated values of signal and noise levels in the first and second halves of the message element, at that with two parallel frequency-spaced channels the decision is taken in favour of the channel for which the estimate of (signal+noise)/noise ratio has bigger value.

EFFECT: improvement of reliability of correct reception of message through the short-wave communication channel in the presence of fading and station interference focused on the spectrum.

3 dwg

Description

The invention relates to the field of radio engineering and is intended for demodulation of frequency-manipulated (FM) signals.

A known method of transmitting with a large deviation of the frequency of the FM signals to increase the coefficient of serviceability (KID) of the communication channel. Reception of such signals is carried out as reception of two signals with amplitude manipulation (2 AM).

A disadvantage of the generally accepted method for transmitting signals by the FM method with low deviation is that due to selective fading or when station interferences are simultaneously located on the frequency axis of both signal subcarriers, the reliability of message reception is significantly reduced.

To reduce the likelihood of simultaneous fading and damage by station interference, the parameters of which differ from the parameters of the transmitted signal, it is advisable to perform frequency manipulation of signals at subcarrier frequencies with large deviation, and accept subcarrier signals individually as 2 AM. In this case, the properties of station interference should differ from the parameters of the transmitted signal. A disadvantage of the known methods for receiving AM signals is the difficulty in choosing the optimal threshold based on which a decision is made. In order to optimize the thresholds, an absolute bi-pulse signal (ABS) is used to receive AM signals [1].

An analogue of the proposed method for ABS demodulation is a demodulator of conventional FM signals, in which the signal "squeeze" and "press" are separately received, after which they are compared and a decision is made in favor of a signal with a larger amplitude [2].

In the proposed method, the levels of two neighboring message elements are compared in a similar way and the decision is made in favor of the element whose signal has a higher level.

The prototype of the proposed demodulator is a demodulator, with which it is taken ABS [1]. The ABS described by the authors is obtained by encoding the transmitted binary information sequence (IDP) according to the following algorithm: each transmitted packet (Fig. 1 a) is divided into two parcels that are equal in time, the first parcel coinciding with the original, and the second is the inverse of the first (Fig. .1 b). The received signal is fed to the input of the frequency modulator. From its output, the FM signal with a large deviation is transmitted through the communication channel. Figure 1 b shows the signal modulating the subcarrier in the channel "pressing", and figure 1 c - the signal modulating the subcarrier in the channel "squeeze". In the demodulator, with the help of narrow-band filters, the received signal is divided into two: the “press” signal and the “squeeze” signal. To make a decision about the received bit, weighted addition of signals is performed. The weight of each signal is determined by its level. The signal quality is determined by comparing the bursts on the subcarrier frequencies corresponding to one bit of the transmitted DID. If they are the same, then the signal is delayed and does not go to the input of the adder, if they are different, then the signal goes to the adder, the output of which is connected to the solution circuit. The decision scheme determines the sign of the received bit, taking into account the zero threshold.

The disadvantage of this demodulation method is that when making a decision, the ratio (signal + interference) / interference is not taken into account when comparing signals received at subcarrier frequencies. The data presented in the above article showed that the CT-ABS demodulator described in it loses to the ideal demodulator 2.22 dB.

The objective of the invention is to increase the reliability of the correct reception of the message in the short-wave communication channel in the presence of fading and concentrated on the spectrum of station interference.

The technical result is achieved using the ABS, thanks to which the transmitted FM signal with a large base is received as two independent, duplicating each other channels with AM signals. In the first channel, transmission is carried out at a frequency f _n , in the second - at a frequency f _from , a channel with the maximum ratio (signal + interference) / interference is selected from them. Decisions about the bit value in each channel are made in the order of changing the active and passive sending. In the first channel, the acceptance of the active premise, and then the passive, corresponds to the adoption of the logical “unit”, and the arrival of the passive and then active premise corresponds to the adoption of the logical “zero”. In the second channel, everything happens the other way around: the acceptance of an active premise, and then the passive, corresponds to the adoption of a logical “zero”, and the arrival of a passive and then active premise corresponds to the adoption of a logical “one”.

Figure 2 shows the structural diagram of the demodulator. The demodulator consists of a demodulator of the "press" signal 1, a demodulator of the signal "squeeze" 2 and a decision circuit 3. The demodulator of the signal "press" 1 consists of multipliers of the received and quadrature reference signals 4 and 5, integrators 6 and 7, two half-period detectors 8 and 9 , addition devices 10. The demodulator of the "squeeze" signal 2 consists of the multipliers of the received and quadrature reference signals 11 and 12, integrators 13 and 14, two half-period detectors 15 and 16, the addition device 17. The decision circuit 3 consists of delay lines 18 and 19, determinant I stress levels and the decision of 20 and 21, the decision selecting circuit 22.

The demodulator works as follows. The input signal is input to the demodulator of the “press” signal 1 and the demodulator of the “squeeze” signal 2, is multiplied in elements 4 and 5 by sin (2 · π · f _n ) and cos (2 · π · f _n ), respectively, and in the elements 11 and 12, respectively, on sin (2 · π · f _off ) and cos (2 · π · f _off ), where

f _n - the frequency of "pressing";

f _from - the frequency of "squeezing".

The signals from the outputs of multipliers 4 and 5 are fed to the inputs of integrators 6 and 7, respectively, and from the outputs of multipliers 11 and 12 to the inputs of integrators 13 and 14, respectively. Integrators are kinematic filters [3]. The integration time is equal to the duration of the ABS symbol. From the integrators, the signals are fed to half-wave detectors 8 and 9, 15 and 16. From the outputs of the half-wave detectors 8 and 9, the signals are fed to the inputs of the addition device 10, and from the outputs of the half-wave detectors 15 and 16, the signals are fed to the inputs of the device addition 17. The signals from the output of demodulators 1 and 2 (from the output of addition devices 10 and 17) are sent to solution circuit 3 and delayed by the duration of the ABS symbol by delay lines 18 and 19, respectively. The signals from the output of the addition device 10 and the delay line 18 are fed to the inputs of the determinant of the relationship between the voltage levels and decision-making 20, where the obtained values of the levels of the neighboring components of the ABS are compared and the decision is made on the received information bit in the channel with the "click" signal. The obtained ratio of the levels is equal to the ratio of the voltage of the active element corresponding to the signal emission time (signal + interference level) to the passive element voltage corresponding to the signal absence time (noise level). Similarly, the signals from the output of the addition device 17 and the delay line 19 are fed to the inputs of the determinant of the relationship of the voltage levels and decision making 21, where the obtained values of the signal levels are compared and the results of their comparison make a decision about the received information bit in the channel with the "squeeze" signal. The obtained values of the ratio (signal + interference) / interference from the outputs of the elements 20 and 21 go to the decision selection circuit 22. The final decision on the adopted bit is made after comparing the levels (signal + interference) / interference in the channel with the “pressing” signal and in the channel with a signal of "release". The decision selection circuit 22 makes a decision that corresponds to the channel with the highest ratio (signal + interference) / interference.

Figure 3 shows a graph of the probability of an error in the reception of one bit by the proposed demodulator depending on the ratio of the energy of the transmitted symbol to the spectral density of noise power for one channel and the potential noise immunity of the communication system with FM signals, calculated by the formula [3]:

P _err = 0,5 · exp (-h ^2/2),

where h ² is the ratio of the energy of the active transmission to the spectral density of the interference.

In Fig. 3, line 1 is drawn through the points corresponding to the error probability values of message elements obtained during the computational experiment, and line 2 corresponds to the potential noise immunity of the FM signal.

Figure 3 shows that the proposed method of ABS demodulation practically does not lose in energy to the ideal demodulator, which provides the potential noise immunity of receiving frequency manipulation signals. Compared to the prototype, the energy gain is 2.22 decibels, which the prototype energetically loses to the ideal FM demodulator.

Using the proposed method of demodulation allows you to evaluate the ratio (signal + interference) / interference in the communication channel and based on this relationship to decide on the sign of the symbol. Thanks to the use of ABS, signals are received through two independent frequency-spaced channels and provides messages when signals are not transmitted on one of the frequency subcarriers due to selective fading or when one of the channels is damaged by powerful station noise.

The effectiveness of the proposed method is expressed in increasing the noise immunity of digital radio communication systems using FM signals in multipath conditions, in a complex jamming environment and with small signal / noise ratios.

Information sources

1. Nikolaev G.M., Saltykov O.V. CT-ABS demodulator, implemented on the basis of the signal processor ADSP-2181. // Radio communication technology. - 2000. - Issue 5. - S.60-72.

2. Steshenko VB, Bumagin A.V., Petrov A.V., Shishkin G.V. Analysis and modeling of digital incoherent quadrature demodulator of low-speed FM signals. // Digital signal processing. - 2001.-№3 - S.37-42.

3. Fink L. M. Theory of discrete message transmission. Ed. 2nd revised and supplemented. - Soviet Radio, 1970. - P.728.

Claims (1)

A method of demodulating frequency-manipulated absolutely bi-pulse signals with large frequency deviation used to transmit information via a short-wave communication channel, which means that the signals are received as two independent frequency-spaced amplitude-manipulated oscillations, characterized in that in each element of the received message evaluate signal levels and interference in the first and second halves of the message element, determine the ratio (signal + interference) / interference and make a decision that corresponds to t “one” or “zero” depending on the values of the obtained estimates of signal levels and interference in the first and second halves of the message element, while from two parallel frequency-spaced channels, the decision is made in favor of the channel in which the ratio is estimated (signal + interference) ) / interference is more important.

Images