RU2475962C2 - Method to transfer and receive digital information in tropospheric communication lines - Google Patents

Abstract

FIELD: radio engineering, communication.

SUBSTANCE: in the method based on diversity of signals by frequency they use time-compacted radiated signals with linear frequency modulation (LFM), with delays that depend on components of a binary information signal, the received signal is processed on matched compression filters, and useful information is identified with the help of selecting strobes and clock pulses induced by an LFM pilot signal.

EFFECT: reduced effect of fading in transfer and reception of information in digital tropospheric communication lines, higher speed of digital information transfer and communication noise immunity with minimum quantity of signal generation and processing channels.

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Description

The invention relates to the field of radio communications and can be used to develop tropospheric radio stations.

A known method of dual spatially separated reception [Narytnik T.N. Radio relay and tropospheric transmission systems: Textbook. Pos. - K .: Concern “Vidavnichy Dim“ In Yure ”, 2003. - 336 p., P. 191], in which a group signal from channel-forming equipment is received at one transmitter of the transmitting station, and the receiving station has two antennas spaced one from the other 50-100 wavelengths in a direction perpendicular to the direction of the correspondent. The fast Rayleigh fading of the radio signals at the output of the antennas in this case is almost uncorrelated. Therefore, one of the receivers connected to the diversity antennas, but operating at a common load, is almost always in better reception conditions than the other. Space diversity is achieved using a single transmitter, however, two antenna-feeder systems are required, which are expensive because of the high antenna gain and complexity.

A known method of angular diversity of signals [Tropospheric communication. / L.I. Yakovlev, G.V.Dedyukin, E.S. Kagramanov and others. - M .: Military Publishing House, 1984. - 256 p., P.31], which is carried out using two irradiators, shifted relative to the focal plane single parabolic mirror. As a result of this, the directivity pattern of such an antenna has a two-leaf structure, which makes it possible to obtain two re-emission volumes spaced in space, the presence of which leads to uncorrelated fading of the signals in each of them. However, when implementing angular diversity, it is necessary to combine a reasonable compromise between reducing the signal level due to the removal of the irradiator from the focal plane and the departure of the reemission volume from the line connecting the receiving and transmitting points, and the increase in correlation with a decrease in the diversity angle. This affects the complexity of the equipment and its price.

A known method of automatic selection of the optimal operating frequency, implemented in the tropospheric station AN / TRC-105WX, developed by Motorola [Tropospheric communication / L.I. Yakovlev, G.V.Dedyukin, E.S. Kagramanov and others. - M .: Military Publishing House, 1984. - 256 p., P.67]. At this station, the propagation conditions of each of the 16 possible frequencies are analyzed 33 times per second. The spectrum analyzer determines which of the analyzed frequencies the signal has the least attenuation. The selected frequency is fixed at the points of transmission and reception, and work is carried out at this frequency. Auto-selection of the optimal frequency allows you to do with one antenna, one transmitter and one receiver. However, the transition time to optimal frequencies affects the speed of information transfer in the direction of lowering the latter.

Closest to the proposed is a reception method with diversity in frequency [Narytnik T.N. Radio relay and tropospheric transmission systems: Textbook. Pos. - K .: Concern “Vidavnichy Dim“ In Yure ”, 2003. - 336 p., P.192], adopted as a prototype. In dual reception with frequency diversity per antenna, a signal from two transmitters operating at different frequencies is fed through a diplexer to which a group signal from channel-forming equipment is received through a common modulator. The receiving station also has one antenna, to which two receivers are connected through a diplexer, tuned to the corresponding frequency transmitters. With a certain separation between these preset frequencies, the fading of the radio signals at the inputs of the receivers is almost uncorrelated, which ensures a significant reduction in the effect of fast fading of the radio signal on the reception quality. Twice the frequency diversity allows you to bypass one antenna, but you need two transmitters and two receivers with separate local oscillators.

To increase noise immunity, R-multiple (R> 2) frequency diversity is required; but then R transmitters and R receivers are required. Powerful transmitters and sensitive receivers are always complex, and very high demands are made on the stability of the frequency of their pathogens and local oscillators.

The objective of the proposed method is to reduce the effect of fading during transmission and reception of information in digital tropospheric communication lines, increasing the transmission speed of digital information and noise immunity of the communication with a minimum number of channels for generating and processing signals.

To solve the problem, in a method based on frequency diversity of signals, time-compressed linearly modulated (LFM) emitted signals are used, with delays depending on the components of the binary information signal, the received signal is processed on agreed compression filters and useful information is extracted with using selective gates and clock pulses induced by the LFM pilot signal.

The proposed method is as follows.

Suppose that on the transmitting side, when transmitting logical “1” (Fig. 1), the start pulse of the LFM signal (see Fig. 2) is shifted by the delay Δτ ¹ to the right along the time axis, and when transmitting logical “0” (Fig. 1) ) - on Δτ ^{0 to the} left along the time axis (see figure 2).

Then, the analytical expression for the information signal (see figure 3) takes the form

when transmitting a logical "1", or

when transmitting a logical "0",

where ω ₀ = 2πF ₀ is the initial angular frequency of the information LFM signal;

- скорость перестройки частоты;

- frequency tuning frequency;

and

Δω = 2π · ΔF is the frequency deviation;

τ _AND is the duration of the information LFM signal, which is selected from the condition

T is the duration of the logical "0" (logical "1") binary information signal (figure 1).

The laws of changing the frequency of the emitted signals are shown in figure 3.

When receiving information LFM radio pulses (see Fig. 4), time-compressed signals (see Fig. 5) appear at the output of the compression filter, having time shifts relative to the autocorrelation function [C. Cook, M. Bernfeld. Radar signals. Theory and application. Translation from English edited by V.S. Kelson. - M .: “Owls. Radio ”, 1971, Fig. 6.2, p.151].

The impulse response of the compression filter g (t) (see Fig. 4) has the form

In real conditions, the information LFM signal arriving at the input of the receiving side is not an exact copy of the emitted signal due to fading. However, due to the fact that fading is uncorrelated for frequencies spaced 2 MHz apart [Tropospheric communication. / L.I. Yakovlev, G.V.Dedyukin, E.S. Kagramanov et al. - M .: Voenizdat, 1984. - 256 p., P. 65], the probability of simultaneous deep fading of the signal at these frequencies is much less than the probability equally deep fading on each of them individually.

Based on [C. Cook, M. Bernfeld. Radar signals. Theory and application. Translation from English edited by V.S. Kelson. - M .: “Owls. radio ”, 1971, Fig. 1.8 and (1.19), p.24] when using a compression filter, the ratio of the peak powers of the input and output signal is equal to the compression parameter τ _AND · ΔF. That is, for example, when a 50% loss of the useful signal band due to fading occurs, the level of the compressed pulse decreases by only 1.41 times. Therefore, through the use of a broadband (LFM) signal with a band (corresponding to frequency deviation ΔF) of tens of MHz, reliable communications are ensured.

The selection of the information signal (logical "1" and "0") is carried out using selective pulses (gates). For this purpose, the input of the receiving side receives (see FIG. 3) a LFM synchronizing radio pulse (“PS” pilot signal) to guide the gates to information signals (see FIG. 4).

Pilot parameters

are consistent with the impulse response of the compression filter (3). Duration of compressed pilot, logical “0” and logical “1” (see FIG. 5) when using a matched compression filter

The beginning of the formation of the first gate (τ _SI ⁽¹⁾ ) is delayed (see Fig.6) relative to the pilot signal for the duration of the form (T _BL )

The duration of the first gate is selected from the condition

The leading edge of the second strobe (τ _SI ⁽²⁾ ) is delayed relative to the trailing edge of the first strobe (see Fig. 7).

The duration of the second gate (τ _SI ⁽³⁾ ) is selected from the condition

The first gate is designed to highlight the logical signal "0", and the second - logical "1" (see Fig.8).

To extract an information signal (with a bit duration T) from the signal shown in Fig. 8, clock pulses shifted by 772 relative to the pilot signal (see Fig. 9) with a repetition period equal to T. The resulting information signal received at the receiver output presented in figure 10.

One bit of the binary information signal (Fig. 1) after the compression filter corresponds to the time interval XY (Fig. 11). Between the end Y of this segment and the beginning Z of the next segment (Fig. 11) corresponding to the next bit of the binary information signal (Fig. 1), it fits (Fig. 11) K of lengths XY (K corresponds to the integer part of the ratio of the segments YZ and XY).

Therefore, it is possible to organize the transmission and reception of N binary information signals at the same time (i.e., to temporarily compress the signals) by increasing the number of channels for generating and processing signals by N times:

Consider the question of the practical implementation of the method.

The transfer of the binary modulated LFM signal into the microwave range with subsequent amplification and radiation, as well as the reception of the microwave signal with its subsequent amplification and conversion to an intermediate frequency can be implemented in a standard way (for example, as in the prototype method).

The received and amplified signal at an intermediate frequency for further processing is digitized using an analog-to-digital converter (ADC). An intermediate frequency signal prepared for transmission can be converted using a digital-to-analog converter (DAC).

The formation of the chirps for launching the chirp signals, the formation of chirp signals, matched filtering and compression of the received signal at an intermediate frequency, the formation of selective gates and clock pulses can be implemented on programmable logic integrated circuits (FPGAs).

Here is an example of the used elemental base: LTC2207IUK ADC (Linear Technology), AD9786BSV DAC (Analog Devices), EP3C16Q240C8N FPGA (Altera).

Thus, the use of a set of time-shifted broadband (LFM) signals and its processing allows us to reduce the effect of fading during transmission and reception of information in digital tropospheric communication lines.

In addition, it is known [Radio-electronic systems: Fundamentals of construction and theory. Directory. Ed. 2nd, rev. and add. / Ed. J.D. Shirman. - M .: Radio engineering, 2007. - 512 p.: Ill., P. 314], that the extension of the frequency band of signals (note, in particular, the use of the LFM signal) allows to obtain

- increasing the information content of radio electronic means (RES) of information transmission, immunity from interference, electromagnetic compatibility of RES and secrecy of radiation;

- Reducing the likelihood of pursuit of military distribution zones.

The proposed method is new, since neither methods nor devices are known from publicly available information that allow for the simplest (one antenna, one receiver, one transmitter) implementation, but through the use of broadband signals at an acceptable information transfer rate, effectively reduce the effect of fading on quality reception.

Claims (1)

A method of transmitting and receiving digital information in tropospheric communication lines, based on the separation of signals in frequency, characterized in that digital binary information with a clock frequency T is transmitted in time by linear frequency modulated (LFM) signals, which serve for frequency separation, which are triggered by strobe pulses start with delays depending on the components of the binary information signal, process the received signal on a compression filter coordinated with the LFM pilot signal, isolate the signal with logical levels “0” and “1” with the help of selective gates, from which an information binary signal with a clock frequency T is formed using clock pulses.

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