RU2752876C1 - Method and apparatus for transmitting and receiving phase-shift keying in command control radio link using ofdm technology - Google Patents

Abstract

FIELD: data transmission.

SUBSTANCE: invention relates to the field of systems for transmission of information over a radio channel and can be used in constructing command control radio links (CCRLs) operating with binary phase-shift keying signals. To realize the described result, the method is characterized by the fact that the CCRL control command is transmitted in form of an OFDM symbol formed on the transmitting side after signal modulation using the inverse discrete Fourier transform (IDFT), and on the receiving side, DFT is executed in the extended frequency band before demodulation, considering the frequency instability of reference generators of the receiver and transmitter, and an m-channel scheme is used in the receiving apparatus to minimize the impact from the fractional frequency shift ε_ƒ, wherein the OFDM symbol with an excess DFT length is analyzed in the decoding apparatus to correctly retrieve information about the presence of the transmitted control command therein.

EFFECT: providing increased interference resistance of the command control radio link.

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Description

The invention relates to the field of systems for transmitting information over a radio channel and can be used in the construction of command radio control lines (KRU) operating with binary phase shift keying signals.

Known radio links, providing the transmission of telemetry, communication, command and other information using various types of modulation and coding [1]. Also known are a set of quickly deployable technical means of physical protection BSF3-04.10, an automated complex of reconnaissance and signaling means AKRSS and electronic means RP3-8 for detonating charges of explosives (explosives) and engineering ammunition (UPS), imitation of artillery fire and air strikes during military exercises [2, 3, 4], the main disadvantage of which is the work with signals of frequency telegraphy (FT). The most effective, from the point of view of noise immunity, are phase-shift keying (PM) signals, the work with which allows one to approach potential noise immunity [5]. Working with FM signals places high demands on the system in terms of frequency instability and synchronization of signal reception. Switchgears as a rule operate at a low signal-to-noise ratio, which does not allow considering classical synchronization schemes when receiving PM signals.

The closest in technical essence to the claimed invention is a method and device for receiving frequency stabilized signals with binary phase shift keying at an unknown initial phase [6], selected as a prototype. This device allows you to receive phase-shift keyed signals that do not require frequency adjustment, but only phase adjustment.

The device for receiving frequency-stabilized signals with binary phase shift keying with an unknown initial phase contains an analog-to-digital converter, the output of which is connected to the input of a digital matched filter, the outputs of which are connected to three structurally identical phase control channels, each of which consists of a series-connected multiplier signals, a low-pass filter, an amplitude limiter, a comparison circuit, an integrator and a threshold device, the second input of each signal multiplier is connected to the output of the reference generator, the second input of each comparison circuit is connected to the output of the code combinations storage unit, the outputs of the threshold devices of all channels are connected to the input of the logical devices OR.

The most significant disadvantage of this device is that its implementation requires the use of highly stabilized quartz oscillators, in particular, thermostated ones, which have a high current consumption, which is unacceptable for portable switchgear.

The technical result of the invention is to provide the possibility of receiving phase-shift keyed signals without synchronization and without performing sufficient stabilization of the reference oscillators of the receiver and transmitter. The relevance of removing restrictions on the use of highly stabilized reference oscillators of the receiver and transmitter is due to the need to implement a portable switchgear with minimal current consumption, while ensuring high noise immunity of the system, which can be achieved when receiving phase-shift keyed signals without synchronization.

This technical result is achieved by:

1) The control command is transmitted in the form of an OFDM symbol, which is formed on the transmitting side after modulating the signal using inverse discrete Fourier transform (IDFT), and on the receiving side, before demodulation, DFT is performed in the extended frequency band, taking into account the frequency instability of the receiver and transmitter reference oscillators.

2) The receiving device uses a channel m diagram to minimize the effect of fractional frequency offset ε _ƒ.

3) The decoding device analyzes an OFDM symbol that has an excess DFT length to correctly extract information about the presence of a transmitted control command (CC) in it.

Recently, high-speed wireless radio communication systems using OFDM signals (orthogonal frequency division multiplexing) have been intensively developed [7]. OFDM technology implies that the information signal is initially represented in the frequency domain, and not in the time domain as in conventional communication systems. It is this feature that will allow realizing the reception of signals with binary phase shift keying without synchronization in the switchgear. First, the signal is modulated and then subjected to IDFT, as if transforming it into the time domain and transmitted over the air. On the receiving side, the signal is DFT, as if converted back from the time domain to the frequency domain and demodulated.

To reproduce the transmitted OFDM signal, the receiver must be synchronized with the transmitter in frequency, phase and time [8]. The shift of the carrier frequency of the received signal can occur due to the Doppler effect in mobile communications and the frequency mismatch of the reference oscillators in the transmitter and receiver [9]. In the received signal y [n], this causes a phase shift:

where ε = ƒε / Δƒ is the normalized carrier frequency offset, equal to

the ratio of the carrier frequency shift ƒ _ε to the distance between the subchannels Δƒ;

h [n] - channel impulse response;

w [n] - additive white Gaussian noise (AWGN);

N is the character length in samples.

The frequency shift ε can be represented in the form of two parts: ε _{i of the} multiple distance between subchannels and the remainder ε _ƒ [8, 9].

When the carrier frequency is shifted by ε _i, orthogonality is not violated and, thus, interchannel interference (ICI) does not occur, however, this makes demodulation impossible, since information is incorrectly extracted from subchannels [10].

A small amount of information is transmitted to KRU, in contrast to communication systems. In radio control lines, a statistical criterion for optimal signal detection is used - the Neumann-Pearson criterion, according to which, first of all, a given and sufficiently small probability of false reception of a command P _l = const is provided, and then all measures are taken to obtain the maximum probability of correct reception of a control command P _k = max. Let's say that we have a single-command radio link, so the decoding device constantly analyzes the input sequence for the presence of the received CU in it, which ensures time synchronization.

Таким образом принятый вектор данных Y_i получим вычислением R - точечного ДПФ от этого вектора:To correctly extract information from subchannels with a multiple frequency shift ε _i, it is necessary to have the DFT length slightly larger than the length N of the OFDM symbol, which will provide signal reception in a wider frequency band, taking into account the frequency instability of the reference oscillators

Thus, the received data vector Y _i will be obtained by calculating the R - point DFT from this vector:

- минимальная длина ДПФ, позволяющая корректно извлекать последовательность данных из принятого OFDM символа при наличии кратного сдвига частоты ε_i, возникающего в условиях нестабильности опорных генераторов приемника ξ₁ и передатчика ξ₂;where

- the minimum DFT length that allows to correctly extract the data sequence from the received OFDM symbol in the presence of a multiple frequency shift ε _i arising under the conditions of instability of the reference oscillators of the receiver ξ ₁ and transmitter ξ ₂ ;

T _k - the duration of the control command (OFDM symbol);

N is the number of subcarriers;

ƒ _c - carrier frequency of the signal.

Let's consider an example that explains the essence of the proposed approach. Let KU with duration T _to be an OFDM symbol consisting of N information bits modulated with binary phase shift keying, presented in figure 1. The signal is received without synchronization, in the extended frequency band of the OFDM symbol Δƒ, taking into account the maximum possible frequency deviation of the reference oscillators. Then, the received OFDM symbol will have the form shown in figure 2, from which it can be seen that the information bits N are not distorted due to the excess DFT length, but only have a shift, in our case to the right, due to the instability of the frequencies of the reference oscillators and the lack of tuning of the receiving device by frequency. In our switchgear, one KU is transmitted, which is known in advance on the receiving side and is analyzed in the comparison scheme. Thus, it is necessary to pass the redundant data sequence obtained after the DFT of length R through an additional shift register of length R to the comparison circuit to ensure correct identification of the QU of length N.

Fractional frequency shift ε _ƒ leads to interchannel interference (ICI) and loss of orthogonality between subcarriers. This is due to the fact that each subchannel is formed by an IDFT, which has the form of a sinc function and, with a frequency shift, leads to interference from neighboring subchannels. In [11], a relation is given that allows one to determine the value of the maximum degradation of SNR (signal-to-noise ratio) Δγ _max for each specific γ value in the ABGN channel at a given value of ε _ƒ :

In [10], the effect of the frequency shift on the signal phase in the time domain is shown, namely, that the phase difference increases linearly with time, and for ε _ƒ > 0.5, the phase difference exceeds π within one symbol, which results in a phase ambiguity. The implementation of the switchgear threshold device, which makes decisions in direct and inverse modes, allows us to avoid phase ambiguity.

In a switchgear, as a rule, the probability of a bit error is of the order of P _b = 0.04 ... 0.3, which corresponds to a low SNR of ≈-10 ... 2 dB for binary phase shift keying. Effect of the fractional frequency offset ε _ƒ on reception quality according to expression 4 shown in Figure 3, which shows that the implementation of the three reception channels spaced by an amount ε _ƒ = 0.125 ensuring that degradation of reception quality is not more than 0.4 dB, which is acceptable.

Figure 4 shows a diagram of a device for receiving and transmitting phase-shift keying signals in a command radio link using OFDM technology, containing an information source 1, a modulator 2, an inverse discrete Fourier transform unit 3, a digital-to-analog conversion unit 4, an I / Q modulator-frequency converter 5 , transmitting antenna 6, receiving antenna 7, I / Q-demodulator-frequency converter 8, analog-to-digital conversion unit 9, digital band-pass filter 10, code combination storage unit 18, OR gate 17 and three identical signal processing channels, each of which consists of a discrete Fourier transform block 11, a demodulator 12, a shift register 13, a comparison circuit 14, an integrator 15 and a threshold device 16.

The device for receiving and transmitting phase shift keying signals in a command radio link using OFDM technology operates as follows. On the transmitting side, from the information source 1, the QC is read, which is a binary codeword of length N and duration T _k . Next, the QS is modulated by binary phase shift keying in the modulator 2 and is fed to the input of the inverse discrete Fourier transform 3, where the input symbols are converted into a set of subcarrier samples, thus the QC is converted into one OFDM symbol. Next, discrete samples are fed to the digital-to-analog conversion unit 4, where the quadrature components of the signal are converted into analog form. In the I / Q-modulator-frequency converter 5, the spectrum of the OFDM signal is transferred to the required carrier frequency ƒ _c in accordance with the specified frequency range, after which it is amplified and emitted into the air through the transmitting antenna 6.

From the receiving antenna 7, the received OFDM signal is fed to the input of the I / Q-demodulator-frequency converter 8, in which preliminary frequency selection, amplification and transfer of the signal spectrum to the zero frequency are performed with the extraction of the I and Q quadrature components, which are then sequentially fed to the block analog-to-digital conversion 9, where they are periodically sampled and converted into digital form. Discretized samples with a sampling rate ƒ _cch1 are fed to the input of a digital bandpass filter 10, which has a bandwidth equal to the bandwidth of the OFDM symbol N / T _k , expanded taking into account the instability of the reference oscillators of the receiver ξ ₁ and transmitter ξ ₂ , and will be Δƒ _cf = N / T _k + (ξ ₁ + ξ ₂ ) ƒ _c . Next, the signal is fed to the input of the discrete Fourier transform 11, each of the three parallel signal processing channels. Each channel is characterized in that a discrete Fourier transform unit 11 has a frequency shift of complex exponentials, in accordance with the expression 5, ensuring reduction effect fractional frequency offset ε _ƒ to an acceptable level.

where m is the number of the receiving channel;

- минимальная длина дискретного преобразования Фурье, позволяющая корректно извлекать последовательность данных из принятого OFDM символа при наличии кратного сдвига частоты ε_i, возникающего в условиях нестабильности опорных генераторов приемника ξ₁ и передатчика ξ₂ при передаче команды управления длительностью Т_к на частоте несущего сигнала ƒ_c.

is the minimum length of the discrete Fourier transform, which makes it possible to correctly extract the data sequence from the received OFDM symbol in the presence of a multiple frequency shift ε _i arising under the conditions of instability of the reference oscillators of the receiver ξ ₁ and transmitter ξ ₂ when transmitting the command to control the duration T _k at the frequency of the carrier signal ƒ _c ...

The excessive minimum length of the discrete Fourier transform R allows the multiple frequency shift ε _i to be taken into account. The redundant data sequence of length R from the output of the discrete Fourier transform block 11 is fed to the input of the demodulator 12, where binary phase demodulation is performed, and the output stream of information bits is fed to the input of the shift register 13 of length R. At the output of the shift register 13, binary sequences of N samples are formed, having a shift by one sample, which are read with a frequency of ƒ _сч2 = f _сч1 * (R-N + 1) for R> N, and are fed to the input of the comparison circuit 14. The received binary codeword of length N is compared with the reference codeword arriving from the block storing code combinations 18, which is common to all channels. The result of the comparison of the control command is accumulated in the integrator 15 and the result is fed to the input of the threshold device 16. The operation of the threshold device 16 of each channel is organized in direct and inverse modes, which ensures that the effect of the fractional frequency shift in the range of no more than ε _ƒ ≤0.5 is taken into account. A positive decision on the reception of the KU in the channel, when the required number of elements of the KU block coincides or the same number of errors is made, which will correspond to the inverse reception of the KU. All outputs of the processing channels are reduced to one logical block OR 17, where the final decision is made on the reception of the CU in accordance with a positive decision on the reception of the CU in at least one of the channels.

Thus, the device for receiving and transmitting phase shift keying signals in the command radio control line using OFDM technology allows the reception of binary phase shift keying signals without synchronization and without high-precision stabilization of the receiver and transmitter reference oscillators, which ensures that there is no need for thermostated crystal oscillators and the transition to low-consumption reference generators with sufficient frequency instability, while providing increased noise immunity of the command radio control line in comparison with systems using frequency telegraphy signals and signals with relative phase shift keying.

Literature

1. Teplyakov N.M., Kalashnikov I.D., Roshchin B.V. Radio links of space information transmission systems. Ed. THEM. Teplyakov. Textbook for universities. - M .: "Sov. Radio ", 1975. - 400 p.

2. BSF3-04.10. Manual. DAKZH.421452.004 RE.

3. AKRSS. Manual. DAKZH.421452.005 RE.

4. Electronic means of detonation of charges RP3-8. - M .: VIU, 2000 .-- 52 p.

5. Ashimov N.M. // Noise immunity and noise immunity of radio control lines. Moscow: VIU, 2000.372 p.

6. Method and device for receiving frequency stabilized signals with binary phase shift keying with an unknown initial phase: US Pat. 2714302 Rus. Federation / Leushin A.V., Komyakov A.V., Feduro V.V. Publ. 02/14/2020, Bul. No. 5.

7. John A.C. Bingham. "Multicarrier Modulation for Data Transmission: An Idea Whose Time Has Come", IEEE Communications Mag., May 1990, p. 5-14.

8. OFDM technology. Textbook for universities / M.G. Bakulin [and others]. - Hot line - Telecom, 2019.-352 p., Ill.

9. W. Aziz, E. Ahmed, G. Abbas, S. Saleem, Q. Islam. Performance Analysis of Carrier Frequency Offset (CFO) in OFDM using MATLAB // Journal of Engineering (JOE). - 2012. - Vol. 1, no. 1. - P. 5-10.

10. Batyrev I.A. Estimation of the influence of the carrier frequency shift on the quality of the received OFDM signal // Ohm. scientific. vestn. 2015. No. 3 (123). S. 259-262.

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Claims (2)

1. A method for receiving and transmitting phase shift keying signals in a command radio link using OFDM technology, based on transmitting a control command in the form of an OFDM symbol using binary phase shift keying, as well as parallel processing of an OFDM signal in an m channel system, the operation of a threshold device in a forward and in the inverse mode and making a positive decision on receiving a control command with a positive receipt of a control command in at least one of the m channels, characterized in that a three-channel system for receiving OFDM signals with binary phase shift keying without synchronization is used, where the discrete Fourier transform blocks of each channel have different fractional frequency shifts of complex exponentials with an interval of 0.125⋅Δƒ _{raise the} distance between subcarriers

, which ensures the reduction of the influence of the fractional frequency shift, which takes values of 0≤ε _ƒ ≤0.5, to the minimum permissible level of no more than 0.4 dB when the command radio control lines operate with a signal-to-noise ratio of no more than SNR = 2 dB, as well as the fact that excess minimum length of the discrete Fourier transform, equal to

, provides the reception of the OFDM symbol taking into account the multiple frequency shift ε _i , which is due to the instability of the frequencies of the reference oscillators of the receiver ξ ₁ and transmitter ξ ₂ when transmitting the command to control the duration T _to at the frequency of the carrier signal ƒ _c , where the leveling of the multiple frequency shift ε _i is performed in counting the run and comparison of the expected control command of length N over the entire length R of the received sequence discretely with a shift step equal to one sample. 2. A device for receiving and transmitting phase-shift keying signals in a command radio control line using OFDM technology containing an analog-to-digital conversion unit, the output of which is connected to the input of a digital band-pass filter, the output of which is connected to the input of each of the three signal processing channels, in each signal processing channel the output of the comparison circuit is connected to the input of the integrator, the output of which is connected to the input of the threshold device, the output of the threshold device of each of the signal processing channels is connected to the first, second and third inputs of the OR logic gate, respectively, and the second inputs of the comparison circuits of each of the channels are connected to the output of the storage unit code combination, characterized by the presence of a transmitting device that includes a series-connected information source, a modulator, an inverse discrete Fourier transform, a digital-to-analog conversion unit, an I / Q modulator-frequency converter and a transmitting antenna, a receiving device equipment, including a series-connected receiving antenna, an I / Q-demodulator - a frequency converter, the output of which is fed to the input of an analog-to-digital conversion unit, and each of the signal processing channels includes a series-connected block of discrete Fourier transform, a demodulator and a shift register, the output of which is connected the first input of the comparison circuit, wherein the codewords of length N read from the shift register R the length of frequency ƒ _sch2 = ƒ _sch1 * (R-N + 1) R> N, where ƒ _sch1 - sampling rate analog-to-digital converter, input of the discrete Fourier transform of each channel is connected to the output of the digital bandpass filter, and the blocks of the discrete Fourier transform of each of the different channels differ fractional frequency offset of complex exponentials intervals 0.125⋅Δƒ _lifted distance between subcarriers

, where T _k is the duration of the control command.

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