Reliable covert communication method based on SCFDM-TDCS system

Abstract

The invention provides a reliable covert communication method based on an SCFDM-TDCS system. The reliable covert communication method comprises the following steps: S1, carrying out coding interleaving on information needing to be communicated; s2, sampling an electromagnetic environment to obtain a primary function amplitude; s3, generating a primary function by using the primary function amplitude, completing the modulation of the coded and interleaved data through CCSK modulation and subcarrier distribution based on the primary function, and generating a TDCS sending signal; s4, performing SCFDM modulation on the TDCS sending signal by using an SCFDM sending module; and S5, after passing through the channel, inputting the received signal into an SCFDM receiving module to carry out SCFDM demodulation, and sequentially carrying out CCSK demodulation, de-interleaving and decoding on the SCFDM demodulation signal to obtain sent useful information. According to the invention, narrow-band interference such as sweep frequency interference and narrow-band noise interference in a communication channel can be effectively resisted.

Description

Technical Field

[0001] The present invention relates to the technical field of hidden communication, in particular to a reliable hidden communication method based on an SCFDM-TDCS system. Background Art

[0002] Covert communication is also known as low probability of detection (LPD) communication. In military communications, covert communication relies on the concealment of communication signals to ensure communication security, hindering enemy positioning, tracking, and surveillance. Covert communication requires that the transmission itself is difficult to detect, making it even more difficult to intercept the signal and obtain the information it carries. Typically, covert communication methods in wireless environments involve embedding information within interfering signals or hiding information within noise. Transform Domain Communication System (TDCS) is a well-established covert communication system. The concept of TDCS dates back to 1988, when German proposed using known spectrum occupancy information to modify the waveform of a direct sequence spread spectrum signal to avoid interference. In 1991, Andren proposed a low probability of intercept (LPI) communication system. This system uses transform domain signal processing techniques to hide the transmitted signal within noise, making it difficult for the enemy to detect. Subsequently, researchers proposed TDCS technologies based on the wavelet domain and fractional Fourier transform domain. In 2006, Han Chuan and others from the University of Electronic Science and Technology of China proposed a transform domain communication system based on Orthogonal Frequency Division Multiplexing (OFDM). Since then, more and more research has been conducted on OFDM-TDCS systems.

[0003] The basic modulation waveform used by TDCS to transmit information is not designed in the time domain, but synthesized in the transform domain (frequency domain or other abstract domain). Its basic concept is to dynamically change the spectrum occupied by the transmitted information based on the ambient spectrum, which has good anti-interference and concealment. However, as a single-carrier transmission system, TDCS has low communication efficiency, and the waveform shaping and filter design in implementation are relatively difficult. Therefore, OFDM technology and TDCS technology are combined. In addition to maintaining the characteristics of OFDM technology, the OFDM-TDCS system can avoid interference and select available frequency bands for data communication, thereby improving spectrum utilization. The OFDM-TDCS system ensures normal communication when subcarriers are not continuously available, and has better transmission performance gains.

[0004] Currently, research on OFDM-TDCS systems covers aspects such as coding, interleaving, synchronization, modulation and demodulation, as well as the impact of channels and random versus continuous subcarriers. However, few studies have examined the impact of interference signal types on OFDM-TDCS systems. During communications, narrowband interference has a much greater impact on communication systems than noise. For interference environments such as swept-band interference and narrowband noise, the OFDM-TDCS solution, which estimates amplitudes and assigns random phases across the entire frequency band, wastes system resources, making the OFDM-TDCS system's signals more easily detectable, less discreet, and less effective at preventing interference. Summary of the Invention

[0005] The present invention aims to provide a reliable covert communication method based on the SCFDM-TDCS system. Compared with the traditional OFDM-TDCS system, the method can solve narrowband interference such as sweep frequency interference and narrowband noise interference in the communication channel.

[0006] The present invention provides a reliable covert communication method based on an SCFDM-TDCS system, comprising the following steps:

[0007] S1. Encode and interleave the information to be communicated;

[0008] S2, the transmitter and receiver sample the electromagnetic environment and obtain the basis function amplitude;

[0009] S3. Generate a basis function using the basis function amplitude, modulate the coded interleaved data based on the basis function through CCSK modulation and subcarrier allocation to generate a TDCS transmit signal;

[0010] S4, using the SCFDM transmission module to perform SCFDM modulation on the TDCS transmission signal;

[0011] S5. After passing through the channel, the received signal is input into the SCFDM receiving module for SCFDM demodulation. The SCFDM demodulated signal undergoes CCSK demodulation, deinterleaving and decoding in sequence to obtain the useful information sent.

[0012] In some embodiments, in step S1, convolutional coding with a code rate of 1 / 2 is used when performing coding interleaving.

[0013] In some embodiments, step S2 includes the following sub-steps:

[0014] S21. After the transmitter and receiver sample the electromagnetic environment, the power spectrum density of the environment spectrum is calculated;

[0015] S22. Compare the calculated power spectrum density of the ambient spectrum with the set threshold to obtain the frequency band position occupied by the interference, thereby obtaining the basis function amplitude.

[0016] In some embodiments, in step S21, the Welch spectrum estimation algorithm is used to calculate the ambient spectrum power spectrum density.

[0017] In some embodiments, in step S22 , a flat amplitude shaping method is selected to generate basis function amplitudes.

[0018] In some embodiments, step S3 includes the following sub-steps:

[0019] S31, generating a random phase vector having the same length as the basis function amplitude vector by a random phase generator, and then performing a dot product between the random phase vector and the basis function amplitude vector to obtain a frequency domain expression of the basis function;

[0020] S32, CCSK modulation is to perform a cyclic shift on the signal in the time domain. In the frequency domain, the binary bit information in the coded interleaved signal is first converted into a decimal number as a transmission symbol, and the transmission symbol is mapped to the basis function to complete the subcarrier allocation;

[0021] S33. After the transmission symbols and the basis functions are allocated to subcarriers, the power of the basis functions is adjusted, and then the time domain expression of the TDCS transmission signal is obtained through inverse Fourier transform.

[0022] In some embodiments, in step S4, the processing of the TDCS transmit signal after it is input into the SCFDM transmit module includes serial-to-parallel conversion, FFT, subcarrier mapping, IFFT, insertion of cyclic prefix or pilot, and parallel-to-serial conversion.

[0023] In some embodiments, in step S4, the subcarrier mapping is divided into centralized frequency division multiplexing and distributed frequency division multiplexing; assuming that there is Q frame information, the centralized frequency division multiplexing and distributed frequency division multiplexing are specifically as follows:

[0024] Centralized frequency division multiplexing allocates a continuous subcarrier to each frame of information, and the bandwidth of each frame of information is 1 / Q of the system bandwidth;

[0025] Distributed frequency division multiplexing is to allocate a segment of discontinuous subcarriers to each frame of information, where the distance between the allocated subcarriers is Q_t subcarriers, satisfying Q_t≤Q; if Q_t=Q, it is interleaved frequency division multiplexing.

[0026] In some embodiments, in step S5, after passing through the channel, the processing of the received signal input into the SCFDM receiving module includes serial-to-parallel conversion, removal of cyclic prefix or pilot, FFT, subcarrier demapping, IFFT, and parallel-to-serial conversion.

[0027] In some embodiments, in step S5, CCSK demodulation is to perform correlation demodulation on the SCFDM demodulation signal using the basis function; the correlation demodulation adopts a correlation demodulation method of multi-element orthogonal modulation.

[0028] In summary, due to the adoption of the above technical solution, the beneficial effects of the present invention are:

[0029] There are various narrowband interferences in the electromagnetic environment, and TDCS can perform covert communications based on these interference signals. The OFDM+TDCS system has a high communication efficiency, but when faced with the presence of frequency sweep interference and narrowband noise interference in the environment, it will waste resources and be easily detected. The SCFDM+TDCS solution proposed in the present invention has a lower PAPR and is not easily detected because it does not occupy all available frequency bands. Since SCFDM has three different subcarrier mapping methods, experiments have shown that when the bandwidth is the same, the communication performance of these three methods is almost the same. Therefore, it is proposed that the appropriate subcarrier mapping method can be selected according to the actual situation to carry out safe and reliable covert communications. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 This is the SCFDM-TDCS system architecture diagram.

[0031] Figure 2a This is the OFDM flow chart,

[0032] Figure 2b This is the SCFDM flow chart.

[0033] Figure 3 Schematic diagram of two subcarrier mapping methods for the SCFDM-TDCS system.

[0034] Figure 4 The figure shows the relationship between SNR and BER for three subcarrier mapping methods of the SCFDM-TDCS system. DETAILED DESCRIPTION

[0035] To make the objectives, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Generally, the components of the embodiments of the present invention described and shown in the drawings herein can be arranged and designed in various different configurations.

[0036] Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the invention as claimed, but rather merely represents selected embodiments of the present invention. All other embodiments derived by persons of ordinary skill in the art based on the embodiments of the present invention without creative effort are intended to fall within the scope of protection of the present invention.

[0037] Single-carrier frequency division multiplexing (SCFDM) is based on OFDM. Compared to OFDM, SCFDM has a lower peak-to-average power ratio (PAPR). This lower PAPR facilitates signal transmission in mobile terminals and extends battery life. SCFDM offers the dual advantages of low PAPR for a single carrier and the robustness of multiple carriers.

[0038] The SCFDM system operates similarly to the OFDM system, except that an FFT (Fast Fourier Transform) and an IFFT (Inverse Discrete Fourier Transform) are added at the transmitter and receiver, respectively. The signal in the SCFDM-TDCS system only occupies a portion of the available frequency band, and three different subcarrier mapping methods can be selected within the continuously available frequency band. Depending on the actual frequency band conditions, the communicating parties agree on a specific mapping method for communication, or transmit the three mapping methods according to a specific pattern. This confuses the enemy, making the signal difficult to detect, increasing the stealth and reliability of the communication, and achieving reliable covert communication.

[0039] Based on the above design principles, Figure 1 Figure 1 is a diagram of the SCFDM-TDCS system architecture. The functions of the SCFDM-TDCS system include encoding and interleaving, TDCS modulation, SCFDM transmission, framing, channeling, synchronization, deframing, SCFDM reception, TDCS demodulation, deinterleaving, and decoding. Therefore, this embodiment proposes a reliable covert communication method based on the SCFDM-TDCS system, including the following steps:

[0040] S1. Encode and interleave the information to be communicated. In some embodiments, convolutional coding with a code rate of 1 / 2 is used during the encoding and interleaving.

[0041] S2, the transmitter and receiver sample the electromagnetic environment and obtain the basis function amplitude;

[0042] S3. Generate a basis function using the basis function amplitude, and modulate the coded interleaved data based on the basis function through CCSK (Cyclic Code Shift Keying) modulation and subcarrier allocation to generate a TDCS transmit signal;

[0043] S4, using the SCFDM transmission module to perform SCFDM modulation on the TDCS transmission signal;

[0044] S5. After the SCFDM modulated signal undergoes framing, channelization, synchronization, and deframing, the received signal is input into the SCFDM receiving module, which performs SCFDM demodulation. The SCFDM demodulated signal undergoes CCSK demodulation, deinterleaving, and decoding in sequence to obtain the useful information sent.

[0045] In some embodiments, step S2 includes the following sub-steps:

[0046] S21. After the transmitter and receiver sample the electromagnetic environment, the power spectrum density of the environment spectrum is calculated;

[0047] Methods for calculating the power spectrum density of the ambient spectrum include the Welch spectrum estimation algorithm, the AR model method, etc. In this embodiment, the Welch spectrum estimation algorithm is used to calculate the power spectrum density of the ambient spectrum, and the calculation expression is:

[0048]

[0049] Where, Represents the power spectrum density of the ambient spectrum, and U is the normalization factor. In order to make the power spectrum estimation asymptotically unbiased, the normalization factor U is indispensable. The Welch spectrum estimation algorithm transforms the signal x N (n) is divided into L segments, each segment is of length M, and a rectangular window d1[n] is added to each segment. In the Welch spectrum estimation algorithm, the choice of L and M directly affects the effect of spectrum estimation.

[0050] S22. Compare the calculated power spectrum density of the ambient spectrum with the set threshold to obtain the frequency band position occupied by the interference, thereby obtaining the basis function amplitude.

[0051] In this embodiment, the simplest flat amplitude shaping method is selected to generate the basis function amplitude. Its mathematical model is:

[0052]

[0053] Among them, TH is the set threshold, A(w) is the ambient spectrum estimation result, that is, the ambient spectrum power spectrum density mentioned above A'(w) is the amplitude of the basis function.

[0054] In some embodiments, step S3 includes the following sub-steps:

[0055] S31. Generate a random phase vector with the same length as the basis function amplitude vector through a random phase generator, and then perform a dot product between the random phase vector and the basis function amplitude vector to obtain a frequency domain expression of the basis function.

[0056] S32, CCSK modulation is to perform cyclic shift on the time domain of the signal. In the frequency domain, the binary bit information in the coded interleaved signal is first converted into a decimal number as the transmission symbol S i , will send symbol S i Mapping with the basis function completes the subcarrier allocation, which is expressed as:

[0057]

[0058] S33, send symbol S i After subcarrier allocation with the basis function, the power of the basis function is adjusted, and then the time domain expression of the TDCS transmission signal is obtained through inverse Fourier transform:

[0059]

[0060] Where N is the number of subcarriers, is the spectral vector A k The number of 1s in the scale value The power is normalized, m k is a random phase, M=2 r It reflects the multi-access capability, S i is the transmitted symbol, i.e. the number after CCSK modulation of the i-th signal.

[0061] In some embodiments, in step S4:

[0062] The processing of the TDCS transmit signal after it enters the SCFDM transmit module includes serial-to-parallel conversion, FFT, subcarrier mapping, IFFT, insertion of cyclic prefix (CP) or pilot, and parallel-to-serial conversion.

[0063] The subcarrier mapping is divided into two modes: centralized frequency division multiplexing (LFDM) and distributed frequency division multiplexing (DFDM). Assuming that there is Q frame information, the specific details of centralized frequency division multiplexing and distributed frequency division multiplexing are as follows:

[0064] Centralized frequency division multiplexing allocates a continuous subcarrier to each frame of information, and the bandwidth of each frame of information is 1 / Q of the system bandwidth;

[0065] Distributed frequency division multiplexing is to allocate a segment of discontinuous subcarriers to each frame of information, where the distance between the allocated subcarriers is Q_t subcarriers, satisfying Q_t≤Q.

[0066] In addition, interleaved frequency division multiplexing (IFDM) is a special example of distributed frequency division multiplexing DFDM, which satisfies Q_t=Q.

[0067] In some embodiments, in step S5:

[0068] After passing through the channel, the received signal is input into the SCFDM receiving module and the processing includes serial-to-parallel conversion, removal of cyclic prefix or pilot, FFT, subcarrier demapping, IFFT, and parallel-to-serial conversion.

[0069] The CCSK demodulation method uses the same basis functions as the transmitter at the receiver to perform correlation demodulation on the SCFDM demodulated signal. This method employs correlation demodulation using multi-element orthogonal modulation. The SCFDM demodulated signal is correlated with M reference signals, and the value with the largest correlation amplitude is selected as the signal estimate.

[0070] Figure 2a This is the OFDM flow chart, Figure 2b The following is a flow chart of SCFDM. Compared to OFDM, SCFDM adds an FFT at the transmitter and an IFFT at the receiver, based on OFDM modulation and demodulation. Because the added FFT and IFFT allow the information on a single subcarrier to be spread across all subcarriers within each symbol, each subcarrier contains the information of the entire symbol. The data on all subcarriers is integrated and cannot be demodulated separately. Therefore, it is called single-carrier modulation technology.

[0071] Figure 3 Figure 2 shows two types of subcarrier mapping in an SCFDM-TDCS system. SCFDM subcarrier mapping is categorized into centralized frequency division multiplexing (LFDM) and distributed frequency division multiplexing (DFDM), with interleaved frequency division multiplexing (IFDM) being a special case of DFDM. Figure 3 The subcarrier mapping method of SCFDM is analyzed from the perspectives of time domain and frequency domain respectively.

[0072] Figure 4 The figure shows the relationship between SNR (Signal to Interference plus Noise Ratio) and BER (Bit Error Ratio) for the three subcarrier mapping methods of the SCFDM-TDCS system. Figure 4As shown in the figure, the bit error rates of the three subcarrier mapping methods are almost the same when the occupied bandwidth is equal. Therefore, based on the actual situation of the frequency band, the communicating parties can agree on a certain subcarrier mapping method for communication, or transmit the three subcarrier mapping methods according to a certain pattern to confuse the enemy and conduct covert communication.

[0073] In summary, there are various narrowband interferences in the electromagnetic environment, and TDCS can perform covert communications based on these interference signals. The OFDM+TDCS system has a high communication efficiency, but when faced with the presence of swept frequency interference and narrowband noise interference in the environment, it will waste resources and be easily detected. The SCFDM+TDCS solution proposed in the present invention has a lower PAPR and is not easily detected because it does not occupy all available frequency bands. Since SCFDM has three different subcarrier mapping methods, experiments have shown that when the bandwidth is the same, the communication performance of these three methods is almost the same. Therefore, it is proposed that the appropriate subcarrier mapping method can be selected according to the actual situation to perform safe and reliable covert communications.

[0074] The foregoing description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art will readily appreciate that various modifications and variations of the present invention are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention are intended to be within the scope of protection of the present invention.

Claims

1. A reliable covert communication method based on SCFDM-TDCS system, characterized in that: The steps include: S1. Encode and interleave the information to be communicated; S2, the transmitter and the receiver sample the electromagnetic environment and obtain the basis function amplitude; S3, using the basis function amplitude to generate a basis function, based on the basis function and through CCSK modulation and subcarrier allocation to complete the modulation of the coded interleaved data, to generate a TDCS transmission signal; S4, using the SCFDM transmission module to perform SCFDM modulation on the TDCS transmission signal; S5. After passing through the channel, the received signal is input into the SCFDM receiving module for SCFDM demodulation. The SCFDM demodulated signal undergoes CCSK demodulation, deinterleaving and decoding in sequence to obtain the useful information sent.

2. The reliable covert communication method based on the SCFDM-TDCS system according to claim 1, characterized in that: In step S1, convolutional coding with a code rate of 1 / 2 is used when performing coding interleaving.

3. The reliable covert communication method based on the SCFDM-TDCS system according to claim 1 is characterized in that: Step S2 includes the following sub-steps: S21, after the transmitter and the receiver sample the electromagnetic environment, calculate the power spectrum density of the environment spectrum; S22. Compare the calculated power spectrum density of the environment spectrum with the set threshold to obtain the frequency band position occupied by the interference, thereby obtaining the basis function amplitude.

4. The reliable covert communication method based on the SCFDM-TDCS system according to claim 3 is characterized in that: In step S21, the Welch spectrum estimation algorithm is used to calculate the ambient spectrum power spectral density.

5. The reliable covert communication method based on the SCFDM-TDCS system according to claim 3 is characterized in that: In step S22, a flat amplitude shaping method is selected to generate basis function amplitudes.

6. The reliable covert communication method based on the SCFDM-TDCS system according to claim 1, characterized in that: Step S3 includes the following sub-steps: S31, generating a random phase vector having the same length as the basis function amplitude vector through a random phase generator, and then performing a dot multiplication of the random phase vector and the basis function amplitude vector to obtain a frequency domain expression of the basis function; S32, CCSK modulation is to perform cyclic shift on the time domain of the signal. In the frequency domain, the binary bit information in the coded interleaved signal is first converted into a decimal number as a transmission symbol, and the transmission symbol and the basis function are mapped to complete the subcarrier allocation; S33, after the transmission symbol and the basis function are allocated to subcarriers, the power of the basis function is adjusted, and then the time domain expression of the TDCS transmission signal is obtained through inverse Fourier transform.

7. The reliable covert communication method based on the SCFDM-TDCS system according to claim 1, characterized in that: In step S4, the processing of the TDCS transmission signal after it is input into the SCFDM transmission module includes serial-to-parallel conversion, FFT, subcarrier mapping, IFFT, insertion of cyclic prefix or pilot, and parallel-to-serial conversion.

8. The reliable covert communication method based on the SCFDM-TDCS system according to claim 7 is characterized in that: In step S4, the subcarrier mapping is divided into centralized frequency division multiplexing and distributed frequency division multiplexing; assuming that there is Q frame information, the centralized frequency division multiplexing and distributed frequency division multiplexing are specifically as follows: Centralized frequency division multiplexing is to allocate a continuous subcarrier for each frame of information, and the bandwidth of each frame of information is 1 / Q of the system bandwidth; Distributed frequency division multiplexing is to allocate a discontinuous subcarrier to each frame of information, where the distance between the allocated subcarriers is Q_t subcarriers, satisfying Q_t≤Q; if Q_t=Q, it is staggered frequency division multiplexing.

9. The reliable covert communication method based on the SCFDM-TDCS system according to claim 1, characterized in that: In step S5, after passing through the channel, the received signal is input into the SCFDM receiving module and the processing includes serial-to-parallel conversion, removal of cyclic prefix or pilot, FFT, subcarrier inverse mapping, IFFT, and parallel-to-serial conversion.

10. The reliable covert communication method based on the SCFDM-TDCS system according to claim 1, characterized in that: In step S5, CCSK demodulation is to use the basis function to perform correlation demodulation on the SCFDM demodulation signal; the correlation demodulation adopts a correlation demodulation method of multi-element orthogonal modulation.

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