An affine frequency division multiplexing based covert communication method and system
By leveraging the principle of channel reciprocity and simulated radio frequency multiplexing technology, the transmitter and receiver adaptively generate Chirp parameter codebooks, solving the problem of insufficient security for covert communication in high-speed mobile scenarios in 6G mobile communication. This enables dynamic covert communication without key exchange, improving spectrum utilization and communication reliability.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- SOUTH CHINA UNIV OF TECH
- Filing Date
- 2025-11-25
- Publication Date
- 2026-07-28
AI Technical Summary
In 6G mobile communication systems, the security of covert communication in high-speed mobile scenarios is insufficient, codebook updates are inflexible and there is a risk of key leakage, which affects communication reliability and spectrum utilization.
Based on the principle of channel reciprocity, the transmitter and receiver can adaptively generate the same Chirp parameter codebook without key exchange or privacy information transmission. They can achieve covert communication using simulated radio frequency division multiplexing (AFDM) modulation technology, including channel estimation, codebook generation and signal modulation stages, to ensure signal security and spectral efficiency for both parties.
It enables dynamic covert communication without key exchange in high-speed mobile environments, improving communication security and spectrum utilization. It is particularly suitable for aviation communication, high-speed rail and vehicle networking scenarios, enhancing communication stability and anti-eavesdropping capabilities.
Smart Images
Figure CN121530686B_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the field of wireless communication technology, specifically relating to a covert communication method and system based on simulated radio frequency multiplexing. Background Technology
[0002] Next-generation wireless networks, especially sixth-generation mobile communication systems (6G), will fully support new application scenarios of the "Internet of Everything" (IoE), such as aviation communications, high-speed rail systems, and vehicle-to-everything (V2X). In these highly mobile scenarios, wireless channels exhibit rapidly time-varying characteristics. Due to the significant relative motion between the transmitter and receiver, substantial Doppler frequency shifts are introduced, leading to problems such as carrier frequency offset, inter-carrier interference, and channel estimation errors. These factors severely affect the system's transmission performance and communication reliability, becoming key bottlenecks restricting the development of high-speed mobile communication services. To address these challenges, researchers have proposed several novel waveform design methods suitable for high-speed mobile scenarios. Among them, Affine Frequency Division Multiplexing (AFDM), a modulation technique designed for high Doppler channel characteristics, is considered a highly promising physical layer solution in 6G systems due to its superior performance in resisting time-frequency spread and channel reversibility. AFDM can achieve affine transform-based signal mapping in the time-frequency plane, effectively improving signal robustness and spectral efficiency under high-speed mobile conditions.
[0003] However, with the increasing openness and complexity of 6G networks, user privacy and communication security issues are becoming increasingly prominent. How to achieve secure data transmission in high-speed mobile environments without sacrificing system efficiency has become a research hotspot. In recent years, researchers have proposed various data encryption schemes for high-speed mobile communications. Existing research shows that by adjusting the Chirp parameters at the transceiver end of an AFDM system, a certain degree of covert encryption can be achieved at the physical layer. However, these methods typically rely on a pre-agreed Chirp parameter codebook between the transceiver and receiver, leading to a rigid codebook update mechanism, insufficient flexibility, and increased risks of key leakage and cracking. Therefore, there is an urgent need for a dynamic and secure covert communication scheme that can achieve this without explicit information exchange, providing a new solution for data security in high-speed mobile communication environments. Summary of the Invention
[0004] To address the issues of insufficient security, inflexible codebook updates, and potential leakage risks in covert communication under high-speed mobile communication scenarios in existing technologies, this invention proposes a covert communication method and system based on simulated radio frequency division multiplexing. Utilizing the principle of channel reciprocity, i.e., the propagation characteristics of wireless signals are the same in both directions, the two communicating parties can adaptively generate the same encryption book without the need for the transmitting and receiving ends to share keys or exchange privacy information. This enables secure and efficient covert data transmission, significantly improving the system's spectrum utilization and anti-eavesdropping capabilities.
[0005] To achieve the objective of this invention, a covert communication method based on simulated radio frequency multiplexing is provided, comprising the following steps:
[0006] 1. Channel estimation stage: The transmitter and receiver estimate the wireless channel without exchanging any information, and obtain their respective Channel State Information (CSI).
[0007] 2. Channel Matrix Construction Phase: Both the transmitting and receiving parties construct their respective frequency domain channel matrices based on their CSIs to characterize the current time-frequency channel characteristics.
[0008] 3. Codebook Generation Stage: Utilizing the reciprocity of wireless channels, the transmitter and receiver can independently calculate the same Chirp parameter codebook. Since the channel at the eavesdropper's location is different from the channels of the transmitter and receiver, the same codebook cannot be reconstructed, thus achieving physical layer concealment.
[0009] 4. Signal modulation and transmission stage:
[0010] The transmitter generates the corresponding modulation symbol based on the input modulation bits; according to the amplitude of the diagonal elements of the frequency domain channel matrix, it selects the index bits to map to the starting position of the Chirp parameter codebook; the transmitter performs simulated radio frequency division multiplexing (AFDM) modulation based on the Chirp parameters selected by the index bits to generate a covert signal and transmit it.
[0011] 5. Signal Reception and Decryption Stage: After receiving the signal, the receiver reconstructs a Chirp parameter codebook consistent with the transmitter using its own CSI; based on this codebook, it performs matched decoding and modulation decryption on the received signal, recovering the index bits and modulation bits, and finally obtaining the original data. This improves the system's spectral efficiency and anti-eavesdropping capabilities.
[0012] Furthermore, the channel state information includes channel delay, Doppler, and channel impulse response.
[0013] Furthermore, the channel state information obtained by the transmitter and receiver through estimation of the wireless channel is used to construct a frequency domain channel matrix, which is the time domain channel estimation result under the action of the discrete Fourier transform matrix.
[0014] Furthermore, the Chirp parameter codebook is obtained by quantizing the amplitude of the main diagonal elements of the frequency domain channel matrix, and the quantization step size is determined by dividing the difference between the maximum and minimum quantized values by the quantization level.
[0015] Furthermore, the first... Each element is determined by the following formula:
[0016]
[0017] in and These are the maximum and minimum quantization values, respectively. To quantize the level, the frequency domain channel matrix is analyzed. No. OK The main diagonal elements of the column Quantization is performed to obtain the corresponding Chirp parameter codebook. :
[0018]
[0019] in It is an arbitrary irrational constant. The number of subcarriers for the transmitted signal.
[0020] Furthermore, the mapping relationship between the index bits and the Chirp parameter codebook arrangement is determined based on the magnitude of the diagonal elements of the frequency domain channel matrix. Different magnitude sequences correspond to different combinations of index bits, thereby achieving dynamic covert mapping.
[0021] Furthermore, the steps for generating modulation symbols and covert signals include:
[0022] The input bit sequence is divided into modulation bits and index bits;
[0023] The modulation bits are mapped to constellation symbols, and the index bits are mapped to the permutation in the Chirp codebook vector. The modulation symbols are obtained through mapping, and the permuted Chirp codebook vector is obtained based on the index bits.
[0024] The modulation symbols are subjected to radio frequency multiplexing modulation to obtain the time-domain transmit signal. A chirp cyclic prefix is added to generate the covert signal.
[0025] Furthermore, the receiver performs a discrete affine Fourier transform on the received signal and performs maximum likelihood detection based on the reconstructed Chirp parameter codebook to estimate the modulation symbols and index permutation.
[0026] Furthermore, the modulation bits and index bits are recovered using channel knowledge and bit correspondence.
[0027] Furthermore, the method is applicable to high-speed mobile communication scenarios, including but not limited to aviation communications, high-speed railways, and vehicle-to-everything (V2X) communication, and can maintain stable covert communication performance in high Doppler environments.
[0028] This invention provides a covert communication system based on simulated radio frequency division multiplexing, comprising a transmitter module and a receiver module; the transmitter module is used to perform the transmitter operation of the method; the receiver module is used to perform the receiver operation of the method; the transmitter module and the receiver module are connected through a wireless channel, and implicit synchronization of the Chirp parameter codebook is achieved by utilizing channel reciprocity.
[0029] Compared with the prior art, the present invention has the following advantages and effects:
[0030] (1) This invention utilizes the principle of channel reciprocity to enable both the sender and receiver to adaptively generate the same encryption book without the need for key exchange or privacy information transmission, thus fundamentally avoiding the security risks of key distribution in traditional encryption methods.
[0031] (2) This invention fully develops the degrees of freedom of waveform parameters by introducing an index modulation mechanism into the simulated radio frequency division multiplexing (AFDM) waveform and changing the starting position of the index modulation mapping according to the channel state information. This method not only realizes the integration of covert communication and information transmission, but also significantly improves the spectrum utilization efficiency of the system. At the same time, since eavesdroppers cannot accurately obtain the channel state information of the transmitting and receiving parties, it is difficult to reconstruct the correct Chirp parameter codebook and know the starting position of the index modulation mapping, thereby effectively improving the security and covertness of communication.
[0032] (3) This invention is particularly applicable to high-speed mobile communication scenarios such as aviation communication, high-speed railway, and V2X. It can maintain stable and reliable covert transmission performance in complex Doppler environments, and provides an efficient and feasible solution for data security in 6G and future wireless communication systems. Attached Figure Description
[0033] Figure 1 This application provides a flowchart of a covert communication method and system based on simulated radio frequency multiplexing.
[0034] Figure 2This is a schematic diagram of an implementation model provided in this application.
[0035] Figure 3 This is a transceiver structure block diagram of a covert communication method and system based on simulated radio frequency multiplexing. Detailed Implementation
[0036] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the illustrations provided in the following embodiments are only schematic representations of the basic concept of the present invention. Unless otherwise specified, the following embodiments and features can be combined with each other. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.
[0037] Reference Figure 3 This invention considers a classic covert communication scenario. The transmitter and receiver are deployed in a high-speed mobile communication system, such as an aviation communication link, a high-speed railway communication system, or a vehicle-to-everything (V2X) communication scenario. Due to the significant relative motion between the transmitter and receiver, the channel exhibits obvious time-frequency dual-selection characteristics and rapid time-varying features. To achieve secure and efficient data transmission in such a high-Doppler environment, this invention employs a covert communication method based on simulated radio frequency division multiplexing. In this scenario, the transmitter dynamically generates the transmission waveform based on channel characteristics and determines the Chirp parameters using channel state information and index bits to achieve physical layer encryption. The receiver utilizes channel reciprocity to reconstruct the same Chirp parameter codebook, performs matching decoding and modulation decryption on the received signal, and recovers the original information data. Because the eavesdropper is located in a different spatial position, their channel characteristics are inconsistent with those of the legitimate transmitter and receiver, making it impossible to generate the same codebook, thus making it difficult to crack the communication content and achieving covert and secure data transmission. This method enables secure transmission of private data and improves spectrum efficiency in high-speed mobile communication scenarios.
[0038] The following is combined with Figure 3 The specific steps for implementing this method are as follows:
[0039] Step 1: The transmitter and receiver estimate the wireless channel respectively to obtain their respective Channel State Information (CSI), which includes channel delay, Doppler, and channel impulse response.
[0040] Channel estimation serves as the initial step to estimate the reciprocal wireless channel between the transmitter (end) and receiver (end). This stage allows both parties to extract channel features, a shared set of chirps, that will be used to derive chirp parameters. Based on coherence time and bandwidth, the estimated signal is transmitted with each block or every few blocks to maintain new channel knowledge for generating and detecting new chirp parameters. Based on the estimated channel delay, Doppler response, and channel impulse response, the time-domain channels estimated by the transmitter and receiver can be constructed, denoted as follows: and ,in The number of subcarriers for transmitting the signal. Represented as dimension size A complex matrix.
[0041] Step 2: The transmitter and receiver construct corresponding frequency domain channel matrices based on their respective Channel State Information (CSI) to characterize the current time-frequency channel characteristics, i.e.:
[0042]
[0043] in and These are the frequency domain channel matrices reconstructed by the transmitter and receiver based on the estimated channel state information, respectively. It is the discrete Fourier transform matrix.
[0044] Step 3: Utilizing the reciprocity of the wireless channel, the transmitter and receiver independently calculate the same Chirp parameter codebook.
[0045] Imperfect channel estimation, such as noise, can lead to a difference between the estimated and actual channel, potentially causing a mismatch in the Chirp parameter codebook between the transmitter and receiver. To address this issue, a quantization rule, as described below, is employed to eliminate interference when generating the Chirp parameter codebook based on channel state information.
[0046] At the transmitting end, the reconstructed frequency domain channel matrix The main diagonal elements are extracted and represented as ,in
[0047]
[0048] in Represented as a frequency domain channel matrix No. OK The main diagonal element of the column.
[0049] Subsequently, the quantization step size is defined. :
[0050]
[0051] in and These are the maximum and minimum quantization values, respectively. To quantify the level. Through the analysis of
[0052] For quantization, the corresponding Chirp parameter codebook can be represented as:
[0053]
[0054] in It is an arbitrary irrational constant used to guarantee the Chirp parameter codebook in AFDM modulation. The requirement is that it must be an irrational number.
[0055] According to equation (5), the Chirp parameter codebook vector can be obtained as follows: .
[0056] The receiver also obtains the frequency domain channel matrix based on the estimated channel state information. Then, by performing the operations in equations (3)-(5), the same Chirp parameter codebook vector can be obtained without any information exchange. Since the eavesdroppers are in different spatial locations, it is difficult for them to obtain the channel state information of both the transmitter and receiver, and they cannot deduce the same Chirp parameter codebook, thus achieving physical layer concealment.
[0057] Step 4: The transmitter generates the corresponding modulation symbol based on the input modulation bits; it selects the index bits based on the amplitude of the diagonal elements of the frequency domain channel matrix and maps them to the starting position of the Chirp parameter codebook, so that the bit stream [0 0 00…] to the bit stream [1 1 1 1…] can be sequentially mapped to the predefined Chirp parameter set; the transmitter selects the corresponding arrangement of the Chirp parameter codebook based on the input index bits; finally, it performs analog radio frequency division multiplexing (AFDM) modulation on the modulation symbol to generate a covert signal and transmit it.
[0058] In one embodiment, the transmitter uses an analog radio frequency multiplexing (AFDM) modulation scheme for signal transmission. Traditional AFDM systems include two main modulation parameters: the pre-chirp parameter... and the post-Chirp parameter The subsequent Chirp parameter The parameters are set based on the statistical characteristics of the delay-Doppler dual-select channel to ensure strong orthogonality of AFDM subcarriers in the delay-Doppler domain. In contrast, the pre-Chirp parameters... As a configurable parameter, it has a relatively small impact on subcarrier orthogonality. Based on this characteristic, this embodiment uses the Chirp parameter codebook vector generated in step 3 as the pre-Chirp parameter for AFDM modulation to achieve effective modulation and encryption of the signal.
[0059] Considering that each AFDM symbol contains One chirp subcarrier. The transmitter performs a bit mapping operation based on the input bit sequence. In one embodiment, the input bit sequence is first divided into modulation bits. and index bits Modulation bits Mapped to constellation symbols (such as QPSK, QAM, etc.), while index bits Mapped to Chirp codebook vector The arrangement in the sequence. Therefore, it is possible to calculate... , ,in The modulation order, express factorial, This indicates a round-down operation.
[0060] Furthermore, due to redundancy between the number of pattern permutations and the number of possible index bits, The arrangement of these numbers is illegal in classic indexed modulation systems. However, in this embodiment, this redundancy is provided based on the main diagonal elements of the frequency domain channel matrix. The possibility of activating various permutations is used for further data encryption. For clarity, a specific example is given in Table 1 of one embodiment: , . The index order of the magnitudes of the elements in the vector is as follows: This means the first index bit sequence Corresponding Chirp parameter vector The arrangement in is The next index bit sequence The next permutation pattern is followed until all index bits have a corresponding pattern.
[0061] Table 1
[0062] Index bit mapping rules, when ,
[0063]
[0064] Therefore, through the above operations, the modulation symbol can be obtained. And the Chirp codebook vector obtained by arranging the index bits. The modulation symbols are modulated using analog radio frequency division multiplexing (AFDM) to obtain the time-domain transmit signal. :
[0065]
[0066] in and It is a diagonal matrix, and its specific form can be represented as:
[0067]
[0068] in, For the previous Chirp parameter, For the first The post-Chirp parameters used by the subcarrier Represented as an imaginary number, This is the conjugate transpose.
[0069] Finally, add the Chirp cyclic prefix to generate a covert signal and transmit it.
[0070] Step 5: After receiving the signal, the receiver reconstructs the Chirp parameter codebook consistent with the transmitter using its own channel state information, performs matching decoding and modulation decryption on the received signal, recovers the index bits and modulation bits, and obtains the original data.
[0071] After transmission through the channel, the receiving end receives the signal and removes the Chirp cyclic prefix to obtain the time-domain signal. for
[0072]
[0073] in This is the noise vector. Using the discrete affine Fourier transform, i.e. The signal is then transformed into the discrete affine Fourier transform domain for demodulation and decryption.
[0074]
[0075] in For receiving signals in the discrete affine Fourier transform domain, ,
[0076] = Represents the equivalent channel matrix. This is the noise vector.
[0077] Subsequently, the receiving end can obtain the same Chirp parameter codebook vector without any information exchange by following the operation in step 3, and then use maximum likelihood detection to analyze the Chirp codebook vector used by the transmitting end. and modulation symbols Make an estimate:
[0078]
[0079] in and They represent the modulation symbols respectively. and Chirp codebook vector All possible sets and The estimated modulation symbols are respectively and Chirp codebook vector , .
[0080] Finally, the modulation bits and index bits can be recovered using channel knowledge and bit correspondence.
[0081] In one embodiment, a covert communication system based on simulated radio frequency multiplexing is provided, including a transmitter module and a receiver module; the transmitter module is used for transmitter operation of the method described in the foregoing embodiment; the receiver module is used for receiver operation of the method described in the foregoing embodiment; the transmitter module and the receiver module are connected through a wireless channel, and implicit synchronization of the Chirp parameter codebook is achieved by utilizing channel reciprocity.
[0082] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.
Claims
1. A covert communication method based on simulated radio frequency multiplexing, characterized in that, Includes the following steps: The transmitter and receiver estimate the wireless channel respectively, obtain their respective channel state information, and construct the corresponding frequency domain channel matrix. Taking advantage of the reciprocity of the wireless channel, the transmitter and receiver independently calculate the same Chirp parameter codebook; The transmitter generates the corresponding modulation symbol based on the input modulation bits, and selects index bits to map to the starting position of the Chirp parameter codebook based on the amplitude of the diagonal elements of the frequency domain channel matrix. The mapping relationship between the index bits and the arrangement of the Chirp parameter codebook is determined according to the magnitude of the amplitude of the diagonal elements of the frequency domain channel matrix. Different amplitude sequences correspond to different combinations of index bits, thereby realizing dynamic covert mapping. The transmitter performs radio frequency multiplexing modulation on the modulation symbol according to the Chirp parameter selected by the index bit, generates a covert signal and transmits it; The steps for generating modulation symbols and covert signals include: The input bit sequence is divided into modulation bits and index bits; The modulation bits are mapped to constellation symbols, and the index bits are mapped to the permutation in the Chirp codebook vector. The modulation symbols are obtained through mapping, and the permuted Chirp codebook vector is obtained based on the index bits. The modulation symbols are subjected to radio frequency multiplexing modulation to obtain the time-domain transmit signal, and a Chirp cyclic prefix is added to generate the covert signal; After receiving the signal, the receiver reconstructs the Chirp parameter codebook consistent with the transmitter using its own channel state information, performs matching decoding and modulation decryption on the received signal, recovers the index bits and modulation bits, and obtains the original data.
2. The covert communication method based on simulated radio frequency multiplexing according to claim 1, characterized in that, The channel state information obtained by the transmitter and receiver through estimation of the wireless channel is used to construct a frequency domain channel matrix, which is the time domain channel estimation result under the action of the discrete Fourier transform matrix.
3. The covert communication method based on pseudo-RF multiplexing according to claim 1, characterized in that, The Chirp parameter codebook is obtained by quantizing the amplitudes of the main diagonal elements of the frequency domain channel matrix. The quantization step size is determined by dividing the difference between the maximum and minimum quantized values by the quantization level.
4. The covert communication method based on pseudo-RF multiplexing according to claim 1, characterized in that, The first of the Chirp parameter codebook Each element is determined by the following formula: in and These are the maximum and minimum quantization values, respectively. To quantize the level, the frequency domain channel matrix is analyzed. No. OK The main diagonal elements of the column Quantization is performed to obtain the corresponding Chirp parameter codebook. : in It is an arbitrary irrational constant. The number of subcarriers for the transmitted signal.
5. A covert communication method based on simulated radio frequency multiplexing according to claim 1, characterized in that, The receiver performs a discrete affine Fourier transform on the received signal and performs maximum likelihood detection based on the reconstructed Chirp parameter codebook to estimate the modulation symbols and index permutation.
6. The covert communication method based on pseudo-RF multiplexing according to claim 5, characterized in that, The modulation bits and index bits are recovered using channel knowledge and bit correspondence.
7. A covert communication method based on pseudo-RF multiplexing according to any one of claims 1-6, characterized in that, The method is applicable to high-speed mobile communication scenarios, including aviation communication, high-speed rail, and vehicle-to-everything (V2X) communication.
8. A covert communication system based on simulated radio frequency multiplexing, characterized in that, It includes a transmitter module and a receiver module; the transmitter module is used to perform the transmitter operation of the method according to any one of claims 1 to 7; the receiver module is used to perform the receiver operation of the method according to any one of claims 1 to 7; the transmitter module and the receiver module are connected through a wireless channel, and implicit synchronization of the Chirp parameter codebook is achieved by utilizing channel reciprocity.