A non-coherent spread spectrum covert communication method based on UQPSK

Abstract

The present invention discloses a non-coherent spread spectrum covert communication method based on UQPSK, which relates to the field of covert communication and anti-interference communication systems for airborne, vehicle-mounted, and ship-mounted platforms in satellite communications. The present invention mainly adopts the concept of UQPSK modulation to simultaneously transmit two low-speed spread spectrum signals with different rates and powers. The demodulation end distinguishes the different spread spectrum signals by capturing the received signal, and uses the demodulation of one signal as the main body to assist in the correct demodulation of the other signal, thereby achieving reliable transmission of two different business data. It can be used for the transmission of multiple services in a covert communication system. It is simple to implement, occupies few resources, and is highly operational and portable. It is very suitable for FPGA implementation and can be used for anti-interference communication and covert communication on different platforms such as broadband and low-orbit.

Description

Technical Field

[0001] The present invention relates to the field of satellite communications, and in particular to a UQPSK-based non-coherent spread spectrum covert communication method, which can improve the concealment and anti-interference communication capabilities in broadband satellite communications. Background Art

[0002] With the continuous advancement of anti-interference technology, increasing spread spectrum code rates and developing broadband and even ultra-wideband spread spectrum measurement and control communication systems are key future trends. However, high spread spectrum code rates require wider signal bandwidths, which, especially for ultra-wideband direct-spectrum measurement and control communication signals, dramatically increases the sampling cost, design difficulty, and implementation complexity of the receiving system.

[0003] The drone data link system is a critical component of any drone system, primarily responsible for transmitting various information exchanges between drones and between drones and ground stations. Drones designed for different application scenarios transmit different data, each with varying requirements for transmission rate, bandwidth, and accuracy. Using traditional QPSK signals, if the system is designed for high-bandwidth data transmission, would waste valuable bandwidth resources when used to transmit low-speed signals.

[0004] UQPSK can modulate signals of different rates and transmit powers in the in-phase and quadrature branches. It can also select different spreading ratios in the in-phase and quadrature branches to achieve different spreading gains, meeting different bit error rate requirements while achieving covert communication. Summary of the Invention

[0005] The present invention aims to provide a covert communication method based on UQPSK non-coherent spread spectrum. This method ensures the simultaneous transmission of two different service data while maintaining covert communication, solves the problem of frequency bandwidth waste caused by traditional modulation methods, and improves communication efficiency.

[0006] The technical solution adopted in the present invention is:

[0007] A non-coherent spread spectrum covert communication method based on UQPSK comprises the following steps:

[0008] (1) At the modulation end, two channels of information carrying different services are transmitted simultaneously, one channel mapped to the I channel and the other to the Q channel. Different spreading codes are then used for spreading, followed by shaping filtering, D / A conversion, up-conversion, and RF processing before transmission. The two channels have different symbol rates and amplitudes, and the first spreading code spreads the channel with the larger amplitude.

[0009] (2) At the receiving end, the signal is received through the receiving antenna, and then frequency conversion, filtering and A / D processing are performed to obtain a digital baseband signal. After preprocessing and digital matched filtering to remove inter-symbol interference, the signal amplitude is adjusted according to the preset threshold;

[0010] (3) performing digital down-conversion on the amplitude-adjusted signal and performing code capture on the digital down-converted signal using two-way spread spectrum codes;

[0011] (4) After acquiring synchronization, the channel with the largest amplitude is determined as the first channel. The first channel spreading code is used to perform correlation despreading on the digital down-converted signal, and the code tracking error is estimated to adjust the code clock phase.

[0012] (5) performing sliding correlation despreading on the digital down-converted signal using the first spreading code, performing timing error estimation on the sliding correlation despreading result, and performing symbol timing using the timing error estimation result to generate timed symbol data I;

[0013] (6) Using the symbol data I to perform frequency offset estimation, and compensating the estimation result to the digital down-conversion, after the frequency offset estimation is completed, the signal carrier phase is recovered, and after the carrier phase recovery is completed, frame search and decoding processing are performed;

[0014] (7) Using the second spreading code, the signal after digital down-conversion is subjected to sliding correlation despreading, and the timing error estimation result of step (5) is used to perform sampling rate conversion and then used as the timing error of this path for symbol timing. The symbol timing is adjusted using the timing error estimation result of step (5) to generate the symbol data Q after timing;

[0015] (8) Using the symbol data Q to perform signal carrier phase recovery, after which frame search and decoding are performed;

[0016] Complete non-coherent spread spectrum covert communication based on UOPSK.

[0017] Furthermore, in step (1): the I and Q signals are spread using spreading codes of the same length, the spreading code length is not less than the maximum spreading ratio, and the two spreading codes are selected to have autocorrelation characteristics and cross-correlation characteristics.

[0018] Furthermore, the code capture in step (3) is completed using two channels. The spread spectrum codes of the I and Q channels captured by one channel correspond to spread spectrum code 1 and spread spectrum code 2, and the spread spectrum codes of the 1 and Q channels captured by the other channel correspond to spread spectrum code 2 and spread spectrum code 1. After the capture is synchronized, the spread spectrum signal with the largest amplitude is inferred based on the synchronized capture.

[0019] Furthermore, in step (4), the code clock phase adjustment is to adjust the two code clock phases simultaneously according to the code tracking error estimation result of the first spread spectrum signal; the first code clock phase is adjusted according to the code tracking error estimation result, and the second code clock phase is adjusted synchronously in proportion according to the ratio of the two code chip rates; that is, assuming that the ratio of the first code chip rate to the second code chip rate is N, then the second code clock phase adjustment is N times the first code clock phase adjustment.

[0020] Compared with the background technology, the present invention has the following advantages:

[0021] 1. The present invention adopts UQPSK modulation to simultaneously realize the covert transmission of two-way service data, improves communication efficiency, and avoids excessive waste of bandwidth resources.

[0022] 2. The present invention is suitable for the simultaneous transmission of two relatively low-speed data services and is very suitable for FPGA implementation. It has the characteristics of fast synchronization, low power consumption, and simple implementation.

[0023] 3. By adopting the concept of the present invention, one channel is used to transmit pilot data and the other channel is used to transmit data, which can realize signal demodulation in a high dynamic environment and has universality. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is a flow chart of the non-coherent spread spectrum covert communication method of the present invention. DETAILED DESCRIPTION

[0025] The present invention is further described in detail below with reference to the accompanying drawings.

[0026] The present invention adopts the idea of ​​UQPSK modulation to simultaneously transmit two low-speed spread spectrum signals with different rates and powers. The demodulation end distinguishes different spread spectrum signals by capturing the received signal, and uses the demodulation of one signal as the main body to assist in the correct demodulation of the other signal, thereby achieving reliable transmission of two different business data. It can be used for the transmission of multiple services in covert communication systems.

[0027] A non-coherent spread spectrum covert communication method based on UQPSK specifically comprises the following steps:

[0028] (1) At the modulation end, two channels of information carrying different services are transmitted simultaneously, one channel mapped to the I channel and the other to the Q channel. Different spreading codes are then used for spreading, followed by shaping filtering, D / A conversion, up-conversion, and RF processing before transmission. The two channels have different symbol rates and amplitudes, and the first spreading code spreads the channel with the larger amplitude.

[0029] The I and Q signals are spread using spreading codes of the same length, the spreading code length is not less than the maximum spreading ratio, and the two spreading codes are selected to have autocorrelation and cross-correlation characteristics.

[0030] (2) At the receiving end, the signal is received through the receiving antenna, and then frequency conversion, filtering and A / D processing are performed to obtain a digital baseband signal. After preprocessing and digital matched filtering to remove inter-symbol interference, the signal amplitude is adjusted according to the preset threshold.

[0031] (3) performing digital down-conversion on the amplitude-adjusted signal and performing code capture on the digital down-converted signal using two-way spread spectrum codes;

[0032] Among them, code capture is completed in two ways. The spread spectrum codes of the I and Q channels captured by one code channel correspond to spread spectrum code 1 and spread spectrum code 2, and the spread spectrum codes of the 1 and Q channels captured by the other code channel correspond to spread spectrum code 2 and spread spectrum code 1. After capture synchronization, the spread spectrum signal with the largest amplitude is inferred based on the synchronized capture.

[0033] (4) After acquiring synchronization, the channel with the largest amplitude is determined as the first channel. The first channel spreading code is used to perform correlation despreading on the digital down-converted signal, and the code tracking error is estimated to adjust the code clock phase.

[0034] Among them, the code clock phase adjustment is to simultaneously adjust the phases of the two code clocks based on the code tracking error estimation result of the first spread spectrum signal; the phase of the first code clock is adjusted according to the code tracking error estimation result, and the phase of the second code clock is synchronously adjusted proportionally according to the ratio of the two code chip rates; that is, if the ratio of the first code chip rate to the second code chip rate is N, then the phase adjustment of the second code clock is N times the phase adjustment of the first code clock.

[0035] (5) Using the first spreading code, the signal after digital down-conversion is subjected to sliding correlation demodulation, the timing error of the sliding correlation demodulation result is estimated, and the timing error estimation result is used to perform symbol timing to generate the timed symbol data I.

[0036] (6) Using the symbol data I to perform frequency offset estimation, and compensating the estimation result to the digital down-conversion, after the frequency offset estimation is completed, the signal carrier phase is recovered, and after the carrier phase recovery is completed, frame search and decoding processing are performed;

[0037] (7) Using the second spreading code, the signal after digital down-conversion is subjected to sliding correlation despreading, and the timing error estimation result of step (5) is used to perform sampling rate conversion and then used as the timing error of this path for symbol timing. The symbol timing is adjusted using the timing error estimation result of step (5) to generate the symbol data Q after timing;

[0038] (8) Using the symbol data Q to perform signal carrier phase recovery, after which frame search and decoding are performed;

[0039] Complete non-coherent spread spectrum covert communication based on UOPSK.

[0040] This invention primarily utilizes the concept of UQPSK modulation to simultaneously transmit two low-speed spread-spectrum signals with different rates and powers. The demodulator captures the received signal, distinguishes the different spread-spectrum signals, and uses the demodulation of one signal as the primary tool to assist in correctly demodulating the other, thereby achieving reliable transmission of two different service data channels. This technology can be used to transmit multiple services within covert communication systems. It is simple to implement, requires minimal resources, and is highly operational and portable. It is well-suited for FPGA implementation and can be used for interference-resistant and covert communications across various platforms, including broadband and low-orbit orbits.

[0041] The contents not described in detail in the present invention are well known in the art.

[0042] The above is a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent replacement or modification made by a person skilled in the art based on the technical solution and inventive concept of the present invention should be included in the scope of protection of the present invention.

Claims

1. A non-coherent spread spectrum covert communication method based on UQPSK, characterized in that: The following steps are involved: (1) At the modulation end, two channels of information carrying different services are transmitted simultaneously, one channel mapped to the I channel and the other to the Q channel. Different spreading codes are then used for spreading, followed by shaping filtering, D / A conversion, up-conversion, and RF processing before transmission. The two channels have different symbol rates and amplitudes, and the first spreading code spreads the channel with the larger amplitude. (2) At the receiving end, the signal is received through the receiving antenna, and then frequency conversion, filtering and A / D processing are performed to obtain a digital baseband signal. After preprocessing and digital matched filtering to remove inter-symbol interference, the signal amplitude is adjusted according to the preset threshold; (3) performing digital down-conversion on the amplitude-adjusted signal and performing code capture on the digital down-converted signal using two-way spread spectrum codes; (4) After acquiring synchronization, the channel with the largest amplitude is determined as the first channel. The first channel spreading code is used to perform correlation despreading on the digital down-converted signal, and the code tracking error is estimated to adjust the code clock phase. (5) performing sliding correlation despreading on the digital down-converted signal using the first spreading code, performing timing error estimation on the sliding correlation despreading result, and performing symbol timing using the timing error estimation result to generate timed symbol data I; (6) Using the symbol data I to perform frequency offset estimation, and compensating the estimation result to the digital down-conversion, after the frequency offset estimation is completed, the signal carrier phase is recovered, and after the carrier phase recovery is completed, frame search and decoding processing are performed; (7) Using the second spreading code, the signal after digital down-conversion is subjected to sliding correlation despreading, and the timing error estimation result of step (5) is used to perform sampling rate conversion and then used as the timing error of this path for symbol timing. The symbol timing is adjusted using the timing error estimation result of step (5) to generate the symbol data Q after timing; (8) Using the symbol data Q to perform signal carrier phase recovery, after which frame search and decoding are performed; Complete non-coherent spread spectrum covert communication based on UQPSK.

2. The UQPSK-based non-coherent spread spectrum covert communication method according to claim 1, characterized in that: In step (1): the I and Q signals are spread using spreading codes of the same length, the spreading code length is not less than the maximum spreading ratio, and the two spreading codes are selected to have autocorrelation characteristics and cross-correlation characteristics.

3. The UQPSK-based non-coherent spread spectrum covert communication method according to claim 1, characterized in that: The code capture in step (3) is completed in two ways. The spread spectrum codes of the I and Q channels captured by one way are spread spectrum code 1 and spread spectrum code 2 respectively, and the spread spectrum codes of the I and Q channels captured by the other way are spread spectrum code 2 and spread spectrum code 1 respectively. After the capture is synchronized, the spread spectrum signal with the largest amplitude is inferred based on the synchronized capture.

4. The UQPSK-based non-coherent spread spectrum covert communication method according to claim 3, characterized in that: Step (4) of the code clock phase adjustment is to adjust the phases of the two code clocks simultaneously according to the code tracking error estimation result of the first spread spectrum signal; the phase of the first code clock is adjusted according to the code tracking error estimation result, and the phase of the second code clock is adjusted synchronously in proportion to the ratio of the two code chip rates; That is, assuming that the ratio of the first code chip rate to the second code chip rate is N, the phase adjustment of the second code clock is N times the phase adjustment of the first code clock.

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