UQPSK-based incoherent spread spectrum covert communication method

Abstract

The invention discloses an incoherent spread spectrum covert communication method based on UQPSK (Unscented Quadrature Phase Shift Keying), and relates to the field of covert communication and anti-interference communication systems of airborne, vehicle-mounted, ship-borne and other platforms in satellite communication. According to the method, the idea of UQPSK modulation is mainly adopted, two paths of low-speed spread spectrum signals with different rates and different powers are transmitted at the same time, a demodulation end captures received signals and distinguishes the different spread spectrum signals, demodulation of one path of signals serves as a main body, correct demodulation of the other path of signals is achieved in an auxiliary mode, and therefore the demodulation efficiency of the low-speed spread spectrum signals is improved. Therefore, reliable transmission of two paths of different service data is achieved, and the method can be used for transmission of various services in a covert communication system. The method is easy to implement, few in occupied resources, operable and high in transportability. The method is very suitable for FPGA implementation, and can be used for anti-interference communication and covert communication of different platforms such as broadband and low orbit.

Description

Technical Field

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

[0002] With the continuous development of anti-interference technology, improving the spread spectrum code rate and developing broadband or even ultra-wideband spread spectrum measurement and control communication systems are important trends in the future. However, high spread spectrum code rates mean broadband signal bandwidth, especially for ultra-wideband direct spread measurement and control communication signals, the sampling cost, design difficulty and implementation complexity of the receiving system will increase dramatically.

[0003] The UAV data link system is a key part of the UAV system. It is mainly responsible for transmitting various information interactions between UAVs and between UAVs and ground stations. UAVs developed for different application scenarios transmit different data back. Different return data have different requirements for transmission information rate and transmission bandwidth, as well as different accuracy requirements. If the traditional QPSK signal is used, if the system design is to meet the requirements of large bandwidth data transmission, it will waste precious frequency band resources when used to transmit low-speed signals.

[0004] UQPSK can modulate signals of different rates and different transmission powers in the in-phase and quadrature branches respectively. It can also select different spreading ratios through the in-phase and quadrature branches to obtain different spreading gains, meet different bit error rate requirements, and achieve the purpose of covert communication. Summary of the invention

[0005] The purpose of the present invention is to provide a non-coherent spread spectrum covert communication method based on UQPSK. The present invention can ensure the simultaneous transmission of two different service data under the premise of covert communication, solve the problem of frequency band resource waste caused by the traditional modulation method, and improve communication efficiency.

[0006] The technical solution adopted by 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 is mapped to the I channel, and the other channel is mapped to the Q channel. Different spreading codes are then used for spreading, and shaping filtering, D / A conversion, up-conversion, and RF processing are performed before transmission. The two channels have different symbol rates and amplitudes, and the first spreading code spreads the channel with a 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 capturing synchronization, the channel with the largest amplitude is determined as the first channel, and the first channel spreading code is used to perform correlation despreading on the digital down-converted signal, and code tracking error estimation is performed 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 symbol data I after timing;

[0013] (6) frequency offset estimation is performed using the symbol data I, and the estimation result is compensated for the digital down-conversion. After the frequency offset estimation is completed, the signal carrier phase is recovered. After the carrier phase is recovered, frame search and decoding are performed;

[0014] (7) Using the second spreading code to perform sliding correlation despreading on the digital down-converted signal, and using the timing error estimation result of step (5) to perform sampling rate conversion as the timing error of this path to perform symbol timing, and using the timing error estimation result of step (5) to perform data adjustment on the symbol timing to generate symbol data Q after timing;

[0015] (8) Using the symbol data Q to perform signal carrier phase recovery, after the carrier phase recovery is completed, frame search and decoding processing 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 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 1 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.

[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 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, having 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, signal demodulation in a high dynamic environment can be achieved, and it 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 in conjunction with 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 of the signals 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.

[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 is mapped to the I channel, and the other channel is mapped to the Q channel. Different spreading codes are then used for spreading, and shaping filtering, D / A conversion, up-conversion, and RF processing are performed before transmission. The two channels have different symbol rates and amplitudes, and the first spreading code spreads the channel with a larger amplitude.

[0029] The I and Q signal spread spectrum uses the same length of spread spectrum code, the spread spectrum code length is not less than the maximum spread spectrum ratio, and the two spread spectrum codes are selected to have autocorrelation characteristics 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 I and Q captured by one code channel correspond to spread spectrum code 1 and spread spectrum code 2, and the spread spectrum codes of 1 and Q 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 capturing synchronization, the channel with the largest amplitude is determined as the first channel, and the first channel spreading code is used to perform correlation despreading on the digital down-converted signal, and code tracking error estimation is performed to adjust the code clock phase;

[0034] Among them, 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.

[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 symbol data I after timing.

[0036] (6) frequency offset estimation is performed using the symbol data I, and the estimation result is compensated for the digital down-conversion. After the frequency offset estimation is completed, the signal carrier phase is recovered. After the carrier phase is recovered, frame search and decoding are performed;

[0037] (7) Using the second spreading code to perform sliding correlation despreading on the digital down-converted signal, and using the timing error estimation result of step (5) to perform sampling rate conversion as the timing error of this path to perform symbol timing, and using the timing error estimation result of step (5) to perform data adjustment on the symbol timing to generate symbol data Q after timing;

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

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

[0040] The present invention mainly adopts the idea of ​​UQPSK modulation, and transmits two low-speed spread spectrum signals with different rates and powers at the same time. The demodulation end distinguishes different spread spectrum signals by capturing the received signal, and uses the demodulation of one of the signals as the main body to assist in the correct demodulation of the other signal, thereby achieving reliable transmission of two different business data, which can be used for the transmission of multiple services in covert communication systems. It is simple to implement, occupies few resources, and is highly operable 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.

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

[0042] The above is a specific implementation of the present invention, but the protection scope of the present invention is not limited thereto. Any equivalent replacement or change 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 protection scope 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 is mapped to the I channel, and the other channel is mapped to the Q channel. Different spreading codes are then used for spreading, and shaping filtering, D / A conversion, up-conversion, and RF processing are performed before transmission. The two channels have different symbol rates and amplitudes, and the first spreading code spreads the channel with a 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 capturing synchronization, the channel with the largest amplitude is determined as the first channel, and the first channel spreading code is used to perform correlation despreading on the digital down-converted signal, and code tracking error estimation is performed 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 symbol data I after timing; (6) frequency offset estimation is performed using the symbol data I, and the estimation result is compensated for the digital down-conversion. After the frequency offset estimation is completed, the signal carrier phase is recovered. After the carrier phase is recovered, frame search and decoding are performed; (7) Using the second spreading code to perform sliding correlation despreading on the digital down-converted signal, and using the timing error estimation result of step (5) to perform sampling rate conversion as the timing error of this path to perform symbol timing, and using the timing error estimation result of step (5) to perform data adjustment on the symbol timing to generate symbol data Q after timing; (8) Using the symbol data Q to perform signal carrier phase recovery, after the carrier phase recovery is completed, frame search and decoding processing are performed; Complete non-coherent spread spectrum covert communication based on UOPSK.

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 1 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 adjusting the code clock phase 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, the second code clock phase adjustment is N times the first code clock phase adjustment.

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