CN112398774A - Spread spectrum communication method based on orthogonal time frequency expansion - Google Patents
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- H04B1/707—Spread spectrum techniques using direct sequence modulation
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Abstract
The invention provides a spread spectrum communication method based on orthogonal time frequency expansion, which is formed by a discrete Fourier transform and a multi-carrier multiplexing system, wherein a sending end firstly transforms a time-frequency domain to a time delay-Doppler domain, and carries out modulation in the time delay-Doppler domain, and finally maps modulation symbols in the time delay-Doppler domain to the time-frequency domain for transmission, and a receiving end is opposite. The invention adopts the technical means of combining the spread spectrum technology with the OTFS, not only can improve the transmission performance under the high dynamic condition, but also can effectively reduce the probability of intercepting the signal, can be applied to secret communication, and can be cracked without prior information after the conventional communication mode is intercepted.
Description
Technical Field
The invention relates to the field of orthogonal time frequency expansion (OTFS), in particular to wireless security communication by using the combination of the OTFS and a spread spectrum technology.
Background
The low signal interception means that signals are difficult to detect or intercept by a receiver of a non-partner party through some means, and spread spectrum communication is widely applied to various anti-interference low interception systems. The method is divided into three types of direct sequence spread, frequency modulation and time hopping, wherein the direct sequence spread the frequency spectrum through a pseudo-random sequence, the power spectral density of a signal is reduced, the signal is under the noise, and the purpose of low interception is achieved.
Orthogonal Frequency Division Multiplexing (OFDM) is widely used in current communication modes, such as 4G communication, and it transmits data through multiple orthogonal subcarriers, which improves the utilization rate of frequency bands compared to frequency division multiple access (fdma) technology, but has the disadvantage that OFDM is affected by frequency offset, which also causes the transmission performance of OFDM to be limited in high dynamic transmission. With the high-speed development of the unmanned aerial vehicle technology, the application scene of high dynamic communication is more and more common.
OTFS may be considered as an improved technique over OFDM, which improves transmission performance under highly dynamic conditions by converting the conventional time-frequency domain into the delay-doppler domain. By combining the spread spectrum communication of the OTFS, the anti-interception capability of signals can be improved, and the method can be more suitable for high-dynamic environments. Meanwhile, the time delay-Doppler domain is used as a non-traditional transform domain and can also play a role in resisting interception to a certain extent.
Disclosure of Invention
In order to overcome the defects of the prior art, the invention provides a spread spectrum communication method based on orthogonal time frequency expansion, and aims to solve the common problem of high-dynamic wireless covert communication in an unmanned aerial vehicle.
OTFS is different from OFDM and is a two-dimensional expansion mode, and comprises discrete Fourier transform and a multi-carrier multiplexing system, wherein a sending end firstly transforms a time-frequency domain into a delay-Doppler domain and carries out modulation in the delay-Doppler domain, and finally, a modulation symbol in the delay-Doppler domain is mapped to the time-frequency domain for transmission, and a receiving end is opposite.
The technical scheme adopted by the invention for solving the technical problem mainly comprises the following steps:
step 1: assuming that each data packet sends non-return-to-zero code as a [ K ], K is 0,1,2, …, K-1;
step 2: by non-return-to-zero pseudo-random code c of length Lk[l]To b [ k ]]Expansion to b [ l, k ]]Wherein L is 0,1,2, …, L-1;
and step 3: M-PSK modulation is carried out on each path of serial data of the information code block to obtain modulated discrete symbols x [ l, r ]]R is 0,1,2, …, R-1, where R represents the number of modulation symbols, there is a relationship R K/log2(M), M representing a modulation order of the data;
and 4, step 4: defining an inverse discrete symplectic fourier transform denoted ISFFT (X [ L, r ]), obtaining an inverse discrete symplectic fourier transform X [ p, q ] of an information code block, wherein p, L is 0,1, …, L-1; q, R ═ 0,1, …, R-1;
and 5: obtaining a sending baseband signal s (t) of a transmitting end through Heisenberg conversion;
step 6: assuming that the impulse response of a time-varying channel is h (tau, v), wherein tau is time delay, v is Doppler frequency shift, and a receiver receives a signal r (t);
and 7: calculating cross-ambiguity function at receiving end matched filter
The received signal is obtained by Wigner transformation:
and 8: then for Y [ p, q ]]Performing the SFFT to obtain:
denoted SFFT (Y [ p, q ]])；
And step 9: recovering the modulation symbols x [ l, r ] through MAP detection;
step 10: for x [ l, r]Performing M-PSK demodulation to obtain signal bit information after spread spectrum transmission
Step 11: according to pseudo-random code ck[l]To pair
Performing despreading, calculating for the k bit
If it is not
Is greater than 0, then the bit is 1, if less than 0, then the bit is 0, by pairing
Is calculated and judged to obtain the transmitted information sequence a [ k ]]Thereby completing the transmission of the information.
In the step 2, the expansion mode is b [ l, k ]]＝a[k]×ck[l]I.e. serial information code a k]By means of pseudo-random code ck[l]Converting into L-path parallel information code block b [ L, k ]]And represents that the number of subcarriers is L.
Said inverse discrete cosine Fourier transform
The transmitting base of the transmitting terminal is obtained through Heisenberg conversionSignal s (t) of band, expressed as
Wherein, gtx(T) is the transmit pulse function, Δ f is the subcarrier frequency spacing, T is the symbol period, and Δ f is 1/T.
The receiver receives a signal r (t) represented by: r (t) ═ jj (τ, ν) s (t- τ) ej2πν(t-τ)dτdν。
The Wigner transformation is to
Wherein g isrx(t) is a function of the received pulses,
representing the conjugate function of the received pulse function.
The invention has the advantages that the transmission performance under high dynamic condition can be improved and the probability of signal interception can be effectively reduced due to the adoption of the technical means of combining the spread spectrum technology and the OTFS. The invention can be applied to secret communication, the conventional communication mode can be cracked without prior information after being captured, and even if the signal is captured by an eavesdropper, the signal cannot be cracked if the conditions of the frame format, the modulation order and the like of the signal are not clear.
Drawings
FIG. 1 is a block diagram of a system model of the present invention.
Fig. 2 is a schematic diagram of the spreading method of the present invention.
Fig. 3 is a diagram illustrating the bit error rate of the OTFS-based spread spectrum communication technique according to the present invention with different snr.
Detailed Description
The invention is further illustrated with reference to the following figures and examples.
According to the spread spectrum communication method based on the OTFS, the invention takes the example that each data packet sends a 16-bit return-to-zero binary code, namely K is 16. The pseudo random code length L is 8, the modulation scheme is QPSK, that is, M is 4, and each serial data after modulation includes 8 modulation symbols. As shown in fig. 1, the present invention provides a spread spectrum communication method based on OTFS, and the specific implementation manner is as follows
The method comprises the following steps: the non-return-to-zero code a k, k being 0,1, …,15, which 16 bits need to be transmitted, is generated.
Step two: generating 16 sets of length 8 pseudorandom code sets denoted ck[l]The expansion of the kth information code in the l path is shown and used for a [ k ] k]Performing two-dimensional expansion, obtaining a spread code block b according to the spreading mode shown in fig. 2, where b is an 8 × 16 matrix, and each element in the matrix can pass b [ l, k ]]＝a[k]×ck[l]And calculating to show that one information code block is transmitted by 8 carriers, and each carrier transmits 16-bit information.
Step three: QPSK modulating each path of serial data of the information code block to obtain 8 x 8 modulated symbol code block x, wherein each element is represented as x [ l, r ], and the modulated code block is in time delay-Doppler domain.
Step four: the X of the time delay-Doppler domain is converted into the X of the time-frequency domain through ISFFT, and the specific conversion formula is
Step five: converting X [ p, q ] by Heisenberg]The conversion into the transmission baseband signal s (t) of the transmitting end can be specifically expressed as
Wherein, gtx(T) is the transmit pulse function, Δ f is the subcarrier frequency spacing, T is the symbol period, and Δ f is 1/T.
Step six: assuming that the impulse response of the time-varying channel is h (τ, v), where τ is the time delay and v is the doppler shift, the receiver received signal r (t) can be expressed as: r (t) ═ jj (τ, ν) s (t- τ) ej2πν(t-τ)d τ d v + w (t). w (t) is white Gaussian noise with an average value of 0. Assuming that the number of multipath signals is P ═ 4, then h (τ, ν) is rewritten to
Step seven: calculating cross-ambiguity function at receiving end matched filter
Defining a Wigner transform:
wherein g isrx(t) is the received pulse function. The received signal is subjected to Wigner transformation:
the signal at this time appears in the time-frequency domain.
Step eight: combining Y [ p, q ] of time-frequency domain]Y [ l, r ] transformed to the delay-Doppler domain by SFFT]From this equation, it can be calculated:
step nine: the modulation symbols x [ l, r ] are recovered from y [ l, r ] by MAP detection.
Step ten: according to constellation point mapping pair x [ l, r ]]QPSK demodulation to obtain recovered spread-spectrum transmitted signal bit information
Step eleven: according to pseudo-random code ck[l]To pair
De-spread to obtain the bit information a [ k ] of the transmitted signal]. For the k bit calculation
If the error rate is greater than 0, the bit is 1, and if the error rate is less than 0, the bit is-1, so that the error can be further corrected, and a non-partner receiver can not correctly intercept the transmitted information.
Fig. 3 shows a bit error rate diagram of the OTFS-based spread spectrum communication technology with different signal to noise ratios, 10000 monte carlo experiments are performed, and each time, the data is 16 bits. As can be seen from fig. 3, compared to the simple OTFS, the spread spectrum communication technology based on the OTFS has a lower error rate due to the fact that error correction is performed during despreading, thereby enhancing the reliability of wireless communication.
Claims (6)
1. A spread spectrum communication method based on orthogonal time frequency expansion is characterized by comprising the following steps:
step 1: assuming that each data packet sends non-return-to-zero code as a [ K ], K is 0,1,2, …, K-1;
step 2: by non-return-to-zero pseudo-random code c of length Lk[l]To b [ k ]]Expansion to b [ l, k ]]Wherein L is 0,1,2, …, L-1;
and step 3: M-PSK modulation is carried out on each path of serial data of the information code block to obtain modulated discrete symbols x [ l, r ]]R is 0,1,2, …, R-1, where R represents the number of modulation symbols, there is a relationship R K/log2(M), M representing a modulation order of the data;
and 4, step 4: defining an inverse discrete symplectic fourier transform denoted ISFFT (X [ L, r ]), obtaining an inverse discrete symplectic fourier transform X [ p, q ] of an information code block, wherein p, L is 0,1, …, L-1; q, R ═ 0,1, …, R-1;
and 5: obtaining a sending baseband signal s (t) of a transmitting end through Heisenberg conversion;
step 6: assuming that the impulse response of a time-varying channel is h (tau, v), wherein tau is time delay, v is Doppler frequency shift, and a receiver receives a signal r (t);
and 7: calculating cross-ambiguity function at receiving end matched filter
The received signal is obtained by Wigner transformation:
and 8: then for Y [ p, q ]]Performing the SFFT to obtain:
denoted SFFT (Y [ p, q ]])；
And step 9: recovering the modulation symbols x [ l, r ] through MAP detection;
step 10: for x [ l, r]Performing M-PSK demodulation to obtain signal bit information after spread spectrum transmission
Step 11: according to pseudo-random code ck[l]To pair
Performing despreading, calculating for the k bit
If it is not
Is greater than 0, then the bit is 1, if less than 0, then the bit is 0, by pairing
Is calculated and judged to obtain the transmitted information sequence a [ k ]]Thereby completing the transmission of the information.
2. The spread spectrum communication method based on orthogonal time-frequency expansion according to claim 1, characterized in that:
in the step 2, the expansion mode is b [ l, k ]]＝a[k]×ck[l]I.e. serial information code a k]By means of pseudo-random code ck[l]Converting into L-path parallel information code block b [ L, k ]]And represents that the number of subcarriers is L.
3. The spread spectrum communication method based on orthogonal time-frequency expansion according to claim 1, characterized in that:
said inverse discrete cosine Fourier transform
4. The spread spectrum communication method based on orthogonal time-frequency expansion according to claim 1, characterized in that:
the sending baseband signal s (t) of the transmitting end is obtained through Heisenberg conversion and is expressed as
Wherein, gtx(T) is the transmit pulse function, Δ f is the subcarrier frequency spacing, T is the symbol period, and Δ f is 1/T.
5. The spread spectrum communication method based on orthogonal time-frequency expansion according to claim 1, characterized in that:
the receiver receives a signal r (t) represented by: r (t) ═ jj (τ, ν) s (t- τ) ej2πν(t-τ)dτdν。
6. The spread spectrum communication method based on orthogonal time-frequency expansion according to claim 1, characterized in that:
the Wigner transformation is to
Wherein g isrx(t) is a function of the received pulses,
representing the conjugate function of the received pulse function.
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Cited By (3)
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| CN113098818A (en) * | 2021-04-01 | 2021-07-09 | 北京交通大学 | Method for interleaving and mapping orthogonal spread spectrum data |
| CN114884787A (en) * | 2022-04-22 | 2022-08-09 | 华中科技大学 | Waveform controllable multi-carrier communication method, device and system suitable for fast time-varying channel |
| CN115150230A (en) * | 2022-06-01 | 2022-10-04 | 北京理工大学 | Orthogonal time-frequency space modulation system and method for improving spectrum efficiency |
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| CN114884787A (en) * | 2022-04-22 | 2022-08-09 | 华中科技大学 | Waveform controllable multi-carrier communication method, device and system suitable for fast time-varying channel |
| CN114884787B (en) * | 2022-04-22 | 2024-06-04 | 华中科技大学 | Waveform controllable multi-carrier communication method, device and system applicable to fast time-varying channel |
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| CN115150230B (en) * | 2022-06-01 | 2023-10-31 | 北京理工大学 | Orthogonal time-frequency space modulation system and method for improving frequency spectrum efficiency |
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