A double three-dimensional memory hyperchaos-based SCMA-OFDM transmission and verification system

Abstract

The application discloses a kind of based on dual three-dimensional memory hyperchaos SCMA-OFDM transmission system, the system includes sending end and receiving end, the user of sending end possesses respective codebook, J user shares K subcarrier, initial bit information and after the calculation of hyperchaos model of memristor triangle mapping chaotic codebook enter SCMA encoder and carry out codebook mapping processing, obtain encrypted SCMA-OFDM signal, for carrier synchronization, it is inserted pilot, after pilot, obtain frequency domain signal and information data using fast inverse fourier transform after the calculation of hyperchaos model of memristor triangle mapping, frequency domain signal is changed into time domain signal;Then to time domain signal is increased cyclic prefix CP and cyclic suffix CS to eliminate intercarrier interference, then parallel-serial conversion parallel signal is changed into serial signal, finally through up-conversion DUC complex signal mixing becomes real signal into transmission channel.The transmission system of regular hexagon chaotic codebook, its performance is improved by 2.5dB than traditional codebook.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of optical transmission technology in communication technology, and particularly relates to a SCMA-OFDM transmission and verification system based on double three-dimensional memory hyperchaos. BACKGROUND

[0002] With the rapid development of 5G and 4K, explosive mobile data traffic growth, large capacity demand has become a hot spot for research in today's communication direction, various types of multiplexing technology and coding technology have appeared, and the communication capacity has been improved in different ways. Passive optical network (PON) has attracted widespread attention due to its low power consumption, high speed and wider access range. Due to the high spectral efficiency and anti-interference performance of orthogonal frequency division multiplexing (OFDM), it has become the core technology of 4G, and 5G uses non-orthogonal multiple access (NOMA), which has the advantage of being able to allocate one resource to multiple users. SCMA proposed by Huawei has great development prospects, and its biggest feature is to combine modulation and spread spectrum to achieve higher overload rate and carry more user quantity. In addition, the space division multiplexing (SDM) based on multi-core optical fiber uses weakly coupled multi-core optical fiber with low inter-core crosstalk, and its transmission performance is better than that of using multiple single-mode optical fibers at the same time, which has good development prospects in future large-capacity transmission systems.

[0003] In recent years, many researchers have explored sparse code division multiple access (SCMA) endlessly, and the idea of SCMA technology comes from the LDS technology proposed by Hoshyar R. et al. in 2008. While maintaining the high overload rate of SCMA, the transmission performance and gain effect are improved in each dimension, but in the case of large transmission data volume, data transmission security has become a hot issue, and various eavesdropping means are common, and user information leakage leads to serious resource loss. Therefore, a transmission system with high security performance is more popular. Physical layer digital domain chaos encryption technology of high-speed multi-carrier optical communication system has attracted widespread attention. Physical layer encryption is widely used in digital signal processing (DSP) due to its low cost, high flexibility and compatibility. Physical layer encryption based on chaos system has ergodicity, pseudo-randomness and parameter sensitivity, which can protect the transmitted information from brute force attacks. Many scholars have conducted extensive research on the physical layer chaos encryption of OFDM short distance optical transmission system. Most of the research only focuses on SCMA and OFDM, and only a few studies the security of SCMA-OFDM. SUMMARY

[0004] The purpose of the present application is to overcome the shortcomings of the prior art and provide a SCMA-OFDM transmission system based on double three-dimensional memory hyperchaos, and a SCMA-OFDM verification system based on double three-dimensional memory hyperchaos.

[0005] Technical solution: In one aspect, the application provides a SCMA-OFDM transmission system based on double three-dimensional memory hyperchaos, which comprises a sending end and a receiving end. Users of the sending end have their own codebooks. J users share K subcarriers. Initial bit information and chaotic codebooks calculated by a memristor triangular mapping hyperchaotic model enter an SCMA encoder for codebook mapping processing to obtain encrypted SCMA-OFDM signals. A pilot is inserted for carrier synchronization, and the pilot obtains a frequency domain signal and information data calculated by a memristor triangular mapping hyperchaotic model. The frequency domain signal is converted into a time domain signal by using an inverse fast Fourier transform. Then, a cyclic prefix CP and a cyclic suffix CS are added to the time domain signal to eliminate inter-subcarrier interference. Parallel signals are converted into serial signals by parallel-serial conversion. Finally, complex signals are mixed into real signals by up-conversion DUC and enter a transmission channel.

[0006] After the receiving end receives the signals of the transmission channel, the received signals are processed in reverse according to the process of the sending end, and the initial bit information is finally solved by using a message passing algorithm based on spherical decoding in an SCMA decoder. This algorithm only calculates constellation points in the spherical region, thereby reducing the data calculation amount and increasing the calculation efficiency.

[0007] Further, it comprises:

[0008] The codebook mapping processing comprises:

[0009] The bit information sent by the user is mapped into an N-dimensional constellation point c in a constellation set C, c∈C, and the constellation set C formed by the constellation point is called a mother codebook, and C∈C N The mapping of the mother codebook is represented as:

[0010] g: g logM →C

[0011] Wherein, M represents the number of code words contained in each codebook, B is a binary set, and g is the mapping of the mother codebook.

[0012] The N-dimensional constellation point is mapped onto K-dimensional sparse code words through a binary mapping matrix V∈B K×N Here, K is not the same as the K subcarriers in the right 1, because the K-dimensional sparse code word is described in the code domain, and the subcarrier is described in the frequency domain. However, the K-dimensional sparse code word can carry J users, and can be regarded as a K-dimensional subcarrier carrying J users, and has the same function size. In this process, the SCMA encoder is redefined as f=Vg, where g is the mapping of the mother codebook. The time-frequency resources occupied by the code word sent by the user j are determined by the mapping matrix V jTo select, with a K row J column sparse matrix F to represent the encoding structure of SCMA, F is represented according to the codebook mapping principle:

[0013] F=[diag(V1V1 T ),diag(V2V2 T )...diag(V J V J T )

[0014] Wherein, j [1,2,3...J].

[0015] Further comprising:

[0016] The codebook mapping principle comprises:

[0017] First, assume that the number of users is U, each user has its own corresponding code table, each code table has M code words, each code word occupies K subcarriers, of which P is a non-empty subcarrier;

[0018] Then the binary sequence data is mapped according to the non-empty subcarriers in the codebook, specifically, the corresponding code word X1 in the code table 1 is selected according to the data input by the first user, and the corresponding code word XU in the code table U is selected according to the data input by the Uth user. U Finally, the code words obtained by each user are all directly added to obtain the encoded data Y, Y=X1+X2+...+X U Since only P carriers in each code table are non-empty, (U-P-1) users' data will be mixed together on each carrier, although the (U-P-1) users use the frequency resources of T subcarriers together, but use different code tables between each other.

[0019] Further comprising:

[0020] The memristor triangular mapping superchaotic state model is represented as:

[0021]

[0022] Wherein, x, y, q are state variables, a, k1, k2, k3, k4, k5, k6 are parameters, fixed a=100π, k1=1, k3=1, k4=0.1, k5=0.1, k6=0.1, k2=10 is the model in the first memristor triangular mapping superchaotic state, k2=0.1 is the model in the second memristor triangular mapping superchaotic state, Lyapunov exponent is an important parameter to describe the sensitivity of chaotic system to initial value;

[0023] According to the calculation of the memristor triangular mapping super chaotic state model, when k2 is 0.1 and 10, the vectors composed of variables x, y and q in the super chaotic state are respectively subjected to T times of iteration to obtain chaotic sequences X1, X2, X3, Y1, Y2, Y3 with a length of T, and the specific corresponding relationship is: (k2=10, x)→X1, (k2=10, y)→X2, (k2=10, q)→X3, (k2=0.1, x)→Y1, (k2=0.1, y)→Y2, (k2=0.1, q)→Y3.

[0024] The six chaotic sequences are respectively subjected to data preprocessing to generate masking factors A, B, C, D, E and F.

[0025] Further, comprising:

[0026] The calculation process of the chaotic codebook is as follows: first, the masking factors A, B,

[0027] A=round{10 5 *[(10 5 *X1)-(10 5 *X1)]}

[0028] B=round{10 5 *[(10 5 *Y1)-(10 5 *Y1)]}

[0029] Where, round is a rounding function,

[0030] Then, B is mapped as follows:

[0031]

[0032] Where, mod is a remainder operation function,

[0033] Finally, the direction key * (A / 180) is calculated to generate a chaotic codebook.

[0034] Further, comprising:

[0035] After the initial bit information is processed by the codebook mapping, it is sequentially subjected to constellation rotation, subcarrier replacement and symbol permutation, and then a pilot is inserted for carrier synchronization, the input data required for the constellation rotation includes information calculated by the first memristor triangular mapping super chaotic state model, and the input data required for the subcarrier replacement and symbol permutation includes information calculated by the second memristor triangular mapping super chaotic state model.

[0036] Further, comprising:

[0037] The masking factors C, D, E, and F are represented as follows:

[0038]

[0039] Where mod is the remainder operation function, Tra is the transpose transformation algorithm, the transpose operation is to interchange the rows and columns of a given matrix to obtain a new matrix called the transpose matrix, sort is the sorting function from smallest to largest, and the superscript T is the symbol for matrix transpose;

[0040] The frequency domain signal obtained after piloting and the information data calculated by the memristor triangulation hyperchaotic model are expressed using the inverse fast Fourier transform as follows:

[0041] signal_after_IFFT=signal_after_IFFT(timeX,:)

[0042] This formula is a code operation in MATLAB. The result is to shuffle and replace the original matrix according to the shuffled sequence timeX, thereby achieving the effect of encryption perturbation.

[0043] Furthermore, including:

[0044] The information data calculated by the initial value through the first memristor triangular mapping hyperchaotic state model includes: rounding and expanding the chaotic sequence X3 to obtain the radian sequence D in the interval [-π,π] as the constellation point rotation encryption masking factor, adding it to the phase angle of each constellation point to complete the constellation point rotation encryption and obtain the constellation point matrix.

[0045] Furthermore, including:

[0046] The subcarrier replacement is performed by multiplying the transpose transformation matrix E with the constellation point matrix obtained after rotating the constellation points, thus obtaining the replaced information. The symbol permutation is performed by multiplying the transpose transformation matrix F with the replaced information, as shown below:

[0047] complex_carrier_matrix1=matrix_after_encoder(:,frequency)

[0048] complex_carrier_matrix2=complex_carrier_matrix1(symbol,:)

[0049] This formula is a code operation in MATLAB. The result is to randomly replace the positions of the original matrix based on the randomized sequence frequency and symbol, thereby achieving the effect of encryption perturbation.

[0050] In another aspect, the application also provides a SCMA-OFDM verification system based on double three-dimensional memory hyperchaos, comprising: using a continuous wave laser as a light source and setting a power, at an optical line terminal (OLT) transmitter, an encrypted SCMA-OFDM signal is generated by an offline digital signal processing (DSP), and a digital-to-analog conversion is performed on the encrypted signal using an arbitrary waveform generator (AWG) with a first sampling rate; an electrical signal is injected into a Mach-Zehnder modulator through an electrical amplifier to complete intensity modulation and electro-optical conversion, and before an optical signal is coupled to a seven-core optical fiber through a 1:7 beam splitter and a fan-in device, further amplification of the laser is required by an erbium-doped fiber amplifier (EDFA).

[0051] At the receiving end, an optical network unit (ONU) is divided into legal access and illegal access, in the case of a legal ONU, the received optical power is adjusted by using a variable optical attenuator (VOA) and converting the received optical signal into an electrical signal through a photodiode (PD), then the obtained electrical signal is passed through a mixed signal oscilloscope (MSO) with a second sampling rate, after analog-to-digital conversion, the received data is decrypted by using the same key as the transmitter, users with illegal access will not be able to obtain correct data through an offline digital signal processing (DSP) without the key, finally the number of signal subcarriers is set, the total number of SCMA-OFDM symbols contained by each user subcarrier is set, and the total transmission bits are obtained by adding the number of code words of each codebook.

[0052] Beneficial effects: the application proposes a high-security low-complexity SCMA-OFDM transmission system based on double three-dimensional memory hyperchaos, due to the overload rate of SCMA and space division multiplexing, the number of users connected by the proposed system scheme is increased by 10.5 times. In addition, the performance of the transmission system using a regular hexagonal chaotic codebook is improved by 2.5 dB compared with a traditional codebook. The number of signal subcarriers is set to 128, the total number of SCMA-OFDM symbols contained by each user subcarrier is set to 300, and the total transmission bits are 128*300*log24=76800 bits by adding the number of code words of each codebook. Experiments prove that the encrypted SCMA-OFDM signal transmission of 34.9 Gb / s effective information on a 2 km seven-core optical fiber, the key space of the encryption scheme reaches 10215, and the security and robustness of the transmission system are verified. BRIEF DESCRIPTION OF DRAWINGS

[0053] Figure 1 A SCMA-OFDM transmission system structure diagram based on double three-dimensional memory hyperchaos (MTM) according to the application;

[0054] Figure 2 A user code table and SCMA encoding schematic diagram according to the application, wherein (a) is a subcarrier constellation diagram after SCMA encoding;

[0055] Figure 3 Factor graph of user node and core node according to the application;

[0056] Figure 4 Phase diagram of dual three-dimensional memory hyperchaotic MTM attractor according to the application;

[0057] Figure 5 SCMA-OFDM verification system schematic diagram based on dual three-dimensional memory hyperchaotic MTM according to the application;

[0058] Figure 6 BER curve comparison schematic diagram of encrypted SCMA-OFDM signal, unencrypted SCMA-OFDM signal, BTB mode and illegal ONU according to the application;

[0059] Figure 7 SCMA-OFDM transmission system receiver algorithm performance comparison schematic diagram according to the application;

[0060] Figure 8 BER curve comparison schematic diagram of encrypted SCMA-OFDM signal, unencrypted SCMA-OFDM signal, BTB mode and illegal ONU according to the application;

[0061] Figure 9 Sensitivity diagram of dual three-dimensional memory MTM hyperchaotic model encryption according to the application. Specific embodiments

[0062] The application will be further described below in conjunction with the drawings and specific embodiments.

[0063] The application is aimed at SCMA in weakly coupled seven-core optical fiber, and proposes a high-security low-complexity physical layer encryption scheme by using OFDM to improve the link quality of a single user. A memristor triangular mapping (MTM) hyperchaotic model is used, and its chaotic sequence is used to disturb the SCMA codebook, constellation diagram, subcarrier, symbol and time slot after data preprocessing, so as to perform multi-dimensional scrambling to prevent illegal users from eavesdropping. A message passing algorithm based on sphere decoding is used at the receiving end to solve the problem of high computational complexity. In addition, the detection effects of different multi-user detection algorithms in the SCMA-OFDM multi-core optical fiber transmission system are also studied.

[0064] SCMA is a new non-orthogonal multiple access (NOMA) scheme derived from the special code division multiple access-Low-Density Signature CDMA (LDS-CDMA) technology. SCMA combines data modulation and spreading into a joint coding process, i.e., the initial bits are directly mapped into a multi-dimensional code word in the complex domain codebook. Through the sparse multiplexing of the code domain, the data of different users can occupy partially repeated time-frequency resources, which has higher constellation shaping gain and can carry more users. The number of users that can be accessed by the traditional orthogonal frequency division multiple access (OFDMA) system is limited by the frequency resources, and the use of SCMA can break through this limitation to achieve massive user access. The SCMA encoder can be defined as: M represents the number of code words contained in each codebook, X represents a K-dimensional complex domain code word, K represents the number of orthogonal resources, and the codebook specification |X| = M. There are K-N (N < K) zero elements in each K-dimensional code word. The K-dimensional complex domain code word X is a sparse vector containing N < K non-zero elements.

[0065] The SCMA-OFDM encryption flowchart based on the dual three-dimensional memory hyperchaotic MTM is as shown in Figure 1 The sending end user has a respective codebook, J users share K subcarriers, the initial bit information enters the SCMA encoder for codebook mapping processing, a pilot is inserted for carrier synchronization, then the inverse fast Fourier transform (IFFT) is used on the frequency domain signal to convert the frequency domain signal into a time domain signal. Then, the cyclic prefix (CP) and the cyclic suffix (CS) are added to the time domain signal to eliminate the inter-carrier interference (ICI), and then the parallel-to-serial conversion is performed to convert the parallel signal into a serial signal. Since the complex number signal cannot be directly transmitted in the short distance intensity modulation / direct detection (IM / DD) system, the complex number signal is finally mixed into a real number signal by the up-conversion (DUC) to enter the transmission channel. After receiving the signal of the transmission channel, the receiving end reverses the signal according to the process of the sending end to perform data processing, and finally uses the message passing algorithm based on the sphere decoding in the SCMA decoder to solve the initial bit information. The algorithm of the sphere decoding is to only calculate the constellation points in the spherical region, which reduces the data calculation amount and increases the calculation efficiency.

[0066] The SCMA codebook mapping process is that the bit information sent by the user is mapped into an N-dimensional constellation point c (c ∈ C) in the constellation set C. The constellation set C formed by the constellation point is called a mother codebook, and C ∈ CN. The mapping of the mother codebook can be represented as:

[0067] g: B logM → C (1)

[0068] and then through the binary mapping matrix V ∈ B K×NThe N-dimensional constellation points are mapped to K-dimensional sparse codewords, in the process, the SCMA encoder is redefined as: f = Vg, the time-frequency resources occupied by the codeword sent by user j can be selected by the mapping matrix V corresponding to it j The encoding structure of SCMA can be indicated by a sparse matrix F with K rows and J columns, F can be represented as:

[0069] F = [diag (V1V1 T ), diag (V2V2 T )…diag (V J V J T ) (2)

[0070] Take a code table with K = 4 and J = 6 as an example, the specific process of user code table and SCMA encoding is shown in Figure 2 .

[0071] The 6 users all have their own corresponding code table, and each code table has 4 codewords, each of which corresponds to an input bit stream. In addition, each codeword occupies 4 subcarriers, only 2 of which are non-empty, and this sparsity can reduce the algorithm complexity of the demodulation process. In the specific encoding process, first, the binary sequence is mapped according to the codebook. The data input by user 1 is (0, 0), and the first codeword of code table 1 is selected, the data input by user 2 is (0, 1), and the second codeword of code table 2 is selected, and so on. The codewords obtained by each user are all directly added to obtain the encoded data. Since only 2 carriers in each code table are non-empty, the data of 3 users will be superimposed on each carrier. Although the 3 users share the frequency resource of 1 subcarrier, they can be distinguished from each other by using different code tables. Figure 2 The subcarrier constellation diagram after SCMA encoding is shown in , a regular hexagonal chaotic codebook is used, compared with the traditional codebook proposed by Huawei, the performance of the transmission system is improved by 2.5 dB. Using SCMA encoding, only 4 subcarriers are used to transmit the data of 6 users, so that the number of access users is increased by 1.5 times, and the system capacity is also improved. The corresponding factor graph matrix F is:

[0072]

[0073] As shown in Figure 3 , the user node and the core node factor graph, the information of 6 users only needs 4 cores for transmission. In addition, the overload rate of the codebook can be improved by changing the design structure of the codebook, so as to further improve the number of access users. The formula for calculating the overload rate is λ = J / K > 1.

[0074] Hyperchaotic model has been applied more and more in recent years, and using discrete memristor to construct chaotic mapping has become a research hotspot. The application adopts a discrete memristor model, which is coupled with a triangular graph to obtain a three-dimensional memristor hyperchaotic phase diagram. It embodies the dynamics of multiple parameter dependence. It has a large hyperchaotic range under different parameter changes, and has two positive Lyapunov exponents, high randomness and initial value sensitivity. In the encryption scheme, the SCMA codebook, subcarrier, symbol, constellation diagram and time slot are disturbed by the MTM hyperchaotic system. The mathematical expression of the chaotic model is as follows:

[0075]

[0076] Where, x, y, q are state variables, a, k1, k2, k3, k4, k5, k6 are system parameters, a is fixed as 100π, k1 is 1, k3 is 1, k4 is 0.1, k5 is 0.1, k6 is 0.1, and k2 is 0.1 or 10. The system is in a hyperchaotic state, and the Lyapunov exponent is an important parameter for describing the sensitivity of the chaotic system to the initial value. Two positive Lyapunov exponents mean that even if the initial values of the two orbits differ by a very small amount, the difference will separate exponentially over time, making the system locally unstable and globally stable. The dynamic behavior of the double three-dimensional memory hyperchaotic MTM is more complex and suitable for data encryption processing. When k2=0.1, the phase diagram of the MTM hyperchaotic state of different dimensional attractors is shown as (a) in FIG. 4, and when k2=10, the phase diagram of the MTM hyperchaotic state of different dimensional attractors is shown as (b) in FIG. 4. With the change of parameter k2, the attractor changes from a hollow cylinder to a square body, which is uniformly distributed in the whole phase space. Figure 4

[0077] When receiving signals encrypted by these complex chaotic trajectories (chaotic sequences), only the correct initial value (private key) can complete decryption, thereby ensuring high security performance. Around the six chaotic sequences X1, X2, X3, Y1, Y2, Y3 generated by the double three-dimensional memory MTM hyperchaotic model (X1, X2, X3, Y1, Y2, Y3 are vectors composed of variables x, y and q when k2 takes values of 0.1 and 10 in the double three-dimensional memory MTM hyperchaotic model, in a hyperchaotic state, after T iterations, a chaotic sequence with a length of T is obtained), data preprocessing is performed on the six chaotic sequences to generate masking factors A, B, C, D, E, F. These masking factors are used to encrypt the SCMA codebook, constellation point rotation, subcarrier disorder in the frequency domain and symbol permutation, and time slot disturbance. The specific rules are as follows:

[0078]

[0079] ​As shown in formula (5), the masking factor generation rule is used for the chaotic codebook, where round is the rounding function and fix is ​​the rounding function to zero. Then, B is mapped as follows:

[0080]

[0081] After the mapping relationship in (6), the direction key*(A / 180) is calculated, and then it is spread and mapped to generate a chaotic codebook for SCMA codebook encryption, such as... Figure 1 The process is shown in step a.

[0082]

[0083] As shown in (7), this is the masking factor generation rule, where mod is the remainder operation function, Tra is the transpose transformation algorithm, sort is the sorting function from smallest to largest, and the superscript T is the symbol for matrix transpose.

[0084] The transformation matrix C (masking factor) of the chaotic sequence X2 can be obtained through the operation of (7). In order to ensure the randomness of the sequence, the ten decimal places of the chaotic sequence X2 are taken for data processing. After taking the modulo operation of 1, it is multiplied by the derivative of the transposed sorted X2, and finally transposed to obtain the result.

[0085] As shown in (8), the time slot perturbation is finally achieved by multiplying it by the time-domain matrix after IFFT transformation. Figure 1 The process e is shown in the diagram.

[0086] signal_after_IFFT=signal_after_IFFT(timeX,:)(8)

[0087] The rotation transformation matrix X3 (masking factor) of the chaotic sequence D can be obtained through the operation of (7). The chaotic sequence X3 is rounded and expanded to obtain the radian sequence D in the interval [-π,π] as the constellation point rotation encryption masking factor. It can be added to the phase angle of each constellation point to complete the constellation point rotation encryption, such as Figure 1 The process is shown in step b.

[0088] The transpose transformation matrices E and F (masking factors) of the chaotic sequences Y2 and Y3 can be obtained through the same operations as C, and are essentially identity matrices after multiple elementary transformations. Subcarrier substitution in the frequency domain can be accomplished by multiplying the transpose transformation matrix E and the constellation point matrix, as shown in (9). Figure 1The process is shown in step c). Symbol substitution can be achieved by multiplying the transpose transformation matrix F and the constellation point matrix, as shown in formula (10). This process involves transforming the initial data into symbols after constellation operations and mapping the symbols onto the constellation diagram. In MATLAB, symbol substitution can be achieved by multiplying the transpose transformation matrix F and the constellation point matrix, as shown in formula (10). Figure 1 The process d in the diagram is shown.

[0089] complex_carrier_matrix1=matrix_after_encoder(:,frequency)(9)

[0090] complex_carrier_matrix2 = complex_carrier_matrix1(symbol,:) (10)

[0091] Because the dual three-dimensional memory MTM hyperchaotic model is highly sensitive to initial values, and the data processing is highly confidential and complex, it is very difficult to crack even if the parameters and initial values ​​of the chaotic model change very little at the illegal receiving end.

[0092] To verify the high-security, low-complexity SCMA-OFDM transmission system, the PON architecture from short-distance optical communication was selected for the experiment. For example... Figure 5 The IM / DD system was deployed as shown. In the experiment, a 1550nm continuous-wave laser was used as the light source, with a power setting of 14.5dBm. At the optical line terminal (OLT) transmitter, the encrypted SCMA-OFDM signal was generated by an offline DSP. The encrypted signal underwent digital-to-analog conversion using an arbitrary waveform generator (AWG, TekA WG70002A) with a sampling rate of 10GSa / s. The electrical signal, after being amplified, was injected into the MZM to complete intensity modulation and electro-optical conversion. Further amplification of the laser by an EDFA was required before the optical signal was coupled into the seven-core fiber via a 1:7 splitter and fan-in device. At the receiver, optical network units (ONUs) were categorized as legitimate and illegitimate. In the case of legitimate ONUs, the received optical power was adjusted by converting the received optical signal into an electrical signal using a VOA and a PD. The obtained electrical signal was then passed through a mixed-signal oscilloscope (MSO, TexMSO73304DX) with a sampling rate of 50GS / s. After analog-to-digital conversion, the received data is decrypted using the same key as the transmitter. Unauthorized users will not be able to obtain the correct data via the offline DSP without the key. Ultimately, with the number of signal subcarriers set to 128, the total number of SCMA-OFDM symbols per user subcarrier set to 300, and the number of codewords per codebook set to 4, the total transmitted bits are 128 × 300 × log24 = 76800 bits.

[0093] Figure 5 The experiment of high security and low complexity SCMA-OFDM transmission based on double three-dimensional memory hyperchaos in seven-core fiber, wherein, AWG: arbitrary waveform generator; EA: electrical amplifier; MZM: Mach-Zehnder modulator; EDFA: erbium-doped fiber amplifier; MCF: multi-core fiber; VOA: variable optical attenuator; PD: photodiode; MSO: mixed signal oscilloscope.

[0094] Figure 6 The bit error rate (BER) performance of SCMA-OFDM signal after 2km transmission in seven-core fiber is shown, from which Figure 6 It can be seen that the measured BER curves of the seven cores almost overlap, which proves that the seven-core fiber transmission system used in the experiment has strong stability within the range of 2km. When the system BER is 3.8×10 -3 , it can be seen from the figure that the difference in received optical power between the best core and the worst core is less than 0.5

[0095] dB, which proves that the seven-core fiber transmission system used in the experiment has good uniformity within the range of 2km. In addition, when the received optical power is greater than -15dB, the BER performance of the seven cores is within the FEC threshold, and with the continuous increase of optical power, the BER performance of each core shows an obvious improvement trend, which proves that the proposed SCMA-OFDM transmission scheme has good transmission performance in the seven-core fiber transmission system.

[0096] We tested the performance comparison of the proposed SCMA-OFDM transmission scheme based on different receiver demodulation algorithms, in Figure 6 which it has been proved that the 2km seven-core fiber transmission system used has good uniformity and stability, so when analyzing the performance of the receiver demodulation algorithm, only the performance in the same core is analyzed, and the experimental results are shown in Figure 7 , we compared the BER performance of the proposed SD-MPA algorithm, the traditional MPA algorithm and the Log-MPA algorithm, it can be seen that the SD-MPA algorithm sacrifices part of the BER performance compared with the other two algorithms, but when the received optical power is greater than -15dB, the BER performance of the three algorithms is within the FEC threshold, and there is no obvious difference, but the computational complexity of the proposed SD-MPA algorithm is much smaller than that of the MPA algorithm and the Log-MPA algorithm, which proves the superiority of the proposed SCMA-OFDM transmission scheme.

[0097] To study the impact of the encryption scheme adopted on the SCMA-OFDM signal and the impact of the seven-core optical fiber transmission system adopted on the SCMA-OFDM signal, we test the BER performance of the SCMA-OFDM signal without encryption and the SCMA-OFDM signal in the BTB mode, and select the core BER performance after the transmission effect of the 2km seven-core optical fiber transmission system is best, and finally we test the BER performance of the illegal ONU in the case of eavesdropping or brute force decryption at the receiving end without the correct private key of the receiving end. The experimental results are shown in Figure 8 As can be seen from the experimental error, it can be concluded that the encryption scheme adopted has little effect on the BER performance of the SCMA-OFDM signal when the received optical power is greater than -15dB, and the difference between the BER of the 2

[0098] km seven-core optical fiber transmission system and the transmission in the BTB mode is also small within the FEC error correction threshold, which is within the acceptable range, proving the feasibility of the proposed SCMA-OFDM transmission scheme based on dual three-dimensional memory MTM hyperchaos. In addition, the BER value of the forced decryption received at the illegal ONU end is 0.49, which confirms the security of the proposed encryption scheme.

[0099] To verify the security performance of the transmission system, the key space of the dual three-dimensional memory MTM hyperchaotic system is calculated precisely, as shown in Figure 9 The key includes the parameters in the MTM dual hyperchaotic state, i.e. {a, k1, k2, k3, k4, k5, k6}. The key space can be calculated by experiment as MTM0.1 key space x MTM10 key space. 13 ×10 15 ×10 16 ×10 15 ×10 17 ×10 16 ×10 14 )×(10 13

[0100] ×10 15 ×10 15 ×10 15 ×10 17 ×10 17 ×10 17 )=10 215 Since the key space is too large, it takes a long time to find the correct key, thereby effectively preventing the hijacker from obtaining the key.

[0101] In the present application, we propose a high security and low complexity SCMA-OFDM transmission system based on double three-dimensional memory hyperchaos in seven-core fiber, which can effectively improve the transmission rate, at the same time, the security performance of the system is guaranteed by using physical layer chaos encryption and the operation complexity of the system is reduced by using SD-MPA algorithm, and the use of hexagonal chaotic codebook significantly improves the transmission performance of the system. Experiments prove that the scheme can realize 34.9 Gb / s effective data transmission in 2 km seven-core fiber, when the received optical power is greater than-15 dB, the BER performance of the seven cores is within the FEC threshold, the received optical power difference is less than 0.5 dB, which verifies the superiority of the low complexity of the SD-MPA algorithm, and the influence of the hyperchaos encryption algorithm on the BER performance of the system is very small and can be ignored, and the key space calculation is 10 215 The security performance of the system is guaranteed.

[0102] The above only describes the preferred embodiments of the present application, and it should be pointed out that for ordinary skilled in the art, without departing from the technical principles of the present application, a number of improvements and modifications can be made, and these improvements and modifications should be considered as the protection scope of the present application.

Claims

1. A SCMA-OFDM transmission system based on dual three-dimensional memory hyperchaos, characterized in that, The system comprises a sending end and a receiving end, users of the sending end have respective codebooks, J users share K subcarriers, initial bit information and chaotic codebooks calculated through a memristor triangular mapping hyperchaotic model enter an SCMA encoder for codebook mapping processing to obtain encrypted SCMA-OFDM signals, pilots are inserted into the signals for carrier synchronization, frequency domain signals obtained after the pilots and information data calculated through the memristor triangular mapping hyperchaotic model are inverse fast Fourier transformed to convert the frequency domain signals into time domain signals; then, cyclic prefixes CP and cyclic suffixes CS are added to the time domain signals to eliminate inter-subcarrier interference, parallel signals are converted into serial signals through parallel-serial conversion, and finally, complex signals are mixed into real signals through up-conversion DUC to enter a transmission channel; An overload rate calculation formula is λ = J / K > 1; After receiving the signals of the transmission channel, the receiving end reversely processes the received signals according to the flow of the sending end, and finally, the initial bit information is solved through a message passing algorithm based on spherical decoding in an SCMA decoder; The codebook mapping processing comprises: The bit information sent by the user is mapped to an N-dimensional constellation point c in a constellation set C, c∈C, and the constellation set C formed by the constellation point is called a mother codebook, and C∈C N The mapping of the mother codebook is represented as: wherein M represents the number of code words contained in each codebook, B is a binary set, and g is a mapping of a mother codebook; through a binary mapping matrix V e B K×N The N-dimensional constellation points are mapped to K-dimensional sparse code words, in the process, the SCMA encoder is redefined as: f = Vg, g is the mapping of the mother codebook, and the time-frequency resources occupied by the code word sent by user j are selected by the mapping matrix V j to select a K-row J-column sparse matrix F to represent the coding structure of SCMA, and F is represented according to the codebook mapping principle: wherein .

2. The transmission system of claim 1, wherein, The codebook mapping principle comprises: Firstly, it is assumed that the number of users is U, each user has a corresponding code table, each code table has M code words, and each code word occupies K subcarriers, of which P are non-empty subcarriers; Then the binary sequence data is mapped according to the non-empty sub-carriers in the codebook, specifically, the corresponding code word X1 in the code table 1 is selected according to the data input by the first user, and the corresponding code word XU in the code table U is selected according to the data input by the Uth user. U Finally, the code words obtained by each user are all directly added to obtain the coded data Y, Since only P carriers in each code table are non-empty, the data of (U-P-1) users will be mixed together on each carrier, although the (U-P-1) users commonly use the frequency resource of T sub-carriers, but use different code tables among each other.

3. The transmission system of claim 1, wherein, The memristor triangular mapping hyperchaotic state model is represented as: wherein x, y, and q are state variables, a, k1, k2, k3, k4, k5, and k6 are parameters, a = 100π, k1 = 1, k3 = 1, k4 = 0.1, k5 = 0.1, k6 = 0.1, and k2 = 10 are the model in the first memristor triangular mapping hyperchaotic state, k2 = 0.1 is the model in the second memristor triangular mapping hyperchaotic state, and Lyapunov exponent is an important parameter for describing the sensitivity of a chaotic system to initial values; According to the memristor triangular mapping superchaotic state model, when k2 is 0.1 and 10, the vectors composed of variables x, y and q in the superchaotic state are subjected to T times of iteration, respectively, to obtain chaotic sequences X1, X2, X3, Y1, Y2, Y3 with a length of T, and the specific corresponding relationship is as follows: (k2=10, x) X1, (k2=10, y) X2, (k2=10, q) X3, (k2=0.1, x) Y1, (k2=0.1, y) Y2, (k2=0.1, q) Y3; Six chaotic sequences are respectively preprocessed to generate masking factors A, B, C, D, E, and F.

4. The transmission system of claim 3, wherein, The calculation process of the chaotic codebook is as follows: firstly, the masking factors A and B are calculated, wherein round is a rounding function, then, B is mapped as follows: direction key wherein mod is a remainder operation function, finally, direction key*(A / 180) is calculated to generate the chaotic codebook.

5. The transmission system of claim 3, wherein, After the initial bit information is processed through codebook mapping, it is sequentially subjected to constellation rotation, subcarrier replacement, and symbol permutation, and then, pilots are inserted into the signals for carrier synchronization, the input data required for the constellation rotation comprises information calculated through the first memristor triangular mapping hyperchaotic state model, and the input data required for the subcarrier replacement and symbol permutation comprises information calculated through the second memristor triangular mapping hyperchaotic state model.

6. The transmission system of claim 5, wherein, The masking factors C, D, E, and F are represented as: Where mod is the remainder operation function, Tra is the transpose transformation algorithm, the transpose operation is to interchange the rows and columns of a given matrix to obtain a new matrix called the transpose matrix, sort is the sorting function from smallest to largest, and the superscript T is the symbol for matrix transpose; The frequency domain signal obtained after piloting and the information data calculated by the memristor triangulation hyperchaotic model are expressed using the inverse fast Fourier transform as follows: This formula is a code operation in MATLAB. The result is to shuffle and replace the original matrix according to the shuffled sequence timeX, thereby achieving the effect of encryption perturbation.

7. The transmission system of claim 6, wherein, The information data calculated by the initial value through the first memristor triangular mapping hyperchaotic state model includes: rounding and expanding the chaotic sequence X3 to obtain the radian sequence D in the interval [-π,π] as the constellation point rotation encryption masking factor, adding it to the phase angle of each constellation point to complete the constellation point rotation encryption and obtain the constellation point matrix.

8. The transmission system of claim 7, wherein, The subcarrier replacement is performed by multiplying the transpose transformation matrix E with the constellation point matrix obtained after rotating the constellation points, thus obtaining the replaced information. The symbol permutation is performed by multiplying the transpose transformation matrix F with the replaced information, as shown below: This formula is a code operation in MATLAB. The result is to randomly replace the positions of the original matrix based on the randomized sequence frequency and symbol, thereby achieving the effect of encryption perturbation.

Images