Photon terahertz communication system based on layered grid shaping technology

Abstract

The invention discloses a photon terahertz communication system based on a layered grid shaping technology, and belongs to the field of terahertz communication. Comprising the following steps: introducing hierarchical modulation and grid shaping technologies into a transmitting end, and generating terahertz signals by using photon beat frequency, and specifically comprises the following steps: firstly, generating a pseudo-random binary sequence by a TX-DSP module of the transmitting end, and dividing the pseudo-random binary sequence into a most significant bit (MSB) sequence and a low significant bit (LSB) sequence; converting the MSB sequence, combining with a control word y to obtain a sign bit sequence z, mapping the sign bit sequence z to a constellation point, and combining the sign bit sequence z and the LSB sequence to obtain a grid-shaped output sequence; and then, carrying out layered modulation on the output sequence, and modifying the proportional relation between the basic layer and the additional layer by adjusting a parameter lambda to obtain a terahertz signal. The signal is transmitted to a receiving end through an optical fiber channel, and an original signal is recovered through digital signal processing and inverse mapping. The non-linear influence is reduced while the sensitivity of a system receiver is improved, and the bit error rate performance is excellent.

Description

Technical Field

[0001] The present invention belongs to the field of terahertz communication, and particularly relates to a photonic terahertz communication system based on hierarchical grid shaping technology. Background Art

[0002] With the large-scale deployment and in-depth application of 5G technology, emerging services such as large data transmission, virtual reality, vehicle-to-everything, and ultra-high-definition video have shown explosive growth, posing higher requirements for the transmission rate, capacity, and bandwidth of communication networks. In this context, terahertz (THz) communication technology, with its wide frequency band characteristics of 0.1 - 10 THz (corresponding to wavelengths of 3000 - 30 μm), provides a new technical path for solving the problem of tight spectrum resources and is widely regarded as one of the key technologies for future 6G communication systems.

[0003] In a photonic terahertz communication system, due to the nonlinear effects of optical fiber transmission media and optoelectronic devices, signal nonlinear distortion will occur during transmission, severely restricting the improvement of system performance.

[0004] Probabilistic Shaping (PS) technology can effectively alleviate the above-mentioned nonlinear distortion problem. As an advanced modulation format optimization scheme, PS technology adjusts the probability of uniformly distributed signals through a Distribution Matcher (DM), making the constellation point distribution approximate a Gaussian distribution while keeping the shape of the constellation diagram unchanged. This technology increases the probability of signals near the center point of the constellation diagram and decreases the probability of signals far from the center point, thereby reducing the average signal power and improving the system transmission performance.

[0005] In traditional optical communication systems, the Constant Composition Distribution Matcher (CCDM) technology is often used to achieve constellation point distributions with arbitrary probabilities. However, the probability distribution accuracy of CCDM is restricted by the coding block length, resulting in accuracy loss in the case of short coding block lengths, and it has a high computational complexity. Grid shaping, as a short coding block length probabilistic shaping technology, although its probability distribution is fixed, does not have probability accuracy loss in the case of short coding block lengths.

[0006] Hierarchical Modulation technology is a constellation diagram geometry optimization technology. By adjusting the ratio of the Euclidean distances between constellation points, it realizes the differential bit error rate performance allocation among signals at different levels. Although existing research has applied hierarchical modulation technology to electronic terahertz systems to improve the transmission capacity, there is currently no research on organically combining hierarchical modulation and grid shaping technology in terms of the probability distribution of constellation points and the geometry of the constellation diagram to jointly optimize the bit error rate performance and apply it to photon terahertz communication systems.

[0007] Based on the above analysis, the innovative integration of hierarchical modulation and grid shaping technology and the construction of a 450 GHz photon terahertz communication system are expected to significantly improve the system's nonlinear effects and reduce the system's transmission bit error rate. It not only meets the urgent needs of the current development of terahertz communication technology but also has important theoretical value and practical significance for promoting the practical process of photon terahertz communication systems. Summary of the Invention

[0008] The present invention provides a photon terahertz communication system based on hierarchical grid shaping technology. By simultaneously introducing hierarchical modulation technology and grid shaping technology, the performance of the photon terahertz communication system is optimized, the sensitivity of the system receiver is improved while reducing the nonlinear influence, and it has excellent bit error rate performance.

[0009] The photon terahertz communication system includes a transmitting end and a receiving end. The transmitting end introduces hierarchical modulation and grid shaping technology, uses the method of photon beating to generate terahertz signals, transmits them through a single-mode optical fiber to the receiving end, and uses digital signal processing and inverse mapping to recover the original signals.

[0010] The transmitting end includes: two external cavity lasers ECL1 and ECL2, a TX-DSP module, an optical coupler, a single-mode optical fiber, an erbium-doped fiber amplifier, and a single-carrier photodetector;

[0011] The external cavity laser ECL1 is connected to the IQ modulator, providing a continuous optical carrier input to the IQ modulator; the IQ modulator is simultaneously connected to the TX-DSP module, which applies hierarchical modulation and grid shaping technology to generate a HMTS-64QAM baseband signal, driving the IQ modulator to modulate the input optical carrier. The modulated optical carrier and the continuous optical wave output by the external cavity laser ECL2 are coupled through an optical coupler and transmitted through a single-mode optical fiber. The coupled optical signal is transmitted to the erbium-doped fiber amplifier for amplification and finally input to the single-carrier photodetector for beating to obtain terahertz signals.

[0012] The receiving end includes: a mixer, a local oscillator, and an RX-DSP module;

[0013] The terahertz signal is input into a mixer to be mixed with the local oscillator signal generated by the local oscillator, realizing the down-conversion of the signal to obtain a baseband signal; then it is input into the RX-DSP module, and operations such as downsampling, blind equalization, frequency offset estimation, phase recovery, layered demodulation, and grid shaping are performed in sequence, and finally the original data is obtained.

[0014] The specific working processes of the grid shaping and layered modulation according to the present invention are as follows:

[0015] Step 1: The PRBS in the TX-DSP module generates a pseudo-random binary sequence [b1b2…b n , which is divided into a most significant bit (MSB) sequence and a least significant bit (LSB) sequence;

[0016] The MSB sequence is [b1], and the LSB sequence is [b2…b n ;

[0017] Step 2: The MSB sequence is used as the input of the transformation matrix (H -1 ), and through transformation, a transition sequence x is obtained; T That is:

[0018] x = s(H

[0019] ), -1 ), T

[0020] where s is the input MSB sequence;

[0021] Step 3: The transition sequence x is XOR-operated with the control word y to obtain a sign bit sequence z, which is mapped to constellation points, corresponding to obtaining two constellation point results;

[0022] That is:

[0023]

[0024] where the control word y = mG; G is the generating matrix; m is selected from the candidate input sequences {0, 1} by the Viterbi algorithm through the Euclidean distance; so the corresponding values of the control word y are y1 and y2 respectively; the sign bit sequence z correspondingly includes results z1 and z2;

[0025] Because there are two cases for the input of m, resulting in two cases for the mapped constellation points; so the part in the red dotted box is used to select the best case, that is, to determine m = 0 / m = 1;

[0026] The symbol mapping formula is:

[0027] SymMap is the symbol mapping function;

[0028] Constellation point 1 = SymMap([z1, b2…bn ; Constellation point 2 = SymMap([z2, b2…b n ;

[0029] Step Four: Combine the sign bit sequence z and the LSB sequence [b2…b n to obtain the output sequence z, b2…b of grid shaping n ;

[0030] Step Five: Perform 4 / 16QAM hierarchical modulation on the output sequence [z, b2…b n of grid shaping, adjust the proportional relationship between the basic layer and the additional layer by adjusting the parameter λ, obtain the HMTS-64QAM signal and send it to the optical fiber channel for transmission;

[0031] The principle of combining grid shaping and hierarchical modulation is as follows:

[0032] I 64QAM +1i*Q 64QAM = amp1*(SymMap(z, 4QAM)) + amp2*(SymMap([b2…b n , 16QAM))

[0033] Basic layer: I 4QAM +1i*Q 4QAM = amp1*SymMap(z, 4QAM)

[0034] Additional layer: I 16QAM +1i*Q 16QAM = amp2*SymMap([b2…b n , 16QAM)

[0035] λ = amp2 / (amp1 - 3*amp2)

[0036] Where: I and Q represent the in-phase component and the quadrature component of the constellation point respectively, (I 4QAM +1i*Q 4QAM ) is the constellation point of the basic layer of sign bit mapping, (I 16QAM +1i*Q 16QAM ) is the constellation point of the additional layer of LSB mapping; amp1 and amp2 are the corresponding amplitudes of the basic layer and the additional layer respectively; the basic layer refers to the modulation level with a larger Euclidean distance interval; the additional layer refers to the modulation level with a smaller Euclidean distance interval generated around the constellation point of the basic layer.

[0037] Step Six: The receiving end performs DSP processing on the received signal, including downsampling, blind equalization, frequency offset estimation, and phase recovery, and then performs hierarchical demodulation on the transmitted HMTS-64QAM signal to convert it into a TS-64QAM constellation point bit stream;

[0038] Step 7: Demap the TS-64QAM constellation point bitstream and perform reshaping to restore the original transmitted sequence.

[0039] The advantages of the present invention are as follows:

[0040] 1. A photonic terahertz communication system based on hierarchical trellis shaping technology adjusts the probability distribution of constellation points through the hierarchical trellis shaping method, reduces the probability of external constellation points, increases the probability of internal constellation points, thereby reducing the average power of the system, alleviating the nonlinear influence of the system, and improving the bit error rate performance of the communication system.

[0041] 2. A photonic terahertz communication system based on hierarchical trellis shaping technology adjusts the Euclidean distance between the basic layer and the additional layer through the hierarchical modulation method, optimizes the shape of the constellation diagram, and improves the receiver sensitivity and robustness of the communication system.

[0042] 3. A photonic terahertz communication system based on hierarchical trellis shaping technology maps the symbol bits and amplitude bits in the trellis shaping to the basic layer and the additional layer respectively through the hierarchical modulation method, reducing the deterioration of the bit error rate performance caused by symbol bit changes. Description of the Drawings

[0043] Figure 1 It is a structural diagram of a photonic terahertz communication system based on hierarchical trellis shaping technology according to the present invention;

[0044] Figure 2 It is a flow chart of hierarchical modulation and trellis shaping according to the present invention;

[0045] Figure 3 It is a mapping relationship diagram of the bit sequence and constellation points according to the present invention;

[0046] Figure 4 It is a structural schematic diagram of hierarchical modulation and trellis shaping adopted by the present invention;

[0047] Figure 5 It is an enlarged schematic diagram of the hierarchical modulation constellation point structure adopted by the present invention;

[0048] Figure 6 It is a comparison diagram of bit error rate curves under different optical signal-to-noise ratio conditions according to the present invention;

[0049] Figure 7 It is a constellation point diagram recovered at the receiving end when the optical signal-to-noise ratio is 18 dB after optical fiber transmission according to the present invention. Detailed Embodiments

[0050] To facilitate the understanding and implementation of the present invention by those of ordinary skill in the art, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the scope of protection of the present invention.

[0051] Based on the probability shaping technology of trellis shaping (TS) and the constellation geometry distribution modulation technology of hierarchical modulation (HM), the present invention proposes a photon terahertz communication system based on hierarchical trellis shaping technology. The trellis shaping technology is introduced at the transmitting end to achieve the effect of increasing the system transmission capacity and improving the bit error rate performance. At the same time, hierarchical modulation is used to improve the system noise resistance performance, and the symbol bits of the trellis shaping are hierarchically protected to further improve the system robustness.

[0052] As Figure 1 shown, the photon terahertz communication system includes a transmitting end and a receiving end. The transmitting end introduces hierarchical modulation and trellis shaping technologies, uses the method of photon beating to generate terahertz signals, transmits them through a 20 km single-mode optical fiber to the receiving end, and uses digital signal processing (DSP), de-hierarchical modulation, de-trellis shaping and inverse mapping to recover the original signals.

[0053] The transmitting end includes: two external cavity lasers ECL1 and ECL2, a TX-DSP module, an optical coupler, a 20 km single-mode optical fiber, an erbium-doped fiber amplifier (EDFA) and a uni-travelling-carrier photodetector (UTCPD);

[0054] The external cavity laser ECL1 is connected to the IQ modulator to provide a continuous optical carrier input to the IQ modulator. The IQ modulator is simultaneously connected to the TX-DSP module, which applies hierarchical modulation and trellis shaping technologies to generate an HMTS-64QAM baseband signal, drives the IQ modulator to modulate the input optical carrier. The modulated optical carrier is coupled with the continuous optical wave output by the external cavity laser ECL2 through the optical coupler OC, transmitted through a 20 km single-mode optical fiber, the coupled optical signal is amplified by the erbium-doped fiber amplifier (EDFA), and finally input to the uni-travelling-carrier photodetector (UTC-PD) for beating to obtain terahertz signals.

[0055] Two parallel electrical amplifiers EA are used to amplify the HMTS-64QAM signal generated by the TX-DSP.

[0056] An optical coupler OC is used to couple the optical wave output by the IQ modulator with the continuous optical wave output by ECL2.

[0057] A uni-traveling carrier photodetector (UTC-PD) is used to beat the optical signal to obtain a terahertz signal.

[0058] The receiving end includes: a mixer, a local oscillator (LO), and an RX-DSP module;

[0059] The mixer is used to down-convert the received signal to obtain a fundamental-frequency terahertz signal.

[0060] The local oscillator (LO) is the input of the mixer.

[0061] The RX-DSP module is applied to the DSP processing of the receiving-end signal, and the signal recovery is completed through hierarchical demodulation and grating shaping.

[0062] The terahertz signal is input into the mixer and mixed with the local oscillator signal generated by the local oscillator to realize the down-conversion of the signal and obtain a fundamental-frequency terahertz signal; then it is input into the RX-DSP module, and operations such as downsampling, blind equalization, frequency offset estimation, phase recovery, hierarchical demodulation, and grating shaping are performed in sequence, and finally the original data is obtained.

[0063] As Figure 2 and Figure 4 shown, the specific working process of the grating shaping and hierarchical modulation of the present invention is as follows:

[0064] Step 1: The PRBS in the TX-DSP module generates a uniformly distributed pseudo-random binary sequence [b1b2…b n , after grating shaping, according to the mapping rule of the grating shaping coding, the shaping of the input constellation points is completed to obtain the output sequence [z,b2…b n ;

[0065] Specifically:

[0066] Step 101: Divide the pseudo-random binary sequence [b1b2…b n bit stream into the most significant bit (MSB) and the least significant bits (LSB);

[0067] The MSB sequence is [b1], and the LSB sequence is [b2…b n ;

[0068] As Figure 3As shown, it is the mapping relationship between the bit sequence and the constellation points; the 16QAM on the left is the constellation points for LSB mapping, and the quadrants on the right are determined by mapping with 2-bit symbol bits.

[0069] Step 102: Transform the MSB sequence through the transformation matrix (H -1 ) T to obtain the intermediate sequence x;

[0070] That is: x = s(H -1 ) T ; s is the input MSB sequence;

[0071] Step 103: Perform an exclusive OR operation on the intermediate sequence x and the control word y to obtain the constellation point symbol bit sequence z, and the LSB is mapped to the 16QAM constellation points according to the Gray coding method.

[0072] That is: where the control word y = mG; G is the generating matrix; m is selected from the candidate input sequences {0, 1} by the Viterbi algorithm through the Euclidean distance; so the corresponding values of the control word y are y1 and y2 respectively; the symbol bit sequence z correspondingly includes the results z1 and z2; because there are two cases for the input of m, there are two cases for the mapped constellation points; so the part in the red dashed box is used to select the best case, that is, to determine m = 0 / m = 1;

[0073] The symbol mapping formula is:

[0074] SymMap is the symbol mapping function;

[0075] Constellation point 1 = SymMap([z1, b2…b n ; Constellation point 2 = SymMap([z2, b2…b n ;

[0076] Step 104: Combine the symbol bit sequence z and the LSB sequence [b2…b n to obtain the output sequence after trellis shaping as [z, b2…b n ;

[0077] Step Two: Perform 64QAM constellation mapping on the output sequence [z, b2…b n after trellis shaping to obtain the TS-64QAM signal, perform 4 / 16QAM hierarchical modulation, and adjust the proportional relationship between the basic layer and the additional layer by adjusting the parameter λ to obtain the HMTS-64QAM signal and send it to the optical fiber channel for transmission;

[0078] The process of hierarchical modulation is as follows:

[0079] Step 201: Divide the 64QAM constellation points into two layers of 4 / 16QAM. Use the symbol bit sequence z of the TS-64QAM constellation points as the basic layer data for layered modulation, and modulate it onto the basic layer constellation points, i.e., 4QAM; use the LSB part as the additional layer data for layered modulation, and modulate it onto the additional layer constellation points, i.e., 16QAM.

[0080] Step 202: Adjust the layered modulation ratio λ to select the appropriate ratio of layered constellation points.

[0081] Where: λ = amp2 / (amp1 - 3 * amp2), and amp1 and amp2 are the corresponding amplitudes of the basic layer and the additional layer respectively; as Figure 5 shown;

[0082] Step 203: Superimpose the amplitudes of the basic layer constellation points and the additional layer constellation points to obtain the HM-TS-64QAM constellation points, i.e.:

[0083] I 64QAM +1i*Q 64QAM = amp1 * (I 4QAM +1i*Q 4QAM ) + amp2 * (I 16QAM +1i*Q 16QAM )

[0084] The basic layer constellation points mapped by symbol bits: I 4QAM +1i*Q 4QAM = amp1 * SymMap(z, 4QAM)

[0085] The additional layer constellation points mapped by LSB: I 16QAM +1i*Q 16QAM = amp2 * SymMap([b2…b n , 16QAM) That is, the principle of combining grid shaping and layered modulation is:

[0086] I 64QAM +1i*Q 64QAM = amp1 * (SymMap(z, 4QAM)) + amp2 * (SymMap([b2…b n , 16QAM))

[0087] Where: I and Q respectively represent the in-phase component and the quadrature component of the constellation points;

[0088] The basic layer refers to the modulation level with a larger Euclidean distance interval, such as 4QAM modulation; the additional layer refers to the modulation level with a smaller Euclidean distance interval generated around the basic layer constellation points, such as 16QAM modulation.

[0089] Step 3: The receiving end performs DSP processing on the received signal, including downsampling, blind equalization, frequency offset estimation, and phase recovery, and then performs hierarchical demodulation on the transmitted HMTS-64QAM signal to convert it into a TS-64QAM constellation point bit stream;

[0090] The steps of hierarchical demodulation are as follows:

[0091] Step 301: Perform 4QAM demodulation on the basic layer of the received signal to obtain the symbol bit sequence z;

[0092] Step 302: According to the symbol bit sequence z, migrate the center coordinates of the additional layer constellation points in the quadrant where they are located and perform 16QAM demodulation on the additional layer to obtain the additional layer data LSB;

[0093] Step 4: Perform degridding shaping on the TS-64QAM data to restore the original transmission sequence.

[0094] The steps of degridding shaping are as follows:

[0095] Step 401: Pass the symbol bit sequence z through the matrix H T to transform it into a 1-bit MSB sequence;

[0096] where the matrix H T has the following expression: H T = [1 + D + D 2 , 1 + D 2 T

[0097] Step 402: Concatenate the additional layer data LSB and MSB to restore the original data sequence.

[0098] By simultaneously introducing hierarchical modulation technology and gridding shaping technology in the photon terahertz communication system, the performance of the system is optimized, the sensitivity of the system receiver is improved, and the nonlinear effect is reduced, and it has excellent bit error rate performance.

[0099] Example:

[0100] The basic structure of the terahertz communication system mainly consists of a laser, an optical modulator, an arbitrary waveform generator AWG, a UTC-PD, and a receiver. At the transmitting end, the baseband signal generated by the arbitrary waveform generator is modulated by the optical modulator onto the continuous optical wave generated by the laser, and then sent to the UTC-PD to perform beat frequency with another continuous optical wave to generate a terahertz signal with a bandwidth equal to the frequency difference between the two continuous optical waves. At the receiving end, the received signal is first down-converted to the base frequency by the local oscillator signal, and then corresponding offline digital signal processing is performed to recover the original signal.

[0101] This embodiment is mainly built using two software, namely VPIphotonics Design Suite 11.1 and MATLAB R2022b. The overall structure of the system is built on VPI, and signal transmission and reception processing are carried out on MATLAB, making digital signal processing highly flexible and realizing the transmission, transmission, and reception of HMTS-64QAM signals.

[0102] Specifically: At the transmitting end, an external cavity laser (ECL1) generates a continuous optical carrier CW1 with a wavelength of 1555.412 nm, an output power of 11.8 dBm, and a linewidth of 100 kHz. ECL2 generates a continuous optical wave CW2 with a wavelength of 1551.791 nm, an output power of 9.5 dBm, and a linewidth of 100 kHz. The frequency difference between CW1 and CW2 is 450 GHz. The TX-DSP part jointly tunes VPI and MATLAB through a co-simulation module, and a baseband signal is generated by the MATLAB side and sent into the VPI system; at the receiving end, the received signal is directly mixed with a local oscillator signal of the same frequency, down-converted to the baseband, and then sent to the MATLAB side for digital signal processing, saving one down-conversion operation.

[0103] TX-DSP includes grid shaping and hierarchical modulation, and the specific steps are as follows:

[0104] Step 1: Generate a pseudo-random binary sequence, and calculate the number of bits and the total number of symbols of the pseudo-random binary sequence in the 64QAM modulation mode.

[0105] Step 2: Input the pseudo-random binary sequence into a grid shaping matcher for grid shaping (TS).

[0106] Specifically:

[0107] The pseudo-random binary sequence is divided into two data streams, MSB and LSB, according to the frame structure. The MSB passes through a matrix (H -1 ) T transform to obtain a 2-bit data stream x; after performing an exclusive OR operation between x and a control word y, a symbol bit sequence z is obtained. Combining z with the LSB results in a mapped constellation point bit sequence;

[0108] y is a control word generated through a generator matrix G U generate; the Viterbi Algorithm is used to select and generate the optimal control word y with the minimum energy and the most as the judgment criterion.

[0109] The output bit stream sequence after combining z and the LSB is the TS-64QAM bit stream obtained after completing grid shaping.

[0110] Among them: G U =[1 + D2 , 1 + D + D 2 ;

[0111] The specific process is as follows:

[0112] Step 101: The frame structure division method is to group every n bits, where the MSB contains 1 bit and the LSB contains n - 1 bits, and n = log2(N), where N is the constellation point modulation order;

[0113] Step 102: The matrix transformation is to input 1 bit MSB into the (H -1 ) T matrix to obtain a 2 - bit sequence x

[0114] where: (H -1 ) T = [D, 1 + D];

[0115] Step 103: The Viterbi algorithm selects the optimal control word y according to the principle of minimum energy. After performing modulo - 2 addition operation between y and x, the optimal symbol bit sequence z is output.

[0116] Step 3: Perform hierarchical modulation on the TS - 64QAM bit stream to obtain the HMTS - 64QAM signal.

[0117] Specifically: First, find a suitable hierarchical ratio λ to adjust the Euclidean distance between the basic layer and the additional layer, as Figure 5 shown. Modulate z with 4QAM to obtain the basic layer constellation points, and modulate the LSB with 16QAM to obtain the additional layer constellation points. The final output 64QAM constellation points obtained by superimposing the basic layer and the additional layer constellation points are:

[0118] I 64QAM + 1i * Q 64QAM = amp1 * (I 4QAM + 1i * Q 4QAM ) + amp2 * (I 16QAM + 1i * Q 16QAM )

[0119] Step 4: Transmit the signal after trellis shaping and hierarchical modulation into the optical fiber channel for transmission.

[0120] After the 20 GHz baseband signal generated by the TX-DSP module is sent into the system, it is divided into two paths of I and Q. After being amplified by two parallel electrical amplifiers (EAs) with a gain of 25 dB, it is modulated onto the optical carrier generated by CW1 in the IQ modulator, and then coupled with CW2. The coupled signal is amplified by an erbium-doped fiber amplifier (EDFA) after passing through a 20-km single-mode optical fiber, and then a terahertz wave signal with a bandwidth of 430 GHz - 470 GHz is obtained by beating with a UTC-PD. In this embodiment, 450 GHz is selected.

[0121] Subsequently, the signal is sent into the RX-DSP for mixing with the local oscillator signal of 450 GHz and down-converted to the baseband frequency. Then, resampling and corresponding demodulation and recovery processing are performed on the signal, including channel equalization, frequency offset estimation, phase recovery, layered demodulation, and degridding shaping.

[0122] The specific process of layered demodulation is as follows:

[0123] Step 1: Demodulate the received signal processed by the DSP in the 4QAM mode to obtain the 2-bit basic layer z;

[0124] Step 2: Perform scaling and translation transformation on the constellation points according to the z demodulation type and the set λ value to transform the coordinate origin;

[0125] The specific constellation point transformation rule is as follows:

[0126]

[0127] Step 3: Demodulate the additional layer constellation points after coordinate translation transformation in the 16QAM mode to obtain the additional layer data LSB.

[0128] The specific process of degridding shaping is as follows:

[0129] Step 1: Transform and recover z obtained by layered demodulation through the matrix H T to obtain the MSB; the matrix expression is as follows:

[0130] H T =[1 + D + D 2 , 1 + D 2 T

[0131] Step 2: Connect the MSB and the LSB to recover the original bit sequence.

[0132] The present invention simultaneously applies the degridding shaping and layered modulation technologies to the photon terahertz communication system, which has a good effect on improving the system bit error rate performance. When the OSNR continuously increases, the relationship between the measured BER performance and the OSNR magnitude is as Figure 6 shown. It can be seen from the figure that as the OSNR increases, the system BER gradually decreases. Figure 7 ​The signal constellation diagram recovered at the receiving end when the optical fiber length is 20 km and the OSNR is 18 dB is shown. (a) is the constellation diagram without adding TS and HM, and (b) is the constellation diagram after adding TS and HM. Compared with the system without grid shaping and layered modulation, the present invention improves the system bit error rate performance by 18.9 dB and the receiver sensitivity by 3.5 dB.

Claims

1. A photonic terahertz communication system based on layered grid shaping technology, characterized in that: It includes a transmitting end and a receiving end. The transmitting end introduces layered modulation and grid shaping technology, uses the photon beat frequency method to generate terahertz signals, transmits them to the receiving end via a single-mode optical fiber, and uses digital signal processing and inverse mapping to restore the original signal. The specific working process of the grid shaping and layered modulation is as follows: Step 1: The TX-DSP module at the transmitter generates a uniformly distributed pseudo-random binary sequence [b1b2…b n ], after grid shaping, 64QAM constellation mapping is performed according to the mapping rule of grid shaping coding to obtain TS-64QAM signal; Step 2: Perform 4 / 16QAM hierarchical modulation on the grid-shaped output TS-64QAM signal, modify the proportional relationship between the basic layer and the additional layer by adjusting the parameter λ, obtain the HMTS-64QAM signal and send it to the optical fiber channel for transmission; The principle of combining grid shaping and hierarchical modulation is: I 64QAM +1i*Q 64QAM =amp1*(SymMap(z,4QAM))+amp2*(SymMap([b2…b n ],16QAM)) SymMap is a symbolic mapping function; z is a sequence of symbolic bits; Basic layer: I 4QAM +1i*Q 4QAM =amp1*SymMap(z,4QAM) Additional layer: I 16QAM +1i*Q 16QAM =amp2*SymMap([b2…b n ],16QAM) λ=amp2 / (amp1-3*amp2) Where: I, Q represent the in-phase component and quadrature component of the constellation point respectively, (I 4QAM +1i*Q 4QAM ) is the basic layer constellation point for the sign bit mapping, (I 16QAM +1i*Q 16QAM ) is the additional layer constellation point mapped by LSB; amp1 and amp2 are the corresponding amplitudes of the basic layer and the additional layer respectively; Step 3: After the receiving end performs DSP processing on the received signal, the transmitted HMTS-64QAM signal is hierarchically demodulated and converted into a TS-64QAM constellation point bit stream; Step 4: De-grid and reshape the TS-64QAM constellation point bit stream to restore the original transmission sequence.

2. A photonic terahertz communication system based on layered grid shaping technology as claimed in claim 1, characterized in that: The transmitting end comprises: two external cavity lasers ECL1 and ECL2, a TX-DSP module, an optical coupler, a single-mode optical fiber, an erbium-doped fiber amplifier and a single-line carrier photodetector; The external cavity laser ECL1 is connected to the IQ modulator to provide a continuous optical carrier input to the IQ modulator; the IQ modulator is also connected to the TX-DSP module, which uses layered modulation and grid shaping technology to generate HMTS-64QAM baseband signals, drives the IQ modulator to modulate the input optical carrier, and the modulated optical carrier is coupled with the continuous light wave output by the external cavity laser ECL2 through an optical coupler through a single-mode optical fiber. The coupled optical signal is transmitted to an erbium-doped fiber amplifier for amplification, and finally input into a single-line carrier photodetector for beat frequency to obtain a terahertz signal.

3. A photonic terahertz communication system based on layered grid shaping technology as claimed in claim 1, characterized in that: The receiving end comprises: a mixer, a local oscillator and an RX-DSP module; The terahertz signal is input into the mixer and mixed with the local oscillator signal generated by the local oscillator to achieve signal down-conversion and obtain the baseband signal; then it is input into the RX-DSP module and sequentially performs down-sampling, blind equalization, frequency offset estimation, phase recovery, hierarchical demodulation and de-gridding to obtain the original data.

4. A photonic terahertz communication system based on layered grid shaping technology as claimed in claim 1, characterized in that: The specific process of step 1 is: Step 101: The PRBS in the TX-DSP module generates a pseudo-random binary sequence [b1b2…b n ], divided into the most significant bit MSB sequence and the least significant bit LSB sequence; The MSB sequence is [b1], and the LSB sequence is [b2…b n ]; Step 102: Use the MSB sequence as the conversion matrix (H -1 ) T The input is transformed to obtain the transition sequence x; Right now: x=s(H -1 ) T s is the MSB sequence of the input; Step 103, perform an XOR operation on the transition sequence x and the control word y to obtain a sign bit sequence z, map it to a constellation point, and obtain two corresponding constellation point results; Right now: Wherein, control word y=mG; G is the generator matrix; m is selected from the candidate input sequence {0,1} by the Viterbi algorithm through the Euclidean distance; so the corresponding control word y takes values ​​y1 and y2 respectively; the sign bit sequence z includes the results z1 and z2; The symbol mapping formula is: Constellation point 1 = SymMap([z1,b2…b n ]; Constellation point 2 = SymMap([z2,b2…b n ]; Step 104: Combine the sign bit sequence z and the LSB sequence [b2…b n ] to obtain the grid-shaped output sequence [z, b2…b n ]; Step 105: Output the sequence [z, b2…b n ]According to the mapping rules, 64QAM constellation mapping is performed to obtain TS-64QAM signal.

5. The photonic terahertz communication system based on layered grid shaping technology according to claim 1, characterized in that: The base layer refers to a modulation level with a larger Euclidean distance interval; the additional layer refers to a modulation level with a smaller Euclidean distance interval generated around the base layer constellation point.

6. A photonic terahertz communication system based on layered grid shaping technology as claimed in claim 1, characterized in that: In the step three, the DSP processing includes down sampling, blind equalization, frequency offset estimation and phase recovery.

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