A photonic terahertz communication system based on layered grid shaping technology

Abstract

The present invention discloses a photonic terahertz communication system based on layered grid shaping technology, which belongs to the field of terahertz communication. The system includes introducing layered modulation and grid shaping technology at the transmitting end and using photon beat frequency to generate a terahertz signal. Specifically, the system comprises: first, the TX-DSP module at the transmitting end generates a pseudo-random binary sequence, which is divided into a most significant bit (MSB) sequence and a least significant bit (LSB) sequence; the MSB sequence is transformed and combined with a control word (y) to obtain a sign bit sequence (z), which is mapped to a constellation point; the sign bit sequence (z) and the LSB sequence are combined to obtain a grid-shaped output sequence; then, the output sequence is layered modulated, and the proportional relationship between the basic layer and the additional layer is modified by adjusting the parameter λ to obtain a terahertz signal. The system is transmitted to the receiving end via an optical fiber channel, and the original signal is restored through digital signal processing and inverse mapping. The present invention improves the sensitivity of the system receiver while reducing nonlinear effects, and has excellent bit error rate performance.

Description

Technical Field

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

[0002] With the large-scale deployment and extensive application of 5G technology, emerging services such as big data transmission, virtual reality, connected vehicles, and ultra-high-definition video are experiencing explosive growth, placing higher demands on the transmission speed, capacity, and bandwidth of communication networks. In this context, terahertz (THz) communication technology, with its wideband bandwidth of 0.1 to 10 THz (corresponding to a wavelength of 3000 to 30 μm), provides a new technical path to addressing the shortage of spectrum resources and is widely considered a key technology for future 6G communication systems.

[0003] In photonic terahertz communication systems, due to the nonlinear effects of optical fiber transmission media and optoelectronic devices, signals will produce nonlinear distortion during transmission, which seriously restricts the improvement of system performance.

[0004] Probabilistic Shaping (PS) technology can effectively alleviate the aforementioned nonlinear distortion issues. As an advanced modulation format optimization scheme, PS uses a distribution matcher (DM) to adjust the probability of uniformly distributed signals, making the constellation points approach a Gaussian distribution while maintaining the shape of the constellation. This technology increases the probability of signals near the center of the constellation while reducing the probability of signals far from the center, thereby reducing average signal power and improving system transmission performance.

[0005] In traditional optical communication systems, the Constant Composition Distribution Matcher (CCDM) technology is often used to achieve arbitrary probability constellation point distribution. However, the accuracy of CCDM probability distribution is constrained by the coding block length, resulting in accuracy loss when the coding block length is short, and the computational complexity is high. Grid shaping, as a short coding block length probability shaping technology, has a fixed probability distribution but does not suffer from probability accuracy loss when the coding block length is short.

[0006] Hierarchical modulation is a constellation geometry optimization technique that achieves differentiated bit error rate performance distribution among signals at different levels by adjusting the Euclidean distance ratio between constellation points. While previous studies have applied hierarchical modulation to electronic terahertz systems to improve transmission capacity, no research has yet organically combined hierarchical modulation with grid shaping techniques to optimize bit error rate performance based on the probability distribution of constellation points and constellation geometry, and applied this to photonic terahertz communication systems.

[0007] Based on the above analysis, the innovative integration of hierarchical modulation and grid shaping technology to construct a 450GHz photonic terahertz communication system is expected to significantly improve the system's nonlinear effects and reduce the system's transmission bit error rate. This 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 application of photonic terahertz communication systems. Summary of the Invention

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

[0009] The photonic terahertz communication system includes a transmitting end and a receiving end. The transmitting end introduces layered modulation and grid shaping technology, uses a photon beat frequency method to generate a terahertz signal, transmits it to the receiving end via a single-mode optical fiber, and uses digital signal processing and inverse mapping to restore the original signal.

[0010] 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;

[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 also connected to the TX-DSP module, which uses layered modulation and grid shaping technology to generate an HMTS-64QAM baseband signal, driving the IQ modulator to modulate the input optical carrier. The modulated optical carrier and the continuous light wave output by the external cavity laser ECL2 are coupled 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, generating a terahertz signal.

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

[0013] 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, layered demodulation and de-gridding operations to finally obtain the original data.

[0014] The specific working process of the grid shaping and layered modulation described in the present invention is as follows:

[0015] Step 1: 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;

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

[0017] Step 2: Use the MSB sequence as the conversion matrix (H -1 ) T The input is transformed to obtain the transition sequence x;

[0018] Right now:

[0019] x=s(H -1 ) T

[0020] s is the MSB sequence of the input;

[0021] Step 3: Perform an XOR operation on the transition sequence x and the control word y to obtain a sign bit sequence z, which is mapped to a constellation point to obtain two corresponding constellation point results;

[0022] Right now:

[0023]

[0024] Where 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 the values ​​y1 and y2 respectively; the sign bit sequence z includes the results z1 and z2;

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

[0026] The symbol mapping formula is:

[0027] SymMap is a symbolic mapping function;

[0028] Constellation point 1 = SymMap([z1,b2…bn ],64QAM); constellation point 2 = SymMap([z2,b2…b n ],64QAM);

[0029] Step 4: Replace the sign bit sequence z and the LSB sequence [b2…b n ] to obtain the grid-shaped output sequence z,b2…b n ];

[0030] Step 5: Reshape the output sequence [z,b2…b n ] performs 4 / 16QAM layered modulation, modifies the ratio between the basic layer and the additional layer by adjusting the parameter λ, obtains the HMTS-64QAM signal and sends it to the optical fiber channel for transmission;

[0031] The principle of combining grid shaping and layered modulation is:

[0032] I 64QAM +1i*Q 64QAM

[0033] =amp1*(SymMap(z,4QAM))+amp2*(SymMap([b2…b n ],16QAM))

[0034] Basic layer: I 4QAM +1i*Q 4QAM =amp1*SymMap(z,4QAM)

[0035] Additional layer: I 16QAM +1i*Q 16QAM =amp2*SymMap([b2…b n ],16QAM)

[0036] λ=amp2 / (amp1-3*amp2)

[0037] Where: I, Q represent the in-phase component and quadrature component of the constellation point respectively, (I 4QAM +1i*Q 4QAM ) is the base layer constellation point of 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; the basic layer refers to the modulation layer with a larger Euclidean distance interval; the additional layer refers to the modulation layer with a smaller Euclidean distance interval generated around the basic layer constellation point.

[0038] Step 6: The receiving end performs DSP processing on the received signal, including downsampling, blind equalization, frequency offset estimation, and phase recovery. Then, the transmitted HMTS-64QAM signal is hierarchically demodulated and converted into a TS-64QAM constellation point bit stream.

[0039] Step 7: De-gridding and reshaping the TS-64QAM constellation point bit stream to restore the original transmission sequence.

[0040] The advantages of the present invention are:

[0041] 1. A photonic terahertz communication system based on layered grid shaping technology adjusts the probability distribution of constellation points through layered grid shaping, reducing the probability of external constellation points appearing and increasing the probability of internal constellation points appearing, thereby reducing the system average power, alleviating the impact of system nonlinearity, and improving the bit error rate performance of the communication system.

[0042] 2. A photonic terahertz communication system based on layered grid shaping technology adjusts the Euclidean distance between the basic layer and the additional layer through a layered modulation method, optimizes the constellation shape, and improves the sensitivity and robustness of the communication system receiver.

[0043] 3. A photonic terahertz communication system based on layered grid shaping technology maps the sign bit and amplitude bit in the grid shaping to the basic layer and additional layer respectively through a layered modulation method, reducing the bit error rate performance degradation caused by the change of the sign bit. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 This is a structural diagram of a photonic terahertz communication system based on layered grid shaping technology of the present invention;

[0045] Figure 2 This is a flow chart based on hierarchical modulation and grid shaping of the present invention;

[0046] Figure 3 This is a mapping relationship diagram between the bit sequence and constellation points of the present invention;

[0047] Figure 4 This is a schematic diagram of the structural principle of the hierarchical modulation and grid shaping adopted by the present invention;

[0048] Figure 5 This is an enlarged schematic diagram of the hierarchical modulation constellation point structure adopted by the present invention;

[0049] Figure 6 This is a comparison diagram of bit error rate curves under different optical signal-to-noise ratio conditions of the present invention;

[0050] Figure 7 This is the constellation point diagram restored by the receiving end when the optical signal-to-noise ratio (OSNR) of the optical fiber transmission is 18 dB. DETAILED DESCRIPTION

[0051] To facilitate understanding and implementation of the present invention by those skilled in the art, the present invention is further described below in detail with reference to the accompanying drawings and embodiments. It is apparent that the embodiments described are merely partial embodiments of the present invention, not all embodiments. All other embodiments derived by those skilled in the art based on the embodiments of the present invention without creative effort shall fall within the scope of protection of the present invention.

[0052] Based on the probability shaping technology of trellis shaping (TS) and the constellation geometric distribution modulation technology based on hierarchical modulation (HM), the present invention proposes a photonic terahertz communication system based on hierarchical trellis shaping technology. Trellis shaping technology is introduced at the transmitting end to increase the system transmission capacity and improve the bit error rate performance. At the same time, hierarchical modulation is used to improve the system's anti-noise performance, and hierarchical protection is performed on the trellis shaping symbol bits to further improve the system robustness.

[0053] like Figure 1 As shown, the photonic terahertz communication system 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 a terahertz signal, and transmits it to the receiving end through a 20km single-mode optical fiber. Digital signal processing (DSP), de-layered modulation, de-grid shaping and inverse mapping are used to restore the original signal.

[0054] The transmitting end includes: two external cavity lasers ECL1 and ECL2, a TX-DSP module, an optical coupler, a 20km single-mode optical fiber, an erbium-doped fiber amplifier (EDFA) and a single-pass carrier photodetector (UTCPD);

[0055] 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 also connected to the TX-DSP module, which uses layered modulation and grid shaping technology to generate an HMTS-64QAM baseband signal, driving the IQ modulator to modulate the input optical carrier. The modulated optical carrier and the continuous light wave output by the external cavity laser ECL2 are coupled through an optical coupler OC and transmitted through a 20km single-mode optical fiber to an erbium-doped fiber amplifier (EDFA) for amplification. Finally, it is input into a single-carrier photodetector (UTC-PD) for beat frequency, generating a terahertz signal.

[0056] Two parallel electrical amplifiers EA are used to amplify the HMTS-64QAM signal generated by TX-DSP.

[0057] The optical coupler OC is used to couple the light wave output by the IQ modulator with the continuous light wave output by ECL2.

[0058] The single-column charge carrier photodetector (UTC-PD) is used to beat the optical signal to obtain a terahertz signal.

[0059] The receiving end includes: a mixer (Mixer), a local oscillator (LO) and an RX-DSP module;

[0060] The mixer is used to down-convert the received signal to obtain a baseband terahertz signal.

[0061] The local oscillator (LO) is the input to the mixer.

[0062] The RX-DSP module is used for DSP processing of the receiving end signal as well as hierarchical demodulation and de-gridding to complete signal recovery.

[0063] 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 terahertz signal; then it is input into the RX-DSP module and sequentially performs down-sampling, blind equalization, frequency offset estimation, phase recovery, layered demodulation and de-gridding operations to finally obtain the original data.

[0064] like Figure 2 and Figure 4 As shown, the specific working process of the grid shaping and layered modulation of the present invention is as follows:

[0065] Step 1: The PRBS in the TX-DSP module generates a uniformly distributed pseudo-random binary sequence [b1b2…b n ], after grid shaping, the input constellation points are shaped according to the mapping rules of the grid shaping code to obtain the output sequence [z, b2…b n ];

[0066] Specifically:

[0067] Step 101: Substitute the pseudo-random binary sequence [b1b2…b n ]The bit stream is divided into the most significant bit (MSB) and the least significant bits (LSB);

[0068] The MSB sequence is [b1], and the LSB sequence is [b2…b n ];

[0069] like Figure 3As shown in FIG, the mapping relationship between the bit sequence and the constellation point; the 16QAM on the left is the LSB mapping constellation point, and the quadrant on the right is determined by the 2-bit sign bit mapping.

[0070] Step 102: Transform the MSB sequence through the conversion matrix (H -1 ) T The transformation of , we get the transition sequence x;

[0071] That is: x=s(H -1 ) T ; s is the MSB sequence of the input;

[0072] Step 103: Perform an XOR operation on the transition sequence x and the control word y to obtain a constellation point sign bit sequence z. The LSB is mapped to the 16QAM constellation point according to Gray coding.

[0073] Right now: Where control word y = mG; G is the generator matrix; m is selected from the candidate input sequence {0, 1} by the Viterbi algorithm using 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; because there are two cases for the m input, there are two mapping constellation points; the red dotted box part is used to select the best case, that is, to determine m = 0 / m = 1;

[0074] The symbol mapping formula is:

[0075] SymMap is a symbolic mapping function;

[0076] Constellation point 1 = SymMap([z1,b2…b n ],64QAM); constellation point 2 = SymMap([z2,b2…b n ],64QAM);

[0077] Step 104: Combine the sign bit sequence z and the LSB sequence [b2…b n ] are merged to obtain the grid-shaped output sequence [z,b2…b n ];

[0078] Step 2: Reshape the output sequence [z,b2…b n ] Perform 64QAM constellation mapping to obtain TS-64QAM signal, perform 4 / 16QAM layered modulation, modify the ratio between the basic layer and the additional layer by adjusting the parameter λ, obtain HMTS-64QAM signal and send it to the optical fiber channel for transmission;

[0079] The hierarchical modulation process is as follows:

[0080] Step 201: Divide the 64QAM constellation points into two layers: 4QAM and 16QAM. Use the symbol bit sequence z of the TS-64QAM constellation point as the base layer data for layered modulation and modulate it onto the base layer constellation point, i.e., 4QAM. Use the LSB portion as the additional layer data for layered modulation and modulate it onto the additional layer constellation point, i.e., 16QAM.

[0081] Step 202: Adjust the hierarchical modulation ratio λ to select hierarchical constellation points of appropriate ratio;

[0082] Where: λ = amp2 / (amp1-3*amp2), amp1 and amp2 are the amplitudes of the basic layer and the additional layer respectively; Figure 5 As shown in;

[0083] Step 203: The basic layer constellation point and the additional layer constellation point amplitude are superimposed to obtain the HM-TS-64QAM constellation point, namely:

[0084] I 64QAM +1i*Q 64QAM =amp1*(I 4QAM +1i*Q 4QAM )+amp2*(I 16QAM +1i*Q 16QAM )

[0085] Base layer constellation point of sign bit mapping: I 4QAM +1i*Q 4QAM =amp1*SymMap(z,4QAM)

[0086] Additional layer constellation points for LSB mapping: I 16QAM +1i*Q 16QAM =amp2*SymMap([b2…b n ], 16QAM) that is, the principle of combining grid shaping and hierarchical modulation is:

[0087] I 64QAM +1i*Q 64QAM

[0088] =amp1*(SymMap(z,4QAM))+amp2*(SymMap([b2…b n ],16QAM))

[0089] Where: I, Q represent the in-phase component and quadrature component of the constellation point respectively;

[0090] The base layer refers to a modulation layer with a larger Euclidean distance interval, such as 4QAM modulation; the additional layer refers to a modulation layer with a smaller Euclidean distance interval generated around the base layer constellation point, such as 16QAM modulation.

[0091] Step 3: The receiving end performs DSP processing on the received signal, including downsampling, blind equalization, frequency offset estimation, and phase recovery. It then performs hierarchical demodulation on the transmitted HMTS-64QAM signal and converts it into a TS-64QAM constellation point bit stream.

[0092] The steps of hierarchical demodulation are as follows:

[0093] Step 301: Perform 4QAM demodulation on the base layer of the received signal to obtain a symbol bit sequence z;

[0094] Step 302: according to the sign bit sequence z, the center coordinates of the additional layer constellation point in the quadrant are shifted and 16QAM demodulation is performed on the additional layer to obtain the additional layer data LSB;

[0095] Step 4: Degrid and reshape the TS-64QAM data to restore the original transmission sequence.

[0096] The steps to solve the grid reshaping are as follows:

[0097] Step 401: Pass the sign bit sequence z through the matrix H T Transform to obtain 1-bit MSB sequence;

[0098] The matrix H T The expression is as follows: T =[1+D+D 2 ,1+D 2 ] T

[0099] Step 402: Concatenate the LSB and MSB of the additional layer data to restore the original data sequence.

[0100] The present invention optimizes system performance by simultaneously introducing layered modulation technology and grid shaping technology into the photonic terahertz communication system, improves the sensitivity of the system receiver while reducing nonlinear effects, and has excellent bit error rate performance.

[0101] Example:

[0102] The basic structure of a terahertz communication system consists of a laser, an optical modulator, an arbitrary waveform generator (AWG), a UTC-PD, and a receiver. At the transmitter, the baseband signal generated by the AWG is modulated by the optical modulator onto a continuous lightwave generated by the laser. This signal is then sent to the UTC-PD to beat with another continuous lightwave, generating a terahertz signal with a bandwidth equal to the frequency difference between the two continuous lightwaves. At the receiver, the received signal is first down-converted to the baseband frequency using the local oscillator signal, and then undergoes offline digital signal processing to restore the original signal.

[0103] This embodiment is primarily built using VPIphotonics Design Suite 11.1 and MATLAB R2022b. The overall system architecture is built on VPI, and signal transmission and reception processing is performed on MATLAB. This provides high flexibility in digital signal processing and enables the sending, transmission, and reception of HMTS-64QAM signals.

[0104] Specifically, at the transmitting end, an external cavity laser (ECL1) generates a continuous optical carrier wave (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 component uses a co-simulation module to jointly debug VPI and MATLAB. MATLAB generates a baseband signal and sends it to 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 baseband, and then sent to MATLAB for digital signal processing, saving one down-conversion operation.

[0105] TX-DSP includes grid shaping and hierarchical modulation. The specific steps are as follows:

[0106] 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 under the 64QAM modulation mode.

[0107] Step 2: Input the pseudo-random binary sequence into the grid shaping matcher for grid shaping (TS).

[0108] Specifically:

[0109] The pseudo-random binary sequence is divided into two data streams, MSB and LSB, according to the frame structure. The MSB passes through the matrix (H -1 ) T After the transformation, a 2-bit data stream x is obtained; after performing a modulo-2 addition operation on x and the control word y, a sign bit sequence z is obtained, and z is combined with the LSB to obtain the mapping constellation point bit sequence;

[0110] y is the control word, generated by the matrix G U Generate; use the Viterbi Algorithm to select the optimal control word y with minimum energy and the most judgment criteria.

[0111] The bit stream sequence after outputting z and LSB is the TS-64QAM bit stream obtained after completing grid shaping.

[0112] Among them: G U =[1+D2 ,1+D+D 2 ];

[0113] The specific process is:

[0114] Step 101: The frame structure is divided into groups of n bits, where the MSB contains 1 bit and the LSB contains n-1 bits, where n=log2(N), and N is the constellation point modulation order;

[0115] Step 102: The matrix is ​​transformed into 1-bit MSB input (H -1 ) T The matrix gets the 2-bit sequence x

[0116] Among them: (H -1 ) T =[D,1+D];

[0117] Step 103: The Viterbi algorithm selects the best control word y according to the energy minimization principle, performs a modulo-2 addition operation on y and x, and outputs the best sign bit sequence z.

[0118] Step 3: Perform hierarchical modulation on the TS-64QAM bit stream to obtain the HMTS-64QAM signal.

[0119] Specifically, we first find a suitable layer ratio λ, and adjust the Euclidean distance between the basic layer and the additional layer through λ, such as Figure 5 As shown. Perform 4QAM modulation on z to obtain the base layer constellation point, and perform 16QAM modulation on LSB to obtain the additional layer constellation point. The base layer and additional layer constellation points are superimposed to obtain the final output 64QAM constellation point:

[0120] I 64QAM +1i*Q 64QAM =amp1*(I 4QAM +1i*Q 4QAM )+amp2*(I 16QAM +1i*Q 16QAM )

[0121] Step 4: Send the signal after grid shaping and layered modulation to the optical fiber channel for transmission.

[0122] The 20 GHz baseband signal generated by the TX-DSP module is sent into the system and divided into two IQ paths. It is amplified by two parallel electrical amplifiers (EAs) with a gain of 25 dB, and then modulated onto the optical carrier generated by CW1 in the IQ modulator. It is then coupled with CW2. The coupled signal passes through a 20 km section of single-mode optical fiber and is amplified by an erbium-doped fiber amplifier (EDFA). The UTC-PD beat frequency is then used to generate a terahertz wave signal with a bandwidth of 430 GHz to 470 GHz. In this embodiment, 450 GHz is selected.

[0123] The signal is then fed into the RX-DSP, mixed with a 450GHz local oscillator signal, and down-converted to the baseband. The signal is then resampled and subjected to the corresponding demodulation and recovery processing, including channel equalization, frequency offset estimation, phase recovery, layered demodulation, and degridding.

[0124] Hierarchical demodulation is specifically as follows:

[0125] Step 1: Demodulate the received signal after DSP processing according to the 4QAM method to obtain the 2-bit basic layer z;

[0126] Step 2: Scale and translate the constellation points to transform the coordinate origin according to the z demodulation type and the set λ value;

[0127] The specific constellation point transformation rules are:

[0128]

[0129] Step 3: Perform 16QAM demodulation on the additional layer constellation points after coordinate translation transformation to obtain the additional layer data LSB.

[0130] The specific solution of grid shaping is:

[0131] Step 1: Pass the hierarchical demodulated z through the matrix H T The transformation is restored to get the MSB; the matrix expression is:

[0132] H T =[1+D+D 2 ,1+D 2 ] T

[0133] Step 2: Concatenate the MSB and LSB to restore the original bit sequence.

[0134] The present invention applies grid shaping and layered modulation technology to the photonic terahertz communication system, which has a good effect on improving the system bit error rate performance. When the OSNR continues to increase, the relationship between the measured BER performance and the OSNR size is as follows: Figure 6 As shown in the figure, it can be seen that as the OSNR increases, the system BER gradually decreases. Figure 7The figure shows the signal constellation diagram recovered at the receiving end when the fiber length is 20 km and the OSNR is 18 dB. (a) is the constellation diagram without 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: The system includes a transmitter and a receiver. The transmitter introduces layered modulation and grid shaping technology, uses photon beat frequency to generate terahertz signals, transmits them to the receiver 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 rules of grid shaping coding to obtain TS-64QAM signal; The specific process 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, which is mapped to a constellation point to obtain two corresponding constellation point results; Right now: Where 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 the 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 ],64QAM); constellation point 2 = SymMap([z2,b2…b n ],64QAM); Step 104: Combine the sign bit sequence z and the LSB sequence [b2…b n ] are combined to obtain the grid-shaped output sequence [z, b2…b n ]; Step 105: Output sequence [z,b2…b n ] According to the mapping rules, 64QAM constellation mapping is performed to obtain TS-64QAM signal; Step 2: Perform 4 / 16QAM layered modulation on the grid-shaped output TS-64QAM signal. Modify the ratio between the base layer and the additional layer by adjusting the parameter λ to obtain an HMTS-64QAM signal and send it to the optical fiber channel for transmission. The principle of combining grid shaping and layered modulation is: I 64QAM +1i*Q 64QAM =amp1*(SymMap(z,4QAM))+amp2*(SymMap([b2…b n ],16QAM)) SymMap is a symbolic mapping function; 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 base layer constellation point of the sign bit mapping, (I 16QAM +1i*Q 16QAM ) is the additional layer constellation point of LSB mapping; amp1 and amp2 are the corresponding amplitudes of the base layer and 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-gridding and reshaping the TS-64QAM constellation point bit stream to restore the original transmission sequence.

2. The photonic terahertz communication system based on layered grid shaping technology according to 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, providing 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 an HMTS-64QAM baseband signal, driving the IQ modulator to modulate the input optical carrier. The modulated optical carrier and the continuous light wave output by the external cavity laser ECL2 are coupled 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, generating a terahertz signal.

3. The photonic terahertz communication system based on layered grid shaping technology according to claim 1, characterized in that: The receiving end includes: 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, layered demodulation and de-gridding operations to finally obtain the original data.

4. 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 constellation point of the base layer.

5. The photonic terahertz communication system based on layered grid shaping technology according to claim 1, characterized in that: In the step three, the DSP processing includes downsampling, blind equalization, frequency offset estimation and phase recovery.

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