A low-complexity system structure design method based on high-order optical DPSK system

Abstract

The present invention claims protection for a low-complexity system structure design method based on a high-order optical DPSK system, and belongs to the field of free-space optical communication technology. This design is based on the principle of high-order optical DPSK demodulation, and according to the corresponding distribution of each bit sequence on the constellation diagram, selects the bit with the most complex demodulation process for constellation reconstruction, and disperses the 0 and 1 of the bit as much as possible to simplify the demodulation structure. At the same time, an auxiliary bit pair is introduced to transform the constellation point mark, which is transmitted to the receiving end with the optical signal through amplitude modulation, so as to achieve the purpose of assisting the demodulation of the remaining bits and further simplify the system structure. The higher the corresponding modulation order of the system, the higher the degree of simplification of the optimized system after constellation mapping reconstruction, and thus the higher the split light power of each branch of the demodulation, the higher the optical power of each branch of the demodulation, and the high-order optical DPSK optimization system of the present invention can save laser emission power and have higher energy efficiency.

Description

Technical Field

[0001] The present invention belongs to the technical field of free space optical communication (Free Space Optics, FSO), and relates to a low-complexity system structure design based on high-order Differential Phase Shift Keying (Differential Phase Shift Keying, DPSK) of optical communication. Background Art

[0002] In recent years, with the explosive growth of network traffic and devices, the traditional radio frequency (RF) spectrum has become very crowded and can no longer meet the growing network needs. As one of the key technologies of my country's space-ground integrated development strategy, free space optical communication technology combines optical communication technology with wireless communication technology, uses laser as carrier and atmosphere as transmission medium to achieve high-speed information transmission. It has the advantages of rich spectrum resources, strong anti-electromagnetic interference ability, low power consumption, high security, strong directionality, and flexible installation. It is often used in military and other fields with strict requirements on information security. Especially in areas where optical fiber cannot reach, such as deserts and forests, FSO does not need to lay optical fiber lines, which reduces the cost and time of construction. Therefore, it also has broad application prospects in the field of emergency communications.

[0003] FSO is a line-of-sight transmission technology that is very sensitive to weather conditions and atmospheric turbulence, which puts forward more stringent requirements on phase noise. Weather conditions such as rain, fog, snow, and haze will affect the optical signal during transmission. The absorption and scattering of atmospheric gas molecules and aerosols will cause the power attenuation of the optical signal; and the uneven air refractive index caused by atmospheric turbulence will cause random fluctuations in the optical refractive index, causing the optical signal to produce light intensity flicker, beam deviation, beam expansion, and arrival angle fluctuations during transmission, thereby causing phase fluctuations. In order to resist channel fading, academia and industry are studying new modulation schemes to reduce the impact of channel conditions on transmission performance. DPSK technology has been widely used in optical communications due to its high tolerance to fiber nonlinearity and 3dB improvement in receiver sensitivity compared to OOK technology. Due to its higher spectrum efficiency, high-order DPSK technology is often replaced by low-order DPSK technology and is studied in depth in optical communications on the premise of ensuring communication quality.

[0004] With the increase of the modulation order of optical DPSK signal, the complexity of communication system structure shows a proportional growth trend, especially in the demodulation end structure, which often increases the cost of system structure implementation. In view of the problem of increasing complexity of high-order system structure, existing research mainly uses the method of amplitude-phase joint modulation to obtain the same information rate, so as to reduce the complexity of system structure. However, for FSO channel, its atmospheric turbulence and attenuation characteristics have a great influence on the power of optical signal, and the communication quality of optical signal is easily affected by atmospheric conditions and weather conditions. Therefore, compared with high-order DPSK modulation, when transmitting the same information, the method of amplitude-phase joint modulation cannot well resist the atmospheric influence brought by medium and strong turbulence. And as the modulation order increases further, in order to control the system structure not to be too complicated, when using joint modulation, the signal order of phase modulation is not easy to be too high, so it is often balanced by increasing the order of amplitude modulation, which further leads to reduced system reliability. In addition, there are studies showing that appropriate changes to the constellation diagram can achieve the purpose of simplifying the system structure, but the cost is often also a reduction in system reliability. Therefore, for optical communication high-order DPSK systems, it is of certain research significance to find a system with low structural complexity while meeting certain reliability requirements of the system. Summary of the invention

[0005] The present invention aims to solve the above problems of the prior art. A constellation reconstruction design method based on a high-order optical differential phase shift keying DPSK system is proposed. The technical solution of the present invention is as follows:

[0006] A low-complexity system structure design method based on a high-order optical DPSK system comprises the following steps:

[0007] According to the corresponding distribution of each bit sequence on the constellation diagram, the bit position in the demodulation process is selected for constellation reconstruction, and the 0 and 1 of the bit position are dispersed as much as possible. According to the demodulation principle of the system, the mapping relationship of the constellation points is changed, so that the constellation points where the bit positions are located are more concentrated according to certain rules, so as to simplify the demodulation structure and then simplify the demodulation rules, and the constellation points where the mapping relationship changes are marked with auxiliary bits, thereby simplifying the judgment; through amplitude modulation, they are transmitted to the receiving end along with the optical signal, and the remaining bits are demodulated to assist, so as to realize high-order optical DPSK signal transmission with low system structure complexity.

[0008] Furthermore, the analysis steps of the conventional high-order optical DPSK constellation diagram are also included, including:

[0009] 1) Observe the traditional high-order optical DPSK constellation diagram. The number of bits corresponding to each symbol in the DPSK constellation diagram is determined by the modulation order, and the bit position refers to the bit position in each symbol; find the bit position with more 0 and 1 crossings in the constellation diagram, that is, the bit position with more 0 and 1 boundaries; 2) Transform the position of the constellation point corresponding to the bit position so that the 0 and 1 boundaries are reduced as much as possible; 3) If there is a bit position with a higher 0 and 1 distribution complexity than the bit position, continue to step 2 until all bits are traversed; at the same time, ensure that the number of 0 and 1 boundaries of each bit in the constellation diagram is based on 2 n Increase successively, where n is a non-negative integer, to obtain the reconstructed constellation diagram.

[0010] Furthermore, the method also includes the step of transforming the constellation symbols at the transmitting end, specifically including: transforming the constellation points that need to be transformed in position by transforming the differences between adjacent symbols after the constellation points are pre-coded, thereby transforming the mapping relationship of the constellation symbols; and distributing the constellation points of these bits on the constellation diagram with the least 0 and 1 dividing lines.

[0011] Furthermore, the constellation points that have undergone mapping relationship transformation are marked with auxiliary bits, specifically including: 1) marking the symbols that have undergone constellation symbol mapping relationship transformation; 2) intensity modulating the auxiliary marking information, and transmitting the marking bit information through the intensity information of the optical signal; 3) completing the constellation reconstruction at this time, and obtaining the reconstructed optical signal for transmission on the FSO channel.

[0012] Furthermore, the method also includes the steps of demodulating the constellation reconstruction system, specifically including: 1) the optical splitter distributes the optical power of the received optical signal in proportion to the number of demodulation branches; 2) the demodulation of the optical signal is mainly related to the MZI phase shift The value of is related to In the constellation diagram, it is reflected as rotating the constellation diagram counterclockwise by a corresponding angle, which is equivalent to rotating the horizontal axis of the constellation diagram clockwise by a corresponding angle; the system after constellation reconstruction starts from the bit with the least 0-1 boundary line according to the distribution of 0 and 1 bits of the reconstructed constellation diagram, and sets the phase shift of MZI The horizontal axis of the constellation diagram is rotated clockwise to the 0, 1 dividing line, and the demodulation of the bit is realized through the phase demodulation branch; 3) The number of 0, 1 dividing lines theoretically corresponds to the number of phase demodulation branches, and the bits are demodulated in the order of increasing number of 0, 1 dividing lines. For the system after constellation reconstruction, there are two bits with the number of 0, 1 dividing lines being 1, and at least two phase demodulation branches are required. As the number of 0, 1 dividing lines increases, simple logic operation demodulation can be performed through the existing phase demodulation branches; 4) For the bit with the largest number of 0, 1 dividing lines, the auxiliary information restored by the intensity demodulation branch is demodulated by the corresponding logic operation with the demodulated bit; 5) The demodulated parallel bits are converted into serial output to restore the original signal and complete the information transmission of the constellation reconstruction system.

[0013] Furthermore, for the optical 8DPSK system, the system can be operated as follows:

[0014] A1: After differential precoding, the differences between adjacent symbols are exchanged to achieve the following symbol mapping relationship: The constellation points with the same c bits are concentrated together;

[0015] A2: At the same time, the symbol that has undergone mapping transformation is marked as 1, and vice versa as 0. The information of the marking bit is transmitted through the amplitude of the optical signal;

[0016] A3: At this time, the system demodulation end only needs the first and second MZIs to participate in demodulation, and the phase shift of the MZI remains unchanged, the first path can be used to directly demodulate a bit of information, the difference is that the phase shift is The second MZI is no longer used to demodulate b-bit information, but c-bit information. The third branch of the demodulation end needs to use a photodiode PIN for direct detection to demodulate the auxiliary bit information, which is represented by s.

[0017] A4: After low-pass filtering and judgment, the b-bit information can be get.

[0018] Furthermore, for the optical 16DPSK system, the system can be operated as follows:

[0019] B1: In order to simplify the system structure, the following two constellation mapping relationship transformations can be performed after system differential precoding:

[0020]

[0021] In the first transformation, the 0 and 1 of the c-bit are concentrated in the semicircle area, which can be judged by positive and negative. The second transformation is based on the first transformation, so the 0 and 1 information of the d-bit are concentrated in 1 / 4 circle and distributed diagonally, so as to make judgments by XOR;

[0022] B2: At the same time, the constellation point of the first transformation is marked as 1, otherwise it is 0; similarly, it is transmitted by modulating the amplitude of the optical signal;

[0023] B3: The demodulation end structure is further simplified at this time, which is similar to the optimized optical 8DPSK system, but the phase shift is different. The phase shift of the first MZI is Used to demodulate a bit of information; the second MZI phase shift is Used to demodulate c bits of information; the third path uses direct detection to demodulate amplitude information s;

[0024] B4: The first and second paths can directly restore a and c bit information after low-pass filtering and positive and negative judgment; the third path restores the mark bit s information after low-pass filtering and positive and negative judgment, and can restore b bit information by ORing it with c bit information; at the same time, the obtained a and c bit information can restore d bit information through simple logical operations. The advantages and beneficial effects of the present invention are as follows:

[0025] The innovation of the low-complexity system structure design method based on high-order optical DPSK proposed in the present invention is mainly: according to constellation analysis, constellation mapping is reconstructed for the traditional high-order optical DPSK system, the demodulation complexity of the bit is cleverly changed, and the demodulation rules of the bit are further simplified in combination with the auxiliary marking method, so as to obtain a more simplified demodulation process, and finally achieve the purpose of reducing the complexity of the system structure. At the same time, with the increase of the modulation order, such as the modulation order of DPSK increases from 8 to 16, the system structure of the high-order optical DPSK system proposed in the present invention at the demodulation end is more obviously simplified than the traditional system structure, the splitting branches are significantly reduced, and the optical power divided by each branch is significantly increased compared with the traditional system, which is beneficial to the signal demodulation of the path. When the performance improvement of the bit error rate (BER) brought by the simplification of the system demodulation rules is greater than the performance reduction of the BER brought by the auxiliary bit transmission, the entire optimization system simplifies the complexity of the system structure while also improving the reliability of the system. At the same time, under the condition of meeting the same communication quality, the low-complexity system of the present invention has lower transmission power and higher energy efficiency, so the transmission power of the laser can be saved. It has certain application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is the traditional optical 8DPSK and optical 16DPSK modulation system structure;

[0027] Figure 2 This is a simplified diagram of the MZM structure;

[0028] Figure 3 It is the MZM transmission characteristic curve and example;

[0029] Figure 4 It is the structure of the traditional optical 8DPSK demodulation system;

[0030] Figure 5 It is the structure of the traditional optical 16DPSK demodulation system;

[0031] Figure 6 It is the internal structure of MZI;

[0032] Figure 7 It is the constellation diagram of optical 8DPSK signal and optical 16DPSK signal;

[0033] Figure 8 Optimize system structure for optical 8DPSK and optical 16DPSK based on constellation reconstruction;

[0034] Fig. 9 This is the optical 8DPSK constellation diagram after constellation reconstruction;

[0035] Fig.10 This is the optical 16DPSK constellation diagram after constellation reconstruction;

[0036] Fig.11 Comparison of BER of different bits with received optical power before and after optimization for optical 8DPSK system;

[0037] Fig.12 Comparison of BER of different bits with received optical power before and after optimization for optical 16DPSK system;

[0038] Fig.13 Comparison of system BER with received optical power before and after optimization for optical 8DPSK system and optical 16DPSK system;

[0039] Fig.14 Comparison of system BER changing with laser emission power under different turbulence intensities before and after optimization of optical 16DPSK system;

[0040] Fig.15 Comparison of system BER for optical 16DPSK optimization system with amplitude modulation modulation index under different turbulence intensities. DETAILED DESCRIPTION

[0041] The following will describe the technical solutions in the embodiments of the present invention in detail in conjunction with the accompanying drawings in the embodiments of the present invention. The described embodiments are only part of the embodiments of the present invention.

[0042] The technical solution of the present invention to solve the above technical problems is:

[0043] In view of the deficiencies in existing research on high-order optical DPSK systems, the present invention proposes a constellation reconstruction method for a high-order DPSK optimization system based on optical communication, and has been verified in optical 8DPSK and optical 16DPSK systems. Based on the demodulation principle of a high-order optical DPSK system, this method designs and implements a reconstruction scheme for a constellation diagram of a high-order DPSK signal at the transmitting end, by centralizing the constellation points where the same bits are located according to certain rules to achieve the purpose of simplifying the decision. At the same time, the reconstructed constellation points are marked by an auxiliary bit marking method. This assists the decision. Combining the two methods, the simplification of the system structure is finally achieved. And with the increase of the modulation order, the reduction in the complexity of the system structure becomes more and more obvious, and the improvement in the error performance is also more and more significant.

[0044] Figure 1 This is the structure of the traditional optical 8DPSK and optical 16DPSK modulation system. The initial bit sequence is input into the differential precoding module after serial-to-parallel conversion. The module needs to differentially encode the input absolute code, encode the absolute code into relative code information representing the difference between adjacent symbols, and then load it onto the electrical signal through a pulse generator. Then, through the Mach-Zehnder Modulator (MZM), the electrical signal carrying the relative code information is loaded onto the phase of the optical signal to realize the process of converting the electrical signal into an optical signal. Since the MZM can only realize the phase change of 0 and π, the output signal of the MZM also needs to pass through a phase modulator (PM) with different phase shifts to obtain the required optical signal. Among them, optical 8DPSK modulation needs to divide the bit sequence into three paths a, b and c through serial-parallel conversion for parallel transmission, and output three paths of information I, Q and D to carry phase information after differential precoding; optical 16DPSK modulation needs to divide the bit sequence into four paths a, b, c and d through serial-parallel conversion for transmission, and output four bit sequences I, Q, D and E carrying differential information after entering the differential precoding module. The differential information is generated into an electrical signal by a pulse generator, and then modulated onto an optical carrier by MZM. Optical 8DPSK modulation needs to pass through phase shifts of π / 2, π / 4, PM in sequence, and then output as an optical 8DPSK signal. Optical 16DPSK modulation needs to pass through three PMs with phase shifts of π / 2, π / 4, and π / 8 in sequence to obtain an optical 16DPSK signal.

[0045] The most important device in the entire high-order optical DPSK modulation system is the MZM, which realizes the conversion from electricity to light. Figure 2 This is the structure diagram of MZM, which uses lithium niobate (LiNbO3) as the optical waveguide material. inAfter passing through the first Y branch, the optical signal is output with equal power, and then transmitted through the upper and lower arms respectively. When there is a voltage difference between the upper and lower arms of the MZM, the refractive index of the LiNbO3 crystal changes with the magnitude of the applied voltage, so that the light waves of the upper and lower arms have a phase difference when they merge in the second Y branch. The phase of the optical signal is modulated by changing the refractive index of the crystal. The transmission function of MZM is:

[0046]

[0047] Where E0 is the external electric field strength, V bias The DC bias voltage satisfies V bias =V1-V2, V1 and V2 are the voltages applied to the upper arm and lower arm respectively. When the external voltage satisfies When , it is called half-wave voltage, that is, the minimum driving voltage that can make the optical signal produce a phase difference of π. According to the transmission function, the transmission characteristic curve of MZM can be obtained as follows Figure 3 As shown. The figure shows the four working points of MZM, -Quad and +Quad are called quadrature points, Null is called the minimum point, and Peak is the maximum point. When the bias voltage, drive voltage and half-wave voltage meet a certain relationship, the output format of the optical signal can be controlled to be phase modulation or intensity modulation, or it can be an intensity-phase combined modulation signal.

[0048] After the high-order optical DPSK signal is transmitted through the FSO channel, it is amplified and filtered at the receiving end, and then enters the demodulation end, where it is coherently demodulated by the Mach-Zehnder Interferometer (MZI) and converted into amplitude information. Then, it is output as an electrical signal through a balanced detector, and a differential signal is obtained through a subtractor. Then, the original information is restored through low-pass filtering and judgment. The number of MZIs in the traditional demodulation system N is the same as the modulation order M (M=2 n , n≥2) satisfies like Figure 4 and Figure 5 They are the traditional optical 8DPSK system receiving end structure and the traditional 16DPSK system receiving end structure. Among them, the most important device is MZI, and its internal structure is as follows Figure 6 As shown, it consists of two 3dB couplers, a delay device and a phase shifter. Optical signal E(t) = Ae jφ(t) The output of MZI is converted into an electrical signal through two photodiodes PIN to the output of the subtractor:

[0049]

[0050] Among them, Δφ=φ(t)-φ(t-τ) is the optical phase difference, is the phase shift of the lower arm of the MZI, so the demodulation of the optical signal is mainly related to the phase shift The phase shift In the constellation diagram, rotating the constellation diagram counterclockwise by a corresponding angle is equivalent to rotating the horizontal axis of the constellation diagram clockwise by a corresponding angle. Figure 7 is the constellation diagram of optical 8DPSK and optical 16DPSK, where the a-bit information of optical 8DPSK can be obtained through the coordinate axis That is to say, if a=1, otherwise a=0. Similarly, b bits of information can be obtained through the coordinate axis The positive or negative value is determined by b=1, otherwise b=0. The c-bit information needs to be represented by the coordinate axis and the coordinate axes Determine together. If Set c1=1, otherwise c1=0. We set c2 = 1, otherwise c2 = 0. Then we can get Similarly, the demodulation principle of optical 16DPSK is similar. Depending on the constellation diagram, the corresponding phase shift There are also differences.

[0051] But through Figure 4 and Figure 5 It can be clearly seen that with the increase of the modulation order, the structure of the system demodulation end becomes more and more complex, which has a great impact on the implementation cost of the system and the transmission quality of the signal. Therefore, the present invention proposes a constellation reconstruction concept suitable for high-order optical DPSK systems. Through constellation reconstruction, the bits are concentrated according to certain rules, so as to achieve the purpose of simplifying the judgment. At the same time, auxiliary marking bits are used to mark the constellation reconstruction points, which are transmitted to the receiving end through amplitude modulation, thereby simplifying the demodulation structure. And with the increase of the modulation order, the simplification of the system demodulation structure becomes more and more obvious, the demodulation end splits more branches, and the optical power divided by each branch increases significantly, so that the system error performance is improved more than the cost brought by amplitude modulation, and it has strong scalability for high-order optical DPSK systems.

[0052] The present invention has been verified in an optical 8DPSK system and an optical 16DPSK system. Figure 8 The optical 8DPSK and optical 16DPSK optimization system structures using the constellation reconstruction concept will be described using these two systems. The present invention will be described using these two modulation orders as examples.

[0053] For the optical 8DPSK system, according to Figure 4From the demodulation principle, it can be seen that the demodulation of the c bit is more complicated because the corresponding constellation points are more scattered, making the demodulation complexity greater, such as Figure 7 (a). Therefore, according to the idea of ​​constellation reconstruction, the system can be operated as follows:

[0054] 1: After differential precoding, the differences between adjacent symbols are exchanged to achieve the following symbol mapping relationship: The constellation points with the same c bit positions are concentrated together.

[0055] 2: At the same time, the symbols that have undergone mapping transformation are marked as 1, and vice versa. The information of the marking bit is transmitted through the amplitude of the optical signal. At this time, the optical signal constellation diagram transmitted by the FSO channel is as follows: Fig. 9 shown.

[0056] 3: At this time, the system demodulation end only needs the first and second MZIs to participate in demodulation, and the phase shift of the MZI remains unchanged, the first path can be used to directly demodulate a bit of information, the difference is that the phase shift is The second MZI is no longer used to demodulate b-bit information, but to demodulate c-bit information. The third branch of the demodulation end needs to use a photodiode PIN for direct detection to demodulate the auxiliary bit information, which is represented by s.

[0057] 4: Finally, after low-pass filtering and judgment, the b-bit information can be get.

[0058] For the optical 16DPSK system, according to Figure 4 The demodulation principle shows that the demodulation principle of the c-bit and d-bit is more complicated. The traditional constellation diagram is as follows: Figure 7 As shown in (b), according to the idea of ​​constellation reconstruction, the system can be operated as follows:

[0059] 1: In order to simplify the system structure, the following two constellation mapping relationship transformations can be performed after system differential precoding:

[0060]

[0061] In the first transformation, the 0 and 1 of the c-bit are concentrated in the semicircle area, which can be judged by positive and negative. The second transformation is based on the first transformation, so the 0 and 1 information of the d-bit are concentrated in 1 / 4 circle and distributed diagonally, so as to make judgments by XOR.

[0062] 2: At the same time, the constellation point of the first transformation is marked as 1, otherwise it is marked as 0. Similarly, it is transmitted by modulating the optical signal amplitude. The optical 16DPSK constellation diagram after constellation reconstruction is as follows: Fig.10 shown.

[0063] 3: The demodulation end structure is further simplified at this time, which is similar to the optimized optical 8DPSK system, but the phase shift is different. The phase shift of the first MZI is Used to demodulate a bit of information; the second MZI phase shift is Used to demodulate c bits of information; the third path uses direct detection to demodulate amplitude information s.

[0064] 4: The first and second paths can directly restore a and c bit information after low-pass filtering and positive and negative judgment; the third path restores the mark bit s information after low-pass filtering and positive and negative judgment, and can restore b bit information by ORing with c bit information; at the same time, the obtained a and c bit information can restore d bit information through simple logical operations.

[0065] from Figure 8 It can be seen that the optical 16DPSK system structure after constellation reconstruction is similar to the optical 8DPSK system structure. The solid line connection is the optical 8DPSK optimized system structure. It can be clearly seen that at the modulation end, the optimized system structure only uses one more amplitude modulator, but at the demodulation end, the optimized system structure saves 50% of MZI, 37.5% of PIN, 50% of electrical subtractors, 25% of low-pass filters and 25% of decision devices. The dotted part is the structure that needs to be added to the transmission optical 16DPSK system. For the optical 16DPSK system, the optimized system after constellation reconstruction saves 66.7% of MZI, 58.3% of PIN, 50% of adders and subtractors, 50% of low-pass filters and 37.5% of decision devices. For the entire system, with the increase of the modulation order, the system structure complexity of the present invention has a significant trend of reduction.

[0066] At the same time, according to the parameter settings in Table 1, the traditional optical 8DPSK system, the optical 16DPSK system, the optical 8DPSK optimization system using the constellation reconstruction idea proposed in the present invention, and the optical 16DPSK optimization system were simulated by Optisystem software and their system performances were tested.

[0067] Fig.11 and Fig.12 The BER of different bit information before and after the optimization of the optical 8DPSK and optical 16DPSK systems are shown respectively. -15 m -2 / 3 , when the amplitude modulation index is 0.33, it can be found that due to the reduction in the number of optical splitter branches at the demodulation end of the optimization system, when the received optical power (Pr) is the same, the equally divided optical power of each branch can be increased, so the BER of some bits can be improved. Fig.11It can be seen that the BER of the bit information is proportional to the complexity of the decision. As the received optical power decreases, the bit error rate of the c-bit information after the simplified decision is significantly lower than the BER of the traditional system, but at the expense of an increase in the BER of the b-bit information. This is because the AM modulation format used to transmit the s-bit information has a lower ability to resist channel fading than the phase modulation format. The b-bit information is obtained by XORing the s-bit information and the c-bit information, which leads to an increase in the bit error rate of the b-bit information. Fig.12 It is further shown that the structural complexity of the optimized optical 16DPSK system is reduced more significantly. Since the number of branches of the demodulation end splitter is further reduced compared with the traditional structure, and the number of branches is reduced by half, the optical power allocated to each branch is doubled, which directly leads to the improvement of the BER performance of each bit of the optical 16DPSK optimization system over the optical 8DPSK optimization system. At the same time, since the optical signal undergoes constellation reconstruction, the demodulation complexity of the changed c-bit is lower, so the system significantly reduces the BER of the c-bit information. Although the optimized system causes an increase in the BER of the b-bit, the BER performance improvement brought by the remaining bits is sufficient to offset this cost.

[0068] Fig.13 It shows that when p r <-33dBm, the system BER of the optical 8DPSK optimized system is lower than that of the traditional system, but when p r When the received optical power is higher than -33dBm, the opposite is true. This is because, when the received optical power is high, the anti-attenuation capability of the amplitude modulation technology used in the optimization system is lower than that of the phase modulation technology, resulting in a decrease in the BER performance of the bit information related to the auxiliary marker bit of the demodulation method. However, the performance improvement brought about by the simplification of the system structure is not enough to offset these costs. Therefore, when the communication quality requirements are not high, the optical 8DPSK optimization system can be used to simplify the system structure and reduce the system cost. For the optical 16DPSK optimization system, since the optical splitting branches are reduced by half and the optical power of each branch is doubled, the BER performance of each branch signal is theoretically improved more than that of the optical 8DPSK optimization system. The BER performance improvement brought about by it is greater than the cost of amplitude modulation to the system, so the overall BER performance of the system is improved. Therefore, for the optical 16DPSK system, under low turbulence conditions, the optimized system not only greatly simplifies the system structure, but also significantly improves the reliability of the system, which achieves higher transmission quality at a lower cost in practical applications.

[0069] Fig.14 The system BER of the conventional 16DPSK system and the optimized system at different laser output powers are compared. At the same time, the system BER of the two systems at high and low turbulence intensities are compared. When the turbulence intensity is 1×10 -15 m -2 / 3, under the condition of meeting the same BER, the optimized system in the present invention has lower laser output power and higher energy efficiency. -15 m -2 / 3 When the BER of the optimized system is p o When the turbulence is less than 15.5dBm, it is lower than the traditional system. Therefore, when the atmospheric turbulence is in medium and low turbulence, the optimized system has a higher BER performance. As the turbulence increases, the BER performance of the traditional system does not change significantly, while the proposed system reduces the anti-fading ability due to amplitude modulation, which leads to a decrease in the BER performance of the system.

[0070] Fig.15 The BER under different turbulence intensities and AM modulation indices are compared. The laser output power and channel attenuation are set to 20dBm and 22dB / km, respectively. The figure shows the lowest BER under four turbulence intensities and the corresponding amplitude modulation index. As shown in the figure, under the same turbulence conditions, as the amplitude modulation index increases, the BER curve shows a trend of first decreasing and then increasing. This is because when the amplitude modulation index is relatively low, the difference between the two amplitudes is very small, which can easily lead to misjudgment. As the amplitude modulation index increases, the judgment threshold of the partial symbol based on AM increases, resulting in a decrease in the probability of misjudgment. As the amplitude modulation index continues to increase, the optical power loss of the lower amplitude is obvious, causing the low-amplitude optical signal to be seriously affected by the atmospheric channel, which in turn increases the BER. Therefore, the BER curve shows a trend of first decreasing and then increasing. In addition, it can be seen that as the turbulence intensity decreases, the minimum BER of the system decreases, and the corresponding amplitude modulation index also decreases. When the turbulence intensity is relatively low, the amplitude modulation is less affected by turbulence, so the average symbol power of the system can be increased by appropriately reducing the amplitude modulation index. When the turbulence intensity is large, the amplitude modulation is greatly affected by turbulence. By properly improving the modulation efficiency, the decision domain of the amplitude information can be increased, thereby improving the BER performance of the system.

[0071] In summary, the method proposed in the present invention is applicable to high-order optical DPSK systems to simplify the complexity of the system structure. Based on the demodulation principle of the high-order optical DPSK system, the constellation points can be flexibly reconstructed and transformed for specific modulation orders, and the constellation reconstruction schemes performed in the above examples are not unique. Different constellation reconstruction schemes can be constructed as long as the constellation reconstruction scheme proposed in the present invention is satisfied. That is, by changing the mapping relationship, the constellation points corresponding to the bits with higher demodulation complexity are reasonably exchanged according to the principle of the same value concentration of the bit, and the exchange information is marked by the auxiliary bit to achieve the purpose of auxiliary demodulation. The structure of the system is simplified, and as the modulation order increases, the degree of simplification of the system structure increases. In medium and low turbulence, the BER performance of the system can also be improved to a certain extent. Since the degree of simplification of the system is higher as the modulation order of the optical signal increases, the BER performance that can be obtained by the system may be further increased. Therefore, the present invention also has certain scalability.

[0072] Table 1

[0073]

[0074]

[0075] It should also be noted that the terms "include", "comprises" or any other variations thereof are intended to cover non-exclusive inclusion, so that a process, method, commodity or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, commodity or device. In the absence of more restrictions, the elements defined by the sentence "comprises a ..." do not exclude the existence of other identical elements in the process, method, commodity or device including the elements.

[0076] The above embodiments should be understood to be only used to illustrate the present invention and not to limit the protection scope of the present invention. After reading the contents of the present invention, technicians can make various changes or modifications to the present invention, and these equivalent changes and modifications also fall within the scope defined by the claims of the present invention.

Claims

1. A low-complexity system structure design method based on a high-order optical DPSK system, characterized in that: The following steps are involved: According to the corresponding distribution of each bit sequence on the constellation diagram, the bit position in the demodulation process is selected for constellation reconstruction, the 0 and 1 of the bit position are dispersed, the mapping relationship of the constellation point is changed according to the demodulation principle of the system, and the constellation point where the mapping relationship changes is marked with auxiliary bits, thereby simplifying the judgment; it is transmitted to the receiving end with the optical signal through amplitude modulation, and the remaining bits are demodulated to assist in realizing high-order optical DPSK signal transmission with low system structure complexity; The method further includes a step of transforming the constellation symbol at the transmitting end, specifically comprising: transforming the constellation point that needs to be transformed by the difference between adjacent symbols after the constellation point is pre-coded, thereby transforming the constellation symbol mapping relationship; making the constellation points of these bits distributed on the constellation diagram with the least 0 and 1 dividing lines; The auxiliary bit marking of the constellation point where the mapping relationship transformation occurs specifically includes: 1) marking the symbol where the constellation symbol mapping relationship transformation occurs; 2) intensity modulating the auxiliary marking information, and transmitting the marking bit information through the intensity information of the optical signal; 3) completing the constellation reconstruction at this time, and obtaining the reconstructed optical signal for transmission on the FSO channel; For the optical 16DPSK system, the system is operated as follows: B1: In order to simplify the system structure, the following two constellation mapping relationship transformations can be performed after system differential precoding: In the first transformation, the 0 and 1 of the c-bit are respectively concentrated in the semicircle area, which can be judged by positive and negative. The second transformation is based on the first transformation, so the 0 and 1 information of the d-bit are respectively concentrated in 1 / 4 circle and distributed diagonally, so as to make judgment by XOR. B2: At the same time, the constellation point of the first transformation is marked as 1, otherwise it is 0; similarly, it is transmitted by modulating the amplitude of the optical signal; B3: The demodulation end structure is further simplified at this time, and the phase shift of the first MZI is Used to demodulate a bit of information; the second MZI phase shift is Used to demodulate c bits of information; the third path uses direct detection to demodulate amplitude information s; B4: The first and second paths can directly restore a and c bit information after low-pass filtering and positive and negative judgment; the third path restores the mark bit s information after low-pass filtering and positive and negative judgment, and can restore b bit information by ORing it with c bit information; at the same time, the obtained a and c bit information can restore d bit information through simple logical operations.

2. The low-complexity system structure design method based on a high-order optical DPSK system according to claim 1, characterized in that: For the optical 8DPSK system, the system can be operated as follows: A1: After differential precoding, the differences between adjacent symbols are exchanged to achieve the following symbol mapping relationship: The constellation points with the same c bits are concentrated together; A2: At the same time, the symbol that has undergone mapping relationship transformation is marked as 1, and vice versa. The information of the marking bit is transmitted through the amplitude of the optical signal. A3: At this time, the system demodulation end only needs the first and second MZIs to participate in demodulation, and the phase shift of the MZI remains unchanged, the first path can be used to directly demodulate a bit of information, the difference is that the phase shift is The second MZI is no longer used to demodulate b-bit information, but c-bit information. The third branch of the demodulation end needs to use a photodiode PIN for direct detection to demodulate the auxiliary bit information, which is represented by s. A4: After low-pass filtering and judgment, the b-bit information can be get.

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