Optical PPM time slot synchronization method under atmospheric turbulence fading channel

Abstract

The present application relates to a kind of atmospheric turbulence fading channel under light PPM time slot synchronization method, belong to communication technical field.The present application is to improve early-late gate tracking synchronization performance, it is proposed to the ratio of the average information photon number of current PPM symbol and background photon number Weighted scheme to the timing error estimation value of early-late gate.Then the initial phase position of early-late gate is estimated by photon pulse peak position.Finally, for the problem that early-late gate needs to recapture when signal interruption recovers, it is proposed to use the timing error before interruption as the timing error estimation value after recovery, and simulation analysis and experimental data verification.

Description

Technical Field

[0001] This invention belongs to the field of communication technology and relates to a method for optical PPM time slot synchronization under atmospheric turbulent fading channels. Background Technology

[0002] Clock synchronization is a prerequisite for reliable communication between the transmitter and receiver in a communication system; therefore, achieving accurate synchronization is crucial. Clock synchronization is divided into time slot synchronization and frame synchronization. For time slot synchronization, there are mainly phase-locked loops, early-late gates, FFT-based clock synchronization methods, and synchronization methods based on guard slots, etc.

[0003] Because laser signals are transmitted over long distances through atmospheric channels, the signal-to-noise ratio of the laser signal reaching the receiving end is extremely low, and under strong fading conditions, it can even cause signal transmission interruption. Furthermore, the optical pulses received by MPPC exhibit significant pulse broadening, severe tailing, and sharp, symmetrical pulse characteristics. Summary of the Invention

[0004] In view of this, the purpose of the present invention is to provide a method for optical PPM time slot synchronization under atmospheric turbulence fading channels.

[0005] To achieve the above objectives, the present invention provides the following technical solution:

[0006] A method for optical PPM time slot synchronization under atmospheric turbulent fading channels, the method comprising the following steps:

[0007] S1: Optical PPM Slot Synchronization Method Based on Early-Later Gate Tracking

[0008] If the initial sampling point position is i1, the advance sampling point position is i1. - =i1-t0, where i is the position of the lag sampling point. + =i1+t0, if the sampled values ​​of the early sampling point and the late sampling point are not equal, then the difference between the two contains timing error information;

[0009] When sampling time i1 is advanced, then x(i - )<x(i + The next sampling time needs to be delayed;

[0010] When sampling time i1 is delayed, then x(i - )>x(i + The next sampling time needs to be advanced;

[0011] Based on the early-late gate principle, the difference between the early integration gate and the late integration gate is used as the timing error. The estimated timing error value for the i-th time slot is:

[0012] err i =(x(mk) i-1 -1)-x(mki+1 +1))*x(mk i )

[0013] In the formula, x is the sampled data sequence; mk i This refers to the sampling time corresponding to the current time slot;

[0014] Choosing an integrator filter as the loop filter, the estimated frequency offset for the i-th time slot is:

[0015]

[0016] In the formula, ω n Let ξ be the angular frequency of the free oscillation of the loop, and T be the damping coefficient. s The time slot period is represented by Go_Gd, and the loop gain is represented by the loop free oscillation frequency.

[0017]

[0018] In the formula, BL is the tracking loop bandwidth.

[0019] The estimated sampling time value for the i-th time slot is:

[0020] ipos i =ipos i-1 +(1-path i )s

[0021] In the formula, s is the number of sampling points per time slot;

[0022] Rounding the estimated sampling time to the nearest integer, we obtain the sampling time for the i-th time slot as follows:

[0023] mk i =round(ipos i )

[0024] Each time slot uses the amplitude at the selected sampling time as the current time slot signal amplitude. The signal amplitude of the i-th time slot is:

[0025] tdata(i)=x(mk i )

[0026] Based on whether the synchronization loop is locked within the expected range, after several simulations, the loop parameters are selected and time slot synchronization is completed.

[0027] S2: Timing error weighting

[0028] The ratio of the average information photon count to the background photon count of the current PPM symbol is used as the weighting coefficient for the timing error estimate; the timing error estimate for the i-th time slot is:

[0029]

[0030] In the formula, N s N represents the average number of information photons for the current PPM symbol. b The average background photon number;

[0031] S3: Estimating the initial sampling time

[0032] The initial sampling time is estimated by selecting the peak positions of multiple photon pulses, and the calculation formula is as follows:

[0033]

[0034] In the formula, L represents the number of photon pulse peak positions; k i is the peak position of the i-th photon pulse; S is the sampling factor.

[0035] Optionally, S3 is followed by S4: early-late gate tracking optimization after signal interruption;

[0036] The average of the timing error estimates before the interruption is used as the initial timing error value after transmission is resumed. The first timing error estimate after the interruption...

[0037]

[0038] In the formula, Δi is the number of interrupted time slots. L1 represents the estimated timing error value before the interrupt, and L2 represents the number of estimated timing errors before the interrupt.

[0039] The beneficial effects of this invention are as follows: To improve the tracking synchronization performance of the early-late gate, this invention proposes a weighted scheme for the timing error estimate of the early-late gate based on the ratio of the average information photon count to the background photon count of the current PPM symbol. Then, the initial phase position of the early-late gate is estimated using the photon pulse peak position. Finally, addressing the problem of the early-late gate needing to re-acquire during signal interruption recovery, this invention proposes using the timing error before the interruption as the estimated timing error after recovery, and verifies this through simulation analysis and experimental data.

[0040] Other advantages, objectives, and features of the invention will be set forth in part in the description which follows, and in part will be apparent to those skilled in the art from the following examination, or may be learned from practice of the invention. The objectives and other advantages of the invention can be realized and obtained through the following description. Attached Figure Description

[0041] To make the objectives, technical solutions, and advantages of the present invention clearer, the preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings, wherein:

[0042] Figure 1 The diagram illustrates early and late gate sampling; (a) represents optimal sampling; (b) represents early sampling; and (c) represents late sampling.

[0043] Figure 2 Diagram of early and late gate structure;

[0044] Figure 3 Tracking error curves with loop bandwidth BL = 1000 and loop gain Go_Gd = 0.1;

[0045] Figure 4 Comparison of tracking errors for different tracking schemes under different turbulence intensities; (a) l = 5 km; (b) l = 10 km; (c) l = 50 km;

[0046] Figure 5 This is a schematic diagram of the sampled data;

[0047] Figure 6 The following is a comparison of the tracking errors of different schemes under different turbulence intensities; (a) for l = 5 km; (b) for l = 10 km.

[0048] Figure 7 A comparison of timing error estimates under different synchronization schemes when 200,000 time slots are interrupted;

[0049] Figure 8 A comparison of timing error estimates under different synchronization schemes when 500,000 time slots are interrupted;

[0050] Figure 9 Implementation flowchart for the system;

[0051] Figure 10 Comparison of timing error estimates under different synchronization schemes to interrupt the third frame of data;

[0052] Figure 11 Comparison of timing error estimates under different synchronization schemes to interrupt the data of frames 3 and 4;

[0053] Figure 12 The graph shows the bit error rate performance of different methods under experimental data. Detailed Implementation

[0054] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the illustrations provided in the following embodiments are only schematic representations of the basic concept of the present invention. Unless otherwise specified, the following embodiments and features can be combined with each other.

[0055] The accompanying drawings are for illustrative purposes only and are schematic diagrams, not actual pictures. They should not be construed as limiting the invention. To better illustrate the embodiments of the invention, some parts in the drawings may be omitted, enlarged, or reduced, and do not represent the actual product dimensions. It is understandable to those skilled in the art that some well-known structures and their descriptions may be omitted in the drawings.

[0056] In the accompanying drawings of the embodiments of the present invention, the same or similar reference numerals correspond to the same or similar components. In the description of the present invention, it should be understood that if terms such as "upper," "lower," "left," "right," "front," and "rear" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, they are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, the terms used to describe positional relationships in the drawings are only for illustrative purposes and should not be construed as limiting the present invention. For those skilled in the art, the specific meaning of the above terms can be understood according to the specific circumstances.

[0057] 1. Optical PPM time slot synchronization method

[0058] 1.1 Optical PPM Slot Synchronization Method Based on Early-Later Gate Tracking

[0059] Early-late gates utilize the symmetry of the upper portion of photon pulses to synchronize sampling data in time slots. For example... Figure 1 The diagram shows the sampling pattern for early and late gates, where i0 is the optimal sampling position and i1 is the non-optimal sampling position. - To determine the location of the sampling points in advance, i + Let t0 be half a sampling clock cycle to indicate the location of the lagging sampling point. Because the maximum output value is found at the optimal sampling position after integrating the sampled signal, and this value is symmetrical about the optimal sampling position, such as... Figure 1 As shown in (a), under ideal conditions, the sampled values ​​of the early sampling point and the late sampling point are equal, and the midpoint between the positions of the early sampling point and the late sampling point is the optimal sampling position.

[0060] If the initial sampling point position is i1, then the advance sampling point position i - t0, and the lag sampling point position i + = i1 + t0, the sampled values ​​of the early sampling point and the late sampling point are not equal, but the difference between the two contains timing error information. For example Figure 1 As shown in (b), if the sampling time i1 is advanced, then x(i - )<x(i + If so, then the next sampling time needs to be delayed; for example... Figure 1 As shown in (c), when the sampling time i1 is delayed, then x(i - )>x(i + If the sampling time is too early, then the next sampling time needs to be advanced. By comparing the advanced sampling value and the lagging sampling value at the current sampling point position, the sampling clock is continuously adjusted until the two are equal, so that the current sampling point position is the optimal sampling point position.

[0061] like Figure 2 The early-late gate, loop filter, and numerically controlled oscillator constitute an early-late gate system. In the closed-loop synchronization system, the pre-processed sampled data is divided into two inputs to the early-integrating gate and the late-integrating gate, respectively. The early-integrating gate integrates within half a sampling clock cycle t0, and the late-integrating gate integrates within t0 after a delay of half a sampling clock cycle. Therefore, according to the principle of early-late gates, the difference between the early-integrating gate and the late-integrating gate is used as the timing error. The estimated timing error for the i-th time slot is:

[0062] err i =(x(mk) i-1 -1)-x(mk i+1 +1))*x(mk i )

[0063] In the formula, x is the sampled data sequence; mk i This represents the sampling time corresponding to the current time slot.

[0064] Because the ideal integrator filter has a large DC gain and a very small fixed phase error after loop locking, this paper selects the ideal integrator filter as the loop filter. Therefore, the estimated frequency offset value for the i-th time slot is:

[0065]

[0066] In the formula, ω n ξ is the angular frequency of the free oscillation of the loop; ξ is the damping coefficient; T s is the time slot period; Go_Gd is the loop gain.

[0067] The free angular frequency of the loop can be expressed as:

[0068]

[0069] In the formula, BL is the tracking loop bandwidth.

[0070] Therefore, the estimated sampling time for the i-th time slot is:

[0071] ipos i =ipos i-1 +(1-path i )s

[0072] In the formula, s is the number of sampling points per time slot.

[0073] Therefore, rounding the estimated sampling time to the nearest integer gives the sampling time of the i-th time slot:

[0074] mk i =round(ipos i )

[0075] Each time slot uses the amplitude at the selected sampling time as the current time slot signal amplitude. Therefore, the signal amplitude of the i-th time slot is:

[0076] tdata(i)=x(mk i )

[0077] 1.2 Simulation Analysis

[0078] In optical PPM time-slot synchronization based on early-late gate tracking, loop parameters have a significant impact on the performance of the early-late gate; therefore, it is necessary to determine the loop parameters through simulation. First, determine whether the loop parameters can lock the synchronization loop within the expected range, and then search for the optimal loop parameters. Initially, set the damping coefficient in the loop to ξ = 0.707, and then use the following formula:

[0079]

[0080] Calculate the free angular frequency ω of the loop. n It is determined by the tracking loop bandwidth BL and the damping coefficient ξ.

[0081] Then, according to the following formula:

[0082]

[0083] The frequency offset estimate is calculated from the loop free oscillation angular frequency ω. n And the loop gain Go_Gd is determined.

[0084] When determining the loop parameters through simulation, the simulation parameters are set to 64-PPM, with each PPM symbol containing two protection time slots and a time slot period T. s =32ns, signal pulse duty cycle T t =0.5T sThe number of sampling points per time slot is S = 4, and the average number of background photons per time slot is N. b =0.01. Using a mean of 0 and a standard deviation of 0.2T. s The Gaussian jitter distribution model is used, and the actual single-photon waveform obtained in the experiment is used as the photon pulse waveform in the simulation data. Band-limited Gaussian noise with a mean of 0 and a standard deviation of 0.3 times the single-photon peak value is used as the signal noise. To reflect the performance of the synchronization scheme of each time slot, the simulation data is preprocessed. First, to reduce the impact of Gaussian noise on the data, the maximum amplitude of Gaussian noise is estimated based on the amplitude of the first 5000 sampling points. The maximum amplitude of Gaussian noise is used as a threshold, and values ​​in the sampling data less than the threshold are set to 0. Then, because the photon pulse is severely broadened, adjacent time slots of the signal time slot are easily misjudged as having photons. Therefore, only the peak value and the amplitude of the two sampling points on the left and right are retained for the pulse, and the amplitude of all other sampling points is set to 0.

[0085] After multiple simulations, by comparing the acquisition time required by the optical PPM time slot synchronization method based on early-late gate tracking under various set parameters, and determining the loop bandwidth BL=1000 and loop gain Go_Gd=0.1 based on the tracking error change during the tracking phase, good synchronization performance can be obtained. Figure 3 The figure shows the tracking error curve under this parameter setting.

[0086] 2. Timing error weighting

[0087] 2.1 Timing Error Weighting

[0088] Atmospheric turbulence limits the transmission distance and signal quality of atmospheric laser communication. The greater the fluctuations in light intensity due to atmospheric turbulence, the greater the fluctuations in transmitted signal quality. To investigate the synchronization performance of early-late gate tracking in an atmospheric turbulent fading channel, this paper uses numerical simulation to weight the signal pulses based on time-varying light intensity fluctuations, thus simulating the impact of atmospheric turbulence on the laser.

[0089] Atmospheric turbulence causes amplitude variations in light pulses, increasing the probability of information bit errors. Furthermore, the greater the attenuation of the signal light pulse due to atmospheric turbulence, the greater the influence of Gaussian white noise on the signal pulse, resulting in a smaller photon pulse peak value and altered pulse shape. Therefore, variations in the photon pulse peak value in the sampled data affect the calculated timing error estimate, err. i The errors differ. During the early and late gate tracking phase, when the estimated timing error is too large, a "lack of lock" phenomenon will occur.

[0090] To address the above issues, this section proposes weighting the timing error based on the reliability of the timing error estimate and extending the early / late gate tracking duration. Because the signal pulse intensity varies, the reliability of the timing error estimate differs for each signal pulse. Therefore, this section uses the ratio of the average information photon count to the background photon count of the current PPM symbol as the weighting coefficient for the timing error estimate. Thus, the timing error estimate for the i-th time slot can be obtained as follows:

[0091]

[0092] In the formula, N s N represents the average number of information photons for the current PPM symbol. b The average background photon number.

[0093] 2.2 Simulation Analysis

[0094] Using tracking error as an indicator, the tracking performance of the early-late gate scheme with and without weighted processing is simulated and analyzed. The simulation parameters are the same as in the previous section, set to 64-PPM, with each PPM symbol containing 2 guard slots and a slot period T. s =32ns, signal pulse duty cycle T t =0.5T s Average background photon number N per time slot b =0.01. And using a mean of 0 and a standard deviation of 0.2T. s A Gaussian jitter distribution model was used. The actual single-photon waveform obtained in the experiment was used as the photon pulse waveform in the simulation data, and band-limited Gaussian noise with a mean of 0 and a standard deviation of 0.3 single-photon peak value was used as the signal noise. The normalized value of the light intensity fluctuation was used to weight the average number of signal light over time. The simulation data was also pre-processed. In the simulation, the atmospheric turbulence intensity was changed by varying the distance *l* between the transmitter and receiver. Figure 4 The figure shows the weighted and unweighted tracking error curves under a specific turbulence intensity. Figure 4 The tracking errors of different tracking schemes under different turbulence intensities are compared: (a) l = 5 km; (b) l = 10 km; (c) l = 50 km.

[0095] Figure 4 To compare the tracking errors of different tracking schemes under varying turbulence intensities, both schemes maintain a dynamic equilibrium during the early-late gate tracking phase under different turbulence intensities. However, as the distance between the transmitter and receiver increases, atmospheric turbulence intensifies, and the fluctuations in the tracking errors of both schemes become less predictable. Furthermore, compared to the unweighted scheme, the tracking error fluctuation amplitude of the timing error weighted scheme, which estimates the timing error based on the ratio of the average information photon number to the background photon number of the current PPM symbol, is reduced to some extent, indicating that the weighted scheme has better tracking performance.

[0096] 3. Estimate the initial sampling time

[0097] 3.1 Estimating the initial sampling time

[0098] Since the received signal has not only frequency deviation but also initial sampling deviation, the acquisition time required for early-late gate synchronization will vary depending on the selected initial sampling time when using the early-late gate to synchronize the sampled data. Due to the different selected initial sampling times, two situations will occur: (1) If the peak point of the photon pulse and the two points on the left and right are all within the early-late gate at the current sampling time, then the timing error detector will detect the entire photon pulse within the current early-late gate and obtain a timing error with a small error; (2) If the three sampling points of the photon pulse are not completely within the early-late gate at the current sampling time, the timing error detector will detect the pulse in the current early-late gate and obtain an inaccurate timing error, making the early-late gate acquisition time longer.

[0099] To address the above issues, the selection of the initial sampling time is optimized to reduce the acquisition time of early and late gate synchronization. Sampling data is acquired using four times the sampling data. Assuming that signal fading, latency jitter, and other issues are not considered, such as... Figure 5 The diagram shows the first segment of sampled data. The peak positions of the three information photon pulses are all the third sampling point of the time slot. When the third sampling point of the time slot is used as the initial sampling time, the calculated timing error estimate is more accurate. However, when other sampling points of the time slot are used as the initial sampling time, the signal amplitudes of the early and late sampling points are significantly different, resulting in a larger deviation in the timing error estimate. Multiple early and late gate captures are required to obtain a more accurate timing error estimate.

[0100] Therefore, to ensure the photon pulse falls within the early / late gate of the current sampling time, the initial sampling time can be selected based on the peak position of the photon pulse. However, due to atmospheric turbulence and fading, some photon pulses undergo shape changes, and under random time delay jitter, they broaden. Furthermore, the location of background light pulses is also random. Estimating the initial sampling time using the peak position of a single photon pulse results in a large error. Therefore, selecting the peak positions of multiple photon pulses to estimate the initial sampling time is calculated using the following formula:

[0101]

[0102] In the formula, L represents the number of photon pulse peak positions; k i is the peak position of the i-th photon pulse; S is the sampling factor.

[0103] Because the sampled data has frequency shifts, estimating the initial sampling time using a large number of photon pulse peak positions results in significant errors; conversely, estimating the initial sampling time with a small number of photon pulse peak positions also leads to large errors. Therefore, an appropriate number of peak positions needs to be selected, L = 20.

[0104] 3.2 Simulation Analysis

[0105] Using tracking error as an indicator, the tracking performance of the early-late gate scheme with and without estimated initial sampling time is simulated and analyzed. The simulation parameters are the same as in the previous section, set to 64-PPM, with each PPM symbol containing 2 guard slots and a slot period T. s =32ns, signal pulse duty cycle T t =0.5T s Average background photon number N per time slot b =0.01. Using a mean of 0 and a standard deviation of 0.2T. s A Gaussian jitter distribution model was used. The actual single-photon waveform obtained in the experiment was used as the photon pulse waveform in the simulation data, and band-limited Gaussian noise with a mean of 0 and a standard deviation of 0.3 single-photon peak value was used as the signal noise. The normalized value of the light intensity fluctuation was used to weight the average number of signal light over time. The atmospheric turbulence intensity was varied by changing the distance *l* between the transmitter and receiver in the simulation. Figure 6 The figure shows the tracking error curves comparing the tracking errors of different initial sampling times under a specific turbulence intensity.

[0106] Depend on Figure 6 It can be seen that the tracking errors of both schemes can reach a dynamic equilibrium after a certain period of time. The scheme that randomly selects any position in the first time slot as the initial sampling time has an uncertain initial tracking error due to the randomness of the initial sampling time. The optimized scheme uses the average of the peak values ​​of the first few photon pulses at their respective time slot positions as the initial sampling time, resulting in a smaller initial tracking error. The larger the initial tracking error, the longer the early / late gate acquisition time. For the scheme with a random initial sampling time, where… Figure 6 (a) relative Figure 6 (b) has a smaller initial tracking error, while (a) has a shorter early-late gate capture time. Furthermore, the method of estimating the initial sampling time using the peak position of the photon pulse results in a shorter early-late gate capture time due to the small initial tracking error.

[0107] Based on the simulation analysis above, it can be shown that, under turbulent conditions, the scheme proposed in this section for estimating the initial sampling position based on the peak position of the photon pulse has a significant performance improvement effect on early and late gate time slot synchronization.

[0108] 4. Optimization of early and late gate tracking after signal interruption

[0109] Under the influence of various meteorological factors, atmospheric wind speed changes randomly, generating atmospheric turbulence. The previous section discussed extending the early-late gate tracking duration using a weighted approach when the impact of atmospheric turbulence on the signal was relatively small. Excessive atmospheric turbulence causes deep fading of the optical signal during transmission, even leading to signal interruption at the receiving end. Typically, strong atmospheric turbulence causes signal fading on the order of 10ms. In this paper, with each frame containing 2520 PPM symbols, each PPM symbol containing 66 time slots, and a time slot width Ts = 32ns, strong atmospheric turbulence fading would likely cause signal interruption for several frames.

[0110] During signal interruption, the early-late gate cannot detect photon pulses, disrupting the dynamic balance of tracking errors and causing a "lost-lock" phenomenon. After signal transmission resumes, the early-late gate needs to re-acquire the signal. Furthermore, during the signal interruption, the difference between the estimated and actual timing error accumulates, forming the initial phase deviation after signal transmission resumes. This results in a longer re-acquisition time for the early-late gate after transmission resumes, leading to lower synchronization performance. Therefore, to address this issue, the early-late gate scheme during transmission resumption is optimized.

[0111] Doppler shifts due to changes in relative velocity. Given that the maximum acceleration currently does not exceed 10g, and using a sampling period of 8ns in this paper, the change in Doppler frequency shift is far less than 1ppm. Furthermore, the experimental equipment used in this paper maintained a frequency offset of approximately 13ppm over multiple experiments. Therefore, the change in Doppler frequency shift is far less than the frequency shift caused by the hardware. Thus, we ignore the Doppler frequency shift in this paper and assume that the frequency offset remains essentially constant.

[0112] Therefore, to reduce the re-acquisition time of the early-late gate, the timing error estimate before the interruption is considered as the initial timing error value after transmission recovery. As the principle of the early-late gate shows, the timing error estimate is in a dynamic equilibrium state during the early-late gate tracking phase, exhibiting fluctuations. Furthermore, the accuracy of the timing error estimate is low when signal fading is severe. This indicates that the timing error estimate is random within a certain range. If the timing error estimate before the interruption is directly used as the initial timing error value after transmission recovery, a large error may occur. Therefore, to improve the accuracy of the timing error estimate, based on the characteristics of the timing error estimate, the average of the timing error estimates in the segment before the interruption is used as the initial timing error value after transmission recovery. Thus, the first timing error estimate after the interruption...

[0113]

[0114] In the formula, Δi is the number of interrupted time slots. L1 represents the estimated timing error value before the interruption, and L2 represents the number of estimated timing errors.

[0115] 4.2 Simulation Analysis

[0116] To reduce the reacquisition time required by the early-late gate after signal interruption, further optimization was performed on the early-late gate. The simulation parameters remained the same as before, set to 64-PPM, with each PPM symbol containing two guard slots and a slot period T. s =32ns, signal pulse duty cycle T t =0.5T s Average background photon number N per time slot b =0.01. Using a mean of 0 and a standard deviation of 0.2T. s A Gaussian jitter distribution model was used. The actual single-photon waveform obtained in the experiment was used as the photon pulse waveform in the simulation data, and band-limited Gaussian noise with a mean of 0 and a standard deviation of 0.3 times the single-photon peak value was used as the signal noise. The normalized value of the light intensity fluctuation was used to weight the average number of signal light pulses over time, and a segment of data was selected where all signal photon pulses were lost to simulate signal interruption caused by excessive atmospheric turbulence. To prevent the different timing error estimates of the two schemes from affecting the timing error estimate after transmission recovery during the interruption period, both schemes used the mean of the timing error estimates before the interruption as the timing error estimate during the interruption period. Figure 7 and Figure 8 The figure shows the offset at a specific frequency. Below is a comparison of the timing error curves of the optimized and unoptimized schemes.

[0117] Figure 7 and Figure 8 The timing error estimation results for the two methods are shown under interruption conditions of 6.4ms and 16ms, respectively. Firstly, after signal transmission resumes, the timing error estimate of the optimized scheme is basically at a dynamic equilibrium, while the timing error estimate of the unoptimized synchronization scheme requires multiple iterations to reach dynamic equilibrium. For example, using a re-reaching actual timing error of 15ppm as a standard... Figure 7 The optimized scheme requires approximately 19,000 time slots to resynchronize, while the unoptimized scheme requires 51,000 time slots. The unoptimized scheme takes approximately 2.68 times longer to acquire the early / late gate. This demonstrates that using the average of the estimated timing error before the interruption as the initial timing error value for resuming transmission improves the performance of the early / late gate.

[0118] 4.3 Experimental Verification

[0119] The system consists of two parts: a transmitter and a receiver. The transmitter encodes the raw bit data using a computer and an FPGA, and controls the laser to transmit signals based on the data. The receiver receives the optical signal using an MPPC, samples the signal using an oscilloscope, performs clock synchronization and decoding on the computer. The system flowchart is shown below. Figure 9 As shown.

[0120] Specific experimental steps:

[0121] Step 1: The computer encodes the data using a Serially Concatenated Pulse-Position Modulation (SCPPM) encoding system, adds the required frame synchronization symbols to the data, and then transmits the data to a Field Programmable Gate Array (FPGA) via a data connection line.

[0122] Step 2: The laser is driven by the FPGA, so that the laser outputs the modulated PPM light pulse sequence.

[0123] The third step is to use MPPC to receive laser data in photon detection mode at the receiving end, and at the same time convert the optical signal into an electrical signal. Then, a Butterworth filter is used to filter the electrical signal to reduce the interference of signal noise on the optical pulse signal.

[0124] Step 4: Sample the signal using an oscilloscope and transfer the data to the computer using a USB flash drive.

[0125] Step 5: In the computer, clock synchronization and photon count recovery of the sampled data are completed, and the data is decoded and output through SCPPM.

[0126] The experimental parameters were set as follows: 64-PPM, 2520 symbols per frame, 2 guard slots per PPM symbol, and slot period T. s =32ns, S=4 sampling points per time slot. Due to the limited storage space of the oscilloscope, in order to obtain the longest possible continuous sampling data, and since frame synchronization is not considered in this chapter, when sending data, a frame header consisting of 10 consecutive "1" symbols (10 PPM symbols) is added every 10 frames of data. When the oscilloscope captures and saves data, the position of the captured data can be confirmed by observing the position of the frame header. In MATLAB, based on the position of the frame header, 10 frames of continuous sampling data are extracted, and then the sampling data is initially processed. The processing method is still the same: first, Gaussian noise interference is removed, and then only the peak point and the two points to its left and right of each photon pulse are retained.

[0127] Firstly, to verify the performance of the two schemes after a signal interruption, the amplitude of a segment of sampled data was set to 0 to represent the interruption condition. For example... Figure 10 and Figure 11The figure shows a comparison of timing error curves for different synchronization schemes under experimental data, specifically for interruptions of one frame and two frames. Consistent with the conclusions drawn from simulation data, after transmission is resumed, the optimized scheme reaches dynamic equilibrium faster than the unoptimized scheme, regardless of whether the interruption is one or two frames.

[0128] To verify the impact of early and late gate acquisition time on the bit error rate of the current frame after signal interruption, all data from approximately the third frame in the sampled data were set to 0, and the bit error rate of the first iteration of the fourth frame was used as the indicator. Figure 12 The figure shows the bit error rate performance of the two methods under experimental data. Both methods can correctly decode the first frame after resuming transmission following a one-frame interruption. However, the optimized scheme has a lower bit error rate. At a bit error rate of 5*10... -2 Under these circumstances, the unoptimized solution has a loss of approximately 0.1 dB compared to the optimized solution.

[0129] This invention selects the early-late gate synchronization method as the PPM time slot synchronization scheme. Addressing the issue that atmospheric turbulence causes variable amplitude of optical pulses, resulting in varying reliability of timing errors for different photon pulses, this invention proposes a weighted average of the estimated timing error based on the ratio of the average number of information photons to the average number of information photons in the current PPM symbol. Then, considering the difference in initial sampling times leading to varying acquisition times for the early-late gate, this invention proposes estimating the initial sampling time based on the positions of the first 20 photon pulses, reducing the early-late gate synchronization acquisition time. Furthermore, for signal interruptions caused by excessive atmospheric turbulence, the sampling time is re-estimated after transmission resumes, using the timing error estimate before the interruption as the current initial timing error value, significantly reducing the early-late gate re-acquisition time. Finally, the performance of the re-synchronization scheme after signal interruption is experimentally verified using an MPPC-based optical PPM communication experimental platform. The improved early-late gate time slot synchronization method of this invention not only boasts high tracking accuracy and fast computation speed but is also suitable for PPM time slot synchronization in turbulent fading channels.

[0130] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for optical PPM time slot synchronization under atmospheric turbulent fading channels, characterized in that: The method includes the following steps: S1: Optical PPM Slot Synchronization Method Based on Early-Later Gate Tracking Asynchronous sampling sequence at the receiving end If the initial sampling point position is i 1. Location of pre-sampling points i - = i 1- t 0, t 0 represents half a sampling clock cycle, lagging behind the sampling point position. i + = i 1+ t 0. If the sampled values ​​of the advance sampling point and the lag sampling point are not equal, then the difference between the two contains timing error information. When sampling time i If 1 is advanced, then there is x ( i - )< x ( i + The next sampling time needs to be delayed; When sampling time i 1. Lag, then there is x ( i - )> x ( i + The next sampling time needs to be advanced; Based on the early-late gate principle, the difference between the early integration gate and the late integration gate is used as the timing error. i The estimated timing error for each time slot is: In the formula, x The sampled data sequence; This refers to the sampling time corresponding to the current time slot; Selecting an integrator filter as the loop filter, the first... i The estimated frequency offset for each time slot is: In the formula, The angular frequency of the free oscillation of the loop. The damping coefficient is... For the time slot period, The loop gain is expressed as: The free oscillation frequency of the loop is expressed as: In the formula, BL is the tracking loop bandwidth; No. i The estimated sampling time for each time slot is: In the formula, s Number of sampling points per time slot; Rounding the estimated sampling time value to the nearest integer, we get the first... i The sampling times for each time slot are: Each time slot uses the amplitude at the selected sampling time as the current time slot signal amplitude, the 1st time slot... i The signal amplitude for each time slot is: Based on whether the synchronization loop is locked within the expected range, after several simulations, the loop parameters are selected and time slot synchronization is completed. S2: Timing error weighting The ratio of the average information photon count to the background photon count of the current PPM symbol is used as the weighting coefficient for the timing error estimate; the first... i The estimated timing error for each time slot is: In the formula, The average number of information photons for the current PPM symbol; N b The average background photon number; S3: Estimating the initial sampling time The initial sampling time is estimated by selecting the peak positions of multiple photon pulses, and the calculation formula is as follows: In the formula, L This represents the number of peak positions of the photon pulse. For the first i The peak position of each photon pulse; S This is the sampling multiple; S3 is followed by S4: early and late gate tracking optimization after signal interruption; The average of the timing error estimates before the interruption is used as the initial timing error value after transmission is resumed. The first timing error estimate after the interruption... : In the formula, The number of time slots interrupted. This is the estimated timing error value before the interruption. L 1 represents the number of estimated timing errors before the interruption.

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