A method and system for time slot synchronization of scppm signals based on photon detection

Abstract

The present application relates to the field of optical communication technology, especially to a kind of SCPPM signal time slot synchronization method and system based on single-photon detection, the method first acquires photon timing and completes initial synchronization of time slot integration, solves initial time slot offset;Reconstruction time slot boundary realizes PPM symbol reorganization, and the log-likelihood ratio is revised using neighborhood weighting method in combination with pulse leakage characteristics;Fixed offset sampling point is set to carry out finite number of iterations decoding, in combination with CRC check to determine the validity of decoding, and the optimal decoding result is output after iteration termination. The present application forms the closed-loop compensation logic of photon timing measurement, offset iteration optimization and decoding result judgment, adapts to various single-photon detector distorted pulses, has low algorithm complexity and small iteration overhead, can effectively reduce the bit error rate and improve the communication receiving sensitivity, and is suitable for deep space weak light, long-distance single-photon optical communication scene.

Description

Technical Field

[0001] This invention relates to the field of optical communication technology, and in particular to a method and system for SCPPM signal time slot synchronization based on photon detection. Background Technology

[0002] With the development of deep space exploration, high-speed inter-satellite links, and low-light free-space optical communication technologies, single-photon detection technology has gradually become an important means of realizing ultra-long-distance optical communication. Under extremely low photon count conditions, traditional photodetectors struggle to obtain sufficient signal energy. Superconducting nanowire single-photon detectors (SNSPDs), with their high detection efficiency, extremely low dark count rate, and picosecond-level time resolution, have become one of the most promising core devices in current single-photon communication systems. SNSPDs typically possess detection efficiencies exceeding 80%, dark count rates on the Hz level or even lower, and time jitter on the order of tens of picoseconds. These characteristics make them widely applicable in deep space optical communication, quantum communication, and low-light detection. In single-photon detection-based optical communication systems, pulse position modulation (PPM) is a typical high-photon-efficiency modulation method. It carries information by selecting specific positions to send light pulses within multiple time slots of a symbol period, thus maintaining high communication efficiency under extremely low signal power conditions.

[0003] like Figure 1 The diagram illustrates a reference schematic of 4PPM modulation. The information in PPM modulation depends on the position of the pulse slot within the symbol period, and the determination of the photon arrival time determines the pulse intensity. Therefore, in a PPM system, the receiver must accurately identify the slot boundaries within the symbol period; otherwise, even if a photon event is successfully detected, it is difficult to determine its corresponding symbol position, severely impacting the system's bit error rate performance. Thus, slot synchronization becomes one of the key technologies in single-photon optical communication systems.

[0004] In existing time-slot synchronization methods, a synchronization strategy based on cumulative statistics of the symbol period is commonly used to estimate the system's time offset. This method typically uses the symbol length as the integration window, accumulates the received signal at different candidate time offsets, and determines the most likely symbol boundary by comparing the magnitudes of the statistics corresponding to each offset position. Since the optical pulse energy in PPM modulation is mainly concentrated within a specific time slot, the cumulative statistics at the corresponding position are usually the largest under correct alignment conditions, thus achieving the estimation of the symbol start position. This method has a simple structure and low computational complexity, and is widely used in single-photon communication systems where time jitter has a relatively small impact, such as communication systems based on single-photon avalanche diode (SPAD) detectors.

[0005] However, in high-speed optical communication systems, to further improve communication rates, it is necessary to continuously reduce the time slot width. When the time slot width gradually approaches or even reaches the level of detector time jitter, the time uncertainty introduced by the jitter will significantly affect the statistical distribution of photon arrival times, causing the originally ideal pulse distribution to expand on the time axis.

[0006] like Figure 2 The diagram illustrates the energy distribution characteristics of the received optical pulse in the time domain by the SNSPD detector. It can be seen that in a real system, the photon detection event generated by a single optical pulse may span multiple adjacent time slots, thus violating the fundamental assumption of the ideal Poisson model that "photons are mainly concentrated in a single time slot." Under the combined effects of detector arrival time jitter and dead time, the originally ideal rectangular pulse undergoes significant distortion in the time domain, and its distribution gradually evolves into an asymmetric form. Figure 2 The pulse structure appears as a narrower pulse on the left and gradually widens on the right, extending into adjacent time slots. Meanwhile, factors such as the detector's time response function, system bandwidth limitations, and channel transmission effects further alter the statistical distribution characteristics of the received pulses in the time domain, making it difficult to accurately describe the probability distribution function of the actual received signal using simple models.

[0007] In this context, the likelihood function constructed based on the ideal Poisson statistical assumption fails to accurately reflect the statistical characteristics of the real signal. As the system time slot width gradually decreases, the aforementioned time diffusion effect significantly increases the probability of photon events overlapping between adjacent time slots, leading to increased uncertainty in time slot decision-making and further limiting the improvement of system communication rate. Therefore, in high-speed single-photon optical communication systems based on SNSPD, it is necessary to improve existing time slot synchronization algorithms to address the impact of time jitter and the actual pulse statistical distribution characteristics. Summary of the Invention

[0008] The purpose of this invention is to provide a time slot synchronization method and system for SCPPM signals based on photon detection. While maintaining the main framework of the traditional time slot synchronization algorithm, it innovatively introduces a decoding mechanism based on sampling point offset. By tolerating and compensating for sampling point offset caused by time synchronization deviation, it reduces the impact of pulse leakage caused by time jitter and dead time on decoding performance, thereby achieving time slot synchronization of SCPPM signals based on photon detection.

[0009] To achieve the above objectives, the present invention provides the following technical solution: According to one aspect of the present invention, a method for SCPPM signal time slot synchronization based on photon detection is provided, comprising the following steps: S1. The receiver collects the optical pulse signal, obtains the photon arrival timestamp through a time-to-digital converter, and reconstructs the photon counting time sequence. S2. Perform time slot synchronization integration on the photon counting time sequence, complete the initial synchronization according to the time slot energy distribution, and solve for the initial time slot offset. S3. Reconstruct the time slot boundary based on the initial time slot offset to complete the PPM symbol reassembly; S4. Combining the pulse energy leakage characteristics of photon detectors, the log-likelihood ratio of time slot photon counting is corrected using a neighborhood weighting method; S5. The corrected log-likelihood ratio sequence is fed into the SCPPM decoder to complete the decoding, and the validity of the decoding result is verified.

[0010] According to an embodiment of the present invention, step S5, which verifies the validity of the decoding result, specifically includes: If the decoding verification fails, the time slot offset is updated iteratively and the PPM symbol reassembly, likelihood ratio correction, and decoding process are repeated. If the decoding verification passes or the number of iterations reaches a preset threshold, the iteration is terminated, and the optimal decoding result is selected for output.

[0011] According to one embodiment of the present invention, the preset threshold for the number of iterations is set to 3.

[0012] According to one embodiment of the present invention, the offset search sampling points are set to -1, -2, and +1 in sequence, and the optimal offset traversal search is completed within a finite interval based on the initial time slot offset.

[0013] According to one embodiment of the present invention, the validity of the decoding result is determined by CRC cyclic verification. After the iteration is completed, the result with the best error performance is selected from all the iterative decoding results as the optimal decoding result.

[0014] According to an embodiment of the present invention, in step S4, the neighborhood weighting method fuses the photon counts of the current time slot and the adjacent time slots before and after. The weighting coefficients are pre-calibrated based on the background photon noise level and the detector response characteristics, and are used to quantify and suppress demodulation interference caused by pulse energy leakage.

[0015] According to one embodiment of the present invention, the decoding architecture of the decoder in step S5 adopts a serial iterative decoding architecture or a parallel decoding architecture. The parallel decoding architecture reduces the decoding processing latency by synchronously processing time slot data under different offsets through multiple channels.

[0016] According to one embodiment of the present invention, the photon detector is any one of a superconducting nanowire single-photon detector, a single-photon avalanche diode, a photomultiplier tube, and a superconducting edge sensor.

[0017] According to one embodiment of the present invention, the decoder employs serial concatenated pulse position modulation encoding, or LDPC code, or RS code.

[0018] On the other hand, the present invention also provides a time slot synchronization system for SCPPM signals based on photon detection, the system comprising a single photon detection module, a photon timing acquisition module, a time slot synchronization processing module, a likelihood ratio correction module, an iterative decoding module, and a result decision output module; The single-photon detection module is used to receive weak light pulse signals and complete photon detection. The photon timing acquisition module uses a time-to-digital converter to acquire photon timestamps and generate a photon counting timing sequence. The time slot synchronization processing module completes the time slot integration and initial offset calculation; The likelihood ratio correction module corrects the log-likelihood ratio using a neighborhood weighting method. The iterative decoding module combines a preset offset sequence to complete iterative decoding; The result judgment output module selects the optimal decoding data based on the verification result and completes the output.

[0019] The present invention provides a time-slot synchronization method and system for SCPPM signals based on photon detection. Compared with the prior art, the beneficial effects of the present invention are as follows: This invention significantly improves the sensitivity of high-speed single-photon communication under jitter, while maintaining simple logic and low complexity. Traditional time-slot synchronization algorithms only address latency, but fail to provide accurate latency estimates after introducing jitter, resulting in inaccurate decoding due to jitter. Monte Carlo simulations show that after obtaining latency estimates, using adjacent sampling points as correction delays in the photon counting sequence significantly improves communication reliability. This invention solves the time-slot synchronization error problem caused by time jitter and clock deviation in photon detectors (e.g., SNSPDs) through a closed-loop design of timing measurement, initial synchronization, iterative offset correction, and decoding verification. Specifically, with... Using the initial offset benchmark, the offset optimization is completed within a limited computational complexity through iterative search with a fixed step size. The correctness of the decoding result is used as the termination condition for the iteration, which ensures the reliability of the decoding while avoiding the high complexity of full-range offset search. Finally, the overall performance of the SCPPM signal communication system based on photon detection is improved by selecting the best output of the results of multiple iterations. Attached Figure Description

[0020] The accompanying drawings, which are included to provide a further understanding of the invention and form part of this invention, illustrate exemplary embodiments of the invention and are used to explain the invention, but do not constitute an undue limitation of the invention. In the drawings: Figure 1 This is a reference schematic diagram of 4PPM modulation in the prior art; Figure 2 This is a schematic diagram of the probability distribution characteristics of the light pulses received by the SNSPD detector in the time domain; Figure 3 This is a schematic diagram of an optical communication system based on a photon detector using SCPPM signals, according to an embodiment of the present invention. Figure 4 This is a schematic diagram illustrating the use of Poisson approximation to achieve time slot synchronization and decoding offset compensation time slot synchronization according to an embodiment of the present invention; Figure 5 This is a flowchart of the time slot synchronization algorithm for jitter compensation according to an embodiment of the present invention; Figure 6 This is a flowchart of Monte Carlo simulation based on the time slot synchronization algorithm according to an embodiment of the present invention; Figure 7 This is a schematic diagram of the bit error rate performance of the offset compensation combined with time slot synchronization algorithm under a background photon number of nb=0.01 according to an embodiment of the present invention; Figure 8 This is a schematic diagram of the bit error rate performance of the offset compensation combined with time slot synchronization algorithm under a background photon number of nb=0.1 according to an embodiment of the present invention; Figure 9This is a schematic diagram of the frequency distribution of the optimal decoding position under a background photon number of nb=0.01 according to an embodiment of the present invention; Figure 10 This is a schematic diagram of the frequency distribution of the optimal decoding position under a background photon number of nb=0.1 in an embodiment of the present invention. Detailed Implementation

[0021] To facilitate a clear description of the technical solutions in the embodiments of the present invention, the terms "first" and "second" are used to distinguish identical or similar items with essentially the same function and effect. For example, the first threshold and the second threshold are merely used to distinguish different thresholds and do not limit their order. Those skilled in the art will understand that the terms "first" and "second" do not limit the quantity or execution order, and that the terms "first" and "second" are not necessarily different.

[0022] It should be noted that in this invention, the terms "exemplary" or "for example" are used to indicate examples, illustrations, or descriptions. Any embodiment or design described as "exemplary" or "for example" in this invention should not be construed as being more preferred or advantageous than other embodiments or designs. Specifically, the use of terms such as "exemplary" or "for example" is intended to present the relevant concepts in a concrete manner.

[0023] In this invention, "at least one" refers to one or more, and "more than one" refers to two or more. "And / or" describes the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A alone, A and B simultaneously, or B alone, where A and B can be singular or plural. The character " / " generally indicates that the preceding and following related objects are in an "or" relationship. "At least one" or similar expressions refer to any combination of these items, including any combination of singular or plural items. For example, "at least one of a, b, or c" can represent: a, b, c, a combination of a and b, a combination of a and c, a combination of b and c, or a, b, and c, where a, b, and c can be single or multiple.

[0024] In high-speed single-photon optical communication systems based on SNSPD, the synchronization accuracy under narrow time slot conditions can be effectively improved and the communication reliability of the system can be further enhanced by introducing reasonable statistical models or constructing robust decision strategies. For example... Figure 3 The diagram shows a schematic of the basic architecture of a communication system based on an SNSPD detector. According to... Figure 3As can be seen, the signal pulses arriving after passing through the channel exhibit significant distortion. In PPM communication systems, time slot synchronization is achieved by accumulating energy in the received signal within the symbol period. Specifically, the PPM symbol modulation includes a guard slot of a certain length with all low levels. Kevin J. Quirk proposed an integral-based time slot synchronization algorithm in his paper "OPTICAL PPM SYNCHRONIZATION FOR PHOTON COUNTING RECEIVERS". At the receiver, the received signal is first accumulated using a sliding accumulation method with the symbol period length as a window, and the symbol boundary position is estimated by comparing the accumulation results under different time offsets. When the system has a known frame header or synchronization sequence, the periodic structure of the sequence can be used to determine the most likely synchronization start position through correlation detection or cumulative statistics.

[0025] Let the discrete-time receiver counting sequence be... The transmission time slot length for one symbol period is... With N sampling points and a total symbol length of N sampling points, the symbol period accumulation of the received sequence at the candidate time offset τ can be expressed as: The cumulative value was calculated using different time offsets τ. The receiver can find the time position that maximizes the statistic, thereby obtaining the symbol synchronization estimate: This time-slot synchronization method is simple to implement and has low computational complexity, thus it is widely used in traditional optical communication systems. Since the received signal manifests as discrete photon arrival events, its statistical characteristics can typically be modeled as a Poisson process. When the time jitter is much smaller than the time slot width, the arriving photon pulses can be approximated as rectangular pulses in the system. This time-slot synchronization method can achieve good performance, with synchronization accuracy down to approximately a few percent.

[0026] While this method can achieve high-precision time-slot synchronization under photon sparse conditions, it still has certain limitations. This scheme primarily relies on multi-symbol cumulative statistics and correlation detection for synchronization estimation. When the system communication rate is high and the time slot width is significantly reduced, the receiver requires a higher sampling and processing rate. Simultaneously, the statistical accumulation process introduces a significant processing delay, making it difficult to implement in high-communication-rate scenarios. The theoretical model of this method is mainly based on an ideal Poisson photon counting model, assuming that the detector response time is much smaller than the time slot width, without considering non-ideal factors such as time jitter and detector dead time commonly found in practical photon counting receivers. When the optical communication system operates under high-speed or high-photon-flux conditions, time jitter and dead time may produce coupling effects, significantly impacting the statistical characteristics of photon arrival and thus reducing the accuracy of synchronization estimation.

[0027] exist Figure 3 In the optical communication system shown, the transmitter takes the raw data to be transmitted (i.e., modulation information) as input to the system. The raw data is first encoded using SCPPM (Serially Concatenated Pulse Position Modulation), and then mapped into a sequence of pulse symbols at specific time slots, completing the joint processing of channel coding and modulation. The encoded signal is then converted into an optical domain pulse signal, i.e., the transmitted signal. Ideally, the transmitted signal is a narrow pulse (e.g., ...). Figure 3 As shown in the rectangular pulse diagram, the energy is concentrated within a specified time slot.

[0028] The channel transmission and detection stage (PPM and SNSPD channels) is a critical stage for signal distortion and consists of the following steps: The first step, Poisson channel transmission: The optical pulse propagates in the channel and is affected by Poisson noise, which is superimposed with the basis noise n. b (Including dark counts, background light noise, etc.), when the photon count reaches the receiving end, the number of photons follows a Poisson distribution; The second step involves the introduction of SNSPD detection and non-ideal effects: When the arriving photon enters the SNSPD (superconducting nanowire single-photon detector), the non-ideal effect of the detector's dead time directly leads to signal distortion. Thirdly, the time uncertainty of the demodulation system's time-to-digital converter itself causes signal distortion: Timing jitter: The electrical pulse output from the demodulation end is randomly offset in time position relative to the incident photon, and the originally ideal rectangular pulse is broadened and diffused; Dead Time: After a detector is triggered, there is a "dead time" during which it cannot respond to subsequent photons, causing subsequent photons to be unable to be counted correctly, resulting in distortion and energy loss at the leading / trailing edge of the pulse.

[0029] The combined effect of these two non-ideal effects causes the received electrical signal to no longer be an ideal narrow pulse, but a broadened, distorted, asymmetric waveform.

[0030] At the receiving end, a TDC (Time-to-Digital Converter) is performed: the photon arrival event output by the SNSPD is recorded by the TDC (Time-to-Digital Converter) to record the arrival timestamp of each photon, converting the analog time information into a digital signal.

[0031] For the output signal of the multi-pixel SNSPD, pixel alignment and integration processing are performed: alignment compensates for the time offset between different pixels; integration merges the counting results of multiple pixels in the same time slot to restore the energy distribution of the distorted pulse and output a complete distorted signal waveform. Figure 3 The distortion signal corresponding to the filled curve.

[0032] Based on the statistical characteristics of the distorted signal, time slot synchronization is completed, and the time slot boundary of each PPM symbol is determined, providing a time reference for subsequent decoding. According to the synchronized time slot count results, the LLR (Log-Likelihood Ratio) of each time slot is calculated to quantify the confidence level of the bits carried in that time slot. The LLR sequence is fed into the SCPPM decoder, and combined with the encoding rules, the original modulation information is iteratively recovered, outputting the decoded user data (i.e., the decoded information).

[0033] like Figure 4 As shown, for the received signal considering the effects of time jitter and dead time, the average photon arrival rate in different time slots can be expressed as: in, represents the number of background photons, represents the number of signal photons, This represents the normalized photon arrival probability density function after considering the effects of dead time and time jitter.

[0034] For a Poisson channel, its LLR can be expressed as: To simplify computational complexity, the logarithmic terms of each time slot are normalized and represented as a unified weighting coefficient, defined as: Then LLR can be rewritten as: in This represents the normalized weight after signal energy leakage into adjacent time slots due to timing jitter and detector dead time, reflecting the contribution of each time slot to the current decision time slot. For example... Figure 4 As shown, examples comparing the normalized weights of adjacent time slots before and after the Poisson approximation are presented.

[0035] like Figure 5 As shown, a flowchart of a time slot synchronization algorithm for jitter compensation is presented. This time slot synchronization algorithm is based on... Figure 3 The optical communication system shown, combined with Figure 4 The time slot synchronization and decoding offset compensation time slot synchronization method shown addresses the communication reliability problem caused by pulse distortion. The definitions of related symbols are given below: This represents the average number of signal photons per time slot at the receiving end; This represents the average number of background photons per unit time slot at the receiving end; This represents the time delay estimate after the time slot synchronization algorithm, i.e., the initial time slot offset, in units of sampling points (bin). This indicates the offset used when renormalizing the photon counting sequence, in units of sample points (bin). This represents the offset iteration count, used to control the end of the iteration; This represents the photon timestamp sequence after TDC reception, i.e., the statistical number of photons within each sampling point; y This represents the photon count in each time slot after synchronization and resetting; LLR Indicates the relationship between y The processed soft information sequence.

[0036] The time slot synchronization algorithm itself is concentrated at the communication demodulation end. Figure 5 The illustrated process targets the Serial Concatenated Pulse Position Modulation (SCPPM) signal received by photon detection, achieving time slot synchronization, offset iterative correction, and optimal decoding output. The core is a closed-loop processing logic of photon timing measurement - offset iterative optimization - decoding result decision, specifically including the following steps: Step S501: Perform timing acquisition and integration preprocessing on the received photon signal; After receiving the photon signal, the system receiver first uses a time-to-digital converter (TDC) to accurately measure the photon arrival time (in units of sampling point time), completing the digital measurement of the photon arrival time and outputting a timestamp sequence with an accuracy of unit time length of sampling points. The photon count of each sampling point is r[n]. Based on the PPM (Pulse Position Modulation) symbol period, the photon timing data output by the TDC is periodically integrated and statistically analyzed to obtain an integral sequence of length N (N is the number of sampling points included in one symbol period). Thus, the photon count distribution of each time slot can be obtained, completing the energy aggregation within a single symbol period.

[0037] Step S502: Perform time slot synchronization calculation to determine the initial offset; Based on the integrated time slot count distribution, the time slot synchronization algorithm is executed, and the initial time slot offset of the photon sequence is calculated according to equations (1) and (2). This offset represents the time offset of the receiver's synchronization reference position relative to the actual signal time slot boundary. The initial offset size k=0, based on the initial time slot offset. Set the shift position (shift) to... .

[0038] Step S503: PPM symbol processing and iterative correction; The first iteration is initiated based on the initial offset count variable k=0 and the initial current delay estimate shift. The received signal is recombined using PPM symbols to obtain the statistical photon count sequence y[m] of each time slot, with a length of N. The PPM symbol recombination corrects the time slot boundaries and reconstructs the time slot photon count distribution corresponding to each PPM symbol.

[0039] The recombined PPM symbols are subjected to a log-likelihood ratio (LLR) correction. Unlike the conventional log-likelihood ratio correction, a weighted LLR correction is used to account for out-of-band leakage. This is a restriction on the range of values ​​for parameter i in formula (3), and an additional point y in the left and right shifts. The values ​​of are explained to ensure the mathematical rigor of formula (3).

[0040] In formula (3), It is a weighting coefficient for low background photon count and high background photon count, which normalizes the logarithmic terms of each time slot into a unified weighting coefficient, defined as: .

[0041] For example, this parameter is currently determined in the simulation based on the set SNSPD parameter. The exemplary parameter values ​​are as follows: This corresponds to a typical low background photon number situation; This corresponds to a typical case of high background photon number.

[0042] This parameter under different background photon numbers It can be flexibly adjusted to adapt to the system and to suit different detector parameters and channel conditions.

[0043] LLR correction combined with system parameters The process of (number of signal photons to floor noise ratio) is performed, quantizing the confidence level of each bit to provide reliable soft information for decoding.

[0044] The corrected LLR sequence is fed into the SCPPM decoder, where a serial concatenated decoding algorithm is executed to obtain the decoding result at the current offset. The SCPPM decoding module employs soft-decision decoding of serial concatenated pulse position modulation signals.

[0045] Step S504: Iterative offset optimization judgment; Iteration count judgment: Check if the current offset count k is less than 3 (k<3). If it is not satisfied (k≥3 at this time), exit the iteration loop and enter the optimal result selection stage; if it is satisfied (k<3), then execute the decoding correctness judgment.

[0046] Decoding correctness judgment: Verify whether the current SCPPM decoding result is correct (using CRC cyclic check). If the decoding is correct, directly retain the current result as the decoded result; if the decoding is incorrect, perform shift iteration optimization, update the offset (shift iteration update: adjust the shift parameter shift in the order of -1, -2, +1) and the value of k. The offset shift is cyclically adjusted in the order of -1, -2, +1, k = k+1. Repeat step S503, re-perform PPM symbol recombination, LLR correction and decoding, and start the next round of iteration.

[0047] When the number of iterations reaches the upper limit (k=3) or the decoding result cannot be verified, compare the decoding results of all iterations and select the version with the best performance as the final decoding result output.

[0048] like Figure 6The flowchart of Monte Carlo simulation based on the time slot synchronization algorithm is shown. The simulation process adopts a parallel decoding architecture, which reduces decoding latency by synchronously processing time slot data at different offsets through multiple channels. In the actual simulation process, a total of 7 sampling points were selected from the estimated number of sampling points given by the time slot synchronization algorithm. From the simulation results, the three sampling points on the left and the three sampling points on the right did not improve the performance, so they were omitted in the subsequent figures.

[0049] like Figure 7 As shown, the bit error rate performance of the offset compensation combined with time slot synchronization algorithm under a background photon number of nb=0.01 is presented.

[0050] like Figure 8 As shown, the error rate performance of the offset compensation combined with time slot synchronization algorithm under a background photon number of nb=0.1 is presented.

[0051] Comparison of different background photon numbers Figure 7 and Figure 8 Performance data shows that under low background noise, ideal time slot synchronization is prone to performance degradation when only time jitter exists. Offset time slot compensation can effectively mitigate the performance loss caused by jitter, and the improvement is more significant when the compensation range is expanded to ±2 sampling points. When the background noise increases tenfold, the system requires a higher signal-to-noise ratio to achieve the same bit error rate. Even then, ±2 sampling point offset compensation still significantly improves bit error rate performance, outperforming the ±1 sampling point compensation scheme. Therefore, reasonably increasing the offset compensation range can significantly reduce the performance degradation caused by jitter and background noise.

[0052] like Figure 9 As shown, the frequency distribution of the optimal decoding position is given under a background photon number of nb=0.01.

[0053] like Figure 10 As shown, the frequency distribution of the optimal decoding position is given for a background photon number of nb=0.1.

[0054] Based on the frequency distribution of the optimal decoding position under different background photons, it can be seen that the leftmost sampling point of the output position of the time slot synchronization algorithm is the most important choice for the optimal decoding position, followed by the second sampling point from the left. The optimal proportions of the rightmost and rightmost sampling points are extremely low (less than 2×10 under different background noise conditions). −3 ).

[0055] From the perspective of algorithm complexity and implementation efficiency, the optimal decoding positions are highly concentrated at the first and second sampling points from the left. When the initial decoding position is set to the first sampling point from the left, the expected number of decoding operations is 1.556, which is lower than the 1.581 operations at the second sampling point from the left, effectively reducing the average decoding overhead. Therefore, fixing the initial decoding offset to the first sampling point from the left simplifies the process and reduces the number of iterations while ensuring performance, making it the optimal implementation. Based on the frequency distribution of decoding positions, the second from the left, the original position, and the first from the right will continue to be selected.

[0056] By limiting the traversal range of offset sampling points and performing iterative decoding only at the optimal candidate positions (the first and second sampling points from the left), the algorithm complexity is only reflected in a slight increase in the average number of decoding operations, which is approximately 1.56, maintaining a low overall complexity. In terms of performance, this scheme achieves an improvement of approximately 1–2 dB in receiver sensitivity, and near the sensitivity operating point, it achieves a significant improvement of approximately two orders of magnitude in bit error rate performance, effectively mitigating the performance degradation caused by time slot jitter and background noise, thus achieving a good trade-off between complexity and performance.

[0057] Although the invention has been described herein in conjunction with various embodiments, those skilled in the art will understand and implement other variations of the disclosed embodiments by reviewing the accompanying drawings, disclosure, and other materials. In this specification, the word "comprising" does not exclude other components or steps, and "a" or "an" does not exclude multiple components. A single processor or other unit can implement several functions listed in the specification. While certain measures are described in different embodiments, this does not mean that these measures cannot be combined to produce good results.

[0058] Although the invention has been described in conjunction with specific features and embodiments, it is obvious that various modifications and combinations can be made therein without departing from the spirit and scope of the invention. Accordingly, this specification and drawings are merely illustrative of the invention and are considered to cover any and all modifications, variations, combinations, or equivalents within the scope of the invention. Clearly, those skilled in the art can make various alterations and modifications to the invention without departing from its spirit and scope. Thus, if such modifications and modifications fall within the scope of the invention and its equivalents, the invention is also intended to include such modifications and modifications.

Claims

1. A time-slot synchronization method for SCPPM signals based on photon detection, characterized in that, Includes the following steps: S1. The receiver collects the optical pulse signal, obtains the photon arrival timestamp through a time-to-digital converter, and reconstructs the photon counting time sequence. S2. Perform time slot synchronization integration on the photon counting time sequence, complete the initial synchronization according to the time slot energy distribution, and solve for the initial time slot offset. S3. Reconstruct the time slot boundary based on the initial time slot offset to complete the PPM symbol reassembly; S4. Combining the pulse energy leakage characteristics of photon detectors, the log-likelihood ratio of time slot photon counting is corrected using a neighborhood weighting method; S5. The corrected log-likelihood ratio sequence is fed into the SCPPM decoder to complete the decoding, and the validity of the decoding result is verified.

2. The method according to claim 1, characterized in that, Step S5 involves validating the decoding result, specifically including: If the decoding verification fails, the time slot offset is updated iteratively and the PPM symbol reassembly, likelihood ratio correction, and decoding process are repeated. If the decoding verification passes or the number of iterations reaches a preset threshold, the iteration is terminated, and the optimal decoding result is selected for output.

3. The method according to claim 2, characterized in that, The preset threshold for the number of iterations is set to 3.

4. The method according to claim 2, characterized in that, The offset search sampling points are set to -1, -2, and +1 in sequence. The optimal offset traversal search is completed within a limited interval based on the initial time slot offset.

5. The method according to claim 2, characterized in that, The validity of the decoding result is determined by CRC cyclic verification. After the iteration is completed, the result with the best error performance is selected from all the iterative decoding results as the optimal decoding result.

6. The method according to claim 1, characterized in that, In step S4, the neighborhood weighting method integrates the photon counts of the current time slot and the adjacent time slots before and after it. The weighting coefficients are pre-calibrated based on the background photon noise level and the detector response characteristics, and are used to quantify and suppress demodulation interference caused by pulse energy leakage.

7. The method according to claim 1, characterized in that, In step S5, the decoder adopts either a serial iterative decoding architecture or a parallel decoding architecture. The parallel decoding architecture reduces decoding processing latency by synchronously processing time slot data at different offsets through multiple channels.

8. The method according to claim 1, characterized in that, The photon detector is any one of the following: superconducting nanowire single-photon detector, single-photon avalanche diode, photomultiplier tube, or superconducting edge sensor.

9. The method according to claim 1, characterized in that, The decoder uses serial concatenated pulse position modulation encoding, or LDPC code, or RS code.

10. A time-slot synchronization system for SCPPM signals based on photon detection, used to implement the method according to any one of claims 1 to 9, characterized in that, The system includes a single-photon detection module, a photon timing acquisition module, a time-slot synchronization processing module, a likelihood ratio correction module, an iterative decoding module, and a result decision output module. The single-photon detection module is used to receive weak light pulse signals and complete photon detection. The photon timing acquisition module uses a time-to-digital converter to acquire photon timestamps and generate a photon counting timing sequence. The time slot synchronization processing module completes the time slot integration and initial offset calculation; The likelihood ratio correction module corrects the log-likelihood ratio using a neighborhood weighting method. The iterative decoding module combines a preset offset sequence to complete iterative decoding; The result judgment output module selects the optimal decoding data based on the verification result and completes the output.

Images