Optical power estimation method, gain control method, system and terminal

Abstract

The present invention relates to an optical power estimation method, a gain control method, a system and a terminal. The method comprises: receiving an output signal; the output signal comprises a plurality of sequentially transmitted data frames, each of which comprises a frame header and a data sequence, and both the frame header and the data sequence are measured values; defining the frame header in a data frame transmitted by a transmitter as a reference frame header, and the reference frame header is a fixed value. Constructing a cross-correlation function between the reference frame header and the output signal, the maximum value of the cross-correlation function within a data frame time is a correlation peak. Multiple data frames correspond to multiple correlation peaks, each of which obeys a normal distribution, and an estimated value of optical power is calculated based on multiple correlation peaks. The present invention accurately estimates the power of an optical signal in a low-power FSOC system and controls the optimal bias voltage of an APD and the optimal gain voltage of an MA in a receiver according to the estimated value, thereby improving the signal-to-noise ratio and stabilizing the output signal of the receiver.

Description

Technical Field

[0001] The present invention relates to the technical field of automatic gain control, and in particular to an optical power estimation method, a gain control method, a system and a terminal. Background Art

[0002] Free Space Optical Communications (FSOC) refers to a communication method that uses light waves as carriers and does not require any wired channels as transmission media. Compared with traditional microwave communications, the light waves used by FSOC have high frequencies, strong directivity, and do not require frequency use licenses due to their richer spectrum resources. Compared with fiber-optic communications, the locations of FSOC transmitters and receivers are more flexible, with a wider range of application scenarios, and lower costs for long-distance transmission. FSOC combines the advantages of microwave communications and fiber-optic communications, and has become an emerging broadband wireless access method that has attracted widespread attention. At present, FSOC technology has been applied to underwater communications, ground communications, and deep space communications.

[0003] However, in the FSOC system, due to the lack of optical fiber protection, the channel is usually unstable, which causes the signal strength transmitted to the receiver to fluctuate greatly. At this time, it is impossible to ensure the stability of the signal amplitude output by the receiver to the analog-to-digital converter (ADC). In view of this, automatic gain control (AGC) has become a key technology in the FSOC field. The AGC subsystem is configured in the FSOC system. The function of the AGC subsystem is to automatically adjust the gain of the amplifier in the receiver according to the received optical power, thereby stabilizing the output signal of the receiver. The stable output signal is transmitted to the demodulator and decoder for demodulation and decoding.

[0004] In some FSOC scenarios such as laser communication, deep space laser communication, and ground-based confidential optical communication, the received power is very low due to excessive transmission medium attenuation, long transmission distance, limited or specific transmission power requirements, etc. At this time, in order to ensure the performance of the communication system at a lower received power, most low-power FSOCs will use avalanche photodiodes (APDs) as photodetectors, which use the "avalanche" process to improve the signal-to-noise ratio of the signal. At the same time, the low-power FSOC system will use pulse position modulation (PPM), in which the signal pulses are relatively sparse, and each pulse can carry more information, that is, the PPM modulation method improves energy efficiency by reducing bandwidth efficiency, becoming the preferred modulation method in low-power FSOC systems.

[0005] APD converts the received light signal into a photocurrent signal and provides signal-to-noise ratio gain and amplitude gain for the signal through the "avalanche" process. However, as a price, the "avalanche" process will also introduce a certain amount of avalanche noise into the signal. The avalanche factor M (i.e., amplitude gain) and avalanche noise of the APD are both affected by the bias voltage V of the APD. bias Control. The function of the transimpedance amplifier TIA is to convert the photocurrent signal into a small voltage signal and provide a certain gain for the signal. There is a capacitor C after the TIA, which is used to isolate the DC noise and the DC component of the signal. The main amplifier MA amplifies the small voltage signal into a voltage signal of normal amplitude so that the ADC can sample it. The gain multiple of MA is affected by the gain voltage V gain The ADC transmits the sampled signal to an external receiver for demodulation and decoding. It needs to be further explained that, because the signal power is extremely low in the low-power FSOC system, the amplitude of the signal amplified by the TIA still cannot meet the ADC sampling requirements. Therefore, it is usually necessary to add a main amplifier MA after the TIA. The function of the MA is to amplify the output signal of the TIA to an amplitude that the ADC can sample. The gain multiple of the MA is usually variable, and its gain is affected by its gain voltage V gain A capacitor C is usually inserted between TIA and MA to isolate DC noise and the DC component of the signal. The signal amplified by MA will be sampled and converted into a digital signal by ADC, and the subsequent signal processing will be completed by the demodulation module and decoding module.

[0006] Since the bias voltage of the APD determines both the multiplication factor M and the avalanche noise, the bias voltage of the APD can affect the signal-to-noise ratio of the signal. In order to make the signal-to-noise ratio of the signal as high as possible, the bias voltage of the APD needs to be at the optimal position, that is, the optimal bias voltage of the APD. However, the optimal bias voltage of the APD is determined by two factors: the optical power received by the receiver and the absolute temperature of the APD. In order to find the optimal bias voltage, the optical power received by the receiver needs to be estimated.

[0007] The optical signal received by the receiver is a PPM signal, in which only one position among multiple positions has a pulse signal. Due to the sparse pulse characteristics of the PPM signal and the extremely low signal power and signal-to-noise ratio, traditional power estimation methods (such as mean estimation method, mean square estimation method, peak estimation method) cannot accurately estimate the power of the optical signal received by the receiver.

[0008] In summary, if the power of the PPM optical signal received by the receiver cannot be accurately estimated through the new method, the optimal bias voltage control of the APD cannot be completed, and it is difficult to obtain the optimal signal-to-noise ratio.

[0009] In view of this, it is urgent to propose an optical power estimation method, which needs to accurately estimate the power of the received optical signal in a low-power FSOC system, and further adaptively control the optimal bias voltage of the APD and the gain of the amplifier (transimpedance amplifier or main amplifier) ​​in the receiver according to the estimated value of the optical power. Summary of the invention

[0010] The object of the present invention is to provide an optical power estimation method, a gain control method, a system and a terminal, which are used to accurately estimate the power of a received optical signal in a low-power FSOC system, and further adaptively control the optimal bias voltage of an APD and the optimal gain voltage of an MA in a receiver according to the estimated value of the power of the received optical signal, thereby improving the signal-to-noise ratio of the signal and stabilizing the output signal of the receiver.

[0011] In a first aspect, the present invention provides an optical power estimation method for accurately estimating the power of an optical signal in a low-power FSOC system, characterized in that the low-power FSOC system comprises a transmitter, a channel and a receiver; the transmitter comprises an encoder, a modulator and a laser light source; the receiver comprises an avalanche photodiode, a transimpedance amplifier, a filter module, a main amplifier and a signal sampler connected in sequence; the avalanche photodiode has an input port; the signal sampler has an output port;

[0012] The optical power estimation method comprises the following steps:

[0013] S1. Establishing an amplification model of the avalanche photodiode in the receiver, specifically including: calculating the breakdown voltage, avalanche coefficient and temperature coefficient of the avalanche photodiode; the amplification model is realized by fitting the multiplication factor of the avalanche photodiode with the bias voltage and temperature relationship;

[0014] S2. When the main amplifier is a variable gain amplifier and the transimpedance amplifier is a fixed gain amplifier, establish a gain model of the main amplifier to determine the relationship between the gain and the gain voltage of the main amplifier; when the transimpedance amplifier is a variable gain amplifier and the main amplifier is a fixed gain amplifier, establish a gain model of the transimpedance amplifier to determine the relationship between the gain and the gain voltage of the transimpedance amplifier;

[0015] S3, the signal sampler receives the output signal of the main amplifier; the output signal includes a plurality of data frames transmitted sequentially, each of the data frames includes a frame header and a data sequence, and the frame header and the data sequence are both measured values; the frame header in the data frame transmitted by the transmitter is defined as a reference frame header, and the reference frame header is a fixed value;

[0016] S4, constructing a cross-correlation function between the reference frame header and the output signal, and determining a maximum value of the cross-correlation function within a data frame time, wherein the maximum value is a correlation peak; determining a distribution function of the correlation peak and determining an expression of an optical power estimation value according to the distribution function;

[0017] S5, repeatedly executing S4 to obtain correlation peaks corresponding to K data frames; the value range of K is 1 to 1024;

[0018] S6. Calculate the estimated value of the optical power based on the multiple correlation peaks, specifically including: S61. Take the average value of the correlation peaks corresponding to the K acquired data frames; S62. Calculate the optical power to be estimated using the average value of the correlation peaks and the expression of the optical power estimation value determined in S4.

[0019] As a possible implementation method, constructing a cross-correlation function between the reference frame header and the output signal, and determining a maximum value within a data frame time according to the cross-correlation function, wherein the maximum value is a correlation peak; determining a distribution function of the correlation peak and determining an expression of an optical power estimation value according to the distribution function, specifically includes:

[0020] S40. Determine the distribution type of the correlation peak;

[0021] S41. Determining the distribution parameters of the correlation peaks according to the distribution type of the correlation peaks, specifically comprising the following steps:

[0022] S410. Construct a frame header function of the frame header, wherein the frame header function is the sum of a signal component function, a DC noise component function and an AC noise component function; wherein the independent variables of the signal component function include the true value of the positively correlated optical power, the multiplication factor of the avalanche photodiode, the transimpedance amplifier gain, the main amplifier gain, the peak-to-average voltage ratio of the pulse position modulated electrical signal and the reference frame header; the independent variables of the DC noise component function include the noise coefficient of the positively correlated main amplifier and the main amplifier gain; the AC noise component function is a Gaussian noise function, which is related to the standard deviation of the Gaussian noise;

[0023] S411. Calculate the mutual correlation value between the frame header function and the reference frame header; the mutual correlation value is the sum of the first sub-mutual correlation value, the second sub-mutual correlation value and the third sub-mutual correlation value, wherein the first sub-mutual correlation value is the mutual correlation value between the signal component function and the reference frame header, the second sub-mutual correlation value is the mutual correlation value between the DC noise component function and the reference frame header, and the third sub-mutual correlation value is the mutual correlation value between the AC noise component and the reference frame header;

[0024] S42. Construct a cross-correlation function between an output signal including a frame header and a data sequence and the reference frame header, use the cross-correlation function to determine a distribution function of a correlation peak, and determine an expression for an optical power estimation value according to the distribution function of the correlation peak.

[0025] As a possible implementation, S4 is repeatedly executed to obtain correlation peaks corresponding to K data frames; the value range of K is 1 to 1024; each of the correlation peaks extracted by S4 and S5 obeys the following normal distribution:

[0026]

[0027] Where C1 and C2 are constants in the correlation peak distribution function, P s is the true value of the optical power, M APD is the multiplication factor of the avalanche photodiode, G MA is the main amplifier gain, F is the reference frame header, and σ is the standard deviation of the Gaussian noise function.

[0028] As a possible implementation method, the average value E(max{r sF (τ)}), the optical power is inverted as follows to obtain the estimated value of the optical power

[0029]

[0030] Among them, V bias is the bias voltage of the avalanche photodiode, V gain is the gain voltage of the main amplifier or transimpedance amplifier, T is the temperature of the avalanche photodiode, max{r sF (τ)} is the correlation peak.

[0031] In a second aspect, the present invention further provides a gain control method, which is applied to a receiver included in a low-power FSOC system, wherein the receiver is the receiver provided in the first aspect; the gain control method comprises the following steps:

[0032] Estimate optical power using the optical power estimation method provided in the first aspect;

[0033] determining an optimal bias voltage of the avalanche photodiode based on an estimated value of the absolute temperature and optical power of the avalanche photodiode;

[0034] The optimal gain voltage of the main amplifier is determined according to the absolute temperature of the avalanche photodiode, the optimal bias voltage of the avalanche photodiode calculated in the current cycle, and the estimated value of the optical power.

[0035] As a possible implementation manner, determining the optimal bias voltage of the avalanche photodiode according to the estimated value of the absolute temperature and optical power of the avalanche photodiode includes:

[0036] Establish a noise model of an avalanche photodiode, wherein the noise model specifically includes a signal component model, a thermal noise model and an avalanche noise model; wherein the signal component model is specifically implemented by constructing a relationship model between the signal component of the avalanche photodiode and the true value of the optical power and a multiplication factor; the thermal noise model is specifically implemented by constructing a relationship model between the thermal noise and temperature of the avalanche photodiode, and the avalanche model is specifically implemented by constructing a relationship model between the avalanche noise and the bias voltage and temperature;

[0037] Calculating the signal-to-noise ratio of the output signal of the avalanche photodiode according to the signal component model, the thermal noise model and the avalanche noise model;

[0038] Constructing a multiplication factor model according to the maximum signal-to-noise ratio, wherein the multiplication factor model is specifically implemented by constructing a relationship model between the multiplication factor corresponding to the maximum signal-to-noise ratio and the true value of the optical power and the temperature;

[0039] Construct an optimal bias voltage model, wherein the optimal bias voltage model is specifically: in, is the multiplication factor corresponding to the maximum signal-to-noise ratio, T is the temperature, P s is the true value of the optical power, which can be specifically obtained by the optical power estimation method described in the first aspect.

[0040] As a possible implementation, determining the optimal gain voltage of the main amplifier according to the absolute temperature of the avalanche photodiode, the optimal bias voltage of the avalanche photodiode calculated in the current cycle, and the estimated value of the optical power includes:

[0041] Constructing an output signal amplitude model of the main amplifier, wherein the output signal amplitude model is specifically implemented by constructing a relationship model between the output signal of the main amplifier and the true value of the optical power, the multiplication factor of the avalanche photodiode, the transimpedance amplifier gain, the main amplifier gain, and the ratio of the peak voltage to the average voltage of the transmitter signal;

[0042] The output signal amplitude model of the main amplifier is run to keep the output signal amplitude within the upper and lower limits of the preset output signal. At this time, a main amplifier gain model is constructed; the main amplifier gain model is specifically implemented by constructing a relationship model between the main amplifier gain and the preset output signal of the main amplifier, the output signal and the true value of the optical power, the multiplication factor of the avalanche photodiode, the transimpedance amplifier gain, and the ratio of the peak voltage to the average voltage of the transmitter signal;

[0043] The relationship between the main amplifier gain and the gain voltage is determined by the characteristics of the main amplifier itself, that is, there is a linear relationship between the main amplifier gain and the gain voltage; based on the linear relationship between the main amplifier gain and the gain voltage, the optimal gain voltage model is inferred, and the optimal gain voltage model is specifically: in, For the optimal gain of the main amplifier, k1 and k2 are constants determined in S2.

[0044] In a third aspect, the present invention further provides an automatic gain control system, comprising:

[0045] a receiver having an input port for receiving the optical signal sent by the pulse position modulator and an output port for outputting the signal;

[0046] A temperature sampler, used to obtain a temperature signal from a photodetector in the receiver;

[0047] A processor, the processor comprising an optical power estimation unit and an optimal parameter determination unit; wherein the optical power estimation unit executes the optical power estimation method provided in the first aspect to estimate the optical power; the optimal parameter determination unit receives the estimated value of the optical power and a temperature signal, and executes the automatic gain control method provided in the second aspect to obtain an optimal bias voltage and an optimal gain voltage of the receiver.

[0048] In a fourth aspect, the present invention further provides a terminal, comprising:

[0049] A signal acquisition unit, the signal acquisition unit comprising a digital signal collector and a temperature signal collector;

[0050] A chip, the chip comprising a programmable logic controller, a register and a central processing unit; wherein the programmable logic controller comprises a cross-correlation calculation module and an optical power estimation module, the cross-correlation calculation module receives the received digital signal and calculates a correlation peak, and the optical power estimation module receives the correlation peak and calculates an estimated value of the optical power;

[0051] The registers include four and are all connected to the programmable logic controller for communication, and are respectively used to store the temperature signal, the estimated value of the optical power, the bias voltage and the gain voltage;

[0052] The central processing unit includes an initialization module, which is used to determine the number of correlation peaks involved in estimating optical power by setting parameters to determine the frequency of estimating optical power. The initialization module is also used to set the initial bias voltage and gain voltage according to the temperature signal; the central processing unit also includes an update module, a judgment module, a calculation module and a setting module; the update module is used to update the temperature signal and the estimated value of optical power, and send it to the corresponding register; the judgment module determines whether to adjust the bias voltage and gain voltage based on the threshold value of temperature change and the threshold value of optical power estimated value change; the calculation module is used to determine the optimal bias voltage and the optimal gain voltage; the setting module is used to set the optimal bias voltage and the optimal gain voltage to the receiver.

[0053] Compared with the prior art, the AGC system of the present invention can accurately estimate the received power. Under the condition of -70dBm, when the AGC frequency is 20.58Hz, the error standard deviation of the calculated optical power is less than 0.1dBm. In an environment without an AGC system, the amplitude of the output signal varies greatly, and the variation between -70dBm and -50dBm can reach 2 orders of magnitude. When equipped with the AGC system of the present invention, the amplitude mean swing of the output signal is less than 5%. In addition, when the power is low, due to the high bias voltage and large shot noise, the amplitude of the output signal will jump to a certain extent, but it will not have a significant impact on the macro level, nor will it affect ADC sampling. In a receiver without an AGC system, the bit error rate is high when it is from -70dBm to -64dBm; when it is from -64dBm to -50dBm, due to unreasonable receiver parameter configuration, the signal is distorted, and the bit error rate gradually rises to 1; the dynamic range of its receiver communication is only 6-8dBm. If the AGC system of the present invention is configured, the demodulation bit error rate gradually decreases to 10 at -70dBm to -63dBm. -7 The AGC system of the present invention enables the dynamic range of the receiver to reach at least 20 dBm, and can work quickly with high sensitivity in a low-power FSOC system. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] The drawings described herein are used to provide a further understanding of the present invention and constitute a part of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the drawings:

[0055] Figure 1 Application of AGC in FSOC provided by existing technology;

[0056] Figure 2 A structural block diagram of an internal receiver of a FSOC provided by the prior art;

[0057] Figure 3 An AGC system based on analog mean value detection provided by the prior art;

[0058] Figure 4 A rectifier circuit diagram provided by the prior art;

[0059] Figure 5 An integral circuit diagram provided for the prior art;

[0060] Figure 6 An AGC system based on a digital mean value detection method provided by the prior art;

[0061] Figure 7 The average value and pulse amplitude of the 16PPM signal provided by the prior art vary with the received power;

[0062] Figure 8 An AGC system with analog peak detection provided by the prior art;

[0063] Fig. 9 AGC system based on digital peak detection is provided for the prior art;

[0064] Fig.10 It is the baseband signal at -70dBm in low power FSOC using the peak detection method;

[0065] Fig.11 The structural block diagram of the all-optical AGC provided by the prior art;

[0066] Fig.12 A block diagram of the automatic gain control system provided for the implementation of the present invention;

[0067] Fig.13 A noise model of the APD provided in an embodiment of the present invention;

[0068] Fig.14 It is the frame structure of the correlation detection method;

[0069] Fig.15 A block diagram of a terminal structure provided by an embodiment of the present invention;

[0070] Fig.16 The error of estimating optical power by applying the automatic gain control system provided by the embodiment of the present invention;

[0071] Fig.17 A measured diagram of maintaining output stability of the automatic gain control system provided by an embodiment of the present invention;

[0072] Fig.18 This is a measured diagram of the improvement of the demodulation bit error rate by the automatic gain control system provided in an embodiment of the present invention. DETAILED DESCRIPTION

[0073] In order to make the technical problems, technical solutions and beneficial effects to be solved by the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0074] It should be noted that when an element is referred to as being "fixed to" or "disposed on" another element, it can be directly on the other element or indirectly on the other element. When an element is referred to as being "connected to" another element, it can be directly connected to the other element or indirectly connected to the other element.

[0075] In addition, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" and "second" may explicitly or implicitly include one or more of the features. In the description of the present invention, the meaning of "multiple" is two or more, unless otherwise clearly and specifically defined. The meaning of "several" is one or more, unless otherwise clearly and specifically defined.

[0076] In the description of the present invention, it is necessary to understand that the directions or positional relationships indicated by the terms "up", "down", "front", "back", "left", "right", etc. are based on the directions or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific direction, be constructed and operated in a specific direction, and therefore cannot be understood as a limitation on the present invention.

[0077] In the description of the present invention, it should be noted that, unless otherwise clearly specified and limited, the terms "installed", "connected", and "connected" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, it can be the internal connection of two elements or the interaction relationship between two elements. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0078] In order to facilitate an optical power estimation method, a gain control method, a gain control system and a terminal provided by the embodiments of the present invention, the prior art related to the embodiments of the present invention is briefly described below.

[0079] See also Figure 1,In the FSOC system, the AGC system adaptively optimizes the parameters by estimating the optical power. ,The parameters to be optimized are related to the AGC system, ,for example, including the optimal bias voltage of the APD and the optimal gain voltage of the ,amplifier (transimpedance amplifier or main amplifier), etc. The output signal is then transmitted to the ,demodulator and decoder for decoding and decoding.

[0080] See also Figure 2 , APD converts the received light signal into a photocurrent signal, and provides signal-to-noise ratio gain and amplitude gain for the signal through the "avalanche" process, but as a price, the "avalanche" process will also introduce a certain amount of avalanche noise to the signal. The avalanche factor M (i.e., amplitude gain) and avalanche noise of the APD are both affected by the bias voltage V of the APD. bias Control. The function of the transimpedance amplifier TIA is to convert the photocurrent signal into a small voltage signal and provide a certain gain for the signal. There is a capacitor C after the TIA, which is used to isolate the DC noise and the DC component of the signal. The main amplifier MA amplifies the small voltage signal into a voltage signal of normal amplitude so that the ADC can sample it. The gain multiple of MA is affected by the gain voltage V gain The ADC transmits the sampled signal to an external receiver for demodulation and decoding. It needs to be further explained that, because the signal power is extremely low in the low-power FSOC system, the amplitude of the signal amplified by the TIA still cannot meet the ADC sampling requirements. Therefore, it is usually necessary to add a main amplifier MA after the TIA. The function of the MA is to amplify the output signal of the TIA to an amplitude that the ADC can sample. The gain multiple of the MA is usually variable, and its gain is affected by its gain voltage V gain A capacitor C is usually inserted between TIA and MA to isolate DC noise and the DC component of the signal. The signal amplified by MA will be sampled and converted into a digital signal by ADC, and the subsequent signal processing will be completed by the demodulation module and decoding module.

[0081] In view of the above technical problems, the prior art provides the following technical solutions:

[0082] First, the AGC system based on the analog average detection method. Figure 3 ,The output signal of the main amplifier MA is processed by the analog mean detection circuit, ,and the output signal is averaged, and sent to the FPGA for processing after ADC sampling. ,After optimizing the parameters, the bias voltage and gain voltage are set to APD and MA through DAC, ,and the parameter adaptive optimization is completed.

[0083] In the analog mean detection circuit, the analog signal is firstly half-wave rectified or full-wave rectified (rectifier circuit such as Figure 4 As shown), the pulsating DC signal output by the rectifier is then integrated (the integration circuit is shown in Figure 5As shown in the figure, a smooth DC signal is obtained, and then the received optical power is estimated by analyzing the amplitude of the DC signal, the multiplication factor of the APD, and the gains of all amplifiers in the link.

[0084] Second, the AGC system based on the digital average detection method. Figure 6 In the digital average detection method, the digital signal sampled by the ADC is processed by the digital average detection module in the FPGA, and the average value of the output signal is then analyzed in combination with the multiplication factor of the APD and the gain of all amplifiers in the link to estimate the received optical power. After the parameter optimization module, the bias voltage and gain voltage are adjusted to complete the adaptive adjustment of the parameters.

[0085] The above two methods can be summarized as the AGC system of the mean detection method, which has the following technical defects:

[0086] On the one hand, when using the analog average detection method to calculate power, it will introduce certain interference to the signal itself, which will affect the subsequent demodulation and decoding performance, and may lead to an increase in the bit error rate. If a current mirror is introduced to reduce the interference to the signal, there are two disadvantages. First, it will increase the cost of the design; second, it will greatly increase the delay in calculating power, thereby greatly increasing the response time of the AGC subsystem and reducing the real-time performance of the AGC.

[0087] On the other hand, both the analog average detection method and the digital average detection method are only applicable to signals with dense signal pulses and high duty cycle, but not to PPM signals used by low-power FSOC. If the analog average power detection method is used to detect the power of the PPM signal, the average value of the PPM signal will not be obvious to the received power change due to the sparse pulses of the PPM, and thus the accurate power cannot be detected.

[0088] To verify the existence of the above technical defects, see Figure 7 , under 16PPM signal, in the power range of -63 to -55dBm, the pulse amplitude of the signal changes significantly with the received power, but the average value of the signal changes very little with the received power. That is, the two mean detection methods cannot complete the AGC task in the AGC system of low-power FSOC using PPM modulation because they cannot accurately detect the optical power.

[0089] Third, the AGC system based on analog peak detection method. Figure 8 ,The AGC system based on the analog peak detection method performs ,peak detection on the output signal of the MA. The analog peak detection ,circuit outputs the signal peak value, which is ,sampled by the ADC and transmitted to the FPGA. According to the amplitude of the pulse and the ,gain of the APD and the amplifier, the received optical power is ,estimated, and then the bias voltage and the gain voltage are ,adjusted.

[0090] The specific detection method of the analog peak detection circuit is to use the analog signal passing through the APD and the amplifier to charge and discharge a specific capacitor, and use the parameters of the resistor and capacitor to control the charging time and discharging time. Generally speaking, the shorter the charging time, the faster the charging speed. The longer the discharging time, the slower the discharging speed. If the received signal is a positive pulse, the signal can be directly peak detected. If the received signal is a negative pulse, it needs to pass through a reverse voltage follower before the signal is peak detected.

[0091] Fourth, the AGC system based on digital peak detection method. Fig. 9 The content of the digital peak detection method is to first use ADC to sample the signal, and then calculate the maximum value of the sampled signal within a certain period in a programmable device such as FPGA; then average the maximum value, combine the gain of APD and amplifier, estimate the received optical power, and complete the parameter adaptive optimization after adjusting the bias voltage and gain voltage through the parameter optimization module.

[0092] The above-mentioned prior art can be summarized as an AGC system of the peak detection method. According to the analysis of the inventor, the following technical defects exist:

[0093] The analog peak detection method also has the disadvantage of interfering with the original signal, that is, the analog detection circuit will interfere with the signal itself, affecting the accuracy of subsequent demodulation and decoding signals.

[0094] The disadvantage of the digital peak detection method is that it is highly dependent on the sampling accuracy of the ADC. If the sampling accuracy of the ADC is not high enough, the sampling point is likely not to coincide with the peak value, that is, the sampled maximum value is often too small, and there is a system error. To improve the ADC sampling accuracy, a higher system clock frequency and a more expensive ADC are required, which will greatly increase the cost.

[0095] In addition to the above-mentioned shortcomings, both peak detection methods have the problem of being unable to determine the pulse position of the PPM signal. Fig.10 As shown in the figure, the baseband signal at -70dBm in low-power FSOC cannot distinguish between noise and signal pulses after being amplified by APD and amplifier. Therefore, both peak detection methods cannot detect the received optical power under the PPM signal condition of low-power FSOC. The AGC subsystem based on the peak detection method is not suitable for low-power FSOC system.

[0096] Fifth, all-optical AGC system based on EDFA. Fig.11The content of the all-optical AGC system is to use a controllable gain erbium-doped fiber amplifier (EDFA) to directly amplify the optical signal before the APD, and use negative feedback to make the EDFA output stable power without gain control of the APD and the subsequent amplifier. After research, the following technical defects exist:

[0097] On the one hand, while amplifying the signal power, EDFA will introduce amplifier noise and reduce the signal-to-noise ratio. On the other hand, the input sensitivity of EDFA is low, far from the level of -70dBm, and does not meet the sensitivity requirements of low-power FSOC. Thirdly, the additional introduction of low-noise EDFA is not only expensive, but also increases the size and power supply requirements of the receiver, thus greatly increasing the cost of the entire product.

[0098] In view of the problems existing in the prior art, the purpose of the embodiments of the present invention is to adaptively optimize the receiver parameters in a low-power FSOC system using an APD photodetector and a PPM modulation method at extremely high sensitivity (-70dBm to -50dBm) to offset the impact of channel changes and make the output signal amplitude of the receiver stable and the signal-to-noise ratio as high as possible.

[0099] See also Fig.12 In the AGC subsystem model provided by the embodiment of the present invention, the photodetector APD (specifically, it can be an avalanche photodiode) converts the received optical signal into a photocurrent, the transimpedance amplifier TIA converts the photocurrent into a small voltage signal, and the signal filter after the TIA (specifically, it can be a capacitor) isolates all DC components before the node to block DC noise and the DC component of the signal. The main amplifier MA amplifies the small voltage signal into a normal voltage signal for sampling by the signal sampler ADC-S that collects the signal. On the one hand, the sampled digital signal is sent to the FPGA for signal processing. In the FPGA, the correlation peak calculation module calculates multiple correlation peaks, and the optical power estimation module estimates the signal strength to obtain an estimated value of the received power. On the other hand, since the noise and multiplication factor of APD are related to temperature, a thermistor is integrated inside the APD. The ADC-T that collects the temperature samples the resistance of the thermistor and sends the output signal to the FPGA for analysis to obtain the temperature T. The signal-to-noise ratio optimization module in the FPGA and T analysis to calculate the optimal bias voltage and the optimum gain voltage And output to APD and MA respectively through D / A converter, voltage amplifier 1 and voltage amplifier 2.

[0100] In the present invention, the purpose of controlling the APD is to obtain a higher signal-to-noise ratio. Fig.13is the noise model of APD. At the output of APD, there are three components: the PPM signal component I s , and the second is the thermal noise component I n,th , and the third is the shot noise component I n,shot , these three are:

[0101]

[0102] Where P s is the received optical power, M APD is the multiplication factor of APD, is the APD response rate when the multiplication factor is equal to 1, k is the Boltzmann constant, T is the absolute temperature of the APD, B is the passband bandwidth, and R L is the load resistance of the APD, q is the charge of the elementary charge, and x is the excess shot noise factor of the APD, which is between 0 and 1. Therefore, the signal-to-noise ratio Γ(M APD )for

[0103] When SNR(M APD ) is maximum, M APD Recorded as for

[0104]

[0105] In the formula, due to q,B,x,k,R L can be considered as constants, so Can be P s and T decides.

[0106] And M APD With V bias The relationship (i.e. the enlarged model) is:

[0107]

[0108] Where k0 and n are constants determined by the characteristics of the APD. T is the absolute temperature of the APD (K), V BD is the breakdown voltage of the APD at 25 degrees Celsius. So the optimal bias voltage for

[0109]

[0110] The purpose of the present invention to control MA is to keep the MA output signal amplitude stable to ensure accurate ADC sampling. The output signal amplitude of MA is V o :

[0111]

[0112] In the formula, G TIA ,G MA are the gains of TIA and MA respectively, and γ is the ratio of the peak voltage to the average voltage of the PPM signal. o Keep in Then, the optimum MA gain is can be expressed as

[0113]

[0114] Due to G MA With V gain The relationship is determined by the characteristics of MA itself, which is:

[0115] G MA (dB)=k1V gain +k2

[0116] In the formula, k1 and k2 are constants, which are determined by the characteristics of the MA itself. Therefore, the optimal gain voltage is for:

[0117]

[0118] In a first aspect, an embodiment of the present invention provides an optical power estimation method for accurately estimating the power of an optical signal in a low-power FSOC system, characterized in that the low-power FSOC system includes a transmitter, a channel and a receiver; the transmitter includes an encoder, a modulator and a laser light source; the receiver includes an avalanche photodiode, a transimpedance amplifier, a filter module, a main amplifier and a signal sampler connected in sequence; the avalanche photodiode has an input port; the signal sampler has an output port;

[0119] The optical power estimation method includes the following steps:

[0120] S1. Establish an amplification model of the avalanche photodiode in the receiver, specifically including: calculating the breakdown voltage, avalanche coefficient and temperature coefficient of the avalanche photodiode; the amplification model is realized by fitting the multiplication factor of the avalanche photodiode with the bias voltage and temperature.

[0121] The enlarged model is as follows:

[0122]

[0123] Where k0 (temperature coefficient) and n (avalanche coefficient) are constants determined by the characteristics of the APD. T is the absolute temperature of the APD (K), V BD is the breakdown voltage of the APD at 25 degrees Celsius.

[0124] S2. When the main amplifier is a variable gain amplifier and the transimpedance amplifier is a fixed gain amplifier, a gain model of the main amplifier is established to determine the relationship between the gain and the gain voltage of the main amplifier; when the transimpedance amplifier is a variable gain amplifier and the main amplifier is a fixed gain amplifier, a gain model of the transimpedance amplifier is established to determine the relationship between the gain and the gain voltage of the transimpedance amplifier.

[0125] S3, the signal sampler receives the output signal of the main amplifier; see Fig.14 The output signal includes multiple data frames transmitted sequentially, each data frame includes a frame header and a data sequence, and the frame header and the data sequence are both measured values; the frame header in the data frame transmitted by the transmitter is defined as a reference frame header, and the reference frame header is a fixed value.

[0126] S4. Construct a cross-correlation function between the reference frame header and the output signal, and determine the maximum value within a data frame time according to the cross-correlation function, the maximum value is the correlation peak; determine the distribution function of the correlation peak and determine the expression of the optical power estimation value according to the distribution function.

[0127] S5. Repeat S4 to obtain correlation peaks corresponding to K data frames; the value range of K is 1 to 1024.

[0128] S6. Calculate the estimated value of optical power based on multiple correlation peaks, specifically including: S61. Take the average value of the correlation peaks corresponding to the K acquired data frames; S62. Calculate the optical power to be estimated using the average value of the correlation peaks and the expression of the optical power estimated value determined in S4.

[0129] As a possible implementation method, each frame header is composed of the reference frame header component, the DC noise component, and the AC noise component. The independent variables of the reference frame header component include the true value of the optical power, the multiplication factor of the photodetector, the transimpedance amplifier gain, the main amplifier gain, the peak-to-average voltage ratio of the pulse position modulated electrical signal, and the reference frame header. The independent variables of the DC noise component include the noise factor of the main amplifier and the main amplifier gain; the AC noise component is Gaussian noise and is related to the standard deviation of the Gaussian noise.

[0130] Constructing a cross-correlation function between the reference frame header and the output signal, and determining a maximum value within a data frame time according to the cross-correlation function, wherein the maximum value is a correlation peak; determining a distribution function of the correlation peak and determining an expression of an optical power estimation value according to the distribution function, specifically comprising:

[0131] S40. Determine the distribution type of the correlation peak;

[0132] S41. Determining the distribution parameters of the correlation peaks according to the distribution type of the correlation peaks, specifically comprising the following steps:

[0133] S410. Construct a frame header function of the frame header, wherein the frame header function is the sum of a signal component function, a DC noise component function and an AC noise component function; wherein the independent variables of the signal component function include the true value of the positively correlated optical power, the multiplication factor of the avalanche photodiode, the transimpedance amplifier gain, the main amplifier gain, the peak-to-average voltage ratio of the pulse position modulated electrical signal and the reference frame header; the independent variables of the DC noise component function include the noise coefficient of the positively correlated main amplifier and the main amplifier gain; the AC noise component function is a Gaussian noise function, which is related to the standard deviation of the Gaussian noise;

[0134] S411. Calculate the mutual correlation value between the frame header function and the reference frame header; the mutual correlation value is the sum of the first sub-mutual correlation value, the second sub-mutual correlation value and the third sub-mutual correlation value, wherein the first sub-mutual correlation value is the mutual correlation value between the signal component function and the reference frame header, the second sub-mutual correlation value is the mutual correlation value between the DC noise component function and the reference frame header, and the third sub-mutual correlation value is the mutual correlation value between the AC noise component and the reference frame header;

[0135] S42. Construct a cross-correlation function between an output signal including a frame header and a data sequence and the reference frame header, use the cross-correlation function to determine a distribution function of a correlation peak, and determine an expression for an optical power estimation value according to the distribution function of the correlation peak.

[0136] As a possible implementation, S4 is repeatedly executed to obtain correlation peaks corresponding to K data frames; the value range of K is 1 to 1024; each of the correlation peaks extracted by S4 and S5 obeys the following normal distribution:

[0137]

[0138] Where C1 and C2 are constants in the correlation peak distribution function, P s is the true value of the optical power, M APD is the multiplication factor of the avalanche photodiode, G MA is the main amplifier gain, F 2 is the vector product of the reference frame header, and σ is the standard deviation of the Gaussian noise function.

[0139] As a possible implementation method, the average value E(max{r sF (τ)}), the optical power is inverted as follows to obtain the estimated value of the optical power

[0140]

[0141] Among them, V bias is the bias voltage of the avalanche photodiode, V gainis the gain voltage of the main amplifier or transimpedance amplifier, T is the temperature of the avalanche photodiode, max{r sF (τ)} is the correlation peak.

[0142] In order to facilitate understanding of the above optical power estimation method, a specific example is used as an example for explanation below. It should be understood that the following explanation is only for explanation and not for limitation.

[0143] See also Fig.14 , F is the reference frame header, s is the output signal of MA in the receiver, including the frame header h and the data sequence.

[0144] g, d, and a are the signal component, DC noise component, and AC noise component of the frame header h, respectively. Their relationship can be expressed as:

[0145] h=g+d+a

[0146] Definition xy is the cross-correlation value of x and y, then the cross-correlation values ​​of g, d, a and the reference frame header F are expressed as:

[0147] r hF =T gF +r dF +r aF ,

[0148] For the signal component, the ideal signal component is obtained by amplifying the ideal PPM signal through APD, TIA, and MA, so g(t) is calculated by the optical power P s and the gain of each level, this term and its cross-correlation value with the reference frame header F can be expressed as:

[0149] g=P s M APD G TIA G MA γF,

[0150] r gF =P s M APD G TIA G MA γF 2

[0151] For the DC noise component, due to the existence of the DC blocking capacitor C, the DC noise component is only related to MA and is related to the gain G of MA. MA is proportional to the gain G of the MA. MA The term and its cross-correlation value with the reference frame header F can be expressed as:

[0152] d=k d G MA I,

[0153] r dF =k d G MA F.I.

[0154] In the formula, k d is the noise coefficient of MA, which is a constant.

[0155] For the AC noise component, the AC noise can be regarded as Gaussian noise, σ is its standard deviation, then a and its cross-correlation value with the reference frame header F are only related to σ and the reference frame header F, which can be expressed as:

[0156]

[0157] Therefore, if the output signal of MA is cross-correlated with the reference frame header F, the peak value of the cross-correlation can be obtained to obey the following normal distribution:

[0158]

[0159] In the formula, C1 and C2 are constants of several constant parameters. According to the above formula, P can be calculated. s Estimator The values ​​are:

[0160]

[0161] It can be seen that according to the bias voltage V bias , gain voltage V gain , temperature T and cross-correlation value r sF (τ), we can find the estimated value of power Among them, the bias voltage V bias , gain voltage V gain can be obtained from the previous AGC state cycle. The temperature T is obtained by sampling the thermistor. The cross-correlation value r sF (τ) can be obtained by processing the signal in FPGA.

[0162] In a second aspect, the present invention further provides a gain control method, which is applied to a receiver included in a low-power FSOC system, wherein the receiver is the receiver provided in the first aspect; the gain control method comprises the following steps:

[0163] Estimate optical power using the optical power estimation method provided in the first aspect;

[0164] determining an optimal bias voltage of the avalanche photodiode based on an estimated value of the absolute temperature and optical power of the avalanche photodiode;

[0165] The optimal gain voltage of the main amplifier is determined according to the absolute temperature of the avalanche photodiode, the optimal bias voltage of the avalanche photodiode calculated in the current cycle, and the estimated value of the optical power.

[0166] As a possible implementation manner, determining the optimal bias voltage of the avalanche photodiode according to the estimated value of the absolute temperature and optical power of the avalanche photodiode includes:

[0167] Establish a noise model of an avalanche photodiode, wherein the noise model specifically includes a signal component model, a thermal noise model and an avalanche noise model; wherein the signal component model is specifically implemented by constructing a relationship model between the signal component of the avalanche photodiode and the true value of the optical power and a multiplication factor; the thermal noise model is specifically implemented by constructing a relationship model between the thermal noise and temperature of the avalanche photodiode, and the avalanche model is specifically implemented by constructing a relationship model between the avalanche noise and the bias voltage and temperature;

[0168] Calculating the signal-to-noise ratio of the output signal of the avalanche photodiode according to the signal component model, the thermal noise model and the avalanche noise model;

[0169] Constructing a multiplication factor model according to the maximum signal-to-noise ratio, wherein the multiplication factor model is specifically implemented by constructing a relationship model between the multiplication factor corresponding to the maximum signal-to-noise ratio and the true value of the optical power and the temperature;

[0170] Construct an optimal bias voltage model, wherein the optimal bias voltage model is specifically: in, is the multiplication factor corresponding to the maximum signal-to-noise ratio, T is the temperature, P s is the true value of the optical power, which can be specifically obtained by the optical power estimation method described in the first aspect.

[0171] As a possible implementation, determining the optimal gain voltage of the main amplifier according to the absolute temperature of the avalanche photodiode, the optimal bias voltage of the avalanche photodiode calculated in the current cycle, and the estimated value of the optical power includes:

[0172] Constructing an output signal amplitude model of the main amplifier, wherein the output signal amplitude model is specifically implemented by constructing a relationship model between the output signal of the main amplifier and the true value of the optical power, the multiplication factor of the avalanche photodiode, the transimpedance amplifier gain, the main amplifier gain, and the ratio of the peak voltage to the average voltage of the transmitter signal;

[0173] The output signal amplitude model of the main amplifier is run to keep the output signal amplitude within the upper and lower limits of the preset output signal. At this time, a main amplifier gain model is constructed; the main amplifier gain model is specifically implemented by constructing a relationship model between the main amplifier gain and the preset output signal of the main amplifier, the output signal and the true value of the optical power, the multiplication factor of the avalanche photodiode, the transimpedance amplifier gain, and the ratio of the peak voltage to the average voltage of the transmitter signal;

[0174] The relationship between the main amplifier gain and the gain voltage is determined by the characteristics of the main amplifier itself, that is, there is a linear relationship between the main amplifier gain and the gain voltage; based on the linear relationship between the main amplifier gain and the gain voltage, the optimal gain voltage model is inferred, and the optimal gain voltage model is specifically: in, For the optimal gain of the main amplifier, k1 and k2 are constants determined in S2.

[0175] In a third aspect, the present invention further provides an automatic gain control system, comprising:

[0176] a receiver having an input port and an output port, the input port receiving the optical signal sent by the pulse position modulator, and the output port outputting the output signal;

[0177] A temperature sampler, used to obtain a temperature signal of a photodetector in a receiver;

[0178] A processor, the processor includes an optical power estimation unit and an optimal parameter determination unit; wherein the optical power estimation unit executes the optical power estimation method provided in the first aspect to estimate the optical power; the optimal parameter determination unit receives the estimated value of the optical power and the temperature signal, executes the gain control method provided in the second or third aspect to obtain the optimal bias voltage and the optimal gain voltage of the receiver, or executes the gain control method provided in the fourth or fifth aspect to obtain the optimal gain voltage of the receiver.

[0179] As a possible implementation method, the receiver includes an avalanche photodiode, a fixed gain transimpedance amplifier, a capacitor, a variable gain main amplifier and a signal sampler connected in sequential communication; the optimal parameter determination unit receives the estimated value of the optical power and the temperature signal, executes the gain control method provided in the second aspect to obtain the optimal bias voltage and the optimal gain voltage of the receiver, and then loads the optimal bias voltage to the avalanche photodiode through a D / A converter and a voltage amplifier, and loads the optimal gain voltage to the variable gain main amplifier.

[0180] As a possible implementation method, the receiver includes an avalanche photodiode, a variable gain transimpedance amplifier, a capacitor, a fixed gain main amplifier and a signal sampler connected in sequential communication; after executing the gain control method provided by the third aspect to obtain the optimal gain voltage of the receiver, the optimal bias voltage is loaded to the avalanche photodiode through a D / A converter and a voltage amplifier, and the optimal gain voltage is loaded to the variable gain transimpedance amplifier.

[0181] As a possible implementation, the receiver includes a PIN, a variable gain transimpedance amplifier, a capacitor, a fixed gain main amplifier, and a signal sampler connected in sequential communication, and after executing the gain control method provided in the fourth aspect to obtain the optimal gain voltage of the receiver, the optimal gain voltage is loaded to the variable gain transimpedance amplifier through a D / A converter and a voltage amplifier; or,

[0182] As a possible implementation method, the receiver includes a PIN, a fixed gain transimpedance amplifier, a capacitor, a variable gain main amplifier and a signal sampler connected in sequential communication. After executing the gain control method provided in the fifth aspect to obtain the optimal gain voltage of the receiver, the optimal gain voltage is loaded to the variable gain main amplifier through a D / A converter and a voltage amplifier.

[0183] In a fourth aspect, the present invention further provides a terminal, comprising:

[0184] A signal acquisition unit, the signal acquisition unit comprising a digital signal collector and a temperature signal collector;

[0185] A chip, the chip comprising a programmable logic controller, a register and a central processing unit; wherein the programmable logic controller comprises a cross-correlation calculation module and an optical power estimation module, the cross-correlation calculation module receives the received digital signal and calculates a correlation peak, and the optical power estimation module receives the correlation peak and calculates an estimated value of the optical power;

[0186] The registers include four and are all connected to the programmable logic controller for communication, and are respectively used to store the temperature signal, the estimated value of the optical power, the bias voltage and the gain voltage;

[0187] The central processing unit includes an initialization module, which is used to determine the number of correlation peaks involved in estimating optical power by setting parameters to determine the frequency of estimating optical power. The initialization module is also used to set the initial bias voltage and gain voltage according to the temperature signal; the central processing unit also includes an update module, a judgment module, a calculation module and a setting module; the update module is used to update the temperature signal and the estimated value of optical power, and send it to the corresponding register; the judgment module determines whether to adjust the bias voltage and gain voltage based on the threshold value of temperature change and the threshold value of optical power estimated value change; the calculation module is used to determine the optimal bias voltage and the optimal gain voltage; the setting module is used to set the optimal bias voltage and the optimal gain voltage to the receiver.

[0188] Compared with the prior art, the AGC system of the present invention can accurately estimate the received power. Under the condition of -70dBm, when the AGC frequency is 20.58Hz, the error standard deviation of the calculated optical power is less than 0.1dBm. In an environment without an AGC system, the amplitude of the output signal varies greatly, and the variation between -70dBm and -50dBm can reach 2 orders of magnitude. When equipped with the AGC system of the present invention, the amplitude mean swing of the output signal is less than 5%. In addition, when the power is low, due to the high bias voltage and large shot noise, the amplitude of the output signal will jump to a certain extent, but it will not have a significant impact on the macro level, nor will it affect ADC sampling. In a receiver without an AGC system, the bit error rate is high when it is from -70dBm to -64dBm; when it is from -64dBm to -50dBm, due to unreasonable receiver parameter configuration, the signal is distorted, and the bit error rate gradually rises to 1; the dynamic range of its receiver communication is only 6-8dBm. If the AGC system of the present invention is configured, the demodulation bit error rate gradually decreases to 10 at -70dBm to -63dBm. -7 The AGC system of the present invention enables the dynamic range of the receiver to reach at least 20 dBm, and can work quickly with high sensitivity in a low-power FSOC system.

[0189] See also Fig.15 As an example, the AGC system is implemented as follows: The temperature ADC-T and the signal ADC-S sample the temperature and digital signal respectively and transmit them to the FPGA for processing.

[0190] Programmable Logic is called the PL side of FPGA. Since the PL side can perform a large amount of data operations quickly, the cross-correlation value r is calculated. sF (τ) and estimated power Both modules are placed on the PL side to ensure that the AGC system has the shortest possible response time.

[0191] Microblaze is a processor core of Xilinx chip, which can complete more complex processing. Therefore, it is used for process control with simple calculation but complex control logic. In Microblaze, you need to complete the system initialization first, set the AGC system frequency; then enter the main loop. In the update state, the estimated values ​​of temperature and received power need to be updated. In the judgment state, it is necessary to determine whether the bias voltage and gain voltage need to be adjusted based on two threshold values. In the calculation state, the calculation of the optimal bias voltage and the optimal gain voltage needs to be completed. In the setting state, the optimal bias voltage and the optimal gain voltage need to be set to APD and MA. In the entire FPGA, 4 registers are used to store and record temperature T and optical power estimation values. Bias voltage V bias , gain voltage V gain , these four values ​​can be accessed and modified by both the PL side and the Microblaze side.

[0192] The present invention has been verified by experimental verification, which proves that the present invention can indeed bring beneficial effects. By testing in the receiving power range of -70dBm to -50dBm, the signal modulation mode is 16PPM modulation mode, and the performance of optical power estimation, output amplitude stability, and demodulation bit error rate are detected respectively. The results are divided into the following three items:

[0193] (1) Accurately estimate received power

[0194] like Fig.16 As shown, the AGC system of the present invention can accurately estimate the received power. Under the condition of -70dBm, when the AGC frequency is 20.58Hz, the error standard deviation of the calculated optical power is less than 0.1dBm. The performance of estimating the received power under other frequency and optical power conditions has been shown in the figure.

[0195] (2) Maintaining stable system output

[0196] like Fig.17 As shown, in an environment without an AGC system, the amplitude of the output signal varies greatly, and the variation between -70dBm and -50dBm can reach 2 orders of magnitude. When equipped with the AGC system of the present invention, the mean amplitude swing of the output signal is less than 5%. In addition, when the power is low, due to the high bias voltage and large shot noise, the amplitude of the output signal will fluctuate to a certain extent, but it will not have a significant impact on the macro level, nor will it affect ADC sampling.

[0197] (3) Reduce the system bit error rate and improve the receiver dynamic range

[0198] like Fig.18 As shown in the figure, in a receiver without an AGC system, the bit error rate is high when it is between -70dBm and -64dBm; when it is between -64dBm and -50dBm, the signal is distorted due to unreasonable receiver parameter configuration, and the bit error rate gradually rises to 1; the dynamic range of its receiver communication is only 6-8dBm. However, if the AGC system described in the present invention is configured, the demodulation bit error rate gradually drops to 10 when it is between -70dBm and -63dBm. ―7 The AGC system of the present invention enables the dynamic range of the receiver to reach at least 20 dBm, and can work quickly with high sensitivity in a low-power FSOC system.

[0199] In summary, compared with the prior art, the AGC system of the present invention can accurately estimate the received power. Under the condition of -70dBm, when the AGC frequency is 20.58Hz, the error standard deviation of the calculated optical power is less than 0.1dBm. In an environment without an AGC system, the amplitude of the output signal varies greatly, and the variation between -70dBm and -50dBm can reach 2 orders of magnitude. When equipped with the AGC system of the present invention, the amplitude mean swing of the output signal is less than 5%. In addition, when the power is low, due to the high bias voltage and large shot noise, the amplitude of the output signal will jump to a certain extent, but it will not have a significant impact on the macro level, nor will it affect ADC sampling. In a receiver without an AGC system, the bit error rate is high when it is from -70dBm to -64dBm; when it is from -64dBm to -50dBm, due to unreasonable receiver parameter configuration, the signal is distorted, and the bit error rate gradually rises to 1; the dynamic range of its receiver communication is only 6-8dBm. If the AGC system of the present invention is configured, the demodulation bit error rate gradually decreases to 10 at -70dBm to -63dBm. -7 The AGC system of the present invention enables the dynamic range of the receiver to reach at least 20 dBm, and can work quickly with high sensitivity in a low-power FSOC system.

[0200] In the description of the above embodiments, specific features, structures, materials or characteristics may be combined in a suitable manner in any one or more embodiments or examples.

[0201] The above is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art can easily think of changes or substitutions within the technical scope disclosed by the present invention, which should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims.

Claims

1. An optical power estimation method for accurately estimating the power of an optical signal in a low-power FSOC system, characterized in that: The low-power FSOC system includes a transmitter, a channel and a receiver; the transmitter includes an encoder, a modulator and a laser light source; the receiver includes an avalanche photodiode, a transimpedance amplifier, a filter module, a main amplifier and a signal sampler connected in sequence; the avalanche photodiode has an input port; the signal sampler has an output port; The optical power estimation method comprises the following steps: S1. Establishing an amplification model of the avalanche photodiode in the receiver, specifically including: calculating the breakdown voltage, avalanche coefficient and temperature coefficient of the avalanche photodiode; the amplification model is realized by fitting the multiplication factor of the avalanche photodiode with the bias voltage and temperature relationship; S2. When the main amplifier is a variable gain amplifier and the transimpedance amplifier is a fixed gain amplifier, establish a gain model of the main amplifier to determine the relationship between the gain and the gain voltage of the main amplifier; when the transimpedance amplifier is a variable gain amplifier and the main amplifier is a fixed gain amplifier, establish a gain model of the transimpedance amplifier to determine the relationship between the gain and the gain voltage of the transimpedance amplifier; S3, the signal sampler receives the output signal of the main amplifier; the output signal includes a plurality of data frames transmitted sequentially, each of the data frames includes a frame header and a data sequence, and the frame header and the data sequence are both measured values; the frame header in the data frame transmitted by the transmitter is defined as a reference frame header, and the reference frame header is a fixed value; S4, constructing a cross-correlation function between the reference frame header and the output signal, and determining the maximum value of the cross-correlation function within a data frame time, wherein the maximum value is the correlation peak; determining a distribution function of the correlation peak and determining an expression of an optical power estimation value according to the distribution function; specifically comprising: S40. Determine the distribution type of the correlation peak; S41. Determining the distribution parameters of the correlation peaks according to the distribution type of the correlation peaks, specifically comprising the following steps: S410. Construct a frame header function of the frame header, wherein the frame header function is the sum of a signal component function, a DC noise component function and an AC noise component function; wherein the independent variables of the signal component function include the true value of the positively correlated optical power, the multiplication factor of the avalanche photodiode, the transimpedance amplifier gain, the main amplifier gain, the peak-to-average voltage ratio of the pulse position modulated electrical signal and the reference frame header; the independent variables of the DC noise component function include the noise coefficient of the positively correlated main amplifier and the main amplifier gain; the AC noise component function is a Gaussian noise function, which is related to the standard deviation of the Gaussian noise; S411. Calculate the mutual correlation value between the frame header function and the reference frame header; the mutual correlation value is the sum of the first sub-mutual correlation value, the second sub-mutual correlation value and the third sub-mutual correlation value, wherein the first sub-mutual correlation value is the mutual correlation value between the signal component function and the reference frame header, the second sub-mutual correlation value is the mutual correlation value between the DC noise component function and the reference frame header, and the third sub-mutual correlation value is the mutual correlation value between the AC noise component and the reference frame header; S42. Constructing a cross-correlation function between an output signal including a frame header and a data sequence and the reference frame header, using the cross-correlation function to determine a distribution function of a correlation peak, and determining an expression for an estimated optical power value according to the distribution function of the correlation peak; S5, repeatedly execute S4 to obtain correlation peaks corresponding to K data frames; the value range of K is 1 to 1024; each of the correlation peaks obeys the following normal distribution: Where C1 and C2 are constants in the correlation peak distribution function, P s is the true value of the optical power, M APD is the multiplication factor of the avalanche photodiode, G MA is the main amplifier gain, F is the reference frame header, and σ is the standard deviation of the Gaussian noise function; S6. Calculating an estimated value of optical power based on the multiple correlation peaks, specifically comprising: S61, taking an average value of the correlation peaks corresponding to the K acquired data frames; S62, using the correlation peak average value and the expression of the optical power estimation value determined in S4 to calculate the optical power to be estimated, that is, the estimated value of the optical power; specifically expressed as: using the correlation peak average value E(max{r sF (τ)}), the optical power is inverted as follows to obtain the estimated value of the optical power Among them, V bias is the bias voltage of the avalanche photodiode, V gain is the gain voltage of the main amplifier or transimpedance amplifier, T is the temperature of the avalanche photodiode, max{r sF (τ)} is the correlation peak.

2. A gain control method, characterized in that: The gain control method is applied to a receiver included in a low-power FSOC system, and the receiver is the receiver included in the low-power FSOC system described in claim 1; the gain control method comprises the following steps: Estimate optical power using the optical power estimation method according to claim 1; Determine the optimal bias voltage of the avalanche photodiode according to the estimated values ​​of the absolute temperature and optical power of the avalanche photodiode; specifically include: establish a noise model of the avalanche photodiode, the noise model specifically includes a signal component model, a thermal noise model and an avalanche noise model; wherein the signal component model is specifically implemented by constructing a relationship model between the signal component of the avalanche photodiode and the real value of the optical power and a multiplication factor; the thermal noise model is specifically implemented by constructing a relationship model between the thermal noise and temperature of the avalanche photodiode, and the avalanche noise model is specifically implemented by constructing a relationship model between the avalanche noise and the bias voltage and temperature; calculate the signal-to-noise ratio of the output signal of the avalanche photodiode according to the signal component model, the thermal noise model and the avalanche noise model; construct a multiplication factor model according to the maximum signal-to-noise ratio, and the multiplication factor model is specifically implemented by constructing a relationship model between the multiplication factor corresponding to the maximum signal-to-noise ratio and the real value of the optical power and the temperature; construct an optimal bias voltage model, and the optimal bias voltage model is specifically: Where k0 is the temperature coefficient, V BD is the breakdown voltage of the avalanche photodiode at 25 degrees Celsius, is the multiplication factor corresponding to the maximum signal-to-noise ratio, T is the temperature, P s is the true value of the optical power, which can be obtained by the optical power estimation method according to claim 1; Determine the optimal gain voltage of the main amplifier according to the absolute temperature of the avalanche photodiode, the optimal bias voltage of the avalanche photodiode calculated in this cycle, and the estimated value of the optical power; specifically include: constructing the output signal amplitude model of the main amplifier, the output signal amplitude model is specifically realized by constructing a relationship model between the output signal of the main amplifier and the true value of the optical power, the multiplication factor of the avalanche photodiode, the transimpedance amplifier gain, the main amplifier gain, and the ratio of the peak voltage of the transmitter signal to the average voltage; run the output signal amplitude model of the main amplifier to keep the output signal amplitude within the upper and lower limits of the preset output signal, at this time, construct the main amplifier gain model; the main amplifier gain model is specifically realized by constructing a relationship model between the main amplifier gain and the preset output signal of the main amplifier, the output signal and the true value of the optical power, the multiplication factor of the avalanche photodiode, the transimpedance amplifier gain, and the ratio of the peak voltage of the transmitter signal to the average voltage, specifically: Among them, P s is the received optical power, M APD is the multiplication factor of APD, is the APD response rate when the multiplication factor is equal to 1, G TIA is the gain of TIA, γ is the ratio of the peak voltage to the average voltage of the PPM signal, is the optimal output amplitude; the relationship between the main amplifier gain and the gain voltage is determined by the characteristics of the main amplifier itself, that is, the main amplifier gain and the gain voltage have a linear relationship, specifically: G MA (dB)=k1V gain +k2, where k1 and k2 are constants, which are determined by the characteristics of the main amplifier. The optimal gain voltage model is deduced based on the linear relationship between the main amplifier gain and the gain voltage. The optimal gain voltage model is specifically: in, Optimum gain for the main amplifier.

3. An automatic gain control system, characterized in that: include: a receiver having an input port for receiving the optical signal sent by the pulse position modulator and an output port for outputting the signal; A temperature sampler, used to obtain a temperature signal from a photodetector in the receiver; A processor, the processor comprising an optical power estimation unit and an optimal parameter determination unit; wherein the optical power estimation unit executes the optical power estimation method described in claim 1 to estimate the optical power; the optimal parameter determination unit receives the estimated value of the optical power and a temperature signal, and executes claim 2 to obtain an optimal bias voltage and an optimal gain voltage of the receiver.

4. A terminal, characterized in that: include: A signal acquisition unit, the signal acquisition unit comprising a digital signal collector and a temperature signal collector; They are used to obtain digital signal quantity and temperature quantity respectively and send them to the chip; A chip, the chip comprising a programmable logic controller, a register and a central processing unit; wherein the programmable logic controller comprises a cross-correlation calculation module and an optical power estimation module, which are used to execute the optical power estimation method according to claim 1, that is, the cross-correlation calculation module receives the digital signal quantity and calculates the correlation peak, and the optical power estimation module receives the correlation peak and calculates the estimated value of the optical power; The registers include four and are all connected to the programmable logic controller for communication, and are respectively used to store the temperature, the estimated value of the optical power, the bias voltage and the gain voltage; The central processing unit includes an initialization module, which is used to determine the number of correlation peaks involved in estimating optical power by setting parameters to determine the frequency of estimating optical power. The initialization module is also used to set the initial bias voltage and gain voltage according to the temperature quantity; the central processing unit also includes an update module, a judgment module, a calculation module and a setting module; the update module is used to update the estimated values ​​of the temperature quantity and the optical power, and send them to the corresponding registers; the judgment module determines whether to adjust the bias voltage and the gain voltage based on the threshold value of the temperature change and the threshold value of the optical power estimation value change; the calculation module executes the gain control method described in claim 2 to determine the optimal bias voltage and the optimal gain voltage; the setting module is used to set the optimal bias voltage and the optimal gain voltage to the receiver.

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