CN116781173B - A hybrid digital-analog transmission method for space optical wireless systems - Google Patents

Abstract

The invention relates to a hybrid digital-analog transmission method for a space optical wireless system and belongs to the field of optical communications. Aiming at the low spectrum efficiency problem of existing mobile fronthaul, a hybrid digital-analog transmission method for space optical wireless systems is proposed. The main idea is to adaptively change the inserted analog bandwidth at the zero point of the digital signal spectrum according to changes in free-space optical channel state information, thereby improving spectrum utilization. By analyzing the effects of atmospheric attenuation and turbulence on free-space optical channels, an analytical expression of the analog signal bandwidth that can be inserted into the first zero point of the digital signal spectrum is derived. Based on this analytical expression, a hybrid digital-analog transmission method for space optical wireless systems is further designed to improve the spectral efficiency of free-space optical transmission systems under different channel states by optimizing the maximum insertable analog bandwidth.

Description

A hybrid digital-analog transmission method for space optical wireless systems

Technical field

The invention belongs to the field of optical communications and relates to a hybrid digital-analog transmission method for a space optical wireless system.

Background technique

With the advent of the 5th Generation (5G) mobile communications era, large-bandwidth and high-speed traffic communication services are booming. With the advent of the era of the Internet of Everything, the number of large-scale user equipment has increased exponentially, which has a great impact on communication systems. The capacity puts forward strict requirements. However, the existing 5G low-frequency radio frequency (RF) bandwidth has almost been completely occupied, and it is difficult to further tap the available bandwidth, so the efficiency of the spectrum needs to be improved. The attenuation of radio frequency transmission in the air is very large and ultra-long-distance transmission cannot be achieved. Therefore, using optical fiber as a medium for long-distance transmission has become the best choice. So scholars proposed Radio over Fiber (RoF) technology, which loads RF signals onto optical fibers for transmission, thereby making full use of the low attenuation, high ductility, and low-cost characteristics of optical fibers. The current 5G Centralized Radio Access Network (C-RAN) includes three parts: a centralized unit, a distribution unit and a radio frequency unit. Among them, the RoF that carries digital signals is called digital RoF (Digital-RoF, D-RoF); and the RoF that carries analog signals is called analog RoF (Analog-RoF, A-RoF). D-RoF can achieve high signal fidelity by using high quantization bits, but is generally considered to be less spectrally efficient. A-RoF is susceptible to noise and inherent nonlinearity during optical transmission, but it can achieve higher spectral efficiency. Current commercial RoF transmission carries digital signals, also known as public radio interfaces, but faces the problem of low spectrum efficiency and is difficult to meet the task of large-capacity transmission in the future. In order to combine the advantages of digital and analog signal transmission, hybrid digital and analog signal transmission technology emerged as the times require.

Although optical fiber links have many advantages, such as: low cost, high bandwidth, long transmission distance, good transmission quality, immunity to electromagnetic interference, etc., link expansion or repair in cities where fiber optics have been laid will not only lead to The interruption will affect the user's use, and the ground will need to be excavated, which will increase labor costs. Therefore, FreeSpace Optical (FSO) has become a substitute link for optical fiber. At present, many scholars have studied hybrid transmission technology, such as through spectrum zero filling, but related work mainly focuses on fiber optic channels, and the link loss of FSO channels is more complex than that of fiber optic channels, including atmospheric attenuation and turbulence. More importantly, the previously inserted analog signal bandwidth is fixed and cannot adaptively adjust the analog bandwidth according to the channel status. In order to avoid link interruption, the method of fixing the analog bandwidth range will consider the worst channel status, so the final mixed digital-analog transmission capacity is not maximized.

The present invention realizes mixed transmission of digital and analog signals by inserting analog signals into zero points of the digital signal spectrum, thereby making it compatible with the transmission of digital and analog signals. Based on this system, the present invention proposes an expression for the maximum insertable bandwidth of an analog signal under the conditions of atmospheric attenuation and turbulence. According to this expression, the channel conditions can be adapted to change the inserted analog bandwidth. Therefore, the advantage of the present invention is that it can modulate the maximum insertable bandwidth of the analog signal according to the link status information of the channel; thereby maximizing the transmission capacity of the hybrid system.

Contents of the invention

In view of this, the object of the present invention is to provide a hybrid digital-analog transmission method for a space optical wireless system.

In order to achieve the above objects, the present invention provides the following technical solutions:

A hybrid digital-analog transmission method for a space optical wireless system. The method is:

Step S1: The digital signal adopts Pulse Code Modulation (PAM), and the analog signal adopts Orthogonal Frequency Division Multiplexing (OFDM) modulation;

Step S2: Collect the atmospheric attenuation factor and atmospheric refractive index structure constant information of the FSO channel through the sensor;

Step S3: According to the link status information, solve the maximum insertable simulation bandwidth through analytical expressions;

Step S4: Determine the number of sidebands aggregated by the OFDM signal for analog transmission according to the maximum insertable analog bandwidth;

Step S5: Convert the analog transmission OFDM to the first spectrum zero point of the digital transmission PAM signal through frequency upconversion, and combine the two signals through a power combiner to achieve digital-analog hybrid transmission;

Step S6: Perform intensity modulation/direct detection (IM/DD) on the mixed digital and analog signals;

Step S7: The receiving end obtains the analog signal through a band-pass filter (BPF), and further subtracts the analog signal from the mixed signal to obtain a digital signal, thereby achieving demodulation of the digital-analog mixed signal without DSP processing.

Further, S1 is specifically: the digital signal is modulated by PAM signal, and the analog signal is modulated by OFDM. Since digital signals have spectrum zeros, analog signals can be inserted into the spectrum zeros of digital signals, thereby increasing spectrum utilization. The power spectral density function of digital signals is:

In the formula, sinc(x)=sin(x)/x, M is the modulation order of the PAM signal, A _d is the amplitude of the digital signal, R is the bit rate of the PAM signal, and f is the frequency of the signal. When the modulation order M=2, the PAM signal is converted into an NRZ signal.

Furthermore, S2 is specifically: Compared with radio frequency channels, due to the shorter transmission distance of FSO channels, the link status information of the receiving end and the transmitting end is basically consistent, so the status information measured by the transmitting end can be used to estimate the status of the entire link. Channel status changes. The link status information of the FSO channel mainly includes atmospheric attenuation and atmospheric turbulence. Atmospheric attenuation can be quantified through the atmospheric attenuation factor, which can be predicted through cloud weather or obtained from humidity sensors; atmospheric turbulence can be measured through the atmospheric refractive index structure. Quantitatively, the atmospheric refractive index structure constants can be obtained through temperature sensors. This feedback-free approach can avoid the overhead of the feedback channel and avoid link feedback errors.

Further, S3 is specifically: According to the link status information obtained by the sending end, the maximum insertable simulation bandwidth can be solved through analytical expressions. Specifically:

S3.1: In optical communications, signals are usually received through a photodetector (PD) at the receiving end. However, due to the influence of the receiving aperture, interference such as background noise will be introduced. Therefore, the optical channel is usually modeled as

y＝R ₀ hx+n (2)

where x is the transmission symbol with average optical power, h is the channel attenuation coefficient, y is the electrical signal after PD, n is the photocurrent noise, and R ₀ is the responsivity of the photodetector. h here includes two parts: atmospheric attenuation h _l and atmospheric turbulence h _a . Assuming that these two parts act on the channel independently of each other, the channel state can be expressed as

h＝h _l h _a (3)

In the formula, h _l is a deterministic function, which is mainly related to the transmission distance and attenuation factor, while h _a is a random process.

S3.2: h _l is usually modeled as an exponential function related to the atmospheric attenuation factor, and h _a is usually modeled as a Gamma-Gamma channel model related to the atmospheric refractive index structure constant to describe changes in the intensity of turbulence.

S3.3: In order to solve the range of insertable analog bandwidth, the expression of the signal-to-noise ratio (SNR) of the analog bandwidth can first be obtained, and then solved through the error vector amplitude (Error Vector Amplitude, EVM) threshold equation. For analog signals, they are mainly affected by digital crosstalk and power attenuation caused by FSO links. Therefore, the signal-to-noise ratio of the analog signal after PD detection is:

In the formula, R ₀ represents the responsivity of the photodetector, P _A is the power of the analog signal, _PD is the power of the digital signal within the analog bandwidth range, P _N represents the noise power distributed within the photodetector bandwidth B _PD , H _FSO (f) is the transfer function of the FSO link, including the effects of atmospheric turbulence and atmospheric attenuation. Since the power of the signal is equal to the integral of the power spectral density (PSD) within the bandwidth range, since the PSD expression of the digital signal is obtained in formula (1), the PSD of noise and analog signals can be approximately considered to be uniform. Distribution. Therefore, the optical signal-to-noise ratio (Optical SNR, OSNR) can be used to quantify the impact of noise, which is defined as (A _a +A _d ) ² /P _N , where P _N is the noise power distributed within the photodetector bandwidth B _PD , A _a is the maximum amplitude of the analog signal, and A _d is the maximum amplitude of the digital signal. Therefore, the noise power spectral density can be expressed as

In the formula, B _PD is the bandwidth of the photodetector, A _a is the maximum amplitude of the analog signal, A _d is the maximum amplitude of the digital signal, and OSNR is the optical signal-to-noise ratio. Assuming that the OFDM signal is approximately uniformly distributed on the spectrum with a bandwidth of B, the PSD of the analog signal can be simplified as

In the formula, P _PAPR is the peak-to-average power ratio of the OFDM signal.

Substituting formula (6) into formula (4), we can get

In the formula, S _D (f, M) is the power spectral density function of the PAM signal, and B is the maximum insertable simulation bandwidth that needs to be solved. Since the SNR expression of the analog signal is obtained, the maximum insertable analog bandwidth can be solved through the EVM thresholds of different modulations. The expression of EVM is as follows

By substituting SNR _A (·) in formula (7) into formula (8), the EVM expression EVM _A (·) of the analog signal can be obtained. Only when the communication system is lower than the EVM threshold corresponding to its modulation order, correct decoding can be guaranteed. The EVM threshold value corresponds to the modulation order of M-ary Quadrature Amplitude Modulation (M-QAM). So we can get the expression

f(B)＝EVM _th -EVM _A (B,l,M,f) (9)

Three situations can be obtained from formula (9), f(B)>0, f(B)=0, f(B)<0. When f(B)>0, it means that the EVM of the bandwidth at this time is less than the EVM threshold, and the analog bandwidth can be further increased; when f(B)=0, it means that the bandwidth at this time is the maximum insertable analog bandwidth; When f(B)<0, it means that the EVM of the bandwidth at this time cannot meet the EVM threshold requirement, and the bandwidth needs to be reduced. The optimal bandwidth can be quickly searched through the dichotomy method. The optimal bandwidth at this time is the maximum insertable analog bandwidth B. By adjusting the EVM threshold, the maximum insertable analog bandwidth under different modulation orders can be obtained.

Further, S4 is specifically: the number of sidebands aggregated by the simulated OFDM signal is determined by the maximum insertable simulation bandwidth. By rounding the bandwidth solved in S3:

Among them, B is the maximum insertable analog bandwidth obtained by formula (9), B ₀ is the bandwidth of OFDM sideband, Indicates rounding down.

Further, S5 is specifically: PAM signal (digital signal) and OFDM signal (analog signal) are combined through a power combiner, where the analog signal is inserted into the spectrum zero point of the digital signal. Because digital signals naturally have spectrum zeros, and their spectrum zeros are determined by the baud rate of the PAM signal, upconverting the analog signal to the spectrum zeros of the digital signal can achieve a mixture of digital and analog signals in the frequency domain.

Further, S6 is specifically: perform IM/DD on the mixed digital and analog signals, and load the mixed digital and analog signals at the sending end to the MZM (Mach Zehndermodulator) through the Arbitrary Waveform Generator (AWG), where the MZM needs to be loaded. DC bias, and then the optical signal is sent to the FSO channel through the optical collimator. At the receiving end, the optical signal is collected through the optical collimator, and then the optical signal is converted into an electrical signal through the PD.

Further, S7 is specifically: the electrical signal obtained in S6 is divided into two signals by a beam splitter, one of which is an analog signal through a band-pass filter, and the other is a mixed signal. At this time, another beam splitter is used to divide the obtained analog signal into two channels, one of which is used for OFDM signal demodulation, and the other analog signal is used to separate the mixed signal. In order to reduce the interference of the analog signal to the digital signal, the mixed signal output by the first beam splitter is passed through a 3dB attenuator (ATT) to ensure that the power of the analog signal and the digital signal is equal. Finally, the digital signal can be decomposed by removing the analog signal from the mixed signal through a subtractor, and then the demodulation of the analog signal and the digital signal is completed respectively.

The beneficial effect of the present invention is that: in view of the deficiencies in existing research on the FSO cooperative transmission system, the present invention proposes a hybrid digital-analog transmission method for a space optical wireless system. For different atmospheric channel states, the maximum insertable analog bandwidth can be solved using analytical expressions to maximize the spectrum efficiency of the hybrid digital-analog transmission system.

The innovation of the hybrid digital-analog transmission method of the space optical wireless system proposed by the present invention is that the inserted analog signal bandwidth can be adaptively changed according to the channel state information of the link, thereby maximizing the spectrum efficiency of the system.

Other advantages, objects, and features of the present invention will, to the extent that they are set forth in the description that follows, and to the extent that they will become apparent to those skilled in the art upon examination of the following, or may be derived from This invention is taught by practicing it. The objects and other advantages of the invention may be realized and obtained by the following description.

Description of the drawings

In order to make the purpose, technical solutions and advantages of the present invention clearer, the present invention will be described in detail below in conjunction with the accompanying drawings, in which:

Figure 1 shows the channel state information acquisition scheme without feedback link;

Figure 2 is a schematic diagram of a hybrid digital-analog transmission system;

Figure 3 shows the variation of analog signal EVM with frequency under different optical signal-to-noise ratios;

Figure 4 shows the impact of different link lengths on the maximum insertable analog signal bandwidth;

Figure 5 shows the impact of different atmospheric attenuation factors on the maximum insertable analog signal bandwidth;

Figure 6 shows the impact of different attenuation factors on the BER of each sideband of the digital signal.

Detailed ways

The following describes the embodiments of the present invention through specific examples. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments. Various details in this specification can also be modified or changed in various ways based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the illustrations provided in the following embodiments only illustrate the basic concept of the present invention in a schematic manner. The following embodiments and the features in the embodiments can be combined with each other as long as there is no conflict.

The drawings are only for illustrative purposes, and represent only schematic diagrams rather than actual drawings, which cannot be understood as limitations of the present invention. In order to better illustrate the embodiments of the present invention, some components of the drawings will be omitted. The enlargement or reduction does not represent the size of the actual product; it is understandable to those skilled in the art that some well-known structures and their descriptions may be omitted in the drawings.

In the drawings of the embodiments of the present invention, the same or similar numbers correspond to the same or similar components; in the description of the present invention, it should be understood that if there are terms "upper", "lower", "left" and "right" The orientation or positional relationship indicated by "front", "rear", etc. is based on the orientation or positional relationship shown in the drawings, and is only for the convenience of describing the present invention and simplifying the description, and does not indicate or imply that the device or element referred to must be It has a specific orientation and is constructed and operated in a specific orientation. Therefore, the terms describing the positional relationships in the drawings are only for illustrative purposes and cannot be understood as limitations of the present invention. For those of ordinary skill in the art, they can determine the specific position according to the specific orientation. Understand the specific meaning of the above terms.

Current commercial RoF transmission carries digital signals and faces the problem of low spectrum efficiency, making it difficult to meet the task of large-capacity transmission in the future. In order to combine the advantages of digital and analog signal transmission, some scholars have proposed a hybrid digital-analog signal transmission scheme. This technology was first used in the integration of fixed network access and wireless networks. It integrates fixed networks and wireless networks in a special way to improve spectrum efficiency and reduce hardware investment costs. Although some scholars have studied hybrid transmission technology, such as through spectrum zero filling, related work mainly focuses on fiber optic channels, and the link loss of FSO channels is more complex than that of fiber optic channels, including atmospheric attenuation and turbulence. More importantly, the bandwidth of the analog signals inserted by these scholars is fixed and cannot be adaptively adjusted according to changes in channel status. The fixed insertion method of analog bandwidth range is designed based on the worst link status to avoid link interruption, so the final mixed digital-analog transmission capacity is not maximized.

Therefore, the present invention proposes a hybrid digital-analog transmission method for a space optical wireless system in order to solve the problem of low spectrum efficiency of existing digital-analog hybrid transmission. It mainly adaptively changes the inserted analog bandwidth at the zero point of the digital signal spectrum based on changes in free-space optical channel status information, thereby improving spectrum utilization. By analyzing the effects of atmospheric attenuation and turbulence on free space optical channels, an analytical expression of the analog signal bandwidth that can be inserted into the first zero point of the digital signal spectrum is derived. By optimizing the maximum insertable analog bandwidth, the performance of the FSO transmission system in different environments can be improved. Spectral efficiency under channel conditions.

In summary, the present invention proposes a hybrid digital-analog transmission method for a space optical wireless system that adaptively changes the inserted analog signal bandwidth to adapt to changes in free-space optical channel status information, thereby maximizing system transmission efficiency. capacity.

The present invention proposes a digital-analog hybrid transmission scheme for the FSO mobile fronthaul system, and realizes adaptive analog bandwidth insertion on this basis. The digital-analog hybrid transmission system designed by the present invention can adaptively change the range of incoming and outgoing analog bandwidths according to changes in atmospheric channel status, thereby improving the spectrum efficiency of the system.

The present invention is verified in a hybrid digital analog FSO communication system, that is, the simulated multi-sideband OFDM signal is inserted into the spectrum zero points of the digital signal. The specific implementation process of the hybrid digital-analog transmission method of the space optical wireless system proposed by the present invention is as follows:

1. Through the channel state information acquisition scheme without feedback link shown in Figure 1, that is, the system is based on the results under no feedback channel conditions. The channel state information required in the algorithm can be obtained through sensors, and the atmospheric attenuation factor can be obtained through Real-time weather updates in the cloud or humidity sensors can be used to estimate the atmospheric refractive index structure constants through temperature sensors. For actual scenarios, since the FSO transmission link distance is usually large, the link status information does not change drastically within this small range, so the link status information can be obtained through the sensor at the sending end. The sensor sends electrical signals to the distribution unit to complete the capture of link status information.

2. As shown in Figure 2, part (a) is the system block diagram. The digital signal processing (DSP) at the transmitting end is as shown in part (b). The original binary data is filtered after serial to parallel conversion, inverse Fourier transform, parallel to serial conversion and adding cyclic prefix. Although limiting operation will reduce the peak-to-average power ratio, it will also introduce large out-of-band leakage, so a raised cosine filter is used to reduce out-of-band leakage. Finally, the multi-sideband OFDM is carrier aggregated through an intermediate frequency (IF) multiplexer, and the analog signal is converted into the required intermediate frequency signal through up-conversion. The frequency of the up-conversion is required to be the first spectral zero point of the PAM signal. The other channel of data performs PAM modulation, and then passes through the power combiner to obtain a mixed digital and analog signal. The mixed digital and analog signals are modulated onto the optical carrier through the MZM. The MZM operates at an orthogonal bias point to ensure the linearity of the device. The optical signal is then sent to the FSO channel through the optical collimator. In particular, the theoretically derived formula (10) is used in the adaptive sideband allocation module of the transmitter DSP.

In the RU, the optical signal is collected by the optical collimator, and then converted into an electrical signal by the PD. The obtained electrical signal is divided into two signals by the beam splitter, one of which is an analog signal through a band-pass filter, and the other is a mixed signal. At this time, another beam splitter is used to divide the obtained analog signal into two channels, one of which is used for OFDM signal demodulation, and the other analog signal is used to separate the mixed signal. In order to reduce the interference of analog signals to digital signals, the mixed signal output by the first beam splitter is passed through a 3dB attenuator to ensure that the power of the analog signals and digital signals is equal. Finally, the digital signal can be decomposed by removing the analog signal from the mixed signal through a subtractor, and then the demodulation of the analog signal and the digital signal is completed respectively. OFDM signal demodulation is shown in part (c), which is the inverse process of modulation in part (b).

3. Use theoretical derivation to establish the relationship between channel state information and the maximum insertable analog bandwidth. Through this relationship, an adaptive insertion analog bandwidth algorithm can be designed. The specific steps are as follows: Step 1: The channel state information obtained according to process 1 is used as known information. Obtain the SNR expression of the analog signal; Step 2: Establish the relationship between EVM and the maximum insertable analog bandwidth through the requirements of the EVM threshold and the relationship between SNR and EVM; Step 3: Solve the maximum insertable analog bandwidth through the dichotomy method, that is When the EVM of the inserted analog bandwidth is equal to the EVM threshold, the bandwidth at this time is the maximum insertable analog bandwidth.

Table 1 OptiSystem optical simulation software parameter settings

parameter Value/Form unit OFDM bandwidth 10 MHz laser frequency 193.1 THz Laser power 0 dBm Number of OFDM subcarriers 600 - Digital signal baud rate 600 MBaud Transmitter aperture 0.05 m Receiver aperture 0.2 m Transmission divergence angle 2 mrad Responsiveness of photodetector 1 A/W Bandpass filter roll-off factor 0.2 -

According to the parameter settings in Table 1, the maximum insertable simulation bandwidth under the FSO link was simulated through the OptiSystem optical simulation software, and the results of the maximum insertable simulation bandwidth in the theoretical derivation were compared.

Figure 3 shows the impact of frequency changes on EVM under different optical signal-to-noise ratios when the analog-to-digital mixing ratio is 1:1. The attenuation factor set here is 10dB/km, and the FSO link length is 0.5km. The selection of OSNR under different modulation orders in the formula is determined by setting the fixed insertion analog bandwidth value. The analog bandwidth inserted in Figure 3 is preset to 50MHz. The frequency range is [300, 1300] MHz. The horizontal lines in the figure represent the thresholds of EVM under different modulation methods. That is, reliable communication can be achieved only below this threshold. The solid black line in the figure represents the situation without noise interference, that is, the situation when OSNR tends to infinity, which is the theoretical upper limit. In this frequency band, two points with zero EVM value can be found, namely 600 and 1200MHz, which exactly correspond to the zero point of the spectrum of the PAM signal. We can find that as the OSNR increases, at 600MHz, the concave degree of the curve gradually increases; and as the frequency increases, it finally approximates a straight line. In particular, the intersection range of the EVM threshold and OSNR is the insertable analog bandwidth. For example, for 4QAM, when OSNR=30dB, the insertable analog bandwidth is 50MHz; while for 16QAM, the OSNR needs to be increased to 37dB. Because 4QAM is used as an example in subsequent simulations, the OSNR in the theoretical formula is set to 30dB.

Figure 4 is a comparison of the maximum insertable simulation bandwidth under the conditions of a fixed atmospheric attenuation of 10dB/km and different link lengths ([4001000]m). The horizontal axis represents the FSO link length, and the vertical axis represents the maximum insertable bandwidth of the analog signal. The thick line in the figure is the actual result, the thin line is the result of rounding; the dotted line is the result of software simulation, and the continuous line is the result of theoretical analysis. From the curve in Figure 4, there is a negative exponential relationship between the FSO link length and the maximum insertable analog bandwidth. As the transmission distance increases, the insertable analog bandwidth first decreases sharply. When the link length exceeds 600m, the slowing down trend of the ordinate gradually becomes flat. When the transmission distance is greater than 600 meters, the results obtained from the analytical expression deviate greatly from the simulation results. Since the received optical power is very small at this time, it is difficult for the receiving end to demodulate the analog signal, so the results of the analytical expression are no longer applicable.

Figure 5 shows the impact of different atmospheric attenuation factors on the simulated signal filling bandwidth under a fixed transmission distance of 0.5km. The horizontal axis represents the atmospheric attenuation factor, and the vertical axis represents the maximum insertable simulation bandwidth. It can be observed that the atmospheric attenuation factor is approximately linearly related to the interposable analog signal bandwidth. Comparing the results in Figure 5 and Figure 4, it can be seen that the analytical expression has a better fitting effect on the changes in atmospheric attenuation compared with the transmission link distance. For actual scenarios, the distance between the transmitter and the receiver is basically fixed, so the derived analytical expression has certain guiding significance for actual system optimization.

Figure 6 shows the impact of different atmospheric attenuation factors on BER under weak turbulence when the transmission distance is 0.5km. Through Optisystem software, simulate the situation where the inserted analog bandwidth is 60MHz. Figure 6 shows how the BER of each of the six sidebands changes with the atmospheric attenuation factor. The black histogram represents the BER change when the six sidebands are fixed and no adaptive scheme is used. The light histogram represents the adaptive insertion simulation bandwidth. The change in BER corresponding to the number of sidebands. It can be found from Figure 6 that when the link attenuation is greater than 10dB/km, the analog bandwidth that can be inserted cannot exceed 60MHz, because the sixth sideband no longer meets the FEC error correction threshold, and the inserted analog bandwidth needs to be reduced at this time . Therefore, it is necessary to change the bandwidth range that can be inserted into the simulation according to the attenuation of the link and the size of the turbulence. As shown in the light bar chart in Figure 6, these sidebands can meet the requirements of the FEC error correction threshold.

Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not limiting. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified. Modifications or equivalent substitutions without departing from the purpose and scope of the technical solution shall be included in the scope of the claims of the present invention.

Claims (4)

1. A mixed digital-analog transmission method of a space optical carrier wireless system is characterized in that: the method comprises the following steps:

s1: solving the maximum pluggable analog bandwidth through an analytic expression according to the free space optical FSO link state information;

s2: determining the sideband number of the aggregation of the OFDM signals of the analog transmission according to the maximum pluggable analog bandwidth;

s3: the OFDM of analog transmission is converted to the first frequency spectrum zero point of the Pulse Amplitude Modulation (PAM) signal of digital transmission through up-conversion, and the two paths of signals are combined through a power combiner, so that digital-analog hybrid transmission is realized;

s4: the receiving end obtains an analog signal through a band-pass filter (BPF), and subtracts the analog signal from the mixed signal to obtain a digital signal, so that demodulation of the digital-analog mixed signal without Power Spectral Density (PSD) processing is realized;

the S1 specifically comprises the following steps: solving the maximum pluggable analog bandwidth through an analytic expression according to the link state information acquired by the transmitting end; firstly, obtaining an expression of a signal-to-noise ratio (SNR) of an analog bandwidth, and then solving an equation through an EVM threshold; the signal-to-noise ratio of the analog signal after detection by the photodetector PD is:

wherein R is ₀ Indicating the responsivity of the photodetector, P _A Is the power of the analog signal, P _D Is the power of the digital signal in the analog bandwidth range, P _N Represented at photodetector bandwidth B _PD Internally distributed noise power, H _FSO (f) Is the transfer function of the FSO link, including the effects of atmospheric turbulence and atmospheric attenuation; the effect of noise is quantified using OSNR, defined as (a _a +A _d ) ² /P _N Wherein P is _N Is within the bandwidth B of the photoelectric detector _PD Internally distributed noise power, A _a Is the maximum amplitude of the analog signal, A _d Is the maximum amplitude of the digital signal; the noise power spectral density is expressed as

Wherein B is _PD Is the bandwidth of the photodetector, A _a Is the maximum amplitude of the analog signal, A _d Is the maximum amplitude of the digital signal and OSNR is the optical signal to noise ratio; assuming that the OFDM signal is approximately uniformly distributed over the spectrum with bandwidth B, the PSD of the analog signal is reduced to

P in the formula _PAPR Peak-to-average power ratio for OFDM signals;

substituting the formula (3) into the formula (1) to obtain

Wherein S is _D (f, M) is a power spectral density function of the PAM signal, B is the maximum pluggable analog bandwidth that needs to be solved; solving the maximum pluggable analog bandwidth through EVM thresholds of different modulations; the expression of EVM is as follows

SNR in equation (4) is calculated _A (. Cndot.) substitution into equation (5) to obtain EVM expression EVM of analog signal _A (. Cndot.); EVM when communication system _A (. Cndot.) is below the EVM threshold EVM corresponding to its modulation order _th Decoding is error-free when in use; according to EVM threshold value corresponding to modulation order of multi-system QAM, obtaining expression

f(B)＝EVM _th -EVM _A (B,l,M,f) (6)

M is the modulation order of the PAM signal, f is the frequency of the signal;

three cases were obtained from equation (6), f (B) > 0, f (B) =0, f (B) < 0; when f (B) is more than 0, the EVM corresponding to the analog bandwidth is smaller than the threshold value, and the analog bandwidth is increased; when f (B) =0, it is explained that the analog bandwidth at this time is the maximum pluggable analog bandwidth; when f (B) is less than 0, the EVM of the analog bandwidth does not reach the threshold requirement, and the bandwidth needs to be reduced; the optimal bandwidth is obtained by quick search through a dichotomy, and the optimal analog bandwidth at the moment is the maximum pluggable analog bandwidth B; and obtaining the maximum pluggable analog bandwidth under different modulation orders by adjusting the EVM threshold.

2. The method for hybrid digital-to-analog transmission of a spatial light load wireless system of claim 1, wherein: the step S2 is specifically as follows: the number of sidebands aggregated by the analog OFDM signal is determined by the maximum pluggable analog bandwidth; by rounding the bandwidth solved in S1:

wherein B is the maximum pluggable analog bandwidth obtained by the formula (6), B ₀ Is the bandwidth of the OFDM sideband,representing a rounding down.

3. The method for hybrid digital-to-analog transmission of a spatial light load wireless system of claim 2, wherein: the step S3 is specifically as follows: the PAM signal and the OFDM signal are combined through a power combiner, wherein the analog signal is inserted into a frequency spectrum zero point of the digital signal; up-converting the analog signal to a frequency spectrum zero point of the digital signal, and realizing the mixing of the digital signal and the analog signal in a frequency domain; the mixed signal is modulated onto an optical carrier by an MZM modulator and finally transmitted to an FSO channel by an optical collimator.

4. The method for hybrid digital-to-analog transmission of a spatial light load wireless system of claim 1, wherein: the step S4 specifically includes: the receiving end receives the optical signal through the optical collimator, and then converts the optical signal into an electric signal through the photoelectric detector PD; the electric signal is divided into two paths of signals by a beam splitter, wherein one path of the electric signal passes through a band-pass filter to obtain an analog signal, and the other path of the electric signal is a mixed signal; at this time, the obtained analog signal is divided into two paths by using another beam splitter, wherein one path is used for OFDM signal demodulation, and the other path is used for separating the mixed signal; the mixed signal output by the first beam splitter passes through a 3dB attenuator ATT to ensure the power equalization of the analog signal and the digital signal; and removing the analog signal from the mixed signal through a subtracter to decompose the digital signal, and completing demodulation of the analog signal and the digital signal.