阅读说明：本技术 基于串行中继自由空间光通信系统的自适应协议选择方法 (Adaptive protocol selection method based on serial relay free space optical communication system ) 是由 王平 李爽 黄超 庞维娜 王炜 陈雯雯 于 2020-06-08 设计创作，主要内容包括：本发明公开了一种基于串行中继自由空间光通信系统的自适应协议选择方法,包括：构建基于放大转发协议的串行中继自由空间光通信系统的模型,分别得到基于放大转发协议的接收端信噪比、功率限制条件下接收端信噪比、基于信道信息放大转发协议的接收端信噪比、固定增益条件下的接收端信噪比和基于信道信息放大转发协议的系统平均误码率ABER；构建自适应协议选择方法。与现有的基于串行中继的自由空间光通信系统性能相比,本方法能在保证系统平均误码率性能要求的同时降低系统的复杂度。(The invention discloses a self-adaptive protocol selection method based on a serial relay free space optical communication system, which comprises the following steps: constructing a model of a serial relay free space optical communication system based on an amplify-and-forward protocol, and respectively obtaining a receiving end signal-to-noise ratio based on the amplify-and-forward protocol, a receiving end signal-to-noise ratio under a power limiting condition, a receiving end signal-to-noise ratio based on a channel information amplify-and-forward protocol, a receiving end signal-to-noise ratio under a fixed gain condition and a system average bit error rate ABER based on the channel information amplify-and-forward protocol; and constructing an adaptive protocol selection method. Compared with the performance of the existing free space optical communication system based on serial relay, the method can reduce the complexity of the system while ensuring the performance requirement of the average bit error rate of the system.)

1. A self-adaptive protocol selection method based on a serial relay free space optical communication system is characterized by comprising the following steps:

1) constructing a serial relay free space optical communication system model based on an amplify-and-forward protocol, and respectively obtaining a receiving end signal-to-noise ratio based on the amplify-and-forward protocol, a receiving end signal-to-noise ratio under a power limit condition, a receiving end signal-to-noise ratio based on a channel information amplify-and-forward protocol, a receiving end signal-to-noise ratio under a fixed gain condition and an average bit error rate of a system under the amplify-and-forward protocol based on the channel;

2) the optimal forwarding protocol in the free space optical communication system based on the serial relay is selected by using a self-adaptive protocol selection method, so that the requirement on the average error rate performance of the system is ensured, and the complexity of the system is reduced; specifically, the method comprises the following steps:

2a) initializing and defining a source node sending signal and all parameters related to a system;

2b) transmitting a premodulation signal x from a source node to a next hop node;

2c) selecting a signal-to-noise ratio threshold value gamma corresponding to the acceptable average bit error rate of the system_th；

2d) Calculating the receiving signal-to-noise ratio gamma of the kth node; if gamma is less than gamma_thThe kth node selects a decoding forwarding protocol to forward to the next hop; if gamma is not less than gamma_thThe kth node selects an amplification forwarding protocol based on channel information and forwards the protocol to the next hop;

2e) and calculating the average bit error rate of the system after reaching the destination node.

2. The method according to claim 1, wherein in step 1), a serial relay free space optical communication system model based on an amplify-and-forward protocol is constructed, and the system model includes a channel model based on a weak atmospheric turbulence condition:

when the intensity of atmospheric turbulence is weak, the turbulence is usually modeled by a lognormal distribution, the probability density function of whichAnd cumulative distribution functionThe form is as follows:

in the formula, I is the received signal strength after the power normalization of the transmitting terminal,is the logarithmic variance of the light intensity,

3. The method for selecting an adaptive protocol based on a serial relay free space optical communication system according to claim 1, wherein in the step 1), the procedure for obtaining the receiving end signal-to-noise ratio based on the amplify-and-forward protocol, the receiving end signal-to-noise ratio under the power limitation condition, the receiving end signal-to-noise ratio based on the channel information amplify-and-forward protocol, and the receiving end signal-to-noise ratio under the fixed gain condition is as follows:

1a) setting the photoelectric response coefficient and the modulation coefficient to be normalized, and obtaining the electric signal y received by the kth node_k；

1b) Setting the modulation format to BPSK and assuming the power normalization of the transmitting terminal, the signal y received by the destination node is obtained_N；

1c) Obtaining receiving end signal-to-noise ratio gamma based on the amplification forwarding protocol_ee；

1d) Meeting repeater powerUnder the constraint, let the amplification gain at the repeater be,then the signal-to-noise ratio gamma of the receiving end under the power limiting condition is obtained_ee1；

1e) Let the amplification gain of the relay node be the inverse of the channel fading coefficient of the previous hop, i.e. g_i＝1/I_iThen obtaining the receiving end signal-to-noise ratio gamma based on the channel information amplification forwarding protocol_ee2；

1f) If the amplification gain of each node is a fixed value, the receiving end signal-to-noise ratio gamma of the target node under the condition of fixed gain_ee3Can be expressed as the product of the squares of N independent lognormal random variables.

4. The adaptive protocol selection method based on the serial relay free space optical communication system according to claim 3, wherein in step 1a), the kth node receives the electrical signal y_kComprises the following steps:

y_k＝g_k-1I_ky_k-1+n_k(3)

n, where k is 1.. N denotes the number of relay nodes, and y denotes the number of relay nodes_kIndicating that the kth node receives a signal; g_k-1Represents the amplification gain of the signaling signal by the node k-1; i is_kRepresents the channel fading coefficients between node k-1 to node k, and

5. The adaptive protocol selection method based on the serial relay free space optical communication system as claimed in claim 3, wherein in step 1b), the purpose is to provide a method for adaptive protocol selectionOf the node received signal y_NExpressed as:

where i 1.. N, j 1.. N, N denotes the number of relay nodes, x denotes a BPSK signal transmitted from a source node, and g_i-1Represents the amplification gain of the signaling signal by node i-1, where g₀Representing the amplification gain of the source node on the transmission signal; i is_iRepresents the channel fading coefficients between node i-1 to node i, andn_jis a mean of zero and a variance ofWhite gaussian noise of (1);

in the step 1c), the receiving end signal-to-noise ratio gamma based on the amplify-and-forward protocol_eeComprises the following steps:

in the step 1d), the signal-to-noise ratio gamma of the receiving end under the condition of power limitation_ee1Comprises the following steps:

in the step 1e), the receiving end signal-to-noise ratio gamma based on the channel information amplification forwarding protocol_ee2Comprises the following steps:

in the formula (I), the compound is shown in the specification,wherein

In the step 1f), the receiving end signal-to-noise ratio gamma of the target node under the condition of fixed gain_ee3Comprises the following steps:

in the formula, N_TIs the accumulated noise power at the destination node.

6. The adaptive protocol selection method based on the serial relay free space optical communication system as claimed in claim 1, wherein in step 1), the average bit error rate p of the system under the amplified forwarding protocol based on the channel information is obtained_eThe method comprises the following steps:

signal-to-noise ratio upper bound gamma of receiving end of amplifying and forwarding protocol based on channel information_bCumulative distribution function ofComprises the following steps:

n, N is the number of relay nodes in the link, I_jThe channel fading coefficient for the j-th hop,

conditional bit error rate p of system when BPSK modulation is adopted_ec(γ) is:

where erfc (-) is a complementary error function, and γ is the signal-to-noise ratio of the receiving end;

then the average bit error rate of the system under the amplifying and forwarding protocol based on the channel information is as follows:

in the formula (I), the compound is shown in the specification,

7. The adaptive protocol selection method according to claim 1, wherein in step 2a), all parameters related to the system include the link length L of each hop of the relay system, the number Numb of signals transmitted from the transmitting end, the premodulation signal x transmitted from the source node, the number N of hops of the link in the system, the receiving aperture size D, and the logarithmic variance of the light intensitySignal to noise ratio threshold gamma_thAnd the order n of the laguerre gaussian function.

8. The adaptive protocol selection method based on the serial relay free space optical communication system according to claim 1, wherein the step 2d) is performed as follows:

calculating the receiving signal-to-noise ratio gamma of the kth node, if gamma is less than gamma_thAnd the node k forwards the signal to the node k +1 through the decoding forwarding protocol, and the signal received by the node k +1 is represented as:

y_k+1＝I_k+1x+n_k+1(22)

wherein X is a signal obtained after decoding of the node k, X belongs to X, and X is a set of constellation points of a signal sent by a source node;

if gamma is not less than gamma_thIf the node k forwards the signal to the node k +1 through the amplification forwarding protocol based on the channel information, the signal received by the node k +1 is represented as:

9. the adaptive protocol selection method according to claim 1, wherein in step 2e), the average bit error rate P of the system available at the receiving end is obtained_eComprises the following steps:

Technical Field

The invention belongs to the technical field of wireless optical communication, and particularly relates to a method for transmitting a signal by a transmitting terminal in a system, wherein the signal is modulated by Binary Phase Shift Keying (BPSK) and then reaches a receiving terminal by forwarding of serial relay nodes, and a forwarding protocol of a next hop is determined at each relay node by using a self-adaptive protocol selection method.

Background

In recent years, the wireless optical communication technology in China is rapidly developed, and the method has important significance for further improving the communication safety and stability. Compared with wired communication, although the wireless optical communication has more obvious advantages, the wireless optical communication is affected more seriously by factors such as barriers, distances, weather and the like, so that the communication channel state is poor, the quality of received signals is reduced, and the expected high-quality communication effect cannot be achieved. For this reason, a relay cooperative communication model is proposed, in which a plurality of nodes are arranged between the transmitting and receiving ends of a long distance to serve as "relay stations" to transmit signals. The nodes decompose a long distance into a plurality of short distances to slow down the fading of the optical signal and reduce the dependence on power, thereby effectively improving the reliability of a free space optical communication (FSO) system.

In addition, it is worth studying how to reduce the complexity of the FSO system as much as possible while improving the reliability of the FSO system. In a general relay protocol, two types are generally used, one type is direct Amplification Forwarding (AF), which performs adjustable gain amplification by using channel information or performs equal gain amplification directly on a signal, and then forwards the amplified signal to a node of a next hop; the second is Decoding Forwarding (DF), in which a relay node does not simply forward a received signal, but decodes the received signal first, re-encodes the decoded signal, and then sends the re-encoded signal to the next node. It has been reported that there have been many works to study the forwarding protocol of the serial relay based FSO system. However, the current problems are:

1) when the AF protocol is adopted, the system complexity is low, and the implementation is relatively easy, but the problem is that the noise is equally amplified in the amplification process, and if the number of system link hops is large, the noise accumulated from end to end will generate great interference.

2) When the DF protocol is adopted, accumulated noise can be effectively eliminated, but each node needs decoding processing, and the complexity and the system delay are larger.

Considering that the existing two forwarding protocols have respective advantages and disadvantages and random variation of channel environment, in order to better compromise complexity and reliability and economically and effectively improve the overall performance of the FSO system, the research on the self-adaptive protocol selection method with high reliability and low complexity actually belongs to the currently very important research direction.

Disclosure of Invention

The invention aims to provide a self-adaptive protocol selection method based on a serial relay free space optical communication system, which can reduce the complexity of the system while ensuring the performance requirement of the average bit error rate of the system.

The invention is realized by the following technical scheme.

A self-adaptive protocol selection method based on a serial relay free space optical communication system comprises the following steps:

1) constructing a serial relay free space optical communication system model based on an amplify-and-forward protocol, and respectively obtaining a receiving end signal-to-noise ratio based on the amplify-and-forward protocol, a receiving end signal-to-noise ratio under a power limit condition, a receiving end signal-to-noise ratio based on a channel information amplify-and-forward protocol, a receiving end signal-to-noise ratio under a fixed gain condition and an average bit error rate of a system under the amplify-and-forward protocol based on the channel;

2) the optimal forwarding protocol in the free space optical communication system based on the serial relay is selected by using a self-adaptive protocol selection method, so that the requirement on the average error rate performance of the system is ensured, and the complexity of the system is reduced; specifically, the method comprises the following steps:

2a) initializing and defining a source node sending signal and all parameters related to a system;

2b) transmitting a premodulation signal x from a source node to a next hop node;

2c) selecting a signal-to-noise ratio threshold value gamma corresponding to the acceptable average bit error rate of the system_th；

2d) Calculating the receiving signal-to-noise ratio gamma of the kth node; if gamma is less than gamma_thKth node selectionThe decoding forwarding protocol forwards to the next hop; if gamma is not less than gamma_thThe kth node selects an amplification forwarding protocol based on channel information to forward to the next hop;

2e) and calculating the average bit error rate of the system after reaching the destination node.

Further, in step 1), a serial relay free space optical communication system model based on an amplify-and-forward protocol is constructed, wherein the system model comprises a channel model based on a weak atmospheric turbulence condition: when the intensity of atmospheric turbulence is weak, the turbulence is usually modeled with a Lognormal distribution (LN). The LN model is obtained based on a first-order Rytov approximation, and a probability density function of the LN model is given

Sum and cumulative distribution functionForm (a).

Further, in step 1), a receiving end signal-to-noise ratio based on the amplify-and-forward protocol, a receiving end signal-to-noise ratio under the power limit condition, a receiving end signal-to-noise ratio based on the channel information amplify-and-forward protocol, and a receiving end signal-to-noise ratio under the fixed gain condition are obtained, and the process is as follows:

1a) setting the photoelectric response coefficient and the modulation coefficient to be normalized, and obtaining the electric signal y received by the kth node_k；

1b) Setting the modulation format to BPSK and assuming the power normalization of the transmitting terminal, the signal y received by the destination node is obtained_N；

1c) Obtaining receiving end signal-to-noise ratio gamma based on the amplification forwarding protocol_ee；

1d) Under the condition of meeting the power constraint of the repeater, the amplification gain at the repeater is set as,then the signal-to-noise ratio gamma of the receiving end under the power limiting condition is obtained_ee1；

1e) Making the amplification gain of the relay node equal to the channel fading coefficient of the previous hopReciprocal number, i.e. g_i＝1/I_iThen obtaining the receiving end signal-to-noise ratio gamma based on the channel information amplification forwarding protocol_ee2；

1f) If the amplification gain of each node is a fixed value, the receiving end signal-to-noise ratio gamma of the target node under the condition of fixed gain_ee3Can be expressed as the product of the squares of N independent lognormal random variables.

Further, in the step 1), the obtained average bit error rate p of the system under the amplifying and forwarding protocol based on the channel information_eThe method utilizes an amplifying and forwarding protocol based on channel information to receive the upper bound gamma of the signal-to-noise ratio of a receiving end_bCumulative distribution function of

And system conditional bit error rate.

Further, in step 2a), all parameters related to the system include the length L of each hop link of the relay system, the number Numb of signals sent by the sending end, the premodulation signal x sent by the source node, the hop count N of the link in the system, the size D of the receiving aperture, and the logarithmic variance of the light intensity

Signal to noise ratio threshold gamma_thAnd the order n of the laguerre gaussian function.

Further, the step 2d) is performed according to the following process:

calculating the receiving signal-to-noise ratio gamma of the kth node, if gamma is less than gamma_thIf the node k forwards the signal to the node k +1 through the decoding forwarding protocol, the node k +1 receives the signal; if gamma is not less than gamma_thAnd the node k forwards the signal to the node k +1 through an amplification forwarding protocol based on the channel information, so that the node k +1 receives the signal.

Further, in the step 2e), the average bit error rate P of the system can be obtained by the receiving end_eComprises the following steps:

the invention has the following advantages:

compared with the forwarding protocol adopted by the existing serial relay system, the method for selecting the self-adaptive protocol based on the serial relay free space optical communication system sends a premodulation signal from a source node to a next hop node by initializing and defining the signal sent by the source node and all parameters related to the system, and selects the signal-to-noise ratio threshold gamma corresponding to the acceptable average bit error rate of the system_thCalculating the receiving signal-to-noise ratio gamma of the kth node; judging the magnitude of the signal-to-noise ratio threshold value and the receiving signal-to-noise ratio, and determining whether to select a decoding forwarding protocol to forward to a next hop; and therefore, the average bit error rate of the system is calculated after the target node is reached. The invention not only controls the influence of accumulated noise, but also reduces the complexity of the system while ensuring the performance requirement of the average bit error rate of the system, thereby better compromising the complexity and the reliability and economically and effectively improving the practical application capability of the system. The simulation result shows that the invention has good performance.

Drawings

FIG. 1 is a model of a serial relay free space optical communication system comprising N hops;

FIG. 2 is a flow chart of an adaptive protocol selection method based on a serial relay free space optical communication system;

FIG. 3 is a graph of the performance of the ABER of the FSO system based on different forwarding protocols;

FIG. 4 is an ABER performance curve based on the AF protocol system and the lower bound of the ABER theory;

fig. 5a) -5 b) are comparisons of system ABER performance for the adaptive protocol selection method and the DF-based, channel information AF-based protocols for receive aperture sizes of 3mm and 25mm, respectively;

fig. 6 is a comparison of adaptive protocol selection method and system ABER performance based on DF protocol and channel information AF protocol under different system link hop counts.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, the following detailed description is further provided in conjunction with the accompanying drawings and the detailed description. The present embodiment is merely illustrative of the principles of the present invention and does not represent any limitation of the present invention.

The invention provides a self-adaptive protocol selection method based on a serial relay free space optical communication system, which comprises the following steps:

step 1), constructing a serial relay free space optical communication system model based on the amplify-and-forward protocol, and respectively obtaining a receiving end signal-to-noise ratio based on the amplify-and-forward protocol, a receiving end signal-to-noise ratio under a power limit condition, a receiving end signal-to-noise ratio based on the channel information amplify-and-forward protocol, a receiving end signal-to-noise ratio under a fixed gain condition and an average bit error rate of the system under the amplify-and-forward protocol based on the channel information.

The system model comprises a channel model based on a weak atmospheric turbulence condition:

when the intensity of atmospheric turbulence is weak, the turbulence is usually modeled with a Lognormal distribution (LN). The LN model is obtained based on a first-order Rytov approximation, and the probability density function thereofSum and cumulative distribution function

The form is as follows:

wherein I is the power intensity of the received signal after the power normalization of the transmitting terminal,

is the logarithmic variance of the light intensity,

for intensity flicker index, erf (·) is the error function.

The process of obtaining the receiving end signal-to-noise ratio based on the amplify-and-forward protocol, the receiving end signal-to-noise ratio under the power limit condition, the receiving end signal-to-noise ratio based on the channel information amplify-and-forward protocol and the receiving end signal-to-noise ratio under the fixed gain condition is as follows:

1a) if both the photoresponse coefficient and the modulation coefficient are assumed to be normalized, the electrical signal received by the kth node can be represented as:

y_k＝g_k-1I_ky_k-1+n_k(3)

n, where k is 1_kIndicating that the kth node receives a signal; g_k-1Represents the amplification gain of the signaling signal by the node k-1; i is_kRepresents the channel fading coefficients between node k-1 to node k, and

y_k-1is a signal forwarded by the node k-1 to the node k; n is_kIs a mean of zero and a variance ofWhite gaussian noise.

And (4) carrying out recursion on the formula (3), wherein the process is as follows:

in the formula, y₀X is a signal transmitted by the source node;

1b) setting the modulation mode to BPSK and assuming the power normalization of the transmitting end, the signal y received by the destination node_NThe expression is as follows:

where i 1.. N, j 1.. N, N denotes the number of relay nodes, x denotes a BPSK signal transmitted from a source node, and g_i-1Represents the amplification gain of the signaling signal by node i-1, where g₀Representing the amplification gain of the source node on the transmission signal;I_irepresents the channel fading coefficients between node i-1 to node i, and

n_jis a mean of zero and a variance ofWhite gaussian noise.

1c) Therefore, when the relay node adopts the AF protocol, the signal-to-noise ratio gamma of the receiving end_eeComprises the following steps:

the amplification gain of each node cannot be arbitrary, because the power amplifier circuit in the actual system must work in an amplification state, so if the amplification gain is too large, the actual circuit cannot generate a signal meeting the power requirement. When the node sends a BPSK modulation signal x to the next node with the normalized transmission power, the signal received by the next node is:

y＝Ix+n (7)

with a power of

P＝I²+n²(8)

Therefore, the amplification gain should satisfy the following constraint:

g²P≤1 (9)

the upper bound of the amplification gain is therefore

1d) Setting the amplification gain at the repeater toThen the signal-to-noise ratio gamma of the receiving end under the power limiting condition is obtained_ee1Comprises the following steps:

1e) let the amplification gain of the node be the inverse of the channel fading coefficient of the previous hop, i.e. g_i＝1/I_iEqualizing the ith hop link, compensating the channel fading by amplifying the gain, ensuring that the ith hop relay has enough instantaneous output power and neglecting the noise interference under the condition that the gain of the channel of the previous hop is smaller, redefining the signal-to-noise ratio expression of the receiving end on the basis of the formula (6), namely, the signal-to-noise ratio gamma of the receiving end based on the channel information amplifying and forwarding protocol_ee2Comprises the following steps:

in the formula (I), the compound is shown in the specification,

wherein

In order to express the receiving end signal-to-noise ratio gamma in a form which is easier to process mathematically_ee2Using the known inequality between the geometric mean and the harmonic mean:

if and only if x₁＝...＝x_NWhen the equal sign in the inequality is established, the upper bound gamma of the signal-to-noise ratio of the receiving end under the amplification forwarding protocol based on the channel information can be obtained_bComprises the following steps:

as can be seen from the nature of the LN profiles,

is a desireSum varianceLognormal random variable of (1), wherein μ_jAndrespectively, channel fading coefficients I_jExpectation and variance of, then γ_bProbability density function ofAnd cumulative distribution function

Comprises the following steps:

wherein gamma is the signal-to-noise ratio of the receiving end based on the channel information amplification forwarding protocol system,and N is the number of system links, namely the average signal-to-noise ratio of a receiving end.

When determining the amplification gain at each node, it is necessary to estimate the channel fading coefficient of the previous hop by using a channel estimation technique, which also brings some extra complexity and makes it difficult to perform effective estimation under a fast fading channel. To further reduce overhead, the amplification gain at each node is set to a fixed value, and the upper bound of the amplification gain is analyzed, which should be:

the total fading gain at the destination node is:

1f) it is assumed that the destination node can estimate the amplitude value of the product of N channel coefficients well, but there is no or no resource for estimating the channel coefficients for each hop. In this case, the whole end-to-end communication system can be regarded as a whole formed by connecting N subsystems, so that the destination node receives the end signal-to-noise ratio gamma under the condition of fixed gain_ee3Can be expressed as the product of the squares of N independent lognormal random variables:

in the formula, N_TIs the accumulated noise power at the destination node.

When BPSK modulation is adopted, the conditional bit error rate of the system is as follows:

where erfc (-) is a complementary error function, and γ is a signal-to-noise ratio of the receiving end, the lower bound expression of the ABER of the serial relay FSO communication system is as follows:

wherein the content of the first and second substances,represents gamma_bIs determined by the probability density function of (a),represents gamma_bCumulative distribution function of a_mIs a Laguerre polynomialM heavy root of (2), H_mIs a_mCorresponding rightAnd (4) weight coefficient.

And 2), obtaining the optimal forwarding protocol at each relay node by using a self-adaptive protocol selection method, thereby realizing the purpose of reducing the complexity of the system while ensuring the average error rate performance requirement of the system.

The method for selecting the optimal forwarding protocol at each relay node by using the self-adaptive protocol selection method is obtained by the following steps:

2a) the source node is initialized and defined to transmit signals and all parameters related to the system. The parameters are the length L of each hop of the link of the relay system, the number Numb of signals sent by a sending end, the pre-modulation signals x sent by a source node, the hop number N of the link contained in the system, the size D of a receiving aperture and the logarithmic variance of light intensitySignal to noise ratio threshold gamma_thAnd the order n of the laguerre gaussian function.

2b) Transmitting a premodulation signal x from a source node to a next hop node;

2c) selecting a signal-to-noise ratio threshold value gamma corresponding to the acceptable average bit error rate of the system_th；

2d) Calculating the receiving signal-to-noise ratio gamma of the kth node, if gamma is less than gamma_thWhen the node k forwards the signal to the node k +1 through the decoding forwarding protocol, the signal received by the node k +1 is represented as

y_k+1＝I_k+1x+n_k+1(22)

Wherein X is a signal obtained after decoding the node k, and X belongs to X; x is the set of constellation points at which the signal is transmitted at the source node.

If gamma > gamma_thIf node k forwards the signal to node k +1 through the amplify-and-forward protocol based on the channel information, the signal received by node k +1 is represented as

2e) The method comprises the following steps And judging whether the algorithm reaches a termination condition. If the termination condition is met, calculating the average bit error rate of the system; otherwise, return to step 2 d).

Average bit error rate P of system available at receiving end_eComprises the following steps:

the correctness and advantages of the invention can be further illustrated by comparing the following theoretical results:

in the method, analog simulation verification is carried out through MATLAB.

Firstly, accurately describing the working principle of the serial relay free space optical communication system; then, simulating and researching the ABER performance of the proposed relay system based on the DF protocol and the system based on the AF protocol; thirdly, simulating the provided self-adaptive protocol selection method, analyzing and comparing the performance of the self-adaptive protocol selection method with the system ABER performance of the existing method; and finally, analyzing the influence of the actual transmission distance and the size of the receiving aperture on the performance of the system adopting the self-adaptive protocol selection method.

Theoretical and simulation results

FIG. 1 shows a model of a serial relay free space optical communication system including N hops; fig. 2 is a flow chart of an adaptive protocol selection method based on a serial relay free space optical communication system. Table 1 gives the parameters of the system simulation.

TABLE 1 simulation parameters table of system

Fig. 3 shows the ABER performance of FSO systems based on different forwarding protocols when BPSK modulation is used, and the ABER and SNR relation curves of serial relay FSO communication systems based on LN fading channel model including five hops (i.e., N ═ 5). In this simulation, the sizes of the receive aperture sizes are set to D3 mm and D25 mm, respectively, and the corresponding channel parameters are set to DAndand normalizing the transmission power of the source node. As can be seen from fig. 3, under the condition that the received signal-to-noise ratio is the same, the relaying system based on the DF protocol has the best ABER performance and the lowest ABER performance; the ABER performance of the relay system based on the fixed gain AF protocol is the worst, and the ABER is the highest; whereas the ABER performance of the AF protocol system based on channel information is intermediate. Further, the better the channel condition, the smaller the ABER performance gap between the fixed gain AF protocol based system and the channel information AF protocol based system, but the larger the performance gap between the DF protocol based system and the channel information AF protocol based system. This is because the better the channel conditions, the less the turbulence-induced fading and errors, and the absence of cumulative noise by the DF protocol, so the performance advantage is more pronounced in this case; on the other hand, good channel conditions also means that the fluctuation of each hop link is small, and the more the ABER curve of the system based on the fixed gain AF protocol approaches the ABER curve of the system based on the channel information AF protocol. Fig. 4 shows the simulated ABER curve and the lower bound of the theoretical ABER for the system under the power constrained AF protocol and the channel information based AF protocol. The system link hop number N is 5, and the channel parameters are respectivelyAndas can be seen from fig. 4, the larger the receive aperture, the closer the ABER curve of the channel information based AF protocol system is to its theoretical lower bound ABER curve. This is because a larger aperture means an expected E of the channel fading coefficient<I>The larger, the variance

The smaller the ABER performance gap between the fixed gain AF protocol system and the channel information based AF protocol system. Fig. 5a) and fig. 5b) show the performance comparison of the ABER forwarding system using the adaptive protocol selection method, based on DF protocol forwarding and based on channel information AF protocol forwarding under different channel parameters. Wherein, the hop count N of the system link is equal toFig. 5a), the receiving aperture D is 25mm, the channel parametersIn fig. 5b), the receiving aperture D is 3mm, the channel parametersAs can be seen from fig. 5a) and 5b), the ABER performance of the FSO system based on adaptive protocol selection and the ABER performance of the DF-based system are consistent at low signal-to-noise ratios; at high signal-to-noise ratio, the performance of the system ABER based on the self-adaptive protocol selection is consistent with the performance of the system ABER based on the channel information AF protocol. While at intermediate signal-to-noise ratios, the system ABER performance of adaptive protocol selection is intermediate between systems based on the channel information AF protocol and systems based on the DF protocol. It can also be observed that the system ABER for adaptive protocol selection is not smooth, i.e. it does not decrease with increasing signal-to-noise ratio, but fluctuates over a period of the signal-to-noise ratio interval. This is because of the critical signal-to-noise ratio threshold γ_thNearby if the received signal-to-noise ratio is less than gamma_thThen the node adopts the DF protocol to obtain better ABER performance; if the received signal-to-noise ratio is greater than gamma_thThe node will adopt the AF protocol to reduce the complexity of the system, so that although the received signal-to-noise ratio is higher than the point, the ABER performance of the system will be reduced. However, despite fluctuations, the ABER performance of the adaptive protocol is always higher than the system ABER performance of the channel information AF-based protocol. A comparison of the ABER performance of serial relay systems with different relay link hop counts based on different forwarding protocols is given in fig. 6. The system link hop number is respectively set as N-3 and N-10, the size of the receiving aperture is set as D-25 mm, and the threshold signal-to-noise ratio gamma is_th10 dB. As can be seen from fig. 6, as the number of link hops increases, the performance gap between the AF and DF protocols becomes larger, and at this time, the adaptive protocol algorithm can effectively improve the ABER performance of the system while ensuring low complexity.