Free space optical communication digital fountain code encoding and decoding method supporting multi-user access

Abstract

The application discloses a free space optical communication digital fountain code encoding and decoding method supporting multi-user access, which comprises the following steps: each client encodes data by using a fountain code; each client performs channel sensing and listens to channel idle conditions; a communication client sends a handshake signal to reserve a channel; a receiving end sends a response signal; the communication client sends fountain code encoding information; the receiving end broadcasts decoding state information in real time; a hidden station and a non-hidden station dynamically update a deferred access time; the communication client continues to send encoding information and updates a timer; the receiving end sends a confirmation signal and stops sending decoding state information if decoding is successful; the communication client receives the confirmation signal or times out and judges that the receiving end satisfies decoding conditions, and communication ends. In the application, the fountain code algorithm is adopted in data transmission of multi-user access, reliable transmission in a high dynamic and large attenuation environment is realized, and the receiving end broadcasts decoding state information to dynamically update the deferred access time, so that channel fairness is ensured.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of optical communication technology, and in particular to a free space optical communication digital fountain code encoding and decoding method supporting multi-user access. BACKGROUND

[0002] Free space optical communication (FSO) refers to a communication technology that uses light waves as a carrier to transmit information in deep space, atmosphere or underwater. According to the carrier transmission channel, it can be divided into atmospheric, inter-satellite, satellite-ground and underwater optical communication. Free space optical communication has the advantages of ultra-high bandwidth, strong anti-interference capability, good security, small device size, light weight, frequency application exemption, low power consumption, strong mobility, low cost, and flexible networking, etc. It is an important research and development direction in the future wireless communication field.

[0003] In traditional free space optical communication applications, it is extremely susceptible to the influence of atmosphere, seawater, aerosol, organic matter and load platform jitter, etc., thereby causing problems such as signal attenuation, multipath effect, flicker and beam drift, etc., ultimately affecting the stability and reliability of the system. Even in deep space optical communication, atmospheric optical communication, or underwater optical communication, there is a high probability of interruption. In atmospheric optical communication, when using the atmosphere as the channel, the characteristics of the atmosphere pose multiple challenges to the reliability and stability of the FSO communication system: when laser propagates in the atmosphere, it will be affected by the absorption and scattering effects of gas molecules and aerosol particles, resulting in optical attenuation; in addition, atmospheric turbulence due to the non-uniformity of temperature and pressure in the air causes beam drift, flicker, angle of arrival fluctuation and wavefront distortion, especially in strong turbulence conditions, the optical signal can be severely disturbed or even deviated from the target, thereby causing high bit error rate and short-time communication interruption.

[0004] Digital fountain code, also known as rateless code (RC), is a code without coding rate constraint. Its adaptive link rate adaptation and rateless property (stream property) enable it to automatically eliminate the influence of interference without requiring the transmitting and receiving parties to know any channel state information and easily adapt to link dynamic changes. Research shows that using digital fountain code in free space point-to-point optical communication can greatly improve the bit error performance of free space optical communication and further improve the system throughput in long-distance and high-dynamic environments, providing a new technical approach to effectively counteract time-domain disturbances on the channel and establish a channel compensation mechanism.

[0005] However, the digital fountain code needs to be improved in terms of coding and transmission method when applied to the free space optical communication network supporting multi-user access, because in the existing digital fountain code scheme, the receiving end sends feedback information to the sending end only after decoding is successful, and the sending end starts encoding and sending the next information segment after receiving the feedback information, which leads to that the sending and receiving ends cannot know the time delay of communication in real time. This problem that the sending and receiving ends do not know any channel state information does not have too much impact on point-to-point optical communication.

[0006] However, in the free space optical communication network supporting multi-user access, the unknown and dynamically changing sending time will lead to that other users cannot predict the access time, so they cannot effectively judge when to compete for the channel, thereby affecting the fairness of channel competition. In a wireless communication network, there are mainly four types of users: communication clients, receiving ends, hidden stations and non-hidden stations. In a wireless communication network, the method for solving the problem of hidden stations is to broadcast through the request to send / clear to send mechanism (RTS / CTS) to inform the hidden stations of the specific deferred access time. Due to the directivity of the light beam, the problem of hidden stations is particularly prominent in free space optical network communication. When the fountain code algorithm is used in the data transmission part to ensure transmission, the hidden stations cannot receive any information sent by the communication clients, so they cannot receive the information required for updating the encoded data, thereby causing data conflict and reducing transmission efficiency. SUMMARY

[0007] To solve the above problems, the present application provides a free space optical communication digital fountain code coding and decoding method supporting multi-user access. The present application uses the fountain code algorithm to solve the problem of data transmission quality decline caused by high dynamics and large loss in the communication environment of free space optical communication, while supporting multi-user access, informing other clients of the deferred access time in real time through the broadcast of decoding state information by the receiving end, to ensure the fairness of channel competition, and solving the problem of hidden stations which is particularly prominent in multi-user free space optical communication.

[0008] The technical scheme adopted by the present application to solve the technical problems is:

[0009] A free space optical communication digital fountain code coding and decoding method supporting multi-user access, comprising the following steps:

[0010] S1, each client divides the data unit to be sent into K data subunits, and encodes the K data subunits by using the fountain code;

[0011] S2, each client performs channel sensing to confirm the idle state of the channel;

[0012] S3, the client sensing that the channel is idle starts to perform the backoff algorithm, the client whose backoff ends first is the communication client, sends the handshake signal to reserve the channel; the hidden station client or the non-hidden station client sensing that the channel is busy freezes the backoff counter, performs the deferred access operation, ensures the fairness of multi-user access, and returns to S2 to sense the channel again after the deferring ends;

[0013] S4, after the receiving end receives the handshake signal, immediately sends the response signal; after the non-hidden station client receives the handshake signal, performs the deferred access;

[0014] S5, after the hidden station client and the non-hidden station client receive the response signal, update the deferred access time and perform the deferred access operation; if the communication client does not receive the response signal within the timeout, the communication client performs the backoff algorithm and returns to S2 to sense the channel again; if the communication client receives the response signal before the timeout, the communication client establishes the communication connection with the receiving end and starts to send the fountain code encoded information;

[0015] S6, after the receiving end receives the fountain code encoded information sent by the communication client, determines the number X of information to be correctly received according to the number N of correctly received encoded information required for decoding, X < N, and broadcasts the current decoding state information in real time and in all directions;

[0016] S7, the hidden station client and the non-hidden station client dynamically update the deferred access time through the decoding state information;

[0017] S8, the communication client updates the timeout timer through the decoding state information, if the communication client does not receive the decoding state information within the timeout, and judges that the receiving end does not reach the decoding condition according to the X value in the decoding state information received last time, the communication connection established by the communication client and the receiving end is disconnected, the communication client stops sending the encoded information and returns to S2 to start sensing the channel again;

[0018] S9, if the decoding of the fountain code fails, the number N of encoded information required for decoding is increased, the encoded information is continuously received, and the next decoding is waited for; if the decoding succeeds, the recovered data is accumulated and verified, if the accumulation and verification pass, the confirmation information is immediately sent, and the decoding state information is no longer sent;

[0019] S10, after the communication client receives the confirmation signal, stops sending the fountain code encoded information, and the communication ends; if the confirmation signal is not received, the fountain code encoded information is continuously sent, if the timeout occurs and the receiving end meets the decoding condition according to the X value in the decoding state information received last time, the encoded information is stopped, and the communication ends.

[0020] Technical features and remarkable effects of the application:

[0021] (1) The present invention introduces the fountain code algorithm to solve the problem of unreliable data transmission in the high dynamic and high loss free space optical communication environment. By dynamically generating redundant data packets, the anti-interference ability and data transmission reliability of the system are enhanced. Even if the beam is interfered with or some data is lost, the data can be successfully recovered.

[0022] (2) The present invention dynamically updates the delayed access time of other clients by means of the decoding status information of the receiver, and further solves the problem of hidden stations, which is particularly prominent in multi-user optical communication, by means of broadcasting, and ensures the fairness of the channel; the decoding status information is determined by the amount of encoded information received by the receiver and the decoding status of the fountain code. By means of the received encoded information and the decoding status of the fountain code, the duration of the encoded information that the two parties in the current communication need to transmit in the next period of time can be predicted, and it is updated in real time by means of broadcasting. Attached Figure Description

[0023] Figure 1 This is a schematic diagram illustrating the communication between the communication client, the covert station, the uncovered station, and the receiving end in an embodiment of the present invention.

[0024] Figure 2 This is a schematic diagram of time slicing for multi-user access in an embodiment of the present invention;

[0025] Figure 3 This is a schematic diagram of the fountain code encoding and decoding method in free-space optical communication supporting multi-user access in an embodiment of the present invention. Detailed Implementation

[0026] The present invention will be further described below with reference to the embodiments and accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0027] Example:

[0028] like Figure 1 As shown, a system supporting multi-user access in free-space optical communication digital fountain code encoding and decoding includes receiver D, client A, client B, and client C. Figure 1The dashed gray box represents the optical signal emitted by the communication client. Both the client and the receiving end covered by the dashed gray box can receive the information sent by the communication client. The solid gray box represents the optical signal emitted by a non-communication client. To better illustrate the content of this invention, client A is the communication client, client B is the covert station, and client C is the non-covert station. The relative positions of communication client A, non-covert station client C, and covert station client B are such that the light beam of communication client A can illuminate the receiving surface of receiving end D and the receiving surface of non-covert station client C, but cannot illuminate the receiving surface of covert station client B. Under this environment, the communication process between communication client A and receiving end D, and the retreat of other stations, is as follows:

[0029] like Figure 2 Figure 3 As shown, the encoding and decoding method for digital fountain codes in free-space optical communication that supports multi-user access includes the following steps:

[0030] Step 1: Clients A, B, and C all have data waiting to be sent to receiver D. Clients A, B, and C divide their respective data into K equal-sized data sub-units, then encode these K data sub-units using fountain codes, and store the encoded data in a buffer, waiting to be sent.

[0031] Step 2: Clients A, B, and C perform channel sensing to confirm the channel's idle status;

[0032] Step 3: After sensing that the channel is idle for a period of time, clients A, B, and C begin executing the backoff algorithm. Figure 2 Once client A's backoff ends, it sends a handshake signal to reserve the channel, thus becoming the communication client. Clients B and C detect that the channel is busy. Client B becomes a hidden station and client C becomes a non-hidden station. The backoff counter is frozen, and access is delayed to ensure fairness for multiple users. After the delay ends, the process returns to step 2 to re-detect the channel.

[0033] Step 4: After receiving the handshake signal, the receiving end D immediately broadcasts a response signal; after receiving the handshake signal, the non-covert station client C delays access for a period of TS+N*T. D Where TS is the time between receiving the handshake signal and receiving the response signal at the non-covert station, N is the number of encoded information that the receiver needs to receive for fountain code decoding, and T D The time for transmitting, receiving, and processing encoded information;

[0034] Step 5: After receiving the broadcast response signal, the covert station client B and the non-covert station client C update the delayed access time and perform a delayed access operation. The delayed access time is N*T. DIf the communication client A does not receive the response signal, the communication client A performs a backoff algorithm and returns to step 2 to re-perceive the channel; if the communication client A receives the response signal, a communication connection with the receiving end D is established, and the fountain code encoded information is started to be transmitted, the fountain code encoded information is continuously generated, the first transmitted encoded information is marked as encoded information 1, and the N+nth transmitted encoded information is marked as encoded information N+n;

[0035] Step 6, when the receiving end D receives the fountain code encoded information transmitted by the communication client A, the number X of remaining information to be correctly received is determined according to the number N of correctly received encoded information required for decoding, X < N, and the current decoding state information is broadcast in real time and in all directions, Figure 2 wherein when the number N+n of correctly received encoded information is greater than or equal to N, n is a positive or negative number, X in the decoding state information is always 0;

[0036] Step 7, when the concealed station client B or the non-concealed station client C receives the broadcast decoding state information, the deferred access time is dynamically updated; as shown in Figure 2 , the deferred access time is related to the number X of information in the received decoding state information, Y is a threshold value of the number of encoded information preset according to the allowed interruption time between the communication client A and the receiving end D, the size of Y reflects the length of the interruption time, when Y < X < N, it indicates that the remaining transmission time between the communication client A and the receiving end D is greater than the interruption time, at this time, the deferred time is mainly determined by the remaining transmission time, and the deferred time is X*T D ; when 0 < X ≤ Y, it indicates that the remaining transmission time between the communication client A and the receiving end D is less than or equal to the interruption time, and the deferred time is determined by the interruption time, the deferred time of the concealed station and the non-concealed station is Y*T D ; when X = 0, it indicates that the number of received encoded information of the receiving end D meets the decoding condition, and decoding is started, after the concealed station client B and the non-concealed station client C receive the decoding state information, the updated value of the deferred access time is always TE, TE refers to the time from the reception of the last decoding state information to the reception of the confirmation signal by the concealed station or the non-concealed station;

[0037] Step 8, the communication client A updates the timeout timer through the correct reception of the decoding state information before the timeout, if the communication client A does not receive the decoding state information within the timeout, and the number X of information in the last received decoding state information is not 0, it indicates that the number of received encoded information of the receiving end does not meet the decoding condition, the communication connection between the communication client A and the receiving end D is disconnected, the communication client A stops transmitting the encoded information, and returns to step 2 to start channel perception again;

[0038] Step 9, if the receiving end D fails to decode the fountain code, the number of encoded information required for decoding N is increased, the encoded information is continuously received, the number of information X remaining to be correctly received is updated, the decoding state information is filled in, and real-time omnidirectional broadcasting is performed, and the next decoding is waited for; if the decoding is successful, the recovered data is accumulated and checked, and if the accumulation and checking is passed, the confirmation information is immediately sent, and the decoding state information is no longer sent;

[0039] Step 10, when the communication client A receives the confirmation signal, the sending of the fountain code encoded information is stopped, and the communication is ended; if the confirmation signal is not received, the sending of the fountain code encoded information is continuously performed, and after the timeout, according to the saved X=0 in the receiving decoding state information, it is judged that the receiving end D satisfies the decoding condition, the communication client A stops sending the encoded information, and the communication is ended.

[0040] The preferred embodiments of the application disclosed above are only used to illustrate the application, and do not limit the application to the specific embodiments described. Obviously, according to the content of the specification, many modifications and changes can be made. The specification selects and specifically describes these embodiments in order to better explain the principles and practical applications of the application, so that those skilled in the art can well understand and utilize the application.

Claims

1. A method for encoding and decoding digital fountain codes for free space optical communication supporting multi-user access, characterized in that, The method comprises the following steps: S1, each client divides the data unit to be sent into K data subunits, and encodes the K data subunits by using a fountain code; S2, each client performs channel sensing to confirm a channel idle state; S3, a client sensing a channel idle state starts to perform a backoff algorithm, a client first ending the backoff as a communication client sends a handshake signal to reserve a channel; a hidden station client or a non-hidden station client sensing a channel busy state freezes a backoff counter and performs a deferred access operation, and after the deferred access operation ends, the client returns to S2 to re-sense the channel; S4, after receiving the handshake signal, the receiving end immediately sends a response signal; After receiving the handshake signal, the non-hidden station client performs a deferred access operation; S5, after receiving the response signal, the hidden station client and the non-hidden station client update a deferred access time and perform a deferred access operation; if the communication client does not receive the response signal within a timeout period, the communication client performs a backoff algorithm and returns to S2 to re-sense the channel; if the communication client receives the response signal before the timeout period, the communication client establishes a communication connection with the receiving end and starts to send fountain code encoded information; S6, after receiving the fountain code encoded information sent by the communication client, the receiving end determines a number X of remaining information to be correctly received according to a number N of correctly received encoded information required for decoding, X < N, and broadcasts current decoding state information in real time in all directions; S7, the hidden station client and the non-hidden station client dynamically update a deferred access time according to the decoding state information; S8, the communication client updates a timeout timer according to the decoding state information; if the communication client does not receive the decoding state information within a timeout period and judges that the receiving end does not meet a decoding condition according to an X value in the last received decoding state information, the communication client disconnects the communication connection with the receiving end, stops sending encoded information, and returns to S2 to start to re-sense the channel; S9, if the receiving end fails to decode the fountain code, the receiving end increases the number N of encoded information required for decoding, continues to receive encoded information, and waits for the next decoding; If the decoding is successful, the recovered data is accumulated and verified; if the accumulation and verification are passed, confirmation information is immediately sent, and no decoding state information is sent any more; S10, after receiving the confirmation signal, the communication client stops sending the fountain code encoded information, and the communication ends; if no confirmation signal is received, the communication client continues to send the fountain code encoded information; if a timeout period is exceeded and the receiving end meets the decoding condition according to the X value in the last received decoding state information, the communication client stops sending the encoded information, and the communication ends.

2. The method of claim 1, wherein, In S4, the non-cloaked station client receives the handshake signal and performs deferred access, and the deferred time is TS+N*T D , wherein TS is the time between the non-cloaked station receiving the handshake signal and receiving the response signal, N is the number of encoded information required to be received by the receiving end for decoding the fountain code, and T D is the transmission, reception and processing time of one encoded information.

3. The method of claim 1, wherein, In S5, the concealed station client and the non-concealed station client receive the broadcasted response signal, update the deferred access time, and perform the deferred access operation, with the deferred time being N*T D , N being the number of encoded information required to be received by the receiving end for decoding the fountain code, and T D being the transmission, reception and processing time of one encoded information. If the communication client does not receive the response signal within a timeout period, the communication client performs a backoff algorithm and returns to S2 to re-sense the channel; If the communication client receives the response signal, the communication client establishes a communication connection with the receiving end and starts to send fountain code encoded information; fountain code encoded information is continuously generated, and the first encoded information is marked as encoded information 1, and the N+nth encoded information is marked as encoded information N+n.

4. The method of claim 1, wherein, In S7, when the covert station client or the non-covert station client receives the broadcasted decoding state information, the access delay time is dynamically updated; the access delay time is related to the number X of information in the received decoding state information; when YXN, it indicates that the remaining transmission time between the communication client and the receiving end is greater than the interruption time, at this time the delay time is mainly determined by the remaining transmission time, and the delay time is X*T D , N is the number of encoded information required to be received by the receiving end for decoding the fountain code, T D is the transmission, reception and processing time of an encoded information, and Y is a threshold value of the number of encoded information preset according to the allowed interruption time between the communication client and the receiving end; when 0XY, it indicates that the remaining transmission time between the communication client and the receiving end is less than or equal to the interruption time, the delay time is determined by the interruption time, and the delay time of the covert station and the non-covert station is Y*T D ; When X=0, it means that the number of received encoded information has met the decoding condition, and decoding is started. After the client of the concealed station and the client of the non-concealed station receive the decoding status information, the updated value of the access time is always TE, which refers to the time between the reception of the last decoding status information by the concealed station or the non-concealed station and the reception of the acknowledgement signal.

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