CN103701562A - Error control protocol in free space optical communication system - Google Patents

Abstract

The error control protocol in the efficient and feasible free space optical communication system of the present invention, at the sending end: extract the messages to be sent from the host and group them, number the data frames in the group, and generate the random sequence generator The random number randomly selects multiple data frames for simple modulo 2 addition operation to obtain the encoded frame; at the receiving end: through photoelectric conversion, demodulates the output encoded frame, performs modulo 2 plus rolling decoding, restores the message and sends the packet to receive correctly ACK response. Its beneficial effects are: (1) No need to pre-estimate channel parameters, no need for feedback retransmission; solve the huge waste of channel resources and communication time caused by feedback retransmission in HARQ or ARQ protocols; (2) Perform simple modulo 2 addition operation on multiple data frames to realize the superposition of multiple frames of information, so that the transmitted coded frames have correlation; (3) Automatically eliminate interference and adapt to dynamic changes in the link, ensuring that the data is completely reliable transmission.

Description

Error Control Protocols in Free Space Optical Communication Systems

technical field

The present invention relates to the technical field of free-space optical communication transmission control, in particular to an error control protocol in a long-distance, high-speed laser communication system.

technical background

In recent years, with the rapid development of high-power laser manufacturing technology and fast and accurate acquisition, tracking and targeting (ATP) technology, satellite-ground high-speed laser communication technology has made significant progress. Currently, NASA is starting to implement the Lunar Laser Communication Demonstration Project (LLCD). And in 2013, NASA used laser beams to transmit the famous painting "Mona Lisa's Smile" to the "Lunar Reconnaissance Orbiter" flying around the moon. This is the first time that humans use lasers to transmit image data between stars, thus The feasibility of the two-way laser communication technology on the moon has been confirmed. Moreover, in the actual verification carried out recently, the LLCD device has achieved a download speed of 622 megabytes per second and an upload speed of 20 megabytes per second at a distance of 385,000 kilometers, thus creating a new record for the speed of interplanetary data communication . This download speed is more than 6 times faster than the maximum transmission rate of the most advanced radio communication means at present, and its upload speed is more than 5000 times faster than the current equipment. Laser communication will replace radio communication and become an inevitable trend of high-speed interstellar free space communication in the future.

Laser communication has become the first choice for high-speed interstellar free space communication in the future due to its advantages of high speed, long distance, and small devices, and channel error control technology is an extremely important and inseparable part of it. At present, the theoretical upper limit of the free space optical communication rate can reach 50G, but due to the influence of atmospheric turbulence, the link interruption during the communication process even reaches the millisecond level. Therefore, the forward error correction code (FEC) technology is limited by the code word length (the longest error correction code is only a few thousand bits, and the error correction capability is no more than 100 bits) and the coding rate is fixed. Automatic retransmission (HARQ, hybrid-ARQ) partly solves the above problems, but this solution has problems such as insufficient flexibility of the code rate, insufficient real-time support for channel changes, high decoding complexity, and the need for pre-estimation of channel parameters. Moreover, for long-distance laser communication, when the channel conditions are relatively bad, the number of retransmissions will increase greatly, and each retransmission will waste microseconds or even milliseconds of time, which means that about tens of megabytes or even several The loss of gigabit data, and the correctness of the data cannot be guaranteed every retransmission, this result is almost a nightmare for high-speed data communication. Therefore, designing an appropriate error control protocol is one of the most important technical issues that need to be solved urgently in long-distance laser communication.

the

Contents of the invention

The invention proposes an efficient and feasible error control protocol in the free space optical communication system aiming at the problem of communication interruption caused by factors such as atmospheric turbulence and optical transceiver platform shake in the free space optical communication.

Technical scheme of the present invention comprises:

On the sender side:

1) Extract the message to be sent from the host and group it,

2) The data frames in each group are numbered sequentially: 1, 2, ..., K,

3) Generate random seeds in K data frames, and assign random seed identification values,

4) Modulo 2 encoding is performed on the information field, address field and frame number identification of each random seed,

5) Afterwards, frame assembly is performed to obtain encoded frames,

6) Carry out optical carrier modulation on the coded frame and send it;

On the receiving end:

7) The optical receiver demodulates and outputs coded frames through photoelectric conversion,

8) Perform error correction decoding and information verification on the coded frame, if it passes the verification, keep the coded frame, otherwise discard it,

9) Extract and retain the random seed ID of the coded frame, cache the coded frame with the random seed ID value > 1,

10) Perform modulo 2 plus rolling decoding,

11) At the same time, judge whether all the data frames sent in the group are received correctly, if not, repeat the following steps 7) until all the data frames are received correctly,

12) According to the received data frame, restore the message and send an ACK response;

13) After receiving the ACK response, the sender stops sending the current packet data frame and retransmits the next group of message information.

In this solution, in order to obtain performance gain, the number of data frames in the packet needs to be selected to a certain extent. Extract the message to be sent from the host and group it, set the group number as: A1, A2,..., An, and the data frames in the group are sequentially numbered: 1, 2,..., K. For the message data obtained from the host, the data size is determined. If the number of data frames K in the packet is too small, the number of packets will be greatly increased. After sending each packet, the receiving end needs to send an ACK response. In this way This causes a huge increase in the number of responses in communication, thereby wasting communication time and channel resources. If the number K of data frames in the group is too large, the computing power and memory requirements of the system are too high, and it is difficult to realize. Therefore, in step 2), the value range of K is 100-50000.

In step 3), the process of random seed generation is: the random sequence generator generates a random positive integer a1 as the random seed identification value, and then selects evenly distributed random a1 data frames in the group as random seeds;

Among them, 1≤a1≤K.

In step 4), if the random seed identification value is equal to 1, modulo 2 encoding is not performed.

In step 5), the frame assembly includes: adding the random seed identification value and the control field to the data frame, then performing cyclic redundancy check error detection coding on the data frame, and then adding the data frame except the frame header and frame tail Error correction coding is performed on all other bits, and finally the frame header and frame tail are added to the data frame.

In step 10), firstly, find the encoded frame whose random seed identification value is 1, directly recover the encoded frame as the corresponding numbered data frame, and subtract 1 from the random seed identification value;

Secondly, for the coded frames whose random seed identification value > 1 in the cache, the coded frames associated with the random seed identification value of 1 are processed as follows: A. The data in the coded frame is updated as its own coded frame and the corresponding The value after the coded frame modulo 2 is added with a random seed identification value of 1; B. The random seed identification value of the encoded frame is subtracted by 1; finally, it is judged whether the random seed identification value of all encoded frames is 1, and if it is 1, the decoding is completed; otherwise , continue to receive optical signals, and repeat the above steps until the decoding is completed.

In this solution, in order to obtain performance gain, the frame structure needs to be set, and in the encoding frame structure,

Frame header: identifies the starting position of the data frame;

Random seed identification: a positive integer generated by the random sequence generator, which identifies how many data frames in this group are coded modulo 2 for this frame;

Frame sequence number identification: After the message data is grouped, the frame sequence identification in each group;

Control field: control information in the frame transfer protocol;

Address field: reserved address information;

Information field: the data information to be transmitted;

Error detection coding: perform error detection coding on other bits of the data frame except the frame header and frame tail;

Error correction coding: After detection and coding, error correction coding is performed on other bits of the data frame except the frame header and frame tail;

End of Frame: Identifies the cut-off position of the data frame.

The beneficial effects of the present invention are:

(1) It is suitable for free-space optical communication environments with high speed, harsh channel environment, high data loss and high error rate. It is characterized by no need for pre-estimation of channel parameters and no need for feedback retransmission.

(2) Multiple (greater than or equal to 1) data frames are randomly selected through the random number generated by the random sequence generator for simple modulo 2 addition operation to realize the superposition of multiple frames of information, so that the last encoded frames sent have a Correlation. When the channel condition is relatively bad, the use of hybrid automatic retransmission (HARQ, hybrid-ARQ) protocol will greatly increase the number of retransmissions, and each retransmission will waste microseconds or even milliseconds of time, which means About dozens of megabits or even several gigabits of data are lost, and the correctness of each retransmission data cannot be guaranteed; and the solution of the present invention does not require feedback retransmission, which solves the problem caused by the need for feedback retransmission in the HARQ or ARQ protocol. Huge waste of channel resources and communication time.

(3) Aiming at the complex space-time-varying characteristics and space-time domain related constraints faced by free-space optical communication, as well as the actual scene where the channel status of the transmitting and receiving ends is not completely known to each other, it can automatically eliminate the influence of interference and easily adapt to the link Dynamic changes ensure completely reliable data transmission.

Description of drawings

Fig. 1 is a flow chart of the error control protocol of the present invention.

Fig. 2 is a frame structure diagram of the present invention.

the

Detailed ways

Extract the message to be sent from the host and group it, and then number the data frames in each group as follows: 1, 2, ..., K, and the value of K is between 100 and 50000;

According to the random seed identification value a1 generated by the random sequence generator, use a uniformly distributed random sequence generator between 1 and K to generate a1 random numbers B1, B2,..., Ba1, select B1, B2,... , Ba1. Modulo 2 encoding is performed on the information field, address field (reserved) and frame number identifier in the a1 data frames (if the random seed identifier value a1 is equal to 1, modulo 2 encoding is not performed). Then add the random seed identification value a1 and the control field to the data frame, then perform cyclic redundancy check error detection coding on the data frame, and then perform error correction coding on all bits of the data frame except the frame header and frame tail, Finally, add the frame header and frame tail to the data frame to obtain the encoded frame.

The sending end performs optical carrier modulation on the coded frame and then sends it;

The optical receiver at the receiving end demodulates and outputs encoded frames through photoelectric conversion. Then extract the frame for error correction decoding and cyclic redundancy check, if it passes the check, keep the coded frame, otherwise discard it;

Next, extract the random seed ID of the coded frame. If the random seed ID value is 1, the message data frame can be directly recovered, and enter modulo 2 plus rolling decoding; otherwise, the coded frame is cached.

The modulo 2 plus rolling decoding process is as follows: First, find the coded frame whose random seed identification value is 1, and directly recover the coded frame as the data frame with the corresponding number, and subtract 1 from the random seed identification value. Secondly, for the coded frames whose random seed identifier value is greater than 1 in the cache, the coded frames associated with the random seed identifier value of 1 are processed as follows: A. The data in the coded frame is updated as its own coded frame and the corresponding The coded frame modulo 2 plus the value of the random seed identification value of 1; B. The random seed identification value of the coded frame minus 1. Finally, it is judged whether the random seed identification value of all encoded frames is 1: if it is 1, the decoding is completed; otherwise, the optical receiver continues to receive the optical signal, and repeats the above steps until the decoding is completed.

While performing modulo 2 plus rolling decoding, the receiving end judges whether all data frames in the group are received correctly. If not, the receiving end continues to receive signals and repeats the above steps until all data frames in the group are received correctly. According to the received data frame, the receiving end sends the recovery message to the upper host and sends an ACK response. After the sender receives the response, it stops sending the current packet data frame, and retransmits the next group of message information, repeating the above steps until the communication ends.

Claims (7)

1. the error control agreement in free-space optical communication system, comprising:

At transmitting terminal:

1) from main frame, extract and need the message sending and divide into groups,

2) to the Frame number consecutively in each grouping, be: 1,2,, K,

3) in K Frame, produce random seed, and give random seed ident value,

4) information field corresponding to each random seed, address field and frame number sign are carried out to mould 2 and add coding,

5) afterwards, carry out frame assembling, obtain coded frame,

6) coded frame is carried out sending after light carrier modulation;

At receiving terminal:

7) optical receiver passes through opto-electronic conversion, demodulation output encoder frame,

8) this coded frame is carried out to error-correcting decoding and information checking, if by verification, retains this coded frame, otherwise abandon,

9) extract the random seed sign that retains coded frame, the coding frame buffer of random seed ident value > 1 got off,

10) carry out mould 2 and add rolling decoding,

11) simultaneously, judge that whether all Frames that send in this grouping all correctly receive, if do not have, repeat 7) following steps, until all correctly receive,

12), according to the Frame receiving, recover message and send ACK to reply;

13) after transmitting terminal receives ACK and replys, stop the transmission of current grouped data frame, send next group message information elsewhere.

2. according to the error control agreement of claim 1, it is characterized in that: in step 2), the span of K is 100～50000.

3. according to the error control agreement of claim 1, it is characterized in that: in step 3), the process that random seed produces is: random sequence generator produces a random positive integer a1, as random seed ident value, then in this grouping, choose an equally distributed random a1 Frame as random seed;

Wherein, 1≤a1≤K.

4. according to the error control agreement of claim 1, it is characterized in that: in step 4), if random seed ident value equals 1, do not carry out mould 2 and add coding.

5. according to the error control agreement of claim 1, it is characterized in that: in step 5), described frame assembling comprises: add random seed ident value and control field to Frame, then Frame is carried out to cyclic redundancy check (CRC) error detection coding, then to Frame, all bits of other except frame head, postamble carry out error correction coding again, finally add frame head, postamble to Frame.

6. according to the error control agreement of claim 1, it is characterized in that: in step 10), first, find the coded frame that random seed ident value is 1, directly recover the Frame that this coded frame is reference numeral, and random seed ident value is subtracted to 1;

Secondly, coded frame to random seed ident value > 1 in buffer memory will be wherein that 1 coded frame being associated is done following processing with random seed ident value: the value after the coded frame mould 2 that in A. coded frame, Data Update is 1 for itself coded frame with the corresponding random seed ident value being associated adds; B. coded frame random seed ident value subtracts 1; Finally, judge whether all coded frame random seed ident values are 1, if 1 decoding completes; Otherwise, continue receiving optical signals, repeat above step, until decoding completes.

7. according to the error control agreement of claim 1, it is characterized in that: in coded frame structure,

Frame head: the original position of identification data frame;

Random seed sign: the positive integer that random sequence generator produces, identifies this frame and added to encode by how many Frame moulds 2 in this grouping and form;

Frame number sign: after message data divides into groups, the frame sequential sign in every grouping;

Control field: the control information in frame host-host protocol;

Address field: reserved address information;

Information field: the data message that needs transmission;

Error detection coding: other bits to this Frame except frame head, postamble carry out error detection coding;

Error correction coding: detect after coding, other bits to this Frame except frame head, postamble carry out error correcting coding;

Postamble: the rest position of identification data frame.

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