CN115276796A - Ladder code based atmospheric optical transmission method with super-Nyquist rate - Google Patents

Abstract

The invention belongs to the technical field of super-Nyquist transmission technology and channel coding, and particularly relates to a super-Nyquist-rate atmospheric optical transmission method based on a step code. At a transmitting end, step code coding adopts BCH codes as component code words, and then super-Nyquist signals are sent to an atmosphere channel after the super-Nyquist signal is obtained by interleaving, PAM modulation and a super-Nyquist shaping filter; at a receiving end, after the received signal is subjected to super-Nyquist sampling, deinterleaving and PAM demodulation, iterative decoding is carried out through a ladder code decoder, and then binary bit stream is recovered. Compared with an uncoded super-Nyquist atmosphere optical communication system, the method and the device have the advantages that the error code performance of the system is obviously improved under the condition of lower complexity, and the communication quality of the system is improved.

Description

A Super-Nyquist Rate Atmospheric Light Transmission Method Based on Echelon Codes

technical field

The invention belongs to the field of super-Nyquist transmission technology and channel coding technology, and specifically relates to a super-Nyquist rate atmospheric light transmission method based on ladder codes.

Background technique

The 6G era has officially kicked off. In order to meet the diverse business needs of various communication scenarios, people have put forward higher requirements for communication quality and data transmission rate. Wireless optical communication, also known as free space optical communication (Free Space Optical, FSO), is a two-way communication technology that uses light to transmit information in atmospheric channels. It has the advantages of large bandwidth, license-free spectrum, and high data rate, and has been widely used. Applications. However, the transmission performance of the system is seriously deteriorated due to the effect of atmospheric turbulence. Super Nyquist (Faster than Nyquist, FTN) transmission technology, as a new type of non-orthogonal transmission technology, compresses the transmission symbol interval in the time domain and overlaps and sends multi-stream data within one symbol period to obtain higher data Transmission rate. For this reason, the FTN technology is introduced to improve the transmission performance of the free space optical communication system.

The current research on FTN technology has achieved fruitful results. Its research mainly focuses on adopting some techniques to eliminate the inter-symbol interference (Inter-Symbol Interference, ISI) that is inevitably introduced due to the destruction of the orthogonality between signals, such as interference elimination, Turbo equalization and minimum mean square error (Minimum Mean SquareError, MMSE) equalization and other technologies. However, in actual transmission channels, FTN technology is not only interfered by ISI, but also by channel noise, especially in turbulent channels. Channel coding can greatly improve the reliability of communication systems. Therefore, the FTN system can use channel coding technology to ensure reliable signal transmission.

At present, the commonly used channel coding mainly focuses on low-density parity-check (LDPC) codes. Although LDPC codes have better performance, they also have the cost of high decoding complexity, corresponding to high delay , high power consumption and high cost, which also limit the application of LDPC codes in high-speed and low-cost optical communication systems. In this context, Staircase Codes (SCC) with low complexity and high decoding performance have attracted attention rapidly as soon as they appeared in the field of optical communication.

Therefore, the present invention provides an atmospheric light transmission method based on ladder codes at a super-Nyquist rate.

Contents of the invention

In order to make up for the deficiencies of the prior art and solve the above-mentioned problems, the present invention proposes an atmospheric light transmission method based on ladder codes at a super-Nyquist rate.

The technical scheme adopted by the present invention to solve its technical problems is: a kind of method of atmospheric light transmission based on ladder codes at a super-Nyquist rate according to the present invention, combining ladder code technology with super-Nyquist technology, in The transmission rate of the atmospheric optical communication system is increased under the exponential Weibull turbulent channel, and the computational complexity is reduced under the condition that the bit error performance of the system remains unchanged. The specific steps are as follows:

Step 1: At the sending end, the binary bit sequence is first encoded with ladder code, the binary bit sequence is divided into several groups, loaded into several blocks of the ladder code, and each row in the block is encoded by the component code word of the BCH code , so as to obtain the check bit of each row, form a complete ladder code block including information bits and check bits, and complete the ladder code encoding. After passing through the interleaver, the signal is PAM modulated, and the modulated signal is sent to the super-Nyquist shaping filter to obtain the transmitted signal s(t), namely:

In the formula, s _n represents the nth information symbol after mapping, g(t) represents the impulse response of the super-Nyquist shaping filter, T represents the cycle time of each pulse, τ(0<τ≤1) represents Super Nyquist acceleration factor; load the formed super Nyquist signal on the laser carrier, and send it into the atmospheric turbulence channel obeying the exponential Weibull distribution through the laser diode;

Step 2: At the receiving end, after the optical signal passes through the turbulent channel, it is converted into an electrical signal by the photodetector. Assuming that the received signal is y(t), there is

y(t)=ξhs(t)+z(t), (2)

In the formula, h is the channel fading coefficient, ξ is the photoelectric conversion efficiency, z(t) is the additive white Gaussian noise with a mean value of 0 and a variance of ^σ2 ; the signal is sampled by a super-Nyquist sampler, and then passed PAM demodulation and deinterleaving; then use the sliding window decoding method to decode the ladder code, and use the bounded-distance decoding (Bounded-Distance Decoding, BDD) in the sliding window to iterate the row corresponding to each component codeword Decoding; assuming that the length of the sliding window is L, first assist an all-zero block, and then receive L-1 stepped code blocks from the channel, and combine them into a sliding window with a length of L for iterative decoding. When the number of iterations reaches the maximum When , output the first block in the sliding window. Then, receive a new ladder code block, continue to combine into a sliding window of length L for iterative decoding; and so on, continue this process, output all ladder code blocks in turn, and restore them to binary after decoding sequence of bits.

The beneficial effects of the present invention are as follows:

1. A kind of super-Nyquist rate atmospheric optical transmission method based on ladder code of the present invention has improved FTN-FSO system bit error performance; FTN technology can improve the transmission rate of FSO system, but because the signal of this technology The orthogonality among them is destroyed, and intersymbol interference is inevitably introduced, which seriously affects the reliability of the system and reduces the bit error performance of the system. The invention combines ladder code and FTN technology and applies it in FSO system. Compared with the uncoded FTN-FSO system, the present invention effectively improves the bit error performance of the system, and reduces the influence of intersymbol interference and channel noise interference on the system.

2. A kind of super-Nyquist rate atmospheric light transmission method based on ladder codes of the present invention can realize complexity with lower system; Commonly used soft decision forward error correction (Forward Error Correction, FEC) technology Although the performance of LDPC code is better than that of hard-decision FEC technology under the same code rate and code length, it also pays the price of high decoding complexity, corresponding to problems such as high delay, high power consumption and high cost, which limits LDPC. The application of codes in high-speed and low-cost optical communication systems. As a new type of hard-decision FEC technology, ladder codes have the advantages of low complexity and high performance. The invention utilizes the BDD algorithm at the receiving end to iteratively decode the row corresponding to each component code word, and the BDD is a hard-decision decoder for linear block codes, thereby having lower system implementation complexity.

Description of drawings

The present invention will be further described below in conjunction with accompanying drawing.

Figure 1 is a block diagram of the SCC-FTN-FSO system;

Figure 2 is a comparison diagram of bit error performance of FTN-FSO system with or without SCC coding;

Figure 3 is the bit error performance diagram of the FTN-FSO system with or without SCC coding under different turbulence intensities;

Figure 4 is a bit error performance diagram of the SCC-FTN-FSO system under different receiving apertures;

Figure 5 is a bit error performance diagram of the SCC-FTN-FSO system under different acceleration factors;

Detailed ways

In order to make the technical means, creative features, goals and effects achieved by the present invention easy to understand, the present invention will be further described below in conjunction with specific embodiments.

As shown in Fig. 1 to Fig. 5, a kind of step code-based super-Nyquist rate atmospheric light transmission method described in the present invention combines step-code technology with super-Nyquist technology, and in exponential Weibull turbulence The transmission rate of the atmospheric optical communication system is increased under the channel, and the computational complexity is reduced under the condition that the error performance of the system remains unchanged. The specific steps are as follows:

Step 1: At the sending end, the binary bit sequence is first encoded with ladder code, the binary bit sequence is divided into several groups, loaded into several blocks of the ladder code, and each row in the block is encoded by the component code word of the BCH code , so as to obtain the check bit of each row, form a complete ladder code block including information bits and check bits, and complete the ladder code encoding. After passing through the interleaver, the signal is PAM modulated, and the modulated signal is sent to the super-Nyquist shaping filter to obtain the transmitted signal s(t), namely:

In the formula, s _n represents the nth information symbol after mapping, g(t) represents the impulse response of the super-Nyquist shaping filter, T represents the cycle time of each pulse, τ(0<τ≤1) represents Super Nyquist acceleration factor; load the formed super Nyquist signal on the laser carrier, and send it into the atmospheric turbulence channel obeying the exponential Weibull distribution through the laser diode;

Step 2: At the receiving end, after the optical signal passes through the turbulent channel, it is converted into an electrical signal by the photodetector. Assuming that the received signal is y(t), there is

y(t)=ξhs(t)+z(t), (2)

In the formula, h is the channel fading coefficient, ξ is the photoelectric conversion efficiency, z(t) is the additive white Gaussian noise with a mean value of 0 and a variance of ^σ2 ; the signal is sampled by a super-Nyquist sampler, and then passed PAM demodulation and deinterleaving; then use the sliding window decoding method to decode the signal ladder code, and use the limited distance decoding in the sliding window to iteratively decode the row corresponding to each component code word; assuming the sliding window The length is L, an all-zero block is first assisted, and then L-1 stepped code blocks are received from the channel, and combined into a sliding window with a length of L for iterative decoding. When the number of iterations reaches the maximum, the output in the sliding window first block. Then, receive a new ladder code block, and continue to combine it into a sliding window of length L for iterative decoding; and so on, continue this process, output all ladder code blocks in turn, and restore them to binary after decoding sequence of bits.

The invention is an atmospheric light transmission method based on ladder codes with a super-Nyquist rate. The present invention describes the specific implementation of the SCC-FTN-FSO system, and other FTN-FSO systems can be implemented according to this principle. The present invention will be described in detail below with specific embodiments in conjunction with the accompanying drawings.

The present invention achieves through following technical measures:

1. Basic assumptions:

The present invention assumes that the channel state obeys exponential Weibull distribution, assumes that background light has been filtered out by a filter, and only considers additive Gaussian white noise. This assumption is typical for such systems and not a specific requirement of the present invention.

2. Specific implementation steps:

Step 1: At the sending end, after step code encoding and interleaving are performed on the binary bit sequence, the continuous m bits are mapped to the symbol s _n at time n to form an M-order pulse amplitude modulation signal (that is, M-PAM modulation, M= ^2m ). After passing through the FTN shaping filter, the FTN signal s(t) is obtained, and sent to the atmospheric turbulence channel by the laser diode. Assume that the sent signal is

In the formula, s _n represents the nth information symbol after mapping, g(t) represents the impulse response of the FTN shaping filter, T represents each pulse time, and τ (0<τ≤1) represents the acceleration factor.

Step 2: At the receiving end, after the optical signal passes through the turbulent channel, it is converted into an electrical signal by the photodetector. Assuming that the received signal is y(t), there is

y(t)=ξhs(t)+z(t), (4)

In the formula, h is the channel fading coefficient, ξ is the photoelectric conversion efficiency, z(t) is the additive white Gaussian noise with a mean value of 0 and a variance of ^σ2 . In the atmospheric turbulence channel, h obeys the exponential Weibull distribution, and its probability density function and cumulative function are respectively

In the formula, α and β are the shape parameters related to the scintillation index SI, and η is the scale parameter related to the average value of the irradiance. After y(t) is sampled by FTN, it is restored into a binary data bit stream after PAM demodulation, deinterleaving and SCC decoding.

Simulation

In order to further illustrate the correctness of the present invention and the impact of parameters such as turbulence intensity, receiving aperture, and other parameters on the bit error performance of the system, the Monte Carlo method is used to simulate and analyze the bit error performance of the present invention;

Simulation parameters: Assume that the receiving end knows the channel state information, and the photoelectric conversion efficiency ξ=0.5. In weak turbulence, the receiving aperture 0mm corresponds to α=4.89, β=1.03, η=0.46; 25mm corresponds to α=3.67, β=1.97, η=0.73; 60mm corresponds to α=1.69, β=8.27, η=1.00; 80mm corresponds to α=1.01, β=19.27, η=1.03; in medium turbulent flow, receiving aperture 0mm corresponds to α=5.93, β=0.46, η=0.11; 25mm corresponds to α=5.37, β=0.81, η=0.33; 60mm corresponds to α= 3.47, β=2.20, η=0.77; 80mm corresponds to α=2.52, β=4.06, η=0.92; in strong turbulent flow, receiving aperture 0mm corresponds to α=5.94, β=0.46, η=0.11; 25mm corresponds to α=5.50, β=0.74, η=0.29; 60mm corresponds to α=4.80, β=1.08, η=0.48; 80mm corresponds to α=4.39, β=1.34, η=0.58. The ladder code rate is 0.5, and the corresponding component code C is BCH(88,66,3); the ladder code decoding window length is 9, and the number of iterations is 7.

Simulation results

Figure 2 is a comparison chart of the bit error performance of the FTN-FSO system with or without SCC coding under weak turbulence, where the abscissa indicates the signal-to-noise ratio in decibels (dB), and the ordinate indicates the bit error rate. The solid line with the symbol "●" represents the bit error performance of the SCC-FTN-FSO system; the solid line with the symbol "■" represents the SCC-FTN-FSO system.

It can be seen from Figure 2: Compared with the performance of the uncoded FTN-FSO system, the introduction of the ladder code effectively reduces the bit error rate of the received signal. When BER=10 ^-5 , SCC can produce 11.053dB gain.

Figure 3 shows the influence of different turbulence intensities on the bit error performance of the system, where the abscissa indicates the signal-to-noise ratio in decibels (dB), and the ordinate indicates the bit error rate. The solid line with symbol "▼" represents the bit error performance of SCC-FTN-FSO system under the influence of weak turbulence; the solid line with symbol "■" represents the bit error performance of SCC-FTN-FSO system under the influence of moderate turbulence; The solid line of "●" represents the bit error performance of the SCC-FTN-FSO system under the influence of strong turbulence; the dotted line with the symbol "▼" represents the bit error performance of the FTN-FSO system under the influence of weak turbulence; the dotted line with the symbol "■" Represents the bit error performance of the FTN-FSO system under the influence of moderate turbulence; the dotted line with the symbol "●" represents the bit error performance of the FTN-FSO system under the influence of strong turbulence.

It can be seen from Figure 3 that 1) no matter whether the FTN-FSO system is coded or not, the bit error performance of the system is the best under weak turbulence, followed by medium turbulence, and the worst performance under strong turbulence. This is because the stronger the turbulence intensity, the more noise interference is introduced. 2) The BER performance of the SCC-FTN-FSO system is better than that of the uncoded FTN-FSO system at any turbulence intensity. When BER=10 ^-5 , compared with the uncoded system, the step code obtains a performance gain of 16.01dB under moderate turbulence intensity.

Figure 4 shows the influence of different receiving apertures on the bit error performance of the SCC-FTN-FSO system under strong turbulence, where the abscissa represents the signal-to-noise ratio in decibels (dB), and the ordinate represents the bit error rate. The solid line with the symbol "▼" represents the bit error performance of the system under the influence of the receiving aperture of 80mm; the solid line with the symbol "■" represents the bit error performance of the system under the influence of the receiving aperture of 60mm; the solid line with the symbol "●" Represents the bit error performance of the system under the influence of the receiving aperture of 25mm; the solid line with the symbol "◆" represents the bit error performance of the system under the influence of the receiving aperture of 0mm (point receiving).

It can be seen from Figure 4 that for the SCC-FTN-FSO system: 1) the bit error rate performance when the receiving aperture is 25mm, 60mm, and 80mm is better than that of point receiving (0mm), and the larger the aperture, the better the performance the better. 2) Under the same bit error rate, the signal-to-noise ratio required by the system decreases with the increase of the receiving aperture. When BER=10 ^-5 , compared with point reception, the performance gains obtained by 25mm, 60mm and 80mm are 5.615dB, 7.674dB and 8.345dB respectively.

的实线代表加速因子τ＝0.7下系统的误码性能；带符号

的实线代表加速因子τ＝0.67下系统的误码性能；带符号

的实线代表加速因子τ＝0.63下系统的误码性能；带符号

的实线代表加速因子τ＝0.6下系统的误码性能。Figure 5 shows the influence of different acceleration factors on the bit error performance of the SCC-FTN-FSO system under weak turbulence, where the abscissa indicates the signal-to-noise ratio in decibels (dB), and the ordinate indicates the bit error rate. The solid line with the symbol "■" represents the bit error performance of the system under the acceleration factor τ = 1; the solid line with the symbol "●" represents the bit error performance of the system with the acceleration factor τ = 0.9; the solid line with the symbol "▲" Represents the bit error performance of the system under the acceleration factor τ=0.8. The solid line with symbol "▼" represents the bit error performance of the system under the acceleration factor τ = 0.76; the solid line with the symbol "◆" represents the bit error performance of the system under the acceleration factor τ = 0.73;

The solid line of represents the bit error performance of the system under the acceleration factor τ=0.7;

The solid line of represents the bit error performance of the system under the acceleration factor τ=0.67;

The solid line of represents the bit error performance of the system under the acceleration factor τ=0.63;

The solid line of represents the bit error performance of the system under the acceleration factor τ=0.6.

It can be clearly seen from Figure 5 that for the SCC-FTN-FSO system, the bit error rate performance decreases with the decrease of the acceleration factor, and this phenomenon is more obvious when the acceleration factor τ is less than 0.67, because the acceleration factor The reduction of will lead to more serious ISI, resulting in serious loss of system performance.

The above is the specific implementation manner and simulation verification of the present invention. Those skilled in the art may implement the methods described in the embodiments of the present invention through software or hardware for the contribution of the present invention to the prior art.

The basic principles, main features and advantages of the present invention have been shown and described above. Those skilled in the industry should understand that the present invention is not limited by the above-mentioned embodiments. What are described in the above-mentioned embodiments and the description only illustrate the principle of the present invention. Without departing from the spirit and scope of the present invention, the present invention will also have Variations and improvements are possible, which fall within the scope of the claimed invention. The protection scope of the present invention is defined by the appended claims and their equivalents.

Claims (1)

1. A step code based atmospheric optical transmission method with super-Nyquist rate is characterized in that: combining the step code technology with the super Nyquist technology, improving the transmission rate of an atmospheric optical communication system under an exponential Weibull turbulence channel, and reducing the calculation complexity under the condition that the error code performance of the system is not changed, wherein the method specifically comprises the following steps of:

step 1: at a transmitting end, firstly, step code coding is carried out on a binary bit sequence, the binary bit sequence is divided into a plurality of groups and is loaded into a plurality of blocks of step codes, and each row in the blocks is coded by utilizing component code words of BCH codes, so that check bits of each row are obtained, a complete step code block containing information bits and check bits is formed, and step code coding is completed; and PAM modulation is carried out on the signal after the interleaving device is further carried out, the modulated signal is sent to a super-Nyquist shaping filter, and a sending signal s (t) is obtained, namely:

in the formula, s_nRepresents the n-th information symbol after mapping, and g (t) represents the pulse of the super-Nyquist shaping filterImpulse response, T represents the cycle time of each pulse, τ (0 < τ ≦ 1) represents the faster-than-Nyquist factor; loading the formed super-Nyquist signal onto a laser carrier, and sending the laser carrier into an atmospheric turbulence channel which obeys exponential Weibull distribution through a laser diode;

step 2: at the receiving end, the optical signal passes through a turbulent flow channel and is converted into an electric signal by a photoelectric detector, and the received signal is assumed to be y (t), namely

y(t)＝ξhs(t)+z(t)， (2)

Wherein h is a channel fading coefficient, ξ is a photoelectric conversion efficiency, z (t) is a mean value of 0, and a variance σ is²Additive white gaussian noise of (1); the signal is sampled by a super-Nyquist sampler and then is demodulated and deinterleaved by PAM; then, step code decoding is carried out on the signal in a sliding window decoding mode, and iterative decoding is carried out on the line corresponding to each component code word in the sliding window by using limited distance decoding; supposing that the length of a sliding window is L, firstly assisting an all-zero block, then receiving L-1 ladder code blocks from a channel, combining the L-1 ladder code blocks into a sliding window with the length of L to carry out iterative decoding, and outputting a first block in the sliding window when the iteration times reach the maximum; then, a new ladder code block is received, and a sliding window with the length of L is continuously combined for iterative decoding; and by parity of reasoning, continuously performing the process, sequentially outputting all ladder code blocks, and recovering the ladder code blocks into a binary bit sequence after decoding.

Images