CN109525314B - Coding and decoding method for reducing bit error rate in LACO-OFDM system - Google Patents

Coding and decoding method for reducing bit error rate in LACO-OFDM system Download PDF

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CN109525314B
CN109525314B CN201811577532.6A CN201811577532A CN109525314B CN 109525314 B CN109525314 B CN 109525314B CN 201811577532 A CN201811577532 A CN 201811577532A CN 109525314 B CN109525314 B CN 109525314B
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刘武
杨超
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Wuhan Research Institute of Posts and Telecommunications Co Ltd
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    • H04—ELECTRIC COMMUNICATION TECHNIQUE
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    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/11—Arrangements specific to free-space transmission, i.e. transmission through air or vacuum
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    • H04B10/116—Visible light communication
    • H—ELECTRICITY
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    • H04L1/0001—Systems modifying transmission characteristics according to link quality, e.g. power backoff
    • H04L1/0009—Systems modifying transmission characteristics according to link quality, e.g. power backoff by adapting the channel coding
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L1/00—Arrangements for detecting or preventing errors in the information received
    • H04L1/004—Arrangements for detecting or preventing errors in the information received by using forward error control
    • H04L1/0056—Systems characterized by the type of code used
    • H04L1/0061—Error detection codes
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    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
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    • H04L27/2601—Multicarrier modulation systems
    • H04L27/2647—Arrangements specific to the receiver only
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Abstract

A coding and decoding method for the error rate in an LACO-OFDM system relates to the field of the LACO-OFDM system, and comprises the following steps: carrying out channel coding on user data at a sending end to generate redundancy check data, and then respectively carrying out symbol coding on the user data and the redundancy check data; in a multilayer frame structure of a system, redundancy check data symbols are preferentially placed on partial low-frequency subcarriers of a layer 1, and user data symbols are placed on other subcarriers; and decoding layer by layer at a receiving end by taking the subcarrier as a unit, correcting the user data according to the redundancy check data, and repeating the decoding and correcting process layer by layer from the lowest layer of the corrected symbol until the decoding result of the layer by layer is consistent with the channel decoding result. The invention reduces the error rate through cyclic error correction and layer-by-layer decoding, and reduces error transmission to the maximum extent.

Description

Coding and decoding method for reducing bit error rate in LACO-OFDM system
Technical Field
The invention relates to the field of LACO-OFDM (layer amplified orthogonal frequency Division Multiplexing) systems, in particular to a coding and decoding method for an error rate in an LACO-OFDM system.
Background
In short-range optical communication systems, low-cost intensity modulation/direct detection schemes are typically employed. The intensity modulation/direct detection scheme requires a modulation format in which the signal is always positive, such as PAM (Pulse amplitude modulation), DCO-OFDM (direct current biased optical-Orthogonal Frequency Division Multiplexing), ACO-OFDM (asymmetric multiplexed optical-Orthogonal Frequency Division Multiplexing), and the like. DCO-OFDM adds a bias dc signal to an OFDM (Orthogonal Frequency division multiplexing) signal to obtain a signal of a full positive value, which can directly modulate an optical device; the ACO-OFDM directly zeroes the negative OFDM signal to obtain a signal with a full positive value, and signal interference caused by the zero-resetting of the negative value is mainly concentrated on the empty subcarriers with even numbers, but the spectral efficiency is reduced by half.
In order to improve spectral efficiency, an improved LACO-OFDM scheme is provided, and the technical key point is that the carrier-chopping interference of subcarriers in ACO-OFDM coding always regularly falls on other channels, so that at a receiving end, part of subcarriers without superimposed interference are decoded, and the interference brought to other subcarriers by the part of subcarriers due to negative value zero setting can be determined. The interference on all the subcarriers can be eliminated step by step without being influenced by the zero setting of the negative value by repeating the decoding process, so that the subcarriers which are vacant because the negative value is set to zero in the ACO-OFDM can also be provided with information by step decoding, and the spectral efficiency is improved. Research has shown that the LACO-OFDM scheme can achieve spectral efficiency close to that of DCO-OFDM, but due to the smaller bias current, better signal quality can be achieved at the same luminous intensity, which means that the optical device can modulate signals with higher modulation depth, and the optical signals can be transmitted further.
The layer-by-layer decoding method of the LACO-OFDM scheme is different from the traditional OFDM decoding method, namely, part of subcarriers which are not interfered are firstly selected for decoding, and other subcarriers can be decoded after the interference of other subcarriers is eliminated. The decoding layer by layer can cause the decoding error of the previous layer to influence the next layer, so as to form error transmission, and especially under the condition of lower SNR (Signal-to-noise ratio), the Signal error rate of the sub-carrier wave after decoding can be obviously increased. This extra bit error due to erroneous transmission reduces the availability of the LACO-OFDM system at low SNR.
Disclosure of Invention
Aiming at the defects in the prior art, the invention aims to provide a coding and decoding method for the error rate in the LACO-OFDM system, which reduces the error rate through cyclic error correction and layer-by-layer decoding and reduces error transmission to the maximum extent.
In order to achieve the above object, in one aspect, a method for coding and decoding an error rate in an LACO-OFDM system is adopted, which includes the steps of:
carrying out channel coding on user data at a sending end to generate redundancy check data, and then respectively carrying out symbol coding on the user data and the redundancy check data;
in a multilayer frame structure of a system, redundancy check data symbols are preferentially placed on partial low-frequency subcarriers of a layer 1, and user data symbols are placed on other subcarriers;
and decoding layer by layer at a receiving end by taking the subcarrier as a unit, correcting the user data according to the redundancy check data, and repeating the decoding and correcting process layer by layer from the lowest layer of the corrected symbol until the decoding result of the layer by layer is consistent with the channel decoding result.
Preferably, the system adopts an M-ary quadrature amplitude modulation (M-QAM) modulation format, the user data is subjected to symbol coding according to the M-QAM, the redundancy check data is subjected to symbol coding according to 4-ary quadrature amplitude modulation (4-QAM), the amplitude is normalized to the corresponding maximum amplitude in the M-QAM, M is the element number of the M-QAM, and M is 2mAnd m is 1,2, … and N, wherein N is an integer.
Preferably, the channel coding is performed by selecting the user data with L x m bits to generate the redundancy check data with r bits, and the number of the redundancy check data symbols and the user data symbols is
Figure GDA0002297166770000032
Less than the number of available subcarriers at the transmitting end.
Preferably, when the length of the redundancy check data is odd, zero padding is performed to change the length of the redundancy check data to an even number, and then symbol encoding is performed according to 4-QAM.
Preferably, the number of subcarriers of the multi-layer frame structure corresponds to the number of FFT (fast fourier transform) points, and the number of the subcarrier corresponding to each layer is 2x-1(2n +1), where n is {0,1,2,3, … }, x is the number of layers, and each layer of subcarriers is independently subjected to FFT or inverse fast fourier transform IFFT.
Preferably, the receiving end layer-by-layer decoding and correcting process includes: the receiving end decodes layer by layer according to the multilayer frame structure to obtain all symbols, all symbols become 01 sequences and frame according to the position corresponding to the transmitting end, and then channel decoding is carried out; and correcting the inconsistent symbols in the layer-by-layer decoding by using the symbols decoded by the channel, and starting from the lowest layer where the inconsistent symbols are located, and decoding and correcting again until the results obtained by the layer-by-layer decoding and the channel decoding are consistent.
Preferably, the symbol correction using channel decoding for the inconsistent symbols in layer-by-layer decoding includes: and comparing the 01 sequence obtained by layer-by-layer decoding with the 01 sequence obtained by channel decoding, and when the inconsistent 01 sequence appears, replacing the symbol corresponding to the inconsistent 01 sequence obtained by layer-by-layer decoding by adopting the symbol corresponding to the 01 sequence in the channel decoding.
Preferably, a threshold is set, and when the number of times of layer-by-layer decoding and channel decoding results being inconsistent reaches the threshold, the decoding is ended.
Preferably, the step of decoding layer by the receiving end according to the multi-layer frame structure to obtain all symbols includes:
carrying out FFT on a received signal to obtain a multilayer OFDM frame of a frequency domain; extracting and decoding data on the x layer subcarrier; carrying out symbol coding, IFFT operation and wave chopping on the extracted data in sequence to obtain the component of the extracted data in the received signal; subtracting the component of the x layer from the received signal to obtain a residual signal containing data of x +1, x +2 … and x + p layers;
and repeating the steps until the x + p layer data is decoded, wherein x is greater than 0, and x + p is less than N.
Preferably, the size of the data block decoded and input by the receiving end channel, the adopted channel coding and the sending end convention are the same.
One of the above technical solutions has the following beneficial effects:
channel coding is carried out on user data at a sending end, a proper subcarrier is selected to place a check result (namely redundancy check data), and the bandwidth is effectively utilized; at a receiving end, cyclic error correction and layer-by-layer decoding are carried out by taking the sub-carrier as a unit, so that error codes are effectively reduced, and error transmission is reduced.
Drawings
FIG. 1 is a multi-layer data frame structure of valid data in an LACO-OFDM system according to an embodiment of the present invention;
FIG. 2 is a flow chart of layer-by-layer decoding at a receiving end according to an embodiment of the present invention;
fig. 3 is a flowchart of a cyclic error correction at a receiving end according to an embodiment of the present invention.
Detailed Description
The present invention will be described in further detail with reference to the accompanying drawings and examples.
The embodiment of the invention discloses a coding and decoding method of an error rate in an LACO-OFDM system, which comprises the following steps:
s1, at a sending end, carrying out channel coding on user data to generate redundant check data, and then respectively carrying out symbol coding on the user data and the redundant check data.
S2, in a multi-layer frame structure of the system, redundant check data symbols are preferentially placed on partial low-frequency subcarriers of a layer 1, and user data symbols are placed on other subcarriers.
And S3, decoding layer by layer at a receiving end by taking the subcarrier as a unit, correcting the user data according to the redundancy check data, and repeating the decoding and correcting process layer by layer from the lowest layer of the corrected symbol until the decoding result layer by layer is consistent with the channel decoding result.
In step S1, the normal LACO-OFDM system adopts M-QAM (M-ary quadrature amplitude modulation) modulation format, where M is 2mAnd m is 1,2, … and N, wherein N is an integer. In the system, user data is subjected to symbol coding according to an M-QAM modulation format, redundant check data is subjected to symbol coding according to a 4-QAM modulation format, and the amplitude is normalized to the maximum amplitude corresponding to the M-QAM so as to improve the SNR (signal to noise ratio) of the system and reduce error codes.
Preferably, the user data with L x M bits is selected to perform channel coding, r bits of redundancy check data are generated, and symbol coding is performed on the user data with L x M bits and the r bits of redundancy check data according to the format, so as to obtain L user data symbols with M-QAM modulation format.
Preferably, when the length of the redundancy check data (r bits) is odd, zero padding is performed to change the length of the redundancy check data into even, and then symbol encoding is performed according to the 4-QAM modulation format.
In the step S2, as shown in fig. 1, the system adopts a multi-layer frame structure, the number of subcarriers corresponds to the number of FFT points, the subcarriers are divided into multiple layers, and the number of the subcarrier corresponding to each layer is 2x-1(2n +1), where n is {0,1,2,3, … }, x is the number of layers, and each layer is independently subjected to FFT (fast Fourier Transform) or IFFT (Inverse fast Fourier Transform)Transform).
The redundant check data symbols of the 4-QAM modulation format are preferably placed on a portion of the low frequency subcarriers of layer 1 to minimize interference and attenuation. The L user data symbols of the M-QAM modulation format are then placed on other subcarriers, which may be other subcarriers of the first layer or subcarriers of other layers. And, all redundancy check data symbols and user data symbols are in number
Figure GDA0002297166770000061
Less than the number of available subcarriers at the transmitting end.
In the step S3, the receiving end decodes layer by layer according to the multi-layer frame structure to obtain all symbols, and the symbols become 01 sequences and frame according to the position corresponding to the sending end, and then performs channel decoding; and correcting the inconsistent symbols in the layer-by-layer decoding by using the symbols decoded by the channel, and starting from the lowest layer where all the inconsistent symbols are located, and decoding and correcting again until the results obtained by the layer-by-layer decoding and the channel decoding are consistent.
As shown in fig. 2, the process of layer-by-layer decoding by the receiving end according to the multi-layer frame structure includes:
a301. and performing FFT on the received signal to obtain a multi-layer OFDM frame of a frequency domain.
a302. And extracting and decoding data on the x layer subcarrier, wherein x > 0.
a303. And sequentially carrying out symbol coding, IFFT operation and wave chopping on the extracted data to obtain the component of the data on the x-th layer subcarrier in the received signal.
a304. The x layer component is subtracted from the received signal to yield a residual signal containing x +1, x +2 … x + p layer data, with x + p < N.
a305. Judging whether x is smaller than p, if so, indicating that the last layer of data is not decoded, and turning to a 301; if not, the decoding is performed to the last layer of data, and a306 is entered.
a306. And outputting all layer subcarrier data.
As shown in fig. 3, the cyclic error correction process in step S3 specifically includes:
b301. and changing all symbols obtained by layer-by-layer decoding into 01 sequences, and framing according to the position corresponding to the transmitting end. The size of the data block input by the receiving end channel decoding, the adopted channel coding and the sending end convention are consistent.
b302. And performing channel decoding on the framed 01 sequence.
b303. Comparing the 01 sequence obtained by layer-by-layer decoding with the 01 sequence obtained by channel decoding, judging whether the sequences are consistent, if so, entering b 306; if not, go to b304.
b304. Judging whether the iteration times are smaller than a threshold value, if so, entering b 305; if not, go to b306. The threshold value is set in advance according to the requirement, and may be set to 5 in the present embodiment.
b305. And (b) replacing the symbols corresponding to the inconsistent 01 sequences obtained by layer-by-layer decoding by the symbols corresponding to the corresponding 01 sequences in the channel decoding, and turning to b301.
b306. And outputting data of all the subcarriers.
In the above steps, the interleaving adopts layer-by-layer decoding and cyclic error correction, which can eliminate error transmission to the maximum extent and effectively reduce error codes.
The present invention is not limited to the above-described embodiments, and it will be apparent to those skilled in the art that various modifications and improvements can be made without departing from the principle of the present invention, and such modifications and improvements are also considered to be within the scope of the present invention. Those not described in detail in this specification are within the skill of the art.

Claims (10)

1. A coding and decoding method for bit error rate in LACO-OFDM system is characterized by comprising the following steps:
carrying out channel coding on user data at a sending end to generate redundancy check data, and then respectively carrying out symbol coding on the user data and the redundancy check data;
in a multilayer frame structure of a system, redundancy check data symbols are preferentially placed on partial low-frequency subcarriers of a layer 1, and user data symbols are placed on other subcarriers;
and decoding layer by layer at a receiving end by taking the subcarrier as a unit, correcting the user data according to the redundancy check data, and repeating the decoding and correcting process layer by layer from the lowest layer of the corrected symbol until the decoding result of the layer by layer is consistent with the channel decoding result.
2. The method for coding and decoding a bit error rate in a LACO-OFDM system as claimed in claim 1, wherein: the system adopts an M-ary quadrature amplitude modulation (M-QAM) modulation format, user data is subjected to symbol coding according to the M-QAM, redundant check data is subjected to symbol coding according to 4-ary quadrature amplitude modulation (4-QAM), amplitude values are normalized to the corresponding maximum amplitude value in the M-QAM, M is the element number of the M-QAM, and M is 2mAnd m is 1,2, … and N, wherein N is an integer.
3. The method of claim 2, wherein the coding and decoding method for the bit error rate in the LACO-OFDM system comprises: selecting user data of L multiplied by m bits to carry out channel coding, generating redundancy check data of r bits, the number of redundancy check data symbols and user data symbols is
Figure FDA0002297166760000011
Less than the number of available subcarriers at the transmitting end.
4. The method of claim 2, wherein the coding and decoding method for the bit error rate in the LACO-OFDM system comprises: and when the length of the redundancy check data is odd, zero padding is carried out to change the length of the redundancy check data into an even number, and then symbol coding is carried out according to 4-QAM.
5. The method for coding and decoding a bit error rate in a LACO-OFDM system as claimed in claim 1, wherein: the number of the sub-carriers of the multi-layer frame structure corresponds to the number of Fast Fourier Transform (FFT) points, and the number of the sub-carriers corresponding to each layer is 2x-1(2n +1), where n is {0,1,2,3, … }, x is the number of layers, and each layer of subcarriers is independently subjected to FFT or inverse fast fourier transform IFFT.
6. The method of claim 1, wherein the receiver layer-by-layer decoding and correcting process comprises:
the receiving end decodes layer by layer according to the multilayer frame structure to obtain all symbols, all symbols become 01 sequences and frame according to the position corresponding to the transmitting end, and then channel decoding is carried out;
and correcting the inconsistent symbols in the layer-by-layer decoding by using the symbols decoded by the channel, and starting from the lowest layer where the inconsistent symbols are located, and decoding and correcting again until the results obtained by the layer-by-layer decoding and the channel decoding are consistent.
7. The method of claim 6, wherein the step of correcting the non-uniform symbols in the layer-by-layer decoding by using the symbols decoded by the channel comprises:
and comparing the 01 sequence obtained by layer-by-layer decoding with the 01 sequence obtained by channel decoding, and when the inconsistent 01 sequence appears, replacing the symbol corresponding to the inconsistent 01 sequence obtained by layer-by-layer decoding by adopting the symbol corresponding to the 01 sequence in the channel decoding.
8. The method of claim 6, wherein the coding and decoding method for the bit error rate in the LACO-OFDM system comprises: and setting a threshold, and finishing decoding when the number of times of layer-by-layer decoding and channel decoding results which are inconsistent reaches the threshold.
9. The method for coding and decoding an error rate in an LACO-OFDM system as claimed in claim 6, wherein the step of decoding layer by layer according to the multi-layer frame structure at the receiving end to obtain all symbols comprises the steps of:
carrying out FFT on a received signal to obtain a multilayer OFDM frame of a frequency domain;
extracting and decoding data on the x layer subcarrier;
carrying out symbol coding, IFFT operation and wave chopping on the extracted data in sequence to obtain the component of the extracted data in the received signal;
subtracting the component of the x layer from the received signal to obtain a residual signal containing x +1 and x +2 … x + p layer data;
and repeating the steps until the x + p layer data is decoded, wherein x is greater than 0, and x + p is less than N.
10. The method for coding and decoding a bit error rate in a LACO-OFDM system as claimed in any of claims 1-9, wherein: the size of the data block input by the receiving end channel decoding, the adopted channel coding and the sending end convention are consistent.
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