CN107682122B - Iterative demodulation decoding method of wireless optical communication multilevel coding modulation system - Google Patents

Iterative demodulation decoding method of wireless optical communication multilevel coding modulation system Download PDF

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CN107682122B
CN107682122B CN201710981782.5A CN201710981782A CN107682122B CN 107682122 B CN107682122 B CN 107682122B CN 201710981782 A CN201710981782 A CN 201710981782A CN 107682122 B CN107682122 B CN 107682122B
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胡昊
王红星
张骁
毛忠阳
刘敏
杨凡
刘锡国
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Naval Aeronautical University
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Abstract

The invention discloses an iterative demodulation decoding method for wireless optical communication multilevel coding modulation, which comprises the steps of constructing a full feedback structure from decoding to receiving estimation, obtaining an iteration initial value, and estimating the receiving estimation through the full feedback structure
Figure DDA0001439616750000011
And decoded output
Figure DDA0001439616750000012
And performing iteration updating, iteration stopping and the like. In the multi-stage code modulation wireless optical communication system, when the component code configuration is not proper, the error propagation is caused, the method has better error performance compared with the multi-stage demodulation decoding method, and when the component code configuration is not in error propagation, the method is equivalent to the multi-stage demodulation decoding method in error performance.

Description

Iterative demodulation decoding method of wireless optical communication multilevel coding modulation system
Technical Field
The invention relates to a demodulation and decoding method of a wireless optical communication multi-level coding modulation system, in particular to an iterative demodulation and decoding method of multi-level coding modulation and an implementation thereof.
Background
Channel conditions of wireless communication systems are generally severe, and the use of channel error correction coding is one of the commonly used methods for improving communication reliability. In general, a good coded modulation design is to construct long code words and implement random coding, and approach or implement maximum likelihood demodulation and decoding at a receiving end. However, the maximum likelihood decoding of the long code word is often complex in algorithm and difficult to implement. The multi-stage coding modulation technology can construct a plurality of short code words into a long code word, realize random coding through simple interleaving in modulation, and simultaneously, adopt a multi-stage demodulation decoding method at a receiving end, can obtain the performance approaching maximum likelihood demodulation decoding, and greatly reduce the complexity.
The schematic diagram of the multi-level coded modulation structure is shown in fig. 1, and consists of three parts: serial-parallel conversion, multi-level coding and signal modulation of an input source, wherein the number of serial-parallel conversion paths, the number of coding levels and the modulation order of the signal modulation are equal. For an M-order modulation system, source data are firstly converted into M paths of parallel data, the M paths of parallel data are respectively sent to M-level encoders for encoding, the code word lengths of component codes output by the encoders are the same, the component codes at each level are combined by taking one bit according to the output sequence to form a modulation packet, and the modulation packet is modulated into a transmitting signal in a modulator for transmitting.
Let i (1 ≦ i ≦ M) encoder have information bit length kiEncoder C with encoding length ni，C1、C2、...、CMRespectively denoted by v1＝(a1,1,a1,1,....,a1,n)，v2＝(a2,1,a2,2,....,a2,n)，…，vM＝(aM,1,aM,2,....,aM,n) The code word may be regarded as a sequence of n symbols after modulation, where f (-) represents the mapping of the combined sequence to the transmitted signal, and "+" represents the simple interleaving of the signal formed by the modulated packet.
At the receiving end, after the transmitted signal f (-) is received and detected, the original transmitted information bits are recovered, which is called demodulation and decoding of multi-level code modulation. A typical demodulation and decoding method is a multi-stage demodulation and decoding method, which starts with a first-stage component code, and includes two steps of reception estimation (i.e., demodulation) and decoding one component code at a time, and decoded information obtained at a previous stage is transferred to a next stage for a next-stage reception estimation, and ends with a last stage. The schematic block diagram of the principle is shown in fig. 2, and the specific method is as follows: the photoelectric receiving signal is taken as a vector R, and the vector R is taken as a known condition, and is firstly sent to a first-stage receiving estimation (demodulation) link to demodulate a first-stage component code v1Evaluation of
Figure BDA0001439616730000011
(referred to as receive estimation) and then enters a first-stage decoding stage
Figure BDA0001439616730000012
Into a corresponding decoder D1Decoding to obtain the final value of the first-level component code
Figure BDA0001439616730000013
(referred to as decoding estimation); then by R and
Figure BDA0001439616730000021
demodulating R to obtain the second-stage component code v as the known condition2Receive estimation of
Figure BDA0001439616730000022
And fed into a decoder D2Obtaining a decoding estimate of a second-level component code
Figure BDA0001439616730000023
Then R is used,
Figure BDA0001439616730000024
And
Figure BDA0001439616730000025
obtaining a third-stage received estimate for R demodulation as a known condition
Figure BDA0001439616730000026
Is sent to a decoder D3Obtaining a decoded estimate thereof
Figure BDA0001439616730000027
And so on until the last stage is obtained
Figure BDA0001439616730000028
Finally, according to the coding and decoding rules of each level of component code, the information bits are coded from the code block
Figure BDA0001439616730000029
And separating the information sequence and performing parallel-serial conversion according to the reverse order of transmission to obtain the final information sequence. The advantages of this method are: compared with the maximum likelihood method, the method greatly reduces the complexity of demodulation and decoding, and can obtain better error code performance.
It can be seen that in the multi-stage demodulation decoding method, the ith component code word v is processediBy means of previous estimations
Figure BDA00014396167300000210
Such as
Figure BDA00014396167300000211
Under the condition of large probability with v1,v2,...,vi-1Equal, a certain protection effect can be formed on the demodulation estimation of the later stage, thereby improving
Figure BDA00014396167300000212
The accuracy of the method is improved, and better error code performance is obtained. However, e.g. evaluation
Figure BDA00014396167300000213
V with actual transmission1,v2,...,vi-1Large difference also easily results in
Figure BDA00014396167300000214
Inaccuracies result in increased error rates. Since the multi-stage demodulation and decoding are performed sequentially from the first-stage component code, when a large number of errors occur in the estimation of the current stage, the decoding of all the subsequent component codes is affected and error accumulation is formed, which is called an error propagation phenomenon of the multi-stage demodulation and decoding. Error propagation can cause serious reduction of error code performance of system communication, and the current method is to analyze the sub-channel capacity corresponding to each level of component code and correspondingly configure channel codes with different error correction capabilities respectively on the basis. This method needs to accurately estimate the channel state information of each sub-channel, which is obviously difficult to achieve in the actual communication process。
Disclosure of Invention
The invention aims to invent a novel demodulation decoding method based on a multi-stage demodulation decoding basic method by utilizing a posterior probability Bayes expansion principle and an iteration technology, and inhibit error propagation in multi-stage demodulation decoding so as to improve error performance.
In order to achieve the above object, the present invention provides a demodulation and decoding method based on iterative multi-level coded modulation. The method starts from the basic idea of reducing the errors of the previous-stage codes, utilizes the diversity of Bayesian expansion of posterior probability and the characteristic of the protection effect of the previous stage to the next stage in multi-stage demodulation and decoding, designs the receiving and estimating process of each stage of sub-channels into a full feedback structure of the decoding result, and carries out parallel updating on the output of each stage of receiving, estimating and decoding links through iteration, so that the one-way protection effect of the previous stage to the next stage in the original multi-stage demodulation and decoding is converted into the mutual protection effect among the sub-channel codes of each stage, thereby improving the accuracy of the estimation. The single iteration demodulation decoding structure principle of the method is shown in figure 3.
The invention is achieved by the following technical measures:
①, constructing a full feedback structure from decoding to receiving estimation, wherein the M-order multi-stage coding modulation system corresponds to M-stage demodulation and decoding sub-channels, each sub-channel comprises two links of receiving estimation and decoding, and the decoder output of each sub-channel
Figure BDA0001439616730000031
Is sent to other receiving and estimating links at all levels except the current level to form a full feedback structure from the decoding result to the receiving and estimating so as to lead the output of each receiving and estimating link at each level
Figure BDA0001439616730000032
The estimation is carried out by the decoding output of other stages and the received signal together.
② obtaining initial value of iteration, for M-order multi-stage code modulation system, each stage of decoding output
Figure BDA0001439616730000033
The initial value of (2) can be directly obtained by a multi-stage demodulation decoding method.
③ iteration begins, receiving estimates for each stage through a full feedback structure
Figure BDA0001439616730000034
And decoded output
Figure BDA0001439616730000035
And (6) updating. Let the output code word of each level component encoder of M-level multi-level code modulation be v1＝(a1,1,a1,2,....,a1,n)，v2＝(a2,1,a2,2,....,a2,n)，…，vM＝(aM,1,aM,2,....,aM,n) The received signal is represented as a vector R, the iteration starts, for the ith stage therein
Figure BDA0001439616730000036
When updating with the estimated value of (2), all other values are used
Figure BDA0001439616730000037
And R is a known quantity, calculated separately
Figure BDA0001439616730000038
J (j) th bit a of 1ijLikelihood metric of (2):
Figure BDA0001439616730000039
obtained (Λ (a)i1),Λ(ai2),...,Λ(ain) Is that
Figure BDA00014396167300000310
Updated confidence measure, pair (Λ (a)i1),Λ(ai2),...,Λ(ain) ) make a binary decision, i.e. get an updated reception estimate
Figure BDA00014396167300000311
Maximum likelihood decoding or updating of confidence measures
Figure BDA00014396167300000312
Algebraic decoding to obtain updated decoded estimate
Figure BDA00014396167300000313
In parallel to all
Figure BDA00014396167300000314
And updating to finish the iteration.
④ stopping when the iteration stop criterion reaches a set value
Figure BDA00014396167300000315
The information bits are separated according to the specific composition rule of each level of component code words, and the obtained information bits at each level are subjected to parallel-serial conversion according to the transmitted reverse order, so as to recover the final information sequence.
Compared with the prior art, the invention has the following beneficial effects:
① when the multi-stage coding modulation system adopts multi-stage demodulation and decoding method to generate error propagation, the invention can obtain better error performance for the wireless optical communication multi-stage coding PPM modulation system with weak turbulence atmospheric channel with flicker index of 0.1 and error rate of 10-4Under the condition, the gain (figure 4) of more than 0.5dB is obtained in the three iterations compared with the gain obtained in the non-iteration process, and the output updating of each stage in the iteration is synchronous updating, so that the iteration time cost is saved.
Secondly, when each level of component codes of multi-level coding modulation are configured properly, namely, when multi-stage demodulation decoding is adopted, error propagation phenomena hardly occur, the error code performance is the same when the invention is adopted to carry out three times of iteration and no iteration, and the error performance of the system is not changed at the moment. (FIG. 5).
Drawings
Fig. 1 is a schematic diagram of a multi-level coded modulation architecture.
Fig. 2 is a functional block diagram of multi-stage demodulation decoding.
Fig. 3 is a schematic block diagram of single iteration demodulation and decoding.
Fig. 4 is a comparison curve of error performance obtained by adopting a multi-stage demodulation decoding method and three iterations of the method in a weak turbulence air channel in a wireless optical communication 3-order PPM multi-level coding modulation system (three-level component codes are configured according to the sequence of a BCH (127, 113) code, a BCH (127, 99) code and a BCH (127, 85) code, and error propagation exists).
Fig. 5 is a comparison curve of error performance obtained by adopting a multi-stage demodulation decoding method and three iterations of the method in a wireless optical communication 3-order PPM multilevel coded modulation system (three-level component codes are configured according to the sequence of BCH (127, 85) code, BCH (127, 99) code and BCH (127, 113) code, and no error propagation) in the second embodiment.
Detailed Description
Example one
In this embodiment, the wireless optical communication system adopts 3-level PPM modulation, and correspondingly needs three-level channel coding, the three-level component codes are sequentially configured as BCH (127, 113) code, BCH (127, 99) code and BCH (127, 85) code, respectively, and the PPM modulation mapping mode is as shown in table 1, where (a)1,a2,a3) Represents a modulated packet, mi(i-0, 1.. 7.) denotes the position of the modulated light pulse on the slot of the PPM symbol, and PPM modulation characterizes information by the difference in the position of the slot within the symbol in which the light pulse is located.
TABLE 1 PPM mapping scheme
Figure BDA0001439616730000041
Firstly, three groups of information source sequences are generated at a sending end, the three groups of information source data are respectively sent to a three-level channel encoder for encoding, then are modulated into optical PPM signals for sending, through channel transmission, a receiving end obtains a receiving vector R consisting of 127 PPM signals, wherein the t-th PPM sending group (a) is set1,t,a2,t,a3,t) The received signal of (r) is (r ═ r)0,r1,...,r7)，ri(i ═ 0.. 7.) represents the photoelectric conversion current value of the PPM reception slot, and the corresponding position information is recorded as m0,m1,...,m7。
According to the method, an initial estimation value is first obtained by a multi-stage demodulation decoding method
Figure BDA0001439616730000051
Iteration begins and updates
Figure BDA0001439616730000052
Assuming that the probability density of the converted current received over the time slot with optical pulses is f1，
Figure BDA0001439616730000053
Is "00". From formula (1) and table 1, it is possible to obtain:
Figure BDA0001439616730000054
taking into account the current probability density function f1Is an increasing function, so that r can be compared directly0And r1Size, r1＞r0Time of flight
Figure BDA0001439616730000055
r1＜r0Time of flight
Figure BDA0001439616730000056
Figure BDA0001439616730000057
The same applies to the case of other values. Finally, the data is updated through a decoder
Figure BDA0001439616730000058
The communication channel of the embodiment is set to be cascade of weak turbulence atmosphere and Gaussian channel, the flicker index of the turbulence is 0.1, and the mean value of the Gaussian channel noise is zeroThe difference is 2 × 10-25The channel attenuation factor is 1, the light wave working wavelength is 1.55um, the detector quantum efficiency is 0.5, the multiplication gain is 100, and the information source rate is 10 MBit/s. When the average transmitter power is-39.3 dBm, the implementation of the method is as follows.
Three sets of source data to be transmitted are generated S1, S2, S3:
Figure BDA0001439616730000059
Figure BDA0001439616730000061
the three groups of data are modulated into PPM signals containing eight time slots after being coded by each subchannel coder, and the received data are obtained at a receiving end after being transmitted by an atmospheric turbulence channel as shown in table 2 (note that the noise introduced by the channel is a random process, so the result obtained by each experiment is different). Wherein, the sequence number represents the received PPM signal sequence number, r0～r7The number of received photocurrents detected for each time slot of the PPM signal is 1e-11A。
Table 2 reception data of the receiving end
Figure BDA0001439616730000062
Figure BDA0001439616730000071
Figure BDA0001439616730000081
Figure BDA0001439616730000091
Figure BDA0001439616730000101
The initial estimation value obtained by adopting multi-stage demodulation decoding on the received data is as follows:
S1(0):
Figure BDA0001439616730000102
S2(0):
Figure BDA0001439616730000103
S3(0):
Figure BDA0001439616730000104
the result after the first iteration is:
Figure BDA0001439616730000105
Figure BDA0001439616730000111
the result after the second iteration is:
Figure BDA0001439616730000112
the results after the third iteration were:
Figure BDA0001439616730000113
Figure BDA0001439616730000121
comparing the source data, it can be seen that the number of errors after different iterations is as shown in table 3. It can be seen that the number of errors is 3 when there is no iteration, and after the first iteration, the number of errors becomes 0.
TABLE 3 number of errors at different iterations
Figure BDA0001439616730000122
The method is repeated, and the error rate condition is calculated through large data volume under different transmitting average powers, so that a system error rate performance curve can be obtained as shown in fig. 4. The abscissa is average power, the ordinate is error rate, ITE represents iteration times, ITE is 0, and a multi-stage demodulation decoding method is adopted when iteration is not performed. It can be seen from the figure that the error performance curve of the system is improved after iteration. In which, 1 iteration (ITE ═ 1) obtains a more obvious performance improvement than the non-iteration (multistage demodulation decoding method, ITE ═ 0), and the error rate is 10-4In comparison, 1 iteration obtains about 0.5dB of gain, the error code performance curves of 2 iterations and 3 iterations are slightly better than that of 1 iteration, but the two curves are almost overlapped, and the improvement is not obvious when the iteration is continued on the basis, namely the performance improvement cannot be obtained when the iteration is continued. Meanwhile, because each iteration of component code output at each level can be updated in parallel, the time overhead of iteration is also small.
Example two
In this embodiment, the wireless optical communication system still uses 3-order PPM modulation, and the corresponding three-level component codes are arranged in the order of BCH (127, 85), BCH (127, 99) and BCH (127, 113), and in this arrangement, the multi-stage demodulation decoding has the minimum error propagation. At this time, error performance curves of iteration (when ITE is 1,2, 3) and non-iteration (when ITE is 0, a multi-stage demodulation decoding method is used) are shown in fig. 5. As can be seen from the figure, the four curves almost coincide, i.e. the gain improvement is not obtained by the present invention, but the system performance is not degraded.

Claims (1)

1. An iterative demodulation decoding method of a wireless optical communication multilevel coding modulation system is characterized in that: setting the multi-level coding modulation system as M order, and the code words output by the component coders of each level are v respectively1＝(a1,1,a1,2,....,a1,n)，v2＝(a2,1,a2,2,....,a2,n)，…，vM＝(aM,1,aM,2,....,aM,n) The received signal is represented as a vector R, comprising the steps of:
1) constructing a full feedback structure from decoding to reception estimation
M-level multi-level coding and modulation system is correspondingly provided with M-level demodulation and decoding subchannels, each level of subchannel comprises two links of receiving estimation and decoding, and the decoder output of each level of subchannel
Figure FDA0002477560800000011
Is sent to other receiving and estimating links at all levels except the current level to form a full feedback structure from the decoding result to the receiving and estimating, and the output of each receiving and estimating link is
Figure FDA0002477560800000012
The decoded output and the received signal of other stages are jointly estimated;
2) obtaining initial estimation value of each stage of decoding output by multi-stage demodulation decoding method
Figure FDA0002477560800000013
3) The iteration starts, and the i (i-1, 2.. multidot.m) th stage is estimated by using a full feedback structure decoded to the received estimation
Figure FDA0002477560800000014
And decoding output estimation
Figure FDA0002477560800000015
When updating, all the other modules are used
Figure FDA0002477560800000016
And R is a known amount based on formula
Figure FDA0002477560800000017
Respectively calculate to obtain
Figure FDA0002477560800000018
J (j) th bit a of 1ijThe likelihood metric of (a) obtained (Λ (a)i1),Λ(ai2),...,Λ(ain) Is updated)
Figure FDA0002477560800000019
Confidence measure, pair (Λ (a)i1),Λ(ai2),...,Λ(ain) ) make a binary decision, i.e. get an updated reception estimate
Figure FDA00024775608000000110
Maximum likelihood decoding or updating of confidence measures
Figure FDA00024775608000000111
Algebraic decoding to obtain updated decoded estimate
Figure FDA00024775608000000112
In parallel to all
Figure FDA00024775608000000113
Updating to complete the iteration;
4) stopping the iteration stopping criterion after the iteration stopping criterion reaches a set value
Figure FDA00024775608000000114
According to the specific composition rule of each level of component code words, information bits are separated, and the obtained information bits at each level are subjected to parallel-serial conversion according to the transmitted reverse order, so that the final information sequence is recovered.
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