CN110324065B - Multi-user underwater acoustic communication method based on cyclic shift keying spread spectrum modulation - Google Patents

Multi-user underwater acoustic communication method based on cyclic shift keying spread spectrum modulation Download PDF

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CN110324065B
CN110324065B CN201910488731.8A CN201910488731A CN110324065B CN 110324065 B CN110324065 B CN 110324065B CN 201910488731 A CN201910488731 A CN 201910488731A CN 110324065 B CN110324065 B CN 110324065B
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景连友
殷洪玺
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Dalian University of Technology
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    • H04B1/69—Spread spectrum techniques
    • H04B1/707—Spread spectrum techniques using direct sequence modulation
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Abstract

The invention belongs to the technical field of underwater acoustic communication, and particularly relates to a multi-user underwater acoustic communication method based on cyclic shift keying spread spectrum modulation. The first step is as follows: transmitting end, using bit interleaver pi of mth usermTo cmInterleaving, then carrying out CSK modulation to improve the communication rate, and then interleaving to distinguish users; the second step is that: modulating data by using an underwater acoustic communication transmitter through a carrier wave and then transmitting; the third step: and the receiving end is used for carrying out passive time reversal processing on the preprocessed multipath received signals so as to compress channels and improve the signal-to-interference ratio. And then, a CSK soft demodulation mode is adopted, and CSK modulation and a traditional IDMA iterative demodulation system are perfectly combined, so that the detection performance of the system can be effectively improved. The method of the invention can effectively improve the communication rate of each user and can improve the number of users at the same time.

Description

Multi-user underwater acoustic communication method based on cyclic shift keying spread spectrum modulation
Technical Field
The invention belongs to the technical field of underwater acoustic communication, and particularly relates to a multi-user underwater acoustic communication method based on cyclic shift keying spread spectrum modulation.
Background
With the increasing demand of ocean development for underwater communication networks, underwater acoustic communication has become a research hotspot in the current ocean field. However, the development of underwater acoustic communication is also limited by the characteristics of long delay spread, limited bandwidth, fast time variation, high background noise and the like of the underwater acoustic channel. For a multi-user underwater acoustic communication system, the influence of the channel characteristics on the performance of the communication system is more obvious. Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), and Code Division Multiple Access (CDMA) are three commonly used Multiple Access techniques in wireless communication, but the applications of these three techniques in multi-user underwater acoustic communication systems face their respective problems. For FDMA, the number of users that can be supported by the system is limited due to the limited bandwidth available for the underwater acoustic channel, and is less applicable in underwater acoustic communications. TDMA systems require better time synchronization and have a lower channel utilization, which limits their use in underwater acoustic communications. CDMA distinguishes users by using spreading codes, different users can share time and spectrum, and spreading gains can also improve the performance of the system in high background noise underwater acoustic channels. Therefore, CDMA is widely used in multi-user underwater acoustic communication. However, due to the long delay spread of underwater acoustic channel multipath, the number of users supported by the existing CDMA multi-user underwater acoustic communication system is relatively small, and is far lower than that of users in land multi-user wireless communication. In addition, due to the spread spectrum communication characteristics, the communication rate of each user of the CDMA system is very low, which is far from meeting the current rapidly-increasing communication demand.
In recent years, researchers have proposed an Interleave Division Multiple Access (IDMA) technique for distinguishing users by an interleaver. The IDMA system not only has the multiple access interference resistance and fading resistance of the CDMA system, but also can effectively improve the number of multiple access users, and has a wider research prospect in wireless communication, but the application of the IDMA technology in underwater acoustic communication is less at present.
The Cyclic Shift Keying (CSK) spread spectrum modulation technique uses the shift size of the spreading code to represent information, can effectively improve the communication rate of spread spectrum communication, and has been widely applied to underwater acoustic communication in recent years.
In the patent "IDMA system communication method based on CCSK modulation" (CN 107612861 a "), it is mentioned to apply CSK technique to multi-user IDMA system. However, in the invention, a CSK modulation is additionally added on the basis of the IDMA system, so that although the anti-interference capability is increased, the rate of the communication system is further reduced, and therefore, the invention is not suitable for being used in the underwater acoustic communication system with a low communication rate. In addition, the CSK demodulation in the invention belongs to hard decision, and certain performance is lost.
Disclosure of Invention
The invention aims to solve the technical problems that fewer accessible users are available and the communication rate of each user is low in the current multi-user underwater acoustic communication system.
The technical scheme of the invention is as follows:
a multi-user underwater acoustic communication method based on cyclic shift keying spread spectrum modulation comprises the following steps:
the first step is as follows: transmitting terminal
1.1 there are M users, the original data information of the mth user is bmRepresenting by channel coding at rate RTo the coded signal is cm；m＝1、2、3……M。
1.2 bit interleaver with mth user pimTo cmInterweaving to obtain data dm(ii) a The bit interleaver functions to break up concentrated burst errors.
1.3 pairs of data dmCarrying out CSK modulation to obtain a sequence sm；
1.4 chip interleaver pi with mth usermTo smInterweaving to obtain data xm. The chip interleaver is used for distinguishing users;
the second step is that: using an underwater acoustic communication transmitter to transmit data xmAnd transmitting after carrier modulation.
The third step: receiving end
And 3.1, preprocessing signals received by each receiving transducer, wherein the preprocessing comprises synchronization, down-conversion, sampling processing and the like.
3.2, channel estimation is carried out on the preprocessed received signals;
3.3, carrying out passive time reversal processing on the preprocessed multipath received signals so as to compress channels and improve the signal-to-interference ratio;
3.4 initializing the mean value and the variance of each user signal and the corresponding CSK symbol probability, namely presetting an initial value of a signal to be demodulated;
3.5, calculating the mean value and the variance of the signal of the ith user after passive time reversal processing;
3.6, calculating the mean value and the variance of the interference signal of the ith user;
3.7 computing the outer likelihood ratio of the ith user's transmitted signal and using the decoding chip interleaver of the ith user
Figure GDA0002730508980000031
Performing de-interleaving;
3.8 carrying out CSK soft demodulation on the deinterleaved signal external likelihood ratio to obtain demodulated soft information;
the CSK soft demodulation comprises the following steps: calculating a transmission signal using the deinterleaved transmission signal outer likelihood ratioAn estimate of (d). The obtained signal estimation value is correlated with a spreading sequence used by CSK modulation to obtain a correlation result thetai。θiThe value of (c) is affected by two factors: the estimated error of the transmitted signal and the sidelobes of the autocorrelation result of the spreading sequence. Assuming that both factors fit into a Gaussian distribution and the respective variances are calculated, θ can be derivediA posterior probability density function of. And calculating the outer likelihood ratio of the demodulated coded data by combining the prior probability of the CSK symbols. At this point, the CSK soft demodulation process is completed.
3.9 Debit interleaver with ith user
Figure GDA0002730508980000032
De-interleaving the outer likelihood ratio after CSK soft demodulation, and then performing channel decoding by using a decoder to obtain the outer likelihood ratio of estimated information bits and decoded coded data;
3.10 bit interleaver with the ith useriAnd interleaving the decoded outer likelihood ratio output by the decoder, and performing CSK soft mapping again. Re-inputting the obtained CSK symbol probability into a CSK soft demodulator for next CSK soft demodulation;
the CSK soft mapping is as follows: and calculating the probability of each coded bit by using the outer likelihood ratio output by the decoder after interleaving. And (3) setting each code bit to be independent, and calculating the probability of the CSK symbol mapped by the code bits according to the mapping rule of CSK modulation.
Furthermore, in the CSK soft mapping process, the probability that the CSK symbol cannot be mapped is removed, so that the system performance is improved.
3.11 repeating (3.8) - (3.10) until reaching the upper limit of the iteration number of CSK soft demodulation;
3.12 chip interleaver pi with the ith useriRe-interleaving the result of CSK soft mapping, and re-calculating the mean value and variance of the ith user signal;
3.13 calculate the next user signal. And (3.5) to (3.12) are repeated until all users are calculated.
3.14 repeat (3.5) - (3.13) until the set upper limit of IDMA iteration is reached.
The invention has the beneficial effects that:
the communication method of the invention utilizes CSK modulation to improve the communication rate on the basis of the traditional IDMA. At the receiving end, a passive time reversal technology is firstly adopted to compress the channel, and meanwhile, the signal-to-interference ratio of each user can be effectively improved. And then, a CSK soft demodulation mode is adopted, and CSK modulation and a traditional IDMA iterative demodulation system are perfectly combined, so that the detection performance of the system can be effectively improved. Compared with the existing multi-user underwater acoustic communication method, the method of the invention can effectively improve the communication rate of each user and can improve the number of users at the same time.
Drawings
Fig. 1 is a schematic diagram of the transmitting end structure of the present invention.
Fig. 2 is a schematic diagram of a receiving end structure of the present invention.
Fig. 3(a) is the equivalent channel of user 1 after passive time reversal.
Fig. 3(b) shows the interference channel from user 2 after passive time reversal.
Fig. 3(c) shows the interference channel from user 3 after passive time reversal processing.
Fig. 3(d) shows the interference channel from user 4 after passive time reversal.
Fig. 3(e) shows the interference channel from user 5 after passive time reversal.
Fig. 3(f) shows the interfering channel from user 6 after passive time reversal.
Fig. 3(g) shows the interference channel from user 7 after passive time reversal.
Fig. 3(h) shows the interference channel from user 8 after passive time reversal.
Detailed Description
The following detailed description of the invention refers to the accompanying drawings.
The present invention relates only to the baseband modulation stage in a communication system, and therefore emphasizes this part of the method, ignoring the content for the carrier modulation and demodulation stages.
The transmitting end structure is shown in fig. 1. Set the number of users of the system as MFor the mth (M ═ 1,2, 3 … … M) user, its transmitting end mainly consists of 5 modules: channel encoder, bit interleaver pim(for breaking concentrated burst errors), CSK modulator, chip interleaver pim(to distinguish users) and a carrier modulator. The following takes the specific implementation of the transmitting end of the mth user as an example.
The specific implementation mode of the transmitting terminal comprises the following steps:
t1. information data b of mth usermAfter channel coding, the coded signal is cm. Bit interleaver pi with mth usermTo cmInterweaving to obtain data dm(ii) a T2. using α ═ α1,α2,…,αG]To indicate a spreading sequence, and if the length is G, the corresponding CSK modulation order is Q log2G. For data dmGrouped by length Q. The total number of the groups is Z, and Q bit binary data in each group is converted into decimal deltaz。
T3. for Z spread spectrum sequences alpha, respectively, the cyclic shift delta to the right is carried outzBit, after parallel-to-serial conversion, the sequence s is obtainedm，smI.e. data dmA CSK modulated signal.
T4. use chip interleaver pi of mth usermTo cmInterweaving to obtain data xm。
T5. for data xmAnd carrying out carrier modulation and sending the carrier modulated signals into an underwater acoustic channel.
The receiving end structure is shown in fig. 2. The receiving end includes: pre-processing module, channel estimator, passive time reversal processor, basic signal estimator, decoding chip interleaver
Figure GDA0002730508980000061
CSK soft demodulator, de-bit interleaver
Figure GDA0002730508980000062
Channel decoder, bit interleaver pimCSK soft mapper and chip interleaver IImWherein the template is pretreatedIncluding synchronization, down-conversion, and sampling, which mainly convert the received passband signal into a baseband signal, and will not be described here.
The specific implementation mode of the signal processing of the receiving end baseband comprises the following sub-steps:
r1. N receiving hydrophones are arranged, and h is used for channels from the mth user to the nth receiving hydrophonen,m＝[hn,m(0),hn,m(1),…,hn,m(L-1)]TWhere L represents the channel impulse response length, then the nth receiving hydrophone receives a signal of
Figure GDA0002730508980000063
Wherein wn(j) Is additive white Gaussian noise with a variance of
Figure GDA0002730508980000064
Performing channel estimation on the received signal to obtain
Figure GDA0002730508980000065
The channel estimation method may employ a conventional channel estimation algorithm.
R2. initialize the mean value E (x) of all user signalsi(j) 0 and variance Var (x)i(j) 1, bit d)i(q) prior probability P (d)i(q)＝0)＝P(di(q)＝1)＝1/2，i＝1,2,…M。
And R3, initializing system parameters. Setting the number of IDMA iterations to Nouter，i outer1. Set i to 1.
And R4, setting the iteration number of the CSK soft demodulator to be Ninter，i inter1. For received signal rn(j) And carrying out passive time reversal processing. For the ith user, get
Figure GDA0002730508980000071
Wherein
Figure GDA0002730508980000072
Which represents a convolution operation, the operation of the convolution,
Figure GDA0002730508980000073
Figure GDA0002730508980000074
according to the passive time-reversal nature, Qi,i(t) is an approximate Sinc function and has a gain much greater than Qm,i(t) (m ≠ i). Fig. 3 shows the equivalent channel after the underwater acoustic communication experiment of 8 users, in which the user 1 has undergone passive time reversal processing, as can be seen from the figure, the expected equivalent channel gain is much higher than the interference channel gain of other users, and the expected channel has been compressed, and the energy is concentrated at the main path position. Therefore, the passive time reversal technology can effectively compress channels, improve the signal-to-interference ratio of each user and further improve the detection performance.
R5. calculating signal y(i)Mean and variance of (t):
Figure GDA0002730508980000075
Figure GDA0002730508980000076
wherein
Figure GDA0002730508980000077
Is the equivalent channel Q after passive time reversal processingi,i(l) Length of (d).
R6. calculating the interference signal of the ith user on the ith path
Figure GDA0002730508980000078
The mean and variance of (a), l ═ 1,2,…
Figure GDA0002730508980000081
Figure GDA0002730508980000082
Figure GDA0002730508980000083
r7., calculating the obtained extrinsic information of the ith user after the basic signal estimator:
Figure GDA0002730508980000084
Figure GDA0002730508980000085
r8. use of the decoded slice interleaver for the ith user
Figure GDA0002730508980000086
To eESE(xi(j) De-interleaving to obtain eESE(si(j))。
R9. pairs eESE(si(j) By length G) and calculating the signal si(g) Mean and variance of
Figure GDA0002730508980000087
Figure GDA0002730508980000088
Figure GDA0002730508980000089
R10. utilization of
Figure GDA00027305089800000810
Correlating with the local spread spectrum sequence alpha to obtain the result after the correlation
Figure GDA00027305089800000811
Where, represents the elements in the vector by dot multiplication, represents the conjugate, F is a Fourier transform matrix, F-1Is the inverse of the fourier transform matrix.
R11. calculating the variance of the autocorrelation of the local spreading sequence alpha
Figure GDA00027305089800000812
Figure GDA0002730508980000091
Figure GDA0002730508980000092
R12. calculating d of encoded datai(q) prior information:
Figure GDA0002730508980000093
r13. calculation of di(q) posterior information
Figure GDA0002730508980000094
Wherein P (d)i) Is based on bit di(q) a priori information calculation of,
Figure GDA0002730508980000095
Figure GDA0002730508980000096
corresponding to a binary sequence of value delta-1 after conversion to decimal.
R14. calculation of di(q) external likelihood ratio eCSK(di(q))＝Lposter(di(q))-Lpriori(di(q))，eCSK(di) Is the result of the CSK soft demodulation.
R15. Debit interleaver with ith user
Figure GDA0002730508980000097
To eCSK(di) De-interleaving and inputting to channel decoder to obtain information bit estimation of ith user
Figure GDA0002730508980000098
And outer likelihood information e of coded bitsDEC(ci)。
R16. bit interleaver with ith user piiTo eDEC(ci) Interleaving to obtain eDEC(di)。
R17. according to eDEC(di) Calculating P (d)i(q)＝0)，P(di(q)＝1)，
Figure GDA0002730508980000101
P(di(q)＝0)＝1-P(di(q)＝1)
R18. if iinter＜Ninter，iinter＝iinter+1, return to step R12;
r19. calculating the signal obtained after CSK soft mapping
Figure GDA0002730508980000102
The probability of (a) of (b) being,
Figure GDA0002730508980000103
wherein the content of the first and second substances,
Figure GDA0002730508980000104
Figure GDA0002730508980000105
is that
Figure GDA0002730508980000106
And (3) a sequence obtained after CSK modulation.
R20. chip interleaver pi with ith useriTo pair
Figure GDA0002730508980000107
Interweave to obtain
Figure GDA0002730508980000108
The corresponding mean and variance are calculated.
Figure GDA0002730508980000109
Figure GDA00027305089800001010
R21. if i < M, calculate the next user, let i ═ i +1, go back to step R4;
r22. if iouter＜NouterLet iouter＝iouter+1, i ═ 1, return to step R4;
and R23. ending the demodulation.
From the above description, it can be seen that the method of the present invention is vastly different from the conventional multi-user underwater acoustic communication method. At the transmitting end, CSK modulation is used to replace conventional direct sequence spread spectrum modulation, the communication rate of each user can be effectively improved, and the chip interleaving technology in the IDMA system is adopted to improve the number of users; at a receiving end, a passive time reversal technology is firstly adopted to carry out channel compression and improve the signal-to-interference ratio of each user, then a basic signal estimator is adopted to estimate symbol probability, and then iterative CSK soft demodulation is carried out by utilizing the estimated symbol probability. The method can be well combined with the iterative detection algorithm of the traditional IDMA, and can effectively improve the communication quality and the number of users of the existing underwater sound multi-user communication system.

Claims (3)

1. A multi-user underwater acoustic communication method based on cyclic shift keying spread spectrum modulation is characterized by comprising the following steps:
the first step is as follows: transmitting terminal
1.1 there are M users, the original data information of the mth user is bmMeaning that the signal after channel coding at rate R is cm；m＝1、2、3……M；
1.2 bit interleaver with mth user pimTo cmInterweaving to obtain data dm；
1.3 pairs of data dmCarrying out CSK modulation to obtain a sequence sm(ii) a The CSK represents cyclic shift keying;
1.4 chip interleaver pi with mth usermTo smInterweaving to obtain data xm；
The second step is that: using an underwater acoustic communication transmitter to transmit data xmTransmitting after carrier modulation;
the third step: receiving end
3.1, preprocessing the signals received by each receiving transducer;
3.2, channel estimation is carried out on the preprocessed received signals;
3.3, carrying out passive time reversal processing on the preprocessed multipath received signals so as to compress channels and improve the signal-to-interference ratio;
3.4 initializing the mean value and the variance of each user signal and the corresponding CSK symbol probability, namely presetting an initial value of a signal to be demodulated;
3.5, calculating the mean value and the variance of the signal of the ith user after passive time reversal processing;
3.6, calculating the mean value and the variance of the interference signal of the ith user;
3.7 computing the outer likelihood ratio of the ith user's transmitted signal and using the decoding chip interleaver of the ith user
Figure FDA0002730508970000011
Performing de-interleaving;
3.8 carrying out CSK soft demodulation on the deinterleaved signal external likelihood ratio to obtain demodulated soft information;
the CSK soft demodulation comprises the following steps: calculating an estimated value of the transmission signal by using the deinterleaved transmission signal outer likelihood ratio; the obtained signal estimation value is correlated with a spreading sequence used by CSK modulation to obtain a correlation result thetaiDerivation of thetaiThe posterior probability density function is combined with the CSK symbol probability to calculate the external likelihood ratio of the demodulated coded data;
3.9 Debit interleaver with ith user
Figure FDA0002730508970000021
De-interleaving the outer likelihood ratio after CSK soft demodulation, and then performing channel decoding by using a decoder to obtain the outer likelihood ratio of estimated information bits and decoded coded data;
3.10 bit interleaver with the ith useriInterweaving the decoded outer likelihood ratio output by the decoder, and performing CSK soft mapping again; re-inputting the obtained CSK symbol probability into a CSK soft demodulator for next CSK soft demodulation;
the CSK soft mapping is as follows: calculating the probability of each coded bit by using the outer likelihood ratio output by the decoder after interleaving; setting each code bit to be independent, and calculating the probability of the CSK symbol mapped by the code bit according to the mapping rule of CSK modulation;
3.11 repeating (3.8) - (3.10) until reaching the upper limit of the iteration number of CSK soft demodulation;
3.12 chip interleaver pi with the ith useriRe-interleaving the result of CSK soft mapping, and re-calculating the mean value and variance of the ith user signal;
3.13 calculating the next user signal; repeating (3.5) - (3.12) until all users are calculated;
3.14 repeat (3.5) - (3.13) until reaching the set IDMA iteration upper limit, the IDMA is the interleaving multiple access which distinguishes users by an interleaver.
2. The method of claim 1 wherein the probability of failing to map into CSK symbols is removed during the CSK soft mapping procedure of step 3.10 to improve system performance.
3. The method of multi-user underwater acoustic communication based on cyclic shift keying spread spectrum modulation according to claim 1 or 2, wherein in step 3.1, the preprocessing comprises synchronization, down-conversion and sampling processing.
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