Underwater sound OFDM downlink communication multiple access method based on polarization code superposition coding

Abstract

The invention belongs to the technical field of underwater acoustic communication, and relates to an underwater acoustic OFDM (Orthogonal Frequency Division Multiplexing) downlink communication multiple access method based on polarization code superposition coding. The method comprises the following steps: determining an optimal signal-to-noise ratio according to a required code rate, and calculating a Bhattacharyya parameter logarithm value of each sub-channel under the optimal signal-to-noise ratio; performing ascending sorting on the sub-channels according to the Bhattacharyya parameter logarithm value, and selecting an information bit position distribution set; dividing the sub-channels in the set into a plurality of subsets according to a recording sequence, and respectively distributing the subsets to different users; carrying out interleaving scrambling, constellation mapping and OFDM modulation on code words coded by a user to obtain time domain signals of the code words; superposing the time domain signals of the two users to obtain a sending signal of the main node; and decoding the received signal at a receiving end. According to the method provided by the invention, the fairness of the communication quality of a plurality of users is realized, the bit error rate is reduced, and under the condition that the signal-to-noise ratio difference is large enough, fair resource allocation is realized among the plurality of users, and the data transmission efficiency is improved.

Description

Technical Field

[0001] This invention belongs to the field of underwater acoustic communication technology, and relates to a method for multiple access in underwater acoustic OFDM downlink communication based on polar code superposition coding. Background Technology

[0002] OFDM (Orthogonal Frequency Division Multiplexing) is a type of MCM (Multi-Carrier Modulation). It achieves high-speed parallel transmission of serial data through frequency division multiplexing, exhibiting good resistance to multipath fading and supporting multi-user access.

[0003] In digital communication systems, improving the reliability and efficiency of communication transmission has always been a key research focus. Channel coding technology is an important means to solve communication reliability problems. Applying channel coding technology can effectively improve the system's anti-interference capability and enhance the quality of wireless channel data transmission. Severely limited communication bandwidth is one of the main bottlenecks to improving the data transmission rate of underwater networks. How to improve the spectrum utilization of physical layer multi-user communication within limited bandwidth is also a key problem to be solved. To address these issues, research can be conducted on many problems, including the core technologies of underwater multi-user communication, by combining polar code coding technology with orthogonal frequency division multiplexing (OFDM) technology, which has strong anti-multipath capability and extremely high spectrum utilization.

[0004] Polar codes, proposed by Arikan in 2008, are a novel channel coding scheme with a deterministic construction method. Among numerous coding schemes, polar codes are the first and only known channel coding method that can be rigorously mathematically proven to achieve the required channel capacity. Polar codes are a coding method specifically designed for binary-input discrete memoryless channels (B-DMC). Represents any B-DMC channel Channel input symbol set The output symbol set is , Let be any real number, The transition probability is .make express If it is reused a few times, then for The channel switching probability.

[0005] The existing multiple access technologies can be divided into orthogonal multiple access (OMA) technology and non-orthogonal multiple access (NOMA) technology. Due to various types of orthogonal multiple access, various resources can only be allocated to one user, thereby limiting the overall system throughput and the number of accessible users. Under such conditions, the non-orthogonal multiple access technology under the 5G communication technology emerges as the times require. The research on multi-user underwater communication technology has gradually developed since 1996. This technology studies the spatial diversity characteristics of the underwater acoustic channel to reduce the inter-symbol interference or mutual interference between underwater users. The practical application research of underwater acoustic multi-user communication in China has entered the initial stage. The research on underwater acoustic communication network is relatively lagging behind the development of foreign countries, and there are fewer long-duration sea trials. SUMMARY

[0006] In view of the defects and deficiencies in the prior art, in the framework of the underwater acoustic OFDM communication system, the polar code is selected as the channel coding scheme of the system, the code rate flexibility and other characteristics are utilized to combine the channel coding scheme with the non-orthogonal multiple access technology of the downlink communication under the premise of ensuring the reliability of the system communication, the access capacity of the system is improved, and a multi-user data high-speed stable transmission channel is opened up. For the downlink underwater acoustic communication system with strong and weak users, the optimal resource allocation scheme is to superimpose and send the multi-user data in the power domain, control the power allocation between users through the power factor, and realize the data recovery of each user through linear detection (weak user) or successive interference cancelation (SIC) detection mode (strong user) at the receiving end.

[0007] The technical scheme provided by the present application is a polar code superposition coding underwater acoustic OFDM downlink communication multiple access method, comprising the following steps: (1) according to the required code rate determining the optimal signal-to-noise ratio, calculating the log value of the B parameter of each subchannel under the optimal signal-to-noise ratio; (2) according to the log value of the B parameter, sorting each subchannel in ascending order, using a mark array to record the original index value, and selecting an information bit position distribution set ; (3) dividing the subchannels in the set into several subsets according to the recording order in the mark array, and allocating them to different users; the data subset of the first user is represented as ; (4) the code word after user coding is: ; wherein, represents the input information sequence of the th user, represents a polar code generation matrix , in which the rows are composed of rows in the information bit position distribution set ; represents a frozen bit sequence, represents a polar code generation matrix , in which the rows are composed of rows in the frozen bit position distribution set ; After interleaving scrambling, constellation mapping and OFDM modulation are performed on them respectively, the time domain signals of the users are obtained ; (5) The time domain signals of the users are superimposed to obtain the transmission signal of the master node; (6) Fourier transform is performed on the received signal at the receiving end to obtain frequency domain data; and decoding is performed.

[0008] Preferably, in the step (5), the calculation formula of the transmission signal of the master node is: ; wherein, is a power factor, satisfying .

[0009] Preferably, in the step (6), the near-end user adopts a successive interference cancellation decoding scheme, decodes the data of the far-end user first under the premise of its own interference, then subtracts it from the received signal, and then decodes its own data.

[0010] Preferably, in the step (6), the far-end user adopts a linear detection method to decode its own data.

[0011] The application proposes a non-orthogonal multiple access scheme based on polar code superposition coding for the scene in which the channel difference between multiple nodes is large in the downlink underwater acoustic multi-user communication system, introduces coding gain for the system by using the allocation mode of the polar code subchannel, so that the non-orthogonal multiple access algorithm in the downlink underwater acoustic communication system can expand the selectable range of the power factor through the double superposition of the coding domain and the power domain. The quality of multiple user communication is fair, the bit error rate is reduced, and in the case of large enough signal-to-noise ratio difference, fair resource allocation is realized among multiple users, and the data transmission efficiency is improved. BRIEF DESCRIPTION OF DRAWINGS

[0012] Figure 1 It is a model of underwater acoustic downlink communication system based on NOMA; Figure 2 It is a master node transmission signal construction process based on polar code superposition coding; Figure 3 The decoding principle block diagram of the receiving end of the polarization code superposition coding based multiple access method; Figure 4 The channel impulse response of two users is simulated. Figure 5 The comparison of the receiving error rates of two users under different power factors in the AWGN channel condition. Figure 6 The comparison of the receiving error rates of two users under different power factors in the underwater acoustic multipath channel condition. Figure 7 The comparison of the error rate performance of the polarization code superposition coding based multiple access and OFDMA. Figure 8 The transducer arrangement diagram of the pool experiment. Figure 9 The measured channel impulse response of the pool experiment. Figure 10 The comparison of the pool experiment picture transmission results of the polarization code superposition coding based multiple access method. DETAILED DESCRIPTION

[0013] In order to facilitate the understanding of the present application, the present application will be described in more detail below in combination with the drawings and specific embodiments. However, the present application can be realized in many different forms, and is not limited to the embodiments described in the present specification. On the contrary, the purpose of providing these embodiments is to make the understanding of the disclosed content of the present application more thorough and comprehensive.

[0014] The polarization code superposition coding based underwater acoustic OFDM downlink communication multiple access method provided by the present application introduces coding gain for the system by using the allocation mode of the polarization code subchannel, so that the non-orthogonal multiple access algorithm in the downlink underwater acoustic communication system can expand the selectable range of the power factor through the double superposition of the coding domain and the power domain. The basic principle is: When there is more than one group of input data in a polarization code codeword, there is a corresponding relationship between each group of input data and the subchannel set, and the coding form of the polarization code is: (1); Wherein, represents the input information sequence of the i-th user, represents the input information sequence of the i-th user, represents the polarization code generation matrix , the number of rows of which is the matrix composed of the rows in the information bit position distribution set . represents the frozen bit sequence, represents the polarization code generation matrix , the number of rows of which is the frozen bit position distribution set The matrix formed by the rows in the matrix.

[0015] A certain bitrate The following is a set of information bit location distributions. Divide into disjoint subsets This is allocated to different users as the set of data information bits used for their encoding, with each user having their own exclusive subset. Data information is transmitted in the middle, while A subset exclusively enjoyed by China Random bit information is transmitted on the other information bits.

[0016] NOMA technology, which performs multi-user data overlay in the power domain, has extremely high spectral efficiency. Figure 1 This describes a common scenario used in underwater acoustic downlink communication. Within the coverage area of ​​a master node, there exists a pair of users with different channel conditions. Assume the users... The distance between the master node and the user is less than Then the user The channel conditions are better than those of the user. The master node's transmission signal superimposes multi-user data on time and frequency resource blocks, and adjusts the transmission power between different users through a power factor.

[0017] Taking this scenario as an example, for a code length of... The process of constructing polar codes used for superposition coding is described in detail: First, based on the designed bitrate Determine the optimal signal-to-noise ratio (SNR) and calculate the logarithmic values ​​of the Bavarian parameters for each sub-channel at the optimal SNR. Sort them in ascending order and select the first few. Each record is a set of information bit location distributions. The remaining The sub-channels corresponding to each index value are a set of frozen bits. The subchannels are divided into two equal-sized subsets according to the recording order. and Because of users The channel conditions are poor, so a set of highly reliable polarized sub-channels is assigned to it. This serves as the set of unique information bit locations for that user. For users The system defines a set of dedicated information bit locations. Each user transmits data information bits within their own dedicated set, while transmitting random bit streams in the sub-channel sets of other users, thus achieving multi-user access in the coding domain. Considering the feasibility of the algorithm and the burden of computational complexity, the random bits transmitted by each user should be known prior information, and can be set to all zero bits.

[0018] Assuming that the random bit stream transmitted in two users is all zero bits , the sending bit data of user and user are respectively and , the encoded code words of user and user are respectively formula (2) and formula (3): (2) ; (3).

[0019] The polarization code encoding code rate of each user is , at this time, in the assumed downlink OFDM underwater acoustic communication system scenario, the actual available information code rate of a single user is . Figure 2 The flow of constructing the primary node sending signal based on the polarization code superposition coding of two users is described, and the flow can be extended to the scenario of more users. After interleaving scrambling, constellation mapping and OFDM modulation are performed on the code words and , the time domain signals thereof are respectively and , the time domain data of the two users are superposed according to a power factor to obtain the sending signal of the primary node as shown in formula (4), and the double superposition of the coding domain and the power domain is completed: (4), and alpha is a power factor.

[0020] The decoding principle of each user in the receiving end in the polarization code superposition coding based multi-access method is shown in Figure 3 . Different users adopt different detection methods for processing the received information in the receiving end. In the sending end, the user is allocated a polarization post-subchannel set with higher reliability, so that the user can decode the own data by using a linear detection method under the premise that there is data interference of the user . The received signal-to-noise ratio of the user is large, and the SIC decoding scheme is adopted, that is, the data of the user is decoded firstly under the premise that there is own signal interference, and then the data is subtracted from the received signal, and then the own data is decoded.

[0021] The following simulation experiment is a specific application example of the method of the application, and the performance of the method of the application can be verified.

[0022] Simulation condition: in the MATLAB simulation platform, a water acoustic OFDM communication system with the number of receiving users being 2 is constructed, the polarization code length is N=2048, the mapping method is QPSK, and the proportion of sub-channels that actually carry information bits in each codeword is... C =3 / 4, due to the presence of random bits, the effective code rate for a single user is 3 / 4. The communication frequency band is 8~16kHz, and the simulated channel impulse response is as follows: Figure 4 As shown.

[0023] First, the impact of power factor selection on the system's bit error rate performance is analyzed. When the signal-to-noise ratio difference between strong and weak users is 8 dB, the influence of different power allocation factors on the system's bit error rate is explained. Figure 5 The distribution of the system bit error rate (BER) curves for two users under different power factors in an additive white Gaussian noise (AWGN) channel is presented. The simulation step size is 0.5. The BER in the figure is... The figure indicates that the system's bit error rate is 0 in this simulation. As can be seen from the graph, with weak users... An increase in the received signal-to-noise ratio (SNR) increases the range of power factors that can meet the requirements of reliable communication, especially for weak users. When the received SNR is 9dB, the selectable power factor range has reached . Figure 6 The distribution of the system BER curves for two users under different power factors in the underwater acoustic multipath channel condition is presented. It can be seen that when the weak user... When the received SNR is 9dB, the selectable power factor range is approximately Therefore, it can be seen that the method proposed in this invention expands the selection range of power allocation factors, enabling weak users to correctly decode their own information even when the power of strong users is only slightly less than their own power. Thus, the power allocation between strong and weak users is more flexible and more suitable for underwater acoustic channels with complex communication environments. This effectively reduces the system's bit error rate, improves the overall system throughput and other performance, and can approach the optimal situation.

[0024] Secondly, an Orthogonal Frequency Division Multiple Access (OFDMA) system was selected as a control group. Simulation analysis was conducted to compare the performance of the polar code-based superposition coding multiple access scheme with OFDMA. Some parameters of both schemes are as follows: polar code superposition coding rate. , The code rate is 3 / 8, and the OFDMA access method code rate is 9 / 16; polar code superposition encoding mapping method. , QPSK, the mapping manners of user 1 and user 2 in the OFDMA access scheme are QPSK and 16QAM respectively; the data rate of the polar code superposition coding and the OFDMA access method , are all 0.45. Figure 7 The system bit error rate performance of the polar code superposition coding-based multiple access and the OFDMA is given when the signal-to-noise ratio difference between users is 8dB. Figure 7 It can be seen from the comparison that, when the polar code is also used as the channel coding scheme, the system bit error rate of the two users based on the polar code superposition coding has a gain of about 4-5dB compared with the OFDMA system. Therefore, the performance of the non-orthogonal multiple access method used in the present application is obviously higher than that of the OFDMA access method, and the performance of the non-orthogonal multiple access method is improved more within a certain signal-to-noise ratio difference. In the orthogonal multiple access method, the transmitting end needs to provide most of the channel resources for the user with poor channel conditions to obtain a data rate close to that of a single user. The non-orthogonal multiple access method can provide full freedom of the channel for the user with a strong channel state. Compared with the OFDMA access method, each channel sub-block of the fading channel can be used by multiple users simultaneously in the present application, and the sub-channel transmits the superimposed data of multiple different users. Although the non-orthogonal multiple access method actively introduces interference between different users, the coding method based on the polar code used in the present application uses the successive interference cancellation algorithm at the receiving end to superimpose the information of multiple downlink users in the power domain, so that the channel capacity can be improved under the condition of ensuring the transmission quality.

[0025] The following is an analysis of the pool experiment results, which can verify the reliability and effectiveness of the present application. The experiment is verified in the channel pool, and the layout of the transmitting transducer and the receiving hydrophone 1 and the receiving hydrophone 2 is shown in Figure 8 , and the power factor of the superimposed transmitted signal is . The measured pool channel impulse response is shown in Figure 9 . Figure 10 The picture transmission results of the pool experiment are given, and it can be seen that, as the received signal-to-noise ratio difference between users decreases, the data of user gradually increases and appears on user , i.e., user can accurately decode the data of user and remove it from the received signal after reconstruction. Therefore, the performance of the method proposed in the present application can be verified.

[0026] In conclusion, the polar code superposition coding based on the method for water acoustic OFDM downlink communication multiple access provided by the application, through the double superposition of coding domain and power domain, the optional range of power factor is expanded, the power distribution between strong and weak users is more flexible, and it is more suitable for the water acoustic channel with more complex communication environment.

Claims

1. A method for multiple access in underwater acoustic OFDM downlink communication based on polar code superposition coding, characterized in that: Includes the following steps: (1) Based on the required bit rate Determine the optimal signal-to-noise ratio (SNR) and calculate the logarithmic values ​​of the Bach parameters for each sub-channel at the optimal SNR. (2) Sort each sub-channel in ascending order according to the logarithmic values ​​of the Parity parameter, use a marker array to record the original index values, and select the information bit position distribution set. ; (3) Set The sub-channels in the array are divided into several subsets according to the record order in the marker array, and each subset is assigned to a different user; A subset of data for each user is represented as ; (4) The polar codewords after user encoding are: ； in, Indicates the first A sequence of input information from each user. Represents the polar code generator matrix In the set, the number of rows is distributed in the information bit position. The matrix formed by the rows in the matrix; Indicates a frozen bit sequence. Represents the polar code generator matrix In the set of row numbers distributed at the frozen bit positions The matrix formed by the rows in the matrix; After interleaving, constellation mapping, and OFDM modulation, the time-domain signal is obtained. ; (5) Superimpose the time-domain signals of each user to obtain the transmission signal of the master node; (6) Perform Fourier transform on the received signal at the receiving end to obtain frequency domain data; and then decode it.

2. The downlink communication multiple access algorithm based on polar code superposition coding underwater acoustic OFDM according to claim 1, characterized in that: In step (5), the formula for calculating the master node's transmitted signal is: ; in, Power factor For the first A time-domain signal, satisfying .

3. The method for downlink communication multiple access based on polar code superposition coding of underwater acoustic OFDM according to claim 1, characterized in that: In step (6), the near-end user adopts a continuous interference cancellation decoding scheme. Under the premise of its own interference, it first decodes the data of the far-end user, then subtracts it from the received signal, and then decodes its own data.

4. The method for multiple access in underwater acoustic OFDM downlink communication based on polar code superposition coding according to claim 1, characterized in that: In step (6), the remote user decodes its own data using a linear detection method.

Images