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Hindawi Journal of Robotics Volume 2022, Article ID 2576303, 11 pages https://doi.org/10.1155/2022/2576303 Research Article Research and Implementation of Turbo Coding Technology in High-Speed Underwater Acoustic OFDM Communication 1 2 Yarang Yang and Yunpeng Li College of Physics and Electrical Engineering, Kashi University, Kashi, Xinjiang 844006, China Qingdao Vocational and Technical College of Hotel Management, Qingdao, Shandong 266100, China Correspondence should be addressed to Yunpeng Li; liyunpeng@qchm.edu.cn Received 28 December 2021; Accepted 17 February 2022; Published 15 March 2022 Academic Editor: Shan Zhong Copyright © 2022 Yarang Yang and Yunpeng Li. )is is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. It is demonstrated that the fully parallel turbo decoding algorithm can achieve an approximate error correction decoding performance when 36 iterations are used and when the log-map algorithm with 6 iterations is used. By comparison, it is shown that it can achieve much higher decoding rates than the log-map algorithm for various frame lengths of LTE standard turbo codes at the cost of higher hardware resource requirements. According to the fully parallel turbo decoding algorithm, this paper proposes a scheme for implementing a fully parallel turbo decoder on FPGA, detailing the overall structure and processing of the decoder hardware implementation, the design of the algorithm block processing unit, and the interleaving module. )e per- formance of the decoder is tested by ﬁxed-point simulation for diﬀerent frame lengths of turbo coding in LTE standard, and it is proved that the fully parallel turbo decoder can be applied to turbo coding of various frame lengths. Both simulation and experimental results show that the distributed cancellation method and the joint estimation cancellation method have good results for both time-domain impulse noise and large-amplitude single frequency noise cancellation, while the joint estimation can- cellation method of large-amplitude single frequency noise cancellation ﬁrst has better performance. carrier frequency. )e problems in hydroacoustic OFDM 1. Introduction communication need to be studied in more depth, such as Hydroacoustic communication is a rapidly developing ﬁeld how to eliminate external noise interference; how to elim- of scientiﬁc research; its engineering applications used to be inate and compensate the impact of UAC on the OFDM limited to military aspects to solve the problem of mine system to ensure high system reliability; how to better use remote control, submarine to submarine, mother ship and the sparsity characteristics of UAC to improve system submarine, or other underwater unmanned combat plat- performance; how to adapt to the hydroacoustic commu- form transmissions to obtain battleﬁeld information, and nication channel by using coding techniques to improve limited bandwidth, multipath, transmission delay, and system performance; and how to improve the utilization of channel structure of the rapid time change are currently frequency band to achieve high-speed hydroacoustic diﬃcult problems [1]. communication. After years of development, OFDM has fully demon- )e hydroacoustic channel is very complex, with strong strated its advantages, but some problems based on the multipath and complex noise interference, and the channel hydroacoustic OFDM communication system are still not parameters are time varying, so it is very diﬃcult to get the well solved. Many communication methods are contro- state information of the hydroacoustic channel at the versial, there is no uniﬁed standard, and diﬀerent experi- transmitter side. )e transmitter side achieves adaptive ments are usually used in diﬀerent system parameters, such transmission based on the obtained CSI, but the hydro- as coding modulation, communication bandwidth, and acoustic channel is time varying and there is a certain delay 2 Journal of Robotics technology to hydroacoustic communication requires in the process of feeding CSI back to the transmitter side. )erefore, this paper investigates the prediction of hydro- overcoming the Doppler shift. Due to the above complex characteristics of the shallow sea hydroacoustic channel, acoustic channels based on the OFDM system and proposes a channel prediction technique based on the sparsity of various OFDM-based communication technologies must be hydroacoustic channels to compensate for the time delay of improved accordingly to apply to the hydroacoustic channel, underwater transmission, so that the obtained channel state and a lot of experimental validation is needed after the information can better reﬂect the current channel condition. improvement to be put into use. Because of the channel characteristics such as space-time frequency variation and narrow-band, high-noise, strong 3. OFDM for Hydroacoustic multipath interference, and large transmission delay, Turbo Communication Systems codes combine convolutional codes and random interleavers to realize the idea of random coding, while soft-output it- 3.1. OFDM for Hydroacoustic Communication Systems. erative decoding is used to approximate the maximum )e basic principle of OFDM is to split the original signal likelihood decoding, which can help achieve the channel into N subsignals by serial-parallel conversion: if the serial coding performance limited by Shannon theory over code rate is R, the converted code rate is R/N, the subsignal Gaussian channels [2, 3]. period is Δt, and the theoretical value of the complete period of the original signal is T � N∗Δt. )en the N subsignals 2. Key Technologies for High-Speed are modulated on N mutually orthogonal subcarriers, and ﬁnally the N modulated signals are summed to get the Hydroacoustic Communication transmit signal. At the receiving end, the input signal is )e key technologies for high-speed hydroacoustic com- divided into N branches, and then the N subcarriers are munications include six main directions, namely, new mixed and integrated to recover the original data. At the modulation methods to reduce the eﬀects of multipath; receiving end, the input signal is divided into N branches, coding techniques, including compression coding required which are mixed and integrated with N subcarriers to re- for image transmission and error correction coding tech- cover the parallel data, and then the original data can be niques that can improve system reliability; receiver archi- recovered through parallel-serial conversion and demodu- tecture, as reﬂected in the use of powerful signal processors lation [4]. and algorithms; underwater network systems; various ex- T − T + k · T< T + Δt<T + T + (k + 1) · T. plicit and implicit diversity techniques applied to fading 1 min 2 1 min channels; and hydroacoustic channel physics. )e topics (1) include the simulation and measurement of channels [2]. )e phenomenon of self-interference exists in the After decades of development, the hydroacoustic com- hydroacoustic communication system, that is, the signal sent munication method has gradually tended to move from by a terminal after mechanical vibration and another short- noncoherent communication to coherent communication, range low-attenuation propagation method to reach its with new technologies and advances in signal modulation receiving transducer, and its amplitude is much larger than and demodulation and signal processing, such as spatial the target signal propagated from other terminals over long modulation technology and blind equalization technology. distances. )erefore, the frequency division multiplexing of According to historical materials, it was Leonardo da Vinci the hydroacoustic system in the simultaneous transmission who pioneered a method of transmitting and receiving and reception of signals, the need to ﬁlter out the self-in- information underwater, which can be traced back to the terference signal, and the system’s ﬁltering performance put earliest hydroacoustic communication system. Several fre- forward high requirements, and engineering implementa- quency points are selected in the frequency band, and the tion is also more diﬃcult; time-division multiplexing system transmission of information works at the transmitting end in the reception of signals also needs to send a signal after a by transmitting carriers of diﬀerent frequencies. )e fre- certain interval, waiting for the elimination of the self-in- quency shift keying system is more stable than the ASK terference component before collecting data. system communication performance, but the same com- munication rate is slower, and the frequency band utilization T + Δt≤ T + T + k · T. (2) 2 1 min is not high, the information of adjacent frequency points is easily aﬀected by Doppler shift, and the demodulation is easy Diﬀerent terminals have diﬀerent requirements for to generate false codes. communication indicators, which include communication Multipath propagation causes intercode interference in type, communication target, communication speed, and single-carrier digital communication systems, introduces communication delay. Take an exploration activity that multipath expansion between symbols, and to some extent includes a diver, an underwater probe, an underwater limits the use of single-carrier communication technology beacon, and a mother ship as an example: the diver and the for high-speed hydroacoustic communication systems. In mother ship communicate with each other through com- shallow sea channels, the time-varying nature of the seawater mands, and the mother ship sends commands to the diver to medium and the relative motion between the transmitter direct him to conduct underwater exploration. )is process and receiver are the two factors that cause Doppler fre- requires a small communication delay; the underwater probe quency shift. )e application of OFDM communication sends data back to the mother ship in one direction, which Journal of Robotics 3 does not require high communication delay, but high pa communication speed; the underwater beacon sends com- h(t, τ) � ξ (t)δ τ + τ (t) , (5) p p mands back to the mother ship in one direction, which p�1 requires high real-time communication and does not require where N denotes the number of paths, τ (t) denotes the am- high communication speed. plitude of the path, and ξ (t) denotes the delay of the amplitude k · T of the path. Coherent communication of hydroacoustic com- v � , 2T + T + (k + 1) · T − 2Δt 1 max munication systems must perform channel estimation and (3) compensation for correct demodulation. Using hydroacoustic 2 2 Sf f f channel characteristics, such as the sparse nature of the channel, a � A − B . 2 2 f − f the design of a simple structure, low computation, and high estimation accuracy of the channel estimation, an equalization )e overlapping time slot allocation method is still algorithm is a key technology for multicarrier hydroacoustic applied under the condition that the propagation time of the communications. acoustic signal is greater than the minimum time slot. )e As much as possible, the bits in the information se- ﬁrst time slot is allocated for the host and the slave to send quence input to the interleaver are disordered so that the signals simultaneously, while the second time slot is allo- information bits nearby before interleaving are permuted cated for the host and the slave to receive signals simulta- to positions farther apart after interleaving, especially not neously. When communication is performed in this way, the to have information bits that are originally adjacent to each highest communication rate per unit time is achieved be- other still in adjacent positions after being permuted. Try cause as many data frames as possible are transmitted, but not to have the bits in the information sequence of an input the communication delay of the system is larger. If the host interleaved, after being permuted by the interleaver and or slave needs to send data at moment t, the earliest it can when it does not go through interleaving, corresponding to receive this signal is at moment 3t, and the longest com- the check bits in the output check sequence of diﬀerent munication delay is twice the propagation time. component encoders that are most correlated with it and are in the position where the censoring operation will be performed. )e theory of ray propagation is expounded, 3.2. Key Technologies of a Hydroacoustic Communication and, on this basis, the sound ray span model and the search System with OFDM. )e key technologies of OFDM’s algorithm for eigenrays in the layered ocean based on the hydroacoustic communication system mainly include spatial model are introduced. )e algorithm can quickly and diversity. accurately search for important eigenrays. )en, the eigenpath model of the underwater acoustic channel is ψ(t) � exp jλt + ct − jβsgn(t)φ(t, a) , given in this paper, and the eigenpath channel is analyzed in detail with mathematical formulas from the aspects of aπ ⎧ ⎪ ⎪ tan , a≠ 1, (4) propagation attenuation and delay, multipath components, φ(t, a) � and Doppler frequency shift, along with the rationality and feasibility of the model. )e length of the interleave is ln t, a � 1. increased as much as possible so that the correlation of the information sequence before and after interleaving is as low Havingλ, c, α, β as four parameters, the characteristic as possible, thus obtaining a turbo coded output sequence function of the steady-state distribution can be obtained by these four parameters. )e accurate measurement of the with a larger minimum Hamming distance and a minimum Hamming distance code word, which can reach a lower pressure expansion ratio of the signal eﬀectively compen- sates for the Doppler of the signal, while the compensation of BER limit. In the practical application of communication systems, the consistent Doppler frequency bias is usually required the choice of interleaver used in turbo encoder requires a again after the variable sampling due to the measurement balance between the error correction decoding performance and compensation accuracy. and the complexity it requires for hardware implementation. Compared with single-carrier systems, OFDM signals In the LTE standard, a quadratic permutation polynomial behave in the time domain as a superposition of N mutually interleaved is chosen for turbo coding. )e redundant bits orthogonal subcarrier signals. When all these N signals are exactly summed at the peak point, the OFDM signal pro- can be removed by the redundancy process because the conventional turbo encoder uses two fractional convolu- duces the maximum peak, which is N times the average power, and the peak-to-average power ratio is corre- tional encoders to obtain two sequences of check bits equal to the length of the information bit sequence as the re- spondingly larger. As the number of subcarriers N increases, the maximum value of peak-to-average power ratio (PAPR) dundant bits of the coded output, which is twice as many as the check bits needed in general, and this extra check bits can also increases linearly, which puts high requirements on the be removed by the redundancy process, which can reduce linear range of the transmitter front-end ampliﬁer, so a the redundancy of the turbo coded output sequence and simple and eﬀective technique to reduce the peak-to-average obtain higher performance. )ese extra parity bits can be power ratio becomes an important research direction for removed through redundancy processing, thus reducing the OFDM technology. 4 Journal of Robotics redundancy of the turbo coding output sequence and appropriate length of the cyclic preﬁx has a certain impact obtaining a higher code rate. on the improvement of the prediction performance. )e Turbo frequency domain equalizer structure is shown in Figure 3. Both demodulator and decoder use 3.3. Advantages and Disadvantages of a Hydroacoustic processing operates a priori on the input and output values, Communication System with OFDM. )e advantages of the providing a Turbo excitation for the next iteration. )e same OFDM system are strong resistance to intercode interfer- Turbo processing principles apply to coded CDMA systems, ence, suitability for high-speed data transmission, high- producing Turbo multiuser detection. frequency utilization, and strong antifading ability. OFDM is Ampliﬁer circuits are divided into Class A, Class B, strong against intercode interference due to the use of cyclic Class A-B, and Class D according to their eﬃciency, of preﬁx [5]. which Class A ampliﬁers have an ampliﬁer tube con- )e shortcomings of OFDM technology include sensi- duction angle of 360 . Class B ampliﬁers have a conduction tivity to frequency bias and phase noise and large power angle of 180 ; the ampliﬁer tube only works in half a cycle peak-to-mean ratio. )e power eﬃciency of the OFDM and can only amplify the positive half-cycle signal [12–14]. signal is lower, and the OFDM signal is summed by multiple Raised cosine ﬁlters (also known as Nyquist ﬁlters) gen- independent modulated subcarrier signals [6]. erate bandwidth-limited signals without intermember interference, making them the most commonly used ﬁlter type for OFDM modulation. In most communication 4. Turbo Code-Based High-Speed systems, the raised cosine overall response is obtained by Hydroacoustic OFDM Communication evenly distributing the ﬁltering between the transmitter and receiver, resulting in a square root raised cosine ﬁlter. Coding Scheme Design For the OFDM modulation scheme, where the amplitude is 4.1. High-Speed Hydroacoustic OFDM-Based Communication constant and the data being transmitted are used to control System Structure Design. )e architecture of the high-speed the carrier frequency, a Gaussian ﬁlter is typically used to hydroacoustic OFDM-based communication system shown modulate the signal (rather than the RF signal). )e in Figure 1 includes coding, interleaving, and modulation common push-pull ampliﬁcation structure is the use of modules. two ampliﬁer tube combinations, respectively, amplifying High-speed hydroacoustic OFDM-based communica- the positive and negative half-cycle signal and then syn- tion system architecture support all mandatory and optional thesizing as the output. Class A and B ampliﬁers are data rates: 6, 9, 12, 18, 24, 36, 48, and 54 Mb/s, including slightly less eﬃcient than class B ampliﬁers, but they solve adaptive modulation and multipath fading channel dis- the problem of crossover distortion of class B push-pull persion, i.e., simulating dynamically changing data rates ampliﬁcation. Class D ampliﬁer tube is switching, and the encoded with a high channel fading rate and allowing data eﬃciency is very high, but the circuit design and imped- rates to change more rapidly, and random data generation at ance matching are very tedious and easy to lead to signal a bit rate simulates a variety of code rate environments that nonlinear distortion. )is system selects the high-eﬃ- hold at diﬀerent data rates for some time so that the required ciency Class A and B ampliﬁer structure to build the power data rate depends on the expected value of the module; it ampliﬁer circuit. uses 1/2, 2/3, and 3/4 convolutional coding code rates, uses data interleaving design, and supports BPSK, QPSK mod- ulation, 16QAM, and 64QAM modulation [7]. See Figure 2 4.2. Design of Turbo Code Coding Scheme in the High-Speed for an OFDM frame fusion module based on the architecture Hydroacoustic OFDM Communication. )e original coding of a high-speed hydroacoustic OFDM communication structure of the Turbo code encoder is Parallel Concatenated system. Convolution Codes (PCCC), as shown in Figure 4 [8], which )e high-speed hydroacoustic OFDM-based commu- cleverly combines convolutional codes and random inter- nication system architecture corrects the number of data leavers to realize random coding while constructing long symbols and omitted bits in each packet and uses an un- codes from short codes through the interleaved. interrupted frame structure, omitting the tail part; the de- )e Turbo code has a much better error correction coder has a reset state and ﬁxes the transmit power level decoding performance than traditional channel coding instead of the average signal-to-noise ratio of the diﬀerent schemes such as packet codes and convolutional codes channels. because, on the one hand, the interleave is used to obtain two In general, a high input SNR implies a high output SNR check sequences with low correlation as the coding output and a high prediction accuracy. However, the length of the and, on the other hand, the decoder uses an iterative cyclic preﬁx determines the size of the input SNR. If the decoding method by exchanging soft information between cyclic preﬁx is too long, fewer noise coeﬃcients will be the component decoders. Since a hard decision will lose removed and the accuracy of the time-domain channel some information in the information sequence, the decoder prediction will be aﬀected. If the cyclic preﬁx is too short, it uses two soft-input and soft-output component decoders to will make the actual channel tap coeﬃcients potentially leaky preserve as much as possible the information of the system and lead to a serious loss of prediction performance [8–11]. information sequence contained in the diﬀerent check )erefore, in time domain channel prediction, choosing the sequences. Journal of Robotics 5 High-speed Orthogonal frequency Architecture Frequency division OFDM- Modulation based Hydroacoustic modules Communication Training Dispersion system Module Multiplexing sequences module Convergence Fading protocol channels Channel Subcarriers parameters Channel Viterbi decoder module characteristics Equalizer module Figure 1: Structure of high-speed hydroacoustic OFDM-based communication system. Pilot1 Pilot2 Pilot3 In 1 5×20 A1 13×20 In 2 B1 6×20 In 3 In C1 15×24 Out A2 13×20 In 4 ... B2 In 5 5×20 data rates In 6 1×20 A3 Prepend training In 7 1×20 Assemble Subcarriers Figure 2: OFDM frame fusion module based on the structure of high-speed hydroacoustic OFDM communication system. Since the output of the two ends of the receiver trans- component of the output signal of the hydroacoustic ducer is a diﬀerential signal with a DC bias, direct ampli- transducer. )e diﬀerential ampliﬁer circuit is built using ﬁcation of this diﬀerential signal with an uncertain reference TI’s low-power instrumentation ampliﬁer INA118. level tends to overload the input of the ampliﬁer circuit [15]. As shown in Figure 5, the Turbo encoder is usually )e error of our simulation and experimental results is designed to consist of two or more conventional decoders small, less than 1%, which can illustrate the accuracy of our connected in parallel. To ensure that the data received by the encoder is statistically independent, an interleaver is added theory. )erefore, a diﬀerential to single-ended ampliﬁer circuit is introduced to convert the diﬀerential signal into a between the two conventional decoders. single-ended signal whose reference level is the ground level )e same is fed to the convolutional encoder in bit in the circuit for poststage processing. )e diﬀerential to format. )is encoder obtains this data, ready to be sent over single-ended circuit also suppresses the common-mode the physical channel, by competing for information, select data blocks 6 Journal of Robotics High-speed hydroacoustic Uninterrupted frame structure Lead to a serious loss Noise Input Communication system Transmit power level instead of the of coefficients SNR architecture corrects average signal-to-noise ratio prediction performance Number of data symbols Different channels Each packet Accuracy Actual channel tap Time-domain channel coefficient prediction Length of the cyclic prefix SNR implies a high output SNR High prediction accuracy determines Figure 3: Frequency domain equalizer structure. Positive and negative Two half-cycle ampliﬁer Ampliﬁers signal tube Interleaver RSC encoder have a Eﬃciency combination conduction s is very high Information sequence Class A and B ampliﬁers are slightly Class D ampliﬁer tube Eﬃcient than class B ampliﬁers Shi Register Impedance Common Amplify the matching Ampliﬁer tube push-pull positive half- Shi Register Two only works ampliﬁcation cycle signal ampliﬁer tube combination Figure 4: Parallel cascaded convolutional code encoder. extracting information systems and recursive bits that can )e two independent check information sequences enter ensure that the data reach the end-user terminal accurately. into the two-component decoders respectively, and each Figure 5 depicts a schematic diagram of a Turbo cyclotron subdecoder decodes the system information sequence and encoder consisting of two separate interleaved encoders. check information sequence respectively to get diﬀerent soft Journal of Robotics 7 Input Bits Encoder is statistically independent Convolutional Interleaver is added between encoder in bit the two conventional Convolutional Encoder Interleaver format decoders Turbo encoder is usually designed to consist Extracting Ensure that the information data received systems End-user terminal Data reaches accurately Figure 5: Structure of Turbo code encoder. An improved decoding method is to use sliding windows information about the information sequence, and then they exchange the relatively independent two sets of output soft for decoding. )e sliding window decoding algorithm di- information as to their respective new inputs, and after vides a long block of code into multiple windows according several iterations, the error bits in the received sequence are to a preset window length and slides to each window in turn reduced to a certain degree, as well as the output soft in- for decoding. In the processing of the sliding window al- formation [16]. After several iterations, the error bits in the gorithm, the initial values of the forward state metric and received sequence are reduced to a certain level, and the backward state metric of each window are used [18]. After output soft information can be hard-judged as the decoded analyzing the characteristics of Circular Permutation Code output so that a good error correction performance is Words (CPC), taking the group interface of a communi- cation private network as an example, the hardware circuit achieved. )e encoded check bits are processed to produce design scheme of the group CPC transceiver is introduced, and the generation of the CPC decoding table is given. )e codewords of diﬀerent code rates. )e component en- coder can be either a recursive systematic convolution algorithm focuses on the module division, function and (RSC) code or a nonsystematic convolution (NSC) code. implementation method of the circuit, and the design and Given an RSC code, it is always possible to ﬁnd an NSC use of registers and brieﬂy analyzes the detection perfor- code that generates polynomials corresponding to it and mance of the circuit. )e design is implemented by a ﬁeld vice versa; see Figure 6 for the structure of the Turbo programmable logic device (FPGA), and the resource oc- receiver of ISI channel. cupancy is low, has good portability, and can be used for Because the calculation of the forward state metric and similar interfaces after simple modiﬁcation. It is a relatively backward state metric of each information bit in the log-map common CPC transceiver circuit. By taking advantage of the convergence of the lattice recursive computation, the various decoding algorithm of turbo code depends on serial re- cursive operations, the processing time of the whole unknown probabilities are in advance. decoding process increases signiﬁcantly when using larger )e basic structure of the Turbo code decoder is shown code blocks and interleaves that can bring better error in Figure 7. correction performance, which causes signiﬁcant processing )e hard judgment module is used to process the soft delays and limits the channel compilation code rate [17]. information of the ﬁnal output of the second component When turbo coding is applied in a practical communication decoder after several iterations to obtain the decoded output system, the turbo decoding algorithm needs to be improved information bit sequence of the decoder [19]. by weighing the processing delay, system throughput, and hardware resource consumption. Common turbo decoding improvement structures include the sliding window 4.3. Simulation Results for the Design of Turbo Code Coding Scheme in a High-Speed Hydroacoustic OFDM decoding algorithm, block parallel decoding structure, and radix-8 algorithm. Communication. )e simulation results of the Turbo code 8 Journal of Robotics Performance e encoded Diﬀerent code Produce limits check bits rates codewords Game match MAP decoder Deinterleaver Change of Component status encoder MAP Good free NSC code demodulator throw shooting Team's e component defensive codes Same lattice structure Recursive systematic convolution Interleaver Generates Non-systematic Free distance Vice versa polynomials convolution corresponding Figure 6: Turbo receiver architecture for ISI channel. Same before the recursive computation Improved decoding Sliding windows for method decoding Taking advantage of the Lattice recursive convergence computation Sliding window decoding algorithm divides Unknown probabilities Forward and backward of the various possible Long block of code into state metrics initial values multiple windows Const State Backward Advance Length raint Preset window length Each window in turn and slides for decoding Initial Processing of the sliding Forward Recursive Turbo Recursive values window algorithm Initial values of the Backward state metric forward state metric of each window Wind Compu Metrics Decoder A ows tation Figure 7: Block diagram of Turbo code iterative decoding structure. Journal of Robotics 9 coding scheme in high-speed hydroacoustic OFDM com- 100 munication designed as described above are as follows. Per- packet BER, shown as a percentage packet error rate. For 85 most of the packets, the BER is zero. See Figure 8 for simulation results of the high-speed hydroacoustic OFDM communication system based on Turbo codes, per packet BER. 55 When using the maximum frame length of N-6144 for turbo coding in the LTE standard, the decoding rate uses the 40 fully parallel decoding algorithm. If shorter frame lengths are used, the decoding rate advantage of the fully parallel 25 decoding algorithm decreases but is still much higher than that of the log-map algorithm. Although the fully parallel 10 decoding algorithm can greatly increase the decoding rate and reduce the decoding latency, it will signiﬁcantly increase 0 5 101520253035404550556065707580859095100 the hardware resources required by the decode, and the fully Per packet parallel turbo decoder requires a large amount of computing BER 1 BER 3 and register resources. System sticks to LTE speed. BER 2 BER 4 See Figure 9 for simulation results of high-speed Figure 8: Simulation results of the high-speed hydroacoustic hydroacoustic OFDM communication system based on OFDM communication system based on Turbo codes, BER per Turbo codes-S/N ratio. packet. )e fully parallel decoding algorithm with 36 iterations of approximate computation is shown in Figure 9 when a longer frame length of 4800 is used. )e decoding perfor- mance of the max-log-map algorithm with 6 iterations and the fully parallel decoding algorithm with 36 iterations of approximate computation are very similar for diﬀerent frame lengths. Our system is relatively perfect, and it has almost no eﬀect on the thermodynamics of water, so we do not need to consider the thermodynamics of water. To avoid that the value of the state metric exceeds the range of the data bit width of the ﬁxed-point quantization, which brings the decoder’s error correction capability down, the hardware implementation adopts the method of nor- malizing the state metric to reduce the value of the forward and backward state metrics to ensure that they are limited to the range supported by the data quantization bit width. Signal-to- BER 1 BER 2 BER 3 BER 4 Within the range supported by the data quantization bit noise ratio width, the backward and forward equalization of the re- ceived signal by the scatter plot gives an idea of the mod- Signal-to-noise ratio BER 3 ulation being used by the system; see Figure 10 for BER 1 BER 4 simulation results of a high-speed hydroacoustic OFDM BER 2 communication system based on unbalanced Turbo codes Figure 9: Simulation results of the high-speed hydroacoustic and Figure 11 for simulation results of a high-speed OFDM communication system based on Turbo codes-signal-to- hydroacoustic OFDM communication system based on noise ratio. balanced Turbo codes. )e formation mechanism of multipath is again very diﬀerent in shallow water environments, which consists of or seaﬂoor reﬂections, and, in the negative gradient shallow possible direct paths and reﬂections from the seaﬂoor, and in sea, the seaﬂoor reﬂection is the main mode of propagation, deep water, which is caused by the bending of sound lines. near the sound waves and can be reached through the )e hydroacoustic communication in shallow sea channels straight line and surface path. In the long-distance, only part is seriously aﬀected by multipath eﬀects and is prone to large of the surface scattered signal and seaﬂoor reﬂections can impacts. In the shallow sea hydroacoustic communication, reach the location; for positive gradient shallow sea, sound the sound velocity proﬁle has a signiﬁcant impact on the waves reach the location of the receiving point. For positive multipath eﬀect, and Figure 10 gives several typical sound gradient shallow sea, the acoustic wave reaches the location velocity proﬁle cases. From the ﬁgure it can be seen that, in of the receiving point through multiple surface reﬂections. the sound velocity of the uniform sea, sound waves will reach )e intercode interference caused by multipath seriously the location of the receiving point through multiple surfaces aﬀects the performance of hydroacoustic communication Values Range 10 Journal of Robotics 5 location should be determined by the space they occupy. For example, allocate three segments of storage space for the operation of the DSP’s built-in Fast Fourier Transform li- brary function. )e input and output arrays of the Fast Fourier Transform of length 16384 are stored in the form of alternating ﬂoating-point arrays, which occupy 13.1072 KB, so if all the input and output arrays of this function are allocated in the cache, the total space required is about 40 KB and should be placed in the L2 cache. -1 )e SNR of the received signal can be estimated at the -2 receiver side, and then the SNR of the received signal can be -3 changed by adding additive Gaussian white noise, and the BER can be calculated under diﬀerent SNRs, so the BER -4 curve with SNR can be obtained, which is called the sem- -5 isimulation experiment. -5 -4 -3 -2 -1 012345 Figure 11 shows the BER performance comparison of the Unequalized signal OFDM system with the precoding module, the black line Figure 10: Simulation results of the high-speed hydroacoustic shows the result of the semisimulation experiment without OFDM communication system based on unbalanced Turbo codes. precoding at the transmitter, the rose line shows the result of the semisimulation experiment with ZF precoding at the transmitter, the green line shows the result of the semi- simulation experiment with MMSE precoding at the transmitter, and the blue line shows the result of the sem- 8 isimulation experiment with ZF-THP. From Figure 11, after using the precoding technique at the transmitter side, only a simple detection process is re- quired at the receiver side to achieve good performance. Moreover, the performance of the system with nonlinear precoding is better than that of the system with linear precoding. )e BER performance of the hydroacoustic STBC- SCFDE system with spatial and temporal diversity is better than that of the hydroacoustic SC-FDE system with a single transmitter and single receiver; in addition, the BER of the system decreases as the number of transmit antennas in- creases. )e hydroacoustic STBC-SCFDE system is feasible 0 20 40 60 80 and eﬀective. Equalized signal Equalized signal Turbo 2 5. Conclusion Turbo 1 Turbo 3 )rough the above introduction of key technologies of high- Figure 11: Simulation results of the high-speed hydroacoustic speed hydroacoustic communication and the study of a OFDM communication system based on balanced Turbo codes. hydroacoustic communication system with OFDM, the key technologies and advantages and disadvantages of OFDM systems and causes great problems to ensure robust and application in a hydroacoustic communication system are high-speed hydroacoustic communication. )e ISI in analyzed, and a high-speed hydroacoustic OFDM com- wireless communication is very short, often only a few code munication coding scheme is designed based on TURBO elements long. In contrast, the ISI of hydroacoustic com- codes. System simulation is conducted to analyze the data munication is much larger, usually tens or even hundreds of rate, unbalanced signal, balanced signal, received signal code elements in length, which makes it more diﬃcult or power spectrum, balanced signal power spectrum, the sig- even impossible to distinguish the verdict at the receiving nal-to-noise ratio, bit rate, and per-packet BER results an- end. )erefore, to ensure robust and reliable hydroacoustic alyzed. Finally, the reliability of TURBO codes in high-speed communication, the intercode interference caused by the hydroacoustic OFDM communication is analyzed with the multipath eﬀect must be reduced or eliminated, and this is a simulation results of BER and SNR relationships under major point and diﬃculty in the ﬁeld of hydroacoustic diﬀerent conditions. Parametric modeling of the hydro- communication. acoustic OFDM system under impulsive noise interference is For variables that are often applied throughout the carried out. 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Journal of Robotics – Hindawi Publishing Corporation
Published: Mar 15, 2022
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