An Improved Pilot-free De-spreading Algorithm for Underwater Acoustic Orthogonal Frequency Division Multiplexing Spread Spectrum Communication

The fast temporal variations, long multipath delay spreads, and severe frequency-dependent attenuations of underwater acoustic communication channels are extremely complex that impedes underwater acoustic (UWA) data transmission. To alleviate this problem, channel coding is indispensable in UWA communication system to increase the reliability. In this paper, performances of LDPC codes with different parameters over different underwater acoustic communication channels are presented, which is to know how to adjust the encoding and decoding parameters according to different underwater acoustic channels. The results indicate that LDPC codes have a good performance of the estimated BER on the order of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> -10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> either in positive sound velocity gradient (PSVG) underwater channel or negative sound velocity gradient (NSVG) one, even invariable sound velocity gradient (ISVG) one. In addition, whatever the encoding or decoding parameters is adopted, performance of the LDPC codes over the NSVG underwater channel is the best, compared with PSVG one and ISVG one. Furthermore, in the desired performance of BER of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> , the encoding and decoding parameters according to the three different underwater channels specifications are summarized.

In this paper, we propose index modulated orthogonal frequency division multiplexing spread spectrum (IM-OFDM-SS) scheme, which combines the techniques of SS and IM under the framework of OFDM. In this scheme, the information bits are jointly conveyed by the indices of spreading codes and the conventional M-ary modulated symbols. A low-complexity maximum ratio combining (MRC) detector is designed, in which the receiver first detects the selected spreading codes and then the corresponding data symbols are de-spread and demodulated. The bit error probability (BEP) upper bound and the achievable diversity order are analyzed according to the union bounding technique. Simulation results verify the analyses and show that the proposed IM-OFDM-SS outperforms the existing OFDM-IM and OFDM-SS schemes significantly.

Under water acoustic (UAC) channel is subjected to large delay spread compared to radio frequency (RF) channels. In order to capture this large delay spread and avoid ISI, the UAC symbol duration needs to be considerably large reducing the transmission data rate. Unlike other RF systems, in Ultra-wideband (UWB) RF systems, small pulse width in the order of nano seconds is used to transmit information. This in turn, gives high resolution multipaths. In a time hopping UWB system, to avoid multipath overlap, the chip duration and the frame duration are kept equal to or larger than the maximum delay spread of the channel. This results in large UWB symbol duration and hence reduces the data rate. In order to increase the data rate, the chip duration, and frame duration can be reduced to less than the maximum delay spread. In addition, to farther increase the data rate, guard interval between consecutive symbols can be reduced/removed allowing consecutive symbols to overlap. This results in intersymbol interference (ISI). Surprisingly, this type of overlap doesn't degrade UWB system performance significantly. In this paper, the same concept is applied to various UAC channels. In order to increase the data rate, consecutive UAC symbols are allowd to overlap. Performance under ISI is then evaluated.

Data transmission over HF channels is subject to phenomena of dispersion, absorption and multiple paths. Performance may be improved by using orthogonal frequency division multiplexing spread spectrum (OFDM-SS) systems. In OFDM-SS systems, transmitting data over slowly varying multi-path channels, both inter-symbol interference (ISI) and multiple-access interference (MAI) arise and should therefore be mitigated. Two adaptive non-linear equalization architectures based on the minimum mean-square error (MMSE) criterion are proposed and examined over such channels. The first receiver architecture is for single-user detection (SUD) consisting of a multiple-layer feed-forward filter (ML-FFF) and a feedback filter (FBF). The ML-FFF acts as a multi-path diversity combiner as well as canceller of precursor ISI and main-cursor MAI while the FBF acts as a canceller of post-cursor ISI. The second receiver architecture is for multi-user detection (MUD) where in addition to the first structure, it employs a centralized FBF (CFBF) for cancelling the MAI of previous symbols. Results indicate that an enhancement in capacity is obtained by the multi-user structure over the single-user structure. Additionally, it is demonstrated that both structures perform multi-path energy combining (Rake-like) when the delay spread is in excess of a symbol interval.

The achievement of efficient data transmissions via underwater acoustic channels, while dealing with large data packets and real-time data fed by underwater sensors, requires a~high data rate. However, diffraction, refraction, and reflection phenomena, as well as phase and amplitude variations, are common problems experienced in underwater acoustic (UWA) channels. These factors make it difficult to achieve high-speed and long-range underwater acoustic communications. Due to multipath interference caused by surface and ocean floor reflections, the process of modeling acoustic channels under the water's surface is of key importance. This work proposes a~simple geometry-based channel model for underwater communication. The impact that varying numbers of reflections, low water depth values, and distances between the transmitter and the receiver exert on channel impulse response and transmission loss is examined. The high degree of similarity between numerical simulations and actual results demonstrates that the proposed model is suitable for describing shallow underwater acoustic communication environments.

Multicarrier acoustic communication has enabled wireless underwater transmission at higher data rates. Several multicarrier modulation systems have been explored in the past for data transmission. Orthogonal frequency division multiplexing (OFDM) fights-off inter-symbol-interference due to orthogonality of the carriers. However, it is susceptible to time variations which introduces inter-carrier-interference. Being double selective, the underwater acoustic (UWA) channel is both time and frequency variant. Time variations and multipath fading makes it a complex channel to estimate. In this paper, we explore linear frequency domain equalization techniques available for radio OFDM systems and compare their performance for a shallow underwater acoustic channel. The underwater channel model used is based on Rician shadowed distribution with Doppler shifts. We compare the performance of a linear equalizer with pilot estimated channel against a zero-forcing equalizer where the channel is assumed to be known. Results collected through Monte Carlo simulations show that for a 128-subcarrier OFDM system and a transmitter-receiver separation of 800 m, a gain of almost 7dB is obtained at a BER of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> when a zero-forcing equalizer is used. Moreover, when the subcarriers are increased to 256, this gain almost doubles. In conclusion, the zero-forcing equalizer outperforms the LS equalizer for an underwater acoustic channel.

Spread spectrum technology has been widely employed for positioning and communicating with autonomous underwater vehicles (AUVs), but conventional spread spectrum sequences lack confidentiality and reliability in UWA channel. Considering the limitations of conventional sequences and the characteristics of underwater acoustic (UWA) channel, a composite chaotic orthogonal sequence (CCOS) based on the UWA channel is proposed. The confidentiality of the CCOS is superior to that of the m‐sequence, while the autocorrelation performance of the CCOS is superior to that of the orthogonal sequence. Moreover, the CCOS can compensate for the imbalance of logistic chaotic sequence when assigned certain initial values. Acquisition is a crucial component of accessing the spread spectrum signal; therefore, the acquisition performance indicates the applicability of the CCOS. The source–target model is established to simulate communication with an underwater moving target. To simulate the acquisition process, a parallel algorithm based on fast Fourier transform is adopted, and the entire simulation process is completed based on the BELLHOP ray acoustic model. Through data processing, the Doppler shift error is less than half of the frequency‐search element. Furthermore, the acquisition probabilities of the CCOS with different numbers of bits are over 90%, which demonstrates the reliability of the CCOS.

The complexity of the underwater environment including long time-delay, large background noise, and excessive transmission loss, leads to two main challenges for the current mid-distance and long-distance underwater acoustic (UWA) communications: limited transmission rate and system reliability. To improve the performance of mid-range and long-range underwater acoustic communication, this paper proposes an M-ary multi-sequence superimposed spread spectrum(M-MSSS) UWA communication scheme based on a virtual time-reversal mirror (VTRM). Multiple sequences with periodic correlation characteristics of the original spread spectrum waveform are superimposed, and M-ary multi-sequence modulation is performed in each data block at the transmitter. A VTRM matched filter is designed for the receiver. The VTRM technology uses the physical characteristics of the typical coherent multipath of the underwater acoustic wave channel to reduce the signal amplitude through its strong spatial focusing ability. The inter-symbol interference is suppressed by reconstructing multipath signals and improved receiver. Simulations show that the scheme not only significantly improves the communication rate, but also ensures the system's better robustness in terms of bit error rate.

In this paper, the estimation of doubly spread underwater acoustic (UWA) channels is investigated. The UWA channels are characterized by severe delay spread and significant Doppler effects, and can be well modeled as a multi-scale multi-lag (MSML) channel. Furthermore, exploiting the sparsity of UWA channels, MSML channel estimation can be transformed into the estimation of parameter sets (amplitude, Doppler scale factor, time delay). Based on this, the orthogonal matching pursuit (OMP) algorithm has been widely used. However, the estimation accuracy of OMP depends on the size of the dictionary, which is related with both delay spread and Doppler spread. Thus it requires high computational complexity. This paper proposes a new method, called wavelet transform (WT) based algorithm, for the UWA channel estimation. Different from OMP algorithm which needs to search in both time domain and Doppler domain, WT-based algorithm only needs to search in time domain by using the Doppler invariant characteristic of hyperbolic frequency modulation (HFM) signal. The performance of the proposed algorithm is evaluated by computer simulations based on BELLHOP. The simulation results show that WT-based algorithm performs slightly better than OMP algorithm in low signal to noise ratio (SNR) while can greatly reduce computational complexity.

Low-density parity-check (LDPC) codes are commonly used in communication systems to improve the system performance, but LDPC codes takes too long for decoding, making communication inefficient and unsuitable for short-frame data transmission methods. In underwater acoustic channels, spread-spectrum communication becomes an effective way to realize long-distance communication. This paper combines short-block LDPC codes with a direct sequence spread spectrum and soft spread spectrum in underwater acoustic communication, addressing the problem of the inapplicability of conventional LDPC codes. The applicability of the proposed method in this paper is verified through simulation tests and pool experiments. The results indicate that the proposed communication system achieves lower bit error rates compared to the classical coding methods used in underwater acoustic spread-spectrum communication systems under the same channel conditions.

We investigate two methods for estimating the matched signal transformations caused by time-varying underwater acoustic channels in orthogonal frequency division multiplexing (OFDM) communication systems. The underwater acoustic channel for this 12-20 kHz medium frequency range OFDM system is best modeled using multipath and wideband Doppler scale changes on the transmitted signal. As a result, our first channel estimation method is based on discretizing the wideband spreading function time-scale representation of the channel output using the Mellin transform. The second method is based on extracting the time-scale features of distinct ray paths in the received signal using a modified matching pursuit decomposition algorithm. We validate and discuss both methods using data from the recent Kauai Acomms MURI 2008 (KAM08) underwater acoustic communication experiment.

An underwater acoustic multimedia communication (UWAMC) system is proposed with 2400 transmission modes according to time-varying multipath underwater acoustic (UWA) channel conditions. The orthogonal variable spreading factor (OVSF) scheme and Gold sequence (GS) scramble code are integrated into multi-input multi-output UWAMC system based on orthogonal frequency-division multiplexing to achieve the quality of service of multimedia transmission in the UWA channel. Binary phase shift keying (BPSK) and quadrature phase shift keying (QPSK) adaptive modulation, direct mapping (DM) or space–time block code (STBC) transmission strategies, convolution channel code with rate 1/2 and 1/3, and a power assignment mechanism were adopted in the proposed system. Simulation results show that the bit error rate (BER) and power saving ratio (PSR) performance of the STBC strategy with transmission diversity is superior to that of the DM strategy without transmission diversity, and the performance of the BERs and PSRs of the transmission scheme with the GS scramble code surpasses that of the scheme without the code. The performance of the BERs and PSRs of BPSK modulation with a channel code rate of 1/3 is better than that of BPSK modulation with a channel code rate of 1/2, and the performances of BERs and PSRs of BPSK modulation with a channel code rate of 1/3 are better than that of QPSK modulation with a channel code rate of 1/3. As the length of the OVSF codes increases, the UWAMC system’s BERs decrease, and its PSRs increase. The UWAMC system can achieve either maximum transmission speed or maximum transmission power efficiency.

Recent advancements in underwater technology have made cooperative exploration, an efficient underwater exploration technique, possible. This technique networks mobile and fixed platforms, sensors and systems, and makes it possible to accomplish underwater tasks efficiently by integrated operations over a wide area and balancing exploration and exploitation. As a networking technique, underwater acoustic (UWA) communication is a technology of choice today, since it can enables mobility and flexibility in cooperative exploration systems. However, UWA communication still has several design challenges which need to be carefully addressed. To provide a reliable UWA modem suitable for cooperative exploration, in this study we designed a Doppler-resilient orthogonal signal division multiplexing with time diversity (D-OSDM-TD) and evaluated its performance in simulations and experiments. We evaluated the D-OSDM-TD in simulations and experiments and found that the D-OSDM-TD achieved a far better bit-error-rate performance in UWA channels with delay and Doppler spreads using low signal power and without using a receiver array. We expect that D-OSDM-TD can become a viable alternative offering highly reliable communication for underwater operations, and we believe it is suitable for underwater cooperative operations.

This paper proposes a selection method to choose a better modulation scheme in a given underwater acoustic communication channel. In this paper, parameters affecting the BER of CDMA and OFDM were calculated from underwater acoustic channels. Deep Neural Network (DNN) with the parameters was utilized to select the better modulation method given an underwater acoustic channel environment. Computer simulations demonstrate the proposed method selects the better modulation scheme by 98.98%.

Direct sequence spread spectrum (DSSS) signal uses phase coherent signals where the information symbols are multiplied with a code sequence, commonly known as chips. The signals are processed at the receiver using the code sequence as a matched filter to extract the information symbols. Taking advantage of the spread processing gain derived from the matched filter, communications can be carried out at low signal levels, which is the preferred method for high quality underwater acoustic (UWA) communication and remote UWA communication. However, in practical applications, DSSS UWA communication will face lots of interferences including multipath interference, Doppler compression interference and phase fluctuation interference, which will seriously affect the performance of DSSS. This paper first analyzes the interferences that DSSS faces in practical applications based on multiple actual received data collected in different water areas. By calculating the gain of the matched filter loss, one can find that influence of phase fluctuation interference has the most serious impact on DSSS. Secondly, the corresponding receiver processing algorithms are given for these interferences, including the differential energy detector and Doppler estimation method. The performance of differential energy detector under the above interferences is analyzed, which shows that the differential energy detector is insensitive to slow phase fluctuation interference and multipath interference and the differential energy detector combined with Doppler compensation can work well based on moving at-sea data. Finally, the parameter design of DSSS system in practical application is discussed and parameter design of DSSS UWA communication system should comprehensively consider the Doppler compression interference and phase fluctuation interference.