Chapter 1 - Introduction

This paper presents performance evaluation and comparison analysis of Direct Sequence Spread Spectrum (DSSS) and Frequency Hopping Spread Spectrum (FHSS) systems. The evaluation and analysis are done based on the systems performance against wideband interferences. The interferences are signals with similar spectrum characteristic to the transmitted signals of DSSS and FHSS systems. Bit Error Ratio (BER) is used as evaluation parameter to assess the performance of both systems. Simulation and mathematical analysis are performed to test and verify the performance of both systems. Mathematical analysis also verifies that increasing Spreading Frequency on certain conditions will reduce the BER. This research also points out that FHSS system has a better performance compared to DSSS system indicated by smaller BER.

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.

The recent growing interest in underwater acoustic communication (UAC) requires an in depth understanding of a vast and diverse water medium. The UAC channel presents many difficulties such as high frequency, space, and time selectivity, frequency dependent noise, and significant range and band limitation on transmission. The existing channel models may not be sufficient to cover the entire UAC channel domain and its many applications. In addition, the choice of channel model from the existing ones may not always be accurate to represent the channel environment. Thus, it is necessary to explore higher communication intelligence in the mapping of UAC channel models according to environment. This paper will present novel cognitive intelligent (CI) algorithm in accurate selection of UAC channel models. To the best knowledge of the authors, cognitive intelligence in the mapping of UAC environment to channel models has not been presented in the literature. Thus this research presents a valuable work.

In recent years, underwater acoustic (UWA) communications have received much attention as their applications have begun to shift from military toward commercial. Digital communications through UWA channels differ substantially from those in other media, such as radio channels, due to severe signal degradations caused by multipath propagation and high temporal and spatial variability of the channel conditions. The design of underwater acoustic communication systems has until recently relied on the use of noncoherent modulation techniques. However, to achieve high data rates on the severely band-limited UWA channels, bandwidth-efficient modulation techniques must be considered, together with array processing for exploitation of spatial multipath diversity. The new generation of underwater communication systems, employing phase-coherent modulation techniques, has a potential of achieving at least an order of magnitude increase in data throughput. The emerging communication scenario in which the modern underwater acoustic systems mill operate is that of an underwater network consisting of stationary and mobile nodes. Current research focuses on the development of efficient signal processing algorithms, multiuser communications in the presence of interference, and design of efficient modulation and coding schemes. This paper presents a review of recent results and research problems in high-speed underwater acoustic communications, focusing on the bandwidth-efficient phase-coherent methods. Experimental results are included to illustrate the state-of-the-art coherent detection of digital signals transmitted at 30 and 40 kb/s through a rapidly varying one-mile shallow water channel.

Underwater acoustic communication is paid more and more attention with the development of marine researches in recent years. Underwater acoustic communication channel is considered to be one of the most complicated channels, it suffers from the less available bandwidth, the more serious multipath effect and the even complex noise. Due to the severe distortion of the received signals, the underwater acoustic communication always experiences a high Bit Error Rate (BER). To solve this problem, Carrier Waveform Inter-Displacement (CWID) modulation was proved to be an effective method to decrease BER in our previous work. The Linear Frequency Modulation (LFM) carrier-waves are used in CWID modulation. The frequency band of LFM carrier-waves has an important effect on the communication performance. The Particle Swarm Optimization (PSO) is introduced to search the optimal frequency band of the LFM carrier-waves in this paper, due to its excellent performance in solving complicated optimization problems. The multiple objective optimization nature of the PSO gives a clear hint to deduce the relationship of upper band and lower band of the LFM carrier-waves. Simulations verify the improved performance and effectiveness of the optimization method.

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.

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%.

Underwater acoustic communications are required for controlling and networking a variety of underwater or sea surface objects. Due to the unique characteristics of the underwater acoustic channel, e.g., complicated multipath propagation, selective frequency fading, extreme Doppler effect and heavy ambient noise, FH-MFSK techniques play a vital role in robust underwater acoustic communication, especially in shallow water. However, based on detecting the frequency of the receiving signals, the demodulation scheme for FH-MFSK may be sensitive to the frequency shifting caused by underwater Doppler shifts. This paper focuses on how to design the frequency demodulation scheme and to improve the performance of underwater FH-MFSK acoustic communication system. By using an original Double-Line Interpolation algorithm with Hanning window, we propose a demodulation scheme to enhance the tolerance of the Doppler shift and improve the detection rate. Simulations and demodulation performance were tested. Experimental results show a promising future for FH-MFSK demodulation in underwater acoustic communication system.

Underwater acoustic communication channels are characterized by a large variety of propagation conditions. Designing a reliable communication system requires knowledge of the transmission parameters of the channel, namely multipath delay spread, Doppler spread, coherence time, and coherence bandwidth. However, the possibilities of its estimation in a real-time underwater communication system are limited, mainly due to the computational time of implementations of the digital signal processing algorithms. The article presents the results of analysis of probe signals measured during a shallow water experiment. The implementation issues of estimation algorithms are discussed, assuming floating point arithmetic, and with both single-core and multi-core digital signal processors as the target platforms.

Underwater acoustic communication (UAC) channel is a typical doubly spread channel with large multipath delay spread and large Doppler spread. The large multipath delay spread will lead the severe inter symbol interference (ISI) in the single carrier communication systems. The usual ISI will affect as much as hundreds of symbols, which will need complex equalization. OFDM can effectively reduce the ISI with guard interval. However due to the slow speed of acoustic propagation (about 1500m/s), the slow movement of underwater sensor nodes caused by ocean current will lead large Doppler spread, which will lead severe inter carrier interference (ICI). Channel estimation in this doubly spread channel will encounter the problem that the channel state information of pilot is affected by the neighboring unknown data symbols, which will largely degrade the performance of OFDM system. The paper studies the iterative OFDM receiver to reduce the performance degradation caused by the ICI.

Communication over doubly selective channels (both time and frequency selective) suffers from significant intercarrier interference (ICI). This problem is severe in underwater acoustic communications. In this paper, a novel partial fractional (PFrFT)-MIMO-OFDM system is proposed and implemented to further mitigate ICI. A new iterative band minimum mean square error (BMMSE) weight combining based on LDLH factorization is used in a scenario of perfect knowledge of channel information. The proposed method is extended from SISO-OFDM configuration to MIMO-OFDM. Simulation results demonstrate that the proposed PFrFT-LDLH outperforms the other methods in the SISO-OFDM scenario and that its performance can be improved in MIMO-OFDM scenarios.

Underwater acoustic (UA) communication channel in the shallow water is fast time-varying and selective fading. Proper de-spreading method is needed for the orthogonal frequency division multiplexing spread spectrum UA communication system to reconstruct the information. The conventional de-spreading algorithm decrease the communication rate due to the using of the pilot for channel estimation. To accurately estimate the channel response without much loss in communication efficiency, an improved pilot-free spread spectrum de-spreading (IPSSD) method is proposed in this paper. This algorithm first recognizes the spread spectrum signal in frequency domain using the sparseness of the underwater acoustic channel. Then a decision feedback estimation of the channel impulse response is achieved according to the recognition results. The least square method is utilized to decrease the estimation complexity, and channel puncturing technique is exploited to correct the estimation error, thus realizing the robust performance of the communication system. The results obtained by Monte Carlo simulation and lake-trial are provided, demonstrating that the new pilot-free algorithm can significantly improve the performance of de-spreading algorithm under the circumstance of limited spreading length, and hence, increase the communication efficiency.

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.

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.

Based on experimental data from the deep-sea sound channel axis in the Western Pacific, the statistical distribution law of cluster structure and channel delay spread characteristics are analyzed for three typical receiving depths near the sound channel axis in this paper. A ray theory-based underwater acoustic channel model is used to explain the variations in channel parameters over time and the receiving depth. The results indicate that the underwater acoustic communication channel at the channel axis depth over a 20-km range exhibits a clustered structure that depends on the emission angles of sound rays. For the amplitude characteristics, the amplitude of each cluster follows an inverse Gaussian distribution, with the maximum average amplitude observed when the receiver and transmitter depths are similar. The amplitude of each cluster fluctuation decreases as the receiving depth increases. Regarding delay spread characteristics, the delay spread of each cluster, as well as the maximum and root mean square delay spread of the channel, conform to a Gaussian mixture distribution. The mean and fluctuation of the delay spread parameters increase with the receiving depth. Variations in the cluster structure and channel delay spread characteristics above are primarily attributed to the time-varying sound speed along the propagation paths of sound rays emitted at small upward angles.