Underwater Communication by using OFDM System

Archives of Acoustics, 2020

The large variability of communication properties of underwater acoustic channels, and especially the strongly varying instantaneous conditions in shallow waters, is a challenge for the designers of underwater acoustic communication (UAC) systems. The use of phase modulated signals does not allow reliable data transmission through such a tough communication channel. However, orthogonal frequency-division multiplexing (OFDM), being a multi-carrier amplitude and phase modulation technique applied successfully in the latest standards of wireless communications, gives the chance of reliable communication with an acceptable error rate. This paper describes communication tests conducted with the use of a laboratory model of an OFDM data transmission system in a shallow water environment in Wdzydze Lake.

Orthogonal Frequency Division Multiplexing (OFDM) is an attractive modulation technique for underwater acoustic communications, where it has proven to be effective in combating the frequency-selective multipath distortion without the need for complex time-domain equalizers, thus offering simplicity of implementation. However, OFDM signals are characterized by large peak-to-average power ratios (PAR), which can reduce the system efficiency. To avoid the non-linear distortion in the transmit amplifier, the peak signal amplitude must be kept under a pre-specified limit, i.e. a back-off must be introduced, which reduces the average (useful) power. Acoustic systems are confined to low frequencies due to the propagation losses, while further band-limiting occurs naturally in the transducer. The transducer thus acts as an analog filter in the final stage of transmission. While otherwise undesirable, this filtering effect can be exploited towards PAR reduction [26], also have considered the challenges in underwater and made a comparison with other communication techniques in attenuation, effective range, propagation delay, bandwidth and data rate, and looking for improvement of such technique to enhance the system.

Wireless Personal Communications, 2020

Acoustic signals are a first choice in underwater communication since sound waves face very low attenuation in water compared to radio frequency (RF). However, the tendency of receiver to detect multipath signals (due to signals bouncing off surface or the bottom) induces large delay spreads and thus strong channel frequency selectivity. Additionally, rapid spatial and temporal variations in underwater acoustic (UWA) channel makes it hostile for communication systems based on single carrier modulation. Multicarrier modulation techniques, on the other hand, can greatly improve bandwidth utilization and help deal with time dispersal effects. Orthogonal frequency division multiplexing (OFDM) is a proven multicarrier communication system having capabilities to cope with frequency selectivity and delay spreads effectively. OFDM has started to get attention for being a simpler alternative to high complexity and high maintenance single carrier systems in UWA communication systems. This work proposes a Matlab model of an OFDM transceiver along with UWA channel characterization based on Rician shadowed fading model as it perfectly characterizes the way in which a shallow UWA channel behaves. Eventually, the proposed design allows implementation of various OFDM modulation methods and to perform Monte Carlo simulations for bit error rate comparisons together with the ability to tune multiple UWA channel parameters.

Journal of Telecommunication, Electronic and Computer Engineering, 2019

The problems of signal loss and poor channel estimation are inherent in underwater communication. In this work, underwater communication system has been improved using Orthogonal Frequency Division Multiplexing (OFDM) system with quadrature phase shift keying (QPSK) based on modulation scheme. The modern choice of underwater communication system, where the available bandwidth was divided into many overlapping sub-channels, such that the symbol duration was compared to the multipath spread of the channel; thus, eliminating inter symbol interference as well as improving bandwidth availability. Matlab tool was used to develop and implement series of simulations of the underwater acoustic communication system model. The proposed OFDM-QPSK underwater wireless acoustic technique achieved a BER of 2.44 × − , approximately equivalent to 1.8%, and a SNR of 42.82 , at a transmission bit rate of 67.93 /. The work showed a positive improvement and a novel achievement as compared to many literatures. Comparing the results of existing literature and the results obtained from this work, it can be logically summarized that a better performance OFDM system is achieved by utilizing the QPSK modulation scheme as presented in this work. The results showed an improved channel characterization, low noise level and suitable modelling for the underwater acoustic applications.

2012

Progress in wireless communications is the result of the development of digital modulation and coding techniques, and improvements in exploring the capabilities of natural radio channels. Also among designers of underwater acoustic communication (UAC) systems, there is a broad interest in multicarrier OFDM technique as a means of better matching to the tough constraints of UAC channels. An efficient adaptation of modern wireless data transmission methods in underwater communications requires knowledge about the channel characteristics described in terms of specific parameters related to channel variability and time dispersion. These parameters, including delay spread, coherence time, coherence bandwidth, and Doppler spread, are estimated on the basis of channel impulse response measurements. The paper describes communication tests conducted in reverberant acoustic channels and inland shallow water channels, with the use of a laboratory model of an OFDM data transmission system. The channel impulse responses were measured using the correlation method based on transmission of MLS sequences. The dynamics of estimated channel transmission parameters was examined, and a procedure was developed of matching the UAC system signaling parameters to varying channel conditions. The results allow for a discussion of the relationship between the channel time-variability and limit values of the data transmission rate.

Orthogonal frequency division multiplexing (OFDM) has been used in underwater communications for high data rate transmission. Although robust against frequency selective fading, OFDM suffers from high peak to average power ratio (PAPR). PAPR is a serious concern for power constrained underwater sensor nodes. This paper evaluates the performance of Walsh-Hadamard (WH) code spread OFDM in terms of bit error rate (BER) improvement and PAPR reduction. Also the space-time and space-frequency diversity are used to further improve the performance of WH-OFDM in underwater acoustic channel. It is shown by simulation that WH-OFDM with space-frequency coding has the best performance at achievable signal to noise ratio in underwater environment.

Wireless Personal Communications, 2019

The supported bandwidth of the underwater communication systems is limited to several kilo hertz, which considers as the main challenge for Underwater Acoustic (UWA) communications. Meanwhile, the Bit-Error-Rate (BER) performance of the UWA systems is degrades as a result of water temperature, water salinity, attenuation, and multi-path propagation. In this paper, we present a modification to the conventional Orthogonal Frequency Division Multiplexing (OFDM) based (FFT) using Fast Walsh-Hadamard transform (FWHT) instead of Fast Fourier Transform (FFT). Also, the proposed algorithm is encoded and decoded using Low Density Parity Check (LDPC) coding algorithm. Simulation results show that the proposed algorithm with LDPC coding can improve the BBER system performance than the corresponding traditional one.

Proceedings of 2014 11th International Bhurban Conference on Applied Sciences & Technology (IBCAST) Islamabad, Pakistan, 14th - 18th January, 2014, 2014

The underwater channel poses various challenges for the wireless information exchange as it offers complex communication design. OFDM, being a best viable solution for high rate and efficient underwater communication scheme, also faces signal degradation problems due to the dynamics nature of Sea. These non-uniform dynamics due to the wind or the platform motion are critical for the acoustic subcarriers (i.e. low frequency and low speed) and may cause inter-carrier interference which results distortion of the entire signal. In order to avoid these erratic distortions, proper tracking of relative change in terms of Doppler factor have to be incorporated in the design. The communication design must be capable enough to track aquatic variations and compensate the effects in an efficient way. In this article, a novel OFDM approach based on steering sinusoids is presented which may be able to trace and rectify the fast oceanic variations incurred with in a symbol length. The steering sinusoids are used to check the dynamic effects imposed by the channel and the recorded variations (either uniform or non-uniform) are further utilized to mitigate the Doppler spread. The algorithm is computationally very efficient and gives low latency period as compare to the two-step approach. The placement of the appended sinusoids among subcarriers is depended upon the measure of relative motion and the level of precise tracking i.e. tradeoff have to be considered. The proposed scheme is analyzed in the MATLAB simulation with selfinduced Doppler and Bellhop ray tracing program is also executed to generate static multipath channel models. The simulation results endorsed the efficient performance of the proposed algorithm and suggested for the practical evaluation. I.

2017

In this paper, the low cost home-made underwater sensors are designed for underwater Acoustics Sensor Network. This system is used to read the values of pressure, temperature and salinity using these sensors under the water and then the values are transmitted from underwater sensor to base station receiver. The sensors are continuously reading these values. Here, the communication channel estimation model is designed using Rayleigh Fading method, where the Gaussian processes for complex values were used. The channel is called Rayleigh Fading since its phase is uniformly distributed in the range of (0,2π) for the zero value of the coefficients of amplitude and phase for time-varying delay line complex channel. Different algorithms and formulas are used for successful transmission of these values from one sensor to another. Acoustic channel is used for data transmission, which is estimated using Scattering function estimation and maximum data transmission can be possible using OFDM te...

Journal of Marine Science and Application, 2012

The major constraint on the performance of orthogonal frequency division multiplexing (OFDM) based underwater acoustic (UWA) communication is to keep subcarriers orthogonal. In this paper, Doppler estimation and the respective compensation technique along with various diversity techniques were deliberated for OFDM-based systems best suited for underwater wireless information exchange. In practice, for mobile communication, adjustment and tuning of transducers in order to get spatial diversity is extremely difficult. Considering the relatively low coherence bandwidth in UWA, the frequency diversity design with the Doppler compensation function was elaborated here. The outfield experiments of mobile underwater acoustic communication (UWAC) based on OFDM were carried out with 0.17 bit/(s•Hz) spectral efficiency. The validity and the dependability of the scheme were also analyzed.