Performance enhancement of UWA‐OFDM communication systems based on FWHT

Abstract

The available bandwidth of underwater environment tends to several kilohertz, which considers the main challenges of communications under sea water. On the other hand, the bit-error-rate (BER) performance degrades because of several reasons such as multipath propagation, time variabilities of the channel, attenuation, and water temperature. In this paper, we aim to improve the underwater acoustic (UWA) BER system performance by using orthogonal frequency division multiplexing (OFDM) based on fast Walsh-Hadamard transform (FWHT) instead off fast Fourier transform (FFT). We proposed a low-complexity equalization and carrier frequency offset (CFO) compensation for UWA-OFDM-based FWHT using banded-matrix approximation concept.Simulation results show that the UWA-OFDM-based FWHT with lowdensity parity check (LDPC) codes give a good improvement performance compared with traditional OFDM in UWA system especially in case of estimation errors.peak-to-average power ratio (PAPR) at the transmitter side 8 and the carrier frequency offset (CFO) at the receiver side. 9 The PAPR results from the limited range of the power amplifier; moreover, the PAPR increases as the number of subcarriers increased. There are different algorithms used for PAPR reduction such as in previous studies. [8][9][10][11] On the other hand, the CFO results from the relative motion of the transmitter and/or receiver (Doppler effect). 12 The CFO induces intercarrier interference (ICI), which destroys the orthogonality and consequently degradation in the biterror-rate (BER) performance. 13,14 There are different algorithms used for CFO estimations such as in previous works. [15][16][17][18][19] The OFDM system lies in its simplicity by using a frequency domain equalizer (FDE) at the receiver side to mitigate the intersymbol interference (ISI) effect and improve the system performance. In the same time, we modify the OFDM system by replacing the inverse fast Fourier transform (IFFT) and fast Fourier transform (FFT) by inverse fast Walsh-Hadamard transform (IFWHT) and fast Walsh-Hadamard transform (FWHT), respectively. Replacing the FWHT/IFWHT with FFT/IFFT has been introduced in wireless radio communications, 20 which can improve the system performance. 20 But this modification requires an FFT and IFFT block to implement the FDE and enhance the system performance when compared with traditional OFDM system. On the other hand, implementation of the FWHT requires fewer computations (it omits the FFT's twiddle factors), and implement on smaller and cheaper hardware can be used to improve the performance of the UWA communication system and give better data transmission. 21 The FWHT transform can be implemented with lower complexity, due to its operation that is built on the subtraction and addition operations, while the FFT transform requires multiplication and division. 22 Also, the FWHT can be used for channel estimation with lower complexity. 22 Yang and Yang 23 show that the FWHT can be highly attentive to deal with the UWA channel impairments, and it is bu...

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“…Thus, even if some sub-carriers are completely lost, it will be possible to recover them as the information they carry is partially loaded on other carriers. One way to accomplish this is to use the Walsh–Hadamard Transform (WHT), which is an orthogonal transformation [ 20 , 21 ].…”

“…Here, u i are the updating coefficients. In the update process, the approximate value of the obtained signal by passing the signal through a low pass filter is obtained by updating the linear combination of the d[k] prediction difference as in Equation (20). These examples contain low frequency components that give approximate information (s[k]) about the signal.…”

A new FWHT‐CMF‐DFE based approach for channel equalization in CP‐free OFDM systems

“…It should be mentioned that, this is the first study in the literature where FWHT and time domain equalizer are combined in the CP‐free OFDM systems [27]. Although there are a few studies in the literature on the FWHT‐OFDM [18, 20, 21], it is worth mentioning that there is no study that combines FWHT with time domain equalizer and implements it to the CP‐free OFDM system.…”

“…Therefore, even if some subcarriers are totally lost, it will be possible to reconstruct since the data they transport is partly burdened on the other transporters. The way to achieve this is to employ a perpendicular conversion, the fast Walsh-Hadamard transform [20,21].…”

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“…Also, several authors [17], [18] have suggested the use of the Fast Walsh Hadamard Transform (FWHT), which has recently garnered increased interest in several fields [19], [20], [21].…”