RESEARCH ARTICLE Performance enhancement of UWA‐OFDM communication systems based on FWHT Mohamed El‐Mahallawy 1 | Adly Tag Eldien 2 1 Department of Electronics and Electrical Communications, Higher Institute of Engineering, El‐Shorouk Academy, Cairo, Egypt 2 Department of Electronics and Electrical Communications, Faculty of Electronic Engineering, Benha University, Shoubra, Egypt Correspondence Mohamed El‐Mahallawy, Department of Electronics and Electrical Communications, Higher Institute of Engineering, El‐Shorouk Academy, Cairo, Egypt. Email: mohmedmahallawy@gmail.com Summary 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 low‐ density parity check (LDPC) codes give a good improvement performance com- pared with traditional OFDM in UWA system especially in case of estimation errors. KEYWORDS FFT, FWHT, LDPC, OFDM, UWA 1 | INTRODUCTION The underwater acoustic (UWA) communication systems support significantly low data rate 1 due to large delay spread and limited the available bandwidth (BW). The UWA channel has a lot of challenges such as attenuation, water salinity, temperature, frequency‐dependent attenuation, and time‐varying nature. Thus, those challenges cause a limitation to communications in sea water. 2 In general, the ocean water can be divided into four horizontal layers: the surface layer (mixed layer), seasonal thermocline layer, permanent thermocline layer, and deep isothermal layer. The sound propa- gation speed differs in each layer because of the variation of the water properties (salinity, pressure, and temperature). 3 The propagation speed differs in each layer according to the water properties (temperature, salinity, and water depth). To establish the communication over the underwater environment, four different communication media have been used. Cables are not suitable to establish the underwater communications (not realistic). The electromagnetic and optical waves support a range that does not exceed 100 m because of the large power loss. 1 Thus, we are forced to use sound waves. Moreover, acoustic waves support a frequency ranging up to tens of kilohertz. The main advantages of the use of acoustic waves are the lower power loss and effective range reaches to several kilometers, and the disadvantage is the low propagation speed (1500 m/s). The orthogonal frequency division multiplexing (OFDM) modulation is a type of modulation that can be used for UWA communications 4-7 .The OFDM communication systems suffer from two main performance degradation: the Received: 31 October 2017 Revised: 26 March 2019 Accepted: 28 March 2019 DOI: 10.1002/dac.3979 Int J Commun Syst. 2019;32:e3979. https://doi.org/10.1002/dac.3979 © 2019 John Wiley & Sons, Ltd. wileyonlinelibrary.com/journal/dac 1 of 14