# Low-Complexity Progressive MIMO-OFDM Receiver for Underwater Acoustic Communication
## 1 Introduction  
Underwater acoustics (UWA) communication has attracted huge attention during the last two decades because of the extension of its applications to different domains. UWA sensor networks are introduced, which increased the design challenges to meet the higher demands [1]. The challenges like high data rate, low bit error rate (BER), and utilization of maximum channel capacity still have a lot of room for improvement. Underwater communication is no match with the terrestrial wireless communication as the sound speed in water is at least five orders of magnitude less than the speed of wireless radio channels. Therefore, alternate ways need to be found in order to improve the efficiency of UWA communication, as the speed of sound is constant inside water and cannot be increased. Moreover, the acoustic wave propagation speed differs from ocean to ocean depending on numerous factors [2; 3]. Channel modeling is the basic step of any communication model, where the effects of the varying channel on the acoustic signal are found out and signal attenuation due to ambient noises, multipath, and Doppler Effect are analyzed. Multipath introduces inter-symbol interference (ISI) and Doppler shift introduces Inter-carrier Interference (ICI) [4; 5]. Modulation scheme plays an importantrole in designing an underwater communication model. No matter how proficient the channel is modeled, without a competent modulation scheme, a reliable and efficient model cannot be designed. Orthogonal Frequency Division Multiplexing (OFDM) is considered to be a low-complexity and highly efficient modulation scheme, which overcomes the problem of ISI to a large extent. OFDM uses multiple carriers, sending some bits on each channel; hence all the sub-channels are dedicated to data source. The carriers are orthogonally spaced in the frequency domain, which helps the demodulator in rejecting frequencies other than its own. The increased symbol duration in OFDM helps in combating ISI. OFDM also has higher data rate and spectral sufficiency, therefore, it becomes the most suitable option for multipath environment [6]. MIMO was introduced recently in UWA communication to efficiently use the acoustic bandwidth and increase the system capacity by exploiting the spatial diversity. The parallel transmission of independent data streams in MIMO improves the data rate performance [7]. The combination of MIMO and OFDM is a tempting low-complexity solution for bandwidth-efficient communications over frequency selective and bandwidth limited UWA channels [8; 9; 10; 11; 12]. Channel estimation, equalization, and de-coding are the key steps in designing any receiver, which get the signal back into the proper shape as it was when transmitted. Channel estimation estimates the channel parameters. The channel estimation of MIMO systems is an intricate job, as each received signal consists of independent information from every transmitter and all the channels need to be estimated at the same time [13; 14; 15]. Equalization removes the effects of the channel on the received signal and removes the ISI, Inter carrier interference (ICI), and other distortions from the signal. Channel coding adds some redundancy in the useful bits in order to protect the data in a noisy channel. The complexity of the UWA receiver has always been pointed out as the main problem in designing any UWA communication system [16]. Different receivers were designed focusing on the performance and complexity of the receiving system [17; 18]. In this paper we propose a low-complexity, novel progressive iterative MIMO-OFDM receiver for the UWA communication system, which outperforms all the previous designs in terms of complexity and performance. The proposed design is compared with another non-iterative receiver, basic Turbo iterative receiver, and also with a Turbo iterative receiver with channel estimation inside the loop.