# Coded-GFDM for Reliable Communication in Underwater Acoustic Channels
## 1 Introduction  
Underwater communication plays an important role in various offshore applications including remote monitoring, fishing industry, exploration, search and rescue, surveillance, and border safety to name a few [1]. Acoustic signals have widely been used since they do not suffer from absorption and scattering the way radio waves and light waves suffer. Low speed of sound severely limits the bandwidth at which the data transmission can take place [2]. A larger delay spread causes inter-symbol-interference (ISI) and even a small change in the position of transmitter and receiver causes Doppler shifts making the channel doubly selective [3]. Multipath properties of the channel vary according to the geometry and physical conditions such as the transmitter-receiver position as well as the sea surface and boundary. With the ever-increasing demand for high data-rate multicarrier acoustic communication, OFDM has widely been studied [4; 5; 6; 7]. It can effectively counter ISI due to a longer symbol period. However, OFDM suffers from a high PAPR and high OOB emissions which makes it less efficient [8].  
Generalized frequency division multiplexing (GFDM) [9] is a non-orthogonal modulation scheme and is among the many next generation 5G contenders proposed to deal with the limitations of the current OFDM systems. Some of its advantages include higher spectralefficiency, low OOB emissions, and flexibility [10]. Each GFDM data-block is divided into subcarriers and sub-symbols, with the subcarriers filtered through a pulse shaping filter to reduce OOB emissions. GFDM is a highly flexible scheme in time-frequency distribution with its ability to collaborate with single carrier frequency domain equalization based systems and filter-bank multicarrier systems [11]. The block-based nature of GFDM allows it to utilize several OFDM techniques, such as cyclic prefix (CP) to avoid inter symbol interference. Thus, low complexity equalizers can be employed in the frequency domain to counter the effects of multipath at the receiver side [12]. As GFDM subcarriers are filtered through a variety of pulse shapes [13] the tradeoff is a lack of orthogonality making them susceptible to self-interference at the receiver. Another disadvantage of utilizing flexible waveform is the associated complexity in implementation. However, with rapidly growing computational power and complexity reduction being an active area of research, it appears to be a viable solution for future next generation multicarrier acoustic communications.  
Channel coding techniques such as forward error correction codes (FEC) have long been used for reliable transmission in wireless communication systems [14; 15]. The data-rate is reduced due to the coding overhead and requires an increase in bandwidth. Communication in DAC is affected by multipath fading, absorption, and frequency dependent noise. In this work, we consider block coding schemes that include Bose, Chaudhuri, and Hocquenghem (BCH) codes and Reed Solomon (RS) codes, and a few convolutional codes for a GFDM transceiver in UAC for error-rate improvements. We compare the Symbol error rate (SER) performance of various coding schemes over varying transmitter-receiver (Tx-Rx) distances. Some contributions of this paper are:  
* A coded-GFDM transceiver modeled in MATLAB for a shallow underwater acoustic channel.
* SER analysis of the proposed architecture for various transmitter and receiver distances along with the comparison of coded and uncoded GFDM modulation.  
The remainder of this paper is organized as follows: Section 2 presents the state-of-art and the literature review. The coding techniques and proposed architecture of a GFDM system are described in Section 3. Section 4 explains the simulation setup and presents the results while the discussion on results is done in Section 5. Finally, Section 6 provides a brief conclusion.