# A Survey on Physical Layer Techniques and Challenges in Underwater Communication Systems
## 2 UWAC Physical Layer Techniques
### Ofdm-Im  
ICI in OFDM becomes a complicated task to deal with in communication systems with a challenging physical layer, such as UWACs [11]. Therefore, inspired by the idea of SM, OFDM-IM has been presented to avoid the ICI issue by transmitting a part of the information bits physically by activated subcarriers out of the available subcarriers. The other part of the information bits is transmitted by the index of those active subcarriers. In low-to-medium data rate systems, OFDM-IM outperforms the conventional OFDM in terms of BER [139; 140]. Unfortunately, like the conventional OFDM, OFDM-IM too suffers from ICI resulting from superimposing all the time-domain subcarrier data and high PAPR. In rapid time-varying channels, OFDM-IM provides better performance in the case when only activating fewer subcarriers, but unfortunately, that is at the expense of the achievable data rate. In OFDM-IM, the ML detector is considered an optimal receiver to detect the information bits transmitted by the index of subcarriers and modulated data jointly. Accordingly, many other schemes related to OFDM-IM have been presented either to improve the BER performance or to increase the achievable data rate [141; 142]. Others are trying to increase the indexed data bits by implementing different modes of indexing instead of setting the subcarriers to only be active and idle. On the other hand, the OFDM-IM has been discussed for the UWAC [143] to provide reliable data transmission. However, the UWA effects and analysis have not been discussed. However, OFDM-IM channel estimation is a challenging task due to the UWA channel features, which cannot be considered as perfectly known.  
OFDM-IM does not offer any diversity degree compared with conventional OFDM; it has only improved the performance as the ICI effect is reduced. Therefore, to propose a higher gain in spectral efficiency in comparison to the OFDM-IM [139], the index modulation OFDM with spread spectrum (IM-OFDM-SS) has been introduced [144]. Unlike the scheme of OFDM-IM, all subcarriers of the IM-OFDM-SS are active. However, every data symbol, by applying a predefined spread code, is spread across numerous subcarriers. The index of the selected spreading code carries a part of the information bits while the rest of it is conveyed through the spread-modulated symbols. Hence, additional diversity is harvested through the data symbols spreading across different subcarriers. The information bits at the receiver end can be detected through a maximal ratio combining (MRC) detector. MRC as compared to the ML detector used in OFDM-IM, proposes improved efficiency for computational complexity with less proneness to error for channel estimation [144].  
In an untrusted environment, such as UWAC, the use of OFDM-IM schemes with an ML detector might be less attractive as the detected information bits mainly depend on the correct detection of the index of active subcarriers [145]. Entirely inaccurate detection of all the bits of information can result from the inaccurate detection of the active subcarriers index. To overcome that issue, an encoder and inter-leaver have been adopted [145; 146; 147]. The use of encoding and/or coordinate interleaving leads to an additional diversity degree where the information bits of the same sub-group used within the IM are spread over different groups. This spreading increases the diversity compared to the OFDM-IM whose diversity degree is always one. Other channel effects should also be discussed in UWA environments, such as the Doppler effects on the detection of IM. However, the diversity degree in the IM-OFDM-SS is harvested since the same information bits are spread over different subcarriers. However, that diversity is at the cost of the attained data rate-making IM-OFDM-SS only suitable for small data rate systems. In summary, both OFDM-IM and IM-OFDM-SS might be promising techniques for achieving reliable UWAC without any extra-required hardware complexity.