# A Survey on Physical Layer Techniques and Challenges in Underwater Communication Systems
## 2 UWAC Physical Layer Techniques
### Spatial Modulation Techniques  
Recently, an alternate group of MIMO systems, identified as space modulation techniques, has been recommended to study the advanced method for dealing with the current problems related to the MIMO systems. In SMTs, receiver computational complexity and energy resources are conserved using a novel spatial constellation diagram to improve spectral efficiency. The elementary approach comes from [118]. With the set of numerous antennas present, currently, a symbol of BPSK is employed to identify an active antenna. The receiver estimates the transmitting antenna and the transmitted symbol of BPSK. [PERSON] et al. suggested the prevalent initial SMT [119; 120] known as SM. The rest of the SMTs are defined as generalized or special cases from the SM. As compared to the customary systems of modulation, information by SM is carried through the multipath method,as an extra constellation map signified as a spatial constellation for the fading channel of MIMO. The spatial symbol of the constellation is modulated by the data bits received. Therefore, the spatial location relevant to one of the existing antennas for communication is specified. It is stimulated at a particular time for transmission of the modulated carrier signal and further processed through extracted complex symbols from an arbitrary diagram of the constellation. The concept of the spatial constellation was primarily explained using the SM system. It proposes to transfer the data using modulating spatial symbols. It is observed that SM can attain a gain in multiplexing by free ICI upholding [120], reduce the complexity of computation for the receiver [121], improve the likelihood of bit error [122], and ensure the use of chain transmitter for a single radio frequency (RF) [123]. Considerable research comprised the theory of SM. In recent years, diverse features of functioning were reflected methodically [124; 125; 126; 127; 128; 129; 130; 131; 132; 133; 134; 135]. Later, multiple different systems relating to similar SM conceptions were projected. A system of space shift keying (SSK) was suggested in [133]. SSK lacks in the transmission of the data symbol, therefore, simply spatial symbols appear. Generalized spatial modulation (GSM) triggers multiple transmit antennas at each time instant for transmission of similar data [135]. Likewise, researchers in [134] recommended generalized space shift keying (GSSK). To transmit the spatial bits, all these layouts formed and applied a single-dimensional spatial diagram for the constellation. References [127; 128] defined an added quadrature spatial constellation diagram. Different spatial symbols are used to transmit the real and imaginary parts of the complex data symbol. After protecting all previous SM benefits, the development of the data rate for two base logarithms of the many transmit antennas is achieved. Quadrature space shift keying (QSSK) and quadrature spatial modulation (QSM) fall under this category of systems. Their generalized forms are known as generalized quadrature spatial modulation (GQSM) and generalized quadrature space shift keying (GQSSK). Similarly, enhanced fully generalized spatial modulation (EFGSM) [136] and FGSM [137] are suggested for the UWAC. For accurate detection of data in SMTs, perfect channel state information (P-CSI) is required by the receiver. However, it is not practical to avail the perfect knowledge of the channel at the receiver. To acquire the CSI, the estimation of the channel is the main concern. Therefore, the adaptive estimation of the channel method, i.e., pilot-based recursive least squares (RLS) in comparison to the time-varying underwater MIMO channel has been suggested. The transmitted data is detected using the maximum likelihood (ML) decoder. The decoder also identifies the estimated UWA-MIMO channel and the antenna indices from the signal received [138]. These are the basic SMTs. The core part includes their function, method of work, limits, practical applications and capacity analysis.