# Editorial for Special Issue: Underwater Acoustics, Communications, and Information Processing
## 3 Underwater Acoustic Communication Systems  
After proper channel characterization, system engineers perform waveform design, modulation selection, further signal shaping against channel, and proper receiver implementation, mainly physically, for the communication system configuration. [PERSON] et al. proposed the constant envelope frequency-modulated orthogonal frequency division multiplexing (FM-OFDM) for underwater acoustic communication systems in [8], and justified experimentally that the system works well in a channel, less than 25 km long, with strong impulse noises and quasi-non-stationary properties. In [9], \"Chaos-Based Underwater Communication with Arbitrary Transducers and Bandwidth\", [PERSON] et al. used a chaotic communication system with an enhanced differential chaos shift keying (DCSK) for underwater acoustic communications. Among various forms of DCSK, such as correlation delay shift keying (CDSK), phase-separated DCSK (PS-DCSK), high-efficiency DCSK (HE-DCSK), and reference-modulated DCSK (RM-DCSK), the new proposal based on a first-order hybrid chaotic system showed the best performance. Time-reversal processes represent a scheme to generate a spatio-temporal focus at a source location by transmitting a time-reversed version of a received signal, reducing inter-symbol interference at the receiver, and improving communication performance, in the harsh environment characterized by significant multipath channels. Time-reversal beamforming is a well-known approach in practice, and [PERSON] et al. proposed in [10] a passive time-reversal communication system for shallow water applications. Specifically, they introduced a new measure Eq of an estimate of how much of the q-function lies within one symbol duration, where the q-function is estimated by a sum of the autocorrelation of the channel impulse response for each channel in the receiver array. As a result, the parameter Eq shows a strong correlation to the bit error rate and the output signal-to-noise ratio obtained after the time-reversal operation. Furthermore, in [11], \"Virtual Source Array-Based Multiple Time-Reversal Focusing\", [PERSON] et al. investigated time-reversal beamforming using virtual sources. For the time reversal to focus correctly, the focusing requires the source for a coherent acoustic focus at its origin, whose requirement is known to be partially relaxed by the introduction of the concept of a virtual source array (VSA). They extended the existing VSA-based single focusing to simultaneous multiple focusing, by resolving the multiple constraint problem derived from a constraint matrix, with appropriately synchronized transfer functions. Through numerical simulations, they showed that simultaneous multiple focusing guarantees a distortionless response at selected multiple locations, yet its performance degrades according to the presence of sound speed mismatch. To further achieve robust multiple focusing in the mismatched sound speed environment, a singular value decomposition algorithm is applied to find the weight backpropagation vector, approximating the column vectors of the constraint matrix [11]. [PERSON] et al. in [12], \"Study on the Structure of an Efficient Receiver for Covert Underwater Communication Using Direct Sequence Spread Spectrum\", considered a special purpose communication system under a hostile underwater environment. Spread spectrum techniques are known to provide a low probability of interception property, and turbo equalization techniques employing a RAKE receiver and a decoding with repetition are employed additionally, to improve system performance even at a low signal-to-noise ratio due to the spread spectrum technique. In [13], \"A Novel Fractional Fourier Transform-Based ASK-OFDM System for Underwater Acoustic Communications\", [PERSON] et al. proposed an alternative underwater system using fractional Fourier transform.