# Investigation of the Scattering Noise in Underwater Optical Wireless Communications
## 1 Introduction  
In underwater environments, where we are witnessing a growing number of fixed and mobile devices, robots, sensors, etc., there is the need for reliable and advanced communication networks offering several data rates, low latency over a short to long transmission range for commercial, scientific and exploration applications [1, 2]. For underwater communications, there are three main technologies. (i) Acoustic waves, where the transmission data rate is limited to tens of kbps over one km link range and <kbps for longer distances due to the surface-induced pulse spreading and frequency-dependent attenuation [3]. In addition, due to the low propagation speed of acoustic waves (i.e., 1500 m/s), the transmission latency is high, particularly for longer ranges; thus, establishing real-time communications and achieving synchronization becomes challenging [4, 5]. (ii) Radio frequency (RF) waves, which suffer from high attenuations (i.e., 3.5-5 dB/m) and offer moderate transmission data rates (up to 100 Mbps over short distances) [3]. Since the seawater with a high salt level is a conductive transmission medium, the RF waves can only propagate a few meters at very low frequencies (i.e., 30-300 Hz). However, in fresh waters, RF wave propagation is much better when utilizing large-sized antennas or higher transmit power [6]. Note, (i) and (ii) are widely used in underwater communications for a range of applications; however, they have limitations, which can be addressed by adopting a third option based on optical wave propagations. The underwater optical wireless communications (UOWC) technology offers much higher data rates (i.e., in the order of Gbps) [1, 7], lower latency (compared with acoustic waves) [8], lower attenuation (at the blue-green (450-580 nm) transmission band), and higher-level of security at the physical layer due to well-confined light beams in point-to-point communications [9, 10]. The Underwater channel suffers from both absorption and scattering. The former is due to the energy dissipation of propagating photons, which are converted into otherforms (e.g., heat, chemical, etc.) along the transmission path. Whereas the latter is due to photons propagating direction, which is changing randomly because of (i) interaction with the water particles with sizes comparable to the carrier wavelength (i.e., diffraction); and/or (ii) changing refractive indices (i.e., refraction). In turbid waters, especially coastal and harbor, the transmitted photons will experience multiple scattering [11; 12]. In [13], it is shown that the multiple scattering and system geometries affected the temporal dispersion. They simulated the channel loss due to the multiple scattering under the different field of views (FoVs) and apertures for the ocean, coastal, and harbor waters; also, the BER performance is depicted. In [14], the impacts of moving microalgae on the received signal in a short-range UOWC link was investigated by means of experimental measurements, where the random contributions of the scattered light on the moving microalgae were summed up at the receiver (Rx), thus resulting in increased total signal variance. In [15], the 500 Mb/s UOWC system performance was investigated by considering the shot noise, dark current noise, thermal noise, and background noise, and it was shown that link spans of 40 and 10 m with the transmit power levels of 47 and 27 dBm for the coastal and harbor waters, respectively were achievable at the forward error correction (FEC) BER limit of \(3\times 10^{-3}\). In [16], it is shown that for the clear and coastal waters, the maximum link ranges achieved were 30 and 18 m, and 41 and 26 m for the transmit power levels of 0.1 and 1 W, respectively for the FEC BER limit of \(3\times 10^{-3}\) and a data rate of 100 Mb/s.