## 2 Experimental Setup  
The experiment setup of the proposed RS-based UWOCC system with PMMA SGOF is shown in Figure 1. The on-off keying (OOK) signal at different data rates is produced by a low-speed arbitrary waveform generator (AWG, Tektronix(r) AFG3252C, Beaverton, OR, USA). The AWG has a sampling rate of 2 GS/s and an analog bandwidth of 240 MHz. It is used to modulate a blue LD directly via an LD driver. The blue LD has an output optical power of 600 mW and a wavelength of 445 nm. Then, the blue OOK optical signal is launched into the PMMA SGOF, which is located inside a water tank with dimensions of 0.3 m \(\times\) 0.73 m \(\times\) 0.88 m. The PMMA SGOF has a diameter of 3 mm and a length of 2 m. The OCC Rx is a mobile phone (ASUS(r) Zenfone3, Taiwan, China) with a CMOS camera with a 30 fps frame rate and a 1920 \(\times\) 1440 pixel resolution. We select a shutter speed of 1/10,000 s and an ISO of 1600 in the experiment. As this mobile phone is not water resistant, it is put inside a waterproof transparent plastic bag, and then the Rx is placed inside the water tank. The Tx includes the PMMA SGOF and the blue LD module. The blue LD module consists of a blue LD with an output power of 600 mW and an LD driver. The driver DC voltage is 12 V, the modulation mode is analog, and the maximum modulation frequency is 10 kHz. An optical collimator is installed in front of the LD to produce the optical beam divergence < 3 mrad.  
In order to increase the applicability of our proposed UWOCC system, we use off-the-shelf components and devices. Hence, a typical mobile-phone-embedded CMOS image sensor can be used. The SGOF is also an off-the-shelf product. There is no special requirement about the dimensions of the water tank, as long as the SGOF and the mobile phone can be immersed for the experiment. The use of a 445 nm wavelength blue LD is because it has low attenuation underwater, as shown in [18].  
Here, the PMMA SGOF is utilized as the optical Tx. It is robust against bending and stretching. It is mainly used for signage lighting, such as advertising light boxes, swimming pools, tunnels, etc. Table 1 shows the parameters of the SGOF used in the experiment. Figure 2a,b show, respectively, the photographs of the PMMA SGOF and the proof-of-concept experiment, illustrating the PMMA SGOF emitting blue light homogeneously around its circumference. It can act as a wide 360\({}^{\circ}\) large FOV \"omni-directional optical Tx\". This large FOV can fascinate the optical detection in turbulent water environments. In the proof-of-concept demonstration, the distance between the mobile phone and the PMMA SGOF is about 20 cm.  
The OCC Rx is a CMOS Image Sensor (Sony(r) IMX298, Tokyo, Japan) embedded in a mobile phone (ASUS(r) Zenfone3). Table 2 shows the parameters of the CMOS image sensor used in the experiment.  
Figure 1: Experiment setup of the proposed RS-based UWOC system with PMMA SGOF. AWG: arbitrary waveform generator; PMMA: poly-methyl methacrylate.  
In the RS-based OCC system reported in this work, when the transmission distance between the SGOF and the camera increases, the occupied area of the SGOF image captured in the CMOS image sensor becomes smaller. As the RS is produced at the CMOS camera, the RS column pixel width is unchanged when the transmission distance increases. Hence, when the transmission distance increases, a complete OCC packet cannot be observed in an image frame, as illustrated in [32], and the BER measurement cannot be performed. In addition to the transmission distance limitation, UWOCC performance could also be affected by water clarity and water turbulence. This is because dirty water will block the light and reduce the received signal-to-noise ratio (SNR), while wavy water could distort the received RS pattern and increase the BER. We believe that it could be employed in real underwater conditions if the received SNR is enough.