Fig. 1. Schematic diagram of the general UWOC setup in a lab experiment. AWG: arbitrary waveform generator; EA: electrical amplifier; ATT: adjustable attenuator; DC: direct current; LD: laser diode; APD: avalanche photodiode; DSA: digital serial analyzer; Tx-DSP: digital signal processing at the transmitter; Rx-DSP: digital signal processing at the receiver.

Fig. 2. Received optical power (ROP) and SNR versus transmission distance under tap water. w/: with; w/o: without; NLE: nonlinear equalization^[39].

Fig. 3. (a) Received SNR versus the volume of added Maalox suspension after a 1 m underwater transmission. (b) Attenuation coefficient versus volume of the added Maalox suspension. (c)–(f) The snapshots of the optical beam passing through water of different turbidities which represent (c) “tap water”, (d) “clear ocean”, (e) “coastal ocean”, and (f) “harbor water”^[39].

Fig. 4. (a) Shannon capacity limit under different underwater transmission distances. (b) Entropy of different subcarriers for 25 m and 35 m underwater transmission distances. (c) Graphical illustrations for bit-power loading and the PCS-256/1024QAM-DMT scheme of three different entropies. Note that the bars denote the probability of each modulation symbol^[42].

Fig. 5. Received constellation diagrams of (a) bit-power loading, (b) PCS-256QAM-DMT for 35 m, and (c) PCS-1024QAM-DMT for 25 m underwater transmissions^[36].

Table 1. Summary of Recent Works in UWOC