## 2 Experiment Setup  
### Underwater Optical Wireless Communications System  
Figure 1 illustrates the block diagram of the proposed UWOC system assisted by DeepESN. 19-bit pseudorandom binary sequences (PRBSs) were generated as data source, followed by the application of (229, 255) Reed-Solomon error correction (RSEC) to enable error-free transmission. The error-free limit of the experimental system was approximately BER = \(10^{-3}\)[53]. Using intensity modulation/direct detection (IM/DD), the intensity of the laser is directly modulated by the baseband pulse-amplitude modulation (PAM) and DC-biased optical OFDM signal [22]. PAM signal could simplify the study on the system's transmission performance with different equalization methods and the selection of DC-biased optical OFDM was a trade-off between the system's stability and communication capacity.  
In the PAM modulation case, the encoded bits were modulated with pulse-amplitude modulation at data rates of 100, 125, and 167 Mbps, respectively. For QAM-OFDM, encoded bits were first modulated with quadrature amplitude modulation (QAM), and the time-domain OFDM signal was obtained by applying the inverse fast Fourier transformation (IFFT) to the Hermitian symmetric QAM symbols following the DC-biased optical OFDM scheme [22]. Data rates for QAM-OFDM were 100.38, 111.70, 121.21, and 130.68 Mbps. A cyclic prefix (CP) was added to the OFDM signal to avoid intersymbol interference. After signal modulation, a synchronization header was inserted into the signal frame, and pre-distortion was applied to the modulated signal to minimize the impact of the nonlinear response of the laser diode (LD). The encoded digital signal was converted to an analogy signal using a 250-Msamples/s arbitrary waveform generator (AWG). A 520-nm (green) laser was employed as the light source, and the analogy signal was internally modulated onto the optical carrier. The wireless optical signal was then transmitted through a 40.5 m underwater optical link established in a water tank.  
At the receiver side, a single-pixel avalanche photodiode (APD) performed the conversion of the signal from the optical domain into the electrical domain, followed by a 250-Msamples/s oscilloscope for analogy-to-digital conversion. DeepESN was first applied  
Figure 1: System configuration for deep echo state network (DeepESN)-assisted underwater wireless optical communication.  
to learn and compensate for channel noise in the received digital signal prior to signal demodulation. The received PAM signal was directly demodulated. In the QAM-OFDM case, the cyclic prefix and guard band were removed from the received signal before performing QAM-OFDM demodulation. The BER was measured after RSES decoding.  
In this system, an arbitrary waveform generator (RS PRO SDG6022X) was utilized to generate the analogy signal. An OSRAM PLT5 520B, yielding an average output power of approximately 16.5 dBm, served as the light source. A water tank, measuring 225 cm \(\times\) 80 cm \(\times\) 60 cm, generated a 40.5 m optical path by generating multiple reflections at both ends. The calculated attenuation of the UWOC channel was around 0.9 dB/m (with an attenuation coefficient of 0.207/m), approximating the conditions of clear ocean water [1]. On the receiver side, a variable attenuator (Thorlabs LCC1620A/M) was employed to attenuate the receiving power. This setup introduced a consistent power loss of 9.23 dBm at the point of least attenuation across all results. Therefore, the actual received power should be adjusted by adding this constant loss of 9.23 dBm to the observed results. The photonic receiver employed was an APD430A(/M) with a responsivity of 12 A/W. Analogy-to-digital conversion was performed using a Tektronix MDO32 oscilloscope.