Experimental Study of Joint Hierarchical Equalization Scheme for AF/DF Relaying in a 1 Gbps Underwater Visible Light Communication System

To overcome limitations imposed by device bandwidth and transmission range in underwater visible light communication (UVLC) systems, this study compares amplify-and-forward (AF) and decode-and-forward (DF) relaying on a relay-assisted UVLC (RA-UVLC) experimental platform. A joint hierarchical equalization scheme (JHES) is proposed, comprising two branches tailored to the different transmission characteristics of AF and DF. For the AF scheme, lightweight cascaded spectral pre-equalization (LCS-PE) performs two-stage spectral compensation to suppress spectral distortion across successive hops. For the DF scheme, hierarchical multi-domain post-equalization (HMD-PE) successively compensates for distortion in the frequency, time, and symbol domains. A 5 m water tank and neutral density filters are used to establish attenuation-equivalent transmission conditions. Experimental results show that both schemes achieve transmission at 1 Gbps over an attenuation-equivalent distance of 1 km, with an end-to-end bit error rate below 3.8 × 10−3. Their performance is further compared under four disturbance conditions. These results provide experimental evidence for selecting equalization methods and relay schemes in multi-hop UVLC systems.