Hybrid LED–LD underwater optical wireless communication system for long-range IoUT applications

Abstract Underwater Optical Wireless Communication (UWOC) has emerged as a key enabling technology for high-speed Internet of Underwater Things (IoUT) networks; however, its performance is severely constrained by wavelength-dependent absorption, scattering, turbulence, beam divergence, and strict alignment requirements in different water environments. To address these challenges, this paper proposes a robust hybrid optical transmission architecture that intelligently integrates light-emitting diode (LED) and laser diode (LD) sources to exploit their complementary characteristics. LEDs provide wide-beam coverage and reliable link establishment at short ranges, while LDs enable high-power, narrow-beam transmission for extended communication distances and high data rates. A comprehensive analytical UWOC channel model is developed incorporating water attenuation, turbulence-induced fading, pointing errors, geometric losses, and receiver noise, together with an adaptive SNR-based switching mechanism for dynamic source selection. The proposed hybrid LED–LD UWOC system is evaluated using received optical power, signal-to-noise ratio (SNR), bit-error rate (BER) (analyzed versus distance, SNR, and transmitted power), achievable data rate (throughput), outage probability, coverage probability, link margin, energy efficiency (bits/J), spectral efficiency (bits/s/Hz), quality factor (Q), and eye-diagram–based signal quality assessment. Extensive simulations are conducted at blue (450 nm) and green (520 nm) wavelengths for multiple water types, including pure sea, clear ocean, coastal, and harbor conditions. Results demonstrate substantial performance gains of LD transmission over LED links. In pure seawater, LD communication distances exceed approximately 80–90 m, whereas LED links remain limited to roughly 15–20 m. In clear ocean conditions, LD links maintain ranges near 50 m compared with about 12–14 m for LEDs. Under a strict reliability constraint of BER ≤ 10⁻ ⁹ LD sources achieve successful transmission at significantly lower SNR requirements, reducing the needed SNR by roughly 8–9 dB relative to LEDs. Furthermore, at a 1 Gbps throughput threshold, LEDs support only short-range links around 10 m, while LD transmission extends beyond 60 m, confirming the strong suitability of laser-based propagation for long-distance high-capacity UWOC. The obtained results verify that the proposed hybrid LED–LD framework significantly enhances communication reliability, achievable range, and spectral efficiency across diverse underwater environments. This adaptive dual-source strategy provides a scalable and practical solution for next-generation high-speed underwater sensing, monitoring, and autonomous IoUT applications.