In this work, underwater visible light communication (UWVLC) vertical links are modeled and evaluated, taking account of the inhomogeneous nature of UW environment. An equivalent simplified model of stratified N layers is employed in which variations in refractive index and attenuation profiles across UW depth are considered. A generalized path loss expression is deduced which allows estimation of the vertical link loss prior to link design. Mathematical expressions of the received power for line-of-sight (LOS) and non-line-of-sight (NLOS) links between transmitters (Tx) and receivers (Rx) are deduced. We evaluate the performance by computing the received power and BER for inhomogeneous underwater links. Numerical examples are used to illustrate the proposed models. Deviations from expected results when considering a homogeneous underwater model are discussed. A simple underwater bilayer model is then introduced, which is considered a rough approximate model in comparison to the stratified N layers model. This model assists in estimating the UW link behaviour without extensive calculations. The conditions necessary for applying this model are discussed and justified. Moreover, the effects of Tx orientation along with narrowing the transmitted light cone on LOS and NLOS vertical links are examined. A Tx power saving of 30%−50% in UWVLC links is proved to be achieved by rotating the Tx and narrowing its emitted light cone.

High-speed, reliable, flexible, and cost-effective underwater wireless communication systems have a wide variety of use in scientific, civilian, and commercial domains. Visible light communication (VLC) seems to be a promising candidate in satisfying majority of the requirements listed above. From this perspective, VLC could easily be deployed in heterogeneous wireless networking scenarios with high quality of service. Therefore, in this paper, an adapted LTE frame structure is implemented for underwater VLC (UVLC) system. The performance of UVLC system employing asymmetrically clipped optical orthogonal frequency division multiplexing modulation technique is practically tested by taking channel estimation and synchronization into account. Also, the effects of varying orthogonal frequency division multiplexing parameters on the system's signal-to-noise ratio and the bit-error-rate (BER) performances are studied. The implementation is tested for quadrature phase shift keying and 16-quadrature amplitude modulation techniques supporting both 128- and 1024-points fast Fourier transform configurations, which correspond to data rates of 1.92 and 15.36 Mbps, respectively. Experimental results show that BER could be reduced down to a level on the order of 10 ^-6 .

This work studies the impact of employing blue filters on the resulting effective modulation bandwidth of visible light communication (VLC) systems using phosphor-coated white light emitting diodes (LEDs) under wide-angle operation. Effective modulation bandwidths are assessed and compared for VLC systems with ideal and non-ideal transmission blue filters at the receiver. Angle tolerance capabilities of the employed filters have also been investigated in the study. Commercially available phosphor-coated white LED and photodetector (PD) models are utilized to obtain realistic results. We show that the choice of the blue filter structure, combined with the white LED and PD, affects the system’s effective modulation bandwidth. We also show that there exists a trade-off between obtaining a relatively large modulation bandwidth and preserving the wide-angle operation of the system. A figure of merit (FOM) is also introduced to represent such a trade-off. Moreover, a simple estimation of the expected effective modulation bandwidth of a VLC system using phosphor-coated white LED is developed as a result of the weighted average of the blue and yellow light responsivity of the filter. A numerical example is also included in which the optical signal to noise ratio (OSNR) is computed in VLC systems with and without the addition of a blue filter. The OSNR shows enhancement when a blue filter with a wide rejection region is inserted in the studied VLC system.

This work presents what we believe is a novel design of a hybrid plasmonic-transmission blue filter for visible light communication systems that employ yellow phosphor-coated blue light-emitting diodes. The proposed filter balances the trade-off between transmission performance and tolerance to variation in angles of incidence (AOIs) while maintaining a low cost with limited complexity design. The designed filter operation is based upon quasi-plasmon mode excitation in a hybrid structure of alternating layers of silver and titanium dioxide over a silica substrate. A primary design approach for a hybrid plasmonic filter of five alternating layers is illustrated in detail. Needle optimization technique is further applied to achieve the required filter performance. The designed filter has an insertion loss of $\sim 1\mathrm{dB}$ over a spectral range of 400–485 nm and a minimal close to zero polarization-dependent loss for a wide range of AOI (slightly above 50°). The tolerance of the proposed design against fabrication errors is also tested. The performances of the proposed filters are tested for individual and simultaneous variations from the designed thicknesses, with a $\pm 10\%$ standard deviation from each layer’s thickness.

This work presents a technique for the design of visible optical filters using a hybrid plasmonic insulator–metal–insulator (IMI) structure. The proposed IMI visible light filter exhibits high transmission ($\sim 91\%$) and an insertion loss of $\sim 0.4\mathrm{dB}$ with almost an omnidirectional field-of-view (0°–70°), a feature that is important for light collection in miniaturized devices. The proposed design also has a minimal polarization dependent loss of 0.2 dB at an angle of incidence of 60°. The effects of design parameters on the filter’s performance are studied. Design rules of the filter are deduced along with physical justifications of the obtained results.

In this work, the performance of an underwater visible light communication system utilizing a vertical communication channel is evaluated. The inhomogeneity of underwater environment is taken into consideration. A mathematical model for the received power is derived and bit error rates (BER) are computed for different underwater environments. The validity of the model is verified by numerical results. Moreover, our results show that there exists an optimum transmitter-receiver separation, where BER is minimum, under a specific transmission orientation angle.