RESUMO
Recent research has shown that an accurate underwater channel characterization is necessary for underwater optical wireless communication (UOWC) in order to improve its current limitations related to the achievable data rate and the link distance, as required in undersea optical networks. This paper presents a new statistical model to characterize the scattering effect in terms of a fading never considered before. In this way, the probability density function of the scattering-induced fading channel is derived by means of a Gamma distribution by using only one degree of freedom in clear ocean and coastal waters. The developed fading model is employed to compute the performance of UOWC systems in terms of bit error rate and outage probability along with turbulence-induced fading modeled by a Weibull distribution. The results prove that smaller diversity order values are achieved when scattering-induced fading is the dominant effect, i.e., when the condition σ s2>1ß 1 is satisfied, where σ s2 and ß1 are parameters related to the Gamma and Weibull distributions, respectively. Moreover, the optical power penalty due to scattering-induced fading is analytically evaluated in several turbulence conditions to provide a deeper insight. Optical power penalty values of up to 6 dB and 9 dB are achieved when compared with no scattering scenarios at moderate distances for clear ocean and coastal waters. As a key feature, scattering should be always considered in terms of fading for future designs of advanced UOWC systems. The analytical results are verified by Monte Carlo simulations.
RESUMO
Point-to-point underwater optical wireless communication (UOWC) links are mainly impaired by scattering due to impurities and turbidity in the open water, resulting in a significant inter-symbol interference (ISI) that limits seriously both channel capacity and the maximum practical information rate. This paper conducts, for the first time, the channel capacity analysis of UOWC systems in the presence of ISI and salinity-induced oceanic turbulence when the undersea optical channel is accurately modeled by linear discrete-time filtering of the input symbols. In this way, novel upper and lower bounds on channel capacity and mutual information are developed for non-uniform on-off keying (OOK) modulation when different constraints are imposed on the channel input. The results show that the capacity-achieving distribution, which is computed through numerical optimization, is discrete and depends on the optical signal-to-noise-ratio (SNR). Moreover, a non-uniform input distribution significantly improves the channel capacity of such systems affected by ISI and oceanic turbulence, especially at low optical SNR. Monte Carlo techniques are employed to test the developed bounds for different undersea optical channels with one, two and three casual ISI coefficients.
