text: abstract—this paper presents a comprehensive study of underwater visible light communications (uvlc), from channel
  characterization, performance analysis, and effective transmission and reception methods. to this end, we first simulate
  the fading-free impulse response (ffir) of uvlc channels
  using monte carlo numerical procedure to take into account
  the absorption and scattering effects; and then to characterize
  turbulence effects, we multiply the aforementioned ffir by a
  fading coefficient which for weak oceanic turbulence can be
  modeled as a lognormal random variable (rv). based on this
  general channel model, we analytically study the bit error rate
  (ber) performance of uvlc systems with binary pulse position
  modulation (bppm). in the next step, to mitigate turbulence
  effects, we employ multiple transmitters and/or receivers, i.e., we
  apply spatial diversity technique over uvlc links. closed-form
  expressions for the system ber are provided, when an equal
  gain combiner (egc) is employed at the receiver side, thanks to
  the gauss-hermite quadrature formula as well as approximation
  to the sum of lognormal rvs. we further apply saddle-point
  approximation, an accurate photon-counting method, to evaluate
  the system ber in the presence of shot noise. both laserbased collimated and light emitting diode (led)-based diffusive
  links are investigated. additionally, in order to reduce the intersymbol interference (isi), introduced by the multiple-scattering
  effect of uvlc channels on the propagating photons, we also
  obtain the optimal multiple-symbol detection (msd) algorithm,
  as well as the sub-optimal generalized msd (gmsd) algorithm.
  our numerical analysis indicate good matches between the
  analytical and photon-counting results implying the negligibility
  of signal-dependent shot noise, and also between the analytical
  results and numerical simulations confirming the accuracy of our
  derived closed-form expressions for the system ber. besides,
  our results show that spatial diversity significantly mitigates
  fading impairments while (g)msd considerably alleviates isi
  deterioration.
  index terms—underwater visible light communications, ber
  performance, lognormal turbulent channels, mimo, spatial diversity, photon-counting methods, (generalized) multiple-symbol
  detection, collimated laser-based links, diffusive led-based links.