Underwater Optical Wireless Communications with Chromatic Dispersion and Time Jitter

Abstract

The obsolete communication systems used in the underwater environment necessitates the development and use of modern telecommunications technologies. One such technology is the optical wireless communications, which can provide very high data rates, almost infinite bandwidth and very high transmission speed for real time fast and secure underwater links. However, the composition and the optical density of seawater hinder the communication between transmitter and receiver, while many significant effects strongly mitigate the underwater optical wireless communication (UOWC) systems’ performance. In this work, the influences of chromatic dispersion and time jitter are investigated. Chromatic dispersion causes the temporal broadening or narrowing of the pulse, while time jitter complicates the detection process at the receiver. Thus, the broadening of the optical pulse due to chromatic dispersion is studied and the influence of the initial chirp is examined. Moreover, the effect of the time jitter is also taken into consideration and for the first time, to the best of our knowledge, a mathematical expression for the probability of fade is extracted, taking into account the influence of both of the above-mentioned effects for a UOWC system. Finally, the appropriate numerical results are presented.

Time Jitter, Turbulence and Chromatic Dispersion in Underwater Optical Wireless Links

“…Thus, by using the empirical model introduced by McNeil [48], and by substituting the wavelength, λ, in nm, of the optical beam, with the corresponding angular frequency, ω, in rad/sec, i.e. λ = 2 × 10 9 πvω −1 , with v being the speed of light in the water in m/sec, the refractive index of the underwater environment will be given through the following expression as a function of the link's parameters [27]:…”

“…where Pr is the water pressure in kg/cm 2 , Te is the temperature in Celsius degrees, while Sa stands for the salinity of the seawater in % , [27,48]. Next, by substituting the refractive index into the propagation constant, i.e., β(ω) = ωn(ω)c −1 , with c being the speed of light in vacuum, in m/sec [28,33], and by expanding it in a Taylor series around the angular frequency ω 0 and keeping terms up to the second order [28], with β q = (d q β/dω q ) ω=ω 0 for q = 1,2, ... [28], the value of β 1 , which represents the inverse group velocity [28], is estimated, while β 2 , which represents the GVD parameter, is given in ps 2 /km as [28]:…”

“…Then, by taking into account (4) and (5), the values of the refractive index, and β 2 are presented in the following Table 1, for various values of the wavelength of the UOWC systems and by assuming typical values of water temperature and salinity, i.e., Sa = 35% , Te = 0 • C [27,48]. It should be mentioned here that for the specific temperature and salinity values, it has been proven that the pressure can be accurately estimated as Pr 0.104d [48], where d represents the underwater depth in meters and takes the value of 100 m. By using the GVD parameter, β 2 , obtained in (5), the evolution of the normalized amplitude, U(z,T), of a pulse propagating along axis z, in km, inside a dispersive medium, can be estimated through the following equation [28]:…”

“…Thus, any delayed arrival or time desynchronization at the receiver can cause the detection of the optical pulse earlier or later than the "correct" time of the detection. This effect results in the performance mitigation of the link, because it increases the negative influence of the scintillation effect, due to the increased intensity of the fluid turbulence and the negative affection on the systems characteristics [21][22][23][24][25][26][27].…”

“…Thus, any delayed arrival or time desynchronization at the receiver can cause the detection of the optical pulse earlier or later than the "correct" time of the detection. This effect results in the performance mitigation of the link, because it increases the negative influence of the scintillation effect, due to the increased intensity of the fluid turbulence and the negative affection on the systems characteristics [21][22][23][24][25][26][27].Another effect that can significantly decrease the performance of the underwater optical communication links is the chromatic dispersion. More specifically, due to the dependence of the refractive index of the seawater on the wavelength and taking into account that the typical optical pulses used in the optical communication systems cannot be monochromatic, the longitudinal pulse shape changes along its propagation, and thus the detection procedure at the receiver becomes more difficult and erroneous [28][29][30][31][32][33].In this work-for the first time and to the best of our knowledge-the simultaneous influence of turbulence, time jitter and chromatic dispersion is investigated for an UOWC link.…”