Free Space Optical (FSO) systems are being actively considered as viable options for seamless integration between backhaul optical fiber links and RF or copper links used for last mile connectivity. However, adverse atmospheric conditions can affect the performance and distance over which the link can operate reliably. Propagation of optical waves through the atmosphere is affected by atmospheric turbulence, scattering of aerosols and atmospheric absorption. Atmospheric turbulence is a result of localized variations of temperature, humidity, and pressure in the atmosphere. Turbulence is by nature a random process, and as such may be described using statistical quantities. Atmospheric turbulence induced fading is one of the main impairments affecting Free Space Optics (FSO). FSO systems can suffer outages in the presence of heavy fog, smog and haze. Many places in India see very heavy rainfall. We have studied the effects of rainfall on propagation characteristics in Dakshina Kannada district, Karnataka, India where Surathkal is located which is affected by heavy rainfall for about four to six months in a year. We observe that the attenuation during heavy rainfall is quite high and leads to extremely low levels of the received signal or complete erasure of the transmitted data. With the validation through simulation in this paper, on the use of Digital Fountain codes in FSO links, we suggest that if such codes are used in areas prone to rainfall, it would help in the recovery of dropped packets and would also improve the BER performance.
Optical fibres provide the best solutions for transmitting high speed, large amounts of data with good power efficiency. However such transmission would also need amplification for transmission over large distances. Erbium Doped Fibre Amplifiers(EDFAs) are currently being used for optical amplification. But good amplification is achievable with multiple stages and considerable length of EDFA fibres. In this paper we compare the use of Silver Split Ring Resonators(SRRs) , Gold Nano Rods and Silver Fishnet structures which give metamaterial properties to be used in optical fibres to give better amplification than EDFA based fibres.
Metamaterials belong to a new class of materials with negative values for permittivity and permeability. Such materials would exhibit negative refractive index leading to these materials being called as left handed media.If such left handed media have an internal structure made of dimensions much smaller than the wavelength but sufficiently thick to exhibit bulk properties, using other optical domains such as plasmonics, it is possible to control light interactions and propagation. Artificial structures smaller than the wavelength of light can be used to enhance electric and magnetic fields.
Surface plasmons can be excited on a metal and this can enhance the electric field at the surface. Our paper proposes the use of this phenomenon of achieving gain at optical frequencies by using SRRs, Fishnet structures , Nano Rods. We compare the performance of these structures and observe that they provide gain which is much more than that provided by EDFAs.
Terrestrial Free Space Optical (FSO) links transmit information by using the atmosphere (free space) as a medium. In this paper, we have investigated the use of Luby Transform (LT) codes as a means to mitigate the effects of data corruption induced by imperfect channel which usually takes the form of lost or corrupted packets. LT codes, which are a class of Fountain codes, can be used independent of the channel rate and as many code words as required can be generated to recover all the message bits irrespective of the channel performance. Achieving error free high data rates with limited energy resources is possible with FSO systems if error correction codes with minimal overheads on the power can be used. We also employ a combination of Binary Phase Shift Keying (BPSK) with provision for modification of threshold and optimized LT codes with belief propagation for decoding. These techniques provide additional protection even under strong turbulence regimes. Automatic Repeat Request (ARQ) is another method of improving link reliability. Performance of ARQ is limited by the number of retransmissions and the corresponding time delay. We prove through theoretical computations and simulations that LT codes consume less energy per bit. We validate the feasibility of using energy efficient LT codes over ARQ for FSO links to be used in optical wireless sensor networks within the eye safety limits.