ReviewA review on channel models in free space optical communication systems
Introduction
In recent years, the increase in the bandwidth and capacity requirements leads to a shift from RF to optical communication. The transmission of optical beams can be carried through free space or in a confined medium which is called as wireless optical communication (WOC) [1]. WOC is classified as Indoor and outdoor wireless optical communication. Indoor wireless optical communication uses wavelength band from 750 nm to 950 nm which is in IR range. Indoor wireless optical communication systems usage is limited with a room.
Outdoor Wireless optical communication is also called as Free Space Optical (FSO) communication is the recent technology with numerous advantages. In free space optical communication the transmitter LED or LD is at the transmitter side sends the digital data like video images, data files through the unguided light beam in free space rather than an optical fibre. In the receiver end these beam of lights are captured by the receiving lens connected to the high sensitive receiver [2]. The advantages like high speed of orders Gbps, large bandwidth, unlicensed spectrum, and high security extends its usage in large applications. Optical carrier operating in IR wavelength is used to establish connection between the terrestrial links within earth and between inter satellite (space optical links).
Free Space Optical communication has various advantages over the Radio Frequency communication. The RF wavelength is larger than the Optical wavelength. This wavelength difference shows FSO is more advantageous than RF.
- 1.
Large Bandwidth: Increase in the carrier frequency causes increase in high data rate transmission. In Optical communication the optical carrier frequency is high when compared with the RF communication.
- 2.
Less power requirement: Because of narrow beam divergence, the optical intensity of the transmitted beam power is more at the receiver than the RF. Smaller wavelength of the FSO leads to the reduction in the size of antenna when compared with the RF.
- 3.
Unlicensed spectrum: Spectrum licensing is the main difference between RF and FSO. FSO requires no spectrum licensing which leads to easy and cost effective deployment. RF requires spectrum licensing to avoid the interference. FSO requires line of sight communication.
- 4.
High security: Optical beam cannot penetrate the walls so information transfer is secure. FSO beams cannot be detected using a spectrum analyser as in case of RF.
In addition to the above advantages FSO has various other advantages also. It can be easily installed when compared with fibre optic cables and it is cost effective. FSO can be used wherever optical fibre cable cannot be used. FSO transceiver system is light weight and easily expandable. FSO has its disadvantages like attenuation and blocking of signal by building. Scintillation causes fading of the signal. Other limiting factors are background noise by the illumination of sun and limit on the laser power transmission due to eye safety limits the link length.
A Free Space Optical communication system (Fig. 1) has three parts: transmitter, atmospheric channel and receiver. In the transmitter section first the data is modulated using modulation schemes and it is converted into optical source by the driver circuit and the light source (LED/LD). The optical beam is transmitted through the transmit optics through the atmospheric channel. In the atmospheric channel the signal is attenuated due to the factors like atmospheric turbulence, scattering, absorption and background noise. In the receiver side the transmitted beam is collected by the receiver lens and converted into electrical source by a photo detector and pre amplifier circuit. The electrical signal is demodulated by a demodulator to obtain the transmitted data.
The paper is organised as in Section 2 the challenges of Free Space Optical communication in terrestrial and space links are discussed. In Section 3 the channel model is discussed in detail. In Section 4 Beam wandering and Beam spreading is discussed separately. Section 5 concludes the paper.
Section snippets
Free space optical communication challenges
In Free Space Optical communication the optical wave is propagating in free space which is subjected to many disturbances. The disturbances like absorption, scattering and turbulence cause the attenuation of the wave. The electromagnetic properties, shape and direction of the beam are affected by these disturbances which in turn affects the overall performance of the optical link. FSO link distance is dependent on the unpredictable weather conditions like fog, rain and haze.
Channel model
This section deals with the channel models for different weather conditions, atmospheric losses and atmospheric turbulence while transmitting an optical signal. The optical signal which is received at the receiver is the convolution of the transmitted signal and the channel impulse response. The channel model is modelling the transmission path between transmitter and receiver. In free space optical communication, the channel is atmosphere. The optical wave experience attenuation in the signal
Mitigation techniques of beam spreading and beam wandering
In this section the mitigation techniques like aperture averaging, relaying and adaptive optics are discussed for beam spreading and beam wandering.
Conclusions
Free Space Optical communication is the emerging technology with lots of advantages. FSO is an alternative of the RF communication because of the merits like high bandwidth, high data rate and unlicensed spectrum. Though FSO has merits to fully utilize the advantages of FSO communication the drawbacks in it has to overcome. The main hindrances in FSO are turbulence, absorption and scattering. To improve the quality of the FSO communication many mitigation techniques are followed. Proper
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