Performance optimisation mechanisms for optical wireless communication systems

Gatri, Aymen (2017) Performance optimisation mechanisms for optical wireless communication systems. Doctoral thesis, Northumbria University.

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Abstract

In this day and age, free space optical (FSO) enable the deployment of a new category of products that can transmit voice, data, and video at bandwidths up to 2.5 Gbps at distances up to 4 km, over any protocol. This optical connectivity doesn’t require expensive fibre-optic cable or spectrum licenses. FSO is reliable due to the capability of FSO systems to provide truly broadband and secure communications, as well as their immunity to interference from other sources of optical radiation. The principal challenge facing FSO technology is to achieve 100% link availability in all weather conditions. While rain, fog, haze, turbulence and aerosols all attenuate the optical signal to a certain level, fog is considered to be the main impairment in FSO systems. Thick fog resulting in over 300 dB/km of signal attenuation can reduce the transmission span from a few kilometres to just 100 m or so. Turbulence (i.e. scintillation) also results in reducing fade margins from 4 to 10 dB for FSO links of 1 km length or less, which is well below the margins for atmospheric attenuation. In the real-world environment, it is very challenging to measure the effect of atmospheric fog under diverse circumstances. This is due to several reasons: (i) the longer observation time required and the lower probability of reoccurrence of dense fog events for visibility V < 0.5 km, and (ii) the difficulty in controlling and characterising aerosols in the atmosphere, due to the inhomogeneous presence of aerosols along the FSO link path. This thesis examines and analyses the performance of a terrestrial FSO system by investigating the impact of a number of modulation techniques on mitigating the atmospheric impairments. A dedicated indoor atmospheric chamber is designed to carry out tests and measurements in a controlled manner and mimic the real outdoor atmosphere. The experimental results are compared with predicted data for the range of modulation techniques tested in the presence of atmospheric turbulence and fog, including binary phase shift keying subcarrier intensity modulation (BPSK-SIM), 2-pulse position modulation (2-PPM), 4-pulse position modulation (4-PPM) and hybrid pulse position modulation binary phase shift keying subcarrier intensity modulation (BPSK-SIM-PPM).The results show that BPSK-SIM-PPM offers a similar performance to 2-PPM, a superior performance to BPSK-SIM, while having the same bandwidth, and an inferior performance to 4-PPM under turbulent conditions. Furthermore, the experimental investigation indicates that 4-PPM is more resistant to turbulence compared to BPSK-SIM. The improvement of the link performance by optimising the beam spot size using combinations of mirrors is also investigated. In addition, the effects of low to high visibility on the FSO link BER performance in the presence of fog are measured and investigated. The obtained results indicate the dependency of the performance of the FSO link on the fog intensity variation. Moreover, the experimental results show that the impact of severe fog induced attenuation is greater on the receiver than the transmitter. Finally, the effect of fog on an FSO system employing quadrature phase-shift keying (QPSK) and operating at various carrier wavelengths is also studied.

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