Average Bit Error Rate at Signal Transmission with OOK Modulation Scheme in Different FSO Channels

In this paper, the Average Bit Error Rate of the signal in the Free Space Optical system modulated with On-Off keying scheme is calculated and analysed. The Average Bit Error Rate is determined in the case of an atmospheric channel modelled with Gamma-Gamma distribution, Log-Normal distribution, K distribution and I-K distribution. The results are presented both analytically and graphically for different lengths of the Free Space Optical link and the strength of the atmospheric turbulence. The quality of the received signal based on the Average Bit Error Rate for weak, moderate and strong atmospheric turbulences, different lengths of the transmission section and different Signal-to-Noise Ratio values was analysed. The operation of the Free Space Optical system in the observed environment was simulated and the transmission quality was analysed based on Bit Error Rate and Q factor

Free-Space Optical Communications Over Lognormal Fading Channels Using OOK With Finite Extinction Ratios

Free-space optical communication links operating over lognormal turbulence channels using on-off keying (OOK) are studied in this work. Such systems can suffer from irreducible error floors that result from the use of demodulation with fixed and unoptimized detection thresholds. The resulting error floors are analyzed for the general case of low and high state offsets (i.e., finite extinction ratios). An electrical signal-to-noise ratio (SNR) optimized detection system is applied. The system uses the electrical SNRs to implement adaptive detection thresholds and eliminate the error floors. The system can accommodate operation with finite extinction ratios, as it uses the method of moments and maximum likelihhod estimation techniques to estimate the low and high state offsets and electrical SNR. Numerical results show that the SNR gap between the electrical-SNR-optimized detection system and the adaptive detection system is 2:3 dB at a bit-error rate of 10-⁵ without a state offset. The SNR gap increases to 4:5 dB with a state offset of = 0:2.Applied Science, Faculty ofEngineering, School of (Okanagan)ReviewedFacultyPostdoctora

Free-Space Optical Communications Over Lognormal Fading Channels Using OOK With Finite Extinction Ratios

Free-space optical communication links operating over lognormal turbulence channels using on-off keying (OOK) are studied in this work. Such systems can suffer from irreducible error floors that result from the use of demodulation with fixed and unoptimized detection thresholds. The resulting error floors are analyzed for the general case of low and high state offsets (i.e., finite extinction ratios). An electrical signal-to-noise ratio (SNR) optimized detection system is applied. The system uses the electrical SNRs to implement adaptive detection thresholds and eliminate the error floors. The system can accommodate operation with finite extinction ratios, as it uses the method of moments and maximum likelihhod estimation techniques to estimate the low and high state offsets and electrical SNR. Numerical results show that the SNR gap between the electrical-SNR-optimized detection system and the adaptive detection system is 2:3 dB at a bit-error rate of 10-⁵ without a state offset. The SNR gap increases to 4:5 dB with a state offset of = 0:2.Applied Science, Faculty ofEngineering, School of (Okanagan)ReviewedFacultyPostdoctora

Mitigation techniques through spatial diversity combining and relay-assisted technology in a turbulence impaired and misaligned free space optical channel.

Doctor of Philosophy in Electronic Engineering. University of KwaZulu-Natal, Durban, 2018.In recent times, spectrum resource scarcity in Radio Frequency (RF) systems is one of the biggest and prime issues in the area of wireless communications. Owing to the cost of spectrum, increase in the bandwidth allocation as alternative solution, employed in the recent past, does no longer offer an effective means to fulfilling high demand in higher data rates. Consequently, Free Space Optical (FSO) communication systems has received considerable attention in the research community as an attractive means among other popular solutions to offering high bandwidth and high capacity compared to conventional RF systems. In addition, FSO systems have positive features which include license-free operation, cheap and ease of deployment, immunity to interference, high security, etc. Thus, FSO systems have been favoured in many areas especially, as a viable solution for the last-mile connectivity problem and a potential candidate for heterogeneous wireless backhaul network. With these attractive features, however, FSO systems are weather-dependent wireless channels. Therefore, it is usually susceptible to atmospheric induced turbulence, pointing error and attenuation under adverse weather conditions which impose severe challenges on the system performance and transmission reliability. Thus, before widespread deployment of the system will be possible, promising mitigation techniques need to be found to address these problems. In this thesis, the performance of spatial diversity combining and relay-assisted techniques with Spatial Modulation (SM) as viable mitigating tools to overcome the problem of atmospheric channel impairments along the FSO communication system link is studied. Firstly, the performance analysis of a heterodyne FSO-SM system with different diversity combiners such as Maximum Ratio Combining (MRC), Equal Gain Combining (EGC) and Selection Combining (SC) under the influence of lognormal and Gamma-Gamma atmospheric-induced turbulence fading is presented. A theoretical framework for the system error is provided by deriving the Average Pairwise Error Probability (APEP) expression for each diversity scheme under study and union bounding technique is applied to obtain their Average Bit Error Rate (ABER). Under the influence of Gamma-Gamma turbulence, an APEP expression is obtained through a generalized infinite power series expansion approach and the system performance is further enhanced by convolutional coding technique. Furthermore, the performance of proposed system under the combined effect of misalignment and Gamma-Gamma turbulence fading is also studied using the same mathematical approach. Moreover, the performance analysis of relay-assisted dual-hop heterodyne FSO-SM system with diversity combiners over a Gamma-Gamma atmospheric turbulence channel using Decode-and-Forward (DF) relay and Amplify-and-Forward (AF) relay protocols also is presented. Under DF dual-hop FSO system, power series expansion of the modified Bessel function is used to derive the closed-form expression for the end-to-end APEP expressions for each of the combiners under study over Gamma-Gamma channel, and a tight upper bound on the ABER per hop is given. Thus, the overall end-to-end ABER for the dual-hop FSO system is then evaluated. Under AF dual-hop FSO system, the statistical characteristics of AF relay in terms of Moment Generating Function (MGF), Probability Density Function (PDF) and Cumulative Distribution Function (CDF) are derived for the combined Gamma-Gamma turbulence and/or pointing error distributions channel in terms of Meijer-G function. Based on these expressions, the APEP for each of the under studied combiners is determined and the ABER for the system is given by using union bounding technique. By utilizing the derived ABER expressions, the effective capacity for the considered system is then obtained. Furthermore, the performance of a dual-hop heterodyne FSO-SM asymmetric RF/FSO relaying system with MRC as mitigation tools at the destination is evaluated. The RF link experiences Nakagami-m distribution and FSO link is subjected to Gamma-Gamma distribution with and/or without pointing error. The MGF of the system equivalent SNR is derived using the CDF of the system equivalent SNR. Utilizing the MGF, the APEP for the system is then obtained and the ABER for the system is determined. Finally, owing to the slow nature of the FSO channel, the Block Error Rate (BLER) performance of FSO Subcarrier Intensity Modulation (SIM) system with spatial diversity combiners employing Binary Phase Shift Keying (BPSK) modulation over Gamma-Gamma atmospheric turbulence with and without pointing error is studied. The channel PDF for MRC and EGC by using power series expansion of the modified Bessel function is derived. Through this, the BLER closed-form expressions for the combiners under study are obtained