17 research outputs found
AVERAGE BER PERFORMANCE OF SIM-DPSK FSO SYSTEM WITH APD RECEIVER
In this paper, average bit error rate (BER) analysis of the free-space optical (FSO) system employing subcarrier intensity modulation (SIM) with differential phase-shift keying (DPSK) and avalanche photodiode (APD) receiver is presented. The atmospheric turbulence is described by the Gamma-Gamma statistical model taking the pointing errors into account. Numerical results are presented and confirmed by Monte Carlo simulations. The effects of atmospheric turbulence, pointing errors and receiver parameters on the average BER performance are observed and discussed. Based on the presented results, it is concluded that the minimum of the average BER exists for an optimal value of APD gain, which is heavily dependent on receiver noise temperature, bit rate and atmospheric conditions
Optical Communication
Optical communication is very much useful in telecommunication systems, data processing and networking. It consists of a transmitter that encodes a message into an optical signal, a channel that carries the signal to its desired destination, and a receiver that reproduces the message from the received optical signal. It presents up to date results on communication systems, along with the explanations of their relevance, from leading researchers in this field. The chapters cover general concepts of optical communication, components, systems, networks, signal processing and MIMO systems. In recent years, optical components and other enhanced signal processing functions are also considered in depth for optical communications systems. The researcher has also concentrated on optical devices, networking, signal processing, and MIMO systems and other enhanced functions for optical communication. This book is targeted at research, development and design engineers from the teams in manufacturing industry, academia and telecommunication industries
Subcarrier intensity modulated free-space optical communication systems
This thesis investigates and analyses the performance of terrestrial free-space optical communication (FSO) system based on the phase shift keying pre-modulated subcarrier intensity modulation (SIM). The results are theoretically and experimentally compared with the classical On-Off keying (OOK) modulated FSO system in the presence of atmospheric turbulence. The performance analysis is based on the bit error rate (BER) and outage probability metrics. Optical signal traversing the atmospheric channel suffers attenuation due to scattering and absorption of the signal by aerosols, fog, atmospheric gases and precipitation. In the event of thick fog, the atmospheric attenuation coefficient exceeds 100 dB/km, this potentially limits the achievable FSO link length to less than 1 kilometre. But even in clear atmospheric conditions when signal absorption and scattering are less severe with a combined attenuation coefficient of less than 1 dB/km, the atmospheric turbulence significantly impairs the achievable error rate, the outage probability and the available link margin of a terrestrial FSO communication system. The effect of atmospheric turbulence on the symbol detection of an OOK based terrestrial FSO system is presented analytically and experimentally verified. It was found that atmospheric turbulence induced channel fading will require the OOK threshold detector to have the knowledge of the channel fading strength and noise levels if the detection error is to be reduced to its barest minimum. This poses a serious design difficulty that can be circumvented by employing phase shift keying (PSK) pre-modulated SIM. The results of the analysis and experiments showed that for a binary PSK-SIM based FSO system, the symbol detection threshold level does not require the knowledge of the channel fading strength or noise level. As such, the threshold level is fixed at the zero mark in the presence or absence of atmospheric turbulence. Also for the full and seamless integration of FSO into the access network, a study of SIM-FSO performance becomes compelling because existing networks already contain subcarrier-like signals such as radio over fibre and cable television signals. The use of multiple subcarrier signals as a means of increasing the throughput/capacity is also investigated and the effect of optical source nonlinearity is found to result in intermodulation distortion. The intermodulation distortion can impose a BER floor of up to 10-4 on the system error performance. In addition, spatial diversity and subcarrier delay diversity techniques are studied as means of ameliorating the effect of atmospheric turbulence on the error and outage performance of SIM-FSO systems. The three spatial diversity linear combining techniques analysed are maximum ratio combining, equal gain combining and selection combining. The system performance based on each of these combining techniques is presented and compared under different strengths of atmospheric turbulence. The results predicted that achieving a 4 km SIM-FSO link length with no diversity technique will require about 12 dB of power more than using a 4 × 4 transmitter/receiver array system with the same data rate in a weak turbulent atmospheric channel. On the other hand, retransmitting the delayed copy of the data once on a different subcarrier frequency was found to result in a gain of up to 4.5 dB in weak atmospheric turbulence channel
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
Subcarrier intensity modulated free-space optical communication systems
This thesis investigates and analyses the performance of terrestrial free-space optical communication (FSO) system based on the phase shift keying pre-modulated subcarrier intensity modulation (SIM). The results are theoretically and experimentally compared with the classical On-Off keying (OOK) modulated FSO system in the presence of atmospheric turbulence. The performance analysis is based on the bit error rate (BER) and outage probability metrics. Optical signal traversing the atmospheric channel suffers attenuation due to scattering and absorption of the signal by aerosols, fog, atmospheric gases and precipitation. In the event of thick fog, the atmospheric attenuation coefficient exceeds 100 dB/km, this potentially limits the achievable FSO link length to less than 1 kilometre. But even in clear atmospheric conditions when signal absorption and scattering are less severe with a combined attenuation coefficient of less than 1 dB/km, the atmospheric turbulence significantly impairs the achievable error rate, the outage probability and the available link margin of a terrestrial FSO communication system. The effect of atmospheric turbulence on the symbol detection of an OOK based terrestrial FSO system is presented analytically and experimentally verified. It was found that atmospheric turbulence induced channel fading will require the OOK threshold detector to have the knowledge of the channel fading strength and noise levels if the detection error is to be reduced to its barest minimum. This poses a serious design difficulty that can be circumvented by employing phase shift keying (PSK) pre-modulated SIM. The results of the analysis and experiments showed that for a binary PSK-SIM based FSO system, the symbol detection threshold level does not require the knowledge of the channel fading strength or noise level. As such, the threshold level is fixed at the zero mark in the presence or absence of atmospheric turbulence. Also for the full and seamless integration of FSO into the access network, a study of SIM-FSO performance becomes compelling because existing networks already contain subcarrier-like signals such as radio over fibre and cable television signals. The use of multiple subcarrier signals as a means of increasing the throughput/capacity is also investigated and the effect of optical source nonlinearity is found to result in intermodulation distortion. The intermodulation distortion can impose a BER floor of up to 10-4 on the system error performance. In addition, spatial diversity and subcarrier delay diversity techniques are studied as means of ameliorating the effect of atmospheric turbulence on the error and outage performance of SIM-FSO systems. The three spatial diversity linear combining techniques analysed are maximum ratio combining, equal gain combining and selection combining. The system performance based on each of these combining techniques is presented and compared under different strengths of atmospheric turbulence. The results predicted that achieving a 4 km SIM-FSO link length with no diversity technique will require about 12 dB of power more than using a 4 × 4 transmitter/receiver array system with the same data rate in a weak turbulent atmospheric channel. On the other hand, retransmitting the delayed copy of the data once on a different subcarrier frequency was found to result in a gain of up to 4.5 dB in weak atmospheric turbulence channel.EThOS - Electronic Theses Online ServiceGBUnited Kingdo
A Free Space Optic/Optical Wireless Communication: A Survey
The exponential demand for the next generation of services over free space optic and wireless optic communication is a necessity to approve new guidelines in this range. In this review article, we bring together an earlier study associated with these schemes to help us implement a multiple input/multiple output flexible platform for the next generation in an efficient manner. OWC/FSO is a complement clarification to radiofrequency technologies. Notably, they are providing various gains such as unrestricted authorizing, varied volume, essential safekeeping, and immunity to interference.