29 research outputs found
Frame Synchronization for FSO Links with Unknown Signal Amplitude and Noise Power
In this letter, we investigate the problem of frame synchronization in a free-space optical (FSO) communications link, where a known synch pattern is periodically embedded in the transmitted bitstream. The modulation format is on-off keying (OOK) and the electrical signal provided by the photo-detector is plagued by a mixture of thermal and shot noise with signal-dependent power. Due to atmospheric turbulence, channel attenuation can exhibit large random fluctuations, so that no prior knowledge of the signal level and noise variances is assumed. These parameters, together with the start-of-frame, are jointly estimated using a simplified maximum likelihood (ML) approach. Numerical simulations indicate that the proposed scheme is able to effectively exploit the presence of shot noise to improve its detection capability, and outperforms the standard frame synchronization method tailored for an AWGN channel with signal-independent noise power
Beyond 5G Fronthaul based on FSO Using Spread Spectrum Codes and Graphene Modulators.
High data rate coverage, security, and energy efficiency will play a key role in the continued performance scaling of next-generation mobile systems. Dense, small mobile cells based on a novel network architecture are part of the answer. Motivated by the recent mounting interest in free-space optical (FSO) technologies, this paper addresses a novel mobile fronthaul network architecture based on FSO, spread spectrum codes, and graphene modulators for the creation of dense small cells. The network uses an energy-efficient graphene modulator to send data bits to be coded with spread codes for achieving higher security before their transmission to remote units via high-speed FSO transmitters. Analytical results show the new fronthaul mobile network can accommodate up to 32 remote antennas under error-free transmissions with forward error correction. Furthermore, the modulator is optimized to provide maximum efficiency in terms of energy consumption per bit. The optimization procedure is carried out by optimizing both the amount of graphene used on the ring resonator and the modulator’s design. The optimized graphene modulator is used in the new fronthaul network and requires as low as 4.6 fJ/bit while enabling high-speed performance up to 42.6 GHz and remarkably using one-quarter of graphene only
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
A High-speed Reconfigurable Free Space Optical Communication System Utilizing Software Defined Radio Environment
Free space optical (FSO) communication allows for high-speed data transmissions while also being extremely cost-effective by using visible or infrared wavelengths to transmit and receive data wirelessly through the free space channel. However, FSO links are highly susceptible to the effects of the atmosphere, particularly turbulence, smoke, and fog. On the other hand, FSO itself does not provide enough flexibility to address the issue of such blockage and obstruction caused by objects and atmospheric conditions. This research investigates, proposes, and evaluates a software defined multiple input multiple output (MIMO) FSO system to ensure link availability and reliability under weather conditions as part of the last mile access in the 5th generation, 6th generation, and beyond. Software defined radio (SDR) technology is adopted in order to provide a certain degree of flexibility to the optical wireless communications system. The scope of this research focuses on the design, validation, implementation, and evaluation of a novel adaptive switching algorithm i.e., activating additional transmitters of a MIMO FSO system using a software defined ecosystem. The main issues are the compactness of the experimental design; the limitation of software-oriented signal generation; robustness; reliability; and the quality of service. As part of the system design, the thresholding method, a decision-making process via the feedback link, and a spatial diversity technique is adopted to carry out the adaptive switching. The adaptive switching is performed via a feedback link in which the atmospheric loss and scintillation index are calculated for fog and turbulence respectively. The initial design is implemented in SDR/ GNURadio for a real-time emulation of the proposed system to enhance the system flexibility of a traditional MIMO FSO system. A bit-by-bit comparison is performed with the GNURadio signal processing block and BERT for a real-time BER estimation. However, based on the initial results, the switching mechanism can only overcome the effect of turbulence at a certain level. A new design to mainly mitigate the varying fog conditions is proposed based on the SDR-based adaptive switching for a gigabit ethernet (GbE) MIMO FSO system and tested in a 5 m dedicated atmospheric chamber. The proposed system is implemented using off-the-shelf components such as a media converter, small form pluggable transceivers, optical switch, and power meter to estimate the channel state information. A new Schmitt trigger-based thresholding method is also introduced. The proposed software defined GbE MIMO FSO with an adaptive switching algorithm is fabricated, implemented, and investigated. The results are also compared with the real-time simulated data. Since the purpose of this Ph.D. is to explain and demonstrate the proof of concept for the proposed SDR-MIMO FSO system, the emphasis has been on the design, evaluation, and minimal performance requirements rather than maximizing the data rate. The outcome of the thesis will be a huge degree of flexibility and mitigation property MIMO FSO can offer with the help of SDR. It will be shown that the designed system has the capability to provide data transmission with 99.999% availability with a packet error rate and data rate of 7.2 ×10−2 and ~120 Mbps respectively, under extremely harsh fog conditions with visibility V of < 11 m
Broadband optical wireless communications for the teleoperation of mining equipment
The current level of technological advancement of our civilization serving more than seven
billion human population requires new sources of biotic and abiotic natural resources. The
depletion and scarcity of high-grade mineral deposits in dry land are forcing the Natural Re-
sources industry to look for alternate sources in underwater environments and outer space,
requiring the creation of reliable broadband omnidirectional wireless communication systems
that allows the teleoperation of exploration and production equipment. Within these ob-
jectives, Optical Wireless Communications (OWC) are starting to be used as an alternative
or complement to standard radio systems, due to important advantages that optical wave-
lengths have to transmit data: potential for Terabit/s bit rates, broadband operation in
underwater environments, energy e ciency and better protection against interference and
eavesdropping. This research focus in two crucial design aspects required to implement
broadband OWC systems for the teleoperation of mining equipment: high bandwidth wide
beam photon emission and low noise omnidirectional Free-Space Optical (FSO) receivers.
Novel OWC omnidirectional receivers using guided wavelength-shifting photon concentra-
tion are experimented in over 100 meters range vehicle teleoperation.Master of Science (MSc) in Natural Resources Engineerin
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