Robust Timing Synchronization for AC-OFDM Based Optical Wireless Communications

Visible light communications (VLC) have recently attracted a growing interest and can be a potential solution to realize indoor wireless communication with high bandwidth capacity for RF-restricted environments such as airplanes and hospitals. Optical based orthogonal frequency division multiplexing (OFDM) systems have been proposed in the literature to combat multipath distortion and intersymbol interference (ISI) caused by multipath signal propagation. In this paper, we present a robust timing synchronization scheme suitable for asymmetrically clipped (AC) OFDM based optical intensity modulated direct detection (IM/DD) wireless systems. Our proposed method works perfectly for ACO-OFDM, Pulse amplitude modulated discrete multitone (PAM-DMT) and discrete Hartley transform (DHT) based optical OFDM systems. In contrast to existing OFDM timing synchronization methods which are either not suitable for AC OFDM techniques due to unipolar nature of output signal or perform poorly, our proposed method is suitable for AC OFDM schemes and outperforms all other available techniques. Both numerical and experimental results confirm the accuracy of the proposed method. Our technique is also computationally efficient as it requires very few computations as compared to conventional methods in order to achieve good accuracy.Comment: Accepted for publication in IEEE ICNS 2015, 10 Pages, 7 fig

Transmission Line Synthesis Approach to Extending the Bandwidth of LEDs for Visible Light Communication

This paper proposes, for the first time, a transmission line synthesis approach to extending the bandwidth of light-emitting diodes (LEDs) in the context of high capacity visible light communications links. As opposed to the more traditional pre-distortion, amplitude equalisation or driver circuitry based approaches, the extension in bandwidth is achieved by incorporating the LED diffusion capacitance into a pseudo-artificial transmission line (p-ATL) cell with significantly improved transmission and cut-off properties. With the proposed technique, we show the possibility of achieving close to 400% improvement in bandwidth with studies based on a verified LED equivalent model. It is envisaged that the proposed approach will enable bespoke driver circuits based on the individual characteristics of LEDs, while combination with existing bandwidth extension schemes can lead to further improvement

Bandwidth Dependency of (O)LEDs on Bias current

This work investigates the modulation bandwidth (Bmod) dependency of organic and non-organic light emitting diodes (OLED\LED) on the applied bias current (IB). The equivalent lumped element transient circuit models are shown with the critical components empirically extracted for both types of device. Four OLEDs of varying sizes are tested in addition to four high power LEDs (white phosphor, red, green and blue). Through analysis of the current-voltage characteristics, the device and dynamic diode resistances are determined as well as the ideality factors. We show that OLEDs have higher ideality factors to the traditional LEDs (almost double) hence the increased turn-on voltage, however have similar AC drive voltage characteristics across the emitting portion of the device between 8-12%. Furthermore, both devices exhibit an increase in Bmod with an increase in IB. It is shown that the OLEDs Bmod increases linearly in relation to IB, reaching =80% of the maximal Bmod at =60% of their maximum IB rating. Conversely, the LEDs display an exponential rise in Bmod in relation to IB, with =80% of the maximal Bmod at =35% of their maximum IB rating, with the red LED showing the greatest results at just 17%

Epitaxial growth of iii-nitride nanostructures and their optoelectronic applications

Light-emitting diodes (LEDs) using III-nitride nanowire heterostructures have been intensively studied as promising candidates for future phosphor-free solid-state lighting and full-color displays. Compared to conventional GaN-based planar LEDs, III-nitride nanowire LEDs exhibit numerous advantages including greatly reduced dislocation densities, polarization fields, and quantum-confined Stark effect due to the effective lateral stress relaxation, promising high efficiency full-color LEDs. Beside these advantages, however, several factors have been identified as the limiting factors for further enhancing the nanowire LED quantum efficiency and light output power. Some of the most probable causes have been identified as due to the lack of carrier confinement in the active region, non-uniform carrier distribution, and electron overflow. Moreover, the presence of large surface states and defects contribute significantly to the carrier loss in nanowire LEDs. In this dissertation, a unique core-shell nanowire heterostructure is reported, that could overcome some of the aforementioned-problems of nanowire LEDs. The device performance of such core-shell nanowire LEDs is significantly enhanced by employing several effective approaches. For instance, electron overflow and surface states/defects issues can be significantly improved by the usage of electron blocking layer and by passivating the nanowire surface with either dielectric material / large bandgap energy semiconductors, respectively. Such core-shell nanowire structures exhibit significantly increased carrier lifetime and massively enhanced photoluminescence intensity compared to conventional InGaN/GaN nanowire LEDs. Furthermore, AlGaN based ultraviolet LEDs are studied and demonstrated in this dissertation. The simulation studies using Finite-Difference Time-Domain method (FDTD) substantiate the design modifications such as flip-chip nanowire LED introduced in this work. High performance nanowire LEDs on metal substrates (copper) were fabricated via substrate-transfer process. These LEDs display higher output power in comparison to typical nanowire LEDs grown on Si substrates. By engineering the device active region, high brightness phosphor-free LEDs on Cu with highly stable white light emission and high color rendering index of \u3e 95 are realized. High performance nickel?zinc oxide (Ni-ZnO) and zinc oxide-graphene (ZnO-G) particles have been fabricated through a modified polyol route at 250?C. Such materials exhibit great potential for dye-sensitized solar cell (DSSC) applications on account of the enhanced short-circuit current density values and improved efficiency that stems from the enhanced absorption and large surface area of the composite. The enhanced absorption of Ni-ZnO composites can be explained by the reduction in grain boundaries of the composite structure as well as to scattering at the grain boundaries. The impregnation of graphene into ZnO structures results in a significant increase in photocurrent consequently due to graphene\u27s unique attributes including high surface area and ultra-high electron mobility. Future research directions will involve the development of such wide-bandgap devices such as solar cells, full color LEDs, phosphor free white-LEDs, UV LEDs and laser diodes for several applications including general lighting, wearable flexible electronics, water purification, as well as high speed LEDs for visible light communications