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%

Investigation of optical wireless for employment within a vehicular environment

The substantial increase in powerful electronic systems and functions has produced significant implications for the vehicular industry, where the amount of wiring infrastructure has increased the vehicle weight, weakened performance, and made adherence to reliability standards difficult. Eventually, connecting the electronics infrastructure was mostly complicated and costly in vehicular domain systems. Thus, little research has been conducted to explore appropriate wireless technologies that may be suitable with the emerging network standard within the context of vehicular networks. This thesis describes an in-depth investigation of deploying an optical wireless communication system within the vehicular environment, particularly in confined spaces. A wide variety of measurements has been performed using tubes of various materials and geometries, in a laboratory setup. The principle objective is to provide a primary knowledge of optical wireless channel characterization within a laboratory vehicular setting. The work presented is a study on directed line-of-sight (LOS) and non-LOS (NLOS) links, and focuses on frequency response, power efficiencies, and path losses in different experimental settings. Further, a variety of experimental settings was used in respect to different receiver/transmitter orientations and various bent tubes angles in order to investigate the channel conditions. The noise analysis, xviii SNR, path loss and the eye pattern for the digital system prototype designed were also analysed. The system requirement for the LOS link were based on the transmission of the sinusoidal signal at a distance of 1 m with 13 MHz signal and approximately 15.6 dB SNR. Successful demonstration of the OWC within smaller size and high reflection coefficient material are promising. In addition to good transmitter and high sensitivity receiver. The NLOS link also demonstrated a good indication, both in straight tube with angled transmitter/receiver orientation and bend tubes. Detail studies on NLOS link with pulse signal transmission, which replicates a digital system transmission with 54.48mW or 44.58 mW/cm2 output power,6 MHz signal transmission with the aim of 10-4 to 10-6 BER. Although, the operational functionality of digital system has successfully demonstrated, however achieving the desired BER is a bit difficult with the designed system. Further improvement on the highly sensitive receiver design, a proper modulation scheme is required in order to improve the quality of the transmitted signal in terms of SNR and BER. The study also suggested that the transmission within the metal tubes is better than in plastic tubes in addition to minimum bend angle, smaller tube diameter and high reflective coefficient. Transmission within 20 mm circular aluminium tube and 35 mm galvanised aluminium tube are the best so far. Finally, based on the initial viability results, it was seen that it is possible to implement an optical wireless communication infrastructure within the vehicular environment. Experimental validation of the system proposed shows that achieving high data rates is not a problem with the use of high brightness, high power LEDs as this system is xix going to be implemented within the vehicle chassis, thus the eye safety constraints should not be a limiting factor. Therefore, in this study, optical wireless transmission within the vehicular environment is proposed, solving the problems of vehicular networking systems

High speed energy efficient incoherent optical wireless communications

The growing demand for wireless communication capacity and the overutilisation of the conventional radio frequency (RF) spectrum have inspired research into using alternative spectrum regions for communication. Using optical wireless communications (OWC), for example, offers significant advantages over RF communication in terms of higher bandwidth, lower implementation costs and energy savings. In OWC systems, the information signal has to be real and non-negative. Therefore, modifications to the conventional communication algorithms are required. Multicarrier modulation schemes like orthogonal frequency division multiplexing (OFDM) promise to deliver a more efficient use of the communication capacity through adaptive bit and energy loading techniques. Three OFDM-based schemes – direct-current-biased OFDM (DCO-OFDM), asymmetrically clipped optical OFDM(ACO-OFDM), and pulse-amplitude modulated discrete multitone (PAM-DMT) – have been introduced in the literature. The current work investigates the recently introduced scheme subcarrier-index modulation OFDM as a potential energy-efficient modulation technique with reduced peak-to-average power ratio (PAPR) suitable for applications in OWC. A theoretical model for the analysis of SIM-OFDMin a linear additive white Gaussian noise (AWGN) channel is provided. A closed-form solution for the PAPR in SIM-OFDM is also proposed. Following the work on SIM-OFDM, a novel inherently unipolar modulation scheme, unipolar orthogonal frequency division multiplexing (U-OFDM), is proposed as an alternative to the existing similar schemes: ACO-OFDMand PAM-DMT. Furthermore, an enhanced U-OFDMsignal generation algorithm is introduced which allows the spectral efficiency gap between the inherently unipolar modulation schemes – U-OFDM, ACO-OFDM, PAM-DMT – and the conventionally used DCO-OFDM to be closed. This results in an OFDM-based modulation approach which is electrically and optically more efficient than any other OFDM-based technique proposed so far for intensity modulation and direct detection (IM/DD) communication systems. Non-linear distortion in the optical front-end elements is one of the major limitations for high-speed communication in OWC. This work presents a generalised approach for analysing nonlinear distortion in OFDM-based modulation schemes. The presented technique leads to a closed-form analytical solution for an arbitrary memoryless distortion of the information signal and has been proven to work for the majority of the known unipolar OFDM-based modulation techniques - DCO-OFDM, ACO-OFDM, PAM-DMT and U-OFDM. The high-speed communication capabilities of novel Gallium Nitride based μm-sized light emitting diodes (μLEDs) are investigated, and a record-setting result of 3.5Gb/s using a single 50-μm device is demonstrated. The capabilities of using such devices at practical transmission distances are also investigated, and a 1 Gb/s link using a single device is demonstrated at a distance of up to 10m. Furthermore, a proof-of-concept experiment is realised where a 50-μm LED is successfully modulated using U-OFDM and enhanced U-OFDM to achieve notable energy savings in comparison to DCO-OFDM