A Study on the Impact of Nonlinear Characteristics of LEDs on Optical OFDM by

“The truly learned man is he who understands that what he knows is but little in comparison with what he does not know.” (Ali Bin Aby-Talib (r.a) ) In the 21 century, the demand for wireless bandwidth will continue to increase to serve bandwidth-intensive applications such as high definition (HD) video streaming via Internet. The existing radio frequency (RF) technology standards, i.e. the IEEE (institute of electrical and electronics engineers) 802.11a/b/g/n standards, suffer from the bandwidth bottleneck. However, optical wireless (OW) technology has the potential for being an attractive complementary to realize broadband wireless access indoors. Recently, indoor OW communication systems based on light emitting diode (LEDs) as light sources and orthogonal frequency division multiplexing (OFDM) as a modulation technique have gained significant attention. For instance, in visible light communication (VLC), in addition to lightening capabilities of white LEDs, they can be modulated to achieve high-speed wireless data transmission. Optical OFDM (O-OFDM) systems are able to support high data rates through parallel transmission of high order multi-level quadrature amplitude modulation (M -QAM) symbols on orthogonal subcarriers. Complex channel equalizers are not required and the time-varying channel can be easily estimated using frequency-domain channel estimation. Adaptive modulation per subcarrier can be easily realized based on the uplink/downlink requested data rates and quality of service (QoS). Also, the inherent robustness of OFDM against multipath effects, and the possibility to combine it with any multiple access scheme, e.g. time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA), makes it an excellent choice for indoor OW links. However, only little research has been done in this area, and many open questions exist such as the performance of O-OFDM systems in the presence of LED nonlinearities. The time domain OFDM signal exhibits a random-like variations which is characterized by a high peak-to-average power ratio (PAPR). Therefore, special attention must be paid to its sensitivity to nonlinearities. This thesis analyzes the performance of indoor OW systems based on OFDM in the presence LED nonlinearities, i.e. amplitude and clipping distortions. The key parameters investigated are the LED DC bias point, the average OFDM signal power modulating the LED, and the constellation orders. The distortion analysis shows that LED nonlinearities can drastically degrade system performance. As a consequence, a linearization technique is proposed. A digital pre-distorter is applied to compensate for LED nonlinearities. It is found that the degradation can greatly be mitigated by using the proposed predistortion technique. In a practical system, the OFDM signal is forcibly clipped at the LED turn-on voltage (TOV) and purposely clipped prior to the LED modulation for voltage levels above the saturation voltage which corresponds to the maximum permissible AC/pulsed current recommended by the LED manufacturer. The OFDM pre-conditioning prior to the LED modulation is to ensure that the LED chip does not overheat, in order to avoid degradation in output light or, in the worst case, total failure. Therefore, there is a need to describe the nonlinear behavior of an LED with a model that fits the LED measured transfer function and to level off at both extremes to represent the TOV at one end and the saturation voltage at the other end. A model offering the desired ”S-shaped” curve is proposed. Analytical methodologies to evaluate error probability of uncompensated O-OFDM system and compensated O-OFDM system are proposed. A comparison with Monte Carlo simulation results are carried out. The simulation curves and the analytical curves show a close match in terms of symbol error ratio (SER). Finally, hardware prototypes to demonstrate online short-range optical transmission are developed. Visible light transmission is considered. Apart from providing proof of concept, experimental measurements for practical scenarios are obtained to analyze the performance in real environments. Realizing the analog optical transmitter and receiver front ends, involves schematic design, layout design, printed circuit board (PCB) fabrication, component assembly and soldering. The O-OFDM model is based on Matlab/Simulink/C programming. Digital signal processing (DSP) development boards are configured to run the O-OFDM model. Defence date: 29.03.2010 Author: Hany Elgala Supervisor: Prof. Dr. Harald Haas I certify that I have prepared this PhD Thesis by my own without any inadmissible outside help. Jacobs University, 29.03.2010