Analysis of OFDM-based intensity modulation techniques for optical wireless communications

Optical wireless communication (OWC) is a promising alternative to radio frequency (RF) communication with a significantly larger and unregulated spectrum. Impairments in the physical layer, such as the non-linear transfer characteristic of the transmitter, the dispersive optical wireless channel and the additive white Gaussian noise (AWGN) at the receiver, reduce the capacity of the OWC system. Single-carrier multi-level pulse position modulation (M-PPM) and multilevel pulse amplitude modulation (M-PAM) suffer from inter-symbol interference (ISI) in the dispersive channel which reduces their capacity even after channel equalization. Multi-carrier modulation such as optical orthogonal frequency division multiplexing (O-OFDM) with multilevel quadrature amplitude modulation (M-QAM) is known to maximize the channel capacity through bit and power loading. There are two general signal structures: bipolar Gaussian signal with a direct current (DC) bias, i.e. DC-biased O-OFDM (DCO-OFDM), or unipolar half- Gaussian signal, employing only the odd subcarriers, i.e. asymmetrically clipped O-OFDM (ACO-OFDM). In this thesis, the signal distortion from the transmitter nonlinearity is minimized through pre-distortion, optimum signal scaling and DC-biasing. The optical front-ends impose minimum, average and maximum optical power constraints, as well as an average electrical power constraint, on the information-carrying signals. In this thesis, the optical signals are conditioned within these constraints through optimum signal scaling and DC-biasing. The presented analysis of the optical-to-electrical (O/E) conversion enables the derivation of the electrical signal-to-noise ratio (SNR) at the receiver, including or excluding the additional DC bias power, which is translated into bit-error rate (BER) performance. In addition, a generalized piecewise polynomial model for the non-linear transfer characteristic of the transmitter is proposed. The non-linear distortion in O-OFDM is translated by means of the Bussgang theorem and the central limit theorem (CLT) into attenuation of the data-carrying subcarriers at the receiver plus zero-mean complex-valued Gaussian noise. The attenuation factor and the variance of the non-linear distortion noise are derived in closed form, and they are accounted towards the received electrical SNR. Through pre-distortion with the inverse of the proposed piecewise polynomial function, the linear dynamic range of the transmitter is maximized, reducing the non-linear distortion to double-sided signal clipping. Finally, the OWC schemes are compared in terms of spectral efficiency and electrical SNR requirement as the signal bandwidth exceeds the coherence bandwidth of the optical wireless channel for a practical 10 dB linear dynamic range. Through optimum signal scaling and DCbiasing, DCO-OFDM is found to achieve the highest spectral efficiency for a target SNR, neglecting the additional DC bias power. When the DC bias power is counted towards the signal power, DCO-OFDM outperforms PAM with linear equalization, approaching the performance of the more computationally intensive PAM with non-linear equalization. In addition, the average optical power in O-OFDM is varied over dynamic ranges of 10 dB, 20 dB and 30 dB. When the additional DC bias power is neglected, DCO-OFDM is shown to achieve the Shannon capacity, while ACO-OFDM exhibits a 3 dB gap which grows with higher SNR targets. When the DC bias power is included, DCO-OFDM outperforms ACO-OFDM for the majority of average optical power levels with the increase of the SNR target or the dynamic range

ELECTRIC DRIVE FOR PHOTOVOLTAIC MODULES DUAL AXIS TRACKING SYSTEM

The paper deals with the implementation of dual axis tracking system for photovoltaic modules in order to demonstrate the advantages of these systems in education. According to the main components – the system includes electronic module for control and two DC motors for each of the axes, respectively.The paper deals with the implementation of dual axis tracking system for photovoltaic modules in order to demonstrate the advantages of these systems in education. According to the main components – the system includes electronic module for control and two DC motors for each of the axes, respectively

Spectrally Efficient Waveforms for the Return Link in Satellite Communication Systems

In this paper, we study the applicability of terrestrial mobile waveforms in the return link of a high throughput satellite (HTS) communication system. These include orthogonal frequency division multiple access (OFDMA), single-carrier frequency division multiple access (SC-FDMA) and filter bank multi-carrier (FBMC). Key solutions to the challenges in a geostationary orbit (GEO) satellite channel, such as synchronization and non-linear distortion, are presented. A global-positioning-system-(GPS)-based approach for synchronization acquisition is proposed, while suitable algorithms are studied for timing/frequency offset estimation and synchronization tracking. The spectral and power efficiencies of the schemes are optimized by means of an intermodulation interference (IMI) cancelling receiver, and these are compared to state-of-the-art time division multiple access (TDMA). Finally, end-to-end simulations validate the system performance

Principles of LED Light Communications: Towards Networked Li-Fi

Balancing theoretical analysis and practical advice, this book describes all the underlying principles required to build high performance indoor optical wireless communication (OWC) systems based on visible and infrared light, alongside essential techniques for optimising systems by maximising throughput, reducing hardware complexity and measuring performance effectively. It provides a comprehensive analysis of information rate-, spectral- and power-efficiencies for single and multi-carrier transmission schemes, and a novel analysis of non-linear signal distortion, enabling the use of off-the-shelf LED technology. Other topics covered include cellular network throughput and coverage, static resource partitioning via dynamic interference-aware scheduling, realistic light propagation modelling, OFDM, optical MIMO transmission and nonlinearity modelling. Covering practical techniques for building indoor optical wireless cellular networks supporting multiple users and guidelines for 5G cellular system studies, in addition to physical layer issues, this is an indispensable resource for academic researchers, professional engineers and graduate students working in optical communications

The influence of period of sowing of winter fodder pea variety of tuber - formation and plant residues content for improving soil fertility

The study of plant - microbial associations is important for the development of modern farming and environmental ecology. In the process of the gradual reduction of import quantities of mineral fertilizers and pesticides to increase the plants productivity and yield it is necessary to activate the agrocenoses biological components, where learning about the legume bean - rizobial symbiosis nitrogen - fixation, mobilization and provision of the plants with environmentally safe biological nitrogen ant its storage in the soil are the reasons for constant researches and experiments. The purpose of this study is to determine the influence of the period of sowing on the development of the root mass, tuber -formation, the amount of post - harvest residues and soil agrochemical composition of winter fodder peas. the experiment is made in the period of 2005 to 2008 in Pleven EFC with the method of random block system. The result show that in the first sampling maximum number of tubers (26.62 g plant) and root dry weight (1.55 g/plant) were formed in a second sowing period (10-15 October), but with the greatest amount of post - harvest residues (435.5 kg/ha) are those of the a third period (25 - 30 October). After the harvest and the secondary growing of peats the indicators value are lower by 76,2, 30.2 and 61.6%. The agrochemical soil analysis shows pronounced trends in mineral nitrogen and organic carbon content under the influence of different sowing periods. The soils where the plants are harvested from later sowing time are characterized with high nitrogen and carbon content determined by the larger root volume and post - harvest residues in these terms. After the pea harvest the current and potential soil acidity values decreased by 2.7 and 5.5 percent compared with the control variant