BER Estimation of Dual Hop QAM OFDM ROFSO over Exponentially Modeled Turbulence and Optical Fiber with Nonlinear Clipping

The radio on free space optical (RoFSO) communication systems are gaining popularity due to their high data rates, license free spectrum and adequate reliability at installation and operational costs which are much lower than comparable technologies. A significant disadvantage of these systems concerns the randomly time varying characteristics of the propagation path mainly caused by the turbulence effect. On the other hand, the optical fiber communication systems offer links with higher data rates but with expensive infrastructure and installation cost. In this work, we study the BER performance of an optical communication system which consists of a RoFSO link that is connected with an optical fiber link through a regenerator node. The signal propagates, in both links, using the OFDM technique with QAM format and the dominant impairments which have been taken into account are the atmospheric turbulence, modelled with the negative exponential distribution, the nonlinear responsivity of the laser diode which can be modelled with a Volterra series and the biasing with nonlinear clipping noise. For this setup, closed form mathematical expression for the estimation of system's BER has been derived and the corresponding numerical results are presented for common link parameters

K Modeled Turbulence and Nonlinear Clipping for QAM OFDM with FSO and Fiber Serial Linked

The free space optical (FSO) and more specifically the radio on free space optical (RoFSO) communication systems are becoming very popular because they can achieve high bit rate transmission with low operational and installation cost. The main disadvantage of such systems is their dependence on the atmospheric conditions and more specifically the randomly time varying characteristics of the propagation path through the atmosphere which is, mainly, caused by the turbulence effect and affects significantly the system's availability and performance. On the other hand, the use of optical fiber systems, offer higher bit-rates and security level but their cost is larger. In this work, the performance of a hybrid dual hop optical communication system is investigated. This system consists of a RoFSO communication link which is connected with an optical fiber link part using a regenerator between them. In both links, the modulation technique that is used is the orthogonal frequency division multiplexing (OFDM) with either a 4 or 16 QAM format. The main phenomena that are taken into account are the atmospheric turbulence, which is modelled with K distribution, the nonlinearities of the laser diode which could be modelled by Volterra series and the biasing with the nonlinear clipping at the optical fiber segment. For this system, closed form mathematical expression for the estimation of its BER is derived and numerical results are presented for realistic parameter values

Advanced DSP Techniques for High-Capacity and Energy-Efficient Optical Fiber Communications

The rapid proliferation of the Internet has been driving communication networks closer and closer to their limits, while available bandwidth is disappearing due to an ever-increasing network load. Over the past decade, optical fiber communication technology has increased per fiber data rate from 10 Tb/s to exceeding 10 Pb/s. The major explosion came after the maturity of coherent detection and advanced digital signal processing (DSP). DSP has played a critical role in accommodating channel impairments mitigation, enabling advanced modulation formats for spectral efficiency transmission and realizing flexible bandwidth. This book aims to explore novel, advanced DSP techniques to enable multi-Tb/s/channel optical transmission to address pressing bandwidth and power-efficiency demands. It provides state-of-the-art advances and future perspectives of DSP as well

OFDM Wireless Optical Communication Systems with Serial Relays Over Exponentially Modeled Turbulence Channels

The optical wireless communication systems attract very significant research and commercial interest, the last years, due to their high availability, and performance characteristics with relatively low installation and operational cost. On the other hand, due to the fact that the optical channel which is used in this technology is the atmosphere, their performance depends strongly on the atmospheric conditions and the link’s characteristics. Thus, the effective distance that these systems can cover is relatively short. Thus, in many cases where the long distance signal propagation using wireless optical systems, is necessary, relay nodes are used. In this work, we study the performance of a wireless optical system, which is using the orthogonal frequency division multiplexing (OFDM) technique and relay nodes which decode and retransmit the received signal, over atmospheric turbulence channels modeled with the negative exponential distribution. For this system, we derive closed form mathematical expressions for its outage probability and average bit error rate (BER). Finally, we present the corresponding numerical results for realistic cases with common parameter values

Voronoi Constellations for Coherent Fiber-Optic Communication Systems

The increasing demand for higher data rates is driving the adoption of high-spectral-efficiency (SE) transmission in communication systems. The well-known 1.53 dB gap between Shannon\u27s capacity and the mutual information (MI) of uniform quadrature amplitude modulation (QAM) formats indicates the importance of power efficiency, particularly in high-SE transmission scenarios, such as fiber-optic communication systems and wireless backhaul links. Shaping techniques are the only way to close this gap, by adapting the uniform input distribution to the capacity-achieving distribution. The two categories of shaping are probabilistic shaping (PS) and geometric shaping (GS). Various methods have been proposed for performing PS and GS, each with distinct implementation complexity and performance characteristics. In general, the complexity of these methods grows dramatically with the SE and number of dimensions.Among different methods, multidimensional Voronoi constellations (VCs) provide a good trade-off between high shaping gains and low-complexity encoding/decoding algorithms due to their nice geometric structures. However, VCs with high shaping gains are usually very large and the huge cardinality makes system analysis and design cumbersome, which motives this thesis.In this thesis, we develop a set of methods to make VCs applicable to communication systems with a low complexity. The encoding and decoding, labeling, and coded modulation schemes of VCs are investigated. Various system performance metrics including uncoded/coded bit error rate, MI, and generalized mutual information (GMI) are studied and compared with QAM formats for both the additive white Gaussian noise channel and nonlinear fiber channels. We show that the proposed methods preserve high shaping gains of VCs, enabling significant improvements on system performance for high-SE transmission in both the additive white Gaussian noise channel and nonlinear fiber channel. In addition, we propose general algorithms for estimating the MI and GMI, and approximating the log-likelihood ratios in soft-decision forward error correction codes for very large constellations

Power efficient subcarrier modulation for intensity modulated channels

We compare formats for optical intensity modulation limited by thermal noise with the assumption of having ideal devices. At the same bitrate and bandwidth, a hitherto unknown format turns out to be more power efficient than known formats. This new modulation, which is a hybrid between on-off keying and phase-shift keying, belongs to the subcarrier modulation family. At asymptotically high signal-to-noise ratios, this hybrid scheme has a 1.2 dB average electrical power gain and 0.6 dB average optical power gain compared to OOK, while it has a 3.0 dB average electrical power gain and 2.1 dB average optical power gain compared to subcarrier QPSK

Baud sampling bit synchroniser for channels with data dependent noise

A bit synchronisation algorithm for channels with data dependent noise which operates with one sample per symbol is presented. The algorithm uses the same information as the Mueller and Muller (M&M) algorithm, and is optimised for operation with data dependent noise. The performance is derived and it is shown that signfkant improvements over the M&M algorithm can be obtained in practical optical channels