A Survey on OFDM-Based Elastic Core Optical Networking <s> 2) M-QAM (M-Quadrature Amplitude Modulation) <s> We show the first experiment of bit and power loading for coherent optical orthogonal frequency-division-multiplexing (CO-OFDM) systems. The data rate of CO-OFDM systems can be dynamically adjusted according to the channel condition. The system performance can be further improved through optimal power loading into each modulation band. <s> BIB001 </s> A Survey on OFDM-Based Elastic Core Optical Networking <s> 2) M-QAM (M-Quadrature Amplitude Modulation) <s> The first book on optical OFDM by the leading pioneers in the field. The only book to cover error correction codes for optical OFDM. It gives applications of OFDM to free-space communications, optical access networks, and metro and log haul transports show optical OFDM can be implemented. It contains introductions to signal processing for optical engineers and optical communication fundamentals for wireless engineers. This book gives a coherent and comprehensive introduction to the fundamentals of OFDM signal processing, with a distinctive focus on its broad range of applications. It evaluates the architecture, design and performance of a number of OFDM variations, discusses coded OFDM, and gives a detailed study of error correction codes for access networks, 100 Gb/s Ethernet and future optical networks. The emerging applications of optical OFDM, including single-mode fiber transmission, multimode fiber transmission, free space optical systems, and optical access networks are examined, with particular attention paid to passive optical networks, radio-over-fiber, WiMAX and UWB communications. Written by two of the leading contributors to the field, this book will be a unique reference for optical communications engineers and scientists. Students, technical managers and telecom executives seeking to understand this new technology for future-generation optical networks will find the book invaluable. William Shieh is an associate professor and reader in the electrical and electronic engineering department, The University of Melbourne, Australia. He received his M.S. degree in electrical engineering and Ph.D. degree in physics both from University of Southern California. Ivan Djordjevic is an Assistant Professor of Electrical and Computer Engineering at the University of Arizona, Tucson, where he directs the Optical Communications Systems Laboratory (OCSL). His current research interests include optical networks, error control coding, constrained coding, coded modulation, turbo equalization, OFDM applications, and quantum error correction. 'This wonderful book is the first one to address the rapidly emerging optical OFDM field. Written by two leading researchers in the field, the book is structured to comprehensively cover any optical OFDM aspect one could possibly think of, from the most fundamental to the most specialized. The book adopts a coherent line of presentation, while striking a thoughtful balance between the various topics, gradually developing the optical-physics and communication-theoretic concepts required for deep comprehension of the topic, eventually treating the multiple optical OFDM methods, variations and applications. In my view, this book will remain relevant for many years to come, and will be increasingly accessed by graduate students, accomplished researchers as well as telecommunication engineers and managers keen to attain a perspective on the emerging role of OFDM in the evolution of photonic networks' - Prof. Moshe Nazarathy, EE Dept., Technion, Israel Institute of Technology. <s> BIB002 </s> A Survey on OFDM-Based Elastic Core Optical Networking <s> 2) M-QAM (M-Quadrature Amplitude Modulation) <s> Fast advancing silicon technology underpinned by Moore's law is creating a major transformation in optical fiber communications. The recent upsurge of interests in optical orthogonal frequency-division multiplexing (OFDM) as an efficient modulation and multiplexing scheme is merely a manifestation of this unmistakable trend. Since the formulation of the fundamental concept of OFDM by Chang in 1966 and many landmark works by others thereafter, OFDM has been triumphant in almost all the major RF communication standards. Nevertheless, its application to optical communications is rather nascent and its potential success in the optical domain remains an open question. This tutorial provides a review of optical OFDM slanted towards emerging optical fiber networks. The objective of the tutorial is two-fold: (i) to review OFDM fundamentals from its basic mathematical formation to its salient disadvantages and advantages, and (ii) to reveal the unique characteristics of the fiber optical channel and identify the challenges and opportunities in the application of optical OFDM. <s> BIB003
M-QAM is a modulation scheme that conveys data by modulating both the amplitude and the phase of a reference signal to increase the bits per symbol. In M-QAM, two M-level amplitude-modulated signals are multiplexed onto two carriers of the same frequency with phase shift of π/2. 4-QAM, 8-QAM, 16-QAM, 64-QAM, and 256-QAM are defined for 2, 3, 4, 8, and 16 bits/symbol, respectively, as illustrated in Fig.  10 . Amplitude phase-shift keying or asymmetric phase-shift keying (APSK) is also a combination of amplitude modulation and phase modulation schemes. It can be considered as a superclass of QAM. Its advantage over conventional QAM is the lower number of possible amplitude levels, resulting in fewer problems with non-linear amplifiers. The multi-level optical modulation formats offer high spectral efficiency at the cost of a reduced tolerance to noise according to Shannon's law (which defines the theoretical maximum bit rate that can be transmitted over a bandwidthlimited channel in the presence of noise). Hence, an adaptive modulation technology (also referred to as bit loading BIB001 ) can be adopted to decide what modulation format to use on which subcarrier, based on channel conditions such as reach and signal-to-noise ratio (SNR). The subcarriers with higher SNR can be loaded with higher-level modulation formats in a compact spectrum, while lower SNR subcarriers use lowerlevel modulation formats in a wider spectrum. Furthermore, adaptive modulation schemes have the advantage of supporting variable bit rates, which is a desirable feature for future optical networks. Instead of performing changes in hardware, modulation formats can be reconfigured in the DSP and DAC/ADC via software BIB002 [66], ultimately bringing more flexibility to the optical network. To summarize this section, optical OFDM has received a lot of interest in recent years, leading to a large body of work on various transmission schemes and implementations. Optical OFDM technology has the advantages of superior tolerance to CD/PMD, high spectrum efficiency, and scalability to everincreasing transmission speeds due to its compact subcarrier multiplexing and adaptive modulation scheme. Moreover, it provides capabilities of multiple data rate accommodation and flexible sub-wavelength bandwidth access. Although realtime O-OFDM implementations are still far from mature due to the lack of high-speed DACs BIB003 , the advantages of O-OFDM show that it is a promising candidate for future optical networks.