A Survey on OFDM-Based Elastic Core Optical Networking <s> 1) Guard Interval and Cyclic Prefix <s> A channel estimation technique is developed for an OFDM system with multiple transmit and multiple receive antennas. The approach exploits the channel structure in the time domain and the frequency domain. The method is appropriate for OFDM systems operating in a time varying channel. A multi-input-multi-output OFDM (MOFDM) system model is defined, followed by a proposed excitation strategy for sufficient identification of the channel. The estimation strategy is shown to be optimal within the class of linear MMSE estimators. Experimental results of the algorithm are provided. <s> BIB001 </s> A Survey on OFDM-Based Elastic Core Optical Networking <s> 1) Guard Interval and Cyclic Prefix <s> Coherent optical OFDM (CO-OFDM) has recently been proposed and the proof-of-concept transmission experiments have shown its extreme robustness against chromatic dispersion and polarization mode dispersion. In this paper, we first review the theoretical fundamentals for CO-OFDM and its channel model in a 2x2 MIMO-OFDM representation. We then present various design choices for CO-OFDM systems and perform the nonlinearity analysis for RF-to-optical up-converter. We also show the receiver-based digital signal processing to mitigate self-phase-modulation (SPM) and Gordon-Mollenauer phase noise, which is equivalent to the midspan phase conjugation. <s> BIB002 </s> A Survey on OFDM-Based Elastic Core Optical Networking <s> 1) Guard Interval and Cyclic Prefix <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> BIB003 </s> A Survey on OFDM-Based Elastic Core Optical Networking <s> 1) Guard Interval and Cyclic Prefix <s> This paper describes coherent optical orthogonal frequency division multiplexing (CO-OFDM) techniques for the long-haul transmission of 100-Gb/s-class channels. First, we discuss the configurations of the transmitter and receiver that implement the optical multiplexing/demultiplexing techniques for high-speed CO-OFDM transmission. Next, we review the no-guard-interval (No-GI) CO-OFDM transmission scheme which utilizes optical multiplexing for OFDM signal generation and the intradyne receiver configuration with digital signal processing (DSP). We examine the transmission characteristics of the proposed scheme, and show that No-GI CO-OFDM offers compact signal spectra and superior performance with regard to tolerance against optical amplifier noise and polarization-mode dispersion (PMD). We then introduce long-haul high-capacity transmission experiments employing No-GI CO-OFDM; 13.4 Tb/s (134 times 111 Gb/s) transmission is successfully demonstrated over 3600 km of ITU-T G.652 single-mode fiber without using optical dispersion compensation. <s> BIB004
One of the enabling techniques for OFDM is the insertion of a guard interval (GI) and cyclic prefix (CP) BIB002 . In optics, the phase velocity of an optical pulse depends on its frequency. Different frequency components of an optical pulse travel with different speeds, so the optical pulse is spread out after transmission (i.e., delay spread). Because of this dispersion phenomenon, an OFDM symbol with a large delay spread after a long-distance transmission may cross its symbol boundary, leading to interference with its neighboring OFDM symbol, which is referred to as inter-symbol interference (ISI). Furthermore, because OFDM symbols of different subcarriers are not aligned due to the delay spread, the critical orthogonality condition for the subcarriers will be lost, resulting in an intercarrier interference (ICI) penalty BIB003 . To deal with the ISI caused by channel delay spread, a guard interval is inserted into the OFDM symbol, as shown in Fig.  4 . It can be shown that, if the maximum delay spread of the transmission channel is smaller than the guard interval, the ISI can be perfectly eliminated. ICI can be reduced by introducing Fig. 4 . Guard Interval (GI) of an OFDM Symbol BIB003 a cyclic prefix into the guard interval. The cyclic prefix is a copy of the past beginning of the current symbol at the end, and ensures that the complete OFDM symbol with the longer delay can also be received with the appropriate DFT window shifting BIB003 . The length of the GI/CP is determined by the maximum delay spread induced by channel dispersion. Since the GI/CP introduces additional overhead, a conventional approach to minimize this overhead is to set a long OFDM symbol interval, namely to use many subcarriers BIB004 . 2) Channel Estimation Similar to single-carrier modulation, time-and frequencyvarying channels affect the performance of OFDM systems. The effect of the channel on the transmitted signal needs to be estimated to recover the transmitted information. Many techniques have been proposed for estimating and adjusting both timing and frequency variation in OFDM systems BIB001 . The channel-state information can be estimated using nonblind channel estimation or blind channel estimation. In non-blind channel estimation, training symbols (TS) containing information known by both the transmitter and the receiver are periodically inserted into data-bearing subcarriers. Channel-state information can be estimated based on these training symbols using channel-estimation algorithms. Sometimes, specific OFDM symbols with known data are inserted into selective subcarriers, called pilot subcarriers. The channelstate information corresponding to the pilot subcarriers is first estimated, and then the channel state corresponding to the data-bearing subcarriers can be obtained by interpolation. The overhead for training symbols or pilot subcarriers depends on the channel dynamics, where, in general, the more stable a channel is, the less overhead is required. Blind channel estimation uses the intrinsic characteristics of the modulated signal, which is independent of the transmitted data, to estimate the channel state. This scheme requires careful design of the channel-estimation algorithm such that the system converges under all conditions. Compared to blind channel estimation, non-blind channel estimation is more straightforward, but introduces additional overhead.