A Survey on OFDM-Based Elastic Core Optical Networking <s> 1) FFT-Based Approach <s> Coherent optical orthogonal frequency division multiplexing is proposed to combat dispersion in optical media. It is shown that optical-signal-to-noise ratio penalty at 10 Gbit/s is maintained below 2 dB for 3000 km transmission of standard-singlemode fibre without dispersion compensation. <s> BIB001 </s> A Survey on OFDM-Based Elastic Core Optical Networking <s> 1) FFT-Based Approach <s> Optical Orthogonal frequency division multiplexing (OOFDM) is shown to outperform RZ-OOK transmission in high-speed optical communications systems in terms of transmission distance and spectral efficiency. The OOFDM in combination with the subcarrier multiplexing offers a significant improvement in spectral efficiency of at least 2.9 bits/s/Hz. <s> BIB002 </s> A Survey on OFDM-Based Elastic Core Optical Networking <s> 1) FFT-Based Approach <s> PDM-OFDM transmission of 10x121.9-Gb/s (112.6-Gb/s without OFDM overhead) at 50-GHz channel spacing is demonstrated over 1,000-km SSMF without any inline dispersion compensation. 8-QAM subcarrier modulation allows transmission of 121.9 Gb/s within a 22.8-GHz optical bandwidth. <s> BIB003 </s> A Survey on OFDM-Based Elastic Core Optical Networking <s> 1) FFT-Based Approach <s> Coherent optical OFDM (CO-OFDM) has emerged as an attractive modulation format for the forthcoming 100 Gb/s Ethernet. However, even the spectral-efficient implementation of CO-OFDM requires digital-to-analog converters (DAC) and analog-to-digital converters (ADC) to operate at the bandwidth which may not be available today or may not be cost-effective. In order to resolve the electronic bandwidth bottleneck associated with DAC/ADC devices, we propose and elucidate the principle of orthogonal-band-multiplexed OFDM (OBM-OFDM) to subdivide the entire OFDM spectrum into multiple orthogonal bands. With this scheme, the DAC/ADCs do not need to operate at extremely high sampling rate. The corresponding mapping to the mixed-signal integrated circuit (IC) design is also revealed. Additionally, we show the proof-of-concept transmission experiment through optical realization of OBM-OFDM. To the best of our knowledge, we present the first experimental demonstration of 107 Gb/s QPSK-encoded CO-OFDM signal transmission over 1000 km standard-single- mode-fiber (SSMF) without optical dispersion compensation and without Raman amplification. The demonstrated system employs 2x2 MIMO-OFDM signal processing and achieves high electrical spectral efficiency with direct-conversion at both transmitter and receiver. <s> BIB004 </s> A Survey on OFDM-Based Elastic Core Optical Networking <s> 1) FFT-Based Approach <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> BIB005 </s> A Survey on OFDM-Based Elastic Core Optical Networking <s> 1) FFT-Based Approach <s> We review coherent-optical orthogonal frequency division multiplexing (OFDM) for long-haul optical transmission systems. Two important aspects of such systems are reviewed: RF-aided phase noise compensation and polarization division multiplexing enabled by MIMO processing. <s> BIB006 </s> A Survey on OFDM-Based Elastic Core Optical Networking <s> 1) FFT-Based Approach <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> BIB007 </s> A Survey on OFDM-Based Elastic Core Optical Networking <s> 1) FFT-Based Approach <s> We discuss optical multi-band orthogonal frequency division multiplexing (OFDM) and show that by using multiple parallel OFDM bands, the required bandwidth of the digital-to-analogue/ analogue-to-digital converters and the required cyclic prefix can significantly be reduced. With the help of four OFDM bands and polarization division multiplexing (PDM) we report continuously detectable transmission of 10 times121.9-Gb/s (112.6-Gb/s without OFDM overhead) at 50-GHz channel spacing over 1,000-km standard single mode fiber (SSMF) without any inline dispersion compensation. In this experiment 8 QAM subcarrier modulation is used which confines the spectrum of the 121.9 Gb/s PDM-OFDM signal within a 22.8 GHz optical bandwidth. Moreover, we propose a digital signal processing method to reduce the matching requirements for the wideband transmitter IQ mixer structures required for PDM-OFDM. <s> BIB008 </s> A Survey on OFDM-Based Elastic Core Optical Networking <s> 1) FFT-Based Approach <s> We demonstrate generation, transmission and reception of a 1.21 Tb/s continuous waveband PDM-OFDM-FDM signal with spectral efficiency of 3.33 bit/s/Hz. After DCF-free transmission over 400-km SSMF a considerable Q-factor margin of 2dB vs. EFEC limit was achieved. <s> BIB009 </s> A Survey on OFDM-Based Elastic Core Optical Networking <s> 1) FFT-Based Approach <s> We propose a novel coherent optical orthogonal frequency-division multiplexing (CO-OFDM) scheme with reduced guard interval (RGI) for high-speed high-spectral-efficiency long-haul optical transmission. In this scheme, fiber chromatic dispersion is compensated for within the receiver rather than being accommodated by the guard interval (GI) as in conventional CO-OFDM, thereby reducing the needed GI, especially when fiber dispersion is large. We demonstrate the generation of a 448-Gb/s RGI-CO-OFDM signal with 16-QAM subcarrier modulation through orthogonal band multiplexing. This signal occupies an optical bandwidth of 60 GHz, and is transmitted over 2000 km of ultra-large-area fiber (ULAF) with five passes through an 80-GHz-grid wavelength-selective switch. Banded digital coherent detection with two detection bands is used to receive this 448-Gb/s signal. Wavelength-division multiplexed transmission of three 80-GHz spaced 448-Gb/s RGI-CO-OFDM channels is also demonstrated, achieving a net system spectral efficiency of 5.2 b/s/Hz and a transmission distance of 1600 km of ULAF. <s> BIB010 </s> A Survey on OFDM-Based Elastic Core Optical Networking <s> 1) FFT-Based Approach <s> This paper presents a new channel estimation/equalization algorithm for coherent OFDM (CO-OFDM) digital receivers, which enables the elimination of the cyclic prefix (CP) for OFDM transmission. We term this new system as the zero-guard-interval (ZGI)-CO-OFDM. ZGI-CO-OFDM employs an overlapped frequency-domain equalizer (OFDE) to compensate both chromatic dispersion (CD) and polarization mode dispersion (PMD) before the OFDM demodulation. Despite the zero CP overhead, ZGI-CO-OFDM demonstrates a superior PMD tolerance than the previous reduced-GI (RGI)-CO-OFDM, which is verified under several different PMD conditions. Additionally, ZGI-CO-OFDM can improve the channel estimation accuracy under high PMD conditions by using a larger intra-symbol frequency-averaging (ISFA) length as compared to RGI-CO-OFDM. ZGI-CO-OFDM also enables the use of ever smaller fast Fourier transform (FFT) sizes (i.e. <128), while maintaining the zero CP overhead. Finally, we provide an analytical comparison of the computation complexity between the conventional, RGI- and ZGI- CO-OFDM. We show that ZGI-CO-OFDM requires reasonably small additional computation effort (~13.6%) compared to RGI-CO-OFDM for 112-Gb/s transmission over a 1600-km dispersion-uncompensated optical link. <s> BIB011
In the FFT-based approach, the OFDM subcarriers are generated in the digital domain using IFFT (Inverse Fast Fourier Transform). The FFT-based O-OFDM transmitter is composed of a radio frequency (RF) OFDM transmitter and a RF-to-optical up-converter, while the receiver is composed of an optical-to-RF down-converter, and a RF OFDM receiver BIB001 . The processing of the electrical OFDM carrier by an RF OFDM transmitter and receiver was described in Section II-B. The function of the optical up-converter and down-converter is to modulate an OFDM baseband signal onto an optical carrier and vice versa. The conversion could be implemented with an intermediate frequency (IF) up-/down-conversion architecture or a direct up-/down-conversion architecture BIB007 , as shown in Fig. 6 . In the intermediate-frequency up-conversion architecture, as shown in Fig. 6(a) , the complex-valued OFDM signal is first up-converted to an intermediate frequency through in-phase (I) and quadrature (Q) modulator, and then modulated onto the optical carrier through a conventional single-ended MachZehnder modulator (MZM). Besides the original baseband signal, an image-band is also generated with this method and stands side by side with the original baseband, and an optical band-pass filter (BPF) is needed to eliminate the imageband. For intermediate frequency down-conversion, the optical signal is first down-converted to an intermediate frequency, and then electrical I/Q detection is performed. In the direct up-conversion architecture, as shown in Fig.  6(b) , the optical transmitter uses a complex Mach-Zehnder modulator, composed of two MZMs with 90-degree phase shift, to up-convert the real/imaginary parts of the complex OFDM signal from the electrical domain to the optical domain. For direct down-conversion, the OFDM optical receiver uses two pairs of balanced receivers and an optical 90-degree hybrid to perform optical I/Q detection. Such an optical conversion performs direct modulation of the OFDM signal onto the optical signal without image-band and thus no optical filtering is required at the transmitter. The electrically-generated subcarriers of OFDM can be modulated on a single optical carrier or on multiple optical carriers. These two schemes are described as single-band O-OFDM and multi-band O-OFDM, respectively BIB008 . a) Single-Band OFDM Single-band OFDM modulates the electrical OFDM subcarriers on a single optical carrier, through the up-/downconversion scheme discussed above. One of the main challenges of this single-band OFDM is that the operation speed of electronic devices such as the DAC/ADC and modulator drivers limits the transmission rate  . b) Multi-Band OFDM One approach to overcome the electrical processing bottleneck is to use the multi-band OFDM scheme BIB008 [26] BIB003 [28], which generates a large number of electrical subcarriers and modulates them on multiple optical carriers, so that the data rate of each subcarrier can be reduced. The basic principle of multi-band OFDM is to split the OFDM signal into multiple subbands, each modulated to an optical carrier, while maintaining their orthogonal property. As shown in Fig. 7 , the entire OFDM spectrum comprises N OFDM bands, each with the subcarrier spacing of Δf and band frequency guard spacing of Δf G . When the guard band spacing (Δf G ) is a multiple of the subcarrier spacing (Δf ), the orthogonality is satisfied not only between subcarriers inside a band, but also between subcarriers from different bands BIB004 . The inter-band interference is avoided through the orthogonality of each band. Consequently, Δf G can be set equal to Δf where no frequency guard band is necessary. Multiple OFDM bands can be generated through a multicarrier optical transmitter and band multiplexing. Upon reception, a filter with bandwidth slightly larger than the bandwidth of each band can be used to select the desired band. This concept is also referred to as orthogonal-band-multiplexed OFDM (OBM-OFDM) BIB004 , subcarrier multiplexing BIB002 , or cross-channel OFDM (XC-OFDM) BIB005 . The band-multiplexed OFDM scheme without enforcing the band orthogonality has also been discussed in BIB009 . BIB004 Using multi-band OFDM, high-speed transmission can be achieved without forcing the subcarriers to be run at extremely high rate, and as such, the DAC/ADC requirements for each subcarrier are significantly relaxed. The FFT-based O-OFDM described in this section has the advantage of simplified optics design. However, this scheme requires guard intervals, training symbols, or pilot carriers, which introduce additional OFDM-specific overhead of around 8-24% compared with single-carrier modulation formats (depending on the detailed system design) BIB006 . To limit the proportion of overhead, a large number of subcarriers (>> 100) is normally used in this scheme. To overcome the large overhead problem of FFT-based O-OFDM, a new Reduced-Guard-Interval (RGI) CO-OFDM scheme was recently introduced BIB010 . In the RGI-CO-OFDM scheme, a reduced GI between adjacent OFDM symbols is used to accommodate the ISI induced by transmitter bandwidth limitations or fiber polarization mode dispersion (PMD), while fiber chromatic dispersion (CD)-induced ISI is compensated at the receiver using electrical dispersion compensation (EDC). As a result, the overhead and OSNR (Optical Signalto-Noise Ratio) penalty due to the GI are dramatically reduced. Further, a Zero-Guard-Interval (ZGI) CO-OFDM scheme BIB011 was proposed to completely remove the GI by performing a joint CD and PMD compensation at the EDC. Comparison of ZGI-CO-OFDM with RGI-CO-OFDM was reported, showing that ZGI-CO-OFDM demonstrates a superior PMD tolerance than the previous RGI-CO-OFDM scheme, with reasonable small additional computation effort BIB011 .