IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS, VOL. 31, NO. 1, JANUARY 2013 1 Guest Editorial Next-Generation Spectrum-Ef ficient and Elastic Optical Transport Networks Gangxiang Shen, Ken-ichi Sato, William Shieh, Ioannis Tomkos, Jennifer Yates, and Eric W. M. Wong I MPROVING capacity utilization of the optical transport network has been an important research challenge for many years. Extensive research efforts have been devoted to devel- oping approaches of grooming subwavelength traffic demand onto large wavelength capacity pipes. These efforts, however, only focus on efficiently utilizing the capacity pipes, without addressing the issue of fixed ITU-T grid and frequency spacing in the DWDM layer. Under the fixed frequency spacing, op- tical spectrum is often over-provisioned for a low-rate optical channel, and this inefficiency becomes even prominent when more advanced modulation formats are employed. Advanced optical transmission and networking techniques are needed to provide flexibilities for optical channel spectrum allocation and to develop the related network control system so as to cater to the bandwidth elasticity of Internet traffic and improve fiber optical spectral usage. Significant attention has been given to develop spectrum- efficient and elastic optical transport networks in both academia and industry in the past few years, and currently there are many research efforts underway targeting at the development of appropriate solutions for future dynamic elas- tic and scalable photonic infrastructures and network archi- tectures, efficient new algorithms that determine how optical frequency resources can be matched to traffic demands in an optimized way, and a more flexible control and management plane. The technical issues that remain open for such elas- tic optical transport networks include efficient architecture, spectrum-efficient transmission technique, reconfigurable op- tical add/drop multiplexer (ROADM), routing and spectrum assignment (RSA), spectrum de-fragmentation, traffic groom- ing, new protocols and control plane, and more. Our objective of this special issue is to identify various challenges posed by the spectrum-efficient and elastic optical transport networks and explore research avenues for address- ing them. We have received a total of 27 papers, and after multi-round careful reviews by both reviewers and guest- editors, we have accepted six papers for this special issue. G. Shen is with Soochow University, P. R. China. K. -i. Sato is with Nagoya University, Japan. W. Shieh is with Melbourne University, Australia. I. Tomkos is with Athens Information Technology Center. J. Yates is with AT&T Labs, USA. E. W. M. Wong is with City University, Hongkong. Moshe Zukerman is the J-SAC Board Representative for this issue of IEEE Journal on Selected Areas in Communications. Digital Object Identifier 10.1109/JSAC.2013.130101 The accepted articles span various topics ranging from routing and spectrum assignment (RSA), traffic grooming, elastic optical network testbeds, and CO-OFDM optical transmission systems. In “Dynamic Traffic Grooming in Elastic Optical Net- works,” S. Zhang, C. Martel, and B. Mukherjee propose a dynamic traffic grooming scheme based on a multi-layer aux- iliary graph that jointly optimize electrical-layer and optical- layer routing and spectrum assignment. In “Time-Varying Spectrum Allocation Policies and Blocking Analysis in Flexible Optical Networks,” K. Christodoulopoulos, I. Tomkos, and E. Varvarigos propose and analyze three spectrum expansion/contraction (SEC) poli- cies for modifying the spectrum allocated to each service connection. They also derive exact formulas for calculating the blocking probability for a connection and for the whole network. In “Elastic Spectrum Allocation for Time-Varying Traffic in FlexGrid Optical Networks,” M. Klinkowski, M. Ruiz, L. Velasco, D. Careglio, V. Lopez, and J. Comellas formulate a Multi-Hour Routing and Spectrum Allocation (MHRSA) optimization problem and solve it by means of both Integer Linear Programming (ILP) and efficient heuristic algorithms. In “Adaptive Spectrum Control and Management in Elastic Optical Networks,” K. Wen, X. Cai, Y. Yin, D. J. Geisler, R. Proietti, R. P. Scott, N. K. Fontaine, and S. J. B. Yoo present an adaptive spectrum control and management scheme, which includes: dynamic on-demand spectral defragmentation, adap- tive combinational quality of transmission (QoT) restoration (ACQR) and supervisory channel-assisted active restoration. In “Design and Experimental Validation of a GMPLS/PCE Control Plane for Elastic CO-OFDM Optical Networks,” R. Casellas, R. Mu˜ noz, J. M. Fabrega, M. S. Moreolo, R. Mar- tinez, L. Liu, T. Tsuritani, and I. Morita design and deploy a GMPLS control plane for flexible optical networks with coherent optical orthogonal frequency division multiplexing (CO-OFDM) transmission; the functional control architecture combines a centralized entity that performs path routing and modulation assignment, with a distributed spectrum allocation. Finally, in “Spectrum-Efficient Coherent Optical OFDM for Transport Networks,” L. Dai, C. Zhang, Z. Xu, and Z. Wang propose a flexible coherent zero padding OFDM (CO- ZPOFDM) scheme with signaling-embedded preambles and polarization-time-frequency (PTF) coded pilots to improve 0733-8716/13/$31.00 c  2013 IEEE