IEEE SIGNAL PROCESSING MAGAZINE [82] MARCH 2014 1053-5888/14/$31.00©2014IEEE Digital Object Identifier 10.1109/MSP.2013.2287021 Date of publication: 12 February 2014 I n this article, we describe advances in digital signal process- ing (DSP) techniques that enable Tb/s transmission, and software-defined flexible transponders that support adaptive modulation formats and elastic optical networks (EONs). INTRODUCTION Over the past decade, advances in optical hardware such as external modulators, narrow-linewidth lasers and 90° hybrids have enabled DSP algorithms from wireless/copper-wire com- munications [1] to be applied in coherent optical communica- tion systems. Coherent transceivers supporting 100 Gb/s per channel using polarization-multiplexed quadrature phase-shift keying (QPSK) were announced by system vendors in 2009 and subsequently deployed in transmission systems several years later. DSP units such as chromatic dispersion (CD) compensa- tion, polarization-mode dispersion (PMD) compensation, timing phase recovery (TPR), frequency offset estimation (FOE), as well as carrier phase estimation (CPE) are standard in commercial coherent receivers. Moving forward, 400 Gb/s and 1 terabit per second (Tb/s) per-channel transmission using 16-QAM and above were demonstrated and commercialized in 2012. The next generation of hardware [e.g., digital-to-analog converters (DACs)] and software (e.g., programmability) advances will enable EONs, networks that are capable of adapting to network conditions to maximize overall efficiencies. One of the key [ Alan Pak Tao Lau, Yuliang Gao, Qi Sui, Dawei Wang, Qunbi Zhuge, Mohamed H. Morsy-Osman, Mathieu Chagnon, Xian Xu, Chao Lu, and David V. Plant ] IMAGE LICENSED BY INGRAM PUBLISHING /JANMIKS [ Toward elastic optical networks ] Advanced DSP Techniques Enabling High Spectral Efficiency and Flexible Transmissions