OFDM versus Single-Carrier Transmission for 100 Gbps Optical Communication

Abstract

Abstract-We analyze the orthogonal frequency division multiplexing (OFDM) technique in long-haul next generation optical communication links and compare it with the well-established single-carrier (SC) data transmission using high-level modulation formats and coherent detection. The analysis of the two alternative solutions is carried out in the 100 Gbps scenario, which is commonly considered to be the next upgrade of existing optical links, with special emphasis on quaternary phase-shift keying (QPSK) modulations. The comparison between OFDM and SC takes into account the main linear and nonlinear impairments of the optical channel, e.g., group velocity dispersion (GVD), polarization mode dispersion (PMD), self-phase modulation (SPM), cross-phase modulation (XPM), and four-wave mixing (FWM), as well as the phase noise due to transmit and receive lasers, their relative frequency offset, other synchronization aspects, the overall complexity, the power and spectral efficiency, and the technological constraints.Index Terms-Cross-phase modulation (XPM), four-wave mixing (FWM), group velocity dispersion (GVD), optical coherent transmission systems, orthogonal frequency division multiplexing (OFDM), phase-shift keying (PSK), polarization mode dispersion (PMD), self-phase modulation (SPM), single-carrier transmissions.

Impact of Phase Noise and Compensation Techniques in Coherent Optical Systems

“…The optional differential phase noise estimation (DPNE) blocks are required for the implementation of the DCE technique and will be introduced in Section IV. As in [9], in the numerical results we will consider classical differentially encoded quadrature phase-shift keying (QPSK) signals. The modulated signal is then launched on two orthogonal states of polarization (SOPs) of a single mode fiber (SMF).…”

“…In the considered SC system, dispersion compensation is performed by a two-dimensional fractionally-spaced linear equalizer which, if properly designed, can perfectly remove the ISI due to GVD and PMD [4]. This two-dimensional equalizer can be suitably implemented in the frequency domain for complexity-saving purposes [9]. The adoption of the asynchronous strategies for detection and for the adjustment of the equalizer taps described in [4] allows to remarkably increase the robustness to PN.…”

“…Instead, in the OFDM scheme, blocks of signal samples undergo FFT, are then processed with a one-tap equalizer and, finally, detection of the transmitted sequences takes place. Details on one possible OFDM receiver architecture with the relevant algorithms can be found in [9].…”

“…In the light of such an upgrade, other transmission techniques, alternative to standard singlecarrier (SC) schemes, have been proposed, since they seemingly allow to more easily scale to higher bit rates, such as orthogonal frequency division multiplexing (OFDM) [5], whose success and popularity in wireless communications several authors push to repeat in optical communications [6]- [8]. Besides more general considerations about the debate between SC and OFDM (see [9]), one of the most severe impairment that affects coherent systems employing high-order modulation formats is the presence of phase noise (PN) [10], [11], introduced by both transmit and receive lasers-the former also present in widespread intensity-modulation direct-detection systems, the latter peculiar to coherent systems.…”

“…Several works have addressed the sensitivity to PN of SC coherent systems with inline dispersion compensation and discussed some countermeasures to reduce its detrimental effects [4], [13]- [15] (see also [16]- [18] and references therein for the effects and countermeasures on wireless systems). In OFDM systems, the effect of PN is even more severe [9] and represents a major comparison aspect between OFDM and SC.…”