Experimental Demonstration and Numerical Simulation of 107-Gb/s High Spectral Efficiency Coherent Optical OFDM

Orthogonal frequency-division multiplexing (OFDM) is a multicarrier modulation format in which the data are transmitted with a set of orthogonal subcarriers. Recently, this modulation format has been actively explored in the field of optical communications to take advantages of its high spectral efficiency and resilience to chromatic and polarization dispersion. However, to realize the optical OFDM at 100 Gb/s and beyond requires extremely high electronic bandwidth for the electronic signal processing elements. In this paper, we investigate orthogonal-band-multiplexed OFDM (OBM-OFDM) as a suitable modulation and multiplexing scheme for achieving bandwidth scalable and spectral efficient long-haul transmission systems. The OBM-OFDM signal can be implemented in either RF domain, or optical domain, or a combination of both domains. Using the scheme of OBM-OFDM, we show the successful transmission of 107 Gb/s data rate over 1000-km standard single-mode fiber (SSMF) without optical dispersion compensation and without Raman amplification. The demonstrated OBM-OFDM system is realized in optical domain which employs 2times2 MIMO-OFDM signal processing and achieves high optical spectral efficiency of 3.3 bit/s/Hz using 4-QAM encoding. Additionally, we perform numerical simulation of 107-Gb/s CO-OFDM transmission for both single-channel and wavelength-division-multiplexed (WDM) systems. We find that the Q -factor of OBM-OFDM measured using uniform filling of OFDM subbands is in fact more conservative, in particular, is 1.2 dB and 0.4 dB lower than using random filling for single-channel and WDM systems, respectively.

I. Introduction

Orthogonal frequency-division multiplexing (OFDM) has been extensively studied to combat RF microwave multipath fading and has emerged as the leading modulation technology for the wireless and wire-line systems in RF domain. Recently, an optical equivalent of RF OFDM called coherent optical OFDM (CO-OFDM) has been proposed [1] and has become a promising technique for high spectral efficiency and dispersion resilient transmission [2], [3]. As the IP traffic continues to grow at a rapid pace, the 100-Gb/s Ethernet is being considered as the next-generation transport standard for IP networks [4]. As the data rate approaches 100 Gb/s and beyond, the electrical bandwidth required for CO-OFDM would be at least 15 GHz [5] and is not cost-effective to implement even with the best commercial digital-to-analog converters (DACs) and analog-to-digital converters (ADCs) in silicon integrated circuit (IC) [6]. To overcome this electrical bandwidth bottleneck, we propose and demonstrate the concept of OBM-OFDM to divide the entire OFDM spectrum into multiple orthogonal bands. Due to the inter-band orthogonality, the multiple OFDM bands with zero or small guard bands can be multiplexed and de-multiplexed without inter-band interference. With this scheme, transmission of 107-Gb/s CO-OFDM signal over 1000 km (10 × 100 km) standard single-mode fiber (SSMF) has been realized using only erbium-doped fiber amplifiers (EDFAs) and without a need for optical dispersion compensation. Although several transmission experiments at 100 Gb/s and above have been demonstrated at longer distances relying on a dispersion-compensation module and Raman amplification (RA) in each span [7], [8], our work has achieved the 1000-km transmission without optical dispersion compensation and without RA beyond 100 Gb/s. The 107-Gb/s OBM-OFDM can be also considered as 5 × 21.4 Gb/s WDM channels without frequency guard band, occupying 32-GHz optical bandwidth, implying a high spectral efficiency of 3.3 bit/s/Hz using only 4-QAM encoding. To the best of our knowledge, we have also achieved a record back-to-back OSNR sensitivity of 15.8 dB at 107 Gb/s for a BER of .