High-order QAM transmission for the future optical transport network beyond 100Gb/s

Higher-order multi-level modulation formats are very attractive for achieving the high spectral efficiency and high speed channels needed to accommodate ultra-high speed client signals on the optical transport network (OTN). In particular, quadrature amplitude modulation (QAM) is a promising modulation technique to achieve the high spectral efficiency with PDM. However, required OSNR is increased and transmission distance is restricted as the number of signal point increase. Moreover, system requirements, such as laser line-width, ADC/DAC resolution, and circuit linearity, become severe. We recently demonstrated the 3000-km-class long-haul transmission of a single channel 160 Gb/s 16-QAM signal. We employed three key technologies; optical 16-QAM signal synthesis by superposing two optical QPSK signals, proposed pilot-less detection scheme with digital PLL-based frequency offset compensator and OSNR improvement by ultra low-loss fiber and EDFA/distributed Raman amplification. In this paper, we review system configurations for higher-order QAM, and then describe the single channel transmission performance of 16-QAM.