Frequency-Domain Hybrid Modulation Formats for High Bit-Rate Flexibility and Nonlinear Robustness

Since the deployment of the first commercial polarization-multiplexed (PM) QPSK 100G systems, the evolution of coherent optical communications in the last decade has been largely dominated by single-carrier quadrature amplitude modulation (QAM) based modulation of increasing constellation size. However, as the data traffic becomes more dynamic and heterogeneous, an efficient use of the optical link requires more flexible modulation schemes, capable of adapting data-rate and distance with fine granularity. While typical multi-carrier modulation schemes composed of hundreds of subcarriers may be inadequate for optical transmission, namely due its high peak-to-average power ratio and increased nonlinearity penalties, it has been recently shown that few-carrier modulation (with symbol rate in the order of 2-4 GBaud) can provide an increased robustness to nonlinear propagation impairments. In this paper, we exploit the concept of frequency-domain hybrid modulation formats (FDHMF) based on electronic subcarrier multiplexing with the use of different QAM formats on each subcarrier to simultaneously enhance the data-rate flexibility and the nonlinear propagation performance of the optical link. In addition, by properly designing the FDHMF signal, an increased tolerance against optical filtering can also be achieved. Using the enhanced Gaussian noise model, we report a comprehensive theoretical study on the performance of FDHMF, considering independent- and joint-subcarrier forward-error correction strategies, optimization of power ratio between subcarriers, and corresponding impact of fiber nonlinearities. These theoretical insights are then validated by a broad range of wavelength-division multiplexing experiments with per-channel bit-rates in the range of 150G to 250G. The obtained results demonstrate that FDHMF is an effective solution for the implementation of flexible transponders capable of adapting the data rate to the lightpath quality of transmission: An enabling technology for the introduction of future elastic optical networks.