Multi-subcarrier flexible bit-loading enabled capacity improvement in meshed optical networks with cascaded ROADMs

We propose to use adaptive bit loading based on time-domain hybrid QAM (TDHQ) to maximize the capacity of subcarrier-multiplexing (SCM) systems in meshed optical networks with cascaded reconfigurable optical add and drop multiplexers (ROADMs). Note that the capacity is defined as the achievable net bit rate at the soft-decision FEC threshold of BER = 2 × 10^-2 in this work. The capacity improvement is first numerically and experimentally demonstrated in a 4-subcarrier SCM system with an aggregate symbol rate of 34.94 Gbaud. Compared with the conventional SCM system using uniform standard QAM, the proposed system can achieve an average capacity increase of 31.75% and 26.1% over various link conditions in simulations and experiments, respectively. Furthermore, we demonstrate that the proposed SCM system can better approach the channel capacity in the presence of narrow inline optical filtering. An average capacity improvement of 7.59% is also reported over all 17 ROADMs cases from 1 to 17 by simulations at OSNR = 21 dB, compared with its single carrier counterpart using TDHQ.