Direct Detection of Pilot Carrier-Assisted DMT Signals With Pre-Phase Compensation and Imaginary Noise Suppression

We propose and experimentally demonstrate a cost-effective pilot carrier assisted direct detection scheme based on discrete multitone (DMT) modulation format. The cost of the proposed scheme can be substantially reduced by two ways. First, we use distributed feedback (DFB) laser with large linewidth (~10 MHz) as the pilot carrier. Secondly, we design a low-complexity pre-phase compensation (PPC) algorithm combined with imaginary noise suppression (INS), in order to mitigate the effect of phase noise and consequently enhance the signal to noise ratio (SNR). Different from the conventional channel equalization schemes for the DMT signal, we pre-compensate the phase noise in time domain before fast Fourier transform (FFT) demodulation at the receiver side. The PPC algorithm is implemented with an average operation, instead of complex multipliers. Since DMT signal is a real-valued signal, the imaginary part of time-domain samples after the PPC process is only noise, which can be removed for the SNR enhancement. Simulation results indicates that 3-dB SNR improvement can be obtained, which agrees well with the theoretical analysis. Meanwhile, our experimental results show that the combination of PPC and INS can significantly improve the laser linewidth tolerance of transmission system, leading to the possible use of DFB laser, instead of external cavity laser (ECL), as the pilot carrier. Finally, we successfully demonstrate 4×50-Gb/s DMT signal transmission over 400-km standard single mode fiber (SSMF) by using DFB lasers as signal and pilot carriers, respectively, with sufficient margin to reach 20% forward error correction (FEC) threshold.