Digital Signal Processing for Optical Access Networks

In this paper,we investigate advanced digital signal process-ing(DSP)at the transmitter and receiver side for signal pre-equalization and post-equalization in order to improve spec-trum efficiency(SE)and transmission distance in an opticalaccess network.A novel DSP scheme for this optical super-Nyquist filtering 9 Quadrature Amplitude Modulation(9-QAM)like signals based on multi-modulus equalization with-out post filtering is proposed.This scheme recovers the Ny-quist filtered Quadrature Phase-Shift Keying(QPSK)signal toa 9-QAM-like one.With this technique,SE can be increasedto 4 b/s/Hz for QPSK signals.A novel digital super-Nyquistsignal generation scheme is also proposed to further suppressthe Nyquist signal bandwidth and reduce channel crosstalkwithout the need for optical pre-filtering.Only optical cou-plers are needed for super-Nyquist wavelength-division-multi-plexing(WDM)channel multiplexing.We extend the DSP forshort-haul optical transmission networks by using high-orderQAMs.We propose a high-speed Carrierless Amplitude/Phase-64 QAM(CAP-64 QAM)system using directly modulated la-ser(DML)based on direct detection and digital equalization.Decision-directed least mean square is used to equalize theCAP-64QAM.Using this scheme,we generate and transmitup to 60 Gbit/s CAP-64QAM over 20 km standard single-mode fiber based on the DML and direct detection.Finally,several key problems are solved for real time orthogonal-fre-quency-division-multiplexing(OFDM)signal transmission andprocessing.With coherent detection,up to 100 Gbit/s 16QAM-OFDM real-time transmission is possible.