Digital signal processing techniques in Nyquist-WDM transmission systems

In single-carrier wavelength-division-multiplexing (WDM) systems, the spectral efficiency can be increased by reducing the channel spacing through digital signal processing (DSP). Two major issues of using tight filtering are crosstalk between channels and inter-symbol interference (ISI) within a channel. By fulfilling the Nyquist criterion, Nyquist spectral shaped WDM systems can achieve a narrow channel spacing close to the symbol rate (Rs) with negligible crosstalk and ISI. In principle, DSP can generate any signals with arbitrary waveforms and spectrum shapes. However, the complexity of DSP is limited by its cost and power consumption. It is necessary to optimize the DSP to achieve the required performance at a minimum complexity. In this paper, we investigated the use of digital finite impulse response (FIR) filters to generate Nyquist-WDM 16-ary quadrature amplitude modulation (16QAM) signals with the raised-cosine (RC) and root-raised-cosine (RRC) shape spectrum. The system performance of both the RC and RRC spectra are also examined. Moreover, we explored the various methods to reduce digital-to-analog-converter (DAC) sampling speed, such as using super Gaussian electrical filters (E-filter), and spectral pre-emphasis.