Design Rules for Pulse Shaping in PDM-QPSK and PDM-16QAM Nyquist-WDM coherent optical transmission systems

In the context of Nyquist-WDM systems, the optimum rolloff factor, and the impulse response truncation length of root-raised-cosine pulse shapes are obtained via numerical simulations for both PDM-QPSK and 16QAM, considering trade-offs between the channel crosstalk and transmitter/receiver impairments. Introduction In long-haul coherent wavelength division multiplexing (WDM) optical communication systems, increasing the spectral efficiency (SE) is a key target in order to respond to higher capacity requirements. High SE can be achieved by using high-order modulation formats and/or reduced channel spacing. As the communication channels suffer from inter-carrier interference (ICI) when the channel spacing is reduced, substantial effort is currently devoted to optimize pulse shaping for high-SE systems [1]. A promising solution is Nyquist-WDM (NWDM). In N-WDM, a "sinc" pulse, with rectangular spectrum as large as the symbol rate, is used. Given ideal sampling, the inter-symbol interference (ISI) is zero, and the maximum SE is achieved. However, N-WDM suffers from three hardware implementation limitations: a) the length of the pulse shaping digital filter is finite, b) the digital to analog conversion (DAC) pulse shaping and the receiver analog to digital conversion (ADC) samplings are not ideally timed [2], and c) the amplitude resolution (in terms of number of bits) of both DAC and ADC are finite. These constraints translate into both ISI and ICI penalty. The sinc pulse transfer function is a raised cosine with zero rolloff.