Parallel polyphase filtering for pulse shaping on high-speed optical communication systems

The requirement of higher data rates motivates the adoption of high-order modulations and pulse shaping in optical systems. This paper proposes an efficient architecture for FPGA-based parallel fractional sample rate conversion that has an optimum computational cost. The architecture supports non-integer oversampling factors, which allow increased flexibility when compared to schemes limited to adopting one or two samples per symbol, for example. The presented example is evaluated for a spectrally-efficient 400G optical supper-channel (three sub-channels with 16-QAM modulation) with different pulse shaping filter implementations using an FPGA working with a clock of 156.25 MHz. The results also illustrate the impact of the pulse shaping when a non-integer oversampling is adopted.

I. Introduction

Fiber optical systems are essential to attend the ever growing traffic demands in today's telecommunication services. A key enabler for achieving high data rates in optical systems is digital signal processing (DSP) for coherent optical communication systems [1]. DSP allows pulse shaping in the electrical domain that improves spectral efficiency in wavelength division multiplexing (WDM) systems [2], provides optimization in the number of users, network capacity, reach and flexibility [3]; besides, it may reduce the effects of fiber nonlinearities. The adoption of coherent optical communication and DSP permits many different pulse shapes other than non-return-to-zero (NRZ) and return-to-zero (RZ), which can be implemented using digital finite impulse response (FIR) filters and digital-to-analog converters (DACs) [4].