Pulse-shaping for spectrally-efficient coherent optical networks: OFDM, Nyquist signaling, and DFT-spread OFDM

Pulse-shaping is a powerful tool to increase the spectral efficiency of signals transmitted over optical fiber. Two main contenders, namely OFDM and Nyquist signaling have emerged as promising candidates for future optical networks. As a third option, DFT-spread OFDM combines the two techniques. Although all techniques are well-known in wireless and wireline communications, the extremely high data rates are challenging when designing real-time optical transmitters and receivers, particularly in view of the special impairments imposed by the optical transmission channel. After giving a theoretical overview on OFDM and Nyquist signals, we show the system design for the three pulse-shaping techniques and discuss the processing requirements and the potential performance in terms of spectral efficiency and out-of-band signal suppression. We further investigate the impact of modulator nonlinearity and limited resolution of DACs and ADCs on the different signals. Finally, we demonstrate real-time OFDM and Nyquist pulse generation with data rates beyond 100 Gbit/s transmitted on a single optical carrier and in a single polarization.