Generate Nyquist-WDM Signal Using a DAC With Zero-Order Holding at the Symbol Rate

We demonstrate the Nyquist spectral shaping using a two-channel interleaved DAC with zero-order holding at the speed of the symbol rate, and investigate the required spectral preemphasis, bandwidth, and resolution. Compared to a conventional DAC at a speed of two times the symbol rate, we find that an extra spectral preemphasis is required for compensating the extra spectral roll-off in the two-channel interleaved DAC. The spectral preemphasis can facilitate Nyquist spectrally shaped signals generation with negligible OSNR penalty (<;0.1 dB). With proper spectral preemphasis, both the required electrical filter bandwidth and DAC resolution are similar to those with a conventional DAC used. To obtain <;0.1 dB OSNR penalty for root-raised-cosine Nyquist spectrally shaped WDM systems with a roll-off factor of 0.1 and a spectral shaping FIR filter tap number of 37, the two-channel interleaved DAC needs a Gaussian electrical filter with the bandwidth between 0.3 and 0.8 times of the symbol rate for PDM-QPSK signal, and between 0.35 and 0.65 times of the symbol rate for PDM-16QAM signal. Meanwhile, the required DAC resolutions are 6 bits and 7 bits for PDM-QPSK and PMD-16QAM, respectively. We also demonstrate how a polyphase FIR filter structure is applied in the transmitter to reduce the calculation by half and to increase the processing parallelism.