We first established the baseline performance by evaluating the back-to-back BER versus OSNR at both 28 and 32 GBaud; see Fig. 2(b). Note that the vertical scale is erfc−1(BER) in order to demonstrate the expected linear relationship with OSNR. The 32 Gbaud PDM-QPSK signal achieves 10−2 BER at an OSNR of 11.6 dB. The 28 Gbaud signal requires 14.1 dB OSNR to achieve a BER of 10−3 . We observe a ∼0.8 dB penalty when increasing the baud rate from 28 to 32 GBaud compared to the expected 0.6 dB increase. We also note that we are able to observe BER to less than 1 × 10−5 . These results demonstrate the good performance of the transmitter, receiver, and demodulation code in the linear regime.