# Partial Pre-Emphasis for Pluggable 400 G Short-Reach Coherent Systems
## 4 Experimental Setup and DSP
### Results and Discussion
#### 4.1.1 Impact of Partial Pre-Compensation on Q-Factor in a Single-Polarization Coherent System  
We first investigate the performance penalty of partial pre-compensation relative to full pre-compensation in a 50 Gbaud 16 QAM single-pol coherent system in back-to-back. Note that we use a low bandwidth Tx (10 GHz) in the experiment to better study the impact of partial pre-compensation on the performance of a low-cost, low-power coherent system. Though this limits the system from working at 60 Gbaud for 400 ZR, the conclusion is still valid. The black (*) and red (+) curves in Figure 7 show the Q-factor penalty of partial pre-compensation versus the reduced frequency peaking of the pre-emphasis filter at 20 GHz obtained from experimental and numerical results, respectively.  
It is consistently found for the two curves that the Q factor penalty increases with increasing attenuation at 20 GHz. The black (*) curve indicates in the experiment that a 0.4 dB Q factor penalty  
Figure 6: Experimental setup.  
Figure 7: Q-Factor penalty versus loss at high-frequency.  
occurs at 6 dB attenuation at 20 GHz. This is due to increased noise enhancement at the receiver, since more Tx frequency response is compensated for by the receiver equalizer. The magenta (o) curve (right vertical axis) shows the PAPR of the RF signal into the modulator for different attenuation values at 20 GHz. Higher attenuation results in lower PAPR, which allows for the use of lower cost AWGs with smaller ENoB and RF amplifiers with less stringent requirements on linearity and smaller gain. The reduction of PAPR is, however, smaller than that found in simulation as we measure it after the AWG and DA, where low pass response of the Tx comes into play.  
We also measured the Q factor penalty associated with partial pre-emphasis for a 50 Gbaud 16 QAM transmission after 80 km on SSMF at different OSNR values. We found that the Q factor penalty increases from 0 at 22 dB OSNR to 0.11 at 18 dB OSNR for an attenuation of 3 dB at 20 GHz. It is worthwhile to note that though the PAPR of the received signal increases due to chromatic dispersion (CD), the Q factor penalty remains stable.  
Figure 8 shows the BER for different levels of partial pre-emphasis in Back-to-Back (B2B) case for two different OSNR values. As mentioned earlier, the BER is similar after 80 km transmission. The BER threshold for the 400 ZR FEC is chosen, as in [1], to be 0.0125.  
1.2 Impact of Partial Pre-Compensation on Q-Factor in a Dual-Polarization Coherent System with PDL and PMD  
Based on 400 ZR specifications from [1], typical link impairments are expected to be handled by the adaptive filter at the Rx. In order to stress-test the Rx equalization capability so it performs further post-compensation while keeping the manageable Q penalty, as shown in Figure 9, we ran a dual-polarization experiment using the setup shown in Figure 7. Using emulators, polarization-dependent loss (PDL) of 3.5 dB and polarization-mode dispersion (PMD) of 28 ps were set. PC1 and PC2 are used to scramble the signal polarization, emulating state-of-polarization (SOP) variations at rotation rates of 1 rad/ms and 50 rad/ms, respectively. Additionally, we included, as typically used in point-to-point DCI connections, a 75 GHz C-band channel mux/demux (CMD) at the ingress and egress of the fiber model with a 3 dB net half bandwidth of 36 GHz. With a modulator attenuation at 20 GHz equal to 3, 6, and 9 dB, when comparing full versus partial pre-emphasis in presence of impairments, the Q-factor penalty is measured to be 0.25, 0.5, and 1.1 dB, respectively. In comparison to the data presented in Figure 6, the results show that adding impairments does not introduce more than 0.1 dB of additional penalty.  
Figure 8: Bit-error rate (BER) versus loss at 20 GHz for 50 Gbd 16 QAM signal.