## 4 Experimental Setup  
Figure 4 shows our experimental setup to investigate single-carrier high-baud-rate and high-order QAM signal transmission in unamplified fiber link. A 92 GS/s digital-to-analog converter (DAC) with 32 GHz analog bandwidth is used to generate electrical signals for the four tributaries of dual-polarization (DP) in-phase and quadrature (IQ) modulator. The trace loss between the DAC and the DP-IQ modulator is compensated in the digital domain by a finite impulse response (FIR) filter [27]. The root-raise cosine filter with a roll-off factor of 0.2 was applied in the following experiment. We used two tunable narrow-linewidth (<100 kHz) lasers with 16 dBm maximal output power, and set them to 1550.12 nm (i.e., 193.4 THz). One is with maximal power for the optical input to the DP-IQ modulator, and the other is set to 12 dBm to act as the local oscillator (LO) for the coherent receiver. The laser to the modulator is separated into two orthogonal polarizations, and each polarization is modulated by the corresponding IQ signals. An automatic bias control (ABC) is used to control the bias voltage applied to the DP-IQ modulator. At the Rx side, the optical signal is first de-modulated by using a 90\({}^{\circ}\) heterodyne coherent detector. Four linear trans-impedance amplifiers (TIAs) with differential outputs are set to automatic gain control (AGC) mode. This minimizes the BER performance dependence on the LO power, and four tributaries of analog electrical signals are generated. Then, an 8-bit analog-to-digital converter (ADC) with 63 GHz analog bandwidth and 4.5 effective number of bits (ENOB) samples the analog signals into discrete digital samples at 160 GS/s for post signal processing. In the DSP, chromatic dispersion (CD) is first compensated. Then polarization mode dispersion (PMD) is compensated and polarization tributaries are de-multiplexed by a blind equalization using the radius directed equalization (RDE). A 21-tap equalizer is implemented in the time domain with a Ts/2 spacing [28].  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**DSP** & **2012** & **2015** & **2017** & **2019** \\ \hline CMOS node & 40 nm & 28 nm & 16 nm & 7 nm \\ Power/100G & \textless{}50 W & \textless{}20 W & \textless{}7 W & \textless{}2 W \\ \hline \hline \end{tabular}
\end{table}
Table 1: Coherent digital signal processing (DSP) power consumption evolution. CMOS: complementary metal-oxide-semiconductor.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Module Power** & **DSP** & **TROSA** & **Other** \\ \hline QSFP-DD & 7 W & 6–7 W & 1–2 W \\ \hline \hline \end{tabular}
\end{table}
Table 2: 400G-ZR QSFP-DD power consumption. TROSA: transmitter and receiver optical subassembly, QSFP-DD: quad small form factor double density.  
Figure 3: Typical architecture of 400G pluggable coherent transceiver. DSP: digital signal processing, TROSA: transmitter and receiver optical subassembly, Tx: transmitter, Rx: receiver.  
Furthermore, the blind estimation algorithm is used to estimate and compensate the carrier frequency offset between the Tx signal and the LO laser, followed by the carrier phase noise [29].  
In our experiment, we tried to emulate the power limited unamplified case, which is being adopted by the industry with 10-12 dB link margin. According to the receiver sensitivity of 64 GBd 16-QAM signal, we adjusted the variable optical attenuator (VOA) to set the transmitter optical output power to a reference point \(P_{Tx\_Ref}\), which is 12 dB greater than the receiver sensitivity at a BER of \(4.2\times 10^{-2}\). In our experiment, we generated 16-QAM and 64-QAM with symbol rates from 45 to 86 GBd. By adjusting the VOA, we also increased the Tx optical power into the single-mode optical fiber (SMF) by 3 dB and 6 dB. Optical 16-QAM and 64-QAM signals at various baud rates were then transmitted over an SMF link with distance from 10 to 110 km. Different combinations of a few fiber sections were used to achieve specific fiber length, and their corresponding losses are illustrated in Table 3.