# DSP-Based 40 GB/s Lane Rate Next-Generation Access Networks +
## 3 Simulation Results
### Transmission Performance  
By adopting the identified optimum transceiver bandwidth, the receiver optical power sensitivity for each modulation scheme is shown in Figure 4 at different fiber distances. The optical power sensitivity is obtained at a FEC threshold BER of 1 \(\times\) 10\({}^{-3}\). The performance comparison considers  
Figure 3: Receiver sensitivity versus fiber dispersion for PAM-4, subject to various (**a**) transmitter bandwidths and (**b**) avalanche photo-detector trans-impedance amplifier (APD-TIA) receiver bandwidths. An Rx bandwidth of 12.5 GHz is used in (**a**), and a Tx bandwidth of 20 GHz is used in (**b**). (De-)MUX filters as shown in Figure 2 are included. Only seven tap feedforward equalizer (FFE) and five tap decision feedback equalizer (DFE) with first-order terms are considered. four receiver signal processing cases: without equalization, which adopts only simple threshold devices in the receiver side [11], linear equalization only, linear and nonlinear equalizations, and MLSE together with (non-)linear equalizations. Figure 2a shows the optical Duobinary system where the optical sensitivity shows little variation with different SMF lengths within 10 km, beyond which the power penalty increases with increasing fiber length. Without equalization, the system shows the worst power sensitivity at all considered fiber lengths. Linear equalization brings about a slightly improvement in power sensitivity, compared to the no equalization case, which agrees with published results in [19]. The optical Duobinary performance is dominated by the nonlinear interaction of MZM modulation response with fiber dispersion. Such a nonlinear effect is further enhanced by the three-level signal to two-level signal transformation upon the square-law direct detection [11]. Not surprisingly, nonlinear equalization enables an approximately 1 dB improvement in sensitivity at different fiber lengths compared to the case without equalization. The MLSE can further improve the power sensitivities for fiber lengths beyond 10 km. Significant improvement is achievable when the number of MLSE states increases from 2 to 4. Only nonlinear equalization and MLSE enable an SMF transmission distance of 20 km, and a 4-state MLSE can further achieve 25 km of SMF transmission. Figure 4b presents the results for the electrical Duobinary system. Unlike optical Duobinary, the benchmark performance of electrical Duobinary without equalization shows that the optical power sensitivity improves with increasing fiber length until approximately 12 km, beyond which the penalty occurs and increases with fiber length. This is attributed to the EAM negative chirp, which can offset certain amounts of fiber chromatic dispersion. By using linear or nonlinear equalization, optical  
Figure 4: Optical power sensitivity at a bit error rate (BER) of 10-3 versus fiber lengths for (**a**) optical Duobinary; (**b**) electrical Duobinary; and (**c**) PAM-4 subject to different post-equalization and detection conditions. For receiver digital signal processing (DSP), the following configurations are considered: without equalization (w/o EQ) where no DSP considered; linear EQ, which uses only 7-tap/5-tap linear FFE/DFE; nonlinear EQ includes both linear FFE/DFE terms and second-order nonlinear FFE/DFE terms; maximum likelihood sequence estimation (MLSE) contains linear and nonlinear EQ, as well as 2- or 4-state MLSE. Unless indicated explicitly, the four DSP configurations apply throughout this work. sensitivity can improve the optical power sensitivity below 10 km and beyond 18 km of SMFs as it further alleviates the negative chirp in the short fiber length case, and equalization is efficient to mitigate the fiber dispersion. Nonlinear equalization outperforms linear equalization, simply because the former is efficient in combating the EAM nonlinearity [11].