## 3 Communication Performance and Analysis
Figure 6: Captured output eye diagrams with equalisers for SM-VCSEL Setup-II: (**a**) 12 Gbps with PAM-8. (**b**) 6 Gbps with PAM-16. non-linearity in the system, resulting in improved eye diagram characteristics. This improvement is also reflected in the BER plots in Figure 5, where the MLPDFE demonstrates superior performance compared to the TRDFE. The results achieved with Setup-II were obtained at an ROP of \(\sim\!-3.1\) dBm. However, misalignments and extensions in the OWC link can diminish the ROP, subsequently impacting the SNR and bit rate. Hence, Figure 7 is presented to illustrate the bit rate attainable at the 7% S-FEC limit for various ROPs ranging from \(-14\) dBm to \(-3\) dBm using PAM-2, PAM-4, PAM-8, and PAM-16 modulation schemes with both the TRDFE and MLPDFE. At lower bit rates and reduced ROPs, PAM-4 requires more power than PAM-2 to achieve similar bit rates. For instance, with the TRDFE at a bit rate of 10 Gbps, PAM-2 and PAM-4 necessitate ROPs of \(\sim\!-13.2\) dBm and \(\sim\!-11.0\) dBm, respectively, indicating an ROP difference of 2.2 dB in favour of PAM-2. However, at a higher bit rate of 25 Gbps, PAM-2 and PAM-4 demand ROPs of \(\sim\!-6.0\) dBm and \(\sim\!-5.4\) dBm, respectively, showcasing a reduced ROP difference of 0.6 dB between the two modulation schemes. PAM-4 offers a higher bit rate of \(\sim\)29 Gbps compared to PAM-2, which gives \(\sim\)28 Gbps at the high ROP of \(-3.1\) dBm. When considering PAM-2, the TRDFE shows a slight advantage over the MLPDFE at lower bit rates with reduced ROPs. However, as the bit rates increase with higher ROPs, the MLPDFE demonstrates superior performance over the TRDFE. For example, at a bit rate of 10 Gbps, the TRDFE and MLPDFE require ROPs of \(\sim\!-13.2\) dBm and \(\sim\!-13.0\) dBm, respectively, indicating a marginal ROP difference of 0.2 dB in favour of the TRDFE. However, at a higher bit rate of 25 Gbps, the TRDFE and MLPDFE demand ROPs of \(\sim\!-6.0\) dBm and \(\sim\!-7.1\) dBm, respectively, showcasing a notable ROP difference of 1.1 dB in favour of the MLPDFE. With PAM-4, the MLPDFE demonstrates superior ROP performance compared to the TRDFE at lower bit rates with reduced ROPs. However, at higher bit rates with increased ROPs, the MLPDFE shows ROP requirements similar to those of the TRDFE. For instance, at a bit rate of 10 Gbps, the TRDFE and MLPDFE require ROPs of \(\sim\!-11.5\) dBm and \(\sim\!-11.0\) dBm, respectively, indicating a marginal ROP difference of 0.5 dB in favour of the MLPDFE. However, at a higher bit rate of 25 Gbps, both the TRDFE and MLPDFE demand an ROP of \(\sim\!-5.4\) dBm, showcasing no ROP difference between the two equalisers. However, increasing the modulation level to PAM-8 and PAM-16 does not improve the bit rate performance due to the limited SNR available in the system [29]. Consequently, no bit rates above 2 Gbps were achieved at the FEC limit with the TRDFE for these modulation  
Figure 7: Plots comparing the bit rates achieved at the 7% S-FEC limit against the received optical power (ROP) with SM-VCSEL Setup-II. Solid and dashed lines denote the results from TRDFE and MLPDFE, respectively. levels. In contrast, the MLPDFE enables bit rates beyond 6 Gbps at ROPs greater than \(-10\) dBm with PAM-8. Notably, with PAM-16, the MLPDFE achieves a consistent bit rate of \(\sim\)5 Gbps at ROPs ranging from \(-5\) dBm to \(-3\) dBm. Compared to PAM-8, which experiences a significant drop in bit rates with a decreasing ROP, PAM-16 shows a milder reduction. This is because PAM-16 requires lower bandwidth and thus experiences lesser noise at lower ROPs for bit rates similar to that of PAM-8. In conclusion, the MLPDFE outperforms the TRDFE when the link SNR exceeds a certain threshold. This threshold is smaller with increasing PAM levels and higher system non-linearity, as indicated by the bit rate results from the two setups (Setup-I and Setup-II) using both VCSELs.