In this paper, we simulated the ADC/DAC resolution tolerance improvement by implementing probabilistic shaping (PS) distributions in PAM and QAM modulation schemes for 400G and 800G transmissions. We compared PS- and uniformdistributed PM-16QAM, PM-32QAM, and PM-64QAM performance in 800G coherent transmission in FR links. The PS distribution scheme is based on Maxwell-Boltzmann distribution, and 0.1378 FEC overhead is chosen. We demonstrate that for any of these modulation schemes the post-BER (post-FEC bit-error rate) performance is more sensitive to ADC resolution. Moreover, the application of PS distribution can offer us 0.5-dB improvement in received optical power (ROP) for LDPC-coded PM-16QAM and 0.7-dB improvement for PM-32QAM, while PM-64QAM can get 0.6-dB improvement in ROP. For PAM transmission, we simulated a 4-channel CWDM 400G transmission system with wavelengths 1271 nm, 1291 nm, 1311 nm, and 1331 nm for 4-PAM modulation scheme. The PS distribution scheme we used in PAM is exponential distribution, and the FEC overhead is also 0.1378. We demonstrated that the pre-SER (pre-FEC symbol-error rate) performance is also more sensitive to the ADC resolution compared to DAC resolution. Therefore, the PS LPDCcoded modulation represents a great candidate to improve the system performance and reduce the cost.
One of the biggest challenges of free-space optical (FSO) communication is the wave-front aberration due to atmospheric turbulence. In FSO links the wave-front distortion manifests as a significant drop in received power, beam wander, information loss, and scintillation effects. The performance of FSO communication system is degraded significantly by the atmospheric turbulence effects. Fortunately, the adaptive optics system offers potential to mitigate the performance degradation, which is relevant for quantum communication applications as well. In our FSO experiment, we perform the transmission of 6.25 GBd QPSK signal over an FSO link without and with adaptive optics, operating at 1550nm. We emulate the atmospheric aberration in our indoor experimental setup by applying random Kolmogorov phase screens on spatial light modulators (SLMs). We demonstrate significant improvements in the power-collected, signal-to-noise-ratio (SNR), and bit-error-rate (BER) performance due to the application of adaptive optics.