Nonlinearity Compensation for Coherent Optical Satellite Communications

Abstract

Optical satellite uplinks rely on high-power optical amplifiers (HPOAs) to overcome free-space attenuation and enable long-distance transmission. However, at high power levels, fiber Kerr nonlinearity becomes significant and degrades system performance. In this work, we develop a realistic model for optical uplinks that accounts for nonlinear effects and analyze their impact, highlighting key differences from conventional longhaul fiber systems. We then introduce low-complexity digital signal processing techniques for nonlinearity compensation, based on constellation shaping via a look-up table (LUT) and a simple nonlinear phase rotation applied at the transmitter and/or receiver. The LUT also enables adaptive rate tuning according to channel conditions, enhancing robustness against link variations. Simulation results show that the proposed techniques increase the maximum acceptable link loss by up to 6 dB with negligible complexity. Finally, we show that, at the system level, propagation in the HPOA can be modeled as a simple nonlinear phase rotation, equivalent to propagation in a zero-dispersion noiseless fiber link, and fully characterized by a single parameter - the characteristic nonlinear power.

Figures (13)

• Figure 1: Ground-to-satellite optical link
• Figure 2: System model
• Figure 3: PSD of the normalized transmitted (left) and received (right) signals ($100$GBd, $\bar{\phi}\approx1.1$) obtained analytically from (\ref{['eq:autocrrelation_evolution']}) or with numerical simulations (MB and sph.sh. $N=4$): (a) without NLPC; (b) with TX-side NLPC; and (c) with split NLPC. The details of the setup are reported in Section \ref{['subsec:System-setup']}.
• Figure 4: Simplified model
• Figure 5: LUT with $N=4$ amplitudes from $\{1,3,5,7\}$ and $32$ entries $(k=5)$, corresponding to a $64$QAM shaped constellation with rate $4.5$bits/2D and sequences with minimum energy.