Deep-space and near-Earth optical communications by coded orbital angular momentum (OAM) modulation

TLDR

Deep‑space optical links require multi‑gigabit rates, motivating pulse‑position modulation, while orbital angular momentum offers orthogonal high‑dimensional modes that are vulnerable to turbulence‑induced mode mixing between spacecraft and Earth stations. The study aims to show that orbital angular momentum modulation, combined with LDPC coding, can satisfy future deep‑space and near‑Earth optical communication bandwidth needs while remaining robust to atmospheric turbulence. The authors employ orbital angular momentum modes together with low‑density parity‑check coding to encode high‑dimensional signals that remain distinguishable under strong atmospheric turbulence. The proposed OAM‑LDPC scheme achieves a spectral efficiency N²/log₂N times higher than conventional pulse‑position modulation.

Abstract

In order to achieve multi-gigabit transmission (projected for 2020) for the use in interplanetary communications, the usage of large number of time slots in pulse-position modulation (PPM), typically used in deep-space applications, is needed, which imposes stringent requirements on system design and implementation. As an alternative satisfying high-bandwidth demands of future interplanetary communications, while keeping the system cost and power consumption reasonably low, in this paper, we describe the use of orbital angular momentum (OAM) as an additional degree of freedom. The OAM is associated with azimuthal phase of the complex electric field. Because OAM eigenstates are orthogonal the can be used as basis functions for N-dimensional signaling. The OAM modulation and multiplexing can, therefore, be used, in combination with other degrees of freedom, to solve the high-bandwidth requirements of future deep-space and near-Earth optical communications. The main challenge for OAM deep-space communication represents the link between a spacecraft probe and the Earth station because in the presence of atmospheric turbulence the orthogonality between OAM states is no longer preserved. We will show that in combination with LDPC codes, the OAM-based modulation schemes can operate even under strong atmospheric turbulence regime. In addition, the spectral efficiency of proposed scheme is N2/log2N times better than that of PPM.