Optical Orthogonal frequency division multiplexing (OOFDM) is shown to outperform RZ-OOK transmission in high-speed optical communications systems in terms of transmission distance and spectral efficiency. The OOFDM in combination with the subcarrier multiplexing offers a significant improvement in spectral efficiency of at least 2.9 bits/s/Hz.

The concept of deep-space optical communications was formulated shortly after the invention of lasers. The promise of laser communications, high data rate delivery with significantly reduced aperture size for the flight terminal, led to the pursuit of several successful experiments from Earth orbit and provided the incentive for further demonstrations to extend the range to deep space. This paper is aimed at presenting an overview of the current status of optical communications with an emphasis on deep space. Future perspectives and applications of optical communications related to near-Earth and interplanetary communications are also addressed.

We experimentally demonstrate and characterize the performance of a 400-Gbit/s orbital angular momentum (OAM) multiplexed free-space optical link over 120 m on the roof of a building. Four OAM beams, each carrying a 100-Gbit/s quadrature-phase-shift-keyed channel are multiplexed and transmitted. We investigate the influence of channel impairments on the received power, intermodal crosstalk among channels, and system power penalties. Without laser tracking and compensation systems, the measured received power and crosstalk among OAM channels fluctuate by 4.5 dB and 5 dB, respectively, over 180 s. For a beam displacement of 2 mm that corresponds to a pointing error less than 16.7 μrad, the link bit error rates are below the forward error correction threshold of $3.8\times {10}^{-3}$ for all channels. Both experimental and simulation results show that power penalties increase rapidly when the displacement increases.

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/log₂ N times better than that of PPM.

Codes on graphs of interest for next generation forward error correction (FEC) in high-speed optical networks, namely turbo codes and low-density parity-check (LDPC) codes, are described in this invited paper. We describe both binary and nonbinary LDPC codes, their design, and decoding. We also discuss an FPGA implementation of decoders for binary LDPC codes. We then explain how to combine multilevel modulation and channel coding optimally by using coded modulation. Also, we describe an LDPC-coded turbo-equalizer as a candidate for dealing simultaneously with fiber nonlinearities, PMD, and residual chromatic dispersion.

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