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.

Current technologies available for deep-space optical data transmission and networking are discussed in this paper, as well as ongoing experiments, future perspectives, and applications.

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Abstract-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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The propagation of an ON-OFF keying modulated optical signal through an optical atmospheric turbulent channel is considered. The intensity fluctuations of the signal observed at the receiver are modeled using a gamma-gamma distribution. The capacity of this channel is determined for a wide range of turbulence conditions. For a zero inner scale, the capacity decreases monotonically as the turbulence strengthens. For non-zero inner scale, the capacity is not monotonic with turbulence strength. Two error-correction schemes, based on low-density parity-check (LDPC) codes, are investigated as a means to improve the bit-error rate (BER) performance of the system. Very large coding gains-ranging from 5.5 to 14 dB, depending on the turbulence conditions-are obtained by these LDPC codes compared with Reed-Solomon error-correction codes of similar rates and lengths.

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.

Ren, Y. et al. (2016) Experimental characterization of a 400 Gbit/s orbital angular momentum multiplexed free-space optical link over 120 m. Optics Letters, 41(3), pp. 622-625.There may be differences between this version and the published version. You are advised to consult the publisher's version if you wish to cite from it.http://eprints.gla.ac.uk/115873/ Free-space optical (FSO) communications has attracted much attention for a variety of applications, such as back-haul and data centers [1, 2]. Given the rapid growth of bandwidth demand for these applications, there is increased interest in utilizing advanced multiplexing of multiple data streams to increase the data capacity and spectral efficiency of an FSO system [3]. Multiplexing in wavelength and polarization, known as wavelength division multiplexing (WDM) and polarization division multiplexing (PDM) respectively, have previously been used for FSO transmission [3, 4]. Another potential approach is to use space division multiplexing (SDM), for which multiple beams each carrying an independent data stream are transmitted through a common medium [5, 6]. Provided these spatially-overlapping beams can be properly demultiplexed with tolerable crosstalk, the total capacity and spectral efficiency of the communication system is increased by a factor equal to the number of transmitted orthogonal modes. An orthogonal spatial modal basis set for SDM that has gained interest is orbital angular momentum (OAM) [5][6][7][8]. OAM beams with different ℓ values (ℓ is an unbounded integer) are mutually orthogonal [8, 9], so that beams carrying different OAM can act as independent data channels for efficiently multiplexing multiple information-bearing signals in an SDM-based communication system [5]. Moreover, similar to any SDM approach, OAM multiplexing is in principle compatible with existing WDM and PDM techniques [6]. We note that compared to other modal sets, such as Hermite-Gaussian (HG) modes that could also be used for SDM, OAM modes might offer the potential advantage of being conveniently matched to many optical subsystems due to their circular symmetry.It is known that the amount of phase change per unit area for an OAM beam is greatest in the beam center, and that collecting sufficient phase changes is critical for ensuring orthogonality among OAM beams [9]. As a result, OAM multiplexing might be more sensitive to system alignment as it relies more critically on a common optical axis to achieve low inter-modal crosstalk [10]. OAM multiplexing has been employed to demonstrate high-capacity FSO transmission links in laboratory settings [6, 7]. These experiments were generally