Coded Orbital Angular Momentum Modulation and Multiplexing Enabling Ultra-High-Speed Free-Space Optical Transmission

Abstract

To simultaneously achieve high throughput and low energy consumption, in this chapter we advocate the use of energy-efficient N-dimensional coded orbital angular momentum (OAM)-based modulation and multiplexing for ultra-high-speed optical transmission over free-space optical (FSO) links. OAM is associated with the azimuthal phase dependence of the complex electric field. Because its eigenvectors are orthogonal, they can be used as basis functions for multidimensional signaling. Because the information capacity is linear in the number of dimensions, we can dramatically improve the overall optical channel capacity through multidimensional signal constellations. On the other hand, energy efficiency can be achieved by properly designing the N-dimensional signal constellation. To deal with time-varying FSO channel conditions, OAM modulation can be combined with rateless coding. Atmospheric turbulence effects can be handled, in addition to FSO-MIMO, through the use of the azimuthal phase correction method, similar to the Gerchberg-Saxton (GS) phase retrieval algorithm. Moreover, distortions introduced by turbulence will be compensated for via adaptive optics approaches. Additionally, LDPC-coded OAM-based free-space optical FSO transmission system is experimentally studied in both the absence and presence of emulator-induced atmospheric turbulence.