Optical Geometric Transformation-Based Orbital Angular Momentum for Indoor Multiuser Communications

This paper presents a new and unique approach that significantly increases the channel capacity of multiuser indoor optical wireless communication systems. In particular, we consider an orbital angular momentum (OAM) based multiuser indoor communication with a unique optical geometric transformation (OGT) technique. We derive the channel impulse response for OAM carrying Laguerre-Gaussian beams using the angular spectrum method. The optical transceiver is designed on the principle of space variance and parallel processing by realizing a geometric transformation in structured light. Specifically, each transmitter transforms transverse positions in the input beams into multiplexed azimuthal positions at the output. The transmitter uses the orthogonality of OAM modes to encode many optical channels on the same wavelength. As a proof of concept, multiple signals are encoded on each transmitter and multicast to various receivers simultaneously. The amplitude function and the spatial dependence of the OAM field vectors are analyzed. We visualize the combined effects of the dispersion, optical channel noise immunity, accuracy of timing extraction, and intersymbol interference on the performance of the proposed system. In addition, we analyze the impact of multiuser interference on performance. The channel equalization condition for an interference-free transmission is also presented. Moreover, we present the impacts of amplification gain and reflectivity on the ripples. The results illustrate that reflectivity has a strong impact on ripples. The proposed transceiver design can also easily distinguish the true mode from other neighboring modes, as we expect the energy of an OAM-carrying beam to spread from the true mode symmetrically to its neighbors. In addition, the proposed transceiver design enables the detection of existing all multiple OAM modes through a single transformation. Finally, it is shown from a series of results and comparative analyses that the proposed system can offer very high channel capacity in indoor optical multiuser communication systems, while maintaining an arbitrary low bit error rate.