## 2 State of the Art  
In recent years, enormous progress in the area of OAM technologies has been made, either in optical communication systems utilizing optical fibers or also in optical wireless propagation in free space environments [5]. It can be considered that OAM deployment is feasible with current technologies. In substance, the OAM technique is a group of higher-order regimes defined on a different base as other mode forms in the optical fiber. Using OAM multiplexing, it is possible to consider the utilization of two different approaches in optical fiber signal transmission. The first approach presents an implementation of the OAM transmission in the common optical FMF (Few-Mode Fiber) transmission medium with a few regimes. The second approach introduces a specially designed vortex fiber that has an interconnection with lower regimes. Thanks to this fact, it is possible to avoid the DSP (Digital Signal Processing) equalizing for relatively short transmission ranges [6]. For the MDM (Mode Division Multiplexing) in common FMF fibers, LP (Linearly Polarized) regimes can be used to achieve impressive results. Each OAM regime presents a linear combination of authentic fiber regimes. Due to faults or other non-idealities, OAM regimes coupled into the FMF fiber can be mutually interconnected. This mutual interconnection of regimes can lead to inter-channel interferences and/or to transmission failures. One possible solution for the regime interconnection effects is the MIMO (Multiple Input Multiple Output) utilization in combination with coherent detection [6]. At the fiber end, received regimes are decomposed based on the OAM (\(l\) = \(+1\) and \(-1\)) regime using the regime sorter. Subsequently, each of the two OAM light components is paired with the PBS (Polarization Beam Splitter) element on the base of polarization fibers. For a reduction in the inter-channel interference, the algorithm with a constant module for blind estimation of the channel crosstalk and its compensation using a linear equalizing is used [6]. In this manuscript, a focus on the proposed WDM-PON architecture involving OAM multiplexing is introduced, and the effective utilization of available wavelength and bandwidth resources in common optical fibers is analyzed. In [7], a novel PON (passive optical network) access architecture based on the OAM multiplexing is proposed and experimentally demonstrated. Multiple data streams can be multiplexed by orthogonal OAM modes on the same wavelength to achieve the efficient utilization of bandwidth resources. This architecture contains the vortex fiber that supports OAM transmission. In the OLT (Optical Line Terminal), the data stream is first modulated onto the optical carrier. The OLT can transmit multi-channel data streams for multiple user access. These multi-channel signals are converted from a fundamental Gaussian beam to the LG (Laguerre Gaussian) beam with various OAM modes. Signals carried by different OAM modes will not interfere with each other [8]. In [9], multiple data channels are multiplexed by mutually orthogonal OAM modes on the same wavelength to realize the efficient utilization of bandwidth resources. The proposed OAM-PON system has the potential to provide flexibility and high bandwidth for large-capacity passive optical networks. In [10], a new hybrid system for high-bandwidth and high-transmission-capacity communication that integrates the MMF (Multi-Mode Fiber) cable with the FSO (Free Space Optics) link using the OAM multiplexing is introduced. This solution can be used for providing services to end users located in remote areas. However, using MMF fiber markedly decreases the transmission distance between the central office and end users' locations. Also, the number of possible subscribers is meaningfully limited compared to common passive optical networks using SMF (Single-Mode Fiber) fibers. In [11], the new-generation PON architecture is proposed and experimentally demonstrated involving the FSO link that also utilizes OAM modes and WDM techniques.