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This paper addresses the RF link budget of a communication system using unusual waves carrying an orbital angular momentum (OAM) in order to clearly analyze the fundamental changes for telecommunication applications. The study is based on a typical configuration using circular array antennas to transmit and receive OAM waves. For any value of the OAM mode order, an original asymptotic formulation of the link budget is proposed in which equivalent antenna gains and free-space losses appear. The formulations are then validated with the results of a commercial electromagnetic simulation software. By this way, we also show how our formula can help to design a system capable of superimposing several channels on the same bandwidth and the same polarization, based on the orthogonality of the OAM. Additional losses due to the use of this degree of freedom are notably clearly calculated to quantify the benefit and drawback according to the case.

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This paper presents a discussion about the RF link budget for a communication system based on unusual waves carrying a non-zero orbital angular momentum (OAM). A typical configuration using circular antenna arrays of isotropic sources to transmit and receive OAM waves is considered. We point out issues when usual assumptions are made to express the link budget. Then, we propose to express the power budget associated with OAM in an elementary MIMO matrix approach. Using a single mode communication configuration, some basic numerical calculations and case studies based on the developed formulations can then be led and discussed, highlighting some intrinsic limitations. Indeed, the asymptotic behavior of the link budget leads to question the concepts of gain and propagation losses.