Discussion about the link budget for electromagnetic wave with orbital angular momentum

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.