Optimizing OAM Side-lobe Levels Using Sparse 2D Array

Abstract

Electromagnetic orbital angular momentum (OAM) as the promise of enhancing spectral efficiency in informational transmission has attracted widespread attention in the fields of radio frequency and optical communications. Hitherto, two main methods have been put forward to generate beams carrying OAM in the field of communication at lower frequencies, which include spiral phase plates (SPP) and uniform circular array (UCA). However, modern multiple-input-multiple-output (MIMO) systems mainly consist of 2D linear array, which is inconvenient to combine with OAM generated by UCA. Furthermore, OAM radio suffers significant loss of due to the divergence of OAM waves, especially in high eigenmodes. In this contribution we will present the results of original work carried out into 2D linear array and sparse 2D linear array which bring benefits of orthogonal OAM modes without change the antenna systems. Work on 2D linear array will report on: i) Numerical derivation for the radiation pattern of a 2D linear array antenna is presented to generate beams carrying OAM is available; ii) An innovative simple and feasible method that utilizes genetic algorithm (GA) to reduce sideband levels by thinning array. The approach also reaches the effect of converging the power to propagating direction. First, OAM beam have been successively generated using 2D arrays. Second, optimizing the performance over different sparse rates, we aim to maintain the main lobe width substantially similar, while reducing side lobe levels to converge the energy in main direction. An example of a 64-element 2D linear sparse array are proposed and peak side lobe lever (SLL) reduces to −19.76 dB with respect to −11.56 dB for similar size ordinary 2D array. Finally, using genetic algorithm, a sparse array of less elements (some antennas are off) are proposed to generate beam which has a regular helical phase-front similar to that generated by a single ring which will help to save energy and increase efficiency. This work will help the application of orbit angular momentum in existing massive MIMO or 2D arrays and also other technology based on radio wave OAM, e.g., secure communication, quantum encryption.