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Orbital angular momentum (OAM) at radio frequency (RF) provides a novelapproach of multiplexing a set of orthogonal modes on the same frequencychannel to achieve high spectrum efficiencies. However, there are still bigchallenges in the multi-mode OAM generation, OAM antenna alignment and OAMsignal reception. To solve these problems, we propose an overall scheme of theline-of-sight multi-carrier and multi-mode OAM (LoS MCMM-OAM) communicationbased on uniform circular arrays (UCAs). First, we verify that UCA can generatemulti-mode OAM radio beam with both the RF analog synthesis method and thebaseband digital synthesis method. Then, for the considered UCA-based LoSMCMM-OAM communication system, a distance and AoA estimation method is proposedbased on the two-dimensional ESPRIT (2-D ESPRIT) algorithm. A salient featureof the proposed LoS MCMM-OAM and LoS MCMM-OAM-MIMO systems is that the channelmatrices are completely characterized by three parameters, namely, the azimuthangle, the elevation angle and the distance, independent of the numbers ofsubcarriers and antennas, which significantly reduces the burden by avoidingestimating large channel matrices, as traditional MIMO-OFDM systems. Afterthat, we propose an OAM reception scheme including the beam steering with theestimated AoA and the amplitude detection with the estimated distance. At last,the proposed methods are extended to the LoS MCMM-OAM-MIMO system equipped withuniform concentric circular arrays (UCCAs). Both mathematical analysis andsimulation results validate that the proposed OAM reception scheme caneliminate the effect of the misalignment error of a practical OAM channel andapproaches the performance of an ideally aligned OAM channel.

Multi-mode OAM Radio Waves: Generation, Angle of Arrival Estimation and Reception With UCAs