Orbital angular momentum (OAM) has attracted considerable attention as a novel solution for wireless communications because its orthogonal modes significantly increase the channel capacity without an additional frequency band. The joint multiplexing between OAM technologies and other modulation techniques has not been thoroughly investigated. In this paper, we first proposed the orthogonal frequency-division multiplexing-orbital angular momentum (OFDM-OAM) multiple-input-multiple-output (MIMO) system. The proposed OFDM-OAM MIMO based on the discrete Fourier transformation (DFT) operations achieves a very high sum-rate and spectrum efficiency (SE). However, the expensive hardware and software overheads for transmitting and receiving OAM waves lead to an unexpected cost for the OFDM-OAM MIMO scheme. A time-switched OFDM-OAM (TOO) MIMO is then proposed to reduce the computational complexity, and the procedure of OAM generations and recoveries has also explicitly been derived. The mathematical derivation shows that the proposed TOO MIMO system based on a simple switching sequence is suitable for small-scale and low-cost wireless broadband communications, and the simulation results demonstrate that the TOO MIMO scheme achieves a considerable SE and much less computational complexity than the OFDM-OAM MIMO scheme.INDEX TERMS Orbital angular momentum (OAM), joint multiplexing, orthogonal frequency-division multiplexing-orbital angular momentum (OFDM-OAM), multiple-input multiple-output (MIMO), spectrum efficiency (SE), time-switched OFDM-OAM (TOO).

The theory of time-modulated circular arrays in respect of generating orbital angular momentum (OAM) beams at harmonic frequencies is analysed. Without using expensive phase shifters in phased arrays, beam steering for OAM beams is achieved using switches connected to array elements. The model of an eight-element time-controlled circular array is derived with the method of time-modulation beam steering techniques. Simulation results show that beam steering for OAM beams is realised, which indicates the effectiveness of the proposed method.

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