Pre-Coding OAM Based MIMO System for Multi-User Communications

Recently, orthogonal Orbital Angular Momentum (OAM) modes of electromagnetic waves have been widely applied to achieve high spectral efficiency in the radio and microwave regimes. This study investigates utilizing multimodal OAM carrying waves in communications between a central unit and distributed receiving units. Accommodated by a planner grid-array of antenna elements and backed by appropriate pre-coding, the central unit can transmit to each receiving unit multi-OAM modes. A receiving unit comprises a simple circular array. Pre-coding at the transmitting unit is a two-fold process: preventing user-to-user interference and enforcing any predetermined combination of OAM modes to reach its destination. Accordingly, the parallel transmission of multiple channels to each receiver is possible. Channels aimed at the same receiver are distinguished by different modes, but the same group of modes can be simultaneously used upon transmission to other receivers. The numerical results show that significant increase in the transmission capacity is achieved. For instance, in a system including 6 receivers, 25 channels are delivered in parallel to each receiver while maintaining an acceptable low bit-error rate. Results are judged by considering the theoretical single-user quadrature phase shift keying modulation performance as a benchmark for comparison. This study also examines channel perturbations when interfering modes exist. In addition, it overcomes many typical limitations with the utilization of OAM-based solutions in radio, such as the need for transmitter-receiver alignment. The proposed system also solves the problem of wave divergence at high OAM modes, thereby avoiding the exclusion of using these modes in transmission.