Deliberate Clipping and Iterative Distortion Recovery for UCA-Based OAM Multiplexing Systems

Deliberate amplitude clipping is a simple and well-known technique to reduce the peak-to-average power ratio of orthogonal frequency division multiplexing (OFDM) systems. In this paper, we propose a clipping technique for peak power reduction in orbital angular momentum (OAM) multiplexing systems with uniform circular array (UCA) antennas. In the proposed technique, clipping is performed on digital baseband signals prior to OAM beamforming and pulse-shaping filtering, which helps to avoid out-of-band radiation and affecting the orthogonality of the OAM modes. In addition, an iterative distortion recovery algorithm is proposed in order to mitigate performance degradation in bit error rate (BER) due to the clipping. The algorithm is derived by unfolding the clipping noise cancellation (CNC) algorithm for OFDM systems into layers and by introducing layer-wise learnable parameters. Simulation results show that for a realistic OAM multiplexing system with 256QAM signaling, the unfolded CNC exhibits excellent BER performance even when the conventional CNC suffers from a high error-floor. The combination of the proposed clipping and distortion recovery schemes provides a significant reduction in the peak power of the OAM signals at the cost of only a slight degradation in BER performance.