Phase Offset Aided Index Modulation for UCA-Based OAM Communication

In this paper, a novel phase offset (PO)-aided index modulation (IM) for orbital angular momentum (OAM) communication, called OAM-PO-IM, is proposed to enhance capacity. The proposed system conveys two types of index information including an active mode combination (AMC) and misalignment of PO states. This method leverages the PO state induced by the selective activation of antenna elements within a uniform circular array (UCA). Specifically, a UCA is divided into multiple uniform circular subarrays (UCSAs) in the transmitter, and additional index bits are allocated according to the activation state of the UCSAs. In addition, the modified Bessel function (MBF) of the first kind is used to derive the channel model to allow a straightforward determination of the optimal AMC for the OAM-IM system. To detect index and symbol bits, the receiver provides a maximum likelihood (ML) detector and a new low-complexity ML detector using the log-likelihood ratio (LLR). The effectiveness of the proposed OAM-PO-IM is verified through comprehensive simulations that benchmark its performance against the existing OAM and OAM-IM in terms of sum capacity, bit error rate (BER), and energy efficiency. Compared with the OAM-IM, the proposed OAM-PO-IM provides a capacity gain of additional log (2)G bps/Hz, where G denotes the number of UCSA. Furthermore, this scheme improves the gain by approximately 1 dB at a BER of 10(-4) for active mode L-A=2 at 6 bps/Hz.