Mode Modulation for Orbital-Angular-Momentum Based Wireless Vorticose Communications

Recently, orbital angular momentum (OAM) based vorticose communication has attracted much attention because of its potential to significantly increase the spectrum efficiency (SE) of wireless communications. However, the multiple radio frequency (RF) chains used for multiple OAM modes lead to an unexpected cost for wireless vorticose communications. To reduce the high cost of RF chains for multiple OAM modes and the high complexity required for signal processing, we first propose the mode modulation (MM) based OAM system to allow multiple OAM modes sharing a common RF chain, which can not only reduce the hardware cost, but also boost the SE by introducing the mode as an additional dimension for data transmission. To solve the problem of how to maximize the SE of MM based OAM systems with the limited RF chains, we develop the equal-probability mode modulation (EMM) scheme, where the OAM modes are selected with equal probability and the signal is transmitted though the activated OAM modes. Moreover, we develop the Huffman coding based adaptive mode modulation (AMM) scheme, which can adaptively choose the OAM modes to further increase the SEs of OAM based vorticose communications. We also develop the OAM-water-filling power allocation policies for both EMM and AMM schemes to achieve the maximum SEs for OAM based vorticose communications. Numerical results are presented to show that the MM can offer the mode dimension for vorticose communications and the AMM scheme can achieve larger SE than the EMM scheme. Also, our developed power allocation policies can further increase the SEs for the MM based OAM communications.