Spatial division multiplexing using conventional orbital angular momentum (OAM) has become a well-known physical layer transmission method over the past decade. The mode-group (MG) superposed by specific single mode plane spiral OAM (PSOAM) waves has been proved to be a flexible beamforming method to achieve the azimuthal pattern diversity, which inherits the spiral phase distribution of conventional OAM wave. Thus, it possesses both the beam directionality and vorticity. In this paper, it's the first time to show and verify novel PSOAM MG based multiple-in-multiple-out (MIMO) communication link (MG-MIMO) experimentally in a line-of-sight (LoS) scenario. A compact multi-mode PSOAM antenna is demonstrated experimentally to generate multiple independent controllable PSOAM waves, which can be used for constructing MGs. After several proof-of-principle tests, it has been verified that the beam directionality gain of MG can improve the receiving signal-to-noise (SNR) level in an actual system, meanwhile, the vorticity can provide another degree of freedom (DoF) to reduce the spatial correlation of MIMO system. Furthermore, a tentative long-distance transmission experiment operated at 10.2 GHz has been performed successfully at a distance of 50 m with a single-way spectrum efficiency of 3.7 bits/s/Hz/stream. The proposed MG-MIMO may have potential in the long-distance LoS back-haul scenario.
Orbital-angular-momentum mode-group multiplexed transmission over a graded-index ring-core fiber based on receive diversity and maximal ratio combining
提出了一种基于渐变折射率环芯光纤（GIRCF）的轨道角动量（OAM）模群复用（MGM）方案，该方案为每个MG信道设计了单输入双输出（或接收分集）结构，并利用简单的数字信号处理（DSP）自适应地抵抗随机群内模串扰引起的模划分噪声。在该方案中不需要复杂的多输入多输出（MIMO）均衡。此外，如果在接收机侧采用简单的最大比合并（MRC）技术以有效地利用接收机的分集增益，则可以提高接收信号的信噪比（SNR）。通过实验分别验证了强度调制直接检测（IM-DD）系统在1 km GIRCF上传输3个OAM模式组，总传输速率为100 Gb/s的离散多音（DMT）信号，以及在18 km GIRCF上传输2个OAM模式组，总传输速率为40 Gb/s的DMT信号，从而验证了所提出的OAM-MGM方案的可行性.
