# An Evaluation of Orbital Angular Momentum Multiplexing Technology  
[PERSON]  
1 NTT Network Innovation Laboratories, NTT Corporation, 1-1 Hikarinooka, Yokosuka-Shi 239-0847, Japan; [EMAIL_ADDRESS] (H.S.); [EMAIL_ADDRESS] (H.F.); [EMAIL_ADDRESS] (Y.Y.); [EMAIL_ADDRESS] (T.S.)  
[PERSON]  
1 NTT Network Innovation Laboratories, NTT Corporation, 1-1 Hikarinooka, Yokosuka-Shi 239-0847, Japan; [EMAIL_ADDRESS] (H.S.); [EMAIL_ADDRESS] (H.F.); [EMAIL_ADDRESS] (Y.Y.); [EMAIL_ADDRESS] (T.S.)  
[PERSON]  
1 NTT Network Innovation Laboratories, NTT Corporation, 1-1 Hikarinooka, Yokosuka-Shi 239-0847, Japan; [EMAIL_ADDRESS] (H.S.); [EMAIL_ADDRESS] (H.F.); [EMAIL_ADDRESS] (Y.Y.); [EMAIL_ADDRESS] (T.S.)  
[PERSON]  
1 NTT Network Innovation Laboratories, NTT Corporation, 1-1 Hikarinooka, Yokosuka-Shi 239-0847, Japan; hirofumi.sasaki.  
and demultiplexing OAM algorithms using computer simulations [4]. This enabled us to clarify the performance and effect of mode-dependent power attenuation. In doing so we generated OAM signals by using a UCA (uniform circular array) that comprises multiple omnidirectional antenna elements.  
The second part of our work was validating the feasibility from beam generation and propagation perspectives. In particular, we expanded our study to the usage of a dielectric lens to examine the feasibility of long distance transmission using OAM [5]. Ideally, all OAM modes are orthogonal to each other due to the unique nature of their phase fronts. However, with these modes, it is difficult to transmit over long distances because their energy rapidly diverges as the beam propagates. To achieve long-distance transmission, we developed and proposed a beam divergence reduction method using the focusing effect of a dielectric lens. We conducted a wave propagation experiment on 28 GHz bands to demonstrate the effectiveness of using such a lens. In the experiment, we were able to generate OAM modes 0, \(\pm\)1, and \(\pm\)2. In addition, we showed the beam divergence reduction effect by making a comparison between conventional OAM beam generation methods and using a dielectric lens with a UCA.  
In the third part of our work, we validated the feasibility from the end-to-end wireless communication perspective. We report experimental results using wireless OAM multiplexing at 28 GHz. One of the major challenges for achieving OAM multiplexing is its intensity variation among different OAM modes. As shown in Figure 7, intensity distributions of OAM signals with different modes are given by the Bessel function of the first kind by their inherent nature [2]. This yields selective reception (Rx) SNR (signal-to-noise ratio) degradation when the Rx antenna is located in a low SNR region (null region). To address this problem, we used multiple UCAs, which are designed to avoid reception in null regions. We obtained successful experimental results using five OAM modes over 28 GHz [6]. We also developed and proposed OAM-MIMO multiplexing using multiple UCAs. Unlike OAM multiplexing, OAM-MIMO multiplexing exploits multiple sets of the same OAM modes with receiver equalizations [7]. This enables the number of concurrently transmitted data streams to be increased without using higher OAM modes that have large beam divergence. We experimentally demonstrated the effectiveness of the proposed OAM-MIMO multiplexing by using 11 OAM modes in total (three OAM modes 0 and two sets each of OAM modes \(\pm\)1 and \(\pm\)2). Experimental results reached a new milestone in point-to-point transmission rates when 100 Gbps was achieved at 10 m transmission distance.