Orbital Angular Momentum: New Physical Resource and Dimension for Future MIMO

The advent of 6G technologies is anticipated to leverage new physical resources of electro-magnetic (EM) waves. Two notable technologies, i.e., multi-input multi-output (MIMO) and orbital angular momentum (OAM) transmissions, have emerged as promising approaches for fully utilizing spatial domain resources and providing new physical resources for wireless communications. However, the contrasting perspectives of physicists and telecommunications engineers have led to a contention regarding the relationship between OAM and MIMO transmissions as well as how OAM can be effectively applied to wireless scenarios beyond MIMO transmission. OAM can generally be classified into two types: intrinsic OAM and extrinsic OAM. This article contends that the quantum OAM transmission system employing intrinsic OAM can harness new physical resources, surpassing the capabilities of traditional MIMO transmission systems. Conversely, statistical OAM beams with extrinsic OAM merely share the spatial dimensions of the conventional space domain and can be considered a specialized form of MIMO transmission systems. Moreover, the article reviews how the additional OAM dimension enables transmission capacity beyond the confines of traditional MIMO limits. Various communications systems that incorporate the OAM dimension are also presented. This study underscores power reuse as the fundamental factor enabling higher capacity rather than the inclusion of independent channels limited by total power. Meanwhile, traditional MIMO systems lacking new physical resources, such as those utilizing only the signal of electric field strength, will ultimately face diminishing marginal effects on capacity even with the increase in the number of orthogonal channels.