Capacity limits of spatially multiplexed free-space communication

Increasing the information capacity per unit bandwidth has been a perennial goal of scientists and engineers1. Multiplexing of independent degrees of freedom, such as wavelength, polarization and more recently space, has been a preferred method to increase capacity2,3 in both radiofrequency and optical communication. Orbital angular momentum, a physical property of electromagnetic waves discovered recently4, has been proposed as a new degree of freedom for multiplexing to achieve capacity beyond conventional multiplexing techniques5–9, and has generated widespread and significant interest in the scientific community10–14. However, the capacity of orbital angular momentum multiplexing has not been established or compared to othermultiplexing techniques. Here, we show that orbital angular momentum multiplexing is not an optimal technique for realizing the capacity limits of a free-space communication channel15–17 and is outperformed by both conventional line-of-sight multi-input multi-output transmission and spatial-mode multiplexing. An orbital angular momentum (OAM) mode carrying angular momentum lħ in free space is a Laguerre–Gaussian (LG) beam given by