Capacity limits of spatially multiplexed free-space communication

Increasing the information capacity per unit bandwidth has been a perennial goal of scientists and engineers(1). Multiplexing of independent degrees of freedom, such as wavelength, polarization and more recently space, has been a preferred method to increase capacity(2,3) in both radiofrequency and optical communication. Orbital angular momentum, a physical property of electromagnetic waves discovered recently(4), has been proposed as a new degree of freedom for multiplexing to achieve capacity beyond conventional multiplexing techniques(5-9), and has generated widespread and significant interest in the scientific community(10-14). However, the capacity of orbital angular momentum multiplexing has not been established or compared to other multiplexing techniques. Here, we show that orbital angular momentum multiplexing is not an optimal technique for realizing the capacity limits of a free-space communication channel(15-17) and is outperformed by both conventional line-of-sight multi-input multi-output transmission and spatial-mode multiplexing.