Orbital angular momentum multiplexing is a physical layer method for multiplexing signals carried on electromagnetic waves using the orbital angular momentum (OAM) of the electromagnetic waves to distinguish between the different orthogonal signals. OAM is one of two forms of angular momentum of light; it is distinct from, and should not be confused with, light spin angular momentum. The latter offers only two orthogonal quantum states, corresponding to the two states of circular polarization, and can be demonstrated to be equivalent to a combination of polarization multiplexing and phase shifting. OAM on the other hand relies on an extended beam of light, and the higher quantum degrees of freedom which come with the extension. OAM multiplexing can thus access a potentially unbounded set of states, and as such offer a much larger number of channels, subject only to the constraint of real-world optics. The constraint has been clarified in terms of independent scattering channels or the degrees of freedom of scattered fields through angular-spectral analysis, in conjunction with a rigorous Green's function method. The degrees of freedom limit is universal for arbitrary spatial-mode multiplexing, which is launched by a planar electromagnetic device, such as antenna, metasurface, etc., with a predefined physical aperture. As of 2013, although OAM multiplexing promises very significant improvements in bandwidth when used in concert with other existing modulation and multiplexing schemes, it is still an experimental technique, and has so far only been demonstrated in the laboratory. Following the early claim that OAM exploits a new quantum mode of information propagation, the technique has become controversial, with numerous studies suggesting it can be modelled as a purely classical phenomenon by regarding it as a particular form of tightly modulated MIMO multiplexing strategy, obeying classical information theoretic bounds. As of 2020, new evidence from radio telescope observations suggests that radio-frequency orbital angular momentum may have been observed in natural phenomena on astronomical scales, a phenomenon which is still under investigation.
History OAM multiplexing was demonstrated using light beams in free space as early as 2004. Since then, research into OAM has proceeded in two areas: radio frequency and optical transmission.
Radio frequency
Terrestrial experiments An experiment in 2011 demonstrated OAM multiplexing of two incoherent radio signals over a distance of 442 m. It has been claimed that OAM does not improve on what can be achieved with conventional linear-momentum based RF systems which already use MIMO, since theoretical work suggests that, at radio frequencies, conventional MIMO techniques can be shown to duplicate many of the linear-momentum properties of OAM-carrying radio beam, leaving little or no extra performance gain. In November 2012, there were reports of disagreement about the basic theoretical concept of OAM multiplexing at radio frequencies between the research groups of Tamburini and Thide, and many different camps of communications engineers and physicists, with some declaring their belief that OAM multiplexing was just an implementation of MIMO, and others holding to their assertion that OAM multiplexing is a distinct, experimentally confirmed phenomenon. In 2014, a group of researchers described an implementation of a communication link over 8 millimetre-wave channels multiplexed using a combination of OAM and polarization-mode multiplexing to achieve an aggregate bandwidth of 32 Gbit/s over a distance of 2.5 metres. These results agree well with predictions about severely limited distances made by Edfors et al. The industrial interest for long-distance microwave OAM multiplexing seems to have been diminishing since 2015, when some of the original promoters of OAM-based communication at radio frequencies (including Siae Microelettronica) have published a theoretical investigation showing that there is no real gain beyond traditional spatial multiplexing in terms of capacity and overall antenna occupation.
Radio astronomy In 2019, a letter published in the Monthly Notices of the Royal Astronomical Society presented evidence that OAM radio signals had been received from the vicinity of the M87* black hole, over 50 million light-years distant, suggesting that orbital angular momentum information can propagate over astronomical distances.
ISAC OAM has recently been explored in the context of integrated sensing and communications (ISAC), where a single electromagnetic waveform is used for both data transmission and environmental sensing. OAM beams, characterized by their helical phase structure and orthogonal modal basis, enable multiplexing of multiple data streams while also producing spatially diverse field distributions that can probe targets. Recent experimental work has demonstrated that reconfiguring combinations of OAM modes can generate composite beam patterns whose reflections encode location-dependent information, enabling simultaneous high-rate communication and target localization in millimeter-wave systems. In parallel, studies in radar and antenna systems have examined the use of structured electromagnetic fields, including OAM modes, within joint radar–communication frameworks, highlighting their potential for enhanced parameter estimation and spatial diversity, while also noting that their performance is fundamentally tied to system geometry and shares similarities with conventional multiple-input multiple-output (MIMO) approaches.
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