Wireless data services have seen an enormous growth over the past two decades: not only has the number of users and connected devices increased dramatically, but also the connection speed has grown by a factor of 1000. All of this had led to an unprecedented variety of wireless services that changes the way that people are working, interacting, and spending their spare time. However, the continued increase in data rates has also created great challenges, since the spectrum used for signal transmission is a limited and precious resource. For this reason, it is essential to both use new spectrum, and to exploit available spectrum as efficiently as possible. Advances in chip manufacturing have opened the possibility for low-cost communications systems using millimeter waves, which have much shorter wavelength than currently used cellular systems, and where a lot of spectrum is available. Still, given the increase in data usage, also this spectrum has to be used efficiently.
One way to drastically improve spectral efficiency is to transmit multiple data streams together, using the same spectrum. However, in a normal setting, the receiver obtains then a mixture of those data streams, and they would interfere with each other. It is thus necessary to transmit the data streams in a way that allows the receiver to disentangle them. This project investigates a recently discovered such transmission method that is especially suitable for wireless links where the transmitter and receiver can see each other, and are fixed - such situations can occur, e.g., for wireless backhaul (essentially, the connection from the base station to the internet), or wireless communication between servers in a data center. The method transmits different data streams on waves that have different orbital angular momenta (OAM), which describe the phase twist of a propagating wave. The current project investigates the fundamental science, as well as potential practical problems of OAM systems, and aims to assess their potential as a revolutionary method for drastically improving speed and efficiency of wireless data connections.
To make the above description more precise, OAM describes a phase twist of a propagating wave, and is different from the well-known polarization. Beams with different OAM are orthogonal to each other when propagating along the same beam axis, so that beams emanating from the same aperture, using the same time-frequency resources, can carry independent data streams. The project investigates the fundamentals as well as the real-world behavior of OAM multiplexing systems operating in the millimeter wave frequency range, specifically, 1) Explore the basic factors limiting OAM communication system capacity, in particular the pathloss of different OAM beams, and investigate measures to optimize the system capacity; 2) Investigate fundamental issues of OAM beam propagation, including the impact of attenuation and mode conversion by specular reflection and diffuse scattering; 3) Evaluate residual crosstalk between OAM channels, arising from various propagation environments; 4) Find new methods for compensating OAM channel degradation, in particular compensating for multipath propagation and atmospheric distortions; and 5) Analyze the connection between OAM and spatial multiplexing, and see how they can be combined in a manner that is most beneficial for implementation.
