Fundamentals of Wireless Communication Systems Using Orbital Angular Momenta

无线数据服务在过去20年中经历了巨大的增长：不仅用户数量和连接的设备数量大幅增加，而且连接速度也增长了1000倍。所有这些都带来了前所未有的各种无线服务，改变了人们的工作、互动和闲暇时间的打发方式。然而，数据速率的持续增长也带来了巨大的挑战，因为用于信号传输的频谱是一种有限而宝贵的资源。因此，既要使用新的频谱，又要尽可能有效地利用可用的频谱。芯片制造的进步为使用毫米波的低成本通信系统打开了可能性，毫米波的波长比目前使用的蜂窝系统短得多，而且有很多频谱可用。尽管如此，考虑到数据使用量的增加，这种频谱也必须得到有效利用。显著提高频谱效率的一种方法是使用相同的频谱一起传输多个数据流。然而，在正常设置中，接收器然后获得这些数据流的混合，并且它们会相互干扰。因此，必须以允许接收器将其解开的方式来传输数据流。本项目研究最近发现的这种传输方法，该方法特别适用于发射机和接收机可以彼此看到并且是固定的无线链路--这种情况可能发生在例如无线回程(实质上是从基站到因特网的连接)或数据中心中的服务器之间的无线通信。该方法在具有不同轨道角动量(OAM)的波上传输不同的数据流，该轨道角动量描述传播波的相位扭曲。目前的项目研究OAM系统的基础科学以及潜在的实际问题，旨在评估其作为一种革命性方法的潜力，以显著提高无线数据连接的速度和效率。为了使上述描述更准确，OAM描述了传播波的相位扭曲，不同于众所周知的极化。具有不同OAM的波束在沿相同的波束轴传播时彼此垂直，因此从相同孔径发出的波束使用相同的时频资源，可以携带独立的数据流。该项目研究了工作在毫米波频率范围内的OAM多路复用系统的基本原理和实际行为，具体地说，1)探索限制OAM通信系统容量的基本因素，特别是不同OAM波束的路径损耗，并研究优化系统容量的措施；2)研究OAM波束传播的基本问题，包括镜面反射和漫反射引起的衰减和模式转换的影响；3)评估不同传播环境引起的OAM信道之间的剩余串扰；4)找到补偿OAM信道衰落的新方法，特别是补偿多径传播和大气失真；5)分析了OAM和空间复用之间的关系，以及如何将它们以最有利于实现的方式结合在一起。

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.