# High-Capacity Free-Space Optical(FSO) Communications Between a Ground Transmitter and a Ground Receiver via a UAV Using Multiplexing of Multiple Orbital-Angular-Momentum(OAM) Beams — PART 1

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Hi, how you doing? So, I'm going to start a series of articles with the objective to help me get my studies going. I'm having difficulties to focus on one thing and get this going. So, for practice my English and also help me get my studies going I'm going to make my notes here so I can learn and also help you guys to understand this paper(hope so). So, in this first article, we are going to explore this paper. Let's go. Important: this article is not following any standard, it has only informative and educational purposes.

Firstly, there is something you need get familiar with to fully(or partially) understand this explanation. Unmanned-aerial-vehicle(UAV) is, for example, the drone used in military operations.

So, the concept of this paper is to ensure, for example, that the link of the stations and UAVs is to minimize the probability of possible interception of data exchange to achieve enhanced privacy and security.

An important thing to highlight in this paper is that maximum crosstalk, i.e. the undesired effect that one circuit is generating on the other circuit that is sharing the same channel, was 19dB while the UAV hovering over the stations.

So, in the abstract we find the following quote:

We explore the use of orbital-angular-momentum(OAM) -multiplexing to increase the capacity of free-space data transmission to moving platforms, with an added potential benefit of decreasing the probability of data intercept.

I thought with myself: What the heck is the use of orbital-angular-momentum(OAM)-multiplexing? And why we got this behavior by consequence?

So I find(or remember) that every beam has a linear momentum and an angular momentum. For such beam, there is SAM (light spin angular momentum) that is strictly related to with the optical polarization and in particular with the so-called circular polarization. And OAM is related with the spatial field distribution, and in particular with the wavefront helical shape. In summary, OAM is related to the size of third column figures below. m could be higher than +/-3 , but if it was the wavefront would be composed of m distinct but intertwined helices.

So, we now understand what is OAM(hopefully). Now what? Since we are talking about multiplexing beams through orbital-angular-momentum we can get in a conclusion that this paper is controlling and sending data by changing the shaped wavefront. Which is a pretty good and complex idea. Now I have some questions in mind. How much is the energy cost in this application? Is it expensive to apply? (probably since is most used in military applications) I think more questions will show up as I read the paper and some of them will be answered.

Reading the introduction I started to get what the problem is related to. For military applications using UAV, the ground stations use lightwave frequencies because due to their great value they bring high capacity to the channel and low probability of intercept(LPI).

A specific technique has gained interest over the years because it could increase significantly capacity for fixed ground-based FSO links. This technique is known as space-division-multiplexing(SDM) and is based on the simultaneous transmission of multiple independent data-carrying beams. Mode-division-multiplexing (MDM) is a subset of SDM, where each of the multiple beams is a unique mode from an orthogonal modal basis set orthogonality minimizes crosstalk among the modes and enables efficient multiplexing at the transmitter, co-propagation of overlapping beams, and low-crosstalk de-multiplexing at the receiver.

For my understanding, basically, we could change the phase of orbitals —by controlling the helical modes(m) showed above — or the use of space for a light beam(similarly to TDM, with space)and modulate the transmission.

Since (until the date of this paper was published) there were little reports using OAM with moving objects there are two major challenges arising from their usage:

# Alignment

Low crosstalk and power-coupling loss rely on accurate axis alignment. If you think about it align two different objects when one is moving around is not a simple task. We need a technique that guarantees a reliable link between the two stations.

# Turbulence

This one strikes me the most. The turbulence generated by the propellers could distort the OAM beam's phase front, increasing power fluctuations, and intermodal crosstalk.

In their experiment, they use a ground transmitter at the same location as the ground receiver and the UAV carrying a retro-reflector. The UAV wasn't carrying any transmitter or receiver. For ease demonstration, they made two approaches: UAV hovering and moving at 0.1m/s. The objective was to explore the capacity of OAM multiplexing to decrease the probability of intercept data and increase the capacity of transmission.

This is the big picture of the article and only the motivation for you to read. The article is pretty interesting and has the potential to help take off studies using moving stations controlled by ground stations over FSO. The results are pretty expressive and when compared to previous studies using ground stations at the same distances. This is only the part 1 of this article and I plan to get through the details to share with you. If you read until here they what you think! Did you like it? Was it easy to understand? Did motivate you? Tell me what you think.