The fundamentals of coherent transmission

Understanding coherent transmission

Have you ever heard of coherent transmission? Are you familiar with the terms QPSK, 16QAM or 64QAM? Does 32Gbaud or 64Gbaud ring a bell? How many times have you wondered what it all means? Conversations about coherent transmission can be extremely confusing, and maybe you never dared to ask for clarifications. There are lots of articles on coherent transmission on the Internet, but unfortunately, most of them are written in a complex technical language and are hard to understand. We have good news for you — this is what you need to read to understand coherent transmission in simple terms. The text will include some jargon, like amplitude modulation, phase modulation or polarisation, but we’ll walk you through it.

On-off keying

What was it like before coherent transmission? Until recently, optical transmission was based on something called on-off keying (OOK). This means that to transmit data, the light was switched on and off. This was possible until we reached the maximum speed limit for switching the light on and off. Thanks to OOK innovations, we could gradually increase this limit, but it was still not enough. What we needed was a huge increase of capacity, which was only possible by complicating data transmission. What does this mean?

Amplitude modulation

Modulation means affecting, or and changing data transmission in some way. It might be helpful to visualise this process with something more tangible than data, such as sea waves. Let’s assume that a wave’s amplitude refers to its height. In an optical communication system based on OOK, we only focused on whether the wave appeared or not. Now, imagine making sea waves for the purpose of sending signals, where big waves denote one thing and small waves another, such as dots and dashes in the Morse Code. By distinguishing among four different sizes of waves, we will be able to send much more information within the same time frame.

Phase modulation

To understand the meaning of phase, let’s use another analogy. Let’s take a project consisting of three basic elements, an idea, execution and result. Similarly, an approaching sea wave has also three elements, the front, the crest and the back. Suppose we are especially interested in its crest —the wave’s highest point — and when it is reached. If the waves appear on the shore at regular time slots, the crest of every new wave will be reached after exactly 60 seconds.

Now, imagine that a person on the opposite shore can affect and change this dynamic. What would be the consequences if the wave crest was reached at second zero and, let’s say, at the 30th second? Similarly, when considering amplitude variations, if we define four phases, the peak of our wave may appear in 0th, 15th, 30th and 45th second.

Polarisation

Like sea waves which can rise and fall, a buoy floating on the water will move up and down. Vertical polarisation works the same way. Now, you will have to stretch your imagination a little further. Supposing we took our sea waves and flipped them so that they could undulate horizontally, not vertically, the person standing on the opposite shore and expecting to see vertical waves would see…nothing. However, if we could simultaneously make use of the two waves, both vertical and horizontal, then we would be able to carry twice as much information.

Now, we have some very good news for you. Imagine if we could combine the three phenomena described above. Notice that when we combine a few waves of different sizes (amplitude modulation) with the times of their appearance (phase modulation) and then add two planes (polarisation), we can carry much more information in the same time frame. So, what is coherent transmission? In simple terms, it is a system that combines amplitude modulation, phase modulation and polarisation to send larger amounts of data in optical transmission than it is possible using a simple OOK system.

Quadrature Amplitude Modulation (QAM) and Gbaud

When defining the type of coherent transmission, we often speak of Quadrature Phase Shift Keying (QPSK) modulation, which, for simplicity, can be just abbreviated to Quadrature Amplitude Modulation (4QAM). Depending on how many phase and amplitude levels the modulation generates, we can denote it as 4QAM, 8QAM, 16QAM, etc.

Another important parameter is Gbaud, which specifies how many times the light may change in a second. So, 32Gbaud means that the light changes 32 billion times per second. Combining QPSK on the four levels, we can send them 32 billion times per second, obtaining more than 100 billion bits per second.

We have now covered the basics of coherent transmission. We hope that the next time you will participate in a discussion on 16QAM, 64Gbaud and other topics related to coherent transmission, you will feel more confident in this subject. If not, feel free to give us a call!

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