Have you ever heard of coherent transmission? Do you know the terms: QPSK, 16QAM or 64QAM? Or maybe, 32Gbaud or 64Gbaud ring a bell? How many times have you wondered what it all means? Or maybe you even felt confused during a conversation on coherent transmission but you were ashamed to ask for clarification?
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. Let me try to clarify this subject without using a great number of sophisticated terms and concepts. Of course, the text will include some professional vocabulary, like amplitude modulation, phase modulation or polarisation, but I will limit it to a minimum.
What was it like before? Well, until now, we have been using optical transmission based on something what is called on-off keying (OOK). It simply means that in order to send data we switch the light on and off. This was possible until we reached the top limit of speed for switching on and off the light. Thanks to OOK innovations, we could gradually move the limit but still it was not enough. What we needed was a huge increase of capacity, and we managed to reach it through complicating the transmission.What does this complication mean?
- Amplitude modulation
Modulation means that we affect something and change it in some way. In order to visualise it, imagine something more tangible, for example, sea waves. Let’s assume that the wave amplitude refers to its height. In an optical communication system based on OOK our focus was only on whether the wave appeared or not. Imagine that you can make sea waves for the purpose of sending signals, and, for example, a big wave denotes one thing and a small wave denotes another thing, like dots and dashes in the Morse Code. If we could distinguish 4 different sizes of waves, we would be able to send much more information within the same time boundaries.
- Phase modulation
In order to understand the meaning of phase, let’s use analogy again and as an example let’s discuss a project. In its basic form, it consists of three elements: an idea, execution and result. Similarly, an approaching sea wave also has three elements: the front, the peak and the back. Suppose we are most interested in its peak and when it is reached. If the waves appear on the shore in equal time slots, the peak of every new wave is reached precisely after every 60 seconds. Now imagine that a person on the other shore can affect it and, in consequence, change it. What would it mean if the wave peak came in 0th second and, let’s say, in 30th second? Analogously to amplitude variations, suppose we have defined 4 phases – then, it means that the wave peak may appear in 0th, 15th, 30th and 45th second
Similarly to sea waves which can rise and fall, a buoy floating on the water will move up and down. It is the same case with vertical polarisation. Now, you will have to strain your imagination a bit more J Supposing we took our sea waves and rolled them over so that they could undulate horizontally, not vertically, then the person standing on the other shore and expecting to see vertical waves would see… nothing. But, 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.
And now it’s time for a very good news. Imagine, that you can combine the three phenomena described above. Notice that when we set together a few waves of different size (amplitude modulation) with the times of their appearance (phase modulation) and then we add two planes (polarisation), then it turns out that we can carry much more information within the same time boundaries. So, what is coherent transmission? In simple words, it is a system which combines amplitude modulation, phase modulation and polarisation in order to send bigger amounts of data in optical transmission than it is possible by a simple OOK system.
Depending on how many phase and amplitude levels the modulation generates, we can denote it as 4QAM, 8QAM, 16QAM, etc. When defining the type of coherent transmission we often speak of QPSK (Quadrature Phase Shift Keying) modulation, which, for simplicity, can be just abbreviated to 4QAM (Quadrature Amplitude Modulation). Another significant parameter is Gbaud which specifies the number of times the light may change in a second. And thus, 32Gbaud means that the light changes 32 billion times per second. Combining QPSK (the four levels ) we can send them 32 billion times per second obtaining more than 100 billion bits per second.
And thus, we have covered the major issues connected with coherent transmission. I 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.