Understanding Coherent Transmission
Have you ever heard of coherent transmission? Have you come across terms like QPSK, 16QAM or 64QAM? Or maybe 32Gbaud or 64Gbaud?
How many times have you wondered what all this means? Perhaps you even felt discomfort in talking about this topic, but did not have the courage to ask?
There are tons of studies on coherent broadcasting on the net, unfortunately most of them are written in heavily technical language and do not help to unravel this mystery. Let me introduce you to this issue without using a whole bunch of complicated concepts or formulations. In this text, expert vocabulary will surely appear, such as amplitude modulation, phase modulation or polarization, but I will keep them to a minimum.
And how did we do it before? Not only before, but today we still use optical transmission based on something we call on-off keying (OOK). This means that in order to transfer the data we turn the light on and off.
For a long time, this approach worked, until we finally reached the limit of the speed at which we can turn the light on and off. Thanks OOK Innovations we are gradually pushing this limit, but it is still not enough. We needed a surge in bandwidth, we managed to get it by “complicating” the transmission.
What is this complication about?
- Amplitude Modulation
Modulating something means we have an impact on it and somehow we change it. To make it easier to imagine, let's relate it to a more tangible reality, that is, to the waves at sea. The amplitude of the wave will be its height.
In the optical communication system based on the OOK, we were only interested in whether there is a wave or not. Suppose we can create waves in the sea to send signals, and for example, a big wave means one thing and a small wave means another. Suppose we can distinguish 4 different wave sizes. In this way we can send much more information at the same time.
- Phase modulation
To understand the essence of phase, let us again use the analogy. Take, for example, any project. In the simplest terms, it will consist of an idea, an implementation and a summary. By analogy, a wave at sea approaching you has a front, a top and a back. Let's just say that we are most interested in the summit and when it will come.
Suppose the waves appear on the shore at equal intervals and exactly every 60 seconds the peak of the next wave occurs. What if the person on the opposite shore could influence it and change it? It would be different if the peak of the wave appeared at 0 second, and something else if, for example, at 30. In a similar way as in the case of amplitude, suppose that we have defined 4 Phases - this means that the peak of the wave can appear in 0, 15, 30 and 45 seconds.
- Polarization
Remaining with the analogy of sea waves, which can rise and fall, if we observe buoyancy on the water, it will move up and down — and so it is with vertical polarization.
Now our imagination will have to be strained a little 😊 What if we took our sea waves like this and “turned them on their side” so that they “wave” horizontally, and not vertically? Someone standing on the opposite shore and expecting a sea undulating vertically would not see anything... That is, if we could use these two seas - both vertical and horizontal - at the same time, then we could move Twice as much information.
And now it's time for some very good news. Imagine that all 3 techniques described above we can connect. So when we put together several wave sizes (amplitude modulation) with several times of their arrival (phase modulation) and add two planes to it (polarization), then it turns out that we are able to transfer much more information at the same time.
So what is coherent transmission?
Simply put, it a system that combines amplitude modulation, phase modulation and polarizationto transmit larger amounts of data in optical transmission than when using a simple OOK system.
Depending on how many phase and amplitude levels the modulation will introduce, we can have 4QAM, 8QAM, 16QAM andtp. Often in the definition of the type of coherent transmission, QPSK modulation appears, in simple terms you can think of it as 4QAM.
Another important parameter is Gbaud, which means how many times per second light can change. For example, 32Gbaud means that light changes 32 billion times per second. And so combining QPSK (4 states) with the ability to transfer them 32 billion times per second, we get more than 100 billion bits per second!
This is how we went through the issues related to coherent transmission. I hope that the next time you talk about 16QAM, 64Gbaud and other issues related to coherent transmission, you will feel confident and more comfortable.
Have you reached this point and are wondering how to put it into practice?
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