Coherent Transmission Explained in Simple Terms

By John Houghton on August 10, 2017 | Leave a Comment

Why Explain Coherent Transmission in Non-Technical Terms?

At the optical industry’s main trade show of the year, OFC (Optical Fiber Conference) 2017, I was scouting for good blog topics for this educational blog and I discovered that many non-technical folks would like a non-technical explanation of coherent transmission. I talked to a sales rep on the exhibition floor who says that everybody uses the term but they don’t quite understand what it means in any detail.

Coherent transmission has been accurately explained dozens of times, but in a technical way, and it’s still not quite clicking for non-techies. So what I’ll do here is explain coherent transmission by using analogies. Now, analogies eventually break down at some point, but they may help you to understand the concepts of coherent transmission which can enable you to be more conversant with the concepts.

What Will I Understand by Going Through this Example?

Have you ever heard anybody talk about 16 QAM or 64 QAM? 64/64? QPSK? Higher order modulation? Amplitude modulation? Phase modulation? Polarization? These all fall into the bucket of Coherent Transmission. When we’re done here, you’ll have a much better idea of what these are and how they work.

What Coherent Transmission is Not

Coherent transmission does not necessarily involve using different wavelengths of light to transmit more data. These different wavelengths are called lambdas, and it is common to put 96 different wavelengths (or more simplistically, colors) of light onto one fiber. Coherent transmission can work in fibers with multiple lambdas, but to keep things simple, let’s explain coherent transmission without talking about lambda.

Why Do We Need Coherent Transmission?

Before we had coherent transmission, we would send data via simple on-off keying (OOK). This means that the light is switched on and off to send data. This was very effective for transmitting data until we couldn’t switch the light on and off any faster. With further innovation OOK was pushed further, but we always hit a limit. What we needed was a big jump in data rates and not just incremental steps. We got this jump in bandwidth via higher order modulation. In this case “higher order” just means something more complicated than OOK. Let’s talk first about amplitude modulation.

What is Amplitude Modulation?

First of all, to modulate something means to exert an influence and change that thing in some way. For example, if you’re singing softly and then you sing louder, you’ve modulated your voice by adding more force (amplitude) to it. Therefore, you have changed (modulated) the power (amplitude) of your voice. Amplitude can be used to convey meaning. For example, when you sing softly, this conveys tenderness, but when you sing loudly, you convey passion or power, etc. In the realm of light, the amplitude of the light can be used to transmit data, as in “on” for a one and “off” for a zero in OOK. Over and above on-off keying (OOK), we might say that if we have four levels of brightness (amplitude) of light, and each of those four levels of brightness can represent its own state of data.

Amplitude modulation is used in radio transmission—you’ve heard of AM radio. It means the data, in this case sound (such as music), is carried on the outer envelope of the radio wave. The outer envelope conveys the information. In AM Radio, the amplitude of the radio wave can be varied very quickly and in sophisticated detail so as to transmit any sound.

For another amplitude analogy, let’s think about waves in the Ocean. The amplitude of the wave, means the wave height (outer envelope). If you could create waves in the ocean to send signals, like smoke signals, then a big wave could mean one thing and a small wave could mean another. Let’s say you had four different break points for wave sizes, each meaning something different. This way, you can send much more data with four different wave sizes, versus just one.

Amplitude modulation can be combined with other types of modulation, to get even more data throughput. Another such type of modulation is called phase modulation. Let’s talk next about that next.

What is Phase Modulation?

First, to define phase, let’s think of it as phases of an IT project at work. Each project has a beginning, middle, and an end. Waves in the ocean also have phases. A wave in the ocean coming at you has a front, peak, and back side. Let’s say we’re most interested in the peak and when it arrives. Let’s say that you’re looking at your watch, and you notice a wave comes into the shore at the top of every minute. If somehow a person on the opposite shore could control when the wave was sent, this could be used to communicate information. If one wave comes at 00, it means one thing, versus the next at 30 seconds, which means something else, versus 45 seconds, and so on.

Just like the example of ocean waves being sent at different times, so light waves can be modulated to carry data by shifting the of the light forward or backward. You could have four pre-defined phases (00, 15, 30, 45) or you could subdivide even further. Hold that thought on phase modulation and we’ll show how to combine everything to make Quadrature Amplitude Modulation (QAM).

So that’s phase modulation and it can be used in combination with amplitude modulation. The final way we can encode more data is another technique that also starts with a “P” and that is polarization. So don’t confuse phase modulation with polarization because both start with a “P.” Let’s talk about polarization next.

What is Polarization?

Think of the North and South Poles of the Earth and how they are aligned vertically. They have an orientation—they have two opposing attributes, polar opposites with a top and a bottom (there is no horizontal side-to-side when we’re thinking about the vertical polar opposites). Now that you have conceptualized what polar is, let’s go back to thinking about waves in the ocean. Because of gravity, the waves are aligned so the water can only go up and down (simplistically), just like a buoy, bobbing up and down in the ocean. We have just described the waves of the ocean as having vertical polarity. Note that my editor points out that Polarity and Polarization are not very similar, so the analogy isn’t great, but I hope you get the idea that waves have an orientation.

When you move away from water and into radio and light waves, you can have not only vertical polarity, but you can have a horizontal polarity as well. With the two polarities, you can use them both to increase transmission rate. To use an interesting radio example, if you take two walkie-talkies with telescopic antennas, both antennas need to have the same polarization (orientation) in order to communicate. If one antenna is held horizontal and the other vertical, the radios can’t communicate with each other (or the signal might be very faint). When they’re both returned to the same orientation, all of a sudden communication is easy. The same is true with light waves. Because the two polarizations of light don’t conflict, they can both be used at the same time.

Therefore, it is possible to double the transmission rate of data via light in a fiber by transmitting it with both a vertical and a horizontal polarization.

Putting it all Together

There is very good news in all of this. All of these techniques we’ve talked about can be combined: amplitude modulation, phase modulation, and polarization. So when you put multiple levels of amplitude modulation together with multiple levels of phase modulation, you get quadrature amplitude modulation (QAM). This just means that both the amplitude and the phase are being changed to represent the data. Polarization is usually already built into data rates involving QAM and not broken out separately, so just assume polarization is built into any discussion about QAM or coherent transmission. Now that we understand these things, we can define coherent transmission.

What is Coherent Transmission?

In simple terms, Coherent Transmission is a system that combines amplitude modulation, phase modulation, and polarization to transmit greater amounts of information through a fiber optic cable than is possible with simple on-off keying.

What is QPSK?

You might come across the acronym QPSK when talking about coherent transmission. QPSK means Quadrature Phase Shift Keying. In fiber optics, it means shifting the phase of the light wave with four different possibilities while keeping the amplitude fixed. QPSK is synonymous with QAM. Simplistically, you can think of QPSK as 4 QAM.

What is Higher Order Modulation?

Higher order modulation is a modulation technique that has more levels of phase and amplitude. Other examples are 8 QAM, 16 QAM, 32 QAM, 64 QAM, etc, where you are merely setting more granular “levels” of phase and amplitude modulation, which allow you to transmit data at higher rates.

What does 64/64 mean?

It means 64 Gbaud at 64 QAM. Baud rate means the number of pulses per second used to transmit data. 64 Gbaud means that the light changes 64 billion times a second. (Standard coherent systems change at half that speed, 32 Gbaud). 64 QAM means a signaling scheme with 64 different signal states. If you combine them together you can transmit more than 600 billion bits per second!

Conclusion

Now that we’ve gone through analogies to explain coherent transmission, next time QAM, QPSK, or 64/64 come up, you can be more confident and conversant on the topics. Thanks for reading!

John Houghton is a Silicon Valley entrepreneur, technology innovator, and head of MobileCast Media. This article was edited by Ferris Lipscomb, Ph.D.

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