**By John Houghton on December 12, 2017** | Leave a Comment

Let’s say you’re not the technical type and someone in a meeting asks, “If we go from 16 QAM to 64 QAM, how much more bandwidth will we get?” (QAM means Quadrature Amplitude Modulation—a way of packing more data into the same space.) 64 divided by 16 is 4, so we get four times the capacity, right? Not so. In order to get the right answer, you have to think about the symbol rate. Understanding symbol rate seemed complicated for me at first, but I’ve figured it out with the help of my NeoPhotonics Ph.D colleagues, so let me show you how symbol rate works. Let’s make it understandable by showing you a table with the answers and then we’ll explain it further below.

The multiplier column above shows how many times faster than On-Off Keying (OOK – some people call this NRZ – non return to zero) is compared to a higher order modulation format. From this you can determine 64 QAM provides 50% more bandwidth than 16 QAM (6 divided by 4). If you’re going from QPSK to 64 QAM you’ll get 300% more bandwidth or 3x. OOK to 64QAM gives you 6x more bandwidth. Dual polarization can further increase the capacity by two fold.

**Symbol Encoding Examples**

As I was writing this article, I couldn’t really see how bit rate related to symbol rate without drawing out symbols, so for those of you who want more detail, let’s go through some examples. Let’s say we wanted to transmit 12 bits, and here they are: 001100110011. Let’s imagine that these 12 bits represent the first part of a jpeg picture of a flower, and we’ll start by transmitting it with OOK.

When we transmit data, we can transmit using bits or symbols. A bit takes the same space as a symbol. Therefore, if we transmit 12 bits using OOK, we will use the equivalent space of 12 symbols. OOK is our baseline.

We said that a bit takes the same space as a symbol, and the other side of that is that a that symbol can represent more information. If we transmit 12 bits using QPSK (a.k.a 4 QAM), we will use 6 symbols. To understand this, let’s dive further. First of all, here is the constellation for QPSK:

When we use QPSK, instead of sending a 0 or 1 we send data via phase modulation, which gives us 4 different symbol possibilities 00, 01, 10, 11 (versus 2 possibilities). To understand phase modulation, click here. Now, let’s now encode our 12 bits: 001100110011.

The first two bits are 00. Do you see a 00 in the above constellation diagram? It’s in the lower right. Therefore, instead of transmitting 00, which is two bits, we transmit the whole symbol, in one bite:

We’ve now encoded the first two bits (00) into a symbol. Let’s encode the next two bits (11). Do you see a symbol that represents 11? This is represented in the graphic below as the upper left quadrant is checked off.

We’ve just encoded the first two symbols. You get the idea, so to save time, here is how our 12-bit “string of numbers” looks:

There are 6 symbols. Therefore, using QPSK, we can transmit 12 bits of information in the space of 6 bits by using 6 symbols. Bandwidth is doubled using QPSK versus OOK. As we said before, to double this again, you can add polarization. For more information about polarization, click here. Now we know that QPSK has four times the data rate of OOK if you use polarization.

16 QAM has four bits per symbol, so to encode our 12 bits, we break them up into three groups of four, thereby only using 3 symbols.

64 QAM has a more dense symbol that represents six bits per symbol, so to encode our 12 bits, we break up the 12-bit string into two groups of six, thereby only using two symbols. We’re transmitting 12 pieces of information, which used to take up 12 spaces, but with the efficiency of 64 QAM, we’re transmitting those 12 pieces of information in two symbols, which take up only two spaces. In the real world, anything that is QPSK (4 QAM) or more, adds polarization on top, making it twice as efficient again; therefore, we can now transmit 12 pieces of information and do it in the bandwidth (space) required for 1 piece of information.

So back to our original question, if you go from 16 QAM to 64 QAM, you get 50% more bandwidth.

As you can see from the table, the higher QAM you go, the more your returns diminish. For example, going from OOK to QPSK gets you 2x more bandwidth, which is a lot, but going from 64 QAM to 128QAM only gets you a 17% improvement, which is not very much. Going from 512 QAM to 1024 QAM only gets you 11%, which is even less. Therefore, QAM is useful only to a point, and we’ll definitely want to keep using it, because 64 QAM gives us 6x (six times) more bandwidth than OOK.

**So How Do We Get More Bandwidth?**

Getting more bandwidth from QAM increasingly becomes an uphill battle as we reach farther into the higher orders. Keep in mind also that the greater the QAM the less distance you’ll be able to transmit because QAM becomes more sensitive to noise. One next step to bandwidth is by using additional wavelengths, not just DWDM, but additional bands, and this is the subject of a future article.

John Houghton is a Silicon Valley entrepreneur, technology innovator, and head of MobileCast Media.