Types of Digital Modulation

Types of Digital Modulation

In this edition of The Wi-Fi Rabbit Hole, I don’t go “down the rabbit hole”. Instead, I just felt inspired to share my love of sending information through the modulation of waves.

To get started, lets go over what a wave has to offer: wavelength, amplitude, and frequency. If we alter any of these aspects of a wave, we can encode information. Consider the following scenario:

Example: We have a door and we want to use it to send secret messages to our roommate. First, we agree that we are going to send messages in binary. This makes it easier to understand, because all we need to differentiate between is one and zero.

The initial schools of thought were akin to slamming the door to transmit messages. First, you need a carrier signal so that they know something is being sent, so let’s start slamming the door once ever second. Then there are two options when slamming the door: slam the door harder (amplitude) or slam the door faster (frequency). Thus, if you are listening to the door, an unchanged slam on the second (unchanged from the carrier) is a zero while a harder slam or four slams in one second would indicate a one.

We have just created very rudimentary forms of Amplitude Shift Keying (slamming the door harder) and Frequency Shift Keying (slamming the door faster). This is how your standard car stereo sends information. Here is a great picture to explain:

Now continuing with the door idea, what happens when you think outside the box? Instead of slamming the door to send information (it was kind of loud anyway) lets utilize the position of the door. In this scenario, our roommate looks at the door every five seconds. When he looks at the door, he compares its current angle to where it was last time he checked.

Example: The door starts closed. At t=5, we leave the door at a right angle to the doorjamb. At t=10 we close the door again. Lastly, at t=15 we leave the door closed.

In this example, because the door changed positions between start and t=5, that is a one. Then it changes again, one. Finally, it stays closed, which is a zero. We have just created Phase Shift Keying and sent the message 011 (assuming we sent LSB first).

Crazy right? It gets better! Once phase modulation was discovered, the thought occurred, “Hey! There are more than two positions on a door, we can encode multiple bits per position!” In laymen’s terms the door can be: closed, a quarter open, half way open, three quarters open, and fully open. Now, to keep things in line with binary, we will only use four positions (because two bits in binary is four), so lets do away with half open.

Keep in mind, this sort of phase modulation measures relative to the previous position. So for this example lets use the following scheme: no change = 00, one position dif = 01, two position dif = 10, three position dif = 11. Also, let’s say we always count upwards with closed being the lowest and full open being the highest.

Example: The door starts closed. At t=5, we leave the door one quarter open. At t=10, we leave the door fully open. At t=15 we leave the door fully open. At t=20 we leave the door three quarters open.

In this example, because the door changed positions by one position at t=5 that is 01. Then it changes by two positions, 10. Next, it stayed in the same position, 00. Finally, the door moved three positions which is 11. In total, we sent 11000110.

To me, modulation schemes are fascinating. It is amazing how far we have come and just how much information is conveyed with minute changes to a wave. It certainly sends me down the rabbit hole. See you next time!

Further Down (extra reading): Just in case anyone is wondering, the modulation used with 802.11n and 802.11ac is called Quadrature Amplitude Modulation (QAM).


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