What is Orthogonal Frequency Division Multiplexing (OFDM)?

Due to the laws of nature, we can only cram so much stuff into our airwaves. The amount of information we can push wirelessly is extremely limited, and because of that, tightly regulated. 

With a touch of voodoo, though, we can harness physics and make our airwaves more efficient. orthogonal frequency division multiplexing (OFDM) is that voodoo.

What is Orthogonal Frequency Division Multiplexing?

OFDM is a variation of frequency division multiplexing (FDM). Just in case you aren’t familiar with FDM, let’s go over that first. It’s important to understand before diving into OFDM. FDM is a technology used in things like radio broadcasts, satellite communications, and even DSL. Here’s how it works. Please excuse our liberal over-extension of some of these terms. 

What is a radio sine wave?

Wireless data transmissions are sent at specific frequencies. These frequencies are signals that ‘vibrate’ in a sine wave. The frequency denotes how fast that sine wave moves each second. A 2.4 GHz Wi-Fi signal vibrates 2.4 billion times a second for example. 

The more hertz in a frequency, the more data it can carry. But the higher the frequency, the more likely it is to bounce off things like walls, trees, or raindrops. 

Spectrum and sub-frequencies

Frequencies are divided into spectrums. For example, Wi-Fi uses a 2.4 or 5 GHz spectrum. Due to physics, we have a limited amount of usable spectrum, and this is also why it is tightly regulated. Because spectrum is limited, it is divided into sub-frequencies. 

If you ever configured a wireless router, you’ll have noticed these devices can be assigned specific channels to operate on. These channels are the sub-frequencies for the 2.4 GHz and 5 GHz frequency bands. 

They are more than very specific frequencies. Channel 6 of the 2.4 GHz spectrum is 2.437 GHz. That channel has a width of 2.426 to 2.448 GHz.

Notice how that channel width is a range of frequencies around that 2.437 GHz. More specifically, there is 11Mhz of cushion on either side of that 2.437 GHz frequency.

Those last two sentences are very important. Pay special attention to them because you now understand what FDM is. 

FDM has cushion

Each FDM sub-frequency has a layer of cushion. Over-exaggerating our Wi-Fi example above, channel 7 of the 2.4 GHz spectrum would be 2.459 GHz. Otherwise, the radio frequencies between 2.437 GHz and 2.459 GHz would be cushion. That cushion is added between the operating frequencies so they don’t bump into each other and cause interference. 

One final note for this section. The real frequency for channel 7 of the 2.4 GHz Wi-Fi spectrum is 2.442 GHz.  You’ll learn why in the next section. Likewise, that 22 MHz of cushion (11 MHz on each side of the frequency) around each Wi-Fi channel above isn’t cushion. That spectrum is actually used for DSSS encoding, and that is a different topic for another day. 

Ready to Learn Wireless Technologies?

There’s so much to learn about how wireless communication technologies work, though. If you’re interested in learning more, start studying for the Certified Wireless Network Administrator (CWNA) exam. 

The material you need to study for the CWNA covers things like the physics of light, how we harness electromagnetism for communication, and why radio waves can be particularly grumpy if they meet up at the wrong time. Go ahead and get started with our CWNA online training course today! 

How Does OFDM Work?

OFDM is a variant of FDM. It stands for orthogonal frequency division multiplexing. Orthogonal is the keyword in that acronym. 

In the section above, we explained that FDM uses padding between sub-frequencies to prevent radio signals from causing interference with each other. OFDM doesn’t use any padding. Here’s how this technical voodoo works. 

Preventing Frequency Collision

Remember how radio frequencies travel in a sine wave? As it turns out, radio signals typically only get cranky when they are near each other if they are at a similar point or travel in their sine waves. 

On the other hand, if one signal is at its topmost peak while the other signal is in a neutral state (in the middle of its topmost and bottommost peaks), both signals will pass by each other. 

This is how OFDM works. Instead of adding padding between sub-frequencies in a spectrum so they don’t collide, each sub-frequency is perfectly timed so that when one sub-frequency is at its topmost peak in a sine wave, both neighboring frequencies are in a neutral state. 

What Does Orthogonal Mean?

Each sub-frequency travels orthogonally to the others. If we look up the definition for orthogonal, it means of or involving right angles. Here’s where right angles fit into this tech voodoo. 

When a radio signal travels in a sine wave, it moves up and down in a wave pattern as it travels forward. It only moves in the up or down direction between peaks in that wave, though. Once that radio signal reaches the top or bottom of its sine wave, it must reverse course so it can move in the other direction. Otherwise, it wouldn’t travel in a sine wave. 

When that radio wave reverses course to move in the opposite direction of its sine wave, there is a very brief and specific moment where it’s going straight instead of up or down. When a radio wave peaks and changes direction, it’s at this very exact moment the radio waves to the left or right of it are halfway through traveling up or down their sine waves. Because all three frequencies are perpendicular to each other, or technically orthogonal since each frequency intersects the others, they don’t cause interference.