[ELI5] For wifi why do we use a whole channel of 20MHz instead of a couple frequencies?

Question

So I get why we can’t use ONE single frequency, it’s sine and we can’t transfer data through it, it’s always gonna be constant. But what about using like 10 or 20 frequencies, something like from 2.4GHz to 2.4GHz10Hz (wrong syntax but you get what I mean). and use it to transfer the data.

I’m gonna give an other potentially wrong example, for radio, don’t we use one single frequency? we set our receiver to that frequency and get the transmitted data from the transmitter? or is it different from wifi because radio is simplex and wifi is half-duplex?

This ended up requiring 2 explanations (wifi and radio) but I’m more interested in the wifi use of a whole 20MHz channel instead of a smaller number of frequencies. Thanks.

In: 2

Answer 1

If you use a single frequency you can only send a constant sine wave. If you just turn the transition on and off the result is that you create signals on other frequencies.

Any signal that changes so you can encode information will contain signal at multiple frequencies, the faster you change the signal the more information you can encode but it results in a signal with a larger frequency range.

Are real radio transmission will have noise in the signal you send, for natural and another radio transmitter,s and noise from the electronics in the receiver. This limits how small changes you can do to the signal and encode data. The higher the single power compare to noise the more data you can transmit. This is called the signal-to-noise ratio.

from noise, signal power, and channel bandwidth you can mathematically calucalte the maximum bandwidth for a channel. It does done by [https://en.wikipedia.org/wiki/Shannon%E2%80%93Hartley_theorem](https://en.wikipedia.org/wiki/Shannon%E2%80%93Hartley_theorem)

the max bandwidth in bits is C= B Log2 (1 + S/N) where B is the bandwidth in hertz. lon2 is the base 2 logarithm and S/N is the signal to noise ratio

So for a given signal to noise ratio, the capacity is directly proportional to the signal bandwidth

So your example with 10Hz channel that starts at 2.4 GHz will have 10/20 000 000= 1/2 000 000 of the capacity of a 20 MHz channel

Because there is an inherent limitation in the amount of data you can transmit on a channel with the noise you need to increase the bandwidth of the channel to be able to transmit more data.