The Physics
All wireless communication including wifi occurs due to electromagnetic interactions which is one of the four fundamental interactions in nature.
In simplest terms this means that a moving charged particle will have a magnetic field associated with it which will be perpendicular to the direction of charge.
[If charge is moving up and down the magnetic force will move sideways. If charged particle is moving sideways magnetic force will move up and down]
Similarly a changing magnetic field will have an electric force associated with it that will be perpendicular to the direction of the magnetic field.
[In the figure above the electric charge represented by E in red moves up and down the magnetic field represented by B in blue moves sideways]
As a result of electromagnetism the charged and magnetic particles move forever. On and on like a wave. An electromagnetic wave.
This wave exhibits both electric and magnetic properties.
So that if you transmit a wave ,generated by an oscillating current, through an antenna at point “a” you can receive it ,as an oscillating current, from another antenna at point “b”.
The Mathematics
Now an electromagnetic wave has certain properties that can be represented mathematically. These are
- Wavelength — Length of the wave between to symmetric points. Such as length between highs and lows.
- Frequency — Number of times the wave oscillates/repeats.
- Amplitude — The max height of the wave. How high/low can it go. Amplitude is related to strength of the wave.
- Phase — The value of a wave at a particular time and a point. A wave is constantly changing. The phase determines it at a particular instant.
These properties allow us to perform calculations on waves. We can add,subtract and compare waves just like we do with real numbers.
The engineering
Engineering solves the practical problem of transmitting useful information on a wireless wave.
It does that by modulating the signal with information before transmitting it via an antenna.
The receiving antenna extracts the current from the wave which can then be demodulated into the information that was transmitted.
Wifi is concerned with providing the physical channel for transmission of information and a mechanism for accessing that channel.
[In osi model this corresponds to layer 1 and layer 2]
Some points to note
- Wifi frequency like 2.4 gHz causes the current to oscillate 2.4 billion times per second before the wave is transmitted. At the receiver this oscillating current is converted into digital bits that can be understood by computer.
- Most modern wifi devices use QAM to modulate (or transmit useful information on) both Amplitude and Phase of the wave.
- Wifi is a shared channel access. Meaning a single channel is shared by multiple devices (known as carrier sense multiple access) but only one at a time (for collision avoidance)
References
Nice video explaining the engineering behind wifi modulation
And some more articles explaining the engineering behind wifi
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