Radio frequency
Some basic terms and definitions
The term radio frequency (RF) is central to understanding wireless technologies. Of course, it applies beyond wireless networking but as we are interested in Wi-Fi in particular, it is also central here.
The radio frequency is the term which refers to a part of the electromagnetic spectrum (see some details in Table: Electromagnetic Spectrum).
Most commonly used Wi‑Fi frequencies are located in the microwave region (300MHz-300GHz) of the electromagnetic spectrum. In general, IEEE 802.11 technologies use several separate, region-dependent bands ranging from sub‑1 GHz to approximately 60 GHz.
| Subrange | 802.11 relevance |
|---|---|
| Sub‑1 GHz | 802.11ah; 802.11af in permitted TV-white-space spectrum |
| 2.4 GHz | Mainstream Wi‑Fi |
| 3.6 GHz | Limited/regional 802.11 use |
| 4.9 and 5 GHz | Mainstream and specialized Wi‑Fi bands |
| 5.9 – 7.1 GHz | 6 GHz Wi‑Fi; exact range depends on region |
| Around 60 GHz | 802.11ad/ay |
It is important to note that the term frequency is closely connected to the term wavelength. They have an inverse relationship expressed by the formula: f = v / λ where v is the wave’s propagation speed. For electromagnetic waves in vacuum, v = c.
So, the higher the frequency, the shorter the wavelength.
An RF communication system uses an electromagnetic carrier wave to carry information. Think about light or sound which can also be considered as carriers of information.
The process of modulation is used to manipulate the carrier to represent digital data.
A simple example of modulation is the following idea:
Imagine a transmitter normally sends a carrier at 2.400 GHz. To transmit digital bits, it could do this:
- 2.400 GHz → means 0
- 2.401 GHz → means 1
The receiver measures the incoming frequency and reconstructs the bits. This example represents binary frequency-shift keying, or BFSK. This is not used by Wi-Fi but represents the idea of modulation in a simple way.
One important distinction is: carrier frequency and modulation are related but different concepts. A Wi-Fi channel might be centered around, say, 5 GHz, while modulation creates carefully controlled changes in the RF signal around that frequency to carry data.
Other modulation methods vary the carrier’s amplitude or phase. Quadrature amplitude modulation (QAM), widely used by Wi‑Fi, represents information using controlled changes in both amplitude and phase. We will go deeper into this question later.
A carrier wave is an electromagnetic wave used to carry information. After modulation, it becomes a modulated carrier and forms an RF signal.