What Is Spectrum
Radio waves are a form of electromagnetic radiation which, like visible light or infrared, make up a portion of the entire spectrum. They cannot be perceived by human eyes or ears, and they are not harmful in the environment. Depending on their frequency (measured in hertz), radio waves can pass through solid objects and travel long distances. This makes them useful for mobile communications, broadcasting and many other wireless applications.
The radio frequency spectrum ranges from very low frequency radio waves at around 10kHz (30 kilometres wavelength) up to 100GHz (3 millimetres wavelength). The radio spectrum is divided into frequency bands reserved for a single use or a range of compatible uses. Within each band, individual transmitters often use separate frequencies, or channels, so they do not interfere with each other.
Band characteristics — capacity and coverage
Not all radio frequencies are equal. In general, lower frequencies can reach further beyond the visible horizon and are better at penetrating physical obstacles such as rain or buildings. Higher frequencies have greater data-carrying capacity, but less range and ability to pass through obstacles. Capacity is also dependent on the amount of spectrum a service uses — the channel bandwidth. For many wireless applications, the best trade-off of these factors occurs in the frequency range of roughly 400MHz to 4GHz, and there is great demand for this portion of the radio spectrum.
How devices communicate
All communication devices that use digital radio transmissions operate in a similar way. A transmitter generates a signal that contains encoded voice, video or data at a specific radio frequency, and this is radiated into the environment by an antenna (also known as an aerial). This signal spreads out in the environment, of which a very small portion is captured by the antenna of the receiving device, which then decodes the information. The received signal is incredibly weak — often only one part in a trillion of what was transmitted.
In the case of a mobile phone call, a caller’s voice is converted by the handset into digital data, transmitted via radio to the network operator’s nearest tower or base station, transferred to another base station serving the recipient’s location, and then transmitted again to the recipient’s phone, which converts the signal back into audio through the earpiece.
There are a number of standards for mobile phones and base stations, such as GSM, WCDMA and LTE, which use different methods for coding and decoding, and ensure that users can only receive voice calls and data that are intended for them.
Because there are so many competing uses for wireless communication, strict rules are necessary to prevent one type of transmission from interfering with the next. And because spectrum is limited — there are only so many frequency bands — governments must oversee appropriate licensing of this valuable resource to facilitate use in all bands.
Governments spend a considerable amount of time allocating particular frequencies for particular services, so that one service does not interfere with another. These allocations are agreed internationally, so that interference across borders, as well as between services, is minimised.