Fixed Frequency vs Spread Spectrum
Generally, industrial wireless communications uses either fixed frequency radio channels or a spread spectrum radio band.
Fixed Frequency, Licensed vs license-free
“Fixed frequency”, as the name implies, uses a single frequency channel – radios initiate and maintain communications on the same frequency at all times.
Fixed frequency channels can be license-free or licensed. A licensed channel is licensed to the operator of the wireless system by a governing body in each country, such as the Federal Communications Commission (FCC) in America. A radio license protects a channel against other users in a specified geographic area, and also specifies the RF power levels which may be used. Generally, licensed channels allow much higher power levels that license-free channels.
In some regions of the world, license-free bands comprise a number of fixed frequency channels – the user can select one of the channels in the band to use. The problem with license-free fixed frequency channels is that if more than one user in close vicinity uses the same channel, the wireless transmissions are corrupted. If a particular channel is heavily used, another channel can be selected, however this extends commissioning times.
In industrial situations, most fixed frequency bands are at a lower frequency than the spread spectrum bands. Combine the lower frequency with the ability to gain licenses for larger RF power, and fixed frequency is the obvious choice for infrastructure radio over large distances.
However, there are some administrative and technical drawbacks to consider. The delay associated with obtaining a license varies from country to country, and it is not uncommon for the granting of a license to take long periods. Furthermore, with a limited number of channels from which a license can be granted, licenses can be difficult to come by in areas of great demand.
Spread spectrum radios use multiple channels within continuous band. The frequency is automatically changed, or transmissions use multiple frequencies at the same time, to reduce the effects of interference.
While there are several types of spread spectrum techniques, the two most common are Frequency Hopping Spread Spectrum (FHSS) and Direct Sequence Spread Spectrum (DSSS).
Spread spectrum allows a large number of wireless systems to share the same band reliably. With a band of fixed frequency channels, the reliability of a system is dependant on no other system using the same channel at the same time. Spread spectrum provides a method of interleaving a large number of users into a fixed number of channels.