Spread spectrum is a radio transmission technique that operates on the principle of intentionally expanding the bandwidth of a narrowband signal before transmission. This expansion is achieved through a process of signal manipulation or scrambling, resulting in the signal occupying a broader portion of the radio frequency (RF) spectrum than strictly necessary. This widening of the signal’s spectrum enhances its resilience against interference and jamming.

To perform this manipulation, a pseudo-random noise code is employed, which, in the original concept, was known only to the communicating parties at each end of the radio link. Spread spectrum technology was originally developed in the 1940s and has since found extensive use, particularly in military and applications where robustness and resistance to jamming or eavesdropping are crucial.

There are three primary methods of spreading the signal:

1. Direct Sequence Spread Spectrum (DSSS): DSSS involves multiplying the original data signal by a much faster pseudo-random noise code, often referred to as the spreading code. This multiplication results in a scrambled signal with a significantly broader spectrum. DSSS offers substantial protection against interfering signals, especially narrowband interference. Additionally, it enables multiple access by employing various spreading codes simultaneously, a concept known as Code Division Multiple Access (CDMA). CDMA is used, for example, in third-generation mobile communications (3G).
2. Frequency Hopping Spread Spectrum (FHSS): FHSS entails rapidly changing the RF frequency of the narrowband transmission within a defined range, guided by a pseudo-random noise code. As a result, a hopping pattern is observable in the spectrum. In terms of coexistence with other systems, FHSS is a technique aimed at avoiding interference. In other words, if a frequency hop coincides with another transmission on the same channel, any collision is limited to the duration of the hop, typically in the order of milliseconds or less. Similar to DSSS, FHSS supports multiple access by utilizing orthogonal hopping codes for different logical communication channels. Bluetooth is one example of a technology that uses FHSS, with Bluetooth devices hopping between 79 available channels at a rate of 1600 times per second.
3. Time Hopping: Time hopping involves transmitting a series of short-duration pulses derived from the narrowband information-carrying signal through modulation with a pseudo-random impulse train. The brief pulse duration generates the spread spectrum profile. Time hopping is utilized as a technique to create specific types of Ultra Wideband (UWB) signals.