Spread spectrum

Spread spectrum is an technique used in radio transmission based on the concept that the narrowband signal is manipulated (scrambled) prior to transmission in such a way that the signal occupies a much larger part of the RF spectrum then strictly needed. This makes the signal more robust against interference and jamming.

The manipulation requires a pseudo random noise code which, in the original concept, was only known to the parties at each end of the radio connection. Spread spectrum technology was invented in the 1940s, and has been used extensively since then for military and other applications that require robustness and resistance to jamming or eavesdropping.

There are three different ways of spreading the signal:

• direct sequence,
• frequency hopping,
• time hopping,

The direct sequence spread spectrum (DSSS) approach is based on multiplication of the original data signal with a much faster pseudo random noise code, which is also called the spreading code. This results in a scrambled signal with a much wider spectrum. DSSS significantly improves protection against interfering signals, especially narrowband interference. It also provides a multiple access capability, when the several different spreading codes are being used simultaneously. The use of DSSS for multiple access is called CDMA, and is used e.g. in the 3^th generation mobile communications.

In case of frequency hopping spread spectrum (FHSS) the RF frequency of the narrowband transmission is quickly changed within a certain range, according to a pseudo random noise code. Hence, a hopping pattern can be observed in the spectrum. In terms of spectral coexistence with other systems, FHSS is an avoidance technique, in other words if the hop coincides with someone else’s transmission on the same channel, the collision will take only the duration of the hop, which is typically in the order of milliseconds or even less. Like DSSS, FHSS also provides a multiple access capability by using orthogonal hopping codes for different (logical) communication channels. FHSS is for instance used by Bluetooth. Bluetooth hoppes 1600 times per second between the 79 available channels.

In case of time hopping a train of short duration pulses is transmitted which is derived from the narrowband information carrying signal through scrambling with a pseudo random modulated impulse train. The short pulse duration generates the spread spectrum profile. Time Hopping is used as a technique to generate a certain type of UWB signals.