A means of communicating by purposely spreading the spectrum (frequency extent or bandwidth) of the communication signal well beyond the bandwidth of the unspread bandwidth. Spread spectrum signals are typically transmitted by electromagnetic waves in free space with usage in both no military and military systems.
Motivation for using spread spectrum signals is based on the following facts:
There are four generic types of spread spectrum signals:
Chirp Modulation: This is an older spread spectrum method that was developed for radar use. The basic idea is to transmit a long rectangular pulse whose carrier frequency is linearly increased from f1 to f2 (f2>f1). The frequency-modulated signal returned from the target passes through a filter in the receiver at a velocity of propagation proportional to frequency. The result is a pulse that is much shorter in time duration than the transmitted pulse with a larger peak power content. Unchirped pulses such as interference or jamming pulses do not "compress" at the receiver, so that this method yields a processing gain or advantage for the chirpped signal.
Time Hopping: Not normally used alone, is a method in which the transmitted pulse occurs in a manner determined by a pseudorandom code which places the pulse in one of n possible positions per frame. If n is sufficiently large, then the pulse is on only 1/n of the time, and again the transmitted pulse has a processing gain against a full frame jamming pulse of equal energy.
Direct Sequence Systems: Direct sequence systems were once the most prevalent method of communicating in spread spectrum communications. Direct sequence modulation is characterized by phase-modulating a sine wave by an unending string of pseudo noise code chips (symbols of much smaller duration than a bit). This unending string is typically based on a pseudo noise code that generates an apparently random sequence of code chips that repeat only after the code period.