Generation and reception of spread-spectrum signals

The term 'spread-spectrum' implies a technique whereby digitized information is added to a pseudo-random number sequence and the resultant bit stream changes some parameter of the carrier frequency in discrete increments. The discrete modulation of the carrier frequency is usually realized either as a multiple level phase shift keyed or frequency shift keyed signal. The resultant PSK-modulated frequency spectrum is referred to as direct sequence spread-spectrum, whereas the FSK-modulated carrier frequency is referred to as a frequency hopped spread spectrum. These can be considered the major subsets of the more general term 'spread-spectrum'. In discussing signal reception, it is pointed out that active correlation methods are used for channel synchronization when the psuedo random sequences are long or when the processing gain is large, whereas the passive methods may be used for either short pseudo-random noise generation codes or to assist in attaining initial synchronization in long sequence spread-spectrum systems.