Conventional radio signals are referred to as narrow-band, which means that they restrict all of its power to a very narrow portion of the radio frequency bandwidth. Due to the relatively small portion of the radio band that an individual radio transmission occupies, the FCC has traditionally favored these conventional radios. However, as a result of the very narrow frequency, these radios are susceptible to interference. That is, a single interfering signal at or near their frequency can easily render the radio inoperable and therefore it is not very reliable.
Another feature of narrow-band transmissions is that any receiver tuned to the particular frequency the transmission is broadcasting on can receive that transmission. While this feature ideal for publicly directed transmissions, such as radio or television programs, this is undesirable in situations where privacy and security are issues.
In general, narrow-band signals are not considered secure or reliable.
Spread Spectrum Radio Operation
Spread spectrum is a technique that takes a narrow band signal and spreads it over a broader portion of the radio frequency band. Widening the signal bandwidth in this fashion increases the probability that received information will closely match the transmitted information.
While traditional modulation techniques have been designed to overcome noise by maximizing the power at the center of the allocated frequency, Spread spectrum technology (SST) modulation is designed to minimize the average power at any given frequency over time, gaining reliability by increasing redundancy. This is accomplished by "spreading" the signal over the entire bandwidth available, and requiring that the receiver "know" where to look for the pieces of the signal. This redundancy has the benefit of minimizing the effects of interfering walls and structures.
The codes used for spreading have low cross-correlation values and are unique to every user. This is the reason that a receiver that has knowledge about the code of the intended transmitter, is capable of selecting the desired signal.
Spread Spectrum techniques possess a few advantages over Narrow-band systems:
Because of the difficulty to jam or detect spread spectrum signals, the first applications were in the military field. Due to unfounded concerns over the increased frequency space it occupies, the FCC until 1985, did not permit commercial use of the technology. Nowadays spread spectrum systems are gaining popularity also in commercial applications.Low power spectral density. As the signal is spread over a large frequency-band, the Power Spectral Density is getting very small, so other communications systems do not suffer interference from this kind of communications. Privacy due to unknown random codes. The applied codes are - in principle - unknown to a hostile user. This means that it is hardly possible to detect the message of another user. Random access possibilities. Users can start their transmission at any arbitrary time. Good anti-jam performance.