1.3. Spread Spectrum

Wireless systems transmit data over a specific, quite narrow frequency band that allows a transmitter and a receiver to differentiate the intended signal from background noise when the signal quality is sufficient. Narrowband interference can be avoided by filtering out any other frequencies except the designated ones at the receiver. The major advantage of narrowband signal transmission is, as the term implies, its efficient use of frequency due to only a small frequency band being used for one signal transmission. Its drawback is evident, though, as it requires well-coordinated frequency allocation for different signals, and it is quite vulnerable to signal jamming and interception.

Spread spectrum takes another approach. Instead of applying all transmitting power to a narrow frequency channel, spread spectrum converts the narrowband signals to signals of a much wider band with comparatively lower power density according to a specific signal spreading scheme. Because of the low power density (power per frequency), the converted signal appears to be background noise to others who are unaware of the spreading scheme; only the designated receiver is able to reconstruct the original signal. For data transmission with multiple channels using FM, narrowband signals in each channel can be applied with the spread spectrum technique. To be able to differentiate these channels and reconstruct those signals afterwards, each channel is assigned a code with sufficiently large distance separating it from others in the code space, and the codes are made known to the designated receiver only. The advantages of spread spectrum are as follows: