Spread-spectrum techniques are methods by which electromagnetic energy generated in a particular bandwidth is deliberately spread in the frequency domain, resulting in a signal with a wider bandwidth. These techniques are used for a variety of reasons, including the establishment of secure communications, increasing resistance to natural interference and jamming, and to prevent detection.

http://en.wikipedia.org/wiki/Orthogonal_variable_spreading_factor
http://en.wikipedia.org/wiki/Weak_convergence_of_measures
http://en.wikipedia.org/wiki/Covariance_matrix

In cryptography, pseudorandom noise (PRN) is a signal similar to noise which satisfies one or more of the standard tests for statistical randomness.

Although it seems to lack any definite pattern, pseudorandom noise consists of a deterministic sequence of pulses that will repeat itself after its period.

In cryptographic devices, the pseudorandom noise pattern is determined by a key and the repetition period can be very long, even millions of years.