Frequency hopping spread spectrum
Frequency hopping spread spectrum (FHSS) uses a technique in which the transmission channels are divided into subchannels, which are then time division multiplexed; users simultaneously hop pseudo-randomly from frequency to frequency within the transmission spectrum. FHSS is thus a unique combination of frequency division multiplexing (FDM) and time division multiplexing (TDM).
Time spent on each frequency is usually quite brief, on the order of five milliseconds or less. As a result, intentional jamming is rendered ineffective, as originally proposed.
With FHSS, a unique pseudo-random code sequence is assigned to each mobile unit in a service area. This sequence is used to generate a unique sequence of frequency hops. (A pseudo-random sequence holds several of the same properties as a random sequence, but is a deterministic code—both the mobile unit and the base station must be able to calculate it.) Because the codes are nearly orthogonal (i.e., non-correlated), there is very little correlation between code sequences. Therefore, multiple users can frequent the spectrum with minimal interference, and each additional user contributes only minimally to the background noise.
Bluetooth is an example of a radio system that uses frequency hopping spread spectrum to provide secure transmissions. Slow frequency hopping is used in digital cellular radio to ensure that a user is not stuck with a low quality channel. For example, consider a system with 40 channels, of which one channel is of low quality. With frequency hopping, the user would spend only 1/40th of the time on the low quality channel.