## Frequency hopped CDMA

Frequency hopping involves taking the narrow bandpass signals for individual users and constantly changing their positions in frequency with time. In a frequency selective fading environment, the benefit of changing frequency like this is to ensure that any one user's signal will not remain within a fade for any prolonged period of time. Clearly for frequency hopping to be effective, the users must hop over a bandwidth significantly wider than notch caused by frequency selective fading. In order to ensure that individual users never (or rarely) hop onto the same frequency slot at the same time, causing mutual interference, the carrier frequencies are assigned according to a predetermined sequence or code. |

Frequency hopping is most effective if a fast hopping rate is used (several thousand times per second) so that the communications are not corrupted by fading or mutual interference for any length of time. This, however, brings problems in the design of fast switching synthesizers and broadband power amplifiers which in practice put an upper limit on the hopping rate. Also, the narrowband channels are susceptible to Doppler shift, local oscillator error and so on, and compensation techniques are struggling with fast hop rates. Hopping also makes a system less vulnerable to discrete narrowband interference and near-far effect problems.