Spread spectrum
Réseaux cellulaires

CDMA is a "spread spectrum" technology, meaning that the information to be transmitted is spread over a wider bandwidth than is strictly necessary. In the North American CDMA system, a signal which, for example, is a data signal with a conventional data rate of 9.6 kbit/s will be "widened" to be transmitted at a rate of 1.23 Mbit/s. Two different techniques are used to expand or modulate the signal: direct sequence or frequency hopping. Direct sequence, as shown in figure 1 , modulates a signal by means of a digital code at a bit rate much higher than that of the information signal to be transmitted. Frequency hopping takes the radio signal from one frequency to another in a fraction of a second. For the...