To send digital signals from a transmitter to a receiver, a carrier frequency is used to carry the digital information. This is accomplished by using the digital signal to modulate the carrier. The carrier is modulated by using amplitude, phase, frequency modulation, or frequency hopping, or a combination of these. Phase modulation is the most common way of digitally modulating the carrier frequency. In addition, spread spectrum is used to achieve process gain, or improved reception in the presence of other signals, such as jammers or noise. Spread spectrum systems use more spectrum than is needed to send the digital information directly. Therefore, they require more bandwidth to achieve jammer protection. The same type of digital modulation techniques are used in a spread spectrum system, but a pseudorandom code at a much higher bit rate is combined with the data before modulation.

Parallel techniques are used to increase the data rate of a system. The data is sent out at the slower rate on multiple channels, for example frequency channels, and the receiver on the other end combines all of the parallel channels for an increase in overall data rate. One common scheme that uses parallel techniques to increase data rates is orthogonal frequency division multiplexing (OFDM). This technique uses orthogonality to allow overlap of the frequency cells to maximize data rates within a given bandwidth.