Baseband signals are information signals at their original frequencies, typically low frequencies. Wireless communication systems typically will upshift the frequency spectrum of baseband signals to a higher range of frequencies to allow transmission through the atmosphere. This high frequency for wireless signal transmission is called the carrier frequency, and the process to shift the baseband signals to carrier frequency is called RF modulation.
Since the carrier frequency is at a higher range than baseband signal frequency, the RF modulation process is also referred to as the "upconversion".
RF modulation is achieved by multiplexing the baseband signal with the carrier signal. Let s denote the baseband signal, denote the carrier signal, is the modulated signal at the carrier frequency. For complex signals, use .
At the receiver side, the opposite process takes place to downshift the received signal back to baseband frequency. This is also called downconversion.Toolbox: RF modulation (upconversion) of a single-frequency cosine waveform
RF modulation of a single-frequency cosine signal at frequency f is shown in example below. If the baseband signal has multiple frequency components, all will be shifted around the carrier frequency fc.
Be careful: if f and fc differs a lot (e.g. by more than 20 times), it will take very long time to process.