Interpolation

The baseband signal that has been processed up to this stage (RRC filtering) has been
constructed at a sample rate that meets the Nyquist criteria for its frequency content.
Ultimately, in this example, the signal will be digitally modulated and the IQ streams
recombined to yield a real digital intermediate frequency. This will then be applied to
a digital-to-analog converter to give a modulated analog IF suitable for upconversion
to the final carrier frequency. Because the digital frequency content is increasing (digital
upconversion and IQ combining), the sample rate must be increased to re-meet the
Nyquist requirement. This is the process of interpolation—the insertion of additional
samples to represent the increased frequency components of the signal.