Digital interpolation in the passband domain

This paper presents techniques for digital interpolation in the passband domain, exemplarily for a carrierless amplitude modulation/phase modulation (CAP) transmission system. Well-known techniques for designing baseband interpolation filters are extended to signal processing at nonzero center frequencies. The resulting filters are either real- or complex-valued. Real-valued interpolation filters are shown to suffer from increasing center frequencies, while their complex-valued counterparts keep their characteristics. For implementations based on the Farrow structure, however, the required polynomial degree grows with increasing center frequency.