Aliased Sinc Bandlimited Interpolation and Digital Modulation

Author(s):

Patrick Jungwirth and W. Michael Crowe

Abstract:

Aliased sinc (asinc) interpolation and digital modulation is based on the bandlimited asinc interpolation kernel. The progressive asinc reconstruction (PAR) algorithm was originally developed to perform real-time reconstruction of continuous-time level-crossing samples. Applying the PAR algorithm to quadrature amplitude modulation (QAM) as time-domain pulse shaping results in a small, controlled amount of signal overshoot and a small, controlled amount of intersymbol interference. Numerical simulations indicate that for 16 QAM, asinc interpolation requires 24% less bandwidth and significantly improves sidelobes compared to Feher, half-cycle raised-cosine interpolation. PAR interpolation for digital modulation is currently patent pending.

Feher–QAM modulation replaces each QAM symbol with a half-cycle raised-cosine function. All symbol waveforms begin and end with zero slope. These zero-slope transitions help improve intersymbol interference and reduce timing jitter problems, but they also introduce a rectangular windowing effect, resulting in sidelobes. While the Feher–QAM sidelobes are significantly lower than those of rectangular time-domain pulse shaping, they are substantially higher than those resulting from the application of PAR. The new asinc interpolation offers a substantial improvement over Feher interpolation.