Digital down conversion in software radio terminals

The idea of software radio requires an expansion of digital signal processing towards the antenna. Hence, for converting the received signal to baseband, the need of efficient high speed digital down converters arises. In [1] digital down conversion was identified as one of the ‘critical functionalities’ because it has to run at a relative high sample rate, and has to provide high resolution. The common approach for digital down conversion (DDC) is the so called ROM table approach where the samples of the input signal are multiplied with amplitude values of the sine- and cosine-function stored in ROM. To achieve high resolution this technique requires a large look-up table which means large chip area, high power consumption, lower speed, and increased costs. In this paper a CORDIC-based digital down converter is described. It enables to reduce the size of the look-up table considerably. Additionally to previous publications, this paper provides an overall worst case quantization error estimation that facilitates the dimensioning of the CORDIC-DDC.