Designing Flexible Software-Defined Radio Architectures

Modern high-performance radio/microwave communications systems utilize digital baseband signal processing (coupled with complex up/down conversion), where very little of the signal processing is performed in the analog/RF domain. Such signal processing includes filtering, phase/frequency de-rotation, channel equalization, pulse shaping, and modulation/demodulation. When sampling rates and numeric precision are accounted for, digital signal processing provides ideal and deterministic functionality. In fact, there exists no more accurate or controllable filtering technique. However, even the most powerful signal processing engines and algorithms are incapable of processing sampled waveforms that have undergone most forms of non-linear distortion. Therefore, even modern digital communications systems still require high-order analog filters to control the non-idealities of the radio and data conversion processes.

Programmable baseband filters, which provide the necessary RF channel selectivity, offer software-defined flexibility. This allows one single radio system to easily adapt to multiple modes (such as the many WiMAX bandwidths) and radio standards, without any change in hardware. This, of course, implies direct conversion operation rather than the superheterodyne approach (which achieves its channel selectivity via intermediate frequency filtering—such filters are expensive, not programmable, and it is difficult to bank multiple filters).

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