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In this article, we present a mixed analog-digital spectrum sensing method that is especially suited to the typical wideband setting of CRs. The next section briefly summarizes existing approaches to CR sensing. The advantages of our system with respect to current architectures are threefold. First, our analog front end is fixed and does not involve scanning hardware. Second, both the analog-to-digital conversion (ADC) and the digital signal processing (DSP) rates are substantially below Nyquist. Finally, the sensing resources are shared with the reception path of the CR, so that the low-rate streaming samples can be used for communication purposes of the device, besides the sensing functionality they provide. Combining these advantages leads to a real-time map of the spectrum with minimal use of mobile resources. Our approach is based on the modulated wideband converter (MWC) system, which samples sparse wideband inputs at sub-Nyquist rates. We report on results of hardware experiments, conducted on an MWC prototype circuit, which affirm fast and accurate spectrum sensing in parallel to CR communication. This can help alleviate one of the current main bottlenecks in wide-spreading deployment of CRs.