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Low-complexity and flexible spectrum sensing is one of the challenging tasks in a cognitive radio (CR). Most of the work on energy-detection-based spectrum sensing presented in the literature employ a filter bank (FB) to split the wideband input signal into several subbands and detect the spectrum by computing the energy of subbands. However, for a mobile CR handset, a filter-bank-based spectrum sensing is not an area- and power-efficient scheme. In this paper, a novel spectrum-sensing scheme based on a reconfigurable digital downconverter (RDDC) and a reconfigurable filter based on coefficient decimation is proposed for mobile CR handsets, which has very low complexity and high flexibility compared to conventional FB approaches. The basic idea is to rotate the spectrum using an RDDC and filter the desired portion of the spectrum using the reconfigurable filter, which then results in very low complexity. The implementation results on Virtex-4 field-programmable gate array show that our method offers significant reductions in gate count, power consumption, and delay over conventional discrete Fourier transform FB-based spectrum sensing.