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Spectrum sensing is a core problem in cognitive radio. Detecting the presence/absence of very weak primary users with a single antenna can be very difficult. Environmental uncertainties result in SNR walls that detectors cannot overcome. Multiple-antenna approaches show some potential gains, but the authors show here that single-user multiple-antenna detection still must suffer from an SNR wall. The reason is that the real-world uncertainty in noise is dominated by the potential presence of an unknown number of low-power and time-varying interference sources in the external environment.
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