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To reduce the influence of interference on the OFDM-based cognitive radio overlaid with narrowband primary users, we consider a scheme called the variable subband nulling (VSN), in which a VSN mode is selected adaptively according to the result of spectrum sensing for the frames following the sensing period. For reliable detection of the VSN mode embedded in the preamble, we consider two types of metrics. The performances of the metrics are analyzed in Gaussian channels with/without narrowband interference (NBI), and are confirmed by simulation results. In fading channels with/without NBI, the detection performance is examined via simulation results. The analysis and simulation results have confirmed that, if the VSN mode is selected appropriately at the transmitter, the proposed metrics provide us with a reliable detection performance even when the NBI is strong.