Superposed multicarrier transmission is known to improve frequency utilization efficiency when several wireless systems share spectrum. In the superposed multicarrier transmission scheme, interference can be suppressed by setting the corresponding initial log likelihood ratio (LLR) for forward error correction (FEC) decoder to zero or proper values determined by the desired to undesired power ratio (DUR). Since this technique needs information of superposed subcarriers and DUR, in this paper we propose a superposed band and interference power estimation technique based on the residual power. Our proposed scheme adopts robust LLR for initial LLR and averages the residual power over all packets even if these contain bit errors. The superposed band is detected by comparing the averaged residual power with a threshold. Simulation results indicate that the proposed method performs faster interference detection than the conventional scheme.