Overlapped FFT based energy detection has been proposed as a signal detection scheme in dynamic spectrum access. The overlapped FFT scheme increases the number of FFT frames to reduce the variance of squared noise and improves the detection performance. As the FFT frames are overlapped, correlation values between the frames affect to the detection performance. This paper proposes the window functions which decrease the correlation values between adjacent FFT bins. Numerical results obtained through computer simulation show that novel window functions generated by upsampling a Hamming window improves the detection performance by 0.09. However, this window function suffers more from adjacent channel interference than a conventional window. Therefore, this paper also proposes a two step detection scheme to achieve higher detection performance and to avoid the influence of the adjacent channel signal. Numerical results also indicate that the proposed scheme improves the detection performance and reduces the effect from the adjacent channel signal.