Time-varying channel gains are approximated by basis expansion model (BEM) to make the tremendous unknowns estimation feasible in orthogonal frequency division multiplexing (OFDM) systems. However, sparse dispersive channel requires overloaded subcarriers for pilot-aided estimation in the framework of BEM. This paper proposes a channel estimation scheme over time-varying and sparse dispersive channel with circular grouped pilot pattern, which occupies much less subcarriers. In this scheme, the sparse tap delays are detected with the interpolated channel frequency response and the modified BEM based estimation deals with only the detected taps, eliminating the noise interference from the null taps. The proposed estimator outperforms the existing methods applied to sparse dispersive channel with higher spectral efficiency. Our claims are verified by simulation results, which are obtained in COST207 typical urban channel model.