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We propose a new channel estimation technique to mitigate interference with the aid of pilot density discrepancy in a cellular OFDM system, especially for 3GPP systems. The proposed estimator applies interference mitigation to pilot symbols, using repeated interference features according to the pilot subcarrier distance. In order to use this property, the proposed estimator employs a specific pilot structure which consists of two types of pilot symbols with different pilot density. The combination of interference alleviation and pilot rearrangement not only makes the channel estimation robust to the time-selectivity of the channel but also reduces the number of pilot subcarriers needed to estimate the channel. To clarify the advantages of the proposed method, the average mean square errors (MSE) of the frequency channel estimate are derived for the proposed estimator with unequal pilot density, and it is compared with general estimators with equal pilot density. Numerical analysis and simulation results confirm that the proposed estimator with unequal density relationship outperforms the estimators over practical time-varying environments in terms of MSE performance and pilot overhead efficiency.