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When orthogonal frequency division multiplexing offset-QAM (OFDM/OQAM), termed OQAM in short, was revealed to possess several advantages compared to OFDM, the channel estimation (CE) technique became a crucial issue for OQAM to make a practical sense. Based on the previous proposal on CE of OQAM, this letter addresses the study of a decisive factor that directly determines the CE performance. The idea is to relax the orthogonality constraint of the prototype filter used for transmitting the preamble. This provides a larger degree of freedom for finding a good balance between its time and frequency localization (TL and FL). Two prototype filter design algorithms are proposed and show that the proposed preamble-based CE method can almost attain the perfect CE.