Single-carrier frequency-division multiple-access (SC-FDMA) with frequency domain equalization (FDE) can provide good performance even in a frequency selective fading channel. In this paper, we focus on a special kind of SC-FDMA, i.e., interleaved frequency-division multiple-access (IFDMA) because of its peak-to-average power ratio (PAPR) property and frequency diversity gain. FDE requires accurate channel estimation. In the case of fast time varying fading channel, decision directed channel estimation (DDCE) is effective. However, in IFDMA, amplitude of each subcarrier may fluctuate depending on the combination of data symbols, then estimation accuracy may degrade. That can be avoided by multiplying information data sequence by random phase sequence. The performance depends on the selection of the sequence. In this paper, selection criterion suitable for IFDMA with DDCE is proposed. The effect of the proposed method is evaluated by computer simulation. The results will show the proposed method can lower the error floor of the system.