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This paper presents an investigation into improving the channel estimation scheme and reducing the effects of symbol timing misalignment when the digital video broadcasting-return channel through terrestrial (DVB-RCT) is used as return path model. Under orthogonal frequency division multiple access (OFDMA) which uses the modulation scheme of DVB-RCT, appropriate symbol length of constant-amplitude zero-autocorrelation (CAZAC) sequences as a preamble could be utilized in accordance with the number of transmitting antenna and channel condition. However, because its capability of channel estimation is limited by the number of transmitting antennas and multipath components to maintain the orthogonality, we propose more flexible preamble design called extended-CAZAC (E-CAZAC). Also, taking into account the effect of multiple access interference (MAI) introduced by a symbol timing misalignment, the bit error rate (BER) and throughput performance are investigated for a typical OFDMA uplink scenario.