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This paper proposes using iterative decision-feedback channel estimation (DFCE) in a turbo soft interference canceller (SIC) for a quasi-orthogonal (QO) - space-time block code (STBC) with four antennas in discrete Fourier transform (DFT)-precoded orthogonal frequency division multiple access (OFDMA). In the proposed method, channel estimation (CE) is updated at each turbo SIC iteration loop by using soft-symbol estimation based on the log-likelihood ratio (LLR) of an extrinsic probability at the Max-Log-MAP (maximum a posteriori probability) decoder output. Hence, the accuracy of the generated residual inter-code interference (ICI) due to non-orthogonality in the QO-STBC improves in the turbo SIC, which leads to improvement in the achievable performance. Computer simulation results show that by applying the proposed iterative DFCE in the turbo SIC, the required average received signal-to-noise power ratio (SNR) at the average block error rate (BLER) of 10-2 is decreased by approximately 1.5 and 1.0 dB compared to those for STBC using reference signal (RS) based CE and DFCE, respectively, for the coding rate of R = 1/3 in turbo code. Hence, the iterative DFCE works cooperatively in the turbo SIC to decrease the ICI in the full rate QO-STBC.