The paper is to resolve the error floor for SUI-5 channel. Since the pilot allocation is solid, we canpsilat modify it exhaustively. So we combine the preamble and pilot carrier to resolve the problem. Here we introduce preamble time-domain LS (Least-squares Estimation) algorithm for consult. Simulation result shows that if we select channel estimation algorithm properly, combing preamble and pilot can improve the performance. We also conclude that pilot only scheme exits error floor when the coherent bandwidth of channel is narrow, and preamble only scheme can decrease the error floor drastically when the coherent bandwidth of channel is narrow, however it canpsilat trace the fast fading channel.