In wireless communications with high mobility, an interpolation-based channel estimation scheme shows the feasibility of windowed orthogonal frequency division multiplexing (OFDM) systems, where severe inter-carrier interference (ICI) is mitigated by a wrap-around based frequency-varying Viterbi detection scheme. In the interpolation-based channel estimation, insertion of pilot and null subcarriers leads to a decrease in available spectral rate. To reduce null subcarriers and thus improve available spectral rate, we propose an iterative channel estimation and signal decoding scheme, which mitigates the ICI introduced by reduction of null subcarriers. In the proposed iterative scheme, an initial coarse channel estimation procedure is conducted before the iterative channel estimation and signal decoding procedure. Two procedures are skillfully designed to prevent error propagation. Furthermore, the properties of raised-cosine roll-off window function are exploited to design the proposed scheme, where repetition data transmissions are proposed to safeguard against decoding errors. Simulation results demonstrate that the available spectral rate increases at the price of tolerable performance degradation.