In this paper, we study the signal coding and processing scheme of the STBC-SC-FDE systems with 2 transmit and 1 receive antenna over frequency-selective fading channels, and develop a least-squares channel frequency response estimation algorithm based on the input-output modal in frequency domain. We analyze the mean square error of the proposed algorithm, and get the result that the algorithm is not only able to achieve the minimum mean square error for channel estimation but also to make a communication system with a relative low peak-to-average power ratio by employing Chu sequences as its optimal training sequences. For SC-FDE systems, the proposed algorithm can save more computing resources than those algorithms that estimate channel impulse response firstly and then transfer it to frequency domain. In the end, the performance of the proposed algorithm is evaluated by Monte Carlo simulations.