In this paper, we investigate how to send space-time codes with full diversity and low decoding complexity for X channels using precoders. First, we assume that we have 2 transmitters and 2 receivers. Each transmitter sends different codewords to each receiver at the same time. We propose an orthogonal transmission scheme that combines space-time block coding and array processing to achieve low-complexity decoding and full diversity for transmitted signals. To our best knowledge, this is the first scheme which can achieve low-complexity decoding and full diversity for any transmitted codeword in X channel when all the users transmit at the same time. Finally, we show that under certain conditions, our proposed scheme can be extended to a general case with any number of transmitters and receivers each with any number of antennas. Simulation results validate our theoretical analysis.