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Recently, relay transmission schemes have been much studied according to increasing demands for ad-hoc wireless communication. In bi-directional relay transmission, the transmission efficiency has been decreased due to the waiting time. To tackle this problem, network coding (NC) has been proposed. The orthogonal frequency division multiplexing (OFDM)-based relay transmission applying analog network coding (ANC) can reduce the number of timeslots per one sequence compared to that without NC and it increases the transmission capacity. However, in the bi-directional relay transmission with ANC, the relay node receives the signals of each transmitting nodes simultaneously and the signal detection must be conducted. To do that, the channel state information of each nodes is needed, and the rate-efficient and accurate channel estimation should be applied for the bi-directional relay, which is not much considered so far. Therefore, in this paper, we propose a new channel estimation scheme for bi-directional two-timeslot OFDM relay transmission where the transmitters send sparse Walsh-code pilots and the receiver detects and estimates each channel information by the correlation processing. The proposed scheme can estimate the channels by less pilot symbols sparsely allocated in OFDM frame. In addition, an accurate interpolation scheme with two-dimensional fast Fourier transform (FFT) is utilized. The performances of the proposed scheme are evaluated by computer simulations.