This paper investigates an attempt to improve performance of in-band full-duplex system in terms of both multiplexing gain and diversity order when time selectivity exists and instantaneous channel information is not available at the transmitter. The system, which utilizes spatial resource bi-directionally with shared time and frequency band, is called a 'bi-directional in-band full-duplex (BFD)' system. The use of full bandwidth and the reciprocity between time and frequency allows the BFD system to reduce the symbol duration. Focusing on this reduction in symbol duration, we can propose a space-time coded BFD (STC-BFD) scheme for improving both system capacity and reliability using Alamouti code. The proposed STC-BFD scheme with N antennas can achieve multiplexing gain of 2 with diversity order of 2(N - 2), whereas conventional half-duplex scheme is only able to achieve multiplexing gains of 1 with diversity orders of N in a fast fading environment without channel state information at the transmitter.