In this paper, a spectrum sharing protocol that combines coordinate-interleaved orthogonal design (CIOD) and space-time block coding is proposed for cognitive radio (CR) networks. Primary system's transmitter (PT) and receiver (PR) are equipped both by one antenna while secondary system's transmitter (ST) and receiver (SR) are equipped by two and one antennas, respectively. CIOD applied by means of PT and ST, provides signal-space diversity at PR and space-time block coding (STBC), especially Alamouti coding applied at two antennas of ST avoids interference at both PR and SR while providing spatial diversity for both receivers. An upper bound on the bit error probability (BEP) of primary system is analytically derived for Rayleigh fading channels and supported via computer simulations. The results show that the new protocol significantly improves the primary system bit error performance compared to the conventional non-cooperative case and the reference cooperative scheme based on the transmission of superimposed primary and secondary signals from ST; while allowing the secondary system's periodic access to the network with an improved performance level.