Most channel access schemes for cognitive radio only consider using the idle periods of the primary users. Such schemes are not using the spectrum efficiently, since transmission opportunities also arise when a primary transmitter is active but its corresponding primary receiver is far away from the cognitive user. By detecting the signal power of the acknowledgments, a cognitive user is able to estimate the distance and channel condition between the primary receiver and itself. Then if the primary receiver is far away, the cognitive user can transmit simultaneously with the primary transmitter without affecting the primary link. Based on this idea, the spatio-temporal channel access scheme, utilizing both idle periods and spatial reuse, can be applied. This paper provides fundamental analysis of the channel access delays and shows the advantage of using the spatio-temporal access scheme in the carrier sense multiple access (CSMA)-based network with bi-directional links.