Channel access probability in unslotted IEEE 802.15.4 CSMA/CA under correlated clear channel assessments

Correlation between successive clear channel assessments in unslotted carrier sense multiple access with collision avoidance (CSMA/CA) is commonly assumed in the literature to be negligible for modeling the medium access control algorithm. However, in previous research, the authors found this assumption to be questionable under certain realistic conditions. In this paper, the correlation between successive clear channel assessments is studied and its effect on the the probability and delay of accessing the medium by a node following the IEEE 802.15.4 Standard is modeled for the unslotted CSMA/CA mode. The results obtained are used for evaluating the delay probability distributions of the three outcome events of the CSMA/CA algorithm: frame transfer success, channel access failure and frame transfer failure. For scenarios in which the duration of contentions is similar to, or shorter than, the duration of the frames propagating in the air, the correlation effect is strong, making classical models to yield results that deviate from a realistic scenario. The theoretical results are validated by simulations. It is shown that neglecting correlation between successive clear channel assessments leads to overestimating the expected delay of frame transfer success and channel access failure by up to 15%. The results also reveal that the probability of frame transfer success can be overestimated by up to 30%, the probability of frame transfer failure by up to 80% and the probability of channel access failure is underestimated by up to a factor 6 when clear channel assessment correlation is ignored.