The number of indoor wireless communication devices and technologies employing the ISM band is increasing day by day. As a consequence, co-existence is a big challenge for tiny, IEEE 802.15.4-based and resource-constrained devices. In this paper, 2.4GHz ISM band aggregated traffic is analyzed and a novel channel access approach is proposed based on the cognitive radio concepts in order to increase free channel access performance. The performance is evaluated by using the real-world RF signal strength measurements of an indoor IEEE 802.15.4 node with the presence of many IEEE 802.11 interferers. The performance evaluation gives promising results considering free channel access performance.