The Research And Implementation Of Time Window Synchronization Protocol For 802.15.4

IEEE 802.15.4 is an industrial standard widely used for interconnection by data communication devices using low data rate, low power and low complexity, short-range radio frequency (RF) transmissions in a wireless personal area network (WPAN). The non-beacon mode of 802.15.4 supports mesh topology. However, the radio on every coordinator in non-beacon network has to remain active to ensure connectivity, which consumes considerable amount of power. Given the fact that data rate is low in such network, it is desired that every radio on the coordinator in the whole network is active in the same time window and stays inactive for the rest of the time to save power. This paper proposed a protocol to achieve synchronization and maintain time window.Based on the study of 802.15.4 protocol and time synchronization technique, a time synchronization scheme was proposed. The scheme is based on synchronization packet broadcast. When broadcasting, MCU load and parameters for CSMA/CA algorithm are controlled to reduce the uncertainty of broadcast delay. Least square linear fitting is also used to achieve further synchronization precision. To synchronize the whole network, a logic synchronization tree is used. Nodes are synchronized alone the edge of the tree.The time window synchronization protocol uses the proposed synchronization scheme. The protocol is also accommodated for physical topology change.The time synchronization scheme was implemented on Freescale 802.15.4 develop kit. A synchronization precision test was run for 2.25 hours with an 8-level logic synchronization tree. The results show that the synchronization precision degenerating as the level of logic synchronization tree growing. At level 8, the average synchronization precision is 86.3us while 98.7% of the time the synchronization precision is within 320us.The time window synchronization protocol was implemented on the same platform. The test shows that it can maintain time window for the whole network while physical topology is changing.