RECEIVER-BASED TIME SYNCHRONIZATION FOR MULTI-HOP WIRELESS NETWORKS

Abstract

Time synchronization is a critical piece of software infrastructure in Wire less Sensor Network (WSN). Sensor nodes require a concept of global time for many of their applications, which include data fusion, internal software stack functioning, networking stack infrastructural support, etc. Time synchronization methods can be grouped under three techniques, namely round-trip synchronization method, Medium Access Control (MAC) layer time-stamping and receiver-based synchronization. Our proposed methods, Hierarchical Reference Synchronization (HRS) and Energy Harvesting Time Synchronization (EH-TS), falls under the last category. </br> HRS is a proactive time synchronization protocol for multi-hop WSN. Unlike a flooding mechanism, HRS dynamically selects a subset of helper reference nodes to broadcast time-sync beacons, reducing unnecessary synchronization messages. The beacons serve as common reference points in time for all sensor nodes to time-stamp using their local timers. The base node will broadcast its global time at each reference point, thus making it possible for service nodes to compare their respective times with that of the base node. </br> Border nodes are sensor nodes that help to further extend time synchronization to other hops. In this thesis, HRS is implemented as an underlying MAC software component to achieve a time-slotted MAC super-frame. In the super-frame, sync-frames are used for synchronization while subsequent data-frames are used for data transmission. Unlike most receiver-receiver synchronization schemes, HRS can synchronize with nodes that are isolated from other peer nodes. HRS is very scalable as spatial diversity is exploited to enable simultaneous synchronization across multiple hops. HRS has been implemented on TelosB motes and has been shown to achieve micro-seconds time accuracy. </br> EH-TS is a receiver-based time synchronization protocol for energy harvesting nodes. A key consideration for energy harvesting sensor network is energy optimization. Thus, unlike HRS, it is a passive scheme as it relies on piggy-backing on packets generated by the application layer. Time synchronization information is encapsulated onto the application packets so no dedicated time sync packets are created. This piggy backing of synchronization packets on data packets helps to conserve energy. A multiple linear regression method is used to determine clock drift between the base node and the various sensor nodes.