$R^{4}Syn$ : Relative Referenceless Receiver/Receiver Time Synchronization in Wireless Sensor Networks

A new time synchronization protocol for wireless sensor networks (WSN) is proposed. It uses the receiver-to-receiver principle introduced by the Reference Broadcast Synchronization (RBS), which reduces the time-critical path compared to the sender-to-receiver approach. The proposed protocol has the advantage of distributing the reference's function among all sensors, which eliminates the single point of failure (reference) shortcomings of RBS. It also allows timestamps to be piggybacked to the regular signals (beacons) and thus eliminates the need of separate transmissions for exchanging timestamps. After local synchronization, a multihop extension is proposed using final local estimates, with no forwarding of synchronization signals. Maximum likelihood estimators (MLE) are derived to estimate relative skew/offset for channels with Gaussian distributed delays. The Cramer-Rao lower bounds (CRLB) are accordingly derived and numerically compared with the MLE's mean square error (MSE). Results show convergence of the proposed estimators' precision to their respective CRLB with the increase of the number of signals.

I. Related Work

The receiver-to-receiver approach introduced with the Reference Broadcast Synchronization (RBS) protocol [1] has the advantage of reducing the time-critical path, and therefore improving synchronization accuracy compared to the sender-to-receiver approach [2, Sec 3.3, p. 294]. It exploits the broadcast property of the wireless communication medium, where receivers- located within listening distance of the same sender-receive a broadcast message at approximately the same time with very little variability due to the reception timstamping at the receivers. RBS uses a sequence of synchronization signals (beacons) from a dedicated sender (reference), which make periodical broadcasts. Reception events are timestamped with local clocks. The timestamps are then exchanged between nodes and used as samples for estimating relative skews/offsets.