Publication

Time agreement in a network is a critical issue for communications. While clock synchronization in wired networks is a well studied problem, the wireless medium presents extra challenges. For example, in wireless sensor networks which may contain several thousands of cheap and small sensors, the scalability and energy conservation are two major considerations in the design of synchronization. Moreover, in wireless ad hoc networks which have no infrastructure or limited infrastructure support, self-organization is an important concern. Besides, in vehicular networks, high mobility will require synchronization to be robust against node failures and dynamic topologies. Clock synchronization in wireless networks can be divided into pairwise and networkwide clock synchronization. Pairwise clock synchronization is designed for two nodes, where the goal is to synchronize one node to the other one by transferring timing messages. In this case, the synchronization problem can be transformed into a parameter estimation problem, since the random delays during the transmission lead to inaccuracy of timing messages. On the other hand, the network-wide clock synchronization is designed for synchronizing multiple nodes in a network, which is inherently a consensus problem. In this formulation, the main considerations are convergence speed and resilience to random delays. In [Paper A], we investigate the pairwise clock synchronization problem based on a two-way message exchange mechanism with exponentially distributed random delays. A novel synchronization scheme is proposed to estimate the clock skew and offset by directly utilizing mean square error as the metric to be optimized. This consideration results in performance improvements compared to the existing synchronization methods, especially for a small number of observations and large standard deviation of random delays. Furthermore, [Paper B] studies the pairwise clock synchronization problem for different random delay distributions. In this paper, for a Gaussian distribution, we derive the Cram´er-Rao lower bound for the joint estimation of clock offset and skew, and propose a linear-biased estimator to improve the accuracy. For an exponential distribution, we extend our work in [Paper A] by deriving the Chapman-Robbins bound. Moreover, a simple and robust estimation algorithm is proposed to handle arbitrary delay distributions, which can achieve a good tradeoff between accuracy and complexity. In [Paper C], we study the network-wide clock synchronization problem. A distributed consensus synchronization algorithm is proposed when using a random access protocol for timing message broadcasts. In the absence of transmission delays, [Paper C] theoretically proves the convergence of the proposed scheme, which is further illustrated by the numerical results. On the other hand, when the transmission delays are also taken into account, the proposed approach still shows resilience.