Distributed Cooperative Synchronization for Large-Scale 4G Wireless Sensor Networks Using CAZAC Sequences

Distributed Cooperative Synchronization for Large-Scale 4G Wireless Sensor Networks Using CAZAC Sequences

Mahdy Saedy (University of Texas at San Antonio, USA) and Brian Kelley (University of Texas at San Antonio, USA)
DOI: 10.4018/jitn.2012010104
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Clock synchronization is an important requirement of wireless sensor networks (WSNs). Synchronization is crucial to maintain data consistency, coordination, and perform fundamental operations. Many application scenarios exist where external clock synchronization may be required because WSN itself may not consist of an infrastructure for distributing the clock reference. In distributed systems the clock of a reference node is synchronized with GPS time tag or UTC as conventional external clock sources. The rest of the nodes estimate the offset and drift based on a synchronization protocol. For vast WSN, where the topology introduces propagation delay and fast drift rate of clock over sampling periods, synchronizing the WSN nodes and maintaining the synchronization is difficult. To maintain an accurate synchronization across the WSN, the authors propose a cooperative synchronization method, which uses Constant Amplitude Zero Auto Correlation (CAZAC) sequences for OFDM symbols. The proposed method is part of a class of distributed methods known as Gossip or Consensus. These protocols are robust and self-correcting to topology changes and link failure. In this paper, the authors introduce a specific type of power-law topology called scale-free and compare the synchronization performance of the proposed method in random and scale-free topologies.
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1. Introduction

A. Background

Recent advances in technology have made the dream of low cost, low power and small wireless sensors come true. Sensor nodes are tiny devices meant for collecting data from the field, timestamping with local clock, and sending them to the base station for interpretation and post processing. For several applications of a wireless sensor network, such as mobile target tracking, event detection, efficient TDMA scheduling, and structural health monitoring, In order to get meaningful information from received data, it is a crucial requirement that the nodes operate in a coordinated and synchronized fashion. In all of these applications, all the nodes of the network need to refer to a common notion of time.

Wireless sensor nodes have not improved considerably since 802.15.4 standard was introduced. As new complicated applications come, the need for coordinating wireless sensor nodes in a cooperative manner within a network becomes more prominent. This implies the need for better quality in sensor nodes functionality both individually and cooperatively which necessitates more advanced communication capabilities. Our proposed system is based on 4G standard with IP-based infrastructure and is bandwidth scalable, OFDMA broadband modulated that offers optional MIMO and iterative decoding methods (Ghash, Ratasuk, Classon, Nangia, Love, Schwent, & Wilson, 2007). Ad Hoc Sensor Networks are characterized by multi-hop forwarding issues such as forwarding delay in nodes that act as routers, mobility challenges, limited energy (potentially mitigated by fuel cell breakthroughs) and bandwidth, large routing tables, and bandwidth overhead. The use of 4G, i.e., 3GPP-LTE resource management and physical layer technology, leads to new protocols, optimizations and other communication capabilities.

The existing clock synchronization methods in WSNs can be classified into two categories, internal and external clock synchronization (Rhee, Lee, Kim, Serpedin, & Wu, 2009). In internal clock synchronization, the nodes synchronize with each other in a peer-to-peer manner (Swain & Hansdah, 2010; Sommer & Wattenhofer, 2009; Apicharttrisorn, Choochaisri, & Intanagonwiwat, 2010). In external clock synchronization, nodes synchronize with a reference clock such as GPS time tag and UTC (Maroti, Kusy, Simon, & Lédeczi, 2004). Another classification could be the way the synchronization task is performed. The synchronization can be either central or distributed. Because WSNs have Ad Hoc type of topology and are dispersed over a wide geographical region, most central synchronization algorithms seem to be not efficient due to the multihop delay time. As opposed, distributed methods are more popular and can be classified into different categories. These types of distributed algorithms mainly use averaging protocols in individual nodes to get an estimate of time offset by cooperating with other nodes and some tend to extract the clock from observation signals by use of Least Error Estimation and Filtering Methods like Iterative Gaussian Mixture Kalman particle filter (IGMKPF) in Kim, Lee, Serpedin, and Qaraqe (2011) and Costas Loop Time and Frequency Recovery. Many papers have so far focused on WSNs with ZigBee 802.15.4 standard whereas here in this paper, we based our framework on 4G OFDMA standard. It can also be tweaked to be customized for other standards too. The synchronization problem has been touched in many papers but only few have recently used the CAZAC sequences with perfect autocorrelation properties which gives close to ideal orthogonality (Meng & Kang, 2010; Hu & Xu, 2008; Wang & Zheng, 2011). Here in this paper we propose a cooperative synchronization algorithm for large-scale 4G WSNs and discuss the topology effect on synchronization behavior. The robustness of such method comes from diversity order which is proportional to the size of the cluster, i.e., WSN.

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