I. Introduction
LOCALIZATION of wireless sensor networks (WSN), which is crucial for environmental, military, and medical applications, has gained increasing attention in the past decade [1], [2]. Taking environmental monitoring sensor networks as example, every node needs to report its measurements together with its location information. Actually, environmental data without the corresponding location is meaningless. Besides, the routing protocol designed for multi-hop WSN can be more efficient if the network topologies are considered [3].