The ever-increasing number of WSN deployments based on a large number of battery-powered, low-cost sensor nodes, which are limited in their computing and power resources, puts the focus of WSN time synchronization research on three major aspects of accuracy, energy consumption, and computational complexity. In the literature, the latter two aspects haven’t received much attention compared to the accuracy of WSN time synchronization. Especially in multi-hop WSNs, intermediate gateway nodes are overloaded with tasks for not only relaying messages but also a variety of computations for their offspring nodes as well as themselves. Therefore, not only minimizing the energy consumption but also lowering the computational complexity while maintaining the synchronization accuracy is crucial to the design of time synchronization schemes for resourceconstrained sensor nodes. In this paper, focusing on the three aspects of WSN time synchronization, we introduce a framework of reverse asymmetric time synchronization for resourceconstrained multi-hop WSNs and propose a beaconless energyefficient time synchronization scheme based on reverse oneway message dissemination. Experimental results with a WSN testbed based on TelosB motes running TinyOS demonstrate that the proposed scheme conserves up to 95% energy consumption compared to the flooding time synchronization protocol while achieving microsecond-level synchronization accuracy.

Radio frequency fingerprint (RFF) identification has become a promising security solution for resource-constrained Internet of Things (IoT) devices, which relies on hardware impairments-induced radio frequency features for identification; among which, a hotspot feature is the carrier frequency offset (CFO). Existing research, however, advocates contradictory perspectives on the usage of CFO: For identification and for compensation; the former employs CFO in the feature space while the latter eliminates the CFO from the feature space, both for improving the RFF identification accuracy. In this review, we first discuss the RFF identification procedures and investigate the origination of the CFO and further its relationship with the clock skew of the crystal oscillator. We then provide a review of the state-of-the-art RFF identification schemes, in two categories, respectively, employing CFO for identification and compensation. Finally, on a real testbed, we experimentally investigate the impact of the usage of CFO on RFF identification accuracy. Experimental results reveal that, the stabilities of CFOs are quite different on hardware platforms from different manufacturers; CFOs can be used for identification when they are relatively distinguishable; compensating CFO alone is inadequate for long-term identification.

Long Range (LoRa) network has been thriving in the IoT era due to its long-range coverage and energy efficiency. Prevalent LoRa applications rely on time synchronization to achieve accurate data ordering and coordination among the LoRa network. The energy-efficient nature of the LoRa network where end nodes in sleep scheduling always initiate communications, however, contradicts most conventional time synchronization methods based on message exchanges. In this paper, we propose a one-way time synchronization scheme tailored for the energy-efficient LoRa network based on the reverse asymmetric framework. We first discuss the delay minimization and compensation for the reverse one-way time synchronization in the LoRa network. We then propose a time synchronization scheme consisting of two time translation methods with different computational complexities and error bounds, respectively for resource-abundant and -constrained LoRa gateway and end nodes. Experiment results on a real LoRa testbed consisting of LoRa gateway and end nodes demonstrate that the proposed scheme could achieve microsecond-level synchronization accuracy in both scenarios between the end node and gateway and between the end node and end node; the latter scenario is advocated in the recent multi-hop LoRa network research.

Energy-efficient time synchronizations for wireless sensor networks (WSNs) have been put under the spotlight for years. A promising technique among which is the timestamp-free approach where no timestamps are required to establish the synchronization, thereby sparing the transmissions of the timing messages for conserving significant transmission energy. In this paper, we first investigate the feasibility of adopting timestamp-free time synchronization in practical resource-constrained WSNs; we then identify the issue of inaccuracy in maintaining the pre-defined response interval which affects the foundations of most existing timestamp-free schemes. Based on the investigation and our previously proposed reverse asymmetric time synchronization framework, we further propose an asymmetric timestamp-free time synchronization scheme with two estimation methods tailored for resource-constrained WSNs. We as well introduce the centralized and distributed multi-hop extension methods for the proposed scheme to cover diverse multi-hop scenarios. Experimental results on a real WSN testbed consisting of TelosB motes running TinyOS demonstrate that the proposed scheme achieves high energy efficiency while maintaining microsecond-level time synchronization accuracy compared to three other conventional schemes.

Node identification based on unique hardware features like clock skews has been considered an efficient technique in wireless sensor networks (WSNs). Spoofing attacks imitating unique hardware features, however, could significantly impair or break down conventional clock-skew-based node identification due to exposed clock information through broadcasting. To defend against Spoofing attacks, we propose a new node identification scheme called node identification against Spoofing attack (NISA). It utilizes the reverse time synchronization framework, where sensor nodes’ clock skews are estimated at the head of a WSN, and the spatially-correlated radio link information to achieve simultaneous node identification and attack detection. We further provide centralized and distributed NISA for covering both single-hop and multi-hop scenarios, the former of which employs a single-input and multiple-output convolutional neural network. With a real WSN testbed consisting of TelosB sensor nodes running TinyOS, we investigate the identifiability of clock skews under temperature and voltage variations and evaluate the performance of both centralized and distributed NISA. Experimental results demonstrate that both centralized and distributed NISA could provide accurate node identification and Spoofing attack detection.

High-precision time synchronization is chased by thriving Long Range (LoRa) research due to its significance for accurate monitoring, detection, and network collaboration. Only a few LoRa time synchronization methods could achieve microsecond-level accuracy, however, at the expense of extensive synchronization message exchanges. In this paper, we propose a microsecond time synchronization for the LoRa network based on low-layer timestamping and asymmetric time translation. Note that, the proposed time synchronization does not induce synchronization message transmissions at the LoRa end nodes, therefore, achieves high energy efficiency. Experimental results on a real LoRa testbed demonstrate that the proposed method achieves microsecond-level synchronization accuracy with significantly fewer synchronization message transmissions compared to the state-of-the-art.