Gaotao Shi

MuZi: Multi-channel ZigBee Networks for Avoiding WiFi Interference

In Cyber-Physical system (CPS), different wireless technologies are used, which results in cross-technology communication interference. Specifically, ZigBee networks share the 2.4GHz ISM band with WiFi but have much lower transmission power. Thus, the ZigBee networks inevitably suffer the interference from WiFi Networks. This paper focuses on the locality of WiFi interference and proposes an interference avoiding approach based on multi-channel for ZigBee networks called MuZi. MuZi has three basic mechanisms: interference assessment, channel switch and connectivity maintenance. The proposed interference assessing approach jointly considers the degree of intensity and density and achieves a much actual relationship between WiFi interference and link quality. Based on this finding, the better working channel for each node is determined. A connectivity maintenance approach is also proposed to ensure the connectivity of the nodes to the sink. Our extensive experiments on a test bed of 802.11 embedded nodes and 802.15.4 TelosB motes show that, under the existence of WiFi interference, MuZi can achieve 3.3 times throughput than the traditional single-channel method.

Exploiting sink movement for energy-efficient load-balancing in wireless sensor networks

Prolonging the lifetime is an important design consideration for battery powered wireless sensor networks. In a network with stationary sink, the sensor nodes located near the sink have to relay data from the rest of the network and thus deplete their energy very quickly. A sink mobility strategy was proposed in [1], which manages the sink to move along the periphery of the network for load-balancing. In this paper, based on the work in [1], we study the relationship of the energy efficiency and load-balancing. A novel mobility scheme for sink has been proposed to achieve the energy efficient load-balancing. By this scheme, the sink is controlled to move along a circle trajectory in a stationary for data buffering. All sensed data are forwarded into the annularity area firstly and then collected by the mobile sink. We find the optimum trajectory of sink movement with consideration of energy consumption and load-balancing and present how to find the location for the buffering area. Compared with the static sink scheme and the existing mobile sink scheme, the proposed is the most energy-efficient and it can reduce the load by 95% and 70%, respectively.

Fueling Wireless Networks perpetually: A case of multi-hop wireless power distribution ∗

Inspired by the recent invention of a high efficiency Wireless Power Transfer (WPT) technique, we propose in this paper the Perpetual Wireless Networks (PWNs) as a novel wireless networking paradigm. Similar to the conventional wireless (data) access point, a PWN has a power access point, from which electrical power is injected and distributed into the network in a form of multi-hop transfer. Consequently, we lay our focus on this new type of multi-hop flow problems concerning not data but power. We formulate and analyze a set of such power flow problems (some are joint with data flow), and we devise algorithms to solve them. The intriguing insights obtained from solving these optimization problems offer instructive guidance for future studies on real PWN constructions.

FAVOR: frequency allocation for versatile occupancy of spectrum in wireless sensor networks

While the increasing scales of the recent WSN deployments keep pushing a higher demand on the network throughput, the 16 orthogonal channels of the ZigBee radios are intensively explored to improve the parallelism of the transmissions. However, the interferences generated by other ISM band wireless devices (e.g., WiFi) have severely limited the usable channels for WSNs. Such a situation raises a need for a spectrum utilizing method more efficient than the conventional multi-channel access. To this end, we propose to shift the paradigm from discrete channel allocation to continuous frequency allocation in this paper. Motivated by our experiments showing the flexible and efficient use of spectrum through continuously tuning channel center frequencies with respect to link distances, we present FAVOR (Frequency Allocation for Versatile Occupancy of spectRum) to allocate proper center frequencies in a continuous spectrum (hence potentially overlapped channels, rather than discrete orthogonal channels) to nodes or links. To find an optimal frequency allocation, FAVOR creatively combines location and frequency into one space and thus transforms the frequency allocation problem into a spatial tessellation problem. This allows FAVOR to innovatively extend a spatial tessellation technique for the purpose of frequency allocation. We implement FAVOR in MicaZ platforms, and our extensive experiments with different network settings strongly demonstrate the superiority of FAVOR over existing approaches.

Monitoring the high-voltage transmission lines based on two-tier wireless networks

Wireless networks have become an effective solution for remote surveillance and data gathering without an existing communication infrastructure. The poster presents a system for monitoring the high-voltage transmission lines. This system implemented a two-tier wireless network to gather the data for the high voltage transmission lines and transmit these data to the control center based on WiFi technology and Zigbee technology.

ART: Adaptive fRequency-Temporal Co-Existing of ZigBee and WiFi

Recent large-scale deployments of wireless sensor networks have posed a high demand on network throughput, forcing all (discrete) orthogonal ZigBee channels to be exploited to enhance transmission parallelism. However, the interference from widely deployed WiFi networks has severely jeopardized the usability of these discrete ZigBee channels, while the existing CSMA-based ZigBee MAC is too conservative to utilize each channel temporally. In this paper, we propose ART (Adaptive fRequency-Temporal co-existing) as a framework consisting of two components: FAVOR (Frequency Allocation for Versatile Occupancy of spectRum) and P-CSMA ( Probabilistic CSMA), to improve the co-existence between ZigBee and WiFi in both frequency and temporal perspectives. On one hand, FAVOR allocates continuous (center) frequencies to nodes/links in a near-optimal manner, by innovatively converting the problem into a spatial tessellation problem in a unified frequency-spatial space. This allows ART to fully exploit the “frequency white space” left out by WiFi. On the other hand, ART employs P-CSMA to opportunistically tune the use of CSMA for leveraging the “temporal white space” of WiFi interference, according to real-time assessment of transmission quality. We implement ART in MicaZ platforms, and our extensive experiments strongly demonstrate the efficacy of ART in enhancing both throughput and transmission quality.

Exploiting temporal and spatial variation for WiFi interference avoidance in ZigBee networks

Specifically, ZigBee networks share the 2.4 GHz ISM band with WiFi but have much lower transmission power. Therefore, the ZigBee networks inevitably suffer the interference from WiFi networks. In this paper, we focus on utilising the temporal and spatial feature of WiFi interference to adjust Zigbee channel over the whole networks. We experimentally examine the spatiotemporal variation of interference and then present a novel interference assessing method which jointly considers the intensity and density of WiFi interference in order to better characterise the relation between interference and link quality. Further considering the interference locality, we propose MuZi Multi-channel ZigBee as an interference avoiding approach for ZigBee networks. It aims to determine a better working channel for each node while taking network connectivity into consideration. Our experiments show that, under the existence of WiFi interference, MuZi can achieve 3.3 times throughput than the traditional single-channel method.

Energy-Efficient Data Gathering in High-Voltage Transmission Line Monitoring System

It is important to protect power supply grid system against various damages for national economics. A high-voltage transmission line monitoring system is an effective way to protect the power supply system. This paper presents an energy-efficient data gathering mechanism for such a system. Our contributions are two-fold: (1) a detailed measurement of the energy consumption for wireless nodes, (2) the collaboration between backbone network nodes and subnets to implement the sleep-wakeup mechanism. To validate the proposed mechanism, we have established an indoor test bed consisting of 8 backbone network nodes. The energy consumption of each node with our mechanism is evaluated and compared to that without sleep. The results show that the energy consumption has decreased by 24% by our scheme.

Understanding link-level characterization of long-distance 802.11g semi-urban links

WiFi-based Long Distance (WiLD) mesh networks have been widely deployed with the intention of bringing extremely low-cost Internet connectivity to the rural areas worldwide. Prior studies do not discuss performance degradation in outdoor 802.11g links caused by faulty driver/card-specific proprietary algorithms and have fewer investigations on the impact of external WiFi interferences in terms of burst loss on WiLD 802.11g links. To the best of our knowledge, this is the first study of long distance 802.11g links in a typical semi-urban environment, concerning the maximum achievable throughput as well as the bursty features of loss ratio. We found that the principle causes for poor link performance are the buggy ANI (Ambient Noise Immunity) algorithm implemented in Atheros chips and the inefficient 802.11 CSMA/CA mechanism for point-to-point links. The former results in massive undecodable frames due to PHY errors while the latter throttles the transmission rate of the sender. OFDM timing and CRC errors caused by external interference contribute most to the burst loss, which appears in the magnitude of several to tens of seconds. Besides, we also document some valuable experiences learned in our experiments.