Shudong Fang

An Overhead Free Clustering Algorithm for Wireless Sensor Networks

An overhead-free, fully distributed clustering algorithm is proposed to decompose wireless sensor networks, where nodes are initialized with either equivalent or different energy capacities, into a two-tier clustered hierarchical structure. Energy-rich nodes are assured to act as cluster heads (CH), and CHs are dispersed evenly over the network. In the new algorithm, a converting function, a multiplicatively increasing CH selection probability, and two backoff strategies are interwoven over three phases during the CH selection and placement. Via simulations, the performance of the proposed algorithm has been demonstrated considering representative network scenarios. The results show that our algorithm outperforms some existing clustering methods in extending the system lifetime and enlarging the network data capacity.

A study on DSSS transceivers using OQPSK modulation by IEEE 802.15.4 in AWGN and flat Rayleigh fading channels

IEEE 802.15.4 defines physical layer specifications for compatible interconnections amongst devices using low-power transceivers. However, error performances of these transceivers must be discreetly studied to investigate the transceivers energy expense on wireless communication. This paper reports a simulation-based study on the transceivers conducting direct sequence spreading spectrum (DSSS) processing and Offset Quadrature Phase Shift Keying (OQPSK) modulation specified by IEEE 802.15.4. The study focuses on evaluating the bit error rate (BER) performances of these transceivers in Additive White Gaussian Noise (AWGN) and Rayleigh flat-fading channels. A Matlab-based simulator is developed to simulate the signal processing components of the DSSS transceivers such that the input/output signals of each component and the transceivers BER curve can be drawn. The derived BER curves are found greatly degrading from the corresponding theoretical BER. The degradation may stem from the peculiar transceiver design and inferior orthogonality of the standard chip sequences. To redeem this degradation without changing the transceiver structure, the standard chip sequences are replaced with w-sequences possessing better orthogonality. Signals transmitted via the transceivers exploiting w-sequences are found to significantly outperform the counterpart via the transceivers using the standard sequences in withstanding noise and fading effects.

Energy consumption evaluations of cluster-based sensor nodes with IEEE 802.15.4 transceiver in flat Rayleigh fading channel

This paper studies the energy consumption of cluster-based wireless sensor networks (WSNs) which comprise sensor nodes carrying the transceivers on the basis of the Direct Spread Sequence Spectrum (DSSS) techniques as per the IEEE 802.15.4 standard to transmit data over the AWGN channel with flat Rayleigh fading. To this end, clarifications are made on the bit error rate (BER) expressions for the considered transceivers in the presence of AWGN and the flat Rayleigh fading. Based on the presented BER expressions, the node energy consumption is evaluated in the cluster-based WSNs that are formed by running representative clustering algorithm via simulations. Parameters for the evaluations take values from the off-the-shelf transceivers in compliance with IEEE 802.15.4. The numerical results manifest the large energy expenditures on compensating the Rayleigh fading to accomplish reliable data transmissions.

Analysis of Neighbor Discovery Protocols for Energy Distribution Estimations in Wireless Sensor Networks

This paper analyses the duration, energy consumption and efficiency of two groups of neighbor discovery protocols dedicated for wireless sensor networks (WSN) with respect to various densities and discovery ratios. The derivation of the mathematical models regarding these parameters can be used for other WSN-oriented neighbor discovery schemes. Numerical results show that significant amounts of time and energy are consumed by the investigated protocols to achieve desirable discovery ratio that may have severe effects on the subsequent network organization, especially in dense scenarios.

Energy efficiency of collaborative communication with imperfect phase synchronization and Rayleigh fading in wireless sensor networks

In this paper it is presented that significant power gain can be accomplished using collaborative communication, despite of imperfect phase synchronization over the Rayleigh fading channel. This paper presents an energy consumption model for the considered collaborative communication system with imperfect phase synchronization and channel fading. With this model, it is easy to find that, as the number of collaborative sensor nodes increases, notable power can be saved from the data transmission at the expense of increased circuit power. Hence, an energy saving model is defined to study the trade-off between the data transmission power gain and required circuit power. The presented numerical results use parameters taking values from the off-the-shelf products. It is shown that significant energy can be saved using collaborative communication as compared to SISO (Single input single output) communication when the transmission distance is greater than the break-even distance.

Performance of a clustering algorithm for high density wireless sensor networks

The present study investigates the performance of a clustering algorithm for sensor networks called SWEET (Slotted Waiting period Energy-Efficient Time-driven clustering algorithm) [1] under a variety of situations. The effectiveness of SWEET has been illustrated by changing both the number and the radio range of nodes considering several performance metrics such as number of selected Cluster Heads (CH) and the time delays of CH selections. It is shown that SWEET can generate desirable number of CHs within an acceptable time delay in densely deployed scenarios. Based on an energy saving model, it is observed that inter-cluster multihop routing schemes are more energy efficient compared to intra-cluster multihop routing schemes for relaying data in a network which is decomposed by SWEET. Further, it has been shown that provided the network density satisfies an analytical lower bound, the distance between adjacent CHs selected by SWEET is bounded, which alternately facilitate the inter-cluster multihop routing. Simulations on high density sensor networks show that SWEET performs satisfactorily in organizing nodes with varying radio ranges into various clusters with neighboring CHs being bounded by a particular distance.

Characterization of hello message exchange to estimate sensor node's neighborhood residual energy distribution in initialization phase

The paper characterizes the hello message exchange (HME) procedure for a sensor node to develop its neighborhood residual energy distribution in the initialization phase of a static wireless sensor network. Because of the lack of coordination on channel access in the initialization phase, hello messages from multiple nodes face a high risk of data collision in the exchange course. A discovery ratio is hereby defined to measure the sufficiency of the HME procedure. The discovery ratio is related to the precision of the parameter estimates for the probability density function of a nodes neighborhood residual energy distribution. To achieve an arbitrarily high discovery ratio within a resolvable time interval, the HME procedure is implemented using Birthday protocol, which results in large node energy consumption. To overcome this flaw, a method termed carrier sensing mini-slot algorithm is proposed to carry out the HME procedure. The time duration and the node energy consumption for the HME procedures based on the Birthday protocol and the carrier sensing mini-slot algorithm, respectively, are theoretically analyzed and verified by simulations. Copyright

Delay diversity for direct sequence spread spectrum system to reduce transmitting power in Rayleigh flat-fading channel

This paper presents a novel technique to save the transmitting power of spread-spectrum communication systems based on the application of delay diversity. The core idea of this delay-diversity system is to delay two spreading sequences that carry the same information of a bit with respect to each other in order to statistically separate the influence of fading on the same bit in time domain. The theoretical bit error rate (BER) expression of the proposed system is derived and verified by simulations. According to the numerical results, it is found that this technique save significant amount of transmitting power, and it may be used to extend the life-time of energy-constrained devices such as sensor nodes in wireless sensor networks.

Characterization of hello message exchange for estimating distribution of network residual energy

This paper investigates the practicability that a sensor node develops the probability density function (pdf) of its local network energy via exchanging hello messages with its neighboring nodes in the context of dense node deployment. The pdf is proven to approach Gaussian and can be used to decentralize a recent clustering algorithm. To alleviate the broadcast storm problem, a node is considered broadcasting hello messages to its neighboring nodes without immediate feedback from the receiving ones. Thus the broadcasting node cannot be guaranteed that its neighboring nodes have received its messages which are at high risk of channel collision. Characterizing hello message exchange becomes nontrivial, as the discovery ratio, which measures the effectiveness and the sufficiency of the message exchange, is identified having decisive effect on the precision of the developed pdf. A set of time asynchronous and slot-based channel access rules is presented for the sufficient and fast exchange of hello messages.

Closed-form average BER expression for chip-interleaved DS-CDMA system conducting M-ary communication and noncoherent demodulation in flat Rayleigh fading channel

Closed-form expression of average Bit Error Rate (BER) is developed for Chip-Interleaved DS-CDMA system that conducts M-ary communication and non-coherent demodulation in AWGN channel with flat Rayleigh fading. The transmitter uses BPSK modulation and employs binary pseudo-random sequences for symbol-spreading. In receiver the non-coherent demodulation is based on the optimal quadrature demodulator to cancel the phase angles of received signals. Numerical results of simulations greatly agree with the analytical results of the developed expression.