Zafer Sahinoglu

UWB Systems for Wireless Sensor Networks

Wireless sensor networks are emerging as an important area for communications. They enable a wealth of new applications including surveillance, building control, factory automation, and in-vehicle sensing. The sensor nodes have to operate under severe constraints on energy consumption and form factor, and provide the ability for precise self-location of the nodes. These requirements can be fulfilled very well by various forms of ultra-wide-band (UWB) transmission technology. We discuss various techniques and tradeoffs in UWB systems and indicate that time-hopping and frequency-hopping impulse radio physical layers combined with simple multiple-access techniques like ALOHA are suitable designs. We also describe the IEEE 802.15.4a standard, an important system that adopts UWB impulse radio to ensure robust data communications and precision ranging. In order to accommodate heterogeneous networks, it uses specific modulation, coding, and ranging waveforms that can be detected well by both coherent and noncoherent receivers.

The Cramer-Rao bounds of hybrid TOA/RSS and TDOA/RSS location estimation schemes

The Cramer-Rao bound (CRB) on location estimation accuracy of two different hybrid schemes, time of arrival/received signal strength (TOA/RSS) and time difference of arrival/received signal strength (TDOA/RSS), is computed. For short-range networks, the hybrid schemes offer improved accuracy with respect to conventional TOA and TDOA schemes, particularly in the proximity of the reference devices.

Ranging in the IEEE 802.15.4a Standard

The emerging IEEE 802.15.4a standard is the first international standard that specifies a wireless physical layer to enable precision ranging. In this article, ranging signal waveforms and ranging protocols adopted into the standard are discussed in a tutorial manner

TOA estimation for IR-UWB systems with different transceiver types

In this paper, performances of stored-reference, transmitted-reference (TR), and energy-detection (ED)-based time-of-arrival estimation techniques are analyzed for impulse-radio ultra-wideband (IR-UWB) systems at sub-Nyquist sampling rates. First, an additive white Gaussian noise channel is considered to emphasize certain fundamental issues related to these different transceivers. In particular, energy collection characteristics and decision statistics are presented. Probability of accurate peak detection is analyzed for each transceiver, and receiver operating characteristics for the leading edge are derived. Effects of number of pulses per symbol and number of averaging symbols are investigated in detail. Then, realistic multipath channels are addressed, and various maximum-likelihood estimation approaches are investigated. A new estimator that jointly exploits the noise statistics and power delay profile of the channel is proposed, and a Bayesian estimator that (ideally) gives a lower bound is analyzed. Simulation results show that while ED and TR have better energy collection capabilities at low-rate sampling, they suffer from distributing the energy over time.

Threshold selection for UWB TOA estimation based on kurtosis analysis

In this letter, a normalized threshold selection technique for time of arrival estimation of ultra-wideband signals is proposed. It exploits the kurtosis of the received signal samples. The dependency between the kurtosis and optimal normalized threshold are investigated via simulations. The proposed technique yields efficient threshold selection, has low complexity and sampling rate requirements, and accounts both the signal to noise ratio and the statistics of individual channel realizations.

Image transmission over IEEE 802.15.4 and ZigBee networks

An image sensor network platform is developed for testing transmission of images over ZigBee networks that support multi-hopping. The ZigBee is a low rate and low power networking technology for short range communications, and it currently uses IEEE 802.15.4 MAC and PHY layers. Both ZigBee networking (NWK) and IEEE 802.15.4 MAC layer protocols are implemented on a single M16C microprocessor. Transport layer functionalities such as fragmentation and reassembly are performed at the application layer, since the ZigBee NWK does not have a fragmentation support. The multiple access scheme is CSMA/CA, therefore only the best effort multi-hop transmission of JPEG and JPEG-2000 images are tested; observations and resulting statistics are presented, and open issues are discussed.

Tree-Based Data Broadcast in IEEE 802.15.4 and ZigBee Networks

This paper studies efficient and simple data broadcast in IEEE 802.15.4-based ad hoc networks (e.g., ZigBee). Since finding the minimum number of rebroadcast nodes in general ad hoc networks is NP-hard, current broadcast protocols either employ heuristic algorithms or assume extra knowledge such as position or two-hop neighbor table. However, the ZigBee network is characterized as low data rate and low cost. It cannot provide position or two-hop neighbor information, but it still requires an efficient broadcast algorithm that can reduce the number of rebroadcast nodes with limited computation complexity and storage space. To this end, this paper proposes self-pruning and forward node selection algorithms that exploit the hierarchical address space in ZigBee networks. Only one-hop neighbor information is needed; a partial list of two-hop neighbors is derived without exchanging messages between neighboring nodes. The ZigBee forward node selection algorithm finds the minimum rebroadcast nodes set with polynomial computation time and memory space. Using the proposed localized algorithms, it is proven that the entire network is covered. Simulations are conducted to evaluate the performance improvement in terms of the number of rebroadcast nodes, number of duplicated receivings, coverage time, and communication overhead

Energy efficient JPEG 2000 image transmission over wireless sensor networks

We propose an energy efficient JPEG 2000 image transmission system over point-to-point wireless sensor networks. The objective is to minimize the overall processing and transmission energy consumption with the expected end-to-end QoS guarantee, which is achieved by jointly adjusting the source coding schemes, channel coding rates, and transmitter power levels in an optimal way. The advantages of the proposed system lie in three aspects: adaptivity, optimality, and low complexity. Based on the characteristics of the image content, the estimated channel conditions, and the distortion constraint, the proposed low-complexity joint source channel coding and power control algorithm adjusts the coding and transmission strategies adaptively, which can approximate the optimal solution with a tight bound.

Multiscale energy products for TOA estimation in IR-UWB systems

In this paper, we consider time of arrival estimation of ultra-wideband signals based on low-rate samples that are obtained after a square-law device. Signal conditioning techniques based on a bank of cascaded multi-scale energy collection filters and wavelets are introduced, where correlations across multiple scales are exploited for edge and peak enhancements towards a more accurate detection. The performances of the discussed algorithms are tested on IEEE 802.15.4a residential line-of-sight (LOS) channels.

Reliable broadcast in ZigBee networks

ZigBee is a new industrial standard for ad hoc networks based on IEEE 802.15.4. It is used for low data rate wireless networks and sensor networks. The data broadcast algorithms for ZigBee networks have been recently studied in (21), where the major task is to reduce the number of rebroadcast nodes. A broadcast algorithm should be robust, which can be achievable by introducing redundant transmissions to some extent. This issue will be studied by this paper. Results presented here extend the forward node selection algorithm in (21) for ZigBee networks. The tree neighbor information is exploited to make sure that every node is covered by at least one of its tree neighbors. A non-random rebroadcast timer is set according to the number of neighbors to be covered, distance and link quality to trigger rebroadcast times. The complexity of the proposed algorithm is polynomial in terms of both computation time and memory space. Simulations are conducted to evaluate the broadcast redundancy, coverage time, and coverage ratio, when the number of nodes varies and there is packet loss. Keywords—ZigBee; broadcast; reliability; ad hoc network