Ai-Chun Pang

An Adaptive GTS Allocation Scheme for IEEE 802.15.4

IEEE 802.15.4 is a new standard uniquely designed for low-rate wireless personal area networks. It targets ultralow complexity, cost, and power for low-rate wireless connectivity among inexpensive, portable, and moving devices. IEEE 802.15.4 provides a guaranteed time slot (GTS) mechanism to allocate a specific duration within a superframe for time-critical transmissions. This paper proposes an adaptive GTS allocation (AGA) scheme for IEEE 802.15.4, which considers low latency and fairness. The scheme is designed based on the existing IEEE 802.15.4 medium access control protocol, and IEEE 802.15.4 devices can receive this AGA service without any modification. A simulation model and an analytical model are developed to investigate the performance of our AGA scheme. The numerical results show that the proposed scheme significantly outperforms the existing IEEE 802.15.4 implementation.

Multiprocessor energy-efficient scheduling with task migration considerations

This paper targets energy-efficient scheduling of tasks over multiple processors, where tasks share a common deadline. Distinct from many research results on heuristics-based energy-efficient scheduling, we propose approximation algorithms with different approximation bounds for processors with/without constraints on the maximum processor speed, where no task migration is allowed. When there is no constraint on processor speeds, we propose an approximation algorithm for two-processor scheduling to provide trade-offs among the specified error, the running time, the approximation ratio, and the memory space complexity. An approximation algorithm with a 1.13-approximation ratio for M-processor systems is also derived (M > 2). When there is an upper bound on processor speeds, an artificial-bound approach is taken to minimize the energy consumption with a 1.13-approximation ratio. An optimal scheduling algorithm is then proposed in the minimization of the energy consumption when task migration is allowed.

Distributed Throughput Optimization for ZigBee Cluster-Tree Networks

ZigBee, a unique communication standard designed for low-rate wireless personal area networks, has extremely low complexity, cost, and power consumption for wireless connectivity in inexpensive, portable, and mobile devices. Among the well-known ZigBee topologies, ZigBee cluster-tree is especially suitable for low-power and low-cost wireless sensor networks because it supports power saving operations and light-weight routing. In a constructed wireless sensor network, the information about some area of interest may require further investigation such that more traffic will be generated. However, the restricted routing of a ZigBee cluster-tree network may not be able to provide sufficient bandwidth for the increased traffic load, so the additional information may not be delivered successfully. In this paper, we present an adoptive-parent-based framework for a ZigBee cluster-tree network to increase bandwidth utilization without generating any extra message exchange. To optimize the throughput in the framework, we model the process as a vertex-constraint maximum flow problem, and develop a distributed algorithm that is fully compatible with the ZigBee standard. The optimality and convergence property of the algorithm are proved theoretically. Finally, the results of simulation experiments demonstrate the significant performance improvement achieved by the proposed framework and algorithm over existing approaches.

A comprehensive analysis of low-power operation for beacon-enabled IEEE 802.15.4 wireless networks

ZigBee, a unique communication standard designed for low-rate wireless personal area networks, has extremely low complexity, cost, and power consumption for wireless connectivity of inexpensive, portable, and moving devices. ZigBee uses the IEEE 802.15.4 standard as its communication protocol for medium access control (MAC) layer and physical (PHY) layer. The IEEE 802.15.4 MAC layer achieves duty-cycle operations by setting two system parameters, macBeaconOrder (BO) and macSuperFrameOrder (SO), to achieve low power consumption for ZigBee devices. This study comprehensively analyzes IEEE 802.15.4 duty-cycle operation. Specifically, a novel analytical model that accommodates a general traffic distribution is developed. An NS-2 based simulation model, which is validated by the developed analytical model is also proposed. Through the experiments conducted by the analytical and simulation models, some important performance-evaluation insights are gained that can be used as guidelines for future low-power ZigBee network deployment.

Dynamic backoff for wireless personal networks

Based on IEEE 802.15.4 low-rate wireless personal area networks (LR-WPANs), this paper proposes a memorized backoff scheme (MBS) with the exponential weighted moving average (EWMA) approach to dynamically adjust the size of the contention window. The proposed scheme can be implemented in the standard IEEE 802.15.4 medium access control (MAC) protocol without adding any new message type and without modifying the communicating procedure. An analytic model and a simulation model are developed to evaluate the performance of IEEE 802.15.4, MBS and MBS+EWMA. The numerical results indicate that in terms of goodput, completion rate, average MAC delay, average queuing delay and average number of collisions for each data frame, our proposed scheme significantly outperforms the standard IEEE 802.15.4 backoff scheme.

An all-IP approach for UMTS third-generation mobile networks

This article describes the UMTS all-IP approach for third-generation mobile systems, with emphasis on the core network architecture. Following the introduction of the core network nodes, we elaborate on application-level registration, circuit-switched call origination, packet-switched call origination, and packet-switched call termination.

Comparing soft and hard handoffs

This paper studies the soft-handoff mechanism and compares its performance with hard handoff. Our study indicates that although a handset may potentially consume extra radio links in soft handoff, the mechanism provides better opportunity to transfer the link successfully in the handoff procedure. Thus, by carefully planning the overlay areas of cells, soft handoff can outperform hard handoff.

A Mobility-Aware Node Deployment and Tree Construction Framework for ZigBee Wireless Networks

ZigBee is a specification formalized by the IEEE 802.15.4 standard for low-power low-cost low-data-rate wireless personal area networks. In ZigBee networks, a tree topology is often used to construct a wireless sensor network for data delivery applications. However, delivery failures constantly occur in ZigBee wireless applications due to node movements and network topology changes. The conventional route reconstruction method is designed to mitigate the effects of topology changes, but it consumes a large amount of resources. In this paper, we exploit the regularity in node mobility patterns to reduce the frequency of route reconstructions and ensure that the transmission of data to mobile nodes is efficient. To increase the data delivery ratio and mitigate the effects of packet loss caused by the node mobility, we propose a ZigBee node deployment and tree construction framework. In particular, the framework considers the regularity in mobility patterns during the construction of the routing tree and deployment of nodes. It also includes an overhearing mechanism for mobile nodes to further improve the data delivery ratio. We present details of the proposed algorithms for node deployment and tree construction in the framework. The effectiveness of network topologies constructed under the framework is demonstrated through comprehensive ns-2 simulations based on two real-world scenarios. The results show that our approach can construct ZigBee tree topologies with a high data delivery ratio and low routing overhead.

Energy-Efficient Video Multicast in 4G Wireless Systems

Layer-based video coding, together with adaptive modulation and coding, is a promising technique for providing real-time video multicast services on heterogeneous mobile devices. With the rapid growth of data communications for emerging applications, reducing the energy consumption of mobile devices is a major challenge. This paper addresses the problem of resource allocation for video multicast in fourth-generation wireless systems, with the objective of minimizing the total energy consumption for data reception. First, we consider the problem when scalable video coding is applied. We prove that the problem is NP-hard and propose a 2-approximation algorithm to solve it. Then, we investigate the problem under multiple description coding, and show that it is also NP-hard and cannot be approximated in polynomial time with a ratio better than 2, unless P = NP. To solve this case, we develop a pseudopolynomial time 2-approximation algorithm. The results of simulations conducted to compare the proposed algorithms with a brute-force optimal algorithm and a conventional approach are very encouraging.

Mobility and session management: UMTS vs. cdma2000

This article describes the mobility and session management mechanisms for UMTS and cdma2000 packet-switched (PS) service domains, and compares the design guidelines for these two third-generation technologies. We first introduce the network architectures and protocols for UMTS and cdma2000, and then elaborate on the PS service domains mobility management, session management, and IP-level mobility mechanisms. Based on the mobility and session management mechanisms of the UMTS and cdma2000 PS service domains, an integrated architecture and intersystem roaming procedures are proposed to show the implementation feasibility of UMTS-cdma2000 IP-level interworking.