Showing papers by "Mihail L. Sichitiu published in 2011"

Optimal Max–Min Fair Resource Allocation in Multihop Relay-Enhanced WiMAX Networks

Abstract: In this paper, an optimal resource allocation scheme is proposed for WiMAX networks that are enhanced with nontransparent relay stations (RSs). We first focus on two-hop relaying networks, because this scenario has the largest throughput gain. Furthermore, several researchers have proposed different resource allocation schemes in two-hop relaying networks, and thus, we can compare the performance of our proposed scheme with the following two well-known schemes in terms of cell throughput, outage rate, and computational time: 1) orthogonal and 2) overlapped . We then explore an extension of the proposed scheme to a general multihop relaying scenario by taking into account spatial reuse in both the access and relay zone periods. The numerical results show that the highest cell throughput can be achieved by the proposed optimal scheme while maintaining the fairness and lower outage performance of the orthogonal scheme at a higher computational cost. Moreover, the cell throughput degradation from increasing the number of hops from two to three hops, under the proposed optimal scheduling scheme, is only 12% and 19% when six and nine RSs are deployed, respectively.

Contact time in random walk and random waypoint: Dichotomy in tail distribution

Abstract: Contact time (or link duration) is a fundamental factor that affects performance in Mobile Ad Hoc Networks. Previous research on theoretical analysis of contact time distribution for random walk models (RW) assume that the contact events can be modeled as either consecutive random walks or direct traversals, which are two extreme cases of random walk, thus with two different conclusions. In this paper we conduct a comprehensive research on this topic in the hope of bridging the gap between the two extremes. The conclusions from the two extreme cases will result in a power-law or exponential tail in the contact time distribution, respectively. However, we show that the actual distribution will vary between the two extremes: a power-law-sub-exponential dichotomy, whose transition point depends on the average flight duration. Through simulation results we show that such conclusion also applies to random waypoint.

Optimal resource allocation in multihop relay-enhanced WiMAX networks

Abstract: Multihop relaying in WiMAX networks is considered an increasingly attractive technology for providing throughput enhancement, coverage extension, or both. In this paper, we consider deploying non-transparent relay stations (RSs) that transmit on the same carrier frequency as the base station (BS) for the purpose of coverage extension. Since it is possible that the RSs and BS are transmitting/receiving simultaneously to/from the subscriber stations (SSs) during the access zone period, it is challenging to schedule resources optimally for serving SSs in a fair manner. We propose an optimal scheduling scheme that preserves fairness among active SSs, and compare its performance with two different resource allocation schemes, namely, the overlapped and orthogonal in terms of cell throughput and outage performance. Our numerical results show that proposed scheme can achieve more throughput than the orthogonal scheme, while maintaining the fairness and lower outage performance of the orthogonal one.

Optimal Placement of Transparent Relay Stations in 802.16j Mobile Multihop Relay Networks

Abstract: WiMAX (IEEE 802.16) has emerged as a promising radio access technology for providing high speed broadband connectivity to subscribers over large geographic regions. New enhancements allow deployments of relay stations (RSs) that can extend the coverage of the base station (BS), increase cell capacity, or both. In this paper, we consider the placement of transparent RSs that maximize the cell capacity. We provide a closed-form approximation for the optimal location of RS inside a cell. A numerical analysis of a number of case studies validates the closed-form approximation. The numerical results show that the closed-form approximation is reasonably accurate. key words: WiMAX, relay networks, optimal placement

Performance evaluation of multihop wireless networks

Abstract: In this thesis, we investigate the performance of protocols for multihop wireless networks, focusing on Wireless Sensor Networks (WSNs) and Wireless Mesh Networks (WMNs). For both type of networks, we design and implement a protocol development and evaluation environment, and propose a new protocol for each multihop network. For WSNs, we design and implement RaPTEX, a rapid prototyping tool for embedded communication systems, consisting of three major subsystems: a toolbox for protocol compositions, an analytical performance estimation for fast exploration of performance trade-offs between different protocols, and a hybrid emulation environment for better accuracy of performance evaluation. Using RaPTEX, we study the performance trade-offs between energy efficiency, end-to-end delay, and throughput for different protocols, configurations, and topologies. Based on the performance study, we propose a multichannel asynchronous scheduled MAC protocol for WSNs, which significantly improves packet delivery ratio and delay without compromising energy efficiency under a wide range of traffic loads. For WMNs, we design and develop a service oriented software platform for a WMN testbed to facilitate protocol development and experimental research. Using the testbed software, we study the spatial throughput unfairness in a saturated WMN. Based on the measurement study, we define a joint channel assignment and routing problem, find the optimum solution using a mixed-integer non-linear programming (MINLP) model, and propose a joint channel assignment and routing algorithm that heuristically balances the channel utilization of bottleneck links to maximize throughout in multi-radio, multi-channel, multi-rate WMNs, while maintaining fairness. Through simulations, we show that the proposed algorithm substantially outperforms the existing schemes.