Performance Optimization Algorithms for Wireless Networks

• Main Authors: Yean-Fu Wen, 溫演福
• Other Authors: Frank Yeong-Sung Lin
• Format: Others
• Language: en_US
• Published: 2007
• Online Access: http://ndltd.ncl.edu.tw/handle/52923795647852828626

博士 === 臺灣大學 === 資訊管理學研究所 === 95 === It is now possible to access data services anywhere, any time, via wireless networks ranging from PWANs (Personal Wireless Area Networks) to office or home area networks, from mesh networks and WiMax to satellite networks. As for the future of network technologies, it is essential that research be directed toward improving person-to-person, person-to-machine, and machine-to-machine communications. Thus, in this dissertation, we focus on wireless networks, as well as the challenges and research avenues presented by network planning and performance. Primarily focusing on network architecture and network layers, the research scope of this dissertation covers various network architectures, such as Wi-Fi hotspots, mesh networks, and ad hoc networks (including sensor networks); and considers various network layers: the application layer, the network layer, media access control (MAC), and the physical layer. Previous related research is discussed in Chapter 1. Both network planning and performance optimization issues are addressed. The following is a brief summary of the presentation of these issues to be addressed in depth in the body of this dissertation: • Wi-Fi hotspots [Chapter 2] In such hotspots, the transmission bit rate for a mobile device (MD) is dependent on its distance from the nearest base-station. A problem arises when fast and slow MDs share Abstract vii a network in that, despite the higher capability of a fast MD, the throughput of that fast MD is the same as that of a slow MD. Therefore, we address this problem and propose an algorithm to achieve channel access time fairness. Our research includes comparative studies of three adaptive MAC parameters: (i) the packet size, (ii) the initial contention window size, and (iii) multiple back-to-back packets. On the basis of that research, we have determined that adjusting the size of the initial contention window provides the most significant optimization of the maximum system throughput. It has been established that determining a global optimal solution is impossible in a reasonable time; therefore, a modified binary search algorithm is implemented to solve the problem. Experiment results show that the system throughput is 5.92 Mbps, which is a 21% improvement over the original MAC protocol. • Mesh networks [Chapter 3-4] In mesh networks, the main issues are the performance and fairness of the system or individual devices due to spatial bias. The issues addressed include: (i) top load-balanced routing; (ii) end-to-end delay fairness; and (iii) backhaul assignment problems, which have proven to be NP-complete. In this dissertation, these problems are formulated as mixed-integer nonlinear programming problems. Lagrangean Relaxation (LR) is used to solve the primal and Lagrangean dual problems, and to obtain upper and lower bounds. Gaps between research issues (i) and (ii) are shown to be less than 5%. Although a larger gap exists between issues (i) and (iii), i.e., 40%, the improvement ratio is still 10% over other modified methods. • Ad hoc networks [Chapter 5] For ad hoc networks, the main concern addressed in this dissertation is the transmission of multicast messages via broadcasting. The advantage of this method is that it obtains the so-called “wireless broadcast advantage”. The same message is sent only once, but it is received by many devices. Based on routing paths, we propose an optimization-based integer- and nonlinear-programming model. The radius of each node is calculated intelligently according to the structure of the broadcast tree, thus minimizing the total power consumption required to broadcast each multicast message to all receivers. This problem has also proven to be NP-complete. We adopt LR methods to solve the problem, and determine the gaps to be within 10%. viii Wen - Performance Optimization Algorithms for Wireless Networks This static network research problem is extended to include mobility issues in mobile networks. The message is broken down into smaller sub-sections. For a mobile node, given the direction and speed, the duration of the current broadcast tree is found. New broadcast trees are constructed to provide coverage to multicast group nodes until the complete message is sent. Like the previous static case, this is also an NP-complete problem. We solve it by LR, which obtains a gap of less than 30%. Our experiment results show that the proposed algorithms outperform the MSPT, Prim MST, BIP, and GIBT heuristics by at least 5%. • Sensor networks [Chapter 6,7] Sensors are typically scattered throughout an area of interest. As they may be located in remote areas, recharging the sensors’ batteries is often infeasible. The network lifetime of a wireless sensor network, which is interrupted when depleted batteries prevent the transmission of environmental information, is dependent on battery capacity and energy consumption efficiency, and has become a crucial issue in sensor network research. Therefore, to prolong network lifetime starting from the physical layer and extending all the way up to the application layer, we focus on: (i) multi-rate routing; (ii) dynamic adjustment of the nodal transmission radius; (iii) duty cycle scheduling; (iv) collision avoidance; (v) routing; and (vi) data aggregation. All combinations of these six issues are considered within multi-sink and cluster-based architectures. These are serial problems, formulated as mixed-integer nonlinear programming problems that have proven to be NP-complete. Thus, the LR approach is used to find solutions to the serial problems. Meanwhile, algorithms, including an O-MAC protocol and a serial DAR (data aggregation routing) algorithm, are proposed to optimize energy consumption. The feasible solution is derived from information provided by the Lagrangean multipliers, and compared with the performance of other heuristics, such as GIT, CNS, or SPT, which are modified to satisfy constraints on the research problem. Our experiment results show that the proposed heuristic outperforms the others approaches by 7%-43%. Conclusions and extensions of the work in this dissertation are presented in the final portion of the dissertation [i.e., Chapter 8], including additional issues that could be addressed in future research, such as scheduling, admission control, and end-to-end delay in Abstract ix IEEE 802.16 broadband wireless area (BWA) networks. Accordingly, these issues are listed as follows: • Mesh networks + Wi-Fi hotspot networks The signal may overshoot, even when the multi-channel is used. As the interference is considered, the transmission error reduces the link capacity C(u,v), so that the traffic flow is limited. In addition, if the interference issue is considered, it increases the number of retransmissions which means increasing the node-to-node delay. Thus, the interference issue is extended as one of our future work. • Ad hoc and sensor networks The proposed maximization of mobile network lifetime may be extended to include balancing the power consumption of all nodes within a multiple session construction. • IEEE 802.16 BWA networks Potential future research in this area includes: (i) optimization of the relative parameters and placing controls on scheduling and admission to minimize delay or maximize performance under quality of service considerations; and (ii) minimization of end-to-end delay with controls on scheduling in the IEEE 802.16j.