Design and optimization of QoS-based medium access control protocols for next-generation wireless LANs

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.In recent years, there have been tremendous advances in wireless & mobile communications, including wireless radio techniques, networking protocols, and mobile devices. It is expected that different broadband wireless access technologies, e.g., WiFi (IEEE 802.11) and WiMAX (IEEE 802.16) will coexist in the future. In the meantime, multimedia applications have experienced an explosive growth with increasing user demands. Nowadays, people expect to receive high-speed video, audio, voice and web services even when being mobile. The key question that needs to be answered, then, is how do we ensure that users always have the "best" network performance with the "lowest" costs in such complicated situations? The latest IEEE 802.11n standards attains rates of more than 100 Mbps by introducing innovative enhancements at the PHY and MAC layer, e.g. MIMO and Frame Aggregation, respectively. However, in this thesis we demonstrate that frame aggregation's performance adheres due to the EDCA scheduler's priority mechanism and consequently resulting in the network's poor overall performance. Short waiting times for high priority flows into the aggregation queue resolves to poor channel utilization. A Delayed Channel Access algorithm was designed to intentionally postpone the channel access procedure so that the number of packets in a formed frame can be increased and so will the network's overall performance. However, in some cases, the DCA algorithm has a negative impact on the applications that utilize the TCP protocol, especially the when small TCP window sizes are engaged. So, the TCP process starts to refrain from sending data due to delayed acknowledgements and the overall throughput drops. In this thesis, we address the above issues by firstly demonstrating the potential performance benefits of frame aggregation over the next generation wireless networks. The efficiency and behaviour of frame aggregation within a single queue, are mathematically analysed with the aid of a M=G[a;b]=1=K model. Results show that a trade-off choice has to be taken into account over minimizing the waiting time or maximizing utilization. We also point out that there isn't an optimum batch collection rule which can be assumed as generally valid but individual cases have to be considered separately. Secondly, we demonstrate through extensive simulations that by introducing a method, the DCA algorithm, which dynamically determines and adapts batch collections based upon the traffic's characteristics, QoS requirements and server's maximum capacity, also improves e ciency. Thirdly, it is important to understand the behaviour of the TCP ows over the WLAN and the influence that DCA has over the degrading performance of the TCP protocol. We investigate the cause of the problem and provide the foundations of designing and implementing possible solutions. Fourthly, we introduce two innovative proposals, one amendment and one extension to the original DCA algorithm, called Adaptive DCA and Selective DCA, respectively. Both solutions have been implemented in OPNET and extensive simulation runs over a wide set of scenarios show their effectiveness over the network's overall performance, each in its own way.This study was supported by the Engineering and Physical Sciences Research Council (EPSRC)

A Comparative Study on Iterative Methods Regarding Cross-Layer Optimization for OFDMA Systems

The key issues in the design of a cross-layer optimization scheme for multiuser orthogonal frequency multiple access (OFDMA) systems are the optimal subcarrier, bit and rate distribution policies. In many recent researches, these allocation strategies are derived either through dynamic programming or via mathematical analysis. When the latter option is available, an additional iterative process is needed for the final solution to be obtained. This paper includes performance comparisons between the analytical and the numerical techniques as well as assessments on which of the existing iterative methods exhibits the highest effectiveness. Deductively, relying on system's evaluations, the semi-implicit root (SIR) mechanism produces the highest rapidity, precision and stability, in terms of convergence rate, root's accuracy and number of iterations

Power-Efficient Cross-Layer Design for OFDMA Systems With Heterogeneous QoS, Imperfect CSI, and Outage Considerations

Reducing power consumption subject to quality-of-service (QoS) provisions is a critical task for next-generation networking. Recent research in resource management for orthogonal frequency-division multiple-access (OFDMA) systems has generally assumed the availability of perfect channel state information at the transmitter (CSIT) or imperfect CSIT with small uncertainty. Nevertheless, such approaches deliver resource scheduling strategies with high transmitting power because, in real environments, large channel feedback delays and estimation errors cause high CSIT imperfectness. Furthermore, most existing works treat various QoS requirements from mobile users as homogenous, although they are heterogeneous in nature. In this paper, we address these issues by proposing a new power-efficient adaptive error-tolerant cross-layer scheduling scheme for OFDMA systems (PE-AETS). Our target is to minimize the transmitting power by considering heterogeneous QoS requirements and data outage due to imperfect CSIT. The proposed scheme adopts a robust power-bit loading (PBL) method that adjusts power and data rates across subcarriers with increased system resilience to channel errors. We develop a statistical queuing model to express the delay limitation of each user with an equivalent cross-layer constraint, and we apply subcarrier time-sharing relaxation to formulate a convex optimization problem. Finally, we utilize Lagrangian optimization to propose a joint power and subcarrier allocation policy with guaranteed convergence to optimal solutions and linear complexity. Various simulation scenarios confirm the superior performance of the proposed PE-AETS over relevant cross-layer approaches