Mohammad M. Rashid

Wireless sensor networks with energy harvesting technologies: a game-theoretic approach to optimal energy management

Energy harvesting technologies are required for autonomous sensor networks for which using a power source from a fixed utility or manual battery recharging is infeasible. An energy harvesting device (e.g., a solar cell) converts different forms of environmental energy into electricity to be supplied to a sensor node. However, since it can produce energy only at a limited rate, energy saving mechanisms play an important role to reduce energy consumption in a sensor node. In this article we present an overview of the different energy harvesting technologies and the energy saving mechanisms for wireless sensor networks. The related research issues on energy efficiency for sensor networks using energy harvesting technology are then discussed. To this end, we present an optimal energy management policy for a solar-powered sensor node that uses a sleep and wakeup strategy for energy conservation. The problem of determining the sleep and wakeup probabilities is formulated as a bargaining game. The Nash equilibrium is used as the solution of this game.

Opportunistic spectrum scheduling for multiuser cognitive radio: a queueing analysis

We develop a queueing analytic framework to study the data link layer quality-of-service performance measures for cognitive radio users in an infrastructure-based dynamic spectrum access environment. In order to allocate the available spectrum white spaces among the cognitive radio users in a spectrum overlay scenario, an opportunistic scheduling scheme is considered. The queueing model considers bursty traffic arrival pattern at the cognitive radio user ends, finite buffer size, activity of primary users (i.e., dynamic channel availability), and correlated channel fading. We present a step-by-step procedure to derive the delay distribution, average throughput, and packet loss rate for the cognitive radio users. The proposed framework facilitates cross-layer design for improved QoS experience in cognitive radio networks. Usefulness of the developed analytical model is demonstrated through example applications.

Medium access control in distributed cognitive radio networks

Since cognitive radio technology can significantly boost spectrum utilization by exploiting radio spectrum unused by licensed users, it is rapidly gaining popularity and inspiring numerous applications. However, many technical issues still need to be addressed for successful deployment of CR networks, especially in the MAC layer. We focus on CR networks that have distributed architecture because they offer ease of deployment, self-organizing capability, and flexibility in design, and are believed to be more practical for future deployments compared to their centralized counterparts. The MAC protocols for distributed CR networks should consider the key features of these networks such as lack of a central unit to coordinate the communication, dynamic topology, requirements to keep interference to primary users minimal, and variation of spectrum availability with time and location. To clarify the relevant research challenges and issues, we provide a detailed study of the critical design issues and an overview of current state-of-the-art MAC protocols proposed for DCRNs. A classification of existing proposals is provided, and their salient features, advantages, and limitations are discussed. We then introduce and study a novel MAC protocol that addresses some of the research issues better than existing solutions. We also highlight important research challenges that could drive future research in this area.

Controlled Channel Access Scheduling for Guaranteed QoS in 802.11e-Based WLANs

The IEEE 802.11e standard developed by IEEE 802.11TGe intends to provide the quality of service (QoS) support in IEEE 802.11-based wireless local area networks (WLANs) through the introduction of hybrid coordination function (HCF). The HCF controlled channel access (HCCA) designed as a part of HCF is the medium access mechanism for parameterized QoS and is suitable for multimedia applications requiring hard QoS guarantees. The standard also defines a reference scheduler to complement HCCA in meeting these guarantees. In this paper, we investigate the performance of the reference scheduler described in the standard by using a novel queueing analytic framework. The analysis reveals the performance deficiencies of the reference scheduler. Afterwards, to overcome these deficiencies, we propose a new scheduling scheme, namely, the prediction and optimization-based HCCA (PRO-HCCA), based on the insights gained from the queueing analysis. Simulation experiments show that the proposed scheme overcomes the problems of the reference scheduler and successfully enables the HCCA to fulfill the QoS guarantees for multimedia applications.

Joint Relay Selection and Power Allocation for Decode-and-Forward Cellular Relay Network with Channel Uncertainty

In this paper, we propose joint relay selection and power allocation algorithms that can work robustly under imperfect channel knowledge for a decode-and-forward (DF) cellular relay network. The objective is to minimize the uplink transmit power of the network taking each users target data rate as the quality of service (QoS) constraint in the presence of imperfect channel state information (CSI). We consider the worst-case optimization approach, in which QoS constraint is satisfied for all users assuming both probabilistic and deterministic channel estimation error models. In this optimization framework, equivalent convex formulations are derived for the nonlinear optimization problems that are often combinatorially hard to solve in their original forms. After relay selection, efficient centralized as well as distributed power allocation algorithms scalable with respect to the size of the networks are developed. We also consider the case of power constrained networks where the objective is to provide QoS in the presence of limited power budgets on source and relays. The robust optimization problem is reformulated accordingly and efficient solution is provided. Numerical results show the effectiveness of the proposed algorithms and demonstrate the implications of ignoring channel estimation errors while developing relay selection and power allocation algorithms.

An analytical approach to providing controllable differentiated quality of service in Web servers

Provisioning quality of service (QoS) in Web servers has gained immense importance because Web servers are a major part of the Internet. To deliver the pledged QoS, Web service providers need control over the allocation of the resources in their Web servers. Control is also necessary for reaching the optimal resource allocation through proper service differentiation. In this paper, we propose and investigate an analytic approach that enables the service providers to deploy a differentiated service policy that offers this control. The proposed service policy is configurable by tunable control parameters. We devise the relationships between the performance measures and these parameters by adopting a unique queuing theoretic approach. Once these relationships are established, we describe how these parameters can be set to their most appropriate values depending on the objectives of the service providers. We illustrate the usefulness of our approach by conducting the analysis on a real Web trace.

OMC-MAC: An Opportunistic Multichannel MAC for Cognitive Radio Networks

Cognitive radio (CR) is a recent innovation based on an intelligent radio design paradigm that can vastly improve utilization of valuable radio spectrum. However, successful de- ployment of CR technology in next generation wireless networks requires many new research challenges to be addressed, especially in the medium access control (MAC) layer. In this paper, we pro- pose a novel MAC design, referred as Opportunistic Multichannel Cognitive MAC (OMC-MAC), for distributed CR networks to address some of these most important design issues. We also develop an analytical framework to study the performance of our proposed protocol in terms of important performance measures. The results achieved from the analytical model and validated by simulation, show that our simple yet efficient design achieves excellent spectrum utilization, shows superb robustness in presence of sensing error and handles multichannel hidden terminal problem very effectively.

Cross-layer analysis of downlink V-BLAST MIMO transmission exploiting multiuser diversity

We develop a queuing analytic model to study cross-layer effects on quality of service (QoS) performance for downlink transmission in a Vertical Bell Laboratories Layered Space-Time Architecture (V-BLAST) multiple input multiple output (MIMO) wireless system exploiting multiuser diversity. As in multiuser single input single output (SISO) systems, multiuser diversity has been shown to have a great potential in improving system throughput in multiuser MIMO systems as well. In a V-BLAST MIMO system, the transmit antennas can carry parallel streams and multiple users can be scheduled for transmissions at the same time. We consider a multiuser diversity scheme that can effectively exploit this extra dimension in multiuser scheduling. To investigate the cross-layer aspect of the performance improvement, we perform a queuing analysis to derive buffer statistics, queuing delay distribution, packet throughput and loss rate experienced in the data link layer when this multiuser diversity technique is deployed in the system. We also present selected numerical results to show how the queuing model can help us to relate these important QoS measures to relevant physical layer and traffic parameters. Usefulness of the developed analytical model is also demonstrated through example applications.

Resource Allocation for Selective Relaying Based Cellular Wireless System with Imperfect CSI

In this paper, we propose robust power allocation and admission control schemes for providing probabilistically constrained quality of service (QoS) in selective relaying based decode-and-forward (DF) cooperative cellular systems. The proposed schemes are robust against imperfect channel state information (CSI) in slow fading while optimizing the total uplink transmit power in these cooperative wireless networks. At first, we derive novel closed-form solutions for the optimization problem, where the objective is to minimize total uplink transmit power while meeting the probabilistic QoS guarantees for a given number of admitted users. This is achieved by approximating the probabilistic optimization problem into a convex deterministic form and then by deriving closed form analytical solutions for power allocation using Karush-Kuhn-Tucker (KKT) conditions. The closed-form property of these solutions allows us later to develop a very low-complexity suboptimal algorithm for joint admission control and power allocation in presence of imperfect CSI and selective relaying. We also conduct comprehensive simulation experiments to demonstrate the effectiveness of our proposed schemes and to highlight the benefits gained from considering channel estimation errors in resource allocation for cooperative cellular systems.

Queueing Analysis of 802.11e HCCA with Variable Bit Rate Traffic

The IEEE 802.11e draft standard currently being developed by IEEE 802.11TGe proposes to enable the much needed Quality of Service (QoS) support for the popular 802.11 based wireless local area networks (WLAN) through the introduction of Hybrid Coordination Function (HCF). The HCF Controlled Channel Access (HCCA) designed as a part of HCF is the medium access mechanism for parameterized QoS and is suitable for multimedia applications requiring hard QoS guarantees. The draft standard also defines scheduling and admission control schemes to complement HCCA in meeting these guarantees. However, most of the popular multimedia applications generate Variable Bit Rate (VBR) traffic that brings challenge to the HCCA and its scheduler and admission controller design. This paper introduces a novel queueing analytic framework that will be useful to analyze the performance of HCCA in provisioning required QoS for VBR traffic applications. The analysis also provides important insights that could be useful to improve the HCCA scheduler and admission controller designs.