M. Majid Butt

Survey on energy harvesting wireless communications: challenges and opportunities for radio resource allocation

Green radio communications has received a lot of attention in recent years due to its impact on telecom business, technology and environment. On the other hand, energy harvesting communication has emerged as a potential candidate to reduce the communication cost by tackling the problem in a contrasting fashion. While green communication techniques focus on minimizing the use of radio resources, energy harvesting communication relies on environment friendly techniques to generate energy from renewable resources and on effective use of the stored energy under the condition that there is always energy available when required. Thus, the focus migrates from minimization of energy to optimal time domain ‘distribution’ of energy, which causes a paradigm shift in radio resource allocation research. This survey summarizes major research work in the area of energy harvesting resource allocation. Instead of just focusing on the power allocation based on average and maximum power constraints, the random energy arrival process and packet/energy buffering interact in a challenging way to open new research problems. First, we present the fundamental concepts in energy harvesting communications and review recent research work in different wireless network applications. We discuss some quantitative results from the existing literature to explain the state of the art work. The energy cooperation aspect of energy harvesting is addressed in detail which has emerged as an interesting area of research recently. Finally, we conclude by summarizing some open challenges for future research and scope for innovation in this emerging area.

Trading wireless information and power transfer: Relay selection to minimize the outage probability

This paper studies the outage probability minimization problem for a multiple relay network with energy harvesting constraints. The relays are hybrid nodes used for simultaneous wireless information and power transfer from the source radio frequency (RF) signals. There is a tradeoff associated with the amount of time a relay node is used for energy and information transfer. Large intervals of information transfer implies little time for energy harvesting from RF signals and thus, high probability of outage events. We propose relay selection schemes for a cooperative system with a fixed number of RF powered relays. We address both causal and non-causal channel state information cases at the relay-destination link and evaluate the tradeoff associated with information/power transfer in the context of minimization of outage probability.

Hard deadline constrained multiuser scheduling for random arrivals

In this work, an opportunistic scheduling scheme for a large multiuser system is proposed. A group of users with good channels are scheduled simultaneously for data transmission and separated by means of superposition coding. The proposed scheduling scheme is analyzed in the large system limit. Random packet arrivals are modeled as constant arrivals with random content size. Transmission thresholds are optimized in such a way that the system energy is minimized while obeying a strict upper bound on the packet delay. We find that the state space representations of systems with either constant or random arrivals are equivalent. Thus, the thresholds optimized for constant arrivals in earlier work are valid for systems with random arrivals as well.

Maximizing System Energy Efficiency by Exploiting Multiuser Diversity and Loss Tolerance of the Applications

We address the problem of energy efficient scheduling over fading channels for the loss tolerant applications. The proposed scheduling scheme allows dropping of a certain predefined proportion of data packets on the transmitter side. However, there is a hard constraint on the maximum number of successively dropped packets. The scheduler exploits average data loss tolerance to reduce the average system energy expenditure while fulfills the hard constraint on the number of successively dropped packets. We explore the effect of average and successive packet loss constraints on the system energy and characterize the regions where one parameter is more critical as compared to the other in the sense of achieving better energy efficiency. The scheme is analyzed using asymptotically large user limit and the optimized channel¿-dependent dropping thresholds are computed by using combinatorial optimization techniques. The numerical results illustrate the energy efficiency of the scheme as a function of the average and successive packet drop parameters.

Dynamic Licensed Shared Access - A New Architecture and Spectrum Allocation Techniques

This paper proposes a new system architecture for Licensed Shared Access (LSA) wireless networks, as well as novel band management techniques for fair and ranking-based spectrum allocation. The proposed architecture builds upon recently standardized and regulatory-accepted LSA systems and stems from the work done in the EU-funded project ADEL. Two new resource allocation algorithms are introduced and their behaviour is validated via system-level simulations.

Individual Packet Deadline Constrained Opportunistic Scheduling for a Multiuser System

In this work an opportunistic scheduling scheme is presented and analyzed for a multiuser system. The objective of the proposed scheme is to minimize the system transmit energy in the presence of a hard deadline delay constraint for the individual packets. In the large system limit, the scheme is modeled and analyzed in the scenario when arriving packets have associated deadlines which vary from packet to packet. We introduce transmission thresholds that depend on channel quality and number of time slots left before a packet reaches its hard deadline. These thresholds are optimized such that they reflect the interaction of deadline delay and channel variation, and result in a minimum system energy. The results demonstrate the saving in energy for a system where the applications have individual packet deadline delay constraints.

RF Energy Harvesting Communications: Recent Advances and Research Issues

Green radio communications has got a lot of attention in recent years due to its telling effects on telecom business and environment. On the other side, energy harvesting (EH) communication has emerged as a potential candidate to reduce the communication cost by tackling the problem in a contrasting fashion. While green communication techniques focus on minimization the use of radio resources, EH communication relies on environment friendly techniques to generate energy (from the renewable resources) and effective use of created energy conditioned on the fact that there is always energy available when required. Thus, the focus migrates from minimization of energy to optimal time domain distribution of energy and this causes a paradigm shift in radio resource allocation research. Instead of just focusing on average and maximum power constraint, the packet/energy arrival processes and packet/energy buffering interact in a challenging way to open new research opportunities. This chapter summarizes the major research work in the area of radio frequency (RF) energy harvesting resource allocation. First, we discuss the fundamental concepts related to energy harvesting communications. Then, we review the recent developments in this area and outline the major research challenges for the research community. We address the cooperation aspect of energy harvesting, which has emerged as an interesting area of research. Wireless powered communication networks allow energy and information transfer from the radio frequency waves and provide sustainable networks. Finally, we discuss a wireless powered relay network in detail and show the performance comparison of different relay selection techniques.

Maximizing Energy Efficiency in Multiple Access Channels by Exploiting Packet Dropping and Transmitter Buffering

Quality of service (QoS) for a network is characterized in terms of various parameters specifying packet delay and loss tolerance requirements for the application. The unpredictable nature of the wireless channel demands for application of certain mechanisms to meet the QoS requirements. Traditionally, medium access control (MAC) and network layers perform these tasks. However, these mechanisms do not take (fading) channel conditions into account. In this paper, we investigate the problem using cross layer techniques where information flow and joint optimization of higher and physical layer is permitted. We propose a scheduling scheme to optimize the energy consumption of a multiuser multi-access system such that QoS constraints in terms of packet loss are fulfilled while the system is able to maximize the advantages emerging from multiuser diversity. Specifically, this work focuses on modeling and analyzing the effects of packet buffering capabilities of the transmitter on the system energy for a packet loss tolerant application. We discuss low complexity schemes which show comparable performance to the proposed scheme. The numerical evaluation reveals useful insights about the coupling effects of different QoS parameters on the system energy consumption and validates our analytical results.

On the power efficiency for cognitive radio networks with multiple relays

In this paper, we study and analyze cooperative cognitive radio networks with multiple secondary users (SUs). Each SU is considered a prospective relay for the primary user (PU) besides having its own data demand. The proposed scheme leverages the spectral efficiency of the system via allowing two SUs to transmit simultaneously thanks to dirty-paper coding. We propose a power allocation policy that minimizes the average transmitted power at each SU. Moreover, we are concerned with enhancing the throughput of both primary and secondary links. Towards this objective, we investigate multiple opportunistic relay selection policies. We develop a mathematical framework for deriving stability conditions for the queues involved in the system using outage probabilities. Results reveal that we achieve better performance in terms of average power and throughput as compared to uniform power allocation schemes proposed earlier in literature. Extensive numerical simulations are conducted to validate our theoretical findings.

Maximizing energy efficiency for loss tolerant applications: The packet buffering case

Energy efficient communication has emerged as one of the key areas of research due to its impact on industry and environment. Any potential degree of freedom (DoF) available in the system should be exploited smartly to design energy efficient systems. This paper proposes a framework for achieving energy efficiency for the data loss tolerant applications by exploiting the multiuser diversity and DoFs available through the packet loss pattern. For a real time application, there is a constraint on the maximum number of packets to be dropped successively that must be obeyed. We propose a channel-aware energy efficient scheduling scheme which schedules the packets such that the constraint on the average packet drop rate and the maximum number of successively dropped packets is fulfilled for the case when a finite number of unscheduled packets can be buffered. We analyze the scheme in the large user limit and show the energy gain due to buffering on the proposed scheme.