Ritesh Madan | Bangladesh National University (original) (raw)

Papers by Ritesh Madan

Research paper thumbnail of The Irrevocable Multi-Armed Bandit Problem

This paper considers the multiarmed bandit problem with multiple simultaneous arm pulls and the a... more This paper considers the multiarmed bandit problem with multiple simultaneous arm pulls and the additional restriction that we do not allow recourse to arms that were pulled at some point in the past but then discarded. This additional restriction is highly desirable from an operational perspective, and we refer to this problem as the "irrevocable multiarmed bandit" problem. We observe that natural modifications to well-known heuristics for multiarmed bandit problems that satisfy this irrevocability constraint have unsatisfactory performance and, thus motivated, introduce a new heuristic: the "packing" heuristic. We establish through numerical experiments that the packing heuristic offers excellent performance, even relative to heuristics that are not constrained to be irrevocable. We also provide a theoretical analysis that studies the "price" of irrevocability, i.e., the performance loss incurred in imposing the constraint we propose on the multiarmed bandit model. We show that this performance loss is uniformly bounded for a general class of multiarmed bandit problems and indicate its dependence on various problem parameters. Finally, we obtain a computationally fast algorithm to implement the packing heuristic; the algorithm renders the packing heuristic computationally cheaper than methods that rely on the computation of Gittins indices.

Research paper thumbnail of Fast algorithms for resource allocation in wireless cellular networks

IEEE/ACM Transactions on Networking, 2010

Page 1. IEEE/ACM TRANSACTIONS ON NETWORKING, VOL. 18, NO. 3, JUNE 2010 973 Fast Algorithms for Re... more Page 1. IEEE/ACM TRANSACTIONS ON NETWORKING, VOL. 18, NO. 3, JUNE 2010 973 Fast Algorithms for Resource Allocation in Wireless Cellular Networks Ritesh Madan, Stephen P. Boyd, Fellow, IEEE, and Sanjay Lall, Senior Member, IEEE ...

Research paper thumbnail of Energy Min imization and Delay Analysis in TDMA-based Sensor Networks

Abstract— We consider sensor networks where,energy is a limited resource so that energy consumpti... more Abstract— We consider sensor networks where,energy is a limited resource so that energy consumption must be minimized while satisfying given throughput requirements. Moreover, energy consumption must take into account both the transmission energy and the circuit processing energy for short-range communications. In this context, we analyze energy-efficient joint routing, scheduling, and link adaptation strategies that maximize the network lifetime. We emphasize that the energy efficiency must be supported across all layers of the protocol stack through a cross-layer design. We start with the joint MAC and link layer optimization, and then extend the model to include routing optimization. The joint optimization minimizes the total energy consumption including both the transmission energy and the circuit processing energy across the network. We propose variable-length TDMA schemes where the slot length is optimally assigned according to the routing requirement while minimizing the energy consumption across the network. We show that the optimization problems can be transformed into or approximated by convex problems that can be efficiently solved using known techniques. The results show that multihop routing schemes are usually more energy-efficient when only transmission energy is considered, but single-hop transmission may be more efficient when,the circuit processing energy is also taken into account. The delay performance of the optimal TDMA scheme is analyzed and an algorithm for minimum-delay scheduling is proposed. The delay analysis result can be applied to general networks based on TDMA transmissions. Index Terms— Energy efficiency, Joint Routing and Schedul-

Research paper thumbnail of Cross-layer Energy Minimization in TDMA-based Sensor Networks

We consider sensor networks where energy is a limited resource so that energy consumption must be... more We consider sensor networks where energy is a limited resource so that energy consumption must be minimized while satisfying given throughput requirements. Moreover, energy consumption must take into account both the transmission energy and the circuit processing energy for short-range communications. In this context, we analyze the energy-efficient joint routing, scheduling, and link ad aptation strategies that maximize the network lifetime and we emphasize that the energy efficiency must be supported across all layers of the protocol stack through a cross-layer design. We start with the joint MAC and link layer optimization, and then extend the model to include routing optimization. The joint optimization minimizes the total energy consumption including both the transmission energy and the circuit processing energy across the network. We propose variable-length TDMA schemes where the slot length is optimally assigned according to the routing r equirement while keeping the total energy consumption minimized. We show that the optimization problems can be transformed into or approximated by convex problems that can be efficiently solved using known techniques. The re sults show that multihop routing schemes are more energy-efficient when only transmission energy is concerne d, but single-hop transmissions may be more efficient when the circuit processing energy is considered. Index Terms Energy efficiency, Joint Routing and Scheduling, Link Adapt ation, Cross-layer. I. I NTRODUCTION

Research paper thumbnail of An Energy-Optimal Algorithm for Neighbor Discovery in Wireless Sensor Networks

Mobile Networks and Applications, 2006

We consider sensor networks in which individual nodes with on-board sensing and low-power transmi... more We consider sensor networks in which individual nodes with on-board sensing and low-power transmitters and receivers establish connections with neighboring nodes. The overall objective is to enable energy-efficient data communication, relayed between arbitrary nodes on the network. We develop a distributed algorithm which minimizes the power required for neighbor discovery. Initially nodes do not have deterministic knowledge of the location of their neighbors, and we model the distribution of the nodes as a two-dimensional Poisson process with known intensity. This corresponds to a situation in which a large number of nodes are randomly distributed over a given area. The process of neighbor discovery is modeled as a Markov decision process, and the resulting control policy is a finite automaton, driven by the underlying probability distribution, that minimizes the average power consumed. This policy can be computed offline and stored in each node with very low requirements for online memory and processor capability.

Research paper thumbnail of Optimal control of distributed Markov decision processes with network delays

We consider the problem of finding an optimal feedback controller for a network of interconnected... more We consider the problem of finding an optimal feedback controller for a network of interconnected subsystems, each of which is a Markov decision process. Each subsys-tem is coupled to its neighbors via communication links by which signals are delayed but are otherwise transmit-ted ...

Research paper thumbnail of Distributed algorithms for maximum lifetime routing in wireless sensor networks

IEEE Transactions on Wireless Communications, 2006

Abstract— A sensor network of nodes with wireless trans-ceiver capabilities and limited energy is... more Abstract— A sensor network of nodes with wireless trans-ceiver capabilities and limited energy is considered. We propose distributed algorithms to compute an optimal routing scheme that maximizes the time at which the first node in the network drains out of energy. The ...

Research paper thumbnail of Energy-delay tradeoffs for data collection in TDMA-based sensor networks

We consider a wireless sensor network where the nodes have limited energy. We first analyze the d... more We consider a wireless sensor network where the nodes have limited energy. We first analyze the delay performance of a transmission scheme based on time division multiple access (TDMA). We propose a simple link scheduling algorithm to find the minimum-delay schedule given the slot lengths for all the links. We then combine these results with our previous work on energy-optimal cross-layer design to minimize the delay in transferring a fixed number of bits from the source nodes to the sink, in an energy-constrained manner. We also study the tradeoff between the total energy consumption and delay. Paretooptimal energy-delay curves are computed by solving a series of convex optimization problems where each objective function is a weighted sum of the delay and the total energy consumption. The computation is done for networks with and without link adaptation capabilities.

Research paper thumbnail of Energy-delay Tradeo for Data Collection in Sensor Networks

Research paper thumbnail of Distributed algorithms for maximum lifetime routing in wireless sensor networks

Abstract— A sensor network of nodes with wireless trans-ceiver capabilities and limited energy is... more Abstract— A sensor network of nodes with wireless trans-ceiver capabilities and limited energy is considered. We propose distributed algorithms to compute an optimal routing scheme that maximizes the time at which the first node in the network drains out of energy. The ...

Research paper thumbnail of Cross-Layer Energy and Delay Optimization in Small-Scale Sensor Networks

IEEE Transactions on Wireless Communications, 2007

The general joint design of the physical, MAC, and routing layers to minimize network energy cons... more The general joint design of the physical, MAC, and routing layers to minimize network energy consumption is complex and hard to solve. Heuristics to compute approximate solutions and high-complexity algorithms to compute exact solutions have been previously proposed. In this paper, we focus on synchronous small-scale networks with interference-free link scheduling and practical MQAM link transmission schemes. We show that the cross-layer optimization problems can be closely approximated by convex optimization problems that can be efficiently solved. There are two main contributions of this paper. First of all, we minimize the total network energy that includes both transmission and circuit energy consumptions, where we explore the tradeoff between the two energy elements. Specifically, we use interference-free TDMA as the medium access control scheme. We optimize the routing flow, TDMA slot assignment, and MQAM modulation rate and power on each link. The results demonstrate that the minimum energy transmission scheme is a combination of multihop and singlehop transmissions for general networks; including circuit energy favors transmission schemes with fewer hops. Secondly, based on the solved optimal transmission scheme, we quantify the best trade-off curve between delay and energy consumption, where we derive a scheduling algorithm to minimize the worst-case packet delay.

Research paper thumbnail of Joint routing, MAC, and link layer optimization in sensor networks with energy constraints

We consider sensor networks where energy is a limited resource so that energy consumption must be... more We consider sensor networks where energy is a limited resource so that energy consumption must be minimized while satisfying given throughput requirements. Moreover, energy consumption must take into account both the transmission energy and the circuit processing energy for short-range communications. We emphasize that the energy efficiency must be supported across all layers of the protocol stack through a cross-layer design. In this context, we analyze energy-efficient joint routing, scheduling, and link adaptation strategies that maximize the network lifetime. We propose variable-length TDMA schemes where the slot length is optimally assigned according to the routing requirement while minimizing the energy consumption across the network. We show that the optimization problems can be transformed into or approximated by convex problems that can be efficiently solved using known techniques. The results show that multihop routing schemes are more energy-efficient when only transmission energy is considered, but single-hop transmissions may be more efficient when the circuit processing energy is considered.

Research paper thumbnail of Cross-layer energy miminization in sensor networks

Research paper thumbnail of Cross-layer design for lifetime maximization in interference-limited wireless sensor networks

We consider the joint optimal design of the physical, medium access control (MAC), and routing la... more We consider the joint optimal design of the physical, medium access control (MAC), and routing layers to maximize the lifetime of energy-constrained wireless sensor networks. The problem of computing lifetime-optimal routing flow, link schedule, and link transmission powers for all active time slots is formulated as a non-linear optimization problem. We first restrict the link schedules to the class of interference-free time division multiple access (TDMA) schedules. In this special case, we formulate the optimization problem as a mixed integerconvex program, which can be solved using standard techniques. Moreover, when the slots lengths are variable, the optimization problem is convex and can be solved efficiently and exactly using interior point methods. For general non-orthogonal link schedules, we propose an iterative algorithm that alternates between adaptive link scheduling and computation of optimal link rates and transmission powers for a fixed link schedule. The performance of this algorithm is compared to other design approaches for several network topologies. The results illustrate the advantages of load balancing, multihop routing, frequency reuse, and interference mitigation in increasing the lifetime of energy-constrained networks. We also briefly discuss computational approaches to extend this algorithm to large networks.

Research paper thumbnail of Cross-Layer Design for Lifetime Maximization in Interference-Limited Wireless Sensor Networks

IEEE Transactions on Wireless Communications, 2006

We consider the joint optimal design of the physical, medium access control (MAC), and routing la... more We consider the joint optimal design of the physical, medium access control (MAC), and routing layers to maximize the lifetime of energy-constrained wireless sensor networks. The problem of computing lifetime-optimal routing flow, link schedule, and link transmission powers for all active time slots is formulated as a non-linear optimization problem. We first restrict the link schedules to the class of interference-free time division multiple access (TDMA) schedules. In this special case, we formulate the optimization problem as a mixed integerconvex program, which can be solved using standard techniques. Moreover, when the slots lengths are variable, the optimization problem is convex and can be solved efficiently and exactly using interior point methods. For general non-orthogonal link schedules, we propose an iterative algorithm that alternates between adaptive link scheduling and computation of optimal link rates and transmission powers for a fixed link schedule. The performance of this algorithm is compared to other design approaches for several network topologies. The results illustrate the advantages of load balancing, multihop routing, frequency reuse, and interference mitigation in increasing the lifetime of energy-constrained networks. We also briefly discuss computational approaches to extend this algorithm to large networks.

Research paper thumbnail of A Distributed Algorithm with Linear Convergence for Maximum Lifetime Routing in Wireless Networks

Research paper thumbnail of Modeling and optimization of transmission schemes in energy-constrained wireless sensor networks

IEEE/ACM Transactions on Networking, 2007

We consider a wireless sensor network with energy constraints. We model the energy consumption in... more We consider a wireless sensor network with energy constraints. We model the energy consumption in the transmitter circuit along with that for transmission. We model the bottom three layers of the traditional networking stack -the link layer, medium access control (MAC) layer, and the routing layer. Using these models, we consider the optimization of the transmission schemes to maximize the network lifetime. We first consider the optimization of a single layer at a time, while keeping the other layers fixed. We make certain simplifying assumptions to decouple the layers and formulate optimization problems to compute a strategy that maximizes the network lifetime. We then extend this approach to cross-layer optimization of time division multiple access (TDMA) wireless sensor networks. In this case, we construct optimization problems to compute the optimal transmission schemes exactly and efficiently. We then consider networks with interference, and propose methods to compute approximate solutions to the resulting optimization problems. We give numerical examples that illustrate the computational approaches as well as the benefits of cross-layer design in wireless sensor networks.

Research paper thumbnail of Fast Algorithms for Resource Allocation in Cellular Networks

We consider a scheduled orthogonal frequency division multiplexed (OFDM) wireless cellular networ... more We consider a scheduled orthogonal frequency division multiplexed (OFDM) wireless cellular network where the channels from the base-station to the mobile users undergo flat fading. Spectral resources are to be divided among the users in order to maximize total user utility. We show that this problem can be cast as a nonlinear convex optimization problem, and describe an ( ) algorithm to solve it. Computational experiments show that the algorithm typically converges in around 25 iterations, where each iteration has a cost that is ( ), with a modest constant. When the algorithm starts from an initial resource allocation that is close to optimal, convergence typically takes even fewer iterations. Thus, the algorithm can efficiently track the optimal resource allocation as the channel conditions change due to fading. We also show how our techniques can be extended to solve resource allocation problems that arise in wideband networks with frequency selective fading and when the utility of a user is also a function of the resource allocations in the past.

Research paper thumbnail of Belief Propagation Methods for Intercell Interference Coordination

Computing Research Repository, 2010

We consider a broad class of interference coordination and resource allocation problems for wirel... more We consider a broad class of interference coordination and resource allocation problems for wireless links where the goal is to maximize the sum of functions of individual link rates. Such problems arise in the context of, for example, fractional frequency reuse (FFR) for macro-cellular networks and dynamic interference management in femtocells. The resulting optimization problems are typically hard to solve optimally even using centralized algorithms but are an essential computational step in implementing rate-fair and queue stabilizing scheduling policies in wireless networks. We consider a belief propagation framework to solve such problems approximately. In particular, we construct approximations to the belief propagation iterations to obtain computationally simple and distributed algorithms with low communication overhead. Notably, our methods are very general and apply to, for example, the optimization of transmit powers, transmit beamforming vectors, and sub-band allocation to maximize the above objective. Numerical results for femtocell deployments demonstrate that such algorithms compute a very good operating point in typically just a couple of iterations.

Research paper thumbnail of Cell Association and Interference Coordination in Heterogeneous LTE-A Cellular Networks

IEEE Journal on Selected Areas in Communications, 2010

Embedding pico/femto base-stations and relay nodes in a macro-cellular network is a promising met... more Embedding pico/femto base-stations and relay nodes in a macro-cellular network is a promising method for achieving substantial gains in coverage and capacity compared to macro-only networks. These new types of base-stations can operate on the same wireless channel as the macro-cellular network, providing higher spatial reuse via cell splitting. However, these base-stations are deployed in an unplanned manner, can have very different transmit powers, and may not have traffic aggregation among many users. This could potentially result in much higher interference magnitude and variability. Hence, such deployments require the use of innovative cell association and inter-cell interference coordination techniques in order to realize the promised capacity and coverage gains. In this paper, we describe new paradigms for design and operation of such heterogeneous cellular networks. Specifically, we focus on cell splitting, range expansion, semi-static resource negotiation on third-party backhaul connections, and fast dynamic interference management for QoS via over-the-air signaling. Notably, our methodologies and algorithms are simple, lightweight, and incur extremely low overhead. Numerical studies show that they provide large gains over currently used methods for cellular networks.

Research paper thumbnail of The Irrevocable Multi-Armed Bandit Problem

This paper considers the multiarmed bandit problem with multiple simultaneous arm pulls and the a... more This paper considers the multiarmed bandit problem with multiple simultaneous arm pulls and the additional restriction that we do not allow recourse to arms that were pulled at some point in the past but then discarded. This additional restriction is highly desirable from an operational perspective, and we refer to this problem as the "irrevocable multiarmed bandit" problem. We observe that natural modifications to well-known heuristics for multiarmed bandit problems that satisfy this irrevocability constraint have unsatisfactory performance and, thus motivated, introduce a new heuristic: the "packing" heuristic. We establish through numerical experiments that the packing heuristic offers excellent performance, even relative to heuristics that are not constrained to be irrevocable. We also provide a theoretical analysis that studies the "price" of irrevocability, i.e., the performance loss incurred in imposing the constraint we propose on the multiarmed bandit model. We show that this performance loss is uniformly bounded for a general class of multiarmed bandit problems and indicate its dependence on various problem parameters. Finally, we obtain a computationally fast algorithm to implement the packing heuristic; the algorithm renders the packing heuristic computationally cheaper than methods that rely on the computation of Gittins indices.

Research paper thumbnail of Fast algorithms for resource allocation in wireless cellular networks

IEEE/ACM Transactions on Networking, 2010

Page 1. IEEE/ACM TRANSACTIONS ON NETWORKING, VOL. 18, NO. 3, JUNE 2010 973 Fast Algorithms for Re... more Page 1. IEEE/ACM TRANSACTIONS ON NETWORKING, VOL. 18, NO. 3, JUNE 2010 973 Fast Algorithms for Resource Allocation in Wireless Cellular Networks Ritesh Madan, Stephen P. Boyd, Fellow, IEEE, and Sanjay Lall, Senior Member, IEEE ...

Research paper thumbnail of Energy Min imization and Delay Analysis in TDMA-based Sensor Networks

Abstract— We consider sensor networks where,energy is a limited resource so that energy consumpti... more Abstract— We consider sensor networks where,energy is a limited resource so that energy consumption must be minimized while satisfying given throughput requirements. Moreover, energy consumption must take into account both the transmission energy and the circuit processing energy for short-range communications. In this context, we analyze energy-efficient joint routing, scheduling, and link adaptation strategies that maximize the network lifetime. We emphasize that the energy efficiency must be supported across all layers of the protocol stack through a cross-layer design. We start with the joint MAC and link layer optimization, and then extend the model to include routing optimization. The joint optimization minimizes the total energy consumption including both the transmission energy and the circuit processing energy across the network. We propose variable-length TDMA schemes where the slot length is optimally assigned according to the routing requirement while minimizing the energy consumption across the network. We show that the optimization problems can be transformed into or approximated by convex problems that can be efficiently solved using known techniques. The results show that multihop routing schemes are usually more energy-efficient when only transmission energy is considered, but single-hop transmission may be more efficient when,the circuit processing energy is also taken into account. The delay performance of the optimal TDMA scheme is analyzed and an algorithm for minimum-delay scheduling is proposed. The delay analysis result can be applied to general networks based on TDMA transmissions. Index Terms— Energy efficiency, Joint Routing and Schedul-

Research paper thumbnail of Cross-layer Energy Minimization in TDMA-based Sensor Networks

We consider sensor networks where energy is a limited resource so that energy consumption must be... more We consider sensor networks where energy is a limited resource so that energy consumption must be minimized while satisfying given throughput requirements. Moreover, energy consumption must take into account both the transmission energy and the circuit processing energy for short-range communications. In this context, we analyze the energy-efficient joint routing, scheduling, and link ad aptation strategies that maximize the network lifetime and we emphasize that the energy efficiency must be supported across all layers of the protocol stack through a cross-layer design. We start with the joint MAC and link layer optimization, and then extend the model to include routing optimization. The joint optimization minimizes the total energy consumption including both the transmission energy and the circuit processing energy across the network. We propose variable-length TDMA schemes where the slot length is optimally assigned according to the routing r equirement while keeping the total energy consumption minimized. We show that the optimization problems can be transformed into or approximated by convex problems that can be efficiently solved using known techniques. The re sults show that multihop routing schemes are more energy-efficient when only transmission energy is concerne d, but single-hop transmissions may be more efficient when the circuit processing energy is considered. Index Terms Energy efficiency, Joint Routing and Scheduling, Link Adapt ation, Cross-layer. I. I NTRODUCTION

Research paper thumbnail of An Energy-Optimal Algorithm for Neighbor Discovery in Wireless Sensor Networks

Mobile Networks and Applications, 2006

We consider sensor networks in which individual nodes with on-board sensing and low-power transmi... more We consider sensor networks in which individual nodes with on-board sensing and low-power transmitters and receivers establish connections with neighboring nodes. The overall objective is to enable energy-efficient data communication, relayed between arbitrary nodes on the network. We develop a distributed algorithm which minimizes the power required for neighbor discovery. Initially nodes do not have deterministic knowledge of the location of their neighbors, and we model the distribution of the nodes as a two-dimensional Poisson process with known intensity. This corresponds to a situation in which a large number of nodes are randomly distributed over a given area. The process of neighbor discovery is modeled as a Markov decision process, and the resulting control policy is a finite automaton, driven by the underlying probability distribution, that minimizes the average power consumed. This policy can be computed offline and stored in each node with very low requirements for online memory and processor capability.

Research paper thumbnail of Optimal control of distributed Markov decision processes with network delays

We consider the problem of finding an optimal feedback controller for a network of interconnected... more We consider the problem of finding an optimal feedback controller for a network of interconnected subsystems, each of which is a Markov decision process. Each subsys-tem is coupled to its neighbors via communication links by which signals are delayed but are otherwise transmit-ted ...

Research paper thumbnail of Distributed algorithms for maximum lifetime routing in wireless sensor networks

IEEE Transactions on Wireless Communications, 2006

Abstract— A sensor network of nodes with wireless trans-ceiver capabilities and limited energy is... more Abstract— A sensor network of nodes with wireless trans-ceiver capabilities and limited energy is considered. We propose distributed algorithms to compute an optimal routing scheme that maximizes the time at which the first node in the network drains out of energy. The ...

Research paper thumbnail of Energy-delay tradeoffs for data collection in TDMA-based sensor networks

We consider a wireless sensor network where the nodes have limited energy. We first analyze the d... more We consider a wireless sensor network where the nodes have limited energy. We first analyze the delay performance of a transmission scheme based on time division multiple access (TDMA). We propose a simple link scheduling algorithm to find the minimum-delay schedule given the slot lengths for all the links. We then combine these results with our previous work on energy-optimal cross-layer design to minimize the delay in transferring a fixed number of bits from the source nodes to the sink, in an energy-constrained manner. We also study the tradeoff between the total energy consumption and delay. Paretooptimal energy-delay curves are computed by solving a series of convex optimization problems where each objective function is a weighted sum of the delay and the total energy consumption. The computation is done for networks with and without link adaptation capabilities.

Research paper thumbnail of Energy-delay Tradeo for Data Collection in Sensor Networks

Research paper thumbnail of Distributed algorithms for maximum lifetime routing in wireless sensor networks

Abstract— A sensor network of nodes with wireless trans-ceiver capabilities and limited energy is... more Abstract— A sensor network of nodes with wireless trans-ceiver capabilities and limited energy is considered. We propose distributed algorithms to compute an optimal routing scheme that maximizes the time at which the first node in the network drains out of energy. The ...

Research paper thumbnail of Cross-Layer Energy and Delay Optimization in Small-Scale Sensor Networks

IEEE Transactions on Wireless Communications, 2007

The general joint design of the physical, MAC, and routing layers to minimize network energy cons... more The general joint design of the physical, MAC, and routing layers to minimize network energy consumption is complex and hard to solve. Heuristics to compute approximate solutions and high-complexity algorithms to compute exact solutions have been previously proposed. In this paper, we focus on synchronous small-scale networks with interference-free link scheduling and practical MQAM link transmission schemes. We show that the cross-layer optimization problems can be closely approximated by convex optimization problems that can be efficiently solved. There are two main contributions of this paper. First of all, we minimize the total network energy that includes both transmission and circuit energy consumptions, where we explore the tradeoff between the two energy elements. Specifically, we use interference-free TDMA as the medium access control scheme. We optimize the routing flow, TDMA slot assignment, and MQAM modulation rate and power on each link. The results demonstrate that the minimum energy transmission scheme is a combination of multihop and singlehop transmissions for general networks; including circuit energy favors transmission schemes with fewer hops. Secondly, based on the solved optimal transmission scheme, we quantify the best trade-off curve between delay and energy consumption, where we derive a scheduling algorithm to minimize the worst-case packet delay.

Research paper thumbnail of Joint routing, MAC, and link layer optimization in sensor networks with energy constraints

We consider sensor networks where energy is a limited resource so that energy consumption must be... more We consider sensor networks where energy is a limited resource so that energy consumption must be minimized while satisfying given throughput requirements. Moreover, energy consumption must take into account both the transmission energy and the circuit processing energy for short-range communications. We emphasize that the energy efficiency must be supported across all layers of the protocol stack through a cross-layer design. In this context, we analyze energy-efficient joint routing, scheduling, and link adaptation strategies that maximize the network lifetime. We propose variable-length TDMA schemes where the slot length is optimally assigned according to the routing requirement while minimizing the energy consumption across the network. We show that the optimization problems can be transformed into or approximated by convex problems that can be efficiently solved using known techniques. The results show that multihop routing schemes are more energy-efficient when only transmission energy is considered, but single-hop transmissions may be more efficient when the circuit processing energy is considered.

Research paper thumbnail of Cross-layer energy miminization in sensor networks

Research paper thumbnail of Cross-layer design for lifetime maximization in interference-limited wireless sensor networks

We consider the joint optimal design of the physical, medium access control (MAC), and routing la... more We consider the joint optimal design of the physical, medium access control (MAC), and routing layers to maximize the lifetime of energy-constrained wireless sensor networks. The problem of computing lifetime-optimal routing flow, link schedule, and link transmission powers for all active time slots is formulated as a non-linear optimization problem. We first restrict the link schedules to the class of interference-free time division multiple access (TDMA) schedules. In this special case, we formulate the optimization problem as a mixed integerconvex program, which can be solved using standard techniques. Moreover, when the slots lengths are variable, the optimization problem is convex and can be solved efficiently and exactly using interior point methods. For general non-orthogonal link schedules, we propose an iterative algorithm that alternates between adaptive link scheduling and computation of optimal link rates and transmission powers for a fixed link schedule. The performance of this algorithm is compared to other design approaches for several network topologies. The results illustrate the advantages of load balancing, multihop routing, frequency reuse, and interference mitigation in increasing the lifetime of energy-constrained networks. We also briefly discuss computational approaches to extend this algorithm to large networks.

Research paper thumbnail of Cross-Layer Design for Lifetime Maximization in Interference-Limited Wireless Sensor Networks

IEEE Transactions on Wireless Communications, 2006

We consider the joint optimal design of the physical, medium access control (MAC), and routing la... more We consider the joint optimal design of the physical, medium access control (MAC), and routing layers to maximize the lifetime of energy-constrained wireless sensor networks. The problem of computing lifetime-optimal routing flow, link schedule, and link transmission powers for all active time slots is formulated as a non-linear optimization problem. We first restrict the link schedules to the class of interference-free time division multiple access (TDMA) schedules. In this special case, we formulate the optimization problem as a mixed integerconvex program, which can be solved using standard techniques. Moreover, when the slots lengths are variable, the optimization problem is convex and can be solved efficiently and exactly using interior point methods. For general non-orthogonal link schedules, we propose an iterative algorithm that alternates between adaptive link scheduling and computation of optimal link rates and transmission powers for a fixed link schedule. The performance of this algorithm is compared to other design approaches for several network topologies. The results illustrate the advantages of load balancing, multihop routing, frequency reuse, and interference mitigation in increasing the lifetime of energy-constrained networks. We also briefly discuss computational approaches to extend this algorithm to large networks.

Research paper thumbnail of A Distributed Algorithm with Linear Convergence for Maximum Lifetime Routing in Wireless Networks

Research paper thumbnail of Modeling and optimization of transmission schemes in energy-constrained wireless sensor networks

IEEE/ACM Transactions on Networking, 2007

We consider a wireless sensor network with energy constraints. We model the energy consumption in... more We consider a wireless sensor network with energy constraints. We model the energy consumption in the transmitter circuit along with that for transmission. We model the bottom three layers of the traditional networking stack -the link layer, medium access control (MAC) layer, and the routing layer. Using these models, we consider the optimization of the transmission schemes to maximize the network lifetime. We first consider the optimization of a single layer at a time, while keeping the other layers fixed. We make certain simplifying assumptions to decouple the layers and formulate optimization problems to compute a strategy that maximizes the network lifetime. We then extend this approach to cross-layer optimization of time division multiple access (TDMA) wireless sensor networks. In this case, we construct optimization problems to compute the optimal transmission schemes exactly and efficiently. We then consider networks with interference, and propose methods to compute approximate solutions to the resulting optimization problems. We give numerical examples that illustrate the computational approaches as well as the benefits of cross-layer design in wireless sensor networks.

Research paper thumbnail of Fast Algorithms for Resource Allocation in Cellular Networks

We consider a scheduled orthogonal frequency division multiplexed (OFDM) wireless cellular networ... more We consider a scheduled orthogonal frequency division multiplexed (OFDM) wireless cellular network where the channels from the base-station to the mobile users undergo flat fading. Spectral resources are to be divided among the users in order to maximize total user utility. We show that this problem can be cast as a nonlinear convex optimization problem, and describe an ( ) algorithm to solve it. Computational experiments show that the algorithm typically converges in around 25 iterations, where each iteration has a cost that is ( ), with a modest constant. When the algorithm starts from an initial resource allocation that is close to optimal, convergence typically takes even fewer iterations. Thus, the algorithm can efficiently track the optimal resource allocation as the channel conditions change due to fading. We also show how our techniques can be extended to solve resource allocation problems that arise in wideband networks with frequency selective fading and when the utility of a user is also a function of the resource allocations in the past.

Research paper thumbnail of Belief Propagation Methods for Intercell Interference Coordination

Computing Research Repository, 2010

We consider a broad class of interference coordination and resource allocation problems for wirel... more We consider a broad class of interference coordination and resource allocation problems for wireless links where the goal is to maximize the sum of functions of individual link rates. Such problems arise in the context of, for example, fractional frequency reuse (FFR) for macro-cellular networks and dynamic interference management in femtocells. The resulting optimization problems are typically hard to solve optimally even using centralized algorithms but are an essential computational step in implementing rate-fair and queue stabilizing scheduling policies in wireless networks. We consider a belief propagation framework to solve such problems approximately. In particular, we construct approximations to the belief propagation iterations to obtain computationally simple and distributed algorithms with low communication overhead. Notably, our methods are very general and apply to, for example, the optimization of transmit powers, transmit beamforming vectors, and sub-band allocation to maximize the above objective. Numerical results for femtocell deployments demonstrate that such algorithms compute a very good operating point in typically just a couple of iterations.

Research paper thumbnail of Cell Association and Interference Coordination in Heterogeneous LTE-A Cellular Networks

IEEE Journal on Selected Areas in Communications, 2010

Embedding pico/femto base-stations and relay nodes in a macro-cellular network is a promising met... more Embedding pico/femto base-stations and relay nodes in a macro-cellular network is a promising method for achieving substantial gains in coverage and capacity compared to macro-only networks. These new types of base-stations can operate on the same wireless channel as the macro-cellular network, providing higher spatial reuse via cell splitting. However, these base-stations are deployed in an unplanned manner, can have very different transmit powers, and may not have traffic aggregation among many users. This could potentially result in much higher interference magnitude and variability. Hence, such deployments require the use of innovative cell association and inter-cell interference coordination techniques in order to realize the promised capacity and coverage gains. In this paper, we describe new paradigms for design and operation of such heterogeneous cellular networks. Specifically, we focus on cell splitting, range expansion, semi-static resource negotiation on third-party backhaul connections, and fast dynamic interference management for QoS via over-the-air signaling. Notably, our methodologies and algorithms are simple, lightweight, and incur extremely low overhead. Numerical studies show that they provide large gains over currently used methods for cellular networks.