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Anthony Ephremides

Publications and source records attributed to Anthony Ephremides.

At least 73 records · Page 4Linked to original sources
arXiv2014

Polynomial Complexity Minimum-Time Scheduling in a Class of Wireless Networks

We consider a wireless network with a set of transmitter-receiver pairs, or links, that share a common channel, and address the problem of emptying finite traffic volume from the transmitters in minimum time. This, so called, minimum-time scheduling problem has been proved to be NP-hard in general. In this paper, we study a class of minimum-time scheduling problems in which the link rates have a particular structure consistent with the assumed environment and topology. We show that global optimality can be reached in polynomial time and derive optimality conditions. Then we consider a more general case in which we apply the same approach and thus obtain approximation as well as lower and upper bounds to the optimal solution. Simulation results confirm and validate our approach.

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arXiv2014

Stability and Performance Issues of a Relay Assisted Multiple Access Scheme with MPR Capabilities

In this work, we study the impact of a relay node to a network with a finite number of users-sources and a destination node. We assume that the users have saturated queues and the relay node does not have packets of its own; we have random access of the medium and the time is slotted. The relay node stores a source packet that it receives successfully in its queue when the transmission to the destination node has failed. The relay and the destination nodes have multi-packet reception capabilities. We obtain analytical equations for the characteristics of the relay's queue such as average queue length, stability conditions etc. We also study the throughput per user and the aggregate throughput for the network.

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arXiv2013

Network-Level Cooperation in Energy Harvesting Wireless Networks

We consider a two-hop communication network consisted of a source node, a relay and a destination node in which the source and the relay node have external traffic arrivals. The relay forwards a fraction of the source node's traffic to the destination and the cooperation is performed at the network level. In addition, both source and relay nodes have energy harvesting capabilities and an unlimited battery to store the harvested energy. We study the impact of the energy constraints on the stability region. Specifically, we provide inner and outer bounds on the stability region of the two-hop network with energy harvesting source and relay.

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arXiv2013

The Stability Region of the Two-User Interference Channel

The stable throughput region of the two-user interference channel is investigated here. First, the stability region for the general case is characterized. Second, we study the cases where the receivers treat interference as noise or perform successive interference cancelation. Finally, we provide conditions for the convexity/concavity of the stability region and for which a certain interference management strategy leads to broader stability region.

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arXiv2013

Channel-Aware Random Access in the Presence of Channel Estimation Errors

In this work, we consider the random access of nodes adapting their transmission probability based on the local channel state information (CSI) in a decentralized manner, which is called CARA. The CSI is not directly available to each node but estimated with some errors in our scenario. Thus, the impact of imperfect CSI on the performance of CARA is our main concern. Specifically, an exact stability analysis is carried out when a pair of bursty sources are competing for a common receiver and, thereby, have interdependent services. The analysis also takes into account the compound effects of the multipacket reception (MPR) capability at the receiver. The contributions in this paper are twofold: first, we obtain the exact stability region of CARA in the presence of channel estimation errors; such an assessment is necessary as the errors in channel estimation are inevitable in the practical situation. Secondly, we compare the performance of CARA to that achieved by the class of stationary scheduling policies that make decisions in a centralized manner based on the CSI feedback. It is shown that the stability region of CARA is not necessarily a subset of that of centralized schedulers as the MPR capability improves.

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arXiv2012

On the Stability of Random Multiple Access with Stochastic Energy Harvesting

In this paper, we consider the random access of nodes having energy harvesting capability and a battery to store the harvested energy. Each node attempts to transmit the head-of-line packet in the queue if its battery is nonempty. The packet and energy arrivals into the queue and the battery are all modeled as a discrete-time stochastic process. The main contribution of this paper is the exact characterization of the stability region of the packet queues given the energy harvesting rates when a pair of nodes are randomly accessing a common channel having multipacket reception (MPR) capability. The channel with MPR capability is a generalized form of the wireless channel modeling which allows probabilistic receptions of the simultaneously transmitted packets. The results obtained in this paper are fairly general as the cases with unlimited energy for transmissions both with the collision channel and the channel with MPR capability can be derived from ours as special cases. Furthermore, we study the impact of the finiteness of the batteries on the achievable stability region.

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arXiv2012

Minimum-Length Scheduling with Finite Queues: Solution Characterization and Algorithmic Framework

We consider a set of transmitter-receiver pairs, or links, that share a common channel and address the problem of emptying backlogged queues at the transmitters in minimum time. The problem amounts to determining activation subsets of links and their time durations to form a minimum-length schedule. The problem of scheduling has been studied under various formulations before. In this paper, we present fundamental insights and solution characterizations that include: (i) showing that the complexity of the problem remains high for any continuous and increasing rate function, (ii) formulating and proving sufficient and necessary optimality conditions of two base scheduling strategies that correspond to emptying the queues using "one-at-a-time" or "all-at-once" strategies, (iii) presenting and proving the tractability of the special case in which the transmission rates are functions only of the cardinality of the link activation sets. These results are independent of physical-layer system specifications and are valid for any form of rate function. We then develop an algorithmic framework. The framework encompasses exact as well as sub-optimal, but fast, scheduling algorithms, all under a unified principle design. Through computational experiments we finally investigate the performance of several specific algorithms.

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arXiv2012

Stable Throughput in a Cognitive Wireless Network

We study, from a network layer perspective, the effect of an Ad-Hoc secondary network with N nodes randomly accessing the spectrum licensed to a primary node during the idle slots of the primary user. If the sensing is perfect, then the secondary nodes do not interfere with the primary node and hence do not affect its stable throughput. In case of imperfect sensing, it is shown that if the primary user's arrival rate is less than some calculated finite value, cognitive nodes can employ any transmission power or probabilities without affecting the primary user's stability; otherwise, the secondary nodes should control their transmission parameters to reduce the interference on the primary. It is also shown that in contrast with the primary's maximum stable throughput which strictly decreases with increased sensing errors, the throughput of the secondary nodes might increase with sensing errors as more transmission opportunities become available to them. Finally, we explore the use of the secondary nodes as relays of the primary node's traffic to compensate for the interference they might cause. We introduce a relaying protocol based on distributed space-time coding that forces all the secondary nodes that are able to decode a primary's unsuccessful packet to relay that packet whenever the primary is idle. In this case, for appropriate modulation scheme and under perfect sensing, it is shown that the more secondary nodes in the system, the better for the primary user in terms of his stable throughput. Meanwhile, the secondary nodes might benefit from relaying by having access to a larger number of idle slots due to the increase of the service rate of the primary. For the case of a single secondary node, the proposed relaying protocol guarantees that either both the primary and the secondary benefit from relaying or none of them does.

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arXiv2011

Neighbor Discovery in a Wireless Sensor Network: Multipacket Reception Capability and Physical-Layer Signal Processing

In randomly deployed networks, such as sensor networks, an important problem for each node is to discover its \textit{neighbor} nodes so that the connectivity amongst nodes can be established. In this paper, we consider this problem by incorporating the physical layer parameters in contrast to the most of the previous work which assumed a collision channel. Specifically, the pilot signals that nodes transmit are successfully decoded if the strength of the received signal relative to the interference is sufficiently high. Thus, each node must extract signal parameter information from the superposition of an unknown number of received signals. This problem falls naturally in the purview of random set theory (RST) which generalizes standard probability theory by assigning \textit{sets}, rather than values, to random outcomes. The contributions in the paper are twofold: first, we introduce the realistic effect of physical layer considerations in the evaluation of the performance of \textit{logical} discovery algorithms; such an introduction is necessary for the accurate assessment of how an algorithm performs. Secondly, given the \textit{double} uncertainty of the environment (that is, the lack of knowledge of the number of neighbors along with the lack of knowledge of the individual signal parameters), we adopt the viewpoint of RST and demonstrate its advantage relative to classical matched filter detection method.

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arXiv2011

Relay-Assisted Multiple Access with Multi-Packet Reception Capability and Simultaneous Transmission and Reception

In this work we examine the operation of a node relaying packets from a number of users to a destination node. We assume multi-packet reception capabilities for the relay and the destination node. The relay node can transmit and receive at the same time, so the problem of self interference arises. The relay does not have packets of its own and the traffic at the source nodes is considered saturated. The relay node stores a source packet that it receives successfully in its queue when the transmission to the destination node has failed. We obtain analytical expressions for the characteristics of the relay's queue (such as arrival and service rate of the relay's queue), the stability condition and the average length of the queue as functions of the probabilities of transmissions, the self interference coefficient and the outage probabilities of the links. We study the impact of the relay node and the self interference coefficient on the throughput per user-source as well as the aggregate throughput.

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arXiv2011

Optimal Utilization of a Cognitive Shared Channel with a Rechargeable Primary Source Node

This paper considers the scenario in which a set of nodes share a common channel. Some nodes have a rechargeable battery and the others are plugged to a reliable power supply and, thus, have no energy limitations. We consider two source-destination pairs and apply the concept of cognitive radio communication in sharing the common channel. Specifically, we give high-priority to the energy-constrained source-destination pair, i.e., primary pair, and low-priority to the pair which is free from such constraint, i.e., secondary pair. In contrast to the traditional notion of cognitive radio, in which the secondary transmitter is required to relinquish the channel as soon as the primary is detected, the secondary transmitter not only utilizes the idle slots of primary pair but also transmits along with the primary transmitter with probability $p$. This is possible because we consider the general multi-packet reception model. Given the requirement on the primary pair's throughput, the probability $p$ is chosen to maximize the secondary pair's throughput. To this end, we obtain two-dimensional maximum stable throughput region which describes the theoretical limit on rates that we can push into the network while maintaining the queues in the network to be stable. The result is obtained for both cases in which the capacity of the battery at the primary node is infinite and also finite.

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arXiv2011

Transmission Control of Two-User Slotted ALOHA Over Gilbert-Elliott Channel: Stability and Delay Analysis

In this paper, we consider the problem of calculating the stability region and average delay of two user slotted ALOHA over a Gilbert-Elliott channel, where users have channel state information and adapt their transmission probabilities according to the channel state. Each channel has two states, namely, the 'good' and 'bad' states. In the 'bad' state, the channel is assumed to be in deep fade and the transmission fails with probability one, while in the 'good' state, there is some positive success probability. We calculate the Stability region with and without Multipacket Reception capability as well as the average delay without MPR. Our results show that the stability region of the controlled S-ALOHA is always a superset of the stability region of uncontrolled S-ALOHA. Moreover, if the channel tends to be in the 'bad' state for long proportion of time, then the stability region is a convex Polyhedron strictly containing the TDMA stability region and the optimal transmission strategy is to transmit with probability one whenever the nodes have packets and it is shown that this strategy is delay optimal. On the other hand, if the channel tends to be in the 'good' state more often, then the stability region is bounded by a convex curve and is strict subset of the TDMA stability region. We also show that enhancing the physical layer by allowing MPR capability can significantly enhance the performance while simplifying the MAC Layer design by the lack of the need of scheduling under some conditions. Furthermore, it is shown that transmission control not only allows handling higher stable arrival rates but also leads to lower delay for the same arrival rate compared with ordinary S-ALOHA.

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arXiv2011

A New Approach to Random Access: Reliable Communication and Reliable Collision Detection

This paper applies Information Theoretic analysis to packet-based random multiple access communication systems. A new channel coding approach is proposed for coding within each data packet with built-in support for bursty traffic properties, such as message underflow, and for random access properties, such as packet collision detection. The coding approach does not require joint communication rate determination either among the transmitters or between the transmitters and the receiver. Its performance limitation is characterized by an achievable region defined in terms of communication rates, such that reliable packet recovery is supported for all rates inside the region and reliable collision detection is supported for all rates outside the region. For random access communication over a discrete-time memoryless channel, it is shown that the achievable rate region of the introduced coding approach equals the Shannon information rate region without a convex hull operation. Further connections between the achievable rate region and the Shannon information rate region are developed and explained.

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arXiv2010

Network-Level Cooperative Protocols for Wireless Multicasting: Stable Throughput Analysis and Use of Network Coding

In this paper, we investigate the impact of network coding at the relay node on the stable throughput rate in multicasting cooperative wireless networks. The proposed protocol adopts Network-level cooperation in contrast to the traditional physical layer cooperative protocols and in addition uses random linear network coding at the relay node. The traffic is assumed to be bursty and the relay node forwards its packets during the periods of source silence which allows better utilization for channel resources. Our results show that cooperation will lead to higher stable throughput rates than conventional retransmission policies and that the use of random linear network coding at the relay can further increase the stable throughput with increasing Network Coding field size or number of packets over which encoding is performed.

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arXiv2007

Fair Scheduling in OFDMA-based Wireless Systems with QoS Constraints

In this work we consider the problem of downlink resource allocation for proportional fairness of long term received rates of data users and quality of service for real time sessions in an OFDMA-based wireless system. The base station allocates available power and bandwidth to individual users based on long term average received rates, QoS based rate constraints and channel conditions. We solve the underlying constrained optimization problem and propose an algorithm that achieves the optimal allocation. Numerical evaluation results show that the proposed algorithm provides better QoS to voice and video sessions while providing more and fair rates to data users in comparison with existing schemes.

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arXiv2007

Practical Resource Allocation Algorithms for QoS in OFDMA-based Wireless Systems

In this work we propose an efficient resource allocation algorithm for OFDMA based wireless systems supporting heterogeneous traffic. The proposed algorithm provides proportionally fairness to data users and short term rate guarantees to real-time users. Based on the QoS requirements, buffer occupancy and channel conditions, we propose a scheme for rate requirement determination for delay constrained sessions. Then we formulate and solve the proportional fair rate allocation problem subject to those rate requirements and power/bandwidth constraints. Simulations results show that the proposed algorithm provides significant improvement with respect to the benchmark algorithm.

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arXiv2007

On the Shannon capacity and queueing stability of random access multicast

We study and compare the Shannon capacity region and the stable throughput region for a random access system in which source nodes multicast their messages to multiple destination nodes. Under an erasure channel model which accounts for interference and allows for multipacket reception, we first characterize the Shannon capacity region. We then consider a queueing-theoretic formulation and characterize the stable throughput region for two different transmission policies: a retransmission policy and random linear coding. Our results indicate that for large blocklengths, the random linear coding policy provides a higher stable throughput than the retransmission policy. Furthermore, our results provide an example of a transmission policy for which the Shannon capacity region strictly outer bounds the stable throughput region, which contradicts an unproven conjecture that the Shannon capacity and stable throughput coincide for random access systems.

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