Showing papers on "Channel allocation schemes published in 2007"

Fundamental limits of spectrum-sharing in fading environments

Abstract: Traditionally, the frequency spectrum is licensed to users by government agencies in a rigid manner where the licensee has the exclusive right to access the allocated band. Therefore, licensees are protected from any interference all the time. From a practical standpoint, however, an unlicensed (secondary) user may share a frequency band with its licensed (primary) owner as long as the interference it incurs is not deemed harmful by the licensee. In a fading environment, a secondary user may take advantage of this fact by opportunistically transmitting with high power when its signal, as received by the licensed receiver, is deeply faded. In this paper we investigate the capacity gains offered by this dynamic spectrum sharing approach when channels vary due to fading. In particular, we quantify the relation between the secondary channel capacity and the interference inflicted on the primary user. We further evaluate and compare the capacity under different fading distributions. Interestingly, our results indicate a significant gain in spectrum access in fading environments compared to the deterministic case

COGNITIVE RADIOS FOR DYNAMIC SPECTRUM ACCESS - The Throughput Potential of Cognitive Radio: A Theoretical Perspective

Abstract: Cognitive radios are promising solutions to the problem of overcrowded spectrum. In this article we explore the throughput potential of cognitive communication. Different interpretations of cognitive radio that underlay, overlay, and interweave the transmissions of the cognitive user with those of licensed users are described. Considering opportunistic communication as a baseline, we investigate the throughput improvements offered by the overlay methods. Channel selection techniques for opportunistic access such as frequency hopping, frequency tracking, and frequency coding are presented. The trade-off between regulation and autonomy inherent in the design and performance of cognitive networks is examined through a simple example, which shows that the optimal amount of licensing is equal to the duty cycle of the traffic arrivals

Power Allocation Schemes for Amplify-and-Forward MIMO-OFDM Relay Links

Abstract: We consider a two-hop MIMO-OFDM communication scheme with a source, an amplify-and-forward relay, and a destination. We examine the possibilities of power allocation (PA) over the subchannels in frequency and space domains to maximize the instantaneous rate of this link if channel state information at the transmitter (CSIT) is available. We consider two approaches: (i) separate optimization of the source or the relay PA with individual per node transmit power constraints and (ii) joint optimization of the source and the relay PA with joint transmit power constraint. We provide the optimal PA at the source (or the relay) with a node transmit power constraint that maximizes the instantaneous rate for a given relay (or source) PA. Furthermore, we show that repeating this separate optimization of the source and the relay PA alternately converges and improves the achievable rate of the considered link. Since the joint optimization of the source and the relay PA is analytically not tractable we use a high SNR approximation of the SNR at the destination. This approximation leads to rates which are quite tight to the optimum.

Analysis of Cognitive Radio Spectrum Access with Optimal Channel Reservation

Abstract: A Markov chain analysis for spectrum access in licensed bands for cognitive radios is presented and forced termination probability, blocking probability and traffic throughput are derived. In addition, a channel reservation scheme for cognitive radio spectrum handoff is proposed. This scheme allows the tradeoff between forced termination and blocking according to QoS requirements. Numerical results show that the proposed scheme can greatly reduce forced termination probability at a slight increase in blocking probability

Cognitive Wireless Communication Networks

Abstract: The proposed book includes a set of research and survey articles featuring the recent advances in theory and applications of cognitive radio technology for the next generation (e.g., fourth generation) wireless communication networks. Cognitive radio has emerged as a promising technology for maximizing the utilization of the limited radio bandwidth while accommodating the increasing amount of services and applications in the wireless networks. A cognitive radio transceiver is able to adapt to the dynamic radio environment and the network parameters to maximize the utilization of the limited radio resources while at the same time providing flexibility in wireless access. Development of cognitive radio technology has to deal with technical and practical considerations as well as regulatory requirements, and therefore, there is an increasing interest on this technology among the researchers and the spectrum policy makers. The contributed articles cover both the theoretical concepts (e.g., information-theoretic analysis) and system-level implementation issues. Therefore, the book provides a unified view on the state of the art of cognitive radio technology. The topics include information-theoretic analysis of cognitive radio systems, challenges and issues in designing cognitive radio systems, architectures and protocols for cognitive wireless networks, distributed adaptation and optimization methods, real-time spectrum sensing and channel allocation, cognitive machine learning techniques, interoperability and co-existence issues, spectrum awareness and dynamic channel selection, cross-layer optimization of cognitive radio systems, cognitive radio test-beds and hardware prototypes, regulatory issues on spectrum sharing, and applications of cognitive radio networks. The book starts with the essential background on cognitive radio techniques and systems (through one/two survey articles), and then it presents advanced level materials in a step-by-step fashion so that the readers can follow the book easily. The rich set of references in each of the articles will be invaluable to the researchers. The book is useful to both researchers and practitioners in this area. Also, it can be adopted as a graduate-level textbook for an advanced course on wireless communication networks.

Clustering-Based Multichannel MAC Protocols for QoS Provisionings Over Vehicular Ad Hoc Networks

Abstract: Making the best use of the dedicated short range communications multichannel architecture, we propose a cluster-based multichannel communications scheme that can support not only public-safety message delivery but also a wide range of future multimedia (e.g., video/audio) and data (e.g., e-maps, road/vehicle traffic/weather information) applications. Our proposed scheme integrates clustering with contention-free and/or -based medium access control (MAC) protocols. In our scheme, the elected cluster-head vehicle functions as the coordinator to collect/deliver real-time safety messages within its own cluster and forward the consolidated safety messages to the neighboring cluster heads. In addition, the cluster-head vehicle controls channel assignments for cluster-member vehicles transmitting/receiving nonreal-time traffics, which makes the wireless channels more efficiently utilized for vehicle-to-vehicle (V2V) nonreal-time data transmissions. Our scheme uses the contention-free MAC within a cluster and the contention-based IEEE 802.11 MAC among cluster-head vehicles such that the real-time delivery of safety messages can be guaranteed. Under our proposed scheme, we develop an analytical model to study the delay for the consolidated safety messages transmitted by the cluster-head vehicles. Based on this analytical model, we derive the desirable contention-window size, which can best balance the tradeoff between the delay of safety messages and the successful rate of delivering safety messages. The extensive simulation results show that, under various highway traffic scenarios, our proposed scheme can efficiently support the nonreal-time traffics while guaranteeing the real-time delivery of the safety messages.

Channel Assignment Strategies for Multiradio Wireless Mesh Networks: Issues and Solutions

Abstract: Next-generation wireless mobile communications will be driven by converged networks that integrate disparate technologies and services. The wireless mesh network is envisaged to be one of the key components in the converged networks of the future, providing flexible high- bandwidth wireless backhaul over large geographical areas. While single radio mesh nodes operating on a single channel suffer from capacity constraints, equipping mesh routers with multiple radios using multiple nonoverlap- ping channels can significantly alleviate the capacity problem and increase the aggregate bandwidth available to the network. However, the assignment of channels to the radio interfaces poses significant challenges. The goal of channel assignment algorithms in multiradio mesh networks is to minimize interference while improving the aggregate network capacity and maintaining the connectivity of the network. In this article we examine the unique constraints of channel assignment in wireless mesh networks and identify the key factors governing assignment schemes, with particular reference to interference, traffic patterns, and multipath connectivity. After presenting a taxonomy of existing channel assignment algorithms for WMNs, we describe a new channel assignment scheme called MesTiC, which incorporates the mesh traffic pattern together with connectivity issues in order to minimize interference in multi- radio mesh networks.

Minimum Interference Channel Assignment in Multi-Radio Wireless Mesh Networks

Abstract: In this paper, we consider multi-hop wireless mesh networks, where each router node is equipped with multiple radio interfaces and multiple channels are available for communication. We address the problem of assigning channels to communication links in the network with the objective of minimizing overall network interference. Since the number of radios on any node can be less than the number of available channels, the channel assignment must obey the constraint that the number of different channels assigned to the links incident on any node is atmost the number of radio interfaces on that node. The above optimization problem is known to be NP-hard. We design centralized and distributed algorithms for the above channel assignment problem. To evaluate the quality of the solutions obtained by our algorithms, we develop a semidefinite program formulation of our optimization problem to obtain a lower bound on overall network interference. Empirical evaluations on randomly generated network graphs show that our algorithms perform close to the above established lower bound, with the difference diminishing rapidly with increase in number of radios. Also, detailed ns-2 simulation studies demonstrate the performance potential of our channel assignment algorithms in 802.11-based multi-radio mesh networks.

Distributed Channel Assignment in Multi-Radio 802.11 Mesh Networks

Abstract: To increase the utilization of the available frequency channel space in 802.11-based wireless mesh networks, recent work has explored solutions based on multi-radio stations. This paper reports on our design and experimental study of a distributed, self-stabilizing mechanism that assigns channels to multi-radio nodes in wireless mesh networks. We take a modular approach by decoupling the channel selection decision from the data forwarding mechanism, which makes our solution readily applicable to real-world operation when used with emerging multi-radio routing solutions. We demonstrate the efficacy of our protocol on a real-world, 14-node testbed comprised of nodes, each equipped with an 802.11a card and an 802.11g card. We show via extensive measurements on our testbed that our channel assignment algorithm improves the network capacity by 50% in comparison to a homogeneous channel assignment and by 20% in comparison to a random assignment.

A novel on-demand cognitive pilot channel enabling dynamic spectrum allocation

Abstract: This paper addresses the implementation of the cognitive pilot channel (CPC), which has been recently proposed as a solution to assist the mobile reconfigurable and cognitive terminals in heterogeneous wireless scenarios with different access networks available and varying spectrum allocations. The paper describes the operation of the CPC and the different approaches existing in the literature depending on how it is mapped onto specific radio resources. Then, it focuses on the implementation of the CPC information delivery and proposes the use of an on-demand CPC, which requires a significantly lower bit rate than the broadcast approach to achieve similar performance.

A Cognitive MAC Protocol Using Statistical Channel Allocation for Wireless Ad-Hoc Networks

Abstract: The MAC protocol of a cognitive radio (CR) device should allow it to access unused or under-utilized spectrum without (or with minimal) interference to primary users dynamically. To fulfill such a goal, we propose a cognitive MAC protocol using statistical channel allocation and call it SCA-MAC in this work. SCA-MAC is a CSMA/CA-based protocol, which exploits statistics of spectrum usage for decision making on channel access. For each transmission, the sender negotiates with the receiver on transmission parameters through the control channel. A model is developed for CR devices to evaluate the successful rate of transmission. A CR device should pass the threshold of the successful transmission rate via negotiation before it can begin a valid transmission on data channels. The operating range and channel aggregation are two control parameters introduced to maintain the MAC performance. To validate our ideas, we conducted theoretical analysis and simulations to show that SCA-MAC does improve the throughput performance and guarantee the interference to incumbents to be bounded by a predetermined acceptable rate. The proposed MAC protocol does not need a centralized controller, as the negotiation between the sender and the receiver is performed using the CSMA/CA-based algorithm.

Non-cooperative resource competition game by virtual referee in multi-cell OFDMA networks

Abstract: In this paper, a distributive non-cooperative game is proposed to perform sub-channel assignment, adaptive modulation, and power control for multi-cell multi-user orthogonal frequency division multiplexing access (OFDMA) networks. Each individual user's goal is to minimize his/her own transmitted power in a distributed manner under the constraints that the desirable rate is achieved and the transmitted power is bounded. The pure non-cooperative game may result in non-convergence or some undesirable Nash Equilibriums with low system and individual performances. To enhance the performances, a virtual referee is introduced to the networks and is in charge of monitoring and improving the outcome of non-cooperative competition for resources among the distributed users. If the game outcome is not desirable, either the required transmission rates should be reduced or some users should be prevented from using some radio resources such as sub-channels, so that the rest of users can share the limited resources more efficiently. Moreover, it can be shown that the introduction of the virtual referee does not increase the complexity of the networks. From the simulation results in a two-cell case, the proposed scheme reduces the transmitted power by 80% and 25% compared with the fixed channel assignment algorithm and the iterative water-filling algorithm in the literature, respectively. The achievable rate can be improved by 10%. In a multi-cell case, the proposed scheme can have up to 40% power reduction compared with the iterative water-filling algorithm when the co-channel interferences are severe.

Dynamic spectrum allocation in cognitive radio using hidden Markov models: Poisson distributed case

Abstract: Cognitive radio networks can be designed to manage the radio spectrum more efficiently by utilizing the spectrum holes in primary users' licensed frequency bands. Recent studies have shown that the radio spectrum is poorly utilized by the licensed users even in urban geographical areas. This spectrum utilization can be improved significantly by making it possible for secondary users (who are not being served by the primary system) to access spectrum holes, i.e., frequency bands not used by licensed users. In this novel work, we use hidden Markov models (HMMs) to model and predict the spectrum occupancy of licensed radio bands. The proposed technique can dynamically select different licensed bands for its own use with significantly less interference from and to the licensed users. It is found that by predicting the duration of spectrum holes of primary users, the CR can utilize them more efficiently by leaving the band, that it currently occupies, before the start of traffic from the primary user of that band. We propose a simple algorithm, called the Markov-based channel prediction algorithm (MCPA), for dynamic spectrum allocation in cognitive radio networks. In this work, we present the performance of our proposed dynamic spectrum allocation algorithm when the channel state occupancy of primary users are assumed to be Poisson distributed. The impact of CR transmission on the licensed users is also presented. It is shown that significant SIR improvements can be achieved using HMM based dynamic spectrum allocation as compared to the traditional CSMA based approach. The results obtained using HMM are very promising and HMM can offer a new paradigm for predicting channel behavior in cognitive radio, an area that has been of much research interest lately.

Channel surfing: defending wireless sensor networks from interference

Abstract: Wireless sensor networks are susceptible to interference that can disrupt sensor communication. In order to cope with this disruption, we explore channel surfing, whereby the sensor nodes adapt their channel assignments to restore network connectivity in the presence of interference. We explore two different approaches to channel surfing: coordinated channel switching, where the entire sensor network adjusts its channel; and spectral multiplexing, where nodes in a jammed region switch channels while nodes on the boundary of a jammed region act as radio relays between different spectral zones. For spectral multiplexing, we have devised both synchronous and asynchronous strategies to facilitate the spectral scheduling needed to improve network fidelity when sensor nodes operate on multiple channels. In designing these algorithms, we have taken a system-oriented approach that has focused on exploring actual implementation issues under realistic network settings. We have implemented these proposed methods on a testbed of 30 Mica2 sensor nodes, and the experimental results show that these strategies can each repair network connectivity in the presence of interference without introducing significant overhead.

Dynamic Wavelength and Bandwidth Allocation in Hybrid TDM/WDM EPON Networks

Abstract: We discuss a wavelength-division-multiplexed-based passive-optical-network (PON) architecture that allows for incremental upgrade from single-channel time-division multiple-access PONs in order to provide higher bandwidth in the access network. Various dynamic-wavelength and bandwidth-allocation algorithms (DWBAs) for wave-division multiplexed PON are presented; they exploit both interchannel and intrachannel statistical multiplexing in order to achieve better performance, especially when the load on various channels is not symmetric. Three variants of the DWBA are presented, and their performance is compared. While the first variant incurs larger idle times (and, hence, poor performance), the other two algorithms achieve better but different performance with critical dissimilarities. Our analysis also focuses on the fair assignment of excessive bandwidth in the upstream direction to highly loaded optical network units. We compare the performance of DWBA to another algorithm that relies on static-channel allocation. Furthermore, a study is presented wherein the number of wavelengths increases, and a comparison with interleaved polling with adaptive cycle time is shown. We use extensive simulations throughout this paper

Power Allocation in OFDM-Based Cognitive Radio Systems

Abstract: In this paper, we investigate the optimal power allocation strategy that aims at maximizing the capacity in OFDM- based cognitive radio systems. We show that the traditional water-filling algorithm applied in general OFDM systems needs to be modified due to the per subchannel power constraints in such systems. An iterative partitioned water-filling algorithm is proposed and proved to be optimal based on the convex optimization theory.

Non-Cooperative Multi-Radio Channel Allocation in Wireless Networks

Abstract: Channel allocation was extensively studied in the framework of cellular networks. But the emergence of new system concepts, such as cognitive radio systems, has brought this topic into the focus of research again. In this paper, we study in detail the problem of competitive multi-radio multi-channel allocation in wireless networks. We study the existence of Nash equilibria in a static game and we conclude that, in spite of the non-cooperative behavior of such devices, their channel allocation results in a load-balancing solution. In addition, we consider the fairness properties of the resulting channel allocations and their resistance to the possible coalitions of a subset of players. Finally, we present three algorithms that achieve a load-balancing Nash equilibrium channel allocation; each of them using a different set of available information.

On the extraction of the channel allocation information in spectrum pooling systems

Abstract: The spectrum pooling strategy allows a license owner to share a part of his licensed spectrum with a secondary wireless system (the rental system, RS) during its idle times. The coexistence of two mobile systems on the same frequency band poses many new challenges, one of which is the reliable extraction of the channel allocation information (CAI), i.e. the channel occupation of the licensed system (LS). This paper presents a strategy for the extraction of the CAI based on exploiting the distinct cyclostationary characteristics of the LS and RS signals and demonstrates, via simulations, its application on a specific spectrum pooling scenario, where the LS is a GSM network and the RS is an OFDM based WLAN system

Fairness-Aware Adaptive Resource Allocation Scheme in Multihop OFDMA Systems

Abstract: We investigate a fairness-aware adaptive resource allocation scheme for the downlink of multihop OFDMA systems. Assuming that the base station has all the channel information, we formulate an optimization problem for an adaptive subchannel-, path- and power-allocation scheme that maximizes system capacity while guaranteeing minimum resources for each user. Since the optimization should be performed in real time, we propose an efficient heuristic algorithm composed of subchannel-allocation, load-balancing and power-distribution steps. The proposed algorithm is simple in that the iterative computations are removed, and accurate in that it performs similarly to the optimum solution

Maximizing Multicell Capacity Using Distributed Power Allocation and Scheduling

Abstract: Joint optimization of transmit power and scheduling in wireless data networks promises significant system-wide capacity gains. However, this problem is known to be NP-hard and thus difficult to tackle in practice. We analyze this problem for the downlink of a multicell full reuse network with the goal of maximizing the overall network capacity. We propose a distributed power allocation and scheduling algorithm which provides significant capacity gain for any finite number of users. This distributed cell coordination scheme, in effect, achieves a form of dynamic spectral reuse, whereby the amount of reuse varies as a function of the underlying channel conditions and only limited inter-cell signaling is required.

Dynamic resource allocation in OFDM systems: an overview of cross-layer optimization principles and techniques

Abstract: Recently, a lot of research effort has been spent on cross-layer system design. It has been shown that cross-layer mechanisms (i.e., policies) potentially provide significant performance gains for various systems. In this article we review several aspects of cross-layer system optimization regarding wireless OFDM systems. We discuss basic optimization models and present selected heuristic approaches realizing cross-layer policies by means of dynamic resource allocation. Two specific areas are treated separately: models and dynamic approaches for single transmitter/receiver pairs (i.e., a point-to-point communication scenario) as well as models and approaches for point-to-multipoint communication scenarios (e.g., the downlink of a wireless cell). This article provides basic knowledge in order to investigate future OFDM cross-layer-optimization issues

Distributed channel allocation method and wireless mesh network therewith

Abstract: A distributed channel allocation method and a wireless mesh network with the same are provided herein. By the distributed channel allocation, interference situations are avoided in a wireless network communication, and the allocated bandwidth can then be fully utilized. Besides, unnecessary depletion of an allocated bandwidth due to the interference can be avoided. By this method, a time division technique is applied for dividing a transmission time of each wireless NIC, and different non-overlapping channels can be assigned to different timeslots. Different from other researches that require a symmetrical number of the NICs between a receiving node and a transmitting node, in this method, a unique wireless NIC may communicate with the wireless NICs. The method provides the feature that the number of the NICs on a certain node can be adjusted to meet a communication requirement, by which the efficiency of a network flow is also significantly improved.

Joint logical topology design, interface assignment, channel allocation, and routing for multi-channel wireless mesh networks

Abstract: A multi-channel wireless mesh network (MC-WMN) consists of a number of stationary wireless routers, where each router is equipped with multiple network interface cards (NICs). Each NIC operates on a distinct frequency channel. Two neighboring routers establish a logical link if each one has an NIC operating on a common channel. Given the physical topology of the routers and other constraints, four important issues should be addressed in MC-WMNs: logical topology formation, interface assignment, channel allocation, and routing. Logical topology determines the set of logical links. Interface assignment decides how the logical links should be assigned to the NICs in each wireless router. Channel allocation selects the operating channel for each logical link. Finally, routing determines through which logical links the packets should be forwarded. In this paper, we mathematically formulate the logical topology design, interface assignment, channel allocation, and routing as a joint linear optimization problem. Our proposed MC-WMN architecture is called TiMesh. Extensive ns-2 simulation experiments are conducted to evaluate the performance of TiMesh and compare it with two other MC-WMN architectures Hyacinth [1] and CLICA [2]. Simulation results show that TiMesh achieves higher aggregated network throughput and lower end-to-end delay than Hyacinth and CLICA for both TCP and UDP traffic. It also provides better fairness among different flows.

A stackelberg game for power control and channel allocation in cognitive radio networks

Abstract: The ongoing growth in wireless communication continues to increase demand on the frequency spectrum. The current rigid frequency band allocation policy leads to a significant under-utilization of this scarce resource. However, recent policy changes by the Federal Communications Commission (FCC) and research directions suggested by the Defense Advanced Research Projects Agency (DARPA) have been focusing on wireless devices that can adaptively and intelligently adjust their transmission characteristics, which are known as cognitive radios. This paper suggests a game theoretical approach that allows master-slave cognitive radio pairs to update their transmission powers and frequencies simultaneously. This is shown to lead to an exact potential game, for which it is known that a particular update scheme converges to a Nash Equilibrium (NE). Next, a Stackelberg game model is presented for frequency bands where a licensed user has priority over opportunistic cognitive radios. We suggest a modification to the exact potential game discussed earlier that would allow a Stackelberg leader to charge a virtual price for communicating over a licensed channel. We investigate virtual price update algorithms for the leader and prove the convergence of a specific algorithm. Simulations performed in Matlab verify our convergence results and demonstrate the performance gains over alternative algorithms.

MAC Protocol Design for Spectrum-agile Wireless Networks: Stochastic Control Approach

Abstract: Widespread deployment of wireless networks under different services including wireless LANs and sensor networks in a shared spectrum has caused many interference and performance issues in dense networks. Specifically, such coexistence poses significant challenges on media access protocol design. This paper addresses this problem by proposing a distributed spectrum-agile MAC (media access control) protocol. It is a multichannel CSMA-based protocol equipped with a dynamic channel selection algorithm. The dynamic channel selection problem is formulated as a multi-armed bandit problem, and the optimal channel selection rules are derived. Finally the advantage of the new protocol is demonstrated through simulation.

Cooperation in a Half-Duplex Gaussian Diamond Relay Channel

Abstract: A half-duplex relay channel consisting of two relays in a diamond topology is studied. In contrast to full-duplex systems, it is shown that time should be optimally allocated among different states. In contrast to the classical three-node relay channel, it is shown that spatial reuse can be utilized to allow communication without interference. Both achievable rate and upper bound on capacity are studied. Several communication schemes such as multihop with spatial reuse, scale-forward, broadcast-multiaccess with common message, compress-forward, as well as hybrid ones are characterized, some of which are novel. It is also shown that simple multihop with spatial reuse achieves the capacity of certain symmetric diamond channels.

Price-Based Spectrum Management in Cognitive Radio Networks

Abstract: A key challenge in operating cognitive radios (CRs) in a self-organizing (ad hoc) network is how to adaptively and efficiently allocate transmission powers and spectrum among CRs according to the surrounding environment. Most previous works address this issue via heuristic approaches or using centralized solutions. In this paper, we present a novel joint power/channel allocation scheme that uses a distributed pricing strategy to improve the network's performance. In this scheme, the spectrum allocation problem is modelled as a non-cooperative game. A price-based iterative water-filling (PIWF) algorithm is proposed, which allows users to converge to the Nash Equilibrium (NE). This PIWF algorithm can be implemented distributively with CRs repeatedly negotiating their best transmission powers and spectrum. Simulation results show that the social optimality of the NE solution is dramatically improved with our price-based strategy. Based on the orders by which CRs take actions, we study sequential and parallel versions of the algorithm. We show that the parallel version converges faster than the sequential version.

Joint Channel Allocation, Interface Assignment and MAC Design for Multi-Channel Wireless Mesh Networks

Abstract: In a wireless mesh network (WMN) with a number of stationary wireless routers, the aggregate capacity can be increased when each router is equipped with multiple network interface cards (NICs) and each NIC within a router is assigned to a distinct orthogonal frequency channel. In this paper, given the logical topology of the network, we formulate the joint channel allocation, interface assignment, and media access control (MAC) problem as a cross-layer non-linear mixed-integer network utility maximization problem. An optimal joint design, based on exact binary linearization techniques, is proposed which leads to a global maximum. A near-optimal joint design, based on approximate dual decomposition techniques, is also proposed which is of more interest in terms of practical deployment. Performance evaluation is given through a number of numerical examples in terms of network utility maximization and aggregate network throughput.

On the power efficiency of cooperative broadcast in dense wireless networks

Abstract: A fundamental problem in large scale wireless networks is the energy efficient broadcast of source messages to the whole network. The energy consumption increases as the network size grows, and the optimization of broadcast efficiency becomes more important. In this paper, we study the optimal power allocation problem for cooperative broadcast in dense large-scale networks. In the considered cooperation protocol, a single source initiates the transmission and the rest of the nodes retransmit the source message if they have decoded it reliably. Each node is allocated an-orthogonal channel and the nodes improve their receive signal-to-noise ratio (SNR), hence the energy efficiency, by maximal-ratio combining the receptions of the same packet from different transmitters. We assume that the decoding of the source message is correct as long as the receive SNR exceeds a predetermined threshold. Under the optimal cooperative broadcasting, the transmission order (i.e., the schedule) and the transmission powers of the source and the relays are designed so that every node receives the source message reliably and the total power consumption is minimized. In general, finding the best scheduling in cooperative broadcast is known to be an NP-complete problem. In this paper, we show that the optimal scheduling problem can be solved for dense networks, which we approximate as a continuum of nodes. Under the continuum model, we derive the optimal scheduling and the optimal power density. Furthermore, we propose low-complexity, distributed and power efficient broadcasting schemes and compare their power consumptions with those-of-a traditional noncooperative multihop transmission.

Improving Transport Layer Performance in Multihop Ad Hoc Networks by Exploiting MAC Layer Information

Abstract: The traditional TCP congestion control mechanism encounters a number of new problems and suffers a poor performance when the IEEE 802.11 MAC protocol is used in multihop ad hoc networks. Many of the problems result from medium contention at the MAC layer. In this paper, we first illustrate that severe medium contention and congestion are intimately coupled, and TCP's congestion control algorithm becomes too coarse in its granularity, causing throughput instability and excessively long delay. Further, we illustrate TCP's severe unfairness problem due to the medium contention and the tradeoff between aggregate throughput and fairness. Then, based on the novel use of channel busyness ratio, a more accurate metric to characterize the network utilization and congestion status, we propose a new wireless congestion control protocol (WCCP) to efficiently and fairly support the transport service in multihop ad hoc networks. In this protocol, each forwarding node along a traffic flow exercises the inter-node and intra-node fair resource allocation and determines the MAC layer feedback accordingly. The end-to-end feedback, which is ultimately determined by the bottleneck node along the flow, is carried back to the source to control its sending rate. Extensive simulations show that WCCP significantly outperforms traditional TCP in terms of channel utilization, delay, and fairness, and eliminates the starvation problem