Cross-layer schemes for performance optimization in wireless networks

Wireless networks are undergoing rapid progress and inspiring numerous applications. As the application of wireless networks becomes broader, they are expected to not only provide ubiquitous connectivity, but also support end users with certain service guarantees. End-to-end delay is an important Quality of Service (QoS) metric in multihop wireless networks. This dissertation addresses how to minimize end-to-end delay through joint optimization of network layer routing and link layer scheduling. Two cross-layer schemes, a loosely coupled cross-layer scheme and a tightly coupled cross-layer scheme, are proposed. The two cross-layer schemes involve interference modeling in multihop wireless networks with omnidirectional antenna. In addition, based on the interference model, multicast schedules are optimized to minimize the total end-to-end delay. Throughput is another important QoS metric in wireless networks. This dissertation addresses how to leverage the spatial multiplexing function of MIMO links to improve wireless network throughput. Wireless interference modeling of a half-duplex MIMO node is presented. Based on the interference model, routing, spatial multiplexing, and scheduling are jointly considered in one optimization model. The throughput optimization problem is first addressed in constant bit rate networks and then in variable bit rate networks. In a variable data rate network, transmitters can use adaptive coding and modulation schemes to change their data rates so that the data rates are supported by the Signal to Noise and Interference Ratio (SINR). The problem of achieving maximum throughput in a millimeter-wave wireless personal area network is studied --Abstract, page iv

Policy-Based Radio Resource Management in Multicast OFDMA Systems

Η ασύρματηφασματική αποδοτικότητα είναι ένας, όλο και περισσότερο, σημαντικός παράγοντας εξαιτίας της ταχείας ανάπτυξης των ασύρματων υπηρεσιών ευρείας ζώνης. Η σχεδίαση ενός συστήματος με πολλά φέροντα, όπως είναι ένα σύστημα OFDMA,επιτρέπει στα συστήματα να έχουν υψηλή χωρητικότητα για να ικανοποιήσουν τις απαιτήσεις των υπηρεσιών ευρείας ζώνης.Αυτή η αυξημένη χωρητικότητα των συστημάτων μπορεί να βελτιστοποιηθεί περαιτέρω εκμεταλλευόμενοι καλύτερα τα χαρακτηριστικά των ασύρματων καναλιών. Ηθεμελιώδηςιδέα ενός σχήματος κατανομής πόρων είναι η αποτελεσματική κατανομή των διαθέσιμων ασύρματων πόρων, όπως είναι οι υποφορείς και η ισχύς εκπομπής, μεταξύ των χρηστών του συστήματος. Σχετικά με τα προβλήματα της κατανομής πόρων σε ασύρματα συστήματα τηλεπικοινωνιών βασισμένα στην τεχνική OFDMA, η περισσότερη έρευνα επικεντρώνεται στην αναζήτηση πολιτικών ανάθεσης υποφορέων και ισχύος. Οι διαθέσιμες τεχνικές της βιβλιογραφίας δεν μπορούν να εφαρμοστούν όπως είναι σε συστήματα πολυεκπομπής. Επιπλέον, οι υπάρχουσες τεχνικές δεν μπορούν να εφαρμοστούν αμετάβλητες σε πραγματικά συστήματα στα οποία υπάρχει μεγάλος αριθμός OFDMυποφορέων, καθώς η υπολογιστική πολυπλοκότητα είναι πολύ μεγάλη. Ο βασικός στόχος της παρούσας διπλωματικής εργασίας είναι η πρόταση ικανών μηχανισμών κατανομής των διαθέσιμων υποφορέων σε ασύρματα συστήματα πολυεκπομπής χρησιμοποιώντας την τεχνολογία OFDMA. Πιο συγκεκριμένα, σχετικά με τα συστήματα πολυεκπομπής, θεωρούμε ότι τόσο ο σταθμός βάσης όσο και κάθε χρήστης είναι εφοδιασμένοι με μοναδική κεραία και η μονάδα κατανομής δεν είναι ο υποφορέας, όπως στα συμβατικά συστήματα OFDMA, αλλά μία ομάδα γειτονικώνυποφορέων, η οποία ονομάζεται τεμάχιο, με σκοπό τη μείωση της μεγάλης υπολογιστικής πολυπλοκότητας. Ένας αποτελεσματικός αλγόριθμος προτείνεται του οποίου ο στόχος είναι η μεγιστοποίηση του συνολικού ρυθμού μετάδοσης δεδομένων με περιορισμούς στη συνολική διαθέσιμη ισχύ, στο BERανά τεμάχιο και στους αναλογικούς περιορισμούς μεταξύ των ρυθμών μετάδοσης δεδομένων των ομάδων χρηστών. Η προσομοίωση και η ανάλυση της πολυπλοκότητας που παρουσιάζονται, υποστηρίζουν τα πλεονεκτήματα της κατανομής πόρων σε συστήματα πολυεκπομπήςOFDMA τα οποία βασίζονται σε κατανομή τεμαχίων και έχουν ως στόχος την εξασφάλιση της αναλογικότητας μεταξύ των ρυθμών μετάδοσης δεδομένων των ομάδων χρηστών.Wireless spectral efficiency is increasingly important due to the rapid growth of demand for high data rate wideband wireless services. The design of a multi-carrier system, such as an OFDMA system, enables high system capacity suited for these wideband wireless services. This system capacity can be further optimized with a resource allocation scheme by exploiting the characteristics of the wireless fading channels. The fundamental idea of a resource allocation scheme is to efficiently distribute the available wireless resources, such as the subcarriers and transmission power, among all admitted users in the system. Regarding the problems of resource allocation in OFDMA-based wireless communicationsystems, much of the research effort mainly focuses on finding efficient power controland subcarrier assignment policies. With systems employing multicast transmission,the available schemes in literature are not always applicable. Moreover, the existing approachesare particularly inaccessible in practical systems in which there are a large numberof OFDM subcarriers being utilized, as the required computational burden is prohibitivelyhigh. The ultimate goal of this Thesis is therefore to propose affordable mechanisms toflexibly and effectively share out the available resources in multicast wireless systems deployingOFDMA technology. Specifically, according to multicast system, it is assumed thatboth the BS and each user are equipped witha single antenna and the allocation unit is not the subcarrier,as in conventional OFDMA systems, but a set of contiguoussubcarriers, which is called chunk, in order to alleviate the heavy computational burden. An efficient algorithmis proposed whose aim is to maximize the total throughput subject to constraints on totalavailable power,BER over a chunk, and proportional data rates constraints among multicast groups. Simulation and complexity analysis are provided to support thebenefits of chunk-based resource allocation to multicast OFDMA systems with targeting proportional data rates among multicast groups

Improvement of indoor VLC network downlink scheduling and resource allocation

Indoor visible light communications (VLC) combines illumination and communication by utilizing the high-modulation-speed of LEDs. VLC is anticipated to be complementary to radio frequency communications and an important part of next generation heterogeneous networks. In order to make the maximum use of VLC technology in a networking environment, we need to expand existing research from studies of traditional point-to-point links to encompass scheduling and resource allocation related to multi-user scenarios. This work aims to maximize the downlink throughput of an indoor VLC network, while taking both user fairness and time latency into consideration. Inter-user interference is eliminated by appropriately allocating LEDs to users with the aid of graph theory. A three-term priority factor model is derived and is shown to improve the throughput performance of the network scheduling scheme over those previously reported. Simulations of VLC downlink scheduling have been performed under proportional fairness scheduling principles where our newly formulated priority factor model has been applied. The downlink throughput is improved by 19.6% compared to previous two-term priority models, while achieving similar fairness and latency performance. When the number of users grows larger, the three-term priority model indicates an improvement in Fairness performance compared to two-term priority model scheduling

Cross-layer design through joint routing and link allocation in wireless sensor networks

Both energy and bandwidth are scarce resources in sensor networks. In the past, the energy efficient routing problem has been extensively studied in efforts to maximize sensor network lifetimes, but the link bandwidth has been optimistically assumed to be abundant. Because energy constraint affects how data should be routed, link bandwidth affects not only the routing topology, but also the allowed data rate on each link, which in turn affects the lifetime. Previous research that focus on energy efficient operations in sensor networks with the sole objective of maximizing network lifetime only consider the energy constraint ignoring the bandwidth constraint. This thesis shows how infeasible these solutions can be when bandwidth does present a constraint. It provides a new mathematical model that address both energy and bandwidth constraints and proposes two efficient heuristics for routing and rate allocation. Simulation results show that these heuristics provide more feasible routing solutions than previous work, and significantly improve throughput. A method of assigning the time slot based on the given link rates is presented. The cross layer design approach improves channel utility significantly and completely solves the hidden terminal and exposed terminal problems --Abstract, page iii

Reliable Physical Layer Network Coding

When two or more users in a wireless network transmit simultaneously, their electromagnetic signals are linearly superimposed on the channel. As a result, a receiver that is interested in one of these signals sees the others as unwanted interference. This property of the wireless medium is typically viewed as a hindrance to reliable communication over a network. However, using a recently developed coding strategy, interference can in fact be harnessed for network coding. In a wired network, (linear) network coding refers to each intermediate node taking its received packets, computing a linear combination over a finite field, and forwarding the outcome towards the destinations. Then, given an appropriate set of linear combinations, a destination can solve for its desired packets. For certain topologies, this strategy can attain significantly higher throughputs over routing-based strategies. Reliable physical layer network coding takes this idea one step further: using judiciously chosen linear error-correcting codes, intermediate nodes in a wireless network can directly recover linear combinations of the packets from the observed noisy superpositions of transmitted signals. Starting with some simple examples, this survey explores the core ideas behind this new technique and the possibilities it offers for communication over interference-limited wireless networks.Comment: 19 pages, 14 figures, survey paper to appear in Proceedings of the IEE

Cross-Layer Framework for Multiuser Real Time H.264/AVC Video Encoding and Transmission over Block Fading MIMO Channels Using Outage Probability

We present a framework for cross-layer optimized real time multiuser encoding of video using a single layer H.264/AVC and transmission over MIMO wireless channels. In the proposed cross-layer adaptation, the channel of every user is characterized by the probability density function of its channel mutual information and the performance of the H.264/AVC encoder is modeled by a rate distortion model that takes into account the channel errors. These models are used during the resource allocation of the available slots in a TDMA MIMO communication system with capacity achieving channel codes. This framework allows for adaptation to the statistics of the wireless channel and to the available resources in the system and utilization of the multiuser diversity of the transmitted video sequences. We show the effectiveness of the proposed framework for video transmission over Rayleigh MIMO block fading channels, when channel distribution information is available at the transmitter

QoS multicast routing protocol oriented to cognitive network using competitive coevolutionary algorithm

The human intervention in the network management and maintenance should be reduced to alleviate the ever-increasing spatial and temporal complexity. By mimicking the cognitive behaviors of human being, the cognitive network improves the scalability, self-adaptation, self-organization, and self-protection in the network. To implement the cognitive network, the cognitive behaviors for the network nodes need to be carefully designed. Quality of service (QoS) multicast is an important network problem. Therefore, it is appealing to develop an effective QoS multicast routing protocol oriented to cognitive network. In this paper, we design the cognitive behaviors summarized in the cognitive science for the network nodes. Based on the cognitive behaviors, we propose a QoS multicast routing protocol oriented to cognitive network, named as CogMRT. It is a distributed protocol where each node only maintains local information. The routing search is in a hop by hop way. Inspired by the small-world phenomenon, the cognitive behaviors help to accumulate the experiential route information. Since the QoS multicast routing is a typical combinatorial optimization problem and it is proved to be NP-Complete, we have applied the competitive coevolutionary algorithm (CCA) for the multicast tree construction. The CCA adopts novel encoding method and genetic operations which leverage the characteristics of the problem. We implement and evaluate CogMRT and other two promising alternative protocols in NS2 platform. The results show that CogMRT has remarkable advantages over the counterpart traditional protocols by exploiting the cognitive favors