Dissertations / Theses on the topic 'Broadcast Control'

The pervasiveness of wireless communications in modern society is unprecedent. Using numerous wireless technologies, including cellular, WiFi, WiMax and others, users gain ubiquitous access to a wealth of information services relevant to their everyday lives. These include communication services (voice, email, text), commercial transactions (e-banking, e-trading), location-based services (points of interest, navigation), social networking, web access, and others. Further proliferation of mobile services and applications provided via wireless communications is contingent upon the network's reliability and security. As this wireless revolution continues to unfold, users have grown increasingly concerned with the amount of sensitive information that leaks to unauthorized parties. They have also become more skeptical to the abilities of current technologies in providing a reliable and secure environment. The problem of ensuring network reliability and security is particularly challenging in the wireless domain. The open nature of the wireless medium leaves it accessible to any device equipped with a transceiver. Wireless networks remain vulnerable to numerous attacks, including eavesdropping, packet modification and injection, replay, impersonation, and jamming. Moreover, the poor physical security of the majority of wireless stations (mobile devices, sensor, etc.) makes them susceptible to physical compromise. Such compromised devices become a launchpad for internal attacks from adversaries with intimate knowledge of network secrets and protocol specifications. Internal attacks are much harder to counter than external ones since the adversary can bypass typical cryptographic methods that authorize user access. In this dissertation, we investigate attacks on wireless communications launched by internal adversaries. In particular, we are concerned with sophisticated intentional interference (jamming) attacks against broadcast communications. Such attacks have been shown to be detrimental to network operation. We further consider the problem of distributed node coordination in dynamic spectrum networks. To address these problems, we design novel communication protocols that protect broadcast communications from internal attacks via a combination of randomization, redundancy and cryptographic techniques. We propose new security metrics to quantify the ability of the adversary to deny access to the broadcast channel. We introduce a randomized distributed scheme that allows nodes to establish and maintain the broadcast channel in the presence of the jammer. Also this scheme uniquely identifies the set of compromised nodes, both when nodes are acting independently and when they are colluding. To protect broadcast communication performed on control channel, we propose a secure broadcast scheme called TDBS, which implements the broadcast operation as a series of unicast transmissions, distributed in frequency and time. Finally, we address the problem of dynamically assigning the control channel in CRNs based on time- and space-varying spectrum opportunities. We propose a cluster-based architecture that allocates different channels for control at various clusters in the network. The clustering problem is formulated as a bipartite graph problem, for which we develop a class of algorithms to implement. Extensive simulations are conducted to verify the validity of the proposed mechanisms.

Red imported fire ant specimens were first collected in Virginia in 1989 from colonies located in Hampton, Va. Now colonies are established throughout the Hampton Roads area of Virginia. Because the fire ant is a new pest information regarding the distribution, biology, and management of the ant in Virgnia are absent. Therefore studies were conducted to compare the efficacies of four broadcast fire ant control products and an individual mound treatment to control fire ants and to determine the impact of Advion and Top Choice on non-target ant species. Additionally, twenty-six fire ant colonies in Virginia were sampled to determine colony social form and to detect biological control organisms. Lastly, the CLIMEX model was used to predict the potential spread of the fire ant in Virginia.

Field trials showed that combination applications of Advion and Top Choice provided the fastest and greatest reduction in foraging three days after treatment and the longest residual control of fire ant colonies for one year. Field tests documenting the impacts of Advion and Top Choice on non-target ants indicate treatments do supress ant foraging but ant populations will rebound 10-12 months after treatment.

Laboratory PCR assays revealed that colonies in Virginia are primarily polygyne. Also, Kneallhazia solenopsae, Pseudaction spp. parasitoid decapitating phorid flies, and Solenopsis invicta virues (SINV) genetic materials were detected in fire ant samples. CLIMEX model results indicate that fire ants will be able to spread as far east as the Greater Richmond area and as far north as Alexandria in Virginia.
Master of Science in Life Sciences

This thesis investigates four FEC (forward error correction) coding schemes for their suitability for a broadcast system where there is one energy-rich transmitter and many energy-constrained receivers with a variety of channel conditions. The four coding schemes are: repetition codes (the baseline scheme); Reed-Solomon (RS) codes; Luby-Transform (LT) codes; and a type of RS and LT concatenated codes. The schemes were tested in terms of their ability to achieve both high average data reception success probability and short data reception time at the receivers (due to limited energy). The code rate (Rc) is fixed to either 1/2 or 1/3. Two statistical channel models were employed: the memoryless channel and the Gilbert-Elliott channel. The investigation considered only the data-link layer behaviour of the schemes. During the course of the investigation, an improvement to the original LT encoding process was made, the name LTAM (LT codes with Added Memory) was given to this improved coding method. LTAM codes reduce the overhead needed for decoding short-length messages. The improvement can be seen for decoding up to 10000 number of user packets. The maximum overhead reduction is as much as 10% over the original LT codes. The LT-type codes were found to have the property that can both achieve high success data reception performance and flexible switch off time for the receivers. They are also adaptable to different channel characteristics. Therefore it is a prototype of the ideal coding scheme that this project is looking for. This scheme was then further developed by applying an RS code as an inner code to further improve the success probability of packet reception. The results show that LT&RS code has a significant improvement in the channel error tolerance over that of the LT codes without an RS code applied. The trade-off is slightly more reception time needed and more decoding complexity. This LT&RS code is then determined to be the best scheme that fulfils the aim in the context of this project which is to find a coding scheme that both has a high overall data reception probability and short overall data reception time. Comparing the LT&RS code with the baseline repetition code, the improvement is in three aspects. Firstly, the LT&RS code can keep full success rate over channels have approximately two orders of magnitude more errors than the repetition code. This is for the two channel models and two code rates tested. Secondly, the LT&RS code shows an exceptionally good performance under burst error channels. It is able to maintain more than 70% success rate under the long burst error channels where both the repetition code and the RS code have almost zero success probability. Thirdly, while the success rates are improved, the data reception time, measured in terms of number of packets needed to be received at the receiver, of the LT&RS codes can reach a maximum of 58% reduction for Rc = 1=2 and 158% reduction for Rc = 1=3 compared with both the repetition code and the RS code at the worst channel error rate that the LT&RS code maintains almost 100% success probability.

The lack of interoperability between Pay‐TV service providers and a horizontally integrated business transaction model have compromised the competition in the Pay‐TV market. In addition, the lack of interactivity with customers has resulted in high churn rate and improper security measures have contributed into considerable business loss. These issues are the main cause of high operational costs and subscription fees in the Pay‐TV systems. This paper presents a novel end‐to‐end system architecture for Pay‐TV systems cooperating mobile and broadcasting technologies. It provides a cost‐effective, scalable, dynamic and secure access control mechanism supporting converged services and new business opportunities in Pay‐TV systems. It enhances interactivity, security and potentially reduces customer attrition and operational cost. In this platform, service providers can effectively interact with their customers, personalise their services and adopt appropriate security measures. It breaks up the rigid relationship between a viewer and set‐top box as imposed by traditional conditional access systems, thus, a viewer can fully enjoy his entitlements via an arbitrary set‐top box. Having thoroughly considered state‐of‐the‐art technologies currently being used across the world, the thesis highlights novel use cases and presents the full design and implementation aspects of the system. The design section is enriched by providing possible security structures supported thereby. A business collaboration structure is proposed, followed by a reference model for implementing the system. Finally, the security architectures are analysed to propose the best architecture on the basis of security, complexity and set‐top box production cost criteria.

Tall ironweed (Vernonia altissima) and Canada thistle (Cirsium arvense) control in cool season grass pastures was evaluated in 2007 and 2008. Tall ironweed was evaluated in Fayette and Boone Counties, KY and Canada thistle was evaluated at Spindletop Research Farm. Herbicides applied selectively with a rope-wick were compared to a broadcast foliar spray. Treatments were a broadcast treatment, of aminopyralid + 2, 4-D and six rope-wick treatments: aminopyralid at three concentrations, glyphosate, triclopyr and clopyralid at one concentration each. The Boone County location had five broadcast foliar treatments: aminopyralid at three rates, triclopyr + fluroxpyr, and 2,4-D + triclopyr. The Canada thistle study consisted of the same six rope-wick treatments as the Fayette County tall ironweed study. A broadcast treatment of aminopyralid at 70 g a.e./ha was included in 2008. Studies were evaluated 1, 2, 3, 4, 8 and 52 weeks after treatment. Aminopyralid plus 2,4-D provided 86% control of tall ironweed 52 WAT. Aminopyralid at 20% v/v controlled 65% of tall ironweed. Canada thistle control 52 WAT ranged from 0 to 25% control for the six ropewick treatments.

A method for coordinating a homogeneous swarm of autonomous mobile robots is presented. The broadcast-based coordination scheme was developed for the Army Ant swarm—a system of small, relatively inexpensive mobile robots that can accomplish complex tasks by cooperating as a team. The primary drawback of the Army Ant system is that the absence of a central supervisor poses difficulty in the coordination and control of the agents. Our coordination scheme provides a global "group dynamic" that controls the actions of each robot using only local interactions. Coordination of the swarm is achieved with signals we call "heartbeats". Each agent broadcasts a unique heartbeat and responds to the collective behavior of all other heartbeats. We generate heartbeats with van der Pol oscillators, which are nonlinear oscillators that modify their output when coupled to other oscillators. Van der Pol oscillators have long been utilized in simulations, particularly to model rhythmic behavior in biological systems. In this application, we use the known properties of coupled van der Pol oscillators to create predictable group behavior. We emphasize the use of this controller to allow agents to simultaneously perform an action such as lifting, steering, or changing speed. For this research we synthesize a three oscillator network to show that we can achieve multi-agent coordination. An inexpensive FM communication link is used to broadcast and receive oscillator signals. We show that the network may be configured to entrain to a leader or to a common frequency. Additionally, we use our coordination scheme to provide global speed control to our three agent system.
Master of Science

An important characteristic of Service-Oriented Architectures is that clients do not depend on the service implementation's internal assignment of methods to objects. It is perhaps the most important technical characteristic that differentiates them from more common object-oriented solutions. This characteristic makes clients and services malleable, allowing them to be rearranged at run-time as circumstances change. That improvement in malleability is impaired by requiring clients to direct service requests to particular services. Ideally, the clients are totally oblivious to the service structure, as they are to aspect structure in aspect-oriented software. Removing knowledge of a method implementation's location, whether in object or service, requires re-defining the boundary line between programming language and middleware, making clearer specification of dependence on protocols, and bringing the transaction-like concept of failure scopes into language semantics as well. This paper explores consequences and advantages of a transition from object-request brokering to service-request brokering, including the potential to improve our ability to write more parallel software.

Thesis (Ph. D.)--Electrical and Computer Engineering, Georgia Institute of Technology, 2005. Directed by John Copeland.
Copeland, John, Committee Chair ; Owen, Henry, Committee Member ; Sivakumar, Raghupathy, Committee Member ; Lanterman, Aaron, Committee Member ; Dos Santos, Andre, Committee Member. Includes bibliographical references.

ITC/USA 2009 Conference Proceedings / The Forty-Fifth Annual International Telemetering Conference and Technical Exhibition / October 26-29, 2009 / Riviera Hotel & Convention Center, Las Vegas, Nevada
The Central Test and Evaluation Investment Program (CTEIP) Integrated Network Enhanced Telemetry (iNET) program is currently developing new standards for wired-wireless local area networking (LAN-WLAN) using the Internet Protocol (IP), for use in telemetry (TM) channels, under the umbrella of the Telemetry Network System (TmNS). Some advantages of TmNS are real-time command and control of instrumentation, quick-look acquisition, data retransmission and recovery ('gapless TM' or 'PCM backfill'), data segmentation, etc. The iNET team is developing and evaluating prototypes, based on commercial 802.x and other technologies, in conjunction with Range Commander's Council (RCC) Inter-Range Instrumentation Group (IRIG) standards and standards developed under the iNET program.

This thesis evaluates the security of Supervisory Control and Data Acquisition (SCADA) systems, which are one of the key foundations of many critical infrastructures. Specifically, it examines one of the standardised SCADA protocols called the Distributed Network Protocol Version 3, which attempts to provide a security mechanism to ensure that messages transmitted between devices, are adequately secured from rogue applications. To achieve this, the thesis applies formal methods from theoretical computer science to formally analyse the correctness of the protocol.

This study explores the representation of crime in the news in relation to expressed emotion and intention for future action. Episodic and thematic framing (Iyengar, 1991) and narrative processing (Singer & Bluck, 2001) served as the theoretical foundations of this study and helped examine how scripted news stories involving crime influence levels of fear, anger, and empathy in individuals, and how these emotions subsequently affect behaviors. To measure these framing effects, an experimental manipulation was employed using three conceptually different news stories all involving gun-related crimes. One news story utilized an episodic format, while the other two stories used a thematic format (one positive and one negative). Emotional responses, levels of narrative engagement, policy support, perceived risk of victimization, and pro-social behavioral intentions were measured, all based on exposure to the specific type of news frame. The results of this study indicated that while types of news frames did not have a direct effect on readers' emotions, there was a significant relationship between emotions and future actions. For example, fear, anger, and empathy were significant predictors of perceived risk of victimization, policy support, and pro-social behavioral intentions, respectively. These findings contribute to the understanding of the role emotions play in predicting behavior, both within and beyond the scope of message framing.

碩士
國立交通大學
網路工程研究所
100
The problem considered in this thesis is how to broadcast the emergency message instantly and successfully to the streets of the hazard zone in Vehicular Ad Hoc Networks (VANET) environment. We propose an adaptive emergency message broadcast control mechanism to solve it. The proposed mechanism can be divided into two parts: (1) determining the relationship between the reliability and the message broadcast frequency; (2) adjusting message broadcast frequency based on traffic load. The simulation results show that our adaptive emergency message broadcast control mechanism can effectively reduce the network overhead under a given reliability constraint.

碩士
國立暨南國際大學
資訊工程學系
100
In recent years, vehicle safety application has played a more important role in Vehicle ad-hoc network (VANET). Multi-hop broadcast method is widely used in these applications to propagate emergent messages to surrounding vehicles. However, it will lead to higher packet loss rate and transmission latency while massive emergent packets make data collision occurs frequently. This paper proposes a position base broadcast module, named Broad Control Unit (BCUnit), in order to reduce the re-broadcast nodes and minimize emergent message conflict. This module can be easily implemented to WAVE/DSRC devices without any additional transmission overhead and explicit coordination among vehicles. The simulation results show significant progress in multi-hop delay.

碩士
義守大學
資訊工程學系
100
Recently, three different physical layers (IEEE 802.11 a/b/g) for the IEEE 802.11 standard, they all provide multi-rate capabilities, but they are no specification in the standard for the opportunity and practice. Although, many researches have proposed to improve that the packet loss in the varied of the wireless or provide the higher throughput in the stable codition. In the IEEE 80211 standard, the multicast and broadcast service is defined as an unreliable service, that is, it does not include the use of ACK frames. Therefore, adjust the suitable the transmission rate is the important issue in the multicast video streaming. We proposed the rate contorl mechanism that applicable the media of multicast video streaming in the embedded system. In the IEEE 802.11 specification, the receiver use the statistics-based mechanism requires the sender adjust appropriate transmission rate for provide the higher throughput or lower packet loss rate in the multicast video streaming.

abstract: Broadcast journalists often report on people dealing with illness or physical hardship, their difficulties and triumphs. But what happens when journalists personally experience those kinds of health-related issues? This study explores how 24 local and national on-air journalists share how they manage life with illness and hardship using personal narratives shared on their professional social media pages, detailing how the journalists navigate sharing a deeply personal experience while maintaining a professional journalistic persona. Thematic analysis found the journalists’ performed three acts when sharing personal health information in a public forum: they reported on their illness, they were transparent, and they justified their actions. Within the three themes a range of expression – from personal to professional – and influences over content were found, leading to the final overarching theme, implications and consequences on content creation. This dissertation finds a complicated struggle to maintain a professional self while acknowledging the urge to connect with others through a deeply personal experience.
Dissertation/Thesis
Doctoral Dissertation Journalism and Mass Communication 2019

Synopsis The aim of this diploma paper is to explain and demonstrate censorial practice of the communist regime in the Czechoslovak Radio. The paper demonstrates via narrators' memories the way the transmission in the Czechoslovak Radio was affected after its takeover by the Communist Party in 1948, namely on the part of both the operational technical staff members and editors. Historical experience of narrators is presented in the context of formerly known events amended by published articles or sources including those of radio archive.

博士
國立清華大學
資訊工程學系
102
Wireless communication is inherently error-prone due to path loss, fading, noise, interference, etc. In such an error-inclined environment with scarce spectrum, it is of paramount importance to achieve reliable data delivery efficiently. In this thesis, we develop efficient schemes for providing reliable broadcast in wireless multihop networks and for providing advanced ARQ in wireless relay networks. We first address the minimum transmission broadcast problem in error-prone wireless networks and present efficient solutions, including an optimal broadcast scheme and a distributed game-based algorithm. The minimum transmission broadcast problems over reliable links and over unreliable links are formulated as two mixed integer linear programming (MILP) problems, respectively. This way, optimal broadcast schemes can be easily obtained using any existing MILP solver, for small-scale networks. For large-scale networks, we propose a distributed game-based algorithm and prove that the game-based algorithm achieves Nash Equilibrium. Using simulation, we confirm that compared with existing algorithms in the literature and optimal solutions obtained by our MILP techniques, the proposed game-based algorithm performs very well in terms of delivery ratio, number of transmissions, and convergence speed. We further address the issue of advanced ARQ in wireless relay networks and develop relay-assisted network-coding (RANC) ARQ protocols, which leverage both opportunistic retransmission and network coding techniques. We introduce the concept of work, which is the number of recoded blocks a relay node will send, and study ways to adjusting the rate at which a relay node increases its work. Based on this concept, we present the unified framework of work-based RANC and then, under the unified framework, we develop a number of RANC ARQ protocols including plain-RANC, Work-based Opportunistic RANC (WO-RANC), Listen-and-Supersede (LS) RANC, and Hold-and-Proceed (HP) RANC. LS offers a fundamental limit to any single-relay RANC ARQ protocol. HP is a simple yet efficient RANC ARQ protocol with near-zero overhead. We analyze saturation throughput and segment delay for both LS and HP. Through extensive analysis and simulation results, we show that HP has a performance close to LS.

Road accidents represent a serious social problem and are one of the leading causes of human death and disability on a global scale. To reduce the risk and severity of a road accident, a variety of new safety applications can be realized through wireless communications among vehicles driving nearby each other, or among vehicles and especially deployed road side units (RSUs), a technology known as a vehicular ad hoc network (VANET). Most of the VANET-enabled safety applications are based on broadcasting of safety messages by vehicles or RSUs, either periodically or in case of an unexpected event, such as a hard brake or dangerous road condition detection. Each broadcast safety message should be successfully delivered to the surrounding vehicles and RSUs without any excess delay, which is one of the main functions of a medium access control (MAC) protocol proposed for VANETs. This thesis presents VeMAC, a new multichannel time division multiple access (TDMA) protocol specifically designed to support the high priority safety applications in a VANET scenario. The ability of the VeMAC protocol to deliver periodic and event-driven safety messages in VANETs is demonstrated by a detailed delivery delay analysis, including queueing and service delays, for both types of safety messages. As well, computer simulations are conducted by using MATLAB, the network simulator ns-2, and the microscopic vehicle traffic simulator VISSIM, in order to evaluate the performance of the VeMAC protocol, in comparison with the IEEE 802.11p standard and the ADHOC MAC protocol (another TDMA protocol proposed for ad hoc networks). A real city scenario is simulated and different performance metrics are evaluated, including the network goodput, protocol overhead, channel utilization, protocol fairness, probability of a transmission collision, and safety message delivery delay. It is shown that the VeMAC protocol considerably outperforms the existing MAC schemes, which have significant limitations in supporting VANET safety applications. In addition to enhancing road safety, in-vehicle Internet access is one of the main applications of VANETs, which aims at providing the vehicle passengers with a low-cost access to the Internet via on-road gateways. This thesis presents a new strategy for deploying Internet gateways on the roads, in order to minimize the total cost of gateway deployment, while ensuring that a vehicle can connect to an Internet gateway (using multihop communications) with a probability greater than a specified threshold. This cost minimization problem is formulated by using binary integer programming, and applied for optimal gateway placement in a real city scenario. To the best of our knowledge, no previous strategy for gateway deployment has considered the probability of multihop connectivity among the vehicles and the deployed gateways. In order to allow a vehicle to discover the existence of an Internet gateway and to communicate with the gateway via multihops, a novel data packet routing scheme is proposed based on the VeMAC protocol. The performance of this cross-layer design is evaluated for a multichannel VANET in a highway scenario, mainly in terms of the end-to-end packet delivery delay. The packet queueing at each relay vehicle is considered in the end-to-end delay analysis, and numerical results are presented to study the effect of various parameters, such as the vehicle density and the packet arrival rate, on the performance metrics. The proposed VeMAC protocol is a promising candidate for MAC in VANETs, which can realize many advanced safety applications to enhance the public safety standards and improve the safety level of drivers/passengers and pedestrians on roads. On the other hand, the proposed gateway placement strategy and packet routing scheme represent a strong step toward providing reliable and ubiquitous in-vehicle Internet connectivity.

This thesis addresses the problem of positional consensus of multi-agent systems. A positional consensus is achieved when the agents converge to a point. Some applications of this class of problem is in mid-air refueling of the aircraft or UAVs, targeting a geographical location, etc. In this research work some positional consensus algorithms have been developed. They can be categorized in two part (i) Broadcast control based algorithm (ii) Distributed control based algorithm. In case of broadcast based algorithm control strategies for a group of agents is developed to achieve positional consensus. The problem is constrained by the requirement that every agent must be given the same control input through a broadcast communication mechanism. Although the control command is computed using state information in a global framework, the control input is implemented by the agents in a local coordinate frame. The mathematical formulation has been done in a linear programming framework that is computationally less intensive than earlier proposed methods. Moreover, a random perturbation input in the control command, that helps to achieve reasonable proximity among agents even for a large number of agents, which was not possible with the existing strategy in the literature, is introduced. This method is extended to achieve positional consensus at a pre-specified location. A comparison between the LP approach and the existing SOCP based approach is also presented. Some of the algorithm has been demonstrated successfully on a robotic platform made from LEGO Mindstorms NXT Robots. In the second case of broadcast based algorithm, a decentralized algorithm for a group of multiple autonomous agents to achieve positional consensus has been developed using the broadcast concept. Even here, the mathematical formulation has done using a linear programming framework. Each agent has some sensing radius and it is capable of sensing position and orientation with other agents within their sensing region. The method is computationally feasible and easy to implement. In case of distributed algorithms, a computationally efficient distributed rendezvous algorithm for a group of autonomous agents has been developed. The algorithm uses a rectilinear decision domain (RDD), as against the circular decision domain assumed in earlier work available in the literature. This helps in reducing its computational complexity considerably. An extensive mathematical analysis has been carried out to prove the convergence of the algorithm. The algorithm has also been demonstrated successfully on a robotic platform made from LEGO Mindstorms NXT Robots.

M.A. University of the Witwatersrand, Faculty of Humanities (Journalism and Media Studies), 2012
This study looks at how short message services (SMS’s) was used at the Public Broadcaster to control decision making during the period November 2008 and April 2009. The main argument of this thesis is: 1. How did mobile phones reconfigured control in the newsroom of the public broadcaster 2. The public broadcaster management and the SABC newsroom in Johannesburg did not fulfil the mandate of the public broadcaster doing the study period because of political pressure via SMS 3. Editors did not protect journalists and producers against those with political power. The reason for doing this study is that there are many studies that focus on the positives of digital technologies but very little research has been done about the possible negatives of this innovation in the newsroom. The study acknowledges the advantages of having digital technology as an added communication tool in the newsroom but argues that people with political power can also use mobile technology as surveillance tool to control the decision-making process in the newsroom. SMS’s as a digital technology contributed to a new form of control in the newsroom and sending and receiving of SMS made it difficult for news editors to follow proper news decision-making processes and editorial guidelines which impacted on the role of the public broadcaster. This resulted in either self-censorship, job insecurity or some people even left the newsroom. The literature review gives an overview of the principles of public broadcasting, such as autonomous, inclusive and balanced with very little interference and how it is realised within the structure and dynamics of the newsroom. The study looks at theories of control, and gatekeeping, gatewatching as an editorial function and the impact of SMS’s on these established roles in the newsroom. The study provides a theoretical understanding of political control and its interplay with and effects on the public broadcaster mandate. The research methodology is grounded in the qualitative paradigm. The methods utilised is a case study and semi-structured interviews supported by archived newspaper articles to answer the question: how short message services (SMS’s) was used at the Public Broadcaster to control decision making.

Due to its significant advantage in spectral efficiency, multiple-antenna communication technology will undoubtedly be a major component in future wireless system implementations. However, the full exploitation of this technology also requires perfect feedback of channel state information (CSI) to the transmitter-- something that is not practically feasible. This motivates the study of limited feedback systems, where CSI feedback is rate limited. This thesis focuses on the optimal design of limited feedback systems for three types of communication channels: the relay channel, the single-user point-to-point channel, and the multiuser broadcast channel. For the relay channel, we prove the efficiency of the Grassmannian codebooks as the source and relay beamforming codebooks, and propose a method for CSI exchange between the relay and the destination when global CSI is not available at destination. For the single-user point-to-point channel, we study the joint power control and beamforming problem and address the channel magnitude and direction quantization codebook design problem. It is shown that uniform quantization of the channel magnitude (in dB scale) is asymptotically optimal regardless of the channel distribution. The analysis further derives the optimal split of feedback bandwidth between the magnitude and direction quantization codebooks. For the multiuser broadcast channel, we first prove the sufficiency of a product magnitude-direction quantization codebook for managing the multiuser interference. We then derive the optimal split of feedback bandwidth across the users and their magnitude and direction codebooks. The optimization results reveal an inherent structural difference between the single-user and multiuser quantization codebooks: a multiuser codebook should have a finer direction quantization resolution as compared to a single-user codebook. It is further shown that the users expecting higher rates and requiring more reliable communication should provide a finer quantization of their CSI. Finally, we determine the minimum required total feedback rate based on users' quality-of-service constraints and derive the scaling of the system performance with the total feedback rate.

Ο ταχύτατα εξελισσόμενος τομέας των δικτύων κινητών επικοινωνιών έχει επιφέρει μία ιδιαίτερα αυξανόμενη απαίτηση για ασύρματη, πολυμεσική επικοινωνία. Στη ραγδαία εξέλιξη του τομέα αυτού συμβάλουν τα μέγιστα και οι απαιτήσεις της σύγχρονης αγοράς για ένα ενοποιημένο και λειτουργικό σύστημα κινητής τηλεφωνίας παρέχοντας παράλληλα πληθώρα ευρυζωνικών υπηρεσιών ψηφιακού περιεχομένου στους πελάτες - χρήστες του. Είναι γεγονός ότι τα τελευταία χρόνια τα δίκτυα επικοινωνιών τρίτης γενιάς (3G) - Universal Mobile Telecommunication System (UMTS) γνωρίζουν μεγάλη άνθηση και η χρήση τους έχει επεκταθεί στις περισσότερες ευρωπαϊκές χώρες, όπως και στην Ελλάδα. Τα νέα αυτά κινητά δίκτυα αντικαθιστούν τα υπάρχοντα κινητά δίκτυα δεύτερης γενιάς και επιπλέον προσφέρουν προηγμένες υπηρεσίες στους κινητούς χρήστες. Ωστόσο, η αδήριτη ανάγκη για μεγαλύτερες (ευρυζωνικές) ταχύτητες πρόσβασης οδήγησε στην περαιτέρω ανάπτυξη των 3G δικτύων και στην υιοθέτηση νέων τεχνολογιών, με κυριότερο εκπρόσωπο τους την τεχνολογία High Speed Packet Access (HSPA). Η τεχνολογία HSPA αποτελεί τη φυσιολογική μετεξέλιξη του UMTS, η οποία πολλές φορές συναντάται και ως 3.5G ή 3G+, προκειμένου να δηλώσει την αναβάθμιση του 3G (UMTS) προτύπου. Ωστόσο, παρά το γεγονός ότι η τεχνολογία HSPA αναμένεται να προσφέρει τη δυνατότητα παροχής πληθώρας ευρυζωνικών υπηρεσιών, το 3rd Generation Partnership Project (3GPP), που αποτελεί τον οργανισμό που προτυποποιεί τις νέες τεχνολογίες και ορίζει τις προδιαγραφές τους, ήδη μελετά και επεξεργάζεται νέες τεχνολογίες που θα επικρατήσουν την αμέσως επόμενη δεκαετία στην αγορά των κινητών επικοινωνιών. Το νέο αυτό project αποκαλείται Long Term Evolution (LTE) και στοχεύει στην επίτευξη ακόμη υψηλότερων ρυθμών μετάδοσης σε συνδυασμό με την αξιοποίηση μεγαλύτερου εύρος ζώνης. Κύρια προοπτική του LTE αποτελεί η διασφάλιση της ανταγωνιστικότητας και η επικράτηση του προτύπου στο χρονικό ορίζοντα της επόμενης δεκαετίας. Κατά συνέπεια, η αγορά κινητών επικοινωνιών σταδιακά μεταλλάσσεται προς τη δημιουργία δικτύων κινητών επικοινωνιών επόμενης γενιάς, με απώτερο σκοπό την επίτευξη της αποκαλούμενης «Κινητής Ευρυζωνικότητας» (Mobile Broadband). Ταυτόχρονα με την εκτεταμένη εξάπλωση των δικτύων κινητών επικοινωνιών επόμενης γενιάς καθώς και τις αυξημένες δυνατότητες των κινητών συσκευών, οι πάροχοι πολυμεσικού περιεχομένου και υπηρεσιών ενδιαφέρονται όλο και περισσότερο για την υποστήριξη της πολυεκπομπής (multicasting) δεδομένων στα δίκτυα αυτά με σκοπό την αποτελεσματική διαχείριση και επαναχρησιμοποίηση των διαθέσιμων πόρων του δικτύου. Επιπρόσθετα, οι χρήστες των κινητών δικτύων έχουν πλέον την απαίτηση να προσπελαύνουν εφαρμογές και υπηρεσίες οι οποίες μέχρι σήμερα μπορούσαν να διατεθούν αποκλειστικά από τα συμβατικά ενσύρματα δίκτυα. Έτσι λοιπόν στις μέρες μας γίνεται λόγος για υπηρεσίες πραγματικού χρόνου όπως mobile TV, mobile gaming, mobile streaming κ.α. Ένα από τα σημαντικότερα βήματα των δικτύων κινητών επικοινωνιών προς την κατεύθυνση της παροχής νέων, προηγμένων πολυμεσικών υπηρεσιών είναι η εισαγωγή της υπηρεσίας Multimedia Broadcast / Multicast Service (MBMS). Η υπηρεσία MBMS έχει σαν κύριο σκοπό την υποστήριξη IP εφαρμογών πανεκπομπής (broadcact) και πολυεκπομπής (multicast), επιτρέποντας με αυτό τον τρόπο την παροχή υπηρεσιών υψηλού ρυθμού μετάδοσης σε πολλαπλούς χρήστες με οικονομικό τρόπο. Η multicast μετάδοση δεδομένων σε κινητά δίκτυα επικοινωνιών είναι μια σχετικά νέα λειτουργικότητα η οποία βρίσκεται ακόμη στο στάδιο των δοκιμών και της προτυποποίησης της. Ένας multicast μηχανισμός μεταδίδει τα δεδομένα μόνο μία φορά πάνω από κάθε ασύρματο σύνδεσμο που αποτελεί τμήμα των μονοπατιών προς τους προορισμούς-κινητούς χρήστες. Το κρισιμότερο σημείο που εντοπίζεται κατά τη multicast μετάδοση δεδομένων στα κινητά δίκτυα επικοινωνιών είναι ο αποτελεσματικός έλεγχος ισχύος. Οι σταθμοί βάσης των κυψελωτών αυτών δικτύων διαθέτουν περιορισμένους πόρους ισχύος, γεγονός που περιορίζει τη χωρητικότητα της κυψέλης (cell) και επιβάλλει τη χρήση μίας βέλτιστης στρατηγικής για την όσο το δυνατόν καλύτερη αξιοποίηση των διαθέσιμων πόρων ισχύος. Ο έλεγχος ισχύος στοχεύει στη μείωση της εκπεμπόμενης ισχύος, στην ελαχιστοποίηση του θορύβου στο κυψελωτό δίκτυο και κατά συνέπεια στη διασφάλιση μεγαλύτερης χωρητικότητας επιπλέον χρηστών. Ένα από τα βασικότερα στοιχεία του ελέγχου ισχύος στα δίκτυα κινητών επικοινωνιών επόμενης γενιάς κατά τη multicast μετάδοση πολυμεσικών δεδομένων αποτελεί η επιλογή του κατάλληλου καναλιού μεταφοράς για τη μετάδοση των δεδομένων στον κινητό χρήστη. Συγκεκριμένα, πρόκειται για ένα κρίσιμο ζήτημα το οποίο είναι ακόμα υπό εξέταση στο 3GPP. Προς την κατεύθυνση αυτή, στο MBMS πρότυπο έχουν αναπτυχθεί διάφοροι μηχανισμοί, με χαρακτηριστικό τον MBMS Counting Mechanism. Στόχος του μηχανισμού αυτού είναι η βελτιστοποίηση της ροής δεδομένων για την υπηρεσία MBMS, όταν αυτά διέρχονται από τις διεπαφές του UMTS/HSPA δικτύου. Ωστόσο, η υπάρχουσα μορφή του μηχανισμού αυτού, καθώς και των αρκετών άλλων μηχανισμών που έχουν προταθεί από το 3GPP, διακρίνεται από πολλές αδυναμίες που δεν επιτρέπουν την αποτελεσματική και μαζική μετάδοση πολυμεσικών δεδομένων. Τα σημαντικότερα προβλήματα των μηχανισμών αυτών είναι η απουσία ευρυζωνικών χαρακτηριστικών καθώς και η σπατάλη σημαντικού τμήματος των ούτως ή άλλως περιορισμένων πόρων ισχύος. Εν γένει, η επιλογή του κατάλληλου καναλιού μεταφοράς των πολυμεσικών δεδομένων στο ασύρματο μέσο είναι μια δύσκολη διαδικασία καθώς μια λανθασμένη επιλογή καναλιού μπορεί να οδηγήσει στην αστοχία μίας ολόκληρης κυψέλης. Γίνεται σαφές λοιπόν, ότι απαιτείται μία βελτιωμένη έκδοση των υπαρχόντων μηχανισμών για την αποτελεσματικότερη και οικονομικότερη μετάδοση πολυμεσικού περιεχομένου σε μεγάλο πλήθος χρηστών. Λαμβάνοντας υπόψη τα παραπάνω, στόχος της παρούσας διδακτορικής διατριβής είναι η μελέτη του ελέγχου ισχύος στα δίκτυα κινητών επικοινωνιών επόμενης γενιάς καθώς και η ανάπτυξη νέων μεθόδων/μηχανισμών για τη βελτιστοποίηση του. Ιδιαίτερο χαρακτηριστικό της διατριβής αυτής είναι η ενσωμάτωση και η «εκμετάλλευση» όλων των ιδιαίτερων χαρακτηριστικών της HSPA τεχνολογίας στην MBMS υπηρεσία. Προς αυτή την κατεύθυνση, στην παρούσα διδακτορική διατριβή αναλύονται και αξιολογούνται όλα τα υπάρχοντα κανάλια μεταφοράς του UMTS και της τεχνολογίας HSPA τα οποία μπορούν να χρησιμοποιηθούν για τη multicast μετάδοση MBMS υπηρεσιών. Η αξιολόγηση γίνεται με βάση την απαιτούμενη ισχύ που πρέπει να ανατεθεί από το σταθμό βάσης για καθένα από αυτά, και κατά συνέπεια με βάση το ρυθμό μετάδοσης τους, τον αριθμό των χρηστών που μπορούν να εξυπηρετήσουν, την ποιότητα υπηρεσιών για κάθε χρήστη, τη μέγιστη δυνατή κάλυψη της κυψέλης κ.α. Οι ιδιαίτερα αυξημένες απαιτήσεις των MBMS υπηρεσιών μας οδήγησαν στη συνέχεια στη διερεύνηση και αξιολόγηση ορισμένων τεχνικών μείωσης της εκπεμπόμενης ισχύος, με απώτερο σκοπό την αποδοτικότερη χρήση των πόρων του συστήματος κατά τη μετάδοση MBMS υπηρεσιών. Ενδεικτικά αναφέρουμε πως μία MBMS υπηρεσία με ρυθμό μετάδοσης 128 Kbps μπορεί να καταναλώσει έως και το 80% των πόρων ισχύος ενός σταθμού βάσης. Επομένως, γίνεται κατανοητό ότι η μετάδοση MBMS υπηρεσιών με τόσο υψηλούς ρυθμούς μετάδοσης καθίσταται δύσκολη έως και αδύνατη. Για την αντιμετώπιση αυτού του προβλήματος έχουν αναπτυχθεί τεχνικές, όπως η FACH Dynamic Power Setting, η Macro Diversity Combining και το Rate Splitting, ικανές να μειώσουν τα επίπεδα ισχύος κατά τη μετάδοση multicast δεδομένων στους MBMS χρήστες. Η διδακτορική αυτή διατριβή εξετάζει την αποδοτικότητα της χρήσης αυτών των τεχνικών εξοικονόμησης ισχύος και παρουσιάζει πειραματικά αποτελέσματα που αποκαλύπτουν το ποσό ισχύος που εξοικονομείται από κάθε μία τεχνική. Η αξιολόγηση τόσο των καναλιών μεταφοράς όσο και των τεχνικών μείωσης ισχύος θα οδηγήσει στην εύρεση ενός κατάλληλου σχήματος/μηχανισμού, ο οποίος θα εξασφαλίζει την αποδοτική εναλλαγή μεταξύ των διάφορων τύπων καναλιών κατά τη μετάδοση MBMS υπηρεσιών. Ο μηχανισμός αυτός, τον οποίο καλούμε MBMS Channel Assignment Mechanism (ή χάριν συντομίας «MCAM»), αναμένεται να εξασφαλίσει βελτιωμένη απόδοση σε σχέση με τους αντίστοιχους μηχανισμούς που έχουν προταθεί από το 3GPP, μείωση της καταναλισκόμενης ισχύος και κατά συνέπεια αύξηση της χωρητικότητας των κινητών δικτύων επόμενης γενιάς. Ωστόσο, το πιο αξιοπρόσεκτο πλεονέκτημα του προτεινόμενου μηχανισμού, που ουσιαστικά τον διαφοροποιεί από τις άλλες προσεγγίσεις, είναι ότι προσαρμόζεται στις αυξημένες απαιτήσεις των κινητών δικτύων επόμενης γενιάς για ταυτόχρονη παροχή πολλαπλών πολυμεσικών συνόδων. Το γεγονός αυτό μπορεί να επιτρέψει τη μαζική μετάδοση πολυμεσικών δεδομένων σε πληθώρα κινητών χρηστών, θέτοντας κατά τον τρόπο αυτό τον MCAM σαν έναν ισχυρό υποψήφιο για τα δίκτυα επόμενης γενιάς.
The rapid growth of mobile communications networks has involved an increasing demand for wireless, multimedia communication. The fast development of this area was mainly motivated by the requirements of modern market for a unified and functional system of mobile communications that, at the same time, may provide numerous broadband services to its users. More specifically, in the recent years, the usage of third generation (3G) - Universal Mobile Telecommunication System (UMTS) cellular networks has begun to rise in most European countries, as in Greece. 3G networks have replaced the second generation mobile networks and moreover, are in position of offering advanced services to mobile users. However, the need for higher (broadband) speeds led to the further development of 3G networks and to the adoption of new technologies, with main representative the High Speed Packet Access (HSPA) technology. HSPA constitutes the evolution of UMTS and is known as 3.5G or 3G+ in order to indicate the upgrade from UMTS. However, despite the fact that HSPA technology is expected to allow the provision of numerous broadband services, the 3rd Generation Partnership Project (3GPP), the authorized organization for the standardization of new mobile technologies, already examines new technologies that will prevail in the mobile communications industry over the next decades. This novel project is known as Long Term Evolution (LTE) and aims at achieving increased data rates and reduced latency compared to UMTS and HSPA networks. Therefore, the mobile communications industry progressively evolves to next generation networks, with main target the achievement of the so called “Mobile Broadband”. Simultaneously, multimedia content and service providers show an increased interest in supporting multicast data in order to effectively manage and re-use the available network resources. Additionally, more and more users require access to applications and services that until today could only be accessed by conventional wired networks. Thus, real time applications and services may face low penetration today; however, they are expected to gain high interest in future mobile networks. These applications actually reflect a modern, future way of communication among mobile users. For instance, mobile TV is expected to be a “killer” application for 3G’s. Such mobile TV services include streaming live TV (news, weather forecasts etc.) and streaming video (such as video clips). All the above constitute a series of indicative emerging applications that necessitate advanced transmission techniques. One of the most significant steps towards the provision of such demanding services is the introduction of Multimedia Broadcast/Multicast Service (MBMS). MBMS is a point-to-multipoint service in which data is transmitted from a single source entity to multiple destinations, allowing the networks resources to be shared. Actually, MBMS extends the existing UMTS infrastructure and efficiently uses network and radio resources, both in the core network and most importantly, in the air interface of UMTS, where the bottleneck is placed to a large group of users. Therefore, MBMS constitutes an efficient way to support the plethora of the emerging wireless multimedia applications and services such as IP video conferencing and video streaming. The main requirement during the provision of MBMS multicast services is to make an efficient overall usage of radio and network resources. This necessity mainly translates into improved power control strategies, since the base stations’ transmission power is the limiting factor of downlink capacity in UMTS networks. Under this prism, power control is one of the most critical aspects in MBMS due to the fact that downlink transmission power in UMTS networks is a limited resource and must be shared efficiently among all MBMS users in a cell. Moreover, power control aims at minimizing the transmitted power, eliminating in this way the intercell interference. However, when misused, the use of power control may lead to a high level of wasted power and worse performance results. In order to have efficient power control in MBMS, one of the most critical aspects is the selection of the transport channel for the transmission of MBMS multicast traffic. MBMS services can be provided in each cell by either multiple point-to-point (PTP) channels or by a single point-to-multipoint (PTM) channel. A wrong channel selection may result to a significant capacity decrease, thus, preventing the mass delivery of multimedia applications. It is worth mentioning that channel selection is still an open issue in today’s MBMS infrastructure mainly due to its catalytic role in MBMS performance. In the frame of MBMS power control and transport channel selection several approaches have been proposed, with main representative the 3GPP MBMS Counting Mechanism. However, none of these approaches performs optimal transport channel selection either due to the fact that some of them do not consider the power consumption as the selection criterion or because of the fact that they do not consider all the available transport channels (or combination of them) for the transmission of the MBMS data. All the above stress the need for an advanced version of these mechanisms that could deliver multimedia content to a large number of mobile users in a more efficient and economic way. After taking into account the above analysis, objective of this dissertation is the study of power control issues in next generation mobile communication networks and the development of new approaches/ mechanisms for its optimization. To this direction, this dissertation analyzes and evaluates all the available UMTS and HSPA transport channels that could be used for the transmission of MBMS multicast services. Moreover, this dissertation investigates and evaluates several power saving techniques that aim at the efficient usage of radio and network resources. Techniques, such as Dynamic Power Setting, Macro Diversity Combining and Rate Splitting are capable of decreasing the power consumption during the provision of MBMS services and may enable the mass market delivery of multimedia services to mobile users. The evaluation of the available transport channels and power saving techniques will lead to the development of a novel scheme/mechanism that will enable the efficient selection of transport channels for the transmission of MBMS services. The proposed mechanism, which we call MBMS Channel Assignment Mechanism (or «MCAM»), is expected to optimally utilize the available power resources of base stations to MBMS sessions running in the network, resulting in that way to an extensive increase on the system’s capacity. Therefore, MCAM may allow the mass provision of multimedia data to a large number of mobile users, which makes MCAM a strong candidate for next generation networks.