Recent information theoretical results indicate that dirty-paper coding (DPC) achieves the entire capacity region of the Gaussian multiple-input multiple-output (MIMO) broadcast channel (BC). This thesis presents practical code designs for Gaussian BCs based on DPC. To simplify our designs, we assume constraints on the individual rates for each user instead of the customary constraint on transmitter power. The objective therefore is to minimize the transmitter power such that the practical decoders of all users are able to operate at the given rate constraints. The enabling element of our code designs is a practical DPC scheme based on nested turbo codes. We start with Cover's simplest two-user Gaussian BC as a toy example and present a code design that operates 1.44 dB away from the capacity region boundary at the transmission rate of 1 bit per sample per dimension for each user. Then we consider the case of the multiple-input multiple-output BC and develop a practical limit-approaching code design under the assumption that the channel state information is available perfectly at the receivers as well as at the transmitter. The optimal precoding strategy in this case can be derived by invoking duality between the MIMO BC and MIMO multiple access channel (MAC). However, this approach requires transformation of the optimal MAC covariances to their corresponding counterparts in the BC domain. To avoid these computationally complex transformations, we derive a closed-form expression for the optimal precoding matrix for the two-user case and use it to determine the optimal precoding strategy. For more than two users we propose a low-complexity suboptimal strategy, which, for three transmit antennas at the base station and three users (each with a single receive antenna), performs only 0.2 dB worse than the optimal scheme. Our obtained results are only 1.5 dB away from the capacity limit. Moreover simulations indicate that our practical DPC based scheme significantly outperforms the prevalent suboptimal strategies such as time division multiplexing and zero forcing beamforming. The drawback of DPC based designs is the requirement of channel state information at the transmitter. However, if the channel state information can be communicated back to the transmitter effectively, DPC does indeed have a promising future in code designs for MIMO BCs.

Ad hoc wireless networks are constructed by several mobile hosts and have a property that its topology is changed as mobile hosts moved. There is no stationary infrastructure or based station to coordinate packets transmissions and advertise the information of network topology or something important. The special networks are used in temporal wireless networks, such as battlefield, disease rescue place, and so on. So without any stationary infrastructure supported, mobile hosts can communicate with others immediately or indirectly. Because topology is often changed while mobile hosts moving, mobile hosts must exchange information to deal with the changed conditions. Mobile hosts often utilize broadcasting to exchange information with their neighbor hosts, but there is high bit error ratio in wireless networks, packet corruption occurs frequently, so that mobile host might lost some important information sent from its neighboring host. In 802.11 standard, lack of acknowledgement, broadcasting is an unreliable transmission, because sender host do not know whether all of it neighboring hosts received broadcasting packets correctly or no. Many proposed papers of reliable broadcast assumed that links between mobile hosts are bidirectional links, but bidirectional link is an ideal assumption. In real environment, links are unidirectional, so host A could send packets to host B immediately, but host B could not because of their transmission range are different. In this paper, we propose a new reliable broadcast scheme, Highly Reliable Broadcast Scheme with Directional Antennas (HRBSDA). HRBSDA can reduce the influence of unidirectional links and reach for highly reliable broadcasting. HRBSDA uses directional antennas and concept of Time Division Multiple Access (TDMA)-like. HRBSDA divide DCF Inter-Frame Space (DIFS) into several minislots, and mobile hosts use these minislots to ask sender for retransmission of lost packets. By the way, HRBSDA can not only reach for highly reliable broadcasting, but also reduce Packet Loss Recovery Time, and avoid causing extra overhead. Using directional antennas HRBSDA can reduce collision, so that improving throughput and channel utilization.

Recent advances in networking technology have led to a paradigm shift in broadcast media. The entry of the Internet as a form of popular media means that traditional media are now seeking to transform themselves in a bid for survival. Changes in media broadcasting methods have made the spread and acquisition of information faster and easier. Today¡¦s youth of the E-generation live in an economically stable and technologically advanced society. Changes in society, politics and the traditional family structure have made youths an increasingly important market as their buying power and influence on family decisions have increased. The goal of this study is to discover how the subjects of this study utilize various types of media. Media utilization includes their media preferences, utilization frequency and duration as well as where they use it most often. Beyond utilization, the study goes on to examine and categorize the subjects¡¦ lifestyle and media utilization motives. The study then seeks to discover if there are any links between the subjects¡¦ lifestyles and their media utilization motives. Finally, the study examines the question of ¡§Do different lifestyles or media utilization motives lead to differences in media utilization?¡¨ The subjects of this study were E-generation youths (i.e. young people aged between 13 and 22). Taking into consideration the ease of conducting the survey, information availability and cost, Taipei and Kaohsiung (including their counties and cities) were selected as the regions to be sampled. As for the types of media, the five common forms of TV, newspapers, magazines, the Internet and radio were selected for use in this study into media preferences and usage. For this study, the collection of samples was carried out in the form of questionnaire surveys in Taipei and Kaohsiung. After the sample data was acquired, the lifestyles and media utilization motives were categorized and labeled. After factor analysis, lifestyle was divided into four categories: ¡§Community Involvement and Socialization¡¨, ¡§Fashionable and Trendy¡¨, ¡§Pro-Active¡¨ and ¡§Internet Home¡¨; media utilization motives were divided into three types, these being: ¡§Need for Entertainment and Peer Acceptance¡¨, ¡§Need to Increase Knowledge and Skills¡¨ and ¡§Need for Fashion Shopping¡¨. Finally the collected data were subjected to statistical analysis using the SPSS 10.0 software in order to validate the study¡¦s various hypotheses. The results of the study indicated that among youths the media preference was TV first, followed by the Internet. As for frequency of usage, 46.3% of those interviewed used the Internet every day while 41.0% watched TV every day. In terms of duration, those who used the Internet for 5 hours or more made up 24.0% while those who used it for 2 to 4 hours made up 64%. Those who watched TV for 5 hours or more made up 11.6% while those who watched for 2 to 4 hours made up 72.1%. This showed that when it comes to media preferences and utilization, despite youth¡¦s preferences and increasing dependence on the Internet, TV remains an indispensable part of their daily life. By differentiating amongst the sampling locations of this study, it was found that youths in Taipei used the Internet most, followed by TV; the exact reverse was true for youths in Kaohsiung. This showed that their location and environment affected youths¡¦ media preferences. When youths were classified according to their lifestyle, different lifestyles resulted in media utilization motive variances; different media utilization motives also in turn affected how youths utilized media. Based on the above conclusions, the Internet and TV currently remain the types of media that youths are willing to use and devote a great deal of time to. Newspaper, magazine and radio by contrast made up a smaller segment with most youths having significantly less contact with these types of media and spending less time even when they do.

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In recent years, there has been a growing interest in wireless ad hoc network. One of the major issues in wireless network is developing efficient routing protocol. Based on the concept of designing protocol model such as OSI model, the designers distilled the process of transmitting data to its most fundamental elements and identified which networking functions had related uses and collected those functions into discrete groups that became the layers. It is not suitable to design wireless ad hoc routing protocol based on OSI model conception because the OSI model is developed from the view point of wired network and there are many different characteristics between wired and wireless environment. The main different characteristics between wired and wireless are the mobility of mobile host and the transmission medium. Such differences have great effect on network performance. Due to the differences between wired and wireless characteristics, we present a comprehensive conception of designing wireless ad hoc routing protocol. In this context, we provide a wireless ad hoc routing protocol based on physical layer characteristics, ex: bit error rate, robust link. Our routing protocol will find out a route in good transmission environment and it is efficient to improve network throughput. Furthermore, our routing protocol will decrease the number of route request packets, the amount of retransmissions, link breakage rate, and increase throughput.

Broadcasting is a fundamental primitive in local area networks (LANs).Operations of many data link protocols, for example, ARP (Address Resolution Protocol) and IGMP (Internet Group Management Protocol), must rely on this LAN primitive. To develop the broadcasting service in mobile ad hoc wireless LANs (WLANs) is a challenge. This is because a mobile ad hoc WLAN is a multi-hop wireless network in which messages may travel along several links from the source to the destination via a certain path. Additionally, there is no fixed network topology because of host moving. Furthermore, the broadcast nature of a radio channel makes a packet be transmitted by a node to be able to reach all neighbors. Therefore, the total number of transmissions (forward nodes) is generally used as the cost criterion for broadcasting. The problem of finding the minimum number of forward nodes in a static radio network is NP-complete. Almost all previous works, therefore, for broadcasting in the WLAN are focusing on finding approximation approaches in a, rather than, environment. In this paper, we propose a novel distributed protocol in WLANs to significantly reduce or eliminate the communication overhead in addition to maintaining positions of neighboring nodes. The important features of our proposed protocol are the adaptability to dynamic network topology change and the utilization of the existing routing protocol. The reduction in communication overhead for broadcasting operation is measured experimentally. From the simulation results, our protocol not only has the similar performance as the approximation approaches in the static network, but also outperforms existing ones in the adaptability to host moving.

Wireless data broadcast is an efficient way to disseminate digital information to clients equipped with mobile devices. It allows a huge number of the mobile clients simultaneously access data at anytime and anywhere in the wireless environments. Applications using wireless data broadcast to disseminate information include accessing stock activities and traffic conditions. Using index technologies on the broadcast file, i.e., selective tuning, can reduce a lot of energy consumption of the mobile devices without significantly increasing client waiting time. Most of the research work for selective tuning assumes that each data item broadcast on the wireless channel is fairly evenly accessed by mobile clients. In real-life applications, more popular data may be frequently accessed by clients than less popular ones, i.e., skewed access patterns. In this dissertation, to support efficiently selective tuning with skewed access patterns in the single-channel wireless environments, we first propose a skewed distributed index, SDI, on the uniform data broadcast, on which each data item is broadcast once in a broadcast cycle. Second, we propose a skewed index, SI, on the nonuniform data broadcast, on which a few popular data items are broadcast more frequently in a broadcast cycle than the others. The first proposed algorithm, SDI, considers the access probabilities of data items and the replication of index nodes. The proposed algorithm traverses a balanced tree to determine whether an index node should be replicated by considering the access probability of its child node. In our performance analysis and simulation results, we have shown that our proposed algorithm outperforms the variant-fanout tree index and the distributed index. The second proposed algorithm, SI, applies Acharya et al.'s Broadcast Disks to generate a broadcast program, in which the popular data items are broadcast more times than the others, in order to reduce client waiting time. Moreover, the proposed algorithm builds a skewed tree for these data items and allocates index nodes for the popular data items more times than those for the less popular ones in a broadcast cycle. From our performance analysis and simulation results, we have shown that our proposed SI outperforms the flexible index and the flexible distributed index.

<p>This report concerns the implementation of a simulation framework to evaluate an emerging peer-to-peer network topology scheme using 8-point hypercircles, entitled HyperCircle. This topology was proposed in order to alleviate some of the drawbacks of current P2P systems evolving in an uncontrolled manner, such as scalability issues, network overload and long search times. The framework is supposed to be used to evaluate the advantages of this new topology. The framework has been built on top of an existing simulator software solution, the selection of which was an important part of the development. Weighing different variables such as scalability and API usability, the selection fell on OverSim, an open-source discreet-event simulator based on OMNET++.</p><p>After formalizing the protocol for easier implementation, as well as extending it for better performance, implementation followed using C++ with OverSim’s API and simulation library. Implemented as a module (alongside other stock modules providing their own protocols such as Chord and Kademlia), it can be used in OverSim to simulate a user-defined network using one of the simulation routine applications provided (or using a custom application written by the user). For the purposes of this thesis, the standard application KBRTestApp was used; an application sending test messages between randomly selected nodes, while adding and removing nodes at specific time intervals. The adding and removing of nodes can be configured with probability parameters.</p><p>Tentative testing shows that this implementation of the HyperCircle protocol has a certain performance gain over the OverSim implementations of the Chord and Kademlia protocols, measurable in the time it takes a message to get from sender to recipient. Further testing is outside the scope of this thesis.</p>

Un réseau ad hoc sans fil est composé d'un ensemble décentralisé d'objets mobiles et auto-organisés. Un tel réseau ne repose sur aucune infrastructure, et est donc complètement autonome et dynamique. Selon ces hypothèses, tout ensemble d'objets équipés d'interfaces de communication adéquates peut spontanément former un tel réseau. Puisque des communications sans fil sont utilisées, seuls les objets suffisamment proches les uns des autres peuvent communiquer ensemble. Les communications de longue portée doivent donc être effectuées via un mécanisme multi-sauts : cela veut simplement dire que les objets intermédiaires doivent faire suivre les messages jusqu'au destinataire.<br /><br />Les réseaux de capteurs sont similaires aux réseaux ad hoc, car ils sont également décentralisés et autonomes. Un capteur est un petit appareil capable de surveiller son environnement. Des cas typiques d'utilisation peuvent être la surveillance de zones militaires (détection de mouvements) ou de forêts (détection d'incendie).<br /><br />Parmi les problèmes communs à ces deux types de réseaux se trouve la diffusion. Dans une telle communication, un message est envoyé depuis un objet donné vers tous les autres du réseau. Les applications de ce processus sont nombreuses : découverte de routes, synchronisation... Comme les objets mobiles utilisent une batterie, il est nécessaire que la diffusion soit la plus économe possible d'un point de vue énergétique. Cela est généralement obtenu en réduisant la quantité de relais nécessaires, ou en limitant la puissance d'émission à chaque relais.<br /><br />Le but de mon travail était d'étudier la diffusion dans les réseaux ad hoc et de capteurs, afin de mettre en lumière les caractéristiques et les défauts des mécanismes existants, puis d'en proposer de nouveaux, plus efficaces. Dans tous ces travaux, nous avons toujours voulu rester dans le domaine des solutions 'réalistes' : beaucoup des précédentes études utilisaient en effet des mécanismes centralisés, où une connaissance globale du réseau est nécessaire pour effectuer la diffusion. Nous nous sommes concentrés sur des solutions fiables et localisés, c'est-à-dire n'utilisant que des informations sur le voisinage de chaque noeud. Ce type de mécanisme permet également un passage à l'échelle simplifié, car la quantité d'informations nécessaire ne varie pas avec la taille du réseau. Nos études montrent de plus que ces solutions peuvent être aussi efficaces que les méthodes centralisées.<br /><br />Puisque l'ajustement de portée est un mécanisme très important dans la conservation de l'énergie, nous avons proposé une méthode de diffusion originale, basée sur le concept de portée optimale de communication. Cette dernière est calculée de manière théorique grâce au modèle énergétique considéré, et représente le meilleur compromis entre l'énergie dépensée à chaque noeud et le nombre de relais nécessaires. Nous avons ainsi proposé deux protocoles différents basés sur ce concept, chacun étant plus spécifiquement adapté soit aux réseaux ad hoc (TR-LBOP), soit aux réseaux de capteurs (TR-DS).<br /><br />Afin de réduire encore plus la consommation énergétique, nous avons étudié le fameux protocole centralisé nommé BIP. Son efficacité est due au fait qu'il considère la couverture obtenue par une seule émission omnidirectionnelle, au lieu de considérer chaque lien séparément. Nous avons proposé une solution localisée basée sur BIP, afin de construire incrémentalement une structure de diffusion. Nous avons montré de manière expérimentale que les résultats ainsi obtenus sont très proches de ceux fournis par BIP, notamment dans les réseaux de forte densité, tout en n'utilisant que des informations locales à chaque noeud.<br /><br />Nous avons finalement considéré la suppression d'une hypothèse forte, largement répandue dans la communauté des réseaux ad hoc et de capteurs : l'utilisation d'un graphe du disque unitaire. Ce dernier définit la zone de communication d'un noeud comme étant un cercle parfait. Nous avons remplacé cette hypothèse par une autre plus réaliste afin d'en étudier les conséquences sur un protocole connu, le protocole de diffusion par relais multipoints (MPR). Nous avons montré que ce dernier ne fournit plus de résultats suffisants dans un tel environnement. Nous avons également proposé quelques modifications afin d'obtenir à nouveau de bons résultats.

This thesis is a study of a new specification for end user interactivity developed by the Open Mobile Alliance, the specification is called OMA BCAST Service Interaction Function. The specification is one part of the OMA BCAST Service Enabler, which enables service delivery to mobile devices, where the most common service is mobile television. The Service Interaction Function enables end user interactivity related to a service, this could be a poll about the current television program or a chat where every message is presented to the users that are watching the same channel. The specification is still of draft version and the scope of this thesis has been to investigate the Service Interaction Function and implement a PC prototype.