Rozprawy doktorskie na temat „Random access efficiency”

Massive MIMO communication systems have been highlighted as the main technology for physical layer of next generation communication standards, like 5G. While conventional communication between BS and its covered users is performed in orthogonal time-frequency resources, the improved interuser interference mitigation capability provided by the large number of BS antennas enables the BS to communicate with several users in the same time-frequency resource. This better usage of available but scarce spectrum elevates the spectral efficiency to very appreciable levels, and has a similar effect on energy efficiency, since the transmit power is not increased. On the other hand, if the objective is to provide a target performance for the users, the required transmit power in both direct and reverse links can be made inversely proportional to the number of BS antennas employed. In this Doctoral Thesis, several important aspects of massive MIMO systems are systematically investigated aiming to improve their energy and spectral efficiencies. We can enumerate our main contributions as follows. Considering a cellular massive MIMO network, we proposed an optimized assignment policy of training sequences to the users, which is then combined with suitable power control algorithms. We have also investigated the adoption of alternative waveforms in this scenario, such as single-carrier transmission, in order to overcome the issues of conventional OFDM. Our contributions in this topic are to derive analytical performance expressions for a time-domain single-carrier equalizer taking advantage of the large number of BS antennas, and to evaluate and compare the total energy efficiency of OFDM versus single-carrier massive MIMO systems. Finally, considering crowded massive MIMO networks, composed by both human users as well as machine-type communication devices, we proposed an improved random access protocol aiming to decrease the average number of access attempts for the users and decreasing the probability of failed access attempts.
Sistemas de comunicação de múltiplas antenas (multiple-input multiple-output - MIMO) têm se destacado como a principal tecnologia para a camada física dos padrões de comunicação da próxima geração, como o 5G. Enquanto a comunicação convencional entre a estação base (base station - BS) e seus usuários atendidos é realizada em recursos ortogonais de tempo-frequência, a grande capacidade de redução da interferência interusuários possibilitada pelo grande número de antenas da BS habilita a BS a se comunicar com diversos usuários no mesmo recurso tempo-frequência. Este melhor uso do escasso espectro disponível eleva a eficiência espectral a níveis muito apreciáveis, e tem um efeito similar na eficiência energética, pois a potência de transmissão não é aumentada. Por outro lado, se o objetivo é fornecer um desempenho desejado para os usuários, a potência de transmissão necessária em ambos os enlaces direto e reverso pode ser feita inversamente proporcional ao número de antenas na BS. Nesta Tese de Doutorado, diversos aspectos importantes de sistemas MIMO massivo são sistematicamente investigados com o objetivo de melhorar suas eficiências energética e espectral. Pode-se enumerar as principais contribuições alcançadas como se segue. Considerando uma rede celular MIMO massivo, propõe-se uma política de atribuição de sequências de treinamento aos usuários otimizada, a qual é depois combinada com apropriados algoritmos de controle de potência. Também investiga-se a adoção neste cenário de formas de onda alternativas, tal como a transmissão de portadora única, visando superar as deficiências da convencional multiplexagem por divisão de portadoras ortogonais (orthogonal frequency-division multiplexing - OFDM). As principais contribuições obtidas neste tema são derivar expressões de desempenho analíticas para um equalizador de portadora única no domínio do tempo que aproveita o grande número de antenas na BS, e avaliar e comparar a eficiência energética total de sistemas MIMO massivo OFDM versus portadora única. Finalmente, considerando redes MIMO massivo sobrecarregadas, compostas por usuários humanos bem como dispositivos de comunicação do tipo máquina, propõe-se um protocolo de acesso aleatório melhorado visando diminuir o número médio de tentativas de acesso para os usuários e diminuir a probabilidade de falhas de tentativa de acesso.

This thesis is concerned with maximizing the coding efficiency, random accessibility and visual performance of scalable compressed video. The unifying theme behind this work is the use of finely embedded localized coding structures, which govern the extent to which these goals may be jointly achieved. The first part focuses on scalable volumetric image compression. We investigate 3D transform and coding techniques which exploit inter-slice statistical redundancies without compromising slice accessibility. Our study shows that the motion-compensated temporal discrete wavelet transform (MC-TDWT) practically achieves an upper bound to the compression efficiency of slice transforms. From a video coding perspective, we find that most of the coding gain is attributed to offsetting the learning penalty in adaptive arithmetic coding through 3D code-block extension, rather than inter-frame context modelling. The second aspect of this thesis examines random accessibility. Accessibility refers to the ease with which a region of interest is accessed (subband samples needed for reconstruction are retrieved) from a compressed video bitstream, subject to spatiotemporal code-block constraints. We investigate the fundamental implications of motion compensation for random access efficiency and the compression performance of scalable interactive video. We demonstrate that inclusion of motion compensation operators within the lifting steps of a temporal subband transform incurs a random access penalty which depends on the characteristics of the motion field. The final aspect of this thesis aims to minimize the perceptual impact of visible distortion in scalable reconstructed video. We present a visual optimization strategy based on distortion scaling which raises the distortion-length slope of perceptually significant samples. This alters the codestream embedding order during post-compression rate-distortion optimization, thus allowing visually sensitive sites to be encoded with higher fidelity at a given bit-rate. For visual sensitivity analysis, we propose a contrast perception model that incorporates an adaptive masking slope. This versatile feature provides a context which models perceptual significance. It enables scene structures that otherwise suffer significant degradation to be preserved at lower bit-rates. The novelty in our approach derives from a set of "perceptual mappings" which account for quantization noise shaping effects induced by motion-compensated temporal synthesis. The proposed technique reduces wavelet compression artefacts and improves the perceptual quality of video.

In this dissertation, analysis and design of the efficient Medium Access Control (MAC) sub-layer schemes are considered for Vehicular Ad hoc Networks (VANE~s). The contributions of this study are three-fold. First, an analytical model based on Markov chain is developed in order to investigate the performance of the MAC sub-layer of the IEEE 802.11p for vehicular communications. The results indicate that single channel MAC sub-layers may not be adequate for the future Intelligent Transportation Systems (ITS). The analytical model is validated with the results from simulation-based analysis. Performance analysis based on simulations is given on MAC metrics such as throughput, access delay, packet delivery. Second, a multi-channel MAC protocol is proposed and comprehensively analyzed in terms of channel utilizing and Quality of service (QoS) differentiation for dense VANETs. It is demonstrated that the proposed scheme, namely Asynchronous Multichannel MAC with Distributed TDMA (AMCMACD), improves the system performance in terms of throughput, packet delivery rate, collision rate on service channels, load balancing, and service differentiation for dense vehicular networks. Third, to cope with the interference from contention with neighbours within two hops in large-scale networks, a Large-scale Asynchronous Multichannel MAC (LS-AMCMAC) is proposed. The proposed scheme outperforms other benchmark multichannel MAC schemes in large-scale networks, in terms of throughput, channel utilization, dissemination of emergency messages, and the collision rates on control and service channels.

The advent of many core architectures has coincided with the energy and power limited design of modern processors. Projections for main memory clearly show widening of the processor-memory gap. Cache capacity increased to help reduce this gap will lead to increased energy and area usage and due to small growth in die size, impede performance scaling that has accompanied Moore's Law to date. Among the dominant sources of energy consumption is the on-chip memory hierar- chy, specically the L2 cache and the Last Level Cache (LLC). This work explores the use of a novel non-volatile memory technology - Spin Torque Transfer RAM (STT RAM)" for the design of the L2/LLC caches. While STTRAM is a promising memory technology, it has some limitations, particularly in terms of write energy and write latencies. The main objectives of this thesis is to use a novel cell design for a non-volatile 1T1MTJ cell and demonstrate its use at the L2 and LLC cache levels with architectural optimizations to maximize energy reduction. The proposed cache hierarchy dissipates significantly lesser energy (both leakage and dynamic) and uses less area in comparison to a conventional SRAM based cache designs.

The gradually widening speed disparity of between CPU and memory has become an overwhelming bottleneck for the development of Chip Multiprocessor (CMP) systems. In addition, increasing penalties caused by frequent on-chip memory accesses have raised critical challenges in delivering high memory access performance with tight power and latency budgets. To overcome the daunting memory wall and energy wall issues, this thesis focuses on proposing a new heterogeneous scratchpad memory architecture which is configured from SRAM, MRAM, and Z-RAM. Based on this architecture, we propose two algorithms, a dynamic programming and a genetic algorithm, to perform data allocation to different memory units, therefore reducing memory access cost in terms of power consumption and latency. Extensive and intensive experiments are performed to show the merits of the heterogeneous scratchpad architecture over the traditional pure memory system and the effectiveness of the proposed algorithms.

Like the rest of the society modern health care has to deal with the ever increasing information flow. Imaging modalities such as CT, MRI, US, SPECT and PET just keep producing more and more data. Especially CT and MRI and their 3D image stacks cause problems in terms of how to effectively handle these data sets. Usually a PACS is used to manage the information flow. Since a PACS often is implemented with a server-client setup, the management of these large data sets requires an efficient representation of medical image stacks that minimizes the amount of data transmitted between server and client and that efficiently supports the workflow of a practitioner.

In this thesis an efficient wavelet representation for large medical image stacks is proposed for the use in a PACS. The representation supports features such as lossless viewing, random access, ROI-viewing, scalable resolution, thick slab viewing and progressive transmission. All of these features are believed to be essential to form an efficient tool for navigation and reconstruction of an image stack.

The proposed wavelet representation has also been implemented and found to be better in terms of memory allocation and amount of data transmitted between server and client when compared to prior solutions. Performance tests of the implementation has also shown the proposed wavelet representation to have a good computational performance.

The overall energy consumption of portable devices has been projected to triple over the next decade, growing to match the total power generated by the European Union and Canada by 2025. The rise of the internet-of-things (IoT) and ubiquitous and embedded computing has resulted in an exponential increase in such devices, wherein projections estimate that 50 billion smart devices will be connected and online by 2020. In order to alleviate the associated stresses placed on power generation and distribution networks, a holistic approach must be taken to conserve energy usage in electronic devices from the component to the circuit level. An effective approach to reduce power dissipation has been a continual reduction in operating voltage, thereby quadratically down-scaling active power dissipation. However, as state-of-the-art silicon (Si) complimentary metal-oxide-semiconductor (CMOS) field-effect transistors (FETs) enter sub-threshold operation in the ultra-low supply voltage regime, their drive current is noticeable degraded. Therefore, new energy-efficient MOSFETs and circuit architectures must be introduced. In this work, tunnel FETs (TFETs), which operate leveraging quantum mechanical tunneling, are investigated. A comprehensive investigation detailing electronic materials, to novel TFET device designs, to memory and logic digital circuits based upon those TFETs is provided in this work. Combined, these advances offer a computing platform that could save considerable energy and reduce power consumption in next-generation, ultra-low voltage applications.
Ph. D.

Graph algorithms are concerned with the algorithmic aspects of solving graph problems. The problems are motivated from and have application to diverse areas of computer science, engineering and other disciplines. Problems arising from these areas of application are good candidates for parallelization since they often have both intense computational needs and stringent response time requirements. Motivated by these concerns, this thesis investigates parallel algorithms for these kinds of graph problems that have at least one of the following properties: the problems involve some type of dynamic updates; the sparsification technique is applicable; or the problems are closely related to communications network issues. The models of parallel computation used in our studies are the Parallel Random Access Machine (PRAM) model and the practical interconnection network models such as meshes and hypercubes. ¶ Consider a communications network which can be represented by a graph G = (V;E), where V is a set of sites (processors), and E is a set of links which are used to connect the sites (processors). In some cases, we also assign weights and/or directions to the edges in E. Associated with this network, there are many problems such as (i) whether the network is k-edge (k-vertex) connected withfixed k; (ii) whether there are k-edge (k-vertex) disjoint paths between u and v for a pair of given vertices u and v after the network is dynamically updated by adding and/or deleting an edge etc; (iii) whether the sites in the network can communicate with each other when some sites and links fail; (iv) identifying the first k edges in the network whose deletion will result in the maximum increase in the routing cost in the resulting network for fixed k; (v) how to augment the network at optimal cost with a given feasible set of weighted edges such that the augmented network is k-edge (k-vertex) connected; (vi) how to route messages through the network efficiently. In this thesis we answer the problems mentioned above by presenting efficient parallel algorithms to solve them. As far as we know, most of the proposed algorithms are the first ones in the parallel setting. ¶ Even though most of the problems concerned in this thesis are related to communications networks, we also study the classic edge-coloring problem. The outstanding difficulty to solve this problem in parallel is that we do not yet know whether or not it is in NC. In this thesis we present an improved parallel algorithm for the problem which needs [bigcircle]([bigtriangleup][superscript 4.5]log [superscript 3] [bigtriangleup] log n + [bigtriangleup][superscript 4] log [superscript 4] n) time using [bigcircle](n[superscript 2][bigtriangleup] + n[bigtriangleup][superscript 3]) processors, where n is the number of vertices and [bigtriangleup] is the maximum vertex degree. Compared with a previously known result on the same model, we improved by an [bigcircle]([bigtriangleup][superscript 1.5]) factor in time. The non-trivial part is to reduce this problem to the edge-coloring update problem. We also generalize this problem to the approximate edge-coloring problem by giving a faster parallel algorithm for the latter case. ¶ Throughout the design and analysis of parallel graph algorithms, we also find a technique called the sparsification technique is very powerful in the design of efficient sequential and parallel algorithms on dense undirected graphs. We believe that this technique may be useful in its own right for guiding the design of efficient sequential and parallel algorithms for problems in other areas as well as in graph theory.

Memory design is a crucial component of VLSI system design from area, power and performance perspectives. To meet the increasingly challenging system specifications, architecture, circuit and device level innovations are required for existing memory technologies. Emerging memory solutions are widely explored to cater to strict budgets. This thesis presents design methodologies for custom memory design with the objective of power-performance benefits across specific applications. Taking example of STTRAM (spin transfer torque random access memory) as an emerging memory candidate, the design space is explored to find optimal energy design solution. A thorough thermal reliability study is performed to estimate detection reliability challenges and circuit solutions are proposed to ensure reliable operation. Adoption of the application-specific optimal energy solution is shown to yield considerable energy benefits in a read-heavy application called MBC (memory based computing). Circuit level customizations are studied for the volatile SRAM (static random access memory) memory, which will provide improved energy-delay product (EDP) for the same MBC application. Memory design has to be aware of upcoming challenges from not only the application nature but also from the packaging front. Taking 3D die-folding as an example, SRAM performance shift under die-folding is illustrated. Overall the thesis demonstrates how knowledge of the system and packaging can help in achieving power efficient and high performance memory design.

La quinta generación de redes móviles (5G) se encuentra a la vuelta de la esquina. Se espera provea de beneficios extraordinarios a la población y que resuelva la mayoría de los problemas de las redes 4G actuales. El éxito de 5G, cuya primera fase de estandarización ha sido completada, depende de tres pilares: comunicaciones tipo-máquina masivas, banda ancha móvil mejorada y comunicaciones ultra fiables y de baja latencia (mMTC, eMBB y URLLC, respectivamente). En esta tesis nos enfocamos en el primer pilar de 5G, mMTC, pero también proveemos una solución para lograr eMBB en escenarios de distribución masiva de contenidos. Específicamente, las principales contribuciones son en las áreas de: 1) soporte eficiente de mMTC en redes celulares; 2) acceso aleatorio para el reporte de eventos en redes inalámbricas de sensores (WSNs); y 3) cooperación para la distribución masiva de contenidos en redes celulares. En el apartado de mMTC en redes celulares, esta tesis provee un análisis profundo del desempeño del procedimiento de acceso aleatorio, que es la forma mediante la cual los dispositivos móviles acceden a la red. Estos análisis fueron inicialmente llevados a cabo por simulaciones y, posteriormente, por medio de un modelo analítico. Ambos modelos fueron desarrollados específicamente para este propósito e incluyen uno de los esquemas de control de acceso más prometedores: access class barring (ACB). Nuestro modelo es uno de los más precisos que se pueden encontrar en la literatura y el único que incorpora el esquema de ACB. Los resultados obtenidos por medio de este modelo y por simulación son claros: los accesos altamente sincronizados que ocurren en aplicaciones de mMTC pueden causar congestión severa en el canal de acceso. Por otro lado, también son claros en que esta congestión se puede prevenir con una adecuada configuración del ACB. Sin embargo, los parámetros de configuración del ACB deben ser continuamente adaptados a la intensidad de accesos para poder obtener un desempeño óptimo. En la tesis se propone una solución práctica a este problema en la forma de un esquema de configuración automática para el ACB; lo llamamos ACBC. Los resultados muestran que nuestro esquema puede lograr un desempeño muy cercano al óptimo sin importar la intensidad de los accesos. Asimismo, puede ser directamente implementado en redes celulares para soportar el tráfico mMTC, ya que ha sido diseñado teniendo en cuenta los estándares del 3GPP. Además de los análisis descritos anteriormente para redes celulares, se realiza un análisis general para aplicaciones de contadores inteligentes. Es decir, estudiamos un escenario de mMTC desde la perspectiva de las WSNs. Específicamente, desarrollamos un modelo híbrido para el análisis de desempeño y la optimización de protocolos de WSNs de acceso aleatorio y basados en cluster. Los resultados muestran la utilidad de escuchar el medio inalámbrico para minimizar el número de transmisiones y también de modificar las probabilidades de transmisión después de una colisión. En lo que respecta a eMBB, nos enfocamos en un escenario de distribución masiva de contenidos, en el que un mismo contenido es enviado de forma simultánea a un gran número de usuarios móviles. Este escenario es problemático, ya que las estaciones base de la red celular no cuentan con mecanismos eficientes de multicast o broadcast. Por lo tanto, la solución que se adopta comúnmente es la de replicar e contenido para cada uno de los usuarios que lo soliciten; está claro que esto es altamente ineficiente. Para resolver este problema, proponemos el uso de esquemas de network coding y de arquitecturas cooperativas llamadas nubes móviles. En concreto, desarrollamos un protocolo para la distribución masiva de contenidos, junto con un modelo analítico para su optimización. Los resultados demuestran que el modelo propuesto es simple y preciso, y que el protocolo puede reducir el con
La cinquena generació de xarxes mòbils (5G) es troba molt a la vora. S'espera que proveïsca de beneficis extraordinaris a la població i que resolga la majoria dels problemes de les xarxes 4G actuals. L'èxit de 5G, per a la qual ja ha sigut completada la primera fase del qual d'estandardització, depén de tres pilars: comunicacions tipus-màquina massives, banda ampla mòbil millorada, i comunicacions ultra fiables i de baixa latència (mMTC, eMBB i URLLC, respectivament, per les seues sigles en anglés). En aquesta tesi ens enfoquem en el primer pilar de 5G, mMTC, però també proveïm una solució per a aconseguir eMBB en escenaris de distribució massiva de continguts. Específicament, les principals contribucions són en les àrees de: 1) suport eficient de mMTC en xarxes cel·lulars; 2) accés aleatori per al report d'esdeveniments en xarxes sense fils de sensors (WSNs); i 3) cooperació per a la distribució massiva de continguts en xarxes cel·lulars. En l'apartat de mMTC en xarxes cel·lulars, aquesta tesi realitza una anàlisi profunda de l'acompliment del procediment d'accés aleatori, que és la forma mitjançant la qual els dispositius mòbils accedeixen a la xarxa. Aquestes anàlisis van ser inicialment dutes per mitjà de simulacions i, posteriorment, per mitjà d'un model analític. Els models van ser desenvolupats específicament per a aquest propòsit i inclouen un dels esquemes de control d'accés més prometedors: el access class barring (ACB). El nostre model és un dels més precisos que es poden trobar i l'únic que incorpora l'esquema d'ACB. Els resultats obtinguts per mitjà d'aquest model i per simulació són clars: els accessos altament sincronitzats que ocorren en aplicacions de mMTC poden causar congestió severa en el canal d'accés. D'altra banda, també són clars en què aquesta congestió es pot previndre amb una adequada configuració de l'ACB. No obstant això, els paràmetres de configuració de l'ACB han de ser contínuament adaptats a la intensitat d'accessos per a poder obtindre unes prestacions òptimes. En la tesi es proposa una solució pràctica a aquest problema en la forma d'un esquema de configuració automàtica per a l'ACB; l'anomenem ACBC. Els resultats mostren que el nostre esquema pot aconseguir un acompliment molt proper a l'òptim sense importar la intensitat dels accessos. Així mateix, pot ser directament implementat en xarxes cel·lulars per a suportar el trànsit mMTC, ja que ha sigut dissenyat tenint en compte els estàndards del 3GPP. A més de les anàlisis descrites anteriorment per a xarxes cel·lulars, es realitza una anàlisi general per a aplicacions de comptadors intel·ligents. És a dir, estudiem un escenari de mMTC des de la perspectiva de les WSNs. Específicament, desenvolupem un model híbrid per a l'anàlisi de prestacions i l'optimització de protocols de WSNs d'accés aleatori i basats en clúster. Els resultats mostren la utilitat d'escoltar el mitjà sense fil per a minimitzar el nombre de transmissions i també de modificar les probabilitats de transmissió després d'una col·lisió. Pel que fa a eMBB, ens enfoquem en un escenari de distribució massiva de continguts, en el qual un mateix contingut és enviat de forma simultània a un gran nombre d'usuaris mòbils. Aquest escenari és problemàtic, ja que les estacions base de la xarxa cel·lular no compten amb mecanismes eficients de multicast o broadcast. Per tant, la solució que s'adopta comunament és la de replicar el contingut per a cadascun dels usuaris que ho sol·liciten; és clar que això és altament ineficient. Per a resoldre aquest problema, proposem l'ús d'esquemes de network coding i d'arquitectures cooperatives anomenades núvols mòbils. En concret, desenvolupem un protocol per a realitzar la distribució massiva de continguts de forma eficient, juntament amb un model analític per a la seua optimització. Els resultats demostren que el model proposat és simple i precís
The 5th generation (5G) of mobile networks is just around the corner. It is expected to bring extraordinary benefits to the population and to solve the majority of the problems of current 4th generation (4G) systems. The success of 5G, whose first phase of standardization has concluded, relies in three pillars that correspond to its main use cases: massive machine-type communication (mMTC), enhanced mobile broadband (eMBB), and ultra-reliable low latency communication (URLLC). This thesis mainly focuses on the first pillar of 5G: mMTC, but also provides a solution for the eMBB in massive content delivery scenarios. Specifically, its main contributions are in the areas of: 1) efficient support of mMTC in cellular networks; 2) random access (RA) event-reporting in wireless sensor networks (WSNs); and 3) cooperative massive content delivery in cellular networks. Regarding mMTC in cellular networks, this thesis provides a thorough performance analysis of the RA procedure (RAP), used by the mobile devices to switch from idle to connected mode. These analyses were first conducted by simulation and then by an analytical model; both of these were developed with this specific purpose and include one of the most promising access control schemes: the access class barring (ACB). To the best of our knowledge, this is one of the most accurate analytical models reported in the literature and the only one that incorporates the ACB scheme. Our results clearly show that the highly-synchronized accesses that occur in mMTC applications can lead to severe congestion. On the other hand, it is also clear that congestion can be prevented with an adequate configuration of the ACB scheme. However, the configuration parameters of the ACB scheme must be continuously adapted to the intensity of access attempts if an optimal performance is to be obtained. We developed a practical solution to this problem in the form of a scheme to automatically configure the ACB; we call it access class barring configuration (ACBC) scheme. The results show that our ACBC scheme leads to a near-optimal performance regardless of the intensity of access attempts. Furthermore, it can be directly implemented in 3rd Generation Partnership Project (3GPP) cellular systems to efficiently handle mMTC because it has been designed to comply with the 3GPP standards. In addition to the analyses described above for cellular networks, a general analysis for smart metering applications is performed. That is, we study an mMTC scenario from the perspective of event detection and reporting WSNs. Specifically, we provide a hybrid model for the performance analysis and optimization of cluster-based RA WSN protocols. Results showcase the utility of overhearing to minimize the number of packet transmissions, but also of the adaptation of transmission parameters after a collision occurs. Building on this, we are able to provide some guidelines that can drastically increase the performance of a wide range of RA protocols and systems in event reporting applications. Regarding eMBB, we focus on a massive content delivery scenario in which the exact same content is transmitted to a large number of mobile users simultaneously. Such a scenario may arise, for example, with video streaming services that offer a particularly popular content. This is a problematic scenario because cellular base stations have no efficient multicast or broadcast mechanisms. Hence, the traditional solution is to replicate the content for each requesting user, which is highly inefficient. To solve this problem, we propose the use of network coding (NC) schemes in combination with cooperative architectures named mobile clouds (MCs). Specifically, we develop a protocol for efficient massive content delivery, along with the analytical model for its optimization. Results show the proposed model is simple and accurate, and the protocol can lead to energy savings of up to 37 percent when compared to the traditional approach.
Leyva Mayorga, I. (2018). On reliable and energy efficient massive wireless communications: the road to 5G [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/115484
TESIS

A wireless local area network (WLAN) provides high bandwidth to users in a limited geographical area. This network faces certain challenges and constraints that are not imposed on their wired counterparts. They are: frequency allocation, interference and reliability, security, power consumption, human safety, mobility, connection to wired LAN,service area, handoff and roaming, dynamic configuration and the throughput. But the wireless medium relies heavily on the features of MAC protocol and the MAC protocol is the core of medium access control for WLANs. The available MAC protocols all have their own merits and demerits. In our research works, we propose a hybrid MAC protocol forWLAN. In the design, we have combined the merits of the TDMA and CDMA systems to improve the throughput of the WLAN in a picocellular environment. We have used the reservation and polling methods of MAC protocols to handle both the low and high data traffics of the mobile users. We have strictly followed the standards specified by IEEE 802.11 for WLANs to implement the designed MAC protocol. We have simulated the hybrid TDMA/CDMA based MAC protocols combined with RAP (Randomly Addressed Polling) for Wireless Local Area Networks. We have developed a closed form mathematical expressions analytically for this protocol. We have also studied the power control aspects in this environment and we derived a closed form mathematical expressions analytically for this power control technique. This hybrid protocol is capable of integrating different types of traffic (like CBR,VBR and ABR services) and compiles with the requirements of next-generation systems.The lower traffic arrival is dealt with the Random Access and the higher traffic arrival is with the Polling methods. This enables us to obtain higher throughput and lowmean delay performance compared to the contention-reservation-based MAC schemes. The protocol offers the ability to integrate different types of services in a flexible way by the use of multiple slots per frame, while CDMA allows multiple users to transmit simultaneously using their own codes. The RAP uses an efficient "back-off" algorithm to improve throughput at higher arrival rates of user's data. The performance is evaluated in terms of throughput, delay, and rejection rate using computer simulation. A detailed simulation is carried out regarding the maximum number of users that each base station can support on a lossy channel. This work has analyzed the desired user's signal quality in a single cell CDMA (Code Division Multiple Access) system in the presence of MAI (Multiple Access Interference). Earlier power control techniques were designed to assure that all signals are received with equal power levels. Since these algorithms are designed for a imperfect control of power, the capacity of the system is reduced for a given BER (Bit-Error Rate). We proposed an EPCM (Efficient Power Control Mechanism) based system capacity which is designed for the reverse link (mobile to base station) considering the path loss, log-normal shadowing and Rayleigh fading. We have simulated the following applications for the further improvement of the performance of the designed MAC protocol:Designed protocol is tested under different traffic conditions. The protocol is tested for multimedia traffic under application oriented QoS requirements. Buffer Management and resource allocation. Call Admission Control (hand-offs, arrival of new users). The adaptability to the variable nature of traffic.The propagation aspects in the wireless medium. The proposed MAC protocol has been simulated and analysed by using C++/MATLAB Programming in IBM/SUN-SOLARIS UNIX environment. The results were plotted using MATLAB software. All the functions of the protocol have been tested by an analysis and also by simulation. Call admission control function of the protocol has been tested by simulation and analysis in a multimedia wireless network topology and from analysis we found that at low traffic the throughput is high and at high traffic the throughput is kept constant at a reasonable high value. The simulation results also justify/ coordinate the analysis results. Dynamic channel allocation function of the protocol was tested and analysed and the coordinated results show that at low traffic, high throughput and at high traffic the throughput is constant. Buffer management function of the protocol simulation shows the results that the packet loss can be controlled to a minimum by adjusting the buffer threshold level at any traffic conditions. Maintenance of data transfer during the hand-offs function was simulated and the results show that the blocked calls are less during low traffic and at high traffic the blocked calls can be kept constant at low value. Thus, the proposed model aimed at having high throughput, high spectral efficiency, low delay, moderate BER and moderate blocking probability. We have considered a pico cell with a maximum of several users and studied the power efficiency of combined channel coding and modulation with perfect power controlled CDMA system. Thus our simulation of the "software radio" has flexibility in choosing the proper channel coders dynamically depending upon the variations of AWGN channel.

博士
國立清華大學
材料科學工程學系
98
A novel memory device based on the magnetoreisitive effect is known as magnetic random access memory (MRAM) and is also called “dream memory”. In this dissertation, we investigate the key issue of the MRAM － writing efficiency. Firstly, in the concept of spin torque transfer MRAM, we can manipulate the damping constant and saturation magnetization of CoFeB by simply adjusting the capping layers. We found that the Ta capping layer caused the intermixing, and increased the damping constant of CoFeB. By inserting various metals, the intermixing can be improved and the lower damping constant with Cu/Ta capping layers was obtained. Moreover, the results of micromagnetic simulation indicate that the optimal switching current density can be achieved by reducing the damping constant with a Cu capping layer. In our simulation results, we demonstrate that by changing the capping layer from Ta to Cu/Ta, we can effectively reduce the critical current density by 27 %. Secondly, in the concept of magnetic domain wall RAM (so called “race-track memory”), we demonstrate that the depinning probability of the transverse-type domain wall strongly depends on the domain wall configuration when the domain wall moves to the notch. We found that when the current density is larger than threshold current density, the domain wall structure periodically changes between transverse-type domain wall and antivortex-type domain wall. Due to this transition behavior, part of the spin torque energy contributes to the transformation of the domain wall structure. Since the antivortex-type domain wall stores more energy, the stored energy can help the domain wall to be depinned from the notch and increases the depinning probability. Finally, we take advantage of the magnetic domain wall and we propose a method which combines either conventional current- or field- driven MRAM with the magnetic domain wall. We demonstrate that the domain wall can be artificially created in patterned antiferromagnetic/ferromagnetic exchange bias system using ion irradiation. The magnetic domain wall was formed by the different switching fields in the irradiated and non-irradiated areas and could assist the magnetization switching. Furthermore, this study proposes a new type of MRAM design. A comparison of the proposed magnetoresistive device with the conventional ellipse one shows that the new design reduces the switching field and the critical current density by about 90.0 % and 68.8 %, respectively, in field- and current- driven cases. In both of these cases, we demonstrated the domain wall assisted magnetization reversal. This study provides an effect way to increase the writing efficiency of MRAM.

碩士
國立清華大學
通訊工程研究所
95
In this thesis, we analyze the efficiency and delay of distributed source coding (DSC) in sensor networks under the random access setting. Consider a network of N sensors that observes correlated information from the environment and sends the local data to a central processor through direct transmission links. Due to the low message rate in sensor networks, we adopt the slotted ALOHA random access protocol where the time is divided into synchronized time slots and each sensor is allowed to access the time slots with independent probabilities. To eliminate the redundancy in the transmission data, the sensors encode the local messages based on the Slepian-Wolf DSC method. Specifically, the network is divided into K clusters and we assume that the sensors are encoded sequentially in a cluster such that the decoding of a particular message is reliant on the successful decoding of all the messages encoded earlier in the sequence. In this case, the loss of one message may result in the failure of other messages and the delay in the successful decoding of a particular message also varies from sensor to sensor. In this article, we analyze the performance of DSC in random access networks in terms of the rate of successful decoding, the average delay and the average energy consumption of each message. Specifically, we propose and compare different transmission probability assignments for DSC in the ALOHA network and emphasize the importance of the MAC design. Finally, we propose an adaptive MAC protocol such that improve the performance of our system.

碩士
國立臺灣科技大學
資訊工程系
105
As the technology evolves, networking technologies become more and more popular.As an increasing amount of devices are connected to a network, the machine-to-machine (M2M) communication has become pervasive. Because of a better coverage solution provided by the cellular network, it is one of networks which can be chosen for the M2M communication. Although the data size is small, the number of M2M devices is perhaps huge. When a lot of devices are trying to access the network in a short period, collisions are inevitable, leading to communication failure and degradation of service. To address such an issue, how to alleviate collisions in the random access of the long-term evolution (LTE) system is considered when M2M devices are added. We try to propose a random access scheme to enhance efficiency. By splitting the preamble into two sets, it is able to reduce the delay for M2M devices suffering from preamble collisions. The two sets include the one of M2M devices which never tried before and the one of backlogged M2M devices. Additionally, priority is considered for devices which can not tolerate a longer delay and the others. Actually, the coexistence M2M and human-to-human (H2H) devices form an issue to be addressed. This allows us to let H2H devices have a priority higher than M2M devices. Finally, our simulation results demonstrate that our proposed scheme can achieve the goal of delay reduction and access efficiency enhancement regardless of traffic loads within the network.

碩士
國立清華大學
電機工程學系
102
The requirements of non-volatile memories for handheld consumer electronics, medical electronics, car electronics, and lots of electronic products become larger and larger, especially for large capacity, low cost, low power and high speed memory. After integrated with micro controller unit (MCU), it can effectively increase the chip performance. Flash memory is the mainstream embedded memory. However, it cannot achieve high speed write operation and be randomly accessed. Furthermore, it is difficult to scale down flash memory into deep nanometer scale. Thus, developing new nonvolatile memories is necessary. Among these emerging nonvolatile memories, Resistive Random Access Memory (ReRAM) is one of the most promising candidates. It has attractive characteristics such as low write power, small area, and logic-process compatibility which can lower the manufacturing cost. Currently, the most common memory cell structure is one transistor and one ReRAM (1T1R), which is suitable for high speed and low supply voltage embedded applications, particularly for devices powered by batteries. As devices shrink, ReRAMs have higher cell resistance (R) and greater variations in write time and R, which reduces the R-ratio (RH/RL) between the high-R state (HRS, RH) and low-R state (LRS, RL). ReRAM also has a high RL, which enables a larger voltage drop across ReRAM to reduce write voltage and cell-switch (CS) size. Thus, ReRAM memory macro designs suffer two major problems： 1. Small sensing margin (SM), limited read-VDDMIN, and slow access time (TAC) due to high-RL and small R-ratio. 2. Increase in energy due to large set DC-current (IDC-SET) resulting from wide set-time (TSET) distribution. Here, we propose swing-sample-and-couple voltage mode sense amplifier (SSC-VSA) and self-boost-write-termination (SBWT) scheme to solve above two major problems, respectively. Proposed SSC-VSA designs the VREF on specific voltage level, after the operation of the circuit, it can increase the usage of ΔVBLS_MIN (by up to 99%) as the SM for lower read VDDMIN and faster TAC. It can achieve 1.7x faster TAC across various VDD compared to conventional differential-input (CD) voltage mode sense amplifier. As for ReRAM write operation, proposed SBWT scheme is a 4T self-detective write-termination circuit. When ReRAM successfully sets (HRS→LRS), large IDC-SET will increase the BL voltage, enabling the positive feedback between BL and SBWT and cutting off the current path, which can save over 99% write power. We fabricated a 28nm 1Mb ReRAM memory macro. Under the 0.85V and 0.27V supply voltage, the measured read access times are 6.8ns and 404.4ns, respectively. Besides, the SBWT scheme has also been demonstrated.

碩士
國立中央大學
電機工程研究所
97
Yield and reliability are two key challenges for designing nano-scale chips. Embedded memory is one key component in modern system-on-chip (SoC) designs. It typically represents a significant portion of the chip area as well. Moreover, it is designed with the smallest transistors and aggressive design rules. Thus, the yield and reliability of embedded memories dominate that of SoCs. Therefore, efficient yield and reliability enhancement techniques for embedded memories are very important for SoCs. Built-in self-diagnosis (BISD) and built-in self-repair(BISR) are two key techniques for improving the yield of embedded memories. Typically, a BISD design exports diagnostic data serially. In the first part of this thesis, a diagnostic data compression technique is proposed to reduce the diagnostic data of a RAM with error correction code (ECC). By reusing the ECC circuit, the proposed approach can compress the diagnostic data efficiently with very low area cost. In the second part of this thesis, a transparent BISR scheme for RAMs with ECC is proposed to enhance the yield and reliability of RAMs. The transparent BISR scheme can perform off-line test/repair for RAMs in production phase. It also can perform online test/repair for RAMs in operation. In online test/repair mode, the transparent BISR scheme performs transparent march tests for the RAM under test and repairs the RAM cells with hard faults if some spares are unused after the off-line test/repair phase. This can prolong the reliability of the RAM. In comparison with existing transparent test approaches, the proposed transparent test approach has the following advantages: fault-location capability and low test complexity. Experimental results show that the area cost of the proposed transparent BISR scheme for RAMs with ECC is low—only about 4.8% for a 4K×39-bit SRAM. In the third part of this thesis, a shared transparent test and repair scheme for multiple homogeneous RAMs without ECC is proposed to reduce the area cost. In the shared transparent test and repair scheme, a shared code memory is designed to store the signature of a RAM under test. Thus, the RAMs sharing the code memory are tested and repaired one by one in a time-multiplexing method. Since the code memory is shared by multiple homogeneous RAMs, the area cost of the transparent test and repair scheme is reduced. In comparison with typical transparent BIST schemes, the proposed scheme has good fault location capability. The signature prediction phase is only 1N for an N×B-bit RAM.

碩士
世新大學
資訊管理學研究所(含碩專班)
99
Over 70% of our Earth's surface is covered by water, and the Underwater Sensor Networks (UWSN) can collection ocean information, monitor ocean environment. Since the UWSN can only use sound waves to transmit messages, it has the following features as opposed to the wireless sensor network, which uses radio waves to transmit messages: long propagation delay, lower available bandwidth and multi-path problem. The sink node’s broadcasting and collecting data from sensor nodes are routine works in the sensor networks, but many MAC protocols of sensor networks fail to consider the characteristics of these routine works to design an more efficient MAC protocol. In this thesis, we propose a cross-layered MAC protocol for UWSN named as Scheduling and Random Access Hybrid (SRH-MAC) protocol. In SRH-MAC channel is divided into several superframes with fixed length. Each superframe contains three periods, broadcast period, routine report period and event report period. During the broadcast and routine report periods, each sensor node broadcasts and gathers data in a predetermined time slot so as to avoid collisions and reduce idle waiting time. The data is received and transmitted from and to a predetermined neighboring source and destination nodes. The predetermined time slot, source, and destination nodes are set according to the broadcasting and gathering trees. During the event report period, the senor node reports the sensed event through the time slots reserved by exchanging RTS/CTS packets. Simulations have shown that the proposed SRH-MAC protocol performs better than existing protocols in terms of network throughput, broadcasting, and gathering latency.

碩士
國立中央大學
電機工程研究所
96
Memory core is one key component in system-on-chip (SOC) designs. Also, memory cores usually represent a significant portion of the chip area. Therefore, the yield of memory cores dominates the yield of the chip. Efficient yield-improvement techniques for memory cores thus are essential for improving the yield of the chip. Diagnosis and repair are two major techniques for improving the yields of memory cores. On the other hand, leakage power issue is another challenge for designing nano-scale SOCs. Drowsy static random access memory (SRAM) is one possible candidate of memory core with low-leakage power consumption. Therefore, we propose efficient diagnosis and repair techniques for drowsy SRAMs in this thesis. First, we propose a March D2 algorithm for distinguishing drowsy faults (DFs) from non-drowsy faults (NDFs). We also propose a March D6 diagnosis algorithm for distinguishing all DFs of drowsy SRAMs. The test complexity of a March D6 algorithm is O((10*log2N+17+9*log2W)*N), where N represents the number of words of the memory under test; W represents the word width of memory. Second, an efficient built-in self-repair (BISR) scheme is proposed to repair defective drowsy SRAMs. A new redundancy analysis (RA) algorithm is proposed to allocate redundancies of the drowsy SRAM with spare rows, spare columns, and drowsy-masking registers (DMRs) [2]. The proposed BISR scheme can repair DFs by disabling the drowsy operation mode of the corresponding rows with DMRs. Simulation results show that the repair rate (the ratio of the number of repaired memories to the number of defective memories) of the proposed RA algorithm is 91%, which is better than that of repair-most algorithm which offers 67% repair rate.

The past decade has seen wide availability of solid-state drives (SSDs) in settings ranging from personal computing to enterprise storage. Their success over the hard disks is driven by performance considerations and cost savings. Besides SSDs based on flash memory, there have been ongoing efforts in developing other non-volatile memory technologies such as phase-change memory and MRAM. All these technologies enable what we refer to as random-access block devices. Unlike hard disks, these devices have fast random accesses; on the other hand, their writes are more expensive than their reads. In this work, we study how to optimize database and storage algorithms for the I/O characteristics of random-access block devices. Specifically, we tackle the following three problems.

The first one is about permuting data out-of-core. While external merge sort is popular for implementing permutation on hard disks, it carries unnecessary overhead in storing and comparing keys. We propose more efficient algorithms for a useful class of permutations called Address-Digit Permutations on random-access block devices.

The second problem is concurrency control for indexes on SSDs. Various indexes have been proposed for these devices, but to make such indexes practical, we must address the issue of concurrency control. We propose a novel indexing and concurrency control scheme which allows concurrent accesses during ongoing index reorganizations, and does so with minimal memory and block-level locking.

The third problem concerns log-structured merge, a popular indexing technique well-suited to random-access block devices. We show how an intelligent partial merge policy, combined with a block-preserving merge procedure, can ­significantly lower write traffic while preserving other advantages of log-structured merge.

Dissertation

碩士
國立交通大學
網路工程研究所
105
Massive Machine Type Communication (mMTC) has attracted increasing attention due to the explosive growth of IoT devices. Random access (RA) for a large number of mMTC user equipments (UEs) is especially difficult since the severe collisions problem between UEs and an eNB may overwhelm the available spectrum resources. To address this issue, we propose a contention-free access scheme and an efficient RA response (RAR) message for mMTC in 5G. The core idea is to avoid collisions happened between UEs and also the one-to-one mapping mechanism applied by 4G-Long-Tern Evolution (4G-LTE). To do so, we exploit the concept of Chinese Remainder Theorem (CRT) and prime {\em factorization} to encode the {\em hint} such that UEs can extract their required information accordingly. Our simulations show that we can reduce the access delay for mMTC UEs up to 80\%. Besides, we can reduce the RAR message size by 20\%--40\%. Such reduction can be translated to around 50\% improvement of spectrum efficiency in LTE-M.

碩士
國立交通大學
電信工程研究所
101
Machine-to-machine (M2M) communication is a type of communication. It makes the automatic information exchange between a machine and other machine or a machine and a machine type communication (MTC) server by integrating various wireless communication techniques to satisfy requirements of many applications. Because M2M communication can provide various automatic applications and open a new market, M2M communication is received public attention. But M2M communication needs to deploy a large number of machines everywhere, and bring the current 4G wireless communication system many challenges to be solved. In this thesis, we propose an energy efficient random access control (E2RAC) in which the fine access quality is provided through estimating the number of machines. As we separate access resources for different types of communication, different orders of the service quality can be provided. Simulation results show that the E2RAC scheme provide the better quality of service than the conventional scheme.