During the last decade, relay networks have attracted a lot of interest due to their numerous benefits. The relaying technique allows extending the coverage zone of wireless networks and offers a higher reliability for communication systems. The performance of relay networks can be improved further by the use of automatic repeat request (ARQ) and hybrid automatic repeat request (HARQ) techniques. ARQ and HARQ are retransmission mechanisms that ensure a good quality of service even in absence of channel state information at the transmitter. We, firstly, study the spectral and energy efficiency of ARQ in Nakagami-m block-fading channels. We maximize both spectral efficiency and energy efficiency with respect to the transmitted power. We derive exact expressions as well as compact and tight approximation for the solutions of these problems. Our analysis shows that the two problems of maximizing spectral efficiency and energy efficiency with respect to the transmitted power are completely different and give different solutions. Additionally, operating with a power that maximizes energy efficiency can lead to a significant drop in the spectral efficiency, and vice versa. Next, we consider a three node relay network comprising a source, a relay, and a destination. The source transmits the message to the destination using HARQ with incremental redundancy (IR). The relay overhears the transmitted message, amplifies it using a variable gain amplifier, and then forwards the message to the destination. This latter combines both the source and the relay message and tries to decode the information. In case of decoding failure, the destination sends a negative acknowledgement. A new replica of the message containing new parity bits is then transmitted in the subsequent HARQ round. This process continues until successful decoding occurs at the destination or a maximum number M of rounds is reached. We study the performance of HARQ-IR over the considered relay channel from an information theoretic perspective. We derive exact expressions and bounds for the information outage probability, the average number of transmissions, and the average transmission rate. Moreover, we evaluate the delay experienced by Poisson arriving packets over the considered relay network. We also provide analytical expressions for the expected waiting time, the sojourn time, and the energy efficiency. The derived exact expressions are validated by Monte Carlo simulations. Finally, we consider a relay network consisting of a source, K relays, and a destination. The source transmits a message to the destination using HARQ-IR. We study the performance of HARQ-IR over dualhop multibranch amplify-and-forward relay channels. We derive exact expression for outage probability of the considered network. We investigate the benefit of relaying and the effect of changing the rate and the maximum number M of rounds on the outage probability.

International Telemetering Conference Proceedings / October 25-28, 1993 / Riviera Hotel and Convention Center, Las Vegas, Nevada / Soft-decision correlation decoding with retransmission requests for block codes is proposed and the resulting performance is analyzed. The correlation decoding rule is modified to allow retransmission requests when the received word is rendered unreliable by the channel noise. The modification is realized by a reduction in the volume in Euclidean space of the decoding region corresponding to each codeword. The performance analysis reveals the typical throughput - reliability trade-off characteristic of error control systems which employ retransmissions. Performance comparisons with hard-decision decoding reveal performance improvements beyond those attainable with hard-decision decoding algorithms. The proposed soft-decision decoding rule permits the use of a simplified codeword searching algorithm which reduces the complexity of the correlation decoder to the point where practical implementation is feasible.

Generation by generation, wireless communication has advanced in various ways and provided reliable communication services at higher and higher data rates to the needs of more and more advanced wireless applications. Two main issues towards future wireless communications are high-speed transmission technique to provide high data rate services and reliable communication to ensure the required performance. This dissertation focuses on these two issues. Since orthogonal frequency division multiplexing (OFDM) has emerged as an enabling technique for high-speed transmission in dispersive environments, major and fundamental issues in OFDM, namely, synchronization, channel estimation, and peak-to-average power ratio (PAPR) reduction are addressed. For the required reliability, automatic repeat request (ARQ) schemes must be applied. Due to large potential performance improvement, adaptive ARQ schemes have recently attracted much attention and are also addressed here. We propose two improved OFDM timing synchronization methods which overcome the drawbacks of existing methods. We present a time-domain-based OFDM channel estimation which outperforms the existing time-domain-based approach and has a similar performance to the linear minimum mean square error estimator but with less complexity. For OFDM systems with transmit diversity, we present a reduced complexity channel estimation which has a comparable performance to the existing method for channels with relatively small delay spreads, but achieves much complexity saving. An approach to find the number of most significant channel taps is described for diverse channel environments. We analyze the effect of non-sample-spaced channel path on the channel estimation and propose a modification for further improvement. Timing synchronization, frequency synchronization and channel estimation are usually addressed separately. Since they can affect each other, the idea of jointly addressing all of them together is much desirable and pursued here. This joint approach reflects the actual performance and gives an opportunity to exploit some information obtained from one task in another, hence promising more improvement. The proposed training preamble-based joint timing and frequency synchronization utilizes some information from the channel estimation. The sync detection is also considered. We design the training symbol to achieve a better coarse timing synchronization. Methods to suppress or circumvent the interference in the frequency estimation caused by timing errors are presented. A new performance measure for OFDM timing synchronization is proposed which leads to obtaining optimal timing estimation setting. Next, we present a joint timing synchronization, frequency synchronization and channel estimation based on training preamble based maximum likelihood realization. Further complexity reduction by an adaptive scheme is also proposed. We address some fundamental questions on peak factors, sampling theorem and sampling series. We present several bounds of bandlimited functions and peak factor ratio bound of a continuous signal and its sampled signal. Some discussion on the requirements of sampling theorems and related aspects on sampling series are presented. We study PAPR behavior of some Reed-Muller codes in OFDM systems in an attempt to find a code with good error correction, low PAPR, efficient encoding/decoding and reasonable code rate. Some regularities of the second and third-order cosets of first order Reed-Muller codes with low PAPR are presented which indicates possibility of finding such code for OFDM. The main issues in adaptive ARQ schemes are how to design the adaptive system parameters and how to effectively sense the channel conditions. We present an approach for designing the adaptive ARQ system parameters based on the through put calculation and optimization. An alternative approach which avoids the tedious throughput calculation is also presented. An effective channel sensing algorithm which utilizes the error correcting capability is proposed. Incorporation of the adaptive frequency hopping concept into the ARQ scheme with adaptive error control is introduced which has a significant throughput improvement in slow fading channels. / Graduate

ITC/USA 2011 Conference Proceedings / The Forty-Seventh Annual International Telemetering Conference and Technical Exhibition / October 24-27, 2011 / Bally's Las Vegas, Las Vegas, Nevada / A novel type-II hybrid automatic repeat request (HARQ) algorithm using adaptive modulations and bit-interleaved coded modulation (BICM) is presented. The algorithm uses different optimized puncturing patterns for different transmissions of the same data packet. The proposed approach exploits mapping diversity through BICM with iterative decoding. The modulation order is changed in each transmission to keep the number of symbols transmitted constant. We present new bit error rate and frame error rate analytical results for the proposed technique showing good agreement with simulation results. We compare the throughput performance of our proposed HARQ technique with a reference HARQ technique that uses different mapping arrangements but keeps the modulation order fixed. By using optimized puncturing patterns and adaptive modulations, our method provides significantly better throughput performance over the reference HARQ method in the whole signalto- noise ratio (SNR) range, and achieves a gain of 12 dB in the medium SNR region.

La transmisión asistida por relay o transmisión cooperativa es una nueva técnica de diversidad espacial donde aparece un elemento nuevo (un relay o un usuario cooperativo) en la tradicional transmisión punto a punto (fuente a destino). Ahora en la comunicación intervienen tres enlaces: fuente-relay, relay-destino y fuente-destino. El relay, además de asistir a la fuente en la transmisión de un mensaje, permite combatir las degradaciones que puede sufrir el canal como el shadowing y el pathloss. Aunque esta técnica está basada en el trabajo realizado en los 70 por Van der Meulen, Cover y El Gamal, ha sido en los últimos años cuando se han vuelto a considerar el uso de relays. En realidad, la transmisión asistida por un relay puede verse como un sistema virtual multi-antena (virtual MIMO) donde las antenas están distribuidas en diferentes terminales. Sin embargo, al contrario de los sistemas multi-antena y debido a la limitación de la actual tecnología radio, el relay debe trabajar en modo half-duplex, ya que no puede transmitir y recibir simultáneamente en la misma banda. Este hecho, motiva que la transmisión deba realizarse en dos fases ortogonales en función del modo del relay (recibiendo datos - relay-receive phase o transmitiendo datos - relay-transmit phase). Estas fases pueden implementarse en el dominio de la frecuencia o el tiempo.Esta tesis investiga protocolos y estrategias para la transmisión asistida por relay para mejorar la eficiencia espectral y homogeneizar el servicio para todos los usuarios en un sistema de comunicación celular. La introducción del relay en la comunicación implica la redefinición de muchas técnicas y protocolos considerados en las comunicaciones punto a punto y en los sistemas multi-antena, situados en la capa física y/o superiores.En primer lugar se presentan los achievable rates obtenidos por la transmisión asistida por relay en función del rol del relay (amplifica y retransmite o decodifica y retransmite), el tipo de transmisión (siempre transmite, incremental o selectiva), los datos transmitidos por el relay (repite los símbolos recibidos o son independientes) y el tipo del protocolo half duplex. En función de los terminales activos en cada fase de la comunicación (fuente, destino o relay), existen hasta cuatro protocolos. Otro aspecto considerado es la asignación de recursos (resource allocation) para cada fase de la comunicación, la cual puede estar fijada de antemano o puede ser ajustada dinámicamente en función de los canales de los diferentes enlaces. En el caso de que todos los coeficientes del canal se conocieran perfectamente, los terminales podrían transmitir síncronamente, mejorando la comunicación gracias a la ganancia debida a técnicas de pre-codificación por medio de los autovectores del canal (con antenas distribuidas).Además dos técnicas de transmisión asistida por relay son evaluadas cuando existen múltiples relays por destino. Ambas dependen del tipo de mensajes transmitidos a cada relay (mensajes independientes o uno común). La asignación de recursos para ambas técnicas puede verse como un problema convexo.Tres escenarios resumen diferentes tipos de transmisión asistida por relay para múltiples fuentes y un solo destino: RMAC (Relay-assisted Multiple Access Channel), UC (User Cooperation) and MARC (Multiple Access Relay Channel). Su diferencia se basa en el tipo de relay half- duplex considerado. La transmisión puede hacerse síncrona o asíncronamente. Las fuentes y los relays están limitados en potencia y el acceso de ellos en cada fase de la comunicación pude hacerse por medio de TDMA (time division multiple access), FDMA (frequency division multiple access) or SC (superposition coding multiple access). La asignación de recursos puede ser formulada como un problema convexo en algunos casos y la solución óptima puede ser encontrada. Seguidamente la transmisión asistida por relay y duplexada en tiempo es aplicada a un sistema celular centralizado basado en TDMA en el downlink. Con el objetivo de mejorar la eficiencia espectral se propone el reuso espacial de un slot temporal para las transmisiones de los relays hacia sus respectivos destinos (slot de relay), generando interferencia para todos los restantes destinos activos. Un algoritmo de control de potencia basado en la teoría de juegos es propuesto para combatir la interferencia generada. Bajo esa configuración, un algoritmo de scheduling investiga las posibles ganancias debidas al multi-user gain y mide el overhead introducido.Otra forma de tratar con la interferencia es la de controlar el rate de nuestra transmisión (rate control management). Bajo ciertas condiciones es posible modelar la función de densidad de probabilidad de la potencia interferente. En ese caso, la fuente ajusta el rate para maximizar el throughput de la comunicación. Esta solución es extendida para el caso en el que cada destino es asistido por varios relays. Las dos soluciones propuestas son capaces de proporcionar mejores resultados que la transmisión directa, a pesar de la interferencia existente en el slot de relay. Finalmente, se investiga el control dinámico del enlace para la transmisión asistida por relay con dos diferentes tipos de conocimiento sobre el canal: conocimiento estadístico (statistical knowledge of the channel state) o conocimiento del canal instantáneo (actual information about the current channel state).Estos dos tipos de conocimiento derivan en diferentes estrategias a utilizar para seleccionar la modulación y el esquema de codificación (MCS). En el primer caso, los rates seleccionados no están adaptados al canal actual, por lo que el destino puede recibir erróneamente los mensajes. Los protocolos de retransmisión de mensajes (ARQ - automatic repeat request) son los encargados de asegurarse la correcta recepción y son redefinidos para la transmisión asistida por relay.. En este trabajo, se especifica los códigos espacio-tiempo distribuidos, la codificación en al fuente y el relay y la longitud de las retransmisiones. Cuando la fuente conoce algún parámetro del canal instantáneo puede adaptar el MCS para esa realización del canal. En ese caso se investiga la predicción del error en las transmisiones asistidas por relay, y con ello es posible diseñar el MCS para que maximice la cantidad de información transmitida para una probabilidad de pérdida de paquete o que maximice el throughput. / The relay-assisted or cooperative transmission is a relatively new class of spatial diversity technique where a new element comes up in the conventional source-destination or point-to point communication: an assisting relay or cooperating user. The relay assists to the source in transmitting a message to the destination and allows dealing with the channel impairments like shadowing and pathloss. Although the seminal works were issued in the 70's by van der Meulen, Cover and El Gamal, it has been during the last years when it has re-gained more attention by the researchers. In fact, the relay-assisted transmission can be seen as a virtual MIMO (Multiple Input Multiple Output) with distributed antennas. In contrast to MIMO systems, the transmission requires the use of additional channel resources because of the limitation of the current radio technology: the relay terminal is constrained to work in half-duplex mode, which motivates that the transmission must be carried out in two orthogonal phases (relay-receive and relay-transmit phase), duplexed in time or frequency domains.This dissertation investigates protocols and strategies for the relay-assisted transmission which improve the spectral efficiency and homogenize service in the cellular communication systems. The new element present in the communication, the relay terminal, imposes a redefinition of many techniques and protocols commonly used in the point-to-point and MIMO systems, which are placed at the physical and upper layers.First, achievable rates using the relay-assisted transmission are provided which depend on the role of the relay (amplify-and-forward or decode-and-forward), the type of the transmission (persistent transmission, incremental or selective relaying), the data transmitted by the relays (repetition or unconstrained coding) and the type of half-duplex protocol. There are up to four protocol definition depending on the activity of the terminals on each phase. An additional aspect addressed is the resource allocation for each phase, that is, either it is fixed beforehand (static) or it is adjusted dynamically (dynamic) as a function of the channel quality. For the single-user relay- assisted transmission the resources can be allocated based on the channel quality of the different links. Moreover, if there is complete channel state information about all channel coefficients, source and relay can transmit synchronously enhancing the transmission thanks to the (distributed) eigenvector precoding techniques.Two relay-assisted transmission techniques are evaluated when a destination is assisted by multiple relays. Both depend on the messages intended to each assisting relay (independent or common messaging). The resource allocation for both techniques is shown to be convex. Additionally, three different scenarios illustrate the multi-user relay-assisted transmission with a single destination and different types of half-duplex relays: RMAC (Relay-assisted Multiple Access Channel), UC (User Cooperation) and MARC (Multiple Access Relay Channel). The relay-assisted transmission can be done synchronously or asynchronously. The sources and relays are power limited and access in each phase of the communication by TDMA (time division multiple access), FDMA (frequency division multiple access) or SC (superposition coding multiple access). For those scenarios the allocation of transmitted power and time resources can be formulated as a convex problem under some circumstances, evaluating the optimal solution.Afterwards, the relay-assisted transmission duplexed in time is applied to a centralized cellular system based on TDMA in the downlink. The reuse of one time slot for the transmissions done from the relays to destinations (relay slot) is proposed to improve the spectral efficiency. This solution produces interference for all the destinations active in that time slot. A power control algorithm (at the relays) based on game theory is proposed to combat the generated interference. Under that configuration a scheduler algorithm explores the multi-user gain for the relay-assisted transmission, measuring the introduced overhead.Another way of dealing with the interference is by rate control management. Under some circumstances it is possible to model the probability density function (pdf) of the interfering power. In such a case, the source can tune the transmission rate in order to maximize the throughput. This solution is extended to the case where each destination is assisted by multiple relays. In spite of the interfering power, both proposed solutions are able to provide significant gains over the direct transmission.Finally, the dynamic link control of the relay assisted transmission is investigated under two different assumptions on the knowledge about the channel: statistical knowledge of the channel state and actual information about the current channel state. Both types of knowledge lead to different transmission strategies, in terms of selecting the modulation and coding scheme (MCS). Under the first case, the transmission rates are not adapted to the current channel realization and the destination can decode wrongly the messages. The Automatic Repeat reQuest (ARQ) protocols are redefined for the relayassisted transmission to cope with these events. In this work we specify the (distributed) space-time codes, the coding at the source and relay and the length of the retransmissions. When there is actual information about the channel state the MCS can be adapted to the current channel realization. In such a case, the link error prediction for the relay-assisted transmission is investigated, and thus the MCS can be designed for maximizing the information rate for a given probability of packet loss or maximizing the throughput.

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This dissertation focuses on an efficient cooperative communication method for wireless ad hoc networks. Typically, performance enhancement via cooperative communications can be achieved at the cost of other system resources such as additional bandwidth, transmit power, or more complex synchronization methods between cooperating signals. However, the proposed ooperative transmission scheme in this research utilizes system resources more efficiently by reducing the redundant and wasteful cooperating signals typically required, while maintaining the desired performance improvement. There are four main results in this dissertation. First, an efficient cooperative retransmission scheme is introduced to increase bandwidth efficiency by reducing wasteful cooperating signals. The proposed cooperative transmission method does not require any additional information for cooperation. Furthermore, we ensure good quality for the cooperating signals through a simple yet effective selection procedure. Multiple cooperating nodes can be involved in the cooperation without prior planning via distributed beamforming. The proposed cooperative retransmission scheme outperforms traditional retransmission by the source as well as other cooperative methods in terms of delay and packet error rate (PER). Secondly, the outage probabilities of the cooperative retransmission scheme are analyzed for both the perfect synchronization case and when offset estimation is performed for distributed beamforming. The performance with offset estimation is close to the perfect synchronization case, especially for short data packets. A low-rate feedback channel is introduced to adjust the phase shift due to channel variation and the residual frequency offset. It is shown that substantial gain can be achieved with a low-rate feedback channel, even for long data packets. Third, the throughput efficiency and average packet delay of the proposed cooperative retransmission scheme are analyzed using a two-state Markov model for both a simple automatic repeat request (ARQ) and a hybrid ARQ technique. The benefits of the cooperative ARQ approach are also verified in a multihop network with random configurations when there are concurrent packet transmissions. The average transmit power for the cooperating signals is also investigated in the proposed cooperative transmission scheme with various power control approaches. Finally, cooperative multiple input multiple output (MIMO) systems are examined, mainly focusing on power allocation methods to increase overall channel capacity. An efficient and simple power allocation method at the cooperating node is proposed which can be used for an arbitrary number of antennas without any additional information. / Ph. D.

ITC/USA 2012 Conference Proceedings / The Forty-Eighth Annual International Telemetering Conference and Technical Exhibition / October 22-25, 2012 / Town and Country Resort & Convention Center, San Diego, California / As the integrated Network Enhanced Telemetry (iNET) program prepares for developmental flights tests, refinements are being made to the Radio Access Network Standard that ensures interoperability of networked radio components. One key aspect of this interoperability is the definition of Telemetry Network System (TmNS) RF Data Link Layer functionality for conducting efficient communications between radios in a TDMA (Time Division Multiple Access) channel sharing scheme. This paper examines the overall structure of the TmNS RF Data Link Layer and provides an overview of its operation. Specific topics include Medium Access Control (MAC) scheduling and framing in the context of a burst-oriented TDMA structure, link layer encryption, the priority-enabled Automatic Repeat reQuest (ARQ) protocol, high-level network packet and link control message encapsulation, payload segmentation and reassembly, and radio Link Layer Control Messaging.

L'objectif de cette thèse est de concevoir, implémenter et évaluer les algorithmes cross-layer pratiques. Nous nous concentrons sur la technologie LTE et les réseaux non coordonnés post-LTE où l'interférence est un enjeu majeur compte tenu des nouvelles tendances du trafic. L'objectif est d'allouer les ressources radio d'une manière efficace. Nous développons des modèles d'interférence mathématiques et informatiques qui nous permettent de comprendre le comportement de ces réseaux et nous appliquons une approche basée sur la théorie de l'information à différents scénarios d'interférence et caractéristiques du trafic. Nous avons essayé de s'approcher le plus possible de systèmes réels pour être en mesure de tester la faisabilité des techniques proposées. La thèse porte sur l'évaluation de la performance des scénarios d'interférence dans les réseaux 4G, en particulier celles qui découlent du déploiements de cellules de petite taille ("small cells"). Le travail dans cette thèse s'adresse également à l'analyse de l'allocation des ressources et la requête de répétition automatique hybride (HARQ) à redondance incrémentale pour les interférences sporadiques (de façon plus générale les canaux variables dans le temps) qui permet uniquement des informations partielles de l'état du canal à l'émetteur. Ce travail est ensuite appliquée à la conception d'ordonnanceur pour les stations de base LTE et inclut une analyse de performance pour les modems LTE réels. / The aim of this thesis is to design, implement and evaluate practical cross-layer algorithms. We focus on LTE and post-LTE uncoordinated networks where interference is a key issue given the new traffic patterns. The goal is to allocate the radio resources in an efficient way. We develop mathematical and computational interference models that allow us to understand the behavior of such networks and we apply an information-theoretic approach to different interference scenarios and traffic characteristics. We have tried to remain as close as possible to practical systems to be able to test the feasibility of the proposed techniques. The thesis deals with performance evaluation of interference scenarios in 4G networks, in particular those arising from small-cell deployments. The work in this thesis also deals with analysis of resource-allocation and incremental-redundancy based hybrid automatic repeat request (HARQ) for bursty interference (or more general time-varying channels) which allows for only partial channel state information at the transmitter. The work is then applied to practical scheduler design for LTE base stations and includes performance analysis for real LTE modems.

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