INTERFERENCE MANAGEMENT IN LTE SYSTEM AND BEYOUND

The key challenges to high throughput in cellular wireless communication system are interference, mobility and bandwidth limitation. Mobility has never been a problem until recently, bandwidth has been constantly improved upon through the evolutions in cellular wireless communication system but interference has been a constant limitation to any improvement that may have resulted from such evolution. The fundamental challenge to a system designer or a researcher is how to achieve high data rate in motion (high speed) in a cellular system that is intrinsically interference-limited. Multi-antenna is the solution to data on the move and the capacity of multi-antenna system has been demonstrated to increase proportionally with increase in the number of antennas at both transmitter and receiver for point-to-point communications and multi-user environment. However, the capacity gain in both uplink and downlink is limited in a multi-user environment like cellular system by interference, the number of antennas at the base station, complexity and space constraint particularly for a mobile terminal. This challenge in the downlink provided the motivation to investigate successive interference cancellation (SIC) as an interference management tool LTE system and beyond. The Simulation revealed that ordered successive interference (OSIC) out performs non-ordered successive interference cancellation (NSIC) and the additional complexity is justified based on the associated gain in BER performance of OSIC. The major drawback of OSIC is that it is not efficient in network environment employing power control or power allocation. Additional interference management techniques will be required to fully manage the interference.fi=Opinnäytetyö kokotekstinä PDF-muodossa.|en=Thesis fulltext in PDF format.|sv=Lärdomsprov tillgängligt som fulltext i PDF-format

Recent Advances in Wireless Communications and Networks

This book focuses on the current hottest issues from the lowest layers to the upper layers of wireless communication networks and provides "real-time" research progress on these issues. The authors have made every effort to systematically organize the information on these topics to make it easily accessible to readers of any level. This book also maintains the balance between current research results and their theoretical support. In this book, a variety of novel techniques in wireless communications and networks are investigated. The authors attempt to present these topics in detail. Insightful and reader-friendly descriptions are presented to nourish readers of any level, from practicing and knowledgeable communication engineers to beginning or professional researchers. All interested readers can easily find noteworthy materials in much greater detail than in previous publications and in the references cited in these chapters

Allocation designs for massive multiple access with interference cancellation

In the transition towards the next generation of wireless technology systems, the increasing number of devices curbs the potential of current wireless networks to cope with such increases in network density. Wireless communications via satellite constitute a cost effective option to achieve high transmission reliability in remote areas or to create resilient networks to be used in emergency situations. To counterbalance the growing network density, one of the main goals in the uplink is to increase the spectral efficiency of the network. By working on the application of non-orthogonal multiple access and the exploitation of the collision domain through interference cancellation, this dissertation tackles the problem of massive multiple access. A consensual scheme that meets the main goal and the aim of reducing the interaction between devices and the satellite in the control plane is Enhanced Spread Spectrum ALOHA, which combines spreading-based short-packet transmissions with successive interference cancellation (SIC) on the receiver's side. This combination opens up several design avenues in terms of energy and code allocation to users when a certain amount of channel state information is available to them. Motivated by this scheme, this thesis studies the best allocation strategies when the SIC receiver operates nonideally: firstly, it investigates a system model for a receiver that, inspired by the demodulator adopted in the Enhanced Spread Spectrum ALOHA system, deals with the problems of user ordering and iterative decoding with short packets; and secondly, it delves into the user-asymptotic regime and the application of the calculus of variations to derive the stationary point equations corresponding to the optimal allocation rules.The first part of this thesis investigates the impact of nonideal decoding and imperfect cancellation on the first iteration of a SIC receiver aided by redundancy-check error control. The system model characterises both non-idealities using known functions of the signal-to-interference-plus-noise ratio. The propagation of packet decoding success/failure events throughout the stages of the receiver is circumvented in the user-asymptotic regime, since the model takes a deterministic form. The asymptotically optimal energy and rate allocation is studied for a wide variety of cases. The second part of this thesis investigates an iterative SIC receiver and extends the allocation designs derived previously to iterations beyond the first. The derivation of a system model is challenging, since each iteration of the receiver operates with memory with respect to the previous ones, and due to the fact that the decoding operations for the same user in different iterations are statistically dependent. This thesis motivates and states a system model that solves said difficulties by adding minimal complexity to the one adopted previously. The user-asymptotic regime is investigated to reveal mathematical forms to the above model that allow for a thorough understanding of the adopted receiver. Finally, the chapter exploits the user-asymptotic model and conducts research to designing smooth allocation functions. The third part of this thesis studies the user-ordering problem for a SIC receiver to which the strengths received from all users are unknown. The thesis derives an accurate system model for a large-user SIC receiver, which proceeds to order users after estimating their symbol energies at the initial stage through preamble cross-correlations. Analytical findings are determined in the user-asymptotic regime. The asymptotically optimal energy allocation is shown to obey, in contrast to the practically exponential user-energy distributions obtained before, a piecewise constant function; fact that entails great computational advantages of its application.En la transició cap a la pròxima generació de sistemes tecnològics sense fils, el creixent nombre de dispositius frena el potencial de les xarxes sense fils actuals per fer front a tal augment en la densitat de xarxa. Les comunicacions sense fils via satèl·lit constitueixen una opció rentable per assolir una fiabilitat de transmissió alta en zones remotes o per crear xarxes que puguin ser utilitzades en situacions d'emergència. Per contrarestar la creixent densitat de xarxa, un dels objectius principals en l'enllaç ascendent és augmentar l'eficiència espectral d'aquesta. Aquesta tesi aborda el problema d'accés múltiple massiu combinant l'aplicació de tècniques d'accés múltiple no ortogonal amb esquemes de cancel·lació d’interferència. Un esquema consensuat que acompleix amb l’objectiu principal i amb la fita de reduir la interacció entre dispositius i satèl·lit en el pla de control és Enhanced Spread Spectrum ALOHA, que combina transmissions de paquets curts basades en l'eixamplament del senyal amb la cancel·lació successiva d'interferències (SIC) en recepció. Aquesta combinació obre diverses vies per l'assignació d'energia i codi als diferents usuaris quan aquests disposen d’informació sobre l'estat del canal. Motivat per l'esquema anterior, aquesta tesi estudia les millors estratègies d'assignació quan s'adopta un receptor SIC no ideal: en primer lloc, investiga un model de sistema per un receptor SIC que, inspirat en el desmodulador adoptat en el sistema Enhanced Spread Spectrum ALOHA, aborda els problemes d'ordenació d'usuaris i de descodificació iterativa amb paquets curts; i, en segon lloc, s’endinsa en el règim asimptòtic d'usuaris i en l'aplicació del càlcul de variacions per derivar les equacions de punt estacionari corresponents a les funcions d'assignació òptimes. La primera part d'aquesta tesi investiga l'impacte de la descodificació no ideal i de la cancel·lació imperfecta en la primera iteració d'un receptor SIC assistit per control d'errors. El model de sistema proposat caracteritza ambdues no idealitats fent ús de funcions conegudes de la relació senyal-a-soroll-més-interferència. La propagació dels esdeveniments d'èxit/fracàs en la descodificació de paquets al llarg de les etapes del receptor s'aborda en el règim asimptòtic d'usuaris, ja que el model pren forma determinista. Les funcions d'assignació s'estudien en el règim asimptòtic d'usuaris per varis casos. La segona part de la tesi investiga un receptor SIC iteratiu i estén les assignacions derivades en el capítol anterior per a iteracions del SIC més enllà de la primera. La derivació d'un model de sistema suposa un repte, ja que cada iteració del receptor opera amb memòria respecte a iteracions anteriors i degut a que les operacions de descodificació per a un mateix usuari en iteracions diferents són estadísticament dependents. Es proposa un model de sistema que resol tals dificultats afegint complexitat mínima al model adoptat anteriorment. S'investiga el règim asimptòtic d'usuaris amb l'objectiu d’evidenciar expressions matemàtiques del model que permetin la completa comprensió del receptor adoptat. Per últim, es dissenyen funcions d'assignació contínuament diferenciables fent ús del model asimptòtic anterior. La tercera i última part d'aquesta tesi estudia el problema d'ordenació d'usuaris aplicat a un receptor SIC que desconeix les potències rebudes de tots ells. Es deriva un model de sistema per un receptor que gestiona nombrosos usuaris i els ordena després d'estimar les energies de tots ells en l'etapa inicial mitjançant correlacions de preamble. Els resultats analítics s’obtenen en el règim asimptòtic d'usuaris. Es demostra que, contràriament a les distribucions pràcticament exponencials obtingudes anteriorment, l'assignació d'energia òptima derivada per a infinits usuaris presenta una estructura constant a trossos; fet que comporta grans avantatges computacionals en la seva aplicació.En la transición hacia la próxima generación de sistemas tecnológicos inalámbricos, el creciente número de dispositivos frena el potencial de las redes inalámbricas actuales para hacer frente a esos aumentos en la densidad de red. Impulsadas por las innovaciones en tecnología satelital, las comunicaciones inalámbricas vía satélite constituyen una opción rentable para lograr una alta fiabilidad de transmisión en zonas remotas o para crear redes reservadas para situaciones de emergencia. Para contrarrestar la creciente densidad de la red, uno de los objetivos principales en el enlace ascendente es aumentar la eficiencia espectral de la misma. En favor de este objetivo, se identifican tres técnicas no excluyentes: (i) la aplicación de técnicas de acceso múltiple no ortogonal, para hacer frente a la limitada disponibilidad de recursos ortogonales requeridos en el acceso múltiple convencional, (ii) la explotación del dominio de colisión por el receptor, mediante la cancelación de interferencias, y (iii) la utilización de satélites multihaz, que, usando la tecnología multiantena, permiten una reutilización más eficiente del dominio espacial. Esta tesis aborda el problema de acceso múltiple masivo trabajando en los dos primeros puntos. Un esquema consensuado que cumple con el objetivo principal y con el fin de reducir la interacción entre los dispositivos y el satélite en el plano de control es Enhanced Spread Spectrum ALOHA, que combina transmisiones de paquetes cortos basadas en el ensanchamiento de la señal con la cancelación sucesiva de interferencias (SIC) en recepción. Esta combinación abre diversas vías para la asignación de energía y código a los usuarios cuando estos disponen de cierta información sobre el estado del canal. Motivado por el esquema anterior, esta tesis reexamina resultados previos bajo análisis teóricos de capacidad y cancelación perfecta, y estudia las mejores estrategias de asignación cuando el receptor SIC opera de forma no ideal. Los análisis anteriores se amplían en dos frentes: en primer lugar, adoptando políticas de decodificación y cancelación adaptadas para paquetes cortos; y, en segundo lugar, explorando el desequilibrio de energía, tasa de transmisión y fiabilidad. Con respecto al primer punto, esta tesis investiga un modelo de sistema para un receptor SIC que, inspirado en el demodulador adoptado en el sistema Enhanced Spread Spectrum ALOHA, aborda los problemas de ordenación de usuarios y decodificación iterativa con paquetes cortos. En cuanto al segundo punto, esta tesis se adentra en el régimen asintótico de usuarios y en la aplicación del cálculo de variaciones para derivar las ecuaciones de punto estacionario correspondientes a las funciones de asignación óptimas. Una de las principales contribuciones de esta tesis es el descubrimiento de funciones discontinuas (continuamente diferenciables a trozos) como una clase de distribuciones de energía ordenada para maximizar la eficiencia espectral; un enfoque que ha demostrado ser abrumadoramente exitoso. En concreto, el modelo derivado en la presente tesis incorpora, progresivamente y a lo largo de tres capítulos independientes, aspectos prácticos del cancelador de interferencias adoptado: 1. La primera parte de esta tesis investiga el impacto de la decodificación no ideal y de la cancelación imperfecta en la primera iteración de un receptor SIC asistido por control de errores. El modelo de sistema caracteriza ambas no idealidades utilizando funciones conocidas de la relación señal-a-ruido-más-interferencia (SINR) bajo la suposición de interferencia gaussiana: las funciones tasa de error de paquete (PER) y energía residual. La propagación de los eventos de éxito/fracaso en la decodificación de paquetes a lo largo de las etapas del receptor SIC se sortea en el régimen asintótico de usuarios, puesto que el modelo de sistema adopta expresiones deterministas. La asignación de energía y código se estudia en el régimen asintótico de usuarios para una amplia variedad de casos, incluyendo conjuntos formados por un número finito o infinito de esquemas de modulación y corrección de errores para paquetes de longitud finita e infinita. 2. La segunda parte de esta tesis investiga un receptor SIC iterativo y extiende las asignaciones derivadas anteriormente para iteraciones del SIC más allá de la primera. La derivación de un modelo para tal sistema supone un reto, ya que cada iteración del receptor opera con memoria respecto a las anteriores y porque las operaciones de decodificación para un mismo usuario en distintas iteraciones son estadísticamente dependientes. Esta tesis propone justificadamente un modelo de sistema que resuelve dichas dificultades añadiendo complejidad mínima al adoptado con anterioridad. En concreto, el modelo usa funciones PER multivariable, cuyos argumentos corresponden a las SINRs que experimenta un usuario a lo largo de las iteraciones del receptor, y define biyecciones para relacionar los índices de los usuarios que permanecen decodificados sin éxito en cada iteración. Se investiga el régimen asintótico de usuarios para revelar expresiones matemáticas del modelo anterior que permitan un completo entendimiento del receptor adoptado. Por último, se investiga el diseño de funciones de asignación continuamente diferenciables con extremos libres haciendo uso del modelo asintótico anterior. 3. La tercera y última parte de esta tesis estudia el problema de ordenación de usuarios en un receptor SIC que desconoce las potencias recibidas de todos ellos. La tesis deriva un modelo de sistema para un receptor SIC que gestiona un gran número de usuarios y los ordena tras estimar sus energías en la etapa inicial mediante correlaciones de preámbulo. En el régimen asintótico de usuarios, se obtienen resultados analíticos en los que el rendimiento del sistema se rige por un kernel conocido. Se demuestra que, contrariamente a las distribuciones prácticamente exponenciales obtenidas anteriormente, la asignación óptima de energía derivada para un número infinito de usuarios obedece una función constante a trozos; hecho que conlleva grandes ventajas computacionales en su aplicación.Postprint (published version

Low-Complexity Algorithms for Channel Estimation in Optimised Pilot-Assisted Wireless OFDM Systems

Orthogonal frequency division multiplexing (OFDM) has recently become a dominant transmission technology considered for the next generation fixed and mobile broadband wireless communication systems. OFDM has an advantage of lessening the severe effects of the frequency-selective (multipath) fading due to the band splitting into relatively flat fading subchannels, and allows for low-complexity transceiver implementation based on the fast Fourier transform algorithms. Combining OFDM modulation with multilevel frequency-domain symbol mapping (e.g., QAM) and spatial multiplexing (SM) over the multiple-input multiple-output (MIMO) channels, can theoretically achieve near Shannon capacity of the communication link. However, the high-rate and spectrumefficient system implementation requires coherent detection at the receiving end that is possible only when accurate channel state information (CSI) is available. Since in practice, the response of the wireless channel is unknown and is subject to random variation with time, the receiver typically employs a channel estimator for CSI acquisition. The channel response information retrieved by the estimator is then used by the data detector and can also be fed back to the transmitter by means of in-band or out-of-band signalling, so the latter could adapt power loading, modulation and coding parameters according to the channel conditions. Thus, design of an accurate and robust channel estimator is a crucial requirement for reliable communication through the channel, which is selective in time and frequency. In a MIMO configuration, a separate channel estimator has to be associated with each transmit/receive antenna pair, making the estimation algorithm complexity a primary concern. Pilot-assisted methods, relying on the insertion of reference symbols in certain frequencies and time slots, have been found attractive for identification of the doubly-selective radio channels from both the complexity and performance standpoint. In this dissertation, a family of the reduced-complexity estimators for the single and multiple-antenna OFDM systems is developed. The estimators are based on the transform-domain processing and have the same order of computational complexity, irrespective of the number of pilot subcarriers and their positioning. The common estimator structure represents a cascade of successive small-dimension filtering modules. The number of modules, as well as their order inside the cascade, is determined by the class of the estimator (one or two-dimensional) and availability of the channel statistics (correlation and signal-to-noise power ratio). For fine precision estimation in the multipath channels with statistics not known a priori, we propose recursive design of the filtering modules. Simulation results show that in the steady state, performance of the recursive estimators approaches that of their theoretical counterparts, which are optimal in the minimum mean square error (MMSE) sense. In contrast to the majority of the channel estimators developed so far, our modular-type architectures are suitable for the reconfigurable OFDM transceivers where the actual channel conditions influence the decision of what class of filtering algorithm to use, and how to allot pilot subcarrier positions in the band. In the pilot-assisted transmissions, channel estimation and detection are performed separately from each other over the distinct subcarrier sets. The estimator output is used only to construct the detector transform, but not as the detector input. Since performance of both channel estimation and detection depends on the signal-to-noise power vi ratio (SNR) at the corresponding subcarriers, there is a dilemma of the optimal power allocation between the data and the pilot symbols as these are conflicting requirements under the total transmit power constraint. The problem is exacerbated by the variety of channel estimators. Each kind of estimation algorithm is characterised by its own SNR gain, which in general can vary depending on the channel correlation. In this dissertation, we optimise pilot-data power allocation for the case of developed low-complexity one and two-dimensional MMSE channel estimators. The resultant contribution is manifested by the closed-form analytical expressions of the upper bound (suboptimal approximate value) on the optimal pilot-to-data power ratio (PDR) as a function of a number of design parameters (number of subcarriers, number of pilots, number of transmit antennas, effective order of the channel model, maximum Doppler shift, SNR, etc.). The resultant PDR equations can be applied to the MIMO-OFDM systems with arbitrary arrangement of the pilot subcarriers, operating in an arbitrary multipath fading channel. These properties and relatively simple functional representation of the derived analytical PDR expressions are designated to alleviate the challenging task of on-the-fly optimisation of the adaptive SM-MIMO-OFDM system, which is capable of adjusting transmit signal configuration (e.g., block length, number of pilot subcarriers or antennas) according to the established channel conditions

Radio Communications

In the last decades the restless evolution of information and communication technologies (ICT) brought to a deep transformation of our habits. The growth of the Internet and the advances in hardware and software implementations modified our way to communicate and to share information. In this book, an overview of the major issues faced today by researchers in the field of radio communications is given through 35 high quality chapters written by specialists working in universities and research centers all over the world. Various aspects will be deeply discussed: channel modeling, beamforming, multiple antennas, cooperative networks, opportunistic scheduling, advanced admission control, handover management, systems performance assessment, routing issues in mobility conditions, localization, web security. Advanced techniques for the radio resource management will be discussed both in single and multiple radio technologies; either in infrastructure, mesh or ad hoc networks

Mobile and Wireless Communications

Mobile and Wireless Communications have been one of the major revolutions of the late twentieth century. We are witnessing a very fast growth in these technologies where mobile and wireless communications have become so ubiquitous in our society and indispensable for our daily lives. The relentless demand for higher data rates with better quality of services to comply with state-of-the art applications has revolutionized the wireless communication field and led to the emergence of new technologies such as Bluetooth, WiFi, Wimax, Ultra wideband, OFDMA. Moreover, the market tendency confirms that this revolution is not ready to stop in the foreseen future. Mobile and wireless communications applications cover diverse areas including entertainment, industrialist, biomedical, medicine, safety and security, and others, which definitely are improving our daily life. Wireless communication network is a multidisciplinary field addressing different aspects raging from theoretical analysis, system architecture design, and hardware and software implementations. While different new applications are requiring higher data rates and better quality of service and prolonging the mobile battery life, new development and advanced research studies and systems and circuits designs are necessary to keep pace with the market requirements. This book covers the most advanced research and development topics in mobile and wireless communication networks. It is divided into two parts with a total of thirty-four stand-alone chapters covering various areas of wireless communications of special topics including: physical layer and network layer, access methods and scheduling, techniques and technologies, antenna and amplifier design, integrated circuit design, applications and systems. These chapters present advanced novel and cutting-edge results and development related to wireless communication offering the readers the opportunity to enrich their knowledge in specific topics as well as to explore the whole field of rapidly emerging mobile and wireless networks. We hope that this book will be useful for students, researchers and practitioners in their research studies