Channel Fading in Mobile Broadband Systems: Challenges and Opportunities

High-speed data signals transmitted over mobile broadband channels are seriously distorted by both time-varying effect and frequency-selective fading (FSF). These distortions introduce challenges since channel variances in both time-domain and frequency-domain form a two-dimensional channel matrix which is hard to estimate, but meanwhile provide opportunities for information security since all signals are directly encrypted by the channels which are adequately random over time, frequency and space. These challenges and opportunities are studied in this thesis as two parts. In the first part, we propose a novel time-varying channel estimation (TVCE) algorithm named piece-wise time-invariant approximation (PITIA) to estimate a typical type of mobile broadband channels - the high-speed train (HST) channels. PITIA customizes general time-varying channel models according to HST channels' specific features, and outperforms conventional TVCE algorithms by about 3-dB in terms of estimation error. In the second part, we propose the first physical-layer challenge-response authentication mechanism (PHY-CRAM) which uses the mobile broadband channels to prevent eavesdropping during authentication. Since pilots and reference signals are eliminated, eavesdroppers cannot demodulate credential information, while legitimate receivers use the channels' reciprocal property to cancel FSF. PITIA is evaluated by computer based simulations, and the effectiveness of PHY-CRAM is validated by prototyping and real-world experiments. Both pieces of works are built upon a unified system model and orthogonal frequency-division multiplexing (OFDM) modulation.Ph.D.College of Engineering and Computer ScienceUniversity of Michigan-Dearbornhttp://deepblue.lib.umich.edu/bitstream/2027.42/106584/1/Dissertation_Dan_Shan.pd

Contribution to the optimization of 4G mobile communications by means of advanced carrier aggregation strategies

Mobile broadband subscriptions and data traffic have increasingly grown in the past years with the deployment of the 3G and 4G technologies and the massive use of mobile devices. In this sense, LTE-A has been presented as the next step in wireless communications where higher data rates are targeted and fully packet switched services are held. The ultimate goal of 4G and the forthcoming 5G technology is to increase the Quality of Experience (QoE) of users. In this context, several challenges open up to face the increased bandwidth demands in both uplink (UL) and downlink (DL). To this end, LTE-A has proposed the use of Carrier Aggregation (CA) which allows the simultaneous data transmission in separate fragments of spectrum. The improvements brought by CA in the DL can be almost straightforward appreciable, since the evolved Node B (eNB) is in charge of transmissions, and power availability is not typically an issue. Conversely, the UL presents many open challenges to introduce aggregated transmissions, since it relies on the user terminal for transmission procedures. Lower transmission power and increased interference variability turn the UL more complex than the DL. For this reason, this Ph.D. thesis provides a contribution to the field of CA for UL mobile systems. The novelties here presented address the main limitations the UL encounters when introducing CA; new methods and strategies are proposed with the final aim of enhancing the UL communications with the use of increased bandwidth transmissions, and reducing the unbalanced data rate between the UL and DL. Throughout an exhaustive literature review, the main research opportunities to successfully implement CA in the UL were identified. In particular, three main blocks can be recognized. First, the need for introducing intelligent Radio Resource Management procedures that provide the user with increased QoE, specially in the cell edge, where users are more likely to be power limited, and CA is typically discarded. Consequently, the first part of this dissertation places emphasis on topics related to scheduling and the power limitations to face the increased bandwidth. In this sense, mechanisms that tackle the throughput improvement are proposed and scheduling schemes that specifically assess the gain or deterioration of CA are designed. Indeed, these strategies strongly rely on an accurate Channel State Information (CSI); it is of utmost importance to possess precise CSI to effectively support these assessments. In this line, the second part deals with the imperfect CSI where the efficient use of reference signals provides a high value. Channel prediction techniques have been proposed with the use of the splines method. However, the increased variability of interferences and the high delay in measurements still impairs the CSI accuracy. In this manner, interference management methods are introduced to support the CSI acquisition process. Finally, since CA constitutes the most transverse topic of the new features added to the 4G standard, the last block of research focuses on the opportunities that emerge with the use of CA in the context of heterogeneous networks, and new system designs are addressed. It is proposed to use dual connectivity in the form of decoupled uplink and downlink connections in a CA context, where aggregated carriers may have different coverage footprints. An analysis of two different cell association cases that arise has been driven. Stochastic geometry is used to study the system analytically, propagation conditions in the different tiers and frequencies are considered and the different association cases are compared to a classical downlink received power association rule. Conclusions show that decoupling the uplink provides the system with outstanding gains, however, being connected to the cell that receives the highest received power may not always be profitable, since issues like interferences or load conditions shall be also considered.El número de usuarios móviles y el tráfico de datos generado han aumentado en los últimos años con el despliegue de redes 3G y 4G y el uso masivo de dispositivos móviles. De este modo, LTE-A surge como el siguiente escalón de las comunicaciones móviles, dónde se apunta a mayores velocidades de transmisión y los servicios se basan en la conmutación de paquetes. El objetivo principal de las redes 4G y de la inminente red 5G es mejorar la experiencia del usuario. En este contexto, se presentan nuevos retos para hacer frente a las demandas de incrementar el ancho de banda en ambos enlaces: ascendente (UL) y descendente (DL). Por ello, LTE-A propone el uso de portadoras agregadas (Carrier Aggregation (CA)), tecnología que permite la transmisión simultánea en dos fragmentos del espectro. Las mejoras que aporta CA en el DL son casi inmediatas dado que las transmisiones corren a cargo de la base, la cual no sufre la falta de potencia. Al contrario, el UL presenta más retos para introducir CA, ya que es el terminal quién se encarga de la transmisión. La baja disponibilidad de potencia y la alta variabilidad de la interferencia lo convierten en un entorno mucho más complejo. Por ello, esta disertación presenta una contribución al campo de CA en el UL de comunicaciones móviles. Las novedades presentadas tratan las principales limitaciones para incorporar CA; se proponen nuevos métodos y estrategias con el objetivo de mejorar las comunicaciones en el UL mediante el uso de CA; todo ello, para reducir el desajuste que existe entre la velocidad de transmisión del UL y DL. Mediante una extensa revisión de la literatura, se han detectado las principales líneas de investigación y potenciales mejoras para incorporar CA exitosamente. Se han identificado tres grandes bloques de investigación. Primero, la necesidad de introducir estrategias de gestión de recursos inteligentes, que proporcionen al usuario una mejora de la experiencia, especialmente en el límite de la celda. Es allí donde los usuarios tienen una mayor probabilidad de estar limitados en potencia, razón por la que se les aparta de CA. Consecuentemente, la primera parte de esta tesis pone énfasis en la asignación de recursos y las limitaciones en potencia por parte del usuario para hacer frente a un incremento del ancho de banda. Se proponen mecanismos que mejoran la velocidad de transmisión evaluando las ganancias o pérdidas de incorporar CA a la transmisión. Para apoyar el funcionamiento de estas estrategias de asignación, y asegurar su máximo rendimiento, es necesario un método que proporcione un conocimiento preciso y fidedigno del estado del canal (Channel State Information (CSI)). De este modo, la segunda parte de la investigación lidia con el CSI, donde el uso eficiente de las señales de referencia es de gran importancia. Se proponen técnicas de predicción de señal mediante el uso de Splines; sin embargo, la alta variabilidad de las interferencias y el gran retardo entre dos muestras de CSI perjudican la precisión. Por ello, se introducen métodos de gestión de interferencias que apoyan el proceso de adquisición del CSI. Finalmente, dado que CA es una de las funciones más transversales de las introducidas por el estándar 4G, la última parte de investigación se centra en las oportunidades que surgen con su uso en las redes heterogéneas. Se propone el uso de la conectividad dual, desacoplando el UL del DL junto con CA, donde el área de cobertura de las portadoras puede ser diferente. Se analizan dos escenarios de asociación posibles. Con el uso de geometría estocástica se estudia analíticamente el sistema, considerando diferentes condiciones de propagación en los distintos tipos de celda y frecuencias; los escenarios de asociación se comparan a uno tradicional, en el cual los usuarios se asocian en función de la potencia recibida de las bases. Las conclusiones destacan que el desacoplo aporta mejoras en el UL. Sin embargo, temas como interferencias o carga deben también considera