Beamforming Coordination Techniques in OFDM Multi-hop Cellular Networks

English: Design and evaluate multi-hop cellular networks assuming coordination among multiantenna base stations and relay stations using linear precoders .Castellano: Diseñar y evaluar redes celulares multisalto asumiendo coordinación entre estaciones base y repetidores multiantena usando precodificadores lineales .Català: Dissenyar i avaluar xarxes cel·lulars multisalt assumint coordinació entre estacions base i repetidors multiantena fent servir precodificadors lineals

Beamforming Coordination Techniques in OFDM Multi-hop Cellular Networks

English: Design and evaluate multi-hop cellular networks assuming coordination among multiantenna base stations and relay stations using linear precoders .Castellano: Diseñar y evaluar redes celulares multisalto asumiendo coordinación entre estaciones base y repetidores multiantena usando precodificadores lineales .Català: Dissenyar i avaluar xarxes cel·lulars multisalt assumint coordinació entre estacions base i repetidors multiantena fent servir precodificadors lineals

Els graus de llibertat del canal interferent de 3 usuaris MIMO (p,p+1)

Characterizing the degrees of freedom for the 3-user interference channel in case of channel coefficients do not vary along time and each terminal is equipped with p antennas (transmitters) and p+1 antennas (receivers). To this end, interference alignment concepts, time extension and asymmetric complex signalling concepts are implemented. Furthermore, the achievability of the DoF outer bound by using the proposed scheme is analytically proved.[ANGLÈS] This work is focused on the study of the fundamental limits of wireless channels. In this context, the degrees of freedom (DoF) are a measure to analyze those limits at the high SNR regime. The contribution of this work is to characterize the DoF for the interference channel by using linear transmitters and receivers, and assuming that the channel coefficients are constant along the transmission time, thus establishing a result previously unknown.[CASTELLÀ] Este trabajo se centra en el estudio de los límites fundamentales de las redes inalámbricas. En este sentido, los grados de libertad suponen una medida para analizar estos límites cuando la relación senyal a ruido es muy alta. La contribución de este trabajo es caracterizar los grados de libertad para el canal interference usando transmisores y receptores lineales, y asumiendo que el canal es constante a lo largo de la transmisión, obteniendo resultados hasta el momento desconocidos.[CATALÀ] Aquell treball se centra en l'estudi dels límits fonamentals de les xarxes sense fils. En aquest sentit, els graus de llibertat suposen una mesura per a analitzar aquests límits quan la relació senyal-soroll és molt alta. La contribució d' aquest treball és caracteritzar els graus de llibertat per al canal interferent fent servir transmissors i receptors lineals i assumint que el canal és constant al llarg de la transmissió, obtenint resultats fins al moment desconeguts

Els graus de llibertat del canal interferent de 3 usuaris MIMO (p,p+1)

Characterizing the degrees of freedom for the 3-user interference channel in case of channel coefficients do not vary along time and each terminal is equipped with p antennas (transmitters) and p+1 antennas (receivers). To this end, interference alignment concepts, time extension and asymmetric complex signalling concepts are implemented. Furthermore, the achievability of the DoF outer bound by using the proposed scheme is analytically proved.[ANGLÈS] This work is focused on the study of the fundamental limits of wireless channels. In this context, the degrees of freedom (DoF) are a measure to analyze those limits at the high SNR regime. The contribution of this work is to characterize the DoF for the interference channel by using linear transmitters and receivers, and assuming that the channel coefficients are constant along the transmission time, thus establishing a result previously unknown.[CASTELLÀ] Este trabajo se centra en el estudio de los límites fundamentales de las redes inalámbricas. En este sentido, los grados de libertad suponen una medida para analizar estos límites cuando la relación senyal a ruido es muy alta. La contribución de este trabajo es caracterizar los grados de libertad para el canal interference usando transmisores y receptores lineales, y asumiendo que el canal es constante a lo largo de la transmisión, obteniendo resultados hasta el momento desconocidos.[CATALÀ] Aquell treball se centra en l'estudi dels límits fonamentals de les xarxes sense fils. En aquest sentit, els graus de llibertat suposen una mesura per a analitzar aquests límits quan la relació senyal-soroll és molt alta. La contribució d' aquest treball és caracteritzar els graus de llibertat per al canal interferent fent servir transmissors i receptors lineals i assumint que el canal és constant al llarg de la transmissió, obtenint resultats fins al moment desconeguts

Transmission strategies for interfering networks with finite rate and outdated channel feedback

The emergence of very capable mobile terminals, e.g. smartphones or tablets, has dramatically increased the demand of wireless data traffic in recent years. Current growth forecasts elucidate that wireless communication standards will not be able to afford future traffic demands, thus many research efforts have been oriented towards increasing the efficiency of wireless networks. MIMO technologies, entailing the use of multiple antennas, stand as one of the candidates. This solution allows increasing not only the reliability and robustness (diversity gain), but also the efficiency of the communication (multiplexing gain or degrees of freedom (DoF)). The DoF describe the slope of channel capacity at very high signal-to-noise-ratio (SNR) regime, and for the point-to-point (P2P) channel are equal to the minimum between the number of antennas at the transmitter and the receiver. Consequently, the throughput may be scaled in a promising way. However, the DoF behavior in case of having interference is still an open problem in general. This thesis studies the DoF of interference networks. The most trivial way of tackling this problem is by means of orthogonalization, either in time, frequency or space. However, it does not allow that the scaling of DoF with the number of users. For example, if transmissions are orthogonalized in time each user is served only a fraction of time inversely proportional to the number of users. Likewise, if transmissions are orthogonalized in space, transmitters must be equipped with a large number of antennas, which is costly and not practical. For dimensionally-limited systems, the interference alignment (IA) principle proposes that instead of forcing the design to null the interference terms at the receivers, make the receiver observe them overlapped. This way the number of dimensions occupied by interference is reduced, thus allowing the allocation of more desired signals, thus symbols per user, and also relaxing the constraint on the number of required antennas. Following IA allows that "each user achieves half the cake independently of the number of users", where the cake represents the DoF of the P2P channel. At first, full channel state information was assumed to be available at the transmitter side (full CSIT), i.e. the information is acquired instantaneously, and with perfect quality. The first part of this thesis studies this case and completes the DoF characterization of the 3-user MIMO interference channel for some antenna configurations when channel coefficients are assumed constant. In practice, CSIT should be obtained from channel feedback, thus incurring delays and errors. In this context, and especially intended to scenarios with high mobility, IA concepts were extended to networks where only outdated information of the channels is available, a framework known as delayed CSIT where the channel feedback delay may be larger than the channel coherence time. This form of IA is denoted as retrospective interference alignment, since the transmission is carried out in multiple phases, and signals may be aligned along space and the different phases. The second part of the thesis deepens into the DoF of two network topologies with delayed CSIT, for which linear precoding strategies are proposed. Moreover, it is shown that the proposed strategies are better than state-of-the-art in terms of DoF-delay trade-off, which is relevant as most strategies based on delayed CSIT entail long communication delays. The concluding part of the thesis analyses how one of schemes proposed in the second part performs in terms of DoF with delayed CSIT with errors, and net DoF. This last metric describes the DoF as a function of the coherence time, and taking into account all issues related to channel acquisition at both the transmitter and receiver side: consumption of resources for channel training, for feedback transmission, and feedback waits.La aparición de terminales móviles cada vez más potentes, e.g. smartphones o tablets, ha incrementado dramáticamente la demanda de tráfico de datos en redes inalámbricas en los últimos años. Las previsiones de crecimiento ya avisan que los estándares de comunicación actuales no serán capaces de sostener las demandas de tráfico en un futuro, por lo que se está investigando como aumentar la eficiencia de las redes inalámbricas. La tecnología MIMO, es decir, el uso de múltiples antenas, destaca como uno de los candidatos, permitiendo no sólo aumentar la fiabilidad y robustez (ganancia de diversidad), sino también la eficiencia de la comunicación (ganancia de multiplexado o grados de libertad (DoF)). Los DoF describen la pendiente de la capacidad del canal a muy alta relación señal a ruido (SNR), y para el canal punto a punto (P2P) son igual al mínimo entre el número de antenas en transmisión y recepción. Así pues, el rendimiento de la comunicación inalámbrica se escala de forma muy prometedora. Esta tesis aporta contribuciones en el conocimiento de los DoF para comunicaciones con interferencia (debido a que se sirven múltiples usuarios), cuyo comportamiento es aún un problema por resolver. La forma más trivial de abordar este problema es ortogonalizando las transmisiones, ya sea en tiempo, frecuencia o espacio. Sin embargo, no permite que los DoF escalen con el número de usuarios. Por ejemplo, si se ortogonaliza en tiempo, cada usuario se sirve sólo una fracción de tiempo inversamente proporcional a la cantidad de usuarios. Asimismo, si se ortogonaliza en espacio, se requerirán muchas antenas transmisoras, lo cual es costoso y poco práctico. La técnica de alineación de interferencia (IA) propone que en lugar de diseñar la transmisión para anular la interferencia en los receptores, estos observen todos los términos solapados. Esto permite reducir el espacio ocupado por interferencia, ensanchar el espacio de señales deseadas, pudiendo enviar más símbolos por usuario, y reducir el número de antenas requeridas. Así, IA permitió demostrar que los DoF escalan para el canal interferente con terminales equipado con una sola antena. Al principio, se suponía que los transmisores disponían de toda la información del estado del canal instantáneamente, y con calidad perfecta (CSIT completo). La primera parte de la tesis estudia este caso, y completa la caracterización de los DoF del canal interferente MIMO de 3 usuarios para ciertas configuraciones de antenas, cuando los coeficientes del canal son constantes en tiempo. En la práctica, el CSIT se debe obtener mediante reportes del canal, lo que conlleva retrasos y errores. En este contexto, y sobre todo dirigido a escenarios con alta movilidad, los conceptos de IA se extendieron al caso donde sólo está disponible información obsoleta del canal (delayed CSIT), e.g. cuando el retardo de los reportes es mayor el tiempo de coherencia del canal. Esta forma de IA se denota como retrospective IA, ya que la transmisión se lleva a cabo en múltiples fases, y las señales pueden ser alineadas a lo largo del espacio y las diferentes fases. La segunda parte de la tesis estudia los DoF de dos topologías de red con delayed CSIT, proponiendo estrategias de transmisión. Además, se demuestra que las estrategias propuestas superan el estado del arte en términos del compromiso DoF-retardo, lo que es relevante ya que la mayoría de las estrategias basadas en delayed CSIT requieren muchos usos de canal. Finalmente, la última parte de la tesis describe cómo uno de los esquemas propuestos en la segunda parte trabaja con errores, y en términos de net DOF. Este último indicador describe los DoF en función del tiempo de coherencia, y teniendo en cuenta todas las cuestiones relacionadas con la adquisición del canal, tanto en el transmisor como en el receptor: el consumo de los recursos para estimar el canal en recepción , para la transmisión de los reportes de canal, y las esperas hasta obtener los reportes

Beamforming Coordination Techniques in OFDM Multi-hop Cellular Networks

English: Design and evaluate multi-hop cellular networks assuming coordination among multiantenna base stations and relay stations using linear precoders .Castellano: Diseñar y evaluar redes celulares multisalto asumiendo coordinación entre estaciones base y repetidores multiantena usando precodificadores lineales .Català: Dissenyar i avaluar xarxes cel·lulars multisalt assumint coordinació entre estacions base i repetidors multiantena fent servir precodificadors lineals

Performance analysis of inter-cell interference coordination in small-cell networks with long feedback delays

Current state-of-the-art approaches are evaluated for the downlink (DL) of an LTE-based wireless network when multiple Small-Cells (SC) eNBs coexist in the same area. With the aim of increasing data-rate performance of users, we consider that all SCeNBs reuse the same resources. In this situation, inter-cell interference (ICI) becomes the main impairment in the communication. One of the solution may be the use of time domain eICIC (TDM muting), allowing to avoid the ICI. An alternative strategy is to employ multiple-input multiple-output (MIMO) techniques if channel state information (CSI) at the transmitter (CSIT) can be exploited. Nevertheless, CSI reported by User Equipments (UEs) may suffer large delays, even higher than the channel coherence time, and non-perfect accuracy, where the errors are due to the adopted feedback schemes: digital or analog. In this context, the approach in [1] for the interference channel (IC), and assuming imperfect delayed CSIT is considered, getting insight into the benefits of the MIMO-based over the eICIC-based schemes.Peer ReviewedPostprint (published version

Retrospective interference alignment for the 3-user MIMO interference channel with delayed CSIT

The degrees of freedom (DoF) of the 3-user multiple input multiple output interference channel (3-user MIMO IC) are investigated where there is delayed channel state information at the transmitters (dCSIT). We generalize the ideas of Maleki et al. about Retrospective Interference Alignment (RIA) to be applied to the MIMO IC, where transmitters and receivers are equipped with (M, N) antennas, respectively. We propose a two-phase transmission scheme where the number of slots per phase and number of transmitted symbols are optimized by solving a maximization problem. Finally, we review the existing achievable DoF results in the literature as a function of the ratio between transmitting and receiving antennas ¿ = M/N. The proposed scheme improves all other strategies when ¿ (1/2, 31/32].Peer ReviewedPostprint (published version

On the degrees of freedom of the K-user MISO interference channel with imperfect delayed CSIT

This work investigates the degrees of freedom (DoF) of the K-user multiple-input single-output (MISO) interference channel (IC) with imperfect delayed channel state information at the transmitters (dCSIT). For this setting, new DoF inner bounds are provided, and benchmarked with cooperation-based outer bounds. The achievability result is based on a precoding scheme that aligns the interfering received signals through time, exploiting the concept of Retrospective Interference Alignment (RIA). The proposed approach outperforms all previous known schemes. Furthermore, we study the proposed scheme under channel estimation errors (CEE) on the reported dCSIT, and derive a closed-form expression for the achievable DoF with imperfect dCSIT.Peer Reviewe

Net DoF analysis for the K-user MISO IC with outdated and imperfect channel feedback

The net degrees of freedom (DoF ) describe the efficiency of a protocol in terms of network throughput at high signal-to-noise ratio (SNR). As the conventional DoF, it captures how the achievable user data rate grows as SNR increases, but also takes into account the limited channel state information at the transmitters (CSIT), and required protocol overheads. This work studies the impact of short channel coherence time for the K-user Multiple-Input Single-Output (MISO) Interference Channel (IC) by means of the net DoF. We propose different protocols, combining techniques that exploit different levels of CSIT (delayed, current or no CSIT), and elucidating which performs better as a function of user mobility.Peer Reviewe