Cognitive and signal processing techniques for improved spectrum exploitation in wireless communications (2014)

The increasing demand of wireless data makes the electromagnetic spectrum a scarce and valuable resource. Wireless networks should exploit its available spectrum in the most efficient way to maximize the amount of data they can support. Alternatives to increase the spectral efficiency of wireless networks include the design of new spectrum access paradigms and the operation of existing wireless links in the most efficient way. In this thesis we present two different approaches for increasing the spectral efficiency of wireless networks. The first approach is based on a cognitive-inspired spectrum access paradigm in which a secondary user is allowed to access the frequency bands owned by a primary user. This cognitive access has to be properly designed to avoid any possible degradation on the primary service. We advocate the use of the so-called overlay cognitive radio paradigm, in which primary and secondary users are allowed to cooperate. More precisely, we focus on the cases where the secondary transmitter knows the primary message, and spectrum access is allowed as long as the primary user service is not affected. We analyze overlay access over both point to point and broadcast primary networks, and incorporate some practical impairments into the problem. The second part of the thesis is focused on the design of adaptive transmit strategies under changing channel conditions, in a process that is known as link adaptation. The time varying nature of fading channels causes changes in the maximum rate they can support and, therefore, a transmitter that wants to efficiently exploit the wireless medium should be able to adapt its transmission rate depending on the channel conditions. We first design adaptation techniques for mobile satellite communications, where the main challenge is the inaccuracy of channel state information at the transmitter due to the large propagation delay. We propose the use of adaptation techniques based on statistical knowledge of the channel, and show how to exploit some degree of channel reciprocity even in frequency division duplexing systems. We then move to a different setting, and analyze the link adaptation problem in multiuser multiple input multiple output orthogonal frequency division multiplexing systems with limited feedback of the channel state. Adaptation is performed by combining machine learning techniques, greedy algorithms and interference estimation due to imperfect channel state information.En la actualidad, la creciente demanda de datos en redes inalambricas ha hecho que el espectro radioelectrico sea un recurso escaso y caro. Debido a este hecho, en los ultimos anos ha habido un creciente interes (tanto en el mundo academico como de la industria) en tecnicas que posibiliten un uso del espectro mas eficiente. En esta tesis se estudian distintos tipos de mecanismos que permitan aumentar la eficiencia espectral de las comunicaciones inalambricas. En primer lugar, se trataran tecnicas de acceso dinamico al espectro (Radio Cognitiva), centrandose especialmente en aquellos modos de acceso secundario en los que el usuario cognitivo conoce a priori el mensaje a transmitir por el usuario primario. Dentro de este tipo de acceso, se estudiara con especial interes el caso de permitir acceso secundario en redes de television digital terrestre bajo el estandar DVB-T. En segundo lugar, se estudiara el problema de la comunicacion adaptativa en estandares de ultima generacion, es decir, como modificar diferentes parametros de transmision para adaptarse a las condiciones cambiantes del canal inalambrico de forma que se maximice la tasa binaria manteniendo una calidad de servicio requerida.Na actualidade, a crecente demanda de datos en redes sen fios esta a facer que o espectro radioelectrico sexa un recurso escaso e caro. Debido a este feito, nos ultimos anos houbo un crecente interese (tanto no mundo academico como da industria) en tecnicas que posibiliten un uso do espectro mais eficiente. Nesta tese estudanse distintos tipos de mecanismos que permitan aumentar a eficiencia espectral das comunicacions sen fios. En primeiro lugar, trataranse tecnicas de acceso dinamico o espectro (Radio Cognitiva), centrandose especialmente naqueles modos de acceso secundario nos que o usuario cognitivo conece a priori a mensaxe a transmitir polo usuario primario. Dentro deste tipo de acceso, estuiarase con especial interese o caso de permitir acceso secundario en redes de television dixital terrestre baixo o estandar DVB-T. En segundo lugar, estudiarase o problema da comunicacion adaptativa en estandares de ultima xeracion, e dicir, como modificar diferentes parameteros de transmision para se adaptar as condicions cambiantes do canal sen fios de forma que se maximice a taxa binaria mantendo unha calidade de servizo requerida.