There has been phenomenal interest in applying space-time coding techniques in wireless communications in the last two decades. In general, the benefit of applying space-time codes in multiple-input, multiple-output (MIMO) wireless channels is an increase in transmission reliability or system throughput (capacity). However, such a benefit cannot be obtained in some wireless systems where size or other constraints preclude the use of multiple antennas. As such, wireless relay communications has recently been proposed as a means to provide spatial diversity in the face of this limitation. In this approach, some users or relay nodes assist the transmission of other users information. This dissertation contributes to the advancement of wireless relay communications by investigating the performance of various relaying signal processing methods under different practical fading environments. In particular, it examines two main relaying methods, namely decode-and-forward (DF) and amplify-and-forward (AF).<p> For DF, the focus is on the diversity analysis of relaying systems under various practical protocols when detection error at relays is taken into account. In order to effectively mitigate the phenomenon of error propagation, the smart relaying technique proposed by Wang et al. in [R1] is adopted. First, diversity analysis of a single-relay system under the scenario that only the relay is allowed to transmit in the second time slot (called Protocol II) is carried out. For Nakagami and Hoyt generalized fading channels, analytical and numerical results are provided to demonstrate that the system always obtains the maximal diversity when binary phase shift keying (BPSK) modulation is used. Second, a novel and low-complexity relaying system is proposed when smart relaying and equal gain combing (EGC) techniques are combined. In the proposed system, the destination requires only the phases of the channel state information in order to detect the transmitted signals. For the single-relay system with M-ary PSK modulation, it is shown that the system can achieve the maximal diversity under Nakagami and Hoyt fading channels. For the K-relay system, simulation results suggest that the maximal diversity can also be achieved. Finally, the diversity analysis for a smart relaying system under the scenario when both the source and relay are permitted to transmit in the second time slot (referred to as Protocol I) is presented. It is shown that Protocol I can achieve the same diversity order as Protocol II for the case of 1 relay. In addition, the diversity is very robust to the quality of the feedback channel as well as the accuracy of the quantization of the power scaling implemented at the relay.<p> For AF, the dissertation considers a fixed-gain multiple-relay system with maximal ratio combining (MRC) detection at the destination under Nakagami fading channels. Different from the smart relaying for DF, all the channel state information is assumed to be available at the destination in order to perform MRC for any number of antennas. Upperbound and lowerbound on the system performance are then derived. Based on the bounds, it is shown that the system can achieve the maximal diversity. Furthermore, the tightness of the upperbound is demonstrated via simulation results. With only the statistics of all the channels available at the destination, a novel power allocation (PA) is then proposed. The proposed PA shows significant performance gain over the conventional equal PA.

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Las técnicas de estimación de canal y de adaptación de la transmisión a las condiciones del entorno son temas de interés actual al estudiar la aplicación de técnicas de diversidad en transmisión en la tercera y cuarta generación de sistemas inalámbricos. En esta tesis se realiza un análisis del impacto del error de estimación de canal y la correlación en sistemas OFDM con diversidad en transmisión basados en codificación espacio-tiempo por bloques (STBC), se proponen técnicas de estimación de canal para estos sistemas y se propone una técnica de adaptación de la transmisión mediante la selección de código espacio-tiempo. En primer lugar, una técnica sencilla de mínimos cuadrados en el dominio de la frecuencia permite la estimación de canal en sistemas con dos antenas y constelaciones complejas, y con tres o cuatro antenas y constelaciones reales o complejas, utilizando STBCs ortogonales como bloques de entrenamiento. En segundo lugar, una representación 'sobre-completa' permite hacer una estimación diferencial de canal para un sistema con tres antenas transmisoras mediante la selección a partir de un banco de posibles estimadores, basándose en la redundancia provista por la matriz de transmisión no cuadrada del código ortogonal esporádico de tasa 3/4 para tres antenas transmisoras.En el contexto de sistemas con adaptación del transmisor, la técnica propuesta de diversidad por selección adaptativa de código espacio-tiempo se basa en el estado instantáneo del vector de canal y en un conjunto de niveles umbrales hallados fuera de línea en función del período de realimentación. Los resultados indican que esta técnica proporciona buenas prestaciones en canales correlados e incorrelados. Su aplicación a sistemas OFDM ha sido estudiada, superando a técnicas de selección de antena y a otras técnicas de transmisión adaptativa. / Channel estimation and adaptive transmission techniques are areas of increasing interest these days when considering transmit diversity systems for the 3G and 4G wireless communication systems. In this thesis an analysis of the channel estimation and channel correlation impact on transmit diversity OFDM systems based on space-time block coding (STBC) is presented, two channel estimation techniques are outlined and an adaptive space-time code selection technique is proposed. First, a simple frequency domain least square technique allows channel estimation for two transmitter systems with complex constellation, and three or four transmitter systems with real or complex constellation, using orthogonal STBCs as training blocks. Second, an 'overcomplete' representation allows a di.erential channel estimation for three transmitter systems through the instantaneous selection from a bank of estimators, based on the redundacy provided by the non-square transmission matrix of the sporadic 3/4-rate STBC for three transmitters.In the context of transmit adaptive systems, the proposed adaptive space-time code selection technique is based on both the instantaneous channel vector state and a set of predetermined threshold levels found o.-line as a function of the feedback period. Analytical and simulation results show that the proposed technique has a good performance in the presence of correlated and uncorrelated channels. Its application to OFDM systems has been considered, outperforming classical antenna selection techniques and other closed-loop adaptive transmission techniques.

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