Técnicas de igualização adaptativas com estimativas imperfeitas do canal para os futuros sistemas 5G

Wireless communication networks have been continuously experiencing an exponential growth since their inception. The overwhelming demand for high data rates, support of a large number of users while mitigating disruptive interference are the constant research focus and it has led to the creation of new technologies and efficient techniques. Orthogonal frequency division multiplexing (OFDM) is the most common example of a technology that has come to the fore in this past decade as it provided a simple and generally ideal platform for wireless data transmission. It’s drawback of a rather high peak-to-average power ratio (PAPR) and sensitivity to phase noise, which in turn led to the adoption of alternative techniques, such as the single carrier systems with frequency domain equalization (SC-FDE) or the multi carrier systems with code division multiple access (MC-CDMA), but the nonlinear Frequency Domain Equalizers (FDE) have been of special note due to their improved performance. From these, the Iterative Block Decision Feedback Equalizer (IB-DFE) has proven itself especially promising due to its compatibility with space diversity, MIMO systems and CDMA schemes. However, the IB-DFE requires the system to have constant knowledge of the communication channel properties, that is, to have constantly perfect Channel State Information (CSI), which is both unrealistic and impractical to implement. In this dissertation we shall design an altered IB-DFE receiver that is able to properly detect signals from SC-FDMA based transmitters, even with constantly erroneous channel states. The results shall demonstrate that the proposed equalization scheme is robust to imperfect CSI (I-CSI) situations, since its performance is constantly close to the perfect CSI case, within just a few iterations.Redes sem fios têm crescido de maneira contínua e exponencial desde a sua incepção. A tremenda exigência para altas taxas de dados e o suporte para um elevado número de utilizadores sem aumentar a interferência disruptiva originada por estes são alguns dos focos que levaram ao desenvolvimento de técnicas de compensação e novas tecnologias. “Orthogonal frequency division multiplexing” (OFDM) é um dos exemplos de tecnologias que se destacaram nesta última década, visto ter fornecido uma plataforma para transmissão de dados sem-fio eficaz e simples. O seu maior problema é a alta “peak-to-average power ratio” (PAPR) e a sua sensibilidade a ruído de fase que deram motivo à adoção de técnicas alternativas, tais como os sistemas “single carrier” com “frequency domain equalization” (SC-FDE) ou os sistemas “multi-carrier” com “code division multiple access” (MC-CDMA), mas equalizadores não lineares no domínio de frequência têm sido alvo de especial atenção devido ao seu melhor desempenho. Destes, o “iterative block decision feedback equalizer” (IB-DFE) tem-se provado especialmente promissor devido à sua compatibilidade com técnicas de diversidade no espaço, sistemas MIMO e esquemas CDMA. No entanto, IB-DFE requer que o sistema tenha constante conhecimento das propriedades dos canais usados, ou seja, necessita de ter perfeito “channel state information” (CSI) constantemente, o que é tanto irrealista como impossível de implementar. Nesta dissertação iremos projetar um recetor IB-DFE alterado de forma a conseguir detetar sinais dum transmissor baseado em tecnologia SC-FDMA, mesmo com a informação de estado de canal errada. Os resultados irão então demonstrar que o novo esquema de equalização proposto é robusto para situações de CSI imperfeito (I-CSI), visto que o seu desempenho se mantém próximo dos valores esperados para CSI perfeito, em apenas algumas iterações.Mestrado em Engenharia Eletrónica e Telecomunicaçõe

Performance of MIMO Schemes in Radio-over-fibre-based Distributed Antenna System

The research presented in this thesis has focused on the use of MIMO wireless communications in a RoF-based DAS to improve wireless coverage and capacity performance in an indoor environment. The aim is to analyse the practical issues that cause throughput to deteriorate when commercial MIMO APs are used in a RoF-DAS, and also to verify that improved performance - lower error rates and higher capacities - can be achieved by a large physical separation between the RAUs when specific multi-antenna scheme algorithms are used. The performance of an IEEE 802.11n MIMO-supported AP and IEEE 802.11g spatial-diversity-supported AP are investigated in a RoF-DAS when different fibre lengths are connecting the AP in the central unit to the RAUs, and when the RAUs are widely separated. The analysis indicates that for MIMO, the throughput drops rapidly due to severe ISI caused by differential delay when the fibre-length difference exceeds a certain distance, while for spatial diversity high throughputs can be maintained even at large fibre-length difference. Further, it was observed that largely separated RAU may lead to power imbalances and the throughput drops in specific wireless user's positions when the received power imbalance was above 12-15dB for MIMO-supported AP, while for spatial-diversity-supported AP the power imbalance does not affect the throughput. The majority of previous works on RoF-DAS for improving MIMO systems were based on commercial products and the specific algorithms used within these products are unknown. An investigation was carried out at microwave frequency with SIMO algorithms in RoF-DAS uplink, MISO and MIMO algorithms in RoF-DAS downlink, and compared with the performance of a SISO system. This investigation was later extended to millimetre-wave frequency where larger bands of frequency are available enabling the possibility of wider bandwidth and higher data rates. The result shows significantly reduced error rate and modestly increased capacity for a wireless 1x2 SIMO uplink using MRC algorithm and 2x1 MISO downlink using Alamouti STBC algorithm. Also, error rate was reduced for a wireless 2x2 MIMO downlink using the zero-forcing algorithm while, most importantly, greatly increased capacity was achieved through the spatial multiplexing gain