Time diversity solutions to cope with lost packets

A dissertation submitted to Departamento de Engenharia Electrotécnica of Faculdade de Ciências e Tecnologia of Universidade Nova de Lisboa in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Engenharia Electrotécnica e de Computadores

A random access MAC protocol for MPR satellite networks

Dissertação apresentada para obtenção do Grau de Mestre em Engenharia Electrotécnica e de Computadores, pela Universidade Nova de Lisboa, Faculdade de Ciências e Tecnologia

QoS in LEO satellite networks with multipacket reception

Dissertação apresentada para obtenção do Grau de Mestre em Engenharia Electrotécnica e de Computadores, pela Universidade Nova de Lisboa, Faculdade de Ciências e Tecnologia

High ffficiency transmission techniques for broadband wireless systems

Future broadband wireless systems are expected to cope with severely time dispersive channels, due to multi-path signal propagation between the transmitter and the receiver while having high power and spectral efficiency. Thus, advanced Frequency Domain Equalization techniques are required. The implementation complexity in mobile terminals should be as low as possible to achieve highest possible efficiency. Therefore, most of the signal processing requirements will be shifted to the base station and we will employ signals compatible with an efficient, grossly nonlinear power amplification. For this reason, we will consider offset modulation signals with quasi-constant envelope and develop receivers that will obtain good BER performance. However, these signals require a bandwidth significantly above the Nyquist rate, which can be reduced by an overlap of different frequency channels.

Unraveling intrinsic mechanisms of Nerve Regeneration

Unlike injury to the peripheral nervous system (PNS), where injured neurons can trigger a regenerative program that leads to axonal elongation and in some cases proper reinnervation, after injury to the central nervous system (CNS) neurons fail to produce the same response. The regenerative program includes the activation of several injury signals that will lead to the expression of genes associated with axonal regeneration. As a consequence, the spawned somatic response will ensure the supply of molecular components required for axonal elongation. The capacity of some neurons to trigger a regenerative response has led to investigate the mechanisms underlying neuronal regeneration. Thus, non-regenerative models (like injury to the CNS) and regenerative models (such as injury to the PNS) were used to understand the differences underlying those two responses to injury. To do so, the regenerative properties of dorsal root ganglion (DRG) neurons were addressed. This particular type of neurons possesses two branches, a central axon, that has a limited capacity to regenerate; and a peripheral axon, where regeneration can occur over long distances. In the first paradigm used to understand the neuronal regeneration mechanisms, we evaluated the activation of injury signals in a non-regenerative model. Injury signals include the positive injury signals, which are described as being enhancers of axonal regeneration by activating several transcription factors. The currently known positive injury signals are ERK, JNK and STAT3. To evaluate whether the lack of regeneration following injury to the central branch of DRG neurons was due to inactivation of these signals, activation of the transcription factors pELK-1, p-c-jun (downstream targets of ERK and JNK, respectively) and pSTAT3 were examined. Results have shown no impairment in the activation of these signals. As a consequence, we further proceed with evaluation of other candidates that could participate in axonal regeneration failure. By comparing the protein profiles that were triggered following either injury to the central branch of DRG neurons or injury to their peripheral branch, we were able to identify high levels of GSK3-β, ROCKII and HSP-40 after injury to the central branch of DRG neurons. While in vitro knockdown of HSP-40 in DRG neurons showed to be toxic for the cells, evaluation of pCRMP2 (a GSK3-β downstream target) and pMLC (a ROCKII downstream target), which are known to impair axonal regeneration, revealed high levels of both proteins following injury to the central branch when comparing with injury to their peripheral one. Altogether, these results suggest that activation of positive injury signals is not sufficient to elicit axonal regeneration; HSP-40 is likely to participate in the cell survival program; whereas GSK3-β and ROCKII activity may condition the regenerative capacity following injury to the nervous system.(...)

Multipacket reception in the presence of in-band full-duplex communication

In-Band Full-DupleX (IB-FDX) is defined as the ability for nodes to transmit and receive signals simultaneously on the same channel. Conventional digital wireless networks do not implement it, since a node’s own transmission signal causes interference to the signal it is trying to receive. However, recent studies attempt to overcome this obstacle, since it can potentially double the spectral efficiency of current wireless networks. Different mechanisms exist today that are able to reduce a significant part of the Self- Interference (SI), although specially tuned Medium Access Control (MAC) protocols are required to optimize its use. One of IB-FDX’s biggest problems is that the nodes’ interference range is extended, meaning the unusable space for other transmissions and receptions is broader. This dissertation proposes using MultiPacket Reception (MPR) to address this issue and adapts an already existing Single-Carrier with Frequency-Domain Equalization (SC-FDE) receiver to IB-FDX. The performance analysis suggests that MPR and IB-FDX have a strong synergy and are able to achieve higher data rates, when used together. Using analytical models, the optimal transmission patterns and transmission power were identified, which maximize the channel capacity with the minimal energy consumption. This was used to define a new MAC protocol, named Full-duplex Multipacket reception Medium Access Control (FM-MAC). FM-MAC was designed for a single-hop cellular infrastructure, where the Access Point (AP) and the terminals implement both IB-FDX and MPR. It divides the coverage range of the AP into a closer Full-DupleX (FDX) zone and a farther Half-DupleX (HDX) zone and adds a tunable fairness mechanism to avoid terminal starvation. Simulation results show that this protocol provides efficient support for both HDX and FDX terminals, maximizing its capacity when more FDX terminals are used.