[pt] Os sistemas de rádio acesso em banda larga em freqüências superiores a 20 GHz oferecem soluções de alta velocidade, vídeo, áudio e multimídia sendo, portanto, alternativas aos sistemas cabeados para acesso à última milha. Nestas faixas de freqüências, os sistemas de rádio acesso em banda larga utilizam enlaces do tipo ponto-multiponto na direção de descida e ponto-a-ponto na subida, com configurações semelhantes às encontradas nas redes celulares. Entretanto, devido as altas freqüências utilizadas, os enlaces operam apenas em visibilidade e o principal problema de propagação encontrado é a atenuação por chuvas, que afeta fortemente o nível do sinal recebido reduzindo a cobertura e o desempenho do sistema. Neste trabalho é desenvolvida uma ferramenta computacional com a finalidade de simular o cálculo de cobertura, de capacidade e da disponibilidade de sistemas de rádio acesso em banda larga utilizando os modelos de previsão de atenuação por chuvas do ITU-R e do CETUC, incluindo também efeitos de atenuação diferencial. A inclusão deste último efeito permite tratar de forma completa o problema da atenuação por chuvas em enlaces convergentes e determinar a degradação total na relação sinal-interferência do sistema. / [en] Broadband wireless access systems in frequencies above 20 GHz can provide a simple and effective solution for high speed, video, audio and multimedia service providers being an excellent alternative to cable systems for the last mile. At these frequencies bands, the broadband radio access system uses point to multipoint link in the downlink direction and point to point in the uplink. The configuration is similar to those of cellular networks. However, at these high frequencies the links are line-of- sight and the main problem is rain attenuation, that seriously affects the received signal level reducing coverage and system performance. In this work a computational tool is developed with the purpose of simula- ting coverage prediction, capacity and system availability calculations using models for rain attenuation prediction developed by ITU-R and CETUC. Also considered is the effect of differential attenuation, what allows a complete treatment of rain effects in point to multipoint links and the calculation of the total degradation in the signal to interference ratio.

Multi-carrier systems based on orthogonal frequency division multiplexing (OFDM) are efficient technologies for the implementation of broadband wireless communication systems. OFDM is widely used and has been adopted for current mobile broadband wireless communication systems such as IEEE 802.a/g wireless LANs, WiMAX, 3GPP LTE, and DVB-T/H digital video broadcasting systems. Despite their many advantages, however, OFDM-based systems suffer from potentially high peak-to-average power ratio (PAR). Since communication systems typically include nonlinear devices such as RF power amplifiers (PA) and digital-to-analog converters (DAC), high PAR results in increased symbol error rates and spectral radiation. To mitigate these nonlinear effects and to avoid nonlinear saturation effects of the PA, the operating point of a signal with high peak power must be backed off into the linear region of the PA. This so-called output backoff (OBO) results in a reduced power conversion efficiency which limits the battery life for mobile applications, reduces the coverage range, and increases both the cost of the PA and power consumption in the cellular base station. With the increasing demand for high energy efficiency, low power consumption, and greenhouse gas emission reduction, PAR reduction is a key technique in the design of practical OFDM systems. Motivated by the PAR reduction problem associated with multi-carrier systems, such as OFDM, this research explores the state of the art of PAR reduction techniques and develops new signal processing techniques that can achieve a minimum PAR for given system parameters and that are compatible with the appropriate standards. The following are the three principal contributions of this dissertation research. First, we present and derive the semi-analytical results for the output of asymptotic iterative clipping and filtering. This work provides expressions and analytical techniques for estimating the attenuation factor, error vector magnitude, and bit-error-rate (BER), using a noise enhancement factor that is obtained by simulation. With these semi-analytical results, we obtain a relationship between the BER and the target clipping level for asymptotic iterative clipping and filtering. These results serve as a performance benchmark for designing PAR reduction techniques using iterative clipping and filtering in OFDM systems. Second, we analyze the impact of the selected mapping (SLM) technique on BER performance of OFDM systems in an additive white Gaussian noise channel in the presence of nonlinearity. We first derive a closed-form expression for the envelope power distribution in an OFDM system with SLM. Then, using this derived envelope power distribution, we investigate the BER performance and the total degradation (TD) of OFDM systems with SLM under the existence of nonlinearity. As a result, we obtain the TD-minimizing peak backoff (PBO) and clipping ratio as functions of the number of candidate signals in SLM. Third, we propose an adaptive clipping control algorithm and pilotaided algorithm to address a fundamental issue associated with two lowcomplexity PAR reduction techniques, namely, tone reservation (TR) and active constellation extension (ACE). Specifically, we discovered that the existing low-complexity algorithms have a low clipping ratio problem in that they can not achieve the minimum PAR when the target clipping level is set below the initially unknown optimum value. Using our proposed algorithms, we overcome this problem and demonstrate that additional PAR reduction is obtained for any low value of the initial target clipping ratio. / text