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Percorrer por autor "Belo, Daniel"

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  • Active reflection coefficients characterization system for multiple input multiple output antennas
    Publication . Jordão, Marina; Belo, Daniel; Caldeirinha, Rafael F. S.; Oliveira, Arnaldo S. R.; Carvalho, Nuno B.
    The main objective of this study is to present a characterization system that can be used to measure the active reflection coefficient of each port of the multiple input multiple output (MIMO) antennas, when the main beam is steered to an intended direction. Since MIMO systems are composed of antennas with several elements and each antenna element is connected to a power amplifier, an active component, the beamforming will contribute to the mismatch of each branch. To this extent, a calibrated measurement system has been implemented using off-the-shelf software defined radio and designed to effectively extract such coefficients, when all the ports are simultaneously excited. Results for linear and planar MIMO antenna arrays have been experimentally obtained for several beam directions and the impact of beamforming on each element of the antenna is presented and compared to simulated results. Additionally, their behaviour was also assessed when some of the radiation elements are deliberately disconnected, mimicking those not working properly.
    2021-03-10Artigo científico Acesso aberto Ver mais
  • Over-the-Air Calibration of Active Antenna Arrays Using Multisine
    Publication . Jordão, Marina; Belo, Daniel; Caldeirinha, Rafael F. S.; Oliveira, Arnaldo S. R.; Carvalho, Nuno Borges de
    A novel over-the-air (OTA) beamforming calibration procedure is proposed, which can be applied to different types of multiple-input multiple-output (MIMO) transmitting antennas. The method consists of using two tones on each antenna element, the main tone, and the tickle tone. The latter one operates in a different frequency for each antenna element and is used to obtain the optimum phase that needs to be set for each element to generate a beam in the desired direction, at the main tone frequency. The novel method was applied to linear, planar, 3-D antennas, and noncollocated multiantennas to validate its applicability to different antenna types. Line-of-Sight (LOS) and non-LOS experimental results demonstrate that the method can be used to retrieve the required phase to fully control the antenna radiation pattern. An optimum radiation pattern and lowest error vector magnitude (EVM) can be achieved even under high multipath effects/non-LOS conditions. A comparison between the performance and the application of the array factor (AF) theory is presented for practical indoor scenarios, demonstrating the effectiveness of the method. It is demonstrated that the method may be an alternative to the calibration of nonstandard MIMO antennas, where the AF computation is complex or not possible.
    2021-01-05Artigo científico Acesso aberto Ver mais
  • RF-dc Converter Optimization using MIMO Antennas and OTA Multi-Sine Calibration Method
    Publication . Jordão, Marina; Belo, Daniel; Caldeirinha, Rafael; Oliveira, Arnaldo S. R.; Carvalho, Nuno Borges de
    In this paper, the optimization of the power transmitted from several non-collocated antennas to an RF-dc converter circuit is performed using an Over-The-Air (OTA) multi-sine feedback method. An experimental system is tested in an indoor environment and the OTA multi-sine calibration method is applied to produce constructive interference at the RF-dc converter location. It is shown that its overall performance is improved for different indoor positions. Experimental results demonstrate the effectiveness of using the proposed method for Wireless Power Transfer (WPT) applications since the RF-dc converter presents higher efficiency when the method is applied even in worst scenarios, such as low input power or larger distance.
    2021-01Comunicação em conferência Acesso restrito Ver mais