Abstract-In radio systems linear power amplifiers (class A, B or AB) are usually used as a solution for the power amplifier stage. These amplifiers have high linearity, but suffer from low efficiency when the transmitted signal has low peak-to-average power ratio. The Kahn envelope elimination and restoration (EER) technique is used to enhance efficiency of RF transmitters, by combining highly efficient, nonlinear RF amplifier (class D or E) with a highly efficient envelope amplifier in order to obtain linear and highly efficient RF amplifier. This paper presents a solution for the envelope amplifier based on a combination of multilevel converter and linear regulator. The proposed solution can reproduce any signal with maximal spectral component of 2 MHz and give instantaneous maximal power of 50 W. The efficiency measurements show that when the signals with low average value are transmitted, it has up to 15% higher efficiency than linear regulator that is used as a conventional solution. Additionally, the algorithm for selection of voltage levels for the multilevel converter is explained, as well.
Pervasive devices interacting in open and dynamicspaces with each others require a mechanism that allows them acting autonomously in a secure way and protecting their resources. Trust is fundamental to establish communication with other users, because the identity is often uncertain and on one's own does not provide trust information, for instance, could an unknown user be trustworthy? Nowadays, these devices have a so limited security support. So, we propose a simple trust management model to enhance such support, allowing them interact in ad hoc networks and peer-to-peer applications in a secure way. In this paper, our main contribution is a mathematical and a probabilistic model, as well as demonstrating the model feasibility, since it has been assessed through the prototype implementation, which has been tested in a Pocket PC.
-Due to complex envelope and phase modulation employed in modern transmitters it is necessary to use power amplifiers that have high linearity. Linear power amplifiers (classes A, B and AB) are commonly used, but they suffer from low efficiency especially if the transmitted signal has high peak to average power ratio (PAPR). Kahn's technique based on envelope elimination and restoration (EER) is based on idea that high efficiency power supply (envelope amplifier) could be used to modulate the envelope of high efficient non linear power amplifiers (classes D or E). This paper presents solutions for power amplifier that performs envelope modulation and class E amplifier that is used as a non linear amplifier. The envelope amplifier is implemented as a multilevel converter in series with linear regulator and can provide up to 100 W of instantaneous power and reproduce 2 MHz sine wave. The implemented Class E amplifier can operate at 120 MHz with efficiency near to 85%. The envelope amplifier and class E amplifier have been integrated and efficiency and linearity of the implemented transmitter has been measured and presented.
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