The aim of this paper is to present a high data rate transmission system through the ionospheric channel in the HF band (3-30 MHz). The applications expected in this study are image transmission and real-time videoconferencing. Very high rates are required compared to the standard modems (4.8 kbits/s). Therefore, an array processing performs in the multi channel receiving system. Its main originality stands in the development of a compact device of collocated antennas, the spatial responses of which are different one from each other. Besides, synchronization (Zero Crossing Detector) and spatio temporal equalization (L.M.S. algorithm) as well resort to classical and well-tested techniques involving training sequences. An experimental radio link has been under test with a range of 800 km. The corresponding results underlines the improvement of the bit transfer rate which reaches 20 kbits/s (QAM 16 in a 6 kHz bandwidth) or 30 kbits/s (QAM 16 in a 9 kHz bandwidth). Several transmitted images are presented and appear quite consistent with the original.
The aim of the paper is to estimate the contribution of the polarization diversity in high frequency (3 - 30 MHz) direction finding systems. We first describe the peculiarities of H.F. propagation and the resulting signal model involved in computer simulations. Next, we analyze the behavior of some particular direction finding systems using linear and circular geometries and polarization diversity. Some algorithms (non linear frequential analysis, M.U.S.I.C.) are tested in several conditions (narrowband or broadband signals, polarization filtering reiterated or no, sub-sampling). Theoretical and experimental results show that polarization diversity based upon the knowledge of the antenna complex responses improves greatly the efficiency of direction finding.
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