14. Frequency management system for HF communication link optimization

In this report the procedures for the frequency management and link optimization have been considered. To realize the whole process two tasks are necessary. The first task can be afforded in two main passages, while the last task concerns the data throughput and channel’s quality, evaluable in terms of BER. In Distributed Supervision Function (DSVF) the program for frequency management and link optimization (that contains all the input for long term frequency management) runs. Long term frequency management or frequency forecast is an essential requirement to establish a series of radio links (point-to point) both for transmission and reception in the operating range of frequency from the LUF to the MUF starting in the planned geometric constraints. This task is also necessary to evaluate the transmitting power (Pt), modulation, polarization and the minimum SNR at the receiving point. These are also important for the type of the employed antennas and Low Noise Amplifier (LNA), selection filters, etc. in the receiving devices. Hence, the frequency forecast step to calculate these operating parameters is necessary to plan long term operations. To predict the reflectivity status of the ionospheric layers in those points where the reflection occurs, the ionospheric typical characteristics can be used. For short distances (less than 200-300 km) ground wave propagation [5] can replace sky wave radio link. In this report frequency now casting, or short term forecasting, has also been highlighted. Therefore, the hourly monthly median values are corrected by using methods that introduces new physical inputs or by using real time measurements. These are valid both for LUF and MUF. The first one takes into account also the ionospheric disturbances that strongly affects the radio signal especially in the lower part of the usable range under some particular low ionospheric region disturbances, while the latter can be greatly affected by the higher ionospheric regions. Because other causes can interact with the established radio links, it is useful to adjust the frequencies employed in order to optimize the flux of information evaluating the BER. Link optimization generally concerns the evaluation of both data transfer velocity and BER. Sudden ionospheric disturbances as well as frequency interference on the channel can affect the signal and disrupt the data transfer. To overcome this problems SWING system implements a kind of channel evaluator constituted by the USRP devices. USRP in frequency management system acts itself as a spectrum analyzer estimating from the SNR the channel quality.