19. Realization of a demonstrator constituted by 4- terminals HF network

In this report a practical realization of a demonstrator constituted by 4-terminals HF network has been discussed. After a brief overview of the technologies currently in use in HF band, a detailed description of the SWING network has been given focusing on the main network components (e.g. transmitter and receiver architecture and antenna system).
The design and the realization of the demonstrator took advantage from the most advanced technology available in the market i.e. the software defined radio. The Universal Software Radio Peripheral (USRP) employed is a reconfigurable hardware peripheral that allows general purpose computers to control using a high bandwidth Gigabit Ethernet bus. It was possible to perform all of the operations like waveform generation and baseband processing such as filtering and modulation and demodulation. USRP also performs high-speed digital up and down conversion from baseband to IF using the on board FPGA.

The demonstrator activity has been divided in the following two experiments:

  1. Chirp experiment The chirp system was basically intended to study the background noise in the transmitter and receiver locations. The ionospheric prediction result was used to decide the spectrum sensing range. The identified local noise band using spectral sensing within the frequency range of the ionospheric forecast must be avoided for establishing the radio link. The noise bands at the receiver location must not be used for the radio link because the detection probability of the receiver is very low or zero in those bands. The noise bands at the transmitting location cannot be used for the radio link in order to avoid interference with other radio communications in the area. This chirp system was also used for the study of the ground wave and the sky wave propagation.

  2. PSK experiment This demonstrator regards a possible theoretical and practical implementation of a HF communication link wherein a PSK modulation is used to modulate the data to be transmitted. A reduced scenario has been considered because of the large complexity in modelling of a HF network with more than two terminals. The scenario specifications are: - Point to point connection type; - Single carrier frequency; - PSK modulation type.

During the experiment, the USRP hardware and LabVIEW software have been used; the first represents a low cost and extremely flexible solution for SDR application, while the second represents a general purpose software for data processing and instruments control. The functionality of this system has been experimentally demonstrated in a closed-loop configuration. Moreover the effect of additive white Gaussian noise has been evaluated by using a set of different signal to noise ratio.