8. Definition of the High survival HF radio network technical requirements

The aim of this research activity is to illustrate the technical requirements of a high-survival radio network operating in the high-frequency (HF) band in the South Europe and Mediterranean area. The network will be called SWING and its aim will be that of providing a minimum flux of essential information in case a wide scale terrorist attack puts out of order the Internet connections between some European Critical Infrastructures (ECIs) and their Controlling Governmental Agencies (CGAs). The Medium Access Control (MAC) and Physical (PHY) layer aspects that are required to provide a reliable, secure and robust high survival radio network serving as a back-up communication system will be thoroughly discussed. In particular, three different MAC layer schemes will be investigated and compared: a contention free strategy, a contention based approach and a mixed solution. Among them, the last scheme will be shown to be the most appropriate for the SWING system. Specifically, for communications between ECIs and their home CGA the access will be managed through a contention free token protocol enabling the radio terminal to transmit when it holds the token. The choice of this technique will be motivated by its ability to efficiently manage the access of multiple ECIs to the HF channel in case of the Internet failure over a huge area. For communications between CGAs, a contention based protocol will be proposed, according to which the transmitting radio terminal must sense the channel before sending its data. Regarding the PHY layer specifications, this document will start arguing that reliable interactive communications in the HF frequency range calls for some form of frequency diversity, which can be achieved by using a transmission bandwidth adequately larger than the channel coherence bandwidth. In such a case, the delay spread is expected to span over many signalling periods, thereby producing significant inter-symbol interference. As is known, the latter can be easily handled by a multitone modem operating in the frequency domain with much less complexity than a serial-tone waveform employing a time-domain equalizer. For all these reasons, Orthgonal Frequency-Division Multiplexing (OFDM) will be selected as the air-interface of the SWING network thanks to its advantages in terms of resilience to multipath distortions and the possibility of exploiting the inherent frequency diversity offered by the propagation channel. A preliminary design will be provided for both the voice and data links. The modulation parameters provided by the preliminary system design will be then employed for the link budget analysis. In doing so, a worst-case scenario for ionospheric propagation will be considered.