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Circumventing the Random Access Channel: New Concepts for Accessing a Command & Control Link for Unmanned Aircraft

Unmanned Aircraft (UA) are expected to considerably grow in importance for global aviation during the next years. Although a comparatively high level of autonomy is anticipated for UA, a reliable data link (“Command and Control”, C2-link) will be mandatory for their operation. The C2 link is used to exchange all information between UA and remote pilot, immediately required for operating the UA. This data is called non-payload data; examples are positioning information, telemetry data or flight trajectories, respectively. None of the currently available aeronautical communication standards can fulfill the requirements for such a multi-user, multi-point datalink in terms of reliability, flexibility, data integrity, robustness and latency. The C-Band Digital Aeronautical Communication System (CDACS) is a possible candidate for such a C2 link making use of modern communication techniques like Orthogonal Frequency Division Multiplex (OFDM) and SC-FDMA, [1], [2]. While CDACS development has so far focused on the general requirements and the waveform design, physical layer robustness also known as PHY layer security, e.g. against jamming/spoofing, has not yet been considered yet. One aspect of making CDACS more robust is hardening the login process of a user, in the case of CDACS a UA, into the network against such attacks. This login process is often realized using a Random Access Channel (RACH), that enables users not known to the base station access to the network. In this paper we discuss several concepts that try to avoid the traditional RACH, thus closing one potential vulnerability of the system.