Spectrally Efficient Beaconing for Aeronautical Applications

Spectrally Efficient Beaconing for Aeronautical Applications

In aeronautical surveillance beaconing, each aircraft regularly broadcasts surveillance data such as its own position, speed and heading. By receiving the beacon messages transmitted by other aircraft in vicinity, each aircraft becomes aware of surrounding traffic. Since the density of air traffic is growing, the situational awareness must be increased for future air traffic management. Aeronautical surveillance beaconing is an important method to achieve this increase of situational awareness. In contrast to ground-based radar, aeronautical surveillance beaconing provides situational awareness in the cockpit without any additional ground to air transmission of traffic data, and even works in oceanic and remote areas which are not covered by radar.

In aeronautical communications, the scarcity of unoccupied radio spectrum is a limiting factor already today. For this reason, spectral efficiency is of key importance for future aeronautical surveillance beaconing. Currently, three aeronautical surveillance beaconing systems exist: the SSR Mode S Extended Squitter transmitted on 1090MHz (1090ES), the Universal Access Transceiver (UAT), and the VHF Digital Link Mode 4 (VDL4). The capacity of all three existing systems is known to be inadequate to fulfill the demands of future aeronautical surveillance beaconing. At the same time, there is a lack of research on physical (PHY) layer and medium access control (MAC) layer schemes for aeronautical surveillance beaconing. It is not sufficiently studied which PHY layer and MAC layer schemes achieve a high spectral efficiency in aeronautical surveillance beaconing. Additionally, the joint optimization of PHY and MAC layer parameters is typically neglected in the literature.

In this project, we investigate spectrally efficient PHY layer and MAC layer schemes for aeronautical surveillance beaconing. Initially, the requirements of aeronautical surveillance beaconing are described, the multiple-access channel is explained and a definition of the spectral efficiency of aeronautical surveillance beaconing is given. Subsequently, we review existing PHY layer and MAC layer schemes and assess their suitability for spectrally efficient aeronautical surveillance beaconing. Based on this assessment, we select the two most promising schemes with respect to spectral efficiency. The first scheme, cell-based self-organizing TDMA (CB-SOTDMA), uses self-organizing time-division multiple-access (SOTDMA) within each cell of a cellular reuse pattern. CB-SOTDMA coordinates transmissions such that multiple-access interference is minimized. The second scheme is Aloha MAC with successive interference cancellation (SIC) in the receiver. Aloha with SIC does not attempt to avoid multiple-access interference, but to tolerate it through interference cancellation on the PHY layer.

Both for CB-SOTDMA and for Aloha with SIC, we introduce additional measures needed to overcome challenges specific to aeronautical surveillance beaconing. For CB-SOTDMA, we propose a novel solution to the problem of large power imbalances between signals received from different cells. The existing solution to this problem does not work efficiently in aeronautics due to the long signal propagation delays. For Aloha with SIC, we show that time hopping can mitigate message loss due to received signal outage during the own transmissions of a half-duplex beaconing radio. Time hopping splits up a message into multiple parts which are transmitted with gaps of random length in between.

Considering the additional measures introduced before, we develop semi-analytical models both for Aloha with SIC and for CB-SOTDMA to compute their spectral efficiency under simplifying assumptions. Additionally, we develop such a semi-analytical model for Aloha without any multi-user detection or SIC, since this technique is used by the most common existing systems 1090ES and UAT. The semi-analytical models enable us to jointly optimize PHY and MAC layer parameters for maximum spectral efficiency. This optimization reveals that both Aloha with SIC and CB-SOTDMA can achieve a substantially higher spectral efficiency than Aloha without SIC. Based on the spectral efficiency and on further criteria, we conclude that Aloha with SIC is the most promising PHY and MAC layer concept for aeronautical surveillance beaconing.

In the semi-analytical model of Aloha with SIC, certain PHY layer components are assumed to work ideally. To obtain a more realistic system design, we develop the Interference Canceling Beacon Transceiver (ICBT), a novel aeronautical surveillance beaconing system based on Aloha with SIC and time hopping. ICBT includes realistic solutions for PHY layer components such as message detection, channel estimation and interference cancellation. The design of ICBT does not assume received messages to be synchronous to, e.g., a common symbol clock. Additionally, we optimize the placement of known synchronization symbols in a message such that the Doppler shift can be accurately estimated by the receiver. The resulting structure of synchronization symbols also enables a message detection scheme with reduced computational complexity.

Finally, the spectral efficiency of ICBT is investigated by Monte-Carlo simulations of the complete PHY and MAC layer. The results agree well with the semi-analytical model. Additionally, we derive a scenario of future air traffic based on published predictions for the year 2035. This scenario describes the distribution and movement of aircraft more realistically than the simplifying assumptions of the semi-analytical models. Monte-Carlo simulations of ICBT in the 2035 air traffic scenario demonstrate that the entire beaconing traffic can be handled in a bandwidth which is even smaller than that of existing systems, although both the beaconing range and the packet size are increased in ICBT.