Impact Assessment of LDACS on JTIDS

Abstract

Flying has become an indispensable way to travel and to transport cargo in the last decades. According to [1], 35% of world trade by value is carried by aircraft and, only in 2015, approximately 3.5 billion passengers used air transport. In fact, around 9 917 000 flights over Europe were registered during 2015 [2]. Moreover, flight forecasts for Europe indicate that the number of flights will continuously increase in the following years (e.g. 11 535 000 flights in 2022) [2]. This increasing number of flights threatens to overwhelm the Air-Traffic Management (ATM) system in the near future. In order to further guarantee the safety of flight and the air-traffic efficiency, the ATM system is being modernized under two projects: SESAR (Single European Sky ATM Research) in Europe and NextGen (Next Generation Air Transportation System) in the United States of America. For the modernization of the air-ground communications component, the L-band Digital Aeronautical Communications System (LDACS) has been proposed. LDACS employs a frequency-division duplexing scheme to separate the forward link and the reverse link, allowing for simultaneous transmissions from the ground station and the airborne station. Furthermore, LDACS uses Orthogonal Frequency-Division Multiplexing (OFDM) and Adaptive Coding and Modulation (ACM). Among many other advantages, these technologies allow LDACS to cope with the current and future capacity requirements foreseen for the ATM system. Moreover, because of its scalability and with a suitable frequency allocation, LDACS could also cope with a higher long-term capacity requirement. LDACS is expected to be deployed in the L-band (960 – 1164 MHz), where many other legacy systems can be found, such as the Distance Measuring Equipment (DME), the Joint Tactical Information Distribution System (JTIDS), and the Secondary Surveillance Radar (SSR). Therefore, it must be ensured that the allocation of LDACS in this band will not affect the functionality of these legacy systems. Since the impact of LDACS on DME has already been studied in [3], this work is focused on JTIDS, a military communications system used by the North Atlantic Treaty Organization (NATO) countries and several others. JTIDS employs frequency hopping among 51 frequency channels located between 969 MHz and 1206 MHz. As JTIDS uses most of the spectrum foreseen for the allocation of LDACS, it is likely that LDACS and JTIDS will have to share a part of this spectrum. Thus, as their transmissions may partially overlap in time and frequency, the impact of LDACS on the functionality of JTIDS must be assessed. In this work, we show an assessment of the impact of LDACS on JTIDS obtained through simulations. This impact has been quantified by comparing the performance of JTIDS in different scenarios, with and without LDACS interference. In order to obtain the performance of JTIDS in the different scenarios, simulation models of both systems have been implemented according to their specifications. Whilst the specification used for the LDACS model is public [4], the specification of JTIDS is partly classified. Therefore, our implemented JTIDS model is based on the publicly available parts of JTIDS specification and publicly available literature. Simulation results show that the impact of LDACS on JTIDS mainly depends on the number of frequency channels interfered by LDACS, the transmission parameters used by JTIDS, and the signal-to-interference ratio. As a result, recommendations for the deployment of LDACS are given, under which the impact of LDACS on JTIDS is minimized. Finally, it can be concluded that the deployment of LDACS in the L-band will have no serious impact on JTIDS. With well below 1 dB SNR loss even for the expected worst case scenarios, the impact on JTIDS is almost negligible. [1] International Civil Aviation Organization, “Air Navigation Report 2016 Edition,” Montréal, Canada, 2016. [2] EUROCONTROL, “EUROCONTROL Seven-Year Forecast February 2016. Flight Movements and Service Units 2016-2022,” 2016. [3] B. Haindl, et al., “LDACS1 conformance and compatibility assessment,” in 2014 IEEE/AIAA 33rd Digital Avionics Systems Conference (DASC), 2014. [4] M. Sajatovic, et al., “Updated LDACS1 System Specification”, SESAR JU, Brussels, Belgium, Report EWA04-1-T2-D1, 2011.