Influence of DME Interference onto LDACS1 - Feasibility Study for Europe

Kurzfassung

The aeronautical communication infrastructure is currently undergoing a reorganization process. First, the analogue voice communication system in the VHF band will soon reach its capacity limit. Second, communication needs shift from pure voice towards data transmission. These issues motivate for developing new digital aeronautical communication systems. The future air-ground communication system will be operating in the lower part of the L band from 960 1164 MHz which has been assigned to the aeronautical mobile (route) service (AM(R)S) on a secondary basis at the world radio communication conference (WRC) in 2007. Currently, two candidates for the future L-band digital aeronautical communications system (LDACS) are under consideration. The first candidate termed LDACS1 is a broadband system employing orthogonal frequency-division multiplexing (OFDM) as modulation scheme and separating forward and reverse link by frequency-division duplex (FDD). The second candidate, LDACS2, is a narrowband single-carrier system utilizing time-division duplex (TDD). In this paper, we will focus on LDACS1. A crucial issue when deciding for one of the two candidate systems is to guarantee the co-existence between LDACS and legacy systems already operating in the L band. That is a challenging task since the spectra of the legacy systems and LDACS1 are partially overlapping. This is due to the fact that LDACS1 is designed to be able to operate as an inlay system in the spectral gaps between the legacy systems, for achieving a high transmission capacity. An L-band compatibility study is required for proving proper coexistence. First investigations of both the interference from LDACS1 onto legacy systems and the influence from legacy systems onto LDACS1 have already been presented. In former publications, a theoretical analysis identified the distance measuring equipment (DME) as the most severe source of interference, LDACS1 has to cope with. However, only one worst case and one typical scenario were considered. In contrast, in this paper we will analyze the influence of the actual DME interference situation in Europe onto LDACS1. For assessing the influence of DME signals onto LDACS1, we first describe the basic system parameters of LDACS1, where special focus is put on the physical layer, since the physical received signal is directly affected by DME signals. Next, the DME system and the DME signal shape are described. This enables us to illustrate the DME influence onto LDACS1 when employing it as inlay system. We also present interference mitigation techniques, such as pulse blanking and also pulse blanking compensation, which can be applied to reduce the detrimental influence of interference. In particular, we are interested how the DME influence onto LDACS1 changes when these techniques are applied. For analyzing the realistic DME situation, we have implemented a European interference map, based on the current DME locations and channel allocations. This enables us to assess the particular interference situation for each aircraft position and each chosen LDACS1 forward link carrier frequency. The interference situation is given in terms of the number of visible DME ground stations in the adjacent DME channels, the DME received power at the aircraft receiver, and the DME ground station duty cycles. In the final paper, we will present results of physical layer simulations where we derived bit-error and packet-error rates. The simulation parameters are chosen according to the interference map. The simulation will illustrate the beneficial influence of interference mitigation on the one hand. On the other hand, we are able to identify heavily disturbed areas for certain LDACS1 forward link frequencies and assess if LDACS1 is capable of handling this interference without applying frequency planning. Frequency planning for reducing the DME interference towards LDACS1 is subject to further research.