Mitigation of Impulsive Interference for LDACS1 – Potentials of Blanking Nonlinearity

Abstract

The aeronautical communications infrastructure is currently undergoing a reorganization process. Firstly, the analogue voice communication system in the VHF band will soon reach its capacity limit. Secondly, communication needs to shift from pure voice towards data transmission. These issues motivate for developing new aeronautical communications systems. Currently, VDL Mode 2 is introduced in Europe as a first step toward digital data communications. However, its data link capacity is very limited due to bandwidth restrictions and the choice of the underlying transmission technology which has been standardized approximately 20 years ago. To enable modern ATM concepts as currently developed within SESAR and NextGen, more powerful data link solutions are required. The future terrestrial aeronautical communications system will be operated in the L-band from 960-1164 MHz which was assigned to the aeronautical mobile (route) service (AM(R)S) on a secondary basis at the world radio conference (WRC) in 2007. Currently, the most promising candidate is L-band digital aeronautical communications system type1 (LDACS1) [1]. The aeronautical L-band is already heavily used by both radar-navigation systems and military communications system. To make available significant frequency resources in the L band without the need for changing frequency assignments of existing systems, LDACS1 pursues an inlay approach, deploying the spectral gaps between adjacent channels of the distance measuring equipment (DME). In order to allocate spectrum within these gaps, LDACS1 uses broadband orthogonal frequency-division multiplexing (OFDM) as modulation technique. However, such an approach potentially leads to severe interference from DME onto LDACS1 [2]. To ensure a reliable LDACS1 transmission in the presence of such strong DME interference, appropriate interference mitigation has to be deployed in the LDACS1 receiver. DME signals occur as short impulses, with power typically significantly above the LDACS1 received power. This signal characteristic allows deploying efficient impulsive interference mitigation. A common approach for mitigation such impulsive interference is to apply a memoryless blanking nonlinearity (BN) [3]. Accordingly, a threshold is defined. Any parts of a received signal with a magnitude exceeding this threshold are considered interference and are subsequently blanked. In this paper, we will discuss the advantages and drawbacks of BN when applied in LDACS1. In particular for OFDM, BN leads to specific drawbacks which are explained in the paper. Recent publications dealt with alleviating these drawbacks, e.g. [4]. Firstly, an adaptive threshold calculation was proposed, maximizing the signal quality after BN. Secondly, it was proposed to limit the blanking to spectral parts, actually affected by interference. Thirdly, a recursive neural network equalizer can be deployed in order to suppress inter-carrier interference induced by BN. In former publications, e.g. [4], the LDACS1 performance was assessed for a realistic worst-case DME scenario derived by means of a European interference map. Such a scenario allows evaluating the feasibility of LDACS1 as inlay system for a specific constellation. However, such an approach does not account for future changes in DME allocations. In addition, it does not reveal the degradation of LDACS1 performance as a function of the strength of DME interference. In this paper, we will introduce a generic DME interference scenario. In particular, we vary the DME power at the LDACS1 receiver and the occurrence rate of DME impulse pairs. This allows assessing the potentials and limitations of the applied interference mitigation techniques for LDACS1 more in general. In the final paper, we analyze the effectiveness of the improvements of BN for LDACS1 in the presence of DME interference. Therefore, we present results of physical layer simulations where we derive the bit-error rate performance. This allows us to identify the potentially beneficial influence of the various improvements of BN, resulting in interference mitigation concept, adapted to LDACS1 in the presence of DME interference. References: [1] S. Brandes, U. Epple, S. Gligorevic, M. Schnell, B. Haindl, and M. Sajatovic, “Physical layer specification of the L-band digital aeronautical communications system (L DACS1),” in Proc. Integrated Commun., Navig., and Surveillance Conf., Arlington, VA, USA, 2009, pp. 1–12. [2] U. Epple and M. Schnell, “Overview of interference situation and mitigation techniques for LDACS1,” in Proc. Digital Avionics Syst. Conf., Seattle, WA, USA, 2011, pp. 4C5–1–4C5–12. [3] S. V. Zhidkov, “Analysis and comparison of several simple impulsive noise mitigation schemes for OFDM receivers,” IEEE Trans. Commun., vol. 56, pp. 5–9, Jan. 2008. [4] U. Epple, D. Shutin, and M. Schnell, “Mitigation of impulsive frequency-selective interference in OFDM based systems,” IEEE Wireless Commun. Lett., vol. 1, pp. 484–487, Oct. 2012.