OFDM Receiver Concept for the Aeronautical Communications System LDACS1 to Cope with Impulsive Interference

Kurzfassung

The aim of this thesis is to introduce interference mitigation techniques and an appropriate receiver structure for the aeronautical communications system LDACS1. Due to the proliferation of electronic radio systems, unused spectrum suitable for radio transmissions is scare. This scarcity calls for concepts for allocating spectrum presently used, at least partly, by existing systems also to new systems like LDACS1. This type of spectral deployment poses particular challenges to the design of new systems. On the one hand, it is necessary to ensure that existing systems are not impaired in their functionality by the new, so-called secondary, LDACS1 system. On the other hand, LDACS1 must be designed to become robust against interference from existing systems. In order to achieve these goals, LDACS1 uses OFDM as modulation technique. OFDM is a multi-carrier modulation technique. Its ability to occupy spectrum flexibly predestines OFDM as modulation technique for secondary systems. The proposed interference mitigation algorithms are based on the so-called blanking nonlinearity. Accordingly, a threshold is defined. Any parts of a received signal with a magnitude exceeding this threshold are considered interference and are subsequently blanked. This thesis presents improvements on the blanking nonlinearity aimed at alleviating drawbacks of this method for OFDM signals. It is common knowledge that OFDM signals have a relatively high PAPR. This property makes differentiation of interference impulses from OFDM signal peaks a challenging task. Correspondingly, a poorly chosen threshold may impair the OFDM signal significantly. This thesis presents an algorithm for an adaptive blanking threshold calculation. The algorithm is based on the magnitude distribution of the received signal, and does not require any information about the interference characteristics. Another disadvantage of the blanking nonlinearity is that the entire received signal is discarded during a blanking interval despite the fact that only a fraction of the spectrum of the OFDM signal might be affected by interference. This thesis proposes a new algorithm for estimating the spectral distribution of the interference. Based on this estimate, the received signal is combined with the blanked signal, thus significantly reducing the loss of information caused by the blanking nonlinearity. Furthermore, this thesis investigates concepts for OFDM receiver structures including how the various receiver components must be adapted to interference, and how they can profit from interference mitigation. Finally, this thesis elaborates on iterative receiver structures for further improving the performance of the proposed interference mitigation. Simulations with LDACS1, involving aeronautical channel models and various impulsive interference models, confirm the beneficial influence of the proposed algorithms on performance. In conjunction with an optimized iterative OFDM receiver structure, performance degradation due to impulsive interference can be kept at minimum.