DME Pulse Interference Mitigation Research Based On Smart Antennas For OFDM Receiver

L-Band Digital Aeronautical Communication System1(LDACS1) is an air-ground cellular communication system based on Orthogonal Frequency Division Multiplexing(OFDM), and provides communication services for future Air Traffic Services(ATS) and Aviation Operation Control(AOC). At the World Radio Conference in 2007, the deployment option for LDACS1 as an inlay concept was approved that allows the utilization of frequency gaps between two adjacent channels of distance measure equipment(DME) system in the aeronautical radio navigation band. To mitigate the deleterious influence of DME interference on OFDM receiver of LDACS1 operating as an inlay system, interference suppression researches for LDACS1 system have been carried out based on smart antennas. The main research contents of this paper are as follows:Firstly, the interference of DME system to LDACS1 system is analyzed. The principle and signal mode of DME system are described in the paper. Then the signal to interference ratio(SIR) of the LDACS1 receiving antenna at the typical scene is given after the analysis of the interference link path loss. Furthermore based on the analysis of the RF front end suppression of adjacent channel interference, the SIR budget at the OFDM demodulation is given. Finally the impact to transmission reliability of LDACS1 system from DME signal is verified by simulation.Secondly, a DME pulse interference mitigation method based on joint DOA estimation and main beamforming has been proposed. The DOA of received signals is estimated by2D-MUSIC algorithm. Then the DOA of OFDM direct path, diffuse path and the DME pulse interference signals is determined by frequency-domain power comparison and time-domain intensity comparison methods. Furthermore with the DOA information, the DME pulse interference and diffuse component of OFDM signal is eliminated by Linearly Constrained Minimum Variance(LCMV) beam former. Finally the Doppler shift of OFDM direct path signal is estimated and compensated to eliminate the inter carriers interference(ICI) caused by high-speed motion of aircraft. The computer simulation indicates that the proposed method can overcome DME interference and diffuse interference effectively, and improve the reliability of the LDACS1 system transmission link.Thirdly, a DME pulse interference mitigation method based on subspace projection and blind adaptive beam-forming has been proposed. The subspace projection algorithm is used to suppress the strong DME interference by projecting the array received signals on the orthogonal complement space of interferences. Then blind adaptive beam forming algorithm is utilized to extract the OFDM direct path signals based on inner product maximization criterion between the desired and reference signal. Finally Computer simulation results show that the proposed OFDM receiver can effectively suppress the DME impulse interference and OFDM scattering signal and improve the reliability of the LDACS1 system.