Interference Mitigation Research Based On Spatial Filtering For OFDM Receiver

L-band Digital Aeronautical Communications System 1(L-DACS1) is the candidate technology for the future civil aviation mobile communication system, which mainly provides Air Traffic Control and Airline Operation Management for aircrafts in land route, terminal areas and airports.The L-DACS1 based on Orthogonal Frequency Division Multiplexing(OFDM) modulation, will be deployed as an inlay system and operated in the frequency gaps between adjacent channels of Distance Measure Equipment(DME) system, which brings the problem that DME pulse signals interfere OFDM receiver severely. To mitigate the deleterious influence of DME interference on OFDM receiver of L-DACS1, the interference suppression methods based on spatial filtering are researched. The main contributions are summarized as follows:The DME pulse interference mitigation method based on joint spatial Direction of Arrival(DOA) estimation and main beamforming is proposed. Firstly, by utilizing the spatial characteristics differences in received signals, the DOAs of all received signals are estimated by DOA matrix method. Then, Linearly Constrained Minimum Variance beam forming algorithm is utilized to extract the signals in all directions based on the DOA information.Finally, the OFDM signals are separated by comparing the difference of frequency domain power distribution, and the OFDM direct path signal is finally determinated by calculating the time domain power of all OFDM signals. Computer simulation indicates that the proposed method can overcome DME pulse and OFDM scattering interferences effectively, and improve the reliability of the OFDM receiver.The blind adaptive interference mitigation method based on subspace projection is proposed. This method includes the following three schemes :1)The adaptive interference suppression method based on the symmetrical features of the OFDM cyclic prefix. Firstly,subspace projection algorithm is used to suppress strong interference signals. Then, a novel blind adaptive beamforming algorithm, which uses the symmetrical features of the cyclic prefix, is proposed to extract the OFDM direct path from the interference-free signals.2)The adaptive anti-jamming method based on the spatial DOA estimation of the desired signal.Firstly, the DME interference and noise signals are canceled by the oblique projectionalgorithm. Then, according to the strong self-coherence property of the cyclic prefix and suffix, the direction of the desired signal is estimated by CLEAN algorithm. Finally, the conventional beamforming is formed to extract the OFDM direct path signal.3)The adaptive interference mitigation method based on the estimation of interference signal steering vectors.Firstly, the interferences are eliminated by projecting onto the interference orthogonal complement space, which built by the interference signal steering vectors. Then, the array weight vector is calculated based on the output power ration maximization criterion. Finally,the OFDM direct path signal is extracted from the interference-free signals. Computer simulation results show that the proposed interference mitigation schemes can effectively eliminate the interference impact of the DME impulse and OFDM scattering signal.