OFDM Reference Signal Reconstruction Exploiting Subcarrier-Grouping Based Multi-Level Lloyd-Max Algorithm in Passive Radar Systems

This study proposes a low-complexity subcarrier-grouping-based multi-level Lloyd-Max (SG-ML-LM) algorithm to reconstruct orthogonal frequency division multiplexing (OFDM) reference signal in passive radar systems. The authors’ previously proposed multi-level Lloyd-Max (ML-LM) algorithm is originally intended to blindly estimate the narrowband flat fading channel coefficients for non-constant modulus constellations. The authors extend it into broadband wireless environment to blindly estimate the frequency-selective fading channel coefficients by exploiting OFDM signal structure properties. Then a SG-ML-LM algorithm is further proposed to reduce the computational complexity of ML-LM by utilising OFDM subcarrier correlation. Numerical results validate that: (i) ML-LM and SG-ML-LM algorithms are both feasible schemes to reconstruct OFDM reference signals for passive radar applications; (ii) compared with ML-LM algorithm, SG-ML-LM algorithm significantly reduces the computational complexity at the cost of blind channel estimation error, but the OFDM reference signal reconstruction accuracy is nearly maintained, so that the cross-ambiguity function performance is nearly lossless despite of a negligibly slight increase of main lobe Doppler bandwidth.