Delay and doppler shift estimation for non-constant envelope modulation in OFDM radar system

Estimation of delay and Doppler shift is one of the main applications of radar systems. Particularly in orthogonal frequency division multiplex (OFDM) radar system for estimation of delay and Doppler shift, maximum likelihood (ML) estimator with constant envelope modulation technique is commonly used. However, such estimator does not perform well at low signal to noise ratio (SNR). In this paper, we propose a technique to enhance the SNR threshold of the ML estimator. Instead of using constant envelope modulation technique, we propose to use non-constant envelope modulation technique such as quadrature amplitude modulation (QAM). To reduce the SNR threshold, modulation order of QAM is used as a tuning parameter. To validate correctness of the proposed methodology Cramer-Rao lower bounds for the variance of delay and the Doppler shift are derived. We demonstrate significant improvement in SNR threshold of the estimator through an extensive simulation study. Further, performance improvements of the ML estimator with the proposed technique over conventional matched filter estimator is illustrated.