Time-frequency diffusion analysis for DOA estimation of tropospheric scatter signal
DOA estimation is a hot topic in tropospheric scatter radar target reconnaissance Over-The-Horizon (OTH). The received signal propagated through troposphere OTH is so weak because of strong loss, the classical MUSIC has been unable to effectively estimate angle. So we adopt Time-Frequency (TF) MUSIC, i.e., replacing the array covariance matrix of the classical MUSIC with Time-Frequency Distribution (TFD) matrix, which is suitable for the low Signal-to-Noise Ratio (SNR) condition. In order to effectively processing signal, the troposcatter signal characteristics should be modeled and analyzed. Firstly, we analyze troposcatter propagation loss, and further deduce the receiver output SNR to provide support for algorithm design. Secondly, considering that time, frequency diffusion in energy caused by troposcatter multipath fading, we derive time-delay and frequency-shift diffusion functions based on scatter transmission function. To be more specific, the diffusion characteristics are studied on the variations of the beam width, target distances etc.. Then we deduce the joint time-frequency diffusion function and coefficient from the above two diffusions. Affected by the diffusion, the TFD matrix will be distorted to degrade the algorithm accuracy. To utilize the characteristic, we propose a weighted scattering signal TFD matrix, where the weight matrix is derived from the joint diffusion coefficient, to prominent the scattering signal according to the matched filter theory. Then we can estimate exactly angle for the troposcatter signal using the weighted matrix.