Impact of Nonlinear Distortion in Pulse-Doppler Radar Receivers

Abstract

The aim of the thesis is to study the applicability of Direct Conversion Receiver (DCR) in radar, as these receivers possess the advantage of reduced complexity and fewer bandwidth limiting components compared to the superheterodyne receiver. As the wireless technology, it is also necessary to study about the receiver functionalities in radar using Digital Signal Processing (DSP) techniques. In today’s radar technology, designing the front end part of the radar receiver is more challenging, especially radar employing DCR, as these receivers are more prone to non-idealities such as I/Q Imbalance, non-linear distortion, DC offset and phase noise thereby affecting the dynamic range of the received echo signal. The pulse doppler radar is chosen in the thesis, as these radars are coherent, also capable of multiple target detection and provide large unambiguous range. The objective of the thesis is to analyse the effect of non-linear distortion such as second and third order distortion and observing the effects of these non-linearities in post-processing blocks. In this thesis baseband non-linearities in I and Q branches of the radar receiver are more specifically addressed than the RF non-linearity and blockers. The analysis is carried out in such a way that, the basic mathematical expressions for the RF signal in the transmitter part and received echo signal at baseband considering the effect of doppler are modelled. The second and third order non-linearities are modelled with some special cases by introducing some imbalances in I and Q branches and the effect of increasing the doppler frequency beyond the specified Pulse Repetition Frequency (PRF) is to be analysed. The effects of non-linearities are observed in post-processing blocks of the pulse doppler radar receivers. Thus, the final stage in the radar receiver blocks includes matched filtering or pulse compression and doppler processing as they are capable of separating target and clutter. The matched filtering or pulse compression follows Linear Frequency Modulation technique (LFM) as it is simple to generate the signal and insensitivity to doppler shifts. The pulse compression or matched filtering maximizes the Signal-to-Noise Ratio (SNR) by reducing the sidelobe levels using appropriate weighting functions so as to improve the resolution of the target. The doppler processing is capable of separating the target and clutter thereby improving the Signal-to-Clutter Ratio (SCR). The target signal is recorded and observed in a Range/ Doppler (R/D) matrix where the various parameters such as range, doppler information that includes measuring the doppler shift and radial velocity, location of the target etc. can be estimated. Other than the parameter estimation, the effect of non-linearities is observed in the R/D matrix. Comparisons of ideal scenario and non-ideal scenario and tabulations illustrating the comparison of second and third order distortion profiles are illustrated in the thesis to study about the performance and practicality of the radar receivers.