A Dual-Phase-Term Tracking Method for FMCW Ranging Radar With Elimination of Nonlinear Frequency Chirping and Range Ambiguity

In a conventional frequency-modulated continuous-wave (FMCW) radar, the target distance is estimated by the beat frequency, which is obtained from dechirping the received signal with the transmitted one. The challenge to achieve fine-range sensing is limited by the inaccuracy of beat frequency caused by the nonlinearity of sweeping chirps. In this work, instead of determining the accurate beat frequency, a novel dual-phase-term tracking (DPTT) method is introduced for high-accuracy range sensing even in a severe nonlinear situation. By tracking the phase of the beat signal and observing the phase difference over the up or down period of a triangular chirp, two distinct featured phase terms
$\phi _{o}$
and
$\Phi _{B}$
are extracted. The phase
$\phi _{o}$
represents the carrier round-trip phase delay at
$f_{\mathrm {o}}$
. Although
$\phi _{o}$
is free of the nonlinear chirping effect, the range estimation based on
$\phi _{o}$
suffers from the range ambiguity problem. The phase term
$\Phi _{B}$
represents the phase change over the first half chirp period and it is monotonically increased across the range ambiguity zones of
$\phi _{o}$
. Therefore, by combining these phase terms to determine the target range, both the frequency chirping nonlinearity and the range ambiguity can be effectively eliminated such that the range accuracy of tens of micrometers can be achieved even with an extremely narrow chirp bandwidth. To validate the proposed method, a 24-GHz FMCW radar with a 400-MHz chirp bandwidth and a 10-ms chirp period has been designed and implemented for demonstration. At a distance of 0.6 m, the measured range accuracy is
$40.1~\mu \text{m}$
for the weak nonlinearity factor of 0.3%. For the severe nonlinear chirp case with a 25.3% nonlinearity factor, the range accuracy of
$137.5~\mu \text{m}$
is also successfully obtained, validating the robustness of this proposed DPTT method.