Waveform Bandwidth and Frequency Diversity in FDA-MIMO Range-Angle Estimation: A CRLB Perspective

Abstract

Frequency-diverse array multiple-input multiple-output (FDA-MIMO) radar introduces range-dependent phase diversity through element-dependent carrier-frequency offsets, in addition to the delay information carried by transmitted waveforms. This paper develops a bandwidth-aware Cramer-Rao lower bound (CRLB) framework to characterize how these two mechanisms jointly affect range-angle estimation. Starting from a Nyquist-rate sampled observation model that retains waveform delays and FDA frequency offsets, we derive an equivalent Fisher information matrix (EFIM) after eliminating an unknown quasi-static complex scattering coefficient as a nuisance parameter. We derive closed-form CRLB approximations that reveal the interplay between waveform root-mean-square (RMS) bandwidth and FDA frequency diversity. A three-factor decomposition of the squared normalized cross-information further characterizes range-angle coupling. Results show that when increased independently, both waveform RMS bandwidth and FDA frequency diversity reduce the range CRLB, but FDA frequency diversity increases the angle CRLB at fixed nonzero waveform bandwidth, whereas waveform RMS bandwidth reduces it. Additionally, under a fixed aggregate spectral budget, the range CRLB exhibits an interior maximum, whereas the angle CRLB increases monotonically with the FDA spectral-allocation fraction. Numerical results corroborate the closed-form approximations and the predicted trends.