System-Level Hardware-Waveform Co-Design for Micro-UAV Radar Sensors: Suppressing Noise Folding in Extreme SWaP-C SSDBF Arrays

Micro-Unmanned Aerial Vehicles (micro-UAVs) require compact radar arrays under severe Size, Weight, Power, and Cost (SWaP-C) constraints. Spread Spectrum Digital Beamforming (SSDBF) reduces receiver hardware by multiplexing multiple antenna channels before a shared RF chain, but the receiver-side switching operation folds wideband noise and injects switch-related transient noise before digital demultiplexing. This paper develops a hardware-waveform co-design framework that links the transition density of bipolar spreading sequences to high-frequency code energy and switching-event count, while explicitly distinguishing modulation-induced thermal-noise folding from switch-transient injection. Transition density is used as a physically interpretable design surrogate rather than a sufficient statistic for folded noise, and it is constrained jointly with non-zero-shift cross-correlation to preserve spatial isolation under receiver-side timing skew. An ϵ-constrained Greedy Coordinate Space Search (ϵ-GCSS) algorithm is proposed to synthesize low-transition-density SSDBF code sets. Commercial circuit-level transient simulations and system-level MATLAB R2024b simulations show that the proposed code set reduces the PRN-like transition-density level from about 0.50 to about 0.19, lowers the circuit-simulation-derived integrated IF noise power by 9.63 dB relative to the Gold/PRN-like baseline, and improves the normalized maximum detection range by 8.2 percentage points at K=4 without adding analog front-end hardware.