Frequency modulated continuous wave radar with substrate integrated waveguide beamforming network for multi-target human respiration detection

• Multi-target respiratory identification is performed by observing the phase response obtained from the target. • Errors in reading the phase response can be influenced by radar multipath effects. • Beamforming with a Butler matrix enables the separation of signals from multiple targets by exploiting spatial selectivity. • Implementing Substrate-Integrated Waveguides in millimeter-wave Beamforming Networks reduces radar transmission loss. This study proposes the implementation of Beamforming using a Butler Matrix with Substrate Integrated Waveguide (SIW) on multi-target breathing detection using Frequency Modulated Continuous Wave (FMCW) radar, specifically on mmWave Radar. Although Butler Matrix beamforming has been widely researched for communication systems, its implementation using SIW technology for multi-target breathing detection on mmWave FMCW radar is still limited. The beamforming technique is used to improve the received signal by directing the beam directly to the target, so that the multipath effects from the environment can be suppressed. The test results of the proposed 4 × 4 Butler Matrix (BM) form 4 main radar beams at angles of ±15 and ±40 degrees. The working frequency of the BM is still in the working frequency range of the TE 10 mode, namely 20 - 30.24 GHz. The measurement results show a reflection coefficient below -15 dB and isolation below -10 dB at 24 GHz, which indicates efficient delivery of power. Tests show that the directional beam from the Butler Matrix channel will be directed to each target, thus increasing the extracted phase readings for the respiratory signal above 95% and allowing angular separation of multiple targets. The proposed approach provides a low-complexity passive beamforming solution for multi-target vital signs monitoring in mmWave radar systems.