**5. Conclusions**

A remote vital signs detector based on FSK radar with the cross-correlation method implemented in a single radar architecture is presented for accurately detecting heartbeat signals. The proposed FSK radar can increase the SNR of vital signs and the frequency resolution in the FFT spectrum by using cross-correlation between the phase differences individually obtained at two operating frequencies. An envelope detection method for controlling the frequency-switching signal for FSK operation is proposed for phase-difference discrimination, which is mandatory for obtaining vital signs at each operating frequency. Distance measurements for human subjects shows that the vital signs obtained at each operating frequency of the proposed FSK radar can be correlated with each other while producing an acceptable distance error. The measurement results for the number of heartbeats per minute for subjects at a distance of 1–2.4 m demonstrate that the proposed radar can improve the vital-signs detection accuracy and SNR via cross-correlation.

**Author Contributions:** Conceptualization, J.-R.Y.; methodology, J.Y.S., J.-H.P., and J.-R.Y.; software, J.Y.S. and J.-H.P.; validation, J.Y.S. and J.-R.Y.; formal analysis, J.Y.S., J.-H.P., and J.-R.Y.; investigation, J.Y.S., J.-H.P., and J.-R.Y.; resources, J.-R.Y.; data curation, J.Y.S.; writing—original draft preparation, J.Y.S. and J.-R.Y.; writing—review and editing, J.-R.Y.; visualization, J.Y.S.; supervision, J.-R.Y.; project administration, J.-R.Y.; funding acquisition, J.-R.Y. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was partly supported by the Institute of Information & Communications Technology Planning & Evaluation (IITP) grant funded by the Korea government (MSIT) (No. 2019-0-00138) and the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Korea government (MSIT) (No. 2017R1C1B2002285).

**Acknowledgments:** The authors thank Hyun-Jun Ji, who assisted with the vital-signs detection, and Ju-Hee Son, who designed the patch antennas.

**Conflicts of Interest:** The authors declare no conflict of interest.
