**Jae Young Sim, Jae-Hyun Park and Jong-Ryul Yang \***

Department of Electronic Engineering, Yeungnam University, Gyeongsan, Gyeongbuk 38541, Korea; ja2922@yu.ac.kr (J.Y.S.); bravopark@ynu.ac.kr (J.-H.P.)

**\*** Correspondence: jryang@yu.ac.kr; Tel.: +82-53-810-2495

Received: 9 September 2020; Accepted: 24 September 2020; Published: 26 September 2020 -

**Abstract:** A frequency-shift keying (FSK) radar in the 2.45-GHz band is proposed for highly accurate vital-signs detection. The measurement accuracy of the proposed detector for the heartbeat is increased by using the cross-correlation between the phase differences of signals at two frequencies used by the FSK radar, which alternately transmits and receives the signals with different frequencies. Two frequencies—2.45 and 2.5 GHz—are effectively discriminated by using the envelope detection with the frequency control signal of the signal generator in the output waveform of the FSK radar. The phase difference between transmitted and received signals at each frequency is determined after calibrating the *I*/*Q* imbalance and direct-current offset using a data-based imbalance compensation algorithm, the Gram–Schmidt procedure, and the Pratt method. The absolute-distance measurement results for a human being show that the vital signs obtained at each frequency using the proposed FSK radar have a cross-correlation. The heartbeat detection results for the proposed FSK radar at a distance of < 2.4 m indicate a reduction in the error rate and an increase in the signal-to-noise ratio compared with those obtained using a single operating frequency.

**Keywords:** frequency-shift keying radar; cross-correlation; envelope detection; continuous-wave radar; frequency discrimination; vital-signs monitoring; heartbeat accuracy improvement; heartbeat detection; absolute distance measurement; radar signal processing
