## 4 Discussion  
While CW and FMCW radar systems have been investigated and experimentally verified for use in noncontact subject monitoring for many years [42; 43], demonstrations of the viability of PMCW radar systems in this application space are still sparse. Many studies on PMCW response simulations have been conducted, but the number of realized systems with measured results is low [35]. The multimode radar proposed in this article aims to further research into PMCW subject monitoring and system-level implementation. One challenge faced with PMCW radar measurements is DAQ sampling speed. Without a synchronous digital clock, the radar response and modulation signals need to be oversampled for proper demodulation. A solution to this issue is seen in PMCW radar SoCs, which circumvent this issue by using the same clock for sampling and modulation. For the 1 kHz clock used here, oversampling is achievable using commercial DAQs or oscilloscopes, but this clock speed does not allow for distance measurements through phase delay analysis that is achievable with higher time resolution analysis. Of the tested waveforms, the CW mode is the simplest to implement and measure. Relying only on the Doppler response, target motion is recovered from the baseband signals without needing any additional demodulation or requirements on sampling frequency outside of being high enough to measure the imparted frequency. It is also the most limited mode, since there is no additional information that can be gained from the response without introducing modulation. The PSK response is slightly more flexible in how it can be processed, as it does not require complex demodulation approaches to extract low-frequency information that is modulated by a square wave. It also upshifts the frequency response, meaning DAQs with low cutoff frequencies would still be able to acquire the Doppler response centered around \(F_{CLK}\). Target distance can also be estimated by measuring the phase delay of the transmitted and received phases, but the unambiguous range is limited to one range bin due to the repetition length of the encoding. PMCW modulation, however, is far more flexible for determining range and capable of generating orthogonal signals. As  
Figure 15: PMCW mode spectrograms for repeated hand gestures. mentioned, various binary phase encodings have been studied for their beneficial autocorrelation properties. These allow for unambiguous ranging that is dependent on code length, while range resolution is dependent on \(F_{Clk}\). These benefits require more processing and measurement time to recover and demodulate the PMCW response, and there are limited short PSRB codes. Despite this, as shown in the measurement results, the impact is low enough that respiration responses and gestures can still be recovered. Low-frequency noise also impacts the measurements. The main constant amplitude frequencies seen in the CW mode spectrogram response are noise from the 60 Hz power grid and its harmonics. For the PSK and PMCW modes, since the carrier is a mix of \(F_{C}\) and the modulation wave, the baseband signal is upconverted above this noise level. This frequency shift can be beneficial in many stages of the processing chain, such as in raising the cutoff frequency of a high-pass coupling filter or the improved linearity of baseband amplifiers for higher-frequency signals. This also allows for a variety of DAQ inputs such as computer audio cards, which usually have a cutoff frequency of 20 Hz, to be used for their high resolution and frequency analog-to-digital conversion. The PSK data shows a reduction in background noise due to most of the power being upconverted to the first harmonic of the square wave at \(F_{Clk}\) and then taking a windowed average of the response. The PMCW data ends up with an overall raised noise floor as any noise is spread across the frequency spectrum as opposed to being upconverted to square wave harmonics, but a more directive antenna or a higher power VCO would improve the spectrogram measurements.