## 1 Introduction
Phase codes that satisfy the orthogonality requirements of CDMA MIMO have been adapted from communication systems and include but are not limited to maximum-length sequences (MLS), Hadamard codes, Gold codes, optimal peak-to-sidelobe (OPS) codes, almost perfect autocorrelation sequences (APAS), and zero-correlation-zone sequences (ZCZ) [29; 30]. While many of these codes have been investigated for use in phase-coded FMCW (PCFMCW) MIMO radar systems [31; 32], the benefits gained from each are shared with phase-coded CW radar. From [18; 19], there is clear potential for PMCW radar systems to support low radar-radar interference, provided the phase encoding is unique to each radar in the application space. Moreover, a sufficient knowledge base from decades of communication systems research and implementation, as well as a suitable performance level of commercially available digital components has helped propel the research of digital radar systems. Following years of conceptualization and simulation, PMCW radar systems have started to be explored in depth, mainly through the development of radar systems-on-chip (SoC) with built-in digital modulation [33; 34]. While the main industry targeted by these SoC radar systems is automotive applications, event recognition and vital sign detection are also being explored [35; 36; 37]. Even with the growing interest in exploring PMCW radar systems, few fully realized systems have been demonstrated. In [33], a MIMO PMCW SoC developed by IMEC with 2 transmitters and 2 receivers is detailed along with measurements of stationary corner reflectors. For this device, distance measurements are performed on-chip and transferred to a computer for postprocessing. In [34], a more advanced PMCW MIMO radar SoC developed by Uhnder is detailed, utilizing 12 transmitters and 16 receivers along with internal self-interference cancellation, data acquisition, and on-chip processing. Moving vehicle detection, pedestrian detection, corner reflector response, and digital beamforming sidelobe levels are presented, as well as comparisons to other 77/79 GHz SoCs. In [35], an FMCW radar SoC is compared with a benchtop binary PMCW radar built from high-end evaluation circuits. These systems were used to evaluate current off-the-shelf components for use in automotive radar applications, such as measuring target range, multiple target separation, and moving target response. In [36], various events were classified using a PMCW radar developed by K&G Spectrum Inc. Preliminary results demonstrated the potential for PMCW radar systems for use in identifying stationary subjects and their activities, along with heart rate and respiration measurements. In [37], a PMCW radar response is simulated and compared to the measured response of a PMCW radar prototype also developed by K&G Spectrum Inc. Here, vital sign information was extracted using Time-Frequency Representation (TFR), and possible scenarios were identified using the multiple signal classification (MUSIC) algorithm. PMCW radar systems have some drawbacks compared to FMCW radar systems, such as additional loads being introduced in the processing chain, the incorporation of digital modulation into the carrier wave, and the Doppler shift intolerance of PRBSs when measuring the target range. Advancements in digital signal processing and field programmable gate arrays (FPGA) can help lighten the processing load. High-speed wideband binary phase shifters (BPS) have also been developed, and PMCW radar integrated circuits (IC) have integrated methods of introducing phase modulation [33; 34]. For the Doppler shift intolerance, increasing the length of the PRBS helps to alleviate the loss in the maximum unambiguous range of the target [38]. While there are extensive studies on realized FMCW radar systems for vital sign and gesture detection, most real-world PMCW radar measurements have been focused on automotive applications.