# Short-Range Noncontact Sensors for Healthcare and Other Emerging Applications: A Review
## 3 Recent Technical Advancements
### Hardware System Integration  
In addition to the technical advancements on the system side and the signal processing side, researchers have also made tremendous efforts to develop advanced techniques for integrating radar sensor systems on miniature printed circuit boards or on silicon as a chip substrate. Doppler radar systems built up using instruments [24] or off-the-shelf connectorized RF modules [55] are usually bulky in size and hungry in power consumption, making them inconvenient for portable applications. Research efforts have been made to integrate the radar system on board and on chip. Owing to the advancements of the IC technologies in this century, there are numerous commercially available RF chips in the market, especially in the market of mobile platforms such as smart phones and tablets. In healthcare applications, the vital sign signals of respiration and heartbeat are usually very weak vibration motions with amplitudes of a few millimeters or even less. Unlike the common observation that higher frequency lead to better performance because of deeper phase modulation, it has been demonstrated that radar modulation sensitivity is not proportional to the carrier frequency but is strongly correlated with the ratio between the vibration amplitude and the carrier frequency [62]. It is expected that frequency in the 20 GHz range may result in the optimal heartbeat detection for most people [62]. However, the board level system integration needs to leverage not only the sensing performance but also the chip availability, cost, and form factor. Since various wireless technologies are using the ISM bands of 2.4 GHz and 5 GHz, such as Bluetooth and WiFi, there are ample RF ICs available in the market at reasonably low price. Integrating radar systems at lower frequencies is more cost effective and can meet the performance requirement as long as the system is carefully designed. Although WiFi 802.11n/ac employs 5 GHz, there are a lot more lower-cost RF ICs in the 2.4 GHz band. From a cost perspective, it is a better approach to make the system work at 2.4 GHz. Figure 12 shows the block diagram of a miniature 2.4 GHz radar system designed by researchers at Texas Tech University (Lubbock, TX, USA). The system-on-board has a form factor of 5 cm by 5 cm. Figure 13 is the picture of the fabricated and assembled 2.4 GHz Doppler radar. The radar transceiver was designed as a homodyne architecture with AC coupled baseband. It is a coherent system because both transmitter and receiver share the same free-running voltage controlled oscillator (VCO). The distance between the subject and the radar is usually at most several meters in healthcare applications, which is important for the range correlation effect to stand. Due to range correlation, phase noise from VCO can be cancelled out at receiver so that it does not affect the low-frequency respiration and heartbeat signals. The VCO is free-running so it transmits a single tone signal. The VCO output is divided equally via a surface-mount balun into two parts. One part of the signal is amplified and directly sent out via Tx antenna to the subject. In life applications, such a Tx amplifier is optional depending on the desired detection range. If an Tx amplifier is employed, special attentions should be put on the direct coupling of the strong Tx signal back to Rx front end, which may saturate the Rx chain. The other part of VCO output is fed into the quadrature mixer as the local oscillation (LO) signal to convert the received RF signal down to baseband. The Doppler radar receiver mainly consists of a low noise amplifier (LNA), a band-pass filter, a gain block, and a quadrature mixer. The weak vital sign signal requires that LNA have good noise figure and decent gain to suppress the noise contributions from the following stages. To block out of band interferers, a commercial 2.4 GHz band pass filter with bandwidth of 100 MHz is used. The 2.4 GHz ISM band is full of many other wireless technologies, such as WiFi, Bluetooth, microwave oven, etc.