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The body movement may be removed in digital signal processing if it has a regular motion pattern. However, in practice, the body most likely moves randomly, which challenges the signal processing techniques to predict the precise motion trajectory. Researchers have proposed several approaches to deal with the noise and artifacts such as the random body movement. A multi-radar system was introduced in [ 54 ] to detect the subject from different sides of the body.
Figure 6 shows the setup of using two radars measuring from the back and the front of the body simultaneously. The two radars as shown in Figure 6 are identical and both use patch antennas with linear polarization. To mitigate the interference to each other, antennas on one of the radars can be rotated 90 degrees so that the two sets of antennas on the two radars are orthogonal [ 57 ]. However, for random body movement, when the body leans to one of the radars, it moves away from the other.
That said, the distance change between the body and one radar is opposite as compared to that between the body and the other. By combining the measured signals from the two radars, the noise from random body movement can be mostly canceled [ 57 ]. A similar approach is proposed in [ 10 ] using two injection locking radars array for cancelling the body movement artifacts. Setup of noncontact vital sign detection using two radars with random body movement cancellation.
The sensors array approaches have apparent drawbacks. For example, they inevitably add to the system complexity, cost and power consumption. They are favorable for specific use cases where there is enough space to set up the sensors array. A radar-camera hybrid system using compensation was proposed in [ 69 , 70 ] to cancel the random body movement, the setup of which is shown in Figure 7.
This approach proves the feasibility of using an ordinary smart phone camera to help to cancel the body movement. It works in such a way that the external phase information that is opposite to the phase information of random body movement is added in the radar received signals. Since the radar measured signal includes phase information of both vital sign and body movement, the added information could be used to only compensate the phase information of the body movement but retain the vital sign signals. Three motion compensation strategies were proposed in [ 70 ]: a phase compensation at RF front end using a phase shifter; b phase compensation using baseband complex signals; and c compensation after phase demodulation.
Since the large body movement may produce large signals that may saturate the baseband circuitry, phase compensation using phase shifter at the radar RF front end helps to relieve the radar circuitry from potential saturation. The other two strategies are implemented in the baseband digital domain so they can be more precisely controlled to perform fine tuning to remove the artifacts.
The aforementioned approaches are for compensating the body movement while the radar system is stationary. Researchers have also proposed approaches to tackle the movement of the radar platform itself, which can be useful as radar sensors are expected to be integrated in mobile platforms such as smart phone or tablet. For example, [ 71 ] demonstrated a complete compensated single transceiver radar system for vital sign detection in the presence of platform movement.
By putting a custom designed tag near the subject, it produces a reference signal for the radar platform to compensate the movement of itself.
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Another challenge in Doppler radar sensing is the signal distortion while measuring subjects that are moving slowly. The signal pattern of the slow motions is often distorted. However, due to circuit imperfections and clutter reflections from surrounding stationary objects, the homodyne receiver suffers from DC offset. AC coupling after mixer output is commonly used, probably the simplest way, to remove DC offset.
The AC coupled baseband essentially has high pass characteristics, which results in signal distortion when the target motion has a very low frequency or a stationary moment [ 67 ]. Signal distortion happens due to AC coupling. As radar sensing is based on nonlinear phase modulation, the radar measured signal at baseband output can be expanded as Bessel function.
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As can be seen in Equation 12 , even for an ideal Doppler radar detecting a single-tone sinusoidal movement, harmonics will be created at the baseband output. If the target movement is slow or has stationary moment so that it falls into the stop-band of the active high-pass filter, the harmonics would be subject to different degrees of attenuation, owing to the slope characteristics of the high-pass filter. However, the distorted ribbon trajectory challenges the calibration algorithm to find the accurate DC offset values as well as the center of the arch [ 67 ]. To eliminate the signal distortion, a straightforward method is to avoid using the AC coupled baseband structure and employ the DC coupled architecture for radar hardware [ 61 , 66 ].
The DC coupled baseband is all-pass architecture, not like high-pass in AC coupling, so that all the harmonics from nonlinear phase modulation will undergo the same degree of attenuation or amplification, which ensures no signal distortion. To employ DC coupling, hardware modifications need to be made in order that the DC offset at mixer output will not saturate the following stages. Figure 9 shows the block diagram of the DC coupled Doppler radar system, as proposed in [ 61 ].
The radar system was designed with adaptive DC tuning stages including RF coarse-tuning and baseband fine-tuning. As we know, a major part of the DC offset is from direct coupling from the transmitter to receiver resulting in the mixer self-mixing. To deal with that, the RF tuning was implemented using a path of an attenuator and a phase shifter at the RF front end of the Doppler radar system, as shown in Figure 8. By doing so, it adds a portion of the transmitter signal back to the receiver so as to cancel out most of the DC offset at mixer output.
To further calibrate the remaining DC offset and to minimize the quantization noise impact from ADC, the baseband fine-tuning circuitry is added to adaptively adjust the amplifier bias to the desired level. With the two tuning stages, it allows both high gain amplification and maximum dynamic range the baseband stage. With the proposed DC tuning architectures, the Doppler radar is able to precisely measure the low-frequency movement even if the motion trajectory has stationary moment.
Block diagram of the Doppler radar with DC coupled architectures including RF coarse-tuning and baseband fine-tuning. The proposed DC coupled radar was experimentally verified in the lab environment. Both the DC radar and AC radar were used to measure the actuator that was programmed to perform sinusoidal movement with a short period of stationary moment in between two adjacent cycles.
The experimental results, as shown in Figure 10 , illustrates that the DC radar matches with the actuator very well by successfully preserving the stationary information. However, owing to the fact that the AC coupling capacitors cannot hold the charge for a long time and they tend to discharge over the stationary moment, the stationary information is distorted in AC radar. Experimental results of using both DC radar and AC radar to measure the actuator motion which is sinusoidal with a stationary moment in between two cycles.
Using DC coupled radar inevitably adds to the hardware complexity. Several approaches have been proposed to use AC coupled radar for accurate displacement measurement without adding to radar architecture complexity. The calibration is based on data-based imbalance compensation [ 59 ] and radius correction [ 73 ].
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A more advanced technique of digital post-processing DPoD was proposed to compensate for the signal distortions in the digital baseband domain [ 58 ]. Without any cumbersome hardware modification, the simple quadrature direct-conversion receiver architecture can be used to detect the complete pattern of slow periodic motions.
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In contrast to that, the proposed digital DPoD technique applies signal compensation in the digital baseband to recover the signal information that may be lost in an AC coupled receiver. The signal distortion is compensated in the digital domain by an algorithm whose system response is the inverse function of that of the high-pass filter at AC coupled baseband. Therefore, the signal integrity can be preserved using the low-cost low-complexity AC coupled architecture without any hardware modifications. The DPoD technique was experimentally evaluated by using an AC coupled radar to measure a healthy adult at rest.
It is known that the respiratory motion for an adult at rest includes a short period of stationary moment, which means that the respiration tends to rest for a moment at the end of expiration [ 33 ]. The experimental result in Figure 11 shows that, without DPoD, the information of stationary moment is lost so the respiration signal was distorted. After applying DPoD, the stationary moment is recovered so the complete respiratory patter is restored.
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For example, the quadrature mixer used in [ 58 ], i. However, lower quality radars at reasonable price are still popular for some applications, such as the automotive anti-collision system and the automotive doors. For instance, the radar module used in [ 60 ] has amplitude imbalance of 1. A method using GS to estimate the amplitude and phase imbalances in Doppler radar was proposed in [ 59 ]. However, this method requires modifying the radar hardware by adding an external voltage controlled phase shifter between antenna and radar transceiver.
To fit the ellipse parameters to the data, there are generally two methods: algebraic fitting or geometric fitting, the latter of which is known to be more accurate and robust. The method in [ 60 ] is based on algebraic ellipse-fitting. 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.
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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 From a cost perspective, it is a better approach to make the system work at 2. Figure 12 shows the block diagram of a miniature 2. 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.
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.
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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.