XeThru Pulse Doppler Radar - the backbone technology for respiration monitoring
In this blog post, we take a closer look at the Pulse Doppler principle that the XeThru technology is founded upon. The Pulse Doppler Radar is a highly advanced and powerful system compared to traditional radar systems. Let’s cover some basics first.
From its inception, Pulse-based radar has the ability to determine position to the target and is the measurement principle found in the XeThru sensors. A pulse is beamed towards a target area and an extremely fast (40GSps) receiver is able to detect the reflected energy originating from the pulse interfering with objects in the target area. The energy in each pulse is very low and therefore a large number of pulses is required for good measurement. Figure 1 highlights the issue with pulse radar in presence of large static objects - they will mask away any smaller moving targets.
Figure 1: Pulse radar
On the other hand of the radar spectrum lies Continuous-wave radar, a type of radar system that continuously transmits electromagnetic radiation. CW radar uses Doppler, which renders the radar immune to interference from large stationary objects and slow moving clutter. It also has the capability to measure speed and direction of a moving object. These continuous wave radars are however not capable of measuring distance to the target - and that is one of the key elements needed for successful respiration or presence monitoring.
Figure 2: Doppler radar (CW)
By combining the two types of radar, we are able to obtain the most desirable features: distance measurement from the pulse radar and velocity discriminators from the Doppler signal processing. In figure 3, both principles are put together into one result. With its large bandwidth and good penetration capabilities, the pulse Doppler radar here enables finer range resolution and has the ability to see through duvets and blankets. In a respiration monitoring scenario, large stationary objects like furniture or walls can be suppressed while small chest movements can be accurately measured.
Figure 3:Pulse-Doppler radar
At Novelda, we take the Pulse Doppler Radar technology to the limit with our XeThru sensors. With the best of both types of radars, the sensors are able to do all the above with high accuracy and precision. Fans, curtains, and venetian blinds can in many cases look like a person breathing- however with further signal processing the XeThru radar discriminates all unwanted reflections from these moving objects. The firmware will also not lock onto these. This is shown in figure 4, where all stationary reflections from the room were removed. The only reported targets are the inhale and exhale movements of the chest itself.
Figure 4: Pulse Doppler example - high pass filter
Even when put to the most rigorous test in the industry, our XeThru breath monitoring solution stands tall. You can see more about this in the following video where we compare XeThru with a medical polysomnography sleep study at Trondheim’s St. Olavs Hospital.
You can also check this video for a detailed overview of XeThru's System on Chip, UWB signal processing, and more examples of the Pulse Doppler Radar.