With the range related parameters all set up, we move on to parameters related to sampling rate:
If you need to estimate velocities, that typically means applying an FFT on a frame over sweeps to produce a distance-velocity (a.k.a. range-Doppler) map.
In that case, the
sweeps per frame (SPF)
sets the frequency (velocity) resolution.
For e.g. micro- and macro gesture recognition, typical values range from 16 to 64.
For more accurate velocity measurements, typical values are much higher ranging from 128 to 2048.
If you don’t need to estimate velocities but still need to detect “fast” motions (\(\gtrapprox 500 \text{Hz}\)),
that typically means estimating the energy within a frame.
For such cases, e.g. running, walking, waving, typical SPF:s range from 8 to 16.
If you need to detect “slow” motions or have a mostly static environment, there is no need to use multiple sweeps per frame (SPF), so set it to 1.
Such cases include (inter-frame) presence detection and distance measurements.
For cases where SPF = 1, the sweep rate is not applicable.
What matters then is setting the HWAAS to achieve the needed SNR.
Keep in mind that measurement time linearly increases with HWAAS,
so keeping it as low as possible is important to manage the overall power consumption.
For cases where SPF > 1,
the sweep rate \(f_s\) should be adapted to the range of speeds of interest.
A good rule of thumb is
(3)\[f_s
\approx \frac{10}{\lambda_{RF}} \cdot |v|_\text{max}
\approx 2000 \text{m}^{-1} \cdot |v|_\text{max}\]
where \(|v|_\text{max}\) is the possible maximum relative speed between the radar and the object (in m/s).
In many cases, the most efficient way to achieve the target sweep rate is by adapting the HWAAS.
The sweep rate is inversely proportional to the number of HWAAS.
It is also possible to directly control the sweep rate, letting the sensor idle between sweeps.
However, idling between sweeps is rarely as efficient as idling between frames.
If power consumption is not an issue, start by using the highest possible frame rate.
Otherwise, it is crucial to minimize the frame rate to let the sensor idle in a lower power state as much as possible.
See Table 3 for typical values.