Some of the first considerations for a harmonic radar application are discussed.
Maximum harmonic radar tag range: a 917 MHz radar using uncoded or pulsed CW may get about 10-15 meters range using 10 dBi antennas on the radar and 5 dBi passive tags. Shifting up in frequency increases the gain, narrows directivity (beamwidth) of the antennas, yielding more angular selectivity for a given antenna size.
Active (powered) tags are required for extended ranges into 100+ meters standoff distance. Many years of battery life are possible for such tags, and power harvesting methods and other modern semiconductor techniques may be applied to approach batteryless or battery-assisted tags for decadal service lifetimes.
Depending on the application (what you’re sensing) and the environment the tag is in, the tag could last from a month to a decade or more. Tag lifetime is very system dependent.
This is a topic covered by patents and the open literature, including technology SciVision, Inc. has modeled and brought to fruition. At a high-level, this is a key design parameter of the system. Will classic angular selectivity (antenna beamwidth) be enough? The 3 dB beamwidth might be > 60 degrees, and the 20 degree beamwidth (more important for discrimination) might be well in excess of 90 degrees. Larger antennas, antenna arrays, or networks of radar antennas help increase angular/spatial target discrimination.
The classical multiplexing methods of TDMA, CDMA and FDMA may also be applied in various scenarios, particularly for active tags.
Active tags require a DC power source to bias the diode (or whatever semiconductor structure is chopping or otherwise frequency translating the input RF waveform). This DC bias may well come from power harvesting or trapped charge FETs or the like. Just a wee bit of bias gives a lot of signal gain (via reduced loss) thanks to the non-linear response of the diodic junction that creates the harmonics.
Passive tags inherently get their bias from the exciting RF waveform. Intrinsic power harvesting, to abuse a term (much like how everything is AI/machine learning these days).
To predict performance of a harmonic radar requires a bit of knack and experience at RF systems performance. You might engage with a consultant from SciVision, Inc., which has helped build valuations for several clients to impressive sums via custom model-based designs. Likewise, unfeasible projects were shot down early by modeling and early prototyping.