"The IF signal has a phase that is the difference between transmit and received signal phases."
Yes. That's a neat property of superhetrodyning - phase is preserved. Both the outgoing and incoming signals are down-converted by mixing with the local oscillator. The phase angle difference between out and in is the same at both the transmitted/received frequency and the IF frequency. But down at the IF frequency, you get to work at a lower frequency where it's easier to do A/D conversion and counting. Most software defined radios still have a superhetrodyne front end, so the digital stuff is working at the IF frequency.
This is less necessary than it used to be, now that digital circuits can work well into gigahertz ranges.
Heh, when getting a GPS Disciplined Oscillator set up I was feeding the 10 MHz oscillator into an SDR to use as a reference clock and was calibrating against a local cell tower. 7 Hz beat/offset at 1700 MHz!
>> The time between two consecutive chirps is called the pulse repetition rate (PRT), and plays a key role in the accuracy of doppler velocity estimation.
This is actually known as the pulse-repetition interval (PRI), or time (PRT). A "rate" is describable by a frequency. An interval is described with a unit of time between repetitions. Radar signal characteristics are a rabbit hole of such definitions. They really do matter once one switches from theoretical discussion to actual math. Confuse a rate with a period and your math for calculating ambiguity zones will fall apart.
What will happen if every single car has radar? Wouldn't they interfere? You're stuck in traffic and 300 nearby cars are blasting radars all over the place?
Typical FMCW radars transmit very short ramps (microseconds) at a very long (relatively) intervals (several ten milliseconds), i.e. a duty cycle of less than 0.1%.
In order to create interference between two radars, the ramps have to overlap pretty exactly, within a few nanoseconds of each other. This is very unlikely to happen.
Modern radars employ technologies to detect and/or avoid such collisions.
Overall it is not really an issue, even with many radars in crowded spaces.
This is true for some earlier lofi radars, but as driver assistance and self-driving have developed, so have the requirements and capabilities of the radar systems. Newer systems generally have shorter PRIs for higher doppler bandwidth, and much higher duty cycles for more energy on target - the FCC limits power, so you've got to get energy from the time axis. Both of these things make the interference problem harder.
If you're on the road in a relatively affluent area where people drive late model cars, this is pretty close to already the case. Automakers have started making these systems standard on many/all of their models in the US for several years now. Toyota, for example, started rolling out these systems a decade ago, and have been standard on all US models since 2018.
I'm not sure what these systems use in practice for interference mitigations, but there's a bunch of stuff that could be done, for instance, hopping between different frequencies.
Interference is a real problem with FMCW radars, either maliciously in the case of electronic warfare, or accidentally in the case you mentioned, with many radars in the same space using the same frequency band. Wifi and cell phones use time division or frequency division multiplexing techniques, but radars (at least current-gen) generally do not.
There are mitigation techniques like randomization of chirp frequencies, choosing different idle times between frames, and signal processing techniques to try to detect interference and filter it out. In the general case, FMCW techniques will always have interference problems.
This is one reason amongst many others that military radars do not use FMCW but instead coded pulse compression techniques.
I suspect radar like this needs only a tiny time slices to do its work. Say for example that it's only necessary to get updates about the moving object 100 times per second: every 10 ms. The radar pulse durations necessary to do the job can probably be measured in microseconds, though. A 10 ms separation between pulses measured in microseconds is a large amount of empty space.
There are various ways a radar (or any RF signal) can be designed to recognize its own signal from all the background noise. We don't worry about millions of cell phones or WiFi routers sharing bandwidth either.
Co-channel Wifi interference is real. It really puts a damper on range and throughout compared to how it used to be. It is a largely unmitigated clusterfuck, as is the way with CSMA/CA once density increases enough.
LTE interference isn't an important thing in practice, in part because because all participating devices have very tightly-controlled timings. It isn't a clusterfuck at all because of the mitigations in place, but it does require centralized coordination to be this way.
Radars on cars don't have centralized coordination (do they?). What mechanism prevents their performance from degrading as wifi does?
Nonetheless, even in the very best case, every other radar still increases the background noise floor that each radar has to distinguish its own signal above. It won't ruin the signal completely, but it will affect how much scan time or output power is needed, or the detection resolution it can attain.
Actually, in both cases, we do. Cell towers deliberately have different frequencies allocated from their neighboring towers, and Wi-Fi has multiple channels, several of which do not have any overlap.
> With a PC with Intel Wi-Fi sensing capabilities in sleep mode, the PC Wake-on-Approach is activated as it detects human presence. Even when a user forgets to lock the PC, a count-down to lock starts with no human present. False detection is prevented even with human presence behind and next to the PC.
It would be interesting to see how to attenuate or deflect radar emissions from cars to passively disable automatic braking. Won’t brake if the car believes it’s driving on a flat expanse of nothing.
If you had a chaff dispenser, perhaps you could deploy a chaff cloud that would look like a fixed object and trigger the emergency braking of the cars behind you.
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Yes. That's a neat property of superhetrodyning - phase is preserved. Both the outgoing and incoming signals are down-converted by mixing with the local oscillator. The phase angle difference between out and in is the same at both the transmitted/received frequency and the IF frequency. But down at the IF frequency, you get to work at a lower frequency where it's easier to do A/D conversion and counting. Most software defined radios still have a superhetrodyne front end, so the digital stuff is working at the IF frequency.
This is less necessary than it used to be, now that digital circuits can work well into gigahertz ranges.
>> The time between two consecutive chirps is called the pulse repetition rate (PRT), and plays a key role in the accuracy of doppler velocity estimation.
This is actually known as the pulse-repetition interval (PRI), or time (PRT). A "rate" is describable by a frequency. An interval is described with a unit of time between repetitions. Radar signal characteristics are a rabbit hole of such definitions. They really do matter once one switches from theoretical discussion to actual math. Confuse a rate with a period and your math for calculating ambiguity zones will fall apart.
In order to create interference between two radars, the ramps have to overlap pretty exactly, within a few nanoseconds of each other. This is very unlikely to happen.
Modern radars employ technologies to detect and/or avoid such collisions.
Overall it is not really an issue, even with many radars in crowded spaces.
If you're on the road in a relatively affluent area where people drive late model cars, this is pretty close to already the case. Automakers have started making these systems standard on many/all of their models in the US for several years now. Toyota, for example, started rolling out these systems a decade ago, and have been standard on all US models since 2018.
I'm not sure what these systems use in practice for interference mitigations, but there's a bunch of stuff that could be done, for instance, hopping between different frequencies.
There are mitigation techniques like randomization of chirp frequencies, choosing different idle times between frames, and signal processing techniques to try to detect interference and filter it out. In the general case, FMCW techniques will always have interference problems.
This is one reason amongst many others that military radars do not use FMCW but instead coded pulse compression techniques.
Deleted Comment
Co-channel Wifi interference is real. It really puts a damper on range and throughout compared to how it used to be. It is a largely unmitigated clusterfuck, as is the way with CSMA/CA once density increases enough.
LTE interference isn't an important thing in practice, in part because because all participating devices have very tightly-controlled timings. It isn't a clusterfuck at all because of the mitigations in place, but it does require centralized coordination to be this way.
Radars on cars don't have centralized coordination (do they?). What mechanism prevents their performance from degrading as wifi does?
> With a PC with Intel Wi-Fi sensing capabilities in sleep mode, the PC Wake-on-Approach is activated as it detects human presence. Even when a user forgets to lock the PC, a count-down to lock starts with no human present. False detection is prevented even with human presence behind and next to the PC.
Deleted Comment
https://www.viksnewsletter.com/p/how-automotive-radar-uses-c...
If the author is here I would be curious to know your process and tools to generate the graphs and figures?
https://www.viksnewsletter.com/p/how-i-write-an-engineering-...
The newest weapon in the war against tailgating.
https://owners.honda.com/utility/download?path=/static/pdfs/...
> The velocity of the target also manifests as a frequency shift in the received chirp due to Doppler effect