FMCW Radar timing of transmission and reception

Question

I have a question regarding the implementation of FMCW radars that seems rather basic to me, but for some reason I am a bit unclear about this.

So I understand that in FMCW radar, transmitter and receiver are constantly, simultaneously transmitting and receiving chirp signals, respectively. The received signal is then multiplied with a copy of the tranmsit signal to deramp/dechirp it and a subsequent FFT on this IF/beat signal yields the range estimates. Now what I am wondering: Is there some sort of synchronization going on between the transmission of the chirp and the deramping in the receiver? Lets say, I start transmitting at a certain time instant, does the receiver side start collecting samples at the exact same moment and multiplies the chirp-copy after the corresponding amount of samples have been collected? As there will be a time delay to, lets say, a point target in the scene, the received chirp and the reference copy would have a certain offset when they are multiplied together. Then, if I understand correct, the receiving signal would be similar to the transmit chirp, but in a sense shifted, with the end of it reappearing in the beginning (coming from the next transmitted and received chirp) I guess to summarize my question: Is the "input buffer" of samples at the receiver side cleared out before the beginning of a new chirp transmission, so that data recording and transmission start at the exact same time? Or is it rather that both transmitting and receiving run freely and a running window of the receive signal is collected and multiplied with the reference? From my understanding, it would matter at what point in time we multiply the reference chirp with the received signal, since this would cause a different delay.

I hope my question makes sense, every tip is appreciated !

Cheers Lucas

Answer 1

These are excellent questions and shows that you're really trying to dig into the details.

In FMCW, or most radar systems for that matter, the time of transmission must be known so that you can decide at what time to open the receiver to determine range. In the case of FMCW, theoretically you transmit and receive at the exact same time. In the real world, there might be a slight delay. On top of that, some of the first few samples may have transients from filtering, and are thrown away.

Lets say, I start transmitting at a certain time instant, does the receiver side start collecting samples at the exact same moment and multiplies the chirp-copy after the corresponding amount of samples have been collected?

This is essentially what you want, given the caveats mentioned above. You transmit at some time $t_0$, wait for some period which is usually close to the chirp length $T$, then multiply the signal by the reference to yield the beat frequencies after the DFT.

Is the "input buffer" of samples at the receiver side cleared out before the beginning of a new chirp transmission, so that data recording and transmission start at the exact same time? Or is it rather that both transmitting and receiving run freely and a running window of the receive signal is collected and multiplied with the reference?

This depends. If you want to perform Doppler processing (or simply coherent integration), you commit enough memory to fit the $N$ number of pulses you want to collect. However, for every chirp that is generated and transmitted, the only real time that matters is the difference between the time we transmit $t_0$ and the end of the listening period with is roughly $t_0 + T$. The absolute value of $t_0$ is not all that important. The value can be zero or whatever the current timestamp the system may give, it's the delay that matters.

Whether or not the transmitter and receiver run freely is really a design choice and comes down to the technology. However in actual implementation, the decision to transmit and receive a chirp is discrete, meaning that each chirp is commanded separately as many times as desired. This is really semantics, because you could also design a system that is always running, and you make the decision when to actually sample the return signals. At the end of the day, it all ends up being the same thing.

Answer 2

The transmitter and receiver must be synchronized for the FMCW radar to work. The system master clock is shared between the transmitter and receiver. The receiver must know the start of each sweep, this mechanism is ensured by sharing the sweep trigger signal between the transmitter and receiver. The receiver ignores the initial samples of the received buffer to get rid of the samples from the previous sweep.