Doppler effect in modulated signals

  • #1
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Hello,

If been for a while trying to find information about how Doppler effect affects modulated signals.

When a transmitter is moving relative to a receiver, the receiver will have a shift in the carrier frequency that will vary as the transmitter gets closer or moves away from the transmitter. This variation can be calculated so the receiver knows the exact frequency in order to downconvert it to baseband.

My question is: Once the signal has been downconverted into baseband, has the modulated signal also been affected by the doppler effect ? If so, is there any way to calculate how about 10kHz of doppler has affected a psk or fsk baseband signal and how to reverse the effect?
 

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  • #2
Simon Bridge
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My question is: Once the signal has been downconverted into baseband, has the modulated signal also been affected by the doppler effect ? If so, is there any way to calculate how about 10kHz of doppler has affected a psk or fsk baseband signal and how to reverse the effect?
Yes to both questions.
Usually you would adjust the tuning circuit to extract the signal.
If the signal were, say, audio, it would sound high or low pitched depending on the velocity.
You adjust that by speeding or slowing the playback.

That's usually.

You have something else in mind.
Once you have the doppler shifted signal, you can decompose it and apply the formula to each component separately.
The details will depend on the type of signal and the modulation type.
 
  • #3
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Thank you very much for your answer.

The problem is that I don't know how to apply this to my particular project. I'm trying to receive a 250kHz BPSK modulated signal. The signal is transmitted in UHF (345MHz). If the doppler is, lets say, 10kHz at a certain moment, I know that my carrier frequency will be at 355MHz so I can downconvert it correctly.

If the signal were, say, audio, it would sound high or low pitched depending on the velocity.
You adjust that by speeding or slowing the playback.
As you have stated, I will have doppler effect also in BPSK baseband signal, it will be distorted. As it's not an audio signal but binary file, I don't know how to calculate the doppler in order to recover the original signal.
 
  • #4
Simon Bridge
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OK - so you have a dopplered signal.
Digital should be easier - you change the time-spacing of the pulses.
It's the same maths as how you found out the dopplered carrier frequency.

Actually - a digital signal does not really care about the timing so much as the presence of the highs.
 
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  • #5
Baluncore
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A 345 MHz carrier with a 10 kHz doppler up-shift will move to 345.010 MHz, not to 355 MHz.
At 345 MHz, a 10 kHz doppler shift will represent a differential velocity of about 10 km/sec.

A carrier on 345 MHz that is BPSK modulated at 250k bits per second, will have a signal bandwidth of 500 kHz.
The decoder will probably be a PLL or digital PLL, it will have no trouble locking to, and tracking a 10 kHz doppler shifted carrier.
The 500 kHz bandwidth of the receiver can remain unchanged.

The stability of the receiver crystal reference will be about 1 ppm, the doppler shift is about 30 ppm.
You need make no changes to the system unless you are using it for navigational range measurements.
 
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  • #6
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Thank you for your answer.

I'm trying to receive information from satellites. I'm trying to use different modulations and different frequencies. I'm working at UHF that has a doppler of ~10kHz, and at S band with ~60kHz.

In my system I have a costas loop and pll for the synchronization.

When it comes to carrier synchronization, I don't know if the carrier synchronization could correct a 60kHz shift, but I could correct it myself by changing the frequency of downconversion according to the doppler obtained from a satellite tracking program.

The problem is, I don't know how to reverse the effect of the doppler in the baseband signal as I have no way to tell the shift that I have.

Right know I have FLL bw, timing BW and Phase loop BW variables all set at 62.8m, but I don't know how to properly set this variables in my system. You have told me that in theory I would have to make no changes to my system as it would work as it is, but I'm not sure as I don't know how to configurate the synchronization parameters correctly.
 
  • #7
Baluncore
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Eric C said:
FLL bw, timing BW and Phase loop BW variables all set at 62.8m,
What is "62.8m" ?

I think you will have to simulate or generate a test signal to evaluate your demodulator characteristics. The BW of the loop needs to be wide enough to acquire the carrier and narrow enough to eliminate noise once it is tracking.
 

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