Re: signal detection in doppler

On May 25, 4:32 am, "cpshah99" <cpsha...@xxxxxxxxxxxxxx> wrote:
On May 23, 10:54 am, "cpshah99" <cpsha...@xxxxxxxxxxxxxx> wrote:
On May 19, 9:28 am, "cpshah99" <cpsha...@xxxxxxxxxxxxxx> wrote:
Hello People

I have been scratching my head on this doppler problem:

Because of the doppler effect, the received signal will either be
expanded
or compressed on time axis.

Now, the expansion or compression is sampling rate conversion. So,
I
tried
to use 'resample' function of matlab. The actual length of the
signal
is
30720 and sampling freq is 48000Hz.

Now, when I use y=resample(x,10010,10000) i can find the peak i.e.
start
of my data by correlating the signal with training sequence.

But when I use y=resample(x,10007,10000) i can't find the peak.

So is there any better way to
1. model this exapnsion or compression
or 2. to find peak when the received is corrupted by severe
doppler.

Help in this regard will be highly appreciated.

Chintan

Just clarify - a doppler shift isn't modeled by a frequency shift. It
is really a time dilation. For pure sinusoids it then appears as a
frequency shift. If you signal doesn't have a lot of bandwidth wrt
the
center frequency then you can approximate the doppler shift by a
frequency shift.

Cheers,
David

%%%%%

Hi David

My center freq is 12KHz and BW is 4 KHz.

I understand as u said, the doppler effect is time dilation, but should
I
just shift the center freq or sampling freq to approximate the doppler
shift because if I shift carrier freq then it will just add phase shift
but
it will not expand or compress the signal.

Also I am reading Multirate Digital Signal Processing by Crochiere and
Rabiner to implement Farrow Interpolator, suggested by Julius.

Thanks you again.

Chintan.

What type of system is this? Sonar? What is the speed of propagation
in your medium? What is the format of your signal? Is it a LFM or HFM
pulse?

For these types of signals in sonar they use banks of matched filters,
which are constructed by time dilating the analytic formula for the
pulse. For more detail you can see Neilson's "Sonar Signal
Processing".

For Farrow information I found Harris's book quite good. I also posted
some matlab code on this group to do derive the Farrow spline
coefficients - it doesn't do the online resampling.

Cheers,
David

%%%%%

Hi David

This system is for underwater communications, where speed of acousticwave
is 1500 m/s. And also I am not using LFM or HFM. My signal format is 500
symbols long PN sequence to find the start of that (not to do anything with
DSSS) and after this PN 2000 data symbols.

Can u please post that code here again, if you don't mind.

Thanks,

Chintan

It's on my other computer. I'll try to post it on Monday.

Given you're dealing with probably +- 30 knots doppler (or more) and
the band & bandwidth you should take into account the dilation,
but normally the PN , LFM , HFM signals have fairly broad ambiguity
functions so they still correlate over broad dopplers - you shouldn't
need that many doppler replicas to find a peak. Normally the replicas
are set at the -3dB points in the ambiguity function.

Cheers,
David
.

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