That has an interesting consequence: first of all, the (I)DFT of a Zadoff-Chu sequence is, in fact, also a ZC sequence. However, that doesn't mean that the DFT of a Zadoff-Chu sequence multiplied with a rectangular Window, is one, too:
let
be your Zadoff-Chu sequence, and let 
be your FFT length; judging from your figures, you don't actually use all carriers; let's say there are 
used carriers. The following Fourier transform equalities hold:
, which also is a Zadoff-Chu sequence (i.e. magnitude 
),With your OFDM symbol being but the central
out of 
carriers, I'd model the input of the IDFT as
, which doesn't have Zadoff-Chu properties, so your signal in time domain will not have constant amplitude.In theory, the inverse operation in the receiver should give you the same
, maybe having a bit of noise, and maybe having a constant DC offset if any, IFF transmitter and receiver were synchronized.What I assume is that the DC offset that you're seeing in the center peak is always the same, but the phase of the symbol start isn't; that leads to the DC offset always having the same phase, but the center symbol always a different one; if DC offset and center symbol happen to have the same phase, the peak is high, if they happen to oppose each other, you get cancellation.
Generally, depending on your bandwidth you might do something called offset tuning:
when you use "uhd.tune_request(
,
)" instead of directly using in the tuning frequency of a USRP block, you can use the USRPs capability to digitally tune by in the DSP, moving the DC offset / LO leakage out of band.Best regards,
Marcus
On 10/27/2015 12:15 PM, scott tiger wrote:
MaksimI received the signal in gnu radio sink, then take the file and divide it into 128 length sequences"because I used 128 IFFT in the transmitter". I made FFT to get the signal in frequency domain. Finally, I plotted the results.Best regards
On Tue, Oct 27, 2015 at 12:14 PM, scott tiger <thekiller.scott@gmail.com> wrote:
MaksimI received the signal in gnu radio sink, then take the file and divide it into 128 length sequences"because I used 128 IFFT in the transmitter". I made FFT to get the signal in frequency domain. Finally, I plotted the results.Best regards
On Tue, Oct 27, 2015 at 12:08 PM, Marcus Müller <marcus.mueller@ettus.com> wrote:
That's what I don't understand – how did you put them together?What **are** those steps?
I saved the received signal in a file, then I did further steps in matlab (FFT, gathering ...).
On 10/27/2015 12:02 PM, scott tiger wrote:
MaksimHi MarcusBest regards
of course in gnu, I used a band pass filter. But I have spikes in the center frequency of the signal.
That's what I don't understand – how did you put them together?
I saved the received signal in a file, then I did further steps in matlab (FFT, gathering ...).
On Tue, Oct 27, 2015 at 10:50 AM, Marcus Müller <marcus.mueller@ettus.com> wrote:
Hi Maksim,
In the receiver I made FFT and plot the correspond figure.So, that's pretty clearly frequency domain of the receive signal, right?
So that might answer your question regarding DC offset: If there is DC offset, you'd see a constant spike at the center frequency. That's not really the case here, if I understand correctly.
The last figure which I sent it is the signal in the frequency domain which are repeated with each transmission "I put them all together to compare them."That's what I don't understand – how did you put them together?
Best regards,
Marcus
On 10/27/2015 10:44 AM, scott tiger wrote:
MaksimThank you for your replyI attached the same figure with more explanation "each black block is the signal in the frequency domain", but block 1 .....n is the same signal transmitted in different times.Hi Marcus,I generate ZC sequence "its amplitude equals to 1 in frequency domain" then I made IFFT and transmit the signal using USRP. The environment is a cable. I received the signal from another antenna of the same USRP. In the receiver I made FFT and plot the correspond figure. Since, I am the source file in the transmitter transmit the signal many times"repeat activated". The last figure which I sent it is the signal in the frequency domain which are repeated with each transmission "I put them all together to compare them."
Y-Axis is the amplitude "abs(of the complex signal)".
X-Axis is not pure frequency domain or time domain, because the figure is drown from follow:
Best regards
On Mon, Oct 26, 2015 at 4:47 PM, Marcus Müller <marcus.mueller@ettus.com> wrote:
Hi Maksim,
Could you keep this on the mailing list?
I don't fully understand:
> In fact, the figure shows repeated OFDM signal, each of it in frequency domain.
So you take the OFDM signal, and shift it in frequency domain, and then have N identical OFDM signals transmitted at the same time?
Can you clearly state what your X-Axis and what you Y-Axis are?
For example, I transmitted a zadoff-chu sequence which has a flat characteristic in frequency domain. The environment was a short cable with attenuation. The received signal also showed in frequency domain.
I attached it also "the figure shows the repeated sequences 2Mhz bandwidth in frequency domain". What I am curious about are spikes which appear usually in the center frequency? I thought may it is related some how with dc offset in USRP.
I don't understand this graph:
What is the X-Axis, what is the Y-Axis?
Maybe you meant that you take values from a Zadoff-Chu sequence, IFFT them, thus generating an OFDM signal (which, by the way, is also a ZC sequence), add guard intervals and transmit them?
I'm really getting intrigued by what you observe :) but we'll really have to understand the graphs, which at this point, I'm afraid, I don't.I attached it also "the figure shows the repeated sequences 2Mhz bandwidth in frequency domain". What I am curious about are spikes which appear usually in the center frequency? I thought may it is related some how with dc offset in USRP.
Best regards,
Marcus
No comments:
Post a Comment