[USRP-users] Phase after Freq Hopping

Fabian Schwartau fabian at opencode.eu
Mon Mar 18 07:07:21 EDT 2019


Hi,

why do you need to know/define the absolute phase of the transmitted 
signal? Phase is always relative to something. When impleneting a radar, 
you transmit a signal with an unknwon (absolute) phase. But this is not 
a problem, because after reception you will somehow correlate the 
transmitted with the received signal. In FMCW for example you multiply 
both signals, and get only the relative phase between transmitted and 
received signal - and that is the only thing you are interested in. I 
don't know how this works in OFDM, but you probably have to correlate 
the transmitted with the received signal and this will reveal the 
relative phase between both signals.

Hope that helps,
Fabian

Am 18.03.2019 um 11:29 schrieb Patscheider, Dominik via USRP-users:
> Hey,
> 
> What I wanted to achieve is a stepped OFDM Radar.
> To increase the baseband for the radar, I´m splitting the subcarrier and transmit them on four center frequencies.
> Thus it sends the first symbols on f0 (with an initial phase); afterwards increasing the center freq to f1 and transmit the next symbols,...
> And obviously the initial phase is changing after reconfiguring the frequency. If phase1, phase5, phase9,... phase2,phase6,... isn´t the same, it´s not possible to measure any velocity.
> 
> Secondly, the time commands to change the freq I knew but didn´t use right now.
> 
> Hope this explanation was useful to understand me and thanks for helping :)
> 
> Dominik
> 
> -----Ursprüngliche Nachricht-----
> Von: USRP-users <usrp-users-bounces at lists.ettus.com> Im Auftrag von Piotr Krysik via USRP-users
> Gesendet: Freitag, 15. März 2019 14:52
> An: usrp-users at lists.ettus.com
> Betreff: Re: [USRP-users] Phase after Freq Hopping
> 
> W dniu 12.03.2019 o 17:49, Patscheider, Dominik via USRP-users pisze:
>>
>> Hello ,
>>
>>   
>>
>> For a Radar I´m transmitting and receiving with the USRP X310 samples
>> on different frequency steps.
>>
>>   
>>
>> For instance, after 4 frames I´m coming back to the first center freq
>> and continue this a few times. Hope the following description helps…
>>
>>   
>>
>> f3                   |phase4|       |…|
>>
>> f2                  |phase3|       |…|
>>
>> f1               |phase2|       |phase6|       …
>>
>> f0            |phase1|       |phase5|
>>
>>                  t ------>
>>
>> According to the freq adjust every frame starts with a new phase.
>>
>> Phase 1 ≠Phase 5
>>
>>   
>>
>> Is there any possibility to get the same phase after returning to the
>> same center freq? Phase 1=Phase 5, Phase 2 = Phase 6,…
>>
>>   
>>
>>
> Hi Dominik,
> 
> Can you describe what you want to achieve exactly? Probably you need to know phase relations because you want to do coherent processing of the signal. But from your description I don't know:
> -why on your ASCII art the signals transmitted on different frequencies seems to be overlapped?
> -which you part of it you want to process coherently?
> 
> I also often use USRPs for radar transmissions. Issues with predicting initial phase difference of the received signal, in relation to digital waveform that is being transmitted, are much easier to overcome when you use timed commands to set the frequencies on Rx and Tx side synchronously (if you use UBX or SBX daughter-boards).
> 
> Best Regards,
> Piotr Krysik
> 
> 
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