[USRP-users] mimo sync and acceracy

Marcus Müller marcus.mueller at ettus.com
Tue Sep 9 08:42:43 EDT 2014


Hi Iftah,

On 07.09.2014 21:08, iftah giladi via USRP-users wrote:
> And my new question now is:
>
> If I don't have an external source, and I don't use a GPSDO, so were will
> that pps will come from?
>
The master N2x0.
> Actually what you saying if I got you right is that If I want both usrp to
> have the same time with accuracy butter then 10msec, then I HAVE to either
> use to GPSDO PPS or an external PPS
The time will be exactly the same on both USRPs, but it will have an
offset to the "real" time.

You can of course go the RF communication route and correlate an
arbitrary signal [1] that is sent on a specific time and find the
maximum to *estimate* the temporal offset and account for it in software.
> And what if I won't use either? They will still be sample synced right? Only
> the time base will be biased, right? 
yep, the time will be biased. Now, the harsh truth is: if you want
GPS-synchronous absolute time, you'll have to get it from GPS or
something technically comparable[2].
>
> Plus and that has nothing to do with the time issue the two channel  will
> have a phase difference dew to the different PLL who created the A2D clock,
> right?
Yes, aside from very few daughterboards where the tuner has a mechanism
to avoid that.

Greetings,
Marcus

[1] well, it must not be wide-sense cyclostationary within your
observation length, otherwise you'd get ambiguities. Also, the temporal
resolution of any signal, be it pulse, chirp, pseudorandom noise or
whatever, is inversely proportional to its bandwidth.
[2] GPS employs knowledge about multiple satellites to eliminate the
time the waves travel from satellite to GPSDO. As a completely
unnecessary but fun side effect, you can use this knowledge to estimate
your location in space if you see enough satellites.




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