Re: [Discuss-gnuradio] Synchronize USRP in which level

[Josh Blum]

On 01/31/2013 12:54 AM, Gong Zhang wrote:
> 2013/1/31 11:51, Josh Blum wrote: On 01/30/2013 09:00 PM, Gong Zhang wrote:
>>> Hi,
>>> My last topic "Synchronize multiple USRP-N210s"may be a big one which
>>> gives no reply.If I want to synchronize two USRPs without cable or
>>> GPSDO,I should design a synchronization protocol.Simply A sends some
>>> time-stamped samples to B ,then B record local time when receiving these
>>> samples.Knowing the distance between them,we can synchronize B with
>>> A.Apparently the synchronization accuracy depend on we make the
>>> timestamps in which level.Ethernet involve in 10s of microseconds jitter
>>> shows that we should do it on FPGA.Does UHD have this function?
>>> Any comments will be valuable to me.
>>> Thanks.
>> Start here, you can do this with plain UHD in C++ or with the gnuradio
>> gr-uhd component in python or C++:
>> http://code.ettus.com/redmine/ettus/projects/uhd/wiki
>>
>> The sync docs should explain just about everything:
>> http://files.ettus.com/uhd_docs/manual/html/sync.html
>>
>> Many examples of timed transmission and reception:
>> http://code.ettus.com/redmine/ettus/projects/uhd/wiki#Code-Examples
>>
>> Awesome project/use case for these features:
>> https://github.com/jmalsbury/pre-cog/wiki
>>
>> -josh
> My target is to sample signal at the same time with two USRP connected
> to different PCs placed in different place.The sync accuracy should be
> within microsecond.I can not use cable and GPSDO,which gives no external
> sync signal such as PPS.The only way maybe sync their local time,which
> is usually unsynchronized due to the instability of oscillator.So I need
> a time-stamp exchange based sync protocol.
> Thank you for your time.

Sounds feasible. Suppose you did something simple like a synchronization
pulse. Send out a pulse that all devices could detect. Record the time
the pulse was detected at each device to establish an absolute time
amongst the devices.

The precog project does stuff like this for the frequency hopping and
TDMA engines to sync up the time windows for transmission and hopping,
etc... https://github.com/jmalsbury/pre-cog/wiki

-josh