Re: [Discuss-gnuradio] Re: USRP2 questions

[Matt Ettus]

Michael Ossmann wrote:
> On Thu, Nov 13, 2008 at 11:26:31PM -0800, Matt Ettus wrote:
>   
> > A dual USRP2 system does not need a hub, only a cable.  A quad USRP2 
> > system would need a hub unless you were willing to have 4 ethernet 
> > connections to the host computer.
> >     
>
> I was under the impression that a (synchronized) quad USRP2 system
> would require a MIMO hub even if each USRP2 is connected to a
> dedicated ethernet port.  Am I wrong?  (I would love to be!)  Can
> you tell me how to synchronize them another way?  Also, does anyone
> have a design for a MIMO hub yet?
>   

I think the confusion here comes from the fact that there are multiple 
ways to get MIMO set up.

There are 3 things necessary to get MIMO working.
1 - All systems need to be phase-locked to a common clock (10 MHz in 
this case)
2 - All systems need to have a common time reference (often a 1 PPS 
signal) so that they know which sample in one system corresponds to 
which sample in the other
3 - All the data needs to come from / go to the same place

The MIMO Expansion interface on the USRP2 carries all 3 of those.

Scenario 1 - Two USRP2 systems connected by a MIMO cable
The Master USRP2 is the one connected to both ethernet and the MIMO cable
The Slave USRP2 is the one connected to just the MIMO cable
The master provides a clock to the slave over the MIMO cable
The master provides the time reference to the slave over the MIMO cable
On transmit, the host sends all data over the ethernet to the master. 
The master uses the data which is intended for the master, and passes 
the data for the slave over the MIMO cable
On receive, the slave sends all data over the MIMO cable to the master, 
and the master passes it back to the host over ethernet. It also sends 
its own data over the ethernet to the host.
The firmware and host driver code to do this is not ready yet, but is in 
progress.

Scenario 2 - Up to Four USRP2 systems connected by MIMO cables to the 
not-yet-in-existence HUB
The HUB has a big FPGA on it
The HUB provides clock and time reference to all USRP2 systems
The HUB communicates with the host computer by ethernet or PCI Express, 
or operates standalone
All USRP2 systems send data back and forth to the HUB
The HUB is not ready yet, but the design is in progress. We hope to have 
it ready in 9-12 months.

Scenario 3 - An arbitrary number of USRP2 systems, no MIMO cables, no HUB
Some means of providing the same 10 MHz reference to all USRP2 systems 
must be set up. Typically this would be the 10 MHz output of some lab 
reference like a GPS-locked reference. That reference would be put 
through some sort of active splitter, and fed into all the USRP2s by SMA 
connector on the front panel
Some means of providing the same time reference to all USRP2 systems is 
also needed. This would normally be a 1 PPS signal generated by the same 
reference. It will need to go through some sort of active splitter and 
send into the PPS input of all the USRP2 systems.
ALL the USRP2 systems will need to be connected to the host computer by 
gigabit ethernet. You can probably use an ethernet switch, but there 
will be issues with sharing the bandwidth.
The firmware to do this is ready. The host code to do it is not ready yet.

I hope this has clarified things. Somebody please add this to the wiki.

Matt