USRP2 questions

GNU Radio
1

What does it mean that USRP2 can only do MIMO with eight receivers?
What are the options for doing MIMO with, say 16 receviers?

Why are there two A/D converters if you can only use one RX
daughtercard? Or can you actually use two RX channels with a USRP2?

juha

2

On Wed, Sep 17, 2008 at 01:48:34AM +0300, Juha V. wrote:

What does it mean that USRP2 can only do MIMO with eight receivers?
What are the options for doing MIMO with, say 16 receviers?

Why are there two A/D converters if you can only use one RX
daughtercard? Or can you actually use two RX channels with a USRP2?

juha

Most of the daughterboards do quadrature downconversion and produce
analog I & Q. For those daughterboards we use 1 A/D for I and another
one for Q.

Eric

3

On Wed, Sep 17, 2008 at 01:48:34AM +0300, Juha V. wrote:

What does it mean that USRP2 can only do MIMO with eight receivers?
What are the options for doing MIMO with, say 16 receviers?

You would need to design and build some kind of hub that would provide
the clocks and synchronization for the 16 receivers using their MIMO
interfaces. Depending on how you programmed the hub, you could just
send the control from there, and then send the sample data over the
ethernet interfaces on each of the 16 USRPs.

You’d need some serious computational power to process 16 channels,
each streaming 100MB/s of data. Sounds like fun :slight_smile:

Eric

4

Quoc Lai wrote:

send the control from there, and then send the sample data over the
Is the hub needed for any MIMO systems using USRP2? I want to build a
4x4 MIMO. Do I have the synchronization problem?

Each USRP2 can handle 1 antenna with most daughterboards, or 2 if you
have your own external RF sections connected to the USRP2 at IF.

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.

Matt

5

Michael O. wrote:

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

6

Eric B. wrote:

On Wed, Sep 17, 2008 at 01:48:34AM +0300, Juha V. wrote:

What does it mean that USRP2 can only do MIMO with eight receivers?
What are the options for doing MIMO with, say 16 receviers?

You would need to design and build some kind of hub that would provide
the clocks and synchronization for the 16 receivers using their MIMO
interfaces. Depending on how you programmed the hub, you could just
send the control from there, and then send the sample data over the
ethernet interfaces on each of the 16 USRPs.

You’d need some serious computational power to process 16 channels,
each streaming 100MB/s of data. Sounds like fun :slight_smile:

Eric

Hi Eric,

Is the hub needed for any MIMO systems using USRP2? I want to build a
4x4 MIMO. Do I have the synchronization problem?

Quoc