[USRP-users] Synchronizing Data Streams from multiple N210 boards on FPGA

Josh Blum josh at ettus.com
Mon Jun 11 14:17:53 EDT 2012



On 06/10/2012 03:01 PM, Yasir Javed wrote:
> We are working on a MIMO-OFDM system using two USRP N210s. We want our
> Synchronization algorithm to be implemented on FPGA instead of Software.
> The algorithm needs to process coherent DDC data samples from both boards.
> 
> Is it possible to forward data from one N210 to another where I can
> implement the Synchronization logic?

Yes

> If so, how can I make one FPGA logic wait until I get samples at almost
> exactly the same time instant from the other board?

If you tell all DSP to stream at time X, you simply need to wait until
the same time appears at the front of the FIFO at each channel. In other
words, it will require a little alignment state machine. The time is a
64-bit tick count in the VITA IF packet.

> At what points in the verilog code would you suggest to make changes?
> 

You may want to take a look at the packet router block in the usrp
core.v All the inputs and output data streams from the DSP and MIMO
cable can be found going into this module

> My understanding so far is that I can modify dsp_rx_glue.v to add custom
> processing on DDC out samples (bb_sample and bb_strobe). What I am not
> clear is where would I get samples from the other N210 board. My initial
> guess was to use MIMO cable. But it seems that MIMO cable is used for
> sharing timing information to synchronize capturing time, its not used for
> data transfers  (Is my understanding right?). So the next solution I could
> think of was to send packets from one N210 to other. I am so far unable to
> figure out where I would be able to intercept data in that case. Secondly
> by the time one N210 is waiting for samples from other N210, the DDC out
> samples have to be saved somewhere (in a FIFO).. Is the network delay
> deterministic to decide a safe FIFO size? Thirdly, My understanding is that
> timing information is added in vita_rx_chain.v which comes after
> dsp_rx_glue.v where I can get DDC out samples, how can I figure out the
> timing information of DDC samples from both boards in dsp_rx_glue.v?
> 

The custom DSP is more useful for working at the sample level, but I
think you will want to work at the packet level to handle alignment.

-josh




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