[Discuss-gnuradio] some questions about transmit time-tags

[Francois Quitin]

Hi all,

 

We are currently trying to send time-synchronized packets from several transmitters, and we are having some difficulties with the transmit time-tags. Especially, we have trouble understanding what happens with the flow of samples when transmit time-tags are added to certain symbols:

 

-          When a stream of samples arrives at the FPGA, and sample i has a time-tag corresponding to a future time, what happens? Does the FPGA stop transmitting samples until that future time (and does the FPGA generates an underflow? )

-          When a stream of samples arrives at the FPGA, and sample i has a time-tag correspond to a past time, the FPGA generates an “L”, but is the stream of samples transmitted anyway (ASAP) or are the samples discarded?

-          When a stream of samples arrives at the FPGA, and sample i has a time-tag corresponding to a future time, but corresponding to e.g. the time of sample (i-10). What happens in that case? Does the FPGA transmit all samples and generates a “L” when it gets to sample i, or does the FPGA transmit sample i at the scheduled time and discard the samples (i-10:i-9) ?

 

We are working with USRP-N200 with WBX daughterboards, UHD_003.004.001, and we are coding our custom C-blocks that we integrate in a GRC-flowgraph afterwards.

 

Thanks a lot for your help!

 

Francois

 

François Quitin, Ph.D.

 

BAEF Post-doctoral research fellow

Electrical & Computer Engineering Dept.

University of California

Santa Barbara, CA 93106-9560

Phone: +1-805-883-8599

Email: address@hidden

Home: www.ece.ucsb.edu/~fquitin