Burst transmission USRP N210 + WBX

Hi everyone,

I am writing here because I need some help with GNUradio and the UHD

My problem is:

I want to send burst packet to USRP sink block. If I use the first
flow-graph (see picture #1), all work fine. The “packet_len” tag added
by the “PDU to tagged stream” block is well understood by the UHD driver
(No output characters like “U” or “D”…).

But, I want to use a pulse shape filter before sending my signal over
the air. (see picture #2). I used an “Interpolating FIR filter” block. I
designed a RRC filter with firdes :

firdes.root_raised_cosine(sps,sps, 1, eb,sps*ntaps)
with :
sps = 4
eb = 0.35
ntaps = 44

When I run the second flow-graph with pulse shape filter (see picture
I get characters “tG” on the console window.

1 -
I know this is because of the filter, and more particularly, the filter
delay. I found that the filter delay for a RRC filter of NTAPS long, is
Delay = NTAPS/2.

  • First of all, Am I right with this assumption?

2 -
I set the field “Sample delay” of the “Interpolating FIR Filter” to
NTAPS/2, and added zeros at the end of my signal to flush the FIR
filter. I can see that the tag is then delayed, and the last samples
inside the filter are flushed. But, I always get the “tG” characters on
the console window.

3 -
I also used the two tags “tx_sob” and “tx_eob”. When I do not use pulse
shape filter, all work well. But, when I want to use it, I got the same

I really don’t know where is my mistake so if someone could help me, it
would be amazing.

Thank in advance.