Re: [Discuss-gnuradio] Fwd: Questions on rx_ofdm example in GR 3.7.1

[Marcus Müller]

Hi Han,

You replied to a thread from September 2013 – which is totally OK, but many of us don't have the original mail in their archives, so here it is:

Hi Tim.

The solution I found was to set the Fixed Frame Length parameter to 1 on the OFDM Frame Equalizer block of the "Header Stream".

The original value for this field is set to 0, but I don't exactly know why it works setting it to 1.

a bit of research led to this original question:

On Tue, Sep 10, 2013 at 09:58:16PM +0000, Monahan-Mitchell, Tim wrote:

GR 3.7.1
gr-digital/examples/ofdm/rx_ofdm.grc

1. The OFDM Frame Equalizer block that is downstream from the Header Stream Virtual Source block has the Length Tag Key field set to length_tag_name, yet all other blocks on the diagram with that field are set to length_tag_key (which is the ID of a Variable block at the top of the diagram). Is that a mistake? If someone will confirm, I can file the bug.

Hm. You make me look like a total scrub.

It seems you're right. However, on my machine, length_tag_name doesn't
throw an error. I just can't see why. GRC itself should figure this is a
problem.

2. When I try to run this flowgraph, modified for my target's custom source block, I get this error:
FATAL: Missing a required length tag on port 0 at item #0
thread[thread-per-block[21]: <block ofdm_frame_equalizer_vcvc (22)>]: Missing length tag.

That's good, actually -- it means your block is getting input, which
means it's receiving headers.

To be honest, the {rx,tx}_ofdm.grc files were more of an illustration of
how the OFDM chain works. I know I've used them successfully sometime in
the past, but I've changed some stuff in between.

Using the ofdm_tx and ofdm_rx hier blocks should, however, work. Can you
try those?

My main issues when transmitting OFDM is getting a clean spectrum. OFDM
doesn't like distortions at all.

MB

Which led me to question (1) above. However, changing that block to match the others does not correct the runtime error.

I found a couple of instances of the same error listed in (2) above in the mailing list, but no one replied to them.

So, please note: gr-digital has seen quite some patches since 2013; I wouldn't assume everything said back then still applied today.

You mention

header_eq=digital.ofdm_equalizer_vcvc(

now, did you maybe mean ofdm_frame_equalizer_vcvc

So, before we dig deeper into this, would you mind explaining which question exactly you're referring to when you said

 I have faced the same question in my work,

I think it might be worth giving us a bit of an overview over what you're building, what goes wrong, and how you're trying to fix this.
Right now, I'm a bit confused, sorry :(

Best regards,
Marcus

On 17.05.2016 09:50, hgajiayou wrote:

Hey there:
      I have faced the same question in my work, according to your solution
that set the Fixed Frame Length parameter to 1. I checked the OFDM Frame
Equalizer block in ofdm_txrx.py(the file direction:
gnuradio->gr-digital->python->digital), and i found the next codes:

header_eq=digital.ofdm_equalizer_vcvc(
      header.equalizer.base(),
      cp_len,
      self.frame_length_tag_key,
      True,
      1   #Header is 1 symbol long,
)
According to your solution, which parameter i need to set to 1 in the codes?
I'm a green hand in Gnuradio, hoping for your answer .

Han 





--
View this message in context: http://gnuradio.4.n7.nabble.com/Fwd-Questions-on-rx-ofdm-example-in-GR-3-7-1-tp43833p60074.html
Sent from the GnuRadio mailing list archive at Nabble.com.

_______________________________________________
Discuss-gnuradio mailing list
address@hidden
https://lists.gnu.org/mailman/listinfo/discuss-gnuradio