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

From: Marcus Müller
Subject: Re: [Discuss-gnuradio] Fwd: Questions on rx_ofdm example in GR 3.7.1
Date: Tue, 17 May 2016 12:41:58 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.6.0

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

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

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.


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
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,

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:

      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 .


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