# OFDM cyclic prefixer

**URL:** <https://www.ruby-forum.com/t/ofdm-cyclic-prefixer/241125>\
**Category:** GNU Radio\
**Created:** [May 14, 2014, 3:08am UTC](https://www.ruby-forum.com/t/ofdm-cyclic-prefixer/241125 "2014-05-14T03:08:35Z")\
**Posts on this page:** 6\
**Page:** 1

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**Author:** ![musicdenotation](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/ea666f/32.png) [@musicdenotation](https://www.ruby-forum.com/u/musicdenotation)\
**Post date:** [May 14, 2014, 3:08am UTC](https://www.ruby-forum.com/t/ofdm-cyclic-prefixer/241125/1 "2014-05-14T03:08:35Z")

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Hello,

My name is Yuri.  
​ I very new to GNU Radio, and I’m using GRC 3.7.1.​

I am doing a research, and I need to modulate specific signal in OFDM. I  
would like to insert cyclic prefix (or guard interval in my project, the  
same thing with a different name).

I looked at the examples  
​under gr-digital  
​/examples/ofdm, and I’ve also read some papers about OFDM technics.

My doubt is: what is the parameter “Length Tag Key”? I’ve seen it  
working  
in the examples located in the folder above, I’ve tried to do the same  
in  
my project, but it states a message that this parameter is missing.

By chance, does this cyclic prefixer block only works with other blocks  
of  
the same category? I mean, do I need to use OFDM carrier allocator  
before  
it, or something like that?

Thank you for your attention,

Yuri.

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**Author:** ![Yuri\_P.M](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/y/ea5d25/32.png) [@Yuri\_P.M](https://www.ruby-forum.com/u/Yuri_P.M)\
**Post date:** [May 14, 2014, 4:38am UTC](https://www.ruby-forum.com/t/ofdm-cyclic-prefixer/241125/2 "2014-05-14T04:38:23Z")

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If you’re sending packets, then Length Tag Key should be  
set to the packet length you’re using. If you’re sending  
a stream (like DVB-T), then this field is not used (and  
left blank).

The OFDM cyclic prefixer is used after the (reverse)  
FFT block. The FFT Length in the OFDM Cyclic Prefixer  
block should match the FFT Size in the FFT block.

Ron

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**Author:** ![Yuri\_P.M](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/y/ea5d25/32.png) [@Yuri\_P.M](https://www.ruby-forum.com/u/Yuri_P.M)\
**Post date:** [May 14, 2014, 10:29am UTC](https://www.ruby-forum.com/t/ofdm-cyclic-prefixer/241125/3 "2014-05-14T10:29:58Z")

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On 14.05.2014 04:36, Ron E. wrote:

> If you’re sending packets, then Length Tag Key should be  
> set to the packet length you’re using. If you’re sending  
> a stream (like DVB-T), then this field is not used (and  
> left blank).

It needs an empty string, in this case.

> The OFDM cyclic prefixer is used after the (reverse)  
> FFT block. The FFT Length in the OFDM Cyclic Prefixer  
> block should match the FFT Size in the FFT block.

That’s right. The block doesn’t check if the input is actually OFDM.

Martin

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**Author:** ![Yuri\_P.M](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/y/ea5d25/32.png) [@Yuri\_P.M](https://www.ruby-forum.com/u/Yuri_P.M)\
**Post date:** [May 14, 2014, 10:53pm UTC](https://www.ruby-forum.com/t/ofdm-cyclic-prefixer/241125/4 "2014-05-14T22:53:00Z")

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Hi,

Thanks Mr. Economos and Mr. Braun,

I left this parameter in blank (I guess this is the same as an empty  
string), it seems that it is working now.

Actually I am trying to modulate ISDB-T data stream. I am working with  
vectors, instead of data stream.

Thanks again.

2014-05-14 5:28 GMT-03:00 Martin B. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid):

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**Author:** ![Yuri\_P.M](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/y/ea5d25/32.png) [@Yuri\_P.M](https://www.ruby-forum.com/u/Yuri_P.M)\
**Post date:** [May 15, 2014, 2:29pm UTC](https://www.ruby-forum.com/t/ofdm-cyclic-prefixer/241125/5 "2014-05-15T14:29:39Z")

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Thank you very much, for your help Mr. Economos,

For sure I will take a look at this project,

Yuri.

2014-05-14 21:12 GMT-03:00 Ron E. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid):

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**Author:** ![Yuri\_P.M](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/y/ea5d25/32.png) [@Yuri\_P.M](https://www.ruby-forum.com/u/Yuri_P.M)\
**Post date:** [May 15, 2014, 2:13am UTC](https://www.ruby-forum.com/t/ofdm-cyclic-prefixer/241125/6 "2014-05-15T02:13:55Z")

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If you haven’t already, you may want to take a look  
at this GnuRadio DVB-T implementation.

> **[GitHub - BogdanDIA/gr-dvbt: DVB-T implementation in gnuradio](https://github.com/BogdanDIA/gr-dvbt)**
>
> DVB-T implementation in gnuradio. Contribute to BogdanDIA/gr-dvbt development by creating an account on GitHub.

Ron