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Re: [Discuss-gnuradio] OFDM tx rx error
From: |
Martin Braun |
Subject: |
Re: [Discuss-gnuradio] OFDM tx rx error |
Date: |
Mon, 21 Mar 2016 09:29:39 -0700 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.5.1 |
Jenny,
we have a limit in place for packet sizes, which depends on your system.
If you type sysctl -a, it'll say something like
kernel.shmmax = 18446744073692774399
100 OFDM symbols seems pretty long, though. If your packet length is 96,
as in the example, that means a total packet length of 100 (including
CRC). With 48 useful carriers, you get 6 bytes per OFDM symbol, which
results in 17 OFDM symbols, plus 2 more for the header. You'll need to
give us some more info for us to help you.
Cheers,
Martin
On 03/19/2016 11:44 AM, Jingyi Sun wrote:
> Hi,
>
> I'm using the examples rx_ofdm.grc and tx_ofdm.grc together to transmit
> and receive packets over the air. Both work over a simulated channel,
> but when I actually transmit and receive over the air, I get the error:
>
> *"Detected a packet larger than max frame size (100 symbols)"*
>
> I saw the previous thread on this called "OFDM transmitter receiver"
> which goes as follows, but I don't really know what to make of this...
>
> I tried to run /sysctl kernel.shmmax /in terminal, but I got the message
> "No such file or directory". I'm also not sure what kernel.shmmax is.
>
> These are my parameters:
>
> fft length= 64
>
> cyclic prefix = 16
> packet length = 96
> no. of occupied carriers = 53
> no. of pilot carriers = 4
>
>
> I'm very new to gnuradio, and any insights into this would be appreciated!!
>
>
> Thanks,
> Jenny
>
> ------------------------------------------
>
> *From*: Martin Braun
> *Subject*: Re: [Discuss-gnuradio] OFDM transmitter receiver
> *Date*: Tue, 26 Aug 2014 11:10:53 +0200
> *User-agent*: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101
> Thunderbird/24.6.0
>
> ------------------------------------------------------------------------
>
> Try and not kill the context in a mailing list thread; these are also
> archived and are used by others for referral.
>
> Max frame size depends on max_output_buffer(). 80 symbols at a 64-sized
> FFT would be ~40kB... that doesn't seem unreasonable. Not sure if
> there's a problem here. I suggest you have a look at the buffer sizes to
> track down this problem.
>
> M
>
> On 08/25/2014 03:31 PM, Martin Braun wrote:
>>/What's your kernel.shmmax value? (run sysctl kernel.shmmax)/
>>//
>>/M/
>>//
>>/On 08/25/2014 12:47 PM, sreena p h wrote:/
>>/> Hi/
>>/> I used the ofdm transmitter receiver blocks to create a simple system as/
>>/> shown in the attachment. I used the system parameters as those used in/
>>/> the example transmitter and receiver grcs. Now I get error that/
>>/> 'Detected a packet larger than max frame size (80 symbols)'. I would/
>>/> like to know how the max frame size is set and should I add more/
>>/> information to the grc. /
>>/>/
>>/> My transmitter- receiver parameters/
>>/>/
>>/> fft length= 64/
>>/> cyclic prefix = 16/
>>/> packet length = 96/
>>/> no. of occupied carriers = 48/
>>/> no. of pilot carriers = 4/
>>/>/
>>/>/
>>/>/
>>/> ___________________________________/
>
>
>
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