Re: [Discuss-gnuradio] OFDM tx rx error

[Martin Braun]

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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