# OFDM TX/RX with large packet\_len size

**URL:** <https://www.ruby-forum.com/t/ofdm-tx-rx-with-large-packet-len-size/234747>\
**Category:** GNU Radio\
**Created:** [March 26, 2015, 4:52pm UTC](https://www.ruby-forum.com/t/ofdm-tx-rx-with-large-packet-len-size/234747 "2015-03-26T16:52:21Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![addis\_a](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/c4cdca/32.png) [@addis\_a](https://www.ruby-forum.com/u/addis_a)\
**Post date:** [March 26, 2015, 4:52pm UTC](https://www.ruby-forum.com/t/ofdm-tx-rx-with-large-packet-len-size/234747/1 "2015-03-26T16:52:21Z")

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Hi,

I’ve problem to use a large packet\_len size (ie; 1500 bytes) in OFDM  
Flowgraph as in the digital folder examples. I got this error:

sched: \<block ofdm\_cyclic\_prefixer (34)\> is requesting more input data  
than we can provide.  
ninput\_items\_required = 34  
max\_possible\_items\_available = 31  
If this is a filter, consider reducing the number of taps.

My setup is:

FFT=256  
Packet\_len=1512  
header\_mod=QPSK  
payload\_mod=QAM16  
min output buffer in cyclic prefix=100000

Btw, if i’m using FFT=64, it does works fine when I increase the kernel  
shmax value. I attached the GRC that i’m using. Please kindly advice for  
this problem. My objective is to make this OFDM flowgraph to works with  
UDP/TCP packets. Or maybe there is another workaround to implement this?

Another question, is this OFDM TX/RX example is working good for QAM16?  
I’ve try this with my b200, it seems I got lots of packet lost when I’m  
trying to transmit a file or some random source.

Thanks