# OFDM using USRP

**URL:** <https://www.ruby-forum.com/t/ofdm-using-usrp/243476>\
**Category:** GNU Radio\
**Created:** [November 12, 2015, 5:17pm UTC](https://www.ruby-forum.com/t/ofdm-using-usrp/243476 "2015-11-12T17:17:03Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![Sadaf\_N](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/858c86/32.png) [@Sadaf\_N](https://www.ruby-forum.com/u/Sadaf_N)\
**Post date:** [November 12, 2015, 5:17pm UTC](https://www.ruby-forum.com/t/ofdm-using-usrp/243476/1 "2015-11-12T17:17:03Z")

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Hello.

I’m trying to set up OFDM using TI USRP 2921. At start I wanted to use  
16QAM as modulation, but OFDM Mod block has no options for sync. I  
switched to OFDM Tansmitter and choose QPSK. I have to main problems:

1. I can’t set FFT len \> ~300. It would be nice to set it at about 512  
or higher.

2. Synchronization. For 300 FFT I’m planning to use 20 pilot channels,  
but I have a problem with sync words. Is there any rule how to create  
them?