# Re: How many preambles uesd in ofdm example for frame synchronization, 10 short preambles? **URL:** \ **Category:** GNU Radio\ **Created:** [April 7, 2013, 5:50am UTC](https://www.ruby-forum.com/t/re-how-many-preambles-uesd-in-ofdm-example-for-frame-synchronization-10-short-preambles/228034 "2013-04-07T05:50:38Z")\ **Posts on this page:** 3\ **Page:** 1
**Author:** ![Yingjie\_C](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/y/f14d63/32.png) [@Yingjie\_C](https://www.ruby-forum.com/u/Yingjie_C)\ **Post date:** [April 7, 2013, 5:50am UTC](https://www.ruby-forum.com/t/re-how-many-preambles-uesd-in-ofdm-example-for-frame-synchronization-10-short-preambles/228034/1 "2013-04-07T05:50:38Z")
Thanks for your reply. I still have one question. I try to plot the peak detection data to observe the the shape of the peak, which seems like a plateau(square wave).I have done a simulation before in matlab. If only one preamble exist, when the 1st half of the preamble correlate with 2nd half, the shape of peak should be much sharper(like a triangle), because the repeated pattern is not enough. My question is : If gunradio only use one preamble with 2 repeated pattern, the shape of the peak should look like a triangle rather than a plateau, is that right? 2013/4/6 Tom R. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) ---
**Author:** ![Yingjie\_C](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/y/f14d63/32.png) [@Yingjie\_C](https://www.ruby-forum.com/u/Yingjie_C)\ **Post date:** [April 8, 2013, 9:31am UTC](https://www.ruby-forum.com/t/re-how-many-preambles-uesd-in-ofdm-example-for-frame-synchronization-10-short-preambles/228034/2 "2013-04-08T09:31:57Z")
On Sun, Apr 07, 2013 at 11:49:14AM +0800, Yingjie C. wrote: > Thanks for your reply. I still have one question. I try to plot the peak > detection data to observe the the shape of the peak, which seems like > a plateau(square wave).I have done a simulation before in matlab. If only > one preamble exist, when the 1st half of the preamble correlate with 2nd > half, the shape of peak should be much sharper(like a triangle), because > the repeated pattern is not enough. > My question is : > If gunradio only use one preamble with 2 repeated pattern, the shape of the > peak should look like a triangle rather than a plateau, is that right? No, the plateau is correct. Since your correlation window is 2x half of 1 symbol, the cyclic prefix will increase your peak (in AWGN channels to the length of the cyclic prefix). MB – Karlsruhe Institute of Technology (KIT) Communications Engineering Lab (CEL) Dipl.-Ing. Martin B. Research Associate Kaiserstraße 12 Building 05.01 76131 Karlsruhe Phone: +49 721 608-43790 Fax: +49 721 608-46071 www.cel.kit.edu KIT – University of the State of Baden-Württemberg and National Laboratory of the Helmholtz Association ---
**Author:** ![Yingjie\_C](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/y/f14d63/32.png) [@Yingjie\_C](https://www.ruby-forum.com/u/Yingjie_C)\ **Post date:** [April 8, 2013, 11:35am UTC](https://www.ruby-forum.com/t/re-how-many-preambles-uesd-in-ofdm-example-for-frame-synchronization-10-short-preambles/228034/3 "2013-04-08T11:35:48Z")
Thanks for your reply, Martin. I still have one question. I have done the simulation for PN peak normalization process in matlab. I try to plot the P(refer to correlation result between 1st half and 2nd half) and R(refer to the power of 2nd half symbol) . However, when I plot the P and R in the same picture using gr\_plot\_float in gnuradio, the curve seems different my simulation result. The R curve seems to be up-side-down against P curve. Please check the attached file for reference. Thank you again. 2013/4/8 Martin B. (CEL) [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)