# Why the Qam modulation cannot work well in ofdm benchmark example? **URL:** \ **Category:** GNU Radio\ **Created:** [November 2, 2013, 3:24am UTC](https://www.ruby-forum.com/t/why-the-qam-modulation-cannot-work-well-in-ofdm-benchmark-example/237270 "2013-11-02T03:24:36Z")\ **Posts on this page:** 3\ **Page:** 1
**Author:** ![musicdenotation](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/ea666f/32.png) [@musicdenotation](https://www.ruby-forum.com/u/musicdenotation)\ **Post date:** [November 2, 2013, 3:24am UTC](https://www.ruby-forum.com/t/why-the-qam-modulation-cannot-work-well-in-ofdm-benchmark-example/237270/1 "2013-11-02T03:24:36Z")
Hi guys, Recently, I have conducted a project based on ofdm benchmark. However, when I use the high modulation rate like qam16 and qam64, the packet error rate increase significantly. I guess the reason is that the preamble in gunradio is too weak to do channel estimation, thereby raising the packet error rate. Furthermore, even though I test the example offline without going through the channel, the packet error rate still very high, which make me feel confused. It is supposed to perform normally offline, without any decoding error right? Best, Kay ---
**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:** [November 3, 2013, 11:16am UTC](https://www.ruby-forum.com/t/why-the-qam-modulation-cannot-work-well-in-ofdm-benchmark-example/237270/2 "2013-11-03T11:16:14Z")
On Fri, Nov 01, 2013 at 07:23:33PM -0700, Yingjie C. wrote: > Recently, I have conducted a project based on ofdm benchmark. However, when I > use the high modulation rate like qam16 and qam64, the packet error rate > increase significantly. I guess the reason is that the preamble in gunradio is > too weak to do channel estimation, thereby raising the packet error rate. > > Furthermore, even though I test the example offline without going through the > channel, the packet error rate still very high, which make me feel confused. > It is supposed to perform normally offline, without any decoding error right? In order to do QAM over OFDM, you will need a good equalizer; we currently don’t have good ones implemented. 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:** [November 3, 2013, 10:13pm UTC](https://www.ruby-forum.com/t/why-the-qam-modulation-cannot-work-well-in-ofdm-benchmark-example/237270/3 "2013-11-03T22:13:20Z")
On Sun, Nov 3, 2013 at 5:09 AM, Martin B. (CEL) [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)wrote: > through the > > > channel, the packet error rate still very high, which make me feel > > confused. > > It is supposed to perform normally offline, without any decoding error > > right? > > In order to do QAM over OFDM, you will need a good equalizer; we > currently don’t have good ones implemented. > > MB Thanks for the clarification. I’ve been grappling with the high packet error rates for a while now. 🙂 Aditya