# Rx\_ofdm Does Not Work Past Synchronizer **URL:** \ **Category:** GNU Radio\ **Created:** [March 19, 2014, 12:34am UTC](https://www.ruby-forum.com/t/rx-ofdm-does-not-work-past-synchronizer/239897 "2014-03-19T00:34:51Z")\ **Posts on this page:** 8\ **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:** [March 19, 2014, 12:34am UTC](https://www.ruby-forum.com/t/rx-ofdm-does-not-work-past-synchronizer/239897/1 "2014-03-19T00:34:51Z")
So I have the stock python scripts (tx\_ofdm and rx\_ofdm) located in the gr-digital examples and put USRP sources and sinks in the appropriate spots. I left sampling alone (100K for TX, 3.2M for RX) and had the TX/RX frequency at 500 MHz. The USRPs used are N210s with WBX daughterboards. I tested the USRP used for TX out by outputting a signal to another and looking at it on the GNU Radio spectrum analyzer. Also, this is through SMA cables and a 30 dB attenuator, no over the air transmission. I placed file sinks after each block in the process; each file is a binary file. After multiple runs I felt like something wasn’t working after looking at the binary files. When I execute, only the file sinks placed after the Schmidl & Cox OFDM Synchronizer freq\_offset and detect have data in them, at least their files have some size to them, implying something has been written. The file sinks after FFT (or the header/payload demux) have no data, 0 KB. When I discovered that the receive didn’t work, I took the grc file another user made back in October with both TX mod and RX demod in the same flowgraph to see if that would work (files attached). Again same issue, all the binary files after the FFT have no data so something is getting screwed up in my demod. Has any user on this list run into this issue before or can offer some insight to why the receiver is failing? Thank you, Jon Attached is the python script, grc file, and screen shot of the TXRX combo in GRC. ---
**Author:** ![jfaulkenberry](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/c2a13f/32.png) [@jfaulkenberry](https://www.ruby-forum.com/u/jfaulkenberry)\ **Post date:** [March 19, 2014, 12:54am UTC](https://www.ruby-forum.com/t/rx-ofdm-does-not-work-past-synchronizer/239897/2 "2014-03-19T00:54:09Z")
Hi Jon, Disclaimer: I haven’t run your code, but based on eyeballing the png file: It looks like your “delay” block is not configured properly. The delay should be fft\_len + cp\_len which is 80 (instead of 72). Best, Aditya ---
**Author:** ![jfaulkenberry](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/c2a13f/32.png) [@jfaulkenberry](https://www.ruby-forum.com/u/jfaulkenberry)\ **Post date:** [March 19, 2014, 1:04am UTC](https://www.ruby-forum.com/t/rx-ofdm-does-not-work-past-synchronizer/239897/3 "2014-03-19T01:04:40Z")
And if you are running over the air, try setting the TX and RX sampling rate to have the same value. In any case, I would first run in loopback mode (as shown in your png file) and save the over-the-air transmissions for later. ---
**Author:** ![jfaulkenberry](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/c2a13f/32.png) [@jfaulkenberry](https://www.ruby-forum.com/u/jfaulkenberry)\ **Post date:** [March 19, 2014, 12:51pm UTC](https://www.ruby-forum.com/t/rx-ofdm-does-not-work-past-synchronizer/239897/4 "2014-03-19T12:51:24Z")
On 03/19/2014 12:25 AM, Jonathan F. wrote: > So I have the stock python scripts (tx\_ofdm and rx\_ofdm) located in the > gr-digital examples and put USRP sources and sinks in the appropriate > spots. I left sampling alone (100K for TX, 3.2M for RX) and had the > TX/RX frequency at 500 MHz. The USRPs used are N210s with WBX > daughterboards. I tested the USRP used for TX out by outputting a signal > to another and looking at it on the GNU Radio spectrum analyzer. Also, > this is through SMA cables and a 30 dB attenuator, no over the air > transmission. I placed file sinks after each block in the process; each > file is a binary file. After multiple runs I felt like something wasn’t > working after looking at the binary files. Maybe you’re probably screwing up your tx signal by clipping, although I can’t say for sure from your flow graphs (where exactly do you put the UHD sink?) See [http://video.fosdem.org/2014/AW1125/Sunday/Tutorial\_OFDM\_Packet\_Transceivers.webm](http://video.fosdem.org/2014/AW1125/Sunday/Tutorial_OFDM_Packet_Transceivers.webm), starting around ~27 mins for an explanation, or search the mailing list archive. Martin ---
**Author:** ![jfaulkenberry](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/c2a13f/32.png) [@jfaulkenberry](https://www.ruby-forum.com/u/jfaulkenberry)\ **Post date:** [March 19, 2014, 8:42pm UTC](https://www.ruby-forum.com/t/rx-ofdm-does-not-work-past-synchronizer/239897/5 "2014-03-19T20:42:11Z")
Thanks for the responses. When I use the USRPs the UHD sink is placed after the OFDM Cyclic Prefixer, right after the Mutliply Const (50m). I haven’t changed any settings because I remember reading in a few previous emails that the most common question people ask is “Did you change anything?” Sadly, the delay wasn’t the case, at least for the loopback mode. I reduced the amplitude of the signal by an additional 0.02, so the multiply constant is at 0.03 as opposed to 0.05. It brought the clipping well within +1, -1. Before the signal clipped past -1 but never rose above +1. No effect but I am not ruling out that it wouldn’t help. The video is interesting enough that I am going to continue watching after the first half, maybe I can figure out more from there. Jon ---
**Author:** ![jfaulkenberry](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/c2a13f/32.png) [@jfaulkenberry](https://www.ruby-forum.com/u/jfaulkenberry)\ **Post date:** [March 20, 2014, 10:06am UTC](https://www.ruby-forum.com/t/rx-ofdm-does-not-work-past-synchronizer/239897/6 "2014-03-20T10:06:57Z")
A good way to test is to use the hier blocks for OFDM (you still need to reduce amplitude), they will set up everything in a way that will work for sure. See the loopback example in gr-digital/examples/ofdm. M ---
**Author:** ![jfaulkenberry](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/c2a13f/32.png) [@jfaulkenberry](https://www.ruby-forum.com/u/jfaulkenberry)\ **Post date:** [March 20, 2014, 5:15pm UTC](https://www.ruby-forum.com/t/rx-ofdm-does-not-work-past-synchronizer/239897/7 "2014-03-20T17:15:46Z")
So I found the loopback script, tied the ends together with a multiply const block and was successful in modding and demodding that sinusoidal waveform. Added a UHD source and sink, success again. Then I split the process into two separate GRC files, a TX and a RX script. Everything works. I am extremely happy right now to see OFDM work. Thank you for the nudge in the right direction. So the next question I have is why I have success with the hierarchal blocks in the loopback script but not so much with rx\_ofdm or that custom loopback I posted? Even with a binary file source with the modulated signal, that rx script won’t work. I wouldn’t say too terribly important but it is interesting to learn how OFDM works. But for what I am working on, those hierarchical blocks will suffice. Thanks again. Jon ---
**Author:** ![jfaulkenberry](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/c2a13f/32.png) [@jfaulkenberry](https://www.ruby-forum.com/u/jfaulkenberry)\ **Post date:** [March 20, 2014, 7:47pm UTC](https://www.ruby-forum.com/t/rx-ofdm-does-not-work-past-synchronizer/239897/8 "2014-03-20T19:47:39Z")
On 03/20/2014 05:14 PM, Jonathan F. wrote: > modulated signal, that rx script won’t work. I wouldn’t say too terribly > important but it is interesting to learn how OFDM works. But for what I > am working on, those hierarchical blocks will suffice. Well, it’s hard to say from a distance, but you could check the output on the header path after the header/payload demuxer to see what’s going on. M