# DSRC demodulation with gr-ieee802\_11 **URL:** \ **Category:** GNU Radio\ **Created:** [September 22, 2014, 3:53pm UTC](https://www.ruby-forum.com/t/dsrc-demodulation-with-gr-ieee802-11/234826 "2014-09-22T15:53:03Z")\ **Posts on this page:** 12\ **Page:** 1
**Author:** ![alcina](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/a587f6/32.png) [@alcina](https://www.ruby-forum.com/u/alcina)\ **Post date:** [September 22, 2014, 3:53pm UTC](https://www.ruby-forum.com/t/dsrc-demodulation-with-gr-ieee802-11/234826/1 "2014-09-22T15:53:03Z")
I’m new to Gnu Radio but I’m asked to demodulate a DSRC signal and I don’t know how handle that. DSRC is used in Intelligent Vehicule Systems, it is modulated with OFDM int the 5.9Hz band and is based on the IEEE 802.11p standard. On my first approach, I found some papers about the module gr-ieee802\_11: [https://archive.fosdem.org/2014/schedule/event/towards\_an\_open\_source\_ieee\_802\_11p\_stack/](https://archive.fosdem.org/2014/schedule/event/towards_an_open_source_ieee_802_11p_stack/) > **[bloessl2013decoding.pdf](https://www.ccs-labs.org/bib/bloessl2013decoding/bloessl2013decoding.pdf)** > > 4.52 MB > **[bloessl2013ofdm-tr.pdf](https://www.ccs-labs.org/bib/bloessl2013ofdm-tr/bloessl2013ofdm-tr.pdf)** > > 2.23 MB I installed the module as described here: [GitHub - bastibl/gr-ieee802-11: IEEE 802.11 a/g/p Transceiver](https://github.com/bastibl/gr-ieee802-11) and tried to use the examples/wifi\_rx.grc file to start. Here begin my problem: using the same record\_source\_file, I have several results printed on my console. It’s seems that the module cannot detect OFDM frames. Is there something I missed to use gr-ieee802\_11 module ? I’m using Gnu Radio 3.7.2 with USRP N210, my record\_source\_file is sampled with 5Msps at 5.127 GHz. Do not hesitate to ask if I missed some required information. Thanks ! ---
**Author:** ![Jean-Baptiste\_Truff](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/4da419/32.png) [@Jean-Baptiste\_Truff](https://www.ruby-forum.com/u/Jean-Baptiste_Truff)\ **Post date:** [September 23, 2014, 4:17am UTC](https://www.ruby-forum.com/t/dsrc-demodulation-with-gr-ieee802-11/234826/2 "2014-09-23T04:17:22Z")
Hi, On 09/22/2014 03:51 PM, Jean-Baptiste Truffault wrote: > I installed the module as described here: > [GitHub - bastibl/gr-ieee802-11: IEEE 802.11 a/g/p Transceiver · GitHub](https://github.com/bastibl/gr-ieee802-11) and tried to use the > examples/wifi\_rx.grc file to start. Here begin my problem: using the > same record\_source\_file, I have several results printed on my console. > It’s seems that the module cannot detect OFDM frames. I’m afraid you have to be a bit more verbose… What did you do? And what exactly went wrong? Did you follow the steps in the readme? > I’m using Gnu Radio 3.7.2 with USRP N210, my record\_source\_file is > sampled with 5Msps at 5.127 GHz. IIRC, you need at least 3.7.3 (will add that to the readme). Best, Bastian ---
**Author:** ![Jean-Baptiste\_Truff](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/4da419/32.png) [@Jean-Baptiste\_Truff](https://www.ruby-forum.com/u/Jean-Baptiste_Truff)\ **Post date:** [September 24, 2014, 4:17pm UTC](https://www.ruby-forum.com/t/dsrc-demodulation-with-gr-ieee802-11/234826/3 "2014-09-24T16:17:23Z")
Sorry for the time, I had some trouble with the 3.7.3 install, finally I installed the 3.7.5 thanks to the script [http://www.sbrac.org/files/build-gnuradio](http://www.sbrac.org/files/build-gnuradio). First, I recorded a DSRC signal thanks to Gnu Radio and my USRP N210 (with a CBX daughter card): 5Msps, 5.127 GHz. I now use this file as source. It contends some DSRC exchanges that I would like to decode (I am quite sure that the record is good thanks to the shape of it, cf attached picture [3\_fft.png]). My grc scheme [1\_grc1.png & 2\_grc2.png] is very similar of yours, I just attached it to be sure. The problem is when I read the debug from the OFDM Decode Mac, at the first time I launch the script, I get something like: 80 be 44 b5 e1 71 e5 57 31 ac 58 4e 0b db b8 1f 03 63 c7 b4 1b e5 2c 8c 76 da 20 f5 c2 e0 8e bc 62 ad 12 37 d5 07 1a 5f e6 d4 03 a4 e7 db 1c 9c eb 99 1c 52 e8 83 13 06 ac 08 70 4b bf 71 0c ab 74 52 2c fc f9 ca 05 4a 29 55 c9 c6 cb 7c f8 51 22 60 6f 5f 64 d7 55 f9 32 f6 79 7c 61 ea 89 86 f3 33 ea 85 43 52 12 6e b2 92 e7 3c 81 5c 49 f2 b6 30 03 dc c4 15 f4 cd f0 a1 5a 1b c9 88 a8 94 … the second time I launch the script, I get something completly different that looks like: 00 ee 2b f8 e9 7c d4 5e 93 22 e9 fc d4 1a 9e 1a d5 3e 03 0b 93 da 15 fe 3a 2f 53 fd c0 9c 98 a6 9e 8f 8c 0f 30 cc f6 a6 16 46 81 54 b7 95 3a 1e 20 36 20 b6 91 3c 20 55 ab 02 ea a4 9f e8 69 88 97 77 c8 de 92 25 0e 18 6b e1 29 4f e6 3c 86 65 b0 7a 61 0b 47 d6 22 38 19 4c 0b b9 fb 1b 53 83 aa 4b d7 fe 9f 4d de 0f 8b 99 a7 de a4 09 0d d5 1c d5 6c dc 66 bf 5f 7c dc b7 a3 fa 88 b3 c8 7b 11 1b 04 41 1a 41 5d 51 74 ea 48 11 33 d2 d1 82 … Those results should be the same because they come from the same record file… The debug indicates that the checksum is wrong so the frame is dropped and I do not receive anything on the OFDM Parse MAC Block. My first idea is that it comes from the OFDM frame detection because of the strange shape of it (cf [5\_autocorrelation.png]), it acts like there were always something to decode when the messages come with a burst on the FFT [3\_fft.png]. Another thing that is different from your result is the constellation scope that may have 4 clusters (instead of one on the 4\_constellation.png) I would be very kind if you had some idea where I did something wrong ! Thanks in advance for your time and your help ! 2014-09-23 4:16 GMT+02:00 Bastian B. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid): ---
**Author:** ![Jean-Baptiste\_Truff](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/4da419/32.png) [@Jean-Baptiste\_Truff](https://www.ruby-forum.com/u/Jean-Baptiste_Truff)\ **Post date:** [September 24, 2014, 5:52pm UTC](https://www.ruby-forum.com/t/dsrc-demodulation-with-gr-ieee802-11/234826/4 "2014-09-24T17:52:40Z")
On 09/24/2014 04:14 PM, Jean-Baptiste Truffault wrote: > Sorry for the time, I had some trouble with the 3.7.3 install, finally I > installed the 3.7.5 thanks to the script > [http://www.sbrac.org/files/build-gnuradio](http://www.sbrac.org/files/build-gnuradio). > > First, I recorded a DSRC signal thanks to Gnu Radio and my USRP N210 > (with a CBX daughter card): 5Msps, 5.127 GHz. I now use this file as > source. It contends some DSRC exchanges that I would like to decode (I > am quite sure that the record is good thanks to the shape of it, cf > attached picture [3\_fft.png]). I don’t think so. This does not look like a 10MHz OFDM signal… > My grc scheme [1\_grc1.png & 2\_grc2.png] is very similar of yours, I just > attached it to be sure. This looks OK. > My first idea is that it comes from the OFDM frame detection because of > the strange shape of it (cf [5\_autocorrelation.png]), it acts like there > were always something to decode when the messages come with a burst on > the FFT [3\_fft.png]. The autocorrelation is always high so frame detection is triggered all the time. This might happen when you start the USRP source. Try to drop samples at startup (skip head block). > Another thing that is different from your result is the constellation > scope that may have 4 clusters (instead of one on the 4\_constellation.png) Since you are not synced on a real frame you just see noise. Best, Bastian ---
**Author:** ![Jean-Baptiste\_Truff](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/4da419/32.png) [@Jean-Baptiste\_Truff](https://www.ruby-forum.com/u/Jean-Baptiste_Truff)\ **Post date:** [September 25, 2014, 9:55am UTC](https://www.ruby-forum.com/t/dsrc-demodulation-with-gr-ieee802-11/234826/5 "2014-09-25T09:55:40Z")
OK, I try to record a new signal and I tell you if I get something new. 2014-09-24 17:51 GMT+02:00 Bastian B. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid): ---
**Author:** ![Jean-Baptiste\_Truff](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/4da419/32.png) [@Jean-Baptiste\_Truff](https://www.ruby-forum.com/u/Jean-Baptiste_Truff)\ **Post date:** [September 25, 2014, 11:12am UTC](https://www.ruby-forum.com/t/dsrc-demodulation-with-gr-ieee802-11/234826/6 "2014-09-25T11:12:37Z")
I see there some double used terms. Is it the European ETSI style DSRC, or the US WAVE style? 802.11p? Ralph. From: discuss-gnuradio-bounces+ralph=removed\_email\_address@domain.invalid [[mailto:discuss-gnuradio-bounces+ralph=removed\_email\_address@domain.invalid](mailto:discuss-gnuradio-bounces+ralph=removed_email_address@domain.invalid)] On Behalf Of Jean-Baptiste Truffault Sent: Thursday, September 25, 2014 9:54 AM To: Bastian B. Cc: removed\_email\_address@domain.invalid Subject: Re: [Discuss-gnuradio] DSRC demodulation with gr-ieee802\_11 OK, I try to record a new signal and I tell you if I get something new. 2014-09-24 17:51 GMT+02:00 Bastian B. \: On 09/24/2014 04:14 PM, Jean-Baptiste Truffault wrote: Sorry for the time, I had some trouble with the 3.7.3 install, finally I installed the 3.7.5 thanks to the script [http://www.sbrac.org/files/build-gnuradio](http://www.sbrac.org/files/build-gnuradio). First, I recorded a DSRC signal thanks to Gnu Radio and my USRP N210 (with a CBX daughter card): 5Msps, 5.127 GHz. I now use this file as source. It contends some DSRC exchanges that I would like to decode (I am quite sure that the record is good thanks to the shape of it, cf attached picture [3\_fft.png]). I don’t think so. This does not look like a 10MHz OFDM signal… My grc scheme [1\_grc1.png & 2\_grc2.png] is very similar of yours, I just attached it to be sure. This looks OK. My first idea is that it comes from the OFDM frame detection because of the strange shape of it (cf [5\_autocorrelation.png]), it acts like there were always something to decode when the messages come with a burst on the FFT [3\_fft.png]. The autocorrelation is always high so frame detection is triggered all the time. This might happen when you start the USRP source. Try to drop samples at startup (skip head block). Another thing that is different from your result is the constellation scope that may have 4 clusters (instead of one on the 4\_constellation.png) Since you are not synced on a real frame you just see noise. Best, Bastian ---
**Author:** ![Jean-Baptiste\_Truff](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/4da419/32.png) [@Jean-Baptiste\_Truff](https://www.ruby-forum.com/u/Jean-Baptiste_Truff)\ **Post date:** [September 25, 2014, 3:09pm UTC](https://www.ruby-forum.com/t/dsrc-demodulation-with-gr-ieee802-11/234826/7 "2014-09-25T15:09:18Z")
Finally I’ve got something better concerning the autocorrelation, but it’s still very noisy on the constellation… maybe something I’m still doing wrong ? 2014-09-25 13:05 GMT+02:00 Bastian B. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid): ---
**Author:** ![Jean-Baptiste\_Truff](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/4da419/32.png) [@Jean-Baptiste\_Truff](https://www.ruby-forum.com/u/Jean-Baptiste_Truff)\ **Post date:** [September 25, 2014, 1:07pm UTC](https://www.ruby-forum.com/t/dsrc-demodulation-with-gr-ieee802-11/234826/8 "2014-09-25T13:07:22Z")
Hi Ralph, On 25 Sep 2014, at 11:10, Ralph A. Schmid, dk5ras [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > I see there some double used terms. Is it the European ETSI style DSRC, or the > US WAVE style? 802.11p? IEEE WAVE and ETSI ITS G5 are both based on IEEE 802.11p. The module implements the phy and parts of the mac, which are defined in 11p. So it doesnt matter what you use it for. Best, Bastian ---
**Author:** ![Jean-Baptiste\_Truff](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/4da419/32.png) [@Jean-Baptiste\_Truff](https://www.ruby-forum.com/u/Jean-Baptiste_Truff)\ **Post date:** [September 25, 2014, 3:48pm UTC](https://www.ruby-forum.com/t/dsrc-demodulation-with-gr-ieee802-11/234826/9 "2014-09-25T15:48:43Z")
On 25 Sep 2014, at 15:07, Jean-Baptiste Truffault [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > Finally I’ve got something better concerning the autocorrelation, but it’s still > very noisy on the constellation… > maybe something I’m still doing wrong ? Now it looks like you are just concatenating long and short preamble and miss the actual frame. Best, Bastian ---
**Author:** ![Jean-Baptiste\_Truff](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/4da419/32.png) [@Jean-Baptiste\_Truff](https://www.ruby-forum.com/u/Jean-Baptiste_Truff)\ **Post date:** [September 25, 2014, 4:00pm UTC](https://www.ruby-forum.com/t/dsrc-demodulation-with-gr-ieee802-11/234826/10 "2014-09-25T16:00:13Z")
is there a way to troubleshoot that kind of problem ? 2014-09-25 15:47 GMT+02:00 Bastian B. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid): ---
**Author:** ![Jean-Baptiste\_Truff](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/4da419/32.png) [@Jean-Baptiste\_Truff](https://www.ruby-forum.com/u/Jean-Baptiste_Truff)\ **Post date:** [September 25, 2014, 5:22pm UTC](https://www.ruby-forum.com/t/dsrc-demodulation-with-gr-ieee802-11/234826/11 "2014-09-25T17:22:46Z")
On 25 Sep 2014, at 15:58, Jean-Baptiste Truffault [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > is there a way to troubleshoot that kind of problem ? I think you should read the readme and follow the steps in order to find out what works and at which step things break (for example I dont know if the loopback flow graph works already). If it does not work over the air use graphical sinks and the log / debug options of the blocks to find out at which stage of the receiver things go wron In case you still have problems you should describe your setup in greater detail (including how you generate your frames). Hope it helps, Bastian ---
**Author:** ![Jean-Baptiste\_Truff](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/4da419/32.png) [@Jean-Baptiste\_Truff](https://www.ruby-forum.com/u/Jean-Baptiste_Truff)\ **Post date:** [September 25, 2014, 7:57pm UTC](https://www.ruby-forum.com/t/dsrc-demodulation-with-gr-ieee802-11/234826/12 "2014-09-25T19:57:07Z")
On Thu, Sep 25, 2014 at 09:53:54AM +0200, Jean-Baptiste Truffault wrote: > > > First, I recorded a DSRC signal thanks to Gnu Radio and my USRP N210 > > > (with a CBX daughter card): 5Msps, 5.127 GHz. > > > > > > I don’t think so. This does not look like a 10MHz OFDM signal… Are you trying to capture a 10 MHz wide signal at 5 Msps? That would be a problem. Mike