# NC-OFDM and Cross sub-carrier Interference **URL:** \ **Category:** GNU Radio\ **Created:** [October 17, 2014, 8:30pm UTC](https://www.ruby-forum.com/t/nc-ofdm-and-cross-sub-carrier-interference/232609 "2014-10-17T20:30:28Z")\ **Posts on this page:** 2\ **Page:** 1
**Author:** ![addis\_a](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/c4cdca/32.png) [@addis\_a](https://www.ruby-forum.com/u/addis_a)\ **Post date:** [October 17, 2014, 8:30pm UTC](https://www.ruby-forum.com/t/nc-ofdm-and-cross-sub-carrier-interference/232609/1 "2014-10-17T20:30:28Z")
Hi, I’m currently developing a testbed for spectrum aggregation and fragmentation using GNURadio and USRPs (B200s and B210s). My approach is to emulate aggregation and fragmentation using Non-Contiguous OFDM: Theoretically, each node should be able to transmit in any subset of the sub-carriers available. Everything seems to be fine when tested in a loop back(the GRC OFDM Tx example in gr\_digital was used). And connected to USRPs, a single pair transmission and reception is fine while transmitting in any subset of subcarriers. The problem arises when I have more than one node transmitting in the same OFDM configuration. No matter how wide the OFDM span is (I began from 64 and increased it to 1024), and no matter how far apart the two transmitting channels are, the cross-interference is enough to corrupt both transmissions. I have played with the transmission power of both transmitters to see if lowering it makes any difference, to no avail. I have also thought of applying a bandpass filter before feeding OFDM signals to USRPs to limit their out of band emissions, but as the system is supposed to handle fragmentation and requires complete spectrum agility, a single filter would not solve the problem. I would greatly appreciate any advice on the matter. Many Thanks, Vahid ---
**Author:** ![Vahid\_B](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/v/c68b51/32.png) [@Vahid\_B](https://www.ruby-forum.com/u/Vahid_B)\ **Post date:** [October 18, 2014, 11:24am UTC](https://www.ruby-forum.com/t/nc-ofdm-and-cross-sub-carrier-interference/232609/2 "2014-10-18T11:24:55Z")
-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Hi Vahid, a few things: On one hand, to be really orthogonal, all your nodes would need to exhibit no frequency offset. As long as you don’t have a common clock, be it through a clock distributor, GPSDOs or by doing estimations of the own offset on the nodes and accounting for that, you’d get energy of bins end up in others. Then, it’s intuitive that one tends to drive the DAC with maximum values (ie. using the whole -1,1 magnitude range in GNU Radio). Try adding a factor of 0.7 before! OFDM especially is a bit sensitive to the transmitter clipping, and that might increase the looks of your spectrum. I don’t know your transmitter, so I’m going to point out that OFDM can have quite an ugly PAPR, and although you feed in symbols that themselves as time signal would never lead to complex magnitude values greater than 1, the IFFT of a vector of these quite likely can. The same as for sample values goes for TX gain: tone it down a bit, if possible. Does your time-domain signal on RX look clipped? Turn down the RX gain. Then: Yes, you can gain quite a bit by using a filterbank instead of the good ole IFFT. Tom kind of has _the_ blog post (you’d call it article) to exactly your question: [http://gnuradio.squarespace.com/blog/2014/4/23/peer-review-of-a-dyspan-paper.html](http://gnuradio.squarespace.com/blog/2014/4/23/peer-review-of-a-dyspan-paper.html) And Tom wrote the polyphase filters in GNU Radio. Feed one big tons-of-taps channel filter in, and apply a bit of elegant math, smaller sub-filters and a bit of rotational magic done with an IFFT, and you get nicely filtered channels. You should give it a try, it’s fun 🙂 You might want to grab fred harris’ slides on polyphase filters, if you’re interested in the background of how it works, which should be around somewhere in the internets, but I can’t find that bookmark ☹ Greetings, Marcus On 17.10.2014 20:29, Vahid B. wrote: > single pair transmission and reception is fine while transmitting > OFDM signals to USRPs to limit their out of band emissions, but as > > \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ Discuss-gnuradio > mailing list [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) > [Discuss-gnuradio Info Page](https://lists.gnu.org/mailman/listinfo/discuss-gnuradio) -----BEGIN PGP SIGNATURE----- Version: GnuPG v1 iQEcBAEBAgAGBQJUQjGaAAoJEAFxB7BbsDrL12cH/0fC244WhsK1TX8lvOFPogtQ 8y3xCo26gxVQ01UgmwgybsFE3BukVqG/i8Skk6u0HTgXdzD0oQmC/lJUStYVJwJ6 BwekwvJov7g+OoIpq7QltmHyXcK85SmqCkkRkk/3vShpVXz7CT2SPUdN9GsBSLiN FtOUjD0qNFPgjjli9lEAzFp0KlPe3NzWGoY9OaXswvwMxyNUYZNrxLThVy+5DpPn eeJT/07pIRJkJ4HmwVLXEcMV8AmF6L+lrjeprVZqC+D2tsFm6mpxNyRN6ZzvNU6N Jbl4XEa53q1zw5nDjoIlENBLk7NXY9YZGvRC5UFiErChYxbrgTBy4sK56XPJSEU= =sQIq -----END PGP SIGNATURE-----