Try cranking your RX gain up/down 5dB and see how that affects your constellation.
On 2015-10-27 11:55, Washbourne, Logan wrote: > Ok, that sampling rate mismatch was my fault. The RX spectrum looks better > after that fix. The unpacked to packed bits didn't seem to remove the zero > grouping on the constellation plot. Here are pictures of the setup now. > > Logan Washbourne Electrical Engineering Graduate Student (Electromagnetics) > > On Tue, Oct 27, 2015 at 10:34 AM, Richard Bell <richard.be...@gmail.com> > wrote: > > Logan, make sure you're feeding packed bytes into the DPSK Mod block. That's > probably why you're seeing a constellation point at zero, you're feeding in > unpacked bytes. That might fix everything. > > Rich > > Sent from my iPad > > On Oct 27, 2015, at 8:16 AM, mle...@ripnet.com wrote: > > What does the spectral plot look like? It should look very similar to the TX > side of the house. > > On 2015-10-27 11:10, Washbourne, Logan wrote: > > Thanks for the info guys! They are both using antennas btw. I came in this > morning and turned the RX gain up pretty high, roughly 45dB, and the > constellation plot on the RX side is starting to look pretty square, which I > think is a bad sign. My thoughts are that with the increase in gain, there is > too much noise coming through. > > I've been toying with adding in a frequency offset on the TX side. I've been > adding and subtracting up to 30kHz from the center frequency and I haven't > seen any improvement in the constellation plot on the RX side. > > Another thing that is confusing me, on the TX constellation plot, before it > gets sent to the USRP, there are 3 groupings, -.5, 0, .5. I'm not sure where > the 0 points are coming from. Since this is DBPSK, there should only be -.5 > and .5 groupings right? > > Should this exercise be pretty straightforward? > > Logan Washbourne Electrical Engineering Graduate Student (Electromagnetics) > > On Mon, Oct 26, 2015 at 3:12 PM, <mle...@ripnet.com> wrote: > > Consulting now, the book of armaments, errr, UHD manual: > > http://files.ettus.com/manual/page_dboards.html#dboards_rfx [2] > > The RFX series has no TX RF gain control, output magnitude is entirely > dependent on baseband magnitude. > > There's 70dB of gain range on RX, however. So, crank up the RX gain. > > Also, across 1m, the path loss is about 40dB at 2.5GHz, your TX is probably > putting out 5dBm, so at the RX end, that's maybe as much as -35dBm. If your > RX is set properly, should be enough to see the signal. > > On 2015-10-26 15:46, Marcus Müller wrote: Hi! > > * looking at your constellation, relief comes setting in: It looks pretty > circular to me; notice how the axes are scaled differently. So no > catastrophic IQ imbalance. > * the fact that it's circular and not a line is probably the result of the > constellation sink not being able to achieve timing synchronization, i.e. it > doesn't "know" when a symbol starts > * You're right, RX SNR is terrible. However, RX power is very little indeed > * Rule of thumb: if(not clipping && in doubt) increase gain; > * What exactly is your "RF channel": antennas, direct cable with attenuator? > * The mistake I do every few months: Did you connect the antenna/cable to the > wrong SMA port? > > Best regards, > Marcus > > On 26.10.2015 20:33, Washbourne, Logan wrote: > I looked at the spectrums of both sides and the RX side looks to be inundated > with a lot of noise. I'm not sure if its my environment or my USRPs, but I > really expected to see some resemblance of a signal. I am posting links to > screencaps of the TX and RX sides. > > http://imgur.com/GBTHw8T- [3] TX > http://imgur.com/DBy8gqs [4] - RX > > Logan Washbourne Electrical Engineering Graduate Student (Electromagnetics) > > On Mon, Oct 26, 2015 at 1:31 PM, Marcus Müller <marcus.muel...@ettus.com> > wrote: > > Can you compare the TX spectrum with the RX spectrum? Maybe it'd worthwhile > to first look at at e.g. 500kHz of RX spectrum to see whether, due to > frequency offset, some of the wanted signal simply gets cut off; admittedly, > with 2S/sym that's not too likely... > > Best regards, > Marcus > > On 26.10.2015 19:16, Washbourne, Logan wrote: > I am using 250k! > > Logan Washbourne Electrical Engineering Graduate Student (Electromagnetics) > > On Mon, Oct 26, 2015 at 1:05 PM, Marcus Müller <marcus.muel...@ettus.com> > wrote: > > Hello! > what's the sampling rate you're using? > > Best regards, > Marcus > > On 26.10.2015 18:38, Washbourne, Logan wrote: > > Hello all, > > I'm trying to take a step back from my previous postings and try a simple > approach to try and understand working with over the air communications with > the USRPs. Let me know if it would be better to post this in the USRPs-Users > list. > > I'm trying to have a simple TX and simple RX side so I can eliminate and > unnecessary complications. > > The TX side has the following flowgraph: Vector Source(preamble+data)->DPSK > Mod(DBPSK,2samples/symbol)-> Multiply Const(.707) -> UHD:USRP SINK(2.448GHz > center freq, 10dB default gain) > > RX Side: UHD:USRP Source(2.448GHz center freq, 10dB gain default) -> DPSK > DEMOD(DBPSK, 2 samples/ symbol, rest default values) - > file sink > > I'm checking the progress of the communication by looking at a constellation > plot that is connected directly to the USRP SOURCE block on the receiver > side. I'm getting a very elliptical shape, more spread out in the horizontal > direction but not by much. I'm also looking at the bits from the receiver > side in matlab and my preamble is not showing up. > > These were the same problems I had when trying to use a correlated system for > OTA communications. I feel like I'm missing something really simple. > > Does anyone know of a simple TX, RX grc setup that I can use to test my USRPs? > > Again, I really appreciate the help you guys give. > > Logan Washbourne Electrical Engineering Graduate Student (Electromagnetics) > > _______________________________________________ > Discuss-gnuradio mailing list > Discuss-gnuradio@gnu.org > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio [1] > > _______________________________________________ > Discuss-gnuradio mailing list > Discuss-gnuradio@gnu.org > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio [1] _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org https://lists.gnu.org/mailman/listinfo/discuss-gnuradio [1] _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org https://lists.gnu.org/mailman/listinfo/discuss-gnuradio [1] _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org https://lists.gnu.org/mailman/listinfo/discuss-gnuradio [1] _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org https://lists.gnu.org/mailman/listinfo/discuss-gnuradio [1] _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org https://lists.gnu.org/mailman/listinfo/discuss-gnuradio [1] _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org https://lists.gnu.org/mailman/listinfo/discuss-gnuradio [1] > _______________________________________________ > Discuss-gnuradio mailing list > Discuss-gnuradio@gnu.org > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio [1] _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org https://lists.gnu.org/mailman/listinfo/discuss-gnuradio [1] Links: ------ [1] https://lists.gnu.org/mailman/listinfo/discuss-gnuradio [2] http://files.ettus.com/manual/page_dboards.html#dboards_rfx [3] http://imgur.com/GBTHw8T- [4] http://imgur.com/DBy8gqs
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