Dear Marcus, Some questions on signal cancellation example again, (I have tried other things and am back to this topic again.)
Actually, what I try to do is a signal cancellation in the air. (For example, interference cancellation in the air, so a specific receiver will experience almost zero interference). Could you make some comments on the following experiment what I did? One transmitter (USRP B210) and one receiver (USRP B210) Carrier freq.: 5.89G signal: sine wave having 1000 Hz frequency sampling rate: 5 Mbps office environment / LOS (1m) / assume channel is quite static 1. Sending a signal from TX to RX and recording it into a file (binary data) . Since I turned on RX first, the file has some zero values (not really zero, noise level signal) at the beginning of the file. 2. I repeated the experiment one more and save the samples into another file. 3. Using some tools (GRC / MATLAB), I read both signals (say, A and A') and make time synchronization between them by removing zeros at the beginning of each signal. finally, "result" = A - A' I expected to see almost zero signal (noise level) but still, signals are not canceled. Could the frequency offset be time-variant whenever I tried experiments even if I used the same hardware? (freq_offset1 at t1 / freq_offset2 at t2) Do you think any other reason for that? Thank you, Regard, Inkyu 2018-04-29 18:06 GMT+08:00 Müller, Marcus (CEL) <muel...@kit.edu>: > Hi! > On Sun, 2018-04-29 at 01:17 +0800, Inkyu Bang wrote: > > Dear Marcus > > > > Thank you for the comments. > > > > I am trying to make a beam which is canceled out (i.e., null) at a > > specific location but is not elsewhere. > > > > I am aware of beamforming and half-wavelength things > > (By the way, a wavelength of 6GHz is 5cm) > > oooops :D > > > So I put two Tx antennas and one Rx antenna in proper locations to > > satisfy the condition which you said (i.e., half wavelength distance > > difference). > > > > I do not know much about phase offset and USRP hardware things. > > So, your two TX chains have an unknown phase offset. You could actually > find that by, for example, sending the very same signal on both > antennas, and measuring the angle of the nulls :) > > > > > Can I ask a few more questions? > > (It might be silly questions.) > > No such thing as silly questions! > > > > > I used USRP B210 which has two RF chains and it seems to use the same > > clock for those two RF chains. > > > > Here is a link to brief specification of B210: > > https://www.ettus.com/content/files/b200-b210_spec_sheet.pdf > > > > Q1) In this case, is there a chance of different phase offsets > > between two antennas from the same USRP? > > Yes, there is. The clock's the same, but not necessarily its phase. > > > > > Q2) Then, how can we calibrate those different phase offsets between > > two antennas in the same USRP? > > (What happens in MIMO transmission in real SDR devices?) > > For example, through geometric observation as described above, by some > kind of cable loopback (but that would require phase-length calibrated > cables and some way to switch over to the antennas afterwards), or by > techniques that would send two different (optimally: orthogonal) > signals, one from each TX antenna, and using math after the one RX > antenna to tell the phase difference of these two. A classic for that > would be to use Gold codes, but for your use case, basically any good > pseudorandom bit sequence would do (if you can construct a sufficiently > orthogonal counterpart, but that might be as simple as XOR with > 10101010...). > > Best regards, > Marcus > > > > Thank you. > > Regards, > > > > Inkyu > > > > > > > > > 2018. 4. 27. 오후 10:13, Müller, Marcus (CEL) <muel...@kit.edu> 작성: > > > > > > Dear Inkyu, > > > > > > there's no guarantee that the phases of both TX are exactly > > > identical. > > > In fact, you should expect an unknown offset. > > > > > > Also, you'll notice that your two antennas are at two different > > > positions in the room. You're accidentally building a beamforming > > > system! So, these nulls will not be everywhere in the room, but > > > only in > > > a specific direction in the far field of the TX antenna. And with a > > > carrier frequency of nearly 6 GHz, a wavelength in air is > > > > > > c/f ≈ (3·10⁸ m/s) / (6·10⁹ 1/s) = 0.05 m = 50 cm > > > > > > and half that distance from a (single) transmit antenna would be > > > the > > > distance at which two received signals would be exactly of opposite > > > phase. > > > > > > So, I'm not sure what you want to demonstrate, but unless it's > > > beamforming, you're probably not doing it right :) Which is no > > > shame, > > > but it would be interesting to hear what it is that you want to > > > achieve. > > > > > > Best regards, > > > > > > Marcus > > > > > > On Fri, 2018-04-27 at 20:44 +0800, Inkyu Bang wrote: > > > > Hi, all !! > > > > > > > > I am trying to make a simple example of signal cancellation using > > > > GRC. > > > > However, I did not get any good results. > > > > > > > > Here is my setting > > > > > > > > Plan: sending two sine wave with different phases and receiving > > > > canceled signal (only noise) > > > > Center frequency: 5.89GHz (I am using the antenna supporting this > > > > center frequency) > > > > Sampling rate: 5MHz > > > > Frequency of sine wave: 1kHz > > > > > > > > USRP: B210 > > > > Tx GRC flow graph: signal source block + USRP sink (two channels) > > > > Rx GRC flow graph: USRP sink + QT GUI time sink > > > > > > > > When I send two sine signals, I used two RF chains in the same > > > > USRP. > > > > I received those signals at the other USRP. > > > > > > > > I expected to see canceled signals (i.e., only noise level power > > > > in time domain) > > > > but still, I see sine wave signal. > > > > > > > > I thought frequency offset in the receiving USRP is applied to > > > > both sine waves. > > > > > > > > Do I need to consider frequency offset first? > > > > Could anyone help me to solve this problem? > > > > > > > > Thank you. > > > > Regards, > > > > > > > > Inkyu > > > > _______________________________________________ > > > > Discuss-gnuradio mailing list > > > > Discuss-gnuradio@gnu.org > > > > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio > > > > >
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