Re: [Discuss-gnuradio] USRP's not Communicating

Dear Ravi,

this is no good. We've told you several times now that you need to fix things, and you just come back with your same broken configuration and ask the same question. It's totally OK if you just say "hi, sorry, I did not understand your answer, could you explain this and that", but not addressing what we wrote is really getting frustrating, for me at least.

Let me repeat what Tom and others have tried to explain to you: There's no single "switch" you can flip so that your transmission works. We have pointed out methods to help you optimize your transmission, for example looking at the RX spectrum and adjusting gains until it looks "good". We can't, just by looking at the problems the receiver script has, tell you what you need to do. Really. Please stop asking the same question over and over again, and ask different questions. A clever one would, for example, be: I've got this spectrum (screenshot), and it doesn't look like <modulation I'm expecting>. Increasing the gain just made things worse."

I hope this leads you to a path that promises more success than what you're currently doing.

So: You've gotten a lot of responses. You might need to consider how much consultation costs you would have caused if we weren't doing this out of interest but for money -- so please explain what you did not understand in our answers, we really want to help!

Here's my rather comprehensive answer from last time:

I don't know whether your questions have been solved. I think Tom has extensively discussed [1] a lot of things that can happen here, so you're aware of the fact that transmission-wise, there's the whole world of radio channel that could distort your signal.

So let's talk a bit about what's wrong with the hardware settings you use:

Transmitter

No gain specified.

You should really specify the gain. That defines how much power comes out of your antenna. For a start, you might want to use a moderately high setting, so something like 25 would be good.

UHD Warning:      The hardware does not support the requested TX sample rate:      Target sample rate: 0.050000 MSps      Actual sample rate: 0.250000 MSps

Symbol Rate:         25000.000000  Requested sps:       2.000000  Given sample rate:   250000.000000  Actual sps for rate: 10.000000    Requested sample rate: 50000.000000  Actual sample rate: 250000.000000

I you must really fix that. As I've explained two weeks ago:

as the warning says, the sampling rates you're trying to use on both TX and RX side are lower than the minimum sampling rates. You should fix that. It's not hard, you just need basic understanding of what the program does.

To spell this out: the data rate together with the samples per symbol define your sample rate. For your USRP, this rate must be at least 250kS/s. The effect of you not correcting your flow graph is that the script generates a signal that the USRP converts to an analog signal at five times the speed that you specify -- this means all frequencies with your complex baseband are five times as high!

..............................................................................................U...............................................................

Oh, that's bad. You're seeing an U like "Underrun", which means your PC was too slow at supplying samples to the USRP. This seriously breaks your signal. Now, I don't know your computer, but I know that 250kS/s is really no load for any PC nowadays -- I haven't seen any PC being too slow for this very low rate. How old is your PC? Are you doing something else on it? Or are you running in a virtual machine?

terminate  called after throwing an instance of 'uhd::runtime_error'    what():  RuntimeError: usb tx2 transfer status: 1  Aborted (core dumped).

That's even worse. It can happen if you cancel the script using "ctrl+c", but this looks like there is something seriously wrong with your USB2 controller. Again, what computer are you using?

Receiver

No gain specified.  Setting gain to 56.250000 (from [0.000000, 112.500000])

The same as above. More gain = more signal power, but don't overdo it. You will need to experiment. As Tom explained, letting the transmitter run and observing the spectrum with something like usrp_spectrum_sense will help you determine settings that make sense. First, however, I can not stress this enough, fix your sample rate issue!

UHD Warning:      The hardware does not support the requested RX sample rate:      Target sample rate: 0.050000 MSps      Actual sample rate: 0.250000 MSps    Symbol Rate:         25000.000000  Requested sps:       2.000000  Given sample rate:   250000.000000  Actual sps for rate: 10.000000    Requested sample rate: 50000.000000  Actual sample rate: 250000.000000  

The same as above. Both your RX and your TX are running at 250kS/s instead of the 50kS/s that you specified. Luckily, this means that the effect kind of cancels itself out -- still, fix that!

General

How would I be able to know if the USRP is maintained in the same  channel?

You set both TX and RX to the same frequency, and since there is only one physical medium (the space between the RX and TX antenna), you can be sure. There's no concept like "this specific part of spectrum belongs to channel number 12" in the physical world -- just frequencies, and Maxwell's Equations governing the propagation of radio waves.

What commands do I need to give so that the packet is received  correctly as n_right=1.

Tom has explained enough that it's not like that. You will need to find out yourself.


Best regards,
Marcus

PS: Do you have a preferred name to address you by? "Ravi" is as fine to me as "John", so I'd suggest you pick one and stick with that, because it's less confusing.

[1] "Communication Problems between 2 USRP" email thred

Best regards,

Marcus