Re: [Discuss-gnuradio] Feedback with Transmitters and Receiver

On Thu, Apr 21, 2016 at 8:04 AM, Pavan Yedavalli <address@hidden> wrote:

Hi Derek,

I'll answer your questions in-line, because I think what you are saying is beginning to make me understand what I need:

On Wed, Apr 20, 2016 at 9:03 PM, Derek Kozel <address@hidden> wrote:
Hello Pavan,

Are you trying to create a shared timebase between the two USRPs without having a shared 1PPS or GPS reference? You are still not using enough detail for us to understand fully.

To clarify, my setup is two USRPs connected via MIMO cable, and then another USRP acting as a receiver. So are you asking whether I'm trying to create a shared timebase between the two-USRP unit (because they are MIMO cabled) and the receiving USRP without having a shared 1 PPS or GPS reference? I think my answer to that must be yes, because I have not done anything else but connect them to the computer via ethernet and just have two of them connected via MIMO cable and the other one by itself. I'm assuming I need to have a shared reference between the transmit USRPs and the receive USRP, so how would I be able to do that? This could certainly be one of my problems.

In Figure 5 both USRPs are connected with a MIMO cable and so have both shared frequency and time bases. What is your weight block doing to the sample stream? Is it a time delay block? I don't know what gnuradio would do if you specified 10*sample_rate as the delay there as that's likely to be a very large number of samples.

My weight block is applying a normalized magnitude phase correction to each antenna's transmitted signal, so, yes, it is essentially creating a time delay. Each weight is a complex value with magnitude 1 and a calculated phase. You are saying this could be a problem if it's calculating a value that is too high?

If you have both USRPs connected with a time synchronization (shared 1PPS, GPSDO, or MIMO cable) and have your flowgraph configured correctly, then you can just use timed commands to the USRP_alpha to start transmitting at time X and USRP_beta to start receiving at time X and you will see your signal. You can then move to using burst mode using tags to define the number of samples to send/receive along with timed commands to send/receive bursts of samples. This works because the clocks in both USRPs will be aligned to each other.

I feel like there are two steps here. First, I need to get the transmitting USRPs (which are conneced via MIMO cable) to time sync to each other (which I believe I have done through using USRP sink in GRC and setting the second channels time and clock to MIMO cable?), and second, I need to get the receive USRP to receive at the same time. So, just as above, I need to get my receive USRP to be on the same time as my transmit USRPs? Once I'm able to do that, then I can do burst mode to transmit and receive timed signals, as you are mentioning?

If you do *NOT* have a shared time source for each radio, for instance they are far apart and do not have GPS references, then you need to do some sort of protocol level alignment to create a shared understanding of time between them. A frequently used method is for USRP_alpha to transmit a beacon with a known period (say once every 10 seconds). All other USRPs then receive for longer than 10 seconds to be guaranteed to receive the beacon (assuming they're within range of the transmission). When the receiving USRPs detect the incoming beacon they align their local time to the master (Beacon transmitting) USRP.

I guess a similar question to the above: can I have a shared time source between the transmit USRPs (which are already MIMO cabled to each other) and the receive USRP? It seems like that would be easier to do than going through this protocol level alignment, but maybe it's not possible given my setup.

Here's a quick paper talking about this topic. The technique is widely used.
http://www.ece.uah.edu/~milenka/docs/dc_ssst05_synch.pdf

I hope this helps and is applicable to your need. If you have more questions please try drawing your desired system and maybe include a timeline of events that you expect the radios to do. Attaching your existing flowgraphs, either as photos using GRC's screen capture feature (file>screen capture) or the actual GRC file, also helps us understand what exactly you are working with.

I had to take down the setup because I am moving labs, but I will send some flowgraphs and the diagram of the system next week. Thank you again for being so patient and trying to help me. I think I'm just a bit lost on a few of the simple things, but once those are figured out, then I think it should be smoother sailing.

Derek

On Wed, Apr 20, 2016 at 4:05 PM, Pavan Yedavalli <address@hidden> wrote:
Hi Martin,

I guess I have a few questions:

1.) Are there any examples in the gnuradio codebase/flowgraph repository that show how to do synchronized feedback between two USRPs? In other words, I send a signal from a transmit USRP, and then I receive that signal at the receive USRP, and then I send back something else from the receive USRP back to the transmit USRP, and this would be a sequential process in which they are aligned and know when to transmit and/or receive? I saw a post that I think would be relevant, but I'm not sure how to apply it.

I believe this should be a pretty standard scenario in which you want to have two USRPs communicate with each other synchronously. I guess I'm just having trouble finding an example of how to do this.

2.) Related to the above question, maybe there are no examples to do feedback in one flowgraph, so what I have been doing is the following in my flowgraphs:

Flowgraph A:

The synchronized MIMO flowgraph (Figure 5) from this, so essentially I have two USRPs synchronized and transmitting out two signals that should be offset but frequency aligned. In my own flowgraph's main(), instead of applying a "phase shift" block, I am applying my own "weights" block to both transmissions.

So, I am now sending a signal that has those weights applied to it. So, after I do tb.start(), then I sleep for 10 seconds (by doing sleep(10)) hoping that in the 10 seconds my receiver will catch the signal that I'm transmitting and put it into file.

Flowgraph B:

My own receiver.py in which I have a USRP sink->FFT->Complex to Mag->File sink. I also have a connection from FFT->QT GUI to see a plot of what is being captured.

I now run Flowgraph A in one terminal and Flowgraph B in another terminal. I need to capture A's transmission with the first weights within the 10 seconds (as it's sleeping) into the file sink. Then, A will send a signal with another set of weights applied, and I will need to capture that in the next 10 seconds, and so on. My problem is that I'm often capturing noise because my receive was not aligned with when I was transmitting my desired signal. So, I end up only capturing noise after the transmission stops as opposed to the actual signal when the transmission is happening.

Essentially, I am trying to mimic feedback by doing the above, but I don't know how to align my transmitter and receiver, especially because they are two different blocks. Is there a way to make both the transmission and reception one block so that I can do sleep(rx_time + n_samples_since_tag/sampling_rate) (I think this could be right?) as opposed to my static sleep(10) and pray for the best?

Would it be helpful at all if I showed you my code? I still feel like I'm not being clear. Sorry about that. If there were any examples, then I think that would be the best for me to look at.

Thanks for any help again.

--
Pavan

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--
Pavan

From:	Derek Kozel
Subject:	Re: [Discuss-gnuradio] Feedback with Transmitters and Receiver
Date:	Thu, 21 Apr 2016 13:56:23 -0700