We are using two USRPs and GNU Radio Companion to build an OFDM radar. The first USRP is the N210 which is used as a transmitter through its Tx/Rx port. The second USRP is N200 which is used as a receiver through its Rx2 port. They are connected together through the so-called MIMO cable to synchronize them. The N210 is connected to the host PC through the gigabit Ethernet cable. The samples that feed the transmitter USRP comes from a block "File Source", and the samples that are collected from the receiver USRP goes to a block "File Sink". Initially, an external loopback cable is used between the Tx/Rx port of N210 and the the Rx2 port of the N200. Whenever we run the flowgraph, we expect that for every sample that is transferred from the File Source to N210, there should a corresponding sample with somehow same value that comes from N200 to File Sink. However, we have noticed that the N200 produces a stream of random samples before the awaited samples start to appear!!! The length and the values of this stream of random samples varies each time we re-run the flowgraph!!! Of course this issue constitutes an obstacle for our application because in radar the range of the target to be detected is estimated depending on the delay time. The latter is computed from the number of noise samples at the receiver that precedes the reception of the actual transmitted samples.
The question is: How can we guarantee or force the receiving USRP (N200) not to receive any sample before the transmitter USRP (N210) starts to transmit the required samples? Should not this be the task of GnuRadio?!!!! or we have to do something in GnuRadio to force this to happen? thnx
This sounds like you're reproducing my 2013 bachelor thesis!
No, that would only work when you start and stop both USRPs using the same command times and the same number of samples to be acquired.
Well, over-the air delay, and the state in the DSP chain. This is expected. Use timed commands to make the timing deterministic, and you'll know how many samples to ignore.