In my attempt to create a simple real-time telemetry link I've got stuck in a peculiar and stubborn streaming problem that I hope someone can help me to resolve. I am using two USRP's with GnuRadio 3.3.0 and GRC (since I am still quite new to GnuRadio).
Before going into the details (which unfortunately is necessary in order to explain this issue, therefore I am really sorry for the length of this mail!), let me suggest that the problem seem to be related to stream conversions (rate and type) combined with simultaneous transmission and reception. This results in overflow/underflow problems that I cannot get rid of. Note that it is NOT computer congestion related (> 99% sure...)
So first, let me briefly explain my telemetry link, which I put together in five parts, and then afterwards let me explain and pinpoint the actual problem:
1) At transmitter side I first sample an incoming baseband signal via the LFRX daughterboard at 1 MSa/s into the computer. My test input signal is currently a square pulse (up to some 50 kHz), with a low 200mV amplitude, coming from a tone generator.
First part works well!
2) Now, since this sampled signal has too high data rate (for my over the air link, in part 3) I reduce its bit rate by slicing the amplitude (thus obtaining 1 bit/sample, now the stream flow at 1Mbit/s). The slicing is okay because the telemetry-source is a NRZ-type signal and therefore I am only interested in its sign. To avoid redundant dummy bits after the binary slicing (which produces bytes with "1 significant bit per byte") I pack these "unpacked" bits into fully packed bytes containing all significant bits by using the "Unpacked to Packed" block. I also decimate the stream by a factor two (simply by throwing away every 2nd sample) to obtain a 500 kbit/s bitstream. This digital data-stream which represents the original NRZ signal I want to transmit over-the-air reconstruct it at the receiver side.
Second part works well !
3) The over the air interface is a simple GMSK transmission link, 500 kbit/s (1MHz bandwidth, 2 samples/symbol). I am currently using the RFX2400 board at some 2.45 GHz. GMSK transceiver is used with Packet Encoder & Decoder at Tx and Rx side, respectively. Quite straightforward and nothing fancy.
Third part works well!
4) At the receiver side after the GMSK demodulation I "reconstruct" the original NRZ-signal by interpolate ("sample repeat") by a factor 2 (after the GMSK demodulation). "Unpack K bits" with (K=8) and use "Chunks to Symbols" with "-1.0, 1.0" alphabet. At this point I have reconstructed the original NRZ signal at the Rx side. This reconstruction of the NRZ signal work fine (although the original pulse-timing accuracy is reduced due to the necessary decimation/interpolation process to adapt data rate for air link).
Fourth part works well!
5) The just reconstructed NRZ signal is transmitted via the LFTX daughterboard with 1 MSa/s out to an oscilloscope (to verify the complete telemetry-link).
Fifth part works well !
All of the above parts works very good, but unfortunately not when running the whole chain 1) to 5) all together. That is, for example using 1) together with 5) works excellent, as do 2) with 4) and the air-to-air link 3) by its own, etc.
However, when connecting the whole link 1)-5) I run in to overflow/underflow problems (uU and oU) at the transmitter and receiver sides, which results in a non-continous square waveform coming out at the oscilloscope. Using "real time scheduling" does not improve or change the situation.
Now, the first legitimate suspicion is that the computers are overwhelmed and cannot keep up with all this. But that is NOT the case I am more than 99% convinced (I have made some quite extensive testing). For example, the processors are only running at about 20% of full capacity when running the full chain. Furthermore when I replace 1) and 5) with "Signal Source" and "Scope Sink" (that is, by using an artificial source and sink) together with 2)-3)-4) it also works fine, and even though I simultaneously use the original 1) and 5) parts but don't use them (e.g. connect them to null sinks). So the computers do keep the pace.
Instead the problem seems to be related to my necessary conversion of the signal streams, data rate and type, combined with receiving and transmitting simultaneously with the USRP source and sink blocks. Any ideas or clocks ringing from experienced users?
So, one of my thoughts was that perhaps the stream between the "USRP source" and "USRP sink" do not become perfectly synched when they are both used in tandem, although their rates are perfectly adapted to each other (like to my 500 kbit/s) ?!
For example, if I use a complex "Signal Source" with 1 MSa/s instead of the "original" sampled signal in step 1), I can make the overflow problem at the TX side go away. Correspondingly at the Rx side. That is, anytime I de-connect the USRP source from the USRP sink (still using the explained stream conversion processes in between) or replace the actual USRP source with an artificial "Signal Source" with the same sample rate, the overflow/underflow problem go away.
I thought also that perhaps some stringent timing issues combined with the stream conversion processes lead to the overflow and underflow problems that I experience. Like a "takt time" synch issue on a conveyor belt. Therefore, in an attempt to possibly alleviate such a problem I tried to use "Sample and Hold" circuits between any "USRP source" and "USRP sink" pair, like at the transmitter in between step 1) and 2), respectively. This however didn't improve or change the situation.
So, is there anyone out there who have some qualified guess or hint how to overcome this peculiar overflow/underflow problem? I would be relieved!
Thanks,
Rickard
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