[Discuss-gnuradio] Overflow/Underflow streaming problem (due to rate & type conversions?)

[Rickard Nilsson]

Hi all,

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