I have a GNU radio based software that uses IQ modulation on the sending side and IQ demodulation on the receiving side. In my application, the transmitted signal could be scaled up to +/-5% in time domain. I don't have control to this scaling. So I take two actions to compensate for the scaling: (1) Scale the frequency of the sinusoidal signal used for IQ demodulation. (2) Resample the IQ demodulator output to recover the time scale. In order to detect the time scaling, I added to the transmitted signal an additional fixed frequency sinusoidal signal whose frequency doesn't overlap with the IQ carrier frequency. On the receiving side, I estimate the time scaling by checking the deviation of the addition sinusoidal signal from the expected value. Once the time scale is estimated, I adjust the frequency of the signal source for IQ demodulation and the sampling ratio of the re-sampler accordingly. Debug log shows that the latency from time scaling detected to meaningful data coming out of IQ demodulation is 600-700 ms.
My question for the GNU radio experts is how to reduce the latency. I suspect part of the latency is due to the fact that while I reconfigure the flow graph, data continue to flow through the graph. I need to keep the data in the graph during reconfiguration so I don't want to call lock/unlock. I'm thinking of finding ways to pause/resume the scheduler. Does this approach make sense? Regards, Bolin Hsu
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