[Discuss-gnuradio] Upsampling: Delay and Alignment

[Maximilian Stiefel]

Hello everbody,

upsampling a complex data stream gives me headaches :( In my flowgraph 
the upsampling is done by multiplexing 7 zeros with 1 data sample and 
feeding this into a FIR filter, which is how everybody does it as I 
understood. However, for further processing I have to take the delay 
into account, that is introduced by the filter (length N). This delay 
should be (N-1)/2, as the filter phase is linear. I do this by delaying 
the data stream by one symbol length minus (N-1)/2, since causality 
rules.

       +----+  ------------+
       |Data+--->          |
       +----+   |          |   +---------------------+   +-----+
                |Stream MUX+--->Decimating FIR Filter+--->Delay|
+-----------+   |          |   +---------------------+   +-----+
|Null Source+--->          |
+-----------+  ------------+

If I upsample as depicted above, my transmission kinda works, but I end 
up having alignment problems resulting in a lower SNR. As I understood 
the delay is realized by zero padding, which should work, but there 
seems to be something I am missing out on. When I use the Decimating FIR 
Filter block and I want the same alignment as before (symbol x starts at 
sample y), so it can be further processed, what do I have to do?

Thank you for your ideas.

Regards,

Max