Conceptually: Loops

Hello,

I have an application where I communicate with a device in a tightly time-controlled (and synchronized) manner: In gnuradio I send some data, wait for some data (at precise moment in time) and process the results.

For this I created an OOT module which outputs databits and receives databits (input & output). It is a FSM.

Then I have a hierarchical block which implements the IF part (Bits in, IF-Out, IF-In, Bits out). It includes all signal processing at IF (resampling, filtering, frequency conversion, ...).

Finally I created a block for the analog frontend (IF-In, IF-Out). It includes one USRP Sink and USRP Source. The reason why I have one block is again that USRP Source/Sink must be synchronized and the block abstracts a couple of things of these away. Additionally I created the same block (same I/O, same parameters) for simulation which allows me to quickly switch between simulation and actual hardware. It also provides the flexibility to switch to different hardware or to use a block like gr-radars echotimer.

Hence:

+-------+ +------+ +----------+
| My |------>| |---->| |
| FSM | | IF | | RF |
| block | | | | frontend |
| |<------| |<----| |
+-------+ +------+ +----------+

Clearly this results in a loop but this loop isn't an inherent issue (such as an uncomputable algebraic loop). I can break it by inserting a ZMQ Source/Sink in between but this is of course not optimal.
I can also split up "IF" and "RF frontend" in two parts but then I loose the nice abstraction layer that I created. Or split up "My FSM block". But then I loose the ability to tighly control the transmitted and received bits.

What is the most elegant way to resolve this?

Thanks,
Lukas