[Discuss-gnuradio] Detecting a specific frequency tone and its amplitude

[Kunal Kandekar]

Hi all,

I just started working on Gnu Radio, and am working on a digital
communication project usig the USRP. In this scheme, the transmitter would
send a single frequency tone (not necessarily at the carrier frequency)
for a short time as a preamble before beginning a long, uninterrupted
transmission of bits. The receiver would be listening for this tone to
synchronize with the transmitter, and would only process further samples
from the USRP once this tone is detected. The amplitude of this tone is
also important, as I hope to infer distance to the transmitter based on
this.

Although I am planning to test the scheme with FSK first, I hope to make
it independent of any modulation technique used, hence am not relying on
the default blocks such as gr_simple_framer/correlator and others used in
fsk_tx/rx.py.

Hence if the normal flow graph looks like:
src -> mod -> usrp(tx) ----> usrp(rx) -> demod -> sink

I need a block "sync_tx/rx" such that the flow graph looks like:
src -> mod -> sync_tx -> usrp(tx) ---> (rx)usrp -> sync_rx -> demod -> sink

Sync_tx simply prepends a synchronization tone before any transmission,
and sync_rx does not forward any incoming samples to the subsequent blocks
until it detects this tone and determines its amplitude. Could anyone
please let me know the easiest way to implement such a sync_tx/rx block? I
have started writing a C++ signal processing block, but it's getting a bit
tough. What Gnu radio blocks (gr_fft_*, gr_quadrature_demodulator, etc)
should I look into for code re-use? I was wondering if there was maybe
away to do it in python itself?

Thanks!
Kunal