sdr_rf_to_baseband() processes samples in blocks of size SDR_OVERSAMPLING. If
the total number of samples does not align with this block size, the leftover
samples are lost and phase and timing glitches result.
To mitigate this, sdr_receive() now has an additional parameter that specifies
the alignment of the returned data. The number of samples returned is always a
multiple of this alignment factor. This feature is used to ensure that the
number of returned samples is a multiple of SDR_OVERSAMPLING and therefore no
samples are lost in sdr_rf_to_baseband().
sdr_rf_to_baseband() now has an additional check that makes the function fail
if the alignment is incorrect.
- Ramp-up length increased to 128 symbols (here is room for
improvement!)
- Try to detect the frequency once during ramp-up. To do so, every
second symbol is inverted (to remove the +/-1 symbol toggling) and the
phase difference between neigboring resulting symbols is checked. When
it is low enough for all symbols, the frequency is estimated and
corrected. When frequency estimation was done, it is not retried for a
number of incoming symbols in order to allow the timing estimator to
converge again.
- This approach was verified in a simulated loopback test with frequency
offset and AWGN.
Packets are now read from the TUN device and transmitted. The signal
goes through a channel emulator and is then received + decoded. If
decoding is successful, the result is written back to the TUN device
(which does not make much sense, but works for testing).
Unfortunately, there still seems to be a problem in the receiver: packet
loss is very high even at extremely high SNRs.
Thomas Kolb