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.
- passing argument 2 of 'crc_generate_key' discards ‘const’ qualifier;
a bit ugly but signature of crc_generate_key() is wrong
- variable 'linearized_history' set but not used
- typedef is unused
- superfluous arguments to DEBUG_LOG() and fprintf()
- implicit declaration of function 'memset'
- unused arguments
- (only) time-based end-of-transmission tracking
- removed tx_done flag
- count zero-buffers correctly in time-tracking
- add 10 ms of headroom so the transmission does not stop before buffer was
completely transmitted (race condition)
- fix race condition with tx_start_time in sdr_start_tx()
- simplified packet queuing (no chunking)
- read multiple packets before starting transmission (to fill buffers initially)
Thanks to rudi_s!
If passed on, these cause a segmentation fault in the channel decoder. As
zero-length packets currently make no real sense in this protocol, they are
ignored now.
- 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.
Thomas Kolb
Simon Ruderich