- (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.
The RX chain uses this to acquire an initial estimate of the carrier frequency. The estimate is adjusted on every incoming symbol until a preamble is found.
In the following events, the modem cannot switch to transmit mode for the
given time:
- after a preamble has been received, for 500 ms
- after a packet has been successfully decoded, for 1 ms
- after receive mode was initially enabled, for 500 ms
Note that the times are that long to work around the RX/TX switching of
the hackrf and can be hopefully reduced to <10 milliseconds in a later
version.
- limit frequency adjustment range
- pre-filter the baseband signal to remove out-of-band interference
- reset the frequency acquisition periodically (every 30s) if no preamble is found
Thomas Kolb