Hi all,
as some of you may know, I am currently working on baseband frequency
hopping implementation for osmo-bts-trx (see OS#4546). I've already
submitted more or less final implementation to Gerrit [1], and in
general it works fine [2].
The idea is quite simple: you have several RF carriers (transceivers)
with fixed frequencies, so on hopping timeslots osmo-bts-trx distributes
bursts between them according to the configured hopping sequence.
However, when it comes to hopping timeslots on C0, the burst
distribution algorithm gets a bit more complicated.
The problem is that on C0 the BTS has to maintain constant power level,
even if there is nothing to transmit, so the phones can detect it
reliably during the power scan. This is achieved by sending dummy
bursts. When frequency hopping is not in use, the scheduler in
osmo-bts-trx sends a dummy burst immediately. When it is in use,
sending dummy bursts needs to be postponed until all transceivers are
processed, because some other transceiver might need to send a normal
burst on the frequency of C0, and there would be no need to send a dummy
burst in that case. So I came up with [3] (any comments regarding
potential optimizations and further improvements are welcome).
While working on this, I stumbled upon the burst filling feature in
osmo-trx [4], that is disabled by default. If enabled, osmo-trx would
be sending dummy bursts itself, so there is no need to send them from
osmo-bts-trx, and thus no need to complicate the burst distribution logic.
I am now wondering why don't we enable this feature by default?
From the VTY reference:
> Send a dummy burst when there is nothing to send on C0 (TRX0) and
> empty burst on other channels. Use for OpenBTS compatibility only,
> don’t use with OsmoBTS as it breaks encryption.
it's unclear how sending dummy bursts would break encryption...
[1] https://gerrit.osmocom.org/c/osmo-bts/+/19030
[2] https://www.youtube.com/watch?v=tGMVNYo9s7E
[3] https://gerrit.osmocom.org/c/osmo-bts/+/19029
[4]
http://ftp.osmocom.org/docs/latest/osmotrx-vty-reference.pdf#subsection.1.9…
Best regards,
Vadim.
--
- Vadim Yanitskiy <vyanitskiy at sysmocom.de> http://www.sysmocom.de/
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