osmo-bts-trx/scheduler: implement baseband frequency hopping

The idea behind the baseband frequency hopping is quite simple: we
have several RF carriers (transceivers) transmitting and receiving
on fixed frequencies (just like in a regular multi-trx setup), and
an additional burst routing layer between the schedulear and the
transceiver interface (TRXD over UDP).

Speaking in terms of the proposed implementation:

  - on Downlink, dlfh_route_br() calculates the ARFCN corresponding
    to the current TDMA frame number according to the hopping sequence
    parametets, and picks the transceiver with matching ARFCN;

  - on Uplink, ulfh_route_bi() iterates over the transceiver list of
    of the BTS, calculating hopping ARFCNs for equivalent timeslots,
    and picks the one with ARFCN matching the received burst.

In order to avoid frequent transceiver lookups on the Downlink path,
dlfh_route_br() maintains a "cache" in the timeslot state structure.
Unfortunately, this "cache" seems to be useless on the Uplink path,
so ulfh_route_bi() always needs to lookup the matching transceiver
for each burst received over the TRXD interface.

It may also happen that the scheduler will be unable to route an
Uplink or Downlink burst, e.g. due to inconsistent / incorrect
hopping sequence parameters received from the BSC, or in case
if a transceiver gets RF-locked by the BTS operator.

Such events are logged as "FATAL" and aditionally signalled by the
following osmo-bts-trx specific rate counters:

  - trx_sched:dl_fh_no_carrier (Downlink), and
  - trx_sched:ul_fh_no_carrier (Uplink).

Change-Id: I68f4ae09fd0789ad0d8f1c1e17e17dfc4de8e462
Related: SYS#4868, OS#4546

1 files changed, 2 insertions, 0 deletions

diff --git a/src/osmo-bts-trx/l1_if.h b/src/osmo-bts-trx/l1_if.h
index fb988a65..8c309dba 100644
--- a/src/osmo-bts-trx/l1_if.h
+++ b/src/osmo-bts-trx/l1_if.h
@@ -29,6 +29,8 @@
 /* bts-trx specific rate counters */
 enum {
 	BTSTRX_CTR_SCHED_DL_MISS_FN,
+	BTSTRX_CTR_SCHED_DL_FH_NO_CARRIER,
+	BTSTRX_CTR_SCHED_UL_FH_NO_CARRIER,
 };
 
 /*! clock state of a given TRX */