osmo-pcu: gprs+egprs multiplex: Allow selecting original 8-PSK dl block during retransmission for GMSK-required DL block
- At least 1 GMSK block must be sent every 18th block
- If USF targes the GPRS-only phone, GMSK must be used
That means, for instance, when the scheduler looks for a GMSK DL block candidate to sent (due to above restrictions), it skips selecting DL blocks for TBFs which are waiting for retransmission of 8PSK blocks (MCS>4), meaning it may end up sending a Dummy Block sometimes.
Under that circumstances, we should do a 2nd pass over the TBFs and downgrade the ready-to-be-transmitted 8PSK DL block to a MCS of the same family which is MCS <= 4), that is, in practice, splitting the data in 2 blocks containing same BSN but different SPB to identify first/second subblocks.
This contains a summary of information related to this improvement: