CAT 1 modems causing lots of errors

[avalori1]

Issue Description

When connecting a CAT 1 modem the errors on the connections are always very high

Setup Details

This is visible with different combinations of SDR and OS

Expected Behavior

Assuming good quality signal, the error should stay low

Actual Behaviour

The % of errors is high
If the max_mcs is set appropriately (see "Additional Information") the error goes down, but two problems arise:

• The max bandwidth of the cell is not used (in the situation of several CAT 1 connected)
• if a mixture of CAT 1 and CAT 4 are connected, the CAT 4 modems will be heavily slowed down

Steps to reproduce the problem

Connect a CAT1 modem using the standard default configuration files

Additional Information

• This issue has been partially discussed as a comment in issue Cat-1 sample configuration files  #698
• The category of the modem seem broadcasted (and read by SRS) correctly (from the MAC capture, see comment to the mentioned issue Cat-1 sample configuration files  #698 (comment))
• "limiting" the max_mcs parameters as mentioned at the bottom of the message, the errors disappear, proving that the scheduler does not discriminate correctly

For n_prb=6 (ul=1.8Mbps, dl=3.3Mbps)
[scheduler]
pusch_max_mcs = 28
pdsch_max_mcs = 28

For n_prb =15 (ul=4.9Mbps, dl=9.0Mbps)
[scheduler]
pusch_max_mcs = 20
pdsch_max_mcs = 27

For n_prb =25 (ul=5.0Mbps, dl=9.4Mbps)
[scheduler]
pusch_max_mcs = 14
pdsch_max_mcs = 20

For n_prb=50
[scheduler]
pusch_max_mcs = 6
pdsch_max_mcs = 12