SIB configuration

[mrlnc]

Hi all,

I think there are some problems with SIB configuration. Possible fix included.

Default Config

This is from the SIB1 config in sib.conf:

sched_info =
    (
        {
            si_periodicity = 16;
        si_mapping_info = []; // comma-separated array of SIB-indexes (from 3 to 13). 
                   // Leave empty or commented to just scheduler sib2
        }
);

Expected Behaviour

• only SIB1, SIB2 is transmitted

  65535 LTE RRC DL_SCH SystemInformationBlockType1
  65535 LTE RRC DL_SCH SystemInformation [ SIB2 ]

  (uncomment //#define WRITE_SIB_PCAP in srsenb/src/mac/mac.cc to have SIBs included in PCAP)

  Actual Behaviour

  Despite no optional SIB was configured, srsenb transmits an empty SIB3, which is not announced in SIB1 schedulingInfoList.

65535 LTE RRC DL_SCH SystemInformationBlockType1
65535 LTE RRC DL_SCH SystemInformation [ SIB2 SIB3 ]

Custom Config

First configure SIB4 at end of sib.conf, which is quite simple since all fields are optional

sib4 = {
};

If I attempt to transmit another SIB (assuming SIB4 exists) and use this config:

sched_info =
    (
        {
            si_periodicity = 16;
        si_mapping_info = [4]; // comma-separated array of SIB-indexes (from 3 to 13). 
                   // Leave empty or commented to just scheduler sib2
        }
);

then SIB4 is advertised in SIB1, but still only SIB2 and SIB3 are transmitted.

Expected Behaviour

65535 LTE RRC DL_SCH SystemInformationBlockType1
65535 LTE RRC DL_SCH SystemInformation [ SIB2 SIB4 ]

Actual Behaviour

65535 LTE RRC DL_SCH SystemInformationBlockType1
65535 LTE RRC DL_SCH SystemInformation [ SIB2 SIB3 ]

Custom Config 2

If you transmit si_mapping_info = [4, 4];, then SystemInformation [ SIB2 SIB4 SIB3 ] is transmitted, while SIB1 mapping is SIB4, SIB4

Possible Solution

After some debugging, I think there is a off-by-one, that leads to SIB3 being included and configured SIBs ignored. https://github.com/srsLTE/srsLTE/blob/dafcefb487c1453b1f0bd6e581c1c7f95dd92e79/srsenb/src/upper/rrc.cc#L144 should read

      memcpy(&msg[i].sibs[j+j_start], &cfg.sibs[(int) sched_info[i-1].sib_mapping_info[j].sib_type+2], sizeof(LIBLTE_RRC_SYS_INFO_BLOCK_TYPE_STRUCT));

i and j were not particularly helpful, so I changed some variable names. See 97ef33ca9a8e01cc4d43b3a6863ca2261129b45a for a patch. I've not created a pull request, since I have not yet fully understood SIB scheduling, and cannot tell whether things work as intended.

[mrlnc]

Is this comment still accurate? Multiple SIBs in one SI message seems to work for me https://github.com/srsLTE/srsLTE/blob/4bbcfaa1dcbe347e4c99539bd1d3ec7a92176b4e/srsenb/src/mac/scheduler.cc#L396

[ismagom]

Thanks for spotting this. Can you please create a pull request and I'll merge it?

Can you also remove that comment in the pull request please? Thanks