IoT: LTE-BL/CE(CAT-M1) MIB/SIB1

Compared with legacy MIB, CAT-M1 MIB has one more I.E. on RRC layer named schedulingInfoSIB1-BR-r13. Total length of MIB is 24 bits and legacy MIB has 10 spare bits which 5 most significant bits were used to indicate schedulingInfoSIB1-BR-r13. The value of schedulingInfoSIB1-BR-r13 is used to get the repetition level and TBS for SIB1-BR from 36.213 Table 7.1.6-1 and Table 7.1.7.2.7-1


For SIB1-BR decoding, based on 36.211 6.4.1, if the PDSCH carries SIB1-BR, the PDSCH transmission is repeated periodically in every period of 8 radio frames, where a period starts with a radio frame n_f mod8 = 0 where n_f is the system frame number.
I.E. One period started with sfn=80.
The PDSCH is transmitted N_(SIB1-BR/PDSCH) times in each period of 8 frames.
I.E. Looking at above table, if schedulingInfoSIB1-BR-r13 is 16, the N_(SIB1-BR/PDSCH) is 4. So SIB1-BR will be transmitted 4 times in each period of 8 frames started with sfn mod 8 = 0.



In above equation, let {Sj} be the set of narrowbands, excluding narrowbands oeverlapping with the 72 center subcarriers for N_(DL/RB) > 15, and ordered in increasing order of narrowband index.
I.E. If we have 10MHz DL bandwidth, the N_(DL/RB) is 50 > 15. There is in total 8 narrowbands but two of the central narrowbands are overlapping with center 72 carriers. Excluded these two narrowbands, set {Sj} size is 6 and S0 = n_NB0, S1 = n_NB1, S2=n_NB2, S3=n_NB5, S4=n_NB6, S5=n_NB7.

The PDSCH transmission cycle through the set {Sj} of narrowbands in increasing order of i, starting with i=0 for the first subframe, according to n_NB=Sj, N_(S/NB) is total number of narrowbands in set {Sj}

36.213 7.1.6.4A. The starting OFDM symbol for PDSCH is given by index l_DataStart in the first slot in a subframe. For SIB1-BR, l_DataStart = 3 if N_(DL/RB) > 10 or l_DataStart = 4 if N_(DL/RB) <= 10.

36.321 5.3.1 The RV value for each SIB1-BR transmission is determined by following steps,
 

I.E. 10MHz DL, m = 2. Take CellId=0, starting with i=0, then j = (0 mod 6 + 0*floor(6/2)) mod 6, j = 0 mod 6 = 0; So the first SIB1-BR transmission is S0 narrowband in set {Sj}; i = 1, then j = (0 mod 6 + 1*floor(6/2)) mod 6 = 3, so the second SIB1-BR transmission is the S3 narrowband in set {Sj}, which is narrowband index 5. The third SIB1-BR will be transmitted the same as the first one and the fourth SIB1-BR will be transmitted the same as second SIB1-BR.

Refer to Table 6.4.1-1 and Table 6.4.1-2, if DL bandwidth is greater than 10MHz for FDD. If CellId is even number and repetition level is 4, the starting sfn that transmitted SIB1-BR is sfn mod 8 = 0, I.E. sfn=80; if CellId is odd number and repetition level is 4, the starting sfn that transmitted SIB1-BR is sfn mod 8 = 1. I.E sfn=81

In order to decode SIB2-BR, UE has to interpret content of SIB1-BR correctly. si-Periodicity in SIB1-BR defines the period where SI (which SIB2-BR will be mapped to) period is; si-WindowLength-BR-r13 defines the si-window that SIB2-BR mapped in; si-RepetitionPattern-r13 gives the repetition frequency; subframePattern bitmap defines which subframes are value for DL, which are valid for UL. startSymbolLC or startSymbolBR defines the timing domain starting position of PDSCH. si-Narrowband-r13 indicates the narrowband index to be monitored. si-TBS-r13 indicates the TBS of SI (primarily SIB2-BR currently)
I.E. si-Periodicity=rf8, si-WindowLength-BR-r13=ms20, si-RepetitionPattern-r13=every2ndRadioFrame, subframePattern40-r13 is all 1, startSymbolBR=3, si-Narrowband-r13=8, si-TBS-r13=408 means that SIB2-BR is transmitted in the period of sfn mod 8 = 0 for the first 20ms and in the first radio frame for every subframe started at symbol index 3 in narrowband index 8 of TBS=408 bits with all subframes. These SIB2-BRs will be transmitted with different RVs based on the calculation steps showed above.