IoT: LTE-BL/CE(CAT-M1) Random Access

In terms of PRACH sequence selection and transmission, there is no difference except the preambleIndex that should be used. firstPreamble and lastPreamble in LevelInfo under CELevelList tells UE which preambles to use.

After CAT-M1 UE sent PRACH, instead of monitoring PDCCH and PDSCH in the same subframe. CAT-M1 UE has to monitor narrowband index configured in SIB2-BR for MPDCCH first and after decoded MPDCCH in subframe n, it should monitor subframe (n+2) for random access response, whose MAC header is RAR format.

The RA-RNTI that is used to scramble MPDCCH for RAR is calculated using following formula from 36.321 5.1.4

In a FDD cell, because there is only 1 PRACH resource in a subframe, f_id is always 0 (36.211 5.7.1). If PRACH-r13 config in SIB2-BR indicated that UE can transmit PRACH in subframe=1,  t_id=1.  W_max is fixed 400 for CAT-M1 UEs. If SFN_id is 783, the calculated RA-RNTI is 1+1+0+60*(783 mod 40) = 1382.

DCI format 6-1A is used to transmit MPDCCH and UE should monitor aggregation level up to 8 in common search space. The exact bit mapping can be found in 36.212 5.3.3.1.12
I.E. MPDCCH in HEX is 11588200(29bits), translated in bits is
10001010110001000001000000000
Since it is NOT PDCCH ordered UL re-sync, the interpretation of each field is following:
1 -> value 1 indicates it is DCI format 6-1A
0 -> value 0 indicates frequency hopping is not enabled
For DCI format 6-1A,  is equal to 3 bits, thus, 001 provides the narrowband index, which in INT is 1
01011 -> 5 bits for DL resource allocation type 2 with in indicated narrowband. INT value is 11
Based on 36.213 7.1.6.3 and RIV=11, RBstart and  L_CRBs can be calculated using following equations.
 
RBstart=0 and L_CRBs=6.
0001 -> 4 bits for MCS. INT value is 1
00 -> 2 bits for repetition number. INT value is 0
000 -> 3 bits to indicate HARQ process number for FDD primary cell (for TDD primary cell, 4 bits). INT value is 0
1 -> 1 bit for NDI. First occurrence of 1 means new data.
00 -> 2 bits for redundancy version. INT value is 0 means RV0
00 -> 2 bits for TPC command for PUCCH. Since this MPDCCH is scrambled with RA-RNTI, the MSB is reserved, the LSB is 0 indicates N_(1A/PRB) is 2.
With MCS=1 and N_(1A/PRB)=2, based on Table 7.1.7.2.1-1, TBS of RAR is 56 bits.
From 36.212 Table 5.3.3.1.2-2, DAI has 0 bit
0 -> indicates SRS request. 0 means false
TPMI information is not present
PMI information is not present
00 -> HARQ-ACK resource offset as defined in 36.213 10.1, this value is used later on to calculate parameter N_((1)/PUCCH) which is used to calculate which PRB PUCCH will be transmitted (36.213 10.1.2).
00 -> DCI subframe repetition number as defined in 36.213 9.1.5

After decoded MPDCCH for RAR, UE knows the TBS and the resource allocation for RAR, UE will monitor subframe (n+2) in corresponding narrowband indicated by SIB2 for RAR.
I.E. According to MPDCCH example showed above, the entire RAR length is 56 bits. If HEX value is 73001F02605F8F, the interpretation of each field is as follows:
First 8 bits is MAC header. 73 translated to bits is 01110011
0 -> Extension field is 0 indicates that MAC RAR or padding starts next byte. 0 means at least another set of E/T/RAPID fields follows.
1 -> Type filed indicates whether the MAC subheader contains a Random Access ID or a Backoff Indicator. 0 means Backoff Indicator presented and 1 means Random AccessID presented.
110011 -> Since T=1, 6 bits is used to calculate RAPID. If T=2, 4 bits are used to calculate BI. INT value is 51, which means this RAR is for preambleId=51 (reserved for CAT-M1 in sample config)

Next 48 bits is RAR, first 12 bits is 001 in HEX.
0 -> Reserved bit, set to 0
00000000001 -> 11 bits is Timing Advance Command. It is used to control the amount of timing adjustment that the MAC entity has to apply (36.213 4.2.3). INT value is 1 means 16Ts

For CAT-M1 UE, following 20 bits is UL grant. F0260 in HEX. From 36.213 6.2, interpretation of each field is as follows:
NNB=floor(NUL/RB/6)=8, Nindex/NB=ceiling(log2(NNB))=3
111 -> Msg3 PUSCH narrowband index. INT value is 7 means last narrowband indexed from lowest frequency to highest frequency
1000 -> Msg3 PUSCH resource allocation. For CEmodeA, insert one MSB with value set to 0 and interpret using UL resource allocation type 0 within indicated narrowband according to 36.213 8.1.1

RBstart=2 and L_CRBs=2
00 -> number of repetition for Msg3. From 36.213 Table 6.2-C, 00 means Ya/8. Ya is broadcasted in SIB2-BR value push-maxNumRepetitionCEmodeA-r13=8 in our example. 00 translated to 1.
010 -> MCS for Msg3. INT value is 2.
011 -> TPC. INT value is 3 for PUSCH transmission power control
0 -> CSI request. 0 means not requesting any CSI report.
0 ->  UL dealy.
00 -> Msg3/4 MPDCCH narrowband index. 36.213 Table 6.2-B. 00 means (NBRAR mod NNB2), where NBRAR is signaled in SIB2-BR of parameter mpdcch-NarrowbandsToMonitor-r13 if only 1 narrowband is configured. In our example, it is 2. NNB2=floor((NDL/RB)/6)=8. 2 mod 8 = 2 which means Msg3 and Msg4 should be transmitted in narrowband index 2.

Last 16 bits are for Temporary C-RNTI. 5F8F in HEX and 24463 in DEC