LTE-BL/CE (LTE-M1) - HARQ
In short, HARQ mechanism for LTE M1 downlink is similar to legacy LTE mechanism (except the repetitive transmission), but HARQ mechanism is a little bit different from legacy LTE as described below.
HARQ Operation for Downlink
HARQ Process for Downlink is similar to legacy LTE except that each transmission would happen in repetition in LTE-M1. The rough HARQ sequence would be as follows.
i) UE <-- NW : MPDCCH (DCI for PDCCH) in repetition
ii) UE <-- NW : PDSCH in repetition
iii) UE --> NW : HARQ ACK/NACK
Note : Step i) and ii) happens not in the same subframe. (This is different from legacy LTE)
Following is what is commented in 3GPP
36.321 - 18.104.22.168 HARQ entity describes as follows
HARQ Operation for Uplink
HARQ Process for Downlink of M1 is similar to legacy LTE except that each transmission would happen in repetition. However, HARQ Process for Uplink in M1 is different from legacy LTE. The most critical difference is that eNB does not send any HARQ ACK/NACK for PUSCH (It is understandable because there is no PHICH in LTE M1). Then, you may ask how eNB can handle the case where PUSCH reception fail ? Following process would give you the answer.
The rough HARQ sequence would be as follows.
i) UE <-- NW : MPDCCH (DCI for PUSCH) with UL Grant in repetition
ii) UE --> NW : PUSCH in repetition
iii) One of the following cases happens :
NOTE : If UE does not receive MPDCCH for PUSCH retransmission, UE assumes that PUSCH is properly received by NW. PHICH does not exists to send ACK/NACK for PUSCH in LTE M1.
36.321 - 22.214.171.124 HARQ entity, 126.96.36.199 HARQ process, 7.7 HARQ RTT Timers describes as follows
RV Determination : PDSCH for SystemInformationBlockType1-BR
SystemInformationBlockType1-BR is retransmitted and each retransmission uses different RV (Redundancy Version). RV for each retransmission varies depending on SystemInformationBlockType1-BR retransmission patter as described below : (Refer to 36.321 - 5.3.1 DL Assignment reception for further details)
RV = ceiling(3/2*k) mod 4 // k is determined depending of the number of repetitions
Case 1 : number of repetitions for PDSCH carrying SystemInformationBlockType1-BR is 4
k = floor(SFN/2) mod 4
Case 2 : number of repetitions for PDSCH carrying SystemInformationBlockType1-BR is 8
k = SFN mod 4
Case 3 : number of repetitions for PDSCH carrying SystemInformationBlockType1-BR is 16
k = (SFN * 10 + i) mod 4 , where i = subframe number in the SFN
 3GPP TS 36.321 V13.2.0 (2016-06)