1. For PDSCH transmission, after eNB sent TB to UE at subframe X, UE has to firstly decode C-RNTI scrambled PDCCH to get the location of REs that carries PDSCH data. Combined decoding of PDCCH and corresponding PDSCH usually has allowance of 4ms. UE would send HARQ ACK/NACK to eNB at subframe (X+4).
(1) If eNB sent DCI format 0 to UE at subframe X, eNB would expect UE to send HARQ ACK/NACK on PUSCH at subframe (X+4). However, eNB should also monitor PUCCH at subframe (X+4).
(2) If eNB didn't send DCI format 0 to UE, eNB would only monitor PUCCH resource (This UE specific PUCCH sequence is allocated to UE at attach procedure) at subframe (X+4).
On detection of NACK or DTX at subframe (X+4), eNB would resend TB with RV0 (All systematic bits) with DTX detection and RV2 (Only redundancy bits) with NACK reception after maximum 4ms of processing time.
2. For PUSCH transmission, UE send PUSCH at subframe Y, decoding allowance time for eNB is also 4ms, at subframe (Y+4), eNB send ACK/NACK in PHICH channel with pre-allocated PHICH group and sequence number. For UE to decide whether and how to do PUSCH re-transmission, UE has to monitor PHICH channel as well as C-RNTI scrambled PDCCH channel. (The reason for this is that UL has adaptive and non-adaptive and which one to way is based on the scheduling of PDCCH). This would gave UE 4ms allowance. Thus, for synchronized non-adaptive/adaptive UL re-transmission, UE is supposed to send after 8ms of initial transmission.
3. For PMCH transmission,
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