Question

LTE: Why does HARQ process index include within the DCI format 1 ?

Answer 1

Downlink Control Information Format1 is being used to inform to UE for its downlink assignment.
In other words, DCI format 1 is used to schedule a PDSCH codeword.

It has following fields that carry the information related to the down link assignment:
Field 1: Resource Allocation Type
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There are two types of resource allocation Type "0" and Type "1". when resources are allocated in the terms of RBGs (i.e. Resource Block Groups) it is known as type "0" allocation scheme. And when resources are allocated in the terms of RBs within the RBG, known as type "1" allocation scheme.

Field 2: Resource Block Assignment
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Based on the type "0" or "1". Resource allocation information is sent to UE. The # of bits used to send resource allocation information is constant. I mean to say based on deployed bandwidth of eNodeB, these # of bits are derived.

Field 3: Modulation and Coding scheme
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0-31 MCS index are defined to covey MCS scheme, which is used for downlink assignment.

Field 4: HARQ Process Number
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HARQ process number is used by upper layer for current PDSCH codeword.
In FDD mode "8" and in TDD mode "16" HARQ processes are used. HARQ process is closely associated with NDI and RV fields. NDI stands for New Data Indicator and RV stands for Redundancy version. One bit is enough to indicate that whether the data is new or not. There are four redundancy (i.e. ranges from 0 to 3) versions defined for a PDSCH codeword. Turbo encoder uses those redundancy version while encoding PDSCH codeword.

Field 5: Transmit Power Control
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2 bits are used to inform to an UE about PUCCH and PUSCH power control.

Hope I answered your query along with additional information.
Actually you query was related to field 4.

Answer 2

HARQ Process is use for DCI 1 because of PDSCH Scheduling and RACH procedure using PDCCH. Now question is here, why PDCCH and PDSCH use HARQ in DCI1.

1. HARQ is use for RACH procedure using PDCCH Channels. Use HARQ-ACK process to resolve contention-based resolution.
2. Two-bit field for PUCCH transmit power control (TPC), which will tell the UE to either step up/down or remain at the same PUCCH transmit power for the HARQ indication for the current PDSCH. The TPC field has a special meaning too, when the current DCI carries a SI/P/RA-RNTI, which is as below,

FYI:
The LSB of the TPC field indicates the number of RB, that UE should use to determine the transport block size for this RNTIs. The number of RB will be 2 if the LSB is 0 and 3 if it is

**the DCI 1A is to be used for initiating a non-contention based RACH using PDCCH