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DAI

 

DAI stands for Downlink Assignment Index. When you say 'Index', it usually means a unique number assigned to each elements in an array or set. It usually is assigned to each element in increasing order.

DAI is also a type of index. Then what is the set (or array) to which this index is used for. The set in this case is 'all of the transmitted (scheduled) downlink data(PDSCH) which is bundled to one HARQ ACK/NAC transmission' in TDD operation (If this does not sound clear to you, I would suggest you to read 'HARQ Timing' section first).

 

 

 

Example 01 >

 

Let's take a look at TDD DL/UL Configuration 2 as an example.

Assuming that every DL subframe is scheduled to transmit PDSCH, the DAI is assigned as shown below.

 

 

Just to make it clearer, I would suggest you to describe this illusteration into words. My version of description is as follows.

  • Subframe 4,5,6,8 in a frame are bundled to subframe 2 in next subframe (Lets call the subframe 4,5,6,8 as Set A)
  • Subframe 9,0,1,3 are bundled to subframe 7 (Lets call the subframe 9,0,1,3 as Set B)
  • All of the subframe listed above are scheduled to transmit PDSCH.
  • When scheduling PDSCH in subframe 4, eNB assigns the index(number) 0 to DAI field of the DCI because this is the first PDSCH transmission in a same set(Set A in this case).
  • When scheduling PDSCH in subframe 5, eNB assigns the index(number) 1 to DAI field of the DCI because this is the second PDSCH transmission in a same set(Set A in this case).
  • When scheduling PDSCH in subframe 6, eNB assigns the index(number) 2 to DAI field of the DCI because this is the third PDSCH transmission in a same set(Set A in this case).
  • When scheduling PDSCH in subframe 8, eNB assigns the index(number) 3 to DAI field of the DCI because this is the fourth PDSCH transmission in a same set(Set A in this case).
  • When scheduling PDSCH in subframe 9, eNB assigns the index(number) 0 to DAI field of the DCI because this is the first PDSCH transmission in a same set(Set B in this case).
  • When scheduling PDSCH in subframe 0, eNB assigns the index(number) 1 to DAI field of the DCI because this is the second PDSCH transmission in a same set(Set B in this case).
  • When scheduling PDSCH in subframe 1, eNB assigns the index(number) 2 to DAI field of the DCI because this is the third PDSCH transmission in a same set(Set B in this case).
  • When scheduling PDSCH in subframe 3, eNB assigns the index(number) 3 to DAI field of the DCI because this is the fourth PDSCH transmission in a same set(Set B in this case).

 

 

 

Example 02 >

 

Let's take a look at TDD DL/UL Configuration 2 as an example.  This is the same configuration as in Example 01, but with a little bit of different scheduling.

In this example, some of the subframe does not transmit(not scheduled) to transmit PDSCH and the DAI is assigned as shown below.

 

 

 

 

Just to make it clearer, I would suggest you to describe this illusteration into words. My version of description is as follows.

  • Subframe 4,5,8 in a frame are bundled to subframe 2 in next subframe (Lets call the subframe 4,5,8 as Set A)
  • Subframe 9,3 are bundled to subframe 7 (Lets call the subframe 9,3 as Set B)
  • All of the subframe listed above are scheduled to transmit PDSCH.
  • When scheduling PDSCH in subframe 4, eNB assigns the index(number) 0 to DAI field of the DCI because this is the first PDSCH transmission in a same set(Set A in this case).
  • When scheduling PDSCH in subframe 5, eNB assigns the index(number) 1 to DAI field of the DCI because this is the second PDSCH transmission in a same set(Set A in this case).
  • subframe 6 is NOT secheduled for PDSCH.
  • When scheduling PDSCH in subframe 8, eNB assigns the index(number) 2 to DAI field of the DCI because this is the fourth PDSCH transmission in a same set(Set A in this case).
  • When scheduling PDSCH in subframe 9, eNB assigns the index(number) 0 to DAI field of the DCI because this is the first PDSCH transmission in a same set(Set B in this case).
  • subframe 0 is NOT secheduled for PDSCH..
  • subframe 1 is NOT secheduled for PDSCH.
  • When scheduling PDSCH in subframe 3, eNB assigns the index(number) 1 to DAI field of the DCI because this is the the second PDSCH transmission in a same set(Set B in this case).