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Downlink Assignment Index (DAI)

DAI (Downlink Assignment Index) is an index, which is communicated to UE by eNB to prevent ACK/NACK reporting errors due to HARQ ACK/NAK bundling procedure performed by the UE. To understand how DAI works we need to learn how ACK/NAK reporting used to happen in LTE TDD.
In LTE TDD, UE can send single ACK/NAK of multiple PDSCH sub frame in one bit for each code word CW0 and CW1.

UE perform AND logical operation on each code word CW0 and CW1 (CRC Passed/Failed) of each PDSCH received and report the result in two bits (00, 01, 10, 11) on specific uplink subframe. Below is the table which shows that which all PDSCH subframes need to be bundled for reporting ACK/NAK on which Uplink subframe for each TDD UL DL configurations (Mentioned only for config 1 and config 2 in green color).

For Example:

          UL/DL Configuration 1:
We can see that the k value for 2nd subframe (Uplink) are 7,6 (according to Table 10.1.3.1-1 of specs 36.213 ) hence on this uplink subframe the ACK/NAK of 5th and 6th  subframe (PDSCH, shown in green)  which could be bundled and will be reported on 2nd uplink subframe.

For 3rd uplink subframe the number of bundled subframe would be 1 (for 9th DL subframe of previous radio frame).

  UL/DL Configuration 2:
According to above table, on uplink subframe 2nd, HARQ ACK/NAK   of DL subframe 4, 5, 8 and 6 of previous radio frame can be bundled.

In the same manner, on 7th Uplink subframe,  bundled HARQ ACK/NAK can go for 9th DL subframe of previous radio frame and 0,3rd  and 1st  DL subframe of current radio frame. 

So DAI (Downlink Assignment Index), will ensure that number of HARQ bundled and reported by the UE is exactly for same number of PDSCH/PDCCH subframe received by the UE. Now consider a situation where eNB schedule two subsequent subframe to the UE, but UE misses the first transmission in the first subframe and successfully decodes the second subframe. The UE would transmit one ACK only for second transmission but eNB will interpret that, both transmission is successfully decoded by UE. To prevent such errors DAI will play important role.  

In UE log , you can see what DAI is communicated to UE in DCI Information and you can also check how many subframes are bundled and transmitted on PUCCH or PUSCH in TDD ACK NAK report. If there is a mismatch there will be chance of DAI mismatch.

Example: Suppose for TDD UL/DL configuration 1, the maximum number of Downlink subframe could be bundled either 1 or 2 or 0 (in case no PDSCH or PDCCH scheduled to UE), hence DAI values can be either 1 or 2 or 4 (according to Table 7.3-X: Value of Downlink Assignment Index of specs 36.213), can be seen in LTE DCI information of UE log.


For more information on DAI Mismatch please refer section 7.3 of 3GPP specs 36.213.

Note: This DAI field (2 bits) is present only in TDD operating mode. Above explanation of DAI value only applicable for TDD UL/DL config other than 0. In UL DL config 0 , this DAI field used as an uplink index to signal for which uplink subframe(s) the grant is valid.
  
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Comments

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  2. Unknown24 October 2016 at 11:42

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