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 2 nd subframe (Uplink) are 7,6 (according to Table 10.1.3.1-1 of s...
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