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:
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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ReplyDeleteThanks for the great article! BTW, here’s a IMEI checker tool as a community service intended to validate whether GSM or CDMA phones have LTE feature or not.
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