Ack/Nack bundling and multiplexing in TDD LTE

 

In tdd lte, upper and lower lines are not necessarily symmetric. When the number of downstream sub-frames is higher than that of the upstream sub-frames, the harq of multiple downstream sub-frames is reported in one upstream sub-frame. The number of harq records reported in an uplink sub-frame depends on the configuration of the LTE uplink and downlink MIMO modes. Through upper layer configuration, lte tdd supports two modes of uplink harq report:

(1): ACK/Nack bundling. Multiple ack/Nack generate a bit ack (or NACK) in the uplink sub-frame through logic and operation. The logic and operation are for each MIMO codeword In the downlink sub-frame. When two codewords are reused in space, two bit ack/Nack packets are generated in the uplink sub-frame.

(2): ACK/Nack multiplexing. The ack/Nack multiplexing mode allows ack (or NACK) of up to four downstream sub-frames to be reused. If multiple codewords exist in a downstream sub-frame, a bit ack (or NACK) is generated through logic and computation ). In a special case, the uplink sub-frame only corresponds to one downstream sub-frame, and two codewords exist in the corresponding downlink sub-frame. At this time, the two bit harq feedback in the sub-frame is still used, without binding space (spatial bundling ).

In ack/Nack multiplexing, when the uplink sub-frame corresponds to multiple downlink sub-frames (up to four), one or four bit ack/Nack (section 5.2.2.6, 3GPP 36.212), allows a downstream sub-frame to correspond to a bit. The correspondence between the bit of harq and the downlink sub-frame is described in detail later.

In TDD, whether ack/Nack adopts the bundling or multiplexing mode is determined by the pucch-configcommon: TDD-acknackfeedbackmode in the high-level signaling. this parameter is also applicable to pusch. For TDD configuration 5, LTE can only set this parameter to the bundling format. (3GPP 36.331)

In TDD, for an uplink sub-frame whose sub-frame number is N, one or more downlink sub-frames corresponding to n-k meet the following requirements: (36.213 ), if the transmission of pdsch exists in these sub-frames (including the transmission of pdsch with pdcch indication and the transmission of SPS semi-static scheduling), or there is a pdcch indicating that SPS release, the corresponding ACK or Nack needs to be fed back in the uplink subframe n.

Table 10.1-1: downlink Association SET Index:For TDD

UL-DL Configuration	SubframeN
	0	1	2	3	4	5	6	7	8	9
0	-	-	6	-	4	-	-	6	-	4
1	-	-	7, 6	4	-	-	-	7, 6	4	-
2	-	-	8, 7, 4, 6	-	-	-	-	8, 7, 4, 6	-	-
3	-	-	7, 6, 11	6, 5	5, 4	-	-	-	-	-
4	-	-	12, 8, 7, 11	6, 5, 4, 7	-	-	-	-	-	-
5	-	-	13, 12, 9, 8, 7, 5, 4, 11, 6	-	-	-	-	-	-	-
6	-	-	7	7	5	-	-	7	7	-

For 1-6 of TDD UL-DL configuration, Dai in DCI format 0 (representing grant of pusch) In sub-frame (defined in the following table) is defined, indicates the number of sub-frames that meet the following conditions in all downstream sub-frames corresponding to the uplink sub-frame N: the sub-frame or the transmission of pdsch (including the transmission of pdsch with pdcch indication and the transmission of SPS semi-static scheduling) or the pdcch indicating the release of SPs.

Table 7.3-y: uplink association indexK'For TDD

TDD uL/dl Configuration	DL subframe numberN
	0	1	2	3	4	5	6	7	8	9
1			6	4				6	4
2			4					4
3			4	4	4
4			4	4
5			4
6			7	7	5			7	7

 

Indicates the number of downstream sub-frames that the UE should receive. These Sub-frames may contain pdsch transmission (including pdsch transmission with pdcch indication and SPS semi-static scheduling transmission) or there is a pdcch indicating that SPS release. It is a value of 2 bits. The value is as follows:

Table 7.3-x: Value of downlink assignment Index

Dai MSB, LSB	Or	Number of subframes with pdsch transmission and with pdcch indicating dl sps release
0, 0	1	1 or 5 or 9
0, 1	2	2 or 6
1, 0	3	3 or 7
1, 1	4	0 or 4 or 8

If the UE receives the message, it indicates that the UE will have pusch transmission in the sub-frame n. Otherwise, the UE will not receive the message. At this time, LTE will indicate that the UE should receive n-k, (wherein) downlink sub-frame, there is a pdsch transmission indicated by pdcch or the number of sub-frames indicated by SPS release (note that this does not include the number of semi-static scheduling SPS transmissions ). The Dai in DCI format 1/1a/1B/1D/2/2a/2B represents the current frame, where n-k () downlink sub-frame, the existence of pdcch indicates pdsch transmission or indicates the number of sub-frames released by SPS. Defined as "Dai", where the minimum value of DCI format 1/1a/1B/1D/2/2a/2B is detected by UE in set K. That is to say, the nearest Sub-frame with the corresponding downlink transmission exists.

In tdd lte, it is defined as the number of pdcch sub-frames actually received by UE in the n-k downlink sub-frames, which are used to indicate the transmission of pdsch or the release of SPs. By comparing the actual received value with the expected value, UE can determine whether there is any loss of downstream data transmission.

The value is defined as 0 or 1, indicating the number of SPS transmitted in the n-k sub-frame (the corresponding pdcch indication does not exist. Therefore, it indicates that in the n-k sub-frame, all actually received sub-frames with corresponding downlink transmission exist. When the sub-frame N has an upstream transmission indicated by pdcch (the opposite is the upstream data transmission of SPS), the number of received values and expected values is compared, UE can also determine whether there is any loss of downstream data transmission.

If the UE has pusch transmission in the sub-frame N, In tdd lte, Ack/Nack multiplexing, the number of BITs reported by ack/NACK and each bit in it meet the following requirements:

(1): When the UE receives the message, the UE does not need to send any ACK or Nack unless it is the same. The ack/Nack bit corresponding to the neutron frame is, where Dai (k) is the Dai value of the DCI format of the child frame 1/1a/1B/1D/2/2a/2B. In this case, the bit corresponding to the semi-static scheduling is.

(2) If the UE does not receive the message, M indicates the number of downlink sub-frames corresponding to the uplink sub-frame N. Because ul-dlconfiguration can only be used for ACK and Nack bundling, from table 10.1-1, we can see that in the case of ack/Nack multiplexing, m can be up to 4. Corresponding to the downlink sub-frame, where ack/Nack feedback.

If the UE does not transmit data from pusch in the sub-frame N, the corresponding ack/Nack feedback must be transmitted on pucch. In tdd lte, in the case of ack/Nack multiplexing, the UE transmits two bits in the pucch format 1b on the corresponding pucch resource of the sub-frame n. And select to generate at a certain frequency, as shown in the following table when m = 2: (for more information, see M = 3 or M = 4. See 36.213)

Table 10.1-2: Transmission of ack/Nack multiplexing for m = 2

HARQ-ACK (0), HARQ-ACK (1)
Ack, ack		1, 1
Ack, nack/dtx		0, 1
Nack/dtx, Ack		0, 0
Nack/dtx, nack		1, 0
Nack, dtx		1, 0
Dtx, dtx	N/	N/

It refers to the resources used by pucch based on the sub-frame.

The transmission of pdsch or the release of SPS indicated by pdcch is related to the starting CCE Number of pdcch on the sub-frame. The formula is:, P is selected between 0, 1, 2, and 3 ,. Configured by the upper layer.

There is no corresponding pdcch indication for the transmission of semi-static scheduling SPs. It is specified by the upper layer.

When both ack/NACK and Sr are transmitted at the same time, Ack/Nack must be transmitted on the pucch resource corresponding to the SR. The question is how to select the frequency at this time?