LTE Learning: Introduction to Downlink Harq (controlled by DCI format)

Note: DCI (downlinkcontrolinformation) downlink control information

Transferred from: http://blog.sina.com.cn/s/blog_927cff010101epjc.html

about downlink Harq

Downlink Harq using asynchronous (asynchronous), adaptive (adaptive) means that the emphasis can occur at any point in time and in the frequency domain. The reason for the downstream Harq to use asynchronous, adaptive mode is to avoid collisions with system information, MBSFN, and the like. Avoid overlapping with system information, MBSFN sub-frames on wireless resources by re-dispatching retransmission on the time or frequency domain. In other words, downlink retransmission is always dispatched by PDCCH.

Downstream Pdsch transmission corresponding to the ack/nack on the PUCCH or Pusch sent.

For TDD, dwpts can be used to send pcfich/phich/pdcch and Pdsch. In other words, special sub-frames can be used to send ulgrant and corresponding Pusch data ack/nack and so on. Here's an exception: for special frame configurations with Normaldownlink CP 0 and 5, or special frame configurations with Extendeddownlink CP 0 and 4 (see 36.211 of table4.2-1. ), the dwpts of the special sub-frame is not used for transmitting Pdsch (see 36.213, section 7.1.7. Under these configurations, the Dwpts has a short duration of 3 ofdmsymbol, so it is not used for transport Pdsch).

For TDD, uppts can be used to send SRS and Prach (Preambleformat 4), but cannot be used to send pucch and Pusch (see section 8.2 of 36.213, 36.211 of 5.4, 5.7. The duration of the uppts is very short, so it is not used for transmission of PUCCH and Pusch). In other words, special sub-frames cannot be used to send ack/nack that correspond to Pdsch transmissions. Therefore, for downstream data, only non-special sub-frames can be used to send the corresponding ack/nack.

Usually the decoding time of ENODEB and UE side is at least 3ms.

For downlink data, the UE determines how the HARQ is handled based on the PDCCH received.

Figure 1: Harq related fields in the downstream DCI (take Dciformat 2 A for example)

Figure 1 shows the fields associated with HARQ in the downstream DCI, which I've covered in Lte:harq (iii)---harqinformation, and it's not repeated here.

Figure 2: Harq-related fields included in various downstream DCI (small black dots indicate presence of this field)

Figure 2 lists which fields are included in the different downstream dciformat, which you can see:

(1) Dciformat 1/1a/1b/1d does not support downstream space Division multiplexing, only indicates that 1 Tb;dciformat 2/2a/2b/2c are used to support downstream space division multiplexing, indicating that 2 Tb;dciformat 1C does not support Harq;

(2) Each TB in the Space division multiplexing contains its own independent Mcs/rv/ndi, and is handled using a separate harqprocess. In the downstream space division multiplexing, 2 TB uses different harqprocess, but in the downstream dciformat, there is only one harqprocess number field, which is used to indicate the first TB corresponding harqprocess. The second TB corresponding harqprocess can be obtained indirectly through the Harqprocess number field (for example: the harqprocess Number field + number of HARQ Process is given a second TB corresponding to the harqprocess); (for upstream transmission, Dciformat 0/4 is not harqprocess number field)

(3) Each terabyte in space-division multiplexing has its own Rv/ndi purpose: to allow only one terabyte to be re-transmitted. That is, the 2 TB of empty division multiplexing is irrelevant, and when one of the terabytes fails, another successfully received TB does not need to be re-transmitted, and a successful TB harqprocess can be used for the next new pass. (upstream processing is the same)

Downlink HARQ The maximum retransmission number is determined by the eNodeB, different manufacturers may have different implementations, the UE does not know the maximum number of downstream retransmission (note: UE through MAXHARQ-MSG3TX and maxharq-tx to know the maximum number of retransmissions upstream), The UE will only determine the soft merge (retransmission) or empty harqbuffer (new pass) based on the NDI received.

Unlike an upstream Harq using Mcsindex 29~31 to indicate a non-adaptive retransmission of the RV (from 36.212 of 5.3.3, it can be seen that Dciformat 0/4 has no redundancyversion field), The Redundancyversion field is included in the downstream DCI for direct indication of the redundant version of the downstream transmission. This is also the reason why 36.213 of table7.1.7.1-1 and table8.6.1-1 do not redundancyversion this column.

About downlink Assignment Index (DAI) and TPC command for PUCCH fields are described later in this article on how UE responds to Ack/nack.

Compared to the upstream Harq, the downstream HARQ processing can be summed up into a sentence: "All listen to the following DCI's orders."