LTE 中的 ACK/NACK/DTX

Introduction

If no valid DL-SCH-related control signaling is detected, then nothing is transmitted on the PUCCH (i.e. DTX). Apart from not unnecessarily occupying PUCCH resources that can be used for other purposes, this allows the eNodeB to perform three-state detection, ACK, NAK, or DTX, on the PUCCH received. Three-state detection is useful as NAK and DTX may need to be treated differently. In the case of NAK, retransmission of additional parity bits is useful for incremental redundancy, while for DTX the terminal has most likely missed the initial transmission of systematic bits and a better alternative than transmitting additional parity bits is to retransmit the systematic bits.[1]

Turbo 编码中的systematic bit 比 parity bit 更重要。
在初传中，至少包含所有systematic bit 和 部分parity bit。
在重传中，包含初传中没有的parity bit。
NACK要求重传parity bit。
DTX要求重传system bit。

HARQ功能同时跨越物理层和MAC层。
生成RV、软合并，CRC校验是由物理层负责。
HARQ是通过CRC判断数据包是否出错，检验CRC是在软合并之后。

Algorithm: PUSCHSINR-based PUSCHDTX determination

IF PUSCHSINR>a, NO DTX
ELSE IF a>PUSCHSINR>b
IF（ACK）， NO DTX/ACK
ELSE IF (NACK），Tmp DTX
ELSE IF b>PUSCHSINR
IF(ACK)，NO DTX/ACK
ELSE IF(NACK) DTX

FAQ

1. This msg is sent from UE->BS or BS->UE?
   both

2. If a UE is getting data from multiple carriers, how can it report ACK/NACK?

Option	Comment
Let UE send ‘ACK’ only when is was successful to decode PDSCH from all component carriers and transmit ‘NACK’ if any of PDSCH is failed to be decoded.	This is technically possible, you would clearly see whis would be very inefficient way and cause a lot of unnecessary retransmission. So the method is not considered in real implementation.
Let UE send ‘ACK/NACK’ for each carrier separately	This would not sound perfect way, but we can reuse the existing PUCCH format without much modification.
Let UE send ‘ACK/NACK’ for each carrier on a single PUCCH	This would sound the bets, but you can easily guess we would need a new PUCCH format since all the exsiting PUCCH format is designed to send ‘ACK/NACK’ for one carrier.

[1] 4G LTE/LTE-Advanced for Mobile Broadband,Erik Dahlman, Stefan Parkvall, and Johan Sköld