ACK NACK DTX

ACK/NACK which is carried on uplink HS-DPCCH is sending from UE to inform Node B whether it has received/decoded certain transport block successfully.

The meanings of ACK/NACK/DTX are as obvious as their names express:

• ACK: Transport block is decoded correctly by UE;
• NACK: Transport block is decoded incorrectly;
• DTX: discontinuous transmission.

According to Chapter 6.2.6 ACK/NACK Transmit Power Reduction for HS-DPCCH in Mobile Communication Systems and Security by Man Young Rhee.

In Release 5, the UE always uses DTX in the ACK/NACK field of the HS-DPCCH except when an ACK or NACK is being transmitted in response to an HS-DSCH rtansmission. This means that if the UE fails to detect typical probability 0.01 of the HS-SCCH, the UE will use DTX in the corresponding ACK/NACK field. The Node B must avoid decoding this DTX as ACK if it is to avoid loss of the HS-DSCH TTI at the physical layer.

In summary:

1. If HS-SCCH is not decoded successfully(no matter which part is wrong, part 1 or part 2), UE will not send any ACK/NACK info on HS-DPCCH, which is DTX.
2. If HS-SCCH is successful but HS-PDSCH CRC is failed, NACK will be sent.
3. Only if both HS-SCCH and HS-PDSCH are decoded correctly, UE will send ACK.