What is DTX in LTE?

One of the main issue that I come across every now and then is DTX for the HARQ information expected from the UE, for the corresponding PDSCH TB. As you are aware, for each PDSCH directed to a UE, it is expected to acknowledge the eNB with a ACK or NACK. To do so, the UE may choose PUCCH or PUSCH depending on availability of a uplink grant in the subframe the UE is supposed to transmit the HARQ.

What is DTX?

DTX in general, a situation where eNB finds no HARQ information at the expected frequency resource. eNB calculates the energy at the expected uplink frequency resource to decide if there is energy or a DTX. When the energy at the expected frequency resource is below a certain threshold, the eNB physical layer indicates a DTX for that HARQ.

What causes DTX?

There are tens of reasons for the eNB to decode a DTX but few of the major reasons that I come across most of the times are as below,

DL DCI Decoding Issue

The downlink DCIs are transmitted over PDCCH and it is possible that the UE failed to decode the DCI for PDSCH, which can be due to various reasons such as PDCCH encoding issues, PDCCH configuration issues, genuine PDCCH decoding failures due to poor channel conditions. So when the UE fails to decode the DL DCI, it doesn’t even know that eNB has transmitted a PDSCH to it, where as the eNB also doesn’t know yet that the UE has failed to decode the DL DCI, so the eNB will ask its physical layer to decode the HARQ for the PDSCH it transmitted, but since UE never decoded the DCI, it won’t even attempt to decode PDSCH and not transmit any HARQ for that PDSCH, hence eNB will find a DTX at the HARQ information

Decoding at Incorrect Resource

This happens when the UE and eNB are out of sync with respect to the frequency resource where the HARQ is supposed to be transmitted. Basically the UE is transmitting the HARQ on a different frequency resource than what is expected by the eNB, in which case, the eNB will find low energy at the resource it is trying to decode the HARQ, for obvious reasons because UE has transmitted it at some other frequency resource. This category can also include issues where there is a overlap of CQI and HARQ in the same subframe and eNB is trying to decode either at CQI resource or at HARQ resource while the UE has transmitted otherwise.

Uplink Decoding Issues

Now this is the last category where there is genuine decoding issue in the uplink receiver itself where it has failed to decode the HARQ from the uplink transmitted by the UE.

What are the Consequences of DTX?

The primary consequence of DTX is it will increase the downlink error rate or BLER. So when the eNB fails to get the HARQ for a PDSCH due to DTX, it doesn’t know whether the UE has failed to decode the PDSCH or it was successful, since it cannot assume the later, it will have to retransmit the PDSCH with same redundancy version for which it got DTX.

UE’s Behaviour

Now if the UE had decoded the PDSCH earlier, transmitted a ACK and the DTX was caused due to some issue at the eNB, the UE will simply discard the PDSCH the 2nd time, because it already has the PDSCH. But if the DTX was caused due to DCI decoding failure, the UE will consider this DCI+PDSCH as a fresh transmission and decode the PDSCH.

Let us take few examples, assuming the redundancy version sequence used by the eNB is 0,2,3,1 for the consequent PDSCH re-transmissions, in the below examples case-1 and 2 show two different consequences of DTX, case-3 shows a case of PDSCH failure, where as case-4 shows a normal case, where everything go fine.

Case-1: DCI Decoded by UE, HARQ Decoding failure at eNB

Channel	SFN	SF	HP	RVIdx	NDI	Status at UE	Status at eNB
PDSCH	254	0	4	0	0	CRC ACK
PUCCH	254	4	–	–	–	ACK	DTX
PDSCH	254	9	4	0	0	Discard

Case-2: DCI Decoding Failure at UE

Channel	SFN	SF	HP	RVIdx	NDI	Status at UE	Status at eNB
PDSCH	254	0	4	0	0	DCI failure
	254	4	–	–	–	–	DTX
PDSCH	254	9	4	0	0	CRC ACK
PUCCH	255	3	–	–	–	ACK	ACK

Case-3: PDSCH CRC Failure at UE

Channel	SFN	SF	HP	RVIdx	NDI	Status at UE	Status at eNB
PDSCH	254	0	4	0	0	CRC NACK
PUCCH	254	4	–	–	–	NACK	NACK
PDSCH	254	9	4	2	0	CRC ACK
PUCCH	255	3	–	–	–	ACK	ACK

Case-4: A Normal Case

Channel	SFN	SF	HP	RVIdx	NDI	Status at UE	Status at eNB
PDSCH	254	0	4	0	0	CRC ACK
PUCCH	254	4	–	–	–	ACK	ACK
PDSCH	254	9	4	0	1	CRC ACK
PUCCH	255	3	–	–	–	ACK	ACK

I hope this post gives you fair amount of idea on what is DTX. Now DTX can be decoded when the HARQ is transmitted over PUCCH or PUSCH. If you have specific doubts, please drop a comment and I will try to add the details.