PDCCH principles in 5G

Question 1

Where in the slot are located the PDCCH resources?

Answer 1

PDCCH resources occupy the first, up to the third, symbol in the slot.

Question 2

How are the different PDCCH resource units organized?

Answer 2

In RE, RG, CCE, and CORESETs.

Question 3

What is a resource element (RE)?

Answer 3

The RE is the smallest resource unit, is composed of one subcarrier and one OFDM symbol.

Question 4

What is a resource group (RG)?

Answer 4

A resource group is the second smallest resource unit and is made of twelve REs (i.e., 12 subcarriers and one symbol)

Question 5

What is a control channel element (CCE)?

Answer 5

A control channel element (CCE) is made of six REGs (i.e., 72 subcarriers and one symbol)

Question 6

What is the CORESET?

Answer 6

The CORESET is the spanning of all PDCCH resource elements.

Question 7

What is the content of the DCI?

Answer 7

In general, DCI contains information like:

• MCS;
• Resource assignment of the (DL or UL) shared channel;
• Indication about slot format used;
• Power Control settings;
• H-ARQ information.

Question 8

What are the different DCI formats?

Answer 8

Format_0_0: scheduling of PUSCH in one cell;
Format_0_1: scheduling of one or more PUSCH in one cell;
Format_1_0: scheduling of PDSCH in one cell;
Format_1_1: scheduling of one or more PDSCH in one cell.

Other formats exist but these are those available in Nokia products.

Question 9

In what 3GPP document can I find more details about the DCI formats?

Answer 9

3GPP TS 38.212 Sec. 7.3

Question 10

How are terminals known in the cell?

Answer 10

By means of CRC scrambling the DCI with the UE’s unique RNTI.

The RNTI not only uniquely identifies the terminal but also the type of data to be scheduled.

Question 11

What are the different RNTI types available?

Answer 11

P-RNTI: for paging and system change notification;
SI-RNTI: for broadcast of system information;
RA-RNTI: for random access response;
Temporary C-RNTI: for contention resolution when no valid C-RNTI is available;
Temporary C-RNTI: for Msg3 transmission;
C-RNTI, MSC-C-RNTI: for dynamically scheduled unicast transmission;
C-RNTI: for dynamically scheduled unicast transmission.

Question 12

What is the aggregation level (AL)?

Answer 12

The AL corresponds to the number of CCE used for a DCI transmission:

• AL1 = 1 CCE;
• AL2 = 2 CCE;
• AL4 = 4 CCE;
• AL8 = 8 CCE;
• AL16 = 16 CCE.

Higher AL offer better coverage but consumes more PDCCH resources (, meaning that remaining UEs may not be scheduled in case resources are not available.)

Question 13

Which feature introduced PDCCH configurability?

Answer 13

Release 5G20A feature 5GC002469 Adaptive PDCCH symbol configuration made the PDCCH channel configurable with the inclusion of the BWP, CORESET, and Search Space parameters in the gNB database.

Question 14

Which feature introduced dynamic AL selection?

Answer 14

Release 5G21A feature 5GC000791 PDCCH aggregation level control made it possible to adjust the AL according to the needs of each specific UE . When feature 791 is activated, the AL used by the UE depends on UE’s radio conditions ensuring that PDCCH resources are used efficiently.

Question 15

What CORESET parameters are we able configure?

Answer 15

CORESET size [in terms of PRB], duration [in terms of symbols], and relative position [relative to the lower limit of the synchronization signals and physical broadcast channel (SS/PBCH) bandwidth as defined by the global synchronization channel number (GSCN) and the SCS used in the cell for these signals and channels]

Question 16

What CORESET>0 parameters are we able configure?

Answer 16

We are able to CORESET>0 :

• CORESET ID;
• Size in PRBs;
• Duration in number of symbols;
• CORESET start within the BWP;
• Type of CCE to REG mapping to specify if it’s interleaved or not;
• Interleaver size;
• REG bundle size;

CCE to REG mapping can be interleaved or non-interleaved.

Question 17

Where can I find the CORESET configuration index table?

Answer 17

3GPP TS 38.213 Tab 13-x

Question 18

What is the Search Space (SS)?

Answer 18

The Search Space is a PDCCH configuration element that let’s the UE know where to look for for the DCI.

Search Space (searchSpace 0 \leq s < 40) provides the UE with the following information:

• Associated CORESET (controlResourceSetId - CORESET ID);
• PDCCH slot periodicity and offset monitoring pattern (monitoringSlotPeriodicityAndOffset - (k_s, o_s) slots);
• SS duration in terms of slots (duration)
• PDCCH symbol monitoring pattern (monitoringSymbolWithinSlot);
• Number of candidates for each AL (*nrofCandidates)
• SS type (searchSpaceType);
• DCI Format;

Up to 10 Search Spaces are supported.

Question 19

What is blind decoding in the context of DCI transmission?

Answer 19

Each UE entering the cell is informed about the basic PDCCH settings (BWP, CORESET, and Search Space configuration) but the UE does not know where exactly in frequency domain and with which AL the DCI can be transmitted. The only thing known to the UE is the range of frequency and time resources where PDCCH can be carried. Having known the Search Space and the CORESET configuration, the UE is trying to decode the DCI on a trial-and-error approach. This trial-and-error approach is called Blind Decoding.