5G NR in BULLETS
151
3.5 PHYSICAL DOWNLINK CONTROL CHANNEL
The Physical Downlink Control Channel (PDCCH) is used to transfer Downlink Control Information (DCI). This corresponds to
Physical layer signalling from layer 1, in contrast to RRC signalling from layer 3 or the use of MAC Control Elements from layer 2.
3GPP has specified a set of DCI Formats to accommodate a range of PDCCH payloads:
o DCI Format 0_0 ‘fallback’ DCI format for uplink resource allocations on the PUSCH
o DCI Format 0_1 ‘standard’ DCI format for uplink resource allocations on the PUSCH
o DCI Format 1_0 ‘fallback’ DCI format for downlink resource allocations on the PDSCH
o DCI Format 1_1 ‘standard’ DCI format for downlink resource allocations on the PDSCH
o DCI Format 2_0 provision of Slot Format Indicators (SFI)
o DCI Format 2_1 provision of Pre-emption Indications
o DCI Format 2_2 provision of closed loop power control commands applicable to the PUCCH and PUSCH
o DCI Format 2_3 provision of closed loop power control commands applicable to the SRS
The content of each DCI Format is described in sections 3.5.4 to 3.5.11
DCI Formats 2_0, 2_1, 2_2 and 2_3 are used to provide ‘UE Group Common Signalling’. These DCI Formats are designed to address a
group of UE and can accommodate payloads for each UE within the group. The payload belonging to a specific UE has a specific
position within the DCI so each UE is able to extract its own information while ignoring the information intended for other UE
Figure 145 illustrates the general mapping of DCI onto the PDCCH physical channel. The PDCCH occupies a specific number of
Resource Elements according to its aggregation level. An aggregation level is quantified in terms of the number of Control Channel
Elements (CCE) that it occupies. 1 CCE is equivalent to 6 Resource Element Groups (REG) which is also equivalent to 72 Resource
Elements. The link adaption algorithm within the Base Station assigns a specific aggregation level according to the size of the DCI
payload and the UE coverage conditions. Small payloads can be accommodated using lower aggregation levels. UE which experience
poor coverage are normally allocated higher aggregation levels to allow increased channel coding gain, i.e. higher quantities of
redundancy
Figure 145 – Mapping of DCI Formats onto the PDCCH, Search Space Sets and Control Resource Sets
The PDCCH is mapped onto a specific Search Space Set according to the content of the DCI. For example, if the DCI is being used to
provide a resource allocation for MSG2 during the random access procedure, then the PDCCH is mapped onto a Type 1 Common
Search Space Set. Alternatively, if the DCI is being used to provide a resource allocation for the transfer of application data then the
PDCCH can be mapped onto the UE Specific Search Space Set. Each Search Space Set has got a specific periodicity. This periodicity
impacts latency because it determines the average waiting time for a resource allocation opportunity
Each Search Space Set is mapped onto a specific Control Resource Set (CORESET). The CORESET defines the set of Resource
Blocks and the number of symbols available to the Search Space Set. Various other physical layer characteristics are also defined by
the CORESET. These are described in the next section
A UE is required to deduce which DCI Format is being received at any point in time. This can be achieved using a combination of the
following:
o PDCCH Search Space Set
o type of RNTI used to scramble the CRC bits which are attached to the DCI payload
o size of DCI payload
o information within the DCI payload
For example, if a UE is scanning a Type 2 Common Search Space Set then it knows that it should be checking for DCI Format 1_0
because DCI Format 1_0 is used to provide resource allocations for paging messages. Alternatively, if a UE is scanning a UE Specific
Search Space Set then it knows that it should be checking for DCI Formats 0_0, 0_1, 1_0 and 1_1
CCE: Control Channel Elements
RE: Resource Elements
Downlink Control Information (DCI) PDCCH Search Space Sets CORESET
Format 0_0
Format 0_1
Format 1_0
Format 1_1
Format 2_0
Format 2_1
Format
2_2
Format 2_3
Uplink
Allocations
Downlink
Allocations
UE Group
Common
Signalling
Common
Type 0 SIB1
Type 0A Other SIB
Type 1 MSG2/MSG4
Type 2 paging
Type 3 UE Group Common signalling
& UE Specific signalling
UE Specific UE Specific signalling
Aggregation Level
1 CCE = 72 RE
2 CCE = 144 RE
4 CCE = 288 RE
8 CCE = 576 RE
16 CCE = 1152 RE
Resource Blocks &
Symbols
CCE to REG Mapping
Precoder Granularity
Quasi Co-location
DMRS Scrambling Id
REG: Resource Element Group
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