6.8 Physical downlink control channel
36.2113GPPEvolved Universal Terrestrial Radio Access (E-UTRA)Physical channels and modulationRelease 17TS
6.8.1 PDCCH formats
The physical downlink control channel carries scheduling assignments and other control information. A physical control channel is transmitted on an aggregation of one or several consecutive control channel elements (CCEs), where a control channel element corresponds to 9 resource element groups. The number of resource-element groups not assigned to PCFICH or PHICH is 
. The CCEs available in the system are numbered from 0 to
, where 
. The PDCCH supports multiple formats as listed in Table 6.8.1-1 where PDCCH format 4 is supported only for non-MBSFN subframes in an MBMS-dedicated cell. A PDCCH consisting of 
consecutive CCEs may only start on a CCE fulfilling
, where 
is the CCE number.
Multiple PDCCHs can be transmitted in a subframe.
Table 6.8.1-1: Supported PDCCH formats
|
PDCCH format |
Number of CCEs |
Number of resource-element groups |
Number of PDCCH bits |
|
0 |
1 |
9 |
72 |
|
1 |
2 |
18 |
144 |
|
2 |
4 |
36 |
288 |
|
3 |
8 |
72 |
576 |
|
4 |
16 |
144 |
1152 |
6.8.2 PDCCH multiplexing and scrambling
The block of bits 
on each of the control channels to be transmitted in a subframe, where 
is the number of bits in one subframe to be transmitted on physical downlink control channel number 
, shall be multiplexed, resulting in a block of bits 
, where 
is the number of PDCCHs transmitted in the subframe.
The block of bits 
shall be scrambled with a cell-specific sequence prior to modulation, resulting in a block of scrambled bits 
according to
where the scrambling sequence 
is given by clause 7.2. The scrambling sequence generator shall be initialised with 
at the start of each subframe.
CCE number 
corresponds to bits 
. If necessary, <NIL> elements shall be inserted in the block of bits prior to scrambling to ensure that the PDCCHs starts at the CCE positions as described in TS 36.213 [4] and to ensure that the length 
of the scrambled block of bits matches the amount of resource-element groups not assigned to PCFICH or PHICH.
6.8.3 Modulation
The block of scrambled bits
shall be modulated as described in clause 7.1, resulting in a block of complex-valued modulation symbols
. Table 6.8.3-1 specifies the modulation mappings applicable for the physical downlink control channel.
Table 6.8.3-1: PDCCH modulation schemes
|
Physical channel |
Modulation schemes |
|
PDCCH |
QPSK |
6.8.4 Layer mapping and precoding
The block of modulation symbols 
shall be mapped to layers according to one of clauses 6.3.3.1 or 6.3.3.3 with 
and precoded according to one of clauses 6.3.4.1 or 6.3.4.3, resulting in a block of vectors 
, 
to be mapped onto resources on the antenna ports used for transmission, where 
represents the signal for antenna port 
. The PDCCH shall be transmitted on the same set of antenna ports as the PBCH.
6.8.5 Mapping to resource elements
The mapping to resource elements is defined by operations on quadruplets of complex-valued symbols. Let 
denote symbol quadruplet 
for antenna port
.
The block of quadruplets
, where 
, shall be permuted resulting in 
. The permutation shall be according to the sub-block interleaver in clause 5.1.4.2.1 of TS 36.212 [3] with the following exceptions:
– the input and output to the interleaver is defined by symbol quadruplets instead of bits
– interleaving is performed on symbol quadruplets instead of bits by substituting the terms "bit", "bits" and "bit sequence" in clause 5.1.4.2.1 of TS 36.212 [3] by "symbol quadruplet", "symbol quadruplets" and "symbol-quadruplet sequence", respectively
<NULL> elements at the output of the interleaver in TS 36.212 [3] shall be removed when forming 
. Note that the removal of <NULL> elements does not affect any <NIL> elements inserted in clause 6.8.2.
The block of quadruplets 
shall be cyclically shifted, resulting in 
where
.
Mapping of the block of quadruplets 
is defined in terms of resource-element groups, specified in clause 6.2.4, according to steps 1–10 below:
1) Initialize 
(resource-element group number)
2) Initialize 
3) Initialize 
4) If the resource element 
represents a resource-element group and the resource-element group is not assigned to PCFICH or PHICH then perform step 5 and 6, else go to step 7
5) Map symbol-quadruplet 
to the resource-element group represented by 
for each antenna port 
6) Increase 
by 1
7) Increase 
by 1
8) Repeat from step 4 if 
, where 
corresponds to the number of OFDM symbols used for PDCCH transmission. The quantity is obtained from
– the sequence transmitted on the PCFICH, or
– the higher-layer parameter cfi-SubframeMBSFN-r15 for DCI formats other than DCI format 7 in a MBSFN subframe, or
– the higher-layer parameter cfi-SlotSubslotMBSFN-r15 for DCI format 7 in a MBSFN subframe, or
– the higher-layer parameter cfi-SubframeNonMBSFN-r15 for DCI formats other than DCI format 7 in a non-MBSFN subframe, or
– the higher-layer parameter cfi-SlotSubslotNonMBSFN-r15 for DCI format 7 in a non-MBSFN subframe, or
– the higher-layer parameter cfi-PatternSubframe-r15 for DCI formats other than DCI format 7 in one subframe for frame structure 2, or
– the higher-layer parameter cfi-PatternSlotSubslot-r15 for DCI formats 7 in one subframe for frame structure 2.
9) Increase 
by 1
10) Repeat from step 3 if 
PDCCHs shall not be transmitted in MBSFN subframes with zero-size non-MBSFN region.