6.8C Short physical downlink control channel (SPDCCH)
36.2113GPPEvolved Universal Terrestrial Radio Access (E-UTRA)Physical channels and modulationRelease 17TS
6.8C.1 SPDCCH formats
The short physical downlink control channel (SPDCCH) carries scheduling assignments and other control information for subslot PDSCH, slot-PDSCH, subslot-PUSCH, and slot-PUSCH. A SPDCCH is transmitted using an aggregation of one or several consecutive short control channel elements (SCCEs) where each SCCE consists of multiple short resource element groups (SREGs), defined in clause 6.2.4B. The number of resource elements used for one SPDCCH depends on the SPDCCH format as given by Table 6.8C.1-2 and the number of SREGs per SCCE is given by Table 6.8C.1-1.
Table 6.8C.1-1: Number of SREGs per SCCE, 
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|
CRS based SPDCCH |
4 |
|
DMRS based SPDCCH |
4 for a 2-symbol SPDCCH1 6 for a 3-symbol SPDCCH1 |
|
NOTE 1: see table 6.8C.5-1 and table 6.8C.5-2 |
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Table 6.8C.1-2: Supported SPDCCH formats
|
SPDCCH format |
Number of SCCEs for one SPDCCH, |
|
0 |
1 |
|
1 |
2 |
|
2 |
4 |
|
3 |
8 |
A UE shall monitor multiple SPDCCHs as defined in TS 36.213 [4]. One or two resource sets which a UE shall monitor for SPDCCH transmissions in a slot/subslot can be configured. The SPDCCH can use either localized or distributed transmission. All SPDCCH candidates in SPDCCH set 
use either only localized or only distributed transmission as configured by higher layers (see transmissionType in TS 36.331 [9]). Similarly, all SPDCCH candidates in SPDCCH set 
use either only CRS-based demodulation or only DMRS-based demodulation as configured by higher layers (see spdcch-SetReferenceSig in TS 36.331 [9]). For a resource set with CRS-based SPDCCH, the distributed transmission is implemented at the SREG level, i.e. in the SCCE-to-SREG mapping. For a resource set with DMRS-based SPDCCH, the distributed transmission is implemented at the SCCE level, i.e. in the SPDCCH candidate-to-SCCE mapping.
The number of OFDM symbols spanned by a SPDCCH resource set 
configured with CRS based demodulation can be configured to be one or two.
For slot based transmission, the number of OFDM symbols spanned by each SPDCCH candidate of a resource set 
configured with DMRS-based demodulation is fixed to 2. For subslot based transmission, the number of OFDM symbols spanned by each SPDCCH candidate of a resource set 
configured with DMRS-based demodulation is equal to the number of OFDM symbols used for the subslot based PDSCH transmission (which depends on the starting symbol index, as specified in Table 6.4.2-1).
The physical resource blocks in frequency domain constituting SPDCCH set 
are in this paragraph assumed to be numbered in ascending order from 0 to 
. For a CRS-based SPDCCH, SREGs within an SPDCCH set 
are numbered in a frequency-first time-second manner from 0 to
, where 
is the number of configured OFDM symbols (OS). The frequency-first, time-second mapping of the SREGs within a SPDCCH set is performed from the lowest resource block in frequency domain to the highest resource blocks in frequency domain for the first symbol, and from the highest resource block in frequency domain to the lowest resource block in frequency domain for the second symbol. For a DMRS-based SPDCCH, SREGs within an SPDCCH set
are numbered in a time-first frequency-second manner from 0 to
.
Within CRS-based SPDCCH set 
, the SCCEs available for transmission of SPDCCHs are numbered from 0 to 
where 
for localized mapping and 
for distributed mapping. Within DMRS-based SPDCCH set 
, the SCCEs available for transmission of SPDCCHs are numbered from 0 to 
where 
. The SCCE number 
corresponds
– SREGs numbered 
for localized SPDCCH mapping with CRS and DMRS based demodulation and for distributed SPDCCH mapping with DMRS-based demodulation
– SREGs numbered 
for distributed SPDCCH mapping with CRS-based demodulation.
where 
and 
is the number of SREGs per SCCE.
NOTE:
represents the number of SREGs per each configured OFDM symbol in the SPDCCH resource set 
and 
is the total number of SREGs in SPDCCH resource set 
.
6.8C.2 Scrambling
The block of bits 
to be transmitted on an SPDCCH in a subframe shall be scrambled, resulting in a block of scrambled bits 
according to
where the UE-specific scrambling sequence 
is given by clause 7.2. The scrambling sequence generator shall be initialized with 
where 
is the SPDCCH resource set number.
6.8C.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 6.8C.3-1 specifies the modulation mappings applicable for the physical downlink control channel.
Table 6.8C.3-1: SPDCCH modulation schemes
|
Physical channel |
Modulation schemes |
|
SPDCCH |
QPSK |
6.8C.4 Layer mapping and precoding
In case of CRS based SPDCCH, layer mapping and precoding shall be done according to clause 6.8.4 with PDCCH replaced by SPDCCH.
In case of DMRS based SPDCCH the layer mapping and precoding shall be done according to clause 6.8A.4.
6.8C.5 Mapping to resource elements
The mapping to resource elements is defined by operations on vectors of complex-valued symbols. Let 
denotes symbol vector and 
represents the signal for antenna port
.
The block of vectors 
, 
shall be mapped in sequence starting with 
to resource elements 
on the associated antenna port(s) which meet all of the following criteria:
– they are part of the SREGs assigned for the SPDCCH transmission, and
– they are assumed by the UE not to be used for cell-specific reference signals, where the positions of the cell-specific reference signals are given by clause 6.10.1.2 with the number of antenna ports and the frequency shift of cell-specific reference signals derived as described in clause 6.10.1.2 unless other values for these parameters are provided by clause 9.1.4.3 in TS 36.213 [4], and
– they are assumed by the UE not to be used for transmission of:
– UE-specific reference signal associated with SPDCCH
– zero-power CSI reference signals, where the positions of the CSI reference signals are given by clause 6.10.5.2. The configuration for zero power CSI reference signals is
– obtained as described in clause 6.10.5.2 unless other values are provided by clause 9.1.4.3 in TS 36.213 [4], and
– obtained by higher-layer configuration of up to five reserved CSI-RS resources as part of the discovery signal configuration following the procedure for zero-power CSI-RS in clause 6.10.5.2.
– non-zero-power CSI reference signals for CSI reporting with the configuration for non-zero power CSI reference signals for CSI reporting obtained as described in clause 6.10.5.2.
– The set of indices of 
where the SPDCCH can be mapped to is dependent on if slot or subslot based SPDCCH is used, the subslot number, if CRS or DMRS based SPDCCH is configured and the number of symbols used for PDCCH. In case CRS based SPDCCH is configured, the set of indices is also dependent on the number of symbols, 
, configured by higher layers (see spdcch-NoOfSymbols in TS 36.331 [9]), that the SPDCCH is mapped over.
– For slot-SPDCCH and for frame structure type 1 and 2, the set of indices of 
for the second slot in the subframe is given in Table 6.8C.5-1. It can be noted that no SPDCCH is transmitted in the first slot of the subframe, where the DCI instead is carried in PDCCH, see [3].
– For subslot-SPDCCH and for frame structure type 1, the set of indices of 
for a given downlink subslot number in a subframe is given in Table 6.8C.5-2. It can be noted that for subslot number 0 where no SPDCCH is transmitted, and the DCI is instead carried in PDCCH, see [3].
If 
has been precoded according to clause 6.3.4.3 and if there is an uneven number of resource elements per PRB that fulfil all the above criteria, 
is not mapped to the resource element of the PRB with largest 
.
Resource elements belonging to synchronization signals, the core part of PBCH, PBCH repetitions, or resource elements reserved for reference signals in the mapping operation of PBCH but not used for transmission of reference signals, shall be assumed available in the SPDCCH mapping but not used for transmission of SPDCCH.
NOTE: For DMRS based SPDCCH, the UE is not required to use the PRGs of size 2 (see TS 36.213 [4]) which is/are partially overlapped with PBCH/PSS/SSS for SPDCCH monitoring.
For DMRS based SPDCCH, the mapping to resource elements 
on antenna port 
, meeting the criteria above, shall be in increasing order of first the index 
and then the index 
.
For localized and distributed CRS based SPDCCH, the SREGs of a SPDCCH candidate are first interleaved according to step 1 below and then the modulated symbols are mapped to resource elements 
of the interleaved SREGs according to step 2 below.
– Step 1: Perform a block interleaver on the SREGs building the SPDCCH candidate, where the number of rows equal to the number of SCCEs for the SPDCCH candidate and the number of columns equal to 4 (i.e. the number of SREGs in an SCCE). The SREGs are written into the matrix row by row and read out column by column.
– Step 2: The modulated symbols are mapped in sequence starting with 
to resource elements 
on antenna port 
in increasing order of the index k, meeting the criteria above, over the interleaved SREGs in the order given by the block interleaver of step 1.
Table 6.8C.5-1: Set of indices of 
for slot-SPDCCH
|
DMRS based SPDCCH |
CRS based SPDCCH |
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Table 6.8C.5-2: Set of indices of 
for subslot-SPDCCH
|
Number of symbols used for PDCCH |
Subslot index |
Set of indicies of |
||
|
DMRS based SPDCCH |
CRS based SPDCCH |
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1 or 3 |
1 |
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2 |
1 |
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1, 2 or 3 |
2 |
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1, 2 or 3 |
3 |
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1, 2 or 3 |
4 |
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1, 2 or 3 |
5 |
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