Demodulation reference signals for PDSCHV15.8.0

Toolbox: DMRS for PDSCH signal generator
ParameterValue
l
nSCID
n
cinit8388628
Pseudo-random seq c(n)
DMRS seq r(n)

Sequence generation

The UE shall assume the sequence r(n) is defined by

Note: this is QPSK modulation

where the pseudo-random sequence c(i) is defined in clause 5.2.1. The pseudo-random sequence generator shall be initialized with

where l is the OFDM symbol number within the slot, is the slot number within a frame, and

  • are given by the higher-layer parameters scramblingID0 and scramblingID1, respectively, in the DMRS-DownlinkConfig IE if provided and the PDSCH is scheduled by PDCCH using DCI format 1_1 with the CRC scrambled by C-RNTI, MCS-C-RNTI, or CS-RNTI
  • is given by the higher-layer parameter scramblingID0 in the DMRS-DownlinkConfig IE if provided and the PDSCH is scheduled by PDCCH using DCI format 1_0 with the CRC scrambled by C-RNTI, MCS-C-RNTI, or CS-RNTI;
  • otherwise;

The quantity nSCID ∈ {0, 1} is given by the DM-RS sequence initialization field, in the DCI associated with the PDSCH transmission if DCI format 1_1 in TS 38.212 is used, otherwise nSCID=0.

Note: Slot and symbol number is used to initialize the sequence, thus helps to randomize the downlink interference as a function of time.

Mapping to physical resources

The UE shall assume the PDSCH DM-RS being mapped to physical resources according to configuration type 1 or configuration type 2 as given by the higher-layer parameter dmrs-Type in DMRS-DownlinkConfig.

Note:
  • Type 1: uses every 2nd (50%) Resource Element within the symbols allocated to DM-RS
  • Type 2: uses every third (33%) Resource Element within the symbols allocated to DM-RS

The UE shall assume the sequence r(m) is scaled by a factor to conform with the transmission power specified in TS 38.214 and mapped to resource elements (k, l)p,μ according to

where are given by Tables 7.4.1.1.2-1 and 7.4.1.1.2-2 and the following conditions are fulfilled:

  • the resource elements are within the common resource blocks allocated for PDSCH transmission
What it means:
  • wf(k') implements the Orthogonal Covering coding (OCC) within each Code Division Multiplexing (CDM) group. wt(0) applies to elements 0, 2, 4, ... of the input sequence r, wt(1) applies to elements 1, 3, 5, ... of the input sequence r
  • l' is used for double symbol DM-RS (it equals 0 for single symbol DM-RS). It maps every single value of to two consecutive symbols for DM-RS transmission.
  • Δ ensures orthogonal frequency resources among different CDM groups.
    • For type 1 DM-RS, there are two CDM groups. The first group occupies subcarriers 0, 2, 4,..., the second group occupies subcarriers 1, 3, 5,...
    • For type 2 DM-RS, there are three CDM groups. The first group occupies subcarriers 0, 1, 6, 7,..., the second group occupies subcarriers 2, 3, 8, 9..., the third group occupies subcarriers 4, 5, 10, 11,...
Example: single symbol DM-RS on antenna port 1001,
  • CDM group λ=0; Δ=0; wf(0)=+1, wf(1)=-1; l'=0, wt(0)=+1
  • DM-RS shall be transmitted on symbol 2 and symbol 5. On each symbol, α0,2,4,...=β{r(0), -r(1), r(2), -r(3), ...}

The reference point for k is

  • subcarrier 0 of the lowest-numbered resource block in CORESET 0 if the corresponding PDCCH is associated with CORESET 0 and Type0-PDCCH common search space and is addressed to SI-RNTI;
  • otherwise, subcarrier 0 in common resource block 0

The reference point for l and the position l0 of the first DM-RS symbol depends on the mapping type:

  • for PDSCH mapping type A:
    • l is defined relative to the start of the slot
    • l0=3 if the higher-layer parameter dmrs-TypeA-Position is equal to 'pos3' and l0=2 otherwise
  • for PDSCH mapping type B:
    • l is defined relative to the start of the scheduled PDSCH resources
    • l0=0
Note:
  • PDSCH mapping type A: slot based scheduling. The first DM-RS symbol is on symbol 2 or symbol 3, cannot be earlier than the first PDSCH symbol
  • PDSCH mapping type B: mini-slot based scheduling, can allocate up to 7 symbols (6 for extended CP). The first PDSCH symbol is always used for DM-RS, i.e. Front Loaded. Front loading the DM-RS reduces latency because UE can start channel estimation as soon as the first symbol is received.

The position(s) of the DM-RS symbols is given by and duration ld where

  • for PDSCH mapping type A, ld is the duration between the first OFDM symbol of the slot and the last OFDM symbol of the scheduled PDSCH resources in the slot
  • for PDSCH mapping type B, ld is the duration of the scheduled PDSCH resources

and according to Tables 7.4.1.1.2-3 and 7.4.1.1.2-4. The case dmrs-AdditionalPosition equals to 'pos3' is only supported when dmrs-TypeA-Position is equal to 'pos2'. For PDSCH mapping type A, ld = 3 and ld = 4 symbols in Tables 7.4.1.1.2-3 and 7.4.1.1.2-4 respectively is only applicable when dmrs-TypeA-Position is equal to 'pos2'. For PDSCH mapping Type A single-symbol DM-RS, l1 = 11 except if all of the following conditions are fulfilled in which case l1 = 12:

  • the higher-layer parameter lte-CRS-ToMatchAround is configured; and
  • the higher-layer parameters dmrs-AdditionalPosition is equal to 'pos1' and l0 = 3; and
  • the UE has indicated it is capable of additionalDMRS-DL-Alt
What it means:
  • additional DM-RS symbols can be configured to improve the channel estimation performance,
  • the number of aditional symbols and their positions are determined using a combination of dmrs-AdditionalPosition parameter, and the duration of the PDSCH resource allocation.

For PDSCH mapping type B

  • if the PDSCH duration ld is 2, 4, or 7 OFDM symbols for normal cyclic prefix or 2, 4, 6 OFDM symbols for extended cyclic prefix, and the front-loaded DM-RS of the PDSCH allocation collides with resources reserved for a search space set associated with a CORESET, shall be incremented such that the first DM-RS symbol occurs immediately after the CORESET and until no collision with any CORESET occurs, and
    • if the PDSCH duration ld is 2 symbols, the UE is not expected to receive a DM-RS symbol beyond the second symbol,
    • if the PDSCH duration ld is 4 symbols, the UE is not expected to receive a DM-RS symbol beyond the third symbol,
    • if the PDSCH duration ld is 7 symbols for normal cyclic prefix or 6 symbols for extended cyclic prefix,
      • the UE is not expected to receive the front-loaded DM-RS beyond the fourth symbol, and
      • if one additional single-symbol DM-RS is configured, the UE only expects the additional DM-RS to be transmitted on the 5th or 6th symbol when the front-loaded DM-RS symbol is in the 1st or 2nd symbol, respectively, of the PDSCH duration, otherwise the UE should expect that the additional DM-RS is not transmitted.
  • if the PDSCH duration ld is 2 or 4 OFDM symbols, only single-symbol DM-RS is supported.

The time-domain index l' and the supported antenna ports p are given by Table 7.4.1.1.2-5 where

  • single-symbol DM-RS is used if the higher-layer parameter maxLength in the DMRS-DownlinkConfig IE is not configured
  • single-symbol or double-symbol DM-RS is determined by the associated DCI if the higher-layer parameter maxLength in the DMRS-DownlinkConfig IE is equal to 'len2'.

In absence of CSI-RS configuration, and unless otherwise configured, the UE may assume PDSCH DM-RS and SS/PBCH block to be quasi co-located with respect to Doppler shift, Doppler spread, average delay, delay spread, and, when applicable, spatial Rx parameters. The UE may assume that the PDSCH DM-RS within the same CDM group are quasi co-located with respect to Doppler shift, Doppler spread, average delay, delay spread, and spatial Rx. The UE may assume that DMRS ports associated with a PDSCH are QCL with QCL Type A, Type D (when applicable) and average gain.

The UE may assume that no DM-RS collides with the SS/PBCH block.


Table 7.4.1.1.2-1: Parameters for PDSCH DM-RS configuration type 1.
pCDM group λΔwf(k')wt(l')
k'=0k'=1l'=0l'=1
100000+1+1+1+1
100100+1-1+1+1
100211+1+1+1+1
100311+1-1+1+1
100400+1+1+1-1
100500+1-1+1-1
100611+1+1+1-1
100711+1-1+1-1


Table 7.4.1.1.2-2: Parameters for PDSCH DM-RS configuration type 2.
pCDM group λΔwf(k')wt(l')
k'=0k'=1l'=0l'=1
100000+1+1+1+1
100100+1-1+1+1
100212+1+1+1+1
100312+1-1+1+1
100424+1+1+1+1
100524+1-1+1+1
100600+1+1+1-1
100700+1-1+1-1
100812+1+1+1-1
100912+1-1+1-1
101024+1+1+1-1
101124+1-1+1-1


Table 7.4.1.1.2-3: PDSCH DM-RS positions for single-symbol DM-RS.

ld in symbolsDM-RS positions
PDSCH mapping type APDSCH mapping type B
dmrs-AdditionalPositiondmrs-AdditionalPosition
pos0pos1pos2pos3pos0pos1pos2pos3
2-0-
3l0-
4l00-
5l0-
6l000, 4-
7
8l0l0, 7-
9
10l0l0, 9l0, 6, 9
11
12l0l0, 9l0, 6, 9l0, 5, 8, 11
13l0l0, l1l0, 7, 11l0, 5, 8, 11
14


Table 7.4.1.1.2-4: PDSCH DM-RS positions for double-symbol DM-RS.

ld in symbolsDM-RS positions
PDSCH mapping type APDSCH mapping type B
dmrs-AdditionalPositiondmrs-AdditionalPosition
pos0pos1pos2pos0pos1pos2
<4-
4l0-
5
6l0-0-
7
8l0-
9
10l0l0, 8
11
12
13l0l0, 10
14


Table 7.4.1.1.2-5: PDSCH DM-RS time index l' and antenna ports p.

Single or double symbol DM-RSl'Supported antenna ports p
Configured type 1Configured type 2
single01000 - 10031000 - 1005
double0, 11000 - 10071000 - 1011