5G/NR  -  PDSCH/PUSCH

 

 

 

PDSCH/PUSCH Mapping Type

Both PDSCH and PUSCH has two different types of mapping called Type A and Type B. These types are characterized by DMRS type (PDSCH DMRS type and PUSCH DMRS type) and SLIV table. 38.214 Table 5.1.2.1-1 gives the downlink combinations, and Table 6.1.2.1-1 gives the uplink ones.

Mapping Type A and Type B

Mapping type is not a property of the channel. It is a property of the allocation, and it settles two things at once. The first is where the DMRS sits inside that allocation. The second is which start and length combinations are legal at all, and 38.214 states that as one table per channel.

38.214 Table 5.1.2.1-1 sets the legal downlink combinations. S is the first symbol of the allocation and L is its length, and the third column bounds their sum. The sum is the column to read first. An allocation cannot cross the slot boundary, so a late start caps the length available to it.

< 38.214-Table 5.1.2.1-1: Valid S and L combinations >

38.214 Table 5.1.2.1-1 giving valid S, L and S plus L combinations for PDSCH mapping type A and type B, under normal and extended cyclic prefix

 

38.214 Table 6.1.2.1-1 does the same job for the uplink, and the two tables are not symmetric. The uplink type A row fixes S at 0 rather than allowing 0 to 3. The uplink type B row allows any length from 1 upward, rather than the three fixed lengths the downlink permits.

< 38.214-Table 6.1.2.1-1: Valid S and L combinations >

38.214 Table 6.1.2.1-1 giving valid S, L and S plus L combinations for PUSCH mapping type A and type B, under normal and extended cyclic prefix

PDSCH Mapping Type

The downlink case carries the tighter length rule of the two. Type A is built for an allocation that starts in the first few symbols and covers most of the slot. Type B is built for one that can start almost anywhere inside the slot, and it is short.

Type A :

  • PDSCH DMRS is Type A
    • The DMRS location is fixed to 3rd (pos2) or 4th(pos3)
  • PDSCH Starting Symbol can be 0~3
  • PDSCH Length can be 3~14 in case of normal CP and 3~12 in case of extended CP
  • the DMRS symbol can start only at symbol 2 or 3 regardless of PDSCH start and length. It implies this cannot be used when PDSCH start symbol is greater than 3. This is related to the row 'Type A' in PDSCH SLIV table. This type is used for slot based scheduling.

Type B :

  • PDSCH DMRS is Type B
    • The DMRS location is fixed to the first symbol of the allocated PDSCH
  • PDSCH Starting Symbol can be 0~12 in case of Normal CP and 0~10 in case of extended CP
  • PDSCH Length can only be 2 or 4 or 7 in case of Normal CP and 2 or 4 or 6 in case of extended CP
  • the DMRS symbol can start at the first PDSCH symbol regardless of PDSCH start. This is related to the row 'Type B'  in PDSCH SLIV table. This type is used for mini-slot based scheduling

PUSCH Mapping Type

The uplink rows differ from the downlink ones in both directions at once. Type A is stricter, because the start symbol has no freedom at all. Type B is looser, because its length is not restricted to a short list of values.

Type A :

  • PUSCH DMRS is Type A
    • The DMRS location is fixed to 3rd (pos2) or 4th(pos3)
  • PUSCH Starting Symbol is always 0
  • PUSCH Length can be 4~14 in case of normal CP and 4~12 in case of extended CP

Type B :

  • PUSCH DMRS is Type B
    • The DMRS location is fixed to the first symbol of the allocated PUSCH
  • PUSCH Starting Symbol can be 0~13 in case of Normal CP and 0~12 in case of extended CP
  • PUSCH Length can only be 1~14 in case of Normal CP and 1~12 in case of extended CP
  • Mapping type decides where the DMRS sits : type A places it at symbol 2 or 3, and type B places it at the first symbol of the allocation.
  • Type A is slot based : the allocation starts within the first four symbols and covers most of the slot.
  • Type B is mini-slot based : the allocation may start almost anywhere in the slot, and it is short.
  • The sum column is the real bound : S plus L keeps the allocation inside the slot, which is why a late start limits the length.
  • The uplink is not a copy of the downlink : PUSCH type A fixes S at 0, and PUSCH type B allows any length from 1 to 14.

UE Capability

UE would inform NW of it's supportability about PDSCH Mapping Type via UE Capability Information message. Only the downlink mapping type appears there. There is no matching pusch-MappingTypeA or pusch-MappingTypeB field in the same IE. The uplink mapping types are therefore not declared this way at all.

Following is based on 38.331 v19.3.0 (Release 19)

Phy-ParametersCommon ::=            SEQUENCE {
    ...
    pdsch-MappingTypeA                  ENUMERATED {supported}                      OPTIONAL,
    pdsch-MappingTypeB                  ENUMERATED {supported}                      OPTIONAL,
    ...
    -- the remaining fields of this IE are not mapping type related and are not repeated here
}

Both fields are optional, and both carry the single value supported. The message therefore says what the device can do by including the field rather than by setting it. A device that omits pdsch-MappingTypeB is telling the network not to schedule mini-slot allocations to it.

One further field belongs to the same topic, and it sits in a different information element. Release 16 added a per-band capability for a longer type B allocation. 38.331 records what it means in a comment beside the field.

Following is based on 38.331 v19.3.0 (Release 19)

BandNR ::=                          SEQUENCE {
    ...
    -- R1 14-2: PDSCH Type B mapping of length 9 and 10 OFDM symbols
    pdsch-MappingTypeB-Alt-r16              ENUMERATED {supported}                  OPTIONAL,
    ...
    -- the remaining fields of this IE are not mapping type related and are not repeated here
}

That capability qualifies a number given earlier on this page. 38.331 describes the field as PDSCH Type B mapping of length 9 and 10 OFDM symbols. Where a device signals it for a band, the type B length is no longer confined to 2, 4 or 7 on that band.

  • Only the downlink mapping type is a capability : pdsch-MappingTypeA and pdsch-MappingTypeB exist, and there is no uplink equivalent in this IE.
  • Presence is the signal : the value is always supported, so the field is either present or it is not.
  • Type A is not guaranteed either : both fields are optional, so both have to be checked before scheduling against a mapping type.
  • Release 16 extends type B per band : pdsch-MappingTypeB-Alt-r16 in BandNR adds lengths of 9 and 10 OFDM symbols.

Reference

[1] 38.214 v19.4.0 : NR - Physical layer procedures for data. Table 5.1.2.1-1 and Table 6.1.2.1-1 give the valid S and L combinations quoted on this page.

[2] 38.331 v19.3.0 : NR - Radio Resource Control (RRC) protocol specification. Phy-ParametersCommon and BandNR carry the mapping type capabilities.