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LTE-NB : NPDCCH/DCI

 

If you have been working since the legacy LTE, you would be familiar with the concepts and parameters related to how network notify UE of downlink and uplink resource assignment and you may not need any explanation about the general concept of PDCCH / DCI and you can jump directly in the details. However, if LTE-NB is the first LTE experience, you would need to the basic concept of DCI. For the basic concept, I would suggest you to read the legacy LTE DCI page or at least the first section of the page.

 

 

Basic Terminologies

 

When we are talking about resource allocation mechanism, DCI(Downlink Control Indicator) is the most important part but if you are going a little bit deeper into the implementation of DCI in Transport/Physical layer. You would see a couple of additional parameters that may confuse you in some case.

 

Terminology

Meaning

DCI A Logical Block(Unit) that carries data for NPDSCH(Downlink) or NPUSCH(Uplink) data
NPDCCH The Physical Channel that carries(conveys) DCI
NCCE The basic unit of physical block in NPDCCH where DCI data is allocated. NPDCCH split its area into two subblock. This subblock is called NCCE. In NPDCCH, there are two subblocks called NCCE 0 and NCCE 1.

 

 

NPDCCH / NCCE

 

As in legacy LTE, in LTE-NB as well the DCI is carried by the physical channel called NPDCCH. The basic resource allocation unit for PDCCH is called NCCE. There are two NCCE that are defined as shown below. There are two different types of NPDCCH format (Format 0 and Format 1). NPDCCH Format 0 takes up only one NCCE and NPDCCH format 1 takes up two NCCEs.

 

 

 

DCI(Downlink Control Indicator)

 

Unlike in LTE, in LTE-NB only three types of DCI (DCI Format N0, N1, N2) as described in 36.212 6.4.3 Downlink control information

 

There are three different DCI format defined in LTE-NB as summarized in the table shown below.

 

DCI Format

Bit Length (size)

Purpose / Usage

N0

23

UL Grant (NPUSCH Scheduling)

N1

23

DL Scheduling (NPDSCH Scheduling)

RACH initiated by PDCCH Order

N2

15

Paging and direct indication

 

 

Format N0

 

DCI Format N0 is for UL Grant. It is equivalent to DCI 0 in normal LTE. Each of the field in DCI format N0 is defined as follows.

 

Field

# of Bits

Description

Flag for format N0/format N1 differentiation

1

0 - N0, 1 - N1

Subcarrier indication

6

When Subcarrier Spacing = 15 Khz

      See 36.213 Table 16.5.1.1-1

When Subcarrier Spacing = 3.75 Khz

      n_sc = I_sc

Resource assignment

3

See 36.213 Table 16.5.1.1-2

Scheduling delay

2

See 36.213 Table 16.5.1-1

Modulation and coding scheme

4

See 36.213 Table 16.5.1.2-1

Redundancy version

1

 

Repetition number

3

See 36.213 Table 16.5.1.1-3

New data indicator

1

 

DCI subframe repetition number

2

 

Total Number of Bits

23

 

 

 

 

Format N1

 

DCI Format N1 is for all NPDSCH (user data and SIBs) except NPDSCH carrying Paging and to trigger PRACH(non-contention based).  Each of the field in DCI format N0 is defined as follows.

 

< NPDCCH order = 0,  N1 CRC not masked with RA-RNTI >

This is to assign resources for normal NPDSCH

Field

# of Bits

Description

Flag for format N0/format N1 differentiation

1

0 - N0, 1 - N1

NPDCCH order indicator

1

 

Scheduling delay

3

See 36.213 Table 16.4.1-1

Resource assignment

3

See 36.213 Table 16.4.1.3-1

Modulation and coding scheme

4

See 36.213 16.4.1.5

Repetition number

4

See 36.213 Table 16.4.1.3-2

New data indicator

1

 

HARQ-ACK resource

4

See 36.213 Table 16.4.2-1,  

      36.213 Table 16.4.2-2

DCI subframe repetition number

2

 

Total Number of Bits

23

 

 

 

< NPDCCH order = 0,  N1 CRC masked with RA-RNTI >

This is to assign resources for NPDSCH for RAR(Random Access Response)

Field

# of Bits

Description

Flag for format N0/format N1 differentiation

1

0 - N0, 1 - N1

NPDCCH order indicator

1

 

Scheduling delay

3

 

Resource assignment

3

 

Modulation and coding scheme

4

 

Repetition number

4

 

Reserved

1

 

Reserved

4

 

DCI subframe repetition number

2

 

Total Number of Bits

23

 

 

 

< NPDCCH order = 1 and CRC masked with C-RNTI>

This is to trigger PRACH (in Non-contention based RACH)

Field

# of Bits

Description

Flag for format N0/format N1 differentiation

1

0 - N0, 1 - N1

NPDCCH order indicator

1

 

Starting number of NPRACH repetitions

2

 

Subcarrier indication of NPRACH

6

 

All the remaining fields

13

1

Total Number of Bits

23

 

 

 

Format N2

 

DCI format N2 is used for for paging and direct indication and each of the field in this format is as follows.

 

< Flag = 0 >

Field

# of Bits

Description

Flag for paging/direct indication differentiation

1

0 - Direct Indication, 1 - Paging

Direct Indication information

8

 

Reserved information bits (Padding)

 

 

Total Number of Bits

15

 

 

 

< Flag = 1 >

Field

# of Bits

Description

Flag for paging/direct indication differentiation

1

0 - Direct Indication, 1 - Paging

Resource assignment

3

 

Modulation and coding scheme

4

 

Repetition number

4

 

DCI subframe repetition number

3

 

Total Number of Bits

15

 

 

 

Tables and Figures for Parameter Details

 

 

< 36.213 Table 16.5.1.1-1: Allocated subcarriers for NPUSCH with subcarrier spacing = 15 Khz >

 

 

Following two table shows how the subcarrier indication field in DCI format N0 specifies the specific subcarriers to allocate UL resource.

 

 

 

< 36.213 Table 16.5.1.1-2: Number of resource units (N_RU) for NPUSCH. >  

 

 

< 36.213 Table 16.5.1-1: k0 for DCI format N0 >  

 

 

< 36.213 Table 16.5.1.2-1: Modulation and TBS index table for NPUSCH with N_sc^RU = 1 >

 

 

< 36.213 Table 16.5.1.2-2: Transport block size (TBS) table for NPUSCH >

 

 

< 36.213 Table 16.5.1.1-3: Number of repetitions (N_Rep) for NPUSCH >

 

 

< 36.213 Table 16.4.1-1: k0 for DCI format N1 >

 

 

< 36.213 Table 16.4.2-1: ACK/NACK subcarrier and k0 for NPUSCH with Subcarrier Spacing= 3.75KHz >

 

 

< 36.213 Table 16.4.2-2: ACK/NACK subcarrier and k0 for NPUSCH with Subcarrier Spacing= 15KHz >

 

< 36.213 Table 16.4.1.3-1: Number of subframes (N_SF) for NPDSCH >

 

 

< 36.213 Table 16.4.1.3-2: Number of repetitions (N_Rep) for NPDSCH >

 

 

< 36.213 - 16.4.1.5 Modulation order and transport block size determination >

 

I_MCS and I_TBS mapping gets different depending on whether the NPDSCH carries SystemInformationBlockType1-NB or User Data as follows.

  • Case 1 : When the NPDSCH carries SystemInformationBlockType1-NB
  • Case 2 : When the NPDSCH carries user data
    • I_TBS = I_MCS

 

 

NPDCCH Search Space

 

In order to detect whether there is any data (NPDSCH) sent for it or detect any UL Grant for NPUSCH, it should monitor (try to decode) various regions within downlink subframes. There is no explicit regions notified by the network about exactly which regions UE need to monitor. UE needs to monitor all the possible regions that are allowed for NPDCCH and decode the information in try-and-error based. This kind of process is called 'blind decoding'. However, UE does not try to decode every possible combinations of resource elements within a subframe. There are a certain set of predefined regions in which a PDCCH can be allocated. UE monitor only those predefined regions. These predefined set of regions are called NPDCCH Search Space.  

 

There are a few types of NPDCCH space as shown below (36.213 - 16.6 Narrowband physical downlink control channel related procedures).

 

Type

Sub Type

Description

Common

Type1-NPDCCH common search space

NPDCCH for Paging Message

Type2-NPDCCH common search space

NPDCCH for RAR, Msg3 retransmission and Msg4

UE Specific

NPDCCH UE-specific search space

 

 

 

NPDCCH Location (NPDCCH Subframes)

 

The location of NPDCCH (the subframes transmitting NPDCCH) is determined as follows (described in 36.213-16.6 Narrowband physical downlink control channel related procedures).

 

 

NOTE : In case of Type1-NPDCCH common search space, k = k0 and is determined from locations of NB-IoT paging opportunity subframes

 

 

< 36.213-Table 16.6-1: NPDCCH UE- specific search space candidates >

 

 

< 36.213-Table 16.6-2: Type 1- NPDCCH common search space candidates >

 

 

< 36.213-Table 16.6-3: Type 2- NPDCCH common search space candidates >