5G NR DM-RS(Demodulation Reference Signals)

Venkateshu
Aug 14
11 min read

1. Introduction

DMRS (Demodulation Reference Signals) are UE-specific reference signals in 5G NR that provide essential channel estimation capabilities for coherent demodulation of physical channels. Unlike broadcast reference signals, DMRS are transmitted only when and where needed, accompanying data transmissions to enable accurate channel estimation and data decoding.

Primary Functions of DMRS in 5G NR

Channel Estimation: Enable precise estimation of wireless channel conditions for coherent data demodulation
Precoding Transparency: Since both data and DMRS undergo the same precoding, channel estimation includes both propagation channel and precoding effects
MIMO Support: Facilitate multi-layer MIMO transmission with up to 12 orthogonal antenna ports
Power Estimation: Provide reference for received signal power measurements
Phase Tracking: Support phase correction for high-frequency transmissions

LTE Reference Signal Philosophy

Always-On Transmission: LTE reference signals, particularly Cell-specific Reference Signals (CRS), are continuously transmitted across all downlink subframes regardless of actual need.
Fixed Resource Allocation: Reference signals occupy predetermined resource elements in the time-frequency grid, creating consistent but inflexible overhead.
Cell-Centric Design: Most reference signals are cell-specific, transmitted across the entire cell bandwidth even when only specific UEs require them.
Limitations:
- CRS occupies 4.76% (1 port) to 14.29% (4 ports) of all resource elements
- Always transmitted regardless of UE presence or data activity
- Creates significant interference in dense deployments

5G NR Reference Signal Philosophy

Ultra-Lean Design: NR follows an "only when needed" approach, transmitting reference signals dynamically based on actual requirements.
Flexible Configuration: Reference signals are highly configurable through RRC signaling, allowing adaptation to specific deployment scenarios and service requirements.
Beam-Centric Operation: Supports beamformed reference signals, enabling efficient resource utilization in massive MIMO deployments.
Advantages:
- DMRS overhead ranges from 3.6% to 7% only for allocated resources
- Reference signals transmitted only within scheduled resource blocks
- Approximately 50% overhead reduction compared to LTE

Comparison between reference signals in LTE and 5G NR

Function	LTE	5G NR
PDSCH Demodulation	CRS + UE-specific DMRS	Dedicated DMRS only
PDCCH Demodulation	CRS	Dedicated DMRS
Resource Efficiency	High overhead due to CRS	Lower overhead, UE-specific allocation
MIMO Support	Up to 8 layers	Up to 12 layers with flexible port mapping

2.The NR Demodulated Reference-Signal Landscape

Category	Specific RS	Main Purpose	Typical Density/Ports
Broadcast	PBCH-DMRS	Demodulation of PBCH/MIB	Port 4000
Demodulation	DM-RS (PDSCH, PDCCH, PUCCH, PUSCH)	Channel estimation for coherent demodulation	1–12 ports, Config-T1/T2

2.1 PBCH and PBCH-DMRS

PBCH DMRS in 5G NR serves as the essential reference signal for MIB decoding, replacing LTE's CRS dependency. Its configuration is primarily determined by the MIB's dmrs-TypeA-Position parameter (affecting subsequent PDSCH operations) and physical layer parameters (PCI, SSB index); its demodulation relies on a dedicated DMRS occupying every fourth RE in symbols 1–3, consuming 25% of PBCH REs.

Sequence Generation: Gold sequence initialised with PCI, SSB index and half-frame number yields port-4000 symbols.
Design Goal: Guarantee reliable MIB decode at –6 dB SNR with ≤ 0.1% BLER.

The PBCH DMRS sequence is generated using a Gold sequence with initialization value :

c_init = 2^11 × (i_SSB_bar + 1) × ⌊N_ID^cell/4⌋ + 2^6 × (i_SSB_bar + 1) + (N_ID^cell mod 4)

Where:

i_SSB_bar: Time-dependent parameter derived from SSB index and half-frame
- For Lmax = 4: i_SSB_bar = i_SSB + 4 × n_hf
- For Lmax = 8 or 64: i_SSB_bar = i_SSB
N_ID^cell: Physical Cell ID

The DMRS is allocated to every 4th Resource element and is designed based on PCI mod 4.

Resource Element Mapping of PBCH DMRS

Configuration: 20 MHz carrier, PCI = 123

Resource Allocation:

Total SSB bandwidth: 240 subcarriers (20 RBs)
PBCH symbols: 1, 2, 3
DMRS pattern: v = 123 mod 4 = 3 (frequency offset)

Per Symbol Allocation:

Symbol 1: 20 RBs × 3 DMRS REs = 60 DMRS REs
Symbol 2: 8 RBs × 3 DMRS REs = 24 DMRS REs (center portion)
Symbol 3: 20 RBs × 3 DMRS REs = 60 DMRS REs

Total DMRS REs: 144 Res

2.2. PDCCH DM-RS

Fixed Pattern: DMRS resource elements are located at fixed positions within each REG (Resource Element Group).
Resource Overhead: Occupies 25% of PDCCH resource elements.
Frequency Pattern: Located at specific subcarrier positions (1, 5, 9, 13, 17, 21, ...) within the CORESET.

The PDCCH DMRS sequence uses a Gold sequence with initialization :

text

c_init = (2^17 × (14 × n_s + l + 1) × (2 × N_ID^{cell} + 1) + 2 × N_ID^{cell} + N_cp) mod 2^31

Where:

n_s: Slot number within radio frame
l: OFDM symbol number within slot
N_ID^{cell}: Physical Cell ID
N_cp: Cyclic prefix type (0: normal, 1: extended)
If pdcch-DMRS-ScramblingID is configured, it replaces N_ID^{cell}

Resource Mapping Parameters

Aspect	Configuration	Description
REG Structure	1 PRB × 1 symbol	Basic allocation unit for CORESET
DMRS per REG	3 REs	Fixed at positions (1, 5, 9) within REG
CCE Composition	6 REGs	Each CCE contains 18 DMRS REs total
Total Overhead	25% of CORESET REs	3 DMRS + 9 data per REG

Resource Allocation in CORESET

Configuration:

Bandwidth: 20 MHz (106 PRBs)
CORESET: 24 PRBs across 2 symbols
REGs: 48 REGs total (24 PRBs × 2 symbols)

DMRS Resource Calculation:

text

Total REs in CORESET = 24 PRBs × 12 subcarriers × 2 symbols = 576 REs

DMRS REs per REG = 3 REs

Total REGs = 48 REGs

Total DMRS REs = 48 × 3 = 144 REs

DMRS Overhead = 144/576 = 25%

2.3. PDSCH DM-RS

The Physical Downlink Shared Channel Demodulation Reference Signal (PDSCH DMRS) serves as the primary reference signal for data demodulation in 5G NR systems. Its Key Characteristics include,

UE-Specific Transmission: DMRS is transmitted only when PDSCH is scheduled for a specific UE, reducing interference and overhead.
Flexible Configuration: Supports wide range of configurations to accommodate diverse deployment scenarios and service requirements.
Antenna Port Association: Uses antenna ports 1000-1011, with both PDSCH data and DMRS using the same ports.

Types of Resource Allocation

Frequency Domain Resource Allocation Types

Type	Description	Configuration Method	Use Case
Type 0	Resource Block Group (RBG) based bitmap allocation	resourceAllocation resourceAllocationType0	Non-contiguous allocation, flexible scheduling
Type 1	Contiguous RB allocation using RIV	resourceAllocation resourceAllocationType1	Contiguous allocation, simple scheduling
Dynamic Switch	Dynamic selection between Type 0 and Type 1	resourceAllocation dynamicSwitch	Adaptive allocation based on needs

Time Domain Resource Allocation Types

DMRS for PDSCH has two types of PDSCH Mapping, PDSCH Mapping Types A and B.
The PDSCH Mapping Type has an impact upon the time domain symbols allocated to the PDSCH. This impacts the symbols allocated to the DMRS because the DMRS can only use a subset of the resources allocated to the PDSCH.
PDSCH Mapping Type A: is sometimes referred to as providing ‘Slot’ based scheduling’.
- In PDSCH Type A, first DMRS is located in Symbol 2 or 3 of the slot
PDSCH Mapping Type B: is sometimes referred to as providing ‘Non-Slot’ based Scheduling or ‘Mini Slot' based scheduling’ (Mapping Type B is restricted to allocating up to 7 symbols)
- PDSCH Type B is always located at the first Symbol of PDSCH Allocation. (Called Front Loaded ).
- A front-loaded design supports low-latency transmissions.
- The front-loaded reference signals indicate that the signal occurs early in the transmission. The DMRS is present in each RB allocated for PDSCH.

The resources in the time domain for PDSCH transmission are scheduled by downlink control information (DCI) in the field Time domain resource assignment. This field indicates the slot offset K0, starting symbol S, the allocation length L, and the mapping type of PDSCH. The valid combinations of S and L are shown in Table 1. For mapping type A, value of S is 3 only when the DM-RS type A position is set to 3.

Mapping Type	Start Symbol	DMRS Position	Use Case
Type A	Fixed at symbol 2 or 3	Determined by dmrs-TypeA-Position	Slot-based scheduling, legacy compatibility
Type B	Variable, first symbol of allocation	Always at symbol 0 (relative)	Mini-slot scheduling, low latency

Mapping Types A and B both allow the DMRS to use additional symbols.
Additional DMRS symbols can help to improve the UE channel estimation performance.
If a PDSCH transmission includes 2 DMRS symbols, the propagation channel can be measured at 2-time instants (as shown in below picture) and then interpolated between those time instants.
Increasing the number of DMRS symbols reduces the gap between channel estimates and reduces the requirement for long interpolations.
This is particularly important for high-speed scenarios where the propagation channel can change rapidly and there are large frequency offsets to track.

DMRS Configuration Types

Type 1 Configuration

Frequency Density: 6 REs per PRB per antenna port
Pattern: Every alternate subcarrier (0, 2, 4, 6, 8, 10)
CDM(Code Division Multiplexing) Groups: 2 CDM groups, supporting up to 8 antenna ports (0-7).
- CDM groups provide a mechanism to multiplex different DMRS antenna ports within the same resource elements, maximizing spectral efficiency while maintaining orthogonality.
- Different antenna ports within the same CDM group use orthogonal cover codes
  (OCC) to avoid interference

CDM Group	Antenna Ports	Frequency Pattern	Time Domain OCC
Group 0	1000, 1001, 1004, 1005	Subcarriers 0,2,4,6,8,10	[+1,+1], [+1,-1] for double symbol
Group 1	1002, 1003, 1006, 1007	Subcarriers 1,3,5,7,9,11	[+1,+1], [+1,-1] for double symbol

Usage: Most common configuration, good for SU-MIMO
uses every 2nd (50%) Resource Element within the symbols allocated to DM-RS

Type 2 Configuration

Frequency Density: 4 REs per PRB per antenna port
Pattern: Two groups of consecutive subcarriers (0,1,6,7)
CDM Groups: 3 CDM groups, supporting up to 12 antenna ports (0-11)

CDM Group	Antenna Ports	Frequency Pattern	FD-CDM Codes
Group 0	1000, 1001, 1006, 1007	Subcarriers 0,1	[+1,+1], [+1,-1]
Group 1	1002, 1003, 1008, 1009	Subcarriers 6,7	[+1,+1], [+1,-1]
Group 2	1004, 1005, 1010, 1011	Subcarriers 2,3	[+1,+1], [+1,-1]

Usage: Better for MU-MIMO scenarios, higher spectral efficiency
uses every third (33%) Resource Element within the symbols allocated to DM-RS

Example

A 30 kHz numerology slot (14 symbols) carrying PDSCH for a train UE at 500 km/h may be configured as:

Mapping type B, startSymbol=1, length=10,

DMRS-ConfigType=1, additionalPosition=3 → DMRS in symbols 1,4,7,10

This quadruples pilot time density, sustaining channel tracking under 6 kHz Doppler shift.

Complete RRCReconfiguration with PDSCH DMRS

rrcReconfiguration: {

rrc-TransactionIdentifier 2,

criticalExtensions rrcReconfiguration: {

spCellConfig: {

spCellConfigDedicated: {

downlinkBWP-ToAddModList: {

{

bwp-Id 1,

bwp-Dedicated: {

pdsch-Config setup: {

dataScramblingIdentityPDSCH 456,

dmrs-DownlinkForPDSCH-MappingTypeA setup: {

dmrs-Type type2,

dmrs-AdditionalPosition pos1,

maxLength len2,

scramblingID0 789,

phaseTrackingRS setup: {

frequencyDensity fd2,

timeDensity td1

}

resourceAllocation resourceAllocationType1,

rbg-Size config1,

mcs-Table qam256,

maxNrofCodeWordsScheduledByDCI n2

}

Configuration Analysis:

dmrs-Type type2: 4 REs per PRB, supports up to 12 ports
dmrs-AdditionalPosition pos1: One additional DMRS symbol for mobility
maxLength len2: Allows double-symbol DMRS via DCI signaling
scramblingID0 789: Custom scrambling for interference mitigation

Example: DMRS Sequence Generation

Scenario Parameters:

Physical Cell ID = 123
Scrambling ID0 = 456 (configured)
Slot number = 10
DMRS symbol = 2
Normal CP

Calculation:

c_init = (2^17 × (14 × n_s + l + 1) × (2 × N_ID^DMRS + 1) + 2 × N_ID^DMRS + N_cp) mod 2^31

Where:

- n_s = 10 (slot number)

- l = 2 (DMRS symbol)

- N_ID^DMRS = 456 (scramblingID0)

- N_cp = 0 (normal CP)

Example: DCI-based Dynamic Configuration

DCI Format 1_1 Antenna Ports Field:Using 3GPP TS 38.212 Table 7.3.1.2.2-2 (dmrs-Type=2, maxLength=2):

DCI Value	Interpretation	DMRS Configuration
0	1 layer, port 1000	Single symbol, CDM group 0
1	1 layer, port 1001	Single symbol, CDM group 1
2	2 layers, ports 1000,1001	Single symbol, CDM groups 0,1
3	1 layer, port 1000	Double symbol, CDM group 0

Resource Allocation Impact:

Values 0-2: Single symbol DMRS, data can be frequency multiplexed
Values 3+: Double symbol DMRS, no frequency multiplexing with data

3. Uplink Demodulated Reference Signals

3.1 PUSCH DMRS (Physical Uplink Shared Channel DMRS)

PUSCH DMRS enables the gNB to accurately estimate the uplink channel conditions and demodulate user data transmitted by the UE.

Key Functions:

Channel Estimation: Helps gNB estimate uplink propagation channel for coherent data demodulation
MIMO Support: Enables spatial multiplexing with up to 4 antenna ports for uplink transmission
Precoding Compensation: Accounts for any uplink precoding applied by the UE
Power Estimation: Provides reference for received signal power measurements at gNB

Technical Characteristics

Parameter	Description	Values/Options
Antenna Ports	0-3 (up to 4 ports)	Supports SU-MIMO and MU-MIMO
Waveform Support	CP-OFDM and DFT-s-OFDM	Transform precoding on/off
Mapping Types	Type A and Type B	Similar to downlink concepts
Configuration Types	Type 1 and Type 2	Different frequency domain patterns

The resources in time domain for PUSCH transmission are scheduled by downlink control information (DCI) in the field Time domain resource assignment. This field indicates the slot offset K0, starting symbol S, the allocation length L, and the mapping type of PUSCH. The valid combinations of S and L are shown in Table 1.

Symbol allocation of PUSCH indicates the OFDM symbol locations allocated for the PUSCH transmission in a slot. The mapping type indicates the first DM-RS OFDM symbol location and the duration of OFDM symbols (ld).

For mapping type A, ld is the duration between the first OFDM symbol of the slot and the last OFDM symbol of the allocated PUSCH resources.
For mapping type B, ld is the duration of the allocated PUSCH resources. When intra-slot frequency hopping is enabled, ld is the duration per hop.

The DM-RS symbols are present in each hop when intra-slot frequency hopping is enabled.

When intra-slot frequency hopping is enabled, DM-RS is single-symbol with the maximum number of additional positions either 0 or 1. The DM-RS symbol locations is given by TS 38.211 Tables 6.4.1.1.3-3, 6.4.1.1.3-4, and 6.4.1.1.3-6. Figure 1 shows the DM-RS symbol locations for PUSCH occupying 14 symbols with PUSCH mapping type A, intra-slot frequency hopping enabled, and number of DM-RS additional positions as 1.

The figure shows DM-RS is present in each hop. The locations of DM-RS symbols in each hop depends on the number of OFDM symbols allocated for PUSCH in each hop.

PUSCH DMRS Configuration

The PUSCH DMRS configuration is carried in RRCReconfiguration messages within the PUSCH configuration:

PUSCH-Config ::= SEQUENCE {

dataScramblingIdentityPUSCH INTEGER (0..1023) OPTIONAL,

txConfig ENUMERATED {codebook, nonCodebook} OPTIONAL,

dmrs-UplinkForPUSCH-MappingTypeA CHOICE {

setup DMRS-UplinkConfig

} OPTIONAL,

dmrs-UplinkForPUSCH-MappingTypeB CHOICE {

setup DMRS-UplinkConfig

} OPTIONAL,

transformPrecoding ENUMERATED {enabled, disabled} OPTIONAL

}

DMRS-UplinkConfig ::= SEQUENCE {

dmrs-Type ENUMERATED {type1, type2} OPTIONAL,

dmrs-AdditionalPosition ENUMERATED {pos0, pos1, pos2, pos3} OPTIONAL,

phaseTrackingRS CHOICE {

setup PTRS-UplinkConfig

} OPTIONAL,

maxLength ENUMERATED {len1, len2} OPTIONAL,

transformPrecodingDisabled SEQUENCE {

scramblingID0 INTEGER (0..65535) OPTIONAL,

scramblingID1 INTEGER (0..65535) OPTIONAL

} OPTIONAL,

transformPrecodingEnabled SEQUENCE {

nPUSCH-Identity INTEGER (0..1007) OPTIONAL,

sequenceGroupHopping ENUMERATED {enabled, disabled} OPTIONAL,

sequenceHopping ENUMERATED {enabled, disabled} OPTIONAL

} OPTIONAL

}

3.2 PUCCH DMRS (Physical Uplink Control Channel DMRS)

PUCCH DMRS enables the gNB to demodulate uplink control information (UCI) transmitted by the UE, including HARQ-ACK, CSI reports, and scheduling requests.

Key Functions:

Control Channel Demodulation: Essential for decoding UCI carried on PUCCH
Channel Estimation: Provides channel reference for coherent demodulation of control information
Format Support: Supports PUCCH formats 1, 3, and 4 (long formats requiring coherent demodulation)

PUCCH Format and DMRS Usage

PUCCH Format	Duration	UCI Payload	DMRS Usage	Detection Method
Format 0	Short (1-2 symbols)	1-2 bits	No DMRS	Energy detection
Format 1	Long (4-14 symbols)	1-2 bits	DMRS present	Coherent demodulation
Format 2	Short (1-2 symbols)	>2 bits	No DMRS	Energy detection
Format 3	Long (4-14 symbols)	>2 bits	DMRS present	Coherent demodulation
Format 4	Long (4-14 symbols)	>2 bits	DMRS present	Coherent demodulation

PUCCH DMRS Characteristics

Time-Division Multiplexing: DMRS and UCI symbols are time-multiplexed in long PUCCH formats
DFT-s-OFDM Waveform: Based on DFT spread OFDM for better PAPR characteristics
Sequence Generation: Uses base sequences with cyclic shifts for orthogonality

4. Uplink DMRS vs Downlink DMRS: Key Differences

Design Considerations

Aspect	Downlink DMRS	Uplink DMRS
Power Limitations	gNB has high power budget	UE has limited power budget
PAPR Concerns	Less critical	Critical for UE battery life
Waveform Options	CP-OFDM only	CP-OFDM + DFT-s-OFDM
Antenna Ports	Up to 12 ports	Up to 4 ports (PUSCH)
Sequence Types	Gold sequences	Gold + Zadoff-Chu sequences

Downlink Physical Channels

Physical Channel	DMRS Antenna Ports	Port Range	Maximum Ports	Usage
PBCH	4000	Fixed single port	1	SS/PBCH block transmission
PDCCH	2000	Fixed single port	1	Control channel demodulation
PDSCH	1000-1011	12 ports available	8 (SU-MIMO), 12 (MU-MIMO)	Data channel demodulation

Uplink Physical Channels

Physical Channel	DMRS Antenna Ports	Port Range	Maximum Ports	Usage
PUSCH	0-11	12 ports available	4 typical, 12 maximum	Uplink data demodulation
PUCCH	2000-	Variable range	Format dependent	Uplink control demodulation

Transform Precoding Impact

For PUSCH with Transform Precoding Enabled (DFT-s-OFDM):

Different Sequence Generation: Uses Zadoff-Chu sequences instead of Gold sequences
Group/Sequence Hopping: Supports hopping for interference mitigation
Lower PAPR: Better for UE power amplifier efficiency

5. Conclusion

5G NR replaces LTE’s one-size-fits-all reference-signal scheme with a configurable toolbox that aligns pilot overhead to service needs, frequency band and antenna topology.