Two Configuration Types
Type 1: 6 SCs per port per OFDM symbol (every other SC). Type 2: 4 SCs per port per OFDM symbol (4 of 12). Type 1 supports up to 8 ports; Type 2 supports up to 12 ports. Choice signaled by RRC.
Single-Symbol vs Double-Symbol DMRS
Single-symbol DMRS: 1 OFDM symbol. Lower overhead, lower channel-estimation accuracy. Double-symbol DMRS: 2 consecutive OFDM symbols. Higher overhead, better accuracy in fast-fading channels.
Front-Loaded DMRS
NR places the first DMRS at the start of the slot (symbol 2 or symbol 3) — "front-loaded." This lets the UE start decoding as data arrives, vs LTE where pilots were spread across the slot.
Additional DMRS for High Mobility
For high-mobility UEs (high Doppler), NR adds extra DMRS later in the slot. dmrs-AdditionalPosition: pos0 (no additional), pos1 (1 extra), pos2 (2 extra), pos3 (3 extra). pos3 has the most pilots, used at very high speeds.
CDM Groups · Port Multiplexing
CDM (Code-Division Multiplexed) groups: ports within a group share the same SC positions but use different orthogonal cover codes (length-2 or length-4 OCCs). This lets multiple ports share REs without inter-port interference.
Port Number Space
PDSCH DMRS: ports 1000-1011 (Type 1) or 1000-1023 (Type 2). PDCCH DMRS: port 2000. PUSCH DMRS: ports 0-11 (Type 1). PT-RS DMRS: lives within the same RS family but offset.
How the UE Uses DMRS
The UE computes the channel estimate H_p from the received DMRS REs. Since DMRS is UE-specific (uses a UE-specific scrambling sequence derived from C-RNTI), it doesn't interfere with other UEs' pilots. UE applies H_p to data REs on the same antenna port — that's why "antenna port" is defined relative to its DMRS in TS 38.211.
Frequently Asked Questions
What is DMRS in 5G NR?
DMRS (Demodulation Reference Signal) is the pilot pattern the UE uses to estimate the channel for demodulating PDSCH, PDCCH, PUSCH, and PUCCH. It is UE-specific and beam-specific in 5G NR (unlike LTE's cell-specific RS).
What is the difference between DMRS Type 1 and Type 2?
Type 1 uses 6 subcarriers per port per OFDM symbol (every other SC) and supports up to 8 antenna ports. Type 2 uses 4 subcarriers per port per OFDM symbol (4 of 12) and supports up to 12 antenna ports.
Why does 5G NR use front-loaded DMRS?
Front-loaded DMRS (placed at the start of the slot) lets the UE begin channel estimation immediately as data arrives — reducing decoding latency by ~50% compared to LTE's spread-out pilot pattern.
When is additional DMRS used?
Additional DMRS positions later in the slot are used for high-mobility UEs (high Doppler). The dmrs-AdditionalPosition field signals 0-3 additional positions; pos3 (3 extra) is used at vehicular/high-speed scenarios.