In 5G NR, the Resource Grid is the time-frequency grid over which all downlink physical channels and signals are mapped. One slot contains 14 OFDM symbols (normal cyclic prefix) and a bandwidth-dependent number of Resource Blocks (RBs), each spanning 12 subcarriers. This simulator visualizes how PDSCH data, PDSCH DMRS, CORESET (PDCCH region), PTRS, CSI-RS, and SSB coexist in a single downlink slot, with RE-level accuracy for all reference signals.

Resource Grid Structure (3GPP TS 38.211 §4.4)

A Resource Element (RE) is one subcarrier × one OFDM symbol. A Resource Block (RB) is 12 consecutive subcarriers. The number of RBs in a channel bandwidth is defined by the subcarrier spacing (SCS) according to 3GPP TS 38.101, e.g. 20 MHz / 30 kHz SCS → 51 RBs.

The Bandwidth Part (BWP) is the portion of the channel assigned to a UE: a contiguous set of RBs starting at startRB with numRBs width. The UE maps PDSCH data, DMRS, and PTRS only within this BWP.

PDSCH DMRS Type A (3GPP TS 38.211 §7.4.1.1)

Type A DMRS uses slot-based mapping, anchored at a fixed symbol l₀ within the slot:

• l₀ = 2 or 3: first DMRS symbol position (signalled in MIB/DCI).
• Additional positions (dmrs-AdditionalPosition): 0–3, adds DMRS on extra symbols to combat channel variation across the slot.
• AddPos 0 → symbols [l₀]; AddPos 1 → [l₀, 11]; AddPos 2 → [l₀, 7, 11]; AddPos 3 → [l₀, 5, 8, 11]

Type 1 (comb-2): DMRS occupies every other subcarrier within each RB. CDM group 0: subcarriers {0, 2, 4, 6, 8, 10} + Δ (mod 12). CDM group 1: subcarriers {1, 3, 5, 7, 9, 11} + Δ (mod 12). numCdmGroupsWithoutData = 1 reserves CDM group 0 only (6 DMRS REs/RB); = 2 reserves both groups (12 DMRS REs/RB, no data in DMRS symbols).

DMRS overhead per DMRS symbol per RB = 6 × numCdmGroups / 12

CORESET (3GPP TS 38.211 §7.3.2, TS 38.213 §10)

A CORESET (Control Resource Set) is a rectangular region in which the base station transmits PDCCH. It is defined by:

• Frequency domain: a 45-bit bitmap where each bit represents a 6-RB group. Bit 0 = RBs 0–5, bit 1 = RBs 6–11, …
• Duration: 1, 2, or 3 OFDM symbols starting at startSymbol (typically 0).
• REG / CCE structure: each REG = 1 RB × 1 symbol (12 REs). 6 REGs = 1 CCE. PDCCH candidates are made of 1, 2, 4, or 8 CCEs.
• PDCCH DMRS: on subcarriers 1, 5, 9 within every REG (3 REs/REG, 3/12 = 25%).

CORESET REs are not available for PDSCH (TS 38.214 §5.1.4). If the allocated BWP overlaps the CORESET, those REs are excluded from PDSCH mapping.

PTRS — Phase Tracking Reference Signal (3GPP TS 38.211 §7.4.1.2)

PTRS compensates for oscillator phase noise, which dominates at high SCS (mmWave). It is transmitted at configurable time density (every 1st, 2nd, or 4th OFDM symbol) and frequency density (every 2nd or 4th RB), using one RE per RB group per symbol.

CSI-RS — Channel State Information Reference Signal (3GPP TS 38.211 §7.4.1.5)

CSI-RS is a downlink reference signal used by the UE for channel measurement and feedback (CQI, PMI, RI). Its RE positions within each RB are defined by Table 7.4.1.5.3-1, which specifies rows for different port counts and CDM types:

• Row 1: 1 port, density ρ=3 — 3 REs/RB at k₀, k₀+4, k₀+8 (mod 12), single symbol.
• Row 2: 1 port, density ρ=1 — 1 RE/RB at k₀, single symbol.
• Row 3: 2 ports, FD-CDM2 — 2 adjacent REs {k₀, k₀+1}, single symbol.
• Row 4: 4 ports, FD-CDM2 — 4 consecutive REs {k₀…k₀+3}, single symbol.
• Row 5: 4 ports, FD-CDM2 — 2 REs × 2 symbols {k₀, k₀+1} × {l₀, l₀+1}.
• Row 6: 8 ports, FD-CDM2 — 8 consecutive REs {k₀…k₀+7}, single symbol.
• Row 7: 8 ports, FD-CDM2 — 4 REs × 2 symbols {k₀…k₀+3} × {l₀, l₀+1}.
• Row 8: 8 ports, CDM4-FD2-TD2 — same RE positions as Row 7; port separation achieved via OCC in both frequency and time.

CSI-RS REs are rendered as thin teal marks at their exact subcarrier positions within each RB, so the density and pattern are visible in all grid views. The k₀ slider is automatically limited to the maximum value that keeps all RE offsets within the RB (0–11).

SSB — Synchronization Signal Block (3GPP TS 38.211 §7.4.3)

The SSB occupies 4 consecutive OFDM symbols × 20 RBs (240 subcarriers) and contains three physical channels/signals:

• PSS (Primary Synchronization Signal): symbol 0, subcarriers 56–182 of the SSB (127 REs). Guard subcarriers 0–55 and 183–239 are unused.
• SSS (Secondary Synchronization Signal): symbol 2, subcarriers 56–182.
• PBCH (Physical Broadcast Channel): symbols 1 and 3 (all 240 SCs); symbol 2 outside the SSS window (SCs 0–55, 183–239).
• PBCH DMRS: embedded within PBCH symbols at every 4th subcarrier starting from v = N_ID^cell mod 4, providing 60 DMRS REs across the PBCH.

The SSB start RB and start symbol are configurable. A crimson dashed outline marks the SSB boundary in the BWP Grid and Full Channel views; a shaded band appears in the Spectrum view.

RE Overhead Formula

Overhead = (DMRS + CORESET + PTRS + CSI-RS + SSB REs that overlap BWP) / Total REs

where Total REs = numRBs × 12 × 14 per slot. The stats bar above the legend shows a live RE count breakdown for every signal type.

 

Usage

1. BW / SCS: Select channel bandwidth (5–100 MHz) and subcarrier spacing (15/30/60 kHz). Invalid combinations are automatically disabled. The max-RBs table updates accordingly.
2. Start RB / Num RBs: Define the allocated Bandwidth Part. The slider max adjusts to prevent overflow beyond the channel edge.
3. Preset: Quick-select a complete configuration for common 5G scenarios (20 MHz, 50 MHz, 100 MHz, PTRS demo).
4. DMRS l₀ / AddPos: Control where in the slot DMRS symbols appear. AddPos 0 = single DMRS symbol; AddPos 3 = four DMRS symbols spread across the slot.
5. DMRS Type / CDM groups / Δshift: Type 1 (comb-2) vs Type 2 (comb-3). CDM groups = how many groups of subcarriers are reserved as DMRS-only (no data). Δshift applies a per-cell frequency offset to the DMRS pattern.
6. CORESET: Check “Enable” to overlay a CORESET region. Edit the 45-bit bitmap to select 6-RB groups (each ‘1’ bit = one 6-RB group). Duration and Start Symbol control the time extent. The CORESET tab shows the full CCE/REG breakdown.
7. PTRS: Enable to see phase tracking REs. Adjust time and frequency density to observe the trade-off between overhead and phase tracking capability.
8. CSI-RS Row: Select from Rows 1–8 (TS 38.211 Table 7.4.1.5.3-1) to configure port count, density, and CDM type. k₀ sets the starting subcarrier offset within each RB (range auto-limited per row). l₀ sets the first symbol. CSI-RS REs are drawn as thin teal marks at their exact RE positions rather than colouring the whole RB, so the density pattern is directly visible.
9. SSB: Enable to overlay an SSB. Start RB sets the lowest RB of the 20-RB SSB block; Start Sym sets the first of its 4 symbols. N_ID mod 4 (v) controls the PBCH DMRS subcarrier offset within each PBCH symbol. A crimson dashed outline marks the SSB boundary in the grid views.

Tabs

• BWP Grid: The allocated resource blocks only (Start RB to Start RB + Num RBs − 1), 14 symbols tall. DMRS symbols show a split green/blue cell proportional to the DMRS density. CSI-RS appears as thin teal marks at the exact subcarrier positions within each RB. SSB and CORESET regions are outlined with crimson and red dashed borders respectively.
• Full Channel: All RBs across the full channel bandwidth. A white rectangle highlights the allocated BWP. CSI-RS RE-level marks, SSB colours, and CORESET outlines are all visible in context of the complete channel. Useful for checking guard-band placement and SSB/BWP frequency alignment.
• Detail View: Subcarrier-resolution zoom into the first N allocated RBs (up to 12). Each cell is one RE (1 subcarrier × 1 symbol). Use the “RBs to show” slider to adjust the zoom. All signal types — including individual CSI-RS and SSB REs — are rendered at full RE resolution here.
• Spectrum: Simulated power spectral density. The allocated BWP is shown as a blue fill, the SSB band as a crimson overlay. Green dashed lines = channel BW edges; red dashed lines = allocated BW edges. A text summary shows RB range, allocation bandwidth, and SSB frequency span.
• CORESET: CCE/REG structure of the CORESET. Each cell is one REG (1 RB × 1 symbol). Colors distinguish CCE membership (non-interleaved, sequential). Gold stripes show PDCCH DMRS positions (SC 1, 5, 9 per RB). A dynamic legend below the canvas lists all CCEs.

Key 3GPP References

• TS 38.211 §4.4 — Physical resource grid definition, slot structure, normal CP.
• TS 38.211 §7.3.2 — PDCCH and CORESET RE mapping, PDCCH DMRS positions.
• TS 38.211 §7.4.1.1 — PDSCH DMRS Type A/B, Type 1/2 mapping, CDM groups, OCC.
• TS 38.211 §7.4.1.2 — PTRS for PDSCH, time/frequency density configuration.
• TS 38.211 §7.4.1.5, Table 7.4.1.5.3-1 — CSI-RS RE mapping, rows 1–18, CDM types, density.
• TS 38.211 §7.4.3 — SSB structure: PSS (symbol 0), SSS (symbol 2), PBCH (symbols 1–3), PBCH DMRS pattern.
• TS 38.213 §10 — PDCCH search space, CORESET configuration signalling.
• TS 38.214 §5.1.4 — PDSCH RE mapping order and reserved RE exclusion (CORESET, CSI-RS).
• TS 38.101 — Maximum RBs per channel bandwidth and SCS combination.