In 4G LTE, the Downlink Resource Grid is the time-frequency grid over which all downlink physical channels and signals are mapped. One radio frame contains 10 subframes; each subframe has 14 OFDM symbols (normal cyclic prefix). A Resource Block (RB) spans 12 subcarriers. This simulator visualizes one radio frame (10 subframes) for 6 RB (1.4 MHz) channel bandwidth, showing PDSCH, CRS, PSS, SSS, PBCH, PBCH DMRS, PCFICH, PHICH, and PDCCH with RE-level accuracy.

Resource Grid Structure (3GPP TS 36.211 §6.2)

A Resource Element (RE) is one subcarrier × one OFDM symbol. A Resource Block (RB) is 12 consecutive subcarriers. For 6 RB (1.4 MHz) channel bandwidth, the grid contains 72 subcarriers × 14 symbols per subframe.

Parameters affecting the grid: N_RB^DL × 12 = total subcarriers; N_ID^cell (0–503) controls CRS and PBCH DMRS frequency offsets; CFI (1–3) sets the control region duration.

CRS — Cell-Specific Reference Signal (TS 36.211 §6.10.1)

CRS is used for channel estimation and demodulation. With v = N_ID^cell mod 6, subcarrier positions use k = 6m + v_shift:

• Ports 0 & 1: symbols {0, 4, 7, 11}; v_shift = v (Port 0) and (v+3) mod 6 (Port 1) for frequency orthogonality.
• Ports 2 & 3: symbols {1, 8}; v_shift = (v+1) mod 6 (Port 2) and (v+4) mod 6 (Port 3) for frequency orthogonality.

REs where other antenna ports transmit CRS are shown as CRS Reserved (muted) and are not available for PDSCH.

PSS & SSS (TS 36.211 §6.11)

PSS (Primary Synchronization Signal): symbol 6 in subframes 0 and 5; 62 center subcarriers (k = 5–66).

SSS (Secondary Synchronization Signal): symbol 5 in subframes 0 and 5; same 62 subcarriers. PSS and SSS enable 5 ms timing and cell ID detection.

PBCH (Physical Broadcast Channel, TS 36.211 §6.6)

PBCH carries MIB in subframe 0 only, symbols 7, 8, 9, 10. PBCH DMRS: every 4th subcarrier starting at v = N_ID^cell mod 4, providing 18 DMRS REs per symbol. Remaining REs carry PBCH data.

Control Region (TS 36.211 §6.3)

The first 1–3 OFDM symbols (CFI) form the control region:

• PCFICH: 4 REGs (16 REs) in symbol 0; carries CFI.
• PHICH: carries HARQ ACK/NACK.
• PDCCH: carries DCI scheduling assignments.

1 REG = 4 REs. In 6 RB we have 18 REGs per symbol. Control REs are excluded from PDSCH mapping.

PDSCH (Physical Downlink Shared Channel)

PDSCH occupies all REs not used by CRS, PSS, SSS, PBCH, or the control region. It carries user data and higher-layer control.

RE Overhead Formula

Overhead = (CRS + CRS Reserved + PSS + SSS + PBCH + Control) / Total REs

where Total REs = 72 × 140 = 10080 per radio frame. CRS Reserved counts muted REs for multi-antenna configurations.

 

Usage

1. MIMO: Antenna configuration (1×1, 2×2, or 4×4). Displays one resource grid per antenna port; each port shows its own CRS pattern and muted REs for other ports (TS 36.211 §6.10.1). In the Spectrum tab, a separate PSD plot is shown for each antenna port.
2. CFI: Control Format Indicator (1–3). Sets how many OFDM symbols are used for the control region (PCFICH, PHICH, PDCCH) in each subframe.
3. N_ID^cell: Physical layer cell identity (0–503). Affects CRS subcarrier offset (v_shift derived from v = N_ID^cell mod 6) and PBCH DMRS offset (every 4th subcarrier from v = N_ID^cell mod 4).
4. Magnification: Zoom the grid from 1× to 10×. Scrollbars appear when magnification > 1.
5. Show Channels: Toggle visibility of PDSCH, PSS, SSS, PBCH, CRS, and control region (PCFICH/PHICH/PDCCH) in the grid.
6. Resource Grid / Spectrum: Switch between the time-frequency resource grid view and the Welch-based PSD plot. The Spectrum tab converts the grid to OFDM time-domain via IFFT, then computes power spectral density (Hann window, 50% overlap) for a spectrum-analyzer-style display. Scope: Choose Radio Frame (full frame), Sub Frame (0–9), or OFDM Symbol (0–139). When Sub Frame or OFDM Symbol is selected, use the slider to select which subframe or symbol to analyze. With MIMO, a separate spectrum plot is shown for each antenna port.

Key 3GPP References

• TS 36.211 §6.2 — Physical resource grid, resource elements, resource blocks.
• TS 36.211 §6.3 — Control region, PCFICH, PHICH, PDCCH mapping.
• TS 36.211 §6.6 — PBCH and PBCH DMRS.
• TS 36.211 §6.10.1 — Cell-specific reference signals (CRS).
• TS 36.211 §6.11 — PSS and SSS.
• TS 36.101 — Channel bandwidths and RB counts (6 RB = 1.4 MHz).