This simulator models the complete downlink physical-layer chain for a 5G NR PDSCH transmission: resource grid construction, radio channel propagation (AWGN, TDL, CDL), channel estimation, equalization, and BER measurement using 16-QAM. All signal processing runs entirely in the browser.

5G NR Resource Grid (3GPP TS 38.211 §4.4)

One slot contains 14 OFDM symbols (normal cyclic prefix) and N_RB resource blocks (RBs), each spanning 12 subcarriers. The subcarrier spacing (SCS) determines the number of RBs per channel bandwidth (TS 38.101 Table 5.3.2-1). Within a slot, two RE types coexist:

• PDSCH data REs: carry 16-QAM modulated payload bits.
• DMRS REs: carry known reference sequences used by the receiver to estimate the channel.

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

DMRS Type A anchors the first pilot symbol at l₀ ∈ {2, 3} within the slot. Additional DMRS positions are controlled by dmrs-AdditionalPosition:

• AddPos 0 → 1 DMRS symbol at [l₀]
• AddPos 1 → 2 symbols at [l₀, 11]
• AddPos 2 → 3 symbols at [l₀, 7, 11]
• AddPos 3 → 4 symbols at [l₀, 5, 8, 11]

Two frequency-domain types:

• Type 1 (comb-2): DMRS occupies every other subcarrier. CDM group 0: even subcarriers; CDM group 1: odd. Setting numCdmGroups = 1 reserves 6 DMRS REs/RB; numCdmGroups = 2 reserves all 12 subcarriers in the DMRS symbol.
• Type 2 (comb-3): 4 DMRS REs per CDM group per RB, up to 3 groups. Δshift = 0 or 1 applies a per-cell frequency offset.

Radio Channel Models

The channel between transmitter and receiver is modeled at the resource-grid level by multiplying each RE's complex symbol by a fading coefficient H[s,k] and adding AWGN noise. Three model families are supported:

• AWGN: H[s,k] = 1 ∀ s,k. Only thermal noise is added (no multipath). Serves as ideal baseline.
• TDL (Tapped Delay Line, TS 38.901 §7.7.2): L multipath taps with 3GPP-standardized delay/power profiles (TDL-A through TDL-E). Each tap contributes a complex exponential phase shift Δφ = −2πkΔfτ_n to subcarrier k, producing realistic frequency-selective fading. Doppler spread is modeled as a time-varying phase based on UE velocity and carrier frequency.
• TDL User: Same TDL model with user-defined tap delays and power levels, enabling custom channel profiles.
• CDL (Clustered Delay Line, TS 38.901 §7.7.1): More sophisticated model with clustered scatterers and angular spread. Simulated here with profile-appropriate delay/power sets.

Signal-to-Noise Ratio (SNR)

SNR = E_s/N₀ [dB]  →  σ_noise = 1/√(2 · SNR_linear)

For a given SNR, the complex noise standard deviation is σ = 1/√(2·SNR). At higher SNR, noise variance is small and channel estimates are accurate; at low SNR, estimation errors degrade equalization performance.

Channel Estimation (TS 38.211 §7.4.1.1)

The receiver exploits knowledge of DMRS symbols to estimate the channel at pilot REs:

• LS (Least Squares): &Ĥ;_LS = Y / X at each DMRS RE, where Y is the received symbol and X is the known pilot. Unbiased but noise-limited: σ²_err = σ²/|X|².
• MMSE (Minimum Mean Square Error): &Ĥ;_MMSE = (SNR / (SNR+1)) · &Ĥ;_LS. Diagonal MMSE suppresses noise by trading a small bias for reduced variance. Optimal when the channel covariance is diagonal (i.i.d. taps).

After pilot estimation, channel coefficients are interpolated to all data REs using natural cubic spline interpolation in frequency and nearest-DMRS-symbol copy in time.

Equalization

Given the interpolated channel estimate Ĥ[s,k] and the received RE Y[s,k], the equalizer recovers the transmitted symbol X̂[s,k]:

• ZF (Zero Forcing): X̂ = Y / Ĥ. Inverts the channel exactly but amplifies noise when |Ĥ| is small.
• MMSE: X̂ = Ĥ^* · Y / (|Ĥ|² + σ²). Regularizes the ZF solution, trading residual ISI for reduced noise enhancement. Optimal in the MMSE sense for frequency-flat channels.

16-QAM Modulation and BER

PDSCH data is 16-QAM modulated with Gray coding. The 16 symbols occupy a 4×4 grid in the IQ plane at levels {−3, −1, +1, +3} · (1/√10) for unit average power. BER is measured by hard-decision demodulation after equalization:

BER = (number of incorrectly decoded symbols) / (total data symbols)

Spectral Analysis

When Time-Domain is enabled, the resource grid is converted to a time-domain OFDM signal via per-symbol IFFT (OFDM modulation). A Welch PSD estimate (Hann windowed, 50% overlap) is then computed and displayed as a power spectrum plot and per-symbol spectrogram.

 

Usage

1. BW / SCS: Select channel bandwidth and subcarrier spacing. The Num RBs slider maximum updates automatically according to the TS 38.101 table.
2. Start RB / Num RBs: Define the allocated BWP. Start RB is the lowest allocated RB index; Num RBs is the count of RBs in the allocation.
3. DMRS l₀ / AddPos: Control how many DMRS symbols appear in the slot (1–4). More DMRS symbols give better time-varying channel tracking but reduce data capacity.
4. DMRS Type / CDM groups: Type 1 (comb-2) has lower pilot density per RB; Type 2 (comb-3) has higher density. CDM groups controls how many subcarrier groups are reserved for DMRS only.
5. AWGN (dB): Signal-to-noise ratio (E_s/N₀) in dB. Observe how BER changes as SNR increases (AWGN reference) or how fading makes it BER-floor limited.
6. Est / EQ Method: Compare LS vs MMSE channel estimation and ZF vs MMSE equalization. MMSE is more noise-robust; ZF is simpler but amplifies noise at frequency nulls.
7. Channel Type (Advanced): Switch between AWGN (flat), TDL (standard 3GPP profiles A–E), TDL User (custom taps), and CDL. Higher delay spread profiles (TDL-C, TDL-E) create more frequency-selective channels.
8. Velocity / Carrier Freq (Advanced): Control Doppler spread. High velocity + high carrier frequency (mmWave) creates rapid channel time variation, stressing time-domain interpolation.
9. TDL User taps (Advanced): When TDL User is selected, configure individual tap delays (ns) and power levels (dB) and enable/disable each tap to design a custom multipath profile.
10. Time-Domain (Advanced): Enable to compute the OFDM time-domain signal via IFFT and display PSD and per-symbol spectrogram. This is computationally heavier.
11. Fixed Seed: Check to use a fixed random seed for reproducible results. Uncheck for a new random realization on each run.
12. Run Simulation: Press to run with current parameters. Results are displayed in the active tab instantly.

Visualization Tabs

• Tx Grid (Mag): Resource grid of the transmitted signal. Color maps to |RE symbol| magnitude. Green vertical lines mark DMRS symbol boundaries. DMRS symbols appear uniform (constant-power pilots); PDSCH symbols show random 16-QAM magnitudes.
• Tx Grid (Phase): Same grid, colored by symbol phase (HSL hue). DMRS phases follow a deterministic pattern; PDSCH phases are random QAM states.
• Tx Spectrum: Welch PSD of the OFDM time-domain signal (requires Time-Domain enabled). The top plot shows the power spectral density in dB; the bottom shows a per-symbol spectrogram.
• Rx Spectrum: Same analysis on the received (channel + noise impaired) signal. Compare with Tx Spectrum to visualize channel frequency response and noise floor.
• Rx Grid: Resource grid of the received signal magnitude. Under multipath fading, frequency-selective deep fades appear as dark columns; AWGN adds uniform gray noise.
• Ch Coef (Raw): Channel coefficient magnitudes estimated only at DMRS pilot REs using LS or MMSE. Non-pilot positions are black (no estimate). Shows the sparse pilot grid.
• Ch Coef (Interp): Channel coefficients after cubic spline interpolation across all subcarriers and nearest-neighbor copy across all symbols. This is the Ĥ used by the equalizer.
• Constellation: IQ scatter plots for six signal states. Use sub-tabs to switch view. Tx PDSCH and Tx DMRS show ideal constellations; Rx plots show the effect of channel + noise; EQ plots show how equalization restores the constellation. Reference 16-QAM decision points are shown as gray dots on Tx PDSCH and equalized Rx PDSCH views.

Key 3GPP References

• TS 38.211 §4.4 — Physical resource grid, slot structure, normal cyclic prefix.
• TS 38.211 §7.4.1.1 — PDSCH DMRS Type A/B mapping, CDM groups, additional positions.
• TS 38.214 §5.1 — PDSCH transmission, MCS, modulation order, resource mapping.
• TS 38.101 — Maximum RBs per channel bandwidth and SCS.
• TS 38.901 §7.7 — TDL and CDL channel models, tap delay/power profiles, Doppler.