OFDM System Design

OFDM System Design

When To Use

Use this skill when the user wants to build a custom (non-standard) OFDM transmitter/receiver, configure subcarrier allocation, add noise to OFDM signals, equalize OFDM through fading channels, implement synchronization, or add LDPC coding.

When Not To Use

Do NOT use this skill for standards-specific OFDM (use 5G Toolbox for 5G NR, WLAN Toolbox for Wi-Fi, LTE Toolbox for 4G/LTE, Bluetooth Toolbox for Bluetooth, Satellite Communications Toolbox for satellite links).

GATE — Ask Before Coding

If the user's request does not specify or unambiguously imply ALL of the following, STOP and ask before generating any code. Present unclear items as a numbered list and ask whether the user wants to: (a) specify values, (b) have you derive them from other constraints (e.g., CP from delay spread, SCS from Doppler), or (c) use typical defaults. Do not assume defaults. Do not proceed until the user responds.

1. OFDM parameters — FFT size, CP length, subcarrier spacing, subcarrier allocation
2. Channel model — AWGN only, fading with perfect CSI, or fading with pilot-based estimation
3. Pilots — needed for channel estimation or phase tracking?
4. Coding — uncoded, or coded with a specific rate?
5. Synchronization — perfect sync, or include timing/CFO estimation?

Must-Follow Rules

1. ALWAYS use ofdmmod/ofdmdemod — NEVER implement OFDM by directly calling ifft/fft — The functions handle CP insertion/removal, ifftshift/fftshift for centered-frequency ordering, and pilot/null subcarrier management. Direct IFFT/FFT gets subcarrier mapping wrong (silent error). Input is [nDataSC × nSym] for SISO. Output is a time-domain column vector. Note: sync preambles (e.g., Schmidl-Cox) that require DFT-order subcarrier mapping may use ifft directly, since ofdmmod applies ifftshift which changes the even/odd bin assignment. Windowing: ofdmmod does not support windowing. For raised cosine windowing, use comm.OFDMModulator for the transmitter only (always use ofdmdemod function for demodulation). See references/ofdm-system-guide.md for capability comparison.

2. Always use a symOffset variable in ofdmdemod — when passing nullidx (5th arg), symOffset must be explicitly provided as the 4th arg. The default symOffset value is cpLen (skip entire CP). Note: cpLen/2 is more robust when timing synchronization is imperfect.

3. Use explicit signal power in awgn, NEVER 'measured' or 0 — ofdmmod output power is NOT 1W. It equals nActiveSC / nFFT^2 due to MATLAB's 1/N IFFT normalization. Using awgn(x, snr, 0) adds far too much noise. Compute sigPow = 10*log10(nActiveSC / nFFT^2) once before any loop — it is a constant determined by OFDM parameters.

4. Use ofdmChannelResponse for frequency-domain channel response — do NOT compute H = fft(h, nFFT) directly. ofdmmod uses ifftshift internally, so direct FFT-based channel responses have wrong subcarrier mapping. Pass the result to ofdmEqualize for equalization.

5. When deriving OFDM parameters from physical specs, show the computation with comments — do not just state the values. Show how you calculate subcarrier spacing, FFT size, CP length, sample rate, guard subcarriers, etc. from bandwidth, delay spread, and Doppler spread. Add comments explaining each derivation step.

6. Check MATLAB version before using version-gated APIs — Functions like ofdmChannelEstimate, ofdmPilotConfig (R2026a), and ldpcPCM (R2025a) are not available on older releases. Unless the user specifically asks for code compatible with a previous version, call version('-release') to get the current MATLAB release, then choose the appropriate API path.

Key Functions