Split one wide channel into orthogonal OFDM subcarriers in the sandbox, and predict — before you read it off the screen — the subcarrier spacing, the useful symbol time, and the cyclic-prefix guard that lets a signal survive multipath.
Where FDMA, TDMA and CDMA split a channel by frequency, time or code, OFDM splits it into hundreds of narrow, overlapping-but-orthogonal subcarriers — and the whole numerology follows from just two knobs: the sample rate fs and the FFT size N. The subcarriers sit Δf = fs/N apart, each symbol lasts Tu = 1/Δf, and a cyclic prefix of Tcp = Ncp/fs is prepended as a guard so echoes shorter than it cannot smear one symbol into the next. Reading those three numbers off the two knobs is how you know what an OFDM system costs and what it buys.
Open the sandbox and pick the 20 MHz Wi-Fi (802.11a/g) preset from the dropdown — it reproduces the IEEE 802.11a numerology exactly. Each step names the one readout to read; leave every knob alone.
Preset -> 20 MHz Wi-Fi (802.11a/g) Sample rate fs -> 20 MHz
FFT size N -> 64 Cyclic prefix Ncp -> 16 samples
Watch the numerology readout — the subcarrier spacing Δf, the useful symbol time Tu and the cyclic-prefix time Tcp are each printed there.
With the 20 MHz Wi-Fi (802.11a/g) preset, the sample rate is fs = 20 MHz and the FFT size is N = 64. The subcarriers sit Δf = fs/N apart. Predict that spacing in Hz, then read the Δf readout.
The readout shows Δf = fs/N = 20 MHz / 64 = 312500 Hz (312.5 kHz) — the spacing at which the subcarriers are exactly orthogonal over one symbol.
Each subcarrier carries one symbol per FFT window, and that window lasts Tu = 1/Δf = N/fs. Predict the useful symbol time in microseconds, then read the Tu readout.
The readout shows Tu = 1/Δf = 1/312500 = 3.2 µs — the exact reciprocal of the subcarrier spacing, and the length of the FFT window the receiver integrates over.
The transmitter prepends Ncp = 16 samples as a cyclic prefix — a guard interval so echoes shorter than it cannot smear one symbol into the next. Its duration is Tcp = Ncp/fs. Predict it in microseconds, then read the Tcp readout.
The readout shows Tcp = Ncp/fs = 16 / 20 MHz = 0.8 µs — any multipath echo with a delay under 0.8 µs falls inside the guard and causes no inter-symbol interference.
Everything above is waiting in the sandbox. Drag the FFT size N and watch the spacing Δf and the symbol time Tu move in opposite directions, switch between the Wi-Fi, LTE and 5G NR presets to compare their numerologies, and lengthen the channel's delay spread until an echo spills past the cyclic prefix and inter-symbol interference appears.
- Read the subcarrier spacing Δf = fs/N = 312500 Hz off two knobs
- Found the useful symbol time Tu = 1/Δf = 3.2 µs, the FFT window
- Read the cyclic-prefix guard Tcp = Ncp/fs = 0.8 µs against multipath — so a full symbol is Tsym = Tu + Tcp = 4.0 µs, a 20% overhead
- Every value you predicted is the demo's own OFDM numerology, not a picture