Orthogonal subcarriers · Cyclic prefix · Multipath · 802.11
WiFi doesn't use one radio carrier — it uses hundreds, all transmitting at once. OFDM (Orthogonal Frequency-Division Multiplexing) packs data onto dozens of narrow orthogonal subcarriers simultaneously, achieving very high spectral efficiency while remaining robust to multipath fading.
A set of N sinusoidal subcarriers, each at a different frequency fn = n·Δf where Δf = 1/Tsymbol, are mathematically orthogonal: their cross-products integrate to zero over one symbol period. This means each subcarrier can be decoded independently despite overlapping spectra. The cyclic prefix — a copy of the last part of the IFFT symbol prepended at the start — acts as a guard interval, converting linear convolution (multipath) into circular convolution and allowing a simple 1-tap equaliser per subcarrier. 802.11g (WiFi 4) uses a 64-point FFT, 48 data subcarriers, 4 pilots, 16-sample CP, at 20 MHz bandwidth.
Switch between WiFi modes to see how the 52 active subcarriers are arranged, with null subcarriers around DC and at the band edges (spectral mask). Turn on Multipath delay and increase the delay slider — with CP off, ISI ruins the orthogonality (bar heights distort); with CP on, each subcarrier is still cleanly separated. Increase Noise to watch how 64-QAM needs high SNR (>25 dB) while BPSK works down to ~5 dB.
Every smartphone WiFi chip contains a 64-point FFT/IFFT running at 20 million symbols per second. WiFi 6 (802.11ax) increased to a 1024-point FFT with 980 data subcarriers in an 80 MHz channel. 5G NR also uses OFDM — with configurable subcarrier spacings of 15, 30, 60, 120, or 240 kHz depending on the frequency band. Robert Chang's 1966 Bell Labs paper is considered one of the foundational multicarrier works.