Barker, CCK, OFDM and OFDMA Compared

Four generations of Wi-Fi signals

Barker (802.11, 1 and 2 Mbps) and CCK (802.11b, 5.5 and 11 Mbps) are single-carrier, spread-spectrum methods: one fast stream of short pulses called chips fills the whole 22 MHz channel. OFDM (802.11a/g/n/ac) does the opposite: it splits the channel into dozens of narrow subcarriers, each sending slowly, all at the same time. OFDMA (802.11ax, Wi-Fi 6) keeps OFDM but gives groups of subcarriers, called resource units, to different users in the same transmission.

Pick a method below. The first panel shows how it builds its signal; the spectrum and echo panels compare all four.

2 · Spectrum: how the energy spreads across the channel

Spreading chips at 11 million a second gives Barker and CCK a rounded, 22 MHz-wide hump. OFDM's subcarriers sit side by side, so its spectrum is flat on top with steep edges, and 802.11ax packs four times as many, narrower subcarriers into almost the full 20 MHz.

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3 · Echoes: what multipath does to each method

Reflections from walls, racks and ceilings arrive later than the direct signal. If an echo of one symbol is still arriving when the next symbol starts, the two blur together: inter-symbol interference. Short symbols suffer most; OFDM adds a guard interval in front of each symbol to soak up echoes.

Echo arrives later by

Typical places

802.11ax guard interval

Symbol (direct path); the thinner bar below is the echoGuard interval, echo absorbedEcho overlap handled by the Barker codeInter-symbol interference

4 · Side by side

	Barker	CCK	OFDM	OFDMA
Standard	802.11 (1997)	802.11b (1999)	802.11a (1999), 802.11g (2003); also the basis of 802.11n and ac	802.11ax, Wi-Fi 6 (2021; certified from 2019) and 802.11be, Wi-Fi 7
Band	2.4 GHz	2.4 GHz	5 GHz (a), 2.4 GHz (g)	2.4, 5 and 6 GHz
Data rates	1, 2 Mbps	5.5, 11 Mbps	6 to 54 Mbps (802.11a/g)	Up to 143 Mbps per stream in 20 MHz, shared between users
How bits are sent	DBPSK or DQPSK: 1 or 2 bits per symbol, spread over 11 chips	One of 16 or 256 8-chip codewords: 4 or 8 bits per symbol	BPSK to 64-QAM on each of 48 data subcarriers	BPSK to 1024-QAM (4096-QAM in Wi-Fi 7) on up to 234 data subcarriers
Symbol time	1 µs	0.727 µs	4 µs (3.2 µs + 0.8 µs guard)	13.6 to 16 µs (12.8 µs + 0.8, 1.6 or 3.2 µs guard)
Width on the air	≈ 22 MHz	≈ 22 MHz	16.6 MHz (52 subcarriers × 312.5 kHz)	19.1 MHz (242 tones × 78.125 kHz)
Peak bits per Hz	0.09 (2 Mbps ÷ 22 MHz)	0.5 (11 ÷ 22)	2.7 (54 ÷ 20)	7.2 per stream (143.4 ÷ 20)
Handling echoes	Code correlation: 10.4 dB processing gain suppresses delayed copies	Short symbols, little processing gain; needs an equalizer beyond about 100–200 ns	0.8 µs guard interval (0.4 µs short GI in 802.11n/ac)	0.8, 1.6 or 3.2 µs guard interval
Users per transmission	1	1	1	Up to 9 in 20 MHz (37 in 80 MHz, 74 in 160 MHz)

Notes and simplifications

Chips are sent at 11 Mchip/s (90.9 ns each) for both Barker and CCK. Barker spreads each symbol over 11 chips (1 µs); CCK uses 8 chips per symbol (727 ns), 1.375 million symbols a second. The chip values shown are the complex chips before pulse shaping; phases are differential, starting from a reference phase of 0°, and CCK adds the standard's extra 180° on odd-numbered symbols.
OFDM here is the 802.11a/g 20 MHz symbol: 48 data and 4 pilot subcarriers (at ±7 and ±21) 312.5 kHz apart, a 3.2 µs symbol and a 0.8 µs guard interval that repeats the symbol's last 0.8 µs (cyclic prefix). Scrambling, coding and interleaving are left out, so the subcarrier values are random points of the chosen modulation.
OFDMA shows one 20 MHz 802.11ax downlink transmission with one spatial stream, a 0.8 µs guard interval and the standard resource-unit sizes (26, 52, 106 and 242 tones with 24, 48, 102 and 234 data tones). Here 1024-QAM (MCS 10 and 11) is only offered on the 242-tone RU; the standard allows it on smaller RUs too, but only when the receiving client says it supports that (IEEE 802.11-2024, 26.15.3). The preamble length is not to scale.
Spectra are the ideal, unfiltered shapes. Real transmitters filter the edges to stay inside the dashed spectral mask of each standard.
Echoes are one reflection at the chosen delay. Real channels have many, and receivers use equalizers and rake receivers, so the Barker and CCK verdicts are rules of thumb. Light travels about 0.3 m per nanosecond, so a 400 ns echo has taken a path about 120 m longer.

Four generations of Wi-Fi signals

2 · Spectrum: how the energy spreads across the channel

3 · Echoes: what multipath does to each method

4 · Side by side

Notes and simplifications

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