CCK: complementary code keying

How 802.11b went from 2 to 11 Mbit/s without a wider channel or a faster chip rate: by letting the data choose the code.

1. DefinitionWhat is complementary code keying?

CCK builds an 8-chip code word from the data bits using a fixed equation, then sends the chips at the same 11 million chips a second as the Barker-coded rates. Because the code word itself carries information, each symbol can carry 4 bits (5.5 Mbit/s) or 8 bits (11 Mbit/s).

2. PurposeWhy it was invented

The 1997 standard stopped at 2 Mbit/s. Wi-Fi needed a faster rate that fitted the same channel and the same radios and could coexist with them. CCK did that: 8 chips per symbol instead of 11 gives a faster symbol rate, and the code-word choice carries extra bits.

3. MechanismBuilding a code word

Four phase angles, phi 1 to phi 4, are made from the data bits. A fixed equation then combines them into eight chips:

c = { e^j(p1+p2+p3+p4), e^j(p1+p3+p4), e^j(p1+p2+p4), -e^j(p1+p4), e^j(p1+p2+p3), e^j(p1+p3), -e^j(p1+p2), e^j p1 }

• Phase 1 comes from the first two bits using DQPSK: it rotates the whole symbol relative to the previous one.
• Phases 2, 3 and 4 come from the other bits (two bits each at 11 Mbit/s) and shape the code word.
• The minus signs on chips 4 and 7 are a fixed cover code that improves the correlation properties.
• Every chip ends up as one of four values: 1, j, -1 or -j.

5. MathematicsWhere 11 Mbit/s comes from

• Chip rate 11 Mchip/s / 8 chips per symbol = 1.375 Msym/s.
• 11 Mbit/s: 8 bits per symbol x 1.375 Msym/s = 11 Mbit/s. 5.5 Mbit/s: 4 bits per symbol x 1.375 Msym/s = 5.5 Mbit/s.
• Number of code words: 4 phases of phi 1 x 64 combinations of phi 2 to 4 = 256 at 11 Mbit/s.
• Receiver: correlate against the 64 possible words (phase 1 is removed by taking the magnitude). The right word scores 8. At 11 Mbit/s the best wrong word still scores up to 5.7 (computed over all 64 words), while at 5.5 Mbit/s the four words are completely distinct (0).

6. Real-world behaviourWhat CCK means on a network

• Rate fallback. The 11 Mbit/s words are closer together, so the receiver needs a cleaner signal for them. As the signal weakens a link steps down to 5.5, then 2, then 1 Mbit/s.
• Limited speed. Even 11 Mbit/s is far below modern Wi-Fi rates, and the chips are short (about 91 nanoseconds), so an echo only a few chips late smears into the next symbol.
• Compatibility. 802.11g kept DSSS/CCK so that 802.11b devices keep working, which is why old rates still affect modern 2.4 GHz networks.

Practical validationIn a capture

• Look at the data rate of data frames (wlan_radio.data_rate): 5.5 or 11 Mbit/s frames are CCK.
• Watch a client walk away from an access point: the data rate steps down through the CCK and DSSS rates if OFDM rates are disabled.
• Question to answer: why does disabling the CCK rates help a 2.4 GHz network, and what do you lose?

Check your understandingQuick quiz

ReferencesSources and further reading

• IEEE Std 802.11-2020 (IEEE Xplore)
• IEEE 802.11 Working Group: standards timelines (approval and publication dates)
• IEEE 802.11 Working Group

Facts checked against IEEE Std 802.11-2020: clause 16.3.6.6 (Equation 16-1, the cover code, and Tables 16-8 to 16-11 for the DQPSK and QPSK phase mappings). The correlation figures are computed from the equation over all code words.