CKK modulation

CCK allows for multi-channel operation in the 2.4 GHz band by virtue of using the existing 802.11 1 and 2 Mbps DSSS channelization scheme. The spreading employs the same chipping rate and spectrum shape as the 802.11 Barker Sequence, allowing for three non-interfering channels in the 2.4 to 2.483 GHz band. Thus CCK modulation provides for a spectrum similar to that of the original 802.11 systems, at least at the low bandwidths. This allows interoperability with the original 802.11's DSSS modulation technique and it also allows for 802.11b multi-channel operation in the 2.4 GHz band using the existing 802.11 DSSS channel structure scheme.
CCK modulation consists of a set of 64 eight-bit code words. As a set, these code words have unique mathematical properties that allow them to be accurately distinguished from one another by a receiver even in the presence of substantial noise and multipath interference (e.g., interference caused by receiving multiple radio reflections within a building). The 5.5 Mbps rate uses CCK to encode 4 bits per symbol, while the 11 Mbps rate encodes 8 bits per symbol. Actually, to attain 11 Mbps CCK modulation, 6 bits of the 8 are used to select one of 64 symbols of 8 chip length for the symbol and the other 2 bits are used by QPSK to modulate the entire symbol. This results in modulating 8 bits onto each symbol. The chipping rate is maintained at 11 million chip bits per second for all modes. Both speeds use QPSK as the modulation technique and signal at 1.375 million symbols per second.

Figure 5.4: This graphic shows how the CCK modulation is formed. Graphic courtesy of Intersil.

The FCC regulations for the ISM band require at least 10 decibel (dB) of processing gain (11 dB for 802.11), which is normally achieved with spread spectrum techniques. CCK can achieve this gain too without having to be a conventional spread spectrum signal. Rather than using one or two 11-bit Barker sequences, CCK uses a series of codes called "complementary sequences." Because there are 64 unique code word sets that can be used to encode the signal, up to 6 bits can be represented by any one particular code word (instead of the single bit represented by a Barker symbol).
The wireless radio transmitter device generates a 2.4 GHz carrier wave (2.4 to 2.483 GHz) and modulates that wave using a various techniques, depending on the circumstances. For a 1 Mbps transmission, BPSK is used (one phase shift for each bit). To accomplish 2 Mbps or greater transmission, more sophisticated QPSK is used. QPSK can encode two bits of information in the same space as BPSK encodes one. The tradeoff is the need for increased power or else one must decrease the range to maintain signal quality.
Unfortunately, the FCC regulates the output power of portable radios to just one Watt; therefore, as the 802.11 transceiver moves away from the radio, the radio must adapt to the situation by using a less complex (and slower) encoding mechanism to send data. Ironically, the CCK code word is modulated with the same QPSK technology that was used in 2 Mbps wireless direct spread radios. This enables an additional 2 bits of information to be encoded in each symbol. Eight binary "chip" numbers are sent for each 6 bits, but each symbol encodes 8 bits thanks to the QPSK modulation. So, for a 1 Mbps transmission, 11 million chip bits per second times 2 MHz equals 22 MHz of spectrum. Likewise, for a 2 Mbps transmission, 2 bits per symbol are modulated with QPSK, 11 million chips per second, and thus you need 22 MHz of spectrum. In short, to transmit at a bit rate of 11 Mbps, you need 22 MHz of frequency spectrum.

802.11's Modulation Techniques

The original 802.11 standard specified two different spread spectrum transmission techniques: DSSS and FHSS. All radio equipment use the 2.4 GHz ISM band, and systems based on the original 802.11 standard provide data rates up to 2 Mbps. This is possible because DSSS utilizes an 11-bit chipping code called the Barker Sequence for signal spreading with modulation being achieved using either binary phase shift keying (BPSK) or quadrature phase shift keying (QPSK) techniques. (For FHSS, a modulation technique called Gaussian frequency shift keying or GFSK is employed.) Furthermore, in the U.S. DSSS deployments provide 11 independent channels by using different predefined chipping codes. (FHSS based implementations provide for 78 different logical channels through different hopping patterns, although in reality fewer channels would be actually usable due to frequency separation requirements.)

FHSS was dropped from the 802.11b specification because it was felt that "direct spread" could handle the tradeoff between wireless devices coexisting with other users, while extracting the greatest capacity from systems that are both power and band limited. Later this aspect of 802.11b underwent modification after the FCC indicated in a Notice of Proposed Rule Making from the FCC published in the year 2001: ET 99-231; FNPRM & ORDER 05/11/01 (adopted 05/10/01); FCC 01-158 Amendment of Part 15 of the Commission's Rules Regarding Spread Spectrum Devices, Wi-LAN, Inc. et al. that it would consider relaxing the spread spectrum requirement on the ISM band in order to abandon the peaceful "coexistence of equipment" requirement (interference rejection) in favor of support for greater wireless network capacity (higher bit-rate transmissions). Therefore, for high bit rates above 2 Mbps (5.5 Mbps to 11 Mbps and higher) 802.11b's purely spread spectrum techniques have been supplanted by CCK modulation so as to provide 4 or 8 bits per transmission symbol. The combination of QPSK and CCK is what enables 802.11b's maximum data rate of 11 Mbps. Lower data rates are accommodated through a dynamic rate shifting scheme. Also, the reader should note that 802.11g supports CCK modulation so as to provide backwards compatibility with 802.11b. (As an option for faster link rates, 802.11g also allows packet binary convolutional coding (PBCC) modulation.)