Complementary Code Keying

802.11 direct-sequence systems use a rate of 11 million chips per second. The original DS PHYs divided the chip stream up into a series of 11-bit Barker words and transmitted 1 million Barker words per second. Each word encoded either one bit or two bits for a corresponding data rate of 1.0 Mbps or 2.0 Mbps, respectively. Achieving higher data rates and commercial utility requires that each code symbol carry more information than a bit or two.

Straight phase shift encoding cannot hope to carry more than a few bits per code word. DQPSK requires that receivers distinguish quarter-cycle phase differences. Further increasing the number of bits per symbol would require processing even finer phase shifts, such as an eighth-cycle or sixteenth-cycle shift. Detecting smaller phase shifts is more difficult in the presence of multipath interference and requires more sophisticated (and thus expensive) electronics.

Instead of continuing with straight phase-shift keying, the IEEE 802.11 working group turned to an alternate encoding method. Complementary code keying (CCK) divides the chip stream into a series of 8-bit code symbols, so the underlying transmission is based on a series of 1.375 million code symbols per second. CCK is based on sophisticated mathematical transforms that allow the use of a few 8-bit sequences to encode 4 or even 8 bits per code word, for a data throughput of 5.5 Mbps or 11 Mbps. In addition, the mathematics underlying CCK transforms allow receivers to distinguish between different codes easily, even in the presence of interference and multipath fading. Figure 12-16 illustrates the use of code symbols in CCK. It is quite similar to the chipping process used by the slower direct-sequence layers; the difference is that the code words are derived partially from the data. A static repeating code word such as the Barker word is not used.

Figure 12-16. Code symbols in CCK

Barker spreading, as used in the lower-rate, direct-sequence layers, uses a static code to spread the signal over the available frequency band. CCK uses the code word to carry information, as well as simply to spread the signal. Several phase angles are used to prepare a complex code word of eight bits.