posed to the IEEE by Lucent Technologies and Harris Semiconductor (now part of Intersil Corp.) in 1998, as a means to raise the IEEE 802.11b data rate to 11 Mbps. Instead of using the Barker code, they proposed to use a set of codes called Complementary Sequences, Radio Communication Basics based on the Walsh/Hadamard transforms (see the Section "Code Division Multiple Access, p. 94"). Using CCK, a chipping code word is chosen from a set of 64 unique codes depending on the value of each 6-bit segment of the input data stream. The encoded data sequence comprises a series of code words, and this chip sequence is modulated onto the RF carrier using one of a variety of modulation techniques that will be described in the Section "Digital Modulation Techniques, p. 93". The main advantage of CCK modulation is spectral efficiency, since each transmitted code word represents 6 input data bits instead of the single bit represented by the Barker code. CCK can achieve 11 Mbps using the same 22 MHz bandwidth used to transmit 1 Mbps with the Barker code. However, the price of this high data rate is complexity. A receiver using the Barker code requires just one correlator to pick out the chipping code, while a CCK system needs 64 correlators, one on the lookout for each of the complementary codes. Direct Sequence Spread Spectrum in the 2.4 GHz ISM Band As noted above, in DSSS the data signal is combined with a code word, the chipping code, and the combined signal is used to modulate the RF carrier, resulting in a transmitted signal spread over a wide bandwidth. For example, in the 2.4 GHz ISM band, a spread bandwidth of 22 MHz is specified for IEEE 802.11 networks, as shown in Figure 4-9. The 2.4 GHz ISM band has a total allowed width of 83.5 MHz and is divided into a number of channels (11 in the USA, 13 in Europe, 14 in Japan), with 5 MHz steps between channels. To fit 11 or more 22 MHz 22 MHz channel bandwidth 25 MHz channel separation 2.400 2.410 2.420 2.430 2.440 2.450 2.460 2.470 GHz 11 channels in 5 MHz steps Figure 4-9: