In 802.11 classic DSSS, each data bit is multiplied by quency. The boxes represent the maximum allowed an 11-chip Barker sequence, shown in Figure 5.1-7. power density; the smooth line is a cartoon of a typical (Here data are shown as +1 or -1 both to assist the power spectrum. Observe that although the standard reader in thinking in terms of multiplication and due to allows the spectrum to be as much as 22 MHz wide, the direct translation of the result into binary phase-shift a typical real signal would have a bandwidth measured, keying [BPSK] transmitted chips; a formally equivalent for example, 10 dB from the peak of around 16 MHz. treatment in terms of modulo-2 addition of binary bits is The nominal center frequency for a transmission is also possible.) Thus, a 1 bit becomes the sequence chosen from a set of channel frequencies spaced by (+1-1 +1 -1) and a 0 bit 5 MHz, shown in Table 5.1-1. (In the United States, becomes (-1+1-1-1+1-1-1-1+1+1+1) channels 1-11 are available; channels 1-13 are allowed in The Barker sequence is chosen for its autocorrelation most European jurisdictions, and channel 14 is for properties: if two Barker sequences offset in time are Japan.) Note that if the bandwidth is 16 MHz, 1 In a remarkable example of unexpected ingenuity, frequency hopping was invented by the Austrian-born actress Hedy Lamarr, aided by composer George Antheil (U.S. patent 2,292,387), though she received little credit during her lifetime for this achievement. Wireless local area networks CHAPTER 5.1 Table 5.1-1 802.11 Channels. Channel f (GHz) 16 MHz 2.412 -30 dB -50 dB 2.417 2.422 11 MHz 2.427 22 MHz 2.432 Figure 5.1-8 802.11 spectral mask. 2.437 2.442 transmissions on adjacent channels will interfere with 2.447 each other quite noticeably. A separation of five channels (25 MHz) is needed to remove most overlap; thus, 2.452 there are actually only three nonoverlapping channels 2.457 available in the United States, channels 1, 6, and 11 (Figure 5.1-9). 2.462 Two data rates are supported in 802.11 classic: 1 and