ls (Figure 5.1-9). and a possible channel arrangement in the new bands, are The availability of more than three channels is a sig- shown in Figure 5.1-17. The assumed channel shape is nificant benefit for implementing networks that are based on the spectral mask in Figure 5.1-16. We can see intended to provide complete coverage to a large con- that the old definition had room for as many as 12 non- tiguous area. With four nonoverlapping channels, one can overlapping channels, though operation at the high band construct a network of individual access points according at full power in the lowest and highest channels would to a frequency plan that ensures that access points on the result in out-of-band emissions exceeding FCC same channel are separated by at least four cell radii; with seven channels to work with, a minimum spacing of 5.5 radii is achieved (Figure 5.1-18). Thus, interference be- tween adjacent cells is minimized, and each cell is able to -20 dB provide full capacity to its occupants. With the large number of independent channels now available, in- -28 dB dividual access points can be allocated more than one channel each to increase capacity in heavily used cells -40 dB while still maintaining minimal interference. Coding in 802.11a is somewhat similar to the PBCC 9 MHz option of the 802.11 PHY: after scrambling, incoming 11 MHz bits are processed by rate 1/2 convolutional encoder as 20 MHz shown in Figure 5.1-19. Like the PBCC encoder, the 802.1 code has a six-stage shift register, though the 30 MHz sequence of taps that define the output bits (formally, the generating polynomials of the code) have been Figure 5.1-16 802.11a Transmit spectral mask. changed. Codes with rates of 3/4 and 2/3 (fewer output CHAPTER 5.1 Wireless local area networks 5.725 5.825 30 MHz 30 MHz 20 MHz 20 MHz channel: channel: 149 153 157 161 FCC 03-287 11/03 5.470 5.725 5.825 0 MHz 20 MHz channel: channel: 2004+ Figure 5.1-17 Possible nonoverlapping channel assignments in old and new UNII bands. bits per input bit and thus hi