ISM emission rules and maximum throughput

Spectrum allocation policies are the limiting factor of frequency-hopping 802.11 systems. As an example, consider the three major rules imposed by the FCC in the U.S.:[2] [2] These rules are in rule 247 of part 15 of the FCC rules (47 CFR 15.247). 1. There must be at least 75 hopping channels in the band, which is 83.5-MHz wide. 2. Hopping channels can be no wider than 1 MHz. 3. Devices must use all available channels equally. In a 30-second period, no more than 0.4 seconds may be spent using any one channel. Of these rules, the most important is the second one. No matter what fancy encoding schemes are available, only 1 MHz of bandwidth is available at any time. The frequency at which it is available shifts continuously because of the other two rules, but the second rule limits the number of signal transitions that can be used to encode data. With a straightforward, two-level encoding, each cycle can encode one bit. At 1 bit per cycle, 1 MHz yields a data rate of 1 Mbps. More sophisticated modulation and demodulation schemes can improve the data rate. Four-level coding can pack 2 bits into a cycle, and 2 Mbps can be squeezed from the 1-MHz bandwidth. The European Telecommunications Standards Institute (ETSI) also has a set of rules for spread-spectrum devices in the ISM band, published in European Telecommunications Standard (ETS) 300-328. The ETSI rules allow far fewer hopping channels; only 20 are required. Radiated power, however, is controlled much more strictly. In practice, to meet both the FCC and ETSI requirements, devices use the high number of hopping channels required by the FCC with the low radiated power requirements of ETSI

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