To minimize interference in this crowded spectrum the

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To minimize interference in this crowded spectrum, the Part 15 rules specify that all transmissions with a power level exceeding 0 dBm (1 mW) must utilize either frequency hopping or direct sequence spread spectrum techniques. The FCC is in the process of modifying the Part 15 rules to allow Orthogonal Frequency Division Multiplexing (OFDM) as well. All of these techniques serve the same purpose, to spread the signal’s power over a wider range of frequencies, thus reducing the average power density of the signal at any particular frequency and minimizing interference to other signals occupying the same band. A brief explanation of these signal-spreading techniques follows: Frequency Hopping Spread Spectrum (FHSS) 7 was originally conceived as a means to hide a transmission from unwanted listeners. It is now utilized for another purpose - the reduction of interference. Frequency hopping works by transmitting the signal carrier for a short period of time on one narrow band, then hopping to another, and so on. Over a period of time, the average signal power is thus spread over a very wide band of frequencies. The frequency hops appear random to anyone who doesn’t know the pre-arranged hop pattern. Fifty years ago this made it impossible to tune in and listen to a transmission, because the signal carrier never stayed on one frequency long enough for the listener to locate it and retune the receiver to the new frequency. Today, wireless LANs or PANs that incorporate FHSS do so on predetermined hopping sequences that are not secret, and the technology to follow the hopping pattern and retrieve the signal is available for the cost of a wireless card. Thus, FHSS, as employed by wireless LANs and PANs, no longer offers any inherent security. It does, however, serve to reduce interference to and from other devices. Direct Sequence Spread Spectrum (DSSS) 8 is a more complex technique which spreads the signal’s power across a wider bandwidth by spreading the carrier itself, instead of rapidly moving it around as FHSS does. It does this by directly modulating the carrier
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11 with a high-speed code sequence which has the characteristics of pseudo-random noise (PN). The faster a carrier is modulated, the wider its bandwidth becomes. The spreading sequence is produced by modulating the data stream with a PN spreading code, thus resulting in a signal which has a much higher bandwidth than the information bandwidth alone. For example, with 1 and 2 Mbps 802.11 DSSS, each bit of data is logically combined with an 11-bit Barker code. Because the bit-rate (chip-rate) of the spreading sequence is much higher than that of the data rate, the bandwidth is effectively spread over a much larger area than would otherwise be occupied if the carrier was modulated by the data stream alone. The result is that the signal power is spread over a much larger band and appears to other users as low-power noise. Orthogonal Frequency Division Multiplexing (OFDM) 9 , which utilizes multiple carriers (referred to as subcarriers), is technically not a spread spectrum technique because the subcarriers remain stationary and are not spread, but it serves the same purpose of spreading the signal power over a large band. It does this by breaking the
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  • Spring '12
  • BryanJensen
  • ........., IEEE 802.11, WLANs

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