802.11 s Modulation Techniques

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802.11's Modulation Techniques

The original 802.11 standard specified two different spread spectrum transmission techniques: DSSS and FHSS. All radio equipment use the 2.4 GHz ISM band, and systems based on the original 802.11 standard provide data rates up to 2 Mbps. This is possible because DSSS utilizes an 11-bit chipping code called the Barker Sequence for signal spreading with modulation being achieved using either binary phase shift keying (BPSK) or quadrature phase shift keying (QPSK) techniques. (For FHSS, a modulation technique called Gaussian frequency shift keying or GFSK is employed.) Furthermore, in the U.S. DSSS deployments provide 11 independent channels by using different predefined chipping codes. (FHSS based implementations provide for 78 different logical channels through different hopping patterns, although in reality fewer channels would be actually usable due to frequency separation requirements.)

FHSS was dropped from the 802.11b specification because it was felt that "direct spread" could handle the tradeoff between wireless devices coexisting with other users, while extracting the greatest capacity from systems that are both power and band limited. Later this aspect of 802.11b underwent modification after the FCC indicated in a Notice of Proposed Rule Making from the FCC published in the year 2001: ET 99-231; FNPRM & ORDER 05/11/01 (adopted 05/10/01); FCC 01-158 Amendment of Part 15 of the Commission's Rules Regarding Spread Spectrum Devices, Wi-LAN, Inc. et al. that it would consider relaxing the spread spectrum requirement on the ISM band in order to abandon the peaceful "coexistence of equipment" requirement (interference rejection) in favor of support for greater wireless network capacity (higher bit-rate transmissions). Therefore, for high bit rates above 2 Mbps (5.5 Mbps to 11 Mbps and higher) 802.11b's purely spread spectrum techniques have been supplanted by CCK modulation so as to provide 4 or 8 bits per transmission symbol. The combination of QPSK and CCK is what enables 802.11b's maximum data rate of 11 Mbps. Lower data rates are accommodated through a dynamic rate shifting scheme. Also, the reader should note that 802.11g supports CCK modulation so as to provide backwards compatibility with 802.11b. (As an option for faster link rates, 802.11g also allows packet binary convolutional coding (PBCC) modulation.)

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