IEEE 802.11 Frequency Hopping Spread Spectrum

IEEE 802.11 Frequency Hopping Spread Spectrum
Originally, 802.11 accommodated two transmission strategies: frequency hopping spread spectrum
(FHSS) and direct-sequence spread spectrum (DSSS).
The 802.11 FHSS PHY was meant to operate in the 2.4-GHz band at speeds of 1 or 2 Mbps. The
FHSS system was discarded by the 802.11b standards after it was found that having two transmission
techniques for one standard meant that two kinds of (incompatible) equipment were necessary to
implement the standard, and DSSS turned out to be the more reliable technique [465]. Legacy 802.11
FHSS equipment is not compatible with the current standards. Nevertheless, a discussion of FHSS is
in order here, if only to see why DHSS came to be preferred for 802.11 networks.
When using the FHSS method, a transmitter shifts the center frequency of a signal several times
per second, and both transmitter and receiver remain synchronized because the “hops” take place
according to a pseudorandom pattern, which each device knows. In the United States, the FCC
stipulates that at least 75 discrete frequencies must be employed for each transmission channel, and
that a signal cannot remain on any particular frequency for more than 400 ms. In the 802.11, the
maximum length of a packet is around 30 ms, and the hops are 1 MHz apart from one another [454].
FHSS can be employed for both analog and digital communications, but is currently implemented
primarily for digital transmissions [454]. If 75 contiguous frequencies are used, then the bandwidth
required for a transmission is 75 times larger than when only one frequency is used – the spectrum is
spread over a larger portion of the transmission band (hence “frequency hopping spread spectrum”).
The original motivation for developing this technique was a desire to avoid hostile jamming of a
radio signal. If a transmission hops to a jammed frequency, the data sent in vain on that frequency
are resent after the next hop. For wireless networks, FHSS has a desirable “side effect:” It minimizes
the chances that different transmitters on the network will encounter interference from one another;
otherwise the network could potentially disable itself.
The (original) 802.11 standards called for two-level Gaussian frequency shift key (GFSK) modulation
for transmissions at 1 Mbps and four-level GFSK for 2 Mbps [453].