sequence generator
using the polynomial
x
7
x
4
1 and a 32/33 bias-suppression
encoding algorithm are used to randomize and whiten the data.
The FHSS PMD uses two-level Gaussian frequency shift key (GMSK) modu-
lation to transmit the PSDU at the basic rate of 1 Mbps. Four-level GFSK is an
optimal modulation scheme defi ned in the standard that enables the whitened
PSDU to be transmitted at a higher rate. The value in the PSF fi eld of the PLCP
header is used to determine the data rate of the PSDU.
In
GFSK modulation, the frequency is shifted on either side of the carrier
hop frequency depending on whether the binary symbol from the PSDU is either
a 1 or 0. For two-level GFSK, a binary 1 represents the
upper deviation frequency
(
f
c
f
) from the hopped carrier, and binary 0 represents the lower deviation
frequency (
f
c
f
), where
f
c
is the carrier
hopped frequency and
f
the devia-
tion frequency. The deviation frequency should be greater than 110 kHz for IEEE
802.11 FHSS radios. Four-level GFSK is similar to 2-level GFSK and is used to
achieve a data rate of 2 Mbps in the same occupied frequency bandwidth. The
symbol pairs (1 0, 1 1, 0 1, 0 0) generate four frequency deviations from the
hopped
carrier frequency, two upper and two lower.
A set of hop sequences is defi ned in IEEE 802.11 for use in the 2.4 GHz
frequency band. The channels are evenly spaced across the band over a span of
83.5 MHz. In North America and Europe (except France and Spain) the num-
ber of hop channels is 79. The number of hop channels for Spain and France
are 23 and 35, respectively. In Japan the required number of hop channels is 23.
The hop channels are spaced uniformly across the 2.4 GHz
frequency band occu-
pying a bandwidth of 1 MHz. In North America and Europe (except Spain and
France) the hop channels operate from 2.402 to 2.480 GHz and in Japan, 2.473 to
2.495 GHz. In Spain the hop channels operate from 2.447 to 2.473 GHz, and in
France from 2.448 to 2.482 GHz. Channel 2 is the fi rst
hop channel located at the
center frequency of 2.402 GHz and channel 95 is the last hop frequency channel at
2.495 GHz in the 2.4 GHz band.
Channel hopping is controlled by FHSS PMD. The FHSS PMD transmits the
whitened PSDU by hopping from channel to channel in a pseudo-random fashion
using one of the hopping sequences.
In the United States, FHSS radios hop a mini-
mum 2.5 hops per second for a minimum hop distance of 6 MHz.
PLCP-Preamble
PLCP Header
SYNC
80-bits
SFD
16-bits
PLW
12-bits
PSF
4-bits
Header Error
Check 16-bits
Whitened
PSDU
PPDU
